Hidrológiai Közlöny 1976 (56. évfolyam)
1. szám - Dr. Bogárdi János: Elmélet, oktatás és gyakorlat a hidraulikában és hidromechanikában
2; Hidrológiai Közlöny 1976. 1. sz. Dr_ Bogárdi, L. J.: Theory, education and practice in hydraulics of a professional curriculum and to furnish the student with the necessary advice — alas often without proper regard to the theoretical foundations. In practice the general purpose is to arrive at a rapid and economic solution of the problems encountered without regard, in general, to theory and education. These three approaches to hydraulics are thus seen to follow to a certain extent independent avenues, although these problems are essentially interrelated. This is the circumstance warranting the more detailed treatment of the current problems of research and theory, education and practical applications of hydraulics. Since hydraulics is one of the disciplines in engineering sciences, it appears most appropriate to use one of the "common languages" of these sciences in deriving the relevant relationships. Obviously, this common language must be founded on the accepted and verified theorems of physics. In this way the theorems proved already correct, and even more important, the new relationships, can be interrelated in a substantially simpler manner before the common physical background. The physical approach is a highly complex one, but is rather simple to handle since the essentials thereof have already become familiar in the secondary school and early in the university curriculum. Thus for instance, the concept of physical variable and their limit values —which are of basic practical importance —, the principle of dimensional compatibility, etc. are all familiar. It is widely accepted further, that all theoretical, semi-empirical and rule-of-thumb relationships describe the process of some physical phenomenon. The only differences are, that in a theoretical relationship it is generally possible to formulate exactly the simplifications, approximations and assumptions introduced in the study of physical phenomena, whereas this is generally impossible in the case of semi- and fully empirical relationships. The physical approach is generally suited to obviate these difficulties. There are evidently several methods of approaching engineering processes by a physical philosophy. In this context preference is given to transport theory developed in thermodynamics and applied successfully in several domains of the engineering sciences. The transport theory has been described in detail in several books and papers, which are listed among the references. * For the sake of completeness the substance of this theory will be summarized briefly in this paper as well. The transport theory is founded essentially on the balance equations introduced long ago in hydrology. Changes in the state of engineering systems, in other words the engineering processes are described by the balance equations. In deriving these, two principles are adopted as the starting bases, which may be regarded to be positively demonstrated according to the present state of physical knowledge. * For particulars on the application of transport theory in hydraulics reference is made to the book "Sediment transport in Alluvial Streams" by J. L. Bogárdi (Akadémiai Kiadó, Budapest, 1974). One of these is that the instantaneous condition state of a particular system can be described uniquely by a finite number of state parameters having the property of dimension. These state parameters are referred to as the extensive quantities. In investigations related to hydraulics these are generally the energy, the momentum, the mass and the volume. Any phenomenon is described preferably in terms of changes in these quantities. The other principle is related to the conservation laws of physics. In hydraulics this implies that in any phenomenon under consideration the energy, the momentum, the mass and mostly the volume are conserved. In hydraulics, the state of a body of fluid is positively determined by the extensive quantities mentioned before. In case of subdivision, the extensive quantity is also "subdivided into parts". In principle there exists an infinitely large variety of extensive quantities. For this reason, the first principle can be formulated also by stating that the state of a fluid system is described by a finite number of relevant (dominant) extensive quantities. In the present investigations the second principle relating to the relevant extensive quantities is that in any closed system the magnitude of extensive quantities remains unchanged. Owing exactly to the movement of fluids, interactions occur between fluid systems. To each interaction an extensive quantity can be assigned, the change in which is proportionate to the change in energy taking place during this particular interaction. The relevant "proportionality factors" have two physical implications. On the one hand, these indicate the amount of enei'gy represented by a unit change of the corresponding extensive quantity, on the other hand, these are characteristics which tend towards equalization. In other words, during an interaction energy will flow from one system into the other, as long as the value of the characteristic quantities become balanced. These quantities may be called stresses, or potentials. The common property of all potential quantities tending to become equal —in contrast to the extensive quantities —that they are unrelated to size. In case of subdivision into parts, the value of these quantities in the individual parts remains the same as in the original system. The physical quantities of this nature are called intensive quantities. An infinitely large number of intensive quantities is again conceivable. Examples of the intensive quantities are the energy per unit volume or the energy density, further the mass density q, the hydrostatic pressure, the temperature, or the velocity of flow. All quantities produced as the ratio of two extensive quantities may be intensive. Thus for instance, diverse parameters of the specific fluid gravity, or as will be shown subsequently, the concentration of sediment may Vie regarded intensive, these being the ratios of two extensive quantities. In general, when considering the individual interactions, the relevant intensive quantities must be included, wldcli are always of potential character. As will be perceived from the foregoing, to each interaction a relevant extensive and a relevant intensive quantity pertains. The change in energy resulting from the interaction is proportionate to the change in the relevant extensive quantity. The proportionality factor is the relevant intensive quantity. The equalizing tendency of the relevant intensive quantity plays a fundamental role in the interaction. The extensive quantities will flow from one system into the other, as long as the value of the relevant intensive quantities becomes equal. The criterion of equilibrium between two related systems is that the relevant intensive quantities must be distributed uniformly. The distinctions according to extensive and intensive quantities is not only of formal importance. In fact, these can be used for describing the interactions between two or several systems. Consider a volume element which is acted upon by influences from its environment. The state of the elementary volume can be described by a finite number of extensive quantities x\, . . .X{, . . . r m.
