flow rate vs. flow speed

[shanageena]

Hi everyone, I am confused about the difference between flow rate and flow speed. According to Bernoulli, the smaller the opening, the faster the velocity of the water in the hose. In Exam Krackers, they say, "thicker conduits allow for greater flow." This confuses me. Could someone please explain the difference between flow rate and flow speed and why it is better for a thicker conduit. Thank you so much!

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[mudphudwannabe]

Hi everyone, I am confused about the difference between flow rate and flow speed. According to Bernoulli, the smaller the opening, the faster the velocity of the water in the hose. In Exam Krackers, they say, "thicker conduits allow for greater flow." This confuses me. Could someone please explain the difference between flow rate and flow speed and why it is better for a thicker conduit. Thank you so much!

It's basically just a unit conversion thing. Flow speed would be the linear velocity of the flow (meters/sec). Flow rate is how much volume moves through (meters^3/sec). So, for a hose, the volume in has to be the same as the volume out -- so the volume flow rate will stay the same. But for a smaller opening, that same volume is pushed through a smaller area, so it has to go with a faster linear velocity to be the same flow rate. Let me see if I can find a good illustration of that somewhere... http://home.earthlink.net/~mmc1919/venturi.html Here's one, but it's probably not the best.

 

[OrthoFixation]

Without taking a look, I will comment without promise of accuracy.

Normally, flow rate is a volumetric measure. It can be measured in many units such as ft^3/s, gallons/min, ml/min, pounds/hr, or whatever.

Flow speed is normally related to velocity. Examples are m/s, ft/min, etc.

In Exam Krackers, they say, "thicker conduits allow for greater flow."

That's easy. A thicker conduit has a larger diameter and therefore a larger cross-sectional area. The flow increase with the square of the diameter, so small changes in diameter can represent large changes in flow capabilities.

My responses are conditioned by a engineering education and could differ slightly. Some references use different twists on the same theme, especially between biology and fluid dynamics.

I have the EK review books if you would like to cite the section and page.

 

[Quentin Quinn]

Hi everyone, I am confused about the difference between flow rate and flow speed. According to Bernoulli, the smaller the opening, the faster the velocity of the water in the hose. In Exam Krackers, they say, "thicker conduits allow for greater flow." This confuses me. Could someone please explain the difference between flow rate and flow speed and why it is better for a thicker conduit. Thank you so much!

Flow rate = number of particles or amount of liquid passing by a single point during a given time period (e.g. gallons per minute)

Flow speed = rate of travel of an individual particle (e.g. mph or fps)

A smaller opening increases flow speed -- think of putting your thumb over the end of a hose. However, in doing so, you severely constrict the number of particles that can make it through the opening, reducing the flow rate overall. Also, think of huge storm sewer pipes, which allow for high flow rate, but the water essentially travels in a trickle, since it isn't being "forced" through due to the massive opening.

That is just off the top of my head (not from a book), so hope it is moderately helpful!

 

[shanageena]

Thank you, mudphudwannabe, that was very, very helpful. I do have a follow-up question for you or anyone else. In Exam Krackers, it says, "flow rate depends upon the difference in a property of the reservoirs at either end of the conduit." I thought that flow rate was Av. What am I missing here? Thanks!

 

[shanageena]

Hi orthofixation, thanks for your response. The information I am struggling with is on page 46 of the inorganic chemisty Ek book. It is the sidenotes, written in orange on the left side of the page, and it is everything written under 3-3 Heat. I know they are trying to tie everything together, but I am having trouble understanding exactly what they are saying. Any help is appreciated! Thanks so much!

 

[OrthoFixation]

Thank you, mudphudwannabe, that was very, very helpful. I do have a follow-up question for you or anyone else. In Exam Krackers, it says, "flow rate depends upon the difference in a property of the reservoirs at either end of the conduit." I thought that flow rate was Av. What am I missing here? Thanks!

Most likely, they are referring to head pressure. A one foot column of water will exert a different pressure and therefore a different force upon the water below the column. Greater pressure will result in a greater flow rate in the same pipe.

