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Synchrotron emission is commonly found in relativistic jets from active galactic nuclei (AGNs) and microquasars, but so far its presence in jets from young stellar objects (YSOs) has not been proved. Here, we present evidence of polarized synchrotron emission arising from the jet of a YSO. The apparent magnetic field, with strength of ~0.2 milligauss, is parallel to the jet axis, and the polarization degree increases toward the jet edges, as expected for a confining helical magnetic field configuration. These characteristics are similar to those found in AGN jets, hinting at a common origin of all astrophysical jets. 10.1126/science.1195589

This note presents the results of an experimental study of circular sand jets in air from three nozzles of diameter of 19.2, 31.1 and 63.8 mm. It was found that the frontal speeds of the sand jets and the steady sand jet velocity accelerate due to gravity with negligible air resistance. The sand velocity does not appear to be affected by sand particle sizes for the three sizes tested. The diameters of sand jets, as they travel downwards, decrease and gradually approach an asymptotic value after a distance of 120 times of the initial jet diameter. The sand concentration in the jet decreases as the distance from the nozzle increases. Waves were observed at the periphery of the sand jet and some preliminary results of wave speed and wavelength are reported. DOI: 10.1061/(ASCE)EM.1943-7889.0000161 CE Database subject headings: Sand, material; Velocity; Particle size. Author keywords: Sand jet; Air.

The coalescence of supermassive black holes--a natural outcome when galaxies merge--should produce gravitational waves and would likely be associated with energetic electromagnetic events. We have studied the coalescence of such binary black holes within an external magnetic field produced by the expected circumbinary disk surrounding them. Solving the Einstein equations to describe black holes interacting with surrounding plasma, we present numerical evidence for possible jets driven by these systems. Extending the process described by Blandford and Znajek for a single, spinning black hole, the picture that emerges suggests that the electromagnetic field extracts energy from the orbiting black holes, which ultimately merge and settle into the standard Blandford-Znajek scenario. Emissions along these jets could potentially be observable at large distances. 10.1126/science.1191766

Coherent jets, such as the Jovian banded winds, are a prominent feature of rotating turbulence. Shallow-water turbulence models capture the essential mechanism of jet formation, which is systematic eddy momentum flux directed up the mean velocity gradient. Understanding how this systematic eddy flux convergence is maintained and how the mean zonal flow and the eddy field mutually adjust to produce the observed jet structure constitutes a fundamental theoretical problem. In this work a shallow-water equatorial beta-plane model implementation of stochastic structural stability theory (SSST) is used to study the mechanism of zonal jet formation. In SSST a stochastic model for the ensemble-mean turbulent eddy fluxes is coupled with an equation for the mean jet dynamics to produce a nonlinear model of the mutual adjustment between the field of turbulent eddies and the zonal jets. In weak turbulence, and for parameters appropriate to Jupiter, both prograde and retrograde equatorial jets are found to be stable solutions of the SSST system, but only the prograde equatorial jet remains stable in strong turbulence. In addition to the equatorial jet, multiple midlatitude zonal jets are also maintained in these stable SSST equilibria. These midlatitude jets have structure and spacing in agreement with observed zonal jets and exhibit the observed robust reversals in sign of both absolute and potential vorticity gradient.

Mechanisms leading a synoptic surface cyclone to cross an upper-level zonal jet and its subsequent deepening are investigated using a two-layer model on a [beta] plane. The baroclinic interaction of a low-level circular cyclonic perturbation with an upper-level one is first studied in vertical and horizontal cyclonic or anticyclonic uniform shears. A first nonlinear effect acting on the shape and energetics of the perturbations is analyzed. If the background shear is anticyclonic, the perturbations are stretched horizontally; they lose energy barotropically but gain it baroclinically by a well-maintained westward tilt with height. Conversely, if the shear is cyclonic, perturbations remain quite isotropic, but they do not keep a favorable vertical tilt with time and the baroclinic interaction is thus only transient. The latitudinal motion of the perturbations also results from a nonlinear effect. It is found to depend strongly on the background potential vorticity (PV) gradient. This effect is a baroclinic equivalent of the so-called nonlinear barotropic '[beta] drift' and combines the nonlinear advection and vertical stretching terms. These results are confirmed when the anomalies are initially located south of a confined westerly jet. The poleward shift of the lower cyclonic anomaly occurs faster when the vertically averaged PV gradient is strongly positive, which happens when the jet has a large barotropic component. The lower anomaly crosses the jet from the warm to the cold side and deepens afterward. After a detailed description of this regeneration process with the help of an energy budget, it is shown that linear dynamics are not able to reproduce such behavior.

