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From clinical studies it is well known that atherosclerosis has preferred locations in the vascular system, primarily sited in the carotid arteries, coronary arteries, and in vessels supplying the lower extremities in the arterial system. In the vicinity of bifurcations flow tends to separate forming re-circulation regions. In addition, due to the pulsatile character of blood flow during the deceleration part of the cycle, the flow becomes unstable and transition to turbulence may occur. Vascular stents provide a novel method in treatment of atherosclerotic vessels. Although stents have dramatically decreased the re-stenosis rate of vessels compared to balloon-angioplasty, restenosis still occurs in 25-30% of coronary implanted stents. Understanding how stents influence flow patterns may lead to more hemodynamically compatible stent designs that alleviate thrombus formation and promote endothelialization. The first study employed time-resolved Digital Particle Image Velocimetry (DPIV) to compare the hemodynamic performance of two stents in a compliant vessel. The first stent was a rigid insert, representing an extreme compliance mismatch. The second stent was a commercially available nitinol stent with some flexural characteristics. DPIV showed that compliance mismatch promotes the formation of a ring vortex in the vicinity of the stent. Larger compliance mismatch increased both the size and residence time of the ring vortex, and introduced in-flow stagnation points. These results provide detailed quantitative evidence of the hemodynamic effect of stent mechanical properties. Better understanding of these characteristics will provide valuable information for modifying stent design in order to promote long-term In the second study, DPIV was utilized to explore the fluid dynamics phenomena in a symmetric compliant bifurcation. We studied the effects of the Womersley and the Reynolds numbers under pulsatile flow conditions. New insight of the fluid mechanics is revealed. The flow topology results indicate that the formation of coherent vortices in the vicinity of the bifurcation apex is governed by physical process that dictates the energy and strength of the formed vortices. This is manifested by the identification of a characteristic dimensionless time-scale that combines the impulsive vortex formation with the inertia of the unsteady flow. / Master of Science

Published Article / A stream flow at an outlet of a catchment can be entirely a base flow or direct flow or a combination of both. The base flow component of a stream is mainly contributed from the ground water storage which often is an open aquifer whereas the direct flow component is mainly the result of a direct response of a rainfall event. The Upper Modder river basin catchment is considered to be the origin of the Modder River which supplies water to Rustfontein Dam, situated at the outlet of the C52A with an area of 928 km2. Nine years of daily stream flow showed a continuous none zero discharge throughout the year. During the rainy season the discharge of the stream increases significantly. Thus, it is necessary to separate the direct and base flow of the stream in order to understand the important component that is more likely to be affected by different land use changes in a catchment. The Modder river daily mean flow at the inlet of Rustfontein dam (in Central South Africa) was analysed using four base flow separation methods, the Nathan & McMahon (N&M), the Chapman, Smakhtin & Watkins (S&W) method and the frequency duration analysis. All the methods gave higher percentage of the low flow component, except for the S&W method which underestimated it. The N&M filtering equation gave base flow components greater than 66% in 1999 and increased to 84% in 2007 while the Chapman equation revealed 65% and 74% in 1999 and 2007, respectively. Similarly, the frequency duration analysis gave 62% in 1999 and increased to 79% in 2007. The frequency duration analysis gave up to 13% lower percentage than the N&M (1990) filtering equation. The nine year base flow averages are 69%, 69% and 75% for frequency duration analysis, Chapman (1999) and N&M (1990) filtering equations, respectively. The result revealed that the Modder River is largely supplied by the ground water discharge. The result seemed to concur with the fact that for a semi-arid catchment such as the Modder river basin, with an average annual runoff coefficient of approximately 6%, the contribution of annual rainfall to direct runoff is very minimal.

A systematic approach of testing and validating transition models is developed and employed in testing of a recently developed transition model. The testing methodology uses efficient computational tools and a wide range of test cases. The computational tools include a boundary layer code, single zone Navier Stokes solver, and a multi-block Navier Stokes solver which uses MPI and is capable of handling complex geometries and moving grids. Test cases include simple flat plate experiments, cascade experiments, and unsteady wake/blade interaction experiments. The test cases are used to test the predicting capabilities of the transition model under various effects such as free stream turbulence intensity, Reynolds number variations, pressure gradient, flow separation, and unsteady wake/blade interaction. Using the above test cases and computational tools a method is developed to validate transition models. The transition model is first implemented in boundary layer code and tested for simple flat plate cases. Then the transition model is implemented in single zone Navier Stokes solver and tested for hysteresis effects for flat plate cases. Finally the transition model is implemented in multi zone Navier Stokes solver and tested for compressor and turbine cascade cases followed by unsteady wake/blade interaction experiments. Using the method developed a new correlation based transition model (Menter et al. 2004) which uses local variables is tested and validated. The new model predicted good results for high free stream turbulence and high Reynolds number cases. For low free stream turbulence and low Reynolds number cases, the results were satisfactory.

Like virtually every other human activity, air transport has an impact on the environment and similar to all other industries environmental impacts and economic issues are exerting more pressure on aircraft sector to meet the demands and implicated conditions. Secondly in today’s competitive industrial performance index new modern techniques are being introduced to improve the aerodynamics so that the efficiency of the newly designed aircrafts could be enhanced. The active flow control techniques have been proved vital towards achieving more effective air flow on the aircraft wing and that eventually helps to increase the lift coefficient at full scale flight. Synthetic jet actuators have been experimentally proved a promising technique towards achieving flow separation delay on the surface they have been deployed on. For the operation the synthetic jet actuators offer a unique characteristic in that they make use of the ambient work fluid and that deny the need of any extra fluid from outside the system and that helps in two fold. Firstly the need to make additional arrangements for air supply through the complex piping system has been ruled out completely. Secondly in the system the addition of any new weight that usually associates with the introduction of any new technique has been avoided. In this work firstly the dye visualization technique is used to study the interaction of the synthetic jet with both types of boundary layers that is laminar and turbulent. Secondly PIV measurements are performed to quantitatively analyze the evolution of vortical structures in the boundary layer. The aim is to understand the fluid dynamics involved in the interaction of the vortical structures with the neal wall fluid that ultimately re-attach the flow with the surface. Lastly an artificial flow separation is generated on the deflected flat plate surface and the synthetic jet is deployed to observe the separation delay on the surface. Various vortical structures have been generated by operating the actuator at varying parameters and issued into the boundary layer upstream of the separation line. The effectiveness of each type of vortices has been evaluated quantitatively to work out the optimum parameters at which the actuator must be operated to achieve the best control effect at the given free-stream condition.