Mark M. Murray

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.

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.

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.

Artificial muscle actuators for a robotic fish

Biology is a source of inspiration for many functional aspects of engineered systems. Fish can provide guidance for the design of animal-like robots, which have soft elastic bodies that are a continuum of actuator, sensor, and information processor. Fish respond to minute pressure changes in water, generating thrust and gaining lift from obstacles in the current, altering the shape of body and fins and using sensory nerves in their muscles to control them. Dielectric Elastomer (DE) artificial muscles offer a mechanism for a fish muscle actuator. DE devices have already been shown to outperform natural muscle in terms of active stress, strain, and speed[1-3]. DEs also have multi-functional capabilities that include actuation, sensing, logic and even energy harvesting, all achievable through appropriate control of charge[4, 5]. But DE actuators must be designed so that they provide enough torque to drive the tail and develop useful forward thrust.

Effect of Humpback Whale Inspired Tubercles on Marine Tidal Turbine Blades

The addition of protuberances, inspired by the humpback whale flipper, on the leading edge of lift producing foils has been shown to improve hydrodynamic performance under a certain range of flow conditions. Specifically, finite wing models have displayed delayed stall characteristics at higher angles of attack and increased maximum lift coefficients without significant hydrodynamic penalties. The objective of this project was to investigate the impact that leading edge protuberances (i.e. tubercles) have on the effectiveness of marine tidal turbine blades, especially at lower tidal flow speeds. The experimental results obtained utilizing three different blade designs (baseline and two tubercle modified) are compared. All blades were designed with a 3-D computer aided design software package and manufactured utilizing rapid prototype techniques. The tests were conducted in the 120 ft tow tank at the U.S. Naval Academy using an experimental apparatus that measured flow speed and electrical power generated. Results for power coefficients are presented for a range of tip speed ratios. Cut-in velocity was also used to evaluated the blade designs. For all test criteria, the tubercle modified blades outperformed the smooth leading edge baseline design blades at the lower test velocities, and did not show degraded performance at the higher velocities tested.Copyright

Comparison of real and idealized cetacean flippers.

When a phenomenon in nature is mimicked for practical applications, it is often done so in an idealized fashion, such as representing the shape found in nature with convenient, piece-wise smooth mathematical functions. The aim of idealization is to capture the advantageous features of the natural phenomenon without having to exactly replicate it, and it is often assumed that the idealization process does in fact capture the relevant geometry. We explored the consequences of the idealization process by creating exact scale models of cetacean flippers using CT scans, creating corresponding idealized versions and then determining the hydrodynamic characteristics of the models via water tunnel testing. We found that the majority of the idealized models did not exhibit fluid dynamic properties that were drastically different from those of the real models, although multiple consequences resulting from the idealization process were evident. Drag performance was significantly improved by idealization. Overall, idealization is an excellent way to capture the relevant effects of a phenomenon found in nature, which spares the researcher from having to painstakingly create exact models, although we have found that there are situations where idealization may have unintended consequences such as one model that exhibited a decrease in lift performance.

Demonstration of Heat Transfer Enhancement Using Ferromagnetic Particle Laden Fluid and Switched Magnetic Fields

A convective heat transfer enhancement technique and the experimental methods used to quantify the improvement in heat transfer and subsequent differential pressure are introduced. The enhancement technique employed time varying magnetic fields produced in a pipe to cause the ferromagnetic particles of a particle laden fluid (mineral oil and iron filings) to be attracted to and released from a heated pipe wall. The ferromagnetic particles acted not only to advect heat from the pipe wall into the bulk fluid but they also significantly modified the flow field, disrupted the boundary layer, allowed cooler fluid to reach the high temperature pipe wall, increased thermal energy transfer directly to the fluid, and contributed to the overall improvement in heat transfer rate. The experimental method utilized to quantify an increased effectiveness of convective heat transfer used an apparatus designed to replicate an internally cooled fin, whose surface temperature was measured with an IR camera. These temperature measurements were utilized to calculate the convective heat transfer coefficient (h) of the fluid within the pipe. The enhancement technique demonstrated a 267% increase in heat transfer coefficient with only a corresponding 48% increase in flow differential pressure for an electromagnetic switching frequency of 2 Hz. It is also found that there were optimum magnetic field switching frequencies for both enhancement and differential pressure magnitudes.