Upstream effect on fluid thread

In summary, the conversation discusses adjusting a tap to produce a specific size of water stream, which then breaks up into droplets and forms ripples. The question arises about the presence of ripples both above and below a hair that is touched to the stream. The topic is inspired by a question on a physics forum and the answer involves capillary-gravity waves and their behavior.
  • #1
Keith_McClary
752
1,501
I adjust my tap to produce a thread of water about 2 mm in diameter. It breaks up into droplets about 10 cm below the spout. I touch the thread near the spout with the tip of a single hair (from my head). I see ripples (standing, not travelling) on the thread 1-2 cm below the hair. The strange thing is, there are also ripples 1-2 cm ABOVE the hair.

I Googled up
Break-up dynamics of fluctuating liquid threads.

Just reading the abstract tells me that this is not simple.

Can the upstream ripples be explained on general principles without getting into these complications?

(This was inspired by a question by 23scurtu on http://.com/questions/200815/why-does-a-continuous-water-stream-form-ripples-when-colliding-with-a-surface/200943#200943 with videos of the effect. )
 
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  • #2
The group velocity of capillary-gravity waves exceed the phase velocity, so energy is transported 'upstream' as well as downstream.

http://math.mit.edu/classes/18.376/TermPapers/18376_Term_Paper_Fan.pdf
 

1. How does the upstream effect impact the fluid thread?

The upstream effect refers to the changes in fluid flow that occur due to disturbances upstream of a fluid thread. This can include changes in velocity, pressure, and direction, which can all affect the behavior of the fluid thread.

2. What factors influence the upstream effect on a fluid thread?

Several factors can influence the upstream effect on a fluid thread, such as the shape of the object creating the disturbance, the fluid viscosity, and the fluid's initial velocity. Other factors, such as temperature and surface tension, can also play a role.

3. How does the upstream effect impact the stability of a fluid thread?

The upstream effect can significantly impact the stability of a fluid thread. When disturbances upstream cause changes in the fluid flow, it can lead to instabilities such as breaking, coiling, or separation of the thread. The magnitude and direction of the upstream effect can determine the extent of these instabilities.

4. Can the upstream effect be controlled or manipulated?

Yes, the upstream effect can be controlled and manipulated by altering the conditions upstream of the fluid thread. This can be achieved by changing the shape or size of the object creating the disturbance, adjusting the fluid properties, or using external forces to influence the flow.

5. How is the upstream effect on a fluid thread studied in scientific research?

The upstream effect on a fluid thread is studied through experimental and theoretical research. Experimental studies involve creating disturbances upstream and observing their impact on the fluid thread. Theoretical studies use mathematical models and simulations to analyze the fluid flow and predict the behavior of the thread under different upstream conditions.

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