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OCR for page 927
24~ Symposium on Naval Hydrodynamics
Fukuoka, JAPAN, 8-13 July 2002
Quantitative Visualization (QViz)
Hydrodynamic Measurement Technique of Multiphase Unsteady Surfaces
Deborah Furey and Dr. Thomas C. Fu
NSWCCD
9500 MacArthur Boulevard
West Bethesda, Maryland 20817
Abstract
A non-intrusive optical technique, Quantitative
Visualization (QViz), has been developed to measure the
free surface deformations occurring in the bow and near
wake regions of surface ships. These regions are
generally difficult to quantify due to the multiphase
aspect of the flow as well as their very unsteady nature.
However, the unsteady surfaces, droplets and bubbles in
these regions are effective scatterers and allow for
optical imaging of the extreme deformations in the
surface. With a laser sheet and digital camera, the QViz
system illuminates the surface of interest and collects
digital images representing cross sections in time of the
spray envelope occurring in the bow region and surface
profiles in the steep wake regions. Variations in
illumination and image collection were investigated for
several free surface and multiphase flow conditions in
order to characterize this technique. Data analysis
programs have been developed to extract the surface
boundary data, both time averaged profiles and unsteady
quantities. The results show that the technique is
suitable for multiphase conditions where the boundary is
not rapidly changing over large distances relative to the
field of view. Comparisons were done between
conventional fingerprobe measurements and the QViz
technique.
Introduction
Quantifying surface profiles in the near wake and bow
regions of surface ships has not been easily achievable
using established methods. Other imaging techniques
have been used to evaluate flow characteristics in regions
of limited accessibility and complex free surface flow
regimes [2,3,4,51. The flow in these regions is unsteady
and has discontinuous surfaces making standard methods
such as sonic probes, finger probes or dye traces
unusable. Quantitative visualization, QViz, provides a
method to visualize and quantify these regions of high
unsteadiness and non-uniformity. The fluid in the near
wake and bow regions of surface vessels is commonly
two phase, characterized by both droplets and bubbles
generated from spray sheets forming off the bow and
splashing caused by impacting fluid. Though this makes
using established methods difficult, it provides ideal
surfaces for optical scattering and quantitative imaging.
Using a light sheet and digital camera, the QViz system
illuminates the surface of interest with a specific input
laser power and records an instantaneous digital image.
This paper will discuss the examination of several
technique variables and how they effected the resulting
images. The results being presented include
comparisons with finger probe measurements, parametric
study of QViz variables and data collected from a surface
ship model tests.
For measurement verification, comparisons were done
between the surface displacements measured using QViz
and a conventional fingerprobe method. These results
were analyzed for amplitude comparisons as well as
frequency content. In addition, in an effort to
characterize the QViz method, a parametric investigation
into the effect of system settings and surface roughness
on the resulting images was conducted. Variations in
illumination were achieved through changing incoming
laser intensity and changing the viewing direction for a
given light sheet geometry. Additional variations
include changing CCD exposure times and aperture
settings. An initial set of QViz data was collected using
a surface ship model at NSWCCD to investigate the
feasibility of this method. The images in the bow region
represent an instantaneous cross section in the spray
envelop at a specific axial location and can be used to
extrapolate the entire average spray envelop. The images
in the near field stern region provide instantaneous wake
profiles and can be used to evaluate a time averaged
wake profile.
Quantitative Visualization Method
The QViz system consists of a continuous wave laser and
optics to create a steerable light sheet. The light sheet
and collection optics are mounted at specific orientations
for the flow and conditions being investigated. For the
laboratory set up as well as for the model tests on
Carriage 5 at NSWCCD, the light sheet is directed into
the region of interest from above the water line and
oriented perpendicular to the flat free surface. A digital
camera is oriented at a known angle relative to the light
sheet and free surface being imaged.
For the parametric investigation, the camera and incident
optics were oriented at angles ranging between 0 and 90
degrees relative to the free surface and to each other for
the different surface features being imaged. Figure 1
illustrates the set up in the miniature water basin tests
which is similar to the set up on the Carriage 5 test.
