The trend for decreasing the drag of aircraft is retention of laminar flow in the boundary layer over a large portion of the surface. The laminar boundary layer was studied in a low turbulence wind tunnel for low subsonic velocities. The method used and results of measurements of very low levels of turbulence are presented. Measurements were performed by a constant-resistance thermoanemometer

Demin, V. S.

Morin, O. V.

Polyakov, N. F.

Shcherbakov, V. A.

English

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NASA- Tm 75282
MEASUREMENT OF LOW TURBULENCE LFVELS
WITH A THERMOANEMOMETER
V.S. Demin, O.V. Morin, N.F. Polyakov,
V.A. Shcherbakov
Translation of "tzmereniye nizkikh urovney turbulentnosti
termoanemometro.-,", Izvestiya Sibirskogo Otdeleniya A.adecnii
Neuk SSSR, SPriva Teldinicheskikh Nauk, Vcl. 8, No. 2, June
1972, pp. 21-24'
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(NASA-TM-75282)
	
MEASURFMEN^ (1F 1.04	 N78-26105
TURBULENCE LEVELS WITH A 1HERSUANE40METER
(National Aeronautics and Space
^dministratien)	 11 1) HC AC 2/Af AJ1 CSCL CIA	 Unclas
63/02 23392
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, D.C. 20546	 JUNE 1978
aj
STANDARD TITLE PAGE
1. Report No. 2.	 Government Accession No. Y.	 Recipient's Catalog No.
!ASA TM-75282
4. Title and Subtitle S.	 Report Date	
^,zne	 1978MEASUREMENT OF TAW TURBULENCE =LS WITH A -
6.	 Performing Orgonisotion CodeTHERMOAITEMOMETER
7. Authors) !.	 Performing Orgonisation Report No.
V.S.	 Demin, 0. V. Morin,
,
 N.	 F. Pclyakov, and
10.	 Work Unit No.V. A. Shcherbakov
9. Performing Orgon,of An Nome and Address
Leo Kanner Associates
I1.	 Contract or Grant No.
NA v,1-2790
P. 0.	 Box 5187 13.	 Type of Report end Period Covered
Redwood City, CA	 94063
Translation
12,	 Sponsoring Agency Name oral  Address
i?E.tional Aeronauticc and Space Administration
Washington, D. C. 20546 lI.	 Spo nsorng Agency Code
15. Supplementary Notes
Translation of "Izmereniye nizkikh urovney turbulentnosti
termcane^iometrom", Izvestiya Sibirskogo Ctdeleniya Akademii
Nauk SSSR, Seriya	 ..'ekhnicheskikh Nauk, Vcl. 8, No. 2, June
1972, pp. 21-24 (A73-13666)
16. Absr,ect.
17. Key words(Selected by Author(s))	 18.	 Distribution Stetement
Unclassified - Unlimited
10. Security Clesell. (of this repots) 	 20.	 Security Clossil. (of this page) 21• No. of Pages	 22.
Unclassified	 ^Jnclassified 11
.. ....^ 
__........ ,re..ee► ..•..r». gasser ^,aty^q
"""'M"''R'"M"'F.,.
E'11:ASUREMENT OF LOW TURBULENCE LEVELS
WITH A THE RMOXNEMOML'TER
V.S. Demin, O.V. Morin, N.F. Polyakov,
V.A. Shcherbakov
The most promising trend for decreasing the drag of	 121*
aircraft is retention of laminar flow in the boundary
layer over a large portion of the surface. Achievement of
this goal requires that the laminar boundary layer be studied
in all stags of its development. These studies can be 	 1
p:rformed only in streams with low turbulence [1], created
in special wind tunnels. The low-turbulence wind tunnel
for low subsonic velocities of the Institute of Theoretical
ana Applied 'fechanics, Siberian Affiliate, Academy of
Sciences, USSR, is suitable for such studies. However,
significant difficulties have arisen in studying the para-
meters of the flow in this tunnel, and also in analogous
foreign low-turbulence tunnels [2,3], related to the measure- 	 M'
ment of the low degree of turbulence of the flow. A thermo-
anemometer [hot-wire anemometer -- Tr.] is the most reliable
instrument for measurement of tf,ese low levels of turbulence.
One difficulty is that the level of the output signal, which
corresponds to pulsations in flow velocity, is in this case
comparable to the level of the natural electrical noise in
the thermoanemometer. This article presents a method and
results of measurement of very low (on the order of a few
hundredths of 1%) levels of turbulence.
*`umbers in the margin indicate pagination in the foreign text.
1
iThe experiments were conducted in the low-turbulence,
low subsonic velocity wind tunnel of ^:he Ins<__tute of
Theoretical and Applied Mechanics, Siberian Affiliate,
M
Academy of Sciences, USSR. This is a closed-type wind tunnel
with a closed measurement section 1000 x 10J0 mm in cross
section. The low degree of turbulence of the flow in the
measurement section is achieved by great constriction of the	 i
flow in a confusor (n=1 7 .6), by a set of deturbulizing grids
in the forecliamber, by the small aperture angles of the
diffusors, etc. The measurements were performed by installation	 j
of a grid of 0.3 mm-diameter wire in the forechamber with a
mesh of 1 mm. The measurements were performed by a DISA type
i
5SD00 constant-resistance thermoanemometer and type SS 22
transducers with a filament diameter of 5 um made of gold-
plated tungsten. The transducer i:as installed on a rigid
holder at a fixed point on the axis of the measurement section
of the wind tunnel. Only pulsations in the longitudinal
component of the flo g. velocity were measured. The intensity
	
