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Fluids from two Inlets opposing each other

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Old   February 25, 2010, 04:09
Unhappy Fluids from two Inlets opposing each other
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Arvind
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Hello Every1,

I am trying to run a simulation in which i have a domain as shown in figure. Through Inlet1 i have given velocity( from left to right) and through Inlet2 i want the same fluid to come in the opposite direction(i.e from right to left) but with different velocity. I gave negative values of velocity at Inlet2 thinking it shud work but the solver is terminating with errors.


Pls can any1 tel whether I am heading towards the right direction.
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Last edited by Arvind; February 25, 2010 at 04:12. Reason: Mistake in title
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Old   February 25, 2010, 05:58
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Hi,

An inlet with negative velocity imposed on it is an outlet. Impose positive values with a direction normal to the boundary.

Tijmus
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Old   February 25, 2010, 06:19
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Quote:
Originally Posted by Arvind View Post
Hello Every1,

I am trying to run a simulation in which i have a domain as shown in figure. Through Inlet1 i have given velocity( from left to right) and through Inlet2 i want the same fluid to come in the opposite direction(i.e from right to left) but with different velocity. I gave negative values of velocity at Inlet2 thinking it shud work but the solver is terminating with errors.


Pls can any1 tel whether I am heading towards the right direction.
Hi,
if you set "normal velocity component" on inlet2 the flux will go automatically from right to left, in your case.
CFX knows that from Inlet BC the flux enters into the domain. A negative component is not raccomanded.
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Old   February 25, 2010, 07:10
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@ Timon and Mach000


First of all let me thank u for ur reply.......I tried setting normal speed B.C at inlet 2 (using positive value of velocity) but still the solver is terminated with errors after around 10-15 iterations.


Is there any other thing that i am missing out???????
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Old   February 25, 2010, 07:23
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i suppose yuo have an "overflow".

Is the simulation transient or stationary?

I think you have you have to set a transient one.

You have to try to obtain a small courant number. You can control it modifying the timestep.

Are you initializing the calculation?

Let me know your initial setting

Bye
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Old   February 25, 2010, 08:06
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@ Mach000



Yes u are right. I am getting a fatal overflow error.

I am working with a steady state simulation only. Should i use transient type?

The problem i am trying to solve deals with Hypersonic velocities so my initial Mach is around 5.5 and i hav used supersonic condition at inlet giving cartesian velocity and pressure. The various B.Cs are as follows

INLET1: Supersonic with Cartesian velocity and Pressure

INLET2 : Subsonic with Normal velocity component(as suggested by u and Timon)

OUTLET: Supersonic

OPENING: subsonic with static pressure(Entrain).

Quote:
Are you initializing the calculation?
I din understand what u mean by initialising the calculation. Can u explain it????


Any help in this regard would be very much appreciated.

Thnx in advance!!!!
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Old   February 25, 2010, 08:34
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Hi, whats the timestep your using? try a small timestep in the initial stages and then ramp it up. These issues are discussed in the help files.
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Old   February 25, 2010, 08:34
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Supersonic?
I've not experience in this kind of simulation.
Could you explain me where are the BC "opening" and "outlet"? From your doc I don't see those BC.
Initializing is a step in which you set the initial fluid dynamics features of the flow. Most of cases, the overflow is due to bad initial conditions. In CFX try to find the icon with "t=0" as symbol.
Let me know
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Old   February 25, 2010, 09:01
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@ sans

The time step i am using is 0.1 sec


@mach000

I am working in aerospace domain The model which i have shown is actually a simplified model of a space shuttle .I think now u will be clear why am using Hypersonic velocities.

As u said i have checked the initial conditions ( from that t=o icon) .In that everything is set to automatic with velocity type as cartesian. Should I change that setting from "Automatic" to "Automatic with value" and give ma conditions in der.?


The modified domain showing opening and outlet is attached. Please check it and let me know.
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Old   February 25, 2010, 09:08
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ok,
know it seems clearer.

AS first step i should define symmetry conditions instead of your opening (top and down).

Then, I should change your outlet with opening, but you can try with outlet, too.

About timestep: In my opinion it is too high.

If you want you can send me your pre file (I've cfx v11). I can check it better
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Old   February 25, 2010, 09:13
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@ mach000

Thnx for ur prompt replies.

Actually i have defined the front and back surfaces as the symmetry . If I change ma opening to symmetry( as suggested by you) then what shud i assign for the front and back surface?


Wat difference will it make if i change the opening to symmetry and outlet to opening????? Hope I am not loading u with questions

About timestep: I wil try reducing it.

Quote:
If you want you can send me your pre file (I've cfx v11). I can check it better

Give me ur mail ID so that i can send my pre file.


and one more thing.....Is my simulation a transient one??????
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Old   February 25, 2010, 10:35
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my mail is cannavacciuolociro@gmail.com

in front and back is correct to assign simmetry boundary condition.

If top and down BC are quite far from the object, they are in a not disturbed zone, where the flux is very regular. So, you don't need to simulate something that enter or go out from your domain. There is another kind of solution that yuo can consider but for now i think we can try with the one i've just try to explain to you.

About the opening condition instead of the opening: when you set outlet, cfx will think that from that boundary the flux can only goes out. If there is something different (for exemple a vortex) some flow will "enter" into the domain from that boundary. This could be a problem for the solver. If you set opening, instead, cfx "knows" that from that boundary the flow could enter or go out.
Sorry for my english, i hope you undestand me.
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Old   February 25, 2010, 10:43
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Thnx a ton!!!!

I wil change my outlet to opening and my opening to symmetry as u suggest and try solving my problem.


Moreover I wil mail the CFX-PRE file to ur mail. Pls have a look at it and let me know.
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Old   February 25, 2010, 10:51
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ok!

I'm waiting for your file
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Old   February 25, 2010, 10:59
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I have sent the file to ur mail......Pls check ur inbox and let me knw whether u r able to access it....Thnx
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Old   February 25, 2010, 11:05
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i'm checking your files.

I'll answer you from my private address to your.
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Old   February 25, 2010, 11:26
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i sent you a mail to your private address.
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