OCR for page 928
Set up in the Miniature Water Basin
Mirror
Lens '
Laser
Laser Sheet ~ Camern
Figure 1
12 dewing Angle
For these tests, a Sony DV Cam was used which collect
images at 480 x 640 pixel resolution. The laser was a
Spectra Physics, 5 W Argon laser. The resulting digital
images represent that portion of the flow within the laser
sheet and provide an instantaneous cross section of the
fluid. The images are processed to evaluate the specific
quantity of interest; surface displacement, cross-sectional
area of the spray in the bow region or the displacements
In one tree surface occurring In the wake region.
Processing methods investigated both instantaneous
image analysis as well as averaging techniques to
determine the free surface and spray envelop boundaries.
. .
Initial developments of the QViz analysis averaged
several images together, effectively providing a time-
averaged cross section. From this image the averaged
cross-sectional area and surface profile are determined.
Below (figure 2) is a sample image of the bow wave
spray region from the model tests. The ship model is on
the right side of the image. The illuminated region
represents the bow spray envelope at a particular axial
location.
Figure 2: Spray region illuminated by a laser sheet in the
bow wave of a planing ship model. Station 7, x/1=0.7.
The images were saved in a raw binary format and
processed using Fortran codes to extract the boundary
information. Image filtering and thresholding techniques
24th Symposium on Naval Hydrodynamics
Fukuoka, JAPAN, 8-13 July 2002
were used to evaluate the average profiles while gradient
methods were required to evaluate the unsteady
boundaries. The number of images required to establish a
steady average varied for the different test conditions due
to the nature of the flow generated. The quantities were
converted to spatial coordinates using calibration
conversion routines.
Technique Development:
Calibrations:
The purpose of developing QViz is to spatially quantify
the surface profile in the bow and stern regions of
surface ship models. This requires calibration of the
CCD camera for each laser position and camera setting.
The cameras were calibrated using a calibration target
marked with a 2 inch by 2 inch grid, which was imaged
for each plane of investigation and camera orientation.
This calibration grid was used to map the CCD pixel
information to xyz information. The grid was stabilized
in position and referenced to the free surface using a
plumb bob. The pixel information is transferred to the
grid coordinates and then transferred to the model
coordinates using the grid positional information.
Average Images:
The images collected using the laser sheet of the free
surface profiles and bow spray envelopes show
illuminated regions which are to be quantified spatially
using the CCD calibrations mentioned above. However,
the profiles in the regions of interest, the bow and the
stern, are very unsteady. With this consideration, it was
decided to do preliminary image processing to extract
average profiles from the images.
The digital video was downloaded using an AVID digital
video editing system to a series of binary black and white
images. The binary files were read into a Fortran
program which averages together a specified number of
images. Figure 2 shows an image from the model tests
conducted on Carriage 5. The imaging plane is at model
station 7, x/1=0.7 and the test speed was 30 kts.. A
minimum of 20-30 images is required for this condition
to achieve a steady average for a set threshold pixel
limiting value (refer to Figure 51. From the average
image, the pixels which corresponded to the boundary of
the illuminated regions are identified and mapped back
to spatial coordinates using the calibration data. It was
found that for the different running conditions as well as
camera positions and field of view settings, a different
number of images were required to get a steady average.
(Discussed in the next section).
Analysis Methods:
Thresholding
Two analysis methods were used to extract the data from
the video images. The main purpose was to identify the
edge of the illuminated region. Looking at the average
images, the first approach was to use a certain threshold
pixel value, which corresponds, to the edge of the bright
2
OCR for page 929
illuminated region. This required manual evaluation to
identify the best pixel value and would result in a clearly
identified region of interest (ROI). The analysis
program would then search down each column to
identify the pixel corresponding to the edge of the ROI .
These pixel values are then mapped back to the XYZ
coordinates using the calibration grid information
acquired at the beginning of each run. This method gave
good results for cases with high frequency unsteadiness
Figure 3: Average image at Station 7, all= 0.7 for
model tests on Carriage 5 at DTMB.
Figure 4. Thresholded image for Station 7, x/1= 0.7.
Threshold pixel value of 240.
