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Old   December 23, 2008, 12:20
Default reversed flow over thin plate
  #1
qiaomei
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dear fluent experts,

pls help! i'm modelling 4.5m/s air flow over a thin plate. after setting 0.01 for all residuals and iterating for 2000 iterations, all results diverge and i had reversed flow in outlet from the 1st to the last iterations.

the following are the parameters i used in fluent:

->Solver: pressure-based solver, implicit formulation, steady time, 3D space, absolute velocity formulation, green-gauss cell based gradient option, superficial velocity porous formulation

->viscous model: spalart-allmaras (1 eqn), vorticity-based production, default model constants

->solution controls: using flow & modified turbulent viscosity equations, simple pressure-velocity coupling, default under-relaxation factors, and standard pressure, first order upwind momentum & modified turbulent viscosity discretization

i would really appreciate if someone can tell me where did i go wrong and if any additional information is required.

thank you so much!
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Old   December 25, 2008, 07:33
Default Re: reversed flow over thin plate
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Alagesan
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The possible reason for the divergence and reverse flow is due to poor mesh or improper boundary condition.

Use the search function of this page(top right hand corner), you can get lot of information about the convergence/divergence and reverse flow issues.

For more accurate results use higher order upwind discretization.

Cheers, Alagesan

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Old   December 25, 2008, 11:04
Default Re: reversed flow over thin plate
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qiaomei
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hi alagesan,

thank you so much for your advice! i have already re-meshed with regular quad/hex meshes but they still yield the same problem. i will try searching for help and to try higher order upwind discretization.

thanks!
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Old   December 25, 2008, 11:14
Default Re: reversed flow over thin plate
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Alagesan
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If your mesh is ok then the problem is due to the wrong inlet boundary condition.

Cheers, Alagesan.
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Old   December 26, 2008, 02:49
Default Re: reversed flow over thin plate
  #5
qiaomei
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i am using a velocity inlet BC, with 4.5m/s in x-direction.

my residuals ranged from 0.1 to 0.001 at a run of iterations of 2000. however, my coefficients of lift and drag reached values of order e6.

what could be the possible problems causing such unrealistic values if my reference values are correct?
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Old   December 26, 2008, 07:59
Default Re: reversed flow over thin plate
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Alagesan
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In velocity inlet boundary condition,under turbulent specification method select turbulent viscosity ratio. The default value is 10, Change it to 1 or less than that.

Then initialize your solution and then under report-> reference values select appropriate inlet boundary and reference zone.

Read through the Fluent user guide chapter 7 - Boundary conditions which will explain all boundary conditions and how to calculate the turbulence specification values.

Some useful meshing and fluent tutorials (even for flat palte) available in this below link. http://courses.cit.cornell.edu/fluent/

Try and see.

Cheers, Alagesan.

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Old   December 26, 2008, 15:49
Default Re: reversed flow over thin plate
  #7
qiaomei
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hi alagesan

thank you again for taking time to reply my questions. i have tried your suggestion of turbulent viscosity ratio but the same situation occured. the Cl and Cd values are still very high, to the order of e6. this is the same for using a laminar model.

if my airflow is 4.5m/s, is it justifiable to use a laminar model?

if it ok if i send you printscreens of the steps i used in FLUENT for you to take a closer look?

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Old   December 27, 2008, 09:52
Default Re: reversed flow over thin plate
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Alagesan
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Ok same time give me details of your mesh.

Alagesan
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Old   December 30, 2008, 04:54
Default Re: reversed flow over thin plate
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Anastasios
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i think that since your solution converges you should not worry about reverse flow at outlets. But to be more sure check if the results seem resonable comparing with othwe works. Especially at the case of pressure outlet i think that reverse flow is something that should happen. The reason is that pressure outlets allow fluid to enter or leave the domain according to the flow needs. Hence, if your sollution converges and your results are reasonable you should not worry about reverse flow.

Another alternative would be to increase your flow field so that away from the area of interest in order to minimize possible negative effects from reverse flows at outlets.

Best luck,

Anastasios
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