waterfall rains
TRANSCRIPT
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LEVEV
PROJECT
SQUID
TECHNICAL REPORT S U 3 P U
A
WALL FLOW DIRECTION PROBE FOR
USE
I
N
SEPARATI
N
G AND
R
EATTACHING
F L O W S
BY
~
‘
JOHN
K. EATON ,
ALBERT
H.
JEANS
,
JALAL ASHJAEE and JAMES P. JOHNSTON
LLJ
STANFORD
UNIVERSITY
STANFORD
,
CALIFORN
IA
94305
PROJECT SQUIDHEADQUARTERS
D 0 C
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W E S T
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,
INDIANA 47907
U
DEC
7
1918
NOVEMBER
1978
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Project
SQUID
is
a cooperative program
of
basic research relating
to Jet
Propul
sion. It is
sponsored by the Office of Naval Research and
is
administered
by
Purdue
University
through
Contract NOOO14
~
75.C-1
143,NR-098-038 .
T his
document
has been approved for public release and
sale;
its
distribution
is
unlimited.
78
12
04 1 .1 0 8
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A WALL-FLOW-DIRECTION
PROBE
FOR
USE
IN
SEPARAT I
N
G
AND
REATTACHING FLOWS
by
John
K. Eat
on
*
,
Al bert H.
Jeuns
*
,
Jalal Ashjaee
*
and
James P. Johnston
ABS
T
RA
C
T
The measuranent
of
the
exceedingly
unsteady
behavior of
nominally
two-dimensional , turbulent
flow
s
in regions
of separation and
reattacl’inent
is facilitated by the development
of
a
new instrument;
the wall— flow-direction probe
which
determines
the
instantaneous flow
direction
(upstream
or
downstream
) n
a
thin
layer
of
fluid
very close
to
the wall. The
probe
was tested in
low-speed
,
unsteady,
separating
and reattaching air
flows
. It
appears
to offer considerably more
accuracy
than
other methods for the
determination of
time mean separa-
tion and
reattachment
points.
In
addition
, the probe
and its
control
circuits
are
relatively i nexpensive and easy to
construct
.
ESSION
fo r
______
NTIS
\ h I ’
:
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r
~
Infl
DOC
Li O n
0
UNANN OI
~~
’
~
0
JUSIfI
CA ’
~
BY
—
~
STRIBU
~
H
~
VAt
~
BIUF
tOOtS
~~~
id
or
SPECIAL
-I
~~~
*
Research Assistants
,
Dept.
of
Mechanical Engineering
,
Stanford Univers i ty
.
**
Professor of Mechanical Engineering
,
Stanford
Univers
ity
.
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1
.
INTROD
UC
T
ION
As part of
a
continu i ng program of
research
on
internal flows we
are
currently studying nominally two-dimensiona l
separating and reat-
taching turbulent flows which contain regions where
instantaneou s
flow
reversal frequently
occurs .
The
most notable example of such behavior
occurs
in the transitory stall
regime
of two-dimensiona
l diffuser
flows [1]. In cases
with
unsteady flow reversals
,
the
use
of most
conventional
instrumentation
is
precluded .
A
notable
exception is the
laser
doppler velocimeter
with frequency shifting ,
an expensive and
difficult
technique.
To
facilitate
our research
,
a
simple and
inexpensive
wall-flow -
direction
probe
has
been
developed
which
may be
used
to
determine
the
instantaneous flow direction
(upstream
or downstream
) n a thin
layer
of fluid very near a wall.
Although
the basic idea for this probe is
not new
, we
feel
tha
t other
investigators of
unsteady,
reversing flows
may
find the probe
’
s design
very
useful especially
in
light
of
its
low
cost
,
its
simplicity and
its
relative
ease
of
application in low-speed
air flow experiments
.
2.
BASIC DESIGN
The probe (Figure 1)
consists
of three parallel wires mounted
on
short prongs
which
penetrate
the wall and
are
perpendicular to the flow
.
