| miku11 |
June 28, 2016 07:36 |
2D cylinder drag coefficient
3 Attachment(s)
Hello,
I'm a new user of openfoam and i'm just started my journey with cfd, so my questions might be easy to solve, but i need some help.
I'm trying to validate 2D flow around cylinder and find Drag coefficient. I selected case for Re=1, so Cd should be around 10. Unfortunately i obtained value about 4. I used k omegaSST turbulence model.
I set nu to value 10, because radius of my cylinder is 0.5 m and flow velocity is 20 m/s, so nu should be equal to 10.
I'm not sure about my boundary conditions k and omega:
The problem is, that drag coefficient decrease in every time step
k
Quote:
FoamFile
{
version 2.0;
format ascii;
class volScalarField;
location "0";
object k;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
dimensions [0 2 -2 0 0 0 0];
internalField uniform 0.01;
boundaryField
{
inlet
{
type fixedValue;
value uniform 0.01;
}
outlet
{
type zeroGradient;
}
wall1
{
type zeroGradient;
}
wall2
{
type zeroGradient;
}
ball
{
type zeroGradient;
}
frontAndBackPlanes
{
type empty;
}
}
|
and omega
Quote:
FoamFile
{
version 2.0;
format ascii;
class volScalarField;
object omega;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
dimensions [0 0 -1 0 0 0 0];
internalField uniform 0.01;
boundaryField
{
inlet
{
type fixedValue;
inletValue $internalField;
value $internalField;
}
outlet
{
type zeroGradient;
}
wall1
{
type zeroGradient;
}
wall2
{
type zeroGradient;
}
frontAndBackPlanes
{
type empty;
}
ball
{
type omegaWallFunction;
value $internalField;
}
}
|
Here is my case:
Quote:
FoamFile
{
version 2.0;
format binary;
class dictionary;
location "system";
object controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
application simpleFoam;
startFrom latestTime;
startTime 0;
stopAt endTime;
endTime 4200;
deltaT 20;
writeControl timeStep;
writeInterval 10;
purgeWrite 0;
writeFormat binary;
writePrecision 6;
writeCompression off;
timeFormat general;
timePrecision 6;
runTimeModifiable true;
libs
(
"libforces.so"
);
functions
{
forceCoeffs1
{
type forceCoeffs;
functionObjectLibs ( "libforces.so" );
outputControl timeStep;
timeInterval 1;
log yes;
patches ( ball );
rhoName rhoInf; // Indicates incompressible
rhoInf 1; // Redundant for incompressible
liftDir (0 0 1);
dragDir (1 0 0);
CofR (0 0 0); // Axle midpoint on ground
pitchAxis (0 1 0);
magUInf 20;
lRef 1; // Wheelbase length
Aref 1; // Estimated
}
}
|
turbulenceProperties:
Quote:
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "constant";
object turbulenceProperties;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
simulationType RAS;
RAS
{
RASModel kOmegaSST;
turbulence on;
printCoeffs on;
}
|
Log:
Quote:
smoothSolver: Solving for Ux, Initial residual = 0.000681436, Final residual = 6.04267e-05, No Iterations 12
smoothSolver: Solving for Uy, Initial residual = 0.000724218, Final residual = 6.40965e-05, No Iterations 12
GAMG: Solving for p, Initial residual = 0.000794664, Final residual = 6.97221e-06, No Iterations 8
time step continuity errors : sum local = 7.36926e-05, global = 7.0562e-07, cumulative = 0.0159493
smoothSolver: Solving for omega, Initial residual = 5.51651e-07, Final residual = 4.08989e-08, No Iterations 4
smoothSolver: Solving for k, Initial residual = 0.00124186, Final residual = 0.000112596, No Iterations 3
ExecutionTime = 166.94 s ClockTime = 167 s
forceCoeffs forceCoeffs1 output:
Cm = 3.18034e-21
Cd = 3.91579
Cl = -7.41775e-21
Cl(f) = -5.28535e-22
Cl(r) = -6.88922e-21
Time = 4160
smoothSolver: Solving for Ux, Initial residual = 0.000678821, Final residual = 6.01967e-05, No Iterations 12
smoothSolver: Solving for Uy, Initial residual = 0.000721321, Final residual = 6.38416e-05, No Iterations 12
GAMG: Solving for p, Initial residual = 0.000792302, Final residual = 6.67782e-06, No Iterations 8
time step continuity errors : sum local = 7.03966e-05, global = 6.78587e-07, cumulative = 0.01595
smoothSolver: Solving for omega, Initial residual = 5.46691e-07, Final residual = 4.06206e-08, No Iterations 4
smoothSolver: Solving for k, Initial residual = 0.00123355, Final residual = 0.000112172, No Iterations 3
ExecutionTime = 167.71 s ClockTime = 168 s
forceCoeffs forceCoeffs1 output:
Cm = 3.17389e-21
Cd = 3.90818
Cl = -7.39271e-21
Cl(f) = -5.22465e-22
Cl(r) = -6.87024e-21
Time = 4180
smoothSolver: Solving for Ux, Initial residual = 0.000676188, Final residual = 5.99651e-05, No Iterations 12
smoothSolver: Solving for Uy, Initial residual = 0.000718458, Final residual = 6.35896e-05, No Iterations 12
GAMG: Solving for p, Initial residual = 0.000788874, Final residual = 6.90003e-06, No Iterations 8
time step continuity errors : sum local = 7.25486e-05, global = 6.91636e-07, cumulative = 0.0159507
smoothSolver: Solving for omega, Initial residual = 5.41841e-07, Final residual = 4.0346e-08, No Iterations 4
smoothSolver: Solving for k, Initial residual = 0.00122535, Final residual = 0.000111751, No Iterations 3
ExecutionTime = 168.48 s ClockTime = 169 s
forceCoeffs forceCoeffs1 output:
Cm = 3.16754e-21
Cd = 3.90062
Cl = -7.36866e-21
Cl(f) = -5.16788e-22
Cl(r) = -6.85187e-21
Time = 4200
smoothSolver: Solving for Ux, Initial residual = 0.000673624, Final residual = 5.97395e-05, No Iterations 12
smoothSolver: Solving for Uy, Initial residual = 0.00071561, Final residual = 6.3339e-05, No Iterations 12
GAMG: Solving for p, Initial residual = 0.000786379, Final residual = 6.61115e-06, No Iterations 8
time step continuity errors : sum local = 6.93323e-05, global = 6.64381e-07, cumulative = 0.0159513
smoothSolver: Solving for omega, Initial residual = 5.37108e-07, Final residual = 4.00752e-08, No Iterations 4
smoothSolver: Solving for k, Initial residual = 0.00121725, Final residual = 0.000111333, No Iterations 3
ExecutionTime = 169.29 s ClockTime = 170 s
forceCoeffs forceCoeffs1 output:
Cm = 3.16115e-21
Cd = 3.89311
Cl = -7.34413e-21
Cl(f) = -5.10912e-22
Cl(r) = -6.83321e-21
End
|
I attached some pictures for velocity around ball (after 200, 2000 and 4000 iterations). As you can see after 4000 there is a high range of low velocity close to the cylinder.
Doy you have any ideas how to improve this simulation |
