An improved test section for a supersonic or hypersonic wind tunnel is disclosed wherein the model tested is shielded from the noise normally radiated by the turbulent tunnel wall boundary layer. A vacuum plenum surrounds spaced rod elements making up the test chamber to extract some of the boundary layer as formed along the rod elements during a test to thereby delay the tendency of the rod boundary layers to become turbulent. Novel rod construction involves bending each rod slightly prior to machining the bent area to provide a flat segment on each rod for connection with the flat entrance fairing. Rods and fairing are secured to provide a test chamber incline on the order of 1 deg outward from the noise shield centerline to produce up to 65% reduction of the root mean square (rms) pressure over previously employed wind tunnel test sections at equivalent Reynolds numbers

Beckwith, I. E.

Creel, T. R., Jr.

English

NASA Technical Reports Server

States Patent [191 
Creel, Js. et al. 
[111 4,363,237 
[45] Dec. 14, 1982 
[54] SOUND SHIELD 
[75] Inventors: Theodore R. Creel, Jr., Yorktown; 
Ivan E. Beckwith, Glouscester Point, 
both of Va. 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[73] Assignee: The United States of America as 
[21] Appl. No.: 267,935 
[22] Filed: May 28, 1981 
[51] Int. C1.3 .............................................. GOlM 9/00 
[52] U.S. c1. ...................................................... 73/147 
I581 Field of Search .......................................... 73/147 
1561 References Cited 
U.S. PATENT DOCUMENTS 
3,021,160 4/1962 Holderer ............................... 73/147 
3,602,920 9/1971 Davis .................................... 73/147 
3,853,003 12/1974 Sorensen ............................... 73/147 
3,952,590 4/1976 Howard ................................ 73/147 
FOREIGN PATENT DOCUMENTS 
1575406 9/1980 United Kingdom .................. 73/147 
Primary Examiner-S. Clement Swisher 
Attorney, Agent, or Firm-Wallace J. Nelson; John R. 
Manning 
[571 ABSTRACI' 
An improved test section 16 for a supersonic or hyper- 
sonic wind tunnel 10 is disclosed wherein the model 
tested is shielded from the noise normally radiated by 
the turbulent tunnel wall boundary layer. A vacuum 
plenum 23 surrounds spaced rod elements 28 making up 
test chamber 16 to extract some of the boundary layer as 
formed along the rod elements during a test to thereby 
delay the tendency of the rod boundary layers to be- 
come turbulent. Novel rod construction involves bend- 
ing each rod slightly prior to machining the bent area to 
provide a flat segment 29 on each rod for connection 
with the flat entrance fairing 31. Rods 28 and fairing 31 
are secured to provide a test chamber incline on the 
order of 1" outward from the noise shield centerline to 
produce up to a 65% reduction of the root-mean-square 
(rms) pressure over previously employed wind tunnel 
test sections at equivalent Reynolds numbers. 
5 Claims, 5 Drawing Figures 
SUPERSONIC 
HYPERSONIC ---c 
FLUID 
SOURCE 
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derstood by reference to the following description 
when considered in connection with the accompanying 
drawings, wherein: 
FIG. 1 is a part schematic, part sectional view of a 
The invention described herein was made by employ- 5 supersonic or hypersonic wind tunnel utilizing the im- 
FIG. 2 is a Pictorial view of the novel test chamber 
and accompanying vacuum chamber and conduits em- 
ployed in the wind tunnel shown in FIG. I; 
FIG. 3 is a top plan view showing the interior of the 
novel test chamber for the wind tunnel shown in FIG. 
SOUND SHIELD 
ORIGIN OF T H E  INVENTION 
ees of the United States Government and may be manu- 
factured and used by or for the Government for govern- 
mental purposes without the payment of any royalties 
thereon or therefor. 
BACKGROUND OF T H E  INVENTION 
Proved test chamber of the  Present invention; 
10 
1; 
FIG. 4 is a sectional view of the test chamber as taken 
along the line IV-IV of FIG. 3; and 
FIG. 5 is a part schematic showing of an individual 
rod member making up the test chamber, illustrating the 
angular relationship of the rods forming the test cham- 
ber structural connection thereof to the leading edge 
fairing and the trailing edge exit part, of the wind tun- 
DETAILED DESCRIPTION OF T H E  
INVENTION 
l5 
*’ nel. 
Supersonic and hypersonic wind tunnels presently 
used for ground testing of aircraft concepts are limited 
in their usefulness for purposes of studying boundary 
