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Help understanding BASIC fluid dynamics behind faraday waves 
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#1
Dec911, 07:08 AM

P: 78

Dear Sir and Mam,
So recently, I have been doing an experiment with Faraday waves. I feel like there are some critical things I don't understand about fluid dynamics (they don't offer it as a course where I go to school), so I was wondering if anyone on pf was good with fluid dynamics and help me understand the qualitative side of Faraday Waves. In the experiment we vertically shake water with speakerlike device. At certain critical amplitude and frequency, we observe Faraday waves like in Fig. 1 below. On of the things that really mystify's me is difference the growth of Faraday waves vs. the decay of Faraday waves. For example, let say I introduce a constant sinusodial variation to the speaker. If I plot the acceleration of the speaker vs. the magnitude out put of the Faraday wave, I get something like Fig. 2. Here it easy to see how the onset of the Faraday is quiet sudden vs. the long decrease in Faraday waves. Can anyone explain qualitatively how or why an oscillation grow differently than its decay? I have heard some paper use the word "primary Faraday instability". Could it have anything to do with an instability? Fig. 1: The following youtube video is Fig 2: A picture of the Amplitude vs. acceleration graph. It shows how the growth and decay behavior of a Faraday Wave are different. 


#2
Dec911, 11:38 AM

PF Gold
P: 1,486

Well the whole thing has to do with an instability. The whole mechanism is nonlinear, so without going and doing the math behind it, I would say you are most likely dealing with a hysteresis effect.



#3
Dec1111, 11:02 PM

P: 101

If I had to give you a qualitative explanation without any prior experience with Faraday waves, I would say that during the growth that viscous forces keep the fluid stable, but then upon the sudden growth in amplitude the viscous forces are diffused and this leads to the hysteresis.
Essentially, fluid stays stable as long as it can, but then it becomes unstable and these instabilities "feed themselves." 


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