Another mystery for Decrepit to solve.

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?

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.

How can dry sand blow from the wet sea.

Magic

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.

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

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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.

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

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)

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.

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

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...

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/

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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...

Sine wave