INVESTIGATIONS ON THE CONTROL OF WAKES DOWNSTREAM OF A ROTARY OSCILLATING PLATE

Abstract

The vortex wake of a flutter bridge deck can be simulated by the flow across a forced rotary oscillating plate. Two narrow strips of width ratio b/H = 0.33 are set symmetrically on the upper and lower sides of an oscillating plate of chord to thickness ratio B/H = 5, to suppress synchronized vortex shedding in the wake. The method of numerical simulation and experimental validation is used, and the ranges of amplitude and frequency of oscillation investigated are β = 0° ~ 10° and feH/V∞ = 0 ~ 0.0857 respectively, and the Reynolds number Re = V∞H/ V∞ = 2800, where V∞ is velocity of on-coming flow. Three kinds of stream-wise strip positions, i.e. the front edge, mid-chord and trailing edge of the plate are studied respectively, with transverse location y/H of the strip as varying parameter. The results of experiment demonstrate that, in a certain range of strip location y/H, and β = 0° ~ 7.5°, feH/V∞ = 0 ~ 0.08, the peak to peak ratio of power spectra of fluctuating velocities in the wakes with and without control can be much lower than 1, and the minimum is about 0.3. The results of simulation show that, in β = 0° ~ 7.5° and a certain range of feH/V∞, the root mean square values of fluctuating torque and lift of the plate can be considerably reduced, and the top reductions are 43% and 80% respectively, if the mid-chord strip position is in the vicinity of y/H = ±1. The 1st and 2nd eddy viscosity coefficients are introduced to link the normal and shear turbulent stresses in the wake with the gradients of amplitudes of the perturbation velocities, and a linear stability equation is derived. Stability analysis indicates that, the maximum amplification factor of perturbation ωi max can be drastically reduced, and the frequency range of perturbation with maximum growth rate is substantially narrowed by the control. The application of the strips alters the velocity profiles and promotes the eddy viscosity, therefore weakens the instability of the wake.