Transcript PowerPoint 演示文稿

Application of image processing
method in water impact force
measurement
Reporter: Menghua Zhao
Email:[email protected]
Northwest Polytech Univ,
Xi’an Shaanxi, P.R.China, 710072
School of Mech., Civil Engg.
&Architect.
1
Outlines
 Research Background And Goals
 Studies On Water entry
 Measurement of impact force
 Experimental Apparatus
 Techniques For Impact Force Measurement




Problems analysis
Sub-pixel edge detection
Filter and verification of the method
Filter designed for impact force measurement
 Results
 References
School of Mech., Civil Engg.
&Architect.
2
Research Background
And Goals
• Studies on Water entry
In 1897, Worthington[1] started
the investigation of water impact
phenomenon. According to what
done by von Karman[2],
Wagner[3], Miloh[4], Moghisi[5],
Duez[6], Jeffrey[7], etc, studies of
the impact have been mainly of
two kinds:
1.Formation of cavity and
splash
2.Force of impact on the
object
a
b
Water entry of aluminum sphere
Fig 1 (a) D=50mm,VI=3m/s
(b) D=50mm,VI=5.2m/s
School of Mech., Civil Engg.
&Architect.
3
Research Background
And Goals
• Measurement of impact force
Features of impact force:
1.Transience
Impact force occurs in the very early stage of water entry where
penetration depth is about 0~0.2 radius.
2. Dramatic change
Sphere would experience an acceleration ranging from about 0~50g.
In 2007,Duez’s finding[6] shows that surface properties play an important
role in the formation of cavity and splash, which motivates us to test
impact force under the influence of wettability.
Contacting measurement method
Non-contacting measurement method
School of Mech., Civil Engg.
&Architect.
4
Experimental Apparatus
Release Device
Lights
High Speed Camera
Sphere
Releasing height :
46cm-184cm
Water impact velocity:
3m/s~6m/s
Water
Diffuser
Recording speed:
2000fps
Fig 2
School of Mech., Civil Engg.
&Architect.
5
Techniques For Impact
Force Measurement
• Problem analysis
Impact force
Image sequences
Sub-pixel detection
displacement
Acceleration from video
velocity
Displacement from video
Displacement
Acceleration (a)
Low-pass filter
Error amplified
by A(A=1/Δt2)
Difficult!
Acceleration filtered (af)
Acceleration
School of Mech., Civil Engg.
&Architect.
6
Techniques For Impact
Force Measurement
• Sub-pixel edge detection
Sub-pixel edge detection is achieved by a two-step procedure:
1.Localization by Canny’s method
2.Gaussian fit
X
Difference value of gray intensity
Gauss Fit
60
Difference of I
50
40
30
20
10
0
Y
96
Fig 3 Profile along the movement
98
100
102
104
106
108
110
112
114
i, j
FigSchool
4 Comparison
of Civil
Gaussian
of Mech.,
Engg.fit and difference directly
&Architect.
7
Techniques For Impact
Force Measurement
• Filter and verification of the method
1.4
Filtered Acceleration（ m/s2）
is chosen as digital Butterworth
filter.
1.2
1.Eliminate noise as much as
possible
1
2.Preserve real information
0.8to the best
2
|a(f)|
0
-2
-4
-6
Fig 5(b)
Parameters:
1.Low pass-band
cutoff
frequency:18Hz
2.Stop-band
cutoff
0.6
The aforementioned procedure
is applied to standard sinusoidal
motion:
1.A spherical nose is fixed0.4on a Fatigue Testing Machine frequency:25Hz
3.Maximum
2.Amplitude is 16mm and0.2frequency is 2Hz.
attenuation in
0
0
20
40
60
80
100 pass-band:0.5dB
0.5
1
1.5
Frequency（ Hz）
Time（ s）
4.Mininum
1.4
Fig 6(b)
Fig 6(a)
attenuation in
1.2
stop-band: 20dB
Fig 5(a)
Filter
0
1
1
0.5
|af(f)|
Acceleration（ m/s2）
4
0
0.8
0.6
-0.5
0.4
-1
0.2
0.4
0.6
0.8
Time（ s）
1
1.2
0
School of Mech., Civil Engg.
