# what is converter in OpenLB code?

Hello,

I am Phd student, and my thesis subject concerns about Lattice Boltzmann Method. So I downloaded the Open source codes of Open LB, but in codes I don´t understand the converter expression. Also I read the manual of OpenLB release 0.4 and i looked “lesson 4: Convert between lattice and physical units”. I think, the converter expression in open source code is reletaed between lesson 4. But I don´t understand very much.

So as I mentioned above. I want to ask two questions;

1. What is lattice and physical units?
2. What is the converter function in open source codes of OpenLB? What does converter convert?

Best Regards
Erman ASLAN

Hello,

when you work with an equation (says the Navier–Stokes equations) you study quantities which usually have units. Velocity which is [length/time]. Then you can say that you work in meters per seconds. Then you can say for example that the system you simulate has a 2 meters length. When you a in the discrete world the same system will contain a certain number of nodes for this distance say 100.

YOu therefore have an equivalence between the lattice units [LU] where the distance is 100 LU and the physical world where the distance is 2[m].You can do the same thing for all other quantities like time for example. And this is the work of the converter in OpenLB. You basically tell him what are the caracteristic lengthscales of your system in physical units and in lattice units and it oes the translation.

Hi,

The issue of unit conversion between physics and simulation is explained in the following document:
http://www.lbmethod.org/_media/howtos:lbunits.pdf

This should clarify the meaning of the unit converter in OpenLB.

Thank you for helping. I read document that you send, but I don´t understand very much.

So I try to find physical (real) properties of cavity2d problem which is in OpenLB open source code. In cavity2d problem, I think lattice variables is given. These are;

umax=1e-2
Re=100
N=128
lx=1
ly=1

• As document mentioned, first I must convert from lattice variables (discrete system) to dimensionless variables.

delta(x)= 1/N=1/128= 0,0078125
delta(t)= 7,8125e-5 ( I took this value in olbLog.dat file)

velocity
ulb= [delta(t)/delta(x)]ud ------> ud=[ulbdelta(x)]/delta(t) = [0,01*0,0078125]/7,8125e-5=1=ud

viscosity
vlb=[delta(t)/delta(x)^2][1/Re] -------->vlb=[7,8125e-5/0,0078125^2][1/100]=0,0128=vlb=0,0128
vd=[delta(x)^2/delta(t)]*vlb-------------> vd=[0,0078125^2/7,8125e-5]*0,0128=0,01=vd

• As a second step, I must convert from dimensionless variables to physical variables.

But I can not convert from dimensionless variables to physical variables. Because, I don´t know How can I decide some terms. This terms are;

l0,p
t0,p

Best Regards
Erman ASLAN

The following post may be helpful to clarify the role of unit conversions with OpenLB:

What are these quantities you want to convert?

I only try to find how can convert from lattice variables to physical system. Because, as I understood, in OpenLB open source code, only lattice variables are given, physical (real) variables are not given. Therefore, in my mail that is above, I only tried to understand what is real conditions (inputs) of cavity 2d problem.

Best Regards
Erman ASLAN

OK but what are these :

l0,p
t0,p

?

Thank you very much.

I learn useful knowledge with your messages. Also I understand what are lattice, dimensionless and physical variables.

Best Regards
Erman ASLAN