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Search: Posts Made By: ghorrocks
Forum: CFX August 25, 2013, 20:39
Replies: 45
Views: 5,641
Posted By ghorrocks
No problem Alex, glad for the help. Yes, this...

No problem Alex, glad for the help. Yes, this thread has become a bit of a fluid mechanics lesson.

The choice of boundary condition is driven by what most closely matches your experimental...
Forum: CFX August 22, 2013, 20:32
Replies: 45
Views: 5,641
Posted By ghorrocks
Do a simulation of flow in a laminar pipe, at say...

Do a simulation of flow in a laminar pipe, at say Re=500 (so well laminar) and have a look at how the flow develops.

In turbulent flows it is hard to see the details as it is lost in the...
Forum: CFX August 21, 2013, 19:40
Replies: 45
Views: 5,641
Posted By ghorrocks
:) The diagram you show is wrong. The person who...

:) The diagram you show is wrong. The person who drew that diagram does not understand the details of how fully developed flow forms.

You haven't thought about it yet - how does the flow in the...
Forum: CFX August 21, 2013, 01:36
Replies: 45
Views: 5,641
Posted By ghorrocks
Have a think about it - the flow in the centre...

Have a think about it - the flow in the centre needs to go faster than the initial plug flow in the fully developed flow. So how does it "know" it needs to accelerate?

If the thought experiment...
Forum: CFX August 20, 2013, 20:35
Replies: 45
Views: 5,641
Posted By ghorrocks
If you start a flow with plug flow (ie constant...

If you start a flow with plug flow (ie constant velocity across the whole section) and allow it to develop into fully developed flow, have a think about how it progresses through the development. You...
Forum: CFX August 20, 2013, 20:07
Replies: 45
Views: 5,641
Posted By ghorrocks
1) Mass flow at inlet or outlet - put it where it...

1) Mass flow at inlet or outlet - put it where it best represents the flow you are modelling. You can correct in saying that by default it assumes a constant velocity across the section so that will...
Forum: CFX August 14, 2013, 18:45
Replies: 45
Views: 5,641
Posted By ghorrocks
It is the global conservation of that variable....

It is the global conservation of that variable. The residual is the accuracy the equations are being solved in that specific control volume.

No, there is not a general rule that 1% is good enough....
Forum: CFX August 14, 2013, 04:34
Replies: 45
Views: 5,641
Posted By ghorrocks
have a look at the imbalances under the solver...

have a look at the imbalances under the solver manager. Get the available variables up by right clicking on a graph window and select imbalances.
Forum: CFX August 12, 2013, 07:20
Replies: 45
Views: 5,641
Posted By ghorrocks
The criteria for mesh refinement is until the...

The criteria for mesh refinement is until the variables of interest to you have converged sufficiently to an accuracy you are happy to accept. Any convergence issues which come up you just have to...
Forum: CFX August 12, 2013, 03:46
Replies: 45
Views: 5,641
Posted By ghorrocks
You do not need to do a time step independance...

You do not need to do a time step independance check for a steady state simulation. The residuals, imbalances and/or the stability of variables of important to you (eg drag coefficient, pressure...
Forum: CFX July 31, 2013, 19:57
Replies: 45
Views: 5,641
Posted By ghorrocks
Please post images directly on the forum, don't...

Please post images directly on the forum, don't hide them in word documents. Also a graph comparing one profile directly to another would be useful.

Before going into the details of turbulence...
Forum: CFX July 28, 2013, 08:44
Replies: 45
Views: 5,641
Posted By ghorrocks
y+ is a variable available in the post processor....

y+ is a variable available in the post processor. No need to calculate anything.
Forum: CFX July 27, 2013, 08:12
Replies: 45
Views: 5,641
Posted By ghorrocks
I just tried to find the discussion on this in...

I just tried to find the discussion on this in the documentation but could not find it.

Now you have performed the analysis I would look at the y+ it produced and see what you actually got rather...
Forum: CFX July 26, 2013, 01:41
Replies: 45
Views: 5,641
Posted By ghorrocks
It gets very tricky for low Re and laminar flows....

It gets very tricky for low Re and laminar flows. So the easiest way is to just look at the results from an existing model and estimate from that. But if you are asking what boundary layer resolution...
Forum: CFX July 25, 2013, 22:10
Replies: 45
Views: 5,641
Posted By ghorrocks
This equation is for fully developed high Re...

This equation is for fully developed high Re turbulence flows. You have a low Re flow, it is only just turbulent. So you need a finer mesh as Alex says. And I have already explained twice on this...
Forum: CFX July 25, 2013, 09:00
Replies: 45
Views: 5,641
Posted By ghorrocks
If you plot conservative values you will find...

If you plot conservative values you will find they do not go to zero. This is because they show the velocity at the centroid of the control volume, and these centroids lie off the wall - so they are...
Forum: CFX July 25, 2013, 03:26
Replies: 45
Views: 5,641
Posted By ghorrocks
Have a look at hybrid and conservative variables...

Have a look at hybrid and conservative variables in the documentation and post processor.
Forum: CFX August 24, 2011, 19:39
Replies: 45
Views: 5,641
Posted By ghorrocks
http://www.cfd-online.com/Wiki/Ansys_FAQ#Why_do_I_...

http://www.cfd-online.com/Wiki/Ansys_FAQ#Why_do_I_see_a_non-zero_velocity_at_the_walls.3F
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