Flow separation and wake of a transport aircraft configuration at beginning stall
Creators: name:  M. EHRLE 
name:   T. LUTZ 
name:   E. KRAMER 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLIGHT PHYSICS
COMPUTATIONAL AERODYNAMICS
EXPERIMENTAL AERODYNAMICS

description: In order to characterize the behavior of passenger aircraft beyond the limits of the flight envelope, an understanding of the occurring flow physics is crucial. Since such conditions are difficult to predict, reliable simulation methods are key to these studies. Investigations of the post stall at the CRM [1] showed that scale resolving approaches are mandatory for the prediction of the wake evolution and the flow physics in the wake. At the investigated high angles of attack the use of DES-type methods is facilitated, as the separation is triggered directly at the wing leading edge by the geo-metrical shape and extends over the entire wing span. As wing stall already begins at moderate angles of attack, this region of the polar curve must be considered if the stall behavior is to be understood over the entire ?-range. Such conditions introduce the additional difficulties of variable separation position, varying spanwise stall characteristics, or transient reattachment phenomena. They pose a real challenge for hybrid RANS/LES models, since the boundary layer must be effectively protected against the intrusion of LES content to avoid unphysical grid-induced separation. In the present work, the flow around the NASA Common Research Model (CRM) in cruise configuration at be-ginning stall is investigated by means of hybrid RANS/LES simulations. A robust boundary layer shielding at these challenging conditions is enabled by using the BDES model [2]. In the shielding function of this hybrid RANS/LES model, the boundary layer is defined by the velocity at the boundary layer edge, which in turn is determined from the calculated pressure distribution using the Bernoulli equation. In order to validate the numerical setup, pressure measurements from the ESWIRP project [3] are analyzed and compared to the simulations. From the measured pressure distribution in different spanwise positions, beginning stall can be identified at the wing tip, which rapidly expands inward with increasing ? in a range between ? =13 and ?=15. This behavior can also be observed in the simulation, with the beginning stall shifting to slightly lower angles of attack. As representative example, Figure 1 shows the BDES simulation of the CRM at an angle of attack of 14. Flow separation is visualized by means of slices through the flow field, contours of time averaged cfx0 on the wing surface and time averaged surface streamlines. In the inboard area, attached flow is present on the suction side of the wing. Moving further outward to the midspan area, the flow separates at the leading edge. Near the wing tip, strong spanwise flow is observed. Moreover, the mean flow is attached in the vicinity of the leading edge. Fig. 2 shows the pressure distribution along the three spanwise slices through the wing shown in Fig. 1. It can be seen that the simulation is able to predict the pressure distributions in regions of separated and attached flow in good agreement with the experiment. The present work comprises investigations of the flow physics that drive the stall characteristics by means of flow statistics and spectral analyses. Furthermore, the influence of the angle of attack on the turbulence characteristics in the separation region above the wing and in the wake are investigated. It is shown that using the BDES model in conjunction with effective grey area mitigation, the onset of stall and the flow physics at the swept wing of a transport aircraft configuration can be simulated with good accuracy. References: [1] Waldmann, A., Gansel, P.P., Lutz, T., Kramer, E.: Unsteady wake of the nasa common research model in low-speed stall. Journal of Aircraft 53(4), 10731086 (2016). https://doi.org/10.2514/1.C033413 [2] Weihing, P., Letzgus, J., Lutz, T., and Kramer, E., Development of alternative shielding functions for de-tached-eddy simulations, Progress in Hybrid RANS-LES Modelling, contributions to the 7th Symposium on Hybrid RANS-LES Methods, Berlin 2018, p. 109-118. ). https://doi.org/10.1007/978-3-030-27607-2_8 [3] Lutz, T., Gansel, P.P., Waldmann, A., Zimmermann, D.M., Schulte am Hulse, S.A.: Time-resolved prediction and measurement of the wake past the CRM at high Reynolds number stall conditions. Journal of Aircraft 53(2), 501514 (2016). https://doi.org/10.2514/1.C033351