Prague, Czech Republic
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Roubicek T.,Czech Institute of Thermomechanics
Continuum Mechanics and Thermodynamics | Year: 2017

A rather general model for fluid and heat transport in poro-elastic continua undergoing possibly also plastic-like deformation and damage is developed with the goal to cover various specific models of rock rheology used in geophysics of Earth’s crust. Nonconvex free energy at small elastic strains, gradient theories (in particular the concept of second-grade nonsimple continua), and Biot poro-elastic model are employed, together with possible large displacement due to large plastic-like strains evolving during long time periods. Also the additive splitting is justified in stratified situations which are of interest in modelling of lithospheric crust faults. Thermodynamically based formulation includes entropy balance (in particular the Clausius–Duhem inequality) and an explicit global energy balance. It is further outlined that the energy balance can be used to ensure, under suitable data qualification, existence of a weak solution and stability and convergence of suitable approximation schemes at least in some particular situations. © 2017 Springer-Verlag Berlin Heidelberg


Heczko O.,ASCR Institute of Physics Prague | Straka L.,Aalto University | Seiner H.,Czech Institute of Thermomechanics
Acta Materialia | Year: 2013

The morphology and microstructure of a single, macroscopically straight twin interface with a twinning stress of about 1 MPa was analysed in detail by differential interference contrast optical microscopy and X-ray diffraction. The interface was identified as a Type I macrotwin boundary between two variants with a/b-laminates and constant modulation direction, in contrast with a highly mobile twinned interface consisting of Type II macrotwin boundary segments with changing modulation direction and a/b-laminate reported earlier. Theoretical analysis using elastic continuum theory shows that only pure Type I or Type II boundaries are fully compatible with a/b-laminate. Other hypothetical twin microstructures combining these two mobile interfaces are shown to be incompatible to various degrees. A weakly incompatible combination of Type I and II boundaries was experimentally observed. The large difference in mobility between Type I and Type II macrotwin boundaries created at the same location of the same sample indicates that the mobility depends on the internal structure of these boundaries. A possible origin of this different mobility is discussed. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.


Tesar V.,Czech Institute of Thermomechanics
Flow Measurement and Instrumentation | Year: 2013

This paper presents a simple sensor for measuring shear stress in fluid flow adjacent to a wall. Experiments and numerical flowfield computations have shown no measurable disturbance of the flowfield and the sensor may thus be well suited to conditions in which the invasive character of known sensors causes premature transition to turbulence or flow separation. The sensing part occupies a minimum surface area on the wall. In principle, the sensor is capable of detecting individual vortices in near-wall turbulence. Accuracy and sensitivity are comparable to known sensors of the sublayer-fence type. © 2013 Elsevier Ltd.


Tesar V.,Czech Institute of Thermomechanics
Chemical Engineering Journal | Year: 2010

Flow control valve containing no moving components was developed for switching fluid flow passing through it into a parallel secondary path once the conditions in the main path reach a certain limit. No sensors or actuators are involved; the switching is caused by the inability of the Coanda effect to keep the flow attached to a wall that leads it into a particular outlet once a large resistance is met in this outlet. So far, there has been no method for designing such valves. To provide a guidance, this paper summarises experimental evidence about loading characteristics obtained with a considerable number of tested valve geometries. © 2010 Elsevier B.V.


Benesova B.,Czech Institute of Thermomechanics
Journal of Global Optimization | Year: 2011

This paper presents an approach to numerical solution of problems posed in the framework of quasi-static rate-independent processes. As soon as a problem allows for an energetic formulation there are known methods of its time discretization by time incremental minimization problems, which demand for global optimization of a non-convex functional. Moreover the two-sided energy inequality, a necessary condition for optimization, can be formulated. Here we present an algorithm for finding solutions of rate-independent processes that verifies this condition and uses the strategy of backtracking if it is violated. We present the selectivity of the mentioned necessary condition in general and give numerical examples of the efficiency of such an algorithm, but also of situations that are beyond its limits. For those we propose a second strategy relying on wisely chosen combinations of spatial discretizations. © Springer Science+Business Media, LLC. 2010.


