# Initial velocity condition on a system coupling useless!!!

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 January 8, 2015, 06:00 Initial velocity condition on a system coupling useless!!! #1 Member     Antonio Casas Join Date: May 2013 Location: world Posts: 85 Rep Power: 10 Hi there, I try to apply initial velocity condition over a body on a system coupling between Fluent fluid flow and transient structural. Itīs a body drop from 2 or 3 meters high over water in order to calculate tensions and deformations. I don't want to simulate the whole fall time period, but only the period of time where the impact against water occur. So, what I'm trying to do without success, is to allocate the body just some millimeters above the water, and give it initial vertical velocity condition equivalent as if it was dropped from 2 or 3 meters height. Can anyone tell me why the initial velocity condition does not take effect in my simulation? Am I doing something wrong? Is it ansys coupling system initial velocity condition USELESS?? thankīs

January 8, 2015, 06:46
#2
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Filippo Maria Denaro
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Quote:
 Originally Posted by acasas Hi there, I try to apply initial velocity condition over a body on a system coupling between Fluent fluid flow and transient structural. Itīs a body drop from 2 or 3 meters high over water in order to calculate tensions and deformations. I don't want to simulate the whole fall time period, but only the period of time where the impact against water occur. So, what I'm trying to do without success, is to allocate the body just some millimeters above the water, and give it initial vertical velocity condition equivalent as if it was dropped from 2 or 3 meters height. Can anyone tell me why the initial velocity condition does not take effect in my simulation? Am I doing something wrong? Is it ansys coupling system initial velocity condition USELESS?? thankīs

Hello,
the problem is not very clear to me...you have a rigid body with a mass, right? Is it just subject only to the gravity before the impact, so you can prescribe a simple dropping velocity law

January 8, 2015, 06:56
#3
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Antonio Casas
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Quote:
 Originally Posted by FMDenaro Hello, the problem is not very clear to me...you have a rigid body with a mass, right? Is it just subject only to the gravity before the impact, so you can prescribe a simple dropping velocity law
Itīs a body, a simple shell box made of steel, with the capability to deform under pressure.... so I guess its not rigid.... that is the reason why Iīm using a coupling system, otherwise I would use 1 way FSi and UDF 6DOF in fluent. Yes, its subject only to the gravity before impact. The APROX. velocity before impact if dropped from 2m height, should be SQRT (2*9.81*2)=6.26 m/s . I will investigate how and where to prescribe a simple dropping velocity law, but maybe you could give me some clue?

I attach a picture of the system

Thankīs a lot
Attached Images
 fluent_fluid_flow_transient_structural_system_coupling.jpg (59.7 KB, 40 views) initial_velocity_conditions.jpg (47.0 KB, 49 views)

Last edited by acasas; January 8, 2015 at 07:21. Reason: wanted to add a picture for better understanding of my system

January 8, 2015, 07:02
#4
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Filippo Maria Denaro
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Quote:
 Originally Posted by acasas Itīs a body with the capability to deform under pressure.... so I guess its not rigid.... that is the reason why Iīm using a coupling system, otherwise I would use 1 way FSi and UDF 6DOF in fluent. Yes, its subject only to the gravity before impact. I will investigate how and where to prescribe a simple dropping velocity law, but maybe you could give me some clue? Thankīs a lot
I don't think is relevant considering deformation during drop in air, but just after the impact. So, disregarding the drag of air, you can simply consider, for example, the limit velocity of a body dropping due to gravity

January 8, 2015, 07:26
#5
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Antonio Casas
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Quote:
 Originally Posted by FMDenaro I don't think is relevant considering deformation during drop in air, but just after the impact. So, disregarding the drag of air, you can simply consider, for example, the limit velocity of a body dropping due to gravity
, Yes, I don't care about the drag effect or deformation over the body due to the air effect. I am trying to do that, but I don't want to apply a CONSTANT velocity but initial velocity. I want to get the effect of deceleration due to the impact with water.

