Vortical & Dissipative Motion

An experimental study of the dissipative and vortical motions in turbulent boundary layers

The experimental data of Honkan & Andreopoulos (1997) have been further analyzed and some new statistical results have been obtained. In the present work, particular emphasis has been given to the time-dependent behavior of the kinematic shear stress, vorticity, enstrophy, dissipation rate, vorticity stretching and several of the matrix invariants of the velocity gradient tensor, strain-rate tensor and rotation-rate tensor. The invariants have been linked with terms appearing in the transport equations of enstrophy and dissipation rate. Indicative of the existence of extremely high fluctuations is that all rms values are considerably larger than the corresponding mean values. All invariants exhibit a very strong intermittent behavior, which is characterized by large amplitude of bursts, which may be of the order of 10 rms values. A substantial qualitative agreement has been found between the present experimentally obtained statistical properties of the invariants with those obtained from Direct Numerical Simulations data. Patterns with high rates of turbulent kinetic energy dissipation and high enstrophy suggest the existence of strong shear layers in the near wall region. On many instances, local high values of the invariants are also associated with peaks in the shear stress. Conditional analysis provided some evidence of the existence of streaks of several vortices during strong vortical activities with an average frequency of appearance four times more than the frequency of hairpin vortices.