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Old   August 5, 2008, 10:12
Default flow rate decrement
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I have one question for 3d channel flow simulation. That is the flow rate decrement. I checked the flow rate for each time step. As followed, that decrease continuously.

if nstep = 1, flow rate = 0.839421 if nstep = 5000, flow rate = 0.776719 if nstep = 10000, flow rate = 0.743256

For periodic b.c. I use constant pressure gredient((density*friction velocity^2)/(the half width of channel)) in u-momentum NSE.

I don't know that I look wrong part.........
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Old   August 5, 2008, 12:05
Default Re: flow rate decrement
pankaj saha
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hi, i am also working on the same , facing some problem. Would you like to discuss with me.

Please, tell me the following---- Domain size, grid, Re, time step, and initial condition. i hope you are running a incompressible code.

what is the exact value of presure gradient you are using. i mean the numerical value. If you are using non-dimensional code--and then the value of pressure gradient shoud be 1.0 and it will be added to the x-momentum equation .

Also, in initial condition , are you using random perderbation?

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Old   August 5, 2008, 20:05
Default Re: flow rate decrement
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hi, pankaj saha

my code's details are

domain size : 2(streamwise)*1(normal)*2(spanwise)

staggered grid, 16(streamwise)*64(normal)*32(spanwise)

Retau=395(for friction velocity and the channel half width)

i.c. : u=-1.5*y(y-2) (i don't know generating perturbation....)

i don't use non-dimentionalized NSE.

constant pressure gradient = friction vel.^2/half width

since NSE is devided by density, density is not.

friction velocity used is obtained by averaging all value near the wall(including north and south wall)

i think flow rate decrement may be induced by no slip b.c.

but i don't find my faults near the wall.
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Old   August 6, 2008, 05:18
Default Re: flow rate decrement
pankaj saha
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you are telling that, you are calulating friction velocity by averaging at near wall. that means at every timestep it will vary and your pressure gradient and Retau will vary. i hope this is wrong. you are using dimensional code.then you are supplying seting viscosity not the retau in your code. i think your viscosity=utau*0.5/395. here you need Utau. normally, people take Utau=1 and set viscosity. from that your pressure gradient will have a constant value=2.0 in your case. just check the formulation of getting mean pressure gradient.

also, your x-dimension is very less compare to eddy size. take 2pi=6.3 minimum, othwerwise you will not get anything..also, grid should be large 50x66x60


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