Influence of Thickness and Angle of Attack on the Dynamics of Rectangular Cylinder Wakes
Abstract
Stereoscopic Particle Image Velocimetry measurements were taken in the turbulent wake of two dimensional rectangular cylinders. The influence of post-stall angles of attack and Reynolds number on the flow behind a thin flat plate, and for the normal case, the effect of thickness to chord (t/d) ratio over a family of rectangular cylinders were investigated. At all cases, quasi-periodic vortex shedding is observed, the normal direction Reynolds stress becomes very large just downstream of the mean recirculation zone, and the spanwise motions were uncorrelated to the main vortex shedding process. The data were processed to obtain the mean velocities, Reynolds stresses, and forces on the body. All terms in the turbulent kinetic energy equations were measured with the exception of dissipation which was found by difference. The pressure-related terms were estimated from the numerical solution of the Poisson equation for the instantaneous velocity field. Proper Orthogonal Decomposition modes are related via mean-field theory to construct generalized phase-averaging and low-order models capturing coherent cycle-to-cycle variations. The advection, production and pressure diffusion were all significant and mostly coherent. It is shown that high, average, and low amplitude vortex shedding cycles are different in terms of vortex street dimensions, vortex topology, circulation, and decay rate. It is also shown that these flows experience irregular significant decreases in the shedding amplitude associated with shedding of disorganized vortices in a large wake. Reynolds number was found to have imperceptible effects on the wake of a normal thin plate. A reduction in the angle of attack caused the wake to decrease in size and increase in shedding frequency but the global characteristics vary non-linearly. An increase in thickness from thin plate (t/d=0.05), caused the wake to shrink, low cycles to diminish, and local turbulence increase to a peak at t/d=1.0, identified as a critical thickness. At t/d=1.9, however, turbulent quantities decrease, the wake grows larger and significant cycle-to-cycle variations in the flow reports of a new vortex formation process.
Description
Keywords
Engineering--Aerospace, Engineering--Mechanical
Citation
Mohebi, M. (2016). Influence of Thickness and Angle of Attack on the Dynamics of Rectangular Cylinder Wakes (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/26521