Hi everyone,

I’m running a steady-state simulation of a centrifugal compressor impeller and I keep running into an issue where the residuals settle but don’t properly converge, even after several thousand iterations. The solution looks stuck, and I’d like to understand if the problem is with my setup, mesh, or solver controls.

Case setup

• Geometry: Centrifugal compressor impeller with inlet passage and outlet

• Mesh: ~1.6 million cells, generated in TurboGrid and exported as cgns into TCAE

• Interfaces: Mixing plane, 7 mixing planes total across 2 interfaces

• Fluid: Air, perfect gas; μ = 1.78e-5, ρ = 1.2, p = 1 atm (defaults)

• Rotation: 110,000 RPM (tested both +111000 and −111000, same behavior)

• Flow regime: Transonic option enabled

Boundary conditions

• Inlet:

  • Total pressure = 1 atm

  • Total temperature = 288.15 K

  • Turbulence intensity = 5% (k = 82.14)

  • Turbulence length scale = 0.015 m (blade height) → ω = 604.207

• Outlet:

  • Mass flow = 0.35 kg/s

• Walls: No-slip, adiabatic, standard wall functions

Solver settings

• Turbulence model: k-ω SST (default values)

• Numerics: Second order, non-orthogonal mesh correction enabled

• Under-relaxation factors: Set to 0.15 (started higher but reduced due to stability issues)

• Physical limits:

  • p: 50,000 – 800,000 Pa

  • ρ: 0.1 – 100

  • v: 0 – 800 m/s

  • T: 250 – 700 K

• Initial conditions:

  • p = 1 atm

  • T = 288.15 K

    v = 0 , 0, 148 Got it from a 1D code as the absolute velocity at the inlet.

  • k = 0.01

  • ω = 100

Problem

Residuals drop initially but then flatten out and refuse to converge further. Monitors also don’t stabilize in a meaningful way, I think the monitors get stuck because the residuals do not drop further. I have tried with the fixed mean static pressure outlet condition as well but still the same thing. One thing I suspect is that I think there maybe be some unsteadiness in the flow which maybe causing this, is there a way to set a pseudo time scale or something. Any help would be appreciated!

Hi contributors,

I sincerely appreciate your team's efforts in making your work publicly available online—it has been immensely helpful for OpenFOAM users and developers alike. After reading your article (Node266 - CFD SUPPORT), I followed the provided instructions to configure OpenFOAM to generate a cellZone using snappyHexMesh.

Unfortunately, the attempt was unsuccessful. The number of cells in both the cellZone and faceZones files turned out to be zero, and checkMesh.log confirms this issue. When I check the result via paraFoam, I also find that net0 ~ net4 contain no mesh—they are empty. This is not only contrary to my expectations, but also differs from the result described in the article.

I am using OpenFOAM v2012, and the net0 section in my snappyHexMeshDict was set up following the example from your webpage.

I’ve uploaded the relevant case below. I would greatly appreciate any guidance.

--------------------------------------------------------------------------------------------------

snappyhexmeshdict:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2012                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
#include "../varsSettings" //Include global variables
castellatedMesh true;
snap            true;
addLayers       false;//true;
geometry
{
    floater
    {
        type 	triSurfaceMesh;
        file 	"DeepCwindPlatform_heave.stl";
    }
    
    net1
    {
        type 	triSurfaceMesh;
        file 	"Net1_improved_heave.stl";
    }
    
    net2
    {
        type 	triSurfaceMesh;
        file 	"Net2_improved_heave.stl";
    }
    
    net3
    {
        type 	triSurfaceMesh;
        file 	"Net3_improved_heave.stl";
    }
    
    net4
    {
        type 	triSurfaceMesh;
        file 	"Net4_improved_heave.stl";
    }
    
    net0
    {
    	type	searchableBox;
    	min		(-100	-50	-120);
    	max		(100	50	-80);
    }
    
    refineFreeSurface_L1
    {
        type searchableBox;
        min ($minX  $minY -10);
        max ($maxX  $maxY  10); 
    }
    
    refineFreeSurface_L2
    {
        type searchableBox;
        min (#calc "$minX+$inLen+20"  -70 -8);
        max (#calc "$maxX-$outLen-20"  70  8); 
    }
    
    refineFloater_L1
    {
        type searchableBox;
        min (-51  -49	#calc "-30 + $decay_z + $offset_z");
        max ( 37   49 	#calc "22 + $decay_z + $offset_z"); 
    }
};
castellatedMeshControls
{
    maxLocalCells 100000;
    maxGlobalCells 2000000;
    minRefinementCells 0;
    maxLoadUnbalance 0.10;
    nCellsBetweenLevels 1;
    features
    (
        {
        	file    "DeepCwindPlatform_heave.eMesh";
            level   4;
        }
        
