Relating the Solar Wind Turbulence Spectral Break at the Dissipation Range with an Upstream Spectral Bump at Planetary Bow Shocks

At scales much larger than the ion inertial scale and the gyroradius of thermal protons, the magnetohydrodynamic (MHD) theory is well equipped to describe the nature of solar wind turbulence. The turbulent spectrum itself is defined by a power law manifesting the energy cascading process. A break in the turbulence spectrum develops near-ion scales, signaling the onset of energy dissipation. The exact mechanism for the spectral break is still a matter of debate. In this work, we use the 20 Hz Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) magnetic field data during four planetary flybys at different heliocentric distances to examine the nature of the spectral break in the solar wind. We relate the spectral break frequencies of the solar wind MHD turbulence, found in the range of 0.3–0.7 Hz, with the well-known characteristic spectral bump at frequencies ∼1 Hz upstream of planetary bow shocks. Spectral breaks and spectral bumps during three planetary flybys are identified from the MESSENGER observations, with heliocentric distances in the range of 0.3–0.7 au. The MESSENGER observations are complemented by one Magnetospheric Multiscale observation made at 1 au. We find that the ratio of the spectral bump frequency to the spectral break frequency appears to be r- and B-independent. From this, we postulate that the wavenumber of the spectral break and the frequency of the spectral bump have the same dependence on the magnetic field strength ∣B∣. The implication of our work on the nature of the break scale is discussed.

Whistler-regulated Magnetohydrodynamics: Transport Equations for Electron Thermal Conduction in the High-β Intracluster Medium of Galaxy Clusters

Transport equations for electron thermal energy in the high-β e intracluster medium (ICM) are developed that include scattering from both classical collisions and self-generated whistler waves. The calculation employs an expansion of the kinetic electron equation along the ambient magnetic field in the limit of strong scattering and assumes whistler waves with low phase speeds V w ∼ v te /β e ≪ v te dominate the turbulent spectrum, with v te the electron thermal speed and β e ≫ 1 the ratio of electron thermal to magnetic pressure. We find: (1) temperature-gradient-driven whistlers dominate classical scattering when L c > L/β e , with L c the classical electron mean free path and L the electron temperature scale length, and (2) in the whistler-dominated regime the electron thermal flux is controlled by both advection at V w and a comparable diffusive term. The findings suggest whistlers limit electron heat flux over large regions of the ICM, including locations unstable to isobaric condensation. Consequences include: (1) the Field length decreases, extending the domain of thermal instability to smaller length scales, (2) the heat flux temperature dependence changes from T e 7 / 2 / L to V w nT e ∼ T e 1 / 2 , (3) the magneto-thermal- and heat-flux-driven buoyancy instabilities are impaired or completely inhibited, and (4) sound waves in the ICM propagate greater distances, as inferred from observations. This description of thermal transport can be used in macroscale ICM models.

Field Localization and Density Cavitation in Low-Beta Plasmas

In the present paper, filamentous structure formation, associated turbulent spectrum, and density cavity formation phenomena have been investigated for low- β plasma β ≪ m e / m i applicable to the auroral region. A set of dimensionless equations governing the dynamics of three dimensionally propagating inertial Alfvén wave (3D-IAW) and perpendicularly propagating magnetosonic wave (PMSW) has been developed. Ponderomotive force due to 3D-IAW has been included in the dynamics of the PMSW. Numerical simulation has been performed to study the nonlinear coupling of these two waves. From the obtained results, we found that the field intensity localization takes place which may further lead to the additional dissipation/turbulence process for particle heating and acceleration in space plasma. The associated turbulent spectrum is obtained with scaling nearly k − 4.28 at smaller scales (in the dissipation range). Relevance of the obtained results with the observations reported by various spacecrafts such as Hawkeye and Heos 2 has been discussed. Also, density fluctuations (depletion) of ∼ 0.10 n 0 are calculated, which are consistent with the FAST spacecraft observation reported.