Depth-dependent barotropic instability of short mixed Rossby--gravity (MRG) waves is proposed as a mechanism for the formation of equatorial zonal jets. High-resolution primitive equation simulations show that a single MRG wave of very short zonal wavelength and small to moderate amplitude is unstable and leads to the development of a largely barotropic, zonally symmetric flow, featuring a westward jet at the equator and extra-equatorial jets alternating in direction with latitude. At higher but still moderate amplitude, westward flow still prevails at the equator at depths of maximum horizontal velocity amplitude in the initial wave, but the long-term equilibrated state can also feature eastward 'superrotating' jets at the equator near the depths of zero horizontal velocity in the initial wave. The formation of the superrotating jets in the simulations is found to be sensitive to the inclusion of the nontraditional Coriolis force in the equations of motion. A linear theory is used to demonstrate the existence of exponentially growing horizontally non-divergent perturbations with a dominant zonally symmetric zonal velocity component. An argument for the sense and positioning of the jets relative to the equator is given in terms of inertial instability and the meridional mixing of planetary vorticity by the small zonal-scale components of the linearly unstable modes. In the long time evolution of the flow, if the amplitude of the westward equatorial jet becomes too great, zonally symmetric inertial instability limits the growth of the jets, and inertial adjustment leads to the homogenization of potential vorticity in latitude and depth around the equator.

In this paper, both theoretical analysis and numerical simulations are undertaken to study the parameters that affect the strength of vortex roll-up of synthetic jets. A dimensional analysis reveals that the dimensionless vorticity of vortex roll-up produced by an orifice flow depends on the dimensionless stroke length, Stokes number, and the ratio between the orifice diameter and the thickness of the Stokes layer. Based on the results from a fully developed oscillating laminar pipe flow, the Stokes number is found to play an important role in determining the thickness of the Stokes layer inside the orifice and hence the shape of the velocity profile. Results from the numerical simulations confirm that the Stokes number also determines the strength of vortex roll-up of a synthetic jet issued from an orifice of a finite depth, for the same reason. Finally, a parameter map, which marks the three different regimes of synthetic jets (classified as no jet, jet formation without vortex roll-up, and jet formation with vortex roll-up) is produced based on the numerical simulation results. It is shown that for the synthetic jet actuator used in the present study, a minimum Stokes number of about 8.5 is required to ensure the occurrence of an appreciable vortex roll-up at a dimensionless stroke length greater than 4. In addition, a very low Stokes number can also suppress the formation of synthetic jets. This study provides a further understanding of the behavior of synthetic jets in quiescent conditions, which will be useful for designing more effective synthetic jet actuators in which vortex roll-up is desired.

The mixing of eight isothermal jets issuing in a fully developed circular pipe flow is investigated by means of laser Doppler anemometry, particle image velocimetry, and planar laser induced fluorescence techniques. Two values of the momentum ratio are considered. Unsteady and steady flow patterns are analyzed. Characteristic frequencies are deduced from spectral analysis. Velocity and scalar concentration fields are compared. The mean centerline concentration decay is characterized. The analysis of the flow instabilities is focused on the wake-type structures downstream of the jets and the shear layer structures appearing between the jet and the main flow. The results on the wake-type structures are consistent with previous observations done on a single jet. In particular, the influence of the velocity ratio on the signal-to-noise ratio is confirmed. The Strouhal number associated with the shear layer structure depends on the velocity ratio and on the Reynolds number. The comparison between the velocity and concentration fields confirms the difference in the jet trajectories deduced from these two fields. The detailed analysis of the concentration field gives useful information on the influence of the confinement on the jets' behavior.

Numerous previous works have shown that vertical shear in wind speed and wind direction exist in the atmospheric boundary layer. In this work, meteorological forcing mechanisms, such as the Ekman spiral, thermal wind, and inertial oscillation, are discussed as likely drivers of such shears in the statically stable environment. Since the inertial oscillation, the Ekman spiral, and statically stable conditions are independent of geography, potentially significant magnitudes of speed and direction shear are hypothesized to occur to some extent at any inland site in the world. The frequency of occurrence of non-trivial magnitudes of speed and direction shear are analyzed from observation platforms in Lubbock, Texas and Goodland, Indiana. On average, the correlation between speed and direction shear magnitudes and static atmospheric stability are found to be very high. Moreover, large magnitude speed and direction shears are observed in conditions with relatively high hub-height wind speeds. The effects of speed and direction shear on wind turbine power performance are tested by incorporating a simple steady direction shear profile into the fatigue analysis structures and turbulence simulation code from the National Renewable Energy Laboratory. In general, the effect on turbine power production varies with the magnitude of speed and direction shear across the turbine rotor, with the majority of simulated conditions exhibiting power loss relative to a zero shear baseline. When coupled with observational data, the observed power gain is calculated to be as great as 0.5% and depletion as great as 3% relative to a no shear baseline. The average annual power change at Lubbock is estimated to be -0.5%. [DOI: 10.1115/1.3035818] Keywords: direction shear, speed shear, tall-tower observations, power production, stable boundary layer, FAST, low-level jet, thermal wind, inertial oscillation, Ekman spiral

