# [MITgcm-support] C grid and pressure

Jody Klymak jklymak at uvic.ca
Thu Jul 12 10:43:11 EDT 2012

```Hi Nicolas,

Normally we don't care about the "static" or "background" part of the hydrostatic forces on the topography. The pressure due to the fluid at rest doesn't have a dynamical effect on the fluid, so you can throw those big terms out.  Those terms don't enter into the "form drag" since they don't exert a net force on any fluid parcels. If they did, then a fluid at rest would start moving absent any other external forces.

However, maybe you actually care about the lateral force, including the static, for some reason.

Cheers,   Jody

On Jul 10, 2012, at  15:10 PM, Nicolas Grisouard wrote:

> Hi everyone,
>
> Allow me to follow up on a discussion I started 6 months ago.
>
> On 1/7/12 11:37 AM, Jean-Michel Campin wrote:
>> Hi Nicolas,
>>
>> [...] the pressure P can be obtained as:
>> P(i,j,k)/rhoConst = -g*rC(k) + PH(i,j,k) + PNH(i,j,k)
>>
>> And PHL would be the equivalent of PH (hydrostatic pressure potential anomaly)
>> but at the bottom, so that the hydrostatic pressure at the bottom Pb
>> would be: Pb(i,j)/rhoConst = g*Depth(i,j) + PHL(i,j)
>
> And what about the full bottom pressure? I am trying to compute the force acting on a topography, so I want to integrate something like P*h', where P is the pressure and h'is the topographic slope. The way I compute P is (following the previous notations):
>
> Pb(i,j)/rhoConst = g*Depth(i,j) + PHL(i,j) + PNHbott(i,j),
>
>
> where I created PNHbott by locating the lowermost cells with some water in them and picking up the value of PNH there. I guess my worry is that I am doing something with the PNH fields that was not supposed to be allowed with the PH fields. Am I?
>
> The actual numbers tell that the g*Depth(i,j) term dwarfs the other ones, but not enough when integrated against h' to completely ignore the pressure anomalies.
>
> Thank you for your time and help,
> Nico.
>
> --
> Nicolas Grisouard -- Assistant Research Scientist
> Courant Institute of Mathematical Sciences
> 251 Mercer Street -- New York, NY 10012-1185
> tel: (+1) 212-992-9920 -- room 927
> Office Skype ID: nicolas.grisouard
> http://nicolas.grisouard.free.fr
> http://www.mendeley.com/profiles/nicolas-grisouard/
>
>
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--
Jody Klymak
http://web.uvic.ca/~jklymak/

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