by Ben Evans, CFD Engineer - BLOOHOUND aerodynamics
One of the unique and crucial aspects of the BLOODHOUND SSC aerodynamics is the supersonic nature of the flow, and in particular the presence of shock waves and their interaction with the ground. BLOODHOUND will be the only land-based vehicle in the whole of history to travel for a sustained period of time well above the speed of sound.
Shock waves form because of the nature of the propagation of sound waves through the air. Under normal subsonic conditions, any body, whether it be a jogger or Formula 1 car, transmits pressure disturbances ahead of it in the form of sound waves. These waves carry the ‘information’ to air molecules ahead of it that the body is coming and that they need to start moving out of the way. However, when the body itself is travelling at the same speed (sonic) or faster (supersonic) than the speed of these sound waves, it can no longer transmit this information forward and the waves ‘bunch up’ right in front of the vehicle forming a shock wave and audible ‘sonic boom’.
The flow properties around the vehicle no longer vary smoothly but change rapidly at the shock waves. This is shown in the pressure contours over one of the early Bloodhound concepts travelling at Mach 1.3 - see the picture at the top of the page. An understanding of how these shock waves interact with the vehicle and with the desert surface has been a crucial part of the aerodynamic research for the BLOODHOUND program.
The aerodynamic behaviour of individual components of BLOODHOUND SSC has been studied in conjunction with studies of the vehicle as a whole.
A modelling of the flow around the base of the wheels under various configurations has been very important in understanding how they will interact with the desert surface. The overall dimensions of the wheel had been fixed, at the outset, by structural integrity specifications. Aerodynamic considerations were then applied to design of the finer detail of the wheel profile. Typical flow patterns around the base of the front wheels is indicated in the figure below.
Left: Flow patterns around the ‘side-by-side’ front wheel configuration
Obviously, analysing stand alone components, such as intake, duct, or winglets, in isolation has its merits. However, it is not until all of these components fit together, and the full vehicle aerodynamic behaviour analysed, that we can understand how these components interact with each other aerodynamically.
In addition to the determination of the variation in the lift and drag coefficients for the vehicle over the Mach number range, the aerodynamic response of the vehicle needs to be understood as a function of yaw, pitch and ride-height. The effect of pitch angle on lift for one of the early BLOODHOUND concepts is shown the figure below. This indicates that the slender shape of BLOODHOUND is relatively insensitive to pitch angle over the range that is likely to experienced in the desert runs. This means that the effect of the vehicle’s pitch (or angle of attack) has a relatively small effect on its lift, in comparison with a wing for example. This is an important property as far as stability and, therefore, safety of the vehicle is concerned.
Left: Effect of vehicle pitch angle on lift
The figure below shows the effect of vehicle ride height on lift and drag. Notice that the drag is insensitive to ride height, yet the lower ride height generates an increase in download over the vehicle. This can be explained quite simply by using Bernoulli’s famous theorem, which states that, due to the principle of conservation of energy, the total energy of a lump of fluid moving with the flow must remain constant. If compressibility of the air effects are ignored, this implies that, as the fluid accelerates to pass through the tight space under BLOODHOUND, the corresponding increase in kinetic energy must be compensated for by a decrease in some other form of energy, in this case, pressure. The implication is that the lower the car is to the ground, the faster the flow must accelerate to ‘squeeze’ underneath, the lower the pressure experienced by the base of the car, and hence the greater the suction effect pulling it towards the ground. A bit of download is good, too much is not (at 1000 mph we can generate huge forces!), so we are using this data to find the ideal ride height.
Although the designed maximum speed ofBLOODHOUND is 1050mph, the aerodynamic response of the car has to be such that it is safe and drivable right from its initial roll all the way up to this top speed. This makes the design loop for optimisation of the geometry an extremely complex activity especially since as BLOODHOUND accelerates and decelerates through the ‘transonic’ regime (just below fully supersonic) when parts of the flowfield are supersonic, and parts remain subsonic, conditions (such as positions of shock waves) are changing rapidly. All in all, this makes working on the BLOODHOUND aerodynamics both a massive privilege and challenge!