Tesar V.,Czech Institute of Thermomechanics
Chemical Engineering Research and Design | Year: 2014

Efficient generation of sub-millimetre microbubbles was recently made possible by pulsating the flow of gas supplied into a parallel-exits aerator, using a fluidic oscillator for the purpose. Without moving parts, it can generate oscillation at high frequency, an important factor due to bubble natural frequency rapidly increasing with the desirable decrease of their size. This paper discusses development of an unusual oscillator - a part of an integral oscillator/aerator unit - capable of generating a particularly high driving frequency, in the kilohertz range, in a layout that promotes a strong third harmonic frequency of the basic oscillation. © 2014 The Institution of Chemical Engineers.


Tesar V.,Czech Institute of Thermomechanics
Chemical Engineering Journal | Year: 2014

Microbubbles - gas bubbles of diameter less than 1. mm - became currently of considerable importance for chemical and process engineering applications, mainly because of the recent discovery of an energetically efficient method of their generation with a fluidic oscillator in the gas supply into an aerator. The oscillation should be applied at the microbubble resonant conditions, about which there has been so far known very little. The key problem is the unknown and difficult to evaluate extent of the surrounding liquid that takes part in the microbubble oscillatory motions and represents the inertia term in the governing equation. The oscillation of microbubbles also influences their ascent, which is slow and puts them often into mutual proximity causing their conjunctions. Author evaluated basic data on oscillating microbubbles from high-speed camera frames and used them to setup a simple model, suitable for engineering design purposes. © 2013 Elsevier B.V.


Tesar V.,Czech Institute of Thermomechanics
Chemical Engineering Research and Design | Year: 2011

A wall-jet of a different fluid (or the same fluid at a different thermodynamic state) is blown along the inner surface of a pipe to prevent a contact between the main central flow and the pipe wall. Although motivated by problems encountered in measuring radioactivity, the analogy between heat and mass transfer was utilised for the experiments and concurrent numerical computations which investigated transport of heat from the central air flow towards cool pipe wall, across the protective cool air wall-jet. Similar protective cooling of walls may be actually itself an important application. Another one may be segregation required due to gas chemical aggressiveness. This first part discusses the details of the experiment and the character of the temperature and flow fields evaluated from measured profiles of time-mean velocity and temperature. Analysis of the data, using mutually supporting information from the experiment as well as the computations, is handled in the next, second part. © 2010 The Institution of Chemical Engineers.


Tesar V.,Czech Institute of Thermomechanics
Chemical Engineering Research and Design | Year: 2011

The pump described in this paper is based on an unusual working principle which makes it suitable for handling difficult-to-pump dangerous liquids. It is driven by a two-phase alternating air flow and in its fluidic rectifier part that comes into contact with the liquid has no moving or deformed components. It is thus leak-proof, extremely reliable, and needs no maintenance. The rectifier may be described as a series-connected array of jet-pump units with annular Coanda-effect nozzles. Each odd one of the units is connected to one phase of the driving air flow - and each even unit to the other phase. The air acts periodically on the liquid in vertical displacement tubes on top of each unit. The outflow from these tubes generates in all even jet-pump units an annular synthetic jet, attaching by the Coanda effect to curved wall that diverts it downstream, towards the pump output. At the same time in the odd jet-pump units the orientation of the nozzles makes it easy for the liquid to enter into them from upstream. An important additional pumping effect is generated by travelling waves passing through the whole array. The pump rectifier is robust and may be made of materials withstanding high temperature, chemical aggressiveness of the liquid, or special conditions like exposure to radioactivity. © 2010 The Institution of Chemical Engineers.


Tesar V.,Czech Institute of Thermomechanics
Sensors and Actuators, A: Physical | Year: 2012

Because of nonlinearity of characteristics of nozzles, their effective hydraulic resistance when generating periodically pulsating jet is higher than the steady-flow value. Author investigated this effect in experiments with a nozzle supplied with superposed steady flow and a pulsation generated by piston-type pulsator, and found deviations from the predictions based on mere nonlinearity. The disagreement is explained by fluid accumulation due to its compressibility in the actuator cavity upstream from the nozzle. Two mathematical models of the periodic accumulation processes were set up. The experimental data are located between the results of these models. The final outcome of this study is a law of universal validity for design of actuators with periodic flows in nozzles. © 2012 Elsevier B.V. All rights reserved.

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