Effect of side-edge vortices and secondary induced flow on the wake of normal thin flat plates

Abstract

The wake of a normal thin flat plate of aspect ratio 3.2 was studied in comparison to that of an infinite span (2D) plate at Reynolds number of 1200 (based on the common dimension of the two plates) using Direct Numerical Simulations. The presence of side-edge shear layers suppressed the spanwise instabilities responsible for the three distinct flow regimes in the wake of 2D plates. There was a vortex “peeling” mechanism that detached the vortices in the shear layers on the shorter sides. It increased the Strouhal number to 0.317 from 0.158 for 2D plates. It is also associated with increased entrainment of freestream fluid and a reduction in the mean recirculation length by 40% to 1.6h. Moreover, the peeling mechanism led to formation of interlocked vortex loops outside the base region. The maximum turbulence kinetic energy along the wake centreline for the 3D case was 76% lower than for the 2D flow. Vortex detachment was constrained to the plate sharp corners in contrast to the most common example of thin flat bodies (circular flat disks). This difference lead to a relatively higher vortex shedding frequency for flat plates. The side-edge vortices are associated with a secondary induced flow behind the plate, which formed two interlocked vortex loops and resulted in formation of separating vortex streets in the wake (wake split). Two opposing in-wash flows (in the chordwise direction) were observed accompanied by a similar system of out-wash flows (normal to the plate chord), which increased the freestream fluid entrainment. The in-wash flow contracted the wake in the plane of the chord. The out-wash flow expanded the wake in the spanwise direction and resulted in two counter-rotating vortex streets.