Investigation of Cavity Flows at Low and High Reynolds Numbers Using Computational Fluid Dynamics By

Despite the amount of researchinto the cavity flow problem the prediction of the flow patterns, asso-ciated forces and acoustic phenomenaremains an unsolved problem. The coupling of the shear layerdynamics, the internal vortical structures and the acoustics of the cavity make it a very complex flowdespite the simple geometry. Once doors, stores and release mechanism are added the problem iscompounded, thus accurate prediction methods are a necessity.The cavity has been shown to oscillate in different modes depending on the flow conditions andthe geometry of the cavity. Two modes of oscillation were examined in detail, these being the wakeand 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 byDERA. The cavity geometry was for a UD=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 Machnumber has been varied from 0.3 to 1.0 in order to examine the effect of changing conditions onthis mode of oscillation. The characteristics of this mode of oscillation have been identified and astable region within the varying Mach and Reynolds numbers has been shown. Outside of this stableregion a blended flow has been identified.For the shear layer mode of oscillation the open cavity environment has been examined. This cav-ity is of great interest as examples of it can be found in current airframes, the F-I II for example.This flow type is characterised by intense acoustic noise at distinct frequencies which could causestructural fatigue and damage sensitive electronics. However, this cavity type also has a relativelybenign pressure distribution along the length of the cavity making it ideal for store separation. Theflow cycle predicted shows that the separated shear layer impact on the rear wall generates strongacoustic waves. These waves are further enhanced by the interaction of the wave with the vor-tices and upstream wall of the cavity. The flow conditions of interest for this case are M=0.85 andRe=6.783 million. A study of the effect of time step, grid refinement and turbulence model has beenperformed. It has been seen that the density of the grid and the turbulence model chosen must beconsidered as a pair; if the grid is too fine it may resolve scales being modelled by the turbulencemodel 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-Dimensionalityon the flow. One objective of this work was to try and rectify this. However, it was found that thechoice of solver could play a significant role in the accurate prediction of the 3D cavity flow. Forcases where the acoustic spectrum is broad, typical URANS codes may have difficulty in predictingthese 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 beseen 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 thoughtthat the effect of the third dimension would be to trip the wake mode to shift to another mode ofoscillation. This study has shown that this is indeed the case. The flow cycle shown is more remi-niscent of the blended flows shown in some 2D cases.