Experimental techniques for highly resolved measurements of rotation, strain and dissipation-rate tensors in turbulent flows

A new hot-wire probe with nine independently operating wires was designed, developed and validated in a two-dimensional turbulent boundary layer. The probe is capable of measuring all the elements of the gradient velocity tensor, the rate-of-strain tensor, the rate-of-rotation tensor/vorticity vector and the rate-of-dissipation of kinetic energy tensor with a spatial resolution of seven Kolmogorov viscous length scales. Measurements obtained with the probe were extensively compared with other experimental data and direct numerical simulation results. Scaling of all results with inner wall variables was unsuccessful in collapsing all the data onto one curve. Several of the invariants considered exhibit a very strong intermittent behaviour which is characterized by large amplitude bursts which can be of the order of ten RMS values. It appears that the fluctuating strain field dominates pressure fluctuations over enstrophy.