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Ahmed Body Validation Study – Autodesk CFD

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Old   June 2, 2020, 21:38
Default Ahmed Body Validation Study – Autodesk CFD
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Nik
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I am currently conducting a validation study on Autodesk CFD on the experimental work described in the following paper: https://www.researchgate.net/publica...ns_on_the_aero...


My computational model features a nearly identical setup to the one described in the paper. The only difference is that the paper suggests a slip boundary condition on the ground (bottom of the domain) upstream of the Ahmed body in order to achieve a boundary layer of a set thickness. I found that removing this slip boundary condition in Autodesk CFD helped to achieve the desired boundary layer thickness. Other prominent features of my computational setup are summarized below:

Domain Size (length/height/width): 15m/1.87m/1.4m
Ahmed Body: 25°
Boundary conditions: (1) inlet: velocity = 40 m/s (2) outlet: pressure = 0 Pa (3) Ceiling (top) and sides of the domain: slip/symmetry
Convergence: “tight”
Turbulence model: SST k omega
Boundary layer setup: number of layers = 15, layer factor = 0.8, layer graduation = 1.5, wall blending = on. (Please note: I have tried a variety of different setups but, this combination achieves the lowest y+ values).

Additionally, I have experimented with a variety of refinement boxes and refinement surfaces. I calculate my force values using Autodesk’s built-in wall calculator.


My “best” results are as follows:

1) A surface refinement is applied to the Ahmed body to decrease its element size by a factor 0.15 from the Autodesk’s baseline auto size. This corresponds to 4,468,821 fluid elements. This model achieved y+ values up to 2.3 with the majority of the Ahmed body being below a value of 1. Furthermore, the model achieved a drag force of 47.0 N and a lift force of 15.60 N. This corresponded to Cd of 0.424 and a Cl of 0.141

2) A surface refinement is applied to the Ahmed body to decrease its element size by a factor 0.25 from the Autodesk’s baseline auto size. Additionally, refinement is applied to the Ahmed body’s front and rear panels. This corresponds to 2,784,143 fluid elements. This model achieved y+ values up to 2.2 with the majority of the Ahmed body being below a value of 1 – this is better than the mesh setup discussed in best result #1. Furthermore, the model achieved a drag force of 46.3 N and a lift force of 14.3 N. This corresponded to Cd of 0.418 and a Cl of 0.128

3) Four refinement boxes were applied: 2 are in the streamwise direction to aid the transition from the wall layer to the rest of the domain and 2 are around the Ahmed body. No direct surface refinement was applied. This corresponds to 5,077,823 fluid elements. This model achieved y+ values up to 4.482 with the majority of the Ahmed body being above a value of 1. Furthermore, the model achieved a drag force of 48.3 N and a lift force of 7.53 N. This corresponded to a Cd of 0.435 and a Cl of 0.0679


I should also note that on the Autodesk CFD forum, an Autodesk technical expert provided a geometry file and computational setup. Running this provided geometry following his computational setup and guidance yielded even worse results (drag force = 75.8 N, lift force = 25.4 N, which corresponds to Cd = 0.684 and Cl = 0.228). The Autodesk technical expert states in his post that the forces will be on the high side but, the flow trends will remain the same. Link: https://forums.autodesk.com/t5/cfd-f...rofile.languag...


The experimental values presented in the paper stated above feature a Cd of 0.299 and a Cl of 0.345. Neither my work nor the work suggested by the Autodesk technical expert matches (or even comes close) to the target values.


Any suggestions on how to improve my simulation to match those target values would be greatly appreciated!
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ahmed body, autodesk cfd, drag coefficient, mesh

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