The larger
,
center
wire
is heated
with a
D.C.
current
of
about
1.5 amperes
to create a
hea
t
ed
wake
in
the layer
of
air close to the wall. The two
sensor w i res
,
operated
as
resistance thermometers
,
detect
the
wake
,
and
through a
simple electronic
circuit
give the instantaneou s sign of
the
fluid
velocity In the layer
of
air which
is
within 1 nm
of
the
wall.
J
~
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3. DESIGN
DETAILS
The three
wires
are
mounted
on six copper
plated
sewing needles
(.3
nm diameter),
which
are
rigidl y mounted in
a plexiglas
cylinder.
The
cylinder
fits inside of the outer shell which
itself
fits
into the
wind tunnel Instrument
ports.
Once
installed ,
the surface
of
the probe
appears to be an integ ra l part
of
the test
surface
to
±0.03 mm
.
Th e
needles protrude
throug h clearance holes
in
the
face
of the outer shell.
With this arrangement
,
the needles may be extended about 5 nm from the
probe surface to facilitate
wire
mounting
.
After
the
wires are
mounted
the
needles are
retracted
so that the wires are about
1 mm above
the
surface.
An earlier
version
of
this probe used
a
center
wire
mounted
flush
to
the
surface. Data sets using both
probes
show tha t the two
configurations give identical results to
wi
thin
the
experimenta l uncer-
tainty of
either type
of
probe.
The
sensor
wires
are 5 micron
platinum
plated tungsten and
are
soldered
to
the ends
of
the needles.
The heater
wire
is
125 micron
mone l
,
and it
is soldered
to
the
side of
the
central pair of
needles
just below
their tips.
The solder
joints on the
heater
wire
are coated
with epoxy to prevent oxidation
and
to provide
mechanical
strength.
The wires
are
connected throug h the
needles
to a five
pin connector
mounted in the outer shell. Only
five pins
are needed since
the
sensor
wires
share a
common
ground.
A block diagram
of
the
electronic
circuitry
is
shown
in
Figure
2.
The two
sensor wires
are
operated in
a bridge which
is
balanced
when
both
wi
res are
at
the same temperature
.
If
the temperature
of
sensor
wire 1 is increased ,
its
resistance
w i l l
Increase
and
a
positive voltage
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w i l l
appear
at amplifier A l .
This voltage
is
amplified and
then applied
to
a
comparator (C) which controls switch S The switch
gives an
output of 5 volts or zero volts
depending
on the
flow
direction
.
The
final
amp
lifier
(A2) is used to accurately adjust the
outpu
t
zero
.
The circuit
diagram
is shown in Figure
3.
Power supplies are
needed
to supply
~
15 volts
and
V
ref
which may be any
voltage
from 0
to 12 volts. We chose 5
volts
in order
to facilitate
computer inter-
facing. An additional
power
supply
is needed
to supply
current
to
the
center wire
.
Excluding the power supplies , the
total cost of components
for this
control
circuitry did
not
exceed $40.00.
Before using
the
system , the
probe
should
be
placed in
the
flow and
the i nput offset
adjusted with the heater turned
off. If
the
offset
is
adjusted correctly,
the
output
w i l l dither
between
0 volts and
V
ref
This
adjustment
is
qu i te
stabl e
but
should
be
checked
every
hour or
so
in the
course of
a
run.
4.
APPLICAT
I
ONS
The
wall-flow-d
i
rection
probe
can
be used
for several different
purposes. The
fraction of
the time
that
the local flow
near the
wall is
forward
or backward
can be measured
by time-averaging
the probe
’
s outpu t
using an i ntegrati ng voltmeter
.
When
the
fraction
of
time
that the
flow
is
in
the downstream
direction
henceforth called percent downstream
flow
) Is measured for
a number
of streamwise locations
in a
nominally
two-dimensional separating or reattaching flow
,
the
location of
the
mean
*
detachment or reattacisuent
point can
be determined
.
In addition
to
this
*
We
have defined
the
mean
detachment
or reattachment point
to be the
point
where the
flow very close
to
the wall
is
reversed 50 percent
of
the
time.
This point does
not necessarily
coincide
with the
point of zero mean
shear st
ress
,
although
the two
points
are probably quite
close
together.
