Analysis of boundary layers through computational fluid dynamics and experimental analysis

Boundary layers, although unnoticeable to the naked eye, are a characteristic found where the velocity of a fluid relative to a solid surface is zero, hence in the vicinity of this surface a region of velocity increases rapidly from zero up to main stream velocity. This has the undesirable effect of causing turbulent flow over long surfaces, which reduce the performance of moving solids (examples being cyclists, automobiles, aircrafts) by introducing frictional drag forces. It has been noted that skin-fnction drag is strongly enhanced by the onset of turbulence, however, in turn this turbulence is also greatly enhanced by the separation of the airflow. In combination this laminar flow separation and skin-fnction drag decrease lift effects and/or increase pressure drag on aerodynamic surfaces, resulting in decreased efficiency, that is, increased fuel consumption. This research aims to examine the development of the boundary layer. Specifically the study will look at the effects of separation within the boundary layer and on its impact on the development of the boundary layer and other aspects of the fluid flow. Computational Fluid Dynamics or CFD is the analysis of systems involving such phenomena as fluid flow, by means of computer-based simulation. This technique is very powerful and spans a wide range of applications CFD has lagged behind other simulation techniques due to the complexity of fluid flow behaviour. However, the availability of affordable high performance computational hardware and the introduction of user friendly interfaces, have led to a recent upsurge of interest in the development of CFD simulation packages such as FLUENT. Using CFD a computational model is built to represent a system or device that is under investigation. The advantages of using CFD are to substantially reduce lead times and costs of new designs, also it is a means of predicting certain outcomes before manufacture. To conduct the experimental stages of this research a wind tunnel facility was designed and constructed inhouse. In conjunction with this, 2D CFD models o f the wind tunnel test section were developed. The wind tunnel facility was then used to validate the readings obtained from the CFD models. A number of blunt plate test pieces were placed in the wind tunnel and the resulting boundary layer development across the test pieces was measured A 2D CFD model was also constructed and the results were consistent with experimental findings. Experimental results revealed a reattachment o f the flow, with the surface o f the test pieces, at a point o f x/d = 4 444 from the leading edge of the test piece