Atmospheric refraction, grazing incidence wave impingement, and turbulence interaction effects on sonic boom ground pressure signature

Hubbard, H. H.

Maglieri, D. J.

English

NASA Technical Reports Server

- .. 
Presented a t  t h e  F i f th  International Congress 
on Acoustics (IC&) 
a- i 
https://ntrs.nasa.gov/search.jsp?R=19660020036 2020-03-24T03:19:10+00:00Z
..5' CDN6RES IWTERNATIONAL D'ACOUSTIQIE 
Domenic J. Wglieri" and Harvey H. Hubbard- 
Aerospace Engineer, Head, Acoustics Branch 
NASA Langley Research Center, Hampton, Va., U.S.A. 
** *** 
ABSTRACT 2 9 3 a  
Data from f l igh t  experiments i l l u s t r a t e  the  nature of various atmospheric 
effects  on sonic-boom ground-pressure signatures. Atmospheric refraction, grazing 
incidence wave impingement, and turbulence interaction phenomena are discussed 
along with the i r  significance regarding the community response problem of sonic 
booms. 
The manner i n  which sonic booms affect  community acceptance of such a i r c ra f t  
Shown schematically i n  figure 1 
as the  supersonic transport requires a knowledge of the  effects  of a i rc raf t  opera- 
t i on  and the atmsphere on the ground exposures. 
i s  an airplane f l i gh t  t rack extending from subsonic t o  supersonic speeds. Beneath 
the mght t rack are shown sketches of the shock-wave impingement patterns and the 
associated distributions of N-wave type pressures, both along the track and perpen- 
dicular t o  it. 
s t a t e  of knowledge of sonic-boom phenomena, dealing f i r s t  with the  pressure build- 
ups i n  the transonic speed range ( re fs .  1 and 2) and with the l a t e r a l  extent of the 
pattern i n  steady f l i gh t  fo r  quiescent atmospheric conditions (refs. 3 and 4). 
Also, there w i l l  then be a discussion of recent data from f l igh t - tes t  studies 
re la t ing t o  atmospheric dynamic effects  on the  sonic-boom signatures (ref. 5 ) ,  and 
f ina l ly  brief discussions of the significance of signature shape on the response of 
people and structures ( re f .  6 ) .  
The information of the paper i s  i n  the form of a report on the 
EFFECTS OF ACCELERATED F'LIGEJ! 
A n  extensive ser ies  of ground-pressure measurements has been made fo r  longitu- 
dinal a i rc raf t  accelerations from 0.9 t o  about 1.5 Mach number a t  a constant a l t i -  
tude of 37,200 fee t  with a special  array of microphones extending about 23 miles 
along the ground track. 
f l igh ts  are shown a t  the  bottom of figure 2. 
sent the so-called superboom condition where pressure buildups occur. 
the three separate f l igh ts  were normalized by plotting the highest measured over- 
pressure values at  this zero position. 
t o  right,  as indicated by the sketches a t  the top along with CorrespandrLng tracings 
Of measured signatures. The data points of the  figure represent peak overpressures 
as defined i n  the  sketch. The low-value points t o  the l e f t  of the figure represent 
noise and are observed as rumbles. 
figure correspond t o  measurements that are  very close t o  the so-called focus point, 
and thus represent what are  conventionally described a s  superbooms. 
of the focus point are two dis t inct  se t s  of measurements which re la te  t o  the region 
of multiple booms. For convenience i n  i l lus t ra t ing  the  trends of the data, a sol id  
"he measured data points from three such acceleration 
The data at the zero posit ion repre- 
The data fo r  
The direction of the a i r c ra f t  i s  from left  
The high-value points near the center of the 
To the r ight  
t Material of this paper abstracted from paper presented a t  NASA Conference on 
Aircraft Operating Problems, Langley Research Center, Rampton, Va., May 10-12, 1965. 
L-4607 
I 
2 
I 
ATMOSPHERIC EFFECTS ON SONIC-BOOM SIGNATURES 
1 
* 
and dashed l ine  are  faired through the data points. 
about the  solid curve relate  t o  the f i r s t  signature t o  arr ive i n  a l l  cases, and 