24~ Symposium on Naval Hydrodynamics
Fukuoka, JAPAN, 8-13 July 2002
of low amplitude, and the average was established in few
frames. For cases where the flow was largely unsteady
and rough, 60 images were required to establish an
average cross sectional image. Figure 3 shows the
average image established for the model tests at Station
7, x/1=0.7, 30 knots. This method was considered to be
speculative and too dependent on manual input. Using
thresholding provides clean edges, however, manual
input may cause inaccurate edge definitions. See figure 4
70 - ~
65 - ant
_ 60- fit,
< 55- hi
_ 50 all
In, 45- ii;
40- in
35-)
25
o
5 10 15 20
Number of Images
25 30
Fig 5: Area as function of number of images averaged
together. For the30 kt case, station 7, x/1=0.7, it requires
approximately 20 images at least.
The threshold value was investigated to determine the
correct value for determining the correct pixel value for
establishing the boundary for the region of interest. The
effect of pixel value on area was one indicator. Figure 6
illustrates the change in area as a function of threshold
value. The desired threshold value will be where there is
no change in area for a change in threshold value.
Figures 6a shows that there is a region near a pixel value
of 230 and 250 where the area is least sensitive to
threshold value. Figure 6b illustrates the derivative
directly. At a pixel value of 240 the derivative is a
minimum, approaches zero, and correlates to the pixel
value corresponding to the boundary for that image.
3
OCR for page 930
200 ~
arc 150 ~
n 311
0 50 100 150 200 250 300
Threshold
Figure 6a: Area vs threshold for 2 cameras imaging
station 7, x/l= 0.7.
Gradient Method
As an alternative to the thresholding method, a gradient
technique was developed to utilize the local intensity
information in the image. It had been found that there
was a significant intensity variation across the image for
some test conditions making edge detection more
difficult. Therefore, a localized search method was
developed to search columnwise for the boundaries in
each x location in the image. This approach eliminated
the intensity variation problem and provided a suitable
approach for a detailed unsteady analysis.
The gradient search method interrogates the image
columnwise for changes in pixel intensity. The
boundary is considered to be located where the gradient
is the largest. The average image was calculated using
30, 60, or 120 images depending on the unsteadiness of
the flow field. The average boundary was then identified
using the gradient search technique and defined the ROI
for the cross section. This was considered the average
boundary for that cross section. For extracting unsteady
information, a comparison was made between the
average boundary and the instantaneous boundaries
determined from each image. The set of images used to
establish the average profile are interrogated
individually. Each image is filtered to eliminate noise
and then analyzed using the gradient method in the
established region of interest. The data was stored as a
function of column location, or equivalently x location
relative to the model. The RMS was then calculated as a
function of location. (Refer to Figure 17) This analysis
revealed that the RMS is relatively small and the
boundary is considered to be well established using the
average image.
Real Time Image Analysis
The QViz analysis initially developed analyzed images
downloaded from video tape to discrete files and were
then post processed to compute the desired information.
This procedure requires significant time to allow for the
images to be saved, digitized, and processed
0
-0.524
~ -1 5
·o -2 5
, -3.5 -
-4
-4.5
;)
Threshold
Figure 6b: Change in area for change in threshold.
individually. This original analysis indicated that the
averaging and gradient techniques were reasonable and
were implemented into a real time system. With the
results using the gradient edge detection, a real time
analysis technique was developed using National
Instruments Labview software. The image analysis
software was designed to average several images
together and evaluate the surface profile or to interrogate
a small region, image by image, to track a point in the
flow for varying conditions. The development of the
real-time system greatly increases the practicality of
using QViz for looking at unsteady flows as well as
greatly reducing the data reduction time. A non-real-
time system can still be used, but a substantial effort is
still necessary to process the large number of images
required to examine unsteady flows.
The real time system was developed using Labview
software and image processing toolbox. The reduction
software was written for 2 types of analysis. One
analyzes the entire image, and requires averaging a
minimum of 10 images together. The boundary is
evaluated using the gradient search method discussed
previously. An alternative program interrogates at a
fixed location in the image, this allows for investigating
time varying flows with a moving free surface boundary.
Finger Probe-QViz Comparison using Real Time
analysis
An experimental, in-situ comparison of a finger probe
and QViz system was performed to provide a
comparison between an established measurement
technique and the light sheet method. A mechanical,
servo-motor-controlled finger probe has been used to
quantify the free surface wave heights in the far field
flow regions around towed models. Since the probe
operation is based on the periodic sensing of the water
surface at a fixed location, as the free surface becomes
more turbulent, as in the near field bow and stern
regions, the variance of the "captured" wave height
increases. For QViz technique validation a comparison
was made between data gathered using the conventional
fingerprobe interrogation technique and the QViz
method for various flow conditions. The results were
4
OCR for page 931
mixed but did verify that the QViz method could be used
to extract mean surface data in slowly varying flows or
flows with a small dynamic range.
