It appears to me that everything is periodic, so when you've solved the 10 Hz problem here's another one. When dry loose sand is blown over a wetter base it piles up in ridges with a wavelength of about 20 metres. What's going on to cause this?

Greyhounds cause it!

I dont think its harmonic wavelengths or rebounding air pressure.
I would think it starts on the first mound . As the air moves over the mound ( as on a sail ) the air speeds up , picking up sand behind it. When the air slows down the sand settles creating the next mound. And so on.
I would also think the mound spacing would change depending on wind speed , probably wider the windier .
Something also makes me think the mounds slowly move backwards.
Probably wrong .
Could be the dogs.

Its the wind

How can dry sand blow from the wet sea.

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Hardcarve1 said..
How can dry sand blow from the wet sea.

I think were talking cross shore wind not onshore.

Magic

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log man said..
Greyhounds cause it!

Its the bloody dogs

Aeolian sand deposited on existing beach cusps?

The arcuate shape of the aeolian sand also suggests to me the prevailing wind direction is toward the camera.

It's not the sand piling up, it's the sand being blown away towards Alaska that reveals ancient fossil crop signs !!

^^^^ Crop signs in the shape of dogs !

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Pugwash said..
Aeolian sand deposited on existing beach cusps?

The arcuate shape of the aeolian sand also suggests to me the prevailing wind direction is toward the camera.

Don't think there were pre-existing beach cusps. The ridges appear to all be slowly moving with the wind, in the same direction but just a little bit slower than the 11yo "greyhound".

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Imax1 said...

Hardcarve1 said..
How can dry sand blow from the wet sea.

I think were talking cross shore wind not onshore.

I challenge that assumption, their is some evidence close to the waters edge that sand is blowing slightly on shore. Also the hind quarters of the dog looks to be wind affected.

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Hardcarve1 said..

Imax1 said...

Hardcarve1 said..
How can dry sand blow from the wet sea.

I think were talking cross shore wind not onshore.

I challenge that assumption, their is some evidence close to the waters edge that sand is blowing slightly on shore. Also the hind quarters of the dog looks to be wind affected.

OK got the protractor out . 18 degrees onshore from the camera side.
As for the dog .....too much people food....its just gas .

There looks to be some kind of beach cusp - the water makes nicely-spaced embayments...

Is the wavelength affected by the grain size of the sand ?

Could you go to Bunnings, purchase some different sand and spread it about on the beach ?

may have something to do with boundary layers and turbulence, but I'm thinking about it, (could take a while, this old brain can't do too many thinks at once)

Id say it's that the amount of force to blow the sand is not linear. It starts blowing off the beach at say 18knots. And avg strong winds is about 25-30 knots. If it's like where u live if it's going to be windy it usually Ramos up pretty quickly to the avg 23 knits and stays there for ages. So that's the distance it blows when windy. Extra 5 knots might only get it a bit further. ?

So do you know anybody still smoking? some smoke at ground level could be quite instructive. I have a feeling the wind is doing circles. Typical turbulent scenario, gets slowed at ground level, faster layers just above ground start a revolving motion. I think when the air is rising it's at it's slowest, and will drop sand, when it's descending it's at it's fastest and will collect sand. Once you have an undulation, there will be a wind shadow on the leeward side, any air there will deposit any held sand. So the formation should slowly move downwind.

Now, if you want another one in similar vein. How about those strings of seaweed floating in a current, they're all fairly small pieces but somehow they've organised them selves into very long lines, that are quite often very narrow and well defined.

When the dry sand blows along the beach it is enough for a wavesail. Grovel.
Just
That's all we need to know

Is there any connection in how chop forms on water ? What's the mechanics in that ?

Chop starts from tubulence in the wind at the boundary layer that makes a small ripple. Once there is a ripple the wind going over the top of it speeds up and the static pressure drops which sucks the ripple up, eventually it becomes a wave. over simplified but thats the basic mechanism

I would hazard to guess its the same mechanism that causes other waves to form, I wonder even on bigger beaches if 'sets' begin to form in the sand...

I would think surf waves are a combo of wind and tide.

A bit of 'scientific american' article

"Ripples in sand, found on both beaches and dunes, are one of nature's most ubiquitous and spectacular examples of self-organization. They do not result from some predetermined pattern in the wind that is somehow impressed on the surface, but rather from the dynamics of individual grains in motion across the surface. They arise whenever wind blows strongly enough over a sand surface to entrain grains into the wind. The subsequent hopping and leaping of these grains is called saltation. Saltating grains travel elongated, asymmetric trajectories: Rising relatively steeply off the bed, their path is then stretched downwind as they are accelerated by drag forces. They impact the sand surface centimeters to tens of centimeters downwind, typically at a low angle, around 10 degrees. It is this beam of wind-accelerated grains impacting the sand surface at a low angle that is responsible for ripples."An artificially flattened sand surface will not remain flat for long. (Try it on the beach or on the upwind side of a dune and see for yourself.) Small irregular mottles in the sand surface, perhaps a couple centimeters in wavelength, rapidly arise and grow once the wind starts to blow hard enough to initiate saltation. They then slowly organize themselves into more regular waves whose low crests are aligned perpendicular to the wind direction and begin to march slowly downwind. Typical ripple spacing is about 10 centimeters, whereas the typical height of the crests above the troughs is a few millimeters. The pattern is never perfect, but instead the ripple crests occasionally split or terminate, generating a pattern that looks remarkably like one's fingerprint. In cross section, the ripples are asymmetric, having low-angle upwind (stoss) faces and steeper downwind (lee) faces. Interestingly, the larger grains tend to accumulate on the crests of the ripples, leaving the troughs enriched in smaller grains.

