Learning Forum

[ahmedshafie]

I wanna calculate moment around center of gravity ,for inclined submarine , so to make buoyancy take effect i ve to check gravity option? Is that only ? When do report then choose moment and input the cg coordination , fluent will take into consideration the drag ,lift and buoyancy?

[Amine Ben Hadj Ali]

Yes. Please set the operating density to zero. There is another way which would only affect post processing

[ahmedshafie]

thanks for your generous answer

[ahmedshafie]

if u please can tell me the second way just for more knowledge

[ahmedshafie]

i wanna know why i ve to make operating density zero , before i did change it convergene was to e^-4 with drag 1.5 lift -0.3 but after changing operating density to zero ,convergence reaches to e^-2 drag 2.8 and lift -0.6 . Even in both cases gravity option checked

[ahmedshafie]

is it wrong to use steady?

[Amine Ben Hadj Ali]

Without operating density input Solver will automatically parse it with data from the case and that data might be wrong depending on flow/material properties.
If you put it now to zero then you need to adjust all openings pressure: pressure inlet and outlets as they now require Absolute Pressure Input taking into account the hydro-static pressure.
It is not wrong to use steady if the flow is steady and no transient effects need to be tracked. IF you are doing a multiphase run for your submarine then probably you require unsteady solver

[ahmedshafie]

Thanks for great answer , but can u tell me how can i determine absolute pressure input taking into account the hydro static pressure ? the submarine only moves in water so do i need a multi phase ? isn't multi phase used with more than one fluid , and does making operating density zero more accurate that not ?

[Amine Ben Hadj Ali]

If you just have water there is no need to have mutliphase. Hydro-static pressure is just rho*g*height you need to consider that wherever you are having a pressure inlet or outlet and you need to provide gauge pressure as boundary in the case your operating density is set to something. It is easier to set to zero so that you need to provide the full pressure.

[ahmedshafie]

thanks for your answer sir . But more clarification please . Do u mean i will calculate hydrostaric pressure = Rho *g*h then input this value at inlet and outlet sides ?

[Amine Ben Hadj Ali]

No.
You should add that part of pressure to any pressure input you are providing at inlet /outlet pressure.

[ahmedshafie]

sorry i didnt get it , do u mean if there is source of pressure then i ve to add that value ? . Well i dont think i ve because its only submarine moving underwater .

[Amine Ben Hadj Ali]

Imagine you have a case where you are using a pressure outlet. That outlet is aligned with gravity. You are giving it a value of 5 Pa gauge pressure. Now you want to account for gravity and setting operating density = 0 kg/m┬│. You should now adjust that input into something like 5 Pa +rho*g*z z being the coordinate along the gravity vector.

[ahmedshafie]

thank u sir , i got it , in my case the output pressure i want is zero , so i will add rho *g*h , thanks u very much , lastly the height will be from surface to center of mass right ?

[ahmedshafie]

should i also add r*g*h to inlet pressure ?

[Amine Ben Hadj Ali]

Yes with h the distance along g to a reference point