Drag measurements on bluff-bodies such as bobsleighs using force balance systems have been in prevalence for a few decades. However these studies do not reveal anything on the flow behaviour around the body. In the recent past, various flow visualization techniques have been applied to investigate the flow behaviour around bobsleighs. Even though these studies have highlighted the qualitative flow features, there is a lack of a deep insight into the wake flow topology and the behaviour of the flow structures that are formed in the highly three-dimensional flow. Additionally, when the bobsleigh moves in a curvilinear path, the front cowling rotates with respect to the rear cowling, causing an increase in the frontal area exposed to the wind and in turn a higher drag. However, this perception is not quite straightforward and requires a thorough understanding of the mechanisms and sources of drag generation, in particular at different front cowling angles.

Stereoscopic Particle Image Velocimetry has been used to study the near-wake flow topology of a scaled bobsleigh model. From the velocity field information obtained from PIV, the pressure field in the wake of the bobsleigh model is determined from the Pressure Poisson Equation (PPE) using the methodology outlined by Oudheusden [41]. With the knowledge of the velocity and the pressure fields, the aerodynamic drag is computed using the control volume approach on a wake plane which is at approximately two diameters from the model. The drag obtained from PIV is compared to that obtained from balance measurements. The effects of different bobsleigh configurations on the aerodynamic drag is observed. Secondly, uncertainty quantification of the aerodynamic drag is performed. Finally, a Proper Orthogonal Decomposition (POD) analysis is performed for the velocity fluctuations in order to gain an insight on the motion of the flow structures formed in the wake.

The investigation of the near wake flow topology of the bobsleigh reveals the presence of two counter-rotating vortices with a significant downwash between them in case of the reference configuration. The motion and behaviour of these vortices is obtained by performing the PIV analysis on multiple vertical wake planes. The investigation of the effect of the front cowling misalignment on the drag reveals that there is a reduction of drag for smaller misalignment angles (upto 5°) and a marginal increase in drag for further increase of misalignment (from 5° to 20°). From the analysis of aerodynamic drag conducted on a vertical plane that is approximately two diameters behind the model, it can be seen that the momentum deficit is the highest contributor to the aerodynamic drag (88%) followed by the Reynolds stresses (9%) and pressure (3%). It is also observed that the aerodynamic drag obtained is in good agreement with the results obtained from the balance measurements with a variation of only 2 to 3%. The uncertainty analysis of drag shows that among the different flow variables which affect the drag, the average pressure has the maximum uncertainty. Finally, the POD analysis of the velocity fluctuations shows that the first two fluctuating modes which are the most energetic are not perfectly correlated to each other due to the highly three-dimensional nature of the flow.