The description of coherent features of fluid flow is essential to our understanding of fluid-dynamical and transport processes. A method is introduced that is able to extract dynamic information from flow fields that are either generated by a (direct) numerical simulation or visualized/measured in a physical experiment. The extracted dynamic modes, which can be interpreted as a generalization of global stability modes, can be used to describe the underlying physical mechanisms captured in the data sequence or to project large-scale problems onto a dynamical system of significantly fewer degrees of freedom. The concentration on subdomains of the flow field where relevant dynamics is expected allows the dissection of a complex flow into regions of localized instability phenomena and further illustrates the flexibility of the method, as does the description of the dynamics within a spatial framework. Demonstrations of the method are presented consisting of a plane channel flow, flow over a two-dimensional cavity, wake flow behind a flexible membrane and a jet passing between two cylinders.

https://hal-polytechnique.archives-ouvertes.fr/hal-01020654/document

Dynamic mode decomposition of numerical and experimental data

In this review, we present control methods for flow over a bluff body such as a circular cylinder, a 2D bluff body with a blunt trailing edge, and a sphere. We introduce recent major achievements in bluff-body flow controls such as 3D forcing, active feedback control, control based on local and global instability, and control with a synthetic jet. We then classify the controls as boundary-layer controls and direct-wake modifications and discuss important features associated with these controls. Finally, we discuss some other issues such as Reynolds-number dependence, the lowest possible drag by control, and control efficiency.

Control of Flow Over a Bluff Body

Water electrolysis derived by renewable energy such as solar energy and wind energy is a sustainable method for hydrogen production due to high purity, simple and green process. One of the challenges is to reduce energy consumption of water electrolysis for large-scale application in future. Cell voltage, an important criterion of energy consumption, consists of theoretical decomposition voltage (U-theta), ohmic voltage drop (i*Sigma R) and reaction overpotential (eta). The kinetic and thermodynamic roots of high cell voltage are analyzed systemically in this review. During water electrolysis, bubble coverage on electrode surface and bubble dispersion in electrolyte, namely bubble effect, result in high ohmic voltage drop and large reaction overpotential. Bubble effect is one of the most key factors for high energy consumption. Based on the theoretical analysis, we summarize and divide recent intensification technologies of water electrolysis into three categories: external field, new electrolyte composition and new thermodynamic reaction system. The fundamentals and development of these intensification technologies are discussed and reviewed. Reaction overpotential and ohmic voltage drop are improved kinetically by external field or new electrolyte composition. The thermodynamic decomposition voltage of water is also reduced by new reaction systems such as solid oxide electrolysis cell (SOEC) and carbon assisted water electrolysis (CAWE). (C) 2013 Elsevier Ltd. All rights reserved.

The intensification technologies to water electrolysis for hydrogen production - A review

Closed-Loop Turbulence Control: Progress and Challenges

We review the important general dynamic and thermodynamic properties of structural glass-forming substances and classify them into 12 different categories. Our understanding of glass-forming substances is incomplete until all these properties have been explained. The dynamic properties considered include those relating to the high frequency fast relaxation, the Johari–Goldstein β-relaxation, and the slow structural α-relaxation. Practically all the important relaxation and diffusion processes in the time region extending from 10−13 to 106 s have been discussed. We show from experimental data that the properties in different categories bear some relations to each other. Some issues that impede progress of our understanding of the behavior of glass-forming substances are accentuated by formulating them as questions. Answers to these questions by future experimental and theoretical investigations will improve our understanding. Specific suggestions for future research efforts that can provide some answers to these questions are also made. Within the same property in each category, by examining a large number of glass-formers, a pattern of variation is found which correlates with the departure of the structural relaxation of the glass-former from exponential decay or the quantity, (1−β), where β is the fractional exponent in the Kohlrausch expression, exp[−(t/τ)β]. These patterns, as well as their correlations with (1−β), suggest that the salient dynamic properties (or phenomenology) of glass-forming substances are all governed by the non-exponential nature of the structural relaxation, i.e., the quantity (1−β). Therefore, a theory or model capable of relating the structural relaxation time τ to (1−β) has the potential of explaining many of the salient dynamic properties of glass-forming substances.

Dynamic and thermodynamic properties of glass-forming substances

Turbulentfluidhasoftenbeenconceptualizedasatransientthermodynamicphase. Here, a finite-time thermodynamics (FTT) formalism is proposed to compute mean flow and fluctuation levels of unsteady incompressible flows. The proposed formalism builds upon the Galerkin model framework, which simplifies a continuum 3D fluid motion into a finite-dimensional phase-space dynamics and, subsequently, into a thermodynamics energy problem. The Galerkin model consists of a velocity field expansion in terms of flow configuration dependent modes and of a dynamical system describing the temporal evolution of the mode coefficients. Each mode is treated as one thermodynamic degree of freedom, characterized by an energy level. The dynamical system approaches local thermal equilibrium (LTE) where each mode has the same energy if it is governed only by internal (triadic) mode interactions. However, in the generic case of unsteady flows, the full system approaches only partial LTE with unequal energy levels due to strongly mode-dependent external interactions. The FTT model is

/pdf/a-finite-time-thermodynamics-of-unsteady-fluid-flows-5fq78vfrq7.pdf

A Finite-Time Thermodynamics of Unsteady Fluid Flows

https://www.hzdr.de/FWS/FWSH/EBLC/papers/Weier_et_al_1998_cylinder_wake_stabilization_ETFS.pdf

Experiments on cylinder wake stabilization in an electrolyte solution by means of electromagnetic forces localized on the cylinder surface

On the action of magnetic gradient forces in micro-structured copper deposition

Electrolytic gas evolution is a fundamental phenomenon occurring in a large number of industrial applications. In these processes gas bubbles are formed at the electrode from a supersaturated solution. Since dissolved gases can change the surface tension, a gas concentration gradient may cause the surface tension to vary locally at the interface of the gas bubble. Surface tension gradients may also form due to temperature gradients generated by ohmic heating of the electrolyte. In both cases, the resulting shear stress imposes a convection in the electrolyte and the gas bubble (Marangoni effect). This phenomenon may influence the entire electrolytic gas evolution process, e.g., by an enhanced mass transfer. In this study, the first evidence of the Marangoni convection near growing hydrogen bubbles, generated by water electrolysis, is provided. Microscopic high speed imaging was applied to study the evolution of single hydrogen bubbles at a microelectrode. The convection near the interface of the growing bubble was measured by using a time-resolved Particle Tracking Velocimetry (PTV) technique. The results indicate a clear correlation between the magnitude of the Marangoni convection and the electric current.

Marangoni convection at electrogenerated hydrogen bubbles.

If a fluid is electrically conductive, its flow may be controlled using electromagnetic forces. Meanwhile, this technique is a recognized tool even on an industrial scale for handling highly conductive materials like liquid metals. However, also fluids of low electrical conductivity as considered in the present study, like sea-water and other electrolytes, permit electromagnetic flow control. Experimental results on the prevention of flow separation by means of a streamwise, wall parallel Lorentz force acting on the suction side of inclined flat plates and hydrofoils will be presented.

/pdf/control-of-flow-separation-using-electromagnetic-forces-1otqknrhq4.pdf

Gerd Mutschke

Papers

A Finite-Time Thermodynamics of Unsteady Fluid Flows

Experiments on cylinder wake stabilization in an electrolyte solution by means of electromagnetic forces localized on the cylinder surface

On the action of magnetic gradient forces in micro-structured copper deposition

Marangoni convection at electrogenerated hydrogen bubbles.

Control of Flow Separation Using Electromagnetic Forces