Understanding Bernoulli's Law: Garden Hose Flow, Outlet Squeezing, Pressure & Velocity Changes

Hello.

According to Bernoulli's "principle of flow", in the case of a garden hose, reducing the flow (squeezing) of the outlet causes the water velocity to increase and the stream to reach further. The cross-section of the hose decreases, the pressure also decreases, and thus the speed of the water stream increases, because it is known that the volume of flowing water per unit of time at this end will not change.
However, if we squeeze the hose not at the tip, but somewhere earlier, thus reducing the diameter of the hose, we observe what? Most likely, the pressure drop also at the outlet, and after our "squeezed" hose we already have a normal diameter of the hose, i.e. the same as before the compression. The constriction should not affect the pressure drop behind it, much less at the end of the hose.
But why do we observe a drop in pressure?

__________......................____________
...................\__________/................
p1.................____p2___ ........p3.......
__________/.................\____________

Fig.1 Garden hose with narrowing solid line


It should be, p1 = p3 > p2

link

Moderated By _PREDATOR_:

Pointless posting of links that explain nothing, on a topic you have no idea about.
https://www.elektroda.pl/rtvforum/topic1821801.html#8740625

Well, that's what I described in theory. But why not in the garden hose. Squeeze the hose somewhere in the middle and see if you still have the same pressure at the outlet

link

Moderated By _PREDATOR_:

Pointless posting of links that explain nothing, on a topic you have no idea about.
https://www.elektroda.pl/rtvforum/topic1821801.html#8740625

The Bernoulli equation is for ideal fluids and stationary flows. Real water is compressible and viscous, and the flow can be non-laminar and therefore non-stationary.

The hose has its hydraulic resistance, the narrowing on the hose increases it. The flow of fluid through a resistance causes a pressure drop across that resistance. The analogies between electronic and hydraulic systems are very cool. Current - flow rate, voltage - differential pressure, resistors, capacitors, coils and others are also there. the equivalent of Ohm's law has, unfortunately, minor problems with flow laminarity. Compressibility can sometimes be a problem too, but rather a minor one.

If you put a resistor in series with the output of the power supply, unless you connect the load, the voltage at the output will be the same as without the resistor. It would be the same with the snake. Only when you draw some current, then the voltage will drop. Same in the hose.

mariuszspa wrote:

link

This is about the difference between the pressures in the stenosis and before and after the stenosis, not the difference between the pressure before the stenosis and the pressure after the stenosis.

Taenia_Saginata wrote:

The Bernoulli equation is for ideal fluids and stationary flows. Real water is compressible and viscous, and the flow can be non-laminar and therefore non-stationary.

Thanks ;)

Taenia_Saginata wrote:

The hose has its hydraulic resistance, the narrowing on the hose increases it.

So the aquarium hose has a different resistance. We also have such a narrowing there, because thanks to this faster flow of liquid, we have the effect of sucking air into the water and thus oxygenating it. But after the constriction, there is no noticeable pressure drop than before the constriction. Or is this decline hard to notice?

If we are talking about the flow, it is because the source, i.e. the tap, would have to increase the pressure to overcome the resistance of the orifice.
And the faucet doesn't.
In addition, by squeezing the hose at the end, we reduce the diameter of the outlet and increase the speed of the liquid.

romoo wrote:

If we are talking about the flow, it is because the source, i.e. the tap, would have to increase the pressure to overcome the resistance of the orifice.

What matters is what the source is. Is the fluid compressible or not, viscous or not, and what is the very nature of the flow, i.e. laminarity.
Under the assumptions of pr.Bernoulli, the tap does not have to increase the intensity, because the orifice is overcome by the "faster" flow of liquid, i.e. the sum total, some assumed volume per unit of time is constant. Before the constriction, higher pressure, low velocity, a certain volume per unit of time (some source efficiency) and on the orifice, lower pressure, higher velocity, and thus the same volume of liquid per unit of time.

For conditions deviating from the assumptions, it is just different.

