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March 3, 2005, 01:20 
Mass Balance in CFX5.7

#1 
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Hey Guys
Please, do anyone knows how to obtain both negative and positive mass flow separately at a 2D plane in CFX5.7 post? Thus, say, for a 2D plane, there is backflow occuring at the plane, do anyone knows how to get the estimate of backflow that is occurs at the plane, since if you use the "massflow" function it only gives the overall which I presumed it includes both incoming and outgoing flows in the plane. A quick response to this will be much appreciated. Thanks a lot!! 

March 3, 2005, 14:59 
Re: Mass Balance in CFX5.7

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Create a Contour plot of Mass Flow on your plane with 3 contour levels and your min value equal to the negative of your max value. This will create 2 contour surfaces, one negative, one positive. The flow direction will depend on the normal direction of the surface (which you can visualize by plotting a vector of Normal on the slice plane).
Hide the contour plot and create a User Surface using the "from Contour" method. Set it to the contour plot you created and select the 2nd or 3rd contour level, these will correspond to the negative and positive flow regions respectively. Any calculations done on the User Surface will yield results for the positive or negative flow only. Regards, Robin 

March 4, 2005, 01:03 
Re: Mass Balance in CFX5.7

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Hey 'ere
Thanks! But it seems you didn't get my question. I'm interested in the amount (...kg/s) and not the contour plot. That one just gives the graphical representation and NOT the quantity. Although the lengend shows most of the values but it wouldn't give the amount as if u're using the "calculator" funtion. Anyway, I've solved the problem. I did export the "mass Flow" variable at the specific plane. Then in MS excel, I extracted those backflow masses and sum them up. This did work though, just that is a long process if it can be done in the cfxpost. Regards!! 

March 4, 2005, 11:53 
Re: Mass Balance in CFX5.7

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Hi,
As Robin said, after you have created the 2 User Surfaces (let's call them USmfpos and USmfneg), you may obtain the positive mass flow as massFlow()@USmfpos and the negative mass flow as massFlow()@USmfneg Regards, Rui 

March 4, 2005, 23:24 
Re: Mass Balance in CFX5.7

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thanks!! but it seems I'm missing something here. U mean I shd create 2 surfaces in the same plane or what? I don't get that. Because the plane is a 2D at a specific axis location. I want to know the total backflow or negative mass flow in that 2D plane. You're saying I should divide it into two user surfaces or what? I simply can't get that.
As I said earlier on, I've done it with MS excel. Thanks a lot once again!! 

March 9, 2005, 22:54 
Re: Mass Balance in CFX5.7

#6 
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Yes, that's right.
1. Create the contour plot exactly as described by Robin. 2. Create two User Surfaces using the "From Contour" option. One surface will correspond to the region with negative mass flow and the other to the region with positive mass flow. 3. Use the function calculator as described by Rui to get what you want. No need for Excel (allthough I see how you did this and it works.) Pretty tricky but it will work fine. 

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