0
20 &Architect.
40
60
Frequency (Hz)
80
100
Maximum
error:0.13m/s2 ;
Maximum
relative error:
10%
8
Techniques For Impact
Force Measurement
• Filter designed for impact force measurement
Too few points are available from high speed camera in view of
the transience of impact stage.
Extra points added before impact: NP
Spectrum of theoretical prediction[4]
250
Fig 7
NP=0
NP=3
NP=6
NP=9
NP=12
NP=15
NP=18
200
|a(f)|
150
100
Table 1 Effect of NP on η for VI=5m/s
η
0
3
6
9
12
15
18
0.9606
0.9601
0.9554
0.9553
0.9504
0.9502
0.9499
NP
η
k
50
0
ak  500Hz; aN  1000 Hz
0
200
400
600
Frequency (Hz)
800
1000
Spectrum of impact acceleration under NP
School of Mech., Civil Engg.
&Architect.
 ai
 = iN1
2
 ai
i 1
2
9
Techniques For Impact
Force Measurement
• Filter designed for impact force measurement
Spectrum of impact force differenced directly
VI=4m/s
100
VI=4.6m/s
90
VI=5m/s
80
the parameters
of zero phase low-pass Butterworth filter is set as:
70
|a(f)|
Low pass-band
cutoff frequency is 400Hz,
60
Stop-band cutoff
frequency is 600Hz
50
maximum attenuation
is 0.5dB in pass-band
40
minimum attenuation
is 10dB in stop-band.
30
20
10
0
0
200
600
400
Frequency(Hz)
800
1000
School of Mech., Civil Engg.
&Architect.
10
Results
Alunium Surface Without Coating
Alunium Surface With Olive Oil Coating
Lamb-Cd
1.6
Miloh-Cd
1.4
Numerical Cd by Fluent
1.2
C
d
1
0.8
0.6
0.4
0.2
0
0
0.1
0.2
0.4
0.3
b
b  VI t / R, Cd 
0.5
Experimental results are the
average values of series of
spherical nose impacts with
entry speed ranging from 3-6m/s,
showing that:
1. The dimensionless depth
where peak of Cd occurs
coincides with theoretical and
numerical results
2.The peak of Cd is smaller
possibly because of filtering
3.Surface properties make little
difference on the impact force
before impact peak occurs
0.6
FI
1
AVI2
2
School of Mech., Civil Engg.
&Architect.
11
References
•
[1] Worthington, A. M. & Cole, R. S. 1897 Impact with a liquid surface,
studied by the aid of instantaneous photography. Phil. Trans. R. Soc. Lond. A
189, 137–148.
•
[2] Von Karman, T. 1929 The impact on seaplane floats during landing. Tech
Rep. 321. NACA.
•
[3] H. Wagner, Phenomena associated with impacts and sliding on liquid
surfaces, Z.A.M.M. 12 (1932) 193-235.
•
[4] T. Miloh, On the initial-stage slamming of a rigid sphere in a vertical water
entry, Appl. Ocean Res. 13 (1991) 43-48.
•
[5] M. Moghisi, P. Squire, An experimental investigation of the initial force of
impact on a sphere striking a liquid surface, J. Fluid Mech. 108 (1981)133146.
•
[6] C. Duez, C. Ybert, C. Clanet, Making a splash with water repellency,
Nature Phys. 3 (2007) 180-183.
•
[7]M.A.Jeffrey, W.M.Bush. Water entry of small hydrophobic spheres, J.Fluid
Mech.(2009) vol. 619, pp. 45–78
School of Mech., Civil Engg.
&Architect.
12
Thank you~
School of Mech., Civil Engg.
&Architect.
13