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T. Bacon October 24, 2001 16:14

Fluid Dynamics
Forgive my novice intrusion into your forum, but my friends and I have a question that boggles our untrained minds. We need elucidation from experts.

I'll leave aside our digressions into the unsavory side effects associated with these types of episodes, and get to the question:

What is the cause of split-stream urination?

Originally, we thought Bernoulli might be able to shed some light on the question, but a literature search revealed discussions on the relationship between velocity and pressure that only seemed to explain to us why pissing works, and not why pissing sometimes goes wrong. Then, we dug a bit deeper and found out about something called "Navier-Stokes," which, frankly, was more than we could bear. So, we gave up our own hypothesizing and decided to turn to the pros.

Your advice on this pressing matter, and your contributions to our understanding of fluid dynamics, are greatly appreciated.

alex October 24, 2001 16:22

Re: Fluid Dynamics
Many commercially available codes should be ideally suited for providing highly accurate results for this very intellectually challenging problem. Just keep in mind that you should excercise great caution when applying appropriate boundary conditions.

T. Bacon October 24, 2001 16:26

Re: Fluid Dynamics
Commercially available? Is everything to be commodified?

What happened to the pursuit of science as a way to improve our understanding of the cosmos? Is altruism dead?

We depend upon your generosity.

javad October 24, 2001 17:28

Re: Fluid Dynamics

i have a projects in heat transfer.i must write a

numerical programfor this:

Txx+Tyy=0 T(x,0)=T(0,y)=0 T(1,y)=sin(pi*y) T(x,1)=sin(pi*x) where pi=3.14

any one can help me? thanks

Fred Uckfield October 24, 2001 17:33

Re: Fluid Dynamics
Stop taking the piss.

Altruism isn't dead.

Axel Rohde October 25, 2001 00:42

Re: Fluid Dynamics
A few years ago, I actually gave this phenomenon some thought, after seeing a similar demonstration with a garden hose in a fluid mechanics educational video. I believe the split stream urination can be explained as follows:

If there is enough pressure within your bladder, and if your urethra is unobstructed, the flow of urine within your urethra is turbulent, meaning that the amount of slow moving urine in the vicinity of the urethra wall is relatively small compared to the main stream. As this turbulent stream of urine exits the urethra, the small amount of slow moving fluid, which surrounds the main stream, will adhere to the main stream, and thus you will observe an unsplit stream of urine.

Should the flow of urine be somehow obstructed (e.g. not properly 'unzipped'), the flow within the urethra will be laminar. The velocity profile will be less pronounced, and the amount of slow moving fluid near the wall will be significantly larger compared to the faster moving core flow. As this laminar stream exits, the slower and the faster moving urine separate (gravity), and you will see a split stream.

On my next trip to the bathroom, I will try to find myself a cup and a stop-watch to measure the flow rate, estimate the diameter of my urethra, and hopefully I will come up with a critical Reynolds number at which laminar to turbulent transition occurs. I will let you know about my experimental findings... :)

Perhaps other forum users can participate, too, so we can get a good statistical sample, and perhaps even publish the result. I don't think a full-blown CFD analysis is necessary to get to the bottom of this...(lol)

By the way, did anyone ever solve the mystery of 'morning wood'? (Beavis & Butthead tried once)

Govinder Patel October 25, 2001 00:56

Re: Fluid Dynamics
Don't tell me boss you never once in your life had a split-stream pee!

Govinder Patel October 25, 2001 01:10

Re: Fluid Dynamics
And what pray tell does this project involve?

Fred Uckfield October 25, 2001 02:04

Re: Fluid Dynamics
To ensure that one doesn't 'spray' everywhere (an evolutionary difficiency, obviously) the flow is swirled on development, prior to exit, much like a bullet in a barrel of a gun. I suppose splitting could occur based on some relationship between streamwise and centrifugal momentum.

Yep, you'd certainly need a commercial code with full MCAD integration capability (anyone got a detailed STEP rep of male genitalia, anyone's genitalia been 3D scanned inside and out??). Hey, TexacoBacon, you volunteering ;)


T. Bacon October 25, 2001 12:04

Re: Fluid Dynamics
Mr. Rohde,

My friends and I are heartened by your sense of camaraderie. You give fluid dynamics a good name.

Your insights lead me to paraphrase the British physicist Horace Lamb: when I die and go to heaven there are two matters on which I hope for enlightenment. One is the phenomenon of morning wood, and the other is the turbulent motion of fluids. And about the latter I am rather optimistic.

Warmest regards,

Texaco Bacon

User October 25, 2001 13:53

Re: Fluid Dynamics
Axel your correct. This was actually one of my undergraduate fluid mechanic laboratory tests. (although no references to bodily functions was used)

Also, the paraphrase is an urban myth. So far, I have heard this story with ten different scientists. (although the morning wood is new to me).

Axel Rohde October 25, 2001 17:52

Re: Fluid Dynamics
Thank you! It was my pleasure.

S.P.Asok November 3, 2001 00:50

Re: Fluid Dynamics
Morning wood?What is that?If it is something which can not be discussed here,this query can be neglected.Regards

Axel Rohde November 3, 2001 18:21

Re: Fluid Dynamics
Next time you are at a video store, try to find the Beavis & Butthead video which contains the episode "The Mystery of Morning Wood". You know Beavis & Butthead, the two cartoon characters from MTV (Music Television)? After you watched their 'science experiment' you will know all about it... :)

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