Three-dimensional wake formations behind a family of regular polygonal plates

The experiments were conducted in water to study the initial wake formations behind various polygonal plates placed normal to the starting flow. The models included triangular, square, hexagonal, and octagonal plates. Because of nonuniform self-induced velocity, the initial vortex structure behind each polygonal plate was arched in the axial direction, and pairs of counter-rotating longitudinal vortices induced inward motion of the vortex structure at each corner. The evolution of free polygonal vortex loops was numerically simulated using a simplified three-dimensional vortex segment method to demonstrate the mechanism for the observed vortex deformation. Nomenclature Ap . = acceleration parameter, Deqa/ UQ a = acceleration, dU/dt D = side length of square plate Z)eq = equivalent disk diameter Re = steady-state Reynolds number, Rec = acceleration Reynolds number, T = nondimensional time, U0t/Deq t = time tc = time scale, J(Deq/d) U = freestream velocity UQ = steady freestream velocity X = axial distance v = kinematic viscosity