th i s  eventually develops into the steady-state signature. The data points tha t  
cluster about the dashed curve relate ,  i n  a l l  cases, t o  the second signature t o  
arrive. These values are seen t o  generally decrease as distance increases, and 
eventually th i s  second wave ceases t o  exis t  because of the refraction effects  of 
the atmosphere. 
The data points which cluster  
The highest overpressures are  measured i n  a very localized region. These val- 
ues are noted t o  be as high as  2.5 times the maximum value observed i n  the 
multiple-boom region and are  thus i n  general agreement with the  measured resul ts  
for  lower al t i tude t e s t s  of reference 1. It i s  further noted tha t  the main 
multiple-boom overpressure values are of the  same order of magnitude as  those pre- 
dicted for  comparable steady-state f l i gh t  conditions. Available overpressure pre- 
diction methods give good agreement i n  the multiple-boom region but are  not con- 
sidered reliable i n  the superboom region. 
The locations of the superboom and multiple-boom regions are  predictable with- 
i n  *5 miles, provided such information as f l i gh t  path, a l t i tude,  and acceleration 
rate  of the a i rc raf t  i s  available. 
LATERAL-SPREAD PA!I’IERNS 
With regard t o  the steady-flight conditions, some recent experiments have also 
been conducted i n  an effor t  t o  define more exactly the pressure distribution near 
the extremity of the shock-wave pattern on the ground. Some sample data are  shown 
i n  figure 3. Particular emphasis w a s  placed on the region where a grazing condi- 
t ion exis ts  due t o  atmospheric refraction, as  suggested by the  ray-path sketch a t  
the top of the figure. Flights were made a t  a l t i tudes of 52,200 and 37,200 fee t  
and Mach numbers of 2.0 and 1.5, respectively, during quiescent atmospheric condi- 
tions, and the resul ts  are compared with theory i n  the data plots  a t  the  bottom. 
The results from the f l igh t  a t  52,200 fee t  a l t i tude and Mach number 2.0 show that  
the pressures are  generally highest on the t rack and decrease generally as  distance 
increases (sol id  symbols represent no measured disturbance). The fact  that  meas- 
urements were obtained beyond the theoretically predicted cutoff distance by the 
method of reference 4 led t o  more definit ive studies at 37,200 f ee t  a l t i tude and a t  
Mach number 1.5. 
ious displacement distances of the  a i rc raf t  from the overhead position, are  similar 
and, i n  fact ,  indicate measured signals as  much as 15 miles beyond the predicted 
cutoff distance. 
These data, which were obtained from four f l igh ts  involving var- 
A bet ter  understanding of t h i s  phenomenon may be obtained from examination of 
some sample waveforms based on measurements a t  various distances. It can be seen 
y the calculations. A t  distances beyond the predicted cutoff, the signa- 
at sharply defined shock-wave type signatures exis t  generally for  the region pre- >k 
*uresYose the i r  identity and associated observations indicate the existence of 
rumbles, as described previously. 
OTHER EFFECTS OF THE ATMOSPHERF: 
Measurements of sonic-boom signatures have been made a t  specific measuring 
points for  a large number of supersonic f l igh ts ,  and some sample resul ts  are  shown 
i n  figure 4. A t  the l e f t  of the figure a re  shown fighter a i r c ra f t  sonic-boom sig- 
natures ( ref .  5).  It can be seen that these vary widely from sharply peaked waves 
a t  the  top t o  rounded-off waves of sinusoidal appearance a t  the bottom. The signa- 
tures on the right-hand side of the figure have been recently obtained for  bomber 
ai rcraf t  and have a noticeably longer wavelength or time duration (about 0.18 sec). 
It can be seen that  the main distortions of the waves i n  each case are  associated 
w i t h  the  rapid compression phases and tha t  these distortions are of the same gen- 
