Simulation of Manoeuvring Fighter Aircraft with the Unstructured Chimera Approach

[Abstract] The simulation of manoeuvring fighter aircraft using the Chimera method for the moving control surfaces is discussed . The Chimera, or overset, method consist of combining several component meshes through interpolation boundary conditions. The described approach allows the intersection of component surfaces. For manoeuvre simulations, the time-accurate Favre averaged Navier-Stokes equations are solved using an ALE approach. The simulations are performed using the SimServer multidisciplinary simulation environment, and are fully parallelized. I. Introduction CCURATE pre-flight simulation of highly agile manoeuvring aircraft is a challenging task requiring the simultaneous modeling of the aerodynamic flow field, the flight control system response, the rigid body dynamics and often the structural response of the aircraft. Such simulations thus require a coupled multidisciplinary simulation environment. In this paper, the most computationally expensive component of such an environment, namely the high-fidelity, time-accurate simulation of the aerodynamic interaction of the aircraft including control surface deployment is considered. The evolution of Com putational Fluid Dynamics (CFD) and the rapidly increasing computational resources available, have enabled the aeronautical industry to model the aerodynamic behaviour of a design from the point in the design procedure where the aerodynamically active geometry is known. Traditionally, high-fidelity simulations have been performed assuming steady-state conditions, in which the CFD computations have complemented or replaced static wind-tunnel tests. In the military industry however, the products are often required to perform manoeuvres in which the aerodynamic flowfield can no longer be considered quasi -stationary. The highly unstable nature of agile modern fighter-aircraft also requires rapid control surface deployments to maintain the desired flowpath. This has resulted in the need for simulation software capable of accurate and efficient simulation of dynamic manoeuvres of complex configurations, including the direct simulation of control surface deployment. There are several possible approaches for such problems. One approach is to use the flexibility offered by unstructured mesh generation, to locally regenerate the computational mesh in regions where geometrical differences occur. This approach has been used, mostly in combination with mesh movement algorithms, for inviscid 1 as well as viscous 2 simulations. The advantage of such approaches is the existence of a consistent mesh for each timestep in the solution procedure. There are, however, difficulties with approaches of this class. A robust treatment of the re-meshing of changing geometries in general, and the intersection of control surface geometry definitions with the main aircraft structure in particular, is a potential problem with such approaches. In addition, the unstructured mesh generation must be of high quality to enable the several hundred local remeshings that can be expected for realistic simulations, without failure. The computational cost of these remeshings may also be prohibitive, depending somewhat on the approach used. Remeshing also introduces interpolation errors and issues with geometric conservation which may result in unnecessary fine point distributions for a given accuracy leve l. In this paper, a different approach is considered. The Chimera, or mesh overset, approach 3,4 consists of allocating a separate mesh for each moving component of the aircraft, generated before the simulation is started, and then combining these meshes on the fly during the simulation. This requires an automatic routine to connect the collection of meshes, and thus enable each separate mesh to detect the interaction with the other components. This approach is attractive since it, under a given set of assu mptions, cannot fail to produce a valid computational system