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 Szczepan March 9, 2013 11:11

Symptoms of pressure losses and fluid resistance

I analyse in CFX two models of ventilation pipes.
I have to check, at which model pressure losses and drag (fluid resistance) are lower.
I check the results in CFX post (velocity- and pressure distribudion). I see the differences, but i don't know, how should i interpret the results.
Which phenomenons/symptoms at velocity- and pressure distributions are signs for high pressure losses and drag (high gradient etc.?)?
Thank you.

 oj.bulmer March 9, 2013 13:18

It is difficult to make out from your description, what is the fluid domain and what you problem exactly is. How about sharing some CFD-post snaps to illustrate your question?

You can always create two planes at appropriate locations and measure the average pressures. The difference in the two will give pressure drop between the two locations. You say drag, but on what?

OJ

 Szczepan March 10, 2013 06:36

Helo,

I describe my problem with an easy example: There are two ventilation pipes (see the link below). The first is an elbow without a radius and the second is an elbow with a radius. Its evident that an elbow with a radius is for the flow better than an elbow without a radius. BUT: how to explain thix on basis of velocity and pressure distribudions, see the links below? Which other phenomens are important?

Velocity distribution:
http://postimage.org/image/xpx0rb917/

Pressure distribution:
http://postimage.org/image/t8twmk0xd/

 oj.bulmer March 10, 2013 07:10

Typically, a good design is the one that causes least loss of energy in the fluid domain from inlet to outlet. One way to do it can be to measure average pressures at the two inlets and the outlet, and find out the corresponding pressure drops. And then you'd choose a design that has smaller pressure drop values.

However, it may be deceptive just to rely on pressure because this value is precisely a static pressure and it is reversibly convertible into kinetic energy. I would also try to find out the average velocities at inlets and outlet and calculate the dynamic head and add these values to average static pressure values obtained as stated in earlier paragraph. The drop in total pressure thus obtained is reasonable representative of loss of energy.

Additionally, you may want to see the vector plots at different regions, to make sure there are no unnecessary circulations, which are hungry for energy.

 rene78 March 11, 2013 03:53

Mass flow

Hi Szczepan, I'm quite new to CFD and CFX and to get a first understanding I did a similar study like you.

My inlet boundary condition was: total pressure = 2 bar

Then I checked the mass flow of both systems. Your model 2 should have a higher mass flow.

PS: The mass flow at the inlet/outlet can be checked with the calculator.

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