
[Sponsors] 
October 7, 2014, 05:37 
Lagrangian equation in parcel.C

#1 
New Member
Join Date: Sep 2014
Posts: 11
Rep Power: 11 
Hi everybody,
I'm using OpenFOAM 1.7 with dieselSpray, in order to model gasoline droplets in a turbulent air flow with an Euler approach for the carrier phase and a lagrangian appproach for the droplets. When I look at the lagrangian equation in parcel.C, I see : U() = (U() + (timeRatio*Ucorr)+ gcorr*dt)/(1.0 + timeRatio); (Eq 1) where U() is the droplet velocity, Ucoor the velocity of the gas seen by the droplet and timeRatio=dt/tauMomentum whith tauMomentum the droplet relaxation time. But this looks very different from any classical lagrangian equation which should be something like : dUp/dt = (Ug  Up)/Tp + g (Eq 2) where Up is the velocity of the droplet, Ug the velocity of the gas seen by the droplet ant Tp the droplet relaxation time. Discretized it should be something like : U() = (1  timeRatio)*U() + (timeRatio*Ucorr) + gcorr*dt (Eq 3) So why does OpenFoam use Eq 1 instead of Eq 3 ? Any help would be very appreciated. Thank you. 

February 19, 2015, 11:43 
Lagrangian particle velocity integration

#2 
Member
Emeline Noel
Join Date: Dec 2013
Location: Paris
Posts: 31
Rep Power: 11 
Hi Cluap,
Perhaps, you find the answer but I was asking my self the same question and found the solution so I would like to share with other and also to keep memories of the solution. Perhaps, it is obvious for personn who work in the lagrangian world. First, I should say I was not aware of the tauMomentum so the Euler integration for parcel of openfoam was very strange for me. But I find that the drag force is can be express thanks to the tauMomentum : tauMomentum = mp*UcUp/Fd * Uc : carrier phase velocity * Fd : Drag Force * Up : particle velocity * mp : mass of the particle So, dUp/dt = Fd/mp + Fg/mp (1) Using tauMomentum : Tp dUp/dt = (Ug  Up)/Tp + g (2) From there, I think the key is to know that in (2) OpenFOAM use the particle velocity at t+dt Briefly, taking 1/Tp = A Up(t+dt) = Up(t) + A*dt*(UgUp(t+dt)) + Fg*dt So, Up(t+dt) = Up(t) + (A*dt*Ug+Fg*dt)/(1+A*dt) A*dt = timeRatio So you find the equation in OpenFOAM : U() = (U() + (timeRatio*Ucorr)+ gcorr*dt)/(1.0 + timeRatio); (Eq 1) May it can help someone! Tchuss 

Tags 
dieselspray, lagrangian, parcel, particle 
Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
mass flow in is not equal to mass flow out  saii  CFX  12  March 19, 2018 06:21 
Some problem of "Qcriterion.mcr&  yuyuxuan  Tecplot  9  February 12, 2016 04:27 
Derivation of Momentum Equation in Integral Form  Demonwolf  Main CFD Forum  2  October 29, 2009 20:53 
Need help:about energy equation in CFX  Stein  CFX  4  July 2, 2009 23:31 
Boundary conditions in a Poisson's equation?  vincent  Main CFD Forum  4  April 16, 1999 03:19 