University of Western Ontario Department of Mechanical & Materials Engineering Advanced Fluid Mechanics Research Group Experimental Verification of CFD Modeling of Turbulent Flow over Circular Cavities using FLUENT Presented by: Thomas Hering MESc Candidate Jesse Dybenko Research Engineer Eric Savory Associate Professor May 23, 2006

Overview General overview of experimental parameters CFD grid generation CFD solution procedure Results Key findings Future work (Elliptical Cavities)

Background Cavities may lead to increased noise and drag on an object Main focus on Circular cavities: Resulting asymmetric flow and significant increase in drag at h/D ≈ 0.5 h D Flow direction Experimental Variables Side View Top View

Experimental Data Collected Three cases of h/D ratio = 0.2, 0.47, 0.7 were tested Pressure transducer data was taken to plot surface pressure contours on cavity walls and surrounding plane Hot Wire anemometry was used to examine the wake flow characteristics The free stream velocity during the experiments was 27 ± 0.15 m/s The pressure coefficient was normalized using the tunnel static pressure and free stream velocity

Boundary Conditions and Dimensions Velocity Inlet The inlet velocity was set to 25 m/s, which resulted in a free stream velocity of 26.4 m/s at the free stream reference point Wall Cavity Outflow Wall 32D Y 47.3D 4D X Z 5.5D

Simulation Grid Cavity Side View Top View The computation domain was broken up into several volumes which were meshed using a structured hexagonal cooper meshing scheme

Solution Procedure The solution was first iterated using the k-ε turbulence model After initial solution convergence the Reynolds Stress model was applied and further iterated The tunnel length was used to develop a similar boundary layer as measured in the experiments A steady state solution sought Simulated Data Experimental Data Boundary Layer Parameters

Pressure Distributions 0° TOP VIEW 90° 180° 270° Cavity sidewall coordinate system Flow direction X Z Pressure Coefficient P = Pressure Ps = Free stream static pressure Uo = Free stream velocity = Density of air Due to simulated steady state solution, only the mean values were compared Surface pressure distributions, wake profiles and drag coefficients were compared

Pressure Contours for h/D = 0.7 Ground Plane Simulated Experimental Cavity Base Simulated Experimental Cavity Side Simulated Experimental 0° Flow direction 90° 270° Y X Z Cavity Base Ground Plane 180° Simulated results matched well with experimental data

Pressure Distributions along the Centreline for h/D = 0.7 0° Cavity Wall Pressure Distributions along the Centreline for h/D = 0.7 0° 90° 270° X Z Centreline used in comparison of cavity side walls 180° Y Flow direction 180° Cavity Wall Cavity Base Centreline

Pressure Contours for h/D = 0.2 Ground Plane Experimental Simulated Cavity Base Simulated Experimental Similar trends along the centreline as for the h/D = 0.7 case, but the difference between simulated and experimental data was larger Cavity Side Simulated Experimental

Pressure Contours for h/D = 0.47 (Asymmetric flow) Ground Plane Simulated Experimental Cavity Base Simulated Experimental Asymmetry is much weaker in the simulated results Vortex tube does not completely leave the cavity in the simulated results Cavity Side Simulated Experimental

Comparison between Turbulence Models for h/D = 0.47 k-ε model Ground Plane Reynolds Stress model Cavity Base k-ε model Reynolds Stress model Cavity Side k-ε model Reynolds Stress model Asymmetry is weaker when applying the k-ε turbulence model

Resulting Wake Comparisons h/D = 0.2 Simulated Experimental h/D = 0.47 Simulated Experimental The weak asymmetry can be seen in the wake for h/D = 0.47 case

Drag coefficient Comparison Experimental drag coefficient calculated using pressure distributions along cavity wall Weaker asymmetry the cause of the lower drag coefficient at h/D = 0.47 Drag increment due to presence of cavity CD = Drag coefficient (normalized by the cavity planform area) cf = Skin friction coefficient

Key Findings The simulated results showed the correct flow physics involved in circular cavity flows The asymmetric flow for a symmetric geometry, a distinct feature of this type of flow, was apparent in the simulations The weaker asymmetry led to a lower drag coefficient The simulations constantly under predicted the pressure values for all three configurations tested The Reynolds Stress Turbulence model provided better results than the k-ε Turbulence model when comparing the strength of the asymmetry at h/D = 0.47

Elliptical Cavities h/D =0.47 Cavity Side Simulated Experimental Cavity Base Experimental Simulated Ground Plane Simulated Experimental Cavity Base (modified scaling) Simulated

Discussion and Questions are welcome University of Western Ontario Department of Mechanical & Materials Engineering Advanced Fluid Mechanics Research Group http://www.eng.uwo.ca/research/afm/main.htm Discussion and Questions are welcome An Experimental Investigation of Turbulent Boundary Layer Flow over Surface-Mounted Circular Cavities J. Dybenko and E. Savory, UWO 12:20-12:45 May 24, 2006 (Walker)