© 1998 OPA (Overseas Publishers Association) Amsterdam B.V. Published under license under the Gordon and Breach Science Publishers imprint. Printed in India.
Mar. Fresh. Behav. Physiol., Vol. 31, pp. 55-61 Reprints available directly from the publisher Photocopying permitted by license only
Short Communication VISUALIZATION AND QUANTIFICATION OF BIOLOGICAL FLOW FIELDS THROUGH VIDEO-BASED DIGITAL MOTION-ANALYSIS TECHNIQUES* THOMAS BREITHAUPTa,* and JOSEPH AYERSb a Fakultät für Biologie, Universität Konstanz, Postfach 5560 (M618), D-78457 Konstanz, FRG; b Marine Science Center, Northeastern University, East Point, Nahant, MA 01908, USA
(Received 28 February 1997; In final form 23 April 1997) Keywords: Flow visualization; digital image processing; particle tracking velocimetry; exopodite currents; Procambarus clarkii; Crustacea
The study of the water currents surrounding aquatic animals and the behavior associated with these currents yields a great deal of insight into crucial aspects of the life of aquatic animals. Many behaviors, e.g. locomotion, breathing, filter feeding, odor communication, chemo- and rheotactic orientation, can only be understood with a knowledge about the water currents surrounding the animal. Various techniques have been used so far by biologists to visualize flow pattern in water (for references see Breithaupt and Ayers, 1996). Only recently, new quantitative techniques, Particle Tracking Velocimetry (PTV) and Particle Image Velocimetry (PIV), using digital image processing were introduced into the field of biology (Stamhuis and Videler, 1995). These techniques are based on the analysis of the displacement of *This paper is accompanied by animation on the MFBP web site: http://www.gbhap.com/journals/142/142-top.htm. * Corresponding author. Fax: 49-7531-883018. E-mail:
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tracer particles within a thin illuminated plane in the water. Commercial versions of PIV (available e.g. through TSI Inc., St. Paul, MN 55164, USA) employ pulsed laser sources for illumination and costly computer hardand software for analysis. Such visualization equipment does not fit the budget of most biologists who want to study animal behavior with respect to fluid flow. In addition, the equipment is difficult to handle and relatively rigid in its applications. The high accuracy and resolution offered by these systems is not always needed to answer questions of biological relevance. This paper describes an implementation of PTV that uses simple and low cost off-the-shelf components for both visualization and quantitative analysis of the flow. Light reflecting particles are used as markers for flow visualization. Nearly neutrally buoyant particles are used with specific weights between 1.01 and 1.03 g/cm3. Several particle types have been tested. ABS particles (p= 1.03 g/cm3: BAYER, Leverkusen, Germany) are neutrally buoyant in sea water. Pliolite particles (p = 1.02 g/cm3, e.g. Vestosint™, HULS AG, Germany) gave best results in fresh water. Particles are accelerated by the drag force of the flow, which arises due to the velocity difference between the particle and the fluid. At high accelerations the particle may lag behind the velocity of the fluid. The particle slip velocity \uP — « F | is a function of the applied acceleration uF, the diameter rfP and specific weight pP of the particle and its drag coefficient CD; it can be estimated using (Adrian, 1991; Vogel, 1994): , , 2 pp dp i , ., «F with 1«p - «F = Wr — -~ ' V3 p F CD' '
„ 24 6 CD= — + = + 0.4, Re 1 + VR^
MP = velocity of the particle, MF = velocity of the fluid, d= diameter of the particle, p P = specific weight of particles, p F = specific weight of the fluid. C D is a function of the Reynolds number; Re = (|wP-HF|
