Week 3 - External flow simulation over an Ahmed body.
https://screenrec.com/share/ldjs3xfvLz
Introduction
Flow around the Ahmed body is a well-known CFD benchmark for external aerodynamics. The Ahmed body is a generic car model first proposed by [Ahmed et al., (1984)]. All ground vehicles can be termed as bluff bodies moving close to the road surface. The analyzed body is able to capture the essential features of the flow which characterize modern cars. Most importantly, high-quality test data are available. The aim of the study is to validate results obtained by steady-state, incompressible turbulent flow in SimFlow. The flow around the Ahmed body with the slant angle of 25 degrees and the Reynolds number Re=2,784,000.
Ahmed Body Dimensions - Geometry
The geometry was prepared based on Ahmed et al. (1984), and the sketch is given in Figure 1. The slant angle ϕ is set to 25 degrees. The body is placed inside the wind tunnel of the dimensions:
15 x 1.4 x 1.87 m in X, Y and Z directions, respectively.
The Ahmed body is located inside the wind tunnel with five times its length in front and ten times its length behind the reference point. The cross-sectional area of the Ahmed body in the X direction is 0.112m2, which will be used for calculation of the drag coefficient.
Ahmed Body Simulation Setup
Steady-state, incompressible simulation was conducted in SimFlow. The incompressible solver is using the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm for solving flow equations.
The mesh size was determined based on y+ the dimensionless distance from the wall. The goal was to keep y+ in range 30≤y+≤300 so that k−ωSST turbulence model with the Standard Wall Function could be used.
Reynolds number is based on the length of the Ahmed body (1.044 m) and equals 2 784 000. Velocity convective part of the equation was discretized using a second-order upwind scheme.
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Table 1. Drag Cd and lift Cl coefficients for slant angle ϕ=25o. Comparison between test and simulation results |
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Parameter |
Experiment (Meile et al., 2011) |
Simulation |
|
Drag coefficient Cd |
0.299 |
0.313 |
|
Lift coefficient Cl |
0.345 |
0.355 |
[Ahmed Body and et,al (1984)] described the general wake structure behind the Ahmed body - Figure 7. The characteristic feature is the presence of two recirculation areas marked as regions A and B, situated one over another. It was also noted that those A and B regions do not go to the base surface ("road"), so they are two vortices placed in the so-called "separation bubble" marked by region D. Another characteristic wake structure is the shear layer, coming off the slant side edge, rolls up into a longitudinal vortex - region C. The same structures as indicated in Figure 7 are observed in the simulation results.
Ahmed Body CFD Benchmark Conclusions
The flow around the Ahmed body is a well-known benchmark for CFD solvers. It has been shown that SimFlow is able to provide high-quality results that match the test data in terms of drag coefficient, velocity profiles, and 3D wake structures.
SimFlow predicted the drag coefficient to be Cd=0.313 which matches the test results - Cd=0.299. The difference is at the level of 4.6%. Good agreement was also achieved for lift coefficient Cl=0.345 in test versus Cl=0.355 in simulation, giving the error of 3.2%.
Symmetric boundary condition
It should only be used if the physical object or geometry and the expected flow field pattern of the developed solution are mirrored along that surface. By using this boundary condition, the domain can essentially be halved, reducing the time to achieve a solution.
All the variables have same value and gradients at the same distance from the boundary. It acts as a mirror that reflects all the flow distribution to the other side. The conditions at symmetric boundary are no flow across boundary and no scalar flux across boundary.
In detail, the symmetry condition applies the following constraints on the flow variables:
- The fluxes across the symmetry are zero.
- The normal components of all variables are set to zero.
It can be applied to both planar or non-planar faces/surfaces on the domain boundaries.
Problem solution
Reynolds number = 2,784,000
Coefficient of friction (Cf) = 0.0576*Re^-0.2 = 0.00296
Shear wall = Cf *0.5*Density* free stream velocity^2 = 1.134
Fricition velocity = 0.962
Y= 4.7 mm or 5 mm approximately applies as total thickness of the inflation layer
Y+ = 300
Space claim interface
Upload geometry in space claim
Repair extra edges
Creating two enclosure around Ahmed Body
Share topology to avoid mesh overlap then the conformal boundary layer has been generated.
Meshing Interface
Faces definition
Generate base line mesh by picking first the outer body as it contains only straight lines, and inserting multizone method where Ansys will pick Hexahedral mesh as possible as it can. A hexahedral mesh is a 3D generalization of a 2D quadrilateral mesh, but not necessarily with a cubic (i.e., Cartesian) arrangement of points.
Mesh sizing
Generate mesh for the inner enclosure
Checking cell count
Setup interface
Fluid material is air
Solid material aluminum
Boundary condition (inlet velocity 25 m/s, temperature 300 K, outlet pressure zero gauge, symmetry walls).
Viscous model used k omega SST, plus energy equation and pressure based solver Mach No. is less 0.3.
Create reports
Hybrid initialization & Run for 300 iterations
Pressure contour
Residual
Drag force
Velocity contour
Velocity after Ahmed body 2m & 3m respectively
Velcoity vector
Velocity contour
Literature
- [Ahmed et al., (1984)] S.R. Ahmed, G. Ramm and G. Faltin, Some Salient Features of the Time-Averaged Ground Vehicle Wake, SAE Technical Paper 840300, 1984
- [CFD Online] https://www.cfd-online.com/Wiki/Ahmed_body
- [Meile et al., (2011)] W. Meile, G. Brenn, A. Reppenhagen, B. Lechner, A. Fuchs, Experiments and numerical simulations on the aerodynamics of the Ahmed body, CFD Letters 3(1), 2011
- [ERCOFTAC Test Data AC1-05] ERCOFTAC test data, https://www.kbwiki.ercoftac.org/w/index.php?title=Test_Data_AC1-05