And now it's even different
We have a compressible liquid. She hits a narrowing. Before the constriction, the volume of "pushed liquid" per unit time will be less than at the source, because of the compressibility. The source does a definite work of displacing this particular volume of liquid. This source has a certain capacity. So when liquid compressibility is added, some distance traveled through the initial surface limiting our volume to the final surface will be smaller due to the fact that compressibility is added, which also requires some energy expenditure from the source.
The source is working... the liquid is getting more and more compressed, because we have resistance on the orifice. This results in a situation after some time when all the liquid coming from the source is no longer able to be compressed. But at the expense of source work/efficiency. What I mean is that we eliminated the compressibility of the liquid so that the conditions would be closer to the assumptions of the Bernoulli equation and so that they could be applied. There is of course the issue of laminarity.
In any case, the source would work under a significant load to eliminate the compressibility of the liquid, so that in the area from the source to the venturi we already have a constant elevated pressure. So Bernoulli's formula would have a greater chance of success and application, apart from the already mentioned laminarity. Good reasoning??

simon932 wrote:

What matters is what the source is. Is the fluid compressible or not, viscous or not, and what is the very nature of the flow, i.e. laminarity.
Under Bernoulli's assumptions, the faucet does not have to increase the intensity, because the orifice is overcome by the "faster" liquid flow, i.e. the sum total, some assumed volume per unit of time is constant. Before the narrowing, higher pressure, low velocity, a certain volume per unit of time (some kind of source efficiency) and on the orifice, lower pressure, higher velocity, and thus the same volume of liquid per unit of time.

For an ideal liquid, such a hose connected to the tap behaves senselessly. Like an ideal current source shorted to ground. At the outlet of the hose, the water is at atmospheric pressure. If there is no loss of pressure on the hose, it turns out that the pressure in the tap is also zero.

Quote:

And now it's even different
We have a compressible liquid. She hits a narrowing. Before the constriction, the volume of "pushed liquid" per unit time will be less than at the source, because compressibility is added to it. The source does a definite work of displacing this particular volume of liquid. This source has a certain capacity. So when liquid compressibility is added, some distance traveled through the initial surface limiting our volume to the final surface will be smaller due to the fact that compressibility is added, which also requires some energy expenditure from the source.
The source is working... the liquid is getting more and more compressed, because we have resistance on the orifice. This results in a situation after some time when all the liquid coming from the source is no longer able to be compressed. But at the cost of source work/efficiency. What I mean is that we eliminated the compressibility of the liquid so that the conditions would approximate the assumptions of the Bernoulli equation and be applicable. There is of course the issue of laminarity.
In any case, the source would work under a significant load to eliminate the compressibility of the liquid, so that in the area from the source to the venturi we already have a constant elevated pressure. So Bernoulli's formula would have a greater chance of success and application, apart from the already mentioned laminarity. Good reasoning??

Compressibility can most often be ignored. The bigger problem is viscosity.

Taenia_Saginata wrote:

For an ideal liquid, such a hose connected to the tap behaves senselessly.

I will write it in Polish faucet not in the sense of a physical faucet but as a liquid source Sorry for writing so confusingly.

Taenia_Saginata wrote:

Compressibility can most often be ignored. The bigger problem is viscosity.

Viscosity yes, because then we will certainly have a turbulent nature of the flow. However, compressibility, after all, affects the increase in pressure in the system. I wonder what the relationship is between one and the other?

simon932 wrote:

I will write it in Polish faucet not in the sense of a physical faucet but as a liquid source Sorry for writing so confusingly.

It doesn't change anything. Further, if the hose has zero resistance, the output must have the same pressure as the input, and the output is 0.

Quote:

Viscosity yes, because then we will certainly have a turbulent nature of the flow. However, compressibility, after all, affects the increase in pressure in the system. I wonder what the relationship is between one and the other?

It won't necessarily be turbulent. You need to check the Reynolds number for specific conditions. How compressibility affects pressure is hard to say. At 100 atmospheres, water has about 0.4% less volume than at atmospheric pressure, so the effect will be small.

APPROX. And why do you assume that the liquid at the exit is at atmospheric pressure?

simon932
The law applies if

the liquid is incompressible
the liquid is not sticky
the flow is stationary and irrotational

The viscosity of the water and the laminar flow cannot be neglected in the hose.

Wogule what an idea to the department -- on the border of science.
Such a reducer can be a turbine blade and a hose adit and what huge energy losses are happening there to the advantage because the energy loss of the water column is replaced by another.

simon932 wrote:

APPROX. And why do you assume that the liquid at the exit is at atmospheric pressure?

Because on the one hand it touches the air and this air can only press against the water with atmospheric pressure. On the other hand, this water is pushed by the water deeper in the hose, but nothing of the first water on the other side is holding it so that this pushing could increase its pressure.

It's the same as if you had a block lying on something horizontal without friction. If you push it on the one hand and on the other hand you don't hold it with anything, you won't be able to press it with a force greater than 0 at a constant speed.

