Some aspects in Kelvin-Helmholtz instability with and without Boussinesq approximation

The topic of this paper is the Kelvin-Helmholtz instability, a phenomenon which occurs on the interface of a stratified fluid, in the presence of a parallel shear flow, when there is a velocity and density difference across the interface of two adjacent layers. This paper focuses on a numerical simulation modelled by the Taylor-Goldstein equation, which represents a more realistic case compared to the basic Kelvin-Helmholtz shear flow. The Euler system is solved with new modelled smooth velocity and density profiles at the interface. The flux at cell boundaries is reconstructed by implementing a third order WENO (Weighted Essentially Non-Oscillatory) method. Next, a Riemann solver builds the fluxes at cell interfaces. The use of both Rusanov and HLLC solvers is investigated. Temporal discretization is done by applying the second order TVD (total variation diminishing) Runge-Kutta method on a uniform grid. Numerical simulations are performed comparatively for both Kelvin-Helmholtz and Taylor-Goldstein instabilities, on the same simulation domains. We find that increasing the number of grid points leads to a better accuracy in shear layer vortices visualization. Thus, we can conclude that applying the Taylor-Goldstein equation improves the realism in the general fluid instability modelling.

A bottom-up approach to construct or deconstruct a fluid instability

Fluid instabilities have been the subject of study for a long time. Despite all the extensive knowledge, they still constitute a serious challenge for many industrial applications. Here, we experimentally consider an interface between two fluids with different viscosities and analyze their relative displacement. We designed the contents of each fluid in such a way that a chemical reaction takes place at the interface and use this reaction to suppress or induce a fingering instability at will. This process describes a road map to control viscous fingering instabilities in more complex systems via interfacial chemical reactions.

Nanoparticles suppress fluid instabilities in the thermal drawing of ultralong nanowires

Ultra-long metal nanowires and their facile fabrication have been long sought after as they promise to offer substantial improvements of performance in numerous applications. However, ultra-long metal ultrafine/nanowires are beyond the capability of current manufacturing techniques, which impose limitations on their size and aspect ratio. Here we show that the limitations imposed by fluid instabilities with thermally drawn nanowires can be alleviated by adding tungsten carbide nanoparticles to the metal core to arrive at wire lengths more than 30 cm with diameters as low as 170 nm. The nanoparticles support thermal drawing in two ways, by increasing the viscosity of the metal and lowering the interfacial energy between the boron silicate and zinc phase. This mechanism of suppressing fluid instability by nanoparticles not only enables a scalable production of ultralong metal nanowires, but also serves for widespread applications in other fluid-related fields.

Shear Effects on Scalar Transport in Double Diffusive Convection1

In this article, we examine the effect of shear on scalar transport in double diffusive convection (DDC). DDC results from the competing action of a stably stratified, rapidly diffusing scalar (temperature) and an unstably stratified, slowly diffusing scalar (salinity), which is characterized by fingering instabilities. We investigate, for the first time, the effect of shear on the diffusive and convective contributions to the total scalar transport flux within a confined fluid layer, examining also the associated fingering dynamics and flow structure. We base our analysis on fully resolved numerical simulations under the Oberbeck–Boussinesq condition. The problem has five governing parameters: The salinity Prandtl number, Prs (momentum-to-salinity diffusivity ratio); the salinity Rayleigh number, Ras (measure of the fluid instability due to salinity differences); the Lewis number, Le (thermal-to-salinity diffusivity ratio); the density ratio, Λ (measure of the effective flow stratification), and the shear rate, Γ. Simulations are performed at fixed Prs, Ras, Le, and Λ, while the effect of shear is accounted for by considering different values of Γ. Preliminary results show that shear tends to damp the growth of fingering instability, leading to highly anisotropic DDC dynamics associated with the formation of regular salinity sheets. These dynamics result in significant modifications of the vertical transport rates, giving rise to negative diffusive fluxes of salinity and significant reduction of the total scalar transport, particularly of its convective part.

