Experimental Analysis of Flow Induced Vibration Measurement Using Fiber Optic
Sensor
Jongkil Lee
*<Abstract>
Fiber optic sensor is widely used in measuring acoustic and vibration.
Especially interferometric sensors are more suitable to measure the acoustic signal. In this paper, a Fabry-Perot interferometric fiber optic sensor was used to measure flow induced vibration. This vibration also measured using an accelerometer, and the data was compared to one other. The venture, nozzle, dro p b arrel, and rap id exp ansio n in the p ip eline are the measuring objects. The flow rate is changed from 50 L/min to 150 L/min and the average flow velocity was about 7 m/s. Based on the experimental results the suggested fiber optic sensor detects flow induced vibration effectively. Therefore, this kind of fiber optic sensor can be applied to the monitoring the flow induced noise and vibration such as pipelines, cables, buildings.
Key words : Flow induced vibration, Fiber optic sensor, Fabry-Perot sensor, Interferometer, Rapid expansion, Venture
* Correspondence : [email protected], Professor of Mechanical Engineering Education, College of Education, Andong National University
Ⅰ. Introduction
1. Purpose of this study
Fiber optic sensor is widely used to measure acoustic and vibration. Especially interferometric sensors are preferred in measuring acoustic signal. In this paper, Fabry-Perot interferometric fiber optic sensor was used to measure the flow of the induced vibration. Flow induced vibration was measured at the venture, nozzle, and rapid expansion system on the pipeline using both the accelerometer and the fiber optic sensor. Just one Fabry-Perot sensor was used(Lee, 2009). Therefore, our concerns are wether this kind of fiber optic sensor can be applied to the monitoring of the flow induced noise and vibration or not. Can this kind of fiber optic sensor be applied to the monitoring of the flow induced noise and vibration such as pipelines, cables, and buildings?
2. Previous study
The ever-increasing maturity of fiber-optic technology and its demonstrated practicality have created a growing demand for its use in both military and commercial(Pan, 1983, Foxwell, 1992, Dandridge and Kersey, 1988) Some of the potential advantages of the fiber optic sensors are high sensitivity, high bandwidth, multiplexing capabilities, reduced weight and size, immunity to electromagnetic interference, and in certain configurations, the ability to sense spatially distributed quantities(Davis, 1983, De Paula, 1985). The flow-induced vibration phenomena has been treated by a variety of engineering disciplines. Especially, Flow-induced vibration on the pipe makes many problems to design the smart structure. The instability induced excitation is brought about by a flow instability(Naudascher and D. Rockwell, 1994, Foxwell, 1992, Jackson and Jones, 1986).
A special class of fiber optic sensors makes use of the Fabry-Perot effect(Dakin
and Culshaw, 1988) Integrated optics can be used in two ways in this sensor. In
the conventional design all the optics needed to feed the light into and detect the
light from the sensor can be integrated, thus avoiding a bulky and very unreliable
arrangement. Force, acceleration, acoustic parameters can be detected using this
sensor(Ansari, 1993, Nash and Keen, 1990, Mcdearmon, 1987)
3. Analytical background
The outer signal detection principle of the Sagnac interferometer is as follows.
Laser light splits two parts. One part rotates clockwise direction, the other part rotates counterclockwise. Both signal combines at the directional coupler. Then photo diode shows acoustic signal.
The radius of the closed loop is R, angular velocity is , tangential speed is v, wavelength is , light speed is c, cross sectional area is A. In this case time difference in between CW and CCW wave is
. (1)
light period, T is expressed as
(2)
and light difference is expressed by
. (3)
We can find phase difference, in the loop as
. (4)
When Sagnac loop has m loops, then phase difference, is expressed as
. (5)
Therefore using the equation (5) it can be calculated phase difference, and this
parameter is proportional to the external acoustic signal.
II. Experimental set-up
[Figure 1] shows the experimental set up of the flow measuring system which consists of venture, nozzle, orifice, and sudden expansion. [Figure 2] shows Fabry-Perot interferometer using optical fiber. In this experiment, 1500 nm in wavelength DFB laser source was used and the output signal was detected by a photo-diode.
[Figure 1] Photograph of the experimental set up which is composed of a venture, nozzle, and sudden expansion
In [Figure 2] an accelerometer can be used instead of a microphone. PCB accelerometer was used to measure the flow induced vibration. RION sound level meter was also used to measure the noise level of the flow drop in the box. This kind of noise affects the flow vibration level. A Fabry-Perot interferometric fiber optic sensor is installed as shown in [Figure 2] and [Figure 3]. Just one Fabry-Perot sensor was used. However, an array type sensor can be used in this experiment. Array sensor detects several different signals simultaneously.
3x3 coupler divides the laser light 33% each and it goes through the sensor and
reflect at the mirror which is installed at the end of the sensor. Out laser light
passes the coupler and the oscilloscope detects the signal in real time. RION SA-76
spectrum analyzer displays the frequency spectrum of the detected signal. This
type of interferometer is widely used in measuring sound and vibrations.
P.D.
3×3
fiber optic sensor
Oscilloscope
Spectrum Analyzer
DFB laser
Amp.
microphone
[Figure 2] Schematic diagram of the experiment using fiber optic Fabry-Perot interferometric sensor
[Figure 3] Photograph of the fiber optic senor interferometer
As shown in [Figure 3], rigid board was used to maintain stable background vibration level and the interferometer was installed on the board. When this experiments expands to the undergraduate level clear arrangement of the interferometer is required. To clear understand the light source path and signal path this board is also required. Experiments are as follows. To measure the flow induced vibration accelerometer attached on the surface of the structure directly.
