[Masoud91]

Hi i would like to make the geometry for a pipe with an injector at one end. I would like to have a smaller surface on another surface as outlet/ injector of my pipe after extruding the pipe. The problem is that when i create another small sircle the design modeler wants to to extrude again and creates a hollow cylinder with the inner radius of my second cycle.

Thanks for help

[ghorrocks]

Have you done the designmodeller tutorials? This sounds like a question on the basics of designmodeller. The tutorials are available on the ANSYS customer webpage.

[Christophe]

Design modeler is the worst 3-D modeling software out there. By far. I only use after I bring in the model from a good CAD program, like Creo and want to refine surfaces and bodies for meshing.


Sent from my iPhone using CFD Online Forum mobile app

[ghorrocks]

I agree DesignModeller is not great for doing complex designs from scratch. Definitely you should use a mainstream CAD program for that sort of thing (Solidworks, Creo, UG etc). But DesignModeller is excellent for importing existing complex models and cleaning them up for meshing and modelling. It has excellent defeaturing tools and handles multi-block models nicely (which is very useful for high quality meshes).

So a mainstream CAD program to generate geometry and DesignModeller to prepare it for simulation is a pretty powerful combination.

[Masoud91]

Hi,

I set as reference pressure 0bar. My inlet has 1000 bar total relative pressure. My outlet has 100 bar static rel. pressure.

The steady simulation needs 100 time steps (time step size 0.0001s) to reach the required convergence and runs well.
But when i set a wall as outlet for my transient simulation and my steady simulation as initial condition file my pressure at outlet springs from 100 bar to approx. 2200 bar and starts swinging. Instead, it should fluctuate around 1000bar and this should be the final value. What could be the problem? I have used diesel as material with a variable density expression.

Thanks a lot!!!

[ghorrocks]

Are you saying you stop the flow in your transient simulation by removing the outlet and putting a wall there?

So aren't you seeing pressure transients in this sudden stopping of the flow? They will take quite a while to die down. Also, they will only be realistic if you have the correct fluid compressibility and system compliance in your model.

[Masoud91]

I changed my simulation program to Fluent, because my expression for density is incorrect and i dont reach the final value of 100 bar at outlet.

Does anybody know if it is possible to face the outlet as wall like in CFX. I have only pressure outlet and mass flow outlet. I have already tried to set 0kg/s as mass flow outlet respectively target mass flow but i cant see transients. My report plot is set to area weighted average of pressure and is set to a smaller outlet area as surface.

Thanks

[ghorrocks]

I do not understand what you are saying.

I don't follow how using the wrong expression for density means you need to switch to Fluent.

I don't know what "is it possible to face the outlet as wall like in CFX" means.

If you are running in Fluent then you should try to fluent forum.

[Masoud91]

I used that expression for diesel density: (829/1-((Absolute Pressure -1e5 [Pa])/1.2e9 [Pa])) kg/ m^3.

My bulk modulus is originally a function (Kf= 1.2*10^4(1+0.6*Absolute Pressure(bar)/600)) too, but neither the simulation performs under applying the function well nor under a constant value of 1.2e9[Pa].

I think the problem is the bulk modulus??ß

[ghorrocks]

Your equation starts with: (829/1-((stuff))). This is evaluated as 829/1=829, then the value returned is 829-((stuff)). I do not think this is what you intend.

I highly recommend you enter equations with units for each variable. If you entered with the units you would have 829[kg m^-3]/1-((stuff)) and it would return a units error - because you are subtracting a unitless variable from a density. So you would have caught this error yourself if you entered equations with the variables with correct units.

[ghorrocks]

Your equation starts with: (829/1-((stuff))). This is evaluated as 829/1=829, then the value returned is 829-((stuff)). I do not think this is what you intend.

I highly recommend you enter equations with units for each variable. If you entered with the units you would have 829[kg m^-3]/1-((stuff)) and it would return a units error - because you are subtracting a unitless variable from a density. So you would have caught this error yourself if you entered equations with the variables with correct units.

Also: If you enter your expressions in CFX-Pre, then the CEL editor has a function where it can evaluate or even plot your function. So you could plot your function over a range of absolute pressure and check it is returning values as expected.

[Masoud91]

This is my density expression:
(829[kg m^-3] /(1-((Pressure -1e5 [Pa])/(0.8e9[Pa]))))

It has the desired brackets. When my bulk modulus is 0.8e9 [Pa], i recieve still a strikingly increased outlet pressure of 2000 bar instead of 1000 bar. I think my bulk modulus is the problem. But even 0.8 e9 [Pa] is an unrealistic value. If i use the function for bulk modulus K= 1.2e9*(1[Pa]+0.6*(Pressure*1e-5/600))
i receive a outlet pressure around 6000 bar.

Does anyone know where could be the problem?

[ghorrocks]

Your density function looks OK to me, but you should still check it using CFX-Pre as I suggested.

As for why the outlet pressure is doing unexpected things: First look at the FAQ: https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F

If you need more assistance then you will need to explain what you are doing more thoroughly. Please attach images of your geometry and mesh, and your CCL or output file.