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The distribution and development of air frequency in self-aerated open channel flows were experimentally investigated. A series of experiments was conducted in a rectangular flume with various initial flow velocities (3·50-6·35 m/s) and the test area was in the self-aerated developing region. Analysis of the results shows that the position of maximum air frequency is generally in the relatively low air concentration region in the self-aerated developing region of an open channel flow, and the air concentration with the maximum air frequency becomes greater and gradually approaches 0·50 with the development of self-aeration downstream. Further analysis reveals that the positions of maximum air frequency remain around the centre of the self-aerated region and, with an increase in initial flow velocity, the air frequency distributions over the cross-section gradually tend to a simple normal distribution. The reason for the distribution and development of air frequency in the self-aerated developing region is mainly from the free surface into the limit of self-aeration; the distribution of an air-water flow structure is a developing process involving the coexistence of entrapped air and entrained air, and this air-water structure leads to the difference in air frequency distributions and development in the self-aerated developing region. [ABSTRACT FROM AUTHOR]

We study unconfined homogeneous turbulence with a destabilizing background density gradient in the Boussinesq approximation. Starting from initial isotropic turbulence, the buoyancy force induces a transient phase toward a self-similar regime accompanied by a rapid growth of kinetic energy and Reynolds number, along with the development of anisotropic structures in the flow in the direction of gravity. We model this with a two-point statistical approach using an axisymmetric eddy-damped quasi-normal Markovian (EDQNM) closure that includes buoyancy production. The model is able to match direct numerical simulations (DNS) in a parametric study showing the effect of initial Froude number and mixing intensity on the development of the flow. We further improve the model by including the stratification timescale in the characteristic relaxation time for triple correlations in the closure. It permits the computation of the long-term evolution of unstably stratified turbulence at high Reynolds number. This agrees with recent theoretical predictions concerning the self-similar dynamics and brings new insight into the spectral energy distribution and anisotropy of the flow. [ABSTRACT FROM PUBLISHER]

Direct numerical simulation is used to study the air flow between two vertical plates maintained at different temperatures. The periodic dimensions of the plates are small so as to accommodate only one flow structure, which consists of a convection roll with oblique vorticity braids. At lower Rayleigh numbers, the roll and the braids grow and shrink alternatively following a cyclical process. As the Rayleigh number is increased, the flow becomes temporally chaotic through a period-doubling cascade. Windows corresponding to multiperiodic regimes and interior crises are observed. As the Rayleigh number is further increased, the structure intermittently switches between two vertical positions, which is seen to correspond to an "attractor-merging" crisis. The chaotic flow dynamics are characterized and the corresponding physical mechanisms are identified. We show that some of the flow key features, such as the chaotic oscillation and intermittency, can be captured by a low-order model. [ABSTRACT FROM AUTHOR]

Sommerfeld's affiliation with fluid mechanics started when he began his career as an assistant of the mathematician Felix Klein at Göttingen. He always regarded fluid mechanics as a particular challenge. In 1904, he published a theory of hydrodynamic lubrication. Four years later, he conceived an approach for the analysis of flow instability (the Orr-Sommerfeld approach) as an attempt to account for the transition from laminar to turbulent flow. The onset of turbulence also became a major challenge for some of his pupils, in particular Ludwig Hopf and Fritz Noether. Both contributed considerably to elaborate the Orr-Sommerfeld theory. Heisenberg's doctoral work was another attempt in this quest. When Sommerfeld published his lectures on theoretical physics during World War II, he dedicated one of the six volumes to the mechanics of continuous media. With chapters on boundary layer theory and turbulence, it exceeded the scope of contemporary theoretical physics-revealing Sommerfeld's persistent appreciation of fluid mechanics. He resorted to Prandtl's Göttingen school of fluid mechanics in order to stay abreast of the rapid development of these specialties. [ABSTRACT FROM AUTHOR]

This paper investigates experimentally the correlation and the effect of the turbulence intensity on the pressure drop and the heat transfer mechanism of a heat exchanger with elliptic tubes in a staggered arrangement. The heat exchanger studied in this work is an air–water cross flow heat exchanger with air being the external fluid and water the internal one. The heat exchanger consists of 144 elliptic tubes placed in a staggered arrangement. The experiments were carried out for two setups. The first setup was referring to isothermal conditions for which only air was used, flowing around the tubes. The second setup was referring to non-isothermal conditions with air as the external fluid and water as the internal working fluid, flowing inside the tubes. The Reynolds number for the external air for the isothermal experiments ranged between 3100 and 5200 based on the maximum velocity between the elliptic tubes. The turbulence intensity values varied between 0.9% and 3%. For the non-isothermal measurements the Reynolds number took values from 3100 to 7700 and the turbulent intensity ranged from 0.9% to 3%. The measurements showed that the increase of turbulent intensity led to a decrease in the total pressure drop of the external flow of the heat exchanger together with an enhancement of the heat transfer mechanism. [ABSTRACT FROM AUTHOR]