layer stability and transition by premature transition of 
the model boundary layer. This boundary layer transi- 
tion is caused by noise or pressure fluctuations radiating 
from the turbulent boundary layer on the tunnel wall 
and impinging upon the model. There are two basic 
methods of preventing or reducing the radiated noise. 
First, because it is the turbulent boundary layer that 
radiates noise versus a laminar boundary layer that does 
not radiate noise, a laminar boundary layer on the tun- 
nel wall is desired. This can be accomplished by various particularly 
techniques, one of which consists of using longitudinal 25 to FIG, 1, there is shown portions of a wind tunnel 
to the boundary layer thus maintaining a laminar bound- cluding a conventional baffled entrance 12 in fluid con- 
into a supersonic or hypersonic nozzle 14 directed at a 2566 published July 1972. The second method of reducing the radiated noise is 30 test chamber 16. The exit 18 from test chamber 16 is in 
by shielding the model from the turbulent tunnel wall fluid connection with a vacuum 2o capable of boundary layer. The present invention is directed to a pulling supersonic or hypersonic fluid flow through the system employing this method. wind tunnel 10. In the specific embodiment described It is therefore an object of the present invention to 
provide an improved wind tunnel system for testing 35 herein, vacuum source 20 consists of an evacuated 
Test chamber section 16 of wind tunnel 10 is of rect- flow. 
Another object of the present invention is an im- angular configuration and is surrounded by an essen- 
proved test section for a supersonic or hypersonic wind tially rectangular sound shield 22 which incorporates a 
tunnel wherein the model tested is shielded from the 40 vacuum plenum 23. Four individual ducts or conduits 
layer. serve to provide fluid connection between plenum 23 
Another object of the present invention is to provide and a second vacuum source (not shown). The interior 
a vacuum chamber in fluid communication with and Of test chamber 16 iS of substantially rectangular Config- 
surrounding the test chamber of a wind tunnel to re- 45 uration and formed of a Plurality (eleven On each 
move Some ofthe boundary layer from the test chamber “wide” side and eight on the “narrow” side in one spe- 
as formed and thereby help maintain laminar tunnel cific embodiment) of spaced rod members 28 each of 
wall boundary layers. which is attached at one end thereof to a rectangular 
Another object of the present invention is to provide configured leading edge fairing 31 forming a rectangu- 
a test facility that shields the test model from noise 50 lar configured entrance to test chamber 16. 
radiated by the turbulent tunnel wall boundary layers in The other end of each rod element 28 is attached to 
a supersonic or hypersonic wind tunnel. exit section 18. The corners of test chamber 16 are 
According to the present invention, the foregoing formed of four individual rods 33 (FIG. 4) having inter- 
and additional objects are obtained in the present inven- nally disposed rounded sectional surfaces while the 
tion by providing a slightly tapered rectangular test 55 interiorly disposed surfaces of each rod element 28 is 
chamber in a supersonic or hypersonic wind tunnel circular except near the leading edge fairing end where 
wherein the test chamber walls are formed of spaced a portion 29 of the rods is essentially flat in section as 
rod elements and surrounded by a vacuum chamber. shown more particularly in FIGS. 3 and 5. 
The vacuum chamber provides a vacuum Source to AS shown more particularly in FIG. 5, the individual 
extract some of the boundary layer formed along the 60 rods 28 are first bent to provide a small curvature along 
rod elements within the test chamber through the space a portion of the front end thereof with this tip or curva- 
between the rods and thereby delay the tendency of the ture section being machined to provide a flat surface 29 
rod boundary layers to become turbulent. along this bent area as shown more particularly in FIG. 
3. The thus produced flat end of each individual rod 28 
65 is secured to the flat area of leading edge 31 by a small 
machine screw 34. In order to achieve a flat smooth 
surface leading from the sharp flat leading edge 31 to 
the flat surface portion 29 of rods 28, a flat smooth 
Referring now to the drawings and 
rods for the nozzle walls so that suction can be applied generally designated by reference numeral 10 and in- 