		{
        	file	"Net1_improved_heave.eMesh";
        	level	4;
		}
		
		{
        	file	"Net2_improved_heave.eMesh";
        	level	4;
		}
		
		{
        	file	"Net3_improved_heave.eMesh";
        	level	4;
		}
		
		{
        	file	"Net4_improved_heave.eMesh";
        	level	4;
		}
    );
    refinementSurfaces
    {
		floater
		{
			level (4 4);//(5 5);
			patchInfo
        	{
          		type wall;
 
          		// Optional group
          		// inGroups (meshedWalls);
        	}
		}
          
		net1
    	{
			level (4 4);
			cellZone net1Zone;
/*			cellZoneInside inside;*/
			faceZones net1fZone;
    	}
    	net2
    	{
        	level (4 4);
			cellZone net2Zone;
/*			cellZoneInside inside;*/
			faceZones net2fZone;
    	}
    	net3
    	{
        	level (4 4);
			cellZone net3Zone;
/*			cellZoneInside inside;*/
			faceZones net3fZone;
		}
    	net4
    	{
        	level (4 4);
			cellZone net4Zone;
/*			cellZoneInside inside;*/
			faceZones net4fZone;
    	}
    	
    	net0
    	{
        	level (4 4);
			cellZone net0Zone;
/*			cellZoneInside inside;*/
			faceZones net0fZone;
    	}
    }
    resolveFeatureAngle 45;//30;
    planarAngle 30;
    refinementRegions
    {
        refineFreeSurface_L1
        {
            mode inside;
            levels ((1e-5 1)); //First entry here is distance, which will be ignored unless mode = distance
        }
        
        refineFreeSurface_L2
        {
            mode inside;
            levels ((1e-5 2)); //First entry here is distance, which will be ignored unless mode = distance
        }
        
        refineFloater_L1
        {
            mode inside;
            levels ((1e-5 2)); //First entry here is distance, which will be ignored unless mode = distance
        }
        
/*        net1*/
/*        {*/
/*            mode inside;*/
/*            levels ((1e-5 3));  // STL 内部 refinement 到 level 4*/
/*        }*/
/*        net2*/
/*        {*/
/*            mode inside;*/
/*            levels ((1e-5 3));*/
/*        }*/
/*        net3*/
/*        {*/
/*            mode inside;*/
/*            levels ((1e-5 3));*/
/*        }*/
/*        net4*/
/*        {*/
/*            mode inside;*/
/*            levels ((1e-5 3));*/
/*        }*/
    }
    limitRegions
    {
    }
        locationInMesh (-150 0 0);
        allowFreeStandingZoneFaces true;//false;
        faceZoneControls
        {
        }
    locationsOutsideMesh ((100 100 100));
}
snapControls
{
    nSmoothPatch 5;//3
    tolerance 2.0;
    nSolveIter 50;//30;
    nRelaxIter 10;//5;
        nFeatureSnapIter 20;//10;
        implicitFeatureSnap false;//true;
        explicitFeatureSnap true;//false;
        multiRegionFeatureSnap false;
}
addLayersControls
{
    relativeSizes true;
        expansionRatio 1.25;
        finalLayerThickness 0.64;
    	minThickness 0.1;
    layers
    {
		floater
		{
			nSurfaceLayers    2;//3;
		}
    }
    nGrow 0;
        featureAngle 180;//180;
        maxFaceThicknessRatio 0.5;//0.5;
        nSmoothSurfaceNormals 1;
        nSmoothThickness 10;
        minMedialAxisAngle 90;
        maxThicknessToMedialRatio 0.3;
        nSmoothNormals 3;
        slipFeatureAngle 30;
        nRelaxIter 5;
        nBufferCellsNoExtrude 0;
        nLayerIter 50;
        nRelaxedIter 20;
}
meshQualityControls
{
    #include "meshQualityDict"
    relaxed
    {
        maxNonOrtho 75;
    }
        nSmoothScale 4;
        errorReduction 0.75;
}
mergeTolerance 1e-6;
// ************************************************************************* //

Thank you very much in advance!

Best regards,

Liu Jiaming