ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Wed, 18 Sep 2019 09:31:58 GMT2019-09-18T09:31:58Z50521- Effects of incidence and afterbody shape on flow past bluff cylindershttps://scholarbank.nus.edu.sg/handle/10635/58226Title: Effects of incidence and afterbody shape on flow past bluff cylinders
Authors: Luo, S.C.; Yazdani, Md.G.; Chew, Y.T.; Lee, T.S.
Abstract: In the present paper the effects of the afterbody shape and the angle of incidence on the structure of the flow past a prismatic body are experimentally investigated, both quantitatively in the form of wind tunnel measurement and qualitatively in the form of water tunnel flow visualization. Four cross-sectional shapes with identical upstream facing side and streamwise dimension were chosen in the present investigation. They were a square, two trapeziums, and a triangle. By studying the structure of the flow associated with the above-mentioned shapes, a systematic investigation in which the effects of the afterbody are gradually reduced can be carried out. From the measured magnitude and frequency of the fluctuating aerodynamic forces, the main effect of differences in afterbody shape is the proximity of the two separated shear layers to each other and to their corresponding side faces. This proximity difference, in turn, results in differences in the normal force, the gradient of its variation with the angle of incidence, and, hence, the susceptibility of the shape (with respect) to flow-induced oscillation. Also, due to the difference in interaction between the separated shear layers and the sides of the prismatic structure, the vortex formation length, the base pressure and, hence, the axial force, the rate at which vortices are shed and, hence, the frequency of the aerodynamic loading on the structure and the longitudinal vortex spacing also vary. Changes in the angle of incidence can be viewed as similar to changes in the afterbody shape and, hence, its effects are similar to those caused by changing the afterbody shape. Experiments show that no cross-sectional shape is absolutely stable to galloping oscillation because a shape that is stable to galloping oscillation at a certain mean angle of incidence may become unstable at a different mean angle of incidence. By idealising the vortex wake as two parallel rows of vortices, various vortex street parameters, including the vortex spacing ratio and the strength of each vortex in the wake, are estimated. They show a dependence on the afterbody shape and the angle of incidence. © 1994.
Thu, 01 Dec 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/582261994-12-01T00:00:00Z
- Helical-groove and circular-trip effects on side forcehttps://scholarbank.nus.edu.sg/handle/10635/58345Title: Helical-groove and circular-trip effects on side force
Authors: Lua, K.B.; Lim, T.T.; Luo, S.C.; Goh, E.K.R.
Abstract: When a slender body, such as a missile, is pitched at high angle of attack to an oncoming flow, it may experience a large side force due to the asymmetric shedding of the tip vortices. The side force is well known to be highly detrimental to the performance of the flight vehicle. We assess the effectiveness of two control devices, namely, the circular trips and the helical grooves, in alleviating the side force on a tangent ogive nose cylinder. Simultaneous side force and pressure measurements taken in a wind tunnel show that the circular trip is generally more effective in reducing the side force than the helical grooves over a wide range of angle of attack. Detailed findings of their performances are reported.
Fri, 01 Sep 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/583452000-09-01T00:00:00Z
- Flowfield around ogive/elliptic-tip cylinder at high angle of attackhttps://scholarbank.nus.edu.sg/handle/10635/58297Title: Flowfield around ogive/elliptic-tip cylinder at high angle of attack
Authors: Luo, S.C.; Lim, T.T.; Lua, K.B.; Chia, H.T.; Goh, E.K.R.; Ho, Q.W.
Abstract: We present the results of experimental investigations on the flowfield around a conventional sharp-nose ogive cylinder and an elliptic-tip ogive cylinder. The studies include simultaneous side-force and surface pressure measurements in a wind tunnel as well as flow visualization in a water tunnel. The results show that changes in the direction of the side force are related to changes in the asymmetry of the pressure distribution along the body. Of the two tip shapes investigated, it is found that the variation of the side force with the roll angle for the elliptic tip is more predictable than that for the sharp ogive tip. Although the flow visualization study shows that the elliptic-tip cylinder with the major axis transverse to the freestream is more effective in delaying the onset of flow asymmetry to a higher angle of attack, the maximum side forces for the two tip geometries are almost the same.
Thu, 01 Oct 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/582971998-10-01T00:00:00Z
- Flow past a wind-assisted ship propulsion devicehttps://scholarbank.nus.edu.sg/handle/10635/58296Title: Flow past a wind-assisted ship propulsion device
Authors: Low, H.T.; Luo, S.C.; Winoto, S.H.
Abstract: An experimental investigation has been carried out on a wind-assisted ship propulsion device (WASP), consisting of a cylinder-flap wing with boundary layer suction. Measurements of the pressure distribution, lift and drag have been presented for various angular locations of the suction area and flap. It was found that a lift coefficient of 4.2 is obtained, with suction coefficient of 0.09, when the suction area and flap are symmetrically placed at angular locations of 110° from the relative wind axis; the corresponding lift-drag ratio is 6. Also presented is a polar plot which ascertained that the propulsion force is not significantly affected by small variations (±10°) in the angular position of the flap. However, a corresponding variation in the suction location will significantly affect the propulsion force. © 1991.
Tue, 01 Jan 1991 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/582961991-01-01T00:00:00Z
- Deformation of the upper surface of an airfoil by macro fiber composite actuatorshttps://scholarbank.nus.edu.sg/handle/10635/73312Title: Deformation of the upper surface of an airfoil by macro fiber composite actuators
Authors: Debiasi, M.; Bouremel, Y.; Khoo, H.H.; Luo, S.C.
Abstract: In this follow-on study, macro fiber composite actuators are used to change the shape of the upper surface of an airfoil model with geometry close to that of the NACA 4415 type. In the design discussed, these thin and light piezoelectric actuators are bonded to the inside and become an integral part of the skin of the upper surface of the airfoil. The model used in this study incorporates some structural changes that allow a smoother shaping of the surface closer to the leading edge of the airfoil. Still-air and wind-tunnel measurements in different flow regimes are performed to assess the characteristics of the deformation of the upper surface. The results obtained can be used to design a wing with morphing upper surface for improved aerodynamics, for maneuvering without ailerons, and/or for active control of the flow over the wing. © 2012 by Temasek Laboratories - National University of Singapore.
Sun, 01 Jan 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/733122012-01-01T00:00:00Z
- Galloping of prismatic bodies with different afterbody shapeshttps://scholarbank.nus.edu.sg/handle/10635/92827Title: Galloping of prismatic bodies with different afterbody shapes
Authors: Lee, T.S.; Luo, S.C.; Chew, Y.T.; Yazdani, M.G.
Abstract: Many offshore and polar structures consists of slender prismatic bodies with different cross-sectional shapes. In an ISOPE-93 article by Yazdani et. al (1993) it was shown that at a mean angle of attack [ α ] of zero degrees square cylinder become unstable to galloping oscillation. However, at this same condition, trapezoidal and triangular cylinders exhibit a stable and neutrally stable condition to galloping oscillation respectively. In the present paper, the conditions of galloping stability of cylinders with the same three shapes are examined experimentally at both zero and non zero α. All the experiments were carried out in an open loop wind tunnel. At the experimental wind speed the level of turbulence in the tunnel was no more than 0.5%, which is considered equivalent to a calm sea-state. Dynamic tests, where the cylinder was forced to oscillate in a direction that is transverse to the free stream, were also carried out at amplitude [ a], oscillation frequency [fN] and free stream velocity [UINF]. While a/d (d = cylinder cross-stream dimension) was kept constant at a value of 1 in the experiment, fN and UINF were varied over the range of 1 Hz to 3 Hz and 5 m/s to 20 m/s respectively. The reduced velocity [Ur=UINF/(fNd)] and Reynolds number therefore varied in the range of 33.34 to 400 and 1.5 × 104 to 6.0 × 104 respectively. Dynamic tests were carried out at α of 5°, 10°, 14°, 20° and 30° for the trapezoidal and at α = 5°, 10°, 15°, 22° and 35° for the triangular cylinder. By analyzing the normal force [CN] versus α curves and in particular the sign and magnitude of ∂CN/∂α at different α, it was observed that there exists several ranges of α within which galloping instability for these three shapes becomes possible.
