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-   -   buoyancy - losses estimation through total pressure (http://www.cfd-online.com/Forums/cfx/81545-buoyancy-losses-estimation-through-total-pressure.html)

 clrsgfn October 29, 2010 14:07

buoyancy - losses estimation through total pressure

Hi everyone

As a test, I'm simulating a basic vertical cylinder, with uniform normal water flow at entrance and zero averaged pressure at outlet. Reynolds number is ~2000

- When buyoancy is off, by calculating total pressure difference between inlet and outlet I get losses that corresponds to what can be expected from theory

- when buoyancy is on : computing losses by total pressure difference gives a very different value (*20 !). Computing losses by difference of (abs pressure+cinetic energy) gives non physical results (negative losses...)

I've checked hydrostatic pressure (abs. pressure - ref. pressure - pressure) and this is ok...

what's going on ? and how do I compute losses in case of buoyancy ?

thank you all in advance !

 TommySean October 30, 2010 11:32

If there's setting about reference density, you can make a test with reference density = 0.

 clrsgfn November 1, 2010 14:03

thanks
but I am not considering a multiphase flow, as only water is flowing in. So my density is fixed and constant (i am not considering temperature fluctuation either). This is really a simple test case, and results are puzzling to me...
any other clue ?

 michael_owen November 4, 2010 13:10

Quote:
 Originally Posted by clrsgfn (Post 281420) Hi everyone As a test, I'm simulating a basic vertical cylinder, with uniform normal water flow at entrance and zero averaged pressure at outlet. Reynolds number is ~2000 - When buyoancy is off, by calculating total pressure difference between inlet and outlet I get losses that corresponds to what can be expected from theory - when buoyancy is on : computing losses by total pressure difference gives a very different value (*20 !). Computing losses by difference of (abs pressure+cinetic energy) gives non physical results (negative losses...) I've checked hydrostatic pressure (abs. pressure - ref. pressure - pressure) and this is ok... what's going on ? and how do I compute losses in case of buoyancy ? thank you all in advance !
The absolute pressure is equal to the reference pressure plus the pressure. P_abs - P_ref - P is not P_hydrostatic, it's zero. The reason your pressure difference is so large is that the Pressure includes the hydrostatic pressure. What is the height of the cylinder? You have a hydrostatic pressure difference of rho*g*h. Subtract that from delta P and you will get your losses.

 clrsgfn November 10, 2010 12:05

Quote:
 Originally Posted by michael_owen (Post 282176) The absolute pressure is equal to the reference pressure plus the pressure. P_abs - P_ref - P is not P_hydrostatic, it's zero. The reason your pressure difference is so large is that the Pressure includes the hydrostatic pressure. What is the height of the cylinder? You have a hydrostatic pressure difference of rho*g*h. Subtract that from delta P and you will get your losses.
Hi michael,