 

[OrthoFixation]

Hi orthofixation, thanks for your response. The information I am struggling with is on page 46 of the inorganic chemisty Ek book. It is the sidenotes, written in orange on the left side of the page, and it is everything written under 3-3 Heat. I know they are trying to tie everything together, but I am having trouble understanding exactly what they are saying. Any help is appreciated! Thanks so much!

I found it. They are basically calling your attention to the fact that electricity (electron flow), fluids, and heat all respon in the same manner. The variable and names change, but the relationships are the same. My mind's default is fluid flow because it is easier for me to visualize. Heat is measured in Joules or Btu. But they are all the same.

 

[cfdavid]

Hi everyone, I am confused about the difference between flow rate and flow speed. According to Bernoulli, the smaller the opening, the faster the velocity of the water in the hose. In Exam Krackers, they say, "thicker conduits allow for greater flow." This confuses me. Could someone please explain the difference between flow rate and flow speed and why it is better for a thicker conduit. Thank you so much!

You may wish to revisit EK Physics chapter 5, page 84, where it discusses volumetric flow rate.

Equation Q=Av shows how velocity (v) and Flow Rate (Q) are related.

Of course velocity is typically measured in meters/sec. So, think of the Area in terms of meters squared. Thus, you will end up with Q, in cubic meters/sec.

(Actually fluid flow would be more typcially measured in cc/min or liters/min, but it can be anything. It's all relative)

So, on page 46 of EK gen. chem., the equation delta P=QR, basically is saying that the larger the "pressure drop" or differential across a section of tube or hose, the larger the flow.

Someone mentioned the hose analogy. When the hose nozzle is off, there is no pressure "drop". Thus, no flow. However, when you open the hose you expose the end of the hose to atmospheric pressure, where the pressure at the pump (from the water company) is pushing at around 70 psi. Thus the delta P is 70 psi, and you get that kind of flow.

Now, notice when you just open the nozzle valve up a little bit, so the orifice is very small. The water squirts out very fast (versus just an open ended hose with no nozzle- it just dumps out, but the actual flow is high).

The larger the pressure drop, the larger the flow.

In industry, a "poorman's" flow meter uses two pressure transducers across a section of pipe. If you know the resistance of the inside of the pipe over that length of pipe, between pressure transducers (sensors), you can determine the flow, by using change in P=Flow*Resistance (or Flow=change in pressure/Resistance).

I hope this helps you think of it in real world terms.

In reference to head pressure. Remember that the pressure of a fluid is due to the hieght of the fluid. So, a large fluid height (pressure), and a small open hole at the bottom of a jar of water, causes the water to shoot out. Thus, the larger h (see page 85 EK physics), the larger the pressure differential, and hence the larger the velocity of the fluid coming out.

 

[gujuDoc]

Wow how coincidental. I was just reviewing this chapter in our princeton review books with my roommate.

Flow speed is simply the rate of DISPLACEMENT (change in position)/TIME

Flow rate is the change in VOLUME per TIME

Since Flow rate (f) = Av, we can say that Area and Velocity are inversely proportional to each other and thus

velocity increases while area decreases, and velocity decreases as area increases.

 

[gujuDoc]

Actually I'm gonna add to what I said.

So basically flow rate is the change in amount over time as someone pointed out earlier.

For an ideal fluid flow, we see that flow rate is continous or constant because the liquid is incompressible (has constant density).

We also see that viscosity also known as friction is negligible or zero.

So we may recall that PV = Work or Energy.

We may also recall that total mechanical energy states that

E initial = E final

such that

KE initial + PE initial = Ke final + PE final.

Such that

.5mv^2 + mgh = -PV

If we divide both sides by V then we see that the m/V term is nothing more than density

so our eqn becomes .5 m/V v^2 + m/V g h = -P

So now we can add P to the other side of the eqn. to get

.5 m/V v^2 + m/V g h + P = 0

However since KEi + PEi = KEf + PEf

replace the other side of the eqn with P2 + .5m/V v^2 + m/V g h + P2