By means of a recently installed wind tunnel with a circular free jet, it is shown that requirements concerning the flow parameters (e.g., uniform velocity profile and uniform and low turbulence level in the nozzle exit) can be easily and at moderate expenses fulfilled without changing the settling chamber and the nozzle itself. The installations in the settling chamber are adjustable. The structure is not limited to settling chambers with a circular cross section

Vagt, J. D.

English

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NASA TECHNICAL HEHORANOm1 NASA TH 77326· 
REMARKS ON THE LAYOUT OF THE SUBSONIC FREE JET WIND TUNNELS 
Jorg-Dictcr Vagt 
Translation of "Bemerkungen zur Auslegung cines Unterschall-Freistrahl-
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WASHINGTON, D.C. 20546 NOVEMBER 1983 
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fU:::IARKS ON TilE LAYOUT,OF TilE SUBSONIC 
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By.means of a recently installc.d wind tunnel with.a circular 
free jet, it is shown that requirements concerning the flO\ ... 
parameters (e.q. lIni form velocity profile' and unif~rm and low 
tubulence' level in·.the no;,:zle cxi t) can be casily and at mod-
erate expenses ful filled w~thout changing tllc scttling chamber 
and th~.·nozzle'itsplf. The insta 11a bon!; in the settling 
chamber arc adjustable. The structure is not limited to 
settliny chambers with a circ).llar cr"i)s?-scctipn. 
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REMARKS ON THE LAYOUT OF THE SUBSONIC FREE JET WIND TUNNELS 
Jorg-lJictcr Vagt 
Hermann Foettinger Institute for Flow Technology 
Technical University of Berlin 
I. Introduction 
Wind tunnels often constitute a prerequisite for conducting ex-
periments on flow mechanics. Problems of the aircraft dynamics or also 
problems of the still young sector of building aerodynamics can only be 
worked out in the wind tunnel. In any case however the wind tunnel 
should not generate only a random flow, but exclusively a reproducible 
flow movement defined by the problem and which is characterized by the 
velocity profile, the turbulence profile, etc. Thus for example in one 
case we are interested in the flow around a body in a shear flow, where-
as in another case investigations in a flow with equal velocity vector 
everywhere should be prefered. 
Since experience in building wind tunnels can be transfered only 
rarely, especially when special requirements are imposed on the flow, 
particular importance is attributed to the oplimizing of the channel. 
Therefore this article will report on how by means of a simple con-
struction a wind tunnel may be optimized quickly and economically with 
regard to a uniform velocity profile and a uniform and low degree of 
turbu10nce. But for the sake of clarity a few preliminary remarks will 
first be made on the plant. 
2. Free Jet Plant 
The plant consists of the components blower if air filter collected 
before it, stabilization chamber with diffuser at the entrance, built-in 
units in the chamber and the nozzle. The decrcase of cross-section of 
*Numbcrs in the margin indicate pagination in thc foreign tcxt. 
1 
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ORIGINAL PAG~ t9 
OF POOR QUALllY 
the nozzle over its length was calculated according to /1/. But the 
boundary conditions to achieve a shorter structural length were some-
what modified (compare Figure 1). 
2 
The following requirements were first imposed: 
1. Extensive freedom from dust of the flow, to protect on one 
hand the probes, but on the other hand also to protect the 
built-in units in the stabilization chamber from dirt. 
2. To prevent interferences the sieve should in accordance with 
a study by Bradshaw /2/ should r.ot go below an opening ratio 
of 
of 0.57. Here d is the wire diameter of the round wire sieve /16 
and 1 is the mesh width. 
3. The Reynolds number formed with the wire diaffieter in the 
4. 
chnmber 
should nccording to Pankhurst and Holder /3/ have an upper 
limit of 40, to decrease the intensity of turbulent fluctuations. 
The coefficient for the decrease of pressure on the sieve should 
have a value of approximntely 2 according to /3/ to reduce the 
Inck of uriformity in the flow profile in front of the sieve. 
Sl'~ I (1) 
Figure 1: Stabilization chamber with built-in units and 
nozzle. Nozzle contour: 
r 0_, 
! - .' - l' (J _ tI _ Z , 
", -'.. .), I I. "nil" -U, I~"~l.u (r. - '.' 
Key: (1) sieve; (2) rotating; (3) foam plate. 
· .~. 
In the present case the following values were obtained: 
For 1: The air filter used. absorbs 99.9 percent of all the dust 
particles in the air with a grain diameter of 2.5 IJ m and 80 percent 
of all dust particles with a grain diameter of 0.5 \.I m. 
