This website is currently under development.

This is a reference text, which is to be replaced at a later point. Backwards facing step and cavity flows have historically been popular flow problems for benchmarking computational fluid dynamics models, as they capture the development of a free shear layer, whilst conserving relative simplicity in terms of geometry and thus solver setup. As such, in conjunction with experimental data, the case presents itself as ideal for turbulence model validation. Previously studied extensively with two-equation turbulence models, it is revisited in the present investigation to quantify the fidelity of a large eddy simulation (LES), using time-resolved planar particle image velocimetry (PIV) data as reference.

A transitional cavity with aspect ratio 8.5 was studied at a Reynolds number of 13 000 (cavity depth as reference length), with a nominal freestream velocity of 10.5 m/s. Experimental, LES and k-ω data was compared based on velocity derived quantities such as the mean flow and turbulence kinetic energy fields. Results suggest that the LES performs remarkably well, especially in comparison to k-ω, which was found to replicate the time-averaged velocity field with relative accuracy, but failed to produce sufficient turbulence kinetic energy in the region immediately above the shear layer. Moreover, the stream-wise progression of time integral-scales and the power spectrum, within the shear layer, was reviewed. Both LES and PIV seem to confirm that scales increase in duration towards the trailing edge of the cavity, which was also reflected by a decrease in frequency of the dominant spectral component in that region. Finally, a principal component analysis was conducted, yielding a good match, both in terms of the individual turbulent energy contribution of the first four spatial modes as well as their physical appearance (magnitude and direction).

Concluding, LES is found to reproduce both time-averaged and transient parameters of turbulence adequately. Nevertheless, further investigation of the performance of LES is required, particularly in flow regimes where compressibility and shock-wave phenomena are present. Moreover, the present investigation focuses solely on the planar behavior of the flow. The author recommends that tomographic PIV data be collected to quantify the similarity of three dimensional flow features.