The onset of the Australian monsoon is examined using (i) reanalysis data for seasons when enhanced observational networks were available and (ii) a 15-yr onset composite. Similar to previous findings, onset is characterized by a sudden strengthening and deepening in tropical westerly winds, which are overlain with upper-tropospheric easterlies. All onsets are preceded by up to a 7-day preconditioning period of enhanced vertical motion and moistening. During the transition season, the 6 weeks prior to onset, a number of moist westerly events occur. Generally they are only sustained for short periods and overlain by upper-level westerly winds, suggesting an association with midlatitude troughs, which protrude into the deep Tropics. For individual years and for a 15-yr composite, monsoon onset is associated with major cyclogenesis events over the southwest Indian Ocean in the presence of a subtropical jet over the eastern Indian Ocean. The proposed mechanism for extratropical-tropical interaction is northeastward Rossby wave propagation from the cyclogenesis region toward the Tropics at upper levels. At these levels, westerly winds extend to nearly 10[degrees]S and provide a favorable background flow for such propagation. The process eventually results in the amplification of an equatorward-extending midlatitude upper trough and tropical ridge, which appears to trigger the development of the underlying monsoon trough. To test the hypothesis, the influence of high-latitude cyclogenesis on the tropical circulation is investigated with the aid of an idealized, dry, three-dimensional, baroclinic wave channel model. The initial state consists of (i) a zonally constant baroclinic region centered on 40[degrees]S, from which the high-latitude cyclogenesis develops, (ii) a weak monsoon trough at 15[degrees]S, and (iii) a subtropical jet at 25[degrees]S. The major findings from the simulations are as follows: 1) There is evidence of northeastward Rossby wave propagation from the cyclogenesis region toward low latitudes. 2) Consistent with theoretical studies, the subtropical jet plays a key role by providing a favorable westerly background flow for group propagation into the Tropics. 3) High-latitude cyclogenesis in the presence of a subtropical jet can influence the meridional location, zonal structure, vorticity, and divergence in the monsoon trough. 4) Vorticity and divergence changes are consistent with enhancement of the monsoon trough (increases in low-level cyclonic vorticity) and the potential for triggering a large-scale convective outbreak (changes in upper-level divergence).

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.05.007 Byline: John H. Rogers, Hans-Jorg Mettig, Damian Peach Keywords: Jupiter; atmosphere; Saturn; atmosphere; Atmospheres; dynamics Abstract: Jupiter's eastward jet at 24[degrees] N, which formerly had the fastest winds on the planet, has maintained a less extreme speed of [approximately equal to]135 m/s since 1991, carrying a series of long-lived vortices at 125 m/s. In 2002-2003, as the albedo of the adjacent North Temperate Belt increased, the tracks of the vortices accelerated slightly, and they had disappeared by 2005. In 2005, small tracers had a mean speed of 146.4 ([+ or -]0.9) m/s, significantly faster than the previous mean speed of the jet, suggesting that the jet peak itself has accelerated at cloud-top level, and that the jet is beginning to return to the super-fast state. These changes may resemble the even greater transformations occurring in the equatorial jet of Saturn. Author Affiliation: British Astronomical Association, Burlington House, Piccadilly, London W1J ODU, UK Article History: Received 26 November 2005; Revised 24 April 2006

A computational-fluid-dynamics (CFD) validation workshop for synthetic jets and turbulent separation control (CFDVAL2004) was held in Williamsburg, Virginia, in March 2004. Three cases were investigated: a synthetic jet into quiescent air, a synthetic jet into a turbulent boundary-layer crossflow, and the flow over a hump model with no-flow-control, steady suction, and oscillatory control. This is a summary of the CFD results from the workshop. Although some detailed results are shown, the CFD state of the art for predicting these types of flows is mostly evaluated from a general point of view. Overall, for synthetic jets, CFD can only qualitatively predict the flow physics, but there is some uncertainty regarding how to best model the unsteady boundary conditions from the experiment consistently. As a result, there is wide variation among CFD results. For the hump flow, CFD is capable of predicting many of the particulars of this flow, provided that it accounts for tunnel blockage, but it consistently overpredicts the length of the separated region compared to the experimental results.

Alignment of the most intense dissipation layers at 45 degrees to the flow in the base region of a jet is due to the vortex rotation that entrains air radially inward and in the flow direction. The profile of the mean scalar dissipation rate gives a shape of the turbulent jet in consistence with that of general scaling relation. The addition of coaxial air and swirl, due to local fragmentation of the mixture fraction field, causes an increase in the mean scalar dissipation rate. This increase results from an increase in the number of dissipation layers per unit volume.

Californians call it the Pineapple Express: a weather pattern that zips across the Pacific Ocean from Hawaii, delivering not baskets of tropical fruit, but buckets of rain and snow. In [...]

If a picture is worth a thousand words, then a spectrum can be worth a thousand pictures. That is perhaps an underestimate when dealing with star-like blobs of light that [...]

Wardle and colleagues have detected a subtle feature in the radio emission from the inner jet of the violently variable quasar 3C279, providing images and spectral distributions of circularly polarized emission. This provides evidence that such jets are mostly made of electron-positron pairs, often at low energies. This gives an insight into the mechanisms for jet formation in active galactic nucleus.