Valve Forces in CFdesign

Mike Clapp

Althea,

Since seeing this message on CFD On-line last November I have undertaken a little research into this problem. I contacted the company involved and asked for the Pro/Engineer geometry of the valve that they were simulating. We then ran a CFdesign model of the valve for six different shaft positions. For each position CFdesign produced a summation of the forces on the shaft and the flow-rate through the valve for a given pressure drop.

After running this test "blind" I went down to see the company and compared our results to their experimental results. CFdesign's force predictions were within 15% of the experiment over the entire range and predicted the trends well. The flow rates were up to 20 % out but we would put this down to some slight misunderstandings about how the valve body was connected to the test rig. Assumptions were made about this geometry which turned out to be incorrect.

It turned out that the company involved had tried to simulate this valve with two of the best-known commercial finite volume based CFD codes. One had predicted a force on the shaft of 300 Newton's, at a position where the experiment gave them around 50 Newton's, while the other had given them a value of under 1 Newton. Some of the trend graphs looked like a random number generator produced them. Both of these codes produced very reasonable results for the flow rate.

Unfortunately I cannot give you a simple answer to why these Finite Volume codes are so poor at predicting forces in valves. I would guess that it is something to do with solving for the pressure at the cell centres and then having to interpolate the pressure forces to the walls. You could imagine that this could lead to problems when you have a flow through a small gap where there is only one cell through the width of the feature. CFDesign uses an equal order finite-element technique. That means we are actually solving for pressures on the nodes, some of which are on the walls. Therefore we do not need to do any interpolation to get the forces out.

If anyone else is encountering similar problems with valve forces, or just wants to find a quicker and easier way to run fluid flow problems, please take a look at www.cfdesign.com.

Regards

Mike Clapp

Valve forces Posted By: Althea <adesouza@talk21.com> Date: Fri, 3 Nov 2000, 3:36 a.m. Hello everyone, I am interested in using Fluent to simulate a valve under steady state conditions to predict the forces. The valves are fairly small (approx. 9mm diameter inner shaft). This is a problem which has been tackled previously by people who are no longer available (and unfortunately nor are their simulations). All I know is that their results did not tie up with the experimental data. Actually I think they got close with pressure drop but the forces were way off. What I want to know is this - is it reasonable to expect to be able to use Fluent to predict the forces accurately (within 10%) or would I just be wasting my time? And what complications would I be likely to encounter? Regards Althea