sorry if I do not understand you properly. anyway thanks for your help

January 8, 2015, 09:13
#6
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Filippo Maria Denaro
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Quote:
 Originally Posted by acasas , Yes, I don't care about the drag effect or deformation over the body due to the air effect. I am trying to do that, but I don't want to apply a CONSTANT velocity but initial velocity. I want to get the effect of deceleration due to the impact with water. sorry if I do not understand you properly. anyway thanks for your help Please check the pics I have added to my thread for better understanding

that sounds strange ...if the body at t0=0 has at some position z0 an initial velocity w0=0, then it is sufficient to consider the Newton law along the z axis m*dw/dt = m*g and integrate.
You also know from conservation of total energy that potential and kinetic energy lead to a limit velocity

January 8, 2015, 10:14
#7
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Antonio Casas
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Quote:
 Originally Posted by FMDenaro that sounds strange ...if the body at t0=0 has at some position z0 an initial velocity w0=0, then it is sufficient to consider the Newton law along the z axis m*dw/dt = m*g and integrate. You also know from conservation of total energy that potential and kinetic energy lead to a limit velocity

Hi Filippo, thanks for your prompt answer. Maybe I donīt make myself clear . I am doing as you say, I am applying initial down vertical velocity condition to the body of 6m/sec almost touching the water surface, but it does not take effect at all. Is it like giving a initial velocity condition in Ansys transient structural / fluent fluid flow system coupling was totally useless. Even more, I do believe that the velocity initial condition tool at transient structural, when linked to fluent to perform a coupled system, its a FAKE useless tool. I really really would like to be wrong, so please if somebody could check it out or tell me what is wrong.

thanks

Last edited by acasas; January 8, 2015 at 10:32. Reason: typo

January 8, 2015, 16:50
#8
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Bruno Santos
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Greetings to all!

@Antonio: I came see this thread because you sent me a private message requesting that I came have a look into this.

Well, a first I was going to just write that I don't have experience with Fluent/ANSYS, and that my level of expertise are only mostly related to OpenFOAM...

But then I took a look at the images in post #3 and saw something weird... in the image I've attached, where I placed a red blob, isn't that a checkable box?

Best regards,
Bruno

edit: I've moved this thread to the Fluent forum, since many of the Fluent users possibly won't see this in the Main forum.
Attached Images
 possible check box.jpg (10.1 KB, 23 views)

Last edited by wyldckat; January 8, 2015 at 16:52. Reason: see

January 8, 2015, 17:21
#9
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Antonio Casas
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Quote:
 Originally Posted by wyldckat But then I took a look at the images in post #3 and saw something weird... in the image I've attached, where I placed a red blob, isn't that a checkable box?
Unfortunately, the checkable box is just for parametric purpose ( to make several analysis on the same basis but with different values for this checkable box).
Anyway, thanks, I hope your decision to move my thread to the Fluent forum will increase the CFD forum attention over it.

 January 12, 2015, 12:53 #10 Member     Antonio Casas Join Date: May 2013 Location: world Posts: 85 Rep Power: 10 come on guys, someone must know. Please, help

 February 11, 2015, 13:41 #11 Senior Member     Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 23 Hi Antonio, sorry for delay in answer, I didn't have access to the PC earlier, I hope you have solved the problem. I work with OpenFoam mostly but how are you modelling your initial condition? are the solid and the surface of the liquid touching each other at the beginning of the simulation? I'm eager to such problems, if it has been solved say what you have done. __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked.

 April 1, 2015, 16:31 #12 Senior Member   Join Date: Apr 2009 Posts: 531 Rep Power: 18 A just tested this in R16.0 with a simple falling block. Transient Structural with an initial velocity of (0,-1,0) m/s, coupled to Fluent via System Coupling. Forces and Displacements transferred. It worked fine. The initial velocity of the block was maintained and displacements were passed to Fluent OK. I can share the project if you give me somewhere to send it.