Jacopo Serpieri

Design of a swept wing wind tunnel model for study of cross-flow instability

The need to investigate the cross-flow instability resides on the fact that this is the main cause of laminar to turbulent transition for swept wing flows in free flight. In this paper the procedure to design a wind tunnel model for cross-flow instability investigation is illustrated. The steps that are presented involve the airfoil and the wall liners design and the estimation of the boundary layer growing on the model for the experiment conditions. Furthermore linear stability theory is applied to numerically computed boundary layer profiles in order to evaluate the flow stability to standing cross-flow waves of several wavelengths and for two different angles of attack. A preliminary evaluation of the wavelength of the most amplified cross-flow standing mode and its angle with respect to the free stream direction are presented. Wind tunnel tests encompassing oil flow visualization, surface pressure and boundary layer hot-wire measurements were performed validating the design procedure. The effectiveness of the wall liners to constrain the flow to a spanwise invariant arrangement is showed to be limited for the presented case. The results of linear stability theory are compared to the experimental observations showing good agreement for both the estimated wavelengths and wave angles. The validity of oil flow visualization for qualitative investigation of cross-flow instability is confirmed by hot-wire measurements. Natural occurring transition and forced cross-flow instability flows are investigated giving results in good agreement with published literature.

Swept-wing transition control using DBD plasma actuators

In the present work, laminar flow control, following the discrete roughness elements (DRE) strategy, also called upstream flow deformation (UFD) was applied on a 45◦ swept-wing at a chord Reynold’s number of Rec = 2.1 · 106 undergoing cross-flow instability (CFI) induced transition. Dielectric barrier discharge (DBD) plasma actuation was employed at a high frequency (fac = 10kHz) for this purpose. Specialized, patterned actuators that generate spanwinse-modulated plasma jets were fabricated using spray-on techniques and positioned near the leading edge. An array of DREs was installed upstream of the plasma forcing to lock the origin and evolution of critical stationary CFI vortices in the boundary layer. Two forcing configurations were investigated-in the first configuration the plasma jets were directly aligned against the incoming CF vortices while in the second the CF vortices passed between adjacent plasma jets. Infrared thermography was used to inspect transition location, while quantitative measurements of the boundary layer were obtained using particle image velocimetry. The obtained results show that the plasma forcing reduces the amplitude of stationary CF modes, thus delaying laminar-to-turbulent transition. In contrast to previous efforts [1], the plasma forcing did not introduce unsteady fluctuations into the boundary layer. The mechanism responsible for the observed transition delay appears to leverage more on localised base-flow modification rather than the DRE/UFD control strategy.