Learning Forum

[chakrachand]

Hello,

I am trying to simulate the waves in Ansys Fluent from last 6 months but whatever I try waves are getting damped.

Wave length is 1m and Wave height is 0.05m.  Domain is rectangular 2D having 20m length. Water depth is 1m and air height is 1m. 

I have tried with fine hexahedral cells in the range of 100/150 cells per wavelength and 20/30 cells per waveheight.

 The basic requirements are, deep water conditions, wave shouldn’t get damped, first order airy theory and K-w SST /K-e Standard model for turbulence.

 For wave generation, I tried by using Implicit VOF as well as Explicit VOF I have used open channel wave BC, Implicit body force and Sharp from interface modelling. Interfacial Anti-diffusion is also ticked.

 In order to avoid reflection from opposite wall, numerical beach is provided over 10 wavelengths from opposite wall(Outlet).

Boundary Conditions: Velocity inlet BC at inlet. I also tried with SHM of inlet boundary using UDF. Pressure Outlet BC at the outlet and top air surface. Bottoom of the doamin is slip wall. 

 For implicit VOF:  Pressure- Velocity coupling PISO as well as Coupled scheme is used, Pressure – PRESTO, momentum - second order upwind, Volume fraction – Compressive, Turbulent K.E- second order upwind, Sp. Dissipation rate – second order upwind, transient formulation – Bounded second order implicit.

For Explicit VOF:  Level set was ticked. Pressure- Velocity coupling PISO  scheme is used, Pressure – PRESTO, momentum - second order upwind, Volume fraction – Geo-Reconstruct, Turbulent K.E- second order upwind, Sp. Dissipation rate – second order upwind, transient formulation – First Order Implicit.

The problem is that the waves are losing energy (height) during the propagation phenomena in the domain. I have taken help from Ansys user manual and many other research papers to get waves that are not damping but I failed every time. it will be great help if someone can help me to resolve this issue.

Thank you in advance,

Best Regards

Chakra

 

[Rob]

The numerical beach will start to effect the waves at 10m into the domain, so you may want to reduce that. The other possible issue is cell shape. VOF can be sensitive to aspect ratio, so 100 cells/m length ways and 20cells/5cm height gives an aspect ratio of 1cm: 0.25cm. That should be OK, but try putting a more-or-less uniform square mesh on the domain and then adapt the wave region to improve the resolution. 

Note, 5cm waves aren't very big so you are going to need a good mesh and time resolution to get good results. 

[chakrachand]

Hello Rob,

Thank you for the reply. 

I have reduced the numerical beach to 2 and 4 wavelengths from the outlet in some of previous simulation cases but there was still damping so I thought that the damping may be due to reflection from outlet so I increased the numerical beach length to 10 wave lengths from outlet. But even it did not helped.

I have also increased grids to 30 cells per wave height and aspect ratio of 1 in previous simulation cases but damping was present in every cases.

For the resolution, I have tried various cases T/100,T/200,T/300, T/500 and T/1000 where T is time period. But it did not helped me.

Can there be any spots in setup where I may be missing?

[Rob]

The numerical beach damps waves so whilst you are stopping reflections you're also damping the waves approaching the beach. Try reducing the beach to 1m or if there is an outlet (open channel) turn off the beach. 

[chakrachand]

Length of my domain is 20 wavelengths so I am damping the last 10 wavelengths and my working area is just first 10 wavelengths. Also I have tried by providing no numerical beach as well as 2 numerical beach from outlet still waves were damping. I have open channel BC for pressure outlet and velocity inlet. I have also tried also by changing the initial values of Turbulent Kinetic Energy and Specific Dissipation Rate while initialization but the issue is not solved. I have no idea where I am missing.

[Rob]

Damping tends to cause some effect upstream so try reducing the amount of damping. What is the "sky" boundary condition?