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~~
••
measurement,
spatial
and
temporal
correlations of
the
wall
flow
direction can
be made
usin g
two or more probes
at
the same time
.
Finally,
the wall-flow-d i
rection
probe can
be used
in conjunction with
hot-wire
anem
ometers
and
other
instruments for the purpose of condi-
tional sampl i ng of the anemometer
output.
The probe
has
been
tested in both
a
separating and
a
reattaching
flow.
In
both of these flows
,
the measured value of
percent
downstream
flow was a weak function
of
the
heater current for currents of less than
1 .5
amperes.
Higher
currents
lead to
shorter
heater wire lifetimes
so
a
current
of 1.5
amperes was used for both cases
presented here.
The probe
was first used
in
a
reattaching
flow behind
a
backward
fac i ng step 5.08
cm
h i g
’ ’
with free
stream
air
speed
s
of from 5
to
20
rn/sec. A
typical curve of percent downstream
flow plotted against
streamwise
location
(Figure
4)
shows
the same genera l
features prev i-
ously observ ed
using
flow visualization
,
(e.g.,
Kim
et al. [2]). These
features
include (i)
an
unsteady
reattacIi
~
e
nt
’egion
approximately
3-4
step
heights
l ong , (ii) a
region
of fairl
y steady backilow located close
to
x
=
4
step
heights
,
and ( i i i )
a
resepa ration point located
about 1
step
height downstream of
the
step
.
The
uncertainty
in each
value of percent downstream flow
is
esti-
mated to be
±1
at 20:1 odds.
The
uncertainty
in the location of the
mean reattachment
point
is
then
less than
‘
.1
step
height (Kline and
Mc
Clintock
[3])
which
is
an
order
of
magnitude
better
than
can be
*
obtained wi th most known methods
of
flow visualization
*
The only exception
is
the method of Rothe [4]
which
requires
tedious frame-by-frame
examination of motion picture
film.
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The probe
has
also been used in
a
stra i
ght-walled diffuser operating
in
the
transitory stall
regime.
The
throat
width
of the diffuser
was
7.62
cm
and the total
included angle
between the diverg i
ng walls was
12
degrees
.
In t h i s
case
,
the
i n l e t a i r
speed
was 50 ni/sec
.
The percent downstream flow is plotted against streamwise location
in
Figure 5.
Here
the probe
showed
a s m a l l amount
of
asymmetry
. The
measured
percent downstream flow was
s l i g h t l y
different
when the probe
was rotated 180 degrees thus reversing
the
positions of the sensor wires
.
The
average
of the two measurements was used to determine the mean
detaclmnent
point.
5. CONCLUSION
The
wall-flow-d
i rection probe is prov i ng to be
a
useful
and
easily
applied tool
for studying unsteady
revers ng
flows
It
enables one
to
accurately measure
the
location
of two-dimensional
detachment and reat-
tachment points in a consistent
manner
.
Adoption
of the
method by
future
researchers could enhance our ability to
compare
results
of
different
studies.
For
example
,
current
uncertainties
on distance to reattac
~
wnent
for separated flow
behind
steps
and
fences
are
roughly plus
or
minu s one
step height (H).
Use
of the wall-flow-direction probe can
reduce this
uncertainty to plus or minus
0.1 H
6. ACKNOWLEDGEMENT
We thank Mr. Michael Eaton
who
contributed his time
to design
the
electronic circuitry and
Mr. Robin Birch who
’
s
fine
craftsmanship lent
much to
the success of t h i s project.
Also,
we gratefully acknowl
edge
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—.
~~~~~
-
—
.— ._—‘ ~~
5-.
the support of
Proj
ect SQUID
,
and the National Sc
i ence
Foundation
which
supports the first
two
authors through graduate
fellowshi ps.
REFERENCES
1 .
Fox ,
R.
W.
and
Kline,
S.
J
. (1962).
“Flow
Regime Data and
Design
Method
s
for
Curved Subsonic
Diffusers
,
” TASME J . Bas
~~~~~
in
~~
i
~
a,
Vol. 84,
Series 0,
Sept
.
l96.
~
. pp.
303-312.
2. Kim
,
J.,
K l i n e
,
S.
