# compresible flow

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 March 25, 2003, 02:50 compresible flow #1 Martin Guest   Posts: n/a Hi! Iam begginer user STAR-CD. I dont know, how defining boundry condicion for compresible flow for fluid flow though pipe on end nozzle, (diameter is not constant). Flow is transsonic. Inlet is 50m/s, Preasure 10Mpa and Teperature 2500K. I use Inflow-preasure, but it divergent. Plese help me. Thanks

 March 25, 2003, 02:58 Re: compresible flow #2 Stefan Guest   Posts: n/a You should try "stagnation - pressure", this worked for me (I had to deal with this problem lately, too)

 March 25, 2003, 03:05 Re: compresible flow #3 Martin Guest   Posts: n/a termophyzical models->Liguid gases->initi->default? monitorin -> deffault?

 March 25, 2003, 03:12 Re: compresible flow #4 Stefan Guest   Posts: n/a Well, I guess you have to try this. I put in my initial temperatur and an estimate for the velocity. It would make sense to place your monitoring cell at a point of great changes (throat,...). In compressible flow there is an important connection the reference cell location. I do not know what the best thing location is, I put my reference cell at my outlet and defined my oulet pressure, and therefore all pressure data is relative to that location. Hope that helps.

 March 25, 2003, 03:23 Re: compresible flow #5 Martin Guest   Posts: n/a I try it,init 1000K estimate vel 50m/s ,Stagnation (DU 1, 10Mpa,1000K)- Prasure(0.1Mpa, 293K) but -> negative density #076

 March 25, 2003, 03:29 Re: compresible flow #6 Stefan Guest   Posts: n/a This might be a problem due to the large difference in pressure (two orders of magnitude). I read somewhere that this might lead to problems, and one should place the reference cell accordingly. But actually I just had this one problem, so I might not be a valuable help. Maybe you should place your reference pressure in an area where pressure is around 1 MPa ?

 March 25, 2003, 04:03 Re: compresible flow #7 Martin Guest   Posts: n/a This solution divergent. initial 2000K 50m/s reference 1MPa T 600K stagnation (relative) 10MPa, Tem 2000K, preasure 1Mpa 600K *** WARNING #061 *** RATIO OF STAGNATION PRESSURE AND STATIC PRESSURE AT REGION 1 CELL 5107 LESS THEN 1. TEMPORARILY SET TO 1. *** WARNING #060 *** DENSITY OUT OF BOUND DUE TO PRESSURE CORRECTION AT CELL 16 DENSITY= -1.8100E+03 LIMITER APPLIED

 March 27, 2003, 05:41 Re: compresible flow #8 Nicola Guest   Posts: n/a Hi Martin, I had once a similar problem, I started with a higher density than expected, tried to get convergence, then took this solution as a starting point for the next calculation where I reduced the density a bit. I suppose this should work also with pressure. You have to play also with the residuals to get convergence. It is not at all a scientific way of getting a solution, but if it works...

 March 27, 2003, 22:28 Re: compresible flow #9 cjtune Guest   Posts: n/a My 'beginner's problem' was to not use the 'pseudo-transient' soultion method... after enabling that, although the simulation never really converged, it at least did not diverge. Got to play around with the time step sizes.

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