0
$\begingroup$

Why does water flowing (falling freely) from a tap become turbulent after some distance?

enter image description here

Radius at tap = 0.4 cm Distance from tap where turbulence start = 15 cm approx Radius where turbulence start is 0.1 cm approx. At this flow the tap fills 10 ml in 5 secs approx.

I tried using Reynolds number...but it is not in the turbulent region .

enter image description here

$\endgroup$
1
3
$\begingroup$

fluid jets which are laminar and of small diameter get stretched out into thinner jets as they fall, because the head end of the jet is moving faster than the tail which is still attached to the water source.

When the diameter of the jet becomes small enough, something called Rayleigh instability sets in, in which the jet spontaneously breaks up into individual droplets. All it takes is a very tiny perturbation of the jet to trigger this breakup, which results in the jet disintegrating into a falling stream of droplets with a certain size and spacing between successive droplets.

At no time in this process does the flow become turbulent.

$\endgroup$
2
$\begingroup$

The flow does not become turbulent. It is the interplay between gravity and surface tension that causes the break-up into droplets.

Due to gravity, the fluid will accelerate. We know from mass conservation that the cross-sectional area will diminish. You can also see this in the picture.

Surface tension keeps the water as one stream initially. However, further down the stream the surface tension force in the horizontal direction becomes strong, due to the smaller curvature. Surface tension in the vertical direction is much weaker, and hence the flow breaks up in droplets.

$\endgroup$
6
  • $\begingroup$ Can you please explain the last paragraph, the thing about horizontal surface tension and which curvature is being referred? $\endgroup$ – Navoneel Karmakar Jan 23 at 10:36
  • $\begingroup$ You can also see this in the picture. Blimey, your eyes must be so much better than mine. But the OP isn't asking about the break-up. He's asking about the flow becoming turbulent. $\endgroup$ – Gert Jan 23 at 10:39
  • 1
    $\begingroup$ @NavoneelKarmakar Maybe this link is useful: en.wikipedia.org/wiki/Fluid_thread_breakup So the surface tension in horizontal direction holds the stream together $\endgroup$ – Bernhard Jan 23 at 10:40
  • 1
    $\begingroup$ @Bernhard Yeah, I think the flow is breaking up rather than becoming turbulent. I was unaware of this concept and thought it was due to turbulence. $\endgroup$ – Navoneel Karmakar Jan 23 at 10:51
  • 1
    $\begingroup$ @Gert In the picture it is almost twice as wide near the exit than when it breaks up. You can observe this with any faucet. $\endgroup$ – Bernhard Jan 23 at 11:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.