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Sect 2. 317 tinuity surfaces 1. This suggests that a wake pressure
Pw be associated with each flow past a bluff body, and that a wake
parameter (2. 4) which plays the same role as the cavitation
parameter (2. 1), be defined for the flow. This idea has been made
the basis of a modified wake theory (ef. Sect. 11) which proves to
be in good qu- titative agreement with pressure and drag
measurements. It should be emphasized, however, that un h like the
cavitation number, the wake parameter is a quantity which is not
known a priori, and must be empirically determined in each case.
(3) Jet flows. The problem of jet efflux from an orifice is one of
the oldest in hydrodynamics and the first to be treated by Fig. 3a.
the HELMHOLTZ free streamline theory. Of particular importance for
engineering applications is the discharge coefficient Cd' which is
defined in terms of the discharge Q per unit time, the pressure P,
and the cross-sectional area A of the orifice, by the formula, (2.
5) where e is the fluid density. Two methods of measuring Cd have
been most fre quently adopted. In the first the liquid issues from
an orifice in a large vessel under the influence of gravity _,
-____________ ., (Fig. 3 a), while in the second it 1 L is forced
out of a nozzle or pipe under high pressure (Fig. 3 b)."
343 Whilst this may be so it is also true that this in itself is
not sufficient to deter mine it completely. In fact the extent of
the dead air region and the behaviour of the shear layer are also
of prime importance and in short a unified treatment comprising
external flow, boundary layer, shear layer and dead air region
becomes necessary to complete the investigation. This would take us
outside the scope of the present article and for the substantial
progress that has been made towards such a treatment the reader is
referred to a paper by HOLDER and GADD 1 and its comprehensive list
of references. v. Heat transfer in incompressible boundary layers.
25. Introduction. The term fluid includes gases and liquids. Both
gases and liquids are to some extent compressible but in many
problems of fluid flow the density changes occurring are small.
When they are small enough to be negligible we can regard the flow
as incompressible. In Chap. IV we have established the equations
for compressible flow of gases and these can of course be used to
deter mine when density changes in a gas flow are in fact
negligible. Broadly speaking this will be so when the temperature
changes as determined by the energy equation are small enough."
Am 8. Juni 1988 veranstaltete die Technische Hochschule Darmstadt
un ter der Schirmherrschaft ihres Prasidenten, Professor Dr. HELMUT
BOHME, eine wissenschaftshistorische Tagung zum dreihundertsten
Gedenkjahr der erstmaligen Drucklegung der Philosophiae Naturalis
Principia Mathematica von ISAAK NEWTON am 7. Juli 1687. Ich wurde
mit der Organisation und Durchfiihrung betraut. Dank der
tatkr8.ftigen Unterstiitzung vieler meiner KoHegen ist daraus nicht
nur ein schoner und gediegener AnlaB geworden, sondern ein
interessanter und lehrreicher Zyklus von Vorlesungen zur Ent
wicklungsgeschichte der Mechanik, dessen Herausgabe in Buchform,
wie mir schien, sich lohnte. Das vorliegende Bandchen umfaBt neben
einem Geleitwort von Herrn H. BOHME, dem Prasidenten der
Technischen Hochschule Darmstadt, vier Beitrage von anerkannten
Wissenschaftshistorikern oder Naturwissenschaft lern, die sich der
Geschichte ihres Fachgebietes verpflichtet fiihlen. Alle Beitrage
beleuchten entweder die Entstehungsgeschichte der Principia oder
stellen ihren EinfluB auf die Entwicklung der Mechanik im den
Principia folgenden achtzehnten Jahrhundert dar. G. BOHMEs Beitrag
De Gravitatione klart auf, oder gibt Begriindungen dafiir, daB
NEWTON mit diesem Manuskript eigentlich die Principia schrei ben
und gleichzeitig die Prinzipien der Philosophie von DESCARTES wider
legen wollte. D. SPEISER legt in seinem Aufsatz iiber die
Grundlegung der Mechanik in HUYGENS Horologium Oscillatorium und in
NEWTONs Principia dar, daB NEWTON in HUYGENS einen Vorlaufer fand,
der Teile seiner grundlegenden Axiome bereits vorwegnahm und somit
wegbereitend zu deren Formulierung beitrug."
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