An Improved Synthetic Eddy Method for Generating Inlet Turbulent Boundary Layers

An improved synthetic eddy method (SEM) is proposed in this paper for generating the boundary layer at the inlet of a computational domain via direct numerical simulation. The improved SEM modified the definition of the radius and the velocities of the eddies according to the distance of the eddies from the wall in the synthetic region. The regeneration location of the eddies is also redefined. The simulation results show that the improved SEM generates turbulent fluctuations that closely match the DNS results of the experiments. The skin friction coefficient of the improved SEM recovers much faster and has lower dimensionless velocity at the outer of the boundary layer than that of the traditional SEM.

Observation of a reduced-turbulence regime with boron powder injection in a stellarator

In state-of-the-art stellarators, turbulence is a major cause of the degradation of plasma confinement. To maximize confinement, which eventually determines the amount of nuclear fusion reactions, turbulent transport needs to be reduced. Here we report the observation of a confinement regime in a stellarator plasma that is characterized by increased confinement and reduced turbulent fluctuations. The transition to this regime is driven by the injection of submillimetric boron powder grains into the plasma. With the line-averaged electron density being kept constant, we observe a substantial increase of stored energy and electron and ion temperatures. At the same time, the amplitude of the plasma turbulent fluctuations is halved. While lower frequency fluctuations are damped, higher frequency modes in the range between 100 and 200 kHz are excited. We have observed this regime for different heating schemes, namely with both electron and ion cyclotron resonant radio frequencies and neutral beams, for both directions of the magnetic field and both hydrogen and deuterium plasmas.

Transport in Fusion Plasmas: Is the Tail Wagging the Dog?

Turbulent plasmas notably self-organize to higher energy states upon application of additional free energy sources or modification of edge operating conditions. Mechanisms whereby such bifurcations occur have been actively debated for decades, mostly on the basis of reduced models. Here we unravel a surprising causal chain of events in the onset of an improved confinement state, by applying generic entropy-based and information theoretic measures to the primitive kinetic equations. Interfacial contamination of a large, stable region (the ‘dog’) by locally-borne peripheral turbulent fluctuations (the ‘tail’) is found to be central to explaining transport properties, globally. These results, highly relevant to the quest for magnetic fusion advocate the use of such data-driven methods to many problems in fluids and plasmas where interfacial turbulent contamination is active.

Effects of the large-scale circulation on temperature and water vapor distributions in the Π Chamber

Microphysical processes are important for the development of clouds and thus Earth's climate. For example, turbulent fluctuations in the water vapor mixing ratio, r, and temperature, T, cause fluctuations in the saturation ratio, S. Because S is the driving factor in the condensational growth of droplets, fluctuations may broaden the cloud droplet size distribution due to individual droplets experiencing different growth rates. The small-scale turbulent fluctuations in the atmosphere that are relevant to cloud droplets are difficult to quantify through field measurements. We investigate these processes in the laboratory using Michigan Tech's Π Chamber. The Π Chamber utilizes Rayleigh–Bénard convection (RBC) to create the turbulent conditions inherent in clouds. In RBC it is common for a large-scale circulation (LSC) to form. As a consequence of the LSC, the temperature field of the chamber is not spatially uniform. In this paper, we characterize the LSC in the Π Chamber and show how it affects the shape of the distributions of r, T, and S. The LSC was found to follow a single roll with an updraft and downdraft along opposing walls of the chamber. Near the updraft (downdraft), the distributions of T and r were positively (negatively) skewed. At each measuring position, S consistently had a negatively skewed distribution, with the downdraft being the most negative.

