# Water drag lines - what's it called?

• B

## Main Question or Discussion Point

there's a stationary pebble on the flat shore of the beach. As the last of the wave heads back out to the ocean, it moves over the pebble and creates.. "lines" similar in shape to eg air stream around a jet, etc

Wondering what these 'waves' are called???

Related Other Physics Topics News on Phys.org
Bandersnatch

sophiecentaur
Gold Member
. . . . or bow wave from a moving boat. It's a wave that's continually being formed against the water flow and it falls away on either side, travelling at its wave speed and that forms a vee. Once formed, the wave can travel a long way because there are very few losses (apart from all the other stuff going on).

tech99
Gold Member
. . . . or bow wave from a moving boat. It's a wave that's continually being formed against the water flow and it falls away on either side, travelling at its wave speed and that forms a vee. Once formed, the wave can travel a long way because there are very few losses (apart from all the other stuff going on).
Yes. It is also a supersonic shock wave, not quite the same thing as, say, ordinary flow over a wing.

Gold Member
Streamlines
noun
A line along which the flow of a moving fluid is least turbulent.

From: https://en.oxforddictionaries.com/definition/streamline
I don't know who Oxford hired to write the definition for a streamline, but whoever it was clearly wasn't familiar with fluid mechanics. That is not how a streamline is defined.

A streamline is a line that is everywhere tangent to the velocity field.

. . . . or bow wave from a moving boat. It's a wave that's continually being formed against the water flow and it falls away on either side, travelling at its wave speed and that forms a vee. Once formed, the wave can travel a long way because there are very few losses (apart from all the other stuff going on).
Hmm... is it wake if the object is stationary and the water is moving around it??

Looking to name it in order to find an online model that shows the angles accurately.. in the 'wake' examples, there is usually a single 'V' with quite an acute angle, whereas with the wave and the pebble there are 2-3 'Vs' with increasingly obtuse angles...

sophiecentaur
Gold Member
Hmm... is it wake if the object is stationary and the water is moving around it??
Would you expect a different answer? The motion is relative, just the same.
Multiple wakes would perhaps not be surprising as the water velocity is different at different depths in shallow water. I can't think of an equivalent to the pebble situation for a boat. You could, perhaps observe the Vee formed by a pole in deep water.

Would you expect a different answer? The motion is relative, just the same.
Multiple wakes would perhaps not be surprising as the water velocity is different at different depths in shallow water. I can't think of an equivalent to the pebble situation for a boat. You could, perhaps observe the Vee formed by a pole in deep water.
It's not that I would expect a different answer, it's that I'm looking to find online a model or illustration of the rippling effect that happens on the water surface in that exact situation - a shallow wave moving over an object on a flat surface...

When I look for 'wake', 'streamline' etc online there's some great examples, but they don't look like what happens with the pebble...

A picture would help here.

sophiecentaur
Gold Member
but they don't look like what happens with the pebble...
I have to assume that's because there is a velocity gradient in the water around the pebble and not in a simplified model of a hull moving through deep water.
Also, in shallow water, surface waves are noticeably modified and they can break.
as @mfig says, a picture (with a comparison perhaps) would help. Is it the angle of the V that is different?

A picture would help here.
Can I post a pic from my device? I can only see the URL option...

berkeman
Mentor
Can I post a pic from my device? I can only see the URL option...
Yes, use the Upload button in the lower right corner of the Edit window to Upload a JPEG or PDF copy of your picture.

berkeman
Mentor
I don't know who Oxford hired to write the definition for a streamline, but whoever it was clearly wasn't familiar with fluid mechanics.
LOL

Mister T
Gold Member
Hmm... is it wake if the object is stationary and the water is moving around it??
I don't see why not. When I'm water skiing the boat that pulls me is at rest (relative to me) and the water is in motion. As long as the speed of the boat relative to the water exceeds the speed of the water waves, I'll get bow waves. Or shock waves, or a wake, or whatever you want to call it.

Looking to name it in order to find an online model that shows the angles accurately.. in the 'wake' examples, there is usually a single 'V' with quite an acute angle, whereas with the wave and the pebble there are 2-3 'Vs' with increasingly obtuse angles...
My guess is that any asymmetry in the rock's shape causes bow waves to be created by more than one spot on the rock. I believe that if you happened to leave the ladder hanging off the stern as you sped away in your motor boat, you'd see more than one vee trailing away in the downstream wake.

olivermsun