Fine Scale Features of Turbulent Shear Flows

Abstract

This thesis presents an investigation into kinematic features of fine scale turbulence in free shear flows. In particular it seeks to examine the interaction between the different length scales present in shear flow turbulence as well as the interaction between the strain-rate tensor and the rotation tensor, which are the symmetric and skew-symmetric components of the velocity gradient tensor respectively. A new multi-scale particle image velocimetry (PIV) technique is developed that is capable of resolving the flow at two different dynamic ranges, centred on inertial range scales and on dissipative range scales, simultaneously. This data is used to examine the interaction between large-scale fluctuations, of the order of the integral scale, and inertial and dissipative range fluctuations. The large-scale fluctuations are observed to have an amplitude and frequency modulation effect on the small scales, and the small scales are shown to have a slight effect on the large scales, illustrating the two way nature of the energy cascade. A mechanism whereby integral scale rollers leave behind a wake of intense small-scale fluctuations is proposed. The interaction between strain and rotation is examined with regards to the rate of enstrophy amplification (ωiSijωj). It is found that the mechanism that is responsible for the nature of enstrophy amplification is the alignment tendency between the extensive strain-rate eigenvector and the vorticity vector. This mechanism is also observed to be scale dependent for ωiSijωj > 0, but independent for ωiSijωj < 0. This is subsequently confirmed with new dual-plane stereoscopic PIV experiments performed as part of this study. Finally, computational data is used to examine the effect of experimental noise and variation of spatial resolution on the observation and understanding of this strain - rotation interaction.