For the finger probe/QViz comparison, both systems
were set up in the Miniature water basin at NSWCCD.
The basin is 40 feet long and the water depth was set at
19 inches. The basin is equipped with a manually
controlled paddle type wave maker. Four different wave
settings were used. For changing the surface roughness,
a breakwater 18 inches high was placed upstream of the
test section. This caused the waves to break and the
surface profiles to be irregular.
The finger probe and QViz systems were set up for
tracking the free surface for various waves. The QViz
software was set up to interrogate a narrow region in the
image, or a single x location in the surface, comparable
to the finger probe comparison of a fixed location. This
region of interest (ROI) is defined in the Labview
software. The fingerprobe interrogated the free surface
at a rate of 10 Hz. The QViz data is collected at 30
and does not incorporate any interpolation or smoothing.
The variables investigated include viewing angle, laser
power intensity, wave frequency and breaking/non-
breaking conditions.
For the local QViz measurement, only 8 columns of data
are tracked. For these tests, the ROI was positioned in
column 316 to column 324, and included rows 160 to
2
1.5
on 1
C: 0-5 ~
,= O ..
-0.5
Q i.
-1 =
-1.5
-2
~~ (INS)
Figure 7a: QViz data for 11 degree viewing angle and
laser power set at 2.1 watts. Wave peak to peak
amplitude is 0.15 inches. Wave setting 1.
row 460 of the digitized images. This ROI was chosen
to be in the brightly lit region of the laser sheet, and was
4 inches upstream from the finger probe. To ensure a
true representation of the surface was acquired, the
median of the values extracted was used to represent the
water surface. This would eliminate any erroneous
extreme values caused by data drop outs or extraneous
bright pixels resulting from erroneous reflections. This
proved to be a good technique to reduce the erratic
profiles for cases where the surface was primarily
smooth. However, when there were splashes, i.e.
breaking conditions, there were many spurious data
points. Also, for these measurements, the data was taken
frame by frame, no frame averaging was done.
Figures 7a to 7d show the non breaking wave profiles
collected using QViz and figures 8a to 8d show the
profiles collected using the finger probe. The different
wave settings are referred to as wave setting 1,2,3 and 4
respectively. It is expected the data will have some
inconsistencies because the wave maker is set manually
and data was not collected simultaneously for all data
runs for the finger probe and QViz.
The results show the amplitudes and periods are
comparable. This suggests the calibration and transfer
functions are reliable. Figures 9a to 9d and 10a to 10d
show the frequency content of the data collected by the
QViz and fingerprobe. The frequency analysis was done
using a MATLAB tE l algorithm. Again, the
frequencies are comparable.
2-
1.5 -
°0 1
.
0.5- i
_
As O
.— -0.5 - l
Q
E -1 -
-1.5 - z
-2 -
Time (second)
Figure 7b: QViz data for 11 degree viewing angle and
laser power set at 2.1 watts. Wave peak to peak
amplitude is 0.6 inches. Wave setting 2.
5
OCR for page 932
10 15 20 25
nme(seconds)
Figure 7c: QViz data for 11 degree viewing angle and
laser power set at 2.1 watts. Wave peak to peak
amplitude is 2 inches. Wave setting 3.
2.0
1.5
10
0.5
-1.0 - a.
-1.5- D
-2.0 - ~
20
5 10 15 20
rime (seconds)
Figure 7d: QViz data for 11 degree viewing angle and
laser power set at 2.1 watts. Wave peak to peak
amplitude is 3 inches. Wave setting 4.
25 30 35
nme (seconds)
Figure 8a: Fingerprobe data for wave peak to peak
amplitude of 0.14 inches. Wave setting 1.
20
1.5
1.0
', Q5
~ DO
.' ~.5
E -1.0
-1.5
-2.0
r~ne(se=~s)
Figure 8b: Fingerprobe data for wave peak to peak
amplitude of 0.64 inches. Wave setting 2.
