by neufer » Thu Dec 01, 2011 2:43 pm
Only for the finest glass beads in air
http://adsabs.harvard.edu/abs/2010AGUFM.P53A1510D wrote:
<<Angular materials had time-averaged angles of about 40 deg and rounded materials about 25 deg for all g, except the finest glass beads in air, which is explained by static electricity. For all materials, the static angle of repose increases about 5 deg with decreasing g (from 1 to 0.1 g), whereas the dynamic angle decreases with about 10 deg. Consequently, the avalanche size increases with decreasing g.>>
JohnD wrote:
On the other hand, this theoretical study
http://ascelibrary.org/aso/resource/1/j ... horized=no found no gravity effect in a 2D model.
But this student group found a definite effect - "On Earth, that dust settles at almost a 45-degree angle," Martin said. "In lunar gravity, we were watching the drum and just seeing the angle go up, and up, and up, until it was almost at a 90-degree angle."
I don't think that many 20 kilometer high cliffs are made solely of
dust
http://asterisk.apod.com/viewtopic.php? ... 06#p104706
http://asterisk.apod.com/viewtopic.php? ... 41#p130741
JohnD wrote:
A seems logical that AoR would be changed by gravity. The angle of repose reflects a balance between the shear strength of the material and gravity pulling it downwards. At some angle, the downslope vector of gravity becomes less than the force necessary to pull the particles past each other and slippage stops. As shear strength weakens, the AoR will become less and less - wet, sticky sand piles higher than dry sand - and conversely as gravity weakens, a higher pile should be possible.
If wet, sticky sand is observed anywhere in the solar system
we will immediately send a Viking there to invade!
![Image](http://upload.wikimedia.org/wikipedia/commons/thumb/1/1b/Mars_Viking_11d128.png/320px-Mars_Viking_11d128.png)
http://www.tulane.edu/~sanelson/geol111/masswasting.htm wrote:
Mass-Wasting
Prof. Stephen A. Nelson Tulane University
<<Mass-wasting is the down-slope movement of Regolith (loose uncemented mixture of soil and rock particles that covers the Earth's surface) by the force of gravity
without the aid of a transporting medium such as water, ice, or wind. Still, as we shall see, water plays a key role.
In order for regolith to move in a mass wasting process it must be on a slope, since gravity will only cause motion if the material is on a slope. On a slope, the force of gravity can be resolved into two components: a component acting perpendicular to the slope, and a component acting tangential to the slope. The perpendicular component of gravity, g
p, helps to hold the object in place on the slope. The tangential component of gravity, g
t, causes a shear stress parallel to the slope and helps to move the object in the down-slope direction. On a steeper slope, the shear stress or tangential component of gravity, g
t, increases, and the perpendicular component of gravity, g
p, decreases. Another force resisting movement down the slope is grouped under the term shear strength and includes frictional resistance and cohesion among the particles that make up the object. Down-slope movement is favored by steeper slope angles (increasing the shear stress) and anything that reduces the shear strength (such as lowering the cohesion among the particles or lowering the frictional resistance.
Although water is not directly involved as the transporting medium in mass-wasting processes, it does play an important role. Think about building a sandcastle on the beach. If the sand is totally dry, it is impossible to build a pile of sand with a steep face like a castle wall. If the sand is somewhat wet, however, one can build a vertical wall. If the sand is too wet, then it flows like a fluid and cannot remain in position as a wall. Dry unconsolidated grains will form a pile with a slope angle determined by the angle of repose. The angle of repose is the steepest angle at which a pile of unconsolidated grains remains stable, and is controlled by the frictional contact between the grains. In general, for dry materials the angle of repose increases with increasing grain size, but usually lies between about 30 and 37º. Slightly wet unconsolidated materials exhibit a very high angle of repose because surface tension between the water and the grains tends to hold the grains in place. When the material becomes saturated with water, the angle of repose is reduced to very small values and the material tends to flow like a fluid. This is because the water gets between the grains and eliminates grain to grain frictional contact.>>
[quote="JohnD"]
de Vet et al, http://adsabs.harvard.edu/abs/2010AGUFM.P53A1510D found that,
"static angle of repose increases about 5 deg with decreasing g (from 1 to 0.1 g)"[/quote]
[c][size=125][color=#0000FF][u]Only[/u] for the finest glass beads in air :!: [/color][/size][/c]
[quote=" http://adsabs.harvard.edu/abs/2010AGUFM.P53A1510D"]
<<Angular materials had time-averaged angles of about 40 deg and rounded materials about 25 deg for all g, except the finest glass beads in air, which is explained by static electricity. For all materials, the static angle of repose increases about 5 deg with decreasing g (from 1 to 0.1 g), whereas the dynamic angle decreases with about 10 deg. Consequently, the avalanche size increases with decreasing g.>>[/quote]
[quote="JohnD"]
On the other hand, this theoretical study http://ascelibrary.org/aso/resource/1/jaeeez/v22/i1/p10_s1?isAuthorized=no found no gravity effect in a 2D model.
