Symptoms of pressure losses and fluid resistance

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 March 9, 2013, 11:11 Symptoms of pressure losses and fluid resistance #1 New Member   Michal Join Date: Sep 2012 Posts: 12 Rep Power: 7 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.

 March 9, 2013, 13:18 #2 Senior Member   OJ Join Date: Apr 2012 Location: United Kindom Posts: 475 Rep Power: 13 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

 March 10, 2013, 06:36 #3 New Member   Michal Join Date: Sep 2012 Posts: 12 Rep Power: 7 Helo, thank you for your answer and advises. I check this. 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/

 March 10, 2013, 07:10 #4 Senior Member   OJ Join Date: Apr 2012 Location: United Kindom Posts: 475 Rep Power: 13 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. Last edited by oj.bulmer; March 10, 2013 at 07:11. Reason: Circulation

 March 11, 2013, 03:53 Mass flow #5 New Member   René Join Date: Mar 2013 Location: Switzerland Posts: 5 Rep Power: 6 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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