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 mehul April 12, 2006 03:20

Problem with DES analysis

Hi to all. I have done DES transient analysis of a surge chamber(just like pressure vessel).I have used DES because inlet is fluctuating.The pressure at inlet is fluctuating sinusodially.I have to also do harmonic structutal analysis for that I resuire pressure vs frequency data.so I am getting pressure vs time in cfx and converting into ps vs freq.but after doing transient analysis using DES I am getting same results as RANS model.Actually Actually I a msupplying pr at inlet which is varying sinusodially.In RANS model I am getting same nature at any point inside the pressure.I want some peaks for freq so that I can apply it into structural analysis.That's why I am doing DES .but I am getting same results .Only peaks are coming at upto 0.0165 sec.My total time was 0.075 sec.From 0.0165 to 0.075 sec the nature is same as RANS model. (1) Second thing is that when I have observed residuals of momentum ,mass,velocity then they are also fluctuating after 100 iteration.(Upto 100 ietration it was steady state).and upto end of solution i.e. upto 1000 th iteration it remains same.(I have taken 1000 time steps). (2) My courant no is 0.03 to 0.16.Earlier I was getting 200 and beyond and so I have refined mesh. So what should I do?Should I reduce time step?Or should I use another model.Only thing I require is that I should get some peaks of pressure. Other thing is that can courant no be zero?and DES can only applied to open flows.Because in my case it is closed flow.why I have chosen DES is that it is also applicable to fluctuating inlet.so I have chosen it.Pls help me if anybody can. Thanks all.

 longbow April 12, 2006 09:23

Re: Problem with DES analysis

I don't know the rationale behind using DES because of inlet fluctuation. My understanding is that DES is targeted to massively detached or separated flow. You can check "Blending Function for DES Model" to see in which area LES is used. If LES is never turned on in the domain, DES analysis is acutally just a RANS analysis. Looks like you want to get forces at different harmonics. What's the period of the highest harmonics you are interested in? If you have a strong fluctuation signal, it should not be too difficult to capture frequency information. Certainly, the magnitude of harmonics may be different story.

 mehul April 12, 2006 10:15

Re: Problem with DES analysis

Thanking you Actually i want t oget pressure at some frequncy (this freq i will get from inverse of time).I want some peaks of pressure at some freq.I have rotating compresssore at inlet of my problem.Its speed is 410 rpm.so i want freq which is multiple of 7 .

 longbow April 12, 2006 22:29

Re: Problem with DES analysis

What's the number of your compressor blades? What does "freq which is multiple of 7" mean? Do you want to know pressure magnitude at seventh harmonic or does compressor have seven blades? Basically, you need to run enough physical time to get the information at the frequency you are interested in.

 mehul April 13, 2006 02:40

Re: Problem with DES analysis

No sir actually i am not doing flow analysis of comapressore blade passage.but i am doing analysis of surgae chamber.the comapressor outlet is kept at an inlet of chamber.so the flow frm comapressore effects chamber.The model analysis has been performed for surge chamber and it gives freq in the which is multiple of 7.The freq are like 7,15,21,51 ,110,140 etc.Now the this also matches with freq of comapressore rotation.which is 410 rpm.The compressore is of lobe type comapressor.not rotary but is a positive displacement type.Now for doing harmonic structural analysis of th cham,ber i have to apply max or peak presure at certain freq which i want to get from flow analysis. so the problem is like this.I have got physical time from comp rotation.and applied it tio the surge chamber.Do you think that is it right?why i have done it because comp flow is affecting chamber flow.Any advice will be very helpoful to me. Thaking you.

 longbow April 13, 2006 10:48

Re: Problem with DES analysis

Physical time of CFD analysis should be determined by compressor rotating speed, number of lobe, and freq you are interested in. If compressor has seven blades, the lobe passage freq of excitation force is 47.83Hz and it is very possible that highest energy is contained in the freq. In other words, pressure magnitude at other freq, like 6.83Hz, 13.67Hz, etc., will be smaller than that at 47.83Hz. Now, if you are interesting in pressure fluctuation magnitude at 47.83Hz, one period of signal with that freq is 0.0209[s]. Therefore, your transient CFD simulation should not be less than that period. Actually, you probably need to run several periods to get a stable result that can be used for Foriour Transform.

 mehul April 14, 2006 02:02

Re: Problem with DES analysis

Thak you sir,

I think that you are right.because first i have given total time of 0.15 secc.This i have taken from compressor speed 410 rpm and so 6.83 hz and so 0.15 sec.but i have observed that the results i have got were same as what i have applied.i.e. i have applied sinusodial pressure at inlet and in post i have got same nature.no damping.I have also try to damp it by changing dia and by changing sinusodial nature ,but i have not got that.I have in first trial of DES got some fluctuating nature which was interseting to me to get freq.and this was in the range of 0 to 0.016 sec.so i have reduce tottalk time to 0.016 sec. pls tell me that---"If compressor has seven blades, the lobe passage freq of excitation force is 47.83Hz and it is very possible that highest energy is contained in the freq."hete how you have got 47.83 hz.pls explain me.second thing is that can courant no is zero,because in one iteration i have got avg courant no is 0.and also that can DES can be used closed flows.i.e. flow inside chamber as in m y case? Thanks a lot again.?

 stuart April 25, 2006 08:57

Re: Problem with DES analysis

Hi, I'm not 100% sure I follow the problem set up, but as well as the frequency of the fluctuations at the inlet, will you not also have to take into account the time period for the fluctuations to pass through your domain. This will need to occur and possibly stabalise before you can start to take results of pressure fluctuations on the structure. So you are looking at very small time steps to capture the effects of the Inlet, and then long simulation times for the capturing of the flow through the domain and for compling a time history of the pressure forces. Stuart

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