Three seeding systems have been used in the 4- by 7- Meter Tunnel at NASA Langley Research Center: Kerosene smoke, solid particle dry dispersing, and solid particle liquid dispersing. It is anticipated that the liquid dispersing system will be used in all future applications at this facility because: (1) it has a steady output; (2) it is easy to operate and reconfigure; and, (3) it delivers particles of near uniform size

Elliott, J. W.

Nichols, C. E., Jr.

English

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S E E D I N G  S Y S T E H S  F O R  USE WITH A LASER VELOCIMETER 
I N  L A R G E  S C A L E  WIND T U N N E L S  
Joe W. E l l i o t t  
Rotorcraft  Aerodynamics Office 
A m y  Structures  Lab, USARTL 
NASA Langley Research Center 
Hampton, Virginia  
Cecil E, Nichols 
NASA Langley Research Center 
Hampton, Virginia  
https://ntrs.nasa.gov/search.jsp?R=19860001977 2020-03-20T16:14:04+00:00Z
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I & B Y  7- UETER T U N N E L  
ORIGINAL F1,~4: 12 
- --
OF POOR QUALITY 
The Langley 4- by 7- Meter Tunnel which is shown in f igures  1 and 2 is a closed 
circuit,  atmospheric wind tunnel  which may be operated with a closed o r  open t e s t  
I ~ e c t l o n .  The test sect ion celllng and w a l l s  can be raised to provide t h e  open 
configuration t h a t  is closed on t h e  botto . The test section, o r  t h e  jet entrance i n  the  
open configuration, is 14.5 ft high, by 21.75 it wide. In each  case t h e  test section is 50 ft 
long and Is coniigured in to  t w o  test bays each  equipped with interchangeable model 
support  m echanism s. 
Figure 1 
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Figure 2 
ORIG!?:P,L :-';CZ 1s 
THE LASER VBLOCIHETER OF POOR QUALITY 
The laser velocim eter (LV) la configured to q easure the imtantaneous component of 
velocity in the horizontal (U co m ponent) and vertical (V co m ponent) dlrectlons (ref. 1 ). The 
sy,tam is comprised of three subsystems: optics, traverse, and data acquLsitlon and control. 
0 ptics 
The optics subsystem which is shown schematically in flgure 3 is operated in the 
backscattor mode and a t  high laser power (4 watts in all lines) in  order to acco q m odate the 
long focal lengths required by the wide test section. The co m m ercially available 
Qeansmitting and receiving optics packages are augm ented by a zoom lens system consisting 
of a 3 in. clear aperture negative lev-9 and a 12 in. clear aperture positive lens. Bragg cells 
in each of the optical paths provide a directional measurement capability. 
Traverse 
The traverse subsystem uses a corn bination of mechanical and optical schemes to 
provide five degrees of fPeedom . Translation of the sa m ple volume along the horizontal and 
vertical axes is acco m pllshed by dlaplacirng the entire optics platform. Translation along 
t h e  lateral axes is accomplished by translating tl-!P negative lens located i n  the zoom lens 
assembly thus refocusing the sample volume along the axis of optical transmisslon. The 
other two degrees of freedom, pan and tilt, are implemented by rotatlng the final mirror 
about its vertical and horizontal axis i n  order to change the direction of optical 
transmisslon. The total inclusive range of the traversing system is: Verticak 7 ft; 
Stream wise: 6 ft; Lateral: 16.5 ft;  Pan: 30°; Tilt: 30° 
OPTICS LAYOUT 
SAMRE II[I1w 9 
aim 
SEPARATOR . 
Figure 3 
D A T A  A C Q U I S I T I O N  A N D  C O N T R O L  
The data acquisition and control subsystem is shown schematically in figure 4 and 
performs the following functions: 
1. Interfaces with the optical signal processing equipment to  receive two channels of 
raw LV data and one channel of auxiLiary data. 
2. ~onv;?r t s  that raw data to  engineering units. 
3. Statistically analyses the data and reports the results so tha t  the test  results can 
be evaluated on line. 
4. Stores the test  results on magnetic tape for  subsequent analysis. 
5. Interfacgs with and control8 the five degree of freedom scan system. 
E!? D I S C  
1600 B P I  
9-TRACK TAPE 
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V-COMP COUNTER 
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Figure 4 
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ROTOR 
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SHAFT 
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LASER VELOC IMETER 
(AUTOCOVAR I ANCE) 
BUFFER INTERFACE 
1 6 - B I T  MINICOMPUTER 
