CFX_Force_Calculation
 

Dear Sir,

Pls help for this problem??????

I m solving pressure relief valve. basically its a ball type. Inlet condition-total pressure 3bar and outlet condition-3 bar (back pressure). but i m getting force on the ball is negative. pls tell me why i m getting negative force. and what reference pressure used is used. presently i m using 100000 pa.

Pls help me

What is the problem with negative drag?
Do you have a sketch of your setup?

hi
 

Dear Sir,

I m simulation a pressure relief valve means ball type. flow is going on along X direction, and we are calculation force along X direction. We wand force on the ball along X direction. but i m getting negative force. bcz suction is creating on the ball.how can i avoid this suction on the ball to get positive force. PLs help me .

I guess you have a strong acceleration on the forward face of your ball and this is generating you negative drag. Again, could you please post a sketch (even made with paint is ok) of you geometry?

hi
 

Dear Sir,

PLZ tell me the processor for this simulation. boundary condition at inlet total pressure and outlet pressure.
i took Y+1 along with SST model. but still i did nt get the positive force. plz tell me also setting for this problem.
I attached the pic

I cannot see any picture…

pic
 

Sorry.

PLz find it now

Are you measuring the forces only on the sphere or on the sphere+the plate it is laying on?

hi
 

Dear Sir,

I m measuring the force on spherical ball. force is measuring along the flow direction-X.if u have done like this problem . pls share some information how u setup the problem and how u calculate force etc.

I have never done this before but I guess it is normal to have negative drag on the ball only. If you measure the force on the ball + the plate, then I'm pretty sure you'll find positive drag.

Is there any gap between the ball and the plate? Or are they connected?

hi
 

thanks. sir,

f you measure the force on the ball + the plate, then I'm pretty sure you'll find positive drag.?

How should i measure the force?????????? plz tell me

Dear Sir,

Pls find the attached pic. I shown the ball and flow direction.

hi
 

there is no gap between the ball and plate.

How to measure it depends on the software you used…

hi
 

Dear Sir,

I m using CFX software, and calculated force along the flow direction-X
i used calculator in CFX. force-X()@ ball (Global). and i m doing steady state analysis to calculate the force on the ball. Pls help me i have request to you pls ask your friend who has done like this problem. bcz for me this project is very important.pls

In CFX you use the force_x/y/z() CEL expressions to get the force. I assume you last post means these are the functions you are getting.

Please note that a common problem in hydraulics is that when a high pressure valve first opens the Bernoulli pressure drop caused by the fast flowing fluid in the valve seat causes the valve to close again. Then it opens, then closes, and this repeats very fast until the valve blows itself up. When you design high pressure hydraulics systems you have to design valve systems which do not chatter. And a quick look at your design suggests to me your will chatter badly :)

So if you are getting a negative force (that is the valve wants to close) then I am not surprised and I suspect your modelling is correct - this valve design will chatter.

hi
 

Dear Sir,

thanks for your quick reply.

i m going mad. but i want solution any how. but i tried so much . but everytime i m getting same results. even i know if i do transient analysis. sure i will get the results. but i m working for someone else. he want positive force on the ball.even i suggest him. why we are getting negative force bcz suction is creating on the ball why high velocity.
i m thanks full for your effort...:)

Try this
https://www.youtube.com/watch?v=T1yC9mC-NQw

or this
http://www.daerospace.com/HydraulicS...fValveDesc.php

The client may be god, but even god can be wrong sometimes. Valve chatter is a major issue in hydraulics design and if he does not believe you then let them make it and find out for themselves. Just make sure that when they run it for the first time you and stand well back so you do not get hit by shrapnel and to get a good view of the explosion.

hi
 

Dear Sir,

Thanks for your support.