SST turbulence model

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May 6, 2010, 08:54
#41
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Attesz
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Quote:
 i think the y+-values calculated in CFX do not only depend on number and height of prism layers but also velocities in impeller and thickness of boundary layer. So i dont know how you got the value of y+>20 with 5 prism layers?
you should increase the first cell height for example 2 times. I think thats all...

yes you are right, the "real" surge is a totally transient phenomenon. But I found in every paper, that the operational point is easily attainable with steady simulations. My compressor works at 70000RPM and PR1.65, but we reach "surge" at PR1.4. This "surge" means, that there is a very low mass flow in and out of the domain. The separation mentioned above closes the whole blade passage.

 May 6, 2010, 08:59 #42 Member   Susann Join Date: Apr 2009 Location: Dresden Posts: 33 Rep Power: 10 And how big is the distance between instability/surge-limit and this operational point? Do you have measurements? Often big separations already occur at the design point of a compressor, so that there are strong transient effects even at design point.

May 6, 2010, 09:05
#43
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Attesz
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Quote:
 Often big separations already occur at the design point of a compressor, so that there are strong transient effects even at design point.
Yes, its a good idea! I'm preparing transient simulations present...

I have measurements only on operating point, because this is a gas turbine engine. We have not enough equipment to measure the compressor stage separately.

 May 12, 2010, 04:10 #44 Senior Member     Attesz Join Date: Mar 2009 Location: Munich Posts: 364 Rep Power: 10 Hi, can I use Frozen Rotor fluid interface, when my geometry is periodic, and the Pitch Ratio isnt equals 1? There is nothing about it in the help. I have a 60deg impeller, and a 72deg stator, so the Pitch Ratio is about 1.2. I want to take into account the blade interferences...and I think, I cannot use TRS only when I have the same periodicity angles.. Regards, Attesz

 May 12, 2010, 09:51 #45 Member   Susann Join Date: Apr 2009 Location: Dresden Posts: 33 Rep Power: 10 Hey, when you use Frozen-Rotor-Model and the pitch changes, the fluxes are scaled by the pitch change.

 May 13, 2010, 05:53 #46 Senior Member     Attesz Join Date: Mar 2009 Location: Munich Posts: 364 Rep Power: 10 Hi Susann, I know that, but can it cause high numerical errors? In some papers, I read that in Transient Rotor Stator, or Transient Frozen Rotor simulations, I can use only Pitch Ratio 1. Of course, with Stage option, it is not a problem. What do you think? Thanks, Attila

 May 15, 2010, 09:44 #47 Member   Susann Join Date: Apr 2009 Location: Dresden Posts: 33 Rep Power: 10 Could you maybe give me a link to this papers? Ive never heard about this before...I would follow the recommendation of CFX-help and I found nothing about a pitch change of 1 for frozen rotor or transient simulations...would be very bad because there is seldom a pitch change of 1 for a compressor with vaned diffusor. Just give it a try with the frozen rotor-model, its even faster than stage model...than you will see whats better in your case after comparing it to the experimental results...

 May 15, 2010, 10:07 #48 Senior Member     Attesz Join Date: Mar 2009 Location: Munich Posts: 364 Rep Power: 10 http://www.ansys.com/events/proceedi...PAPERS/252.pdf There is really nothing in help about this, but in this paper, it is described. Have a nice weekend!

 May 17, 2010, 03:43 #49 Member   Susann Join Date: Apr 2009 Location: Dresden Posts: 33 Rep Power: 10 I know this paper...but i find the same description of the interface-models as in CFX-help...i cannot find something about pitch change 1...sorry but can you give me a hit where in the text you find that?

May 17, 2010, 05:01
#50
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Attesz
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Hi Susann,

Quote:
 First the geometry of computational section must be adjusted to minimize the area difference at the interface between the impeller and the diffuser. Because there are 31 impeller passages and 22 diffuser passages the single passage computational geometry used in the steady state simulations had an area difference of more than 40%. This means that the area of the diffuser at the impeller-diffuser interface is 40% greater than the area of the impeller. In the unsteady simulation this is not acceptable because of the significant circumferential variations in the flow at the interface. The ideal setup would be to simulate the entire compressor, but this was judged to be too costly for the computational resources available. Instead the computational domain was defined to include three impeller and two diffuser passages which resulted in an area difference of 6.45%. The CFX solver stretches the solution at the interface during the calculations to remove the area difference and allow the simulation to converge, but this obviously introduces some modelling errors.
Here is what I mean. You are allowed to use not only Pitch 1, but here, they speak about a 6.25% area change which means a pitch change very close to 1, and they mentioned that it can cause errors. It is valid for Frozen Rotor.

So I correct myself, you can use not only pitch ratio 1, but using frozen rotor, it can cause as big numerical error as big the area difference is.

 March 17, 2014, 01:17 Turbulence model in Rotor 37 #51 New Member   Manpreet Join Date: Jan 2014 Posts: 14 Rep Power: 5 Hello Guys, I am working on project Flow field analysis through rotor 37. Cd anyone please let me know about which turbulence model is better to get results and why ? I really appreciate . Thanks Manpreet Singh manpreet_singh_er@yahoo.co.in

March 21, 2014, 04:45
#52
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Song Weimin
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Quote:
 Originally Posted by ghorrocks For a steady run, once the residuals are converging nicely you should increase the physical time scale to quite large compared to the fluid timescales. This should still converge quickly, but should allow the flow to "average" as best as possible.
what u mean by "increase"is continuing to calculate the original results by larger time step?

 March 23, 2014, 16:26 #53 New Member   Manpreet Join Date: Jan 2014 Posts: 14 Rep Power: 5 Hello Guys, Cd anyone please let me know detail about Timescale factor and Convergence RMS. What's significance of these two? In addition, Which solver method is responsible for solving N-stokes equations in CFX. I really appreciate . Thanks With regard Manpreet Singh manpreet_singh_er@yahoo.co.in

 March 23, 2014, 17:49 #54 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,535 Rep Power: 104 Time scale factor is used by the solver to march towards a solution. It is part of the numerical method. The residual is a measure of the accuracy of the solution of the equations.

 March 24, 2014, 00:06 #55 New Member   Manpreet Join Date: Jan 2014 Posts: 14 Rep Power: 5 Thanks..ghorrocks. Cd u please tell me in detail which numerical method CFX used for soling N-S equations such as time marching or anything else. Where can I find detail about this. Thanks Manpreet Singh

 March 24, 2014, 00:07 #56 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,535 Rep Power: 104 It is all in the documentation - see the theory manual.

 June 20, 2017, 15:45 #57 New Member   Michel Dang Join Date: Jun 2017 Posts: 1 Rep Power: 0 \int_S \phi \textbf{} \cdot

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