František Maršík

Homogeneous bubble nucleation in liquids: The classical theory revisited

The classical theory of homogeneous bubble nucleation is reconsidered by employing a phenomenological nucleation barrier in the capillarity approximation that utilizes the superheat threshold achieved in experiments. Consequently, an algorithm is constructed for the evaluation of the superheat temperatures in homogeneous boiling (tensile strengths in cavitation), the critical radii and steady-state nucleation rates. The nucleation theorem is written in this framework and is applied to the classical theory of homogeneous bubble nucleation for the phenomenological nucleation barrier employed. The superheat temperatures calculated show excellent agreement over a wide range of liquid pressures for most of the substances investigated. The steady-state nucleation rates are also altered by many orders of magnitude, in agreement with the results of previous investigators using different approaches.

Coupling Effect between Mechanical Loading and Chemical Reactions

This paper offers a theoretical explanation of the coupling effect phenomenon between mechanical loading and chemical reactions based on linear nonequilibrium thermodynamics and also discusses the classical method of obtaining restrictions on the phenomenological coefficients. The question whether static or dynamic loading influences biochemical processes is addressed-the necessity of dynamic loading as a stimulatory mechanism is shown. Further, the presented paper suggests that chemical and mechanical processes do not only facilitate or support one another but they may also play a triggering role for the other coupled process-some biochemical processes may need mechanical stimulation to run and vice versa as well-chemical reactions may provide energy for some mechanical processes. As an example, a detailed analysis of a model for controlled autocatalytic reproduction is presented, where the coupling effect, i.e. the influence of dynamic loading on reaction kinetics, is demonstrated.

New predictive model for monitoring bone remodeling

The aim of this article was to present a new thermodynamic-based model for bone remodeling which is able to predict the functional adaptation of bone in response to changes in both mechanical and biochemical environments. The model was based on chemical kinetics and irreversible thermodynamic principles, in which bone is considered as a self-organizing system that exchanges matter, energy and entropy with its surroundings. The governing equations of the mathematical model have been numerically solved using Matlab software and implemented in ANSYS software using the Finite Element Method. With the aid of this model, the whole inner structure of bone was elucidated. The current model suggested that bone remodeling was a dynamic process which was driven by mechanical loading, metabolic factors and other external contributions. The model clearly indicated that in the absence of mechanical stimulus, the bone was not completely resorbed and reaches a new steady state after about 50% of bone loss. This finding agreed with previous clinical studies. Furthermore, results of virtual computations of bone density in a composite femur showed the development of a dense cortical bone around the medullary canal and a dense trabeculae bone between the femoral head and the calcar region of the medial cortex due to compressive stresses. The comparison of the predicted bone density with the structure of the proximal femur obtained from X-rays and using strain energy density gave credibility to the current model.

A coupled mechano-biochemical model for bone adaptation.

Bone remodelling is a fundamental biological process that controls bone microrepair, adaptation to environmental loads and calcium regulation among other important processes. It is not surprising that bone remodelling has been subject of intensive both experimental and theoretical research. In particular, many mathematical models have been developed in the last decades focusing in particular aspects of this complicated phenomenon where mechanics, biochemistry and cell processes strongly interact. In this paper, we present a new model that combines most of these essential aspects in bone remodelling with especial focus on the effect of the mechanical environment into the biochemical control of bone adaptation mainly associated to the well known RANKL-RANK-OPG pathway. The predicted results show a good correspondence with experimental and clinical findings. For example, our results indicate that trabecular bone is more severely affected both in disuse and disease than cortical bone what has been observed in osteoporotic bones. In future, the methodology proposed would help to new therapeutic strategies following the evolution of bone tissue distribution in osteoporotic patients.

Binary nucleation of water and sodium chloride

Nucleation processes in the binary water-sodium chloride system are investigated in the sense of the classical nucleation theory (CNT). The CNT is modified to be able to handle the electrolytic nature of the system and is employed to investigate the acceleration of the nucleation process due to the presence of sodium chloride in the steam. This phenomenon, frequently observed in the Wilson zone of steam turbines, is called early condensation. Therefore, the nucleation rates of the water-sodium chloride mixture are of key importance in the power cycle industry.

