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Your question raises more questions than it explains.

CFX and Fluent are both very capable CFD solvers. If they are doing weird things then you set them up wrong. Do more tutorials and training and things will be clearer.

You do not mention chemical reactions, so why are you saying CFX has chemical reactions you do not need.

Alternately if you want specific help on your simulation then post more detail and we will try to help.

hello,

I try to simulate flow between two cylinders.

Normally I should get a constant velocity profile along the length.
But, I found an acceleration along the entire length of the cylinder.

Change speed / length axis> 0

normally it must be = 0

thank you



merci

This effect is probably real - compressible flows accelerate along a duct due to the fluid density decreasing, all flows (incompressible and incompressible) have the centre line velocity accelerate as the boundary layer develops and the centre velocity accelerates to maintain constant volume flow rate.

Hello,


In one study, I found this Fig 1 (that is what I should normally find).
But I find a profile as in Fig 2

Attachment 12766

Attachment 12767

I can not find the trick. Is that it's normal?

I despair

Thank you for the help

You have not described what you are doing so I cannot help you. As I already said, this effect can be due to the boundary layer forming cuasing the centre line velocity to accelerate, or it could be the acceleration of the flow due to fricition in compressible flow. As I have no idea what you are modelling I cannot be more specific.

hello,

You find this link in a description for what I did.
This is another topic that I posted.

http://www.cfd-online.com/Forums/ans...ical-flow.html

If you have an article, a thesis or a study where I can find an explanation which I can pass it to me based retail?

thank you

Your flow is incompressible, so it appears the cause of the effect is development of the boundary layer causing acceleration of the core flow.

So this answers your question, doesn't it?

hello,

yes, certainly for me, but I have to find a literature reference indicating the purpose in order to demonstrate the results that I present, and I can not find.

Thank you for your appreciable help and motivation for my question.

It is simple conservation of volume. No need for a reference, a few lines of maths proves it.

hello,

if it is not too much, can you help me a little more. :confused:

thank you

Have a look at the flow at the inlet and the flow as it develops. See if you can work out why the centre line accelerates.

hello,

how can I do to have a profile as in Figure 1. A constant speed over the entire length of the pipe.
Attachment 12948
The conditions requisent?

thank you

Just let the flow develop over a longer pipe.

hello

I'm working on a configuration such that the length L = 300mm and diameter D = 25.8 mm.
I tried with a length L = 500mm, but I found the same result.
even in simulating with a flow of water.:confused: :confused:

I tried with ANSYS CFX and it is similar, and I even found a constant mass flow along the pipe. :confused: :confused: :confused:

thank you

Have you done a 1D pipe flow analysis to estimate how long the flow development is likely to be?

No, I work with stationary regime, no time in the calculation.

it is important to wear the transient calculation with a time interval? :(

thank you

Flow development occurs both in space and time - the boundary layer needs to grow even for a steady state flow as the flow progresses along the pipe. So work out the pipe lenght required to get developed flow, and I bet you will find your model is a tiny fraction of that.

In other words I think your problem is that your domain needs to be much longer to let the flow develop to fully developed.

hello,

I tried a simulation with ANSYS Fluent (I still have some difficulty with CFX):

Inlet velocity
Outlet pressure with mass flow rate

Time step size: 60 seconds
Number of time step: 5
Number of iterations / time step: 300

I find the same result
I noticed that the solver resolves five blocks of 300 iterations each time, is it recalculates every time?

60 seconds is enough or must I fix a few minutes

thank you very much

Use the Fluent forum for questions on Fluent.

These are pretty basic questions you are asking - surely you know how to determine if the flow is developing in time or not.