Department of Automotive Engineering, Tkayama College,
Shimobayashi 1155, Takayama, Gifu 506, Japan
N. Furuichi, J. Mimatsu, M. Kumada
Department of Mechanical Engineering, Gifu University
Yanagido 1—1, Gifu 501-11, Japan
Department of Mechanical Engineering, Keio University
Hiyoshi 3-14-1, Kohoku-ku, Yokohama 223, Japan
Correspondence to: T. Hachiga
The authors are grateful to Prof. M. Maeda for his useful discussions.
We also thank Dr. H. Morikita for his advice on construction of the
Experiments in Fluids 24 (1998) 70 —76 Springer-Verlag 1998
Development of a multi-point LDV by using semiconductor laser with FFT-based
multi-channel signal processing
T. Hachiga, N. Furuichi, J. Mimatsu, K. Hishida, M. Kumada
An advanced multi-point LDV with FFT-based
multi-channel signal processing, using a 1-bit FFT approach
has been developed. A semiconductor laser light sheet,
a 96-channel plastic optical ﬁber array, 16-Si APDs and
a 16-channel capacious (1 M word) memory system with 1 bit
were used in order to attain 16 individual Doppler frequencies
simultaneously. Several experiments were conducted in this
study to test the performance of the advanced multi-point
LDV. We conﬁrmed that the advanced multi-point LDV could
be a useful, compact and inexpensive optical measuring
instrument for obtaining the information of a velocity ﬁeld as
a function of time and position.
Detailed spatial information of ﬂuid motion is required to
analyze the mechanism of ﬂuid ﬂow and heat transfer in
turbulent ﬂows. Up to now, there have been several techniques
for line sensing in a ﬂow ﬁeld such as an ultrasonic velocity
proﬁler (Takeda 1991, 1995), ﬂow visualization techniques
(Sakakibara et al. 1993), multi-point laser Doppler velocimeter
(LDV) (Nakatani et al. 1978, 1984, 1986, 1987), and scanning
laser Doppler techniques (Durst et al. 1989; Tanaka et al. 1993).
However, such instrumentation is still very expensive and the
processes are time consuming.
The conventional LDV can only furnish an instantaneous
velocity at a single point in a ﬂuid. The present work deals with
the development of a velocity proﬁle monitor as a function of
time and position in an unsteady ﬂow by using an advanced
multi-point LDV. In this study, the software Fourier transform
method using a 1-bit quantization of Doppler signals was
utilized in order to increase the ﬂexibility of the signal
processor of the advanced multi-point LDV, which is a com-
pact and inexpensive optical measuring instrument. The use of
1-bit quantization reduces the size of data storage requirement
signiﬁcantly (Ibrahim et al. 1990). Ibrahim et al. (1990) showed
that signals of SNR at 0 dB can be processed within an accuracy
of 0.1% using the FFT based signal processor. This means the
FFT based signal processor usually provides more accurate
information than the conventional counter type processor.
Furthermore, the software processing algorithm was optimized
without changing hardware.
In the present study, the following improvements on an
advanced multi-point LDV, which enables measurements of
the velocity using the multi-point LDV to be in more points
and in a wider range, were made in order to turn a pointwise
measurement into a linewise measurement with the FFT-based
multi-channel signal processing.
(1) A semiconductor laser light sheet was utilized as
a planar light source by replacing the pointwise laser beam.
A plastic ﬁber array, with output ends installed with an optical
plug, and Silicon Avalanche Photodiodes (Si APD) with an
optical receptacle installed, were used in order to detect the
scattered light signal from tracer particles passing through at
different points separately in a linewise measuring region.
(2) The Fourier transform method using 1-bit quantization
of Doppler signals was utilized in order to accomplish mea-
surements under the low SNR condition and to increase the
ﬂexibility of the signal processor of the advanced multi-point
Several experiments were conducted in this study to test the
performance of the advanced multi-point LDV. Measurement
of the laminar velocity proﬁle of water in an entrance region of
pipe ﬂow and the Karman vortex behavior in the wake behind
a cylinder were taken by using an advanced multi-point LDV.
Furthermore, we examined performance of an advanced
multi-point LDV with FFT-based multi-channel signal pro-
cessing using a 1-bit quantization of Doppler signals which
is compact and inexpensive optical measuring instrumentation
for obtaining the information of a velocity ﬁelds as a function
of time and position in an unsteady ﬂow.
The speciﬁcation of the Semiconductor Laser Unit is given in
Table 1. A Semiconductor Laser unit, which had a rod lens of