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Old   February 7, 2016, 19:18
Question Standard vs Actual Inlet Velocity as BC
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Hi,

I tried to search about it before posting my doubt. But, I could not find anything which would clarify the things that are bugging me. It might seem like stupid question. But, I am new to CFD field.

When one does simulation of internal incompressible flow, then at inlet one should assign Mass flow rate or volume flow rate or velocity and gage pressure 0 at outlet. Lets say I want to assign Velocity at inlet. So this velocity is Standard or Actual Velocity

I have done test of pneumatic check valve. Flow meter gives reading of 6 Standard LPM (Standard Conditions written on flow meter are 20deg C and 1 Atm) and Pressure Drop across valve is 50 hPa. I want to verify same using Simulation. From Q=AV, I can calculate velocity. Pipe Internal Diameter is 4mm. But, people say standard and actual flow rates are different in case of air flow from what I have read. So which flow rate should I use to calculate Velocity so that I can give it as inlet boundary condition?

Thanks in advance!

Best Regards,
Rajdeep Rajput
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Old   February 8, 2016, 04:39
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Filippo Maria Denaro
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As V is the aveaged velocity over the area, which shape of the velocity profile in inlet you prescribe?

However, the BC.s for incompressible flows can be prescribed in terms of either velocity or inlet (both for inlet/outlet), note that we call "pressure" but it actually has no thermodinamic meaning
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Old   February 8, 2016, 09:37
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Thanks for the quick reply! Please the images I have attached to understand the following things I am gonna talk about.

I am not giving any profile input. Actually instead of velocity I want to use mass flow rate as inlet BC. But all tutorials use velocity. In Autodesk CFD software I cannot give profile. I can just give components of velocity one I select inlet surface. In Autodesk CFD, they have one option Environment Scenario. This box is to prescribe environment or reference. So, I put there P=101325Pa and T=293.15K (which is equal to 20deg C and as per flow meter standard). Then when I apply Material Air, then into "Set" option I select "variable" as people from Autodesk recommend to select in case of air flow and also they ask to check in the box "Use Environment Scenario" which means use reference. When I click that, then as you can see in image density changes to 1.20412E-6 g/mm3.

Now, from experiments I know that my flow rate is 6Nl/min. So this volume flow is standard flow. For STD conditions, density is 1.20412E-6 g/mm3. So, I calculated mass flow rate 0.12041g/s and used as BC. The pressure drop I am getting is close to experimental results with these settings. But, when I check density and mass flow at inlet its not matching to given input. Density changes to 1.29228g/mm3 and it is same everywhere in fluid domain. Mass flow rate is calculated using this density. So, I am confused is it supposed to happen like this or not? I am totally new to CFD field. I just want to know what is best setup for air flow. How to implement test data to use in CFD setup? I cannot use pressure at inlet. Because my goal is to study pressure drop using CFD and to check how much deviation it gives from experimental results.

Best Regards,
Rajdeep Rajput
Attached Images
File Type: jpg Scenario.JPG (16.9 KB, 4 views)
File Type: jpg Temp.JPG (37.4 KB, 4 views)
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Old   February 8, 2016, 09:47
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if you are totally new in CFD your problem is to learn the basic, not to use a software ...
I suggest starting from the analysis of your problem... if you work with an incompressible model, you do not have the thermodinamic pressure law in effect. So why are you prescribing density, pressure and temperature?
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