## Fluids

#### how to report the drag force on particles in DPM

• rezazarghani
Subscriber

Hello, everyone!

I am working on a project in which a set of particles is injected into a 2D duct. I want to find out the magnitude and angle (vector) of the drag force exerted from the flow to each particle, at different positions along the duct. I can not find such a report option under the "particle tracks" tab or elsewhere. Does anyone know how to report this when the solution is done?

Thanks

• Rob
Ansys Employee

You'll need to calculate this using the particle & cell velocities and use the equations in the Theory Guide. Or write the drag model into the code using DEFINE_DPM_DRAG (UDF manual) and report that value.

• rezazarghani
Subscriber

dear Rwoolhou,

So you are saying that there is no direct way to get the drag force exerted on each particle, and I should calculate the force for each particle by myself? Actually I was thinking about this method to write the x and y particle velocities (it is a 2D domain)  and the velocity field at the corresponding particle positions into an Excel file to calculate the drag force vector(magnitude and angle), exerted on each particle. However, there are two concerns regarding this suggestion:

1-  I want to depict the drag force vectors on each particle later (like velocity vectors for each particle that is already available in Fluent). Is it possible to import the calculated force vectors back to Fluent and relate them to the corresponding particles?

2- how to relate the vectors to the correct particle? the information is sorted according to particle residence time when I get the outputs to Excel and I wouldn't know which vector would be for a particular particle.

about the DEFINE_DPM_DRAG that you mentioned, I have noticed that before. However, as I understood, it is used to define the drag force correlation and I could not find out how I would be able to report the calculated drag based on this UDF. I tried report definitions, the UDF section, but it didn't work out. Could you show me a way to get a report from this UDF, the problem is almost solved.

Regards, Reza

• Rob
Ansys Employee

The UDF is used to define the drag, but with extra work could then be used to write out the drag coefficient etc.

You could use a custom field function to look at the velocity vector differences but there isn't a way to plot a vector arrow on the particles. We typically want to know where they're going and where they've been, drag force isn't usually reported.

• rezazarghani
Subscriber

dear Rwoolhou

Thank you so much, you have helped me out a lot, so far.

would you please do me a favor and guide me on how to amend my UDF written in the following to get such outputs? (CD or drag force or any other relating issue)

let's consider the default drag UDF provided by the ANSYS website. I'd appreciate it if you give me a hint on this example.

Moreover, do you know how Re is defined in the following code? is it based on the particle velocity relative to the flow? or is it based on absolute particles' velocity?

• rezazarghani
Subscriber

#include "udf.h"

DEFINE_DPM_DRAG(particle_drag_force,Re,p)

{

real w, drag_force;

if (Re < 0.01)

{

drag_force=18.0;

return (drag_force);

}

else if (Re < 20.0)

{

w = log10(Re);

drag_force = 18.0 + 2.367*pow(Re,0.82-0.05*w) ;

return (drag_force);

}

else

/* Note: suggested valid range 20 < Re < 260 */

{

drag_force = 18.0 + 3.483*pow(Re,0.6305) ;

return (drag_force);

}

}