[Tamsu]

I want to define x component of gravity as a sinusoidal function of time, how to do it?
Any UDF? what is Journal file and how do i use it here?
Or is there any way by which I can modify body force term in N-S equations?

[gearboy]

Originally Posted by Tamsu

I want to define x component of gravity as a sinusoidal function of time, how to do it?
Any UDF? what is Journal file and how do i use it here?
Or is there any way by which I can modify body force term in N-S equations?

You can use DEFINE_SOURCE to add your gravity momentum source(N/m3) into x momentum equation.

[Tamsu]

[QUOTE=gearboy;417743]You can use DEFINE_SOURCE to add your gravity momentum source(N/m3) into x momentum equation.

Thanx a lot, got it. used CURRENT_TIME as time variable. UDF Manual also helped.

[nspal1312]

I am facing same problem...
Can u tell me steps to follow?
Plz mail me udf for this
nirajpaliwal1312@gmail.com

[pakk]

Originally Posted by nspal1312

I am facing same problem...
Can u tell me steps to follow?

You can use DEFINE_SOURCE to add your gravity momentum source(N/m3) into x momentum equation.
Use CURRENT_TIME as time variable. UDF Manual can also help.

Plz mail me udf for this
nirajpaliwal1312@gmail.com

This will of course never happen. You can get help on writing your udf here, but I don't think a stranger will do all the work for you and mail it to you. I'd be surprised if these requests on message boards ever work.

[stuart23]

Originally Posted by pakk

This will of course never happen. You can get help on writing your udf here, but I don't think a stranger will do all the work for you and mail it to you. I'd be surprised if these requests on message boards ever work.

Quote Of The Day

[silent2608]

This thread is pretty old but since I found it through google I think others will, too.

I'm a Fluent / UDF beginner myself, but the following works for me. I verified results with manually using gravity in Fluent GUI. It works perfectly in my vof 2phase case. My UDF has a little bit different purpose but it's easy to adjust. Interpret UDF like always, go to cell zone conditions, select "Source Terms" and add it as x/y/z momentum however you like.

#include "udf.h"
DEFINE_SOURCE(gravity_source,c,t,dS,eqn)
{
real gravity;
real source;
real flow_time = RP_Get_Real("flow-time");

    if (flow_time < 0.1)
    {    
        gravity = 0.;
    }
    else
    {
        gravity = 9.81;
    }

/* gravity*cr(ct) is gravity*density = N/m³ = specific weight = F(g) on a unit of volume of a fluid */
source = gravity*C_R(c,t);
return source;
}

In my case it simply activates gravity in +x direction after 0.1s flow time. If you need varying gravity just use a function for gravity instead of a constant. For example remove if / else and use flow_time in sine function for gravity.

[jbarman013]

Originally Posted by silent2608

This thread is pretty old but since I found it through google I think others will, too.

I'm a Fluent / UDF beginner myself, but the following works for me. I verified results with manually using gravity in Fluent GUI. It works perfectly in my vof 2phase case. My UDF has a little bit different purpose but it's easy to adjust. Interpret UDF like always, go to cell zone conditions, select "Source Terms" and add it as x/y/z momentum however you like.

#include "udf.h"
DEFINE_SOURCE(gravity_source,c,t,dS,eqn)
{
real gravity;
real source;
real flow_time = RP_Get_Real("flow-time");

    if (flow_time < 0.1)
    {    
        gravity = 0.;
    }
    else
    {
        gravity = 9.81;
    }

/* gravity*cr(ct) is gravity*density = N/m³ = specific weight = F(g) on a unit of volume of a fluid */
source = gravity*C_R(c,t);
return source;
}

In my case it simply activates gravity in +x direction after 0.1s flow time. If you need varying gravity just use a function for gravity instead of a constant. For example remove if / else and use flow_time in sine function for gravity.

Hey, this didn't work for me. It shows an error that flow_time is an undeclared variable. Could you help me with that please?