4
of pulsations was calculated by the equation
,i
(1)
i
where U is the output voltage of the thermoanemometer (voltage
at the point of the bridge); U' is the mean square value of
the am p litude of the pulsations in the out p ut voltage,E	 p	 I
resulting from pulsations in the flow velocity; U 0 is the	 /22
standard voltage, corresponding to zero velocit y of the
flow (determined from the calibration graph of the trans-
ducer).
Since there is no correlation between the signal caused
by turbulence and the noise of the thermoanemometer, the
value of U' can be determined by the equation
s
W
Ia`
r^
where U^ is the mean square value of the pulsation component
of the output voltage of the thermoanemometer; Un is the level
of natural electric noise in the thermoanemometer.
At low levels of turbulence, the value of W differs
little from Un; therefore, precise determination of Un must
be given particular attention. Curve 1 (Figure 1) shows the
value of UI as a function of the operating ban" of frequencies
of the thermoanemometer, set by the limiting filters, for a
transducer in the stream, while curve 2 shows the level of
noise, uetermined by t he generall y accepted "cap" method.
In the 0-1 and 0-2 kHz bands, U 1 is practicall y equal to Un,
while with frequency bands of 0-5 and 0-10 kllz, UC<U1 1 , which
can only be a result of the incorrectness of this method of
determination of the noise level.
As we know, the noise level of a thermoanemometer is
determined b y many parameters: the noises of the transistors
or tubes, the gain of the amplifiers, the operating frequency
bandwidth, the ratio of the arms of the bridge, the time
constant of the transducer, its heating, the voltage supplied
to the transducer, and its temperature.
The noise level is influenced by induction from electrical
interference, the "microphone effect" of the circuit Iv}len
operating in a noisy room, thermoanemometer power supply
voltage fluctuations, matching of the wave impedance of the
cable of the transducer to the input impedance of the ampli-
fiers, etc. The degree of variation of noise with each of
these factors is determined b y the design of the thermoane-
mometer and its adjustment and conditions of operation. 	 It is
3
oRIGINA1 , PAGE U
Ov PooR QU,'W'I'Y
(2)
I
,J
ik	 U	 111V therefore clear	 that	 the
0.4
s measurement of the noise!
level must be performed
a•' ^r under conditions as close
c L 2To- a f,kHZ as possible to	 the
operating conditions. 	 The
Figure	 1.	 Variation of "cap" method consists	 in
Indications of Thermoanemometer that the transducer of the
as	 a	 Function of Operating; thermoanemometer is
Frequency Band. covered with a cap, 	 the
1.	 Transducer	 in	 stream,	 U=5.5 V;
thermoanemometer	 is	 turned
Transducer under cap,	 U=3.5 V; on and the output	 signal
3,	 Transducer	 installed by the is measured.	 This	 signal
method of	 the authors	 (see is	 assumed	 to be	 the noise
Figure	 2),	 U=5.5	 V. level of the thermoane-
mometer.	 However,	 when
this method	 is	 used,	 the	 transducer power	 supply voltage	 is
different	 from the operating voltage, since	 there	 is	 no stream
of air moving;	 around	 the	 transducer. We also know that	 the
noise of a constant-resistance thermoanemometer depends
essentially on	 the voltage.
i
,j
,i
j
DISH recommends an indirect method of estimating the
natural noise of a thermoanemometer at a voltage equal to the
	 t
output voltage during actual operations of the filament
transducer [4]. The calculation method of DISH is represented