But this student group found a definite effect - "On Earth, that dust settles at almost a 45-degree angle," Martin said. "In lunar gravity, we were watching the drum and just seeing the angle go up, and up, and up, until it was almost at a 90-degree angle." [/quote]
I don't think that many 20 kilometer high cliffs are made solely of [b][u][color=#FF0000]dust[/color][/u][/b] :!:
http://asterisk.apod.com/viewtopic.php?f=9&t=16775&p=104706#p104706
http://asterisk.apod.com/viewtopic.php?f=9&t=20810&p=130741#p130741
[quote="JohnD"]
A seems logical that AoR would be changed by gravity. The angle of repose reflects a balance between the shear strength of the material and gravity pulling it downwards. At some angle, the downslope vector of gravity becomes less than the force necessary to pull the particles past each other and slippage stops. As shear strength weakens, the AoR will become less and less - wet, sticky sand piles higher than dry sand - and conversely as gravity weakens, a higher pile should be possible.[/quote]
[c][b][color=#0000FF]If wet, sticky sand is observed anywhere in the solar system
we will immediately send a Viking there to invade![/color][/b]
[img]http://upload.wikimedia.org/wikipedia/commons/thumb/1/1b/Mars_Viking_11d128.png/320px-Mars_Viking_11d128.png[/img][/c]
[quote=" http://www.tulane.edu/~sanelson/geol111/masswasting.htm"]
Mass-Wasting
Prof. Stephen A. Nelson Tulane University
[float=right][img3=""]http://www.tulane.edu/~sanelson/images/gravslope.gif[/img3]
[img3=""]http://www.tulane.edu/~sanelson/images/drysand.gif[/img3]
[img3=""]http://www.tulane.edu/~sanelson/images/wetsand.gif[/img3]
[img3=""]http://www.tulane.edu/~sanelson/images/slurry.gif[/img3][/float]
<<Mass-wasting is the down-slope movement of Regolith (loose uncemented mixture of soil and rock particles that covers the Earth's surface) by the force of gravity [b][u]without[/u][/b] the aid of a transporting medium such as water, ice, or wind. Still, as we shall see, water plays a key role.
In order for regolith to move in a mass wasting process it must be on a slope, since gravity will only cause motion if the material is on a slope. On a slope, the force of gravity can be resolved into two components: a component acting perpendicular to the slope, and a component acting tangential to the slope. The perpendicular component of gravity, g[sub]p[/sub], helps to hold the object in place on the slope. The tangential component of gravity, g[sub]t[/sub], causes a shear stress parallel to the slope and helps to move the object in the down-slope direction. On a steeper slope, the shear stress or tangential component of gravity, g[sub]t[/sub], increases, and the perpendicular component of gravity, g[sub]p[/sub], decreases. Another force resisting movement down the slope is grouped under the term shear strength and includes frictional resistance and cohesion among the particles that make up the object. Down-slope movement is favored by steeper slope angles (increasing the shear stress) and anything that reduces the shear strength (such as lowering the cohesion among the particles or lowering the frictional resistance.
Although water is not directly involved as the transporting medium in mass-wasting processes, it does play an important role. Think about building a sandcastle on the beach. If the sand is totally dry, it is impossible to build a pile of sand with a steep face like a castle wall. If the sand is somewhat wet, however, one can build a vertical wall. If the sand is too wet, then it flows like a fluid and cannot remain in position as a wall. Dry unconsolidated grains will form a pile with a slope angle determined by the angle of repose. The angle of repose is the steepest angle at which a pile of unconsolidated grains remains stable, and is controlled by the frictional contact between the grains. In general, for dry materials the angle of repose increases with increasing grain size, but usually lies between about 30 and 37º. Slightly wet unconsolidated materials exhibit a very high angle of repose because surface tension between the water and the grains tends to hold the grains in place. When the material becomes saturated with water, the angle of repose is reduced to very small values and the material tends to flow like a fluid. This is because the water gets between the grains and eliminates grain to grain frictional contact.>>[/quote]