# Fluent Basics

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 February 2, 2011, 14:44 Fluent Basics #1 Member   John Join Date: Jan 2011 Posts: 65 Rep Power: 8 Hi Guys, just to say this forums is too useful.. I have some question, let the contest start to you all. Try answering any question you can.. FEEL FREE. I am testing car aerodynamics in Fluent:- 1- How to perform GIS, I know that you have to keep editing your mesh. BUT my question is how to know that its OK. 2- How to do a 2nd order discretization. all I know is that you have to change the solution method (pressure and momentum) to 2nd order, but how to know that your answers are really in 2nd order and to compare it. 3- Drag and Lift (print to console and plot) forces are both ticked in monitors, but where can I see the actual answers of drag and lift. 4- I need to do a comparison between both 1st order and 2nd, how can I do that. And, if you have any car tutorial model from your college, university. Please upload it, it will be very helpful to me. Thanks a lot, John

 February 2, 2011, 15:18 #2 Member   John Join Date: Jan 2011 Posts: 65 Rep Power: 8 For Q3:- I know that my solution will be full converged if the the plotted coefficient remains constant while iterating, my plotted coefficient is not constant now what I am supposed to do in this situation?

 February 2, 2011, 19:59 #3 Member   John Join Date: Jan 2011 Posts: 65 Rep Power: 8 no one in the whole thread? no experts or professionals here?

February 2, 2011, 21:43
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By plotted coefficient, do you mean the residual? If so (perhaps even if not), be careful. Just because the residual remains constant does not necessarily mean the solution is fully converged to a physically realistic answer. Pick the physical parameter you're looking at and ensure that it remains constant to within the error tolerance you're OK with. For instance, in incompressible flow in a pipe w/o mass generation, perhaps you'd want to monitor the mass flow into and out of the system to ensure that they sum to zero. In fact, there are many times I un-check"check convergence" in the residual monitors, and simply watch the fluxes to ensure convergence. This is a very simple case, but can prevent significant rework in the future on a converged, but physically unrealistic answer.

If you're dealing with a parameter that simply won't "settle down" -- that is, it appears to sinusoidally fluctuate, do the following:
1) Review your boundary conditions to ensure they're valid and not inherently transient. The same goes for your physics. Multiphase slug flow has a natural "transient" quality to it which makes convergence to a rock-steady value difficult.
2) Review your mesh to ensure that it's of sufficient quality to capture the physics you're looking for (did you do a grid dependence check?)
3) Ensure your UDF's are behaving properly over the range of temperatures, pressures, etc. that the solver might be trying
4) Change the limits on your solve to maintain "realistic" solver attempts (especially true if you're using udf's -- see #3). For instance, if I have water at 300K flowing into a pipe which has wall temperature of 300, I don't need to have a 5000K upper limit on my temperature. The same goes for pressure.
5) Turn down your under relaxation factors a bit. Don't let the "rapid convergence" fool you, however. Dialing them down will give you a slower, but often more reliable approach to convergence with fewer bumps in the road...It also will often trip the default "convergence" levels (1e-3, etc.) in Fluent.

ComputerGuy

Quote:
 Originally Posted by John222 For Q3:- I know that my solution will be full converged if the the plotted coefficient remains constant while iterating, my plotted coefficient is not constant now what I am supposed to do in this situation?

February 2, 2011, 21:47
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For 2 and 4, you're doing it correctly. To compare, pick a value you're interested in...For instance, pressure or drag on a face. From there, you can simply run two simulations, plot/print/write down or otherwise capture the data you're looking for. As far as I know, Fluent doesn't really have something that will automate the process of solution method comparison. When people to model validation studies (on grid refinement, for example), they are truly making 2,3,4,etc. different grids of various quality, running each simulation, and checking to see when the results stop changing.

ComputerGuy

Quote:
 Originally Posted by John222 Hi Guys, just to say this forums is too useful.. I have some question, let the contest start to you all. Try answering any question you can.. FEEL FREE. I am testing car aerodynamics in Fluent:- 1- How to perform GIS, I know that you have to keep editing your mesh. BUT my question is how to know that its OK. 2- How to do a 2nd order discretization. all I know is that you have to change the solution method (pressure and momentum) to 2nd order, but how to know that your answers are really in 2nd order and to compare it. 3- Drag and Lift (print to console and plot) forces are both ticked in monitors, but where can I see the actual answers of drag and lift. 4- I need to do a comparison between both 1st order and 2nd, how can I do that. And, if you have any car tutorial model from your college, university. Please upload it, it will be very helpful to me. Thanks a lot, John

February 3, 2011, 17:47
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Quote:
 Originally Posted by ComputerGuy For 2 and 4, you're doing it correctly. To compare, pick a value you're interested in...For instance, pressure or drag on a face. From there, you can simply run two simulations, plot/print/write down or otherwise capture the data you're looking for. As far as I know, Fluent doesn't really have something that will automate the process of solution method comparison. When people to model validation studies (on grid refinement, for example), they are truly making 2,3,4,etc. different grids of various quality, running each simulation, and checking to see when the results stop changing. ComputerGuy
Yes but still I didn't really get how to do Grid Independence Study (GIS) in FLUENT. I did an OK mesh please have a look attached.
Is my mesh converged and can be used for 2nd order discretization?
Attached Images
 mesh.jpg (91.6 KB, 14 views)

February 4, 2011, 03:11
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Maxime Perelli
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Quote:
 Originally Posted by John222 Is my mesh converged and can be used for 2nd order discretization?
A mesh cannot converge...A calculation on a mesh can
Discretization order is independant from your grid (so far I know )
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