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Old   May 14, 2002, 07:33
Default resistance & pressure losses
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Dear CFX users, recently I am modelling a subsonic inlet for turboprop engine. I am using CFX 5.5 and I have few problems: 1] As the air input for engine (in fact output from domain = end of inlet canal) I am trying to model screen using fluid subdomain with thickness 6mm, shape is cylindrical. To define pressure losses through screen I use quadratic resistance coefficient QRC { I use followed equation to ensure units [kg/m^4]: QRC = density * 0.5 * (1/thickness) * ksi, ksi= dimensionless loss coefficient for screens geometry by Pope's book Low Speed Wind Tunell Testing }. What is not clear for me is fact that from results I have still huge variation of the velocity values on the boundary after screen (b.c. what I define here is static pressure). Could it caused by fact that resistance is izotropic, so I have to define expression along radial direction where resistance only be applayed? 2] I have tested the equation for computing QRC from ksi (see prolem 1]) by simple model of internal flow in tube with length 3m and dia 0.2m where screen with different thicknesses (fluid subdomain) was located after 2.7m of free flow (b.c.: p.inlet>overpressure 500Pa, p.outlet, walls). I could not obtain better diferences of pressure loses (comparison of p.l. computed from ksi and computed from cfx results) than 6%. For smaller thicknesses were pressure loses differences even greater 10%. How to explain this? 3] And finally last one. Does anybody know how to obtain static or total pressure values in certain cross-section of inlet canal ( shapes of cross-sections are not simple circle or rectang.). When I try to use User define surface from points defined on boundary of cross-section by file, CFX-post lists only values in points which were used for surface definition. One way is to define also inside cross-section points (and their order) not only points on boundary of cross-section to obtain pressure values. Is there any simpliest way how to do that?

Thank you very much for any advice.

I hope that my english is comprehensible.

Jan from Czech Rep.
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