Sat, 01 Jan 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/928271994-01-01T00:00:00Z
- Unsteady fluid force acting on oscillating square-section cylinderhttps://scholarbank.nus.edu.sg/handle/10635/92812Title: Unsteady fluid force acting on oscillating square-section cylinder
Authors: Luo, S.C.; Chew, Y.T.; Goh, E.Y.
Abstract: The difference between experimentally measured lift forces under dynamic condition and quasi-steady condition is termed as an unsteady force, in this technical note. Analysis shows the magnitude of the unsteady force increases with the maximum angle of incidence, αmax encountered by the square cylinder during a cycle of transverse oscillation in a fairly anticipated manner; the phase between the unsteady force and the cylinder displacement only deviates significantly from zero over the range of 8°≤αmax≤16°. In this (αmax) range the unsteady force contributes strongly to the driving force for transverse galloping and cannot be neglected. An attempt had been made to incorporate the unsteady force term into the quasi-steady theory; preliminary results show this leads to an improvement in the agreement between theory and other independent experimental measurements.
Sun, 01 Jan 1995 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/928121995-01-01T00:00:00Z
- Bi-stable flow associated with two tandem unequal diameter cylindershttps://scholarbank.nus.edu.sg/handle/10635/92822Title: Bi-stable flow associated with two tandem unequal diameter cylinders
Authors: Luo, S.C.
Abstract: It was found that just like the case of flow past two tandem cylinders of equal diameter, a bi-stable flow situation where the flow switches intermittently between the reattached type and the co-shedding type also exists when the two tandem cylinders have different diameters. The diameter ratio of the cylinders used in the present investigation is 0.33 with the smaller cylinder upstream. It was also found that when the flow is bi-stable, the probability that the flow is of the co-shedding type Pr varies with the centre to centre spacing between the cylinders l/d2 from 0 to 1 over the bi-stable range and their relation is governed by a normal distribution. The range of the bi-stable flow in l/d2 is Reynolds number dependent. When Reynolds number increases, both the lower and upper limits of the bi-stable flow range reduce.
Tue, 01 Jan 1991 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/928221991-01-01T00:00:00Z
- Vortex wake of a transversely oscillating square cylinder: A flow visualization analysishttps://scholarbank.nus.edu.sg/handle/10635/92814Title: Vortex wake of a transversely oscillating square cylinder: A flow visualization analysis
Authors: Luo, S.C.
Abstract: The flow past a transversely oscillating square cylinder is visualized by the smoke wire technique and studied. In the present investigation, by varying the cylinder oscillating frequency fN in a free stream with constant velocity U, the reduced velocity Ur varies from 7.65 to 97.38. Under the above conditions and at a constant amplitude of oscillation of A/d = 0.675, four different flow structures were identified. They are called lock-on type A, lock-on type B, triple lock-on and quasi-steady in the present paper. The way that vortices are formed and shed are different in these four flow structures. The above results in different wake structures with rather different magnitudes of longitudinal and lateral vortex spacing (a/d and b/d, respectively). It was also found that the vortex shedding mechanism proposed by Gerrard is applicable only to lock-on type B and quasi-steady, but not the other two. The vortex spacing ratio b/a of 0.281 predicted by Von Karman does not apply to the present flow situations. During lock-on, flow visualization reveals that when fN decreases in magnitude at a constant A/d, both a/d and Γ (circulation associated with each vortex in the wake) increase in magnitude. Under certain flow conditions the unconventional thrust type of vortex street exists. The position of the cylinder within its oscillation cycle when vortices are shed was observed to be a factor that affects the magnitude of the lateral vortex spacing and whether the cylinder is susceptible to flow induced oscillation. © 1992.
Tue, 01 Dec 1992 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/928141992-12-01T00:00:00Z
- Effects of Probe Interference on Side Force of an Inclined Ogive Cylinderhttps://scholarbank.nus.edu.sg/handle/10635/85084Title: Effects of Probe Interference on Side Force of an Inclined Ogive Cylinder
Authors: Ng, Y.T.; Lim, T.T.; Luo, S.C.; Lua, K.B.
Abstract: The effects of probe interference on the side force have been investigated and quantified for an ogive cylinder at α=45 deg.. The experiment was carried out in an open-loop suction wind tunnel with a rectangular test section measuring 0.6 m (height)×1.0 m (width). The results show that when the probe was in close proximity to the vortices, there was either an attenuation or amplification of the side forces, which caused an error in the side-force coefficient that can reach as high as 20% of the corresponding value without the probe. The reference values for the side-force coefficient C Fy0 were obtained.
Sun, 01 Feb 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/850842004-02-01T00:00:00Z
- Aerodynamic forces and moments on an ogive cylinder at incidencehttps://scholarbank.nus.edu.sg/handle/10635/84847Title: Aerodynamic forces and moments on an ogive cylinder at incidence
Authors: Luo, S.C.; Ng, Y.T.; Lim, T.T.
Abstract: In flows over an ogive cylinder placed at incidence, it is well documented that a side force acts on the cylinder. However, little is known about the relation between the side force (CFy) and other force and moment components. In this note, the variations of three force and three moment components, acting on the ogive cylinder, with roll angle are measured simultaneously. The model was placed at four different angles-of-attack, namely α = 30°, 45°, 50° and 60° and the results show that in addition to CFy, the variation of CFz, CMx, and CMz with the roll angle also exhibit the square wave like behaviour at α = 45° and 50°, and with the same cross over ø positions, while CFx and CMy remain relatively constant irrespective of the roll angle. The magnitudes of CFx and C My were found to increase with angle of attack and were thought to be due to the increase in normal frontal area.
Tue, 01 Jun 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/848472004-06-01T00:00:00Z
- Discrete vortex simulation of the separated flow around a rotating circular cylinder at high Reynolds numberhttps://scholarbank.nus.edu.sg/handle/10635/92681Title: Discrete vortex simulation of the separated flow around a rotating circular cylinder at high Reynolds number
Authors: Cheng, M.; Chew, Y.T.; Luo, S.C.
Abstract: The separated flow around a rotating circular cylinder is investigated by the discrete vortex method combined with the boundary layer theory. The Keller Box method is used to solve the laminar boundary layer in order to determine the separation points on the upper and the lower sides of the rotating circular cylinder. The nascent vortices are then introduced near the separation points. A discrete circular vortex blob model, which has uniform vorticity distribution, is adopted to simulate the unsteady wake. Numerical experiments are conducted to investigate the cases in which the ratio of the speed at the cylinder surface to the speed at infinity is varied from 0 to 0.3 at a Reynolds number of 6 × 103. The calculated values of the separation positions, the drag and lift force coefficients, the velocuty and the pressure distribution on the cylinder surface are found to agree well with the published experimental data. © 1994.
Thu, 01 Dec 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/926811994-12-01T00:00:00Z
- Side force on an ogive cylinder: Effects of control deviceshttps://scholarbank.nus.edu.sg/handle/10635/58692Title: Side force on an ogive cylinder: Effects of control devices
Authors: Lee, A.S.; Luo, S.C.; Lim, T.T.; Lua, K.B.; Goh, E.K.R.
Abstract: This study is an extension of our earlier work, which examined the effectiveness of using an elliptic tip to control the side force acting on an ogive cylinder. In that study, only one tip was considered, and thus the effect of tip eccentricity on the side force was not known. In the present study, we examine another elliptic tip of a smaller eccentricity to get an insight into how tip eccentricity affects the local and overall side force distribution. Our measurements show that, although the smaller eccentricity tip has a side force distribution similar to that of the larger eccentricity tip, there are some major differences in their flow characteristics. For example, the larger eccentricity tip is found to reduce the onset angle of attack and delay the disappearance of the side force to a higher angle of attack. Furthermore, when α≈60 deg, only the lower eccentricity tip displays a hysteresis effect in its side force distribution. To the best of our knowledge, this phenomenon has not been observed on an elliptic tip before, even though a similar phenomenon has been observed on a conical body with a rounded tip when the cone was subjected to unsteady bleeding.
