We present numerical simulations of internal gravity waves (IGW) in a star with a convective core and extended radiative envelope. We report on amplitudes, spectra, dissipation and consequent angular momentum transport by such waves. We find that these waves are generated efficiently and transport angular momentum on short timescales over large distances. We show that, as in Earth's atmosphere, IGW drive equatorial flows which change magnitude and direction on short timescales. These results have profound consequences for the observational inferences of massive stars, as well as their long term angular momentum evolution. We suggest IGW angular momentum transport may explain many observational mysteries, such as: the misalignment of hot Jupiters around hot stars, the Be class of stars, Ni enrichment anomalies in massive stars and the non-synchronous orbits of interacting binaries.

Many processes in geophysical and industrial settings involve the flow of granular materials down a slope. In order to investigate the granular dynamics, we report a series of laboratory experiments conducted by releasing grains at a steady rate from a localized source on a rough inclined plane. Different types of dense granular flow are observed by varying the flow rate at the source and the slope of the inclined plane. The two cases of steady flow confined by levees and the flow of avalanches down the plane are examined. The width of the steady flow increases linearly with the prescribed flow rate, which does not appreciably affect the characteristic depth or surface velocity of the bulk flow. When the flow rate is just below that required for sustaining the steady flow, avalanches are triggered at regular intervals. The avalanches maintain their shape, size, and speed down the inclined plane. We propose a simple model of steady flow that is consistent with our observations and discuss the challenges associated with the theoretical treatment of avalanche dynamics.

Five avalanches were artificially released at the Vallée de la Sionne test site in the west of Switzerland on 3 February 2015 and recorded by the GEOphysical flow dynamics using pulsed Doppler radAR Mark 3 radar system. The radar beam penetrates the dilute powder cloud and measures reflections from the underlying denser avalanche features allowing the tracking of the flow at 111 Hz with 0.75 m downslope resolution. The data show that the avalanches contain many internal surges. The large or “major” surges originate from the secondary release of slabs. These slabs can each contain more mass than the initial release, and thus can greatly affect the flow dynamics, by unevenly distributing the mass. The small or “minor” surges appear to be a roll wave‐like instability, and these can greatly influence the front dynamics as they can repeatedly overtake the leading edge. We analyzed the friction acting on the fronts of minor surges using a Voellmy‐like, simple one‐dimensional model with frictional resistance and velocity‐squared drag. This model fits the data of the overall velocity, but it cannot capture the dynamics and especially the slowing of the minor surges, which requires dramatically varying effective friction. Our findings suggest that current avalanche models based on Voellmy‐like friction laws do not accurately describe the physics of the intermittent frontal region of large mixed avalanches. We suggest that these data can only be explained by changes in the snow surface, such as the entrainment of the upper snow layers and the smoothing by earlier flow fronts.

. Atmospheric water vapor is unlikely to condense on warm 62 slopes, while groundwater is unlikely to emerge on all sides of isolated peaks 11 . These 63 challenges suggest that we should consider alternative models for RSL. 64 65 Evidence for Granular Flow Processes 66We measured the terminal slopes of 151 RSL at ten well-studied sites (Table S1). 67The results (Fig. 1a) show that in nearly all cases the mean slope near the end of a linea is 68 between 28°-35°. This range matches that of slipfaces for active Martian and terrestrial 69 dunes 23 , interpreted as the range of critical angles where granular flows of sand can 70 terminate (often called the dynamic angle of repose), and is similar to earlier 71 measurements of overall RSL slopes 6, 11, 17, 24 . We avoided clear artifacts or interpolated 72 areas (Methods); the few points outside this slope range are likely due to artifacts in the 73 topographic data. RSL slopes (or fans) are straight to slightly concave (Fig. 1b, Fig. S1), 74 consistent with dry granular flows such as sand dune slipfaces, and unlike the strongly 75 concave slope profiles produced by repeated debris flows or fluvial gullies 25 . Figure S2 76 shows RSL on weakly concave slopes, beginning at >35° and terminating near 30°. 77The terminal slopes of RSL, identical to sand dunes, suggest that movement on 78 those slopes is by dry grainflows. Aqueous flows could occur on such slopes and small 79 volumes of liquid might only produce short lineae and prevent runout onto lower slopes. 80However, it is unlikely that water is only produced near the tops of slopes at these angles 81 or that, if so, it is never able to flow onto lower slopes. RSL at a single site in Eos 82Chasma with widely varying lengths all terminate on similar slopes (Fig. S2). It is 83 unlikely that liquid volume is the controlling variable-this would require the volume of 84 liquid to correspond to the length of slope available, producing more liquid on longer 85 slopes. (If RSL deposit material they could build their own slopes, but saturated flows 86 should be more mobile than dry sandflows.) We therefore consider the primary 87 mechanism of RSL motion to be dry granular flow. 88Flows on a dune slipface at 27ºN provide a useful comparison (Animation S1). The dune slipface setting suggests that they are dry grainflows, particularly since they 96 occur when aeolian transport is strongest (perihelion 26 ) but northern-hemisphere 97 temperatures are low and northern RSL are inactive 10 . We attribute the visibility of these 98 lineae to the presence of a small amount of dust on the surface, as shown by dust devil 99 tracks. The lineae are initially present at the same time as dust devil tracks, and both fade 100 seasonally although the lineae require longer to fade as the dust is removed or 101 redistributed. These tracks and lineae can fade much faster than crater blast zones or 102 slope streaks 27 because they involve only superficial dust on a low-albedo surface. A few 103 microns of dust can markedly brighten a dar...