Laurens E. Howle

Leading-edge tubercles delay stall on humpback whale (Megaptera novaeangliae) flippers

The humpback whale (Megaptera novaeangliae) is exceptional among the baleen whales in its ability to undertake acrobatic underwater maneuvers to catch prey. In order to execute these banking and turning maneuvers, humpback whales utilize extremely mobile flippers. The humpback whale flipper is unique because of the presence of large protuberances or tubercles located on the leading edge which gives this surface a scalloped appearance. We show, through wind tunnel measurements, that the addition of leading-edge tubercles to a scale model of an idealized humpback whale flipper delays the stall angle by approximately 40%, while increasing lift and decreasing drag.

Experimental Evaluation of Sinusoidal Leading Edges

P REVIOUS studies on increasing airfoil lift and improving stall characteristics have addressed various passive and active approaches to modifying the leading and trailing edge shapes. The passive approaches have covered such methods as rippling the trailing edge, applying serrated-edge Gurney flaps, or modifying the leading-edge (LE) profile [1,2]. Other efforts have effectively eliminated the dynamic stall of an NACA 0012 airfoil by perturbing the LE contour as little as 0.5–0.9%of the chord [3]. Levshin et al. [4] demonstrated that sinusoidal LE planforms on an NACA 63-021 airfoil section decreased maximum lift, but extended the stall angle by almost 9 deg. The larger amplitude sinusoids created “softer” stall characteristics by maintaining attached flow at the peaks despite separated flow in the troughs. These tests were performed to simulate the effects of LE tubercles on humpback whale (Megaptera novaeangliae) flippers. Prior work by the authors also reported wind tunnel measurements for idealized scale models of humpback whale flippers [5]. One model had a smooth leading edge and a secondmodel had sinusoidal bumps (tubercles) along the leading edge for the outer 2 3 of the span. It was found that the addition of tubercles to a 3-D idealized flipper increased the maximum lift coefficient while reducing the drag coefficient over a portion of the operational envelope. It is thought that the tubercles on the flipper leading-edge enhance the whale’s ability to maneuver to catch prey [6]. Though the work to date regarding sinusoidal or serrated leading-edge planforms is largely motivated by marine mammal locomotion, the effects of extending the stall point for lifting surfaces at similar Reynolds numbers (Re) may have application to small-UAV (unmanned aerial vehicle) design and the inevitable laminar stall problems [7]. However other relevant applications might benefit from the effects of simulated tubercles such as stall alleviation/separation control on sailboat centerboards or wind turbines, where an expanded operating envelope could improve the overall effectiveness of the blade [8,9]. In the present work, a better understanding is sought of the mechanism of the improvements measured in previous experiments, with a greater applicability in mind. The authors seek to determine whether the performance improvements resulted from enhancements to the sectional characteristics of wings with tubercles (i.e., essentially 2-D effects), or from Reynolds number effects on a tapered planform, or from other 3-D effects such as spanwise stall progression.

The Tubercles on Humpback Whales' Flippers: Application of Bio-Inspired Technology

The humpback whale (Megaptera novaeangliae) is exceptional among the large baleen whales in its ability to undertake aquabatic maneuvers to catch prey. Humpback whales utilize extremely mobile, wing-like flippers for banking and turning. Large rounded tubercles along the leading edge of the flipper are morphological structures that are unique in nature. The tubercles on the leading edge act as passive-flow control devices that improve performance and maneuverability of the flipper. Experimental analysis of finite wing models has demonstrated that the presence of tubercles produces a delay in the angle of attack until stall, thereby increasing maximum lift and decreasing drag. Possible fluid-dynamic mechanisms for improved performance include delay of stall through generation of a vortex and modification of the boundary layer, and increase in effective span by reduction of both spanwise flow and strength of the tip vortex. The tubercles provide a bio-inspired design that has commercial viability for wing-like structures. Control of passive flow has the advantages of eliminating complex, costly, high-maintenance, and heavy control mechanisms, while improving performance for lifting bodies in air and water. The tubercles on the leading edge can be applied to the design of watercraft, aircraft, ventilation fans, and windmills.

Hydrodynamic flow control in marine mammals

The ability to control the flow of water around the body dictates the performance of marine mammals in the aquatic environment. Morphological specializations of marine mammals afford mechanisms for passive flow control. Aside from the design of the body, which minimizes drag, the morphology of the appendages provides hydrodynamic advantages with respect to drag, lift, thrust, and stall. The flukes of cetaceans and sirenians and flippers of pinnipeds possess geometries with flexibility, which enhance thrust production for high efficiency swimming. The pectoral flippers provide hydrodynamic lift for maneuvering. The design of the flippers is constrained by performance associated with stall. Delay of stall can be accomplished passively by modification of the flipper leading edge. Such a design is exhibited by the leading edge tubercles on the flippers of humpback whales (Megaptera novaeangliae). These novel morphological structures induce a spanwise flow field of separated vortices alternating with regions of accelerated flow. The coupled flow regions maintain areas of attached flow and delay stall to high angles of attack. The delay of stall permits enhanced turning performance with respect to both agility and maneuverability. The morphological features of marine mammals for flow control can be utilized in the biomimetic design of engineered structures for increased power production and increased efficiency.