Here's the whole article:

www.scientificamerican.com/article/why-do-regular-wavelike-s/

There u go. Almost nothing the brains trust of seabreeze can't answer. Now if someone can just post up the answer to understanding women I'd be very grateful...

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desertyank said..

Here's the whole article:

www.scientificamerican.com/article/why-do-regular-wavelike-s/

That's a good find. Mostly he talks about ripples with a wavelength of a couple of cm. The formations we've got here are 10 metres or more. In a few places the ~cm ripples were superimposed on the surface of the soft sand accumulations. But maybe our much larger ripples do get a mention in that article right at the end.

"With dry sand, a strong wind begins to wipe out ripples and make larger and regular berms, spaced fairly evenly perpendicular to the wind. These are in reality large ripples. Continued, strong winds can change these rises into undulating dunes that often go far inland."

Another interesting statement from that article, which helps explain why he glossed over our case in that last paragraph.

"There are several possible mechanisms leading to the particular bedforms, and this is an area of legitimate scientific controversy. The most accepted explanation is that the flow of the overlying fluid (water or air) interacts with the moving sediment grains in a manner that results in a stable shape, or bedform."

I've read the early work by Bagnold that always gets quoted. His interesting observations are.

* When sand grains get picked up by the wind the momentum exchange slows down the boundary layer until it doesn't pick up any more. (Well what gets picked up is equalled by what gets stuck on landing)

* It takes several metres for wind suddenly striking loose sand for it to become saturated. i.e. There's no more wind blown sand moving off a 100 metre wide beach than off a 5 metre wide beach.

* Airborn sand will move quicker if it moves onto a harder surface, the particles bounce better.

So putting that all together to try and sort out why's happening in the photo....

The thickness of the sand accumulations is only a couple of cm. I don't think that's going to affect wind strength when spaced by 10 metres.
but it might be something to do with the boundary layer accelerating and decelerating due to the changing looseness of the sand?

Or maybe it's just something like traffic theory, the sand moves quickly in the gaps until it hits another clump?

I think it's still an area of "legitimate scientific controversy"

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myusernam said...
There u go. Almost nothing the brains trust of seabreeze can't answer. Now if someone can just post up the answer to understanding women I'd be very grateful...

I can try to explain by way of a story what women want...

King Arthur, Lancelot, and the Witch: Young King Arthur was ambushed and imprisoned by the monarch of a neighbouring kingdom. The monarch could have killed him but was moved by Arthur's youth and ideals. So, the monarch offered him his freedom, as long as he could answer a very difficult question. Arthur would have a year to figure out the answer and, if after a year, he still had no answer, he would be put to death. The question? What do women really want? Such a question would perplex even the most knowledgeable man, and to young Arthur, it seemed an impossible query. But, since it was better than death, he accepted the monarch's proposition to have an answer by years end. He returned to his kingdom and began to poll everyone: the princess, the priests, the wise men and even the court jester. He spoke with everyone, but no one could give him a satisfactory answer. Many people advised him to consult the old witch, for only she would have the answer. But the price would be high; as the witch was famous throughout the kingdom for the exorbitant prices she charged. The last day of the year arrived and Arthur had no choice but to talk to the witch. She agreed to answer the question, but he would have to agree to her price first. The old witch wanted to marry Sir Lancelot, the most noble of the Knights of the Round Table and Arthur's closest friend! Young Arthur was horrified. She was hunchbacked and hideous, had only one tooth, smelled like sewage, made obscene noises, etc. He had never encountered such a repugnant creature in all his life. He refused to force his friend to marry her and endure such a terrible burden; but Lancelot, learning of the proposal, spoke with Arthur - he said nothing was too big of a sacrifice compared to Arthur's life and the preservation of the Round Table. Hence, a wedding was proclaimed and the witch answered Arthur???s question thus: What a woman really wants, she answered???.is to be in charge of her own life. Everyone in the kingdom instantly knew that the witch had uttered a great truth and that Arthur's life would be spared. And so it was, the neighbouring monarch granted Arthur his freedom and Lancelot and the witch had a wonderful wedding. The honeymoon hour approached and Lancelot, steeling himself for a horrific experience, entered the bedroom. But, what a sight awaited him. The most beautiful woman he had ever seen lay before him on the bed. The astounded Lancelot asked what had happened ?- the beauty replied that since he had been so kind to her when she appeared as a witch, she would henceforth, be her horrible deformed self only half the time and the beautiful maiden the other half. Which would he prefer? Beautiful during the day or night? Lancelot pondered the predicament. During the day, a beautiful woman to show off to his friends, but at night, in the privacy of his castle, an old witch? Or, would he prefer having a hideous witch during the day, but by night, a beautiful woman for him to enjoy wondrous intimate moments? What would YOU do? What Lancelot chose is below. BUT make YOUR choice before you scroll down below. OKAY? Noble Lancelot said that he would allow HER to make the choice. Upon hearing this, she announced that she would be beautiful all the time because he had respected her enough to let her be in charge of her own life Now.what is the moral to this story...

It doesn't matter if a woman is beautiful or ugly... deep down she is still a witch...

Ps sorry for thread hijack...

Gravity and wind speed.

Wave lengths will vary depending on wind speed and type of sand.

Sine wave