# Reason for fluid flow

1. Dec 1, 2008

### venky56

recently i was asked one question in an interview!
How fluids flow? i mean whether it is because of pressure difference or any other reason.

one more thing! could any one explain me what is actually the pressure of a fluid and what is the gravitational force exerted on fluid(i.e weight, as i know).

i am confused bcoz P=row*gh, which itself includes gravitational force right!

2. Dec 1, 2008

### Staff: Mentor

The full form of Bernoulli's equation contains a term for gravitationally induced pressure, but it is often dropped when unnecessary.

Fluids flow when there is a pressure difference between two points in order to equalize that pressure.

3. Dec 1, 2008

### venky56

then wat abt open channel flow where pressure is atmospheric evrywhere

4. Dec 1, 2008

### Astronuc

Staff Emeritus
Fluids flow in response to a net force. Pressure is simply force/area, and the differential pressure is simply a pressure gradient in a pipe and the fluid flows from high pressure to low pressure.

Fluid also flows under the force of gravity, from higher elevation to lower elevation, much the way a mass falls in a gravitational field.

P = $\rho$gh is simply the pressure at some depth, h, of a fluid. At the surface, the pressure between atmosphere and liquid is equal. Note that as one ascends through the atmosphere, the pressure decreases with altitude since there is less atmosphere (mass) above. The density of the atmosphere also decreases with increasing elevation. As on descends into a fluid, e.g. pool, lake, sea, ocean, . . . the pressure increases since the mass above a certain depth is greater (with depth).

Fluid leaving a pipe into the atmosphere must have a hydrostatic pressure approximately equal to the atmosphere, but it also has momentum.

Think about the relationship between momentum and force (and force and pressure).

5. Dec 1, 2008

### venky56

thank u sir
Now, how do u define that pressure force?

like for eg, if u say gravitational force, it is the force by which earth is attracting a body. likewise, what is the source for a static fluid to exert a force, which we call as pressure? is it the kinetic energy of molecules? if so, wat is dynamic pressure?

I dont know sir if i am thinking in correct way!

6. Dec 2, 2008

### ank_gl

fluids flow due to pressure difference. In open channel, water flows due to the difference in head(only you can't see the head, whereas in pipe flow, you can see the energy source)
static pressure is due to gravitational pull only.

Relation between static & dynamic pressure is same as that between potential & kinetic energy.

7. Dec 2, 2008

### venky56

if static pressure is due to gravitational pull, then why there r separate terms for gravity and static pressure in bernoullis equation?
plz explain

8. Dec 2, 2008

### ank_gl

ok you misinterpret me, or maybe i wasn't descriptive enough.

static pressure is due to an external force. think a hydraulic ram lifting something, that is static pressure.

while open channel flow is due to gravitational head.

what i meant was force due to stationary fluid

9. Dec 2, 2008

### venky56

If u see dynamic pressure, they say it is the force exerted by the fluid(over an area) using its bulk K.E.
Dyn pressure= V2/2g. where V= bulk velocity of fluid

Likewise, what is static pressure? what is the energy that is responsible for static pressure?
i was told that it is the average force exerted by the K.E of individual molecules. if it so, why that is not reflected in equation p=row*gh.

Last edited: Dec 2, 2008
10. Dec 3, 2008

### ank_gl

you are not using "your" brain, you are talking about "they". Tell us what you think.

dynamic pressure is due to kinetic energy & static pressure is due to internal energy. When the static pressure of a fluid, like a gas or vapor, is increased, its internal energy increases.

11. Dec 3, 2008

### venky56

this is a learning forum...anyone can participate......but not the persons who dont have minimum manners like u.... finally its not the knowledge that counts...its the character. i guess there should be a separate forum to elighten people like u.....

And for ur kind information.....
i said pressure is due to K.E of individual molecules(i.e K.E of each molecule) which is nothing but Internal Energy
whereas dynamic pressure is due to bulk K.E(i.e K.E of entire fluid)....

i just wanted to know if i was right!

Last edited: Dec 3, 2008
12. Dec 3, 2008

### ank_gl

:zzz:yea right, i can see how that applies to you

13. Sep 8, 2009

### manicks

I was searching the answers for the same question and came across ur question posted... i cud not find very clear answers anywer..

but i wud lik 2 share wat i guess.. i dont know whether my guess is right......

consider a falling object.. the object loses its pot energy and gains KE to reach the earth surface... when this energy cons is applied to a fluid flow, wat we have is a Bernoulli eqn

3 forms of energy will be considered and the reasons why they are considered will be stated...

