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October 6, 2015, 08:25 
Jumping droplets velocity  interFoam

#1 
New Member
Zinedine
Join Date: Sep 2010
Posts: 18
Rep Power: 8 
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.cfdonline.com/Forums/ope...elocity3.html) to evaluate the position of the COG. I made the following changes as indicated in red: forAll(alpha1, cellI){ // Integration of {mass} //////sumM += (1alpha1[cellI])*mesh.V()[cellI]*rho2.value(); sumM += (alpha1[cellI])*mesh.V()[cellI]*rho1.value(); // Integration of (mass*height) ////sumMY += (1alpha1[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. 

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