- #71
T C
- 320
- 8
In this video, the proof of Venturi Effect is much clear and ratio is far better.
I don't see anything in that video about the Venturi Effect or the area/velocity ratio, with the possible exception of an out-of-focus/unreadable CFD snapshot. What results, exactly are you referring to? What ratio are you seeing?T C said:In this video, the proof of Venturi Effect is much clear and ratio is far better.
Just look at the RPM with and without the flaps, that will be good example of velocity increase because RPM will increase in direct proportion to velocity. IMO, the best way get a proper venturi effect demonstration is using plastic or metal cones.russ_watters said:I don't see anything in that video about the Venturi Effect or the area/velocity ratio, with the possible exception of an out-of-focus/unreadable CFD snapshot. What results, exactly are you referring to? What ratio are you seeing?
It's just so disappointing that after all this effort trying to explain it to you that you think that has anything directly to do with the Venturi Effect. I feel like either you aren't trying at all or you are messing with us here.T C said:Just look at the RPM with and without the flaps, that will be good example of velocity increase because RPM will increase in direct proportion to velocity. IMO, the best way get a proper venturi effect demonstration is using plastic or metal cones.
In that case you should re-read this thread from the start and put some real effort into learning the concepts. And for experimenting yourself, I showed you what could be done with a bigger fan, two boxes, a roll of duct tape and virtually no money or effort. If you put in even a small amount of effort and money you could surely do vastly better.T C said:My main motto was to understand why my homemade experiment is unsuccessful and what is necessary to make it successful.
T C said:In case of a wind tunnel, IMO the main purpose is to create a flow at a specific speed. If a open air blower can generate more speed in comparison when fitted at the throat of a convergent nozzle shaped duct, why should one need that?
Conservation of energy, f=ma, etc.T C said:Is there any reason behind it? If so, what's that?
Can you explain how the position of the fan (inlet or exhaust) can affect such laws?russ_watters said:Conservation of energy, f=ma, etc.
It doesn't. The laws govern the flow and the addition of certain elements to the system affects the flow in keeping with the laws. Specifics on particular elements/fittings can be complicated, but much of that has already been discussed.T C said:Can you explain how the position of the fan (inlet or exhaust) can affect such laws?
Why?cjl said:Both a nozzle and a diffuser. Without the diffuser, the pressure requirement to drive the nozzle is much higher, dramatically reducing the flow.
Air doesn't like abrupt direction changes/transitions.T C said:Why?
While Russ's statement is true, I don't think it really explains this very well. This really comes from 3 factors. First of all, you have to understand what a nozzle and diffuser do. A nozzle exchanges pressure for velocity, a diffuser does the opposite. So, at the entry of a nozzle, the pressure will be high and velocity low, while at the exit, the pressure will be lower and the velocity higher. On a diffuser, the entry will be at relatively high velocity and low pressure, and then as it slows the flow down, the pressure increases such that the exit conditions are a higher pressure and lower velocity.T C said:Why?
Seems like it would gain pressure in the forward direction, but maybe lose perpendicular pressure.cjl said:However, now imagine a system with a nozzle and a diffuser. The air enters the nozzle, then loses pressure as it gains velocity.
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.paradisePhysicist said:Seems like it would gain pressure in the forward direction, but maybe lose perpendicular pressure.
Pressure equation is 2d (p=f/a) therefore it can have forward or perpendicular.boneh3ad said: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.
Hmm, NASA says that dynamic pressure is of a moving gas, or in this case I'd assume liquid such as water.cjl said: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.
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,paradisePhysicist said: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.
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, orparadisePhysicist said: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.
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 said: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,
[tex]\vec{F} = p\vec{A} = pA\hat{n}.[/tex]
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
[tex]q = \frac{1}{2}\rho v^2.[/tex]
This is Bernoulli's principle, ##p_0 = p + q##.
paradisePhysicist said: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.