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usefkeyghobadi April 17, 2012 11:36

problem with boundary layer
 
1 Attachment(s)
Hello Foamerís
I want to simulate Boundary Layer in Newtonian and viscoelastic Fluid on 2-D flat plate with openFoam 1.5 dev
My geometry is 2D flat plate
Width: 0.5 meter
Long: 5 meter


Uniform velocity: 1m/s
Velocity BCís
Uperwall
Type fixedvalue;
Value uniform (1 0 0);


Lowerwall
Type fixedvalue;
Value uniform (0 0 0);


Inlet
Type fixedvalue
Value uniform (1 0 0);


Outlet
Type zeroGradient;

Pressure BSís
Uperwall
Type zeroGradient;


Lowerwall
Type zeroGradient;


Inlet
Type zeroGradient;


Outlet
Type fixedValue;
Value uniform 0;


I set zeroGradient boundary condition velocity for uperwall but got the similar result,
After simulation we have velocity profile as below:
According to boundary layer theory in anywhere velocity must lower or equal from uniform velocity
We used icoFoam and ViscoelasticFluidFoam(Giesekus Model) but in the both in some point velocity more than uniform velocity.


Please help me
regards

gonpe April 18, 2012 16:10

Hi

Mass flow must be maintained. If you are setting a uniform inflow and also constraining the top boundary to a certain velocity, then it is normal to expect a higher vel somewhere in your domain.

If you use a symmetryplane, this is alleviated, but could take a while/distance to stabilize.

usefkeyghobadi April 18, 2012 19:59

Hi
Thanks Gonpe
is The boundary conditions like the atmosphere there? For velocity and pressure to flow out and nowhere hasn't more velocity than uniform flow velocity
Thanks

saba_saeb April 21, 2012 15:50

Quote:

Originally Posted by usefkeyghobadi (Post 355483)
Hi
Thanks Gonpe
is The boundary conditions like the atmosphere there? For velocity and pressure to flow out and nowhere hasn't more velocity than uniform flow velocity
Thanks

According to "Gnope", the mass conservation equation, so called, continuity equation should be satisfied during the simulation. So the mass flow rate ( V*A ) through any arbitrary surface in any point of the channel should be same (consider that the area of any cross section is same according to the definition of your problem). In some points, the middle of the channel for instance, due to viscosity effects, fluid velocity next to the walls would be decreased, so fluid moves faster at the center to somehow compensate the velocity deficiency and fulfill the continuity equation. So it seems reasonable and logical if the fluid velocity in some points, gets bigger than the initial velocity that you have set so far.

cheers
Saba

gonpe April 23, 2012 10:25

Sorry for the late reply. Saba has re-iterated my earlier point ... thanks.

Just a last point. If you place a symmetry condition at the top of the domain, this will allow the speed to fluctuate along the top boundary.

Of course, this turns the problem into one of bounded channel flow. In this case, the speed should be highest at the upper boundary.

Goncalo


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