[ghorrocks]

I do not understand your question. Are you having problems calculating the particle Re number? Are problems implementing you drag law?

[Loris88]

Hi ghorrocks!

Since you have said me to read this post, I will write here my question.
I have read the questions and the relative answers but I did not find the solution for my problem.
I have a 2D problem. I created the mesh with ICEM and then I extruded it with the shrewdness of to put as depth, 1 cell like thikness.

After importing the mesh in CFX, I have put boundary condition of symmetry for to simulate the 2D problem.

I have these measures of the rectangular channel:
-Length: 3900 mm
-Heigth: 50 mm (Characteristic length L)
-Depth: 1 cell like thikness
-Re= 80000 (known input of the problem)
-Velocity along x axis= 24 m/s (u) at the inlet. The other velocities are 0 m/s. No swirl component.
-Dynamic viscosity: 1.8058e-05 kg/m*s (alpha)
-Density: 1.20438 kg/m^3 (beta)
-Kinematic viscosity: alpha/beta= 1.4993e-5 (ni)

Re= (u*L)/ni ~ 80000 [for precision 80034]

But Ansys has given me Re=9.2723E+04

How can I do for to solve this problem?
Thanks

[Lance]

CFX doesnt know your characteristic length, so it assumes L = domainvolume^(1/3). Thus, the Reynolds number given by CFX is just an estimation, and nothing else. It never uses the Reynolds number in the calculation.

[Loris88]

Originally Posted by Lance

CFX doesnt know your characteristic length, so it assumes L = domainvolume^(1/3). Thus, the Reynolds number given by CFX is just an estimation, and nothing else. It never uses the Reynolds number in the calculation.

Thanks Lance!
Also ghorrock has said me this.
So, I must trust of my known inputs and to neglect Ansys' results.
Ok!

If I can leave my displeasure, here Ansys with his fictional Re number leaves me perplexed.

[ghorrocks]

The purpose of the Re number reported is just a general guide for the flow, in particular to have a guess as to whether a turbulence model should be used. If it says the Re is high and a laminar flow model is being used it shows a warning saying maybe you should use a turbulence model. That is all the Re calculation does.

[Loris88]

Ok... I thought that Re has influenced also the turbolent model!
Thanks

[rsyc]

Hi everyone

Has anyone checked the velocity that CFX gives in the "Average Scale Information"????

Actually I have made 2 geometries kind of the same with a tiny difference in their thickness. It is made of 3 pipes(one as the inlet pipe bringing the fluid into a big tank and the other 2 pipes let the fluid goes out). the difference is between the thickness of the big tank in these 2 geometries and all other measurements like the inlet velocity, outlet pressure(0) and the dimensions are the same. when I run the CFX solver, I got different Reynolds number ~80 for one and 3.4E+4 for the other one.

As I have checked, what is different among these 2 in the CFX solver is in the "Average Scale Information" --> Global Length, velocity and the Advection time. I can calculate the Global Length as the Ansys do. But my problem is with the Advection time and consequently the velocity that makes my Reynold's number so big!!

Can anyone help me? I appreciate the time you put to read and answer my question. I really could use your experience and knowledge to find out my problem.

Thank you so much
Rojan

[ghorrocks]

Did you read my post #16? The reported Re is not used for anything in the calculation and is just used to generate a warning message about turbulence if a laminar flow is modelled. And it is just a warning which you can ignore if you know is it OK. So ignore it

[rsyc]

Thank you for your response

yes I read it and I ignored it and let the program run. It seems that just the stream line are so messy and turbulent. Basically the streamlines of these two geometries are completely different! As I checked other stuff, all of them are kind of the same for both. May I ignore the streamlines too!!?

Thanks again.
Rojan

[ghorrocks]

The streamlines are the velocity field. If you ignore that - then why bother doing CFD?

[bestniaz]

How to check material properties valve in CFX. It just show the material like air, water.etc...no material propoerties here... Plz help me how to change the material properties...
I am new here and new in CFX.
Thanks in advance

Originally Posted by Loris88

Hi ghorrocks!

Since you have said me to read this post, I will write here my question.
I have read the questions and the relative answers but I did not find the solution for my problem.
I have a 2D problem. I created the mesh with ICEM and then I extruded it with the shrewdness of to put as depth, 1 cell like thikness.

After importing the mesh in CFX, I have put boundary condition of symmetry for to simulate the 2D problem.

I have these measures of the rectangular channel:
-Length: 3900 mm
-Heigth: 50 mm (Characteristic length L)
-Depth: 1 cell like thikness
-Re= 80000 (known input of the problem)
-Velocity along x axis= 24 m/s (u) at the inlet. The other velocities are 0 m/s. No swirl component.
-Dynamic viscosity: 1.8058e-05 kg/m*s (alpha)
-Density: 1.20438 kg/m^3 (beta)
-Kinematic viscosity: alpha/beta= 1.4993e-5 (ni)

Re= (u*L)/ni ~ 80000 [for precision 80034]

But Ansys has given me Re=9.2723E+04

How can I do for to solve this problem?
Thanks

[Lance]

In CFX-pre in the tree to the left you have stuff like Simulation, Analysis Type, Solver, Coordinate Frames, and a thing called Materials. Right click, insert/material. Or from the menu in the top, click insert/material.
This is the easy part with CFD, and if you have problems with this you should at least do all the tutorials before doing anything else.

[bestniaz]

Thanks dear... This forum is really great for the beginner... it help me alot....

Originally Posted by Lance

In CFX-pre in the tree to the left you have stuff like Simulation, Analysis Type, Solver, Coordinate Frames, and a thing called Materials. Right click, insert/material. Or from the menu in the top, click insert/material.
This is the easy part with CFD, and if you have problems with this you should at least do all the tutorials before doing anything else.

[sepidecent]

Originally Posted by ghorrocks

I do not understand your question. Are you having problems calculating the particle Re number? Are problems implementing you drag law?

Hi There,

Yeah I want to define Re Number as an expression in CFX-pre, and then drag coefficient. In that way I can define a new drag model than a common Schiller-Naumann correlation. Thank's a lot.

Regards,
Sep

[ghorrocks]

Have you done the tutorials? They show you have to implement CEL expressions.