Nature-Inspired Fluid Mechanics

The African trypanosome is a protozoan which causes sleeping sickness in mammals. To study the dynamics of this microorganismat low Reynolds number, we implement and investigate three swimmer models, the Taylor sheet, a constanttorque swimmer, and a model for the African trypanosome. The first two swimmers are based on a semi-flexible sheet and the third is a full three-dimensional model. We simulate the viscous fluid environment of the swimmers using a technique called multi-particle collision dynamics. We verify our technique by implementing the Taylor sheet which is activated by a bending wave traveling along the sheet. Its ballistic motion turns into diffusive motion when the sheet becomes passive. For the constant-torque swimmer we apply a torque to the semi-flexible sheet which assumes a cork-screw shape and then generates the thrust force for propelling the swimmer forward.Whereas the angular velocity scales linearly with the torque, the swimming velocity displays a non-linear dependence. Finally, our trypanosome model swims with the help of a beating flagellum attached to the cell body. Since it wraps around the body, the model trypanosome displays a helical swimming trajectory. The swimming velocity displays a non-linear increase with the beating frequency of the flagellum.

The locomotion of microorganisms in a microscopic world, where cells move through a fluid environment without using inertial forces, is a fascinating phenomenon in life science. Nature offers clever and inspiring strategies for self-propelling in an environment of no inertia. The flagellate African trypanosome, which causes African sleeping sickness, moves with help of a flagellum, which is firmly attached to its cell body. The beating flagellum leads to a strong distortion of the cell body and therefore to a swimming agitation of trypanosomes. We have found that trypanosomes use a hydrodynamic mechanism to defend against host’s immune attacks. Owing to continuous and directional swimming, host-derived antibodies attached to surface glycoproteins of the cell are dragged to the posterior cell pole, where they are rapidly internalized and destroyed. In the following we present new methodology and techniques to quantify the movements of proteins and the motility of cells. Moreover trypanosome motility schemes and their influence on cellular lifestyle and survival strategies are characterized.

Understanding the dynamics of force and energy control in flying insects requires the exploration of how oscillating wings interact with the surrounding fluid. In two-winged insects, such as flies, the fluid acceleration fields produced by each wing strongly interact during wing stroke reversals, when the wings reverse their flapping direction. The main finding of this study is that this wing-wake interaction potentially budgets the elevated energy expenditures required for wing flapping by actively lowering the kinetic energy in the wake. This is demonstrated by quantitative flow measurements in flying fruit flies using high-speed particle image velocimetry and measurements in robotic, model wings. Vorticity estimates suggest that, compared to rigid robotic wings, elastic fly wings recycle energy from detached leading edge vortices by a novel mechanism termed vortex trapping. This finding is of great interest in the field of biomimetic aircraft design because it may help to improve the endurance of the next generation of man-made wing-flapping aerial devices such as micro air vehicles.

This study aimed to numerically simulate aerodynamic forces produced by wing motion of small fruit flies maneuvering freely inside a flight chamber. The kinematic data were derived from high-resolution, high-speed video measurements, tracking fluorescent markers on head, body and wings of the animal. We constructed a geometrical model of the fly and applied the kinematic data to simulate free flight. Based on the calculated velocity and pressure fields, we evaluated vorticity and flight forces. Our numerical simulation confirmed experimentally predicted lift enhancing mechanisms such as the leading edge vortex, rotational circulation and wake capture, and thus appears to be a potent tool to study the impact of body motion on forces and moments during the various forms of flight maneuvers.

Owls (Strigiformes) are nocturnal birds of prey that are known for their silent flight. For a long time, the underlying mechanisms were not well understood. In a comprehensive study, we have characterized the flight apparatus of one representative of owls, the barn owl (Tyto alba pratincola), to advance beyond the phenomenological description provided so far. The barn owl wing is adapted to slow flight as indicated by a low wing loading, an elliptical shape, a high camber and a specific thickness distribution. Further, feather specializations can be found: 1.) serrations at the leading edge of the wing, 2.) a velvety dorsal surface texture, and 3.) fringes at the inner vanes of remiges. Quantitative characterizations of these structures revealed that serrations had a uniform shape, but the length depended on their position on the wing. The velvety dorsal surface texture differed between the inner and outer vanes which is a consequence of different functions (air flow control, friction reduction). The fringes were observed to merge into neighboring feather vanes by gliding into grooves at the lower wing surface to create a smooth airfoil. Besides anatomical data, material properties and wearing effects of feather keratin of rachises and barbs were obtained.

