Evaluation of URANS and DES predictions of vortical flows over slender delta wings

[摘要] This thesis presents a study of vortical flows and vortex breakdown over slender delta wings. The formation of leading edge vortices over a slender delta wing provide advantageous aerodynamic characteristics at low velocities and angles of incidence. However, as the incidence of the planform is increased these vortices are affected by changes in the flow behaviour, which causes them to become unstable and breakdown into an incoherent form. This vortex breakdown is detrimental to the aerodynamic characteristics of the wing and can cause instability of the aircraft. Due to this adverse effect, it is important to understand the behaviour of such flows. Computational fluid dynamics is one method which, due to the development of numerical methods and the rapid advances in computer technology, is becoming increasingly valuable for the analysis of vortical flows and vortex breakdown and this thesis assesses the use of CFD to predict these types of flows. To perform this assessment two test cases are considered under different flow regimes. The first test case considers transonic flow and is investigated using steady state calculations and the second test case considers the unsteady subsonic behaviour of the flow. The behaviour of the flow over slender delta wings under transonic conditions is highly complex. With the occurrence of a number of shocks in the flow, vortex breakdown is abrupt and the overall behaviour is quite different to that for subsonic flow. To consider this, the flow over a 65° sharp leading edge delta wing is analysed for a transonic Mach number of M = 0.85 at two incidences: ? = 18.5° and 23°. A Boussinesq based RANS turbulence model is used which has been modified for vortical flows. The flow solutions are compared to existing experimental data and show good agreement for the cases considered and a number of shock systems within the flow are identified. However, a discrepancy with the experimental data is shown where the critical incidence for the onset of vortex breakdown on the wing is under-predicted. A sensitivity study of the flow to a number of computational factors, such as turbulence model and time accuracy, is undertaken. However, it is found that these parameters have little effect on the overall behaviour of the transonic flow and the occurrence of vortex breakdown. From analysis of the solutions, it is determined that the onset of vortex breakdown is highly dependent on the vortex strength and the strength and location of the shocks in the flow. (Abstract shortened by ProQuest.).

[发布机构] University:University of Glasgow

[关键词] Aerospace engineering