Analysis of Radiation Propagation inside a Hierarchical Solar Volumetric Absorber

The solar receiver is a critical component of concentrated solar power technology; it works as a heat exchanger, transforming the concentrated solar radiation into high-temperature heat. Volumetric receiver technologies, using air as a heat transfer fluid, are designed to reach higher temperatures than the current receiver technology, which is limited by material resistance and fluid instability. The higher temperature, up to 1200 K, could be used in high-temperature industrial processes or a high-temperature thermodynamic cycle. A correct radiation propagation is essential to develop their performances, reducing reflection and emission losses and promote the heat transfer to the fluid. In this study, the optical behaviour of a hierarchical volumetric receiver (HVR) developed in Bruno Kessler Foundation (FBK) has been studied using Monte Carlo ray tracing (MCRT) simulations. The simulations have been validated in an experimental setup that evaluates the light transmissivity of the HVR porous structure. Two different HVR structures are evaluated with MCRT simulations that use a real solar dish geometry to configure a complete concentrated solar power (CSP) plant. Results show that frontal and rear losses are, respectively, 12% and 3% of the incoming concentrated radiation. Inside the HVR, 15% of the incoming power is propagated trough the lateral void spaces. Therefore, the power spreading avoids the overconcentration of the centre of the focalized area. The HVR optical behaviour has been investigated, showing an optical efficiency of 85%.

Physical models of streaming instabilities in protoplanetary discs

ABSTRACT We develop simple, physically motivated models for drag-induced dust–gas streaming instabilities, which are thought to be crucial for clumping grains to form planetesimals in protoplanetary discs. The models explain, based on the physics of gaseous epicyclic motion and dust–gas drag forces, the most important features of the streaming instability and its simple generalization, the disc settling instability. Some of the key properties explained by our models include the sudden change in the growth rate of the streaming instability when the dust-to-gas mass ratio surpasses one, the slow growth rate of the streaming instability compared to the settling instability for smaller grains, and the main physical processes underlying the growth of the most unstable modes in different regimes. As well as providing helpful simplified pictures for understanding the operation of an interesting and fundamental astrophysical fluid instability, our models may prove useful for analysing simulations and developing non-linear theories of planetesimal growth in discs.

The structure of the blue whirl revealed

The blue whirl is a small, stable, spinning blue flame that evolved spontaneously in recent laboratory experiments while studying turbulent, sooty fire whirls. It burns a range of different liquid hydrocarbon fuels cleanly with no soot production, presenting a previously unknown potential way for low-emission combustion. Here, we use numerical simulations to present the flame and flow structure of the blue whirl. These simulations show that the blue whirl is composed of three different flames—a diffusion flame and premixed rich and lean flames—all of which meet in a fourth structure, a triple flame that appears as a whirling blue ring. The results also show that the flow structure emerges as the result of vortex breakdown, a fluid instability that occurs in swirling flows. These simulations are a critical step forward in understanding how to use this previously unknown form of clean combustion.

Analytic Solutions as a Tool for Verification and Validation of a Multiphysics Model

Computational physicists are commonly faced with the task of resolving discrepancies between the predictions of a complex, integrated multiphysics numerical simulation, and corresponding experimental datasets. Such efforts commonly require a slow iterative procedure. However, a different approach is available in casesx where the multiphysics system of interest admits closed-form analytic solutions. In this situation, the ambiguity is conveniently broken into separate consideration of theory–simulation comparisons (issues of verification) and theory–data comparisons (issues of validation). We demonstrate this methodology via application to the specific example of a fluid-instability-based ejecta source model under development at Los Alamos National Laboratory and implemented in flag, a Los Alamos continuum mechanics code. The formalism is conducted in the forward sense (i.e., from source to measurement) and enables us to compute, purely analytically, time-dependent piezoelectric ejecta mass measurements for a specific class of explosively driven metal coupon experiments. We incorporate published measurement uncertainties on relevant experimental parameters to estimate a time-dependent uncertainty on these analytic predictions. This motivates the introduction of a “compatibility score” metric, our primary tool for quantitative analysis of the RMI + SSVD model. Finally, we derive a modification to the model, based on boundary condition considerations, that substantially improves its predictions.