Harmonic analysis of pressure data for solid and perforated baffle plates of fuel tank

Abramson, H. N.

English

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S O U T H W E S T  R E S E A R C H  I N S T I T U T E  
B S O O  CULEERA R O A D  5 A N  ANTONIO, TEXAS 76 2 U 6 
April 5, 1965 
Director 
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1 
fl653 July 65 George C .  Marshall Space Flight Center 
National Aeronautics and Space Administration 
Purchasing Office 
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K66-16974 I 
Huntsville, Alabama35812 
Attention: PR-EC 
Reference: Contract No.  NAS8-1555 
a 
9 ( T H R  
c UCCESSION ~ U Y B E R )  
SwRI Project No .  02-1072 
Quarterly Progress  Report No. 16 
Dear Sir: 
The present letter is intended to constitute Quarterly Progress  
Report No. 16 covering the period 1 January 1965 through 31 
March 1965. 
P res su res  on Ring Baffles 
A harmonic analysis of the pressure data is now in progress.  
P re s su re  data reduced by the peak amplitude measurement method 
yielded good results only for the ring section normal to the excita- 
tion amplitude. 
the results of the data taken f o r  peak pressure-force measurements. 
At frequencies corresponding to the peak pressure-force measure- 
ments, the liquid motion is comprised of a combination of the first 
anti-symmetric and symmetrical sloshing modes. 
analysis results a lso indicatetthat these two modes a r e  the dominant 
harmonics . 
The errat ic  pressures  at other ring sections a r e  
The harmonic 
Experimental force measurements for solid and perforated ring 
baffles have been completed and the data reduced. Preliminary 
results indicate that the experimental force measurements on a 
solid ring baffle a r e  in a range bounded by two different theoretical 
force computations. 
dimensional force measurement vs. baffle submergence ds/R for 
three values of translation amplitude. 
Figure 1 presents the experimental non- 
Included in this plot are the 
. .  . 
https://ntrs.nasa.gov/search.jsp?R=19660007685 2020-03-16T23:07:17+00:00Z
Director - NASA 
MSFC 
April 5,  1965 
Page 2 
average theoretical values calculated from the NASA-MSFC Aero- 
Astrodynamics Internal Note 4-64by Frank C. Liu and the average 
values obtained from the Handbook of Astronautical Engineering, 
"Propellant Sloshing", Section 14.14/15, pp. 14-14 to 14-27. 
Experimental slosh heights and resonant frequencies were used in 
the theoretical computations. 
-
Figure 2 presents a comparison of the baffle forces for a solid 
ring baffle and various perforated ring baffles. The force results 
for all the baffles studied in  Technical Report N o ,  5 ,  Southwest 
Research Institute, will be presented ultimately in a new technical 
report  covering the present work. 
Dampinn Coefficients for Mechanical Models 
The object of this program i s  to determine the significance of the 
moment of inertia damping proposed in H. F. Bauer's mechanical 
model for lateral  sloshing (NASA TR R-187). 
only when the tank is excited in pitching motion. If the moment of 
inertia damping is not included, the moment response of the equiva- 
lent mechanical model can be written as @ t terms caused 
by f r ee  surface sloshing (including damping) where Ifluid is the effec- 
tive moment of inertia of the fluid in a closed container. 
to Bauer, however, the moment response should be 
This te rm a r i se s  
According 
t t e rms  caused by free surface sloshing (including 
damping) where Iri id is the moment of inertia of a rigid body of the 
same mass and shgpe as the fluid, ID is the moment of inertia of a 
mass less ,  oscillating disc located at the liquid c o g . ,  and CD is the 
damping coefficient of the viscous damper attached to the disc. 
the damping is zero, this equation reduces to 
If 
(Irgid - I,) t sloshing te rms  
and both models a r e  equivalent. C D  can 'rigid "fluid so  that ID = 
be determined approximately by measuring the damping of the fluid 
in a closed container (no sloshing) which i s  excited in a pitching motion. 
Director - NASA 
MSFC 
April 5, 1965 
Page 3 .  
We had originally planned to determine C D  by pitching a closed 
tank in steady state motion, and then measuring the moment 
response and the phase angle of the moment with respect to the 
