Dirichlet-Neumann boundary condition solution becomes unstable - Pressure Correction Method

Question

I am simulating incompressible flow over a cylinder at Reynold number of 500. I am solving navier stokes equation using pressure correction method. My solution becomes unstable after certain time (approximately 5s).

I have tried refining my mesh, stepsize (0.05) (making sure my CFL < 1, even though I am using implicit methods)

My boundary conditions, mesh and unstable results are shown in the attached figures. The domain is about 25 times larger than the cylinder diameter.

I have tried simulating this problem O grid (which became unstable almost immediately).

The following link contains the pictures of the boundary conditions and results.

http://imageshack.us/photo/my-images/7/boundarycondition.png http://imageshack.us/photo/my-images/32/instability.png

I would be grateful if anyone can share their thoughts/experiences on this problem. Many thanks.

editted:

Apologies for the typing mistake:

I am using the following boundary conditions: Neumann boundary $$\frac{\partial \vec{u}} {\partial n} - \vec{n} p = 0; $$

on Dirichlet Boundary $$ \vec{u} = u_x = 1 $$

editted:

i have applied velocity boundary conditions on the nodes around the dirichlet boundary. Also, top-right and bottom-right corner node is dirichlet boundary with velocity 1.

After, I looked more deeply into the simulation results, I notice that instability starts to creep in at inflow/outflow junction.

Answer 1

How, specifically, are you implementing your boundary conditions? This can make all the difference in a simulation like this.

Answer 2

I have figured out the problem. I had to increase the size of the domain further to remove boundary effects. Moreover, I had to reduce CFL number to around 0.5-1.0

I think CFL number need to be reduced further for higher reynolds number.

Initially, I thought that I had reduced step size enough, but it was not the case.