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-   -   Flow Past Sphere-Pressure Field Behaviour (http://www.cfd-online.com/Forums/cfx/113626-flow-past-sphere-pressure-field-behaviour.html)

Shahr February 23, 2013 09:19

Flow Past Sphere-Pressure Field Behaviour
 
Hello,

I am trying to perform a transient simulation of a standard laminar flow around sphere benchmark problem using CFX 14 from Workbench, but get strange pressure behaviour at second time step (AnimationFrame000003).

Here is a link to transient results:

As you can see pressure in the right side (outlet) is higher than left side (inlet).

The geometry is made according to Turek's benchmark found at:
http://www.mathematik.tu-dortmund.de...rTurek1996.pdf

non-dimensional time step size is 0.1, and Re=100. I cannot think of an explanation for this pressure behaviour. Does anybody have any idea why this happens?

P.S. I used a sphere instead of cylinder positioned at the same location.

ghorrocks February 24, 2013 04:06

Have a think about your initial conditions and what would happen. I am guessing the initial condition is either no flow of just Vinf to the right. So if a sphere magically appeared in this flow then you would expect pressure waves to bounce around until things settle down a bit - and this is exactly what you are seeing.

Shahr February 24, 2013 05:24

Thanks Mr. Horrocks,

I prescribed the inlet velocity and a regression polynominal of pipe flow without the obstacle as the initial condition for the fluid domain. So what you said totally makes sense.

You know, I have to prescribe both velocity and pressure field for initialisation. In the simple case I can use the steady-state results for this purpose. But, what should I do in case this sphere is to be considered as rigid body. I mean, there will be no steady-state result for a mobile rigid body sphere. What should I choose as initial condition then?

ghorrocks February 24, 2013 06:18

It depends on what you are tryign to model. If you only care about the periodic transient pattern (that is shedding vorticies) then don't worry about the initial transient and just keep going. Run it until it achieves a repeating pattern.

If you are interested in the initial startup then you need to specify a start up which is physically possible. Either the flow is initially stationary and a pressure is generated which starts the flow, or the flow is already going and the sphere is swung down into the flow. You cannot have a sphere which magically appears in the middle of a flow as it is not physically possible.

Shahr February 24, 2013 06:42

As you know initial condition should be defined for a transient simulation in cfx like below:
http://img822.imageshack.us/img822/7...13122831pm.jpg

When I input inlet velocity as initial condition to get a fully-developed flow, cfx can't solve with static pressure option set to automatic and returns this error:
| FATAL ERROR : |
| |
| Initial values are required for all variables in TRANSIENT runs. |
| In this simulation, no initial value was set for |
| |
| Variable : Pressure |
| Domain : Fluid

If I use the pressure drop from simple pipe flow, I get pressure waves. This pressure wave created in my simulation domain affects the dynamics of the mobile particle and I don't want to have it. and I cannot use the steady-states results because it doesn't describe the pressure/velocity of a movable particle.

ghorrocks February 24, 2013 16:37

As I said in my previous post, then you need some physically possible way of starting the flow. You could start the velocity from zero and ramp it up, or maybe swing the sphere down from out of the flow. What you are modelling is not physically possible and that is why the results are rubbish.

sajeesh February 27, 2013 05:25

rigid sphere moving in a fluid
 
hi,

I am a beginner in fluent.i want to simulate the sphere moving in a fluid..the sphere is moving by the drag force and pressure force by the fluid initially.and then the motion of sphere modify the flow field ..finally the sphere reaches steady movement ...which method i should follow ...in ansys...plas help me ..or refer any tutorial to start with..somebody is telling VOF method.or particle transport method ..which i should follow pls help me.

ghorrocks February 27, 2013 05:53

Try the fluent forum.

sajeesh February 28, 2013 07:14

sir is this fluent forum

ghorrocks February 28, 2013 16:35

The line
Home > Forums > CFX

Says CFX to me. The fluent forum is here: http://www.cfd-online.com/Forums/fluent/

sajeesh March 1, 2013 06:48

thank you sir

gowtham_donga May 19, 2013 13:49

transient flow past cylinder
 
1 Attachment(s)
Dear friends,
i am also working with the transient simulation of the flow past a cylinder but when i tried to plot the time vs force , i am getting a sudden jump at the end of the time .......i don't know why it is.........pls help me:confused:

ghorrocks May 19, 2013 19:26

How are we meant to be able to work out what is going on when that is all you show? Can you post an image of what you are modelling, and the output file.


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