problems involve so many variables that it is possible only to approximate a solution by mathematical analysis alone.

Consequently, adjustments to mathematical developments must be made on an empirical basis - which results in the often-made accusation* that fluid mechanics is a science of coefficients.

In order to overcome this criticism as much as possible, we have attempted to explain first the characteristics of fluid properties, based upon the fundamentals of molecular structure, and then to develop the basic theory involved in a particular concept of fluid mechanics.

Furthermore, we have attempted to explain and illustrate, on the one hand, the conditions under which the basic theory is directly applicable to an engineering problem and, on the other hand, the conditions under which the mathematical theory must be modified empirically (on the basis of experimental data) in order for it to be applicable to an engineering problem.

In short, ideal theory is indispensable* for basic understanding, and for practical use of such understanding; the large number of variables involved frequently requires experimental data to establish a complete relationship - which includes certain coefficients and the range of application of both the theoretical and empirical aspects.

* laminar flow - ламинарный поток

2 irrotational flow - безвихревой поток

* accusation - обвинение

* indispensable - необходимый

2. FEEDBACK CONTROL SYSTEMS

(Для перевода со словарем)

The essential feature of many automatic control systems Is feedback. Feedback is that property of the system which permits the output quantity to be compared with the input command so that upon the existence of a difference an actuating signal arises which acts to bring the two into correspondence. This principle of feedback is really not new to us; it surrounds every phase of everyday living. It underlies the coordinated motions made by human body in walking and driving an automobile. It plays an equally Important role in the countless applications of control system engineering in the fields of contro of aircraft, special-purpose computers for many types of military equipment and in many other fields.

The distinction between an elementary system and one which is complex lies primarily in the difficulty of the task to be performed. The more difficult the task, the more complex the system. In fact with many present day systems this complexity has reached such proportions that system design has virtually become a

science. The functional behaviour of each system Is treated here in terms of a blocl< diagram and its associated terminology. Following this, attention is focused on the steady-state analysis of the performance of a voltage and a speed control system. Such a study accomplishes two objectives. First, it gives substance to some of the general ideas discussed up to this point, thereby making the operation of the system more vivid. Then, useful results applicable to any feedback control system are developed.

idealized system

ideal vqlue desired output

+ Tsystein i error

Command J

Reference Input

iput-Ч ial /verlable / Tv

ence 1/ /1 Control / Icqiitrolld / ,ut Lo* elefflent* L. aijstem -j

Reference

InpL. element r* J

tpjitrolled variable

Control eleBientt 9,

Controlled system

prlmarv feedbacit

Feedback element ») h

Block diagram of a feedback control system.

Every feedback control system consists of components which perform specific functions. As has been mentioned above, a convenient method of representing this functional characteristic of the system is the block diagram. Basically that is a means of representing the operations performed in the system and the manner in which signal Information flows throughout the system.

The block diagram is concerned not with the physical characteristics of any specific system but only with the functional relationship among various parts in the system. In general, the output quantity of any linear component of the system is related to the input by a gain factor and combinations of derivatives or integrals with respect to time. Accordingly, it is -possible for two entirely different and unrelated physical systems to be represented by the same block diagram, provided that the respective components are described by the same differential equations.

3. REACTORS FOR NUCLEAR-POWERED GAS TURBINES

(Для перевода со словарем)

High-temperature gas-cooled reactors would appear to be particularly well suited to nuclear-powered, closed-cycle gas turbines, particularly if the working gas in the turbine circuit may be passed

through and heated directly In the reactor core. This gives the most compact system possible. A possible disadvantage to such an arrangement is that accidental escape of fission products from the reactor core into the gas stream may contaminate the machinery, and further problems are introduced if the gas used is of a type which can acquire induced radioactivity in passing through the reactor. This may demand light shielding around the turbo-machinery, a safety containment vessel enclosing the entire reactor and turbo-machinery, and will introduce some problems in machinery maintenance if the machinery becomes mildly radioactive. Such problems may not be particularly troublesome, however, except in the event of a major mishap in the reactor core.

These latter problems might be minimized if the reactor gas coolant circuit and the gas turbine circuit are separated from each other, the heat in the hot reactor coolant gas being transferred to the working gas in the turbine by means of an intermediate gas heat exchanger. However, with such an arrangement the intermediary high-temperature heat exchanger would be bulky, heavy and expensive and the potential advantage of compactness might be lost. Moreover, the insertion of a heat exchanger between the reactor gas coolant and the turbine gas inevitably necessitates that the maximum turbine gas temperature is depressed to a value 50-100° С lower than the maximum gas coolant temperature at outlet from the reactor, possibly leading to some sacrifice in the gas turbine thermal efficiency. Furthermore, a separate and independently driven compressor must be inserted in the reactor gas coolant circuit in order to circulate the coolant gas, resulting in additional overall melihanical complexity and further loss of net overall thermal efficiency.

It would seem probable, therefore, that a dual circuit arrangement of reactor and gas turbine of the above form might show little or no advantage over the more conventional reactor-steam turbine systems. Alternatively, such dual circuit arrangements might only prove attractive if high-temperature liquid-cooled reactor systems should prove ultimately to be practicable.

КОНТРОЛЬНЫЕ ВОПРОСЫ

1. На каком месте в предложении стоят обычно слова и словосочетания, служащие для связи частей высказывания (§ 73)?

2. Как переводятся следующие слова и словосочетания, выполняющие функцию средств связи:

to begin with, likewise, on the contrary, similarly, hence, to sum up, conversely, accordingly, alternatively.

3. От чего зависит перевод слов also и again? Укажите возможные варианты перевода каждого из этих слов (§ 74).

4. Какие значения может иметь слово then (упр. 8)?