ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Tue, 24 Nov 2020 21:08:23 GMT2020-11-24T21:08:23Z5041- By-pass mechanism of transition to turbulencehttps://scholarbank.nus.edu.sg/handle/10635/51345Title: By-pass mechanism of transition to turbulence
Authors: Sengupta, T.K.; Chattopadhyay, M.; Wang, Z.Y.; Yeo, K.S.
Abstract: The by-pass mechanism of transition for a wall-bounded shear layer is explained for the case when an infinite row of convecting vortices migrate over a boundary layer at a specific speed range. Such a mechanism is important for noisy flows over bluff bodies, flows inside turbo-machinery and flows over helicopter rotor blades. By solving the Navier-Stokes equation, it is shown that this by-pass transition is a consequence of vortices migrating at convection speeds that are significantly lower than the free-stream speed. This situation is commonly found in flows that are affected by the presence of periodic wakes. Whenever the speed of migrating vortices is in a certain critical range, there is a local instability of the underlying shear layer with a very high-growth rate as compared to the growth of pure Tollmien-Schlichting waves created by wall excitation. The above interpretation is supported by solving the linearized and full Navier-Stokes equation for disturbance quantities under the parallel flow approximation in two dimensions. Various ramifications of such a by-pass route of transition are discussed in this paper. © 2002 Academic Press.
Tue, 01 Jan 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/513452002-01-01T00:00:00Z
- Spatial direct numerical simulation of transitional boundary layer over compliant surfaceshttps://scholarbank.nus.edu.sg/handle/10635/61344Title: Spatial direct numerical simulation of transitional boundary layer over compliant surfaces
Authors: Wang, Z.; Yeo, K.S.; Khoo, B.C.
Abstract: An iterative implicit fractional step method is developed and employed for the simulation of transitional boundary layer over compliant surfaces. The three-dimensional perturbation Navier-Stokes equations are discretized by curvilinear finite volumes on a collocated grid system. A multigrid procedure is used for computations associated with the pressure-Poisson equation, while simulation is carried out by MPI-based parallel computation with domain decomposition. Results in the literature for oblique linear waves and the non-linear breakdown of a wave triad over a rigid wall are repeated to check the accuracy of the codes that had been developed. Oblique linear TS (Tollmien-Schlichting) waves over finite-length compliant membranes are generally found to coexist with CIFI (compliance induced flow instability) or FISI (flow induced surface instabilities) waves. The latter waves usually possess longer wavelengths and thus propagate at a larger oblique wave angles than the TS waves. Simulation reveals that compliant surfaces may slow down the development of secondary instabilities during the early stages of laminar-turbulent transition. However, during the later stages of fundamental wave breakdown, interactions with CIFI and edge-generated waves may increase the amplitudes of the original 2D and 3D TS waves, leading to an earlier breakdown on compliant surfaces. Linear interaction between the flow and compliant membranes has been assumed. © 2004 Elsevier Ltd. All rights reserved.
Tue, 01 Nov 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/613442005-11-01T00:00:00Z
- On two-dimensional linear waves in Blasius boundary layer over viscoelastic layershttps://scholarbank.nus.edu.sg/handle/10635/61003Title: On two-dimensional linear waves in Blasius boundary layer over viscoelastic layers
Authors: Wang, Z.; Yeo, K.S.; Khoo, B.C.
Abstract: This work concerns the direct numerical simulation of small-amplitude two-dimensional ribbon-excited waves in Blasius boundary layer over viscoelastic compliant layers of finite length. A vorticity-streamfunction formulation is used, which assures divergence-free solutions for the evolving flow fields. Waves in the compliant panels are governed by the viscoelastic Navier's equations. The study shows that Tollmien-Schlichting (TS) waves and compliance-induced flow instability (CIFI) waves that are predicted by linear stability theory frequently coexist on viscoelastic layers of finite length. In general, the behaviour of the waves is consistent with the predictions of linear stability theory. The edges of the compliant panels, where abrupt changes in wall property occur, are an important source of waves when they are subjected to periodic excitation by the flow. The numerical results indicate that the non-parallel effect of boundary-layer growth is destabilizing on the TS instability. It is further demonstrated that viscoelastic layers with suitable properties are able to reduce the amplification of the TS waves, and that high levels of material damping are effective in controlling the propagating CIFI. © 2005 Elsevier SAS. All rights reserved.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/610032006-01-01T00:00:00Z
- DNS of low Reynolds number turbulent flows in dimpled channelshttps://scholarbank.nus.edu.sg/handle/10635/59981Title: DNS of low Reynolds number turbulent flows in dimpled channels
Authors: Wang, Z.; Yeo, K.S.; Khoo, B.C.
Abstract: Direct numerical simulation (DNS) is performed to study turbulent flows over dimpled surfaces in a channel. Results on mean field and second-order quantities are obtained. 'Horseshoe' vortices can be observed in the dimples of sparse arrays. As inter-dimple separation is reduced, the 'feet' of the horseshoe vortices are gradually lifted off the dimple surface, and the resulting flow structures in the cavities become flattened and stretched to become something akin to two-dimensional separation bubbles. At the higher dimple density, the stream traces near the surface also develop a distinct formation similar to what had been observed in earlier Reynolds-averaged Navier-Stokes (RANS) simulations (Isaev, S.A., Leont'ev, A.I. and Baranov, P.A., 2000, Technical Physics Letters, 26, 15; Lin, Y.L., Shih, T.I.-P. and Chyu, M.K., 1999, ASME paper, 99-GT-263; Lin, Y.L. Shih, T.I.-P., 2001, International Journal of Transfer Phenomena, 3, 1). Regions of high turbulence intensity are found above the downstream half of the dimples and along their side edges. These regions coincide with the locations of vortex shedding found in the experiments of Ligrani et al. (2001, Physics of Fluids, 13, 3442) and the locations of vorticity concentrations observed in Park et al. (2004, Numerical Heat Transfer, Part A (Applications), 45(1), 1) and Won and Ligrani (2004, Numerical Heat Transfer, Part A (Applications), 46(6), 549). For a fixed mean pressure gradient, it is observed that the flow rates through the channels are reduced by the presence of dimples. This indicates that the dimpled channels we have studied so far have larger drag than flat-wall channels. Computed friction coefficients for dimpled channels also confirmed the conclusion. © 2006 Taylor & Francis.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/599812006-01-01T00:00:00Z