Bio: Charles Henoch is an academic researcher from Naval Undersea Warfare Center. The author has contributed to research in topics: Leading edge & Drag. The author has an hindex of 10, co-authored 16 publications receiving 785 citations.
TL;DR: In this paper, the authors measured lift, drag, and pitching moments of airfoils with leading-edge sinusoidal protuberances in a water tunnel and compared with those of a baseline 63 4 -021 airfoil.
Abstract: Lift, drag, and pitching moments of airfoils with leading-edge sinusoidal protuberances were measured in a water tunnel and compared with those of a baseline 63 4 -021 airfoil. The amplitude of the leading-edge protuberances ranged from 2.5 to 12% of the mean chord length; the spanwise wavelengths were 25 and 50% of the mean chord length. These ranges correspond to the morphology found on the leading edge of humpback whales' flippers. Flow visualization using tufts was also performed to examine the separation characteristics of the airfoils. For angles of attack less than the baseline stall angle, lift reduction and drag increase were observed for the modified foils. Above this angle, lift of the modified foils was up to 50% greater than the baseline foil with little or no drag penalty. The amplitude of the protuberances had a distinct effect on the performance of the airfoils, whereas the wavelength had little. Flow visualization indicated separated flow originating primarily from the troughs and attached flow on the peaks of the protuberances at angles beyond the stall angle of the baseline foil.
••05 Jun 2006
TL;DR: In this article, large-area superhydrophobic test surfaces have been fabricated and tested in a water tunnel, measuring drag in both the laminar and transitional regimes at velocities up to 1.4 m/s.
Abstract: Superhydrophobic surfaces are known to exhibit reduced viscous drag due to "slip" associated with a layer of air trapped at the liquid-solid interface. It is expected that this slip will lead to reduced turbulent skin-friction drag in external flows at higher Reynolds numbers in both the laminar and turbulent regimes. Results are presented from experiments exploring this effect. Large-area Superhydrophobic test surfaces have been fabricated and tested in a water tunnel, measuring drag in both the laminar and transitional regimes at velocities up to 1.4 m/s. Drag reduction of approximately 50% is observed for laminar flow. Lower levels of drag reduction are observed at higher speeds after the flow has transitioned to turbulence.
TL;DR: In this article, the authors used spanwise aligned rows of permanent magnets interlaced with surface-mounted electrodes, segmented to allow the Lorentz force to be propagated in the spanwise direction.
Abstract: Results concerning the design and fabrication of electromagnetic actuators, and their application to affect the wall shear stress in a fully turbulent channel flow are discussed. The actuators utilize a Lorentz force to induce fluid motion due to the interaction between a magnetic field and a current density. The actuators are comprised of spanwise-aligned rows of permanent magnets interlaced with surface-mounted electrodes, segmented to allow the Lorentz force to be propagated in the spanwise direction. Problems commonly associated with electromagnetic flow control—electrolysis, bubble formation, and electrode corrosion are substantially reduced, and in most cases eliminated by the use of a conductive polymer coating. The actuators generate velocity profiles with a penetration depth into the flow of approximately 1 mm (set by the electrode/magnet pitch) and maximum velocities of approximately 4 cm/s. The actuation velocities are found to scale linearly with forcing voltage and frequency. The electrical to mechanical efficiency is found to be very low (≈10−4), primarily due to the limitations on the magnetic field strength and the low conductivity of the working fluid (saltwater). The actuators are used in a fully turbulent low Reynolds number channel flow and their effect on the turbulent skin friction is measured using a direct measurement of drag. Maximum drag reductions of approximately 10% are measured when the flow is forced using a spanwise oscillating Lorentz force. A scaling argument for the optimal amplitude of the current density is developed. The efficiency of this method for drag reduction, and its application at higher Reynolds numbers is also discussed.
TL;DR: In this article, a series of water-tunnel experiments were conducted to determine the effect of sinusoidal leading-edge protuberances on the aerodynamic characteristics of finite span wings.
Abstract: A series of water-tunnel experiments were conducted to determine the effect of sinusoidal leading-edge protuberances on the aerodynamic characteristics of finite span wings. The models consisted of seven rectangular planform wings, two swept-leading-edge wings, and two wings with a planform resembling humpback-whale flippers. All models had an underlying NACA 634-021 profile with protuberance amplitudes of 0.025–0.12 times the chord length. The models were examined at Reynolds numbers up to 4.5×105 and angles of attack up to 30 deg. The lift and drag coefficients were nearly independent of Reynolds numbers above 3.6×105. Specific rectangular-planform models had appreciably greater lift coefficients over a limited angle-of-attack range when compared to the baseline model. However, with the exception of the planform that resembled the humpback-whale flipper, the lift-to-drag ratio of all leading-edge modified models was comparable to or less than the equivalent baseline model. The flipper model had a slight...
TL;DR: In this paper, the authors describe how to make droplets stick to their substrates (even if they are inclined), which is a practical issue in many cases (windshields, window panes, greenhouses, or microfluidic devices).
