How does water flow from a faucet?

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Discussion Overview

The discussion revolves around modeling the flow of water from a faucet, focusing on the physical behavior of the water stream, including phenomena such as necking and droplet formation. Participants explore both theoretical and practical aspects of fluid dynamics as applied to faucet simulations, with references to specific software tools used for modeling.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning
  • Experimental/applied

Main Points Raised

  • One participant asserts that water flow from a faucet necks down due to gravity increasing fluid velocity, but expresses difficulty in simulating this accurately.
  • Another participant challenges the initial assertion, noting that real water from faucets tends to form drops rather than maintaining a continuous stream, suggesting a distinction between theoretical models and actual behavior.
  • Concerns about pressure differences are raised, with one participant questioning whether outside pressure being greater than inside pressure contributes to necking.
  • Surface tension is highlighted as a significant factor in the behavior of the water stream, particularly when it breaks into drops.
  • Participants share experiences with simulation software, specifically Blender, discussing settings and results related to fluid dynamics and necking effects.
  • One participant mentions that their simulation results in a straight-down flow without necking, prompting further inquiries about modeling techniques and settings used by others.
  • Another participant provides insights into the baking time and resolution settings for their Blender simulations, indicating variability in results based on different configurations.
  • Examples of varying fluid viscosity and its effects on simulation outcomes are shared, with acknowledgment of the unpredictability of certain settings leading to unexpected results.

Areas of Agreement / Disagreement

Participants express differing views on the modeling of water flow, particularly regarding the factors influencing necking and droplet formation. There is no consensus on the best approach to accurately simulate these phenomena, indicating ongoing debate and exploration of the topic.

Contextual Notes

Limitations in the discussion include assumptions about pressure dynamics, the influence of surface tension, and the complexity of achieving realistic simulations in software. The discussion does not resolve the mathematical or physical principles underlying the observed behaviors.

Who May Find This Useful

Individuals interested in fluid dynamics, simulation software, and the physical behavior of liquids may find this discussion relevant, particularly those looking to model similar phenomena in their projects.

richengle
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water flow from a faucet will neck down its radius as it flows to the bottom of the sink. It is asserted that this is due to the fact that gravity increases the velocity of the fluid. This is shown at . I am trying to simulate this, and am not having much luck. shows this for a flat surface. Most normal faucets aren't like that. Has anyone seen a good simulation, with explanation of methods for a standard faucet, preferably with flow in the sink as well?
 
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What is described in the first video is a good physics problem to work through, but it does not represent the actual behavior of real water from real faucets. If you look at that, you will see that water rather quickly forms into drops.

So, do you want to describe the "toy" model or something more realistic. A more realistic model probably doesn't have a closed form solution.
 
id like to model water from a faucet. my simulations have it just running straight down, and not necking. it must neck because outside pressure is greater than inside pressure, no?
 
Don't forget surface tension. If the stream breaks into drops, surface tension probably dominates.
 
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richengle said:
id like to model water from a faucet. my simulations have it just running straight down, and not necking. it must neck because outside pressure is greater than inside pressure, no?
What are using to model it?

If I do a quick default setting fluid sim with Blender, I get necking:

tap_flow.png

This is one frame of the simulation.
 
Janus said:
What are using to model it?

If I do a quick default setting fluid sim with Blender, I get necking:

View attachment 276037
This is one frame of the simulation.
cool. blender with manta, right? how long of baking to make 5 sec of simulation?
 
richengle said:
id like to model water from a faucet. my simulations have it just running straight down, and not necking. it must neck because outside pressure is greater than inside pressure, no?

No. Pressure at both sides is equal (except for surface tension effect). The flow necks because it accelerates due to gravity and needs to confirm to the continuity equation (no water is lost).
 
richengle said:
cool. blender with manta, right? how long of baking to make 5 sec of simulation?
On my machine it takes about 3 min total bake time (data and mesh), without particles ( foam, spray or bubbles) for 5 sec. This is with a fluid resolution set at 96.*
This sim is about 3 1/3 sec long, and includes some bubbles and spray. Most of the time is taken in rendering the frames (Over an hour for 80 frames)
water_tap.gif

It is somewhat grainy as I converted it to a compressed GIF in order to get the file down to a size I could directly upload here.

*Your bake times may vary.
 
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Here are a couple of more examples produced by playing around with the viscosity of the the fluid.
thckfld1.gif


A bit thicker and with the "gravity" turned down a bit.
thkfld2.gif

That bit where it kind of hangs there and spurts out was not planned, but this kind of thing can happen with Blender, where a tweak in the settings can produce unintended results.
 

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