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d.h. August 5, 2008 10:12

flow rate decrement

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.........

pankaj saha August 5, 2008 12:05

Re: flow rate decrement
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?

d.h. August 5, 2008 20:05

Re: flow rate decrement
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.

pankaj saha August 6, 2008 05:18

Re: flow rate decrement

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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