Quote:

Such a reducer can be a turbine blade and a hose adit and what huge energy losses are happening there to the advantage because the energy loss of the water column is replaced by another.

It's actually more of an inductance than a resistance. The ideal turbine does not waste energy, but converts it into mechanical energy. If the flow suddenly stops, such a turbine, like a coil, will try to maintain it by increasing the pressure.

simon932 wrote:

Hello.

According to Bernoulli's "principle of flow", in the case of a garden hose, reducing the flow (squeezing) of the outlet causes the water velocity to increase and the stream to reach further. The cross-section of the hose decreases, the pressure also decreases, and thus the speed of the water stream increases, because it is known that the volume of flowing water per unit of time at this end will not change.
However, if we squeeze the hose not at the tip, but somewhere earlier, thus reducing the diameter of the hose, we observe what? Most likely, the pressure drop also at the outlet, and after our "squeezed" hose we already have a normal diameter of the hose, i.e. the same as before the compression. The constriction should not affect the pressure drop behind it, much less at the end of the hose.
But why do we observe a drop in pressure?

__________......................____________
...................\__________/................
p1.................____p2___ ........p3.......
__________/.................\____________

Fig.1 Garden hose with narrowing solid line


It should be, p1 = p3 > p2

This phenomenon can be more easily explained using the analogy of an electrical circuit consisting of a voltage source and three resistances connected in series with it. These resistances are described in the figure p1 (flow resistance from the pump to the place of hose compression), p2 (flow resistance of the compressed part of the hose), and p3 (flow resistance of the remaining section of the hose). At the input we have a constant pressure generated by the pump in the pumping station (voltage of the power source). The voltage drops will correspond to the pressure losses on individual sections of this circuit. Now it will be easy to see that by squeezing the hose somewhere (increasing the flow resistance) we reduce both the amount of flowing water (lower current in the circuit) and the pressure behind the place of compression (voltage at the last resistance p3).

Hello.
I have a question close to this topic, but a slightly different problem. Builds a digital flow recorder. I have flowmeters with a flow of 2100L/h DN15 or ½ if you prefer. I have a source with a capacity of about 5000L/h 2.3bar on a PE32 pipe and I do not want to limit this capacity. Assuming a DN15 flowmeter will definitely limit the flow and here's the question? if I connect 2 flowmeters in parallel to the source, will I get a flow of 2100L*2=4200L at the output? Assuming three flowmeters in parallel, that would be 6300L of max flow by putting it back into one pipe?
The AVR will count the pulses from the flowmeters and add up the result. Is this correct thinking and won't the result be wrong?

The data is very modest

Specification:
Threads: 1/2" male
Material: acetal.
Measurement without reducer: from 60l/h to 2100l/h 1 liter equals 492 pulses
Measurement with a reducer from 30l/h to 600l/h 1 liter is 1200 pulses




Theoretically, the capacity it needs from the source is 20-45L/min, it needs to be applied to the chemical dispenser and mixer.

Probably correct, only close to the maximum flow on such a flowmeter you can have a significant pressure drop. Show me a catalog note for it.

As for the last issue, you can assume the flow through the calibrated cross-section and use a parallel measurement. Of course, it is necessary to calibrate the measuring system.

mczapski wrote:

As for the last issue, you can assume the flow through the calibrated cross-section and use a parallel measurement. Of course, it is necessary to calibrate the measuring system.

That's very interesting, can you elaborate more on that?
how to do it in practice and what impact it has on the flow, pressure.

One more question is looking for a maximally cheap pressure transmitter for aggressive and non-aggressive liquids.

mczapski wrote:

As for the last issue, you can assume the flow through the calibrated cross-section and use a parallel measurement.

By this you mean putting a pipe with a similar flow in parallel and recalibrating the flowmeter. It's a thought, but I'm wondering if at any pressure and flow of such a configuration there will be a correct reading. I assume that the flow will be, for example, 11L/min, won't the water flow faster where it has less resistance?

I'm reviving the topic with a probably trivial, but not quite a question for me (I've always been weak in physics )
Well, does the length of the hose affect the pressure with which water flows out of it? And another question, does the diameter of the hose matter with the pressure of the water flowing out of it?
The questions arose during today's car wash, I connected a longer hose with a smaller diameter to the tap than before and the water pressure was lower, I began to wonder if it was the fault of the hose, tap or the tip attached to the end of the hose

Both factors have an impact. The longer the hose, the lower the outlet pressure. The smaller the diameter of the hose, the lower the outlet pressure will be.