[MITgcm-support] Bottom drag setup question

[Jean-Michel Campin]

Hi Yilang,

There is some confusion regarding these free-slip / no-slip boundary-conditions
in combination with various bottom drag options, so I will try to clarify:

1) no-slip options (default = TRUE):
 (a) no_slip_sides=.TRUE.,
 (b) no_slip_bottom=.TRUE.,
 Here a friction term is computed next to vertical wall (a) or/and in the grid-cell 
 abore the bottom (b). The magnitude of the friction term is function of the 
 horizontal viscosity (a) or vertical viscosity (b).
This is explained in the manual, at the end of section 2.14.5 and 2.14.6 :
 https://mitgcm.readthedocs.io/en/latest/algorithm/algorithm.html#flux-form-momentum-equations
Note that the documentation is found in the Flux-Form sub-section but applies to both 
momentum forumalion, Flux-Form or Vector-Invariant form (in fact the same routines, 
from pkg/mom_commom/, are used in both momentum formulations).

2) Linear or quadratic bottom drag:
 These options (i.e., bottomDragLinear > 0 or bottomDragQuadratic > 0) will increase the
 magnitude of the bottom friction term (from zero if no_slip_bottom=.FALSE. or from non
 zero value otherwise) by adding a friction term that iis simply computed from the 
 velocity near the bottom using a linear or quadratic relation (but independent from 
 vertical viscosity near the bottom).

(1) and (2) can be combined (your second case), and this would result in adding
linear/quadratic bottom drag friction to the vertical viscosity dependent bottom
friction due to no-slip BC.

Cheers,
Jean-Michel

On Tue, Jun 04, 2019 at 10:33:16PM -0400, Yilang Xu wrote:
> I just checked sideDragFactor setup in mom_v_sidedrag.F, scenario 2 is solved for no slip side. 
> 
> Now I mostly wonder the logic behind scenario 1, for the case with free-slip bottom and a specified bottom drag. 
> 
> Any clarification is appreciated. 
> 
>  
> 
> Best,
> 
> Yilang
> 
>  
> 
> From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Yilang Xu <yxu at whoi.edu>
> Reply-To: <mitgcm-support at mitgcm.org>
> Date: Tuesday, June 4, 2019 at 22:12
> To: "mitgcm-support at mitgcm.org" <mitgcm-support at mitgcm.org>
> Subject: [MITgcm-support] Bottom drag setup question
> 
>  
> 
> Hi, 
> 
>  
> 
> I am checking the behavior of the bottom drag, following the old post http://mailman.mitgcm.org/pipermail/mitgcm-support/2011-June/007258.html . 
> 
> My question involves 2 scenarios, and I am wondering if my interpretation is right. 
> 
>  
> 
> 1. Scenario 1 setup: 
> 
> no_slip_sides=.FALSE.,
> 
> no_slip_bottom=.FALSE.,
> 
> bottomDragQuadratic=2.5E-3,
> 
> Although both the bottom and the side are free slip, there are still bottom friction and energy dissipation at the bottom. I saw some papers do this using MITgcm, but it is not very intuitive to me. 
> 
>  
> 
> 2. Scenarios 2 setup: 
> 
> no_slip_sides=.TRUE.,
> 
> no_slip_bottom=.TRUE.,
> 
> bottomDragQuadratic=2.5E-3,
> 
> Now the bottom is no slip with a bottom drag coefficient specified. However, since the side is no slip as well, is there also a drag coefficient that can be specified for the sides? 
> 
>  
> 
> There are some explanations in the documentation http://mitgcm.org/public/r2_manual/latest/online_documents/node58.html , but I am hoping further clarifications about this. 
> 
>  
> 
> Thanks,
> 
> Yilang
> 
>  
> 
> _______________________________________________ MITgcm-support mailing list MITgcm-support at mitgcm.org http://mailman.mitgcm.org/mailman/listinfo/mitgcm-support 
> 

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