Experimental and Flight Investigation of the Laminar Separation Bubble on an Oscillating X-56A Wing Section Near Stall

Abstract

An investigation of laminar separation bubble behavior on an oscillating X-56A wing section has been performed experimentally at Reynolds number 200,000. Wind tunnel results along with Implicit Large Eddy Simulations (CFD) quantify the behavior of the laminar separation bubble. The oscillation parameters were selected based on a scaled flight vehicle at the University of Arizona. Wind tunnel results were validated against theory using static angle of attack sweeps and an unsteady case at an angle of attack of $\alpha = 10$ degrees. The static results show excellent agreement between the experimental data, Thin Airfoil Theory, a computational vortex lattice method (XFLR5), and CFD results in both pressure coefficient and lift coefficient. The unsteady validation case of $\alpha=10$ degrees (nondimensional plunging frequency of $k = \frac{\pi f c}{U_{\infty}}=0.7$, where $f$ is the dimensional plunging frequency, $c$ is the wing section chord, and $U_{\infty}$ is the free-stream velocity, and nondimensional plunging amplitude $h = \frac{amplitude}{chord} = 3.2\%$) also showed agreement for comparison between the experiment, Theodorsen's theory (analytical solution to plunging wing sections), and CFD results. Pressure coefficient behaved similarly between the experiment and CFD with the laminar separation bubble changing pressures at similar times in the cycle. The lift coefficient was found to oscillate sinusoidally, achieving higher lift than the static case with no moment stall. Near static stall angle of attack ($\alpha=12$ degrees, where stall $\alpha=12.25$ degrees), Theodorsen's theory is no longer applicable. Oscillation parameters were $k=0.7$ and $h=4.8\%$ and effective angles of attack reached nearly $16$ degrees. The airfoil continued to produce lift past static stall at the consequence of a moment stall. Pressure measurements indicate that the laminar separation bubble is shed from the leading edge which was confirmed through 2D particle image velocimetry. The shedding behavior was modeled differently in the CFD simulation with a lack of free-stream turbulence. However, pressure coefficient and lift coefficient are in excellent agreement for over $75\%$ of the oscillation cycle. It is shown that the experimental setup is valid and the increased aerodynamic efficiency comes at the consequence of a moment stall for the high angle of attack case ($\alpha=12$ degrees). Additionally, free-flight tests have been completed including maiden flights of the 1/3 scale X-56A vehicle built at The University of Arizona. The flight vehicle is the motivation for the wind tunnel parameters. Flight instruments have been verified against previously collected data including pressure sensors, wing accelerometers (to track the motion), and a stand-alone constant temperature anemometry (CTA) system to measure free-stream turbulence. The instrumentation was flown on a stable platform to compare to historical data (1/5 scale Ximango) and is performing nearly 10 times as fast (data collection frequency) of the expected phenomenon occurring with the laminar separation bubble shedding on the 1/3 X-56A vehicle. This will need to be analyzed in future work as the laminar separation bubble is sensitive to free-stream turbulence conditions.