Investigating the Interaction between Crossflow and Laminar Separation Bubbles

Abstract

This experimental investigation explores crossflow and its interaction with laminar separation bubbles in low-speed flows.The suction side of a modified NACA $64_3-618$ airfoil was tested in conditions relevant to strong crossflow and crossflow instabilities (forced and unforced) in the presence of a laminar separation bubble. Laminar separation bubbles were identified on the model at $Re_c = 600k$ ($AoA > \ang{-2}$) through time-averaged pressure measurements and infrared thermography. Discrete roughness elements were used to promote the most unstable wavelenght of the stationary crossflow instability and obtain measurable disturbance amplitudes for crossflow instabilities within the laminar separation bubble. Infrared thermography was used to confirm the application of the roughness elements by showing the enhancement of the stationary modes at the forced wavelength ($ \lambda = \SI{3.5}{mm}$) at $AoA = \ang{-8}$ and $AoA = \ang{-1}$. Time resolved hotwire measurements provided information about the stationary, primary traveling, and secondary crossflow instabilities. It also provided knowledge of potential Kelvin-Helmholtz instabilities around the transition region of the separation bubble. DREs successfully forced the stationary crossflow mode. However, development of the primary traveling and secondary instabilities are also shown within the boundary layer. In accordance with previous research, the primary instability was seen to displace off of the wall and a set of opposite rotational vortices develops when entering the adverse pressure gradient. It was also shown that multiple secondary instability modes likely contributed to the stationary crossflow mode dominated transition. The dominant frequency bands observed near the typical IR-visualized sawtooth pattern, often associated with crossflow instability-induced transition, appear similar to those previously observed for the secondary instability of the forced stationary mode, having the largest amplitudes when approaching the estimated transition location. In the presence of a laminar separation bubble ($AoA = \ang{-1}$), crossflow was reduced at measurements located within the bubble as the upstream favorable pressure gradient is weaker than at -8 degrees. Growth of a set of opposite-rotating vortices was observed and is consist with the higher frequency modes $\SI{2000}{Hz} \leq f \leq \SI{3500}{Hz}$. As the measurement location approached transition, the crossflow vortices seem to combine with shear layer (K-H) instabilities and eventually leading to a more 2D flow field around reattachment. Higher resolution streamwise measurements between transition and reattachment are needed to corroborate this claim. Spectral analysis shows that the interaction of Kelvin Helmholtz and crossflow instabilities appears to dominate transition. This is postulated since the dominant frequency range near transition is lower than that observed without forced crossflow instabilities. Higher frequency instability modes are also shown in the power spectra which could relate to secondary crossflow instabilities and/or higher order interactions with the Kelvin-Helmholtz instability, but the exact mode could not be identified in the scope of this work. To further this investigation, higher resolution CTA is required, as well as the use of x-wires to collect multi-component velocity data to separate crossflow velocity and chordwise velocity profiles.