Martin Brede

The Salvinia Paradox: Superhydrophobic Surfaces with Hydrophilic Pins for Air Retention Under Water

[*] Prof. W. Barthlott, S. Wiersch, Dr. H. F. Bohn Nees-Institut für Biodiversität der Pflanzen Rheinische Friedrich-Wilhelms-Universität Meckenheimer Allee 170, 53115 Bonn (Germany) E-mail: barthlott@uni-bonn.de Prof. Th. Schimmel, Dr. M. Barczewski, Dr. S. Walheim, A. Weis, A. Kaltenmaier Institute of Applied Physics and Center for Functional Nanostructures (CFN) University of Karlsruhe Karlsruhe Institute of Technology (KIT) 76131 Karlsruhe (Germany) Institute of Nanotechnology and Center for Functional Nanostructures (CFN) Forschungszentrum Karlsruhe Karlsruhe Institute of Technology (KIT) 76021 Karlsruhe (Germany) E-mail: thomas.schimmel@physik.uni-karlsruhe.de Prof. K. Koch Biologie und Nanobiotechnologie Hochschule Rhein-Waal Landwehr 4, 47533 Kleve (Germany)

ON SECONDARY VORTICES IN THE CYLINDER WAKE

The wake of a circular cylinder is investigated for Reynolds numbers between 160 and 500 by means of particle image velocimetry (PIV). For the first time cross‐stream velocity fields are determined for two classes of secondary vortices (A‐mode and B‐mode). The circulation of the A‐mode secondary vortices in this plane is approximately twice the circulation of the B‐mode secondary vortices. The spanwise wavelength of the secondary vortices is four to five cylinder diameters for the A‐mode and one diameter for the B‐mode. The spatio‐temporal development of the wake is analyzed by acquiring a time sequence of PIV images covering several Karman periods. On the basis of the vorticity field, the A‐ and B‐modes can be identified as topologically different vortex structures. Two vortex models are developed to explain the differences between these modes.

Harbor seal vibrissa morphology suppresses vortex-induced vibrations.

SUMMARY Harbor seals (Phoca vitulina) often live in dark and turbid waters, where their mystacial vibrissae, or whiskers, play an important role in orientation. Besides detecting and discriminating objects by direct touch, harbor seals use their whiskers to analyze water movements, for example those generated by prey fish or by conspecifics. Even the weak water movements left behind by objects that have passed by earlier can be sensed and followed accurately (hydrodynamic trail following). While scanning the water for these hydrodynamic signals at a swimming speed in the order of meters per second, the seal keeps its long and flexible whiskers in an abducted position, largely perpendicular to the swimming direction. Remarkably, the whiskers of harbor seals possess a specialized undulated surface structure, the function of which was, up to now, unknown. Here, we show that this structure effectively changes the vortex street behind the whiskers and reduces the vibrations that would otherwise be induced by the shedding of vortices from the whiskers (vortex-induced vibrations). Using force measurements, flow measurements and numerical simulations, we find that the dynamic forces on harbor seal whiskers are, by at least an order of magnitude, lower than those on sea lion (Zalophus californianus) whiskers, which do not share the undulated structure. The results are discussed in the light of pinniped sensory biology and potential biomimetic applications.

Flow sensing by pinniped whiskers.

Beside their haptic function, vibrissae of harbour seals (Phocidae) and California sea lions (Otariidae) both represent highly sensitive hydrodynamic receptor systems, although their vibrissal hair shafts differ considerably in structure. To quantify the sensory performance of both hair types, isolated single whiskers were used to measure vortex shedding frequencies produced in the wake of a cylinder immersed in a rotational flow tank. These measurements revealed that both whisker types were able to detect the vortex shedding frequency but differed considerably with respect to the signal-to-noise ratio (SNR). While the signal detected by sea lion whiskers was substantially corrupted by noise, harbour seal whiskers showed a higher SNR with largely reduced noise. However, further analysis revealed that in sea lion whiskers, each noise signal contained a dominant frequency suggested to function as a characteristic carrier signal. While in harbour seal whiskers the unique surface structure explains its high sensitivity, this more or less steady fundamental frequency might represent the mechanism underlying hydrodynamic reception in the fast swimming sea lion by being modulated in response to hydrodynamic stimuli impinging on the hair.

