[Steeve]

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.


control.txt

ctrl.txt

Scour pattern.png

[Felix_Sp]

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

[Steeve]

Thanks for the reply.

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

Thank you.

[Curiousli]

Originally Posted by Felix_Sp

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

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.

[Felix_Sp]

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