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April 29, 2010, 17:25 |
Low Reynolds Number SST Model
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#1 |
Senior Member
Joshua Counsil
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Location: Halifax, Nova Scotia, Canada
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Hi all -
I am modeling the T106A low-pressure turbine, shown here: I am using the SST turbulence model. Here is my pressure distribution graph: Ignore the orange line (Tu = 1%). Both the red and green lines (my initial results and my results with a quadrupled grid density, respectively) correlate alright with the experimental, but don't capture the laminar separation bubble on the suction surface (the experimental results have a pressure plateau). At a recent fluids conference, a PhD student told me he obtained very good results (in comparison with Stieger's, found here: http://www-g.eng.cam.ac.uk/whittle/T106/Start.html) when he used the SST model with the low Reynolds feature turned on. I know the low Reynolds number feature exists in Fluent, but in CFX-Pre, I am unsure of how to turn off the automatic wall function. After digging through the Help file, I have found many topics about the low Reynolds model, such as the required node refinement (at least 15 within the boundary layer for the low Re model, or 10 for the automatic wall function model), but I can't find any info on actually enabling the low Re model. Does anyone know how to enable the low Re model and/or turn off the automatic wall function? |
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April 30, 2010, 06:33 |
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#2 |
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Glenn Horrocks
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You results are very impressive, very good agreement with experiment, well done.
Rather than change the wall function approach why not turn on the turbulence transition model and try to directly model the laminar to turbulent transition? This will be your best bet to get even better accuracy in my opinion. But if you REALLY want to change the wall modelling you will find the wall function options under the turbulence model in the domain tab. By default the SST mode has the automatic option enabled. |
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April 30, 2010, 20:34 |
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#3 |
Senior Member
Joshua Counsil
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Thanks for the kind remark, Glenn. Unfortunately, my surface velocity and skin friction results aren't nearly as good.
I'll try what you said and post my new results. Thanks! |
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May 4, 2010, 12:05 |
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#4 |
Senior Member
Joshua Counsil
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When I used the SST model without transitional turbulence, my solution did not converge, but it did oscillate very close to the residual values (and produced the above pressure results, which are pretty close to accurate). I didn't expect it to converge fully, as it is a highly three-dimensional, unsteady problem that I am modeling as two-dimensional and steady. When I tried using the transitional Gamma Theta Model, my residuals oscillated once again, but nowhere near the desired residual value.
A plot of y+ reveals maximum y+ values of 2.5 at the trailing and leading edges, and average y+ values < 1. Although CFX recommends a max y+ value of 1, the documentation said that results should not change significantly for 0.001 < y+ < 5, so a max of 2.5 seems reasonable. CFX also recommended an expansion ratio of 1.1 with a hexahedral mesh, which I used. I used the high resolution advection scheme, as recommended. 75-100 nodes in the streamwise direction are recommended. I used more, but that shouldn't drastically affect the results. They found that 30,000 nodes in most turbomachinery cases produces grid-independent results. I used just over 30,000 nodes. I have read and practiced "obtaining convergence" on CFD Online, in the CFX Help file, and from various other sources. I will try a lower order discretization scheme, but I don't feel that will significantly help my solution converge. All I did was switch from the default SST to the SST with the Gamma Theta Model. Any recommendations? |
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May 4, 2010, 18:38 |
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#5 | |
Super Moderator
Glenn Horrocks
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Quote:
The turbulence transition model is often hard to converge in a steady state simulation. This is because you now have a transition, and for many airfoils you will get a laminar separation bubble associated with it (the transition model is very good at picking up laminar separation bubbles). However these bubbles are often unstable and jiggle about, even though the remainder of the flow is stable. This means you are forced to consider transient simulations. |
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May 4, 2010, 20:49 |
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#6 |
Senior Member
Joshua Counsil
Join Date: Jul 2009
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Thanks again, honey (I heard that's what you're getting called these days).
I am trying to capture the laminar separation bubble, and I realize that this is an unsteady, three-dimensional problem in which the LSB likely fluctuates in size and location. However, I read a paper that was able to capture the LSB with steady, 2D simulations, so I thought I'd give it a whirl. |
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May 5, 2010, 06:44 |
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#7 | ||
Super Moderator
Glenn Horrocks
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Quote:
Quote:
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May 5, 2010, 12:24 |
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#8 |
Senior Member
Joshua Counsil
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Hey Glenn -
I tried the simulation with my original, unrefined (though higher quality) mesh, SST Gamma Theta Transition Model, and adaptive timesteps (with the initial, minimum, and maximum timesteps based on experimental data). I captured the LSB (teal line): As for the location/size, we think my geometry is a little off (i.e. the incidence angle is incorrect) and that this is causing added acceleration over the suction surface. We came to this conclusion because: 1) The velocity profiles on the suction surface are too high and show an unexpected spike at about mid-chord. The acceleration should be somewhat constant. 2) My laminar separation bubble is small and delayed, which could be because of added momentum in the boundary layer due to the extra acceleration. Thanks again, Glenn. |
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May 7, 2010, 15:31 |
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#9 |
Senior Member
Joshua Counsil
Join Date: Jul 2009
Location: Halifax, Nova Scotia, Canada
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Just an update for your consideration. I changed the incidence angle from 37.7 degrees to 40.5 degrees and obtained much better results. I believe this is because the stagnation point on my geometry was improperly set.