Wed, 01 Mar 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/586922000-03-01T00:00:00Z
- Stability to translational galloping vibration of cylinders at different mean angles of attackhttps://scholarbank.nus.edu.sg/handle/10635/58722Title: Stability to translational galloping vibration of cylinders at different mean angles of attack
Authors: Luo, S.C.; Chew, Y.T.; Lee, T.S.; Yazdani, M.G.
Thu, 03 Sep 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/587221998-09-03T00:00:00Z
- Vortex street wakes of a rotating circular cylinderhttps://scholarbank.nus.edu.sg/handle/10635/75097Title: Vortex street wakes of a rotating circular cylinder
Authors: Cheng, M.; Chen, Y.T.; Luo, S.C.
Abstract: The evolution of the vortex street behind a rotating circular cylinder in uniform free stream is investigated numerically at high Reynolds number. The ratio of the cylinder surface velocity to the free stream velocity, α is in the range 0 ≤ α ≤ 3. The method used to calculate the flow can be considered as a combination between the diffusion-vortex method and the vortex-in-cell method. The lift and drag forces exerted by the fluid on the cylinder surface as well as the Strouhal number of vortex shedding, are determined together with the flow patterns in the wake. It was found that the vortex shedding and wake development behind the cylinder vary significantly depending on the magnitude of the rotation parameter α. When α ≤2. the vortex street behind the cylinder in the near-wake inclines as a whole towards the direction of rotation as α increases. The Karman vortex street structure begins to deteriorate as soon as α exceeds 2 and finally disappears for α =3.
Sat, 01 Jan 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/750971994-01-01T00:00:00Z
- Aerodynamic stability of square , trapezoidal and triangular cylindershttps://scholarbank.nus.edu.sg/handle/10635/74955Title: Aerodynamic stability of square , trapezoidal and triangular cylinders
Authors: Yazdani, M.G.; Luo, S.C.; Lee, T.S.; Chew, Y.T.
Abstract: The objective of the present study is to investigate the galloping stability of prismatic bodies with different cross-sectional shape ( a square, a trapezium and a triangle) but with the same streamwise and cross-stream wise dimensions, so that the effects of shape can be investigated in isolation.The square cylinder is included in the investigation as a mean of calibrating the entire set-up as there are large numbers of square cylinder data available in the literature.
Fri, 01 Jan 1993 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/749551993-01-01T00:00:00Z
- Shape change of the upper surface of an airfoil by macro fiber composite actuatorshttps://scholarbank.nus.edu.sg/handle/10635/51646Title: Shape change of the upper surface of an airfoil by macro fiber composite actuators
Authors: Debiasi, M.; Bouremel, Y.; Hock Hee, K.; Siao Chung, L.; Tan Zhiwei, E.
Abstract: Macro fiber composite actuators are used to displace inward or outward the upper surface of a NACA 4415 airfoil model. In the design discussed, these thin and light piezoelectric actuators are bonded to the inside and become an integral part of the skin of the upper-surface. Still-air and wind-tunnel measurements in different flow regimes were performed to assess the characteristics of static changes of the shape of the upper surface. The results obtained can be used to design a wing with morphing upper surfaces for improved aerodynamics, for maneuvering without ailerons, or for active control of the flow over the wing. © 2011 by Temasek Laboratories - National University of Singapore. Published by the American Institute of Aeronautics and Astronautics, Inc.
Sat, 01 Jan 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/516462011-01-01T00:00:00Z
- Flow past a finite length circular cylinderhttps://scholarbank.nus.edu.sg/handle/10635/75001Title: Flow past a finite length circular cylinder
Authors: Luo, S.C.
Abstract: In flow past a finite length circular cylinder, experimental data show that the presence of the free end appears to have significant effects on surface pressure only in the range Θ >50°. The wake pressure is more negative near the free end but is normally still less negative than the two-dimensional flow value. The drag coefficient is spanwise position dependent and is larger near the free end. Both the spanwise and overall drag coefficients are lower than the two-dimensional flow drag coefficient. Regular vortex shedding can be detected only at the mounting board half of the h/d = 8 cylinder and the corresponding Strouhal number is slightly smaller than the two-dimensional flow value.
Fri, 01 Jan 1993 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/750011993-01-01T00:00:00Z
- Simulation of flows around a rotating cylinder by a diffusing vortex schemehttps://scholarbank.nus.edu.sg/handle/10635/75081Title: Simulation of flows around a rotating cylinder by a diffusing vortex scheme
Authors: Chew, Y.T.; Cheng, M.; Luo, S.C.
Abstract: The flow development past a rotating circular cylinder in a viscous fluid in investigated. A new diffusion-vortex method is employed and extended to calculate the flow problem. The variations with time of the pressure vorticity distribution, the lift and drag forces exerted by the fluid on the cylinder surface, are determined for the selected values of the Reynolds number and rotation rate, and compared with the published experimental results. The results show a variety of unusual and complex flow patterns around the rotating cylinder.
Fri, 01 Jan 1993 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/750811993-01-01T00:00:00Z
- A Particle Image Velocimetry Study on Elliptical Jets in Cross Flowhttps://scholarbank.nus.edu.sg/handle/10635/73081Title: A Particle Image Velocimetry Study on Elliptical Jets in Cross Flow
Authors: New, T.H.; Lim, T.T.; Luo, S.C.; Soria, J.
Abstract: This paper arises from our earlier flow visualization study of an elliptical jet in a cross flow which showed regular and coherent paring of leading-edge vortices when the elliptical jets are orientated with their major-axes parallel to the cross flow. To obtain a deeper insight into this pairing process, particle image velocimetry technique was used to obtain the instantaneous velocity and vorticity fields. This paper reports our findings.
Mon, 01 Jan 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/730812001-01-01T00:00:00Z
- A visual study on elliptical jets in cross flowhttps://scholarbank.nus.edu.sg/handle/10635/59336Title: A visual study on elliptical jets in cross flow
Authors: New, T.H.; Lim, T.T.; Luo, S.C.
Abstract: Laser-induced fluorescence technique is used to study the near-field of an elliptical jet exiting normally into a cross flow, for elliptic jet having aspect ratio of 2 and 3. Results show that the nonuniform curvature of the elliptic geometry causes the leading-edge vortices at the interface between the jet and the cross flow to behave differently from that of a circular jet. In particular, when the major-axis is aligned with the cross flow there is an intense interaction between the leading-edge vortices which culminates in the pairing of adjacent vortices. The pairing is suppressed when the major axis is perpendicular to the cross flow.
Tue, 01 Jan 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/593362002-01-01T00:00:00Z
- Side force on an ogive cylinder: Effects of freestream turbulencehttps://scholarbank.nus.edu.sg/handle/10635/85625Title: Side force on an ogive cylinder: Effects of freestream turbulence
Authors: Luo, S.C.; Lua, K.B.; Lim, T.T.; Goh, E.K.R.
Abstract: The effects of freestream turbulence on the side forces acting on an ogive cylinder at a high angle of attack were studied. For a fixed angle of attack, an increase in turbulence intensity caused the side force to either decrease or increase, depending on the roll angle position of the surface. A small change in turbulence length scale (Lx/D) greatly influenced the reduction of side forces.
Sat, 01 Dec 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/856252001-12-01T00:00:00Z
- Effects of Strake Position on the Flow past an Inclined Ogive Cylinderhttps://scholarbank.nus.edu.sg/handle/10635/60122Title: Effects of Strake Position on the Flow past an Inclined Ogive Cylinder
Authors: Ng, Y.T.; Luo, S.C.; Lim, T.T.
