Unsteady and time-averaged near-wake flow over the rear of sedan automobiles

2017-11-09T03:07:38Z (GMT) by Brendan Gilhome
The near-wake region over the rear of sedan automobiles is characterised by unsteady flows. Until now, a comprehensive model for the structure of the near-wake has not been found. This thesis will provide a novel near-wake flow model for sedan automobiles. It will also provide both qualitative and quantitative evidence that the near-wake flow structure, over the rear of sedan automobiles has many similar characteristics to basic two-dimensional separating/reattaching flows. Three-dimensional flow in the near-wake region of automobiles provides many challenges for researchers. Investigation of the near-wake region required a novel approach in order to record useful, high quality data. Implementation of a wide range of experimental procedures, and several novel experimental techniques, allowed the determination of the unsteady near-wake structure. Time-averaged skin friction patterns over various sedan automobiles were analysed, and through the application of critical point theory the time-averaged near-wake structure was determined. Detailed time-dependent flow visualisation was performed using real­ time recording techniques of seeded flow and tuft patterns. Time-dependent flow visualisation provided the basis for the construction of an unsteady near-wake flow model. Quantitative measurements included high-frequency simultaneous surface pressure measurements, synchronised with unsteady wake probe measurements. These measurements defined the unsteady characteristics of the near-wake. The unsteady near-wake was found to consist of a large vortex. This vortex is characterised by its hairpin shape and is regularly shed from the near-wake at a Strouhal frequency of approximately 0.41. The separation region was also found to have a low­ frequency unsteadiness at a Strouhal frequency of approximately O.11. The normalisation length of both these Strouhal frequencies is the horizontal separation length. Hence, a new method of separation length determination over sedan automobiles was found. The near-wake flow was also found to consist of reverse flow directed toward the separation location. The near-wake structure, periodicity and characteristic length dimension were found to closely match those found in other separating reattaching flows. The two-dimensional backward facing step, bluff leading edge separations and separating flow from low-aspect ratio aerofoils were all found to consist of similar wake topologies. Direct similarities were also found with time-averaged fastback automobile near-wakes. Hence, suggesting a universal near-wake model. Determination of the near-wake structure over the rear of automobiles will provide the framework for efficient and advanced development of automobile shapes. Minimisation of automobile drag, increased ride comfort and improved automobile stability can now be approached from a position of knowledge. The work contained in this thesis will provide the basis for further detailed investigation, and provide a benchmark for validation of computational codes.