Impact of entrainment-mixing and turbulent fluctuations on droplet size distributions in a cumulus cloud: An investigation using Lagrangian microphysics with a sub-grid-scale model

Entrainment-mixing and turbulent fluctuations critically impact cloud droplet size distributions (DSDs) in cumulus clouds. This problem is investigated via a new sophisticated modeling framework using the CM1 LES model and a Lagrangian cloud microphysics scheme – the “super-droplet method” (SDM) – coupled with sub-grid-scale (SGS) schemes for particle transport and supersaturation fluctuations. This modeling framework is used to simulate a cumulus congestus cloud. Average DSDs in different cloud regions show broadening from entrainment and secondary cloud droplet activation (activation above the cloud base). DSD width increases with increasing entrainment-induced dilution as expected from past work, except in the most diluted cloud regions. The new modeling framework with SGS transport and supersaturation fluctuations allows a more sophisticated treatment of secondary activation compared to previous studies. In these simulations, it contributes about 25%of the cloud droplet population and impacts DSDs in two contrastingways: narrowing in extremely diluted regions and broadening in relatively less diluted. SGS supersaturation fluctuations contribute significantly to an increase in DSD width via condensation growth and evaporation. Mixing of super-droplets from SGS velocity fluctuations also broadens DSDs. The relative dispersion (ratio of DSD dispersion and mean radius) negatively correlates with grid-scale vertical velocity in updrafts, but is positively correlated in downdrafts. The latter is from droplet activation driven by positive SGS supersaturation fluctuations in grid-mean subsaturated conditions. Finally, the sensitivity to model grid length is evaluated. The SGS schemes have greater influence as the grid length is increased, and they partially compensate for the reduced model resolution.

Observation of a novel reduced-turbulence regime with boron powder injection in a stellarator

We report the first observation of a novel confinement regime in a stellarator plasma, characterized by increased confinement and reduced turbulent fluctuations. The transition to this new regime is driven by the injection of sub-millimetric boron powder grains into the plasma. With the line averaged electron density being kept constant, substantial increase of stored energy, electron and ion temperature have been observed. At the same time, the amplitude of the plasma turbulent fluctuations is halved. While lower frequency fluctuations are damped, higher frequency modes in the range 100 ≤ f [kHz] ≤ 200 are excited. The access to this regime has been observed for different heating schemes, namely with both electron and ion cyclotron resonant radio frequency, and neutral beams, for both directions of the magnetic field, and for both hydrogen and deuterium plasmas.

Extension of Harmonic Balance Approach for Large-Eddy Simulation of Unsteady Flows in Cascade

A full scale-resolving simulation of cascades flutter is time consuming because of computational inefficiency owing to its low nondimensional frequencies. To improve the efficiency and reliability of the numerical analyses for such flows, we propose an efficient scale-resolving simulation method dedicated to time-periodic flows by extending the harmonic balance approach to a large-eddy simulation. This method combines convergence calculations of the steady-state problem based on the harmonic balance method for periodic components, and the nonlinear time-marching method for small scale turbulent fluctuations. Using the proposed method, deterministic periodic components and stochastic turbulent fluctuations are calculated simultaneously, and the effect of turbulent fluctuations on deterministic periodic components is directly calculated without using turbulence models. In this paper, we present the algorithm of the simulation technique and the progress of validation calculations for channel flow excited in the streamwise direction.

Jarzynski equality and Crooks relation for local models of air–sea interaction

We show that the most prominent of the work theorems, the Jarzynski equality and the Crooks relation, can be applied to the momentum transfer at the air–sea interface using a hierarchy of local models. In the more idealized models, with and without a Coriolis force, the variability is provided from Gaussian white noise which modifies the shear between the atmosphere and the ocean. The dynamics is Gaussian, and the Jarzynski equality and Crooks relation can be obtained analytically solving stochastic differential equations. The more involved model consists of interacting atmospheric and oceanic boundary layers, where only the dependence on the vertical direction is resolved, the turbulence is modeled through standard turbulent models and the stochasticity comes from a randomized drag coefficient. It is integrated numerically and can give rise to a non-Gaussian dynamics. Also in this case the Jarzynski equality allows for calculating a dynamic beta βD of the turbulent fluctuations (the equivalent of the thermodynamic beta β=(kBT)-1 in thermal fluctuations). The Crooks relation gives the βD as a function of the magnitude of the work fluctuations. It is well defined (constant) in the Gaussian models and can show a slight variation in the more involved models. This demonstrates that recent concepts of stochastic thermodynamics used to study micro-systems subject to thermal fluctuations can further the understanding of geophysical fluid dynamics with turbulent fluctuations.