J., nd
Johns ton
,
J.
P. (1978),
“Investi gation
of
Separation and Reattachuent of a Turbu
l ent Shear
Layer:
Flow
over
a
Backward-Facing
Step
,
”
Report
MD-37 , Thetmiosciences
Div.,
Dept. of Mechanical Engineerin g,
Stanford University.
3.
Kl i
ne,
S. J. and Mc C lintock ,
F
.
A.
(1953),
“Describing Uncertainties
in
Single-Sample Experiments, ” Mechanical
En
~~
neer
i
n
~
, Vol. 75
, pp.
3-8.
4.
Rothe ,
P.
H.
and
Johnston
, J. P
.
(1975),
“The
Effects
of
System
Rotation
on
Separation
,
Reattachment and Performance i n Two-
Dimensional
Diffusers ,” Report
PD-17
,
Thermosciences Div ., Dept.
of
Mechanical Engineering
,
Stanford University .
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EPARATIN G
AND REATTAC
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7.
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OMAN ?
NUM
UA I)
Jalal
Ashjaee
, John K. Eaton
,
Albert H
.
~
1eans
N000 l4
7 5 - C - 1 l 4 3
and James P.
Johnston
NR-098-038
I. PI
~~
FO NMING O R G A N I Z A T I O N
NA M
E
AND ADO
~~
( S
TO.
PROO
~~
AM
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Stanford Un i versity
Stanford
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Califo rnia 90007
II COP 4T
~~
O L L I P i G
O F F I C S N A M E A N D ADD
~~
ISS
~ ~~
PQ
~~~
O A T S
Project SQUID Headquarters
November
1978
Chaffee H a l l ,
Purdue Un
i
vers
i ty “
NuMSE
~~
0F PA0 ES
West Lafayette
,
Indiana
47907
13
F4 MO NIt O
~~
INO
A G E N C Y N A M E S
AD0
~~
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dflf. ,.nt
from Coni,oSUn
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II S E C U R I T Y C L A S S . .1 t A t .
r.po r )
Offi ce of Naval
Research
,
Powe
r Program ,
Code
473
Unclassif Ied
Department of the
Navy
800 No. Quincy Street
l DECL.
A SSIF i CA T I0 N DOWNO
~~
A DING
— -
SCHEDULE
Arlin gton ,
VA
22217
___________________________
1 .
O I T
~~
I
S U T IO N
S T A T E M E N T
(of this
Rsporf)
This document
has
been
approved
for public
release
and
sale;
its
distribution
is u n l i m i
ted.
li oIs T
~~
IS U T I 0 N S T A T E M E N T of A.
bstr ct
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aid. II
n c I a ty
and
Id
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. by
block
n,anb•r)
Probes
Separation
Re
attachment
Unsteady
Flow
20. A S S T
~~
A
C T (C.nttn u.
an
,.v .r. .
aid.
It
ns c asaafl
—
~
id .nHiy
by
block
nu
b.r)
‘
—
~
he
measurement
of the exceedi
ngl
y unsteady behavior of
n tm lna ll y two
-
dimensional
,
turbulent flows In
reg
ions of separation and
rea
‘
achment
Is
facilitated
by
the
development of a new
Instrument;
the wall-flow -direction
probe
which
determines
the Instantaneous
flow
direction upstream or d
own
s
tream
In
a
thin
l ayer of fluid very
close
to
the
wall. The probe was tested
In l
ow-
speed
,
unsteady
,
separating and reattaching air flows
.
It
appears
to
offer
considerably
more
accuracy
than
other
methods for
the
determination of the
t ime
mean separation
and
reattachment
points.
In
addition ,
the
probe
ari
d Its
~
‘
FOAM
~
DD
J A N 7
t4i.i
4
EDITION
OP
NOV SI J
O S S O L ( T I
Unclassified
5/
~~~~
S’N
01 02 .L.F .O (4 .66O1
SICUNITY
CLASSIFICATION
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8/16/2019 Waterfall Rains
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Unclass i
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S C U N I I Y
C L A S S , F R T A T I O N
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‘
/
con Lrol circuits
are
relatively Inexpens ive and easy to
construct.