Browsing by Author "Edwards, John A."
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Item Open Access Investigation of computational techniques for the prediction of supersonic dynamic flows(Cranfield University, 1999-12) Roper, Jeffrey John; Edwards, John A.A computational investigation was undertaken to examine techniques for predicting supersonic dynamic flows, involving unsteadiness over fixed and moving surfaces. The fixed geometries examined were cylinder-flares and compression ramps, and the moving body geometries a pitching aerofoil and a rapidly deployed flap. Investigation into the characteristics of incipient separation of a supersonic cylinder-flare flow revealed that the separated length varied with a power of the flare angle and that the variation in height of the separated region varies in a bi-modal manner with flare angle. For small-scale separations (flare angles less than those which would traditionally have been expected to induce separation) the height of the separated region was seen to vary slowly with flare angle. For larger flare angles, the separation bubble was found to grow rapidly in height and length with increasing flare angle and produce significant deflection of the external flow. Computations of a Mach 5, compression ramp induced unsteady shock boundary layer interaction exhibited self-sustained oscillations at frequencies and amplitudes consistent with experimental data. Large dynamic structures (up to 1.7 boundary layer thicknesses in extent) were observed, and their production, propagation and deformation illustrated. By modifying the turbulent viscosities produced by a non-dimensional implementation of the Baldwin-Lomax turbulence model (using under- relaxation) a turbulence model was produced which accurately predicted separation lengths for a series of Mach 6.85 compression ramp flows encompassing laminar, transitional and turbulent flow regimes (dependent on ramp angle). A technique was developed to enable efficient computation of dynamically moving and/or deforming body flows. This technique was based on hierarchical, adaptive mesh refinement coupled with automatic generation of body surfaces, in which mesh adaption was used to capture the body geometry to within a specified accuracy. This, in conjunction with automatic cell creation and destruction, enabled the derivation of steady and unsteady, time accurate, conservative boundary conditions. This algorithm was used to compute a quasi-steady laminar supersonic pitching aerofoil flow, and an unsteady turbulent supersonic flap deployment. In both cases agreement with experiment was found to be good.Item Open Access Multiple and moving bodies: CFD research in an adaptive environment(2000-11) Ren, Tingrong; Edwards, John A.During this study a new solution adaptive scheme was developed which applied the structured Adaptive Mesh Refinement (AMR) algorithm directly into a Chimera grid system. Two new Computational Fluid Dynamics (CFD) methodologies, the Chimera grid method and the AMR algorithm, have been successfully integrated. The originality of this study is outlined below. 1).This new scheme integrated the Chimera grid method with the AMR algorithm. By using the Chimera grid method to mesh complicated configurations and then applying the AMR algorithm in different Chimera sub-domains to resolve complicated flow features the advantages of these two methods are combined. 2). Both Graph data structure and orthogonal list storage are employed in this new scheme to describing the Chimera sub-domain relationship and improve the region and point searching efficiency. These methods make this new scheme more flexible and more efficient than existing Chimera grid schemes and AMR schemes. 3). This study extended the application of the AMR algorithm. The main limitation of the Quirk's AMR code, poor geometric packing ability, has been replaced by a versatile Chimera grid scheme. 4). This study improved the accuracy of the Chimera inter-grid communication when simulating supersonic or hypersonic flows with strong discontinuities due to applying the AMR algorithm. 5). The combination of the versatile geometric packing ability of the Chimera grid method and the efficient computation ability of the AMR algorithm makes the current scheme capable of simulating multiple and moving body flow problem with strong discontinuities using moderate computational resources. The material in this thesis documents this development of the Chimera grid scheme with an existing AMR code, and the validation and applications of this scheme. Firstly, a Chimera grid scheme has been developed. Most of the tasks associated with the Chimera grid scheme are performed in a fully automated mode. The graph data structure and orthogonal list storage are used to describe the domain relationships. This allows the sub-domains to be arranged in an arbitrary manner and increases efficiency. Secondly, in order to properly integrate the Chimera grid scheme with the AMR, some modification work on the original AMR code had to be done. This work allowed the Chimera grid scheme to be successfully incorporated into the AMR code. Thirdly, this scheme has been employed to solve time-dependent and steady state shock hydrodynamics problems at supersonic and hypersonic speed. Various validation and application cases, such as, shock reflection, shock diffraction, shock/boundary interaction and multiple moving body flows at supersonic speeds are presented, analysed and compared with experimental results or numerical solutions either from the literature or obtained by using another CFD code. The simulations demonstrate the flexibility of the grid generation and the high efficiency and capability to resolve complex flow features. Finally, future research work arising from the present study has also been discussed and highlighted.Item Open Access A study of roughness in turbulent hypersonic boundary-layers(Cranfield University, 1993-12) Babinsky, Holger; Edwards, John A.The influence of large scale regular roughness on a Mach 5 turbulent boundary layer and a compression corner was investigated on axisymmetric wind tunnel models. Three types of roughness were examined; a series of square cavities at two different sizes and a 45 degree sawtooth. Typical sizes ranged from 50% to 100% of an undisturbed boundary layer thickness. The roughness was limited to a short region followed by a smooth surface. Compression corners were formed by 15° and 20° flares located downstream of the roughness. The flow in the wind tunnel was investigated in detail to obtain knowledge on operating conditions and flow quality. Liquid crystal thermography was developed for routine use in hypersonic blow-down wind tunnels with superior spatial resolution and experimental uncertainties in the range of traditional techniques. The effect on flow parameters downstream of the last roughness element were 7, found to differ significantly for the different quantities. Velocity profiles were found i, to be less full and skin friction was found to be reduced for all streamwise "~ distances. Surface heat transfer was increased in a short region limited to 1.5 boundary layer thicknesses behind the roughness whereas surface pressure was not affected. Sawtooth shaped roughness was found to cause a stronger j disturbance than square cavities of twice the size. Little influence of the roughness was noted on the flow over the compression corner. The flow over the 20° compression corner showed an increase in upstream influence for the sawtooth shaped roughness as well as the larger cavities. Surface pressure measurements did not indicate a separation in any case. Heat transfer measurements revealed a peak located approximately 0.25 boundary layer thicknesses behind the corner. No such feature was found in the surface pressure distributions. It is suggested that a small scale separation is located very close to the corner causing the peak in heat transfer at reattachment without any effect on surface pressures. The existence of such a separation has been confirmed by surface flow visualisations for both flares.