Effect of ambient turbulence on trailing vortices

Abstract

The effects of ambient turbulence (generated by a biplanar grid) on the migration and lifespan of trailing vortices are investigated in a towing tank through the use of two NACA-0012 foils moving at a constant angle of attack. The results show that the rise and demise of the vortices are controlled primarily by the rate of dissipation of the background turbulence. The integral scale of turbulence plays only a minor role. In both a quiescent or weakly-turbulent fluid the sinusoidal instability and in a fluid with stronger turbulence, the vortex bursting precedes the subsequent instability events which brings about the eventual destruction of the vortices. Both forms of the large scale instability are often accompanied by the roll of the vortex pair onto its side. Shear is not necessary for the roll but may enhance it under atmospheric conditions. Nomenclature & - aspect ratio of the wing B = wing span 60 = initial vortex core spacing c = chord length of the wing d = width of a square bar in the grid d0 = initial depth of the vortex pair H = vortex rise height H* -H/b0y normalized migration height Ln = integral scale of turbulence M =mesh size of the grid Re = Reynolds number re = effective core radius T* - V0t/bQ, normalized time t = time U = model velocity u =x component of velocity v =y component of velocity K0 =r0/2?rZ?0, initial mutual induction velocity w = z component of velocity x,y = coordinate axes a = model angle of attack F0 = initial circulation of vortex A =d0/b0 e =rate of decay of turbulence energy e* = (eZ?0)1/3 / V0, turbulence parameter v = kinematic viscosity of water p = density of water