Bernoulli principle - negative pressure

maleo7

I will write a program about principle bernoulli, but i have a problem.

My input data:

Pressure(p1) = 1000
Cross-section(A1) = 1
Velocity(V1) = 1

Cross-section(A2) = 0.5

Velocity(v2)= (V1A1)/A2

to simplify: h1=h2

i'm counting the pressure p2 : p2 = p1 + 0.5*v1*v1 - 0.5*v2*v2 , so i getting negative pressure..



This is a problem (negative pressure)

I'm trying to add atmospheric pressure, but it doesn't solve my problem.

Is a mathematical formula is good? Is there any other relationship?

SteamKing

Some units would be helpful in deciphering your problem.

AlephZero

Your equations look OK. You correctly calculated V2 = 2, so you must have made a mistake in the code.

My calculator says 1000 + 0.5*1*1 - 0.5*2*2 = 998.5 whcih isn't negative.

maleo7

My mistake. I forgot write the density of liquid.

Should : p2 = p1 + 0.5*v1*v1*ro - 0.5*v2*v2*ro

ro=density=1000,

so:
p2 = 1000 + 0.5*1*1*1000 - 0.5*2*2*1000 = -500 .

maleo7

I replace Paskal to kPa, but it didn't help

AlephZero

Your "negative outlet pressure" just means the inlet pressure is too low to force the given mass flow through the nozzle.

cjl

Yep, although if the given pressures are gauge rather than absolute, it would work just fine (so long as you weren't trying to exhaust the flow with a negative gauge pressure to the atmosphere).