Abstract: The effectiveness of the use of different strake positions to control vortex asymmetry was investigated. The experiment was performed in an open-loop wind tunnel with a rectangular test section, operating at a freestream velocity. The result shows that a strake placed at the tip of the nose can cause a significant reduction in the side force acting on the ogive cylinder. The oil surface flow visualization which that the presence of a strake alters the position and the shape of the separation line. It was observed that without the strake, the separation line originates slightly downstream of the tip, whereas with the strake placed at the tip, it appears to start at the tip.
Sun, 01 Feb 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/601222004-02-01T00:00:00Z
- Numerical study of a linear shear flow past a rotating cylinderhttps://scholarbank.nus.edu.sg/handle/10635/58546Title: Numerical study of a linear shear flow past a rotating cylinder
Authors: Chew, Y.T.; Luo, S.C.; Cheng, M.
Abstract: The effects of shear rate on flow past a rotating circular cylinder in a linear shear flow have been investigated by using a hybrid vortex method at a Reynolds number of 1000. The velocity gradient of the shear flow K ranges from -0.3 to 0.3 and the rotational to translational speed ratio α is 0.5. The results show that the form of vortex shedding is controlled by the shear parameter K. The drag coefficient decreases with increasing |K|. On the other hand, the lift coefficient and Strouhal number increase as K increases. As K increases, the separation positions shift downstream, the wake becomes narrower and the amplitude of fluctuation of the instantaneous lift and drag coefficients decreases.
Sat, 01 Feb 1997 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/585461997-02-01T00:00:00Z
- Numerical study of flow past a rotating circular cylinder using a hybrid vortex schemehttps://scholarbank.nus.edu.sg/handle/10635/58548Title: Numerical study of flow past a rotating circular cylinder using a hybrid vortex scheme
Authors: Chew, Y.T.; Cheng, M.; Luo, S.C.
Abstract: The vortex shedding and wake development of a two-dimensional viscous incompressible flow generated by a circular cylinder that begins its rotation and translation impulsively in a stationary fluid is investigated by a hybrid vortex scheme at a Reynolds number of 1000. The rotational to translational speed ratio varies from 0 to 6. The method used to calculate the flow is considered as a combination of the diffusion-vortex method and the vortex-in-cell method. The full flow field is divided into two regions. Near the body surface, the diffusion-vortex method is used to solve the Navier-Stokes equations; while the vortex-in-cell method is used in the exterior inviscid domain. The results show that the present method can be used to calculate not only the global characteristics of the separated flow, but also the precise evolution with time of the fine structure of the flow field.
Sun, 01 Jan 1995 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/585481995-01-01T00:00:00Z
- A hybrid vortex method for flows over a bluff bodyhttps://scholarbank.nus.edu.sg/handle/10635/54284Title: A hybrid vortex method for flows over a bluff body
Authors: Cheng, M.; Chew, Y.T.; Luo, S.C.
Abstract: A hybrid vortex method was developed to simulate the two-dimensional viscous incompressible flows over a bluff body numerically. It is based on a combination of the diffusion-vortex method and the vortex-in-cell method by dividing the flow field into two regions. In the region near the body surface the diffusion-vortex method is used to solve the Navier-Stokes equations, while the vortex-in-cell method is used in the exterior domain. Comparison with results obtained by the finite difference method, other vortex methods and experiments shows that the present method is well adapted to calculate two-dimensional external flows at high Reynolds number. It is capable of calculating not only the global characteristics of the separated flow but also the evolution of the fine structure of the flow field with time precisely. The influence of the grid system and region decomposition on the results will also be discussed.
Sat, 15 Feb 1997 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/542841997-02-15T00:00:00Z
- Role of tip and edge geometry on vortex asymmetryhttps://scholarbank.nus.edu.sg/handle/10635/61253Title: Role of tip and edge geometry on vortex asymmetry
Authors: Lim, T.T.; Lua, K.B.; Luo, S.C.
Abstract: Flow visualization studies were performed on an ogive-cylinder to investigate the flow over a delta wing at high incidence. Sharp tip and sharp edges (STSE), Ogive tip and Sharp edges (OTSE) and ogive tip and round edge (OTRE) model were considered for the study. Charge-coupled device (CCD) video cameras were used to capture the flow patterns and asymmetric lifting of the tip vortices from the wing. Investigation results suggested that both axisymmetric ogive tip and edge geometry were responsible for the phenomenon of vortex asymmetry and overall force distribution.
Thu, 01 Mar 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/612532001-03-01T00:00:00Z
- Side force on an ogive cylinder: Effects of surface roughnesshttps://scholarbank.nus.edu.sg/handle/10635/61286Title: Side force on an ogive cylinder: Effects of surface roughness
Authors: Luo, S.C.; Lua, K.B.; Goh, E.K.R.
Abstract: The effects of surface roughness on the side force acting on an ogive cylinder at high incidence were studied. Examination of the circumferential pressure distribution showed revealed that, with the roughened surface at certain roll angles, the boundary layer on both sides of the cylinder appeared turbulent. The results indicted that aluminium powder type of surface roughness used was capable of triggering the laminar turbulent transition in the boundary layers of the ogive cylinders resulting in a side force that fluctuated more with roll angle.
Mon, 01 Jul 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/612862002-07-01T00:00:00Z
- Side force on a rough-surface ogive cylinder: Effects of freestream turbulencehttps://scholarbank.nus.edu.sg/handle/10635/61285Title: Side force on a rough-surface ogive cylinder: Effects of freestream turbulence
Authors: Lua, K.B.; Luo, S.C.; Goh, E.K.R.
Abstract: Side force on a rough-surface Ogive cylinder was discussed. Effects of freestream turbulence were also studied. The turbulent floe was also studied. Results showed that an increase in the turbulence intensity causes the side force to decrease.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/612852003-01-01T00:00:00Z
- Three techniques to control flow separation in an S-shaped ducthttps://scholarbank.nus.edu.sg/handle/10635/61571Title: Three techniques to control flow separation in an S-shaped duct
Authors: Ng, Y.T.; Luo, S.C.; Lim, T.T.; Ho, Q.W.
Abstract: The optimum aerodynamic performance of an S-shaped duct (or, more specifically, aircraft air-intake duct) demands relatively uniform flow in the duct with minimal swirl at the exit. Here, three different flow control methods are examined for a large curvature S-shaped duct of uniform square cross section; they are vortex generators, variable directional tangential blowing, and vortex generator jets. The flow control devices were implemented on a common test rig with the same test parameters and hence provide a more accurate comparison of their performance. While these devices are effective at suppressing flow separation at the inside wall of the first bend and reducing total pressure loss, it is shown that this happens at the expense of higher swirl at the duct exit. The increase in the swirl can be attributed partly to the increase in the radial pressure difference (or imbalance) between the side walls and partly to changes in the configuration and position of streamwise vortices near the outside wall of the second bend. The investigation thus illustrates competing aerodynamic performance when flow control devices are implemented in an S-shaped duct. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc.
Thu, 01 Sep 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/615712011-09-01T00:00:00Z
- On using high-order polynomial curve fits in the quasi-steady theory for square-cylinder gallopinghttps://scholarbank.nus.edu.sg/handle/10635/86131Title: On using high-order polynomial curve fits in the quasi-steady theory for square-cylinder galloping
Authors: Ng, Y.T.; Luo, S.C.; Chew, Y.T.
Abstract: Quasi-steady theory shows that the galloping response of a square cylinder exhibits a hysteresis phenomenon. The equation of motion, which was derived based on a seventh-order polynomial curve fit on the side force (Cy) versus angle of attack (α) curve, shows that the number of positive real roots corresponds to the number of stationary oscillation amplitudes. In this investigation, we use polynomials of even higher order (ninth and eleventh) to curve fit the Cy versus α curve, in an attempt to see if additional positive real roots occur, which may reveal even more flow physics. The results show that only extra negative real roots and/or complex roots are obtained when higher than seventh-order polynomial curve fits are used. Hence, the use of a seventh-order polynomial curve fit in the quasi-steady theory is shown to be sufficient in describing the flow physics which includes the prediction of the hysteresis phenomenon. © 2004 Elsevier Ltd. All rights reserved.
