Three-dimensional Hybrid Simulation Results of a Variable Magnetic Helicity Signature at Proton Kinetic Scales

Three-dimensional hybrid kinetic simulations are conducted with particle protons and warm fluid electrons. Alfvénic fluctuations initialized at large scales and with wavevectors that are highly oblique with respect to the background magnetic field evolve into a turbulent energy cascade that dissipates at proton kinetic scales. Accompanying the proton scales is a spectral magnetic helicity signature with a peak in magnitude. A series of simulation runs are made with different large-scale cross helicity and different initial fluctuation phases and wavevector configurations. From the simulations a so-called total magnetic helicity peak is evaluated by summing contributions at a wavenumber perpendicular to the background magnetic field. The total is then compared with the reduced magnetic helicity calculated along spacecraft-like trajectories through the simulation box. The reduced combines the helicity from different perpendicular wavenumbers and depends on the sampling direction. The total is then the better physical quantity to characterize the turbulence. On average the ratio of reduced to total is 0.45. The total magnetic helicity and the reduced magnetic helicity show intrinsic variability based on initial fluctuation conditions. This variability can contribute to the scatter found in the observed distribution of solar wind reduced magnetic helicity as a function of cross helicity.

Driving Galactic Outflows with Magnetic Fields at Low and High Redshift

Although galactic outflows play a key role in our understanding of the evolution of galaxies, the exact mechanism by which galactic outflows are driven is still far from being understood and, therefore, our understanding of associated feedback mechanisms that control the evolution of galaxies is still plagued by many enigmas. In this work, we present a simple toy model that can provide insight on how non-axisymmetric instabilities in galaxies (bars, spiral arms, warps) can lead to local exponential magnetic field growth by radial flows beyond the equipartition value by at least two orders of magnitude on a timescale of a few 100 Myr. Our predictions show that the process can lead to galactic outflows in barred spiral galaxies with a mass-loading factor η ≈ 0.1, in agreement with our numerical simulations. Moreover, our outflow mechanism could contribute to an understanding of the large fraction of barred spiral galaxies that show signs of galactic outflows in the chang-es survey. Extending our model shows the importance of such processes in high-redshift galaxies by assuming equipartition between magnetic energy and turbulent energy. Simple estimates for the star formation rate in our model together with cross correlated masses from the star-forming main sequence at redshifts z ∼ 2 allow us to estimate the outflow rate and mass-loading factors by non-axisymmetric instabilities and a subsequent radial inflow dynamo, giving mass-loading factors of η ≈ 0.1 for galaxies in the range of M ⋆ = 109–1012 M ⊙, in good agreement with recent results of sinfoni and kmos 3D.

Mechanism and Performance Differences between the SSG/LRR-ω and SST Turbulence Models in Separated Flows

Accurate predictions of flow separation are important for aerospace design, flight accident avoidance, and the development of fluid mechanics. However, the complexity of the separation process makes accurate predictions challenging for all known Reynolds-averaged Navier–Stokes (RANS) methods, and the underlying mechanism of action remains unclear. This paper analyzes the specific reasons for the defective predictions of the turbulence models applied to separated flows, explores the physical properties that impact the predictions, and investigates their specific mechanisms. Taking the Menter SST and the Speziale-Sarkar–Gatski/Launder–Reece–Rodi (SSG/LRR)-ω models as representatives, three typical separated flow cases are calculated. The performance differences between the two turbulence models applied to the different separated flow calculations are then compared. Refine the vital physical properties and analyze their calculation from the basic assumptions, modeling ideas, and construction of the turbulence models. The numerical results show that the underestimation of Reynolds stress is a significant factor in the unsatisfactory prediction of separation. In the SST model, Bradshaw’s assumption imposes the turbulent energy equilibrium condition in all regions and the eddy–viscosity coefficient is underestimated, which leads to advanced separation and lagging reattachment. In the SSG/LRR-ω model, the fidelity with which the pressure–strain term is modeled is a profound factor affecting the calculation accuracy.