excitation. 
by electrical means; hence, these measurements should have been 
sufficient to determine the actual fluid moment of inertia and the 
damping. However, the damping was so small that the inherent 
noise in the system completely invalidated the experimental data. 
After trying a number of corrective procedures, all to no avail, 
it was decided to abandon this method. 
The inertia of the system without fluid was compensated 
A schematic of the new apparatus developed to measure both the 
liquid moment of inertia and damping for the capped rigid tank 
case is shown in Fig. 3 .  
diameter tank which is supported on two sides by sharp pivot points 
near  the top of the tank; the sharp pivot points produce very low 
inherent damping in the system. 
tank near the bottom, s o  that the entire tank forms a single-degree- 
of-freedom, spring -mass system. The natural frequency of the 
system can be varied by changing the attachment point of the springs 
relative to the pivot point. 
The liquid is contained in a 7. 71 inch 
Four springs a r e  attached to the 
The liquid moment of inertia is determined by f i r s t  measuring the 
natural frequency of the tank with liquid, and then without liquid. 
The differente in natural frequencies allows the liquid moment of 
inertia to be calculated about the pivot point. The liquid moment 
of inertia about the c .g .  is then found by the transfer theorem. 
(It was decided to  pivot the tank near  the top, rather than at the c g e , 
since the natural frequency change from empty tank to full tank is 
considerably grea te r  about a pivot point near  the top. 
course,  helps reduce the experimental e r r o r .  ) Tentative data indi- 
cate the liquid moment of inertia about i ts  c .  g. to be about 0. 19 to 
0 .20  of the same mass  of frozen liquid; this agrees  with theory. 
This, of 
Dir e c to r - NASA 
MSFC 
April 5, 1965 
Page 4 
Damping of the liquid is being measured by oscillating the system 
at the natural frequency and then shutting off the excitation and 
observing the decaying response. 
liquid damping is taken a s  the difference of the logarithmic decre- 
ment of the tank with liquid and that for the empty tank weighted 
with an amount of lead equal to the weight of the water. Hence, 
the damping measurements a re  made at the same amplitudes and 
frequencies for both the full  and empty tank. 
The logarithmic decrement of 
Tentative data show the empty tank damping to be about $ = 
0.00270. 
than this at all frequency values which have been measured between 
2 cps and 5 cps. 
by taking the difference of these two values a r e  about 6 = 0.00066, 
which,lof course, is very small indeed. In fact, it  may be questioned 
whether this i s  within the experimental accuracy of the system, except 
that the damping values with liquid are always greater than those for 
the empty tank. 
The full unbaffled tank damping values a r e  slightly greater 
The log decrements due to the liquid alone bbtained 
Preliminary data for the damping of the liquid when one ring baffle 
is located a t  the mid-plane of the tank indicate that the damping is 
substantially larger  than for the 
ing with the baffled tank. 
General 
Both portions of the present wor 
unbaffled tank. Testing is continu- 
L appear to be proceeding satisfac- 
torily. 
been carr ied quite far and therefore we feel that preparation of a 
comprehensive technical report covering this work can be initiated 
shortly . 
The work on ring baffle pressures ,  as discussed above, has 
Director -NASA 
MSFC, 
April 5 ,  1965 
P a g e  5 
We trust  that MSFC technical personnel will visit SwRI in the 
near future for detailed discussions of this work. 
our hope that this general program of research can be conth ied  
for an additional year and that discussions on specific aspects 
of this continuation might be carried out a t  the same time. 
It is also 
Respectfully submitted, 
H. Norman Abramson, Director 
Department of Mechanical Sciences 
HNA:ac 
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Mr.  A.  C.  Hulen, SwRI-w/encls. (2)cc 
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Sloshing studies Quarterly progress report no. 16, 1 Jan. - 31 Mar. 1965

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