Investigation of cavity flows at low and high Reynolds numbers using computational fluid dynamics.
PhD thesis, University of Glasgow.
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Despite the amount of research into the cavity flow problem the prediction of the flow patterns, associated forces and acoustic phenomena remains an unsolved problem. The coupling of the shear layer dynamics, the internal vortical structures and the acoustics of the cavity make it a very complex flow despite the simple geometry. Once doors, stores and release mechanism are added the problem is compounded, thus accurate prediction methods are a necessity.
The cavity has been shown to oscillate in different modes depending on the flow conditions and the geometry of the cavity. Two modes of oscillation were examined in detail, these being the wake and shear layer mode, using computational fluid dynamics and experimental data where available. The flow code used is the in-house CFD solver PMB and the experimental data has been provided by DERA. The cavity geometry was for a 1VD=5 cavity with a W/D ratio of 1 for the 3D investigation.
For the wake mode the Reynolds number has been varied from 5,000 to 100,000 and the Mach number has been varied from 0.3 to 1.0 in order to examine the effect of changing conditions on this mode of oscillation. The characteristics of this mode of oscillation have been identified and a stable region within the varying Mach and Reynolds numbers has been shown. Outside of this stable region a blended flow has been identified.
For the shear layer mode of oscillation the open cavity environment has been examined. This cavity is of great interest as examples of it can be found in current airframes, the H- 111 for example. This flow type is characterised by intense acoustic noise at distinct frequencies which could cause structural fatigue and damage sensitive electronics. However, this cavity type also has a relatively benign pressure distribution along the length of the cavity making it ideal for store separation. The flow cycle predicted shows that the separated shear layer impact on the rear wall generates strong acoustic waves. These waves are further enhanced by the interaction of the wave with the vortices and upstream wall of the cavity. The flow conditions of interest for this case are M=0.85 and Re=6.783 million. A study of the effect of time step, grid refinement and turbulence model has been performed. It has been seen that the density of the grid and the turbulence model chosen must be considered as a pair; if the grid is too fine it may resolve scales being modelled by the turbulence model and result in a double counting of energy resulting in spurious results.
One area of cavity studies that has received only sparse investigation is the effect of 3-Dimensionality on the flow. One objective of this work was to try and rectify this. However, it was found that the choice of solver could play a significant role in the accurate prediction of the 3D cavity flow. For cases where the acoustic spectrum is broad, typical URANS codes may have difficulty in predicting these flows. Under such conditions DES or LES would be more appropriate choices. However, when the frequency spectrum is not as spread out URANS can provide good results. This can be seen in the 3D cavity case where doors are present and aligned vertically.
The wake mode, while identified in 2D. has received little attention in 3D. It is generally thought that the effect of the third dimension would be to trip the wake mode to shift to another mode of oscillation. This study has shown that this is indeed the case. The flow cycle shown is more reminiscent of the blended flows shown in some 2D cases.
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