Sea level has a very small negative slope for large simulations (>100 s)

I am evaluating irregular waves in a 2D domain and measuring the waves approximately 4 m (1 wavelength) from piston. When I plot the surface elevation for 150 s, I notice a very small negative slope (3.08x10^-5) as if the surface elevation is decreasing.

I tried the following to see if this is reduced

  1. Increase Ramp time from 20 to 40 s
  2. Increase domain size with a large damping zone (to prevent reflection)
  3. Increase coefh from 1 to 1.2 and 1.5

For all three methods, the issue persists. I am currently running simulations with DensityDT delta term reduced from 0.15 to 0.1 and another case with artificial viscosity. I am wondering if anyone has encountered this issue before?

I have attached my input as a text file. Please let me know what my issue might be. This is in a 2D domain with about 147k particles.


  • @Alex @Asalih3d would appreciate any inputs. I looked at the forums to see if a similar issue was reported, but could not find any

  • Are you loosing fluid particles?

    Can you share the Run.out with us?


  • Hi, @Alex

    I don't understand the parameter "ramp" in <piston>, even though it is explained as "number of periods to smooth the movement of the piston" in the guide.pdf, can you introduce the following "ramp" effect and use scenario? Thank you so much!

    <piston> % To define the piston-type wavemaker for regular wave generation 
                        <mkbound value="10" comment="Mk-Bound of selected particles" />
                        <waveorder value="2" comment="Order wave generation 1:1st order, 2:2nd order (def=1)" />  % To define the wave order for regular wave generation
                        <start value="0" comment="Start time (def=0)" />   % Instant when the piston-type wavemaker starts to move
                        <duration value="0" comment="Movement duration, Zero is the end of simulation (def=0)" />  % Duration of the piston-type wavemaker moving
                        <depth value="8" comment="Fluid depth (def=0)" />  % Water depth at the wavemaker position
                        <pistondir x="1" y="0" z="0" comment="Movement direction (def=(1,0,0))" />  % Direction of the piston-type wavemaker in X,Y,Z.
                        <waveheight value="2" comment="Wave height" />  % Mean wave hieght to be generated
                        <waveperiod value="10" comment="Wave period" />  % Wave period to be generated
                        <phase value="0" comment="Initial wave phase in function of PI (def=0)" />  % Phase to define the initial movement in the sinusoidal
                        <ramp value="1" comment="Periods of ramp (def=0)" /> % Application of a transient mode(ramp) to the initial displacement (as function of wave period)
                        <savemotion periods="20" periodsteps="20" xpos="50" zpos="-2" comment="Saves motion data. xpos and zpos are optional. zpos=-depth of the measuring point" />  %% Saves theoretical elevation and orbital velocities (for 1st and 2nd order) at a given position (xpos and zpos)                      
  • "ramp" is used to define if the first waves will be a transition to the target waves.

    The same is used in physical pistons to avoid mechanical issues.

    If you define ramp=2, that means that the first two waves will be increasing smoothly and only starting from the third wave you will have the target amplitude

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