Effect of flow separation on discrete gust loads for a free-flying elastic aircraft

ifasd2024 Tracking Number 193

Shock motion and flow separation are aerodynamic nonlinearities, which have a significant effect on dynamic gust loads but are not accounted for in time-linearized aerodynamic models. Shock motion may lead to higher torsional moments (Kaiser, 2019) and even higher loads in case the shock system is dynamically changed (Friedewald, 2023). Gust disturbances are not small and may cause local regions with flow separation during the gust encounter. The detached flow limits the total lift as shown in (Friedewald, 2023) which is promising for a reduction in aerodynamic loads, yielding lighter load carrying structures which in turn may improve overall aircraft performance. This work investigates the reduction of distributed gust loads due to detached flow for the elastic, free-flying aircraft with discrete gust spectra defined by CS25. The DLR TAU-Code is utilized to solve the RANS equations. Different turbulence models (Spalart-Allmaras, RSM SSG/LRR-lnω) are applied, as the problem of predicting the flow separation correctly remains. The time-linearized solution is achieved by scaling the time-marching responses to small gust amplitudes at the same gust gradients. The nonlinear simulation results are compared to the linear solution, to assess the potential of the load reduction. The investigated transport aircraft is the NASA Common Research model (Vassberg, 2008) for which the structural FERMAT model (Klimmek, 2014) is available. The results show that the SA and RSM turbulence model predict similar loads for cases with attached flow. For those gust parameters, the root bending moment of the time-linearized and time-marching calculations deviate by about 2%. For medium to long gust gradients large regions of the outer wing show flow separation during the gust encounter. Here, the RSM turbulence model predicts a root bending moment reduction from 12% to 19% where the SA turbulence model predicts a reduction in the range of 14% to 24%.