by the equation
Co" I	 (3)
which utilizes the follow ing; supplementary experimental data: 	 /23
fCO -- the maximum frequency in the operating band of	 , 1
frequencies of tine thermoanemometer at zero flow speed; fCV
4
s	
^
the maximum frequency of the operating band of frequencies of
the thermoanemometer at a flow speed equal to the speed of the
stream being studied; f ir -- the limiting frequency established
by the operator.
One short,-.oming of this indirect method of determination
of the natural noise of a thermoanemometer is the need for
supplementary measurement operations, each of which is
accompanied by significant experimental errors.
We suggest a method of direct measurement of the noise
level of a thermoanemometer by means of the devices in the
thermoanemometer itself under conditions corresponding to its
operating conditions. The idea of this method is that a
stabilized, fluctuation-free level of heat liberation from the
transducer of the thermoanemometer is achieved, equal to the
heat liberation in the airstream being studied. This means
than the thermoanemometer and transducer are actuall y in their
operating modes, but the variable component of the output
signal is determined only by the natural electrical noise of
the thermoanemometer.
The simplest means of achieving this method is to place
the transducer in a cooled cuvette containing a thermally
stable dielectric p owder (Figure 2). After cleaning, the
transducer can be used again for measurement of turbulence.
If a series of identical transducers is available, it is
more convenient to use one of them onl y for determination of
the noise level of the thermoanemometer. 'To do this, the
sensing portion of the transducer is sealed into a glass or
metal drop, after first applying a thin film of thermally
stable dielectric to the sensing portion. Due to the greater
S
ORiCINA[, PACF h
14
i. 1
f
I	 1
I	 I
A
heat conductivity of the medium and cooling, in comparison to
a gas, sufficient heat liberation of the transducer is achieved,
while the thermal inertia of the medium and the absence of
turbulent movement in the medium assure stability of the heat
liberaticn level. Various heat loss rates of the transducer
can be achieved by changing the temperature of the transducer,
the temperature of the cooling medium, by using substances in
contact with the transducer with different heat conductivities.
Curve 3 of Figure 1 shows the noise level as defined by this
method as a function of operating frequency band of the thermo-
anemometer. A comparison of curves 2 and 3 shows clearly that
the "cap" method yields artifically elevated noise levels,
making it unsuitable for the measurement of low degrees of
turbulence.
=-,	 Figure 3 (curves 1
and 2) shows the noise
level of a DISH 55D00
I.	 tali	 thermoanemometer as a
function of output voltage
I	 _
ilk ;I	 at a fixed frequency.	 Our
attention is drawn by the
noticeable drop in noise
level with increasing
Figure 2. Diagram of installs-	 output voltage U, which
tion of transducer for deter-	 explains the elevated
mination of natural electrical	 noise levels determined
noise of a thermoanemometer.	 by the "cap" method.
1, Capsule; 2, Corundum powder;	 Figure 3 was obtained by
3, Transducer filament; 4, Sealing	 calculation by DISA. We
cap; 5, Output to thermoanemometer	 can see from the graph
that this method yields
a thermoanemometer noise level elevated by approximately 200.
6
.I
epg W44KM.a 1%"*-*
	
