DeltaT Macro, UDF, Time Step Control CFD Simulation

Description

In this project, we performed a numerical simulation using a User-Defined Function (UDF) by ANSYS Fluent software. For this CFD product, we used DeltaT Macro to write UDF programming.

This product is the 4th chapter of the User-Defined Function (UDF) Training Course.

We considered a simple tank where the water flow enters the tank from the inlet opening on the top of the tank. The purpose of this problem is to fill the tank over time.

Since this problem is solved as time-dependent, we performed the current simulation in an unsteady (Transient) state. So for calculations, we need to define the Time Step size. To determine the size of the time step, you must pay attention to one crucial note.

As the time step size becomes smaller, it strengthens the solution’s convergence and increases the solution’s accuracy significantly. But this creates a severe problem. The smaller the time step size, the longer the calculation process, and hence the computational cost of the modeling increases.

If the time step size becomes larger, it increases the solution’s speed and reduces the computational cost. However, the problem is that this work leads to a decrease in the accuracy of the solution or even an increase in the possibility of disruption in the convergence of the solution.

Therefore, in many problems, such as the current simulation where the existence of multiphase flow makes the calculations heavy, we need to control the time step size. Professional CFDs start the solution process with a very small time step size to establish initial convergence and stability.

Then, after ensuring the necessary stability and convergence, step by step and gradually increase the time step size to increase the speed of the solution. But this may be boring. So we intend to provide a better method.

In the first step, we modeled the geometry in 2D with Design Modeler software.

In the next step, we meshed the model with ANSYS Meshing software. Meshing is unstructured, and 135,996 cells are created.

Methodology (UDF)

In this project, we need to define two different phases. At first, the tank is filled with air, and gradually the water flow enters the tank. Then we used the Multiphase model. Since the two phases don’t mix and the separation boundary between the two phases is distinct, we used the Volume of Fluid (VOF) model.

Also, as we said, we intend to control the time step size. So we used a User-Defined function (UDF) to control the time step. We need to use the DeltaT Macro (DEFINE_DELTAT macro) for this UDF.

We ran the calculation for 9 seconds. For this, we use 700-time steps. Before the 5th second, we define the time step size as 0.01 second, and after the 5th second, we increase the time step size to 0.02 second.

Conclusion (UDF)

After completing the calculation, we will review the results. At first, we presented the plot of flow times per time step number. This modeling has been done in 700 time steps. Up to the time step of 500, the time step size equals 0.01 seconds. So the flow time goes up to 5 seconds.

After that, the time step size is equal to 0.02 seconds. So the flow time goes for 4 seconds. ‌This plot correctly shows this time step increase. That is, from the 5th second, the gradient of the flow time increases. We also obtained some contours to show the filling of the water tank. So, we obtained the contours of air and water volume fractions at different simulation times.

The contours show that the water phase inside the tank is increasing, and the air phase inside the tank is decreasing. It means that the process of filling the tank is occurring. We also obtained the plot of water volume fraction changes according to flow time. This plot also shows the filling of the water tank.