OK, there we go. Le me first state what you need to do, and then, why this is the right thing to do. I assume from your post that your converter looks somehow like

LBunits converter(uMax, Re, 40., 5., 1., 1.);

That’s fine, if you agree that your reference length for the definition of the Reynolds number is the short direction (1cm), not the long one. In the latter case, the geometry must be (1., 1./5., 1./5). Now set the lattice velocity to a value which has nothing to do with your 10 cm/s. A good value is for example uMax=0.02 (see below). Run your simulation. Measure the results. For example, compute the velocity at one point through the command

```
T u[3];
lattice.get(x,y).computeU(u);
```

Or, compute the RMS value of the velocity through a command like

```
T uRMS = sqrt( 2. * lattice.getStatistics().getAverageEnergy() )
```

In both cases, you get a velocity value which is measured in lattice units, u_{lu}. Convert this value into a system of dimensionless variables:

```
u_{dl} = u_{lu} * converter.getDeltaX()/converter.getDeltaT();
```

Finally, convert from dimensionless variables into your physical units:

```
u_p = u_{dl} * 10 cm/s
```

And you’re done. Here’s the explanation. The unit converter assumes that the lattice-independent system consists of dimensionless variables (after having read this thread, you could write, for your convenience, a unit converter which performs automatically the conversion from dimensionless to physical variables). In this system, the reference variables have value 1: L_{dl}=1 and U_{dl}=1. In this notation, reference variables are uppercase, whereas free variables, which you typically measure from the simulation, are lowercase. Conversion between the dimensionless and the physical system is straightforward. In physical units, you have L_p = 1 cm and U_p = 10 cm/s. The conversion formula is

```
l_p = l_{dl}*L_p and u_p = u_{dl}*U_p.
```

The variable uMax stands for the velocity in lattice units and is used to control the Mach number. When you simulate an incompressible fluid, this variable has no physical meaning, and is simply a numerical parameter. If you decrease uMax your simulation is more accurate because compressibility effects are smaller, and if you increase uMax, your simulation is faster because dt is larger. It may seem counter-intuitive to you that your value U_p = 10 cm/s is not used to set up the simulation, but only to evaluate the results. However, that’s what it means when people speak of similarity in fluids. The flow structure depends only on the aspect-ratio of the geometry and on the Reynolds number, and not on all individual parameters.