[Faggio11]

Dear all,

I am working on simulating the motion of a leaflet by applying a rigid body motion by means of the DEFINE_CG_MOTION macro.

I wrote a UDF for the movement; I compiled it and tested it in Fluent version 15.0 and it was perfectly reproducing what it was meant to do.

Then, last week I had to upgrade the software and I had version 17.1 installed. When I compiled the UDF this time the motion is completely incorrect although I have used the same structure of the code.

I have searched into the manual and compared the sections regarding the explanation of the macro. It seems that in the new version the software can call multiple times the UDF and that might lead to failure or incorrect updates of the velocity. I checked the example in the manual but I did not really understand it.

Can anybody help me? I also upload the code that I wrote so that you can have a better idea of what I am doing. I really need some suggestions to integrate this concept in the code.

Thank you in advance.

F.

[pakk]

I don't know what caused your problem.
But still, I could see some cleaning-up to do in your code to make it simpler. And simpler usually means that there are fewer places to make mistakes.

#include "udf.h"
#include <math.h>
#include <float.h>

float quadratic(a,b,c,time) {
    return (a*time*time+b*time+c);
}

DEFINE_CG_MOTION(leaflet_1, dt, vel, omega, time, dtime)
{
    real omega_1, omega_2, omega_3, ang_vel;
    const float tau = 0.866;
    const float timefrac=time/tau-(int)(time/tau); /* time = (i-1+timefrac) * tau ;  time-(i-1)*tau = timefrac * tau  */
    float a,b,c;

    NV_S(vel, =, 0.0);
    NV_S(omega, =, 0.0);
    if (timefrac < 0.005773672055427) /*interval 1*/
    {
        a=2159.567580764660; b=-2.407990408052271; c=-0.011976420486241;
    } else if (timefrac < 0.011547344110855) /*interval 2*/
    {
        a=-9353.043658991941; b=112.7181219895138; c=-0.299791701480157;
    } else if (timefrac < 0.017321016166282)) /*interval 3*/
    {
        a=35252.60705520311; b=-779.3948922943873; c=4.160773369939347;
    } else if (timefrac < 0.023094688221709) /*interval 4*/
...
    } else if (timefrac < 1) /*interval 39*/
    {
        a=-85.400423181754974; b=145.5055425427474; c=-61.973216494809272;
    }
    omega_1 =  quadratic(a,b,c,timefrac*tau);
    omega_2 =  quadratic(a,b,c,timefrac*tau+dtime);
    omega_3 =  quadratic(a,b,c,timefrac*tau+dtime/2);
    omega[0] = (omega_1+omega_2+4*omega_3)/6;
}

I left out intervals 4 to 38 in the middle of the code, because they were boring.

[Faggio11]

Dear pakk,

thank you for your quick reply. I really liked the way you coded it. It is simpler and smoother and it was totally understandable.

Unfortunately when I tried this new version in Fluent, it does not solve the problem. Actually, the motion becomes even more chaotic.

I do not think it is a problem of the code because I have just written a similar function in matlab (since I am much more familiar with it rather than C language) keeping the same structure and it perfectly reproduces the behaviour of the angular velocity (a picture in the attachment).

Do you have in mind any other suggestions?

[pakk]

I think you just messed up with the numbers.
If I used your numbers, I sort of see your graph, but with a lot of distortions, especially for t>0.3.

You are putting in numbers for chaotic motion, so Fluent will give that to you!

Your interpolation is a bit unusual.
You use omega = a*t^2 + b*t + c on each interval.
But to get nicer numbers a, b and c, you could use omega = a*(t-t0)^2+b*(t-t0)+c, where t0 is the start of the interval.

[rsaurabh]

Hi,
i am doing a simulation where blade will rotate. i have defined a udf but it is not compiling. it will be great if you look at it.


#include "udf.h"
#include "math.h"


DEFINE_SDOF_PROPERTIES(Valve_6dof, prop, dt, time, dtime)
{


prop[SDOF_MASS] = 0.01467375;
prop[SDOF_IZZ] = 0.00000000008995543548;
prop[SDOF_IXX] = 0.00000000002266435548;
prop[SDOF_IYY] = 0.00000000006729108;
prop[SDOF_ZERO_TRANS_X] = TRUE;
prop[SDOF_ZERO_TRANS_Y] = TRUE;
prop[SDOF_ZERO_TRANS_Z] = TRUE;
prop[SDOF_ZERO_ROT_X] = TRUE;
prop[SDOF_ZERO_ROT_Y] = TRUE;
prop[SDOF_ZERO_ROT_Z] = FALSE;


Message("\n2d : Updated 6DOF properties ");

}

[Faggio11]

Dear rsaurabh,

first of all, I would like to point out that your question is not really related to this thread.
Nevertheless, you should change the second line of your code: it is #include <math.h> and not with inverted commas.

[pakk]

Originally Posted by rsaurabh

Hi,
i am doing a simulation where blade will rotate. i have defined a udf but it is not compiling.

It would help if you could add the error message shown when you try to compile.