Remodelling of living bone induced by dynamic loading and drug delivery-Numerical modelling and clinical treatment

Remodelling is a dynamic process occurring during growth and it includes sensing of environmental changes, tissue resorbance, i.e. the removal of existing old bone, and formation of new tissue. The biomechanical remodelling process is relatively well formulated for bones and can be divided into three stages: (1) bone resorption based on the osteoclast activity, (2) bone deposition based on the osteoblast activity and (3) bone growth control established on RANK/RANKL/OPG pathway-RANKL/OPG balance. The main driving force of remodelling process is a dynamic loading (cyclic compression and expansion, e.g. walking or running), which strongly influences the rate of chemical reactions. The evolution from the homogeneous density distribution to the corticalis and cancellous bone formation is shown. An inevitable influence of a dynamic mechanical loading and osteoprotegerin (OPG) concentration is demonstrated. Deformations were calculated by commercial code ANSYS. The clinical experience indicates that the dynamic loading (above the threshold level 1500-2500microstains/s), especially walking with a characteristic time approximately 1s, influences the whole process of bone remodelling after a time period of approximately 3 months. The numerical simulation shows that the concentration of the new bone and the bone elastic constants substantially depend on history and intensity of the loading, drug delivery and nutrition.

Simulation of cerebrospinal fluid transport

The pulsatile nature of the CSF movement is a result of the cardiac-related pulsations in blood volume in cranial region. According to Monro-Kellie Doctrine, the net inflow of arterial blood during systole is compensated by an equal outflow of venous blood and by caudal displacement of the CSF. Knowledge of the distribution of physical properties (compliance, resistance) along the craniospinal system is crucial for understanding of the CSF hydrodynamics. The synthesis of both invasively and non-invasively obtained data is needed. The aim of our project was to develop a lumped-parameter compartment model of the craniospinal system and, in relation to the cardiac-related blood-volume pulsations, to describe its basic hydrodynamic properties. The model consists of six compartments representing major parts of the craniospinal system. Each compartment has its own set of physical properties which describe its behavior. The pressure transmission from head arteries to the brain compartment serves as a source of pulsations. The simulation tightly mimics pressure waves of the CSF and thus the flow characteristics and magnitudes. The fitted compliance of the spinal compartment in our model was two orders higher (9x10^-^1^0m^3/Pa) then the cranial compartment (5.2x10^-^1^2m^3/Pa): only in this adjustment pulsations were present. It makes 99.5% of compliance related to the spinal canal and 0.5% to the intracranial structures. Our fitting showed that this model might be used in medical education as well as in medical practice.

Visualization and Mass Transfer with a Bistable Two-Slot Impinging Jet

AbsractA two-dimensional air impinging jet with a passive control has been studied experimentally, and smoke visualization, measurement of mean flow characteristics, and mass transfer experiments have been performed. Investigated flow field is intrinsically bistable, and two flow patterns exist under the same boundary conditions. The both patterns differ in a “bubble of separated flow”, either a small or large recirculation area is embraced inside the jet. A change between flow field patterns is hysteretic in character. The large recirculation area bridges the whole nozzle-to-wall spacing, and seems to be very promising for an augmentation of heat/mass transfer.

Pitfalls of Exergy Analysis

Abstract The well-known Gouy–Stodola theorem states that a device produces maximum useful power when working reversibly, that is with no entropy production inside the device. This statement then leads to a method of thermodynamic optimization based on entropy production minimization. Exergy destruction (difference between exergy of fuel and exhausts) is also given by entropy production inside the device. Therefore, assessing efficiency of a device by exergy analysis is also based on the Gouy–Stodola theorem. However, assumptions that had led to the Gouy–Stodola theorem are not satisfied in several optimization scenarios, e.g. non-isothermal steady-state fuel cells, where both entropy production minimization and exergy analysis should be used with caution. We demonstrate, using non-equilibrium thermodynamics, a few cases where entropy production minimization and exergy analysis should not be applied.

The characteristic thickness of polymer electrolyte membrane and the efficiency of fuel cell

A simplified thermodynamic analysis is applied to elucidate the basic feature of fuel cells operation, i.e., the transformation of chemical energy into electricity. To be able to handle this problem analytically we propose a simple model of the chemical reaction kinetics at the electrodes and diffusion in the polymer electrolyte membrane (PEM). The description is based on a set of two mass balance equations involving water and proton transport through the membrane coupled with two reaction equations describing the electrochemical reactions at the electrodes. The transport parameters (water diffusivity, proton conductivity, electro-osmotic drag) involved in the equations for the water and proton flux densities are shown to comply with Onsager reciprocity relations. The resulting form of the transport equations is suitable for a quantitative analysis from the point of view of linear irreversible thermodynamics. In terms of our simplified model, a relation for the characteristic thickness of the PEM is derived that theoretically assures a stable fuel cell operation, and the maximum efficiency of a fuel cell is evaluated, both as functions of the transport properties of the membrane material.