Self-consistent theory of turbulent transport in the solar tachocline. III. Gravity waves

To understand the fundamental physical processes important for the evolution of solar rotation and distribution of chemical species, we provide theoretical predictions for particle mixing and momentum transport in the stably stratified tachocline. By envisioning that turbulence is driven externally in the tachocline (e.g. by plume penetration), we compute the amplitude of turbulent flow, turbulent particle diffusivities, and eddy viscosity, by incorporating the effect of a strong radial differential rotation and stable stratification. We identify the different roles that the shear flow and stable stratification play in turbulence regulation and transport. Particle transport is found to be severely quenched due to stable stratification as well as radial differential rotation, especially in the radial direction with an effectively more efficient horizontal transport. The eddy viscosity is shown to become negative for parameter values typical of the tachocline, suggesting that turbulence in the stably stratified tachocline leads to a non-uniform radial differential rotation. Similar results also hold in the radiative interiors of stars, in general

Evolution of forced shear flows in polytropic atmospheres: A comparison of forcing methods and energetics

Shear flows are ubiquitous in astrophysical objects including planetary and stellar interiors, where their dynamics can have significant impact on thermo-chemical processes. Investigating the complex dynamics of shear flows requires numerical calculations that provide a long time evolution of the system. To achieve a sufficiently long lifetime in a local numerical model the system has to be forced externally. However, at present, there exist several different forcing methods to sustain large-scale shear flows in local models. In this paper we examine and compare various methods used in the literature in order to resolve their respective applicability and limitations. These techniques are compared during the exponential growth phase of a shear flow instability, such as the Kelvin-Helmholtz (KH) instability, and some are examined during the subsequent non-linear evolution. A linear stability analysis provides reference for the growth rate of the most unstable modes in the system and a detailed analysis of the energetics provides a comprehensive understanding of the energy exchange during the system's evolution. Finally, we discuss the pros and cons of each forcing method and their relation with natural mechanisms generating shear flows

Energy- and flux-budget (EFB) turbulence closure model for the stably stratified flows. Part I: Steady-state, homogeneous regimes

We propose a new turbulence closure model based on the budget equations for the key second moments: turbulent kinetic and potential energies: TKE and TPE (comprising the turbulent total energy: TTE = TKE + TPE) and vertical turbulent fluxes of momentum and buoyancy (proportional to potential temperature). Besides the concept of TTE, we take into account the non-gradient correction to the traditional buoyancy flux formulation. The proposed model grants the existence of turbulence at any gradient Richardson number, Ri. Instead of its critical value separating - as usually assumed - the turbulent and the laminar regimes, it reveals a transition interval, 0.1< Ri <1, which separates two regimes of essentially different nature but both turbulent: strong turbulence at Ri<<1; and weak turbulence, capable of transporting momentum but much less efficient in transporting heat, at Ri>1. Predictions from this model are consistent with available data from atmospheric and lab experiments, direct numerical simulation (DNS) and large-eddy simulation (LES).Comment: 40 pages, 6 figures, Boundary-layer Meteorology, resubmitted, revised versio

The Effect of Nonvertical Shear on Turbulence in a Stably Stratified Medium

Direct numerical simulations were performed in order to investigate the evolution of turbulence in a stably stratified fluid forced by nonvertical shear. Past research has been focused on vertical shear flow, and the present work is the first systematic study with vertical and horizontal components of shear. The primary objective of this work was to study the effects of a variation of the angle θ between the direction of stratification and the gradient of the mean streamwise velocity from θ=0, corresponding to the well-studied case of purely vertical shear, to θ=π/2,corresponding to purely horizontal shear. It was observed that the turbulent kinetic energy Kevolves approximately exponentially after an initial phase. The exponential growth rate γ of the turbulent kinetic energy K was found to increase nonlinearly, with a strong increase for small deviations from the vertical, when the inclination angle θ was increased. The increased growth rate is due to a strongly increased turbulence production caused by the horizontal component of the shear. The sensitivity of the flow to the shear inclination angle θ was observed for both low and high values of the gradient Richardson number Ri, which is based on the magnitude of the shear rate. The effect of a variation of the inclination angle θ on the turbulence evolution was compared with the effect of a variation of the gradient Richardson number Ri in the case of purely vertical shear. An effective Richardson number Rieff was introduced in order to parametrize the dependence of the turbulence evolution on the inclination angle θ with a simple model based on mean quantities only. It was observed that the flux Richardson number Rifdepends on the gradient Richardson number Ri but not on the inclination angle θ