Sat, 01 Jan 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/861312005-01-01T00:00:00Z
- Discontinuities in the S-Re relations of trapezoidal and triangular cylindershttps://scholarbank.nus.edu.sg/handle/10635/73366Title: Discontinuities in the S-Re relations of trapezoidal and triangular cylinders
Authors: Luo, S.C.; Eng, G.R.C.
Abstract: From past research, it is known that even for a very slender cylinder with aspect ratio of several hundreds, the flow remains two-dimensional only at low Reynolds number, and will become three-dimensional once a certain critical Reynolds number is exceeded. Most of the works reported are for circular and square cylinders. The present work is intended to look into wake transition phenomena that take place in the wake of two different trapezoidal and one triangular cylinders. With the experiment set up in a water tunnel, the frequency of vortices shed from trapezoidal and triangular cylinders was carefully measured using hot film anemometry. Experimental data revealed that the discontinuities in the S-Re relation, similar to those observed for circular and square cylinders and which mark the transition of the wake to Mode A and Mode B instabilities, are also present in flow past trapezoidal and triangular cylinders. However, the Reynolds numbers at which the discontinuities take place are cross-sectional geometry dependent. For Trapezoidal Cylinder 1 and 2 with a rear face 75% and 50% the length of the front face, respectively, the two discontinuities in the S-Re relation take place at Re = 152 & 173 and 143 & 176, respectively. For the Triangular Cylinder with apex pointing downstream, the first discontinuity occurs at Re = 164. Some preliminary flow visualization was also carried out to confirm the existence of the streamwise vortices associated with Mode A and B instabilities. © 2010 SPIE.
Fri, 01 Jan 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/733662010-01-01T00:00:00Z
- Effects of jet velocity profiles on a round jet in cross-flowhttps://scholarbank.nus.edu.sg/handle/10635/60112Title: Effects of jet velocity profiles on a round jet in cross-flow
Authors: New, T.H.; Lim, T.T.; Luo, S.C.
Abstract: This paper reports the results of an experimental investigation on the effects of jet velocity profiles on the flow field of a round jet in cross-flow (JICF) using laser-induced fluorescence and digital particle-image velocimetry techniques (DPIV). Tophat and parabolic jets were considered, with the momentum ratios (MRs) ranging from 2.3 to 5.8. Results show that the thicker shear layer associated with a parabolic JICF is able to delay the formation of leading-edge and lee-side vortices when compared to the tophat JICF at the corresponding MR. As a result, there is an increase in jet penetration and a reduction in the near-field entrainment of cross-flow fluid by a parabolic JICF. Also, the less coherent nature of the leading-edge and lee-side vortices in a parabolic JICF is more likely to break up sporadically into smaller-scaled vortices. In addition, DPIV results show that a parabolic JICF exhibits not only a faster velocity recovery of cross-flow fluid at the jet lee-side than the corresponding tophat JICF, it also consistently registers a higher magnitude of the peak average vorticity than the tophat JICF for all MR considered. Despite these differences, the time-averaged flow topology for both cases share many salient features.
Thu, 01 Jun 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/601122006-06-01T00:00:00Z
- Ground effect on flow past a wing with a NACA0015 cross-sectionhttps://scholarbank.nus.edu.sg/handle/10635/85248Title: Ground effect on flow past a wing with a NACA0015 cross-section
Authors: Luo, S.C.; Chen, Y.S.
Abstract: Wing in ground (WIG) effects is an important aerodynamics phenomenon as it affects the aerodynamics of aircraft (during take off and landing), marine crafts like wing in ground effect (WIG) crafts and racing cars. In the present investigation, wind tunnel experiment was conducted on a wing with a NACA0015 cross-section, a chord of 0.24m and an aspect ratio of 1.46. The "ground" was simulated by a vertically mounted 0.6m (streamwise)×0.457m (same height as the wind tunnel test section) perspex plate. The wing was mounted vertically on a load cell, and the load cell on a mechanism that could traverse the wing/load cell set-up in a direction transverse to the free stream, thereby varying the distance between the wing and the ground. Measurements carried out include the pressure distribution on the wing surface at mid-span and forces on the wing, and pressure distribution on the ground. Measurements were carried out at various angles of attack, at a constant wind speed of 12m/s. The corresponding chord based Reynolds Number (Re c) is approximately 1.872×10 5.Mid-span surface pressure distributions were measured on the wing at both positive and negative angles of attack α, and at different ground clearance h/. c. It was observed that for both positive and negative angles of attack, the pressure on the upper surface shows a much stronger angle of attack dependence, and only weak h/. c dependence when the angle of attack is in the negative range. On the other hand, the lower surface pressure is influenced by both the angle of attack and h/. c. Within a certain range of positive angle of attack and h/. c, the "ramming effect" which is associated with a local increase in surface pressure that had been reported in the literature was observed. Streamwise pressure distribution on the ground that corresponds to the wing mid-span shows that when the wing is out of ground effect, the effects of wing angle of attack on the pressure distribution on the ground is not significant. When the ground effect are present (small to moderately small h/. c), pressure distribution on the ground especially within the range -0.3 < x/. c< 1 is affected. Different pressure versus x/. c trends were observed, depending on the magnitude and sign of the wing angle of attack. Both lift coefficients obtained from integrating surface pressure distribution and from direct load cell measurements show that as h/. c reduces, the sign of the lift coefficient remains unchanged but its magnitude increases. A temporary reduction in the lift coefficient is observed when h/. c reduces from about 0.3 to 0.15 in the α= 0-6° range, for both the pressure distribution integration and load cell measurement cases. This is believed to be the consequence of the "convergent-divergent channel effect" reported in surface pressure distribution. Generally speaking the lift estimated from integrating surface pressure distribution is slightly larger than the one measured directly from the load cell. This is consistent with the slightly more 2-D nature of the flow at the wing mid-span. The lift slope from both the pressure and load cell data show h/. c dependence, with the former consistently the larger of the two. Their magnitudes generally lie within the magnitude estimated from the Thin Aerofoil Theory (2-D flow) and Prandtl's Lifting Line Theory (3-D flow). © 2012 Elsevier Inc.
Sun, 01 Jul 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/852482012-07-01T00:00:00Z
- Induced parallel vortex shedding from a circular cylinder at Re of O(104) by using the cylinder end suction techniquehttps://scholarbank.nus.edu.sg/handle/10635/85313Title: Induced parallel vortex shedding from a circular cylinder at Re of O(104) by using the cylinder end suction technique
Authors: Luo, S.C.; Tan, R.X.Y.
Abstract: In the present work, the objective is to attempt to induce parallel vortex shedding at a moderately high Reynolds number (=1.578 × 104) by using the cylinder end suction method, and measure the associated aerodynamic parameters. We first measured the aerodynamic parameters of a single circular cylinder without end suction, and showed that the quantities measured are in good agreement with equivalent data in the published literature. Next, by using different amount of end suction which resulted in increasing the cylinder end velocity by 1%, 2% and 2.5%, we were able to show that the above corresponded to the situation of under suction, optimal suction and over suction, respectively. With optimal suction, we demonstrated that the end suction method works at Re = 1.578 × 104. The shape of the primary vortex shed became straighter than when there is no end suction, and parameters like cylinder surface pressure distribution, drag force per unit span, as well as vortex shedding frequency all showed negligible spanwise variation. Further careful analyses showed that when compared to the naturally existing curved vortex shedding, with parallel vortex shedding the mid-span drag per unit span became slightly smaller, but the drag averaged over the cylinder span became slightly larger. For cylinder surface pressure, it was found that cylinder end effects mainly influenced the surface pressure in the angular ranges -180° ≤ β < -60° and 60° < β ≤ 180°. Without end suction, the cylinder surface pressure in the above ranges was found to increase (become less negative) slightly with |z/d|, but such increase disappeared when optimal end suction was applied, and the cylinder surface pressure distribution became spanwise location independent. As for the vortex shedding frequency (Strouhal number), although the Strouhal number showed spanwise variation when there is no end suction and negligible spanwise variation when optimal suction was applied, the difference between the spanwise averaged Strouhal number was quite negligible. With under suction, the spanwise dependence of various aerodynamic parameters existed, but was found to be not as significant as when no end suction was applied at all. With over suction, the flow situation was found to be practically no change from the optimal suction situation. © 2009 Elsevier Inc. All rights reserved.
