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The flow over a round cavity of aspect ratios of 0.2, 0.5 and 2 for a subsonic flow of Mach number 0.6 are numerically investigated using ANSYS FLUENT. Cavity flows find applications in aerospace industry such as interior storage carriages, weapon bay, wheel wells and also in daily life application like car sunroofs, urban street canyons, ship hulls, high-speed trains pantograph recess etc. The objective of the present work is to provide an insight on the flow behaviour in and around the round cavity as the cavity aspect ratio varies. Cavity flows give rise to a complex flow mechanism initiated by a simple geometry. In that way, they are an extensive choice for computational fluid studies. Flow past the rectangular cavities has been numerically studied in the past; nevertheless, very few papers investigate on round cavities. Results reveals that, the cavity geometry changing from rectangular to round eliminates the corner vortices evolved at the leading and trailing corner walls, shear layer formation, its convection, impingement on trailing edge, secondary and primary eddies interaction etc are resolved.
Numerous investigations has been done to investigate the flow dynamics and characteristics over cavities in the past which has got a wide range of applications in automobile industries, gas transport systems, wheel bays and weapon bays of aircrafts and in numerous aerospace applications. The turbulence in the shear layer and the tonal components instigates various components of the cavity noise spectrum. This is induced by two mechanisms: a wake mechanism and a shear-layer mode mechanism. The first one is due to a periodical vortex shedding at the leading edge of the cavity while the other one is due to a feedback looping between the flow field and the acoustic field. Strong acoustic radiations were emitted from rectangular cavity surfaces, when subsonic and supersonic flows pass over the cavity surfaces. The feedback mechanism depending on shadow graphic observations for rectangular cavities of different aspect ratios were described earlier by Rossiter, but Powell earlier related it as a mechanism for edge tones. In reference to experimental results, a semi-empirical formula was derived by Rossiter for the dimensionless frequencies (Strouhal number) of this periodic phenomenon.
Gharib and Roshko conducted experiments in incompressible flow and observed that the flow over rectangular cavity becomes unsteady and more chaotic when the aspect ratio of the cavity was increased. A big vortex is formed at the leading edge of the cavity which entirely fills the cavity and gets ejected out of the cavity at the trailing edge when it gets further bigger. This flow inside the cavity affects the flow above the cavity and the free stream fluid goes in and out of the cavity in a periodic manner.
Experimental researches on cavity flows are dominated by near-field flow feature’s and cavity-wall pressure fluctuation’s studies.
Studies about cavity flows exist in literature as per computational point of view. The two mechanisms of shear-layer and wake modes in the frame of aero acoustic research have been numerically investigated using direct numerical simulation (DNS) by many authors. In such studies the computational domain includes the region of propagation of acoustic waves generated at the cavity.
Neverthless the various modes of oscillation can have implications in several aspects rather than different noise generation patterns. Even though many investigations discusses about the generation of compressive wave, shear layer interaction, vortex formation and generation of acoustic waves, there are only limited studies on flow around round cavities. The present works demonstrate the fluid flow dynamics and its characteristics around the round cavity and acoustic response from the cavity as the aspect ratio varies from 0.2 to 2 and for a subsonic inflow of Mach number 0.6.
Geometries and mesh for computation were created in ICEM CFD and the flow analysis was carried out in ANSYS FLUENT 15. The mesh is refined near the cavity region to resolve the boundary layer and coarse meshes are given at region away from cavity. Cavities of aspect ratio 2,0.5 and 0.2 are considered for a compressible flow of Mach number 0.6. The Reynolds number is taken as 35000 and the viscosity of the air is changed with respect to the Reynolds number. The two-dimensional compressible turbulent flow is solved using Large Eddy Simulation (LES) model in ANSYS FLUENT.A boundary layer profile with a flow velocity corresponding to Mach number 0.6 and boundary layer thickness of 0.15 is given at the inlet using a user defined function.
A grid independence study was carried out by considering three grid levels namely Grid 1(250X330), Grid2 (300X350) and Grid3 (330X350) to obtain an optimised mesh for computation. The stream wise velocity profile was considered at three locations for the grid dependence study. The grid 2 of 300X 350 was found to be the optimised mesh for further analysis of the present work after the study. The computation results of the solver used in the current is validated with the experimental results of Forestier.et.al. for Mach number(M)=0.8 over an open cavity of L/D=2 and Reynolds number based on length Rel=8. 6×105.
The stream wise velocity profile from the simulation matches almost with that from the experimental results which plots for X/L=1.44 of U/U∞ V/S Y/L for M=0.8 & for L/D=2.
The pressure contours for the given aspect ratios 2.0, 0.5 and 0.2 for M=0.6 are depicted in Figure 5.a-c. The maximum pressure was observed at the trailing edge of the cavities due to impingement of vortex. Due to the impingement of this vortex at the trailing edge leads to formation of pressure pulses that travels upstream of the cavity enhancing shear layer at the leading-edge wall. However, as the cavity geometry is reduced the sound pressure level at the leading edge gets reduced due to lack of recirculating flow within the cavity. For subsonic flow as the flow past the leading edge of the cavity, due to sudden geometry change a velocity gradient occurs between the free stream and reverse flow inside the cavity resulting in the shear layer rollup and vortex formation. The formed vortex convects downstream and hits on the trailing edge, splitting up into two vortices-a primary vortex which convects to the downstream of the flow and a secondary vortex moving towards leading edge of cavity and enhances the shear layer making the upstream flow to the cavity more complex. The values and plot shows the acoustic response from the cavity, which reveals that the sound pressure level get reduced as the aspect ratios are reduced to 0.5 and 0.2.
The present study concludes that as the cavity geometry changes from L/D=2 to 0.5 and 0.2,for a subsonic flow, the secondary vortex inside the cavity becomes smaller and thereby less effective in making the upstream flow at the leading edge of the cavity less chaotic. That is the reason why such deep cavities find applications in aircrafts industries like flame holder mechanism in combustion chambers etc. The feedback mechanism in the cavity is similar to that observed in rectangular cavities of similar configuration but with the absence of corner vortices at the cavity bottom edges due to smoothened bottom wall.
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