Questions about the S 13 and S 46
 

Hello everyone,

I'm studying to model scour around cylindrical monopile.
I learned that by setting S 13 and S 46 to the same value, the flow and sediment time steps would be the same.(I would like to be pointed out if my interpretation is incorrect.)

So I would like to know if the values of S 13 and S 46 need to be the same.(Ideally, I would like to set S 46 10 as opposed to S 13 1)

I have attached my control, ctrl files and result for scour pattern.

Thank you.


Attachment 96741

Attachment 96742

Attachment 96743

Hey there,

setting S 13 and S 46 to the same value does not automatically lead to the synchronisation of the sediment time step and the hydrodynamic time step.

S 13 is just the maximum time step of the sediment solver if S 15 with S 14 is fulfilled, i.e. the sediment CFL-criterium (I am not totally sure whether S 13 applies for S 15 1 and S 15 2). For the default value this would essentially mean that nearly nothing is transported in your simulation.

S 46 (together with S 44 and S 48) is the time or for S 44 the iteration in between the application of the sediment transport solver. It is essentially the frequency the bed is updated.

E.g. you have these settings:

S 13 10
S 14 0.3
S 15 0
S 41 2
S 42 2
S 45 5.0
S 46 1.0

The sediment transport starts after 5 seconds of hydrodynamic time. Then it is calculated for the first time. Depending on maximum the amount of transport the maximum sediment time step is calculated based on the sediment-CFL-number (0.3). This might lead to e.g. 0.2 s sediment transport. This means that afterwards your sediment time is at 0.2 s. Now the hydrodynamic solver calculates again for 1.0 s with the updated bed and can therefore adapt to the updated topography. When the hydrodynamic time reaches 6.0 s another iteration of the sediment transport is calculated.

Maybe also read Link and Link for that problem.

If you want to achieve a time accurate sediment transport you need to set S 15 to 1, S 42 to 1 and S 44 to 1. Be aware that this might lead to instabilities as your sediment time step is then not restricted by a CFL-criterium (S 14). Also think about your start of the sediment transport (S 41, S 43/S 45). I guess it would be better for such simulations to couple the sediment time step to the CFL-criterium of the hydrodynamic solver. If you have no problem with coding you might want to that.

Sorry for the long text. Hope it helps.

Cheers

Thanks for the reply.

I found your explanation with specific examples very helpful.
I will also check the related links.

Thank you.

Hi Felix, great to see another great answer from you. Allow me to take the liberty of asking for advice:
Recently I've been learning to code in REEF3D, however, I have not studied programming languages and finding the formulas I wish to understand and modify is a difficult task. Do you have a good method for this, please? For example, I would like to find several formulas in the code for the S 16 option.

Hello,

@Steeve actually I missed something in my explanation. For S 15 1 S 14 is still active in the code. This means that your flow solver and your sediment solver might not result in the same time stepping (have a look into sediment_exner_timestep.cpp). If you want to achieve full coupling (again beware of instability) you also have to set S 14 to 1.0.

@Curiousli No I have no easy way for you. The S 16 options are given in the bedshear.cpp if you want to change something.

Cheers