Sun, 01 Nov 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/853132009-11-01T00:00:00Z
- Elliptic jets in cross-flowhttps://scholarbank.nus.edu.sg/handle/10635/60159Title: Elliptic jets in cross-flow
Authors: New, T.H.; Lim, T.T.; Luo, S.C.
Abstract: Flow structures of an elliptic jet in cross-flow were studied experimentally in a water tunnel using the laser-induced fluorescence technique (LIF), for a range of jet aspect ratio (AR) from 0.3 to 3.0, jet-to-cross-flow velocity ratio (VR) from 1 to 5, and jet Reynolds number from 900 to 5100. The results show that the effects of aspect ratio (or jet exit orientation) are significant only in the near field, and diminish in the far field which depends only on gross jet geometry. For low-aspect-ratio jets, two adjacent counter-rotating vortex pairs (CVP) are initially formed at the sides of the jet column, with the weaker pair subsequently entrained by the stronger pair further downstream. For high-aspect-ratio jets, only one CVP is formed throughout the jet column, but the shear layer develops additional folds along the windward side of the jet. These folds subsequently evolve into smaller scale counter-rotating vortex pairs, which we refer to as windward vortex pairs (WVP). Depending on its sense of rotation, the WVP can evolve into what Haven & Kurosaka (1997) referred to as unsteady kidney vortices or anti-kidney vortices, or, under some circumstances, interconnecting kidney vortices, which have not been reported previously. While Haven & Kurosaka (1997)'s interpretation of the formation of kidney and anti-kidney vortices is topologically feasible, our observation reveals a slightly different formation process. Despite the differences in the near-field flow structures for different jet aspect ratios, the process leading to the formation of the large-scale jet structures (i.e. leading-edge vortices and lee-side vortices) for all cases is similar to that reported by Lim, New & Luo (2001) for a circular jet in cross-flow.
Mon, 10 Nov 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/601592003-11-10T00:00:00Z
- A new numerical simulation method of high Reynolds number flow around a cylinderhttps://scholarbank.nus.edu.sg/handle/10635/92652Title: A new numerical simulation method of high Reynolds number flow around a cylinder
Authors: Guo, T.; Chew, Y.T.; Luo, S.C.; Su, M.D.
Abstract: A new finite-difference is presented to solve the unsteady two-dimensional Navier-Stokes equations in the vorticity stream function form and tested for the flow around a cylinder at Reynolds number Re of 103-104. The simulation uses a body-fitting Cartesian coordinate system in the physical plane which is transformed by conformal mapping to a grid with uniform mesh sizes in the computational domain. A new mixed difference scheme coupling the 3rd-order upwind scheme with the 4th-order central scheme is used for the discretization of the vorticity transport equation, while a 2nd-order central scheme is used for the discretization of the stream function equation. Some numerical results for flow past a circular cylinder at Re=1000 to 9500 and an elliptic cylinder with different angle of attack are given. This numerical method gives results comparable to those of previously published methods but does do using much less memory and computer time. The ease of setting boundary condition is another advantage of the present method. The influence of the initial condition and the grid system, time step is also discussed.
Mon, 01 Jun 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/926521998-06-01T00:00:00Z
- A flow field study of an elliptic jet in cross flow using DPIV techniquehttps://scholarbank.nus.edu.sg/handle/10635/54157Title: A flow field study of an elliptic jet in cross flow using DPIV technique
Authors: New, T.H.; Lim, T.T.; Luo, S.C.
Abstract: The digital particle image velocimetry (DPIV) technique has been used to investigate the flow fields of an elliptic jet in cross flow (EJICF). Two different jet orientations are considered; one with the major axis of the ellipse aligned with the cross flow (henceforth referred to as a low aspect ratio (AR) jet), and the other with the major axis normal to the cross flow (henceforth referred to as a high aspect ratio jet). Results show that the vortex-pairing phenomenon is prevalent in the low aspect ratio jet when the velocity ratio (VR)≥3, and is absent in the high aspect ratio jet regardless of the velocity ratio. The presence of vortex pairing leads to a substantial increase in the leading-edge peak vorticity compared to the lee-side vorticity, which suggests that vortex pairing may play an important role in the entrainment of ambient fluid into the jet body, at least in the near-field region. In the absence of vortex pairing, both the leading-edge and the lee-side peak vorticity increase monotonically with velocity ratio regardless of the aspect ratio. Moreover, time-averaged velocity fields for both AR=0.5 and AR=2 jets reveal the existence of an "unstable focus" (UF) downstream of the jet, at least for VR≥2. The strength and the location of this focus is a function of both the velocity ratio and aspect ratio. In addition, time-averaged vorticity fields show a consistently higher peak-averaged vorticity in the low aspect ratio jet than in the high aspect ratio jet. This behavior could be due to a higher curvature of the vortex filament facing the cross flow in the low aspect ratio jet, which through mutual interaction may lead to higher vortex stretching, and therefore higher peak-averaged vorticity.
Thu, 01 Apr 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/541572004-04-01T00:00:00Z
- Aerodynamic stability of the downstream of two tandem square-section cylindershttps://scholarbank.nus.edu.sg/handle/10635/57866Title: Aerodynamic stability of the downstream of two tandem square-section cylinders
Authors: Luo, S.C.; Li, L.L.; Shah, D.A.
Abstract: The work reported in the present paper consists of three parts. In part one, the velocity distribution in the wake of a square cylinder at different distances from it (2 ≤ x/D ≤ 12) are measured and reported. Analytical expressions for the wake velocity distribution and for the correlation between wake half-width and downstream distance are obtained. The above expressions make it possible to estimate the wake velocity distribution without the availability of the actual experimental data. In part two, the lift and drag acting on the downstream of two cylinders are measured. The results are found to be in reasonable agreement (except in the range L/D = 3-4 and T/D = 2-3) with previous measurements, and are presented as contours of constant quantities, which make them useful to other researchers for quick information retrieval or estimation. Based on these steady flow results, the region where the downstream cylinder will become unstable to transverse galloping (static instability) are estimated and reported. In the next part, data are acquired with the downstream cylinder undergoing transverse oscillation. From the measurement of the phase angle between the body frequency component of the lift force and the cylinder displacement, the region where the downstream cylinder will be (dynamically) unstable to transverse galloping is estimated, and is found to be in good agreement with the estimation based on the steady flow results in the range L/D ≤ 4. The variations of the mean drag as well as the Strouhal number and fluctuating lift and drag of the downstream cylinder with reduced velocity are also measured at different L/D and A/D, and possible explanations for the behaviour of the data are offered.
Fri, 01 Jan 1999 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/578661999-01-01T00:00:00Z
- On the development of large-scale structures of a jet normal to a cross flowhttps://scholarbank.nus.edu.sg/handle/10635/60983Title: On the development of large-scale structures of a jet normal to a cross flow
Authors: Lim, T.T.; New, T.H.; Luo, S.C.
Abstract: It is well known that vortex rings are the dominant flow structures in the near field of a free jet, and this has led many researchers to believe that they also occur in a jet in cross flow (JICF). Previous studies have postulated that these vortex rings deform and fold as they convect downstream, which culminates in the formation of vortex loops at both the upstream and the lee-side of the jet column. In this paper, we take a fresh look at the vortical structures of JICF in water by releasing dye at strategic locations around the jet exit. The results show that there is no evidence of ring vortices in JICF, and the postulation that vortex loops are formed from the folding of the vortex rings does not reflect the actual flow behavior. The presence of a counter-rotating vortex pair (CVP) at the jet exit is found to inhibit the formation of the vortex rings. Instead, vortex loops are formed directly from the deformation of the cylindrical vortex sheet or jet column, without going through the vortex rings, in a process similar to the buoyant jet and wake structures studied by Perry and Lim [J. Fluid Mech. 88, 451 (1978)]. © 2001 American Institute of Physics.
