[ANSYS Meshing] Cfd meshing pls help

khyap96 · January 10, 2018, 17:20

Hi, I am new to CFD and I am currently having a degree assignment to mesh a car. Can anyone tell me how skewness and orthogonal relate to accuracy? And why do we need smaller element size on the car body compare to the enclosure? Lastly, what is the inflation for?

R. Kim Eadies · January 10, 2018, 18:50

Hello,

Skewness and Orthogonality are mesh quality metrics - general ratings that describe how suitable your grid may be to producing an accurate simulation result. There is a great deal of information on mesh metrics available on CFD-online, in the user manuals, and on the rest of the internet, so I will leave it to you to read more about these if you wish. I usually use the Element Quality metric which is a composite metric, a value of 1 being the best and 0 the worst and it typically correlates well with the other metrics for simple problems.

In order to reduce the computational cost of a simulation while keeping the results as accurate as possible, smaller elements are concentrated in regions where gradients of relevant quantities are high (i.e. changing rapidly in space). This increases accuracy because CFD packages set out to solve finite difference equations to linearly approximate derivatives in the flow equations, which become inaccurate if there isn't sufficient resolution in these areas of rapid change. Think back to pre-calculus and approximating integrals with Riemann sums if you aren't familiar. This could be near a heating element where temperature change is rapid, or in a pump where the pressure gradient is large, but in your case of a car or other airfoil, you are most likely concerned with shear stress exerted on the body by the surrounding fluid in order to accurately capture drag. Without sufficient grid resolution, particularly in these areas, you will start to accumulate numerical error.

Inflation is the name of the technique that modifies the mesh as described above, creating an inflation layer of successively denser mesh near a surface of interest.

If you want more advanced detail I'd recommend looking up law of the wall and Y+.

Best of luck with your assignment,
Alan

khyap96 · January 10, 2018, 18:59

thank you so much.. my skewness of the meshing is around 0.89 (max) 0.28 (average) and 0.9 (max) 0.28 (average), is this value acceptable? i have went through some article, saying this range of skewness is poor quality of the cell, does that mean my result is not accurate?

January 10, 2018, 17:20	Cfd meshing pls help	#1
khyap96 New Member Yap Khai Hung Join Date: Jan 2018 Posts: 3 Rep Power: 8	Hi, I am new to CFD and I am currently having a degree assignment to mesh a car. Can anyone tell me how skewness and orthogonal relate to accuracy? And why do we need smaller element size on the car body compare to the enclosure? Lastly, what is the inflation for?

January 10, 2018, 18:50		#2
R. Kim Eadies New Member Join Date: Jan 2018 Posts: 11 Rep Power: 8	Hello, Skewness and Orthogonality are mesh quality metrics - general ratings that describe how suitable your grid may be to producing an accurate simulation result. There is a great deal of information on mesh metrics available on CFD-online, in the user manuals, and on the rest of the internet, so I will leave it to you to read more about these if you wish. I usually use the Element Quality metric which is a composite metric, a value of 1 being the best and 0 the worst and it typically correlates well with the other metrics for simple problems. In order to reduce the computational cost of a simulation while keeping the results as accurate as possible, smaller elements are concentrated in regions where gradients of relevant quantities are high (i.e. changing rapidly in space). This increases accuracy because CFD packages set out to solve finite difference equations to linearly approximate derivatives in the flow equations, which become inaccurate if there isn't sufficient resolution in these areas of rapid change. Think back to pre-calculus and approximating integrals with Riemann sums if you aren't familiar. This could be near a heating element where temperature change is rapid, or in a pump where the pressure gradient is large, but in your case of a car or other airfoil, you are most likely concerned with shear stress exerted on the body by the surrounding fluid in order to accurately capture drag. Without sufficient grid resolution, particularly in these areas, you will start to accumulate numerical error. Inflation is the name of the technique that modifies the mesh as described above, creating an inflation layer of successively denser mesh near a surface of interest. If you want more advanced detail I'd recommend looking up law of the wall and Y+. Best of luck with your assignment, Alan __________________ "The more you do what you're already doing, the more you'll get what you've already got."

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January 10, 2018, 18:59		#3
khyap96 New Member Yap Khai Hung Join Date: Jan 2018 Posts: 3 Rep Power: 8	thank you so much.. my skewness of the meshing is around 0.89 (max) 0.28 (average) and 0.9 (max) 0.28 (average), is this value acceptable? i have went through some article, saying this range of skewness is poor quality of the cell, does that mean my result is not accurate?