# The velocity calculated using simpleFoam exhibits non-physical rise

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May 26, 2024, 01:55
The velocity calculated using simpleFoam exhibits non-physical rise
#1
New Member

Yanjun Tong
Join Date: Jul 2020
Posts: 15
Rep Power: 5
I currently calculating case of wind around buildings and encoutered weired velocity rise. The geometry is as follows, the flow direction is from north to south, with the value of (1 0 0). The mesh contain some grid with high non-orthogonal

Code:
```Checking geometry...
78     Overall domain bounding box (-621.9 0.299993 -126.364) (662.617 50.66    62 175.364)
79     Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
80     Mesh has 3 solution (non-empty) directions (1 1 1)
81     Boundary openness (1.66252e-17 -2.51027e-15 -1.64139e-16) OK.
82     Max cell openness = 2.35866e-16 OK.
83     Max aspect ratio = 11.1171 OK.
84     Minimum face area = 0.000321178. Maximum face area = 103.224.  Face a    rea magnitudes OK.
85     Min volume = 7.6199e-06. Max volume = 324.355.  Total volume = 1.7993    8e+07.  Cell volumes OK.
86     Mesh non-orthogonality Max: 76.2032 average: 17.5263
87    *Number of severely non-orthogonal (> 70 degrees) faces: 17.
88     Non-orthogonality check OK.
89   <<Writing 17 non-orthogonal faces to set nonOrthoFaces
90     Face pyramids OK.
91     Max skewness = 1.00769 OK.
92     Coupled point location match (average 0) OK.
93
94 Mesh OK.
95
96 End```

However, when I do the simulation in OpenFOAM(I tried OpenFOAMv2306, OpenFOAMv2206 and OpenFOAM-7). The max(mag(U)) keeps rising and finally the case blow up. try `cat log.txt | grep "max(mag(U))"` will show this.

I've tried many ways to solve this but still get the problem：

1. I tried the official case settings in OpenFOAM2306, and use laminar to simulate(All the tests are done in laminar mode)
2. I think the intlet and outlet boundary condition matters, the bc in windAroundBuildings are:

# | | p | U |
# | ---- | ---- | -------- |
# | TOP_2D | symmetry | symmetry |
# | GROUND_2D | zeroGradient | noSlip |
# | WEST_2D | symmetry | symmetry |
# | EAST_2D | symmetry | symmetry |
# | SOUTH_2D | totalPressure| pressureInletOutletVelocity |
# | NORTH_2D | zeroGradient | fixedValue |
# | WALLS_2D | zeroGradient | noSlip |

and I also tried:

# method 1, windAroundBuildings in ofv2306
# | inlet | outlet |
# p | zeroGradient | totalPressure
# U | fixedVaule | pressureInletOutletVelocity

# method 2, simpleCar in ofv2306
# | inlet | outlet |
# p | zeroGradient | fixedValue |
# U | fixedValue | pressureInletOutletVelocity

# method 3, turbulentFlatPlate in ofv2306
# | inlet | outlet |
# p | zeroGradient | fixedValue
# U | fixedValue | zeroGradient |

# method 4, airfoil in ofv2306
# | inlet | outlet |
# p | freestreamPressure | freestreamPressure
# U | freestreamVelocity | freestreamVelocity

# method 5, motorBike in ofv2306
# | inlet | outlet |
# p | zeroGradient | fixedValue
# U | fixedValue | inletOutlet

3. I tried to use some bounded fvSchemes

Code:
```divSchemes
{
default         none;

// div(phi,U)      bounded Gauss linearUpwind limited;

// tttt: try-1
div(phi,U)      bounded Gauss limitedLinear 1;

turbulence      bounded Gauss limitedLinear 1;
div(phi,k)      \$turbulence;
div(phi,epsilon) \$turbulence;

div(phi,C)  Gauss linear;
}```
4. I also tried to implement some else matrix solver, but I think this will not influence the velocity?

Code:
```    p
{
solver          GAMG;
smoother        GaussSeidel;
tolerance       1e-6;
relTol          0.01;
}
pFinal
{
\$p;
tolerance       1e-06;
relTol          0.01;
}
"(U|k|epsilon|omega|C)"
{
solver          PBiCGStab;
preconditioner  diagonal;
tolerance       1e-7;
relTol          0.0;
minIter		2;
}

"(U|k|epsilon|omega|C)Final"
{
\$U;
relTol          0.0;
}

}
SIMPLE
{
residualControl
{
p               1e-6;
U               1e-6;
"(k|omega|epsilon)" 1e-6;
}
nNonOrthogonalCorrectors 2;
pRefCell        0;
pRefValue       0;

}```
5. MOST IMPORTANT! The case can be simulated in ANSYS FLUENT. I attempted using ANSYS FLUENT to create an initial velocity field, which is physically correct with a maximum velocity magnitude of 3.2 m/s, just as the picture below. I then converted the FLUENT results to OpenFOAM, but encountered same problem!

Any suggestions or solutions to this problem are welcomed !!!It appears that I am unable to solve this particular issue with my current knowledge...
Attached Files
 log.txt (169.7 KB, 1 views)

 May 30, 2024, 22:59 anyone can help me #2 New Member   Yanjun Tong Join Date: Jul 2020 Posts: 15 Rep Power: 5 anyone can help me - -

 Tags fluent, openfoam, windaroundobuildings

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