Fluids

CFD Fluent Analysis using UDF Coding Problem

• S1d996
Subscriber

I’m working on a CFD simulation of 3D vehicle containing a set of rotors in ANSYS FLUENT. I am trying to move the rigid body using 6-DOF in a straight line while rotors continuously rotate about mentioned axis (origin and direction). I came to know that this can be achieved by writing UDF file and Compiling the file at the start of simulation. I tried writing the UDF file using the DEFINE_CG_MOTION macros but unable to implement.

Is DEFINE_CG_MOTION suitable for my simulation in Fluent. If not, are there any macros that are useful to build a UDF file for this problem definition.

1.       Initially, a set of rotors should start rotating and complete body should translate in particular direction in a time interval by mentioned Angular velocity and Translational velocity.

2.       After some time, the mentioned set of rotors should stop rotating and another set of rotors should start rotating by changing the direction of Translation motion by mentioned Angular velocity and Translational velocity.

3.       Simultaneously, the 6-DOF properties should be followed.

4.       The output of the simulation should be the thrust produced by the individual rotors at the end each flow time.

• DrAmine
Ansys Employee

Do you want to prescribe the motion of the rigid body? If yes then you use DEFINE_CG_MOTION. If the motion is result of hydrodynamic forces then stick to 6DOF. You can prescribe the motion of each grid node by using DEFINE_GRID_MOTION if you know the motion.

• S1d996
Subscriber

Thanks for the response.

I do not want to prescribe the motion. I have a predefined CG location, which will vary as per the flow time due to change in the orientation of the body as a resultant of 6 DOF values. The motion has to be a resultant of the RPM of the individual rotors. So for this, how do i specify varying RPM for rotor for different flow times? and how to calculate Thrust from the RPM of the rotor? Please share any examples if available.

Sid

• DrAmine
Ansys Employee

Your rotors are defined as what? Rotating cell zones? If yes here you can define a profile for angular speed.

• S1d996
Subscriber

Can you please elaborate with an example of how to define a profile for angular speed within a loop

Sid

• DrAmine
Ansys Employee

I do not have any example. You do not require UDF (DEFINE_PROFILE) for that. You can just provide a transient profile for angular velocity or using the new Fluent Expressions.

• S1d996
Subscriber

hie

i am trying initially to move the body using 6DOF udf

but i'am facing the ERROR: 6DOF object must have positive mass

Beat regards,

Sid

• DrAmine
Ansys Employee

Check if you are providing correct and positive mass input sdof_prop[SDOF_MASS].

• S1d996
Subscriber

Thanks for the reponse

The above mentioned problem is solved but now i'm facing new problem.

The Complete simulation is running correctly but  6DOF motion is not happening in the required direction.

I mentioned the force in upward direction but the result is shown as moving downwards.

Can you figure out the mistake

Sid

• Rob
Ansys Employee

Maybe a minus sign is needed somewhere?

• DrAmine
Ansys Employee
• S1d996
Subscriber

But i didnt get the point.

Can u explain it clearly