A Scalable Time-Parallel Solution of Periodic Dynamics for Three-Dimensional Rotorcraft Aeromechanics
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The principal barrier of computational time for rotorcraft trim solution using high-fidelity three-dimensional (3D) structures on real rotor problems was overcome with parallel and scalable algorithms. These algorithms were devised by leveraging the modern supercomputer architecture. The resulting parallel X3D solver was used to investigate advanced coaxial rotors using a notional hingeless rotor test case, Metaltail. This investigation included rotor performance, blade airloads, vibratory hub loads, and three-dimensional stresses.
The technical approach consisted of first studying existing algorithms for periodic rotor dynamics --- time marching, finite element in time (FET), and harmonic balance. The feasibility of these algorithms was studied for large-scale rotor structures, and drawbacks were identified. Modifications were then performed on the harmonic balance method to obtain a Modified Harmonic Balance (MHB) method. A parallel algorithm for skyline solver was devised on shared memory to obtain faster solutions to large linear system of equations. The MHB method was implemented on a hybrid distributed--shared memory architecture to allow for parallel computations of harmonics. These developed algorithms were then integrated into the X3D solver to obtain a new parallel X3D.
The new parallel X3D was verified and validated in hover and forward flight conditions for both idealized and real rotor test cases. A total of four test cases were studied: 1) uniform beam, 2) Frank Harris rotor, 3) UH-60A-like Black Hawk rotor, and 4) NASA Tilt Rotor Aeroacoustic Model (TRAM). The predictions of tip displacements, airloads, and stress distributions from the MHB algorithm showed good agreement with the test data and time marching predictions. The key conclusion is that the new solver converges to the time marching solution 50-70 times faster and achieves a performance greater than 1 teraFLOPS.
The new parallel X3D solver opened the opportunity for modeling advanced rotor configurations. In this work, the coaxial rotor was the selected configuration. Two open access models were developed; 1) a notional hingeless coaxial rotor, and 2) a notional articulated UH-60A-like coaxial rotor. The aerodynamics, structural dynamics, and trim modules of X3D were expanded for coaxial modeling. The coaxial aerodynamics was validated with hover performance data from the U.S. Army model test. The coaxial solver was then used to study rotor aeromechanics in forward flight. The analysis was performed at a low-speed transition flight for which qualitative data is available for the Sikorsky S-97 Raider aircraft for comparison. The UH-60A coaxial airloads showed good agreement with the S-97 data as the twists are likely similar. However, the Metaltail model showed dissimilarities, and the cause was investigated to be its high twist. Vibratory hub loads with advance ratio were studied, and the maximum vibration occurred at the transition flight speed ($\mu = 0.1 - 0.15$), which was consistent with the S-97 data. The effect of the inter-rotor phase was examined for the reduction of vibratory hub loads. Three-dimensional stresses and strains were predicted and visualized for the first time on lift offset coaxial rotors in the blade and the hub.