A Coupled CFD/CSD Investigation of the Effects of Leading Edge Slat on Rotor Performance
Baeder, James D.
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A coupled Computational Fluid Dynamic (CFD) and Computational Structural Dynamics (CSD) methodology is extended to analyze the effectiveness of a leading edge slat (LE-Slat) for mitigating the adverse effects of dynamic stall on rotor blade aerodynamic and dynamic response. This involved the following improvements over the existing CFD methodology to handle a multi-element airfoil rotor: incorporating the so-called Implicit Hole Cutting method for inter-mesh connectivity, implementing a generalized force transfer routine for transferring LE-Slat loads onto the main blade, and achieving increased parallelization of the code. Initially, the structured overset mesh CFD solver is extensively validated against available 2-D experimental wind tunnel test cases in steady and unsteady flight conditions. The solver predicts the measurements with sufficient accuracy for test cases with both the baseline airfoil and that with two slat configurations, S-1 and S-6. As expected, the addition of the slat is found to be highly effective in delaying stall until larger angles for the case of a static airfoil and ameliorating the effects of dynamic stall for a 2-D pitching airfoil. The 3-D coupled CFD/CSD model is extensively validated against flight test data of a UH-60A rotor in a high-altitude, high-thrust flight condition, namely C9017, characterized by distinct dynamic stall events in the retreating side of the rotor disk. The validated rotor analysis tool is then used to successfully demonstrate the effectiveness of a LE-Slat in mitigating (or eliminating) dynamic stall on the rotor retreating side. The calculations are performed with a modified UH-60A blade with a 40%-span slatted airfoil section. The addition of the slat is effective in the mitigation (and/or elimination) of lift and moment stall at outboard stations, which in turn is accompanied by a reduction of torsional structural loads (upto 73%) and pitch link loads (upto 62%) as compared to the baseline C9017 values. The effect of a dynamically moving slat, actuating between slat positions S-1 and S-6, is thoroughly investigated, firstly on 2-D airfoil dynamic stall, and then on the UH-60A rotor. Three slat actuation strategies with upto [1, 3, 5]/rev harmonics, respectively, are considered. However, it is noted that the dynamic slat does not necessarily result in better rotor performance as compared to a static slat configuration. The coupled CFD/CSD platform is further used to successfully demonstrate the capability of the slat (S-6) to achieve upto 10% higher thrust than C9017, which is beyond the conventional thrust limit imposed by McHugh's stall boundary. Stall mitigation due to the slat results in a reduction of torsional load upto 54% and reduction of pitch link load upto 32% as compared to the baseline C9017 flight test values, even for an increase in thrust of 10%.