1 1 	'N"~ .maw -saw" Woo -'Mr -mow	 400-.^ 660 am 'kjMRj*'~ 4t1
In determining the variation of noise level with output
voltage U, the voltage was varied by changing the temperature
of the cooling medium and of the transducer. The noise levels, /24
measured with the same power supply voltage but with different
temperatures of the transducer and medium, coincided. This
indicates that the level of nat,iial noise of the thermoanemo-
meter depends only on voltage U
P
The transducer was calibrated directly 	 in the measurement
portion of the wind tunnel. 	 The turhulence was measured at
flow	 speeds	 of 2-45 m/s	 in
u;m the	 0-1 and 0-5 riHz	 frequency
bands.	 The	 results of the
0.4-
measurements were processed
using equations
	
(1)	 and
o.z `^,	 2
(2)	 and curves	 1 and 2 of
Figure	 3.	 It was assumed
o	 ^.0	 U,v that	 the	 fluctuations	 in	 i
temperature of the stream
Figure 3.	 Thermoanemometer were negligible.	 The
noise as a	 function of output results,	 shown	 in	 Figure	 4,
voltage. show the variation of the
1,	 0-1000 Hz	 operating	 frequency longitudinal	 component of
band,	 method of the authors; the degree of turbulence
2,	 0-5000 Hz operating 	 frequency with mean stream speed.	 Ile
band,	 method of the authors; can see from this graph that
3,	 0-5000 ffz	 operating	 frequency the	 level of turbulence	 in
band,	 calculation method of the	 5--45 m/s	 speed	 range	 is
DIS1 0.02-0.030,	 increasing	 to
0.042%	 as	 the speed decreases
to	 2	 m/s.
i
I l
,r	 i
ORIGENAL PAGE IS
OF POOR QUALITY
7
,w,rrr,^,!t++^ar, ..^,p+^►„w^+.."..r .a.. s.. _. .,r. wwn^	 .teal
	 ifke^
- - - 
-	
- - -	
- -__-` T-	 , ---7--
CA,
0.04
co3	 ^^
o	 ru	 y'r
Figure 4.	 Level of turbulence
as a function of flow speed
in measurement portion of
low-turhulent wind tunnel
of ITAM, SA AS USSR
Since it is difficult
to perform frequency
analysis, due to the low
level of the signal of
the the rmoanemometer, a
rough analysis was performed
using the filters iii the
thermoanemometer.
	 It showed
that the primary portion of
the turbulence occurs in
the infralot. frequency
band at 0.5-20 11z, with an
insignificant portion in
the 20-200 IIz band.
Conclusions
1. Measurement of very low levels of turbulence (below
0.10) requires that a method be used to determine the level
of noise of the thermoanemometer under its operating conditions.
'rhe "cap" method is unsuitable for the measurement of low
degrees of turbulence.
2. The level of turbulence in the measurement section of
the ion:- turbulence Suhsonic Wind tunnel of the Institute of
Theoretical and Applied Mechanics, Siberian .affiliate, Academy
of Sciences, USSR is 0.02-0.030 in the 5-4S m/s speed range.
8
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REFERENCES
1. Struminskiv, V.V., Turbulentnvve Tocheniva [Turbulent
Flows] , Moscow, Nau a Press, 1970, p. 22.
2. Reduction des irregularites do 1'ecoulerienten soufflerie
par Georges Lespinard. attache de Recherches on C.\.R.S.
1965.
3. Gorlin, S.A., I:ks p rimentul'nava :lerome khanika [Experimental
:aeromechanics , MOSCOW, Vyss lava SE ola  Press, 1970.
4. DI SA Instruction Manual for T y pe 55 DO  :lncmometer knit,
1969. 
,i
t
1
i
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Measurement of low turbulence levels with a thermoanemometer

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