# general momentum source/hydraulic pressure jump/positive displacement machine

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

 June 28, 2009, 21:30 general momentum source/hydraulic pressure jump/positive displacement machine #1 Senior Member     George Join Date: Mar 2009 Location: Birmingham, UK Posts: 257 Rep Power: 11 I need some help in setting up a momentum source. I want to model an internal flow system that contains throttles and a positive displacement machine. I want to see the effects of the positive displacement machine in the system, however I'm not interested on modeling the actual machine in 3D but a reduced mathematical model. I have the machine performance curves (speed, pressure ratio, volume flowrate, icentropic efficiency etc. Therefore I have created a general momentum source with a simple array that relates dp with flowrate for testing. As for BC inlet is relative pressure 0 [Pa] and outlet's velocity 5 [m/s] (will be massflow later to match the real problem setup but should be ok for now) FUNCTION: DpQ Argument Units = [Pa] Option = Interpolation Profile Function = Off Result Units = [m^3/s] INTERPOLATION DATA: Data Pairs = 0,0.025,500,0.05,1000,0.075 Extend Max = On Extend Min = On Option = One Dimensional END END #density = 1.284 [kg/m^3] #duct area = 0.05 [m^2] Pin = massFlowAve(p)@F91.89 2 Pout = massFlowAve(p)@F96.89 2 deltaP = abs(Pout-Pin) velocity = volflow/area@F96.89 2 volflow = DpQ(VardeltaP) #the variable VardeltaP is the algebraic expression deltaP scoef5 = -10e5 [kg m^-3 s^-1] svalue5x = scoef5 * (Velocity u - 0[m/s]) svalue5y = scoef5 * (Velocity v - 1[m/s]) svalue5z = scoef5 * (Velocity w - 0[m/s]) When I use the above basic source definition I can get the thing to converge with the specified velocity in the defined subdomain but when I use the equations below there are many issues: scoef5b = Density * volflow / volume()@B89 2 svalue5bx = scoef5b * (Velocity u - 0[m/s]) svalue5by = scoef5b * (Velocity v - velocity) svalue5bz = scoef5b * (Velocity w - 0[m/s]) a) for example cfx-post wont let me evaluate the velocity and volflow scalars so I dont know if my coefficient is calculated correctly. b) the problem converges but velocity has the value from the outlet boundary and pressure's are..... well wrong IN = -1.655e+01 [Pa] OUT= -1.547e+02 [Pa] dp across subdomain = -1.381e+02 [Pa] c) am I doing something wrong with my problem approach? I would appreciate any comments especially if the source coefficient is properly defined and if the expression volflow = DpQ(VardeltaP) is correct. many thanks __________________ Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

 June 28, 2009, 22:55 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,094 Rep Power: 109 Hi, I would not use "velocity" as a CEL variable. It might get corrupted with the internal velocity variable. Use a different variable name. Glenn Horrocks

June 29, 2009, 05:09
#3
Senior Member

George
Join Date: Mar 2009
Location: Birmingham, UK
Posts: 257
Rep Power: 11
Quote:
 Originally Posted by ghorrocks Hi, I would not use "velocity" as a CEL variable. It might get corrupted with the internal velocity variable. Use a different variable name. Glenn Horrocks
didnt make any difference mate - the variable is case sensitive anyway so its should be ok - however I changed it anyway to test it.

However in cfx-post I do get the follwing error "The following unrecognised name was referenced: DpQ." thats the only bit that I'm unsure, the definition of the volflow expression as the function calculates at the pressure diferential at the inlet and outlet of the subdomain and then it should look up the pressure diferential from the array and output the volume flowrate.

I attach the full ccl if anyone has the time to check and comment
Attached Files
 CFX12_source_velocityb_vel.ccl.txt (5.9 KB, 40 views)
__________________
Top 4 tips
1. Knowledge is everything and Ignorance is dangerous.
2. Understand your limitations and try to eliminate them.
3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window.
4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

 June 29, 2009, 18:30 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,094 Rep Power: 109 Hi, Some issues I can see: You refer to "Density" in scoef5b. This is not defined. You define velocityx = volflow/area@F96.89 2. It should be area()@F96.89 2 You have no materials defined. Did you snip that out of the CCL? You need to have some materials defined. Glenn Horrocks

 June 29, 2009, 18:35 #5 Senior Member     George Join Date: Mar 2009 Location: Birmingham, UK Posts: 257 Rep Power: 11 materials is the stanadrd air @ 25 [C] mate (was a bit lazy ) I just copy paste the expression ccl code and forgot materials but use the standard ones for the example Density is the variable name for density of the fluid m8 EDIT: yes stupid of me density but where its located thanks I got it now just checking the area()@ __________________ Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

 June 29, 2009, 18:57 #6 Senior Member     George Join Date: Mar 2009 Location: Birmingham, UK Posts: 257 Rep Power: 11 fixed the mistakes and cfx finds a solution but I still get still wrong results which means there is still something else wrong as I still get the "The following unrecognised name was referenced: DpQ." in cfx post The 1D array is there but maybe there is another way to write the volflow = DpQ(VardeltaP) expression __________________ Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials Last edited by ckleanth; June 30, 2009 at 04:36.

 June 30, 2009, 17:49 #7 Senior Member     George Join Date: Mar 2009 Location: Birmingham, UK Posts: 257 Rep Power: 11 still bugging me this flippin expression __________________ Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

 June 30, 2009, 18:57 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,094 Rep Power: 109 Hi, Sorry, did not read your initial post fully - I don't think CEL interpolation expressions are sent to CFD-Post. The best way to check where on your curve your function is lying is to set up monitor points for the input and output of the expression then you can export them from Solver Manager and check them externally. Glenn Horrocks

 June 30, 2009, 19:03 #9 Senior Member     George Join Date: Mar 2009 Location: Birmingham, UK Posts: 257 Rep Power: 11 thanks mate will try this tomo.. if I remember correcly fluent had this setup as ready made BC ... cant be that hard of a thing to do really... cant see any other way to model loads of fans/pumps inside a system using the machine characteristics... __________________ Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

 August 24, 2012, 16:29 #10 New Member   Philippe Vidori Join Date: Apr 2011 Posts: 8 Rep Power: 8 Hello Sir, I'm going through the same problem at the moment. Just curious to see if your model worked in the end. Best regards, Phil P.S: CFX is a pain with fans !!

 August 25, 2012, 06:23 #11 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,094 Rep Power: 109 Several post on this forum talk about this technique to model fans. It is quite commonly done and works fine when set up correctly.

 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 Mihail CFX 7 September 7, 2014 06:27 alvio CFX 1 November 18, 2008 16:44 Li CFX 2 July 28, 2008 02:33 bioman66 CFX 5 June 3, 2006 01:40 Atit CFX 1 May 31, 2006 09:05

All times are GMT -4. The time now is 14:58.