0 5 10 15
Time (seconds)
Figure 8c: Fingerprobe data for wave peak to peak
amplitude of 1.8 inches. Wave setting 3.
2.0 - ~
1.5 - .
al 1.0
c 0.5-
_ ~
,= 0.0- .
Q -0.5 -
-1 .0 -
-1.5 -
-2.O
5 10
Time (seconds)
. .
15 20
Figure 8d: Fingerprobe data for wave peak to peak
amplitude of 2.8 inches. Wave setting 4.
6
OCR for page 933
figure 9a: QViz frequency analysis for wave 1.
figure 9c: QViz frequency analysis for wave 3
figure 9b: QViz frequency analysis for wave 2.
figure 9d: QViz frequency analysis for wave 4
figure lea: finger probe wave setting 1
figure lob: finger probe wave setting 2
7
OCR for page 934
figure lOc: finger probe wave setting 3
From the frequency analysis, the timing is consistent
between the two methods. However, combining the
amplitude information from the scaled profiles and
frequency information, it would be expected that the
power in the respective frequency components should be
closer. This discrepancy is not yet understood.
Intensity of the incoming laser sheet was also
investigated for the different wave settings, viewing
5 7 9
11 13 15 17 19
Rome (seconds)
Figure lla: QViz data for a 56 degree viewing angle,
laser power at 0.2 watts. Wave setting 2.
~ '~
1 5
-2
5
Figure lib: QViz data for a 36 degree viewing angle,
laser power at 1.1 watts. Wave setting 2.
figure led: finger probe wave setting 4.
angles and breaking conditions. Below, figures lla to
1 1 d show how the wave profiles are effected by
incoming laser intensity. It is evident that for low laser
powers there are more erroneous measurements due to
low contrast in the image. As the power increases, the
wave profiles have less scatter. At the highest laser
setting, the profile is smooth throughout the sample time.
1.25
1
c 0.5
O
-0.5
-1
-1 .5
-2
15 17 19 21 23 25
Time (seconds)
Figure 1 lo: QViz data for a 36 degree viewing angle,
laser power at 0.6 watts. Wave setting 2.
1.5
Tic 1
I 0~5
O
_ -0.5
-1
-1 .5
-2
15 20 25
Time (seconds)
Figure 1 id: QViz data for a 36 degree viewing angle,
laser power at 2.1 watts. Wave setting 2.
8
OCR for page 935
A breakwater was introduced to cause a roughening of
the surface. Figures 12a to 12 d show the QViz profiles
for the breaking and non breaking conditions. For low
amplitude low frequency conditions, QViz technique was
able to track the surface profile reliably for both the
breaking (Figure 12b) and non breaking (Figure 12a)
conditions with some data drop outs and erroneous
A
0 5 10 15
Time (seconds)
Figure 12a: QViz data at an 11 degree viewing angle, 2
watts with no break water for wave setting 2.
1.5
y 0.5
Tic OS
10
14 16 18 20
Time (seconds)
22 24
Figure 12b: QViz data at an 11 degree viewing angle,
1.1 watts with break water for wave setting 2.
The variation in viewing angle did not produce any
unforeseen results. The scaling changed for each case
causing a reduction in resolution for higher viewing
angles. This can be improved by using a smaller field of
view for higher angles giving improved resolution.
From the fingerprobe comparison to QViz, it is evident
that the QViz method is capable of tracking the surface.
For surfaces with a fairly steady boundary, the optical
technique can measure the boundary reliably. However,
if cases where the boundary has a largely dynamic
surface, it is not as reliable. For using QViz in regions
inaccessible to the fingerprobe, the flow must have some
steady character, or a steady average of the boundary
must be established for the QViz technique to be viable.
The results of this study will guide the choice of
techniques for use in various flow field regimes.
Camera Settings
The characterization of the QViz system required
investigating the effects of illumination parameters as
well as image collection parameters. Results showed
that largely bubbly and unsteady free surfaces result in
points in the breaking case. For the higher amplitude
higher frequency waves, figures 12c and 12d, QViz is
unable to track the surface for the breaking condition.
This was due to the large number of scatterers splashing
in the test section which caused the edge detection to
resolve on random droplets in the ROI.
tar
cO OS,
~c'°S,2
~ 2 ! I
10 15 20
Time (seconds)
Figure 12c: QViz data at a 36 degree viewing angle, 2.1
watts with no break water for wave 4.
