An assessment of flow development in a separation bubble subjected to spanwise modulated disturbances using particle image velocimetry

Abstract

The effect of spanwise modulated disturbances on flow development within a separation bubble formed over a flat plate is investigated experimentally using two-component Particle Image Velocimetry (PIV). Two separate PIV configurations are employed to study both streamwise and spanwise aspects of the flow development. Spanwise uniform and non-uniform forcing are considered using novel surface mounted plasma actuators. Characterization of the disturbances is performed in quiescent conditions, showing that actuation produces a streamwise jet within the active region(s) of the actuators and no significant momentum elsewhere, thus enabling spanwise forcing at the desired wavelengths. When subjecting the separation bubble flow to these disturbances applied at the frequency matching that of the most unstable mode in the baseline flow, bubble height reduces and mean reattachment shifts upstreams with decreasing spanwise wavelength, which is a likely consequence of higher momentum coefficients for these cases. However, significant changes in mean topology are also shown to arise for relatively large wavelengths, and hence low momentum coefficients, with these modifications thought to result from the effect of perturbations on the development of the dominant shear layer vortices. The forcing is shown to influence the spanwise wavelengths that develop in the vortex filaments, as when the flow is forced, the spanwise wavelength of the incoming disturbance is shown to exert control over the wavelength that initially develops, while, regardless of the forcing case, secondary undulations develop at a wavelength that matches the streamwise wavelength of the structures.