strange problem with DeLaval nozzle
 

I have simulated a 2D axi-symmetric case of DeLaval nozzle. The divergent is split into two sections. All the things went fine. But, when I computed axial force on the walls. Half of the nozzle wall was positive force and the remaining divergent part is giving me negative force (viscous is positive but pressure force is negative). There is not separation in the flow. The nozzle has area ratio of 25. Outside pressure is 0.4 bar. Why it is giving negative force?

SJ

Fluent uses reference pressure to calculate forces ie. it integrates (Pfluid-Pref) over surface. So when your flow pressure near wall decreases to below 1 bar and your reference pressure is set to 1 bar will give you negative force or force on opposite direction.

if you are talking about Operating Pressure, that is set to zero. While pinlet is 53.5 bar and outlet is also set to zero

nope im talking about reference values.

ok, let me try this and return back to you. Thanks for your help :)

I am still confused, since putting a reference pressure of 53.5 bar gives a lot of axial force. So does this reference pressure has to be the approximate nozzle exit pressure?

@atelcikti1, I should let you know that in a nozzle, due to pressure being applied normal to the surface, the force components are distributed accordingly. The convergent part has a force normal which is tilted to the right hence x +ve while the force on divergent part is tilted left, hence the axial component is in - direction. I did not have had this concept and now it is clear. No need to change reference values.

yes you are right, this is valid for all nozzles but the actual force magnitude will change with reference value, so keep this in your mind if you need numbers.