Abstract: While the behaviour of large amounts of liquid is dictated by gravity, surface forces become dominant at small scales. They have for example the remarkable ability to make droplets stick to their substrates (even if they are inclined), which is a practical issue in many cases (windshields, window panes, greenhouses, or microfluidic devices). Here we describe how this problem can be overcome with super-hydrophobic materials. These materials are often developed thanks to micro-textures, which decorate a solid surface, and we describe the way such textures modify the wettability of that solid. We conclude by showing the unusual dynamics of drops in a super-hydrophobic situation.
TL;DR: A review of the use of the combination of surface roughness and hydrophobicity for engineering large slip at the fluid-solid interface is given in this paper, with an eye toward implementing these surfaces in a wide range of applications.
Abstract: This review discusses the use of the combination of surface roughness and hydrophobicity for engineering large slip at the fluid-solid interface. These superhydrophobic surfaces were initially inspired by the unique water-repellent properties of the lotus leaf and can be employed to produce drag reduction in both laminar and turbulent flows, enhance mixing in laminar flows, and amplify diffusion-osmotic flows. We review the current state of experiments, simulations, and theory of flow past superhydrophobic surfaces. In addition, the designs and limitations of these surfaces are discussed, with an eye toward implementing these surfaces in a wide range of applications.
TL;DR: In this paper, the authors demonstrate that periodic, micropatterned superhydrophobic surfaces, previously noted for their ability to provide laminar flowdrag reduction, are capable of reducing drag in the turbulent flow regime.
Abstract: In this paper, we demonstrate that periodic, micropatterned superhydrophobic surfaces, previously noted for their ability to provide laminar flowdrag reduction, are capable of reducing drag in the turbulent flow regime. Superhydrophobic surfaces contain micro- or nanoscale hydrophobic features which can support a shear-free air-water interface between peaks in the surface topology. Particle image velocimetry and pressure drop measurements were used to observe significant slip velocities, shear stress, and pressure drop reductions corresponding to drag reductions approaching 50%. At a given Reynolds number,drag reduction is found to increase with increasing feature size and spacing, as in laminar flows. No observable drag reduction was noted in the laminar regime, consistent with previous experimental results for the channel geometry considered. The onset of drag reduction occurs at a critical Reynolds number where the viscous sublayer thickness approaches the scale of the superhydrophobic microfeatures and performance is seen to increase with further reduction in viscous sublayer height. These results indicate superhydrophobic surfaces may provide a significant drag reducing mechanism for marine vessels.
TL;DR: A review of the recent trend of plasma actuator design and to summarise aerodynamic control techniques can be found in this article, where the starting vortex that leads to formation of a plasma wall jet is discussed.
Abstract: Flow control using DBD (dielectric-barrier-discharge) plasma actuators is a relatively new, but rapidly expanding area of research. There are a number of review papers available on this subject, but few discuss on their latest developments. The purpose of the present article is to “fill the gap” by reviewing the recent trend of plasma actuator design and to summarise aerodynamic control techniques. Here, we review new plasma actuators, such as plasma synthetic jet actuators, plasma spark jet actuators, three-dimensional plasma actuators and plasma vortex generators, which can induce three-dimensional flows away from the wall. We also review the starting vortex that leads to formation of a plasma wall jet. This is an important subject not only for a better understanding of the flow induced by DBD plasma actuators, but also as a database that can be used to calibrate the numerical models for plasma flow control. Design of DBD plasma actuators to obtain turbulent skin-friction reduction is shown and the modifications to near-wall turbulence structures are summarised. Novel applications of DBD plasma actuators for aerodynamic control are then discussed, including pitch and roll control, plasma jet vectoring, circulation control and plasma flap, showing a potential of DBD plasma actuators for replacing movable, aircraft control surfaces. Finally, vortex shedding control techniques by a number of different plasma actuators are surveyed.
TL;DR: The research on superhydrophobic self-cleaning biological surfaces and the development of similar engineered materials suggests that biomimicry is a matter of multi-stage processes rather than a simple copying of biological developments.
Abstract: The Lotus has been the symbol of purity for thousands of years; contaminations and pathogens are washed off the surfaces of Lotus and some other plants with rain or even dew. After the introduction of scanning electron microscopy, we were able to resolve the mechanism behind this phenomenon. It took some further decades before in-depth studies on self-cleaning with plants were conducted and the effect could be understood in detail. We identified extreme water-repellency ('superhydrophobicity'), characterized by very high contact angles and low sliding angles, as the prerequisite for self-cleaning properties. We could show that the combination of two factors is necessary for obtaining a high degree of water-repellency: (1) low energy surfaces being hydrophobic and (2) surface structures that significantly increase hydrophobicity. It is suggested that this mechanism plays an important role in the protection of plants against pathogens. Our technological application of this effect has resulted in the development of successful, eco-friendly and sustainable industrial products. Another interesting property was found with superhydrophobic surfaces of certain aquatic and semi-aquatic plants and animals: here a layer of air under water is retained. We present a new approach of using this feature for creating structured, air-retaining surfaces for technical underwater applications. It is proposed that such surfaces can reduce significantly the drag of large ships. We conclude that basic biological research is of particular importance for true innovation. Our research on superhydrophobic self-cleaning biological surfaces and the development of similar engineered materials suggests that biomimicry is a matter of multi-stage processes rather than a simple copying of biological developments.