And what country can preserve its liberties if their rulers are not warned from time to time that their people preserve the spirit of resistance? Let them take arms. The remedy is to set them right as to facts, pardon and pacify them. What signify a few lives lost in a century or two? The tree of liberty must be refreshed from time to time with the blood of patriots and tyrants.
Sunday, May 11, 2014
Fluid dynamics and dunes on Mars
Dunes are a fascinating interplay between fluid and granular flow. This satellite photo shows a dune field on Mars, Nili Patera. The dominant direction of wind flow is from the upper right, pushing the dunes themselves slowly toward the left. Many of the dunes along the edge arebarchans, crescent-shaped dunes with a long, gradual slope facing the wind and a steeper leeward side. As the wind blows, it erodes the sand on the windward slope and deposits it on the leeward side. This is how the dune migrates. Check out this close-up of a barchan to see the changes in its ripples and shape over the past couple months. (Photo credit: NASA/JPL/Univ. of Arizona)
Via FYFD
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