
[Sponsors] 
April 24, 2012, 14:03 
How to add velocity in DPM?

#1 
New Member
Ebrahim
Join Date: Mar 2010
Posts: 27
Rep Power: 9 
Dear Freinds
I want track particles in a turbulent flow. for modeling the fluctuating velocities, I use a stochastic model which is not in FLUENT, so I should find fluctuating field, and supply these to the Lagrangian particle tracking unit and telling it to add those fluctuations to the mean velocities when doing trajectory computations! Does anybody know how I can add the fluctuating velocity to the mean velocity? 

January 3, 2014, 20:16 
Adding turbulent fluctuations to the mean velocity in DPM

#2  
New Member
Behtash
Join Date: Jun 2013
Posts: 2
Rep Power: 0 
Quote:


January 4, 2014, 05:33 

#3 
Member
Ali Mohamadi
Join Date: Aug 2012
Posts: 32
Rep Power: 6 
I am not sure about my response, only I guess that maybe it is true.
when you have a turbulent flow it means that you have flow that there are a lot of fluctuations. RANS models define your flow as two parts (mean velocity plus fluctuations) to solve the flow, fluctuations are not solved directly, they are modeled with some equations, such as Kepsilon or Komega. Therefore, in each cells fluctuation velocities are calculated by using specified model. In DPM, in usually case for solving Lagrangian equation only mean velocity is used. whenever you choose stochastic model or cloud model, your solver utilizes both mean and calculated fluctuation velocity together. Therefore, when you chose above mentioned model, it is not necessary to add fluctuation velocity, solver will do it by ownself. 

January 4, 2014, 12:56 
Adding turbulent fluctuations to the mean velocity in DPM

#4  
New Member
Behtash
Join Date: Jun 2013
Posts: 2
Rep Power: 0 
Quote:
You are right. There are options in FLUENT that particles can be tracked considering the flow velocity fluctuations. Though the stochastic models presented in FLUENT are limited to the Discrete Random Walk (DRW) or Cloud models which are based on the normal random or Gaussian Probability Density Function (PDF). I am looking for a way so that I add turbulence fluctuations to the mean velocity seen by the particles, while they are defined using more sophisticated random functions. 

January 5, 2014, 16:04 

#5  
New Member
Ebrahim
Join Date: Mar 2010
Posts: 27
Rep Power: 9 
Quote:
There is no special UDF for defining fluctuating velocities. However, you can use other UDFs such as defining Drag Coefficient (DEFINE_DPM_DRAG) or time step (DEFINE_DPM_TIMESTEP). I recommend you to use DEFINE_DPM_DRAG UDF. When you use such UDF, you can define the drag coefficient in the same way that is defined in FLUENT. Here is an example. in this UDF, the drag coefficient is obtained from the spherical law (which is one of the forms that is used in FLUENT). #include "udf.h" #include "dpm.h" DEFINE_DPM_DRAG(particle_drag_force,Re,p) { real drag_force; ************************************* Here you can write neccessary commands for obtaining the fluctuating velocities and also adding the to the mean flow velocities. At the end write the command below which defines the drag coefficient the same as FLUENT. ************************************* drag_force = SphereDragCoeff(p>Re); return drag_force; } Still having questions, do not hesitate to ask. Bests, Ebrahim 

Tags 
dpm, stochastic, turbulent 
Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Inlet Velocity in CFX  aeroman  CFX  12  August 6, 2009 18:42 
maintaining a logarithmic velocity distribution  Morten Andersen  CFX  1  January 8, 2007 12:37 
DPM velocity at cell face  Szabolcs Varga  FLUENT  1  September 16, 2004 07:30 
How to compute an average slip velocity using DPM  thomas  FLUENT  0  February 9, 2004 09:17 
Terrible Mistake In Fluid Dynamics History  Abhi  Main CFD Forum  12  July 8, 2002 09:11 