Pressure energy:
lets say, a volume of fluid is at rest... but this does not mean that there is no movement of molecules inside it... this volume has some def temp... dep on the temperature, there is random motion of molecules inside that bulk volume of fluid.. these molecules hit the surface and thus exert some pressure on it.. this is static pressure

Kinetic energy:
when the energy due to the random motion of molecules (internal energy) could be converted into kinetic energy, that is, orienting the random motion of molecules in a definite direction, the fluid is said to flow in that particular direction... this energy conservation is explained in Bernoullis eqn... but note that this eqn omits viscous, thermal effects in the flow

so when a part of this pressure energy is converted into K.E., fluid flows.. it is difficult to imagine that all this Press Energy is converted into K.E... bcoz, atleast few molecules would randomly move and hit the surface

Gravitational Potential energy:
similarily to any other object at height, fluid(air) also possesses grav P.E... this is what we express as rho*g*h(similar to m*g*h; but here we consider energy/unit volume)... lets consider 2 pts A and B in a flow with a height diff 'h'... we know atm P dec with alt.. So there is going to be a P diff now... hence there is a flow... At A, the grav Potential is g*h2 (rho is not considered since we are talking abt incomp form of Benoullis eqn; thats we cannot consider flows/convectional cycles in atm - because here density changes with alt) and similarily at B.. we can understand that the diff is goin to be much smaller as long as the flow is between closer vertical dist...

as long as the fluid flow occurs at a level surface, gravitational effects get cancelled on applying Bernoulli eqn to the flow

when static P is always due to gravity, we cannot expect it to be equal in all radial directions when we consider the flow inside a circular pipe... so these 2 are diff... but, as we connect the static and pitot tubes to a manometer, we calculate the P diff by using gravity effect [since we use the formula P=rho*g*(h2-h1)].. this does not mean that the static pressure is actually due to gravity.. we just apply hydrostatic pressure principle to find out the actual static/total pressure

Most of the above is what i assume.. if wrong, feel free to point the mistakes

14. Sep 11, 2009

### girish21

well!!!! fluids are substances which cannot withstand shear stress . if shear stress is applied on them they simply move in the direction of applied force.

15. Sep 11, 2009

### manicks

I was searching the answers for the same question and came across your question posted... i could not find very clear answers anywhere..

but i would like 2 share what i guess.. i do not know whether my guess is right......

consider a falling object.. the object loses its pot energy and gains KE to reach the earth surface... when this energy cons is applied to a fluid flow, what we have is a Bernoulli eqn

3 forms of energy will be considered and the reasons why they are considered will be stated...

Pressure energy:
lets say, a volume of fluid is at rest... but this does not mean that there is no movement of molecules inside it... this volume has some def temp... depending on the temperature, there is random motion of molecules inside that bulk volume of fluid.. these molecules hit the surface and thus exert some pressure on it.. this is static pressure

Kinetic energy:
when the energy due to the random motion of molecules (internal energy) could be converted into kinetic energy, that is, orienting the random motion of molecules in a definite direction, the fluid is said to flow in that particular direction... this energy conservation is explained in Bernoullis eqn... but note that this eqn omits viscous, thermal effects in the flow

so when a part of this pressure energy is converted into K.E., fluid flows.. it is difficult to imagine that all this Press Energy is converted into K.E... because, atleast few molecules would randomly move and hit the surface

Gravitational Potential energy:
similarly to any other object at height, fluid(air) also possesses gravitational P.E... this is what we express as rho*g*h(similar to m*g*h; but here we consider energy/unit volume)... lets consider 2 points A and B in a flow with a height diff 'h'... At A, the gravitational Potential is g*h2 (rho is not considered since we are talking about incompressible form of Benoullis eqn; that is why we cannot consider flows/convectional cycles in atmosphere - because here density changes with alt) and similarly at B.. we can understand that the diff is going to be much smaller as long as the flow is between closer vertical dist...

as long as the fluid flow occurs at a level surface, gravitational effects get canceled on applying Bernoulli eqn to the flow

when static P is always due to gravity, we cannot expect it to be equal in all radial directions when we consider the flow inside a circular pipe... so these 2 are diff... but, as we connect the static and pitot tubes to a manometer, we calculate the P diff by using gravity effect [since we use the formula P=rho*g*(h2-h1)].. this does not mean that the static pressure is actually due to gravity.. we just apply hydrostatic pressure principle to find out the actual static/total pressure

Most of the above is what i assume.. if wrong, feel free to point the mistakes