Experiments on streamwise vortices in curved wall jet flow

Abstract

Hot-wire anemometry and smoke visualizations are used to study the jet on a concave wall. The measurements were performed in the region 91<Go<198, where Go is the Görtler number. It is found that streamwise vortices are amplified on the concave wall. The growth of vortices is dependent on the initial amplitude, i.e. the highest initial amplitude gives the maximum strength of vortices. The amplification occurs in the laminar region while the time-averaged strength of vortices decreases further downstream in the transition region. No merging or splitting of vortex pairs was found. Regular oscillations of the time signal occur at higher Görtler numbers before breakdown. Smoke visualization shows that the horseshoe mode appears with a streamwise wave length of the same order as the triggered spanwise wave length. The streamwise velocity fluctuations indicating the secondary instability were found to coincide better with the total shear distribution rather than streamwise velocity gradients in the spanwise or wall normal directions. Power spectra show distinct peaks at different frequencies. At streamwise positions near the start of curvature, the filtered root-mean-square (rms) levels of the streamwise velocity component around 95 Hz, corresponding to the horseshoe mode, show maximum amplitude between different vortex pairs. The filtered frequency around 22 Hz, which is the dominating frequency further downstream, shows maximum amplitude further away from the wall, where the velocity profile has an inflectional character. However, as the flow proceeds downstream the maximum at 22 Hz moves against the wall.