Active control of Rayleigh–Bénard convection

We report on stabilizing the unstable no-motion state in a moderate aspect ratio one-dimensional Rayleigh–Benard convection experiment. A linear proportional control algorithm uses shadowgraphic convection images to determine heat flux perturbations which are applied to the lower boundary by a network of local heaters. We show that simple linear control stabilizes the otherwise unstable no-motion (conduction) state over a substantial range of supercritical Rayleigh numbers.

Spring stiffness influence on an oscillating propulsor

Abstract We study the propulsive dynamics of a thin foil pitching about its quarter chord and allowed to passively plunge. Specifically, we focus on the effect of variations in translational spring stiffness on propulsor plunge and on the minimum oscillation frequency required to produce positive thrust. Our numerical simulation utilizes a two-dimensional hydroelasticity model of the propulsor–fluid system in a constant velocity free stream. The pitch is forced at the quarter chord by a drive shaft and the dynamics of the fluid–structure interaction coupled to the strength of a translational spring determines the plunge amplitude. We use an unsteady two-dimensional vortex lattice method to model the hydrodynamics of the propulsor producing thrust in a potential flow field. The phase relationship between the driving angle and the plunge displacement is discussed, along with the effects of changing spring stiffness on thrust and efficiency. We show that passive plunge reduces the critical frequency for positive thrust production. This allows simple one-actuator input to compete with more complicated two-actuator systems.

Control of Rayleigh-Bénard convection in a small aspect ratio container

Active control of Rayleigh-Benard convection in small aspect ratio horizontal layer, by perturbation of the lower thermal boundary condition, is shown. These experiments use a novel shadowgraphic system to measure the departure of a high Prandtl number fluid from the no-motion state. A proportional control scheme uses the shadowgraphic information as feedback to determine the lower boundary heat flux distribution. An array of individually controlled heaters imposes the heating distribution. Significant suppression of the convection amplitude is possible to at least 10 times the critical Rayleigh number. This convection control method provides an economical alternative to magnetic convection suppression.

Linear stability analysis of controlled Rayleigh-Bénard convection using shadowgraphic measurement

We conduct a linear stability analysis of Rayleigh-Benard convection in an infinite horizontal layer with active control of the lower boundary heat flux. A simple linear proportional control loop uses a shadowgraph of the convection pattern to actively distribute the constant-mean lower boundary heat flux while the upper boundary is kept at a constant temperature. We find it possible to shift the convection threshold by a factor of approximately 3. This is a companion paper to our recent experimental work.

Lift and drag performance of odontocete cetacean flippers

SUMMARY Cetaceans (whales, dolphins and porpoises) have evolved flippers that aid in effective locomotion through their aquatic environments. Differing evolutionary pressures upon cetaceans, including hunting and feeding requirements, and other factors such as animal mass and size have resulted in flippers that are unique among each species. Cetacean flippers may be viewed as being analogous to modern engineered hydrofoils, which have hydrodynamic properties such as lift coefficient, drag coefficient and associated efficiency. Field observations and the collection of biological samples have resulted in flipper geometry being known for most cetacean species. However, the hydrodynamic properties of cetacean flippers have not been rigorously examined and thus their performance properties are unknown. By conducting water tunnel testing using scale models of cetacean flippers derived via computed tomography (CT) scans, as well as computational fluid dynamic (CFD) simulations, we present a baseline work to describe the hydrodynamic properties of several cetacean flippers. We found that flippers of similar planform shape had similar hydrodynamic performance properties. Furthermore, one group of flippers of planform shape similar to modern swept wings was found to have lift coefficients that increased with angle of attack nonlinearly, which was caused by the onset of vortex-dominated lift. Drag coefficient versus angle of attack curves were found to be less dependent on planform shape. Our work represents a step towards the understanding of the association between performance, ecology, morphology and fluid mechanics based on the three-dimensional geometry of cetacean flippers.

Central Venous Catheter Integrity during Mechanical Power Injection of Iodinated Contrast Medium

PURPOSE To evaluate a widely used nontunneled triple-lumen central venous catheter in order to determine whether the largest of the three lumina (16 gauge) can tolerate high flow rates, such as those required for computed tomographic angiography. MATERIALS AND METHODS Forty-two catheters were tested in vitro, including 10 new and 32 used catheters (median indwelling time, 5 days). Injection pressures were continuously monitored at the site of the 16-gauge central venous catheter hub. Catheters were injected with 300 and 370 mg of iodine per milliliter of iopamidol by using a mechanical injector at increasing flow rates until the catheter failed. The infusion rate, hub pressure, and location were documented for each failure event. The catheter pressures generated during hand injection by five operators were also analyzed. Mean flow rates and pressures at failure were compared by means of two-tailed Student t test, with differences considered significant at P < .05. RESULTS Injections of iopamidol with 370 mg of iodine per milliliter generate more pressure than injections of iopamidol with 300 mg of iodine per milliliter at the same injection rate. All catheters failed in the tubing external to the patient. The lowest flow rate at which catheter failure occurred was 9 mL/sec. The lowest hub pressure at failure was 262 pounds per square inch gauge (psig) for new and 213 psig for used catheters. Hand injection of iopamidol with 300 mg of iodine per milliliter generated peak hub pressures ranging from 35 to 72 psig, corresponding to flow rates ranging from 2.5 to 5.0 mL/sec. CONCLUSION Indwelling use has an effect on catheter material property, but even for used catheters there is a substantial safety margin for power injection with the particular triple-lumen central venous catheter tested in this study, as the manufacturers recommendation for maximum pressure is 15 psig.