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January 6, 2006, 09:49 
Which B.C. i should apply for this problem?urgent

#1 
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Hi to all, I am doing transient flow analysis on a pressure vessel.(More or less it is working as a surge tank).At the inlet of surge tank there is compressor. i.e .outlet of compressor is feeding flow to inlet of surge tank. Inlet known conditions at surge tank are velocity and outlet known conditions are velocity and pressure. Inlet velocity is 22 m/sec .and outlet velocity is 22 m/sec(cross sectional area is same) and pressure at outlet is 3.7 bar. Inlet pressure is not known. Now compressor is giving flow to surge tank inlet and this nature of flow is sinusoidal. i.e. pressure is varying sinusoidal at inlet. i.e pressure variation of flow is sinosodiual. The problem may be considered as compressible, but I have simulated it and found that there is little change in density and also little change in pressure .difference of pressure at inlet and outlet is more or less 0.05 bar. My query is which boundary condition I should apply? I have known conditions at inlet is only velocity (22 m/sec).and outlet known conditions is 22 m/sec and pressure is 3.7 bar. Second thing is whether I should consider the flow compressible or incompressible.? I am thing that following boundary conditiona should be applied.
(1) For steady state velocity at inlet and pressure at outlet. (2) For unsteady state velocity at inlet and pressure at outlet. Now for unsteady anything should vary with respect to time. I have varying thing are pressure at inlet and outlet. but I only know pressure variation at outlet. So should I apply that at outlet. Whether it will create divergence flow at outlet due to varying nature or not? that's why I am afraid of that. Please reply me urgent if anybody knows. Thabkibng you all. 

January 6, 2006, 19:12 
Re: Which B.C. i should apply for this problem?urg

#2 
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If you set velocity at the inlet you by default set mass flow into the domain.
The setting of pressure at the outlet is enough to describe the flow boundary conditions, but I do not believe that you will get a sinusoidal pressure field from this. You could try setting sinusoidal pressures at inlet and outlet , (the inlet being set as an outlet), and not using an inlet directly. or you could try using a compressible fluid and at the inlet for each time step define the density as sinusoidal (from rho=P/RT assuming this applies) with vel=22m/s and set the oulet to vary P sinusoidally over time too. I find it strange that if the inlet surges sinusoidally the velocity stays at 22m/s. Perhaps I misunderstand you. 

January 6, 2006, 19:22 
Re: Which B.C. i should apply for this problem?urg

#3 
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Apologies for mistyping!!
>>The setting of pressure at the outlet is enough to describe the flow boundary conditions, but I do not believe that you will get a sinusoidal pressure field from this. the above should have read: Along with the above, the additional setting of pressure at the outlet...............from this 

January 7, 2006, 03:18 
Re: Which B.C. i should apply for this problem?urg

#4 
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Thakx sir 4 ur suggestion.how d ou belive that velocity should change?because mass flow rate should remain same.because mass flow rate=density*area*velocity .now here if velocity is changing at each cross section then mass flow will also change.If surge is sinusodial then will velocity will also vary sinusodiall or not? regards ravish


February 16, 2006, 05:35 
Re: Which B.C. i should apply for this problem?urg

#5 
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Yes I suspect velocity should change at each time step, so:
1/ Set velocity to vary sinusoidally at the inlet. So it changes for each time step. 2/ Also set the mass coming into the domain to be sinusoidal at the inlet, by using the applied velocity Coval(in1,p1,fixflu,rho*vel)> this sets mass directly not pressure directly, the fixflu coefficient is responsible for this. 3/ set outlet pressure to what you know. Coval(out1,p1,1000.,3.7bar): this sets a condition for fixing Pressure. Of course do not write the 3.7bar literally. Also you may which to set PRESSO="a value" that takes a background pressure away form the calculations so that you can set the outlet pressure to ZERO. PHOENICS will then solve using relative pressure and the absolute pressure will be P1+PRESSO. P1 can set mass or pressure boundary conditions depending on the coefficient used. Read the lecture 'describing the basic functions behind PHOENICS' in the POLIS online: see Polis.html \phoenics\d_polis\d_lecs\GENERAL\INTRLECS.HTM 

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