Updating computer program for determining nozzle admittances to eliminate double-root solution and to fit resultant admittance data curves by statistical mean

Daniel, B. R.

Smith, A. J., Jr.

Zinn, B. T.

English

NASA Technical Reports Server

/ 
NASA GRANT NGL 31-002-085 
BEHAVIOR OF NOZZSES AND ACOUSTIC LINERS IN 
THREE-DIMENSIONAL ACOUSTIC FIELDS 
Quarterly Report for Period 1 June 1970 to 31 August 1970 
by: Ben T. Wnn, Prinipd. 
Allan J. Smith, Jr., 
B. Robert Daniel, 
S ool of Aerospace Engineering 
z r g i a  I ~ t i t U t e  OI Teehn010gy 
Atlanta, Georgia 
https://ntrs.nasa.gov/search.jsp?R=19710004738 2020-03-12T00:07:25+00:00Z
The computer program t h a t  uses experimental data t o  determine 
the nozzle admittances has been updated i n  order t o  (1) eliminate a 
double-root solution and (2) t o  s t a t i s t i c a U y  curve-fi t  the resu l tan t  
admittance data. Pr ior  t o  ac tua l  nozzle t e s t i ng  preliminary t e s t s  
were conducted i n  order t o  determine (1) the effectiveness of the 
i n s t a l l ed  muffler; (2) the  best mode of chamber frequency excitation; 
(3) the chaniber acoustics under flow and no-flow conditions; and (4) 
the qual i ty  of the data measured by various transducers located a t  
d i f fe ren t  locations along the chember. Due t o  the f a i l u r e  of its 
predecessor, a different  tape recorder i s  present2y being added to 
the system, and ac tua l  nozzle tes t ing  t r i l l  take place short ly .  
B, Theoretical Studies 
A considerable amount of trork has been devoted to  f i n a l  
modifications of the computer program that calculates the desired 
nozzle admittances from experimental sound pressure l eve l  and phase 
measurements. Studies conducted during th i s  period showed tha t  f o r  
a given s e t  of input data the computer program may calculate  trio 
solutions for  the nozzle admittance function. I n  order to  eliminate 
t h i s  d i f f i c u l t y  an addit ional subroutine, capable of select ing the 
proper solution,  has been added t o  the coqputer program. I n  i ts  
present form the developed computer program can use available e q e r i -  
mental data t o  compute the desired nozzle admittance function. 
In  addition t o  the aforementioned computer q d a t e ,  a paral lel  
effort  t o  s t a t i s t l c a U y  eusve-fit t h e  adPdittance data has been 
eondue"ced. Buring the test;, f ive axial presswe transducers are 
emleyed to s q p l e  "ce acoustic wave ~rithP_r? the chamber. BUring 
data a n w s i s ,  three pressme measurements are required to de terdne  
the  nozzle adanittarace at thaQpai~ t icu la r  frequency, By using the 
f i ve  pressure measurements i n  combinations of three,  ten v d u e s  
of the  admittance a re  generated a t  each frequency. I n  the event 
tha t  the f i ve  pressure measmements a re  per fec t ,  a l l  ten nozzle 
admittmce values r r i l l  be equal. As a r e s u l t  of experimental 
inaccuracies the computed admittances, a t  each frequency, w i l l  
d i f f e r  from one another. I n  order t o  t r e a t  the data-scatter,  the 
computer program w i l l  apply various s t a t i s t i c a l  methods a t  each 
frequency and then curve-fi t  the admittance data over the frequency 
range. 
Additional e f fo r t s  associated ~ r i t h  the analyt ical  investiga- 
tion of three-dimensional standing wave behavior i n  chambers with 
l ined  t r d l s  as well  as the prediction of the admittance values of 
nozzles with rapidly-converging traLls have continued during th i s  
report  period; the studies have, ho~rever, not progressed t o  the 
point where s ign i f ican t  r e su l t s  can ye t  be presented. 
The in s t a l l a t i on  of the  muffler together with system 
modifications required t o  accommodate the muffler have been com- 
pleted.  Preliminary r e su l t s  of the t e s t s  conducted to  determine 
the mount of chamber noise reduction due t o  the inser t ion of the 
muffler in to  the system indicate  approximately 20 db reduction i n  
the cha-mber fbo~r noise. Based on th i s  "quiclr-look" data, additional. 
t es t ing  of the muffler to  generate more spec i f ic  Information tras 
ecessasy, and the muffles w i l l  be retained as an i n t e g a l  
p a r t  of the f l o ~ r  system. 
The acoustic l i n e r  has been fabricated,  and i t  w 1 U  be 
tes ted once "r;e exhaust nozzles have conrpleted "chelr t e s t  cycles. 
me emerimental efforts  conducted during t h i s  r q o r t  period 
Trerse directed a t  the determination o f  the ;ahi$tance values sf the 
manurf"ae-t;urect convergent-divergent nozra%es, ~rheveas the previow 
report period e f for t s  ?:ere directed a t  understanding the control  
and operatinz charecter is t ics  of the simulated rocket cold-flo~a 
sys tern. A s  a prelude to  the current e f f o r t ,  the follotring questions 
liad to  be answered: 
(1) Could the desired experimental data be obtained by 
using a continuous frequency streep? I n  t h i s  case the frequency of 
the excited three-dimensional traves r.ii.11 be changed i n  a continuous 
pre-assigned r a t e .  An a l te rna t ive  t o  t h i s  method of operation 
trould be t o  run separate t e s t s  a t  various discrete  frequencies 
tha t  a re  of i n t e r e s t  t o  rocket designers. 
(2) Is there a s ign i f ican t  difference between the acoustic 
properties of the cold-flow rocket simulator under flow and no- 
flotr conditions ? 
(3) I s  there a s ignif icant  difference i n  the character is t ics  
