Experimental base flow modification on a swept wing using plasma forcing

Abstract

This work experimentally investigates plasma actuator (PA) forcing effects on the base flow and developing crossflow (CF) instabilities in a swept wing boundary layer. Spanwise-invariant plasma forcing near the leading edge is configured according to the Base Flow Modification (BFM) strategy. A simplified predictive model is constructed by coupling an experimentally-derived plasma body force and Linear Stability Theory and is used to infer the stability characteristics of the boundary layer subject to BFM. The base flow velocity is measured by stereo-PIV at various PA operating conditions. Similarly, the developing CF instabilities, triggered through Discrete Roughness Elements, are quantified by planar-PIV. The results demonstrate that a PA can reduce the boundary layer CF component, whereas the control authority shows a high dependence on the momentum coefficient. The dissimilar reduction between the streamline-aligned velocity and CF component leads to a local re-orientation of the base flow. Spanwise spectral analysis of the time-averaged flow indicates that stationary CF instabilities can be favourably manipulated whereas the BFM reduction effects depend on the corresponding initial amplitudes of stationary instabilities. An evident spanwise shift in the trajectory of stationary CF vortices is observed, which appears to result from the local alteration of the boundary layer stability due to the PA forcing. Despite the overall reduction in amplitude of stationary CF instabilities, unsteady disturbances are found to be enhanced by the PA forcing. The current results shed light into the underlying principles of BFM-based PA operation, in the context of laminar flow control.

Unsteady disturbances in a swept wing boundary layer due to plasma forcing

“…Reconciling previous studies of Yadala et al, 24 Serpieri et al, 8 and Peng et al 41 as well as the current experiment, the stochastic PAinduced disturbances at both low and high frequencies can potentially damage the PA effectiveness for transition delay, at least within the context of BFM. For instance, to achieve the BFM control, the PA generally requires high power input and needs to be positioned close to the leading edge in order to favorably manipulate the CF component.…”

“…The overall transition advancement reported in the present study further highlights the detrimental effects of PA-induced unsteady disturbances and partially resolves the conundrum regarding the effects of unsteady forcing within the context of the BFM method. In the work of Peng et al, 41 the BFM-based PA was found to essentially impose two effects into the boundary layer. One is to weaken stationary CF instabilities (i.e., the intended BFM action), while the other is to enhance unsteady instabilities.…”