Sometimes this method has weak point in detecting clear vibration signals.
III. Experimental results and discussions
The flow induced vibration was measured on the drop box, venture, nozzle, and sudden expansion pipe, using both the accelerometer and fiber optic sensor.
[Figure 4] shows the flow induced vibration measurement of the venture system.
Flow charges were changed to 80 L/min, 110 L/min, and 150 L/min each by each.
Fiber optic sensor
Accelerometer
[Figure 4] Photograph of the experimental set up at the venture system
Fiber optic sensor
Accelerometer
[Figure 5] Frequency spectrum of the flow induced vibration at the venture system
in the case of flow charge, 80 L/min
Fiber optic sensor
Accelerometer
[Figure 6] Frequency spectrum of the flow induced vibration at the venture system in the case of flow charge, 110 L/min
From [Figure 5] to [Figure 7], the fiber optic sensor detected vibration at the frequency of 900Hz according to the increasing flow charges from 80 to 150 L/min. When the flow charge increased, the vibration level was increased. At low frequency region, both the fiber optic and accelerometer signals were coincided each other. Even though the accelerometer detected more frequencies, it can not tell that the accelerometer is better than the fiber optic sensor.
Fiber optic sensor Accelerometer
[Figure 7] Frequency spectrum of the flow induced vibration at the venture system
in the case of flow charge, 150 L/min
[Figure 8] shows the experimental set up at the nozzle. Internal flow through a pipe and nozzle decreases the natural frequency of the pipe(Harris, 1976). As shown in [Figure 9], a fiber optic sensor can not detect the vibration signal at the range of 200~1,000 Hz. This phenomena comes from the thickness of the outer surface of the nozzle. Other possibilities are weak vibration level and bad FOS attachment at the surface.
Fiber optic sensor Accelerometer
[Figure 8] Photograph of the experimental set up at the nozzle
Fiber optic sensor Accelerometer
[Figure 9] Vibration frequency spectrum of the fiber optic sensor and accelerometer
at the outer surface of nozzle
Fiber optic sensor Accelerometer
[Figure 10] Photograph of the experimental set up at the rapid expansion system with two stages
[Figure 10] shows the vibration measurement on the rapid expansion system using both the fiber optic sensor and accelerometer. The frequency spectrum of the experimental results are as shown in [Figure 11]. The flow charge was 80 L/min.
In the low frequency region of 50, 90, and 110 Hz, the fiber optic sensor detected those frequencies. However in the 280 Hz region, the accelerometer detected the vibration frequency. As shown in [Figure 10], air bubbles can be seen near the expansion system. This means that the fluid flows unstably at the rapid expansion.
Fiber optic sensor
Accelerometer
[Figure 11] Vibration frequency spectrum of the fiber optic sensor and
accelerometer at the rapid expansion
[Figure 12] shows the background noise level measurement using a RION sound level meter. [Figure 13] shows the frequency spectrum of both signals. The flow charge was 80 L/min and 150 L/min. From [Figure 13] it can be known that a frequency of approximately 280 Hz is vibration signal and other frequencies are sound signals.
Drop barrel
[Figure 12] Photograph of the measurement of noise level at the drop barrel using RION sound level meter
[Figure 13] Vibration and noise frequency spectrum of the microphone and
accelerometer at the drop barrel
flow induced vibration effectively. Therefore, this kind of fiber optic sensor can be applied to monitoring the flow induced noise and vibration such as pipelines, cables, and buildings.
IV. Conclusions
Fiber optic sensor are widely used to measure acoustic and vibration.
Especially interferometric sensors are preferred to measure the acoustic signals. In this paper, a Fabry-Perot interferometric fiber optic sensor was used to measure flow induced vibration. The flow induced vibration was measured at the venture, nozzle, and rapid expansion system on the pipeline using both the accelerometer and the fiber optic sensor. Only one Fabry-Perot sensor was used. From the experiments it can be concluded as follows:
At the venture, the fiber optic sensor detected vibration at the frequency of 900Hz according to the increasing flow charges from 80 to 150 L/min. When the flow charge increased it surely be increased vibration level. At the low frequency region both fiber optic and accelerometer signals are coincided each other.
At the nozzle, internal flow fiber optic sensor can not detect vibration signal at the range of 200~1,000 Hz. This phenomena comes from the thickness of outer surface of the nozzle. Other possibilities are weak vibration level and bad FOS attachment at the surface.
At the rapid expansion system under the flow charge of 80 L/min, in low frequency region of 50, 90, and 110 Hz, fiber optic sensor detected those frequencies. However, 280 Hz region, the accelerometer detected this vibration frequency. Air bubbles can be seen near the expansion system, which means that fluid flows unstably at the rapid expansion.
At the drop barrel under the flow charge of 80 L/min and 150 L/min it can be known that about a frequency of 280 Hz is vibration signal and other frequencies are sound signals. This vibration also was picked up using accelerometer.
Based on the experimental results, the suggested fiber optic sensor detects flow
induced vibration effectively. Therefore, this kind of fiber optic sensor can be
applied to the monitoring the flow induced noise and vibration such as pipelines,
cables, and buildings.
Acknowledgement
This work was supported by 2008 research grant of the Andong National University.
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*◎ 논문접수: 2009. 1. 30 / 1차수정본 접수: 2009. 3. 13 / 게재승인: 2009. 3. 20
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