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

The experimental data of Honkan & Andreopoulos (1997a) have been further analysed and some new statistical results obtained. In the present work, particular emphasis is given to the time-dependent behaviour 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 are linked with terms appearing in the transport equations of enstrophy and dissipation rate. Indicative of the existence of extremely high fluctuations is that all r.m.s. values are considerably larger than the corresponding mean values. All invariants exhibit a very strong intermittent behaviour, which is characterized by large amplitude of bursts, which may be of the order of 10 times the r.m.s. values. A substantial qualitative agreement is found between the present experimentally obtained statistical properties of the invariants and those obtained from direct numerical simulation 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. In many instances, locally high values of the invariants are also associated with peaks in the shear stress. Conditional analysis provides some evidence of the existence of sequences of several vortices during strong vortical activities, with an average frequency of appearance four times higher than the frequency of appearance of hairpin vortices.