O. R. H. Buxton

Amplification of enstrophy in the far field of an axisymmetric turbulent jet

The amplification of enstrophy is explored using cinematographic stereoscopic particle image velocimetry data. The enstrophy production rate is investigated by observation of the statistical tendency of the vorticity vector (?) to align with the eigenvectors of the rate of strain tensor (ei). Previous studies have shown that ? preferentially aligns with the intermediate strain-rate eigenvector (e2) and is arbitrarily aligned with the extensive strain-rate eigenvector (e1). This study shows, however, that the nature of enstrophy amplification, whether it is positive (enstrophy production) or negative (enstrophy destruction), is dictated by the alignment between ? and e1. Parallel alignment leads to enstrophy production (?iSij?j>0), whereas perpendicular alignment leads to enstrophy destruction (?iSij?j<0). In this way, the arbitrary alignment between ? and e1 is the summation of the effects of enstrophy producing and enstrophy destroying regions. The structure of enstrophy production is also examined with regards to the intermediate strain-rate eigenvalue, s2. Enstrophy producing regions are found to be topologically ‘sheet-forming’, due to an extensive (positive) value of s2 in these regions, whereas enstrophy destroying regions are found to be ‘spotty’. Strong enstrophy producing regions are observed to occur in areas of strong local swirling as well as in highly dissipative regions. Instantaneous visualizations, produced from the volume of data created by Taylors hypothesis, reveal that these ‘sheet-like’ strong enstrophy producing regions encompass the high enstrophy, strongly swirling ‘worms’. These ‘worms’ induce high local strain fields leading to the formation of dissipation ‘sheets’, thereby revealing enstrophy production to be a complex interaction between rotation and strain – the skew-symmetric and symmetric components of the velocity gradient tensor, respectively

The interaction between strain-rate and rotation in shear flow turbulence from inertial range to dissipative length scales

Direct numerical simulation data from the self similar region of a planar mixing layer is filtered at four different length scales, from the Taylor microscale to the dissipative scales, and is used to examine the scale dependence of the strain-rotation interaction in shear flow turbulence. The interaction is examined by exploring the alignment between the extensive strain-rate eigenvector and the vorticity vector. Results show that the mechanism for enstrophy amplification (propensity of which increases when the two vectors are parallel) is scale dependent with the probability of the two vectors being parallel higher for larger length scales. However, the mechanism for enstrophy attenuation, i.e., the probability of the two vectors being perpendicular to each other, appears to be scale independent.

On the Formation Mechanisms of Artificially Generated High Reynolds Number Turbulent Boundary Layers

We investigate the evolution of an artificially thick turbulent boundary layer generated by two families of small obstacles (divided into uniform and non-uniform wall normal distributions of blockage). One- and two-point velocity measurements using constant temperature anemometry show that the canonical behaviour of a boundary layer is recovered after an adaptation region downstream of the trips presenting

Concurrent scale interactions in the far-field of a turbulent mixing layer

150~\%

The triple decomposition of a fluctuating velocity field in a multiscale flow

150% higher momentum thickness (or equivalently, Reynolds number) than the natural case for the same downstream distance (

Effects of multiscale geometry on the large-scale coherent structures of an axisymmetric turbulent jet

x\approx 3\,