TBR FL3 Q8- Hydraulic Press- Poiseuille vs Continuity/Bernouilli SO CONFUSED

[student20000]

Hey Everyone,
I did the TBR FL3, and Passage 2 with the hydraulic press is driving me crazy.
Here is part of the passage and Qs:THe thing on the left is the reservoir (label got cut off)

8.When the flow directions of both one-way valves are reversed while the press chamber is full and the press plate is raised, the press plate:

A. descends at a gradually decreasing rate.

B. descends at a gradually increasing rate.

C. descends at a constant rate.
D. Remains stationary

Ans: A

Q9.
As fluid flows through either of the one-way valves, it

passes through a narrowed pathway. What is true of

fluid passing through the valve?

I. The fluid travels at a faster average speed than it

does through the pipes connecting the chambers.

II. The fluid exerts a greater pressure against the

valve walls than it does against the walls of the

pipes connecting the chambers.

III. The volume flow rate through the valve is the

same as the volume flow rate through the pipes

connecting the chambers.

A. I only

B . I and III only

C. II and III only

D . I , II, and III

Answer: B. Which I got and agree with but I am confused about the comparison btw the method used to solve Q8 vs. Q9, as I describe below:

So, First question:
In Q8 the solution explanation cites poiseuille's law as a way to solve the question...saying based on poiseuille's law flow rate decreases when there is a decrease in the pressure differential. Which I understand. But why is there a decrease in the pressure in this case? I don't get that.
Second Question:
The main things that is confusing me and driving me crazy is that for Q9, the solutions explanation cites Bernoulli's equation and the continuity equation. But for Q8 they used Poiseuille's.
So what gives..?? Why are they using both poiseuille (which I thought was for non ideal fluids or comparing 2 different non connected pipes) and bernoulli/continuity which I thought was for ideal fluids and a single pipe. How can they use both in this case?? In the Hydraulic Press isn't it just one connected pipe..so why does poiseuille's even apply? How do we know when to use which?
THANK YOU SO MUCH TO ANYONE WHO HELPS!

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[student20000]

Can anyone help? Would really appreciate it!

 

[Cawolf]

I can't speak for the reasoning of the author but I can lend my opinion on the solutions.

For the first part there is initially a pressure differential as the height of the fluid in the press chamber would have to be higher than the reservoir for it to be full. The fluid will flow into the reservoir once it can - and the height differential will decrease - slowing the velocity. I would think an application of Bernoulli's equation is the best solution. This does show that the pressure differential decreases with time - making the velocity decrease with time (Pousille's).

It is hard to really answer your questions as I can't tell exactly what you are asking - it seems you got the answers right? There is more than one way to come to a solution. If you could organize your post a little better it would draw more response.

I am happy to help if you can rephrase a little clearer.

 

[student20000]

Hey Cawolf,
Thanks for your reply! Sorry about the lack of clarity. I think I'm just getting frustrated with this question.
Ok, so to clarify my main source of confusion:

I am confused about when to use Poiseuille's law vs. Bernoulli's equation/continuity equation. How do we know when to use which? Is the distinction that Poiseuille's law is used for non ideal fluids and Bernoulli/continuity for ideal? That's what I thought, but then this passage tripped me up b/c it uses Poiseuille's law to solve Q8 and Bernoulli/continuity equation to solve Q9. I attached the explanation for the solutions of Q8 and Q9 if that helps. You'll see that they use Poiseuille's law to Solve Q8 and Bernoulli/continuity to solve Q9.

So, I got Q8 wrong because I didn't use Poiseuille's law to reason it out, rather I used Bernoulli and continuity equations. Why is it wrong to use Bernoulli or the continuity equation for Q8 and assume that the flow rate doesn't change? If I assume the flow rate doesn't change like for ideal fluids in a single pipe, then I would get that question wrong (which I did because I chose c) descends at a constant rate because I thought flow rate was constant). Based on poiseuille's law flow rate changes when the pressure differential changes, but based on the continuity equation flow rate is constant --> So, it seems like poiseuille and continuity equation give different answers about flow rate.

So, to summarize, how do we know that for Q8 we use Poiseuille's law instead of continuity equation/Bernoulli?

I don't know if that's very clear. Thanks again!

 

[Cawolf]

The fluid laws are not as simple as you describe - they all have value and can explain different situations.

All equations can be applied because it seems to be an ideal fluid - did it state otherwise?

You seem to have the right intuition, but in order to compare two spots in this pipe - you need an equation that relates pressure to height - Bernoulli's.

How did you approach question 8 that you got it wrong? It seems that it is clear the press plate will have a higher fluid level under it and it will flow to the lower level (reservoir). As the height difference decreases, the pressure difference will decrease, and therefore the velocity will decrease (reasoned using Bernoulli's).

Intuitively you need to see that in this closed system - flow rate must be constant (which it seems you did).

Choice C for first part does not talk about flow rate - it talks about the velocity of the fluid. The flow rate will be the same throughout, but it will decrease until equilibrium is met.

I think you are focused on the equations - when really you need to see that without applying the concepts of energy from Bernoulli's equation - you can get nothing out of Pousillie's equation.