and quali ty of the data obtained from pressure transducers located 
a t  di f ferent  locations along the tube (e  .g. , transducers located 
a t  the mid-section of' the chamber and near the nozzle entrance)? 
A t o t a l  of seven complete t e s t s  were conducted during t h i s  
report  period,  and the data cras analyzed i n  order t o  anstrer the 
aforementioned questions. The use of an automated frequency sweep 
during the t e s t  i s  the preferred mode of operation because i t  pro- 
vides data a t  many more frequencies from which admittances can be 
determined. This  mode of operation a l so  conserves on the l imited 
st~pply of ai r  available fo r  the blorrdotm t e s t s .  The s w e q  cannot, 
hotrever, be used i f  the srreep-rate exceeds the mtwrimum streep-rate 
t ha t  i s  acceptable t o  the data processing equipment (e ,g. , the 
tracking f i l t e r )  . The f i l t e r  e q l o y e d  for data processing has a 
1.5 Hz biand~ridth when the signal. level  is  3 db dorrn &om i t s  
m & m w  Level end 6.0 Hz bmd~ddttr. lrhen the sign& is 60 cl"B berls~r 
i t s  m a i m m  vd.ue. By ~ecordinli; t he  test data a t  s tape recorder 
speed o f  30 ips (i-e, inches per second) and reducing the data a t  
a tape speed of 1-7/8 i p s ?  the tracking f i l t e r  senses a rrxreq r a t e  
of 0.003 ~ z / s e c  that  is  suf f ic ien t ly  s lov  t o  satisi?y f i l t e r  re -  
quirements. 
Some considerations were given to  investigating the acoustics 
of the system under no-flow conditions a d  then using t h i s  informa- 
t i on  as a guide during ac tua l  flow experiments. To check the 
f e a s i b i l i t y  of t h i s  approach the acoustic properties of the system 
under f l o ~ r  and no-flotr conditions had to  be comgared. Coqarisons 
of  acoustic data obtained during flow and no-flotr t e s t s  revealed 
tha t  the introduction of floxr in to  the system re su l t s  i n  a s ign i f i -  
cant modification of the acoustic properties of the system. then 
no a i r  is  flo~ving i n  the  system, neither the injector  holes nor 
the nozzle throat  are choked. Under these conditions, the portions 
of the system located upstream of the in jec tor  p l a t e  and do~rnstream 
of the nozzle throat can a f f ec t  the acoustic properties of the 
chamber. then f.1011 i s  introduced in to  the system, the influence 
of thzt  por t ion of the system tha t  is  located upstream of the 
insector p l a t e  i s  changed vhexeas the influence of the system dotrn- 
stream of the nozzle throat  i s  eliminated. A s  a r e su l t  the zcoustic 
properties of the system under no-flow conditions are  en t i r e ly  
different from the acoustic proper t ies  of the system during ac tua l  
t es t ing ,  Another difference between f l o ~ s  and no-flov tes t ing  i s  
associated ariCh changes i n  the speed of sound. The electropneumatic 
acoustic dr ivers  used i n  the eqer imenta l  setup require r e l a t i ve ly  
l i t t l e  a i r  f l o t ~ .  Consequently, during no-Ploar tes t ing  the r a t e  of 
discharge from the air supply tanks i s  sm& trhile the s a t e  of 
decrease i n  a i r  t ewera ture ,  due t o  the ewansion of the compsessed 
a i r ,  is negligible. This i s  no"l;the ease during f l ov  testing ~fheaa 
t h e  sir f%~trs t k o u g h  the main chamber and %he tenqersture changes 
be"r;treeaa ad the s t a r o f  the teest and --SOOF a% t h e  end of the 
t e s t .  !T'hzis v ide  terweratwe exews5on resd"e  i n  a rqpicc9 cha%e 
i n  the speed of sound (that in turn resuL"c in a change I n  trave- 
length) during the t e s t ;  a change tha t  p e a t l y  a f fec t s  the acoustic 
properties of the system. Xn viex~ of the above-described obser- 
vations, the previously-mentioned poss ib i l i t y  of us in^ no-flotr 
data t o  determine the acoustic properties of the system should be 
discarded. 
Comparisons of data recorded by various transducers located 
a t  di f ferent  locations along the chamber indicated qua l i ta t ive  
s imi la r i ty  i n  the observed f'requency spectrum as well as the sound 
pressure Level. The recorded signals trere a lso qual i ta t ively 
similar t o  data  obtained i n  re la ted sound-floxr interact ion experi- 
ments. 
Based on the above findings i t  was decided tha t  the invest i -  
gation of the system as trell  as the ac tua l  nozzle t e s t i ng  should 
be conducted under flow conditions. The frequency of the excited 
vmves t r i l l  be eh'mged i n  a continuous manner over a predetermined 
frequency range, and any of the transducer locations can be used 
with the same degree of confidence. It was expected tha t  the 
ac tua l  t e s t i ng  ~rould have been completed a t  t h i s  date; a massive 
f a i l u re  of the  system's tape recorder unfortunately resulted i n  
a teworary postponement of the scheduled tes t ing .  Another tape 
recorder i s  presently being incorporated in to  the system, 2nd 
acquisit ion of the desired data i s  expected to  begin short ly .  
!The computer program ~ri.11 be S ind ized .  The new tape 
recorder w i l l  be inserted in to  the system, and nozzle admitbnce 
t e s t s  - t r i l l  be conducted. Testing of' the acoirstic liner i r i l l  be 
inikiated, and worlr on the other a n d y t i e a l  investigations 1rI3.l 
coni;inue. 


Behavior of nozzles and acoustic liners in three-dimensional acoustic fields Quarterly report, 1 Jun. - 31 Aug. 1970

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