Please register and log-in to create and edit pages

User Tools

    Please register and log-in to create and edit pages

Site Tools


Main Menu

Main menu
Click categories to expand

Other categories



Also see

Importance Ratingsplugin-autotooltip__plain plugin-autotooltip_bigImportance ratings

In a departure from the usual Wiki format, Wikenigma assigns 'Importance Ratings' to some pages.

The idea is to separate articles which are considered (by the editors) to cover exceptionally important unknown issues from those which (although also u…

Newsplugin-autotooltip__plain plugin-autotooltip_bigNews

[ newest at the top ]

• Oct 2020 : A milestone of 500 unknown articles has been reached.

• Aug 2020 : Currently (beta) testing the new 'WYSIWYG' (What You See Is What You Get) page editor. It greatly simplifies the editing process, avoiding the need to learn the…

Contactsplugin-autotooltip__plain plugin-autotooltip_bigContacts

Use this form to send a message to Wikenigma . . . [ * note: all fields must be completed ]

Legalplugin-autotooltip__plain plugin-autotooltip_bigLegal

Content in general

Please note that in common with other publicly editable wikis, this website is not responsible for content posted by the public. Nevertheless, the ongoing editing process should be able to remove unsuitable content in a reasonable time. If you…

Donate/Sponsorplugin-autotooltip__plain plugin-autotooltip_bigDonate to Wikenigma . . .

The best way to donate is by adding new original content !

That can either be by contributing to an existing article, or by creating a new page with an as-yet-unlisted 'Known Unknown'.

Any registered user can create content.

Registerplugin-autotooltip__plain plugin-autotooltip_bigRegistration

At present, only registered users can create and edit articles.

If you'd like to register as a contributor, please request a password.

When filling in the form, make sure you supply a valid e-mail address, as the login password will be sent to that addre…


Wikenigma supports:


Utilities
A-Z listingplugin-autotooltip__plain plugin-autotooltip_bigA-Z listing

This is an alphabetical index of all content pages.

rss / xml feed
sitemap file

Wikenigma - an Encyclopaedia of Unknowns Wikenigma - an Encyclopaedia of the Unknown Science

Sand dune formation

Large swaths of Earth’s surface are covered in loose sediment. The grains that make up this sediment form fascinating bedforms from meandering riverbeds to wavy dunes, whose shapes are constantly changing as water or air currents move the grains. The modeling of sediment transport has significantly advanced in recent years, but researchers lack a unifying framework to describe the transport of grains by different density fluids, like air and water.
Source: APS Physics 13, 62, 2020.

As well as the uncertainty surrounding the formation of dunes, the way in which they disperse and move relies on 'criticality' calculations which are as yet poorly defined with regard to sand grains.

“ […] exactly how and why dunes form in the way they do still eludes us. Now efforts to get to the bottom of this are taking on a new urgency, and not just because they could solve what Nathalie Vriend at the University of Cambridge explains is a “fundamental physics problem”.
New Scientist, Nov 2020

Also see Squeaky Sandplugin-autotooltip__plain plugin-autotooltip_bigSqueaky Sand

Sand on some beaches and dunes emits a distinct 'squeaking' sound when moved under pressure. Videos here

It only affects certain types of sand - more rounded and drier grains generally don't squeak (as much). The underlying acoustic mechanism is as yet un…
and Yardangsplugin-autotooltip__plain plugin-autotooltip_bigYardangs

Yardangs are enigmatic wind-parallel ridges found in arid environments. They often form downwind of rocks in sandy environments. They can be unusually stable structures, given their sandy / dusty composition, and the fact that the wind which forms them can al…


    Share this page :


DOKUWIKI IMPLEMENTATION DESIGN BY UNIV.ORG.UK JANUARY 2021