Thu, 01 Mar 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609832001-03-01T00:00:00Z
- Flow separation of a rotating cylinderhttps://scholarbank.nus.edu.sg/handle/10635/73478Title: Flow separation of a rotating cylinder
Authors: Luo, S.C.; Duong, T.T.L.; Chew, Y.T.
Abstract: The effects of rotating a circular cylinder on the suppression of flow separation were investigated experimentally. Flow separation and vortex shedding were studied by flow visualization to guide the hot-film anemometry measurements that follow. The experiments were conducted for flow past a circular cylinder with an aspect ratio of 16 in a recirculating water channel at Reynolds numbers in the range of 140 to 1,000. The rotational to translational speed ratio, α varied from 0 to 5. The present results show the existence of a critical α value of about 2.3 at which the vortex shedding is suppressed. Below this critical value of α the Kármán vortex street and separation points are observed. Vortex shedding is deflected and separation points are displaced more and more towards the rotation direction of the cylinder as α increases. Above the critical α value, vortex shedding disappears. The two separation points on the cylinder surface seem to move very close to each other at α > 2.3. This issue will also be discussed by analyzing the flow pictures obtained from flow visualization. The flow regime close to the cylinder surface is analyzed at different Reynolds numbers and different values of α to study how the cylinder's rotation affects the flow separation. The effects of rotating circular cylinder to vortex shedding frequency as well as the suppression of vortex shedding and flow separation are studied in order to understand the moving-wall effects in flow separation control. © 2009 Springer Science+Business Media B.V.
Thu, 01 Jan 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/734782009-01-01T00:00:00Z
- On swirl development in a square cross-sectioned, S-shaped ducthttps://scholarbank.nus.edu.sg/handle/10635/85512Title: On swirl development in a square cross-sectioned, S-shaped duct
Authors: Ng, Y.T.; Luo, S.C.; Lim, T.T.; Ho, Q.W.
Abstract: The flow in a uniform square cross-sectioned, S-shaped duct was investigated experimentally, at Reynolds number (Re) = 4.73 × 10 4 and 1.47 × 105, using three S-ducts of different curvature and turning angle. The hydraulic diameter (D) for each S-duct is 150 mm. Besides studying the square cross-sectioned S-duct flow at moderately higher Re than current literature, the S-ducts' geometry used in this study also have larger curvatures and higher turning angles than those reported in the literature. With surface pressure measurement and smoke wire flow visualization, flow separation at the inside wall of the first bend was detected. Using surface oil flow visualization on the bottom wall of the S-duct and cross-wires measurement at the duct exit, it is shown here that the swirl developed in the first bend was partly attenuated in the second bend due to the formation of swirl of opposite direction. The swirl of an opposite sign results in the formation of a clear dividing or separation line on the bottom wall (and top wall) of the duct. Additional flow features include the formation of streamwise vortices on the outer-wall of the second bend. These streamwise vortices can either be a pair of counter-rotating vortices or a single vortex. The formation mechanism of these streamwise vortices is explained using the Squire and Winter [J Aeronaut Sci 18(4):271-277, 1951] formula and it is shown that the said mechanism is applicable to both Re in the present study. © Springer-Verlag 2006.
Fri, 01 Dec 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/855122006-12-01T00:00:00Z
- Hysteresis phenomenon in the galloping oscillation of a square cylinderhttps://scholarbank.nus.edu.sg/handle/10635/85287Title: Hysteresis phenomenon in the galloping oscillation of a square cylinder
Authors: Luo, S.C.; Chew, Y.T.; Ng, Y.T.
Abstract: It is well known that a square cylinder with one side normal to a uniform stream will gallop when a critical flow velocity is exceeded. It is also quite well known that there is a hysteresis phenomenon in the variation of the cylinder's galloping amplitude with the flow velocity. However, little is known about the cause of this hysteresis phenomenon, and the objective of this paper is to study it more closely. In the present study, flow over a stationary square cylinder at different angle of attack (α) and at Reynolds number (Re) of 250 and 1000 was investigated numerically by using a 2-D hybrid vortex computation scheme. The study reveals that the well known point of inflection which exists in the side force (Cy) versus α plots at high Reynolds number only occurs at Re = 1000, α = 4° in the present numerical simulation. Nonlinear analysis further reveals that this point of inflection is the cause of the hysteresis phenomenon. By further analysing the computed flow field, it is noted that at Re = 1000, α = 4°, intermittent flow reattachment takes place at alternate vortex shedding cycle on one side of the cylinder. This results in larger side force fluctuation, and it is conjectured that such large side force fluctuation affects the increasing trend of the side force with angle of attack, resulting in the point of inflection reported earlier. The above-mentioned alternate cycle flow reattachment was much less prominent at α = 2° and 6° (Re = 1000), and was not observed at Re = 250. Finally, dye flow visualization was carried out in a recirculating water tunnel and the results at Re = 1000 confirms the existence of the intermittent flow reattachment. However, in the experiment, flow reattachment does not take place in a very regular alternate cycle manner as in the computation. Instead, it occurs intermittently, possibly due to three-dimensional effects in real flow. © 2003 Elsevier Ltd. All rights reserved.
Fri, 01 Aug 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/852872003-08-01T00:00:00Z
- Scaling of trajectories of elliptic jets in crossflowhttps://scholarbank.nus.edu.sg/handle/10635/61263Title: Scaling of trajectories of elliptic jets in crossflow
Authors: Lim, T.T.; New, T.H.; Luo, S.C.
Abstract: Experiments were conducted in a recirculating water channel in the fluid mechanics laboratory at the National University of Singapore, to identify a scaling parameter for elliptic jet in crossflow (EJICF). Three jet assemblies, which included two elliptic jets of aspect ratios two and three, and one circular jet, all with the same exit area, were used, It was found that within the curves in the same aspect ratio, having different velocity ratio can significantly alter the characteristics of the jet trajectories. This suggests that the increased curvature of the leading-edge vortex loops coupled with the increase in their circulation, plays a decisive role in the jet trajectories. It was also observed that jet exit geometry also plays an important role in determining the resultant jet trajectories.
Fri, 01 Dec 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/612632006-12-01T00:00:00Z
- On the relation between centrifugal force and radial pressure gradient in flow inside curved and S-shaped ductshttps://scholarbank.nus.edu.sg/handle/10635/60992Title: On the relation between centrifugal force and radial pressure gradient in flow inside curved and S-shaped ducts
Authors: Ng, Y.T.; Luo, S.C.; Lim, T.T.; Ho, Q.W.
Abstract: Swirl flow in a curved duct and S-duct is governed by a centrifugal force and radial pressure gradient force between the sidewalls. In the present work, we introduce a dimensionless parameter that relates the ratio of these two forces and the duct's center-line distance and show how the parameter is related to other more familiar dimensionless terms such as the pressure coefficient, Reynolds number, and Dean number. By using published data as well as our own measurements, it is shown that the data collapse reasonably well on a curve when the proposed parameter is plotted against dimensionless distance along the duct. The existence of collapsed curves for ducts of different curvature ratios indicates that the proposed parameter can be used to characterize the flow, although some scatter in the data exists, due to the presence of flow separation and streamwise vortices along the wall of the curved ducts. An attempt to suppress flow separation by using vortex generators in the S-duct leads to some improvement in the collapsed curve at the first bend of the duct. The results reported here are mainly focused on square and circular constant cross-sectioned 90° curved ducts and S-shaped ducts of different curvature ratios. © 2008 American Institute of Physics.
Thu, 01 May 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609922008-05-01T00:00:00Z
- Transition phenomena in the wake of an inclined square cylinderhttps://scholarbank.nus.edu.sg/handle/10635/61615Title: Transition phenomena in the wake of an inclined square cylinder
Authors: Tong, X.H.; Luo, S.C.; Khoo, B.C.
