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passive and active flow control by swimming fishes and mammals

passive and active flow control by swimming fishes and mammals

Annu. Rev. Fluid. Mech.

Annu. Rev. Fluid. Mech. 2006.38:193-224. Downloaded from arjournals.annualreviews.orgby WEST CHESTER UNIVERSITY on 12/20/05. For personal use only.1. INTRODUCTIONAquatic animals have evolved a diversity of propulsive mechanisms to locomote effectivelythrough water. These mechanisms are a result of a long evolutionary historyduring which selection has acted on propulsive systems and generated an array ofnovel anatomical and physiological responses to the problem of moving though water(Fish 2000, Lauder & Drucker 2004). Aquatic animals (e.g., fishes, whales, seals,penguins) produce hydrodynamic thrust by acceleration of water through movementof their body and appendages, while simultaneously reducing the resistance to theirmotion through morphological design, phased kinematics, and behaviors.Interest in animal hydrodynamics is related largely to flow control. Animals havebeen hypothesized to control flow both passively and actively, effecting enhancedperformance in drag reduction, thrust production, and propulsive efficiency. Passivecontrol is related to the anatomy of the animal, with morphology and structuralfeatures dictating flow over the animal’s surface. Controlling flow regimes withinthe boundary layer has been at the center of long-standing arguments related to thepossibility that animals use clever drag reduction mechanisms (Anderson et al. 2001,Gray 1968). In active control, the movements of the animal, under active muscularcontrol, modify flow directly. Vorticity shed from the body or fins as structuredvortices or shear layers is utilized to vector hydrodynamic forces during propulsion,maneuvering, and trim control.This paper is not a comprehensive review of all aspects of aquatic locomotionin fishes and mammals, a topic far too diverse to cover in a meaningful mannerhere; a number of recent works provide overviews of much relevant biological andhydrodynamic literature (Bandyopadhyay 2004; Fish 2004; Lauder 2005; Lauder &Drucker 2004; Triantafyllou et al. 2000, 2004; Webb 1998; Wilga & Lauder 2004a).This discussion focuses on mechanisms of biological flow control by aquatic fishesand mammals, a topic with both a long history and considerable new data that reflectgrowing interest in understanding how organisms control fluid flow around theirbodies and appendages.2. PASSIVE FLOW CONTROL2.1. Gray’s ParadoxOne of the most hotly debated ideas within biofluiddynamics has been the mechanismof viscous dampening for passive flow control to reduce drag in dolphins. Thecontroversy known as “Gray’s Paradox” resulted from the first attempt to evaluateswimming energetics through a simple hydrodynamic model (Fish & Hui 1991, Gray1936, Webb 1975). A rigid-body hydrodynamic model was used to calculate dragpower of a dolphin and a porpoise swimming at high speeds (>7.6 m/s; Reynoldsnumber >10 6 ). The results indicated that the estimated drag power could not bereconciled with the available power generated by the muscles (Gray 1936).Gray’s (1936) calculations assumed that turbulent flow conditions existed in theboundary layer. His resolution to the problem was that the fully laminar boundary194 Fish·Lauder

Annu. Rev. Fluid. Mech. 2006.38:193-224. Downloaded from arjournals.annualreviews.orgby WEST CHESTER UNIVERSITY on 12/20/05. For personal use only.layer was maintained despite accepted hydrodynamic theory. The assertion that dolphinscould maintain a laminar boundary layer remained and became the focusand justification of much of the work on dolphin hydrodynamics for the next 60years (Fish & Hui 1991, Fish & Rohr 1999, Lang & Daybell 1963, Romanenko1995).The basic premise of Gray’s Paradox, however, was flawed because of potentialerrors in estimation of dolphin swimming speed and inconsistencies between dolphinswimming performance and data on muscle power outputs. Gray used an observationof a dolphin swimming along the side of the ship from stern to bow in 7 seconds. Ifthe dolphin was swimming close enough to utilize the flow pattern around the ship,its speed may have been artificially enhanced and energetic effort reduced due tothe pressure distribution close to the ship (Lang 1966, Weihs 2004, Williams et al.1992). In addition, the duration of this high-performance swimming was for a sprintand Gray used measurements for muscle power output of sustained performance (3–5minutes) by human oarsmen (Henderson & Haggard 1925). Muscle fibers specializedfor quick bursts can produce power outputs 2–17 times greater than muscle fibersused for sustained activities (Askew & Marsh 1997).Dolphins have the muscular capacity to swim at high speeds for short durationswhile maintaining a fully attached turbulent boundary layer. The turbulent flow conditionswould delay separation of the boundary layer (Figure 1; Rohr et al. 1998).When the boundary layer separates from the skin surface and interacts with outerflow, this results in a broader wake and increased drag, so delaying separation is beneficialto the dolphin. Separation is more likely to occur with a laminar boundary flow,producing a greater drag penalty compared to turbulent boundary conditions. Thus,the turbulent boundary layer remains attached longer because it has more energythan the laminar boundary layer. The increased drag of a turbulent boundary layerFigure 1Bioluminescence image of gliding dolphin. Courtesy of J. Rohr.www.annualreviews.org • Fish and Mammal Locomotion 195

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