A technical three-dimensional wing model based on the geometry of the wing of a barn owl was designed to investigate the aerodynamic characteristics of this wing, which is known to be perfectly adapted to the requirements of silent flight. This wing model possesses the basic geometry of the barn owl wing. To understand the impact of the owl-based shape plus the owl-specific anatomic elements on the flow field and to further analyze the characteristic flow field that enables the owl to fly at low speeds and thus silently, a prepared natural owl wing was investigated in a wind tunnel. Measurements using particle-image velocimetry were performed on the model and on the natural wing to investigate characteristic flow phenomena such as separation, transition, and reattachment. Additionally, changes of the geometry, i.e., the maximum chord line-to-upper surface distance normalized by the chord length and the deflection of the natural owl wing, are described and discussed in detail to understand the resulting fluid-structure interaction. Unlike the rigid model, the natural owl wing possesses a high flexibility leading to a mutual influence of the wing structure and the surrounding flow field. This has to be investigated to understand the complex physical mechanisms that allow the highly efficient flight of the owl.

This paper describes a new approach for wing shape determination of free flying birds during flapping flight. The optical based measurement method called "image pattern correlation technique" (IPCT) is described as well as modifications of this technique in order to measure wing surfaces formed by feathers. Furthermore a newly developed camera driving system is introduced, which enables a movement of the surface measurement system synchronized with a bird. The application of this system to free flying barn owls is described together with high resolution surface results, obtained during free flapping flight of the bird.

This report is devoted to examining vortex formation and interactions as they occur with isolated and tandem pitching and plunging airfoils. Motivation for understanding such flow phenomena is inspired from dragonflies, which exhibit a wide range of acrobatic flight maneuvers, possibly interesting for the realization of flapping drones known as Micro Air Vehicles (MAVs). Experimental investigations are described in which the kinematics of the forefoil and hindfoil could be independently varied, attaining any combination of pitch and plunge movements up to reduced frequencies of approximately k=0.3 in forward flight. Furthermore, the Reynolds number in these studies was varied between Re=3000 (hover) and Re=30000 (forward flight). Particle Image Velocimetry (PIV) was used to extract information on leading-edge vortex (LEV) and trailing-edge vortex (TEV) circulation, which in turn was useful in examining the growth rate and saturation often referred to as formation number. The following results will be presented: the effect of airfoil kinematics on LEV and TEV development; the influence of hindfoil interaction on the vortex formation and lift generated by the forefoil; and a quantification of the vortex interaction found in the hover condition. Finally, some thoughts regarding a new model of vortex growth will be discussed.

Trout commonly experience unsteady flows such as those caused by a stationary object exposed to running water. Instead of avoiding these flows, trout often use flow fluctuations for station holding. The behaviors associated with station holding are entraining, Kármán gaiting and bow wake swimming. We investigated the swimming behavior of trout in the vicinity of a stationary or moving 2-D shaped cylinder. To uncover the sensory modalities used for station holding, we studied the behavior of intact trout and of trout whose lateral line system was partially or totally impaired in the light or under infrared illumination. We also studied the activity of the axial red swimming muscles of entraining, Kármán gaiting and bow wake swimming trout and the neuronal processing of vortex information in the hindbrain of fish. Further studies showed that small motions of the caudal and/or pectoral fins are necessary to stay in preferred areas irrespective of the unsteadiness imposed by the wake of an object. Computational Fluid Dynamics simulations were carried out to uncover the forces that allow trout station holding with a minimum of energy expenditure.

Transition of an airfoil’s boundary layer from the laminar to the turbulent flow regime increases the overall drag of an airplane significantly. The major origin of this transition are Tollmien-Schlichting (TS) waves. Similar to the dolphin’s skin, a system that is capable to dampen TS waves locally is proposed here. A surface wave can interfere destructively with TS waves and thus delay transition towards the edge of the airfoil. For this purpose, an actuator array is combined with a thin membrane. The presented actuators were developed and improved continuously so that all requirements for the dampening of TS waves are fulfilled. Several actuators are cascaded in a compact manner and combined with a membrane and an array of sensors. The system has proven in wind tunnel experiments to be capable of dampening TS waves successfully and delaying transition.

Control strategies for laminar flow control above a surface are investigated. A flexible membrane displaced by multiple piezo-polymer composite elements is used as actuator in wind-tunnel experiments. Direct methods of damping Tollmien-Schlichting waves are compared to a biomimetic approach imitating the dampingmechanisms of a compliant skin. In both cases, model predictive control algorithms are applied to control the multi-bar actuator segments. For the biomimetic approach, reduced models of compliant surfaces are developed and parametrized by direct optimization and according to numerically generated optimal wall properties. Damping results of up to 85% RMS value are achieved.

Results of linear stability calculations and direct numerical simulations for flow-control experiments are presented. Good agreements between measurements and simulations are shown. Furthermore, the linear stability of the flow over the experimental wing section is investigated. Hereby, also the use of isotropic and anisotropic compliant materials is assessed. The prevailing surface-based compliantwall model of Carpenter was extended to yaw angles, pressure gradients and oblique-traveling disturbances. The influence of the yaw angle is demonstrated for an anisotropy angle of 75°. Also transient-growth of instabilities over the compliant wall was investigated, since the eigenvalue spectrum of the compliant-wall problem turned out to be sensitive to truncation errors. For the parameters investigated, the maximum transient growth of the compliant-wall case is in the same order as the growth of the rigid-wall case.

