Trade off between Y+ and Solution convergence(CFX+WorkBenchMesh)

Aj Nair · November 7, 2015, 00:29

Hello,
I am working with CFX 16.1 in combination with Ansys workbench mesh.And i use SST for most of my simulations. For convergence I mostly look for Residual Max value(velocity, Pressure and energy). I am getting into following combinations,

Bad Mesh quality(1E-04) and more skewed elements + Skewed first layer prism(I use smooth transition for inflation) = good y +(y+<7) + bad convergence (pressure and velocity max residual above 1E-02)

comparatively Good mesh quality(1E-02) and minimum skewed cells + controlled first layer height of prism, which is also the main culprit for reducing mesh quality = Bad Y+(Y+>11) + good convergence(pressure and velocity max residual below 1E-03)

I am aware that first layer is an important factor, but how can the first layer be improved without affecting the mesh quality and its skewness using workbench mesh. so that both Y+ and convergence can be satisfied

.

And in bad convergence Runs(decided by means of max residual), when i look into the RMS Residual values of pressure and velocity i see its below 1E-03. Is that means there are only few places with is running into bad convergence ??

These are my observations. And sorry i may not have results or plots with me.

Ever Seen a Tornado? · November 13, 2015, 10:06

There's an option in CFX-Pre under Output Control->Results->Output Equation Residuals which allows you to plot your residuals in CFX-post like any other variable. This can be quite useful when troubleshooting bad mesh elements that are causing poor convergence.

November 7, 2015, 00:29	Trade off between Y+ and Solution convergence(CFX+WorkBenchMesh)	#1
Aj Nair Member AJ Join Date: Sep 2013 Posts: 30 Rep Power: 12	Hello, I am working with CFX 16.1 in combination with Ansys workbench mesh.And i use SST for most of my simulations. For convergence I mostly look for Residual Max value(velocity, Pressure and energy). I am getting into following combinations, Bad Mesh quality(1E-04) and more skewed elements + Skewed first layer prism(I use smooth transition for inflation) = good y +(y+<7) + bad convergence (pressure and velocity max residual above 1E-02) comparatively Good mesh quality(1E-02) and minimum skewed cells + controlled first layer height of prism, which is also the main culprit for reducing mesh quality = Bad Y+(Y+>11) + good convergence(pressure and velocity max residual below 1E-03) I am aware that first layer is an important factor, but how can the first layer be improved without affecting the mesh quality and its skewness using workbench mesh. so that both Y+ and convergence can be satisfied. And in bad convergence Runs(decided by means of max residual), when i look into the RMS Residual values of pressure and velocity i see its below 1E-03. Is that means there are only few places with is running into bad convergence ?? These are my observations. And sorry i may not have results or plots with me.

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November 13, 2015, 10:06		#2
Ever Seen a Tornado? New Member Join Date: Sep 2015 Posts: 5 Rep Power: 10	There's an option in CFX-Pre under Output Control->Results->Output Equation Residuals which allows you to plot your residuals in CFX-post like any other variable. This can be quite useful when troubleshooting bad mesh elements that are causing poor convergence.