2
1 .
c 0.5
c
~' O
.~ -0.5
1
-1.5
-2
5
10 15
Time (seconds)
20 25
Figure 12d: QViz data at a 36 degree viewing angle, 2
watts with breakwater for wave setting 4.
greater recorded image intensity than for less disturbed
free surfaces for the same incident laser sheet power
Also, as expected, the images become less sharp for
longer exposure settings. The surface is clearly imaged
due to the large number of scatterers as the surface
becomes roughened. However, for the longer exposure
times it is evident that there are also reflections from
internal scatterers as well, obscuring the surface
boundary and making it impossible for the QViz analysis
to decipher the real surface. For high power settings, it is
better to use short exposures when imaging roughened
surfaces to capture scatterers within the ROI. Longer
exposures with lower power settings are suitable for
smooth boundary surfaces with fewer scatterers.
Model Testing Results
One set of experiments utilized the QViz method in the
bow and stern regions of a surface ship model towed on
Carriage 5 at the David Taylor Model Basin [1,6,71.
These regions of the flow are unsteady and are not
accessible by the conventional methods, i.e finger
probes. However, using an optical technique, allows for
9
OCR for page 936
access to these regions. For the initial ship model data,
QViz was utilized to evaluate the average profiles of the
mean cross-sectional area of the spray envelope in the
bow region and a mean surface profiles for the stern
region. Sample data sets will be included here. Figures
15a and l5b show the average image and the
corresponding average profile extracted from the QViz
analysis for station 7, x/l= 0.7. The averaged images
show which portions of the spray envelope are the most
steady, and give an indication of the spatial variability of
the bow spray. Figures 16a and 16b show the resulting
average stern profiles for the model test results.
Figure 15a: Averaged image for the spray region
illuminated by a laser sheet in the bow region of a
planning ship model at station 7, xlI'0.7.
Average Cross Section
Bow Envelope
X (inches)
Figure 15b: Boundary envelope for average image.
Station 7, x/L=0.7.
Figure 16a: Station 13, x/1=1.3 of model test at 30 kts.
A frame by frame analysis was done and the
instantaneous profiles compared to the average profile.
This provided a measure of the RMS about the average,
refer to Figure 17. The unsteady analysis showed that
the variation in the boundary when compared image to
image was small enough and that the average image was
a sufficient measure of the flow profile.
Spoon 13
Y (inC~)
Figure 16b: Station 13 QViz profile at 30 kts
15 -
10 -
07
~ O-
_
-5 -
-10 -
-15 . _
Y (inches)
Figure 17: Average profile of stern profile at x/l=13 with
RMS error bars.
10
OCR for page 937
Conclusions
Quantitative Visualization provides a viable technique to
quantify regions of a surface ship wake, which would
otherwise be inaccessible through established means.
From the initial data set and investigations into the
effects of system components and surface characteristics,
surface profiles can be quantified using the QViz
method. Analyzing time averaged images using the
methods developed provides a realistic measure of the
mean boundary defined by the spray in the bow region
and wake region of surface ship models. It was found
that QViz is not viable for largely dynamic surfaces,
where there are a large number of discrete scatterers.
However QViz is suitable for slowly varying flows or
evaluating boundaries with high frequency low
amplitude changes as well as steep surface slopes where
conventional techniques are less suited. Further, the use
of QViz can potentially provide a viable way to validate
computational predictions in flow regimes where
conventional data is not available.
References:
1. Stahl, R.G., "Ship Model Size Selection,
Faciliteis and Notes on Experimental
Techniques", CRDKNSWC/HD-1448-01, May
1995.
2. Goldstein and Smits, "Flow Vizualization of
the 3-D Time Evolving Structure of a Turbulent
Boundary Layer", Phys Fluids, 6, p. 577, 1994.
3. Rockewell, Lin, Cetiner, Downes and Yang,
" Quantitative Imaging of the Wake of a
Cylinder in a Steady Current and Free Surface
Waves", Journal of Fluids and Structures,
Vol 15, No. 314, Apr 2001, p. 427-443
Logory, Hirsa and Anthony, 'Interaction of
Wake Turbulence with a Free Surface", Phy
Fluids, p. 805, 1996.
