Experimental and computational investigation of flow about low aspect ratio ellipsoids at transcritical Reynolds numbers

As the role of unmanned underwater vehicles expands it becomes increasingly important to understand the nature of the fluid flow around them. This research examines the flow around two ellipsoids with generic shapes representative of streamline unmanned underwater vehicles (UUV). Although a significant body of work, both experimental and computational, exists for flow about spheroids the majority of this is for prolate spheroids with finer aspect ratio. This research examines the flow around a 3‚Äö-1 prolate spheroid and a 4.2‚Äö-2‚Äö-1 scalene ellipsoid. Many of the previous studies have focused on the major crossflow separation that occurs on a 6‚Äö-1 prolate spheroid when placed at medium to large incidences. This study examines the flow around these bluffer bodies at low to moderate incidence in transcritical flow. These are the conditions that many UUV's spend the vast majority of their time operating in, and is thus of importance when assessing their operational envelope. At low to moderate incidence a closed separation on the flank is found to be the dominant flow feature for the 3‚Äö-1 spheroid and the 4.2‚Äö-2‚Äö-1 ellispoid. For the 4.2‚Äö-2‚Äö-1 ellipsoid at lower Reynolds numbers an open separation occurs on the flank upstream of the closed separation. An extended length of attached flow on the suction side of the symmetry plane was observed for these models at incidence. The reasons for this attached flow despite a considerable length of adverse streamwise pressure gradient are identified to be due to the influence of the azimuthal pressure gradient on the boundary layer. Ideally computational fluid dynamics (CFD) could be used to examine the flow about these shapes during the design process. However before this process is useful there needs to be an understanding of the strengths and weaknesses of the techniques being applied. Calculation of the three-dimensional flow around these vehicles presents a number of significant challenges including boundary layer transition and boundary layer separation off smooth doubly curved surfaces. The experimental work has identified flow features and trends with Reynolds number; a considerable amount of quantitative data is also presented. The ability of CFD techniques to calculate the features and trends identified in the experimental work can be used as an indication of their veracity. Numerical studies using two-equation turbulence models modified to allow predetermined regions of laminar flow are presented. Qualitative and quantitative comparisons between the measured and calculated results are presented. Limitations identified in the CFD modelling techniques used include: premature boundary layer separation at the rear of the model, typically on the pressure side; and separation of the laminar region prior to the measured transition region at low Reynolds numbers. A number of experimental techniques were refined during this work. These include a quick and accurate method of applying discrete element boundary layer trip strips, which is particularly suited to three-dimensional shapes; improvements to a fast response total pressure probe; and an oil flow visualisation technique using a mixture that is close to neutrally buoyant and may be formulated to alter the viscosity over a large range.