Superhydrophobic surfaces of the water bug Notonecta glauca: a model for friction reduction and air retention

Summary Superhydrophobic surfaces of plants and animals are of great interest for biomimetic applications. Whereas the self-cleaning properties of superhydrophobic surfaces have been extensively investigated, their ability to retain an air film while submerged under water has not, in the past, received much attention. Nevertheless, air retaining surfaces are of great economic and ecological interest because an air film can reduce friction of solid bodies sliding through the water. This opens perspectives for biomimetic applications such as low friction fluid transport or friction reduction on ship hulls. For such applications the durability of the air film is most important. While the air film on most superhydrophobic surfaces usually lasts no longer than a few days, a few semi-aquatic plants and insects are able to hold an air film over a longer time period. Currently, we found high air film persistence under hydrostatic conditions for the elytra of the backswimmer Notonecta glauca which we therefore have chosen for further investigations. In this study, we compare the micro- and nanostructure of selected body parts (sternites, upper side of elytra, underside of elytra) in reference to their air retaining properties. Our investigations demonstrate outstanding air film persistence of the upper side of the elytra of Notonecta glauca under hydrostatic and hydrodynamic conditions. This hierarchically structured surface was able to hold a complete air film under hydrostatic conditions for longer than 130 days while on other body parts with simple structures the air film showed gaps (underside of elytra) or even vanished completely after a few days (sternites). Moreover, the upper side of the elytra was able to keep an air film up to flow velocities of 5 m/s. Obviously the complex surface structure with tiny dense microtrichia and two types of larger specially shaped setae is relevant for this outstanding ability. Besides high air film persistence, the observation of a considerable fluid velocity directly at the air–water interface indicates the ability to reduce friction significantly. The combination of these two abilities makes these hierarchically structured surfaces extremely interesting as a biomimetic model for low friction fluid transport or drag reduction on ship hulls.

On the Wake Flow Dynamics behind Harbor Seal Vibrissae – A Fluid Mechanical Explanation for an Extraordinary Capability

While hunting for prey in dark and turbid water the harbor seals use their mystacial vibrissae to follow the hydrodynamic trails left by prey fish. Sensing the minute velocity fluctuations in the trail is a challenge. In our research study we will answer the questions how mean and oscillating drag and lift forces are affected by the special body shape of the vibrissa and how the vortex structure in the wake is formed by a vibrissa to suppress self induced vibrations from the wake. For this purpose the wake flow of a harbor seal vibrissa was investigated by Stereo-Micro-PIV and with a detailed 3D direct numerical simulation. Using the proper orthogonal decomposition the most energetic structures of the wake flow could be extracted and evaluated.

Numerical Simulation of the Flow Around a Finite Cylinder with Ground Plate in Comparison to Experimental Measurements

Simulations and experiments were performed to capture the spatio-temporal flow field around a finite circular cylinder mounted on a ground plate. In order to provide a combined database and testcase for future simulations and experiments, the flow is investigated using state-of-the-art techniques with a high resolution in time and space, namely Large-Eddy Simulation and Detached-Eddy Simulation for the numerics and time-resolved PIV as well as LDA for the measurements. The predicted time-averaged and unsteady flow field from simulations corroborate well the experiments, giving new insights into the complex turbulent separated flow behind a quite simple geometry.

Discrete shedding modes of the cylinder wake in a jet with a homogeneous core

The von Karman vortex street behind a circular cylinder in a laminar homogeneous jet core is experimentally investigated. The Strouhal–Reynolds number relationships are measured for various shear‐layer thicknesses and aspect ratios. The experimental Strouhal number values are found to collapse with the discrete vortex shedding modes, which were observed for boundary‐layer end conditions. The results indicate that the shedding modes are independent from the end conditions, but are an intrinsic feature of the shedding process of an infinitely long cylinder. The experimentally assumed shedding mode, however, is strongly dependent of the geometric parameters, like the shear‐layer thickness and the aspect ratio. The observed tendencies can be made physically plausible.

Hydrodynamic Perception in Pinnipeds

The vibrissal system of pinnipeds such as harbor seals (Phoca vitulina) or California sea lions (Zalophus californianus) serves not only for the detection and identification of objects by direct touch, but also detect and analyze water movements (hydrodynamic stimuli). These two species represent two different types of vibrissae, one with an undulated outline (harbor seal) and one with a smooth outline (sea lion). In our recent set of studies, we analyzed the hydrodynamic stimuli generated by stationary fish and by escaping fish, and tested the ability of pinnipeds to analyze artificial hydrodynamic stimuli that share certain features with natural hydrodynamic stimuli. Biomechanical studies of isolated vibrissae in a flow tank show different signal-to noise ratios for the two species that are consistent with their different performance in behavioral experiments, and can be explained by fluid-structure interactions.

Simulating the flow and trail following capabilities of harbour seal vibrissae with the Lattice Boltzmann Method

Abstract Harbour seals ( Phoca vitulina ) are able to follow their prey, even without visible or audible contact. It has been shown that the whiskers, or vibrissae, of harbour seals possess an undulated surface structure, which suppresses vortex-induced vibrations from their own wake and thus minimises the forces that act upon them when they are dragged through the water. The following work uses the Lattice Boltzmann Method, together with off-lattice boundaries and 3D grid refinement, to analyse the flow and trail following capabilities of harbour seal vibrissae from a numerical point of view.