## View Poll Results: Will the circles made by the rock thrown in the stream stay circles?

Voters
13. You may not vote on this poll
• Yes they will

7 53.85%
• No they won't

6 46.15%

# Thread: Rock thrown in water!

1. If you throw a rock in calm water, it will create concentric circles hat many of you will call waves.

Now let's say we have a little stream of water, will the circles created by the rock thrown in the water stay circles or will the change to an elliptic form

Footnote:

William, billco and river_rat, please, I know you can do this very easily (please PM me for the answer)

2.

3. Ummm uhhhhhhhhhhh dohhhh ughhh.... they'll get stretched out. The wave front will get elliptical.

4. Pretty good question LEAP.

Cheers,
william

5. i put yes, but really i think they wont stay circles or eliptical because they would be destroyed.

6. Ripples move out in circles BUT the water does NOT!. I voted circles, purely from first instinct.

7. I know LEAP requested that a certain three of us not answer this but I'll make a vote based only on observation. I fish quite a bit in the river so I'll go with circles.

cheers

8. In my sobre state of mind I'd say circles as well. Moving circles

9. Originally Posted by billiards
In my sobre state of mind I'd say circles as well. Moving circles
Not quite sure whether you mean 'sober' or 'sombre'.... 8)

10. Solution 1 - Laminar Flow:
Stream velocity varies with distance from the bank (lateral) and with water depth (vertical). Therefore the circles will become distorted. This will not be a distortion to ellipses, but rather to a concave/convex lens shape, with the convexity pointing downstream.
The magnitude of this distortion will depend upon the relative velocities of the outgoing ripples and the velocity profile across the stream.

Solution 2 - Turbulent Flow:
The ripples will be rapidly distorted in a random manner, then destroyed.

11. I would say circles that move with the flow of the water

12. Well, it is a good question, because most of you are wrong! :-D

Let's do a normal transformation:

t= x+2, y

Now that transformation does not specify wich point of a graph to transform... When not specified, points move all at the same time and move the same distance. Wich gives us a circle.

Flow does not select wich part of the water to move, so in the case of a translation (flow of water), all the points move at the same time, in the same direction and at the same distance.

I got a few more for you boys!

13. so whats my prize?

14. Originally Posted by river_rat
so whats my prize?
2nd prize you mean... 8)

15. I think you overlooked Ophiolites answer. The circles will remain circles only under the assumption of uniform speed. Given the fact, that no stream in the world shows perfectly uniform speed (not even close), you should have stated this assumption in your question. Otherwise, I'd have to agree with Ophiolite, and add one more thing: Water speed usually varies in streamwise direction as well, due to variations in waterbed slope and stream width. At a location where the water accelerates, you will see the circles stretched in streamwise direction. Where the water decelerates you'll see the circles compressed in the streamwise direction. Hence, they become elliptical, even without the boundary effects or turbulence that were mentioned. I understand you weren'y thinking of accelerating streams, but as that's a more natural condition, you simply have to state your model assumptions to get the answer you expect.

16. I white water paddle a lot and i can tell you that in those kinds of streams the ripples don't stay circles. So please tell us wether the current is strong or not.

17. In theory yes in practise never.

18. I would say the adjective "calm" in the original question excludes white water, but it doesn't exclude calmly accelerating streams or steady boundary layers. (Unless you consider white water calm.) The trouble is, that "calmness" is a very subjective quality. I think it was meant in the sense of "uniform speed". Right?

19. Again it depends what type of whitewater. Some streams I can consider wuite "calm".

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