## Modelling turbulent flow

Numerical methods and mathematical models of Elmer
martijn
Posts: 25
Joined: 05 Oct 2009, 21:30

### Modelling turbulent flow

I have a 2D model of a pipe with an obstruction in it. I am simulating air flow in this structure and would like to see turbulent flow. The pipe width is 5cm, and the average flow speed varies from 0.2 to 10m/s. The Lowest Reynolds number is 684, the highest 34000.
My expectations:
Re < 2500: laminar flow before and after the obstruction
2500 < Re < 4000: laminar flow before, turbulent after the obstruction
Re > 4000: turbulent flow before and after the obstruction.
However: the flow profile just seems to scale with the incident flow speed.
I know the simulation volume is too small to achieve a fully developed flow profile, but my observations are the same for a longer tube. Moreover, in the obstruction the flow speed should certainly be turbulent.
The simulation is steady state, so I a assume that the averaged values of a turbulent flow will be calculated, but then I would still expect that the boundary layer would get thinner for higher Re numbers.

I attached a number of velocity profiles (absolute value). The relevant parts of the SIF file are below

Summarising: Why don't I see any sign of turbulence?

Martijn

SIF file:
Body 1
Target Bodies(1) = 1
Name = "Body 1"
Equation = 1
Material = 1
End

Solver 1
Equation = Navier-Stokes
Procedure = "FlowSolve" "FlowSolver"
Variable = Flow Solution[Velocity:2 Pressure:1]
Exec Solver = Always
Stabilize = True
Bubbles = False
Lumped Mass Matrix = False
Optimize Bandwidth = True
Steady State Convergence Tolerance = 1.0e-5
Nonlinear System Convergence Tolerance = 1.0e-8
Nonlinear System Max Iterations = 25
Nonlinear System Newton After Iterations = 25
Nonlinear System Newton After Tolerance = 1.0e-5
Nonlinear System Relaxation Factor = 1
Linear System Solver = Iterative
Linear System Iterative Method = BiCGStab
Linear System Max Iterations = 500
Linear System Convergence Tolerance = 1.0e-8
Linear System Preconditioning = ILU0
Linear System ILUT Tolerance = 1.0e-3
Linear System Abort Not Converged = False
Linear System Residual Output = 1
Linear System Precondition Recompute = 1
End

Equation 1
Name = "Equation 1"
NS Convect = False
Active Solvers(1) = 1
End

Material 1
Name = "Air (room temperature)"
Viscosity = 1.983e-5
Heat Conductivity = 0.0257
Heat Capacity = 1005.0
Density = 1.205
Compressibility Model = Incompressible
Viscosity = 1.983e-5
Sound speed = 343.0
Heat expansion Coefficient = 3.43e-3
End

Boundary Condition 1
Target Boundaries(1) = 17
Name = "noslip"
Noslip wall BC = True
End

Boundary Condition 2
Target Boundaries(1) = 1
Name = "noslip"
Noslip wall BC = True
End

Boundary Condition 3
Target Boundaries(1) = 21
Name = "noslip"
Noslip wall BC = True
End

Boundary Condition 4
Target Boundaries(1) = 5
Name = "noslip"
Noslip wall BC = True
End

Boundary Condition 5
Target Boundaries(1) = 25
Name = "speed in"
Velocity 1 = 5
End

Boundary Condition 6
Target Boundaries(1) = 26
Name = "noslip"
Noslip wall BC = True
End

Boundary Condition 7
Target Boundaries(1) = 9
Name = "noslip"
Noslip wall BC = True
End

Boundary Condition 8
Target Boundaries(1) = 30
Name = "pressure out"
External Pressure = 0
End

Boundary Condition 9
Target Boundaries(1) = 13
Name = "noslip"
Noslip wall BC = True
End
Attachments 5.0 m/s
velocity-50.png (129.82 KiB) Viewed 3785 times 1.0 m/s
velocity-10.png (129.97 KiB) Viewed 3785 times 0.2 m/s
velocity-02.png (128.63 KiB) Viewed 3785 times
tzwinger
Posts: 94
Joined: 24 Aug 2009, 12:20
Contact:

### Re: Modelling turbulent flow

Martijn,
I am astonished that you get convergence at all, especially with the higher Re-numbers.

perhaps you could take 2 measures:
1) switch in a turbulence model (k-epsilon, for instance)
2) try to resolve (I do not know how fine your mesh is now) finer around the cylinders

And finally, some effects (like von Karman vortices) I guess are rather of transient nature - so perhaps a transient run would also be something you might consider worth trying.

Best wishes,

Thomas
raback