The dolphin skin close to the anisotropic compliant wall design could potentially reduce the friction drag. The goal of this work was to study the relation between local flow conditions around dolphin model and parameters of skin morphology relevant in flow/skin interface. Three-dimensional CAD models presenting authentic geometry of fast-swimming common dolphin Delphinus delphis and low-swimming harbor porpoise Phocaena phocaena were constructed. CFD study of the flow parameters were carried out for the natural range of dolphin swimming velocities. The results of this study allow to conclude that the stream-wise variability of the dolphin skin structure appears to be associated with the streamlined body geometry and corresponding gradients of the velocity and pressure rather than with specific local Re numbers. The hypotheses on different optimal conditions for potential drag-reducing properties of dolphin skin are proposed.

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.

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.

Underwater undulatory swimming describes one of the fastest modes of human aquatic locomotion. The human swimmer can be considered as natural paradigm for technical segmented linkage systems used in robotics that must compensate its anatomical limitations through sophisticated kinetics. In order to reveal and evaluate such mechanisms the flow around and behind the swimmer was measured by tim-resolved particle image velocimetry (TR-2D-PIV) and simulated by computational fluid dynamics (CFD). In comparison to fish, despite of joint asymmetries the swimmers used undulatory waves characterized by very similar absolute amplitude distributions along the body but at much higher Strouhal numbers. The observed 3D-patterns revealed in the CFD helps us to newly interpret experimental findings. Both the experimental flow field as well as that obtained from CFD document the effect of flow preformation and vortex re-capturing. We propose that the use of high Strouhal numbers facilitates the re-capture of vortices unavoidable due the disadvantageous geometry of the human swimmer.

This article reports on a numerical and experimental study of the turbulent drag on riblet surfaces, where the trapezoidal riblet grooves were formed in a wave-like sinusoidal or zigzag pattern. The aim was to enhance the drag-reducing capabilities of conventional, straight riblet grooves by an additional contribution that originates from the induced oscillating lateral flow component. By means of a comprehensive parameter study in an oil channel at Re between 10,000 and 30,000 and DNS simulations at Re _τ =180, suitable waveform parameters are sought, with which wave-like riblets produce a drag reduction larger than that of their straight counterparts. For a riblet cross-section shape that is known to be optimal for straight grooves, no such beneficial drag modification could be demonstrated. With a riblet groove cross-section different from the optimum shape, an augmented attainable drag reduction in comparison to straight riblet grooves was found within a certain range of the waveform amplitude. The improvement amounts up to 1.3%-points in terms of drag reduction. Wave-like riblets with reduced riblet height never outperformed the drag reduction of straight riblet grooves of optimal cross-section form, but exhibit a similar drag reduction in the best cases investigated. It is shown that this favourable influence on the riblet-modified turbulent drag persists under a mild misalignment of the riblets to the main flow direction.

The flapping flight mechanism is expected to provide revolutionary operation capabilities for tomorrow’s Micro Air Vehicles (MAV). The unsteady aerodynamics of the flapping flight is vastly different from traditional fixed-wing flyers. Boundary layers with moving laminar-turbulent transition, three-dimensional wake vortices and fluid-structure interaction with anisotropic wing structure are only a few examples for the challenging problems. To get basic understanding of these effects, the authors develop a computational method that is validated with boundary-layer measurements on flexible and inflexible, flapping wings in a wind-tunnel. The computational method solves the unsteady Reynolds-averaged Navier-Stokes equations and is combined with both transition prediction and fluid structure interaction capability. Using generic airfoils shapes inspired by seagulls and hawks, different aerodynamic, structural and kinematic effects are systematically analyzed on their influence on thrust and propulsive efficiency of the flapping flight mechanism. In particular, we demonstrate that a slight forward-gliding motion during the flapping downstroke can increase significantly thrust and efficiency.Wing elasticity however seems to lower the propulsive efficiency in the investigated cruise flight flapping case. Beyond,we show that the wake structure of 3D flapping wings generates an efficiency loss of about 10% compared to equivalent two-dimensional flapping cases.

Inspired by the results from acoustic measurements on flying owls and prepared owl wings that support the thesis of the silent flight of owls, measurements were conducted on technical airfoils made from open-porous, flow-permeable materials, characterized by their air flow resistivity, in an aeroacoustic wind tunnel. One major objective of these experiments is the identification of porous materials that enable a reduction of aeroacoustic noise. Both the generation of trailing edge noise and the generation of leading edge noise were investigated using microphone array technology and three-dimensional deconvolution beamforming algorithms. The highest trailing edge noise reduction per unit lift force can be achieved by using airfoils with medium to high air flow resistivities, while the highest leading edge noise reduction was measured for airfoils with low air flow resistivities.