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The models shown in figure 5 are representative of those tested in  the 4- by 7- 
Meter Tunnel. The areas that are to be seeded i n  support of such !jests are on the order 
of 1 to 20 square feet. Cach square foot of arra requires 6.0 X 10 particles per minute 
to achieve a unlform data rate of 4000 samples per minute. The shapes of the seeded 
areas which are required to support such tests are vertical or horizontal rectangles, 
squares or circles. 
Figure  5 
SEEDIWG T E C H I I Q U B S  
Three seeding technlques have been utlllzed thus iru, in the 4- by 7-Meter Tunnel: (1) 
kerosene smoke, (2) dry dispensing or solld particlea and (3) Uquid dispensing of solid 
particles. In all three cases seeding ffi aterial was iqjected' into the air streaffi in the 
settllng chamber (flu. 1) because of the relatively low velocity, easy access, up-strea q 
proximity to the te*. section and relatively m'e envlronm ent tha t  exlsts a t  that location 
during tunnel operations. 
K E R O S E N E  S H O K E  SYSTEM 
A schematic of the kerosene smoke seeding system is shown in figure 6. Pressure, 
applied to the storage tank, causes the kerosene to flow to the wand where it is heated al.4 
expelled thrcugh a nozzle that is dzed to maximize the number of 2 micron particles. The 
output of tNs system is a single plume cn the order of 18 in. in diam ef er w Nch can be 
increased to 36 in. with the addition of a diffuser plate located downstream of the nozzle. 
The pressure that is applied to the storage tank and the voltage across the d.c. electric 
heater in the wand are the only control variables on this system. The position of the wand 
i n  the settling chamber and thus the location of the particle plume in the test section is 
changed by raising or lowering it on a system of cables that stretcned from the floor to the 
ceiling. The size of the particles that are generated is dependent on the rate at  w'nici~ 
k e r ~ e n e  flows through the systern and the temperature of the kerosene. Since temperature 
1s critical, anything that cawes a change in that temperature wiU cause the slze of the 
particles 'LO change. For example: 
:. A local tunnel temperature increase during tunnel 09eration wFU increase the 
kerosene tem perature and decrease the size of the particles. 
2. A tunnel velocity increase ;rill decrease the temperature of the kerosene and 
increase the slze of the ~articlcs. 
3. A kerosene tank ppessure increase, i n  order to increase the number of particles, w i l l  
d?crease the temperature of the kerosene and 'increase tne slze of particles. 
Thermocouples located in the wand provide the neces?ary inputs to control the 
tem perature,and therefore thc particle xcput,  but the system still requires a  eat La?? of 
Jperator .?kill and intervention. 
Figure 6 
SOLID PARTICLE DRY DISPEWSWG SYSTEM 
A sche  rn atic of tke solid part icle  dry dispensing system is shown i n  f igure 7. The dry 
part icles  which are contained i n  the hopper are deposited on  t h e  ro ta t ing  toothed 6heel.  
removed a t  t h e  low pressure en t rance  t o  the nozzle and subsequently iqjected i n t o  t h e  
tunnel  i n  a single plume which is on the o rde r  of ! 2 !?. i n  diameter.  The seede r  is installed 
on t h e  cable system in t h e  se t t l ing  cha  q be r  f o r  dispersal of  particles. Presswe applied t o  
t h e  hopper vibrator, pressure applied t o  the nozzle and vol tage applied t o  t h e  motor which 
drives t h e  toothed wheel are t h e  three cont ro l  irlputs t o  t h e  ~ y s t e r n .  The two  rnqjor 
charac te r i s t ics  of t h e  particles,  s i ze  and quantity, are controlled by t h e  se lec t ion  of raw 
part icles  r I be put  in  t h e  hopper and t h e  cont ro l  of th.e speed o f  t h e  toothed wheeL Two 
controls  are provided to optimize the p e r f c m  ance  of t h e  seeder: 
1. The a j r  supply t o  t h e  hopper vibrator is used to vary t h e  frequency of  the  vibratorn. 
This insures t h a t  a continuous supply of par t ic les  w i l l  be  a v d a b l e  ts the  t ~ c t k e d  wheeL 
2. The air supply t o  t h e  nozzie controls  t h e  e f fec t iveness  of t h e  r emova l  process and a t  
higher set t ing3 a c t s  to break up aggio m e r a t ed  m asses of particles. 
The system is fairly easy t o  set up and aQust ,but  does e x p e l e n c e  par t ic le  
agglo m eration. T h i s  condition occurs  when clu m ps of t h e  seeding m a t e r i a l  f a i l  t o  break-up 