Abstract: The division of flow regimes in a square cylinder wake at various angles of attack (α) is studied. This study provides evidence of the existence of modes A and B instabilities in the wake of an inclined square cylinder. The critical Reynolds numbers for the inception of these instability modes were identified through the determination of discontinuities in the Strouhal number versus Reynolds number curves. The spectra and time traces of wake streamwise velocity were observed to display three distinct patterns in different flow regimes. Streamwise vortices with different wavelengths at various Reynolds numbers were visualized. A PIV technique was employed to quantitatively measure the parameters of wake vortices. The wavelengths of the streamwise vortices in the modes A and B regimes were measured by using the auto-correlation method. From the present investigation, the square cylinder wake at various angles of attack undergoes a similar transition path to that of a circular cylinder, although various quantitative parameters measured which include the critical Reynolds numbers, spanwise wavelength of secondary vortices, and the circulation and vorticity of wake vortices all show an α dependence. © 2008 Elsevier Ltd. All rights reserved.
Wed, 01 Oct 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/616152008-10-01T00:00:00Z
- Parallel vortex shedding at Re = 0 (104) - A transverse control cylinder technique approachhttps://scholarbank.nus.edu.sg/handle/10635/85533Title: Parallel vortex shedding at Re = 0 (104) - A transverse control cylinder technique approach
Authors: Luo, S.C.; Xia, H.M.
Abstract: In the present studies, the effects of the end conditions of a circular cylinder on its wake at a fairly high Reynolds number of Re = 1.57 × 104 were studied. The transverse control cylinder technique (TCCT) was previously reported to be able to induce parallel vortex shedding at Re = 0 (102)In the present work, experimental results showed that the TCCT is still effective in inducing parallel vortex shedding at Re = (104). Initially, before the inclusion of the control cylinders, vortices shed by the main cylinder were curved (all shapes referred to are time-averaged shapes) owing to the influence of the cylinder end conditions. Later, two larger control cylinders of diameter D were included and were located normal and upstream of the main cylinder near its ends to change its end conditions. By manipulating the control distance (the gap between the control cylinders and the main cylinder), different vortex-shedding patterns could be induced. With both control cylinders fixed at the optimum control distance of L1 = L2 = L0 = 1.26D), the main cylinder was induced to shed parallel vortices. For the cases of curved vortex shedding (without control cylinders) and parallel vortex shedding (with control cylinders at the optimum distance of L1 = L2 = L0 = 1.26D), various aerodynamic parameters of the main cylinder were measured and compared. Results showed that the inclusion of the control cylinders speeded up the flow velocity at the ends of the main cylinder and led to a more uniform pressure distribution over the central span of the main cylinder, which finally resulted in parallel vortex shedding. Aerodynamic parameters such as drag coefficient and Strouhal number associated with parallel vortex shedding were found to be larger than their curved shedding counterparts. However, extra caution should be exercised in interpreting their implications as these data were under the influence of additional wind-tunnel blockage caused by the presence of the control cylinders. Preliminary and approximate calculations had shown that blockage effects were likely to be responsible for a significant part in the change in the aerodynamic parameters such as the drag coefficient and Strouhal number when the control cylinders were installed. When the control cylinders were symmetrically placed, but not at the optimum distance (L1 = L2 ≠ L0, the vortex-shedding pattern became curved, and was concave or convex downstream at L1 = L2 < L0 or L1 = L2 > L0 respectively. When the control cylinders were asymmetrically placed (L1 ≠ L2), oblique vortex shedding was induced, with the oblique vortex slanting in the same way as the straight line joining the centres of the control cylinders. The relation between the Strouhal numbers for parallel and oblique vortex shedding was found to still follow the cosine law. The present work confirms earlier finding by other workers that a non-uniform spanwise base pressure distribution was the cause of spanwise base flow, which led to curved or oblique vortex shedding. © 2005 Cambridge University Press.
Tue, 25 Oct 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/855332005-10-25T00:00:00Z
- Characteristics of square cylinder wake transition flowshttps://scholarbank.nus.edu.sg/handle/10635/84902Title: Characteristics of square cylinder wake transition flows
Authors: Luo, S.C.; Chew, Y.T.; Ng, Y.T.
Abstract: The wake transition regime in square cylinder flow is investigated experimentally. Mode A and B transitions similar to those captured in circular cylinder flows are found to exist in this regime and by using dye- and laser-induced fluorescence visualization, the spanwise and streamwise vortex structures are captured. Upon comparison with their corresponding modes in circular cylinder flows, some differences are noted. Most notably, the critical Reynolds numbers at which Mode A and B occur for square cylinder flows were estimated to be approximately 160 and 200, respectively, and are lower than those found in circular cylinder flows, which are generally agreed to be approximately 188-190 and 230-260, respectively. Also, the spanwise wavelengths for the two modes in square cylinder flows (5.2 Ds and 1.2 Ds for Modes A and B, respectively) are longer than their counterparts in circular cylinder flows (3-4D0 and 0.8-1 D0, respectively). Furthermore, by using a hot wire to measure the shedding frequency, it was found that the Mode A transition here does not exhibit a hysteresis phenomenon, which is unlike the case in circular cylinder flows. The Mode A to B transition here is also not associated with a sudden increase in Strouhal number, which is again unlike the corresponding situation in circular cylinder flows. However, the vortical structures are noted to be similar between the corresponding modes for the two bluff body shapes, and hence the formation mechanism can be deemed to be similar. © 2003 American Institute of Physics.
Mon, 01 Sep 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/849022003-09-01T00:00:00Z
- Generalized transport vortex methodhttps://scholarbank.nus.edu.sg/handle/10635/60401Title: Generalized transport vortex method
Authors: Zhao, J.; Chew, Y.T.; Luo, S.C.; Pan, L.S.; Wu, J.K.
Abstract: In this paper, a novel vortex method - generalized transport vortex method is introduced. Being a Lagrangian-Eulerian Approach, this method determines the vorticity field through studying the vortex/circulation's generalized transport in an artificial velocity (generalized velocity) field of Lagrangian frame. The velocity field is then determined through the use of Poisson's equation in Eulerian frame. The "generalized transport process" refers the movement and area variation of vortex/circulation, which takes consideration of both diffusion and convection processes. Comparing with traditional vortex-in-cell methods and hybrid vortex methods, it does not use splitting algorithm in math, instead, handle diffusion as a part of the convection process. There is no region decomposition issue in the computation, and its expression is rather simple and easy to realize numerically. Being a numerical application, the present method is used to compute flow past one impulsively started circular cylinder. It is capable of calculating the evolution of the fine structure of the flow field with time precisely. © 2006 Elsevier Ltd. All rights reserved.
Sun, 01 Jul 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/604012007-07-01T00:00:00Z
- PIV investigation of flow past a rotating circular cylinderhttps://scholarbank.nus.edu.sg/handle/10635/73761Title: PIV investigation of flow past a rotating circular cylinder
Authors: Luo, S.C.; Chew, Y.T.; Duong, T.T.L.
Abstract: In the present study, the effects of rotating a circular cylinder on the vortex shedding are investigated by Particle Image Velocimetry measurement. The experiments were conducted for flow past a circular cylinder of aspect ratio 25 in a recirculating water channel at Reynolds numbers in the range of 200 to 1000. The rotational to translational speed ratio α varies from 0 to 5. The present results show the existence of a critical α value in the range of 2 to 3, above which the vortex shedding is suppressed as reported in the literature. The critical α appears to decrease with increase in flow Reynolds number. Below this critical α, the Kármán vortex street is observed. When α increases, both flow separation positions move in the direction of cylinder rotation; and vortex shedding is increasingly deflected towards the rotation direction of the cylinder. Above the critical α, vortex shedding disappears, and a closed streamline is formed around the cylinder. At higher α, only counter clockwise (same direction as cylinder rotation) vorticity was observed around the cylinder. Through this investigation, it is hoped that we can better understand the effects of cylinder rotation on the flow past a slender circular cylinder. © 2010 SPIE.
Fri, 01 Jan 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/737612010-01-01T00:00:00Z