Effect of stabilizing forces on turbulence Page: 2 of 12
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NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
TECHNICAL MEMORANDUM NO. 625
EFFECT OF STABILIZING FORCES ON TURBULENCE*
By L. Prandtl
I have frequently noticed on pleasant summer evenings that
a temperature layer, such as is caused by the evening radiation
and the ensuing cooling off of the air strata near to the
earth's surface, manifests itself in the sudden cessation of
the turbulence of the wind and flows on without any. sign of
forming clouds. There is but one interpretation for this,
namely, the warm air glides in laminas over the cold layers of
air beneath it. The appearance of expressed discontinuity
layers in the free atmosphere, where a warm air mass flows over
a cold mass without perceptible intermingling, belongs in the
same group, so also the flow of fresh water over sea water as
is particularly observed in the polar ocean. The opposite
phenomenon is that of .increased turbulence due to convective
motions ensuing from weight differences when the earth and the
bordering lowest layers of air or water are warmed up. In the
first case the "exchange" as well as the friction of the flow-
ing air or water mass is lowered, in the second case it is
raised. Since these interrogatry .forms were in close relation
to our GSttingen test program on fully' developed turbulence,**
I acquiesced to the request of Wilhelm Schmidt of Vienna to
undertake a study of this subject. In these experiments an
air stream is to be blown )etween a water-cooled and a vapor-
heated plate. According to what preceded it is anticipated
that the exchange will be lowered appreciably or even stopped
altogether under appropriate conditions if the stratum is
staple (warm above, cold below), while in the contrary case
the exchange will be enhanced. Because these experiments had
to be made at relatively low air speeds so as to let the lift-
ing forces become effective, it first required a study of the
test methods, and so we have not yet proceeded beyond the pre-
liminary tests on the most suitable hot-wire connections and
thermocouples for recording speed and temperature. The wish
to predict the phenomena which we want to observe here prompt-
ed me, however, to search into the theory of these phenomena.
*"Einfluss stabilisierender Krafte auf die Turbulenz," from a
reprint of Vortrage aus dem Gebiete der Aerodynamik und
verwandter Gebiete, Aachen, 1929, ..issued by A. Gil:les, L. Hopf
and Th. v. Karman.
**N.A.C.A. Technical Memorandum No. 435, Sept., 1926, "Turbulent
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Prandtl, L. Effect of stabilizing forces on turbulence, report, June 1931; (digital.library.unt.edu/ark:/67531/metadc277441/m1/2/: accessed July 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.