as they are removed from t h e  toothed wheel. These clumps of seeding material,some 
est imated to be  as la rge  as 0.050 in., may not  fc&u t h e  f l o w  and may change t h e  model 
characterst ics  when they  impac t  and adhere  to t h e  surface. Coating of  t h e  seeding 
material  with part icles  of a sub-nicron s i ze  reduces  this problen and aJso improves t h e  
rlow of t h e  seeding material  in th.2 hopper (ref. 2). 
Vibrator --, 1 / 
Air i n  
Variable speed 
motor controller 
toothed wheel I 
F i g u r e  7 
S O L I D  P A R T I C L E  LIQUID L'TSPEISIKC SYSTEM 
A sche m atic of the solid parecle liquid seeding system is shown in figure 8. The 
storage tank is flllec with up to 20 gallons 04 ethyl alcohol in which the particles to be 
dispensed have been mixed. The pump delivers the mixture under pressure to a series of 
nozzles where a source of compressed air acts to atomize the mixture. Up to  40 nozzles 
can be conveniently mounted on the frame that Is 6 ft high and 8 ft wide and is then 
positioned with the sa m e cable arrange m ent as the other syste ms. The pu q ping apoarstus 
w i l l  support up to two arrays of noziles which can be configured to meet the area and 
density reqSIe m en- of the test in progress. Because of its L o w  vapor pressure, alcohol 
evaporatss as it travels ffom the settling chamber to the test section, a distance of 
approximately 85 ft. The particles that were contained i n  the droplets remain in the flow 
to be used in the velocity measure m ent. The array of nozzles which are connected i n  series 
on the liquid supply line is not capable of dispensing the amount of Uqula .,hich is supplied 
by the pump so the excess is returned to the storage tank where it circulates with the 
re.naining fluid and acts to prevent the particles h m  settling out of the mixture. The 
active controls in the syste m are t!lree pressure regulatars: 
1. Nu m ber 1 establishes the prssure of the hlr which wFU be supplied t o  the m a y  of 
nozzles. This pressure is mbnitored a t  the array so that it can be accurately set. 
2. Nu m ber 3 est-ablishes the m axim u m pressure of the circulating alcohol particle 
mixture. The ~egulator also prevents damage to the pump which could be caused Sy 
excessive head presure resulting fro m blocked or disconnected supply lines. 
3. Nula ber 3 establishes the pressure of the alcohol particle mixture that wLU be 
delivered to the array of nozzles. A pressure transduoer is dso used to monitor this 
pressure and is likewise mounted on the array housing to facuta te  accurate settings. 
0 nce the controls on this sy3te.m have been set its output re mains constant and requires 
no attention. The nozzles are com mercially available unit3 which dispense the atomized 
mixture in a wide angle round spray pattern approximately 12  in. in diameter. The 
volumetric flow rate of the iiquid mixture is cont-olled by changing the ratio of air to  iiquid 
pressures,and the droplet size Is controlled by chaliging both pressures. When the pressure 
of the alcohol sixture and the air are increased, the atomized droplets are smaller, 
vaporize faster and contain fewer particles. The droplets should be small so that they w i l l  
corn pletely evapcrate by the time that they arrive a t  the test section and should contain 
only one particle. 
LIQUID SOLID-PAFiTlCLE SEEDING SYSTEM 
1-1 0 u ick disconnect 
Pressure regulator 
Storage tank 
Figure 8 
C O N C L U S I O N S  
Three seeding sys t ems  have been used in t h e  4- by 7- Meter Tunnel: kerosene 
smoke, solid p a c l e  dry dispensing and solid p w t l c l e  liquid dispenstng. It is ant ic ipa ted  
t h a t  t h e  liquid dispensing system will b e  used in a l l  of  the applications at  this facFUty 
because: 
1. It has a s teady  output. 
2. It is easy t o  ope ra t e  and reconfigure. 
3. It delivers par t ic les  of near  uniform size. 
R E F E R E N C E S  
1. Sellers, William L.; and Elliott, J o e  W .: Applications of  a laser Velocim eter in  
t h e  Langley 4- by 7-Meter Tunnel. N A S A  C P  2243, nFlow Visualization, and  Laser 
Velocimetry f o r  Wind Tunnels," pp. 283-294, 1982. 
i 
2. Nichols, C ec i l  E.: Experim e n t s  With Solid Particle Seeding: Wind Tur!!el Seeding i 
S y s t e n 3  f o r  Laser Velocimeters. N A S A  C P-2393, 1985, Pp 77-84. 


Seeding systems for use with a laser velocimeter in large scale wind tunnels

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