Modification of turbulence caused by cationic surfactant wormlike micellar structures in two-dimensional turbulent flow

An experimental study is performed to investigate the effects of the extensional rheological properties of drag-reducing wormlike micellar solutions on the vortex deformation and turbulence statistics in two-dimensional (2-D) turbulent flow. A self-standing 2-D turbulent flow was used as the experimental set-up, and the flow was observed through interference pattern monitoring and particle image velocimetry. Vortex shedding and turbulence statistics in the flow were affected by the formation of wormlike micelles and were enhanced by increasing the molar ratio of the counter-ion supplier to the surfactant, ξ, or by applying extensional stresses to the solution. In the 2-D turbulent flow, extensional and shear rates were applied to the fluids around a comb of equally spaced cylinders. This induced the formation of a structure made of wormlike micelles just behind the cylinder. The flow-induced structure influenced the velocity fields around the comb and the turbulence statistics. A characteristic increase in turbulent energy was observed, which decreased slowly downstream. The results implied that the characteristic modification of the 2-D turbulent flow of the drag-reducing surfactant solution was affected by the formation and slow relaxation of the flow-induced structure. The relaxation process of the flow-induced structure made of wormlike micelles was very different from that of the polymers.

Electromagnetic Scattering of Near-Field Turbulent Wake Generated by Accelerated Propeller

The electromagnetic scattering study of the turbulent wake of a moving ship has important application value in target recognition and tracking. However, to date, there has been insufficient research into the electromagnetic characteristics of near-field propeller turbulence. This study presents a new procedure for evaluating the electromagnetic scattering coefficient and imaging characteristics of turbulent wakes in the near field. By controlling the different values of the net momenta, a turbulent wake was generated using the large-eddy simulation method. The results show that the net momentum transferred to the background flow field determines the development of the turbulent wake, which explains the formation mechanism of the turbulence. Combined with the turbulent energy attenuation spectrum, the electromagnetic scattering characteristics of the turbulent wake were calculated using the two-scale facet mode. Using this method, the impact of different parameters on the scattering coefficient and the electromagnetic image of the turbulence wake were investigated, to explain the modulation mechanism and electromagnetic imaging characteristics of the near-field turbulent wake. Moreover, an application for estimating a ship’s heading is proposed based on the electromagnetic imaging characteristics of the turbulent wake.

Comparative study of the radiative and turbulent energy fluxes during summer and winter at the edge of the Antarctic ice sheet in Dronning Maud Land - East Antarctica

Present study compares the estimated radiative and turbulent energy fluxes at the edge of the Antarctic ice sheet during summer and winter in Dronning Maud land, East Antarctica. Hourly snow meteorological parameters were recorded and analysed during winter months (May, June, July and August) of the year 2007 and summer months (November, December, January and February) of the year 2007-08 using Automatic Weather Station (AWS) on the glacier surface. Snow-meteorological parameters air temperature, relative humidity, wind speed, wind direction, incoming solar radiation, outgoing solar radiation, atmospheric pressure and glacier surface temperature were recorded by the AWS. An energy balance model was used to evaluate the surface energy fluxes from measured meteorological quantities for the summer and winter. Net radiative flux was observed the main heat source during summer with seasonal average of 98Wm-2 while sensible heat flux was observed main heat source during winter with seasonal average of 30 Wm-2. Latent heat flux was observed the main heat sink during both the season with seasonal average values of -86.7 Wm-2 for summer and -65.4 Wm-2 during winter. Sublimation was observed high during summer compare to winter.

Destructiveness of pyroclastic surges controlled by turbulent fluctuations

Pyroclastic surges are lethal hazards from volcanoes that exhibit enormous destructiveness through dynamic pressures of 100–102 kPa inside flows capable of obliterating reinforced buildings. However, to date, there are no measurements inside these currents to quantify the dynamics of this important hazard process. Here we show, through large-scale experiments and the first field measurement of pressure inside pyroclastic surges, that dynamic pressure energy is mostly carried by large-scale coherent turbulent structures and gravity waves. These perpetuate as low-frequency high-pressure pulses downcurrent, form maxima in the flow energy spectra and drive a turbulent energy cascade. The pressure maxima exceed mean values, which are traditionally estimated for hazard assessments, manifold. The frequency of the most energetic coherent turbulent structures is bounded by a critical Strouhal number of ~0.3, allowing quantitative predictions. This explains the destructiveness of real-world flows through the development of c. 1–20 successive high-pressure pulses per minute. This discovery, which is also applicable to powder snow avalanches, necessitates a re-evaluation of hazard models that aim to forecast and mitigate volcanic hazard impacts globally.