Attaining a Steady Air Stream in Wind Tunnels Page: 4 of 40
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N.A.C.A. Technical Meiorandum No. 720 3
fans tn' which the section--of.the passage does not correct-_
..:.i-af6orma with the enlargement of the. volute asing, mam-
ifet- such pressure pulsatiton with a considerable range
of orratig .onditiona. This caT-bo quite often r.ome-
-- 'died"be- closing the exit.:ection of the wheel: for a cer-
thin fraction, say,.by fitting a strip of metal, or oil-
clbth or such, which, of opvrse, must be guarded against
being blown off.
Most electric, motprs: are subject-to speed variations
.due -to-fluet ' -ions in voltage or freqi -cy of the power
suUply; dasid. .from that, the speed ofi.&;c"motors especial-
ly, gradually changes as the motot.raresu-u. Therefore,
it is :necesary-.to provide in some .way for constant spei
of the-mutor, or.better yet, of the wind velocity, rs-scec-
ti'v-y~" Lt dynamic pressure. The latter is e3pecialJy
-r' oas.udIed for air-resistance expcrinrts. (See section
V.)- Aky pressure difference existing Ln the tunnel which
is proprtiocal to the dynamic pr.ossure, can be kept con-
sta:nt herch.7. This may be achieved by micromanomet r,
manual reguLattio- of thelelectri motor, or with an auto-
matic rogulator, which hoids this 2roesure dtff'erenco
constant. his lslatter :ethed .has proved very satisfacto-
ry in the Gottingon laboratory, where it has teen in use
fof:twenty years.. The essential part of the regulator is
a~pressure.scale which can-ba, loaded with weights until
it balances under a certain pressure. Any deviatiQn from
.. his pressure closes an electric contact which actuates a
small booster" motor. The latter in turn adjusts a resist-
anco and through it, changes the spoed of the motor. Pro-
vision must be made for breaking the contact before the
new equilibrium attitude is reached, in order to prevent
permanent regulator oscillations. This can be effected
in various ways as known from other regulator types. In
one method the contacts :;r.e slowly pulled to the side con-
c'.rrently with the regulation stage and resume their orig-
inal'sotting after regulation by means of an oil damper.
Tr scalo itself must alsp be oquippod with'an oilt dazmper
to guard against oscillations dre to shock. DoscriptIons
of:such regulators can be foud in.Z.V.D.I. 1909, p. 1715
(regulator of the Gottingon wind tunnol), also in R. Zron-
erts report; Experimenta in nlargod T'unzels," -torschuzns-
heft ;: 222 des Voroinus Deutscher Inge".ieuro, 1920, page
22 (s: all special wind tunnel); f-trtiher .il Zr:ebzinsen
der Aerodyna:iischen Vepouchs.stalt zr a ttincn. no. 1,
1921, prge 20 (large GottiAgon wind tunnel).4 UPDATA 1975 b
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Prandtl, L. Attaining a Steady Air Stream in Wind Tunnels, report, October 1933; (https://digital.library.unt.edu/ark:/67531/metadc63512/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.