eral nature for waves of short and long wavelengths. The resu l t s  of f igure 4 are  
/ *  
i n  quali tative agreement with similar data for a given f l i gh t  but for measurements 
a t  a number oi stations (see ref. 5 ) .  
Because of the large number of data points available fo r  a range of f l i gh t  
conditions, it was possible t o  make s t a t i s t i ca l  analyses of the variations of over- 
pressure amplitude. 
ure 5 as  re la t ive  curmilative frequency distributions and histogram6 showing proba- 
b i l i t y  of occurrence. 
distributions fo r  a f ighter  a i rc raf t ,  and i n  the right-hand plot are  similar data 
for a bomber a i rc raf t .  
overpressure-ratio value is  plotted on the vertical  scale as a function of the  
r a t i o  of the  mearmred overpressure values divided by the  maximum predicted value 
for the respective f l i gh t  conditions (which occur on the  ground track).  
type of presentation, the data points would all fall  on a s t ra ight  line i f  the log- 
arithms of the data f i t t e d  a normal distribution. 
w e r e  obtained on the ground track and a t  distances up t o  10 miles from it. From an 
inspection of these curves, it can be seen that a wider variation i n  the  amplitudes 
occurred for the  more remote stations.  It was a lso  noted that the  probability of 
encountering high values of pressure was somewhat greater fo r  the Lateral stations.  
In  the  case of the bomber a i rc raf t ,  fewer data points were available for analysis 
and hence the s t a t i s t i c a l  sample i s  not as reliable. 
the variation in amplitude for  the bomber data, which have markedly longer wave- 
lengths, i s  noted t o  be only slightly less than that for the  fighter a i rc raf t .  
is significant t o  note that some of the  pressure buildups due t o  atmospheric 
effects  are of the same order of magnitude as those associated with the superboom 
phenomenon (fig. 2). 
Samples of the  overpressure variation data are  given in f ig-  
In  the  left-hand plot of the figure are shown amplitude 
The probability of equaling or exceeding a given 
For t h i s  
For the f ighter  a i rc raf t ,  data 
Based on the data available, 
It 
The nature of the  acoustic inputs for  two types of exposure si tuations i s  sum- 
In general, 
marized i n  figure 6. 
whereas the  bottom two t races  represent the inside-exposure situation. 
the ear  i s  sensit ive t o  the rapid changes i n  pressure associated with the  two com- 
pressions and i s  not sensit ive t o  the slowly varying pressure i n  between. Studies 
a t  the  University of Southampton which re la te  directly t o  the  outside s i tuat ion 
have shown that the overpressure values and the init ial  r i s e  time were both impor- 
t an t  x i th  regard t o  loudness judgments. 
dominant. 
The top t race represents the  outside-exposure si tuation, 
O f  these, the rise-time factor was 
A microphone inside of a room records a pressure variation in  that room simi- 
lar t o  that of the  middle trace.  
wave, the frequencies of which correspond t o  the fundamental vibration-mode f re -  
quencies of the main framing members of the building. 
sure variation i s  of large amplitude, it usually occurs a t  a characterist ic f re -  
quency a t  which the humin ear is not very sensitive. 
audible acoustic input t o  an inside observer, simultaneous measurements w e r e  made 
with a microphone system having a response similar t o  that of the human ear. The 
objective here was t o  eliminate the dominant low-frequency components that the  ear 
does not normally respond t o  i n  order t o  better define the high-frequency compo- 
nents fo r  which the ear i s  much more sensitive. Such available acoustic signals 
are  believed t o  be associated with the  vibration of smll building components and 
miscellaneous items of furnishings and equipment. The exposure fo r  the inside 
observer i s  strongly shaped by the  response of the  building. 
