Thrust SSC CFD analysis by Swansea University

The Role of CFD in Modern Aerodynamic Design

In the aerospace, automotive and related industries, the aerodynamic performance of new designs has traditionally been investigated by using wind tunnel experiments. In such experiments, a scale model of the vehicle, made to a high degree of accuracy, is held in the working section of the tunnel; air is passed over the model and the forces and moments on the model are measured. When performing the experiments, appropriate scaling factors have to be employed, to ensure that the main aerodynamic parameters are close to those encountered in the real, full scale, flow. Although wind tunnel testing has been a key ingredient in the design of most aircraft in use today, the approach is lengthy and expensive, with a single modern design often utilising thousands of hours of tunnel testing time. The building of models is costly, and minor changes in geometrical shape often require the construction of a new model. The tunnels themselves are expensive to build and operate and they have limited applicability for a full range of flight conditions.

The high speed wind tunnel testing of aircraft in cruise conditions normally involves air being passed over models which are held well away from the tunnel walls. The correct experimental procedure for a vehicle moving on the ground should involve moving a model at the required speed relative to a stationary simulated ground or moving the simulated ground at required speed with respect to a stationary model. Tunnel facilities capable of moving the simulated ground up to 150 mph are used in automotive design and Formula 1 cars. However, this speed falls too short of the World Land Speed target. This meant that, if tunnel testing was to be employed, the best that could be envisaged would be tests in which a model was held at rest close to a simulated stationary ground in a high speed stream. In fact, this approach had already been employed, with a limited degree of success, in the aerodynamic design of THRUST 2. It is now known that this vehicle was operating at the limits of its capability, and it has been estimated that it would have lost contact with the ground if its peak speed had been only seven miles per hour faster.

Over the past thirty years, the aerospace and the automotive industries have been making significant developments in an alternative testing procedure, based upon the use of computer simulation methods for the analysis of the aerodynamic performance of vehicle designs. During this period, as wind tunnel costs have increased, the cost of high performance computers has decreased, and computers capable of performing certain complex flow simulations are now widely available. The process of using computers in this way to simulate realistic flows is termed computational fluid dynamics, CFD.

In computer form, the geometry of vehicle designs can be readily defined and modified and, hence, computational fluid dynamics offers the aerodynamicist a means of exploring a wider range of vehicle shapes than can usually be accomplished, in available time scales, with wind tunnel testing alone. However, computational fluid dynamics has its own associated shortcomings. These are generally related to difficulties in modelling mathematically, and computing, flows involving the complex phenomena associated with extremes of aerodynamic design, such as the prediction of flow separation and turbulence. Lower order mathematical representations of fluid flow, involving simpler flow physics, can avoid some of these difficulties, while still providing useful information for many practical aerodynamic flows. The type of model used in the simulation is often dependent on the accuracy required, the computer power available and the time scale to perform the analysis.

Over the last two decades, Professor Ken Morgan, Professor Nigel Weatherill and Professor Oubay Hassan at Swansea University have lead the development of state of the art techniques for the simulation of complex aerodynamic flows. The procedure has been fully automated and highly tuned for the use in the aerospace design cycle. These procedures form the basis of the original FLITE system, extensions of which are now in production use in AIRBUS, BAE SYSTEMS and Rolls Royce.