[MITgcm-support] Bottom drag setup question

[Jean-Michel Campin]

Hi Yilang,

The answer is "Yes". Any of the three:
 1) bottomDragQuadratic > 0
 2) bottomDragLinear > 0
 3) no_slip_bottom = .TRUE. with non-zero vertical viscosity
will contribute to non-zero friction term at the bottom. 
And if 2 or 3 of the above are used, contributions are added together.

Cheers,
Jean-Michel

On Wed, Jun 05, 2019 at 10:23:30AM -0400, Yilang Xu wrote:
> Hi Jean-Michel, 
> 
> Thanks very much for your explanation. Just to ensure my understanding is correct- for the case with free-slip bottom, if we have bottomDragQuadratic > 0, there is still bottom friction added. Am I right? 
> 
> Best,
> Yilang 
> 
> ???On 6/5/19, 00:48, "MITgcm-support on behalf of Jean-Michel Campin" <mitgcm-support-bounces at mitgcm.org on behalf of jmc at mit.edu> wrote:
> 
>     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
>     > 
>     >  
>     > 
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