Student Doctor Network Forums Greater viscosity should decrease velocity, and in turn increase pressure?
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 05-08-2012, 03:20 AM #1 Senior Member   Status: Medical Student Join Date: Jul 2011 Posts: 315 Greater viscosity should decrease velocity, and in turn increase pressure? SDN Members don't see this ad. (About Ads) http://imgur.com/qb5QS,ScuHA In this problem, a more viscous liquid replaces a less viscous fluid that flows down through point a, and out point b. 1.) The problem notes that, because the pressure difference of the reservoir and at point b is the same, the more viscous liquid should travel slower according to the viscous fluid velocity equation. Viscous Travels Slower. 2.) The height of the second column depends on hydrostatic pressure from the pipe below. 3.) (I think) The pipe should have a "Greater Hydrostatic pressure" according to Bernoulli's equation, because the fluid is traveling slower. ---> So shouldn't the height of the second column be higher than before? The answer is C though, that the column height is unchanged. I am wondering why that is.
05-08-2012, 05:48 AM   #2
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Quote:
 Originally Posted by hellocubed http://imgur.com/qb5QS,ScuHA In this problem, a more viscous liquid replaces a less viscous fluid that flows down through point a, and out point b. 1.) The problem notes that, because the pressure difference of the reservoir and at point b is the same, the more viscous liquid should travel slower according to the viscous fluid velocity equation. Viscous Travels Slower. 2.) The height of the second column depends on hydrostatic pressure from the pipe below. 3.) (I think) The pipe should have a "Greater Hydrostatic pressure" according to Bernoulli's equation, because the fluid is traveling slower. ---> So shouldn't the height of the second column be higher than before? The answer is C though, that the column height is unchanged. I am wondering why that is.
Why is everyone on SDN hearing/seeing viscous and still thinking bernoulli's. Not calling you out... this has legit been the third thread in 24 hours that I've seen this and I want to help everyone out:

ANY MENTION OF BLOOD
ANY MENTION OF VISCOSITY.............--------------------> DO NOT THINK BERNOULLI's (you are falling into a trap)
ANY MENTION OF NOT BEING IDEAL FLUID

Hopefully that get's it through everyone's head . I don't want anyone making that error on the real deal because it is really really avoidable. It's the SOLE "trap" they lay EVERY TIME with viscous fluids and for some reason SDNers are falling in that trap a lot lately.

Anyway, back to the problem.

The pressure before the pipe and at end of the pipe in the ideal fluid is the same for the non-ideal case... rho g h + P_atm on left, P_atm on right.

This is our "voltage" no matter what. NOW for a viscous fluid, we essentially have two resistors on either side of the second vertical tube. The more viscous the fluid, the more resistivity those resistors have so imagine two 5 ohm resistors for less viscous fluid like water and two 100 ohm resistors for honey.

in order to drop the same voltage over either set of resistors what needs to happen?

The current in the highly resistive case must decrease (velocity decreases; flow rate decreases).
BUT if I took the voltage at that middle point, what is it? It's still the same in both cases... always. Why, the flow rate is changed equally over both because all that changed if Nu (viscosity) for BOTH.

Now replace any mention of voltage with pressure, replace any mention of current with flow rate or fluid velocity.

Last edited by EnginrTheFuture; 05-08-2012 at 11:39 AM.

 05-08-2012, 10:56 AM #3 Senior Member     Status: Pre-Medical Join Date: May 2008 Posts: 147 so viscosity affects flow rate but is indepent of the idea's we associate with bernoulii's velocity and pressure combinations? is that correct.
 05-08-2012, 11:25 AM #4 1K Member     Status: Pre-Medical Join Date: Dec 2009 Location: Where the rain grows Posts: 1,837 Pretty much. The ideas behind Bernoulli's law are still aplicable but since they're there is a pressure loss which is also dependent on the shape and size of the flow it becomes really complicated to qualify what the outcome will be, except the cases where the changes are in the same direction. For example, let's consider a pipe with a fixed cross-section which is put on an incline and has a flow of viscous fluid from the bottom to the top: the pressure at the top will be lower than the pressure at the bottom, since both the fact that the fluid is viscous and the increased elevation are creating a change in the same direction. If the flow was from the high to the low end of the pipe, it would be much more complicated to compare the two pressures: there will be increase of pressure because of the lower height being countered by decreased pressure due to viscosity. You'll have to carefully do the math to determine which one has larger magnitude. It gets even more complicated for the case where the cross-section of the pipe changes. I doubt that you'll have to deal with any of these cases on the MCAT. In that light, ETF's advice is quite sound: if there is viscosity in the problem, it is highly unlikely that it is related to application of Bernoulli's law.
 05-08-2012, 12:56 PM #5 SGU MS-2     Status: Medical Student Join Date: Jan 2010 Location: St. George's, Grenada Posts: 4,886 Yes, the "pathway of least resistance" should always be in your mind when thinking of pipes / circuits. Raynaud's number is also useful but might be too much to remember. __________________ You must learn from the mistakes of others. You can't possibly live long enough to make them all yourself.
05-09-2012, 03:46 AM   #6
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Join Date: Sep 2008
Posts: 238

Wow, this pressure stuff is really impossible to understand.

........... so
Quote:
 The ideas behind Bernoulli's law are still aplicable...but there is a pressure loss
...................... But if Bernoulli's laws are applicable, there should be a pressure INCREASE because flow velocity decreases.

........And then we're saying Bernoulli's laws are nonapplicable, so pressure should be the same................

So, Is Bernoulli's applicable or not? Does pressure decrease or increase, or remain the same?
Everyone is saying different things

Last edited by ipodtouch; 05-09-2012 at 04:00 AM.

05-09-2012, 04:24 AM   #7
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Join Date: Jul 2011
Posts: 315

Quote:
 ANY MENTION OF BLOOD ANY MENTION OF VISCOSITY.............--------------------> DO NOT THINK BERNOULLI's (you are falling into a trap) ANY MENTION OF NOT BEING IDEAL FLUID

Sorry i don't know what you guys mean.

Bernoulli's equation obviously has an effect on nonviscous fluids.
Wind traveling over a wing exhibits less pressure because it's traveling faster... a result of Bernoulli's.

I understand IF you are saying that "the effects of increasing viscosity has no effects via Bernoulli's" in the sense that fluid x will behave the same as a more viscous fluid y under identical conditions (even though it is faster/slower).

Even EK states literally, "nonideal fluids behave in the same ways as ideal fluids, just to a deviated extend."

 05-09-2012, 04:30 AM #8 Senior Member   Join Date: Sep 2008 Posts: 238 ...

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