Mesh generation

samraj · August 30, 2016, 15:35

Hello all, I have some problems generating mesh for the attached picture. Its a flow simulation model. two inlets flow into a nozzle and the flow out through outlet. The gas from 2nd inlet flows around the 1st inlet and by the exit of 1st inlet, the gas from 2nd inlet mixes and flows out. I tried meshing but the gas from 2nd inlet dont just flow around 1st but just on the top the 1st inlet walls and move to right hand side of the model. Any help or guidance on how to approach the problem would really help me a lot. Thanks very much

FMDenaro · August 30, 2016, 15:52

and why do you think that the problem is in the mesh?

samraj · August 30, 2016, 15:54

Hallo,
Thanks for the message. Its because the fluid doesnt seem to be flowing around the first inlet. It just stops before the 1st inlet's top line.

FMDenaro · August 30, 2016, 15:56

post the mesh picture around the two inlets

mprinkey · August 30, 2016, 23:50

If you are using mass or velocity inlets, this shouldn't happen--assuming you are using an off-the-shelf CFD code. If you are using pressure inlets, flow reversal at the inlets may occur. The mass/velocity inlets define the face fluxes at the inlets to be in-bound and the flow composition will be fixed based on the BCs. For pressure inlets/outlets, the fluxes at the boundary will computed based on the defined boundary pressure value and the pressure of the cell adjacent to the boundary...that can flow in or out and the composition will be extrapolated from the adjacent cell value (and possibly the cell gradient).

FMDenaro · August 31, 2016, 03:22

I agree ... it is not possible that the mesh causes such problems ..

samraj · August 31, 2016, 06:01

I am using velocity, density, pressure and temperature as inlet boundary conditions. Also the tool i am using is LS dyna.

FMDenaro · August 31, 2016, 06:22

Quote:

Originally Posted by samraj

I am using velocity, density, pressure and temperature as inlet boundary conditions. Also the tool i am using is LS dyna.

Are you modelling a compressible flow problem? consider that for subsonic conditions you cannot set all BC.s as Dirichlet

samraj · August 31, 2016, 06:27

Hi, i am doing mixing of two gases and combustion of gas mixture and outflow

samraj · August 31, 2016, 06:34

yes it is compressible due to significant change in density and pressure.

FMDenaro · August 31, 2016, 06:36

compressible flow with subsonic inlet requires that one condition is from the interior. Do you satisfy that?

samraj · August 31, 2016, 06:40

He,
I am sorry. i didnt understand you question. did you mean i should set one initial boundary condition from interior?

thanks

FMDenaro · August 31, 2016, 07:01

Quote:

Originally Posted by samraj

He,
I am sorry. i didnt understand you question. did you mean i should set one initial boundary condition from interior?

thanks

I mean that one BC cannot be fixed as Dirichlet but has to be let free from the interior. This is because of the direction of one characteristic curve (eigenvalue u-a) coming from the interior.
The opposite happens for subsonic outflow.

samraj · August 31, 2016, 07:02

okay. in this case, i give temp, velocity and density as inputs and dont set pressure value? thanks

FMDenaro · August 31, 2016, 07:03

Quote:

Originally Posted by samraj

okay. in this case, i give temp, velocity and density as inputs and dont set pressure value? thanks

no at all...if you set density and temperature, you are fixing also pressure!

samraj · August 31, 2016, 07:12

Oh okay. Would you recómmend how i can approach this problem? i need to evaluate velocity and temperature conditions for my problem.

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August 30, 2016, 15:52		#2
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,764 Rep Power: 71	and why do you think that the problem is in the mesh?

August 30, 2016, 15:54		#3
samraj New Member Samraj Manickam Palvannan Join Date: Jul 2016 Posts: 25 Rep Power: 9	Hallo, Thanks for the message. Its because the fluid doesnt seem to be flowing around the first inlet. It just stops before the 1st inlet's top line.

August 30, 2016, 15:56		#4
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,764 Rep Power: 71	post the mesh picture around the two inlets

August 30, 2016, 23:50		#5
mprinkey Senior Member Michael Prinkey Join Date: Mar 2009 Location: Pittsburgh PA Posts: 363 Rep Power: 25	If you are using mass or velocity inlets, this shouldn't happen--assuming you are using an off-the-shelf CFD code. If you are using pressure inlets, flow reversal at the inlets may occur. The mass/velocity inlets define the face fluxes at the inlets to be in-bound and the flow composition will be fixed based on the BCs. For pressure inlets/outlets, the fluxes at the boundary will computed based on the defined boundary pressure value and the pressure of the cell adjacent to the boundary...that can flow in or out and the composition will be extrapolated from the adjacent cell value (and possibly the cell gradient).

August 31, 2016, 03:22		#6
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,764 Rep Power: 71	I agree ... it is not possible that the mesh causes such problems ..

August 31, 2016, 06:01		#7
samraj New Member Samraj Manickam Palvannan Join Date: Jul 2016 Posts: 25 Rep Power: 9	I am using velocity, density, pressure and temperature as inlet boundary conditions. Also the tool i am using is LS dyna.

August 31, 2016, 06:27		#9
samraj New Member Samraj Manickam Palvannan Join Date: Jul 2016 Posts: 25 Rep Power: 9	Hi, i am doing mixing of two gases and combustion of gas mixture and outflow

August 31, 2016, 06:34		#10
samraj New Member Samraj Manickam Palvannan Join Date: Jul 2016 Posts: 25 Rep Power: 9	yes it is compressible due to significant change in density and pressure.

August 31, 2016, 06:36		#11
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,764 Rep Power: 71	compressible flow with subsonic inlet requires that one condition is from the interior. Do you satisfy that?

August 31, 2016, 06:40		#12
samraj New Member Samraj Manickam Palvannan Join Date: Jul 2016 Posts: 25 Rep Power: 9	He, I am sorry. i didnt understand you question. did you mean i should set one initial boundary condition from interior? thanks

August 31, 2016, 07:02		#14
samraj New Member Samraj Manickam Palvannan Join Date: Jul 2016 Posts: 25 Rep Power: 9	okay. in this case, i give temp, velocity and density as inputs and dont set pressure value? thanks

August 31, 2016, 07:12		#16
samraj New Member Samraj Manickam Palvannan Join Date: Jul 2016 Posts: 25 Rep Power: 9	Oh okay. Would you recómmend how i can approach this problem? i need to evaluate velocity and temperature conditions for my problem.