A novel laboratory apparatus for simulating isotropic oceanic turbulence at low Reynolds number

A new experimental apparatus to simulate oceanic turbulence at low Reynolds number in the laboratory is described. Actuators located at each corner of a cubic Plexiglas box generated synthetic jets that interacted to create turbulent flow at the center of the saltwater apparatus. Four turbulent intensity levels were established, and velocity measurements were performed using digital particle image velocimetry. The flow characteristics in the center of the apparatus were confirmed to be nearly isotropic and homogeneous. The range of the dissipation rate (10–3 to 1 cm2s–3) and the Kolmogorov microscale (0.03 to 0.2 cm) agreed well with natural oceanic environments in the coastal zone, and the length scales and magnitude of the velocity and strain rate fluctuations were appropriate for zooplankton studies. The flow characteristics compared favorably with previous approaches for generating turbulence in the laboratory, in particular, oscillating-grid apparatus. The current design also possessed increased simplicity, controllability, and transportability. Future objectives include studying the behavioral response of zooplankton to turbulent fluid motions. Acknowledgments The authors thank Mike Sorensen and Andy Udell for constructing the apparatus, L. Prasad Dasi for helping to analyze the data, Wontae Hwang and Prof. John Eaton for providing information about their apparatus, and Dr. David Fields for helpful comments. Thanks also to the National Science Foundation for financial support under the direction of Dr. Alexandra Isern (OCE-0219519). Limnol. Oceanogr.: Methods 2, 2004, 1–12 © 2004, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS Fourth, the grid itself or the lever mechanism often limits physical and optical access to the tank. The oscillating-grid turbulence studies listed above addressed physical oceanography or fundamental fluid mechanics. Oscillating-grid apparatuses have also been used to address biological oceanography issues (e.g., Alldredge et al. 1990; Hill et al. 1992; Saiz and Alcaraz 1992; Saiz and Kiørboe 1995; MacKenzie and Kiørboe 1995; Landry et al. 1995). In this context, the oscillating grid apparatus has the additional disadvantage of placing an oscillating solid object near the creatures, which may be a hazard. Physical and optical access is also an important consideration for these studies because animals need to be taken in and out of the facility, and observations need to be made from several perspectives. Alternative approaches for providing kinetic energy for the turbulent flow include mixing blades (e.g., Semenov 1965), electric fans (e.g., Birouk et al. 1996; Fallon and Rogers 2002), oscillating baffled cylinders (Chamorro 2001), two oscillating grids (Srdic et al. 1996), rotating cylindrical grids located in a strain field (Liu et al. 1999), and a towed grid (Dickey and Mellor 1980). These approaches often suffer from a lack of homogeneity (i.e., the statistical properties of the velocity fluctuations are a function of distance to the blades). Birouk et al. (1996), however, overcame this problem by placing one fan in each corner of a cubic chamber. The flow induced by the eight fans was nearly isotropic and homogeneous at the chamber center. Although data were not presented to support the assertion, Chamorro (2001) also claimed that the oscillating baffled cylinder produced homogeneous and isotropic turbulence. The towed grid approach was distinctly limited to a mode of operation with decaying turbulence in the wake of the grid. In any case, the fans, blades, baffles, or grids create a physical hazard for creatures in the flow. Recently, Hwang and Eaton (2004) designed and constructed a unique apparatus to study solid particle movement in zerogravity, gas-phase, isotropic turbulence. The apparatus consisted of a cubic box with actuators located at each corner. The actuators directed a synthetic jet through a small (1.9 cm) hole, an ejector tube, and a fixed mesh. Thus, a turbulent zero-net-massflux synthetic jet flow was directed from each corner toward the box center. The actuators were a superior driving mechanism for the particle-laden flow because particle contact with an oscillating grid would be an unquantifiable phenomenon during the experiments. The apparatus also needed to be portable to be placed on research aircraft simulating zero-gravity. The purpose of this study is to test a new laboratory apparatus that has been designed to simulate approximately isotropic and homogeneous oceanic turbulence. Turbulence in the ocean occurs over a wide range of scales from large-scale global circulation (order of kilometers) to diffusive microscales (order of less than millimeters). At the scale of a zooplankton, i.e., the order of millimeters, the character of oceanic turbulence is generally isotropic, which means the turbulent fluctuations are, on average, independent of direction (e.g., Jiménez 1997; Yamazaki et al. 2002). To produce turbulent motion with these characteristics, the Hwang and Eaton (2004) approach was adapted to a seawater environment. A set of operating parameters was established to produce turbulent fields with dissipation rates and microscales in the range for the coastal zone and wind-driven turbulent oceanic waters inhabited by plankton. The objective of this paper is to describe the apparatus and report the measured velocity and turbulence characteristics. Materials and procedures Description of the apparatus—The turbulence chamber consisted of a 0.4 m × 0.4 m × 0.4 m Plexiglas box with each corner replaced with an inclined surface to create a nearly spherical internal volume (Fig. 1). The size of the apparatus was large compared to zooplankton, but still relatively compact in the laboratory. Flow was generated in the chamber by synthetic jet actuators located at each of the eight corners (Fig. 2). The tank was filled with 30 ppt saltwater, and air bubbles were removed from the turbulence chamber and each actuator. The specified salinity level is consistent with regions of the ocean inhabited by zooplankton. All components of the apparatus were either plastic or 316 stainless steel to prevent corrosion in the saltwater environment. Optical access was equally available through all sidewalls (Fig. 2). The net-zero-mass-flux actuators induced a synthetic jet toward the center of the turbulence chamber (Glezer and Amitay 2002). The actuator body was constructed with 5.08 cm PVC pipe (Fig. 2). A rubber (0.16-cm thick soft neoprene) diaphragm sandwiched between PVC flanges formed a movable endwall of the actuator chamber. The diaphragm was connected via a stainless steel rod to a woofer speaker (16.5 cm diameter cone) that was mounted at the end of the actuator. The oscillating motion of the diaphragm alternatively drew fluid into and forced fluid out of orifice holes in the turbulence chamber wall. Eight holes (0.32 cm diameter) were equally spaced around a 2.54 cm diameter circle centered on Webster et al. Isotropic oceanic turbulence apparatus 2 Fig. 1. Schematic of the experimental setup