Experimental proof of Venturi Effect

[paradisePhysicist]

Define "perpendicular pressure." Also define "pressure in the forward direction." Given that pressure is a scalar quantity, I have no idea what you mean here.

Pressure equation is 2d (p=f/a) therefore it can have forward or perpendicular.

In the post I quoted there is a cone, nozzle where the water is flowing through, forward pressure I am referring to is the water streaming out the front of the cone, perpendicular is the pressure on the side walls of the cone.

 

[cjl]

Pressure is isotropic though - that is to say, it is the same value in all directions. Yes, the standard definition of pressure is f/a, and that area needs to be in a particular direction, but one of the fun things about (static) pressure is that it acts equally in every direction.

 

[paradisePhysicist]

Pressure is isotropic though - that is to say, it is the same value in all directions. Yes, the standard definition of pressure is f/a, and that area needs to be in a particular direction, but one of the fun things about (static) pressure is that it acts equally in every direction.

Hmm, NASA says that dynamic pressure is of a moving gas, or in this case I'd assume liquid such as water.

"we can define a pressure force to be equal to the pressure (force/area) times the surface area in a direction perpendicular to the surface. If a gas is static and not flowing, the measured pressure is the same in all directions. But if the gas is moving, the measured pressure depends on the direction of motion. This leads to the definition of the dynamic pressure. "
https://www.grc.nasa.gov/WWW/k-12/rocket/dynpress.html

I of course am not a seasoned expert and new to all this but this is just my take.

 

[boneh3ad]

Pressure equation is 2d (p=f/a) therefore it can have forward or perpendicular.

In the post I quoted there is a cone, nozzle where the water is flowing through, forward pressure I am referring to is the water streaming out the front of the cone, perpendicular is the pressure on the side walls of the cone.

Force has direction, but it does not inherit that direction from the pressure, but from the surface being acted on by pressure. In equation form,
\vec{F} = p\vec{A} = pA\hat{n}.
Pressure is a scalar. Force is a vector and, when related to pressure, comes from a scalar pressure acting on a surface that has direction.

Hmm, NASA says that dynamic pressure is of a moving gas, or in this case I'd assume liquid such as water.

"we can define a pressure force to be equal to the pressure (force/area) times the surface area in a direction perpendicular to the surface. If a gas is static and not flowing, the measured pressure is the same in all directions. But if the gas is moving, the measured pressure depends on the direction of motion. This leads to the definition of the dynamic pressure. "
https://www.grc.nasa.gov/WWW/k-12/rocket/dynpress.html

I of course am not a seasoned expert and new to all this but this is just my take.

Dynamic pressure ($q$) is effectively the difference between static (thermodynamic) pressure ($p$) and stagnation pressure ($p_0$). If a fluid is at rest, they are the same. If a fluid is moving, then it must be slowed down in order to reach stagnation, so $p$ rises and $p_0>p$. The difference between them in an incompressible flow is $q$ and is the kinetic energy per unit volume in the fluid, or
q = \frac{1}{2}\rho v^2.
This is Bernoulli's principle, $p_0 = p + q$.

 

[paradisePhysicist]

Force has direction, but it does not inherit that direction from the pressure, but from the surface being acted on by pressure. In equation form,
\vec{F} = p\vec{A} = pA\hat{n}.
Pressure is a scalar. Force is a vector and, when related to pressure, comes from a scalar pressure acting on a surface that has direction.Dynamic pressure ($q$) is effectively the difference between static (thermodynamic) pressure ($p$) and stagnation pressure ($p_0$). If a fluid is at rest, they are the same. If a fluid is moving, then it must be slowed down in order to reach stagnation, so $p$ rises and $p_0>p$. The difference between them in an incompressible flow is $q$ and is the kinetic energy per unit volume in the fluid, or
q = \frac{1}{2}\rho v^2.
This is Bernoulli's principle, $p_0 = p + q$.

Basically I was saying that, if there is a tube of flowing water, the front is going to have more force and thus more pressure. An example would be a flowing river, if you are standing at the front of the river you are going to have much more pressure to deal with, but if you are sitting on the river bank and put your foot in the river the water pressure (the perpendicular pressure) will be much less.

 

[boneh3ad]

Basically I was saying that, if there is a tube of flowing water, the front is going to have more force and thus more pressure. An example would be a flowing river, if you are standing at the front of the river you are going to have much more pressure to deal with, but if you are sitting on the river bank and put your foot in the river the water pressure (the perpendicular pressure) will be much less.

What you are describing is the concept of stagnation pressure. If a fluid (say, water) is flowing at a constant speed throughout, it has both static pressure and dynamic pressure. Static pressure is still a scalar and acts equally in all directions. If you were to non-intrusively measure the pressure in the flow, you would measure static pressure. It's the pressure felt by an object in the flow that is moving along with it and is how you would calculate the force on an immersed body.

Dynamic pressure is also a scalar quantity (it has the magnitude of velocity squared in it). This is the "extra" pressure you get when a fluid is moving, but the key here is that you never "feel" dynamic pressure.

What happens if you put an obstacle in the flow (such as yourself) is that the velocity of the fluid slows to 0 as it encounters you, which converts all of that dynamic pressure back into static pressure. At the very front end of you at the stagnation point, all dynamic pressure has been converted to static pressure. The static pressure at that point is therefore equal to the stagnation (or total) pressure, which is the constant in Bernoulli's principle. In essence, you still feel static pressure, but the static pressure has risen due to the change in flow conditions near your body.