Deriving the full turbulent stress tensor from paired ADCP measurements: application to under-ice convection

<p>The Reynolds stress tensor (RST) is the key characteristic of turbulence describing the paths of turbulent kinetic energy transfer and its anisotropy. Despite recent technical advances in application of multi-beam acoustic Doppler current profilers (ADCPs) to in situ acquiring of the RST components, derivation of the full Reynolds tensor from raw flow measurements remains a challenging problem. We present a method for derivation of the full set of turbulent stresses, based on combined use of two ADCPs with two beams from adjacent devices crossing at some point.  In the proposed framework, two 3-beam ADCPs with vertically aligned axes constitute the minimum configuration sufficient to derive 6 equations for all 6 RST components. <br>The method was applied to studying turbulence in a convectively mixed layer in ice-covered Lake Kilpisjärvi. The calculated dynamics of all six stress components revealed diurnal periodicity along with the variations with the periods of a few hours. The pulsations intensities (diagonal components of RST) remained positive except short outliers; less than 5% of cases did not meet the so-called realizability requirements (positive definiteness of the stress matrix). The off-diagonal stresses demonstrated sign-changing dynamics, mirroring the inter-component energy transfer.<br>The ratio of pulsation intensities along vertical and horizontal axes varied in the range from 0.02 to 0.25. The r.m.s. values of horizontal and vertical pulsations reached diurnal maximums of 4 and 1 mm/s correspondingly, the latter being close to 1/3 of the convective velocity w*, in accordance with the previous studies on free convection. <br>The new approach provides an immediate insight into the internal structure of the turbulent boundary mixing, especially relevant to anisotropic non-stationary flows, like buoyancy-driven convection. The preliminary results on under-ice convection elucidate strong anisotropy of the convective flow — a key to understanding the heat and mass transport in ice-covered waters.</p>

Global Rigidity of 2D Linearly Constrained Frameworks

A linearly constrained framework in $\mathbb{R}^d$ is a point configuration together with a system of constraints that fixes the distances between some pairs of points and additionally restricts some of the points to lie in given affine subspaces. It is globally rigid if the configuration is uniquely defined by the constraint system. We show that a generic linearly constrained framework in $\mathbb{R}^2$ is globally rigid if and only if it is redundantly rigid and “balanced”. For unbalanced generic frameworks, we determine the precise number of solutions to the constraint system whenever the rigidity matroid of the framework is connected. We obtain a stress matrix sufficient condition and a Hendrickson type necessary condition for a generic linearly constrained framework to be globally rigid in $\mathbb{R}^d$.

Effect of Interface Properties on Tensile and Fatigue Behavior of 2D Woven SiC/SiC Fiber-Reinforced Ceramic-Matrix Composites

In this paper, the effect of the fiber/matrix interface properties on the tensile and fatigue behavior of 2D woven SiC/SiC ceramic-matrix composites (CMCs) is investigated. The relationships between the interface parameters of the fiber/matrix interface debonding energy and interface frictional shear stress in the interface debonding region and the composite tensile and fatigue damage parameters of first matrix cracking stress, matrix cracking density, and fatigue hysteresis-based damage parameters are established. The effects of the fiber/matrix interface properties on the first matrix cracking stress, matrix cracking evolution, first and complete interface debonding stress, fatigue hysteresis dissipated energy, hysteresis modulus, and hysteresis width are analyzed. The experimental first matrix cracking stress, matrix cracking evolution, and fatigue hysteresis loops of SiC/SiC composites are predicted using different interface properties.

Depression's Unholy Trinity: Dysregulated Stress, Immunity, and the Microbiome

Depression remains one of the most prevalent psychiatric disorders, with many patients not responding adequately to available treatments. Chronic or early-life stress is one of the key risk factors for depression. In addition, a growing body of data implicates chronic inflammation as a major player in depression pathogenesis. More recently, the gut microbiota has emerged as an important regulator of brain and behavior and also has been linked to depression. However, how this holy trinity of risk factors interact to maintain physiological homeostasis in the brain and body is not fully understood. In this review, we integrate the available data from animal and human studies on these three factors in the etiology and progression of depression. We also focus on the processes by which this microbiota-immune-stress matrix may influence centrally mediated events and on possible therapeutic interventions to correct imbalances in this triune.