5. Huany, Kawall, Keffer, and Ferre, "On the
Entrainment Process in Plane Turbulent
Wakes", , Phy Fluids ,7, 1130, 1995.
6. Zselecszky, J.J., "Resistance and Seakeeping
Model Tests of a Hard Chine Planing Hull",
U.S. Naval Academy, Report EW-17-96, June
1996.
Ratcliffe, T. Mutnick, I, and Rice, J.
"Resistance Characteristics of a Planing Hull as
Respresented by Model 5572 Towed in Calm
Water and In Regular Waves", NSWCCD-50-
TR-2001/025.
11
OCR for page 938
DISCUSSION
T. Waniewski Sur
Science Applications International Corporation,
USA
The QViz system described in this paper appears
to be a simple yet practical technique for making
measurements of unsteady surface locations;
however, a description of its accuracy and
precision would help others to interpret future
measurements. The comparison of QViz and
finger probe measurements taken in the
miniature model basin presented herein (Figures
7, 8, 9, and 10) is a good start and could be
expanded in several ways. For example, the
authors state that the QViz and finger probe data
was "not collected simultaneously for all data
runs," but do not indicate which data was
collected simultaneously and which was not.
Since the distance between the two instruments
is known, it seems possible and useful to adjust
the phase of the simultaneous measurements
accordingly and then compare them. In addition,
data from different runs is presented in the
figures, yet the authors do not indicate the
repeatability of the flows created by the four
"wave settings" of the manually controlled
paddle type wave maker. If the repeatability of
the flows could be demonstrated, it would be
easier to study the repeatability of the QViz
measurement technique.
This paper also presents QViz measurements in
the bow and stern regions of a towed surface
ship model. Free surface data collected in these
regions will be helpful in validating numerical
models. Can QViz resolve a multi-valued free
surface; for example, both the upper and lower
surfaces of a plunging breaking bow wave jet?
OCR for page 939
DISCUSSION
T. Waniewski Sur
Science Applications International Corporation,
USA
The QViz system described in this paper appears
to be a simple yet practical technique for making
measurements of unsteady surface locations;
however, a description of its accuracy and
precision would help others to interpret future
measurements. The comparison of QViz and
finger probe measurements taken in the
miniature model basin presented herein (Figures
7, 8, 9, and 10) is a good start and could be
expanded in several ways. For example, the
authors state that the QViz and finger probe data
was "not collected simultaneously for all data
runs," but do not indicate which data was
collected simultaneously and which was not.
Since the distance between the two instruments
is known, it seems possible and useful to adjust
the phase of the simultaneous measurements
accordingly and then compare them. In addition,
data from different runs is presented in the
figures, yet the authors do not indicate the
repeatability of the flows created by the four
"wave settings" of the manually controlled
paddle type wave maker. If the repeatability of
the flows could be demonstrated, it would be
easier to study the repeatability of the QViz
measurement technique.
This paper also presents QViz measurements in
the bow and stern regions of a towed surface
ship model. Free surface data collected in these
regions will be helpful in validating numerical
models. Can QViz resolve a multi-valued free
surface; for example, both the upper and lower
surfaces of a plunging breaking bow wave jet?
AUTHORS' REPLY
Addressing the first question
phase comparison of the data,
concerning the
the fingerprobe
data was collected on every eighth run for the
QViz data. The camera settings and laser settings
had to be varied for each wave condition making
a much larger run matrix for examining the Qviz
parameter effects. Often, the fingerprobe data
was collected for the first laser setting which was
a very low light condition and not a good data
run for the QViz making that type of comparison
difficult for this data set but your suggestion
could be implemented for future test set ups.
The second question was about repeatability.
The test was set up in the miniature water basin
at NSWC which is instrumented with a manually
controlled wave maker. This type of wavemaker
made it difficult to have the exact same setting
for each run. We had incremental markings on a
continuous pot type control making the exact
wave setting difficult to match. So the results
give wave periods of comparable periods and
amplitudes but the conditions could not be
matched from run to run for this type of
wavemaker.
The last question asked if QViz can resolve
multi-valued surfaces. Yes, the images collected
by the QViz system can resolve both the upper
and lower surfaces for many conditions. The
ability to resolve multi-valued surfaces depends
on the flow conditions, the laser settings and
optics.
Representative terms from entire chapter:
qviz data