ary layer and i’ more described in NASA TMX- nection with a stilling chamber (not shown) and exiting 
models subjected to supersonic and hypersonic fluid sphere Of 8.3 meters diameter. 
noise radiated by the turbulent tunnel wall boundary 24 (only two of which are partially visible in FIG. 1) 
DESCRIPT1oN OF THE 
A more complete appreciation of the invention and 
many of the attendant advantages thereof will become 
more clearly apparent as the same becomes better un- 
4.363.237 
3 
stainless steel surface plate 35 is bonded over screw 34 
by a conventional adhesive. The  merged resulting sur- 
faces (31, 35, 29 and 28) is then machined to assure a 
smooth flat entrance area to  test chamber 16. Rods 28 
are joined to exit 18 in a conventional manner to  pro- 
vide an interior for test chamber 16 free from protru- 
bances. 
The  individual rods 28 are angularly displaced 1” to 
2” relative to  the centerline of test chamber 16 so as to 
provide a chamber of reducing sectional area from en- 
trance to  the end of the rods where they join the exit 18 
as indicated in FIG. 5 by reference line 36 which is 
parallel to  the centerline of test chamber 16. 
Referring to FIG. 3, a schematic representation of the 
sidewall shock generated in a supersonic or hypersonic 
test is shown as it would appear looking at either of the 
walls of test chamber 16 as indicated by the broken 
lines. As shown therein, this sidewall shock pattern 
defines a diamond-shaped region or shielded region that 
defines the desired area for positioning a model to  be 
tested. T h e  present invention provides for suction of 
some of the boundary layer flow on the rods through 
the chamber walls via the spacing between rods 28 and 
under the influence of the vacuum pressure in plenum 
23 and thereby maintain laminar flow along the test 
chamber walls. 
In a specific embodiment of the invention, test cham- 
ber 16 was formed of 0.64 cm diameter rods with 0.102 
cm gaps between each rod and eleven rods of 32.5 cm 
length forming the top and bottom walls of the test 
chamber and eight identical spaced rods forming each 
of the two side walls. The test chamber was designed so 
that the surface of the sharp flat leading edge fairing 31 
was inclined 1” outward from the shield centerline at 
the same angle as the individual rods 28. This test cham- 
ber configuration thus presents a slight taper from an 
enlarged entrance to a smaller exit area. This 1”-2” 
inclined leading edge configuration is in contrast to  
previous shield models wherein sharp leading edges 
were also used but the leading edges thereof were in- 
clined up to  2” inward toward the shield centerline. The  
present invention has shown up to  a 65% reduction of 
the root-mean-square (rms) pressure in the entire 
shielded region when tested at  Mach 5 over a range of 
unit Reynolds number of 0.5 x 107 to  2.0x lO7per meter. 
In the Mach 5 tests, static pressure measurements in 
the nozzle, the test chamber, the model vacuum ple- 
num, and on the rods showed that the flow in the shield 
was fully started over the range of test conditions. 
These data also showed that the inviscid cross flow in 
the gaps was sonic except near the leading edge at the 
lowest Reynolds number where the cross flow was 
subsonic. The suction mass flow in this region was pre- 
sumably reduced below desired values. 
Surveys of mean pitot pressure within the shielded 
region inside the model showed that the leading edge 
shocks were strong, but no centerline focusing effects as 
has occurred in previous tests of a axisymmetric rod- 
wall shield were present. Leading edge shocks (sidewall 
and bottom) were observed to move downstream 
slightly with increasing Reynolds number. A reason- 
ably uniform flow core was found in the shielded region 
of the present model. This core was about 16 cm long 
by 2.5 cm square in cross-section. 
Transition in the rod boundary layers was obtained 
from surface pitot pressure surveys along the windward 
ray of the rods. The  noise field in the model was mea- 
sured with fluctuating pitot pressure probes utilizing 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 
piezoelectric transducers and with hot-wire probes. The  
hot-wire data showed that the dominant disturbance 
mode was acoustic. 
At the lower Reynolds numbers and when the fluctu- 
ating pitot pressure probe “sees” mostly laminar flow 
on the rods, the local freestream noise levels were re- 
duced from about 1.5 percent down to about 0.6 percent 