For 2: 5=0.59. 
For 3: Rekmax=36. 
For 4: For the pressure loss coefficient according to the power 
law of Wieghardt /4/ for the round wire sieve used with d c O.2 rom 
a value of about 1.9 to 2.4 was obtained according to the speed. 
3. Optimization 
The optimization studies refer according to what was stated above 
to the incorporated units in the stabilization chamber. In general the 
number of these units and the place of incorporation are established 
beforehand. The stabilization chamber is subdivided into several parts 
and the site of incorporation is therefore at the connection point of 
two parts of the chamber. A subsequent revision requires considerable 
structural outlay and is therefore costly. 
The purpose was now to organize the structure of the incor~orated 
units in such a simple way that a displacement, exchange or addition of 
a sieve is possible without having to intervene in the stabilization 
chamber itself. In this connection the sieves must be always stretched 
and free from bumps. Fluttering phenomena must be avoided unconditionally 
in the! flow. 
3.1 Construction of the Holders for the Sieve 
The incorporated units extend in this case over a circular cross-
section with a diameter of 2 m. The incorporation of the sieve is first 
discussed. 
3 
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ORIGINAL PAGE i9 
OF POOR QUALITY 
A steel ring of rectangular cross-section is halfed and a rigid 
screw shackle is applied on the two separating points (compare Fig. 2). 
Figure 2: Sieve holder 
Key: (1) spanner; (2) welded threaded rod; (3) after 
incorporation; (4) bonding strip; (5) sieve; (6) initial 
stress in the sieve. 
After their introduction in the stabilization chamber the ring 
halves are drawn apart with the spanners to such an extent that they 
lie on the wall. At the same time the sieve is stretched. If the place 
of t~e sieve has to be altered, the spanners must be loosened to some 
extent. The diameter of the sieve or of the frame is reduced and the 
frame can be shifted. In this connection generally folds occur on the 
sieve, but they disappear again insofar as there was no overextension 
in the first stretching. 
Figure 3 shows two sieves moved several times. The smooth state 
free from folds can be observed particularly in the rear sieve because 
of the illumination in the plane of the sieve. 
4 
OmG1NAt PAGE IS 
OF POOR QUALITY 
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Figure 3: Sieves inside the stabilization chamber. 
The principle of the holding structure is not limited to circular 
cross-sections. For example for a rectangular channel a structure is 
proposed according to Fig. 4. Here four corner frames are joined with /1. 
spanners. The angle between the arms of the frame should be more than 
90 degrees before the connection, to assure sufficient tension in the 
sieve·even in the area of the spanners. If necessary the assembly can 
also be carried out with spacing units, which bring the frames to their 
final form and can be removed after the introduction of the sieve. 
, ____ -- -- 1 
r[---k:l, __ (!L~) 
,[ I .. 1, 
'-= __ ----1_. '.l:~ 
4 ----;--- ------- J 
Figure 4: Construction of the holder for 
rectangular cross-section. Key: (1) spanner. 
The actual optimization consisted in this case of establishing the 
type and number of incorporated units and their position in the stabi-
lization chamber to achieve the required properties of the flow at the 
end of the nozzle. This requires thorough measurements at the nozzle 
outlet, which had to be carried out after each change of the incorporated 
uni ts. 
5 
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The measurements were carried out with a heating wire anemometer of 
the type UDA II of the DFVLR /5, 6/ (Federal German Aeronautics and Space 
Organization) combined with a gauge of the degree of turbulence of the 
type TGM of the DFVLR /7/. The measurement probe for the longitudinal 
intensity of the fluctuations of velocity for the average velocity was 
a so-called single wire probe. To obtain the transversal intensity 
which is not discussed in greater detail here, an X-wire probe was 
introduced according to /8/. 
The following criteria were used for the quality of the flow: 
1. Uniformity of the velocity profile over the nozzle cross-section 
at different levels of the nozzle. 
2. Uniformity of the degree of turbulence or the intensity of the 
velocity fluctuations in the direction of flow in the frequency 
range of 5 Hz up to 10,000 Hz and the extent of the degree of 
turbulence itself. 
3.2 Results of Optimization 
For the arrangement I according to Fig. 5 with two sieves, of which 
one directly behind the diffuser to avoid detachment phenomena in the 
flow, a nonuniformity factor was obtained (Umax - Umin}/Umax of about 
1 percent was obtained measured in the outlet plane of the nozzle. The 
measurement points are obtained from an automatically and continuously 
recorded curve variation. 
