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CFD not matching real world testing on a flow bench

Question asked by Jeff Soucek on Jun 8, 2017
Latest reply on Jun 15, 2017 by Bill McEachern

Hi All,

    Been learning and experimenting with CFD flow mostly as a hobby and a learning experience.  I have access to some nice equipment to validate the results.  Unfortunately the results in Flow simulation are not matching.  I cant seem to figure out why as it is a simple experiment.

 

Let me explain my results and set-up.

 

I have and actual 3d file of a velocity stack that was turned and CNC machined.  It was then put on a flow bench and the results came back as 312CFM at 10" depression

 

18986698_1529552517068612_673037754_o.jpg

 

19022774_1529598743730656_740191160_o.jpg

 

You can see the results above on the flowcom screen

 

I set up a model in Solidworks to represent this test to compare the results

 

Below you can see my model.  The "balloon" is cross sectioned to see the velocity stack mounted to the square plate with a 4" pipe below it just like the actual test.  Even though the actual 4" pipe is only a 6" long I modeled it at 12" to get as much detail as possible

 

18949041_10211269845835033_1502526126_o.png

 

I set up my simulation with the Inside surface of the "balloon" to be environmental pressure opening of 14.6959473 lbf/in^2

 

Screenshot 2017-06-07 22.46.42.png

 

I then set the outlet surface "lid" ( bottom of the 4" pipe) to 14.334676481726 which converts to 10" wc

 

In my model I set the initial mesh to automatic level 3 with advance narrow channel realignment

I then applied a local mesh to the velocity stack and plate surfaces with custom settings of the following

 

Screenshot 2017-06-07 22.56.38.jpg

 

Screenshot 2017-06-07 22.55.54.jpg

 

Screenshot 2017-06-07 22.55.08.jpg

 

I then applied a second local initial mesh over the velocity stack and gave it automatic setting level 6 and picked the solidbody and chose disable solid component.  this can be seen in the picture above as a mushroom looking body

 

One more note, I set my calculation control options to the following:

 

Screenshot 2017-06-07 22.58.52.png

 

 

 

Below you can see my mesh and the results

 

Screenshot 2017-06-07 23.02.17.png

 

Only problem is my results are considerably higher at 346.9 cfm vs 312 on the flow bench -  see below:

 

18947276_10211269449065114_764525433_o.jpg

This is more than 10% off.

 

So does anyone see what I am doing wrong? Or do I simply have to tweak to get it to match?  If I have to tweak it to match, what is the best approach?

My goal is to validate using this simple set-up then move to my cylinder head porting so that I know the changes I am making are valid

 

This is a picture of the head I want to test, print a FDM rapid proto then flow test to compare again.  Eventually I will 5axis port machine the head for my personal track bike

 

18986328_10211265576848311_666970146_o.jpg

Sorry about so many pics but I felt it helps explain where I am at.

 

Any input would be appreciated as I am now stuck and do not know what to do next.

 

Thanks for any help you can provide

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