# Jumping droplets velocity - interFoam

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October 6, 2015, 08:25
Jumping droplets velocity - interFoam
#1
New Member

Zinedine
Join Date: Sep 2010
Posts: 19
Rep Power: 11
Dear foamers,

I am undertaking some OF simulations of droplets coalescence uponm hydrophobic surfaces.

Initially 2 water droplets with the same radius R and contact angle of 160o are set onto the surface. They then coalesce to form a droplet. Owing to energey forces this formed droplet then jumps from the surface.

I attach a snaphot of a simulation of 2 droplets with initial radius R=100micro and contatcd angle = 160o.

This is this jumping velocity I wish to compute.

To do so I compute the is veloocity using 2 methods:

1. I firstly compute the velocity by evaluating the Centre of Gracity (COG) of the droplets. I use the following code baryCenter221.zip (i.e. http://www.cfd-online.com/Forums/ope...elocity-3.html)
to evaluate the position of the COG.
I made the following changes as indicated in red:

forAll(alpha1, cellI){

// Integration of {mass}
//////sumM += (1-alpha1[cellI])*mesh.V()[cellI]*rho2.value();
sumM += (alpha1[cellI])*mesh.V()[cellI]*rho1.value();

// Integration of (mass*height)
////sumMY += (1-alpha1[cellI])*mesh.V()[cellI]*rho2.value()
///// *mesh.C()[cellI][1];

sumMY += (alpha1[cellI])*mesh.V()[cellI]*rho1.value()
*mesh.C()[cellI][1];

} (Eq . 1)

Upon evaluating thre COG at each time step, I plot the COG location vs time.

My velocity is then obtained by taking the slope of the linear part of the plot.

2. I also compute the velocity of the jumping droplet by using Paraview.
I use the following equation to evaluate the velocity:

Udrop = integral_Omega (alpha1*U) dOmega / integral_Omega (alpha1) dOmega (Eq. 2)

I found the same velocity using both methods, however the obtained jumping velocity is TWICE bigger as published experimental and numerical for similar work.

I have been going round and round to find where the error is .... I believe I am missing something silly and very simple while post processing and evaluating this jumping velocity.

Are the equations (Eq . 1) & (Eq . 2) used to compute the velocity correct ?
Am I avaluating the COG correctly ?

I hope you will be able to help me out I am getting mad .....

Kindest regards

Z.
Attached Images
 snapShotsR100.png (37.5 KB, 26 views)