Hydrodynamic and hydromagnetic stability

Although the relative importance of airborne transmission of the SARS-CoV-2 virus is controversial, increasing evidence suggests that understanding airflows is important for estimation of the risk of contracting COVID-19. The data available so far indicate that indoor transmission of the virus far outstrips outdoor transmission, possibly due to longer exposure times and the decreased turbulence levels (and therefore dispersion) found indoors. In this paper we discuss the role of building ventilation on the possible pathways of airborne particles and examine the fluid mechanics of the processes involved.

/pdf/effects-of-ventilation-on-the-indoor-spread-of-covid-19-1kaqm52c7y.pdf

Effects of ventilation on the indoor spread of COVID-19

Internal waves are important physical phenomena on the continental shelf/slope. They are often very energetic, and their breaking provides an important dissipation and mixing mechanism, with implications for biological productivity and sediment transport. Internal waves appear in a variety of forms and can break in a variety of ways. A consequence of their dispersion properties is the breaking of waves reflecting from, or being generated at, near-critical slopes. Breaking mechanisms associated with internal solitary waves include bottom boundary layer instabilities, shear instabilities in the interior of the water column, and wave overturning as they shoal. Shoaling can result in the formation of waves with trapped cores either at the surface or at the bottom. Theoretical, numerical, and laboratory studies have largely focused on simple geometries, whereas recent work has shown that the situation in the ocean is often much more complicated because of more complex geometries and the presence of a full hierarchy of fluid motions.

Internal Wave Breaking and Dissipation Mechanisms on the Continental Slope/Shelf

Bedform-induced hyporheic exchange with unsteady flows

According to energy policies, it is necessary to replace electrical energy or fossil fuels by renewable energy sources. Solar energy systems are recognized as one of these sources. In such systems, high-performance is one of the essential needs. Porous materials have been introduced as one of the most efficient and affordable techniques to improve the heat transfer and energy efficiency in solar energy systems. In this review, the applications of porous materials and their advantages and limitations on different types of solar energy systems are summarized. Moreover, the structures of each system with their applications are briefly introduced. Solar energy systems covered in this paper contains both energy conversion and energy storage systems.

A review on the applications of porous materials in solar energy systems

Large-amplitude internal solitary waves in a stratification comprising a thick, lower, homogeneous layer separated from a thin, upper, homogeneous layer by a broad gradient region are studied using simultaneous measurements of the density and velocity fields. Density field measurements are achieved through synthetic schlieren, operating in an absolute mode to allow efficient and accurate measurements of density in systems with strong curvatures and large perturbations to the density field. The images used for these density measurements are interleaved with images used for particle image velocimetry by phase locking two video cameras (one configured for the density measurements and the other for the velocity measurements) with a computer-driven LCD monitor, allowing the background texture required for synthetic schlieren to be turned off for the particle image velocimetry measurements on the mid-plane of the experimental tank. The simultaneous measurements of both density and velocity fields not only allow greater insight into the internal wave dynamics, but also allow the velocity measurements to be corrected for the normal errors associated with the refractive index variations. As an illustration of the power of this technique, we determine for the first time in an internal solitary wave the spatial structure of the local gradient Richardson number, finding regions where this falls below the limit for linear stability.

https://iopscience.iop.org/article/10.1088/0957-0233/18/3/001/pdf

Simultaneous synthetic schlieren and PIV measurements for internal solitary waves

The stability properties of 24 experimentally generated internal solitary waves (ISWs) of extremely large amplitude, all with minimum Richardson number less than 1/4, are investigated. The study is supplemented by fully nonlinear calculations in a three-layer fluid. The waves move along a linearly stratified pycnocline (depth h2) sandwiched between a thin upper layer (depth h1) and a deep lower layer (depth h3), both homogeneous. In particular, the wave-induced velocity profile through the pycnocline is measured by particle image velocimetry (PIV) and obtained in computation. Breaking ISWs were found to have amplitudes (a1) in the range , while stable waves were on or below this limit. Breaking ISWs were investigated for 0.27 0.86 and stable waves for Lx/λ < 0.86. The results show a sort of threshold-like behaviour in terms of Lx/λ. The results demonstrate that the breaking threshold of Lx/λ = 0.86 was sharper than one based on a minimum Richardson number and reveal that the Richardson number was found to become almost antisymmetric across relatively thick pycnoclines, with the minimum occurring towards the top part of the pycnocline.

/pdf/shear-induced-breaking-of-large-internal-solitary-waves-37wuhme89r.pdf

Shear-induced breaking of large internal solitary waves

Penetrative convection in a two-layer system in which a layer of fluid overlies and saturates a porous medium is simulated via internal heating. The motion in the porous medium is described via Darcy's law and in the fluid layer by the Navier-Stokes equations with a Boussinesq approximation. The lower porous surface is held fixed at a temperature T L , while the upper fluid surface is stress free and held at T U > T L . Internal heating takes place in both layers and allows the model to describe penetrative convection. The strength of heating has a dramatic effect on both the onset of convection and the nature of the ensuing convection cells

/pdf/penetrative-convection-in-a-superposed-porous-medium-fluid-5g83qdjiue.pdf

Magda Carr

Papers

Simultaneous synthetic schlieren and PIV measurements for internal solitary waves

Shear-induced breaking of large internal solitary waves

Penetrative convection in a superposed porous-medium-fluid layer via internal heating

Linear stability of natural convection in superposed fluid and porous layers: Influence of the interfacial modelling

Penetrative convection in a fluid overlying a porous layer