Computational fluid dynamics (CFD) simulations were carried out to find a CFD methodology to predict the critical impeller speed ( N CS) for the complete drawdown of floating solids in a stirred tank with an up- and down-pumping pitched blade turbine (PBTU and PBTD). CFD models along with six mixing parameters predicted that N CS was 7.5 rps and 5.8 rps, respectively, for PBTU and PBTD, when impeller submergence ( S) was equal to half of the liquid height ( H/2). Reasonable agreement was observed between CFD simulations and published empirical correlation. N CS was smaller for PBTD relative to PBTU at S = H/2, which agreed with results in the literature. The drawdown mechanisms based on the mean drag, turbulent fluctuations or large scale eddies failed to explain the difference in N CS between PBTU and PBTD. The flow patterns based on a simulated velocity field were further investigated and successfully accounted for this difference. [ABSTRACT FROM AUTHOR]

The hierarchical shell models of turbulence including a spatial dimension, namely, spatiotemporal tree models, reproduce the intermittent behavior of Navier-Stokes equations in both space and time dimensions corresponding to high Reynolds number turbulent flows. This model is used, for the first time in this paper, in a one-dimensional flow zone containing a dispersed-phase particle that can be used in the study of dispersed-phase flows. In this paper, a straightforward method has been used to introduce discrete phase into the spatiotemporal tree model that leads to an increased amount of turbulent energy dissipation rate in the vicinity of the discrete phase. The effects of particle insertion and particle size on the turbulent energy dissipation rate are demonstrated. Moreover, the space-scale behavior of the time-averaged turbulent energy dissipation rate in the presence of dispersed phase is demonstrated by means of continuous wavelet transform. [ABSTRACT FROM AUTHOR]

We present a full two-fluid magnetohydrodynamic (MHD) description for a completely ionized hydrogen plasma, retaining the effects of the Hall current, electron pressure, and electron inertia. According to this description, each plasma species introduces a new spatial scale: the ion inertial length λi and the electron inertial length λe, which are not present in the traditional MHD description. In the present paper, we seek for possible changes in the energy power spectrum in fully developed turbulent regimes, using numerical simulations of the two-fluid equations in two-and-a-half dimensions. We have been able to reproduce different scaling laws in different spectral ranges, as it has been observed in the solar wind for the magnetic energy spectrum. At the smallest wavenumbers where plain MHD is valid, we obtain an inertial range following a Kolmogorov k-5/3 law. For intermediate wavenumbers such that λi-1≪k≪λe-1, the spectrum is modified to a k-7/3 power-law, as has also been obtained for Hall-MHD neglecting electron inertia terms. When electron inertia is retained, a new spectral region given by k>λe-1 arises. The power spectrum for magnetic energy in this region is given by a k-11/3 power law. Finally, when the terms of electron inertia are retained, we study the self-consistent electric field. Our results are discussed and compared with those obtained in the solar wind observations and previous simulations. [ABSTRACT FROM AUTHOR]

Instantaneous concentrations of reactive species are simultaneously measured in a planar liquid jet with a second-order chemical reaction [ABSTRACT FROM AUTHOR]

The network of interactions among fluid elements and coherent structures gives rise to the incredibly rich dynamics of vortical flows. These interactions can be described with the use of mathematical tools from the emerging field of network science, which leverages graph theory, dynamical systems theory, data science, and control theory. The blending of network science and fluid mechanics facilitates the extraction of the key interactions and communities in terms of vortical elements, modal structures, and particle trajectories. Phase-space techniques and time-delay embedding enable a network-based analysis of time-series measurements in terms of visibility, recurrence, and cluster transitions. Equipped with the knowledge of interactions and communities, the network-theoretic approach enables the analysis, modeling, and control of fluid flows, with a particular emphasis on interactive dynamics. In this article, we provide a brief introduction to network science and an overview of the progress on network-based strategies to study the complex dynamics of fluid flows. Case studies are surveyed to highlight the utility of network-based techniques to tackle a range of problems from fluid mechanics. Towards the end of the paper, we offer an outlook on network-inspired approaches. [ABSTRACT FROM AUTHOR]