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January 20, 2020, 10:59 |
Unstable Stationary FLUENT
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#1 |
New Member
Claudio
Join Date: Jan 2020
Location: Madrid, Spain
Posts: 4
Rep Power: 6 |
Hello everyone,
I am doing simulations on a closed domain. Reynolds number is in the transition region (around 2300) so I am using a laminar model with a fine mesh. The problem is that the stationary is unstable, and the values are oscilating around an average value. Due to the mesh, it is imposible to me to make a transient simulation, so I am asking if there is any way in FLUENT of getting the average value of the monitoring parameters. Also, I would like to know if it is possible to make a selective frequency damping in FLUENT, as I would like to add an extra term to Navier Stokes where the diference between to the calculated the velocity and the average velocity of the latest iterations is dumped by a factor. Thank you for your attention. |
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January 21, 2020, 08:19 |
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#2 |
Member
Andy
Join Date: Jul 2017
Posts: 58
Rep Power: 8 |
Hi,
The oscilation can have various reasons. 1. Your Mesh around inhection is too coarse. 2. What iteration scheme with what order are you using? Is there a pseudo time step, or cfl numer? Then it might be too small. 3. Also the fact that you are around transition-Re makes me suspicious too. I'm not sure if a laminar model can handle this operational point well. But I do not see your geometry. I would try to vary the cell size around the inlet boundaries. For this extra Termin, I would like to put your attention on the under relaxation numbers in the gui. Maybe they are enough for your purpose. |
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January 21, 2020, 08:33 |
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#3 | |
New Member
Claudio
Join Date: Jan 2020
Location: Madrid, Spain
Posts: 4
Rep Power: 6 |
Quote:
1. My actual mesh is of 15 Million elements, I supposed it is fine enough, nevertheless, I have to other finer meshes in case of necessity. 2. I am using a Coupled scheme with CFL of 20. For pressure I´m ussing PRESTO! (as it is a mixed convection problem) and for the other variables I am using a second order discretization. Under relaxation numbers are 0.5 for velocity, pressure, density and temperature. 3. It is a rectangular duct with a bent in the middle, U-shaped. |
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