studies t o  date (ref .  6 )  have suggested s t r o w  tha t  h i l d i n g  vibrations play a 
dominant role  i n  shaping the judgments of community observers i n  sonic-boom expo- 
sure si tuations.  
This has the gross features of a w e d  sine 
Although this inside pres- 
In  order t o  be t te r  define the 
The resu l t s  of 
The significant factors i n  the response of structures t o  sonic-boom signatures 
are i l lus t ra ted  i n  figure 7. Represented by the sketches a t  the top of the figure 
are such features of the input as  the overpressure, impulse, and the period. 
the case of the structure, the most significant feature i s  the period of its first 
natural vibration mode as  indicated by the sketch on the  r ight .  
have suggested that the r a t i o  of the period o f t h e  input t o  the natural  vibration 
I n  
Analytical studies 
4 
A'I'MOSPHERIC EFFECTS ON SONIC-BOOM SIGNATUFZCS 
period of the structure determines the manner i n  which the structure responds. 
plot  at t h e  bottom of the figure represents a brief s m r y  of the si tuation fo r  
various combinations of the period of the input and the natural  period of the 
structure; for instance, for  a short-period input, as i n  the case of a f ighter  air-  
craf t ,  and a long-period response, such a s  fo r  a large structure, the impulse i s  
believed t o  be the significant feature of t he  input. On the other hand, fo r  the 
case of a long-period input or fo r  a large airplane and a short period of the 
structure as i n  the case of the components of a building, the overpressure i s  
believed t o  be the significant feature of the input. Many of the structures which 
are  of concern i n  a comuiunity are  of such a nature that  they do not clearly f a l l  
into either of the two categories of the figure, and hence it must be concluded 
that  both the overpressure and impulse are significant.  
The 
CONCLUDING REMARKS 
The acceleration and lateral-spread phenomena appear t o  be f a i r l y  well under- 
stood and predictable f o r  current and future a i r c ra f t .  Variations i n  the sonic- 
boom signature as a resul t  of the effects of the atmosphere can be expected during 
routine operations. From the data evaluated t o  date, very similar variations i n  
pressure signatures are noted fo r  a range of sonic-boom wavelengths. It i s  i n  the  
area of community acceptance of sonic booms tha t  the greatest questions s t i l l  exist .  
A more definite answer t o  the community-acceptance problem w i l l  have t o  a w a i t  ade- 
quate f l i gh t  experience with larger a i r c ra f t .  
RFFERENCES 
1. Hubbard, Harvey H.; Maglieri, Domenic J.; Huckel, Vera; and Hilton, David A.:  
Ground Measurements of Sonic-Boom Pressures for  the Altitude Range 10,000 t o  
75,000 Feet. NASA TR R-198, July 1964. 
2. Staff of NASA Langley Research Center: Proceedings of NASA Conference on 
Supersonic-Transport Feasibi l i ty  Studies and Supporting Research - September 
17-19, 1963. NASA TM X-905, Dec. 1963. 
3. Maglieri, D. J.; Parrott ,  T. L.; Hilton, D. A. ;  and Copeland, W .  L.: Lateral- 
Spread Sonic-Boom Ground-Pressure Measurements From Aircraft a t  Altitudes t o  
75,000 Feet and a t  Mach Numbers t o  2.0. NASA TN D-2021, Nov. 1963. 
4. Randall, D. G.:  Methods for  Estimating Distributions and Intensi t ies  of Sonic 
Bangs. R & M No. 3113, Brit ish A.R.C.,  1959. 
5 .  Hilton, David A.; Huckel, Vera; Steiner, Roy; and Maglieri, Domenic 5.: Sonic- 
Boom Exposures During FAA Community-Response Studies Over a 6-Month Period i n  
the Oklahoma City Area. NASA TN D-2539, Dec. 1964. 
6. Nixon, Charles W.; and Hubbard, Harvey H.: Results of USAF-NASA-FAA Flight 
Program t o  Study Community Responses t o  Sonic Booms i n  the Greater St.  Louis 
Area. NASA TN D-2703, May 1965. 

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Atmospheric effects on sonic-boom signatures

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