Turbulent model validations with CFD/wind tunnel test and application to SEA for wind noise prediction

Wind noise is becoming to have a higher priority in automotive industry. Several past studies investigated whether SEA can be utilized to predict wind noise by applying a turbulent spectrum model as the input. However, there are many kinds of turbulent models developed and the appropriate model for input to SEA is still unclear. Due to this, this paper focuses on clarifying an appropriate turbulent model for SEA simulation. First, the input turbulent pressure spectrum from five models are validated with wind tunnel tests and CFD. Next, a conventional numerical approach is used to validate models from the aspect of response accuracy. Finally, turbulent models are applied to an SEA model developed for a wind tunnel, and the SEA response is validated with test data. From those input/response validations, an appropriate turbulent model is investigated.

Micron-scale phenomena observed in a turbulent laser-produced plasma

Turbulence is ubiquitous in the universe and in fluid dynamics. It influences a wide range of high energy density systems, from inertial confinement fusion to astrophysical-object evolution. Understanding this phenomenon is crucial, however, due to limitations in experimental and numerical methods in plasma systems, a complete description of the turbulent spectrum is still lacking. Here, we present the measurement of a turbulent spectrum down to micron scale in a laser-plasma experiment. We use an experimental platform, which couples a high power optical laser, an x-ray free-electron laser and a lithium fluoride crystal, to study the dynamics of a plasma flow with micrometric resolution (~1μm) over a large field of view (>1 mm2). After the evolution of a Rayleigh–Taylor unstable system, we obtain spectra, which are overall consistent with existing turbulent theory, but present unexpected features. This work paves the way towards a better understanding of numerous systems, as it allows the direct comparison of experimental results, theory and numerical simulations.

A CFD-PBM coupled model under entire turbulent spectrum for simulating a bubble column with highly viscous media

To account for the effect of liquid viscosity, the bubble breakup model considering turbulent eddy collision based on the inertial subrange turbulent spectrum was extended to the entire turbulent spectrum that included the energy-containing, inertial, and energy-dissipation subranges. The computational fluid dynamics-population balance model (CFD-PBM) coupled model was modified to include this extended bubble breakup model for simulations of a bubble column. The effect of turbulent energy spectrum on the bubble breakup and hydrodynamic behaviors was studied in a bubble column under different liquid viscosities. The results showed that when the liquid viscosity was < 80 mPas, the bubble breakup and hydrodynamics were almost independent on the turbulent spectrum. At liquid viscosity > 80 mPas, the bubble breakup rate and gas holdup were significantly under-predicted when the inertial turbulent spectrum was used, and when using the entire turbulent spectrum the predictions were more consistent with experimental data.

Резонансный отклик масштабно-инвариантных функций случайного процесса с турбулентным спектром

Scale-invariant random processes with large fluctuations are modeled by a system of two stochastic nonlinear differential equations describing interacting phase transitions. It is shown that under the action of white noise, a critical state arises, characterized by a turbulent spectrum and a scale-invariant distribution function. The critical state corresponds to the maximum entropy, which indicates the stability of the process. An external harmonic action on a random process with a turbulent spectrum gives rise to a resonant response of scale-invariant functions.

Asymptotic turbulent friction in 2D rough-walled flows

The friction f is the property of wall-bounded flows that sets the pumping cost of a pipeline, the draining capacity of a river, and other variables of practical relevance. For highly turbulent rough-walled pipe flows, f depends solely on the roughness length scale r, and the f − r relation may be expressed by the Strickler empirical scaling f ∝ r1/3. Here, we show experimentally that for soap film flows that are the two-dimensional (2D) equivalent of highly turbulent rough-walled pipe flows, f ∝ r and the f − r relation is not the same in 2D as in 3D. Our findings are beyond the purview of the standard theory of friction but consistent with a competing theory in which f is linked to the turbulent spectrum via the spectral exponent α: In 3D, α = 5/3 and the theory yields f ∝ r1/3; in 2D, α = 3 and the theory yields f ∝ r.