Increase velocity in tube = decrease in kinetic energy?

[GomerPyle]

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I remember reading in EK that an increase in fluid velocity decreases the random motion of fluid particles (and therefore kinetic energy). The decrease in kinetic energy following the equation KE = 3/2RT means that the temperature is also dropped. If temperature is dropped, than pressure is increased following PV=nRT. Is this the correct way in understanding fluid velocity and it's effects on temp and pressure?

 

[September24]

Okay. So this concept usually throws me and most people off. Someone please correct me if im wrong

The key thing you need to know is that you are dealing with FLUID velocity and not the randomized movement of particles which usually relate kinetic energy to. We need increase temperature, the KE of a particle increases using the 3/2 RT equation you provided. However, this energy adds to the random movement of particles and not a organized laminar flow that ideal fluids provide.

I like to think of it this way. Instead of random chaotic motion which have lots of energy, fluid flow is very linear and organized. The temperature and KE you think of adds to "randomized" velocity and in a sense, fluid velocity is...I guess....not random and chaotic.

So back to the equation. If fluid velocity increases, energy decreases, and so does temperature. According to the gas law, so should pressure. If all this doesn't work, use Bernoullis when thinking of fluids. When fluid velocity goes up, Bernoulli's says that pressure goes down. If pressure goes down, ideal gas law says that temperature should go down.

It's a confusing explanation but it helps me keep fluid velocity in track. I hope the MCAT doesn't try to trick us like that but I have a feeling that it will.

 

[GomerPyle]

Okay. So this concept usually throws me and most people off. Someone please correct me if im wrong

The key thing you need to know is that you are dealing with FLUID velocity and not the randomized movement of particles which usually relate kinetic energy to. We need increase temperature, the KE of a particle increases using the 3/2 RT equation you provided. However, this energy adds to the random movement of particles and not a organized laminar flow that ideal fluids provide.

I like to think of it this way. Instead of random chaotic motion which have lots of energy, fluid flow is very linear and organized. The temperature and KE you think of adds to "randomized" velocity and in a sense, fluid velocity is...I guess....not random and chaotic.

So back to the equation. If fluid velocity increases, energy decreases, and so does temperature. According to the gas law, so should pressure. If all this doesn't work, use Bernoullis when thinking of fluids. When fluid velocity goes up, Bernoulli's says that pressure goes down. If pressure goes down, ideal gas law says that temperature should go down.

It's a confusing explanation but it helps me keep fluid velocity in track. I hope the MCAT doesn't try to trick us like that but I have a feeling that it will.

Thanks, but this is basically the same thing I said, correct? Increase fluid velocity DECREASES kinetic energy which in turn decreases temp (KE = 3/2RT) which in turn decreases pressure (PV=nRT). Same thing with bernoullis equation. Increase velocity = decrease in pressure (K=P + .5pv^2 + pgh) and thus decrease in pressure = decrease in temp due to pv=nrt. Right?

Wait- you are saying KE=3/2RT doesnt apply to fluids since fluids are random motion of particles? So then which equation do you use to determine that decrease in KE decreases Temp? Just intuition?

 

[September24]

Yeah as stated, I'm not an expert so I'm hoping someone will confirm or deny what I said

Mainly for velocity, focus on intuition and bernoullis. What I'm trying to figure out if there are two "velocities" in physics. Random velocity that comes from Kinetic Energy and Temperature and fluid velocity that comes from laminar flow. In theory, the two should be inversely related. Bernoullis would apply to this fluid laminar flow while the 1/2mv^2 and 3/2RT^2 would apply to the random velocity. I think i'm actually going in circles haha.

 

[gettheleadout]

Starting from the beginning here, why would increasing fluid velocity (such as through narrowing of a tube) decrease fluid kinetic energy? For an ideal fluid Bernoulli's equation would show an increase in kinetic energy and a decrease in pressure for increasing fluid velocity through a narrowing tube at constant mean elevation.

 

[GomerPyle]

Starting from the beginning here, why would increasing fluid velocity (such as through narrowing of a tube) decrease fluid kinetic energy? For an ideal fluid Bernoulli's equation would show an increase in kinetic energy and a decrease in pressure for increasing fluid velocity through a narrowing tube at constant mean elevation.

I read from a prep book company that increasing the velocity in the tube decreases the random motion of fluid particles (thus the kinetic energy) and this decreases the temperature...thus this decreases pressure. We need some clarification here.....this is confusing the hell out of me.

 

[The Kraken]

Do you have an exact citation or at least quote exactly what they said?

 

[GomerPyle]

Do you have an exact citation or at least quote exactly what they said?

It's buried in my pile of books and I don't have time right now to go looking through it. I'll try to find it later in the week, but it said the increase of a fluid's velocity increases the laminar flow of the fluid, thus lowering the random movement of fluid, decreasing kinetic energy. This decreases temperature, and thus decreases pressure.

 

[gettheleadout]

See here: http://forums.studentdoctor.net/showthread.php?t=570173

 

[GomerPyle]

See here: http://forums.studentdoctor.net/showthread.php?t=570173

Okay thanks GTLO - that was also the exact EK problem I was referring too. I guess it was a misunderstanding where the fluid was thought to have been an ideal gas, and in that case it makes sense that the KE of random motion would decrease as the gas increases velocity.

So when a the diameter of a piece of tubing increases, the velocity thus decreases (continuity equation) and thus the kinetic density energy (.5pv^2) decreases, and according to bernoulli, the pressure increases (this is assuming ideal fluid where viscosity is not a factor). So if you increase diameter of a tube, the pressure inside of it increases, but that is not intuitive at all because if you vasoconstrict blood vessels, doesn't the decrease in area increase pressure rather than decrease?