Since the fuel crisis of the 1970s, aerodynamic design has become essential to the vehicle design process in order to reduce fuel consumption and lower emissions as well as (in more recent years) increase the range on vehicles with alternative powertrains. Production car manufacturers have developed shape optimisation techniques that have generated significant improvements over the years however, in order to achieve further gains, a deeper understanding of the fundamental flow structures around vehicles must be achieved.
This thesis reports the findings of three studies that aim to understand how the base pressure is manipulated on vehicle like geometries by applying shape optimisations. The base pressure typically contributes approximately 30% of the overall vehicle drag on production vehicles and so presents an opportunity for significant drag saving.
A fundamental One-Box model was used to investigate how changing fore-body drag can effect the base pressure and wake topology at varying ground clearances. It was found that at high ground clearances the total drag changes were generated by base pressure changes however when the model was lowered into ground effect the fore-body and skin friction drag produced significantly larger changes than the base pressure. Analysis of the unsteady results showed that with a thinner boundary layer over the model the unsteadiness in the wake was increased.
A second study was then conducted on a generic vehicle geometry, the Windsor model, where the lower separation was manipulated through the use of different underbody profiles . As the lower boundary layer on the model was thickened the lower recirculation region grew and lowered the pressure on the base. This was also seen to increase the unsteadiness of the pressures recorded on the base when the upper and lower shear layers were of significantly different strengths.
Finally, a rear end optimisation was conducted on the Windsor model using high aspect ratio tapers on the top and bottom trailing edges. It was seen that the amount of downwash or upwash created by the tapers acted on the wake balance which moved the impingement region on the base. This changed the near wall velocities of the wake flow and resulted in changing base pressures.
Overall the work has shown that, by controlling the flow conditions at separation, the base pressure can be modified, particularly by altering the relative strengths of the upper and lower shear layers and the impingement location on the base.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.