http://img208.imageshack.us/i/cpsteadyunsteadytu1.jpg/ Last edited by wyldckat; September 3, 2015 at 17:32. Reason: disabled embedded images |
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May 10, 2012, 03:52 |
Boundary condition values
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#10 |
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Arun
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Location: Vellore,TN,India
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Hi Josh,
I'm doing the same analysis for the project. I checked and different sources i didn't find a good solution for the boundary value. I have my value like given below. Inlet V=2.334 (for 1.6*10^5 re) angle of Attack(37.7 deg) x=2.33 cos(37.7) y=2.33 sin(37.7) but the problem is i didn't get the out let pressure value and the angle(do i need to consider angle on outlet pressure?) i'm doing k and omega sst model And the most worst case is i didn't get any bubble separation still now. my friends suggested the form. I hope i'll find some answer from you |
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May 10, 2012, 11:03 |
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#11 |
Senior Member
Joshua Counsil
Join Date: Jul 2009
Location: Halifax, Nova Scotia, Canada
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Outlet reference pressure is zero if you're specifying an inlet velocity. I don't understand your "outlet angle" question.
You're not seeing bubble separation because you're not using the proper turbulence model. The k-w SST model failed to predict bubble separation (see the above graphs). The SST gamma-Re_theta transition model was what I used to predict the bubble, and an unsteady analysis was required. |
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May 10, 2012, 16:55 |
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#12 |
Member
Arun
Join Date: Mar 2012
Location: Vellore,TN,India
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thank you for setting me to the right track josh.. I'm doing my analysis in fluent and i'm new for CFD.
i calculated x and y by velocity triangle is this correct? x=2.33 cos(37.7) y=2.33 sin(37.7) as the above post i'll change to 40.5 for my angle of attack. Last edited by arunintn; May 10, 2012 at 17:29. |
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May 10, 2012, 17:39 |
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#13 |
Senior Member
Joshua Counsil
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I no longer have access to my simulations, but that looks correct. Post your results once you've obtained them.
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May 10, 2012, 17:42 |
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#14 |
Member
Arun
Join Date: Mar 2012
Location: Vellore,TN,India
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really i was little relaxed
I'll update my result after i finish my computation. |
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May 17, 2012, 09:01 |
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#15 |
Member
Arun
Join Date: Mar 2012
Location: Vellore,TN,India
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Hi Josh,
i completed the solution and it is converged but i did't get bubble separation and i have reverse flow in face(some numbers) and on pressure out (some numbers) i have attached you the jpeg files for reference. I can't figure why i have revers flow. I hope that you can help in this. |
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May 17, 2012, 18:38 |
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#16 |
Super Moderator
Glenn Horrocks
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The reverse flow must be small. I cannot see it on your plots. Are you using Fluent? Any difference when you use CFX?
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May 18, 2012, 07:39 |
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#17 |
Member
Arun
Join Date: Mar 2012
Location: Vellore,TN,India
Posts: 43
Rep Power: 14 |
Hi,
I have not used CFX before. I think it should be same. I showed to my professor and he told it fine but he asked me to make the bubble separation. i tried a little but i can't find it. I need some idea here i need to know where and what change. I'm new to CFD and fluent |
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May 18, 2012, 07:54 |
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#18 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
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This is the CFX forum so we cannot help you specifically with Fluent. I would try this model in CFX as I feel much more comfortable with that solver, but it is your choice. Also note Fluent has lots of different sovlers to try, if you use the coupled solver you will be closer to the CFX solver.
To get laminar separation bubbles you will need the turbulence transition model and a fine, good quality airfoil mesh. |
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April 8, 2013, 14:17 |
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#19 |
New Member
Join Date: Jan 2012
Posts: 5
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Hi all,
I'm trying to do the same T106 analysis, but am not getting a correct velocity contour plot. I'm using inlet velocity of 8.45 m/s at an angle of 37.7. Just wondering if you guys have any tips for what I'm missing. Thanks very much. |
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April 8, 2013, 18:34 |
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#20 |
Super Moderator
Glenn Horrocks
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Location: Sydney, Australia
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