Experimental Verification of CFD Modeling of Turbulent Flow over Circular Cavities using FLUENT

1. 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

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

3. Experimental Variables Top View D Flow direction h Side View 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

4. 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

5. 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

6. 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

7. 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

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

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

10. 0° Cavity Wall Flow direction 90° 0° Y X 180° Z 270° Centreline used in comparison of cavity side walls Cavity Base Centreline 180° Cavity Wall Pressure Distributions along the Centreline for h/D = 0.7

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

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

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

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

15. 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

16. 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

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

18. 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)