Modifications of the near-wall structures in thermally induced three-dimensional turbulent boundary layer

摘要

Statistical analysis is carried out in order to examine the modifications of near wall coherent structures in a thermally induced three-dimensional turbulent boundary layer (3DTBL). The buoyancy forces modify the counter rotating vortices in the near-wall region with different degrees in a similar way to that produced by the pressure and shear driven three-dimensional turbulent boundary layer flows. Evidence of such modifications that is highlighted in the present study may explain the reduction of Reynolds shear stress and turbulence activities in the near-wall region. It has been shown that the induced three-dimensionality weakens the strength of near-wall vortices in such a way that vortices of one sign are weakened to produce ejections and those of the other sign are weakened to produce sweeps. In contrast to the hypotheses explaining the reduction of turbulence activities in pressure and shear driven 3DTBL flows, vortices rotating in the same direction as the background mean spanwise vorticity in the near-wall region are weakened much more than those rotating in the opposite direction.