by the shield. However, the actual nozzle “input” noise 
as measured upstream before reflection at the shield 
wall was not attenuated significantly even when the rod 
boundary layers were laminar. When the rod boundary 
layers were transitional or  turbulent the shield noise 
levels at higher Reynolds numbers were above nozzle 
input levels due at least partly to the increase in high 
frequency energy radiated from the very thin rod 
boundary layers. 
Analysis of theory and data for reflection of noise 
from flat plate laminar boundary layers indicates that 
the lack of significant attenuation when the rod bound- 
ary layers were laminar may be attributed to  the very 
high frequencies of the nozzle input noise. These high 
frequencies are unique to the rapid expansion nozzle 
and are not found in larger conventional nozzles. 
It is thus seen that the present invention provides an 
improved wind tunnel test system for reducing the radi- 
ated noise normally occurring by shielding the test 
model from the noise radiated from the turbulent tunnel 
wall boundary layer. By providing the improved spaced 
rod test chamber and surrounding vacuum chamber, the 
vacuum source removes some of the boundary layer 
from the tunnel test wall as formed and delays the ten- 
dency of this boundary layer to become turbulent. 
There are obviously many variations and modifica- 
tions of the specific embodiment described herein that 
will be readily apparent to those skilled in the art in the 
light of the above teachings. It is therefore to  be under- 
stood that within the scope of the appended claims, the 
invention may be practiced otherwise than as specifi- 
cally described herein. 
What is claimed as new and desired to be secured by 
Letters Patent of the United States is: 
1. A noise shielding system for high speed transition 
research in supersonic and hypersonic wind tunnels 
provided with a source of supersonic or hypersonic 
fluid flow, a nozzle area through which the fluid flows, 
a test section for receiving the fluid flow and subjecting 
a test model positioned therein to  the fluid flow and an 
exit area for the fluid flow, the improvement therewith 
comprising: 
said test section being substantially a rectangular 
configuration; 
a vacuum chamber surrounding each side of the test 
chamber and having conduits leading therefrom to 
a vacuum source; 
a plurality of spaced individual rod elements forming 
the four sides of said test chamber thereby provid- 
ing fluid communication between said test chamber 
and said vacuum chamber; 
one end of each said spaced rod elements terminating 
and merging with a faired entrance shield portion 
of said vacuum chamber at the end of said test 
chamber receiving the fluid flow; 
the other end of each said rod element terminating 
and merging with a rectangular passageway lead- 
ing to  the wind tunnel exit; 
said rod elements and said entrance shield portion 
being slightly inclined inwardly toward the down- 
stream end of said test chamber to provide a taper 
4,363,237 
5 6 
and slightly reduced cross-section dimensions to 
said test chamber at the exit end thereof. 
thereof, means attaching said flat surface area to  said 
faired entrance shield and plate means securely posi- 
tioned Over said means attaching said rod flat surface 
area to provide a smooth surface area formed by said 
entrance shield, said plate means and said rod flat sur- 
face area. 
5. The improved noise shielding system as in claim 4 
wherein each said rod element is provided with a con- 
4. The improved noise shielding system as in claim 1 10 trolled bend along a portion of one end thereof and said 
machined flat surface iS formed along the length of the 
bend to thereby provide a linear surface along the ma- 
chined flat surface area and the remainder of the rod. 
2. The improved noise shielding System as in Claim 1 
wherein the incline of said entrance shield and said rod 
elements is on the order of 1". 
3. The improved noise shielding system as in claim 1 
wherein the substantially rectangular configured test 
section is provided with internally disposed rounded 
corners. 
wherein the end of each of said rod elements merging 
with the faired entrance shield at the end of said test 
chamber receiving the fluid flow is provided with a 
machined flat surface area along a portion of the length * * * * *  
15 
20 
25 
3 0  
35 
4 0  
45  
5 0  
55 
60 
65 


Sound shield

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