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Published: 2006

Total Pages: 119

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Many attempts have been made by researchers, worldwide, to comprehend the physics of separated flows. Study of flow separation is vital as it is encountered in many engineering applications, and is generally detrimental. One such example is flow through a low pressure turbine (LPT) cascade, at relatively low-Re values, where flow separates on the suction surface of the LPT blade, and adversely affects the efficiency of the aircraft engine. Contemporary research is focused on understanding the physics of the separated flow, and identifying control strategies to delay or, if possible at all, prevent the flow separation phenomenon. The main objective of the present research is to study a model separated flow, and identify a control strategy, which can subsequently be applied to manage the flow in the LPT cascade. To achieve this, a model problem of flow past a circular cylinder is considered, as the geometry for this flow is simple and facilitates a focus on the flow itself. Despite of its simple geometry, the flow past a circular cylinder exhibits a variety of complex flow features which make this a challenging problem to solve. As a validation study, the flow for Re = 3,900 is simulated, and the results obtained are compared with the numerical and experimental data available in the literature. For the flow control study, a baseline solution for flow past a circular cylinder at Re = 13,400 is obtained as a first step towards implementation of flow separation control for preventing or delaying the flow separation. The Re value of 13,400 ensures laminar separation and serves to approximate the flow conditions prevailing in a LPT cascade. Later, flow control is incorporated by employing vortex generator jets (VGJs) on the upper surface of the cylinder at about 750 from the stagnation point. The jets are issued into the flow with a blowing ratio of 2.0 and are pitched and skewed by 300 and 700, respectively. A non-dimensional pulsation frequency F+ of 1.0 is used, along with 50% duty cycle. With this understanding, VGJs are then incorporated for the LPT cascade flow. VGJs are placed in a range of 63.5% to 67% Cax. All the jet parameters, i.e., blowing ratio, pitch angle, skew angle and duty cycle ratio, are kept the same as for the cylinder case, while the F+ value of 2.33 is employed for the LPT cascade problem. The three-dimensional, unsteady, full Navier-Stokes equations are solved to obtain accurate prediction of unsteady separated flows governed by the Navier-Stokes (N-S) equations. A fourth-order accurate, compact-difference scheme is used for spatial discretization, with sixth-order filtering to minimize the oscillations in the flow solution. For the cylinder, a multi-block structured grid generated using the grid generation software, GRIDGEN, is used for the present numerical analysis. The grid contains approximately 3.9M grid points, and approximately 70% of the total grid points are concentrated in the wake region to capture the small scales that are expected to exist in this region. A MPI-based higher-order, Chimera version of the FDL3DI flow solver developed by the Air Force Research Laboratory at Wright Patterson Air Force base is used for the numerical computations. PEGSUS a NASA Ames research code is used for storing the connectivity data at the block interfaces. The baseline case for the cylinder flow at Re = 13,400 displays a wide range of vortical structures in the wake region. The separating shear layers are subject to spanwise instability which leads to the formation of an unsteady and three-dimensional wake, with the characteristic features of typical turbulent flow. It is observed that after the jets are being turned on, the pressure on the surface of the cylinder redistributes in a way so as to reduce the pressure drag significantly. The total pressure loss coefficient and momentum thickness are calculated in the wake at x/D = 3.0 and x/D = 5.0, and are found to reduce by 10% and 30%, respectively. The flow control simulation for the LPT cascade flow reveals 27% reduction in total pressure loss coefficient, along with the total elimination of separation upon application of VGJs.