It seems you're encountering issues with generating a consistent surface elevation plot in CFD-Post, possibly due to errors in the wave generation setup or data extraction. Here's a step-by-step approach to ensure the proper generation of a surface elevation vs. time plot for regular waves:

1. Check the Wave Generation in Your CFD Model

Ensure that:

• The wave generation is consistent with Airy wave theory.
• The wave boundary condition (inlet) is implemented correctly and generates regular waves with the specified amplitude and wavelength.
• There are no numerical artifacts in the simulation, such as reflections from the boundaries (even in a tank without structures, ensure the outlet boundary condition can absorb waves properly).

2. Verify the User-Defined Expression

You're using the expression:

plaintextCopy codelengthInt(water.volume.fraction)@line1

This computes the integral of the water volume fraction along a specified line. However:

• The result represents the length of the line submerged in water, which may not directly correspond to surface elevation if the line does not exactly capture the free surface behavior.

To properly calculate surface elevation:

• Use a line probe or point probe at a specific location along the tank.
• The probe should cut through the free surface.
• Create a user-defined expression that extracts the Y-coordinate where the water volume fraction is approximately 0.5 (indicating the free surface): plaintextCopy codesurfaceElevation = yCoordinate(water.volume.fraction=0.5)

3. Setting up the Plot in CFD-Post

4. Debugging Inconsistent Amplitudes

If the waves show varying amplitudes:

• Numerical Diffusion: Check if your mesh resolution and time step are adequate to resolve the wave physics.
• Boundary Conditions: Ensure that the inlet and outlet boundary conditions do not introduce spurious reflections or dissipate energy.
• Wave Absorption: If using a damping zone or numerical beach, verify its setup to prevent wave energy reflection.

5. Validate Against Analytical Solution

For first-order Airy wave theory, the surface elevation is given by: η(x,t)=acos⁡(kx−ωt)

Where:

• a: Wave amplitude
• k: Wave number (2π/λ)
• ω: Angular frequency (2π/T)

Generate the analytical solution for the same location and compare it with the CFD results to validate your setup.

Example User Expression for Surface Elevation:

In CFD-Post, you can define a user expression like:

plaintextCopy codesurfaceElevation = Y(water.volume.fraction=0.5)@line1

Ensure that the line is positioned to capture the free surface. Then plot this expression against time.

If these steps do not resolve the issue, feel free to share more details about your setup (e.g., boundary conditions, mesh size, or time step), and we can troubleshoot further.