Compact International Experiences: Expanding Student International Awareness Through Short-Term Study Abroad Courses With Substantial Engineering Technical Content

Compact International Experience (CIE) courses are investigated as a suitable tool to raise student international awareness while retaining substantial engineering technical content. These courses were developed due to a strong student desire for engineering international studies as well as a drive by the home institution for internationalization of the curriculum. The efficacy of such courses is described through experiences gained from delivering two distinct three-semester-unit engineering elective courses in a three-week time frame in France and Australia. While each of these courses, Topics in Fluid Mechanics and Advanced Electronic Circuit Design, focused on its technical content, the desire for student understanding of the cultural environment and the impact of engineering solutions from a global and societal viewpoint were strong driving factors for each. Assessment validates the hypothesis that CIE courses can successfully deliver substantial engineering technical content while providing an enriching international experience to students

Athleta B Corporation Case Study

As a B Corporation, Athleta has the unique challenge of identifying a competitive strategy that will guide the company to success in the highly competitive athleisure market while also remaining consistent with its mission, vision, and values and supporting its parent company, Gap Inc. This case study was developed to evaluate this challenge through internal and external analysis of the company and to encourage readers to consider Athleta’s future priorities as the company undergoes changes in strategic leadership. The case study begins with a brief overview of the competitive landscape of the Athleisure market and a review of the history of Athleta before diving into the company’s positioning in the market as a company that values sustainability and female empowerment. Athleta’s focus on inclusive sizing, sustainability, partnerships with female athletes, and customer engagement provide strong tools for the company to differentiate itself from competitors. These strengths will be critical for Athleta to utilize in the future as it seeks to become a leading Athleisure brand and a strong brand in Gap Inc.’s portfolio

On the Structure and Dynamics of Sheared and Rotating Turbulence: Direct Numerical Simulations and Wavelet Based Coherent Vortex Extraction

The influence of rotation on the structure and dynamics of sheared turbulence is investigated using a series of direct numerical simulations. Five cases are considered: turbulent shear flow without rotation, with moderate rotation, and with strong rotation, where the rotation configuration is either parallel or antiparallel. For moderate rotation rates an antiparallel configuration increases the growth of the turbulent kinetic energy, while the parallel case reduces the growth as compared to the nonrotating case. For strong rotation rates decay of the energy is observed, linear effects dominate the flow, and the vorticity probability density functions tend to become Gaussian. Visualizations of vorticity show that the inclination angle of the vortical structures depends on the rotation rate and orientation. Coherent vortex extraction, based on the orthogonal wavelet decomposition of vorticity, is applied to split the flow into coherent and incoherent parts. It was found that the coherent part preserves the vortical structures using only a few percent of the degrees of freedom. The incoherent part was found to be structureless and of mainly dissipative nature. With increasing rotation rates, the number of wavelet modes representing the coherent vortices decreases, indicating an increased coherency of the flow. Restarting the direct numerical simulation with the filtered fields confirms that the coherent component preserves the temporal dynamics of the total flow, while the incoherent component is of dissipative nature

Beitrag zur Frage der Stickstoffassimilation durch den Bacillus ellenbachensis α Caron

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Surface Water Quality: Contaminants and Treatment

Extensive population growth and urbanization is affecting surface water quality and infrastructure across the globe. Urbanization was boosted by the advancements of infrastructure for water, which allows treatment and transport of water through large distances from their original surface water sources such as lakes, rivers, and streams. A major concern of the rapid changes experienced by urban areas is the ecological impacts on surface water due to associated anthropogenic processes. Activities such as the overconsumption and extraction of surface water have negatively affected the quality and quantity of those sources. In addition, discharge of treated wastewater may impact the quality of the water source. Prevention of overexploitation and contamination of surface water resources is typically related to community culture and available technologies. Examples of this can include community appreciation of water resources and efforts to limit water usage in households and industries, by implementing new water technologies. This project looks into the effect of knowledge and technology on the quality of surface water sources and the use of those resources. This information is used to devise a comprehensive analysis of water conservation efforts on the quality and quantity of surface waters in Southern California and Israel. The project will involve research by the University of San Diego and Azrieli College of Engineering in Jerusalem