Discontinuities in the S-Re relations of trapezoidal and triangular cylinders

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.