fluid speed vs pressure 2

[Hagar1]

Please someone tell me the Physics Law that states that The Speed of a liquid in a pipe is slow when the pressure is heigh and the speed is fast where the pressure is low.

Keith
kd@tnni.net

 

[sailoday28]

venkatParam
You did not specify if the fluid was compressible.
My response was for a highly incompressible fluid-such as a liquid.
However, if the fluid is compressible, then the approach to solve the pressure-velcity distribution should be done with the method of characteristics (MOC) approach.
And that would be an interesting topic for this forum to explore.

 

[venkatParam]

Thank You guys,

The fluid is incompressible. And frinctional losses are negligible. And I still have some questions. If a flow of low velocity nothing compared to that of sound etc, is stopped in its path. Based on Bernoulli's law,

The stagnation pressure = Static pressure + (rho*V*V)/(2)
So shouldn't the force = area * Stagnation Pressure
Based on
"This pressure must be added to the prevailing static pressure to verify that the pipe is not subjected to excessive tensile stresses" by 25362

25362 : According to you F = (?)(q)(v)/g
Dimensionally speaking
(F)is force (Kg m/s2)
(?)is density(Kg/m3)
(q)is ? (m4/s3) What is this term
(v)is velocity (m/s)
(g)is 9.8 (m/s2)

Also, is the static pressure term (P/?) in the equation, depending on the source or is constant in an environment etc.
Sorry guys if i sound too confused but i am.
I am trying to simulate this flow using FEMLAB.
Thank you for your time

 

[sailoday28]

Bernoulli's equation is for steady state flow.
IT IS NOT APPLICABLE TO :
Stopping the flow as related to sudden or changing the flow velocity.
Now at a point in steady state flow where the velcity is zero, one may apply stagnation properties to determine pressure.

 

[25362]

To venkatParam. You are absolutely right.

q is flow rate, for example, m³/s.

Force can be expressed in various manners. For example kg_f = 9.8 N = 9.8 kg.m/s².

Thus, (kg/m³)(m³/s)(m/s) = kg.m/s².

Dividing by 9.8 one gets kg_f. I should have said that g is a constant = 9.8, and call it g_c to distinguish it from the acceleration of gravity 9.8 m/s².

Otherwise one gets kg which is a measure of mass, not a force.

Another way to express the perpendicular impact force would be [ρ]Av². By dimensions:

(kg/m³)(m²)(m/s)² = kg.m/s², which has the dimensions of force, as given by mass times acceleration.

 

[sailoday28]

25362 (Chemical)states
Another way to express the perpendicular impact force would be ?Av2.

Please note-- for THIS steady state problem the force is related to the change in MOMENTUM. AND therfore for flow perpendicular to a surface one may use the incoming velocity AS the incoming momentum---AND the exit momentum is ZERO. The force from the net change in momentum in the icoming direction is as '25362 (Chemical)states'
I reiterate, if the FLOW, (not an individual path line) is suddenly changed, then the "water hammer" type equation must be used to detrmine pressure, velocity, force distribution.