# Very high or negative pressures

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 July 23, 2009, 16:42 Very high or negative pressures #1 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 9 Dear All, I am modeling a pipe with dynamic mesh. One end is a piston, the other end is an outlet/outflow. Case setup: k-epsilon incompressible: air density = 2.33 kg/m^3 = constant. Operating pressure = 265kPa at piston; also tried 0 Pa. I tried the open pipe end as outlet as well as outflow. The first step of the motion ends always with wrong static pressures. Next to piston they are very high (>1E+6). If the open end is an outflow, then the pressure there becomes negative. Please help me correct the errors in this case.

 July 24, 2009, 01:33 #2 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,137 Rep Power: 32 *switch to pressure-outlet *check your units (scale) *what kind of method are you using for moving mesh (layering/remeshing/...) display the grid while plotting the pressure, and check if the high pressures aren't linked with bad cells __________________ In memory of my friend Hervé: CFD engineer & freerider

 July 24, 2009, 11:26 #3 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 9 Hello mAx, thank you for your time. * switched to pressure outlet, still calculated static/absolute pressure at piston is > 1E+7 * my pressure unit is Pa * I am using layering, it is a hex mesh on a rectangular pipe * equiangle/equivolume skew is < 0.2

 July 24, 2009, 12:19 #4 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,137 Rep Power: 32 *check the scale: grid/scale *how fast is your piston moving? *do you see any discontinuity in the fluid domain (display pressure)? *does the fluid flows out? __________________ In memory of my friend Hervé: CFD engineer & freerider

 July 24, 2009, 13:49 #5 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 9 * grid/scale: all scales = 1, unit = [meter] * piston moves with 32 m/s * no discontinuity: pressure is 265kPa at outlet and grows smoothly to 5E+7 at piston * mass flux at outlet converged to -8.5kg/s Pipe/duct has rectangular cross section, 0.3m x 0.1m, length ~20m

 July 24, 2009, 14:00 #6 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,137 Rep Power: 32 post a picture with pressure and grid. did you check the mesh motion? __________________ In memory of my friend Hervé: CFD engineer & freerider

 July 24, 2009, 14:14 #7 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 9 Yes, the mesh motion is fine. If the time step is dt=1E-4, mass flux at outlet is -8.5kg/s, pressure at piston is 5E+7Pa If the time step is dt=1E-6, mass flux at outlet is -600kg/s, pressure at piston is 3E+11Pa This is totally bogus. Something seems to be conceptually wrong.

 July 24, 2009, 14:21 #8 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,137 Rep Power: 32 disable the MDM, switch to steady solver. Change your moving piston to massflow inlet and give the 8.5kg/s. Iterates and check the pressure distribution. are you working 2d or 3d? __________________ In memory of my friend Hervé: CFD engineer & freerider

 July 29, 2009, 19:43 #9 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 9 Apologies for the delayed reply. Solution: start the simulation with ideal-gas on, run for sufficient number of time steps. Switch to incompressible. It works now. Many thanks, mAx for the useful analysis; it has helped me tackle the problem from many aspects. Cheers, -jpo

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