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

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. Gust disturbances are not necessarily small and may cause local regions with flow separation during the gust encounter. The detached flow limits the total lift which is promising for a passive reduction of aerodynamic loads. This potentially yields lighter load carrying structures which in turn may improve the overall aircraft performance. This work investigates the reduction of distributed gust loads due to detached flow for an elastic, free-flying aircraft in an open-loop simulation with discrete gust disturbances defined by CS25. The DLR TAU-Code is utilized to solve the URANS equations. Different turbulence models (Spalart-Allmaras, RSM SSG/LRR-lnω) are applied, as predicting the flow separation correctly remains a problem. The time-linearized solution serves as reference and is obtained by scaling the time-marching responses to small gust amplitudes at the same gust gradients. The nonlinear simulation results are compared with the linear solution, to assess the potential of the load reduction. The investigated transport aircraft is the NASA Common Research model. The results show that the SA and RSM predict similar loads for cases with attached flow. For medium to long gust gradients large regions of the outer wing show flow separation during the gust encounter. Even though simulations with both turbulence models predict large regions with detached flow, the dynamics of the shock motion and the flow separation are different. The SA turbulence model predicts drastic changes of the aerodynamic loads at the wing tip and thus excites structural oscillations which are not seen with the RSM. A reduction of the maximum root bending moment between −16% to −24% is found for the RSM compared to the time-linearized solution. For the SA turbulence model, this deviation is in the range of −21% to −29%.