# transonic compressor Convrgce pb with transient ..

 Register Blogs Members List Search Today's Posts Mark Forums Read

 August 13, 2008, 15:23 transonic compressor Convrgce pb with transient .. #1 Noureddine Guest   Posts: n/a Hi, I would excute a transient simulation of an isolated transonic Rotor 37 using ANSYS CFX 11 with rotation along one pitch, i did not have any problem to get results for steady fluid flow but i could not converge with transient any way. However, I noticed that the value of pitches displayed by the solver doesnt correspond to that i specified. my setup is as fellow : - Rotor 37 design speed = 17188.7 rpm = 1800 [rad/s] - 36 blades ===> 1 pitch = 2*pi/36 = 2*3.1416/36 = 0.1745333 [rad] - total time to rotate along one pitch = 0.1745333/1800 = 9.7e-5 [s] - if i choose 100 time steps ===> DT = 9.7e-5/100 = 9.7e-7 [s]. I should see in each time step an increase of pitches by 0.01, but it is not the case !!!!!! what it can be the problem ????? any advice is very welcome. Regards,

 November 10, 2014, 11:58 NASA 37 steady simulation #2 New Member   saleh Join Date: Nov 2014 Posts: 16 Rep Power: 5 Dear Noureddine I can not simulate steady case of rotor NASA 37, all my simulation leads to overflow. Can you give me your boundary condition containing: total inlet pressure, mass flow rate or static outlet pressure for steady simulation of one blade(with periodic condition) of NASA 37 in CFX??? I guess that overflow arisen from fault boundary condition.

 November 10, 2014, 17:05 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,195 Rep Power: 109 Have you read the FAQ on overflow error? http://www.cfd-online.com/Wiki/Ansys...do_about_it.3F

November 11, 2014, 03:03
#4
New Member

saleh
Join Date: Nov 2014
Posts: 16
Rep Power: 5
Quote:
 Originally Posted by ghorrocks Have you read the FAQ on overflow error? http://www.cfd-online.com/Wiki/Ansys...do_about_it.3F
Yes, I read it but my sticky problem is not solved. My operating condition is selected according to suggestion of AIAA paper:"Fully Coupled Fluid-Structural Interaction of a Transonic Rotor at Near-Stall Conditions Using Detached Eddy Simulation":
Total inlet pressure=17.7 (psi)
Outlet Mass flow rate= 20.19(kg/s)
Total inlet temp=519 (R)
My grid contain about 600000 element for on blade passage, physical time step in steady simulation assumed 0.0001s,
I used Geometry of NASA 37 which exist in turbogrid tutorial. I test it in different inlet and outlet domain length(by extending original geometry in Bladegen)
In solution procedure, first, Mach Number increased gradually, then Notice:"a wall hase been placed at portion of an outlet..." appears in monitor screen and finally :Overflow!!!!!!

Last edited by sfallah; November 11, 2014 at 07:15.

 November 11, 2014, 17:55 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,195 Rep Power: 109 Well, that's your problem. If you are running near stall conditions you are unlikely to have a steady state solution. You will probably need to run it transient.

November 13, 2014, 07:34
#6
New Member

saleh
Join Date: Nov 2014
Posts: 16
Rep Power: 5
Quote:
 Originally Posted by ghorrocks Well, that's your problem. If you are running near stall conditions you are unlikely to have a steady state solution. You will probably need to run it transient.
ghorrocks
Thank you ghorrocks
Very useful comment.
1 technical question: Does the length of the computational domain in inlet and outlet is important in turbomachinery?
Simulation of Original geometry of NASA67 as exist in turbogrid tutorial(Small inlet and outlet domain) leads to smooth but low slop convergence curve, in the other hand, using geometry with extended inlet and outlet length leads to steep and oscillatory convergence curve. Which of them is correct and optimum?????

November 13, 2014, 18:28
#7
Super Moderator

Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 14,195
Rep Power: 109
Quote:
 Does the length of the computational domain in inlet and outlet is important in turbomachinery?
Yes, it does. This is one of the normal things to check with a sensitivity analysis. A longer domain is more accurate, but will result in a larger model. If the smaller model is converging better then I would be suspicious it is artificially damping the result causing inaccuracy.

 December 23, 2014, 12:48 Outflow boundary condition NASA37 #8 New Member   saleh Join Date: Nov 2014 Posts: 16 Rep Power: 5 Dear All What is the best outlet boundary condition for transonic(subsonic inlet and outlet but transonic passage) compressor and in general transonic turbomachines? why? I would like to have specified inlet mass flow rate. I use total pressure(because of more stable and better convergence behavior than inlet mass flow rate) at inlet but by applying static pressure at outlet, desired mass flow rate is not be obtained. My case is Nasa 37 rotor which outlet length is short. using k-omega sst and steady-state option, I have not converged results but using k-epsilon convergence attainment is easy. I guess that its reason is static outlet boundary condition which forced at non-uniform flow location (outlet). Please help me!!!!

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post jlefevre76 FLUENT 2 February 5, 2013 10:53 audrey CFX 1 September 8, 2011 19:43 siw CFX 5 October 30, 2010 05:45 icesniffer CFX 1 August 8, 2009 07:25 mahesh prakash Main CFD Forum 1 January 21, 1999 14:45

All times are GMT -4. The time now is 10:02.