Bernoulli principle - negative pressure

  • Thread starter maleo7
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  • #1
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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..

1zo7vxc.jpg


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?
 

Answers and Replies

  • #2
SteamKing
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Some units would be helpful in deciphering your problem.
 
  • #3
AlephZero
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i'm counting the pressure p2 : p2 = p1 + 0.5*v1*v1 - 0.5*v2*v2 , so i getting negative pressure..
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.
 
  • #4
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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.
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 .
 
  • #5
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Some units would be helpful in deciphering your problem.
I replace Paskal to kPa, but it didn't help
 
  • #6
AlephZero
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Your "negative outlet pressure" just means the inlet pressure is too low to force the given mass flow through the nozzle.
 
  • #7
cjl
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Your "negative outlet pressure" just means the inlet pressure is too low to force the given mass flow through the nozzle.
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).
 

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