# Flow between two stationary flat plates

Hi,

I’m a newbie and this is my first post.

I’m trying to learn OpenLB by simulating a very simple flow between two flat plates. In the dimensionless system, my flat plates are 4 units long and 1 unit apart. I have chosen a Reynold’s number of 10 (dimensionless) and hence, my relaxation time, tau, is 0.8 (dimensionless). Also, I have decided to choose dx = 0.01 units (in the dimensionless system) and dt = 0.0001 units (in the dimensionless system), and the velocity in lattice units is uMax = 0.01 (in the dimensionless system). I understand that this is not the common approach, but I hope there’s nothing wrong in doing this.

My code is pretty small, so I take the liberty of posting it here. This is the part apart from all the standard declarations.

``````
#define F(i,j,n) for(int i = 0; i < j; i++)
T rho = 1;
T u[2] = {0,0};
F(i,0,N) F(j,0,M) lattice.get(i,j).iniEquilibrium(rho,u);
u[0] = uMax;

F(k,0,100) {
F(j,0,M) lattice.get(0,j).defineU(u);
lattice.collideAndStream(true);
}
imageWriter.writeScaledGif("final.gif",lattice.getDataAnalysis().getVelocityNorm());

``````

Since my time increment is 0.0001 units and I iterate 100 times, I expect that the fluid should have moved a distance of 0.0001 x 100 x uMax = 0.0001 units. Hence, when I generate the velocity image, I expect to see an almost entirely blue rectangle (since the velocity is everywhere zero). But what I see is that the fluid has penetrated quite a distance into the plates.

It does not make any sense to me and I do not know what the problem is. Please help me understand what I’m doing wrong.

Ram