The longitudinal intensity of the fluctuations of velocity assumes 
on the other hand high values (more than 2 percent) is not uniformly 
distributed over the cross-section of the nozzle and depends greatly 
on the speed. 
The incorporation of a third round wire sieve at the end of the 
stabilization chamber, directly in front of the nozzle, caused a decrease 
of the longitudinal intensity by about a factor 4. The nonuniformity 
factor was here increased to 2 to 3 percent. 
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Since the origin of the fluctuations lies certainly in front of the 
fin:t sieve, spin in the ventilator, detaclunents in the diffuser, after 
applying a 5 rom thick foam plate an attempt was made to reduce consid-
erably the intensity of fluctuations and the spin practically at the 
site of their occurrence. The results are shown in Fig. 6. The inten-
sity as improved decisively (less than 0.3 percent) and is distributed 
uniformly. The nonuniformity factor was now about 2 percent, naturally 
only in the measurement plane - 4. 
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Figure 5: Arrangement I. Distribution of the average velocity and the 
intensity. Key: (1) velocity distribution~ (2) stabilization ch~mber; 
(3) diameter~ (4) length. 
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Fig. 6: Arrangement IV. Distribution of the average speed and intensity. 
Key: (1) distribution of veloc~ty; 
Only a displacement of the second and third sieve upstream caused 
an improvement of the nonuniformity factor by about 1.S percent. The /1 
distribution of intensity was not altered in this case (Fig. 7). 
Here we have ~nly a relative optimum, that is decisive improvements 
as was shown by further experiments cannot be achieved directly by 
altering the incorporated units. An analysis of the frequencies occurring 
showed that a large portion of the intensity comes from frequencies 
between 10 and 13 Hz. In the measurement with high pass filters 
connected in between (20 Hz) the intensity in the total speed range is 
about 0.09 percent. 
The more detailed study showed that the wall of the stabilization 
chamber oscillates in operation with a frequency of 10 to 13 Hz in the 
present range of speeds and that thus the further reduction of the 
speeds of oscillation in the flow is possible only by increasing the 
wall mass because of the low frequencies of the wall oscillation. 
8 
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Fig. 7: Arrangement VI. Distribution of the average speed and of the 
intensity. Key: (1) distribution of speed; (2) foam. 
4. Summary 
Wind tunnels often represent the prerequisite for conducting ex-
periments on flow mechanics. This means that a reproducible flow 
movement defined by the problem is needed. But so far the production 
of an exactly defined flow is possible only in the rarest of cases by 
.transferring the values obtained by experience from one tunnel t~ the 
other. The tunnel must be optimized, that is th~ required flow movement 
must be produced by altering the individual components, especially the 
sieves • 
By a flexible consturction of the above described type the opti-
nlization of the wind tunnel can be conducted quickly and cheaply, since 
costly interventions in the external structure of the tunnel are avoided. 
The optimization studies are discussed on the basis of an example of a 
new wind tunnel with round cross-section in the Hermann Foettinger 
Institute for Flow Technology of the Technical University of Berlin. 
The structure is basically available also for noncircular cross-sections. 
9 
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5. Bibliography 
/1/ R. Wille: Contributions to the Phenomenology of Free Jets. DVL 
Report numoer 292 (1963). 
/2/ P. Bradshaw: The Effect of Wind Tunnel Screens on Nominally Two-
Dimensional Boundary Layers. J. Fluid Mech. 22 (1965), pages 
679-687. 
/3/ R. C. Pankhurst and D. W. Holder: Wind Tunnel Technique. Pitman 
and Sons Ltd., London, 1948, page 78. 
/4/ W. Wuest: Wire Sieves in Flow Technology. Draft II (1960), 
number 8, page 417. 
/5/ E. Berger, P. Freymuth, E. Froebel: Application in Regular Tech-
nology for the Development of a Constant Temperature Heating Wire 
Anemometer. Part I. DVL Report number 282 (1963). 
/6/ E. Berger, P. Freymuth, E. Froebel: Application of Regular Tech-
nology in the Development of a Ccnstant Temperature Heating Wire 
Anemometer. Part II. DVL Report number 283 (1963). 
/7/ E. Froebel: Gauge of the Degree of Turbulence TGM I. Internal 
Report of the DFVLR Institute for Turbulence Research, Berlin 
(patent applied for). 
/8/ J. D. Vagt: Hot Wire Measurement Technique. Von Karman Institute 
for Fluid Dynamics, Brussels, Lecture Series 32 (April 26-30, 1971 
Turbulent Jet Flows. 
Received on 24 January 1972. 
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Remarks on the layout of the subsonic free jet wind tunnels

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