Incompressible Fluid Flow (LBM) analysis type is used to simulate the transient effects of external flow around objects using the** PacefishÂ®\(^1\)** Lattice Boltzmann method (LBM). This article answers the following questions regarding the simulation time and timestep in LBM Analysis:

- Can I control simulation time in LBM?
- Is it possible to control the simulation timestep in LBM?

## Solution

The short answer is,

- Yes, you can control the
*simulation time* - No, you cannot control the
*timestep*

Please visit the documentation page to learn more about the LBM Analysis type.

### 1. Simulation Time

*LBM *is a *transient *(time-dependent) analysis. In other words, *LBM *analysis captures the transient behavior of the flow. In a time-dependent analysis, simulation time defines how long a transient simulation runs in terms of time (counted in seconds).

How long should be the simulation time? You can define the simulation time with respect to the n*umber of fluid passes*.

#### 1.1 Number of Fluid Passes

In *transient flow analysis*, initial time steps are *unsteady*. During the initial stage, it is assumed that first fluid particles enter the domain, then leave it. After that, the next party comes and leaves the domain. During the early *fluid passes*, we expect the flow to get developed. We assume that flow reaches a *steady-state* in the final fluid pass. As a result, we can say that 3 fluid passes are sufficient to reach the developed flow. In conclusion, the number of fluid pass defines the simulation time.

#### 1.2 How to Define Simulation Time

Calculate how long does it take a fluid particle to pass though the domain:

$$t_1 = \frac{L}{U} \tag{1}$$

where, \(t_1\) is the time requirement for 1 fluid pass, \(L\) is the length of fluid domain, and \(U\) is the speed of particle.

Multiply the number of fluid passes to find the total simulation time:

$$t_{total} = n\cdot \frac{L}{U} \tag{2}$$

where, \(t_{total}\) is the simulation time, and \(n\) is the number of fluid passes.

### 2. Simulation Timestep

Simulation timestep is the small-time intervals in a transient simulation. The user has no control over the timestep because a special algorithm in the LBM solver calculates the simulation timestep automatically.

## Best Practices

Consider minimum **3 fluid passes** in order to define the simulation time.

## Related Articles

A similar concept is applied to control simulation parameters for pedestrian wind comfort simulations. Have a look at the following article to know more about fluid passes and maximum run time:

Note

If none of the above suggestions solved your problem, then please post the issue on our forum or contact us.

References