The composition and generation of open-wheel racing car wakes
Creators: name: Watts, Matthew
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401201
subject: Aerodynamics (excl. hypersonic aerodynamics)

description: The aerodynamic environment experienced by an open-wheel racing car is often dominated by the wake of upstream vehicles. This environment can be detrimental to the aerodynamic performance of the car, impeding overtaking and as a result, the spectacle of the sport. Simulating this scenario scientifically is difficult, with computational approaches requiring empirical validation which in most cases necessitates wind-tunnel simulations. However the longitudinal length afforded by most existing wind-tunnel test sections limits such multi-vehicle experiments. A possible solution presented in this thesis is to generate the wake upstream of the wind-tunnel contraction, thereby allowing the entire working section for the test vehicle. Implementing this solution required characterisation of both the turbulent wake itself and the influence of a contraction over such wake flows. <br><br>As the first step, the wake of a generic open-wheel racing car was experimentally investigated between a distance of 0.5 and 1 car lengths behind the model thereby characterising the salient features and longitudinal development. The wake was generated over a stationary ground plane and included sensitivity studies to determine the influence of rear wheel rotation. Results agreed with previous studies performed in moving ground facilities with the salient wake features composing a counter rotating vortex pair generated by the rear wing and a velocity deficit produced primarily by the diffuser. The influence of wheel rotation was to the increase the diffusers efficacy although this change was small enough that its effect was primarily qualitative. Within the wake the longitudinal decay rate of vorticity was approximately twice that of both velocity deficit and turbulence intensity, in quantitative terms this indicated that velocity deficit and turbulence intensity were more persistent downstream. This result showed that vorticity did not dominate longitudinal wake development thereby allowing the contraction’s straining influence to be studied using a more generalised flow; a nominally two-dimensional cylinder wake. This case was chosen given the broad similarities to vehicle wakes and its well documented nature. <br><br>The cylinder wake was measured passing through a 3.7:1 contraction and a parallel duct allowing the influence of the contraction to be investigated using comparative analysis. The changes induced by the contraction’s strain field were found to be both dependant on the flow parameter in question and its orientation to the freestream; pressure-based terms, such as velocity, were most affected while turbulence parameters were less influenced. Streamwise and transverse flow components were differentially affected; this was particularly evident for the turbulent stress data which showed a suppression of the streamwise component but amplification of the transverse components. These findings were consistent with literature concentrating on grid and mesh generated turbulence with one exception; published research has shown the amplification of the transverse stresses was such that post-contraction values surpassed those upstream of the contraction, although in this research that was not the case. When comparing the strained and unstrained cylinder wake, amplification of the transverse stresses was evident post contraction, however the transverse stresses measured before the wake entered the contraction were always greater. The ultimate cause of this difference was due to the longitudinal decay rate of the cylinder wake, which was similar to the vehicle wake, and ultimately great enough to mitigate amplification by the contraction. <br><br>The experiments led to a proposed method for characterising the changes induced by the contraction. This consisted of two processes; virtual distance for longitudinal changes and geometric scaling for transverse wake geometry. Virtual distance postulated that the influence of a contraction could be thought of as altering the longitudinal distance the wake has travelled, rather than altering the wake itself. Conversely changes in the transverse wake structure were accounted for by simple geometric scaling using the respective ratio of settling chamber to test section height and width. As a proof of concept the open-wheel racing car wake was passed through the same contraction with encouraging results. The concept of generating a discrete wake profile upstream of a wind tunnel contraction remains promising, it is hoped the results of this research would be used as a basis for methodologies to design distorted wake generators that offsets the influence of the contraction.<br>