Wolfram Hage

Fluid Mechanics of Biological Surfaces and their Technological Application

Abstract A survey is given on fluid-dynamic effects caused by the structure and properties of biological surfaces. It is demonstrated that the results of investigations aiming at technological applications can also provide insights into biophysical phenomena. Techniques are described both for reducing wall shear stresses and for controlling boundary-layer separation. (a) Wall shear stress reduction was investigated experimentally for various riblet surfaces including a shark skin replica. The latter consists of 800 plastic model scales with compliant anchoring. Hairy surfaces are also considered, and surfaces in which the no-slip condition is modified. Self-cleaning surfaces such as that of lotus leaves represent an interesting option to avoid fluid-dynamic deterioration by the agglomeration of dirt. An example of technological implementation is discussed for riblets in long-range commercial aircraft. (b) Separation control is also an important issue in biology. After a few brief comments on vortex generators, the mechanism of separation control by bird feathers is described in detail. Self-activated movable flaps (=artificial bird feathers) represent a high-lift system enhancing the maximum lift of airfoils by about 20%. This is achieved without perceivable deleterious effects under cruise conditions. Finally, flight experiments on an aircraft with laminar wing and movable flaps are presented.

Drag Reduction on Gurney Flaps by Three-Dimensional Modifications

Miniflaps at the trailing edge of airfoils, that is, Gurney flaps, change the Kutta condition and thereby produce higher lift. Unfortunately, because of the flow separation downstream of such trailing edges, the drag also increases. Investigations are described with the aim to stabilize the wake flow to achieve drag reduction. When hot-wire anemometry is used, a tonal component in the spectrum of the velocity fluctuations downstream of the Gurney flap is shown. This points to the existence of a von Karman vortex street. Modifications of the Gurney flap can reduce this flow instability, which results in a drag reduction. Trailing-edge modifications, such as slits or holes in Gurney flaps and vortex generators, were tested in experiments. The experiments were carried out using straight wings and a swept wing at a Re = 1 × 10 6 At lower angles of attack of the airfoils with geometrical modifications a drag reduction was observed. This drag reduction was determined through force measurements. The flowfield behind the Gurney flaps was also investigated numerically, using methods based on Reynolds averaged Navier-Stokes and detached eddy simulation

Separation Control by Self-Activated Movable Flaps

Separation control is an important issue in the physiology of birdflight. Here, the adaption of the separation control mechanism by bird feathers to the requirements of engineering applications is described in detail. Self-activated movable flaps similar to artificial bird feathers represent a high-lift system for increasing the maximum lift of airfoils. Their effect on the unsteady flow around a two-dimensional airfoil configuration is investigated by a joint numerical and experimental study. First, attention is paid to the automatic opening and closing mechanism of the flap. Following this, its beneficial effect on lift is investigated for varying incidences and flap configurations. In-depth analysis of experimental and numerical results provides a detailed description of the important phenomena and the effect of self-adjusting flaps on the flow around the airfoil. In the second part of this paper, a contribution is made to verification of the applicability of unsteady Reynolds-averaged approaches using statistical turbulence models for unsteady flows with particular attention to turbulent time scales with comparison to the results of a hybrid simulation based on unsteady Reynolds-averaged Navier-Stokes equations and large-eddy simulation. Finally, flight experiments are described using an aircraft with movable flaps fitted on its laminar wing.

Turbulent Drag Reduction by Oscillating Riblets

A novel approach using laterally oscillating riblets is investigated to reduce the turbulent drag of wall-bounded flows. The new method is intended to combine the eect of the wellknown stationary riblets with the strong eect of lateral wall-oscillations. Experimental investigations in this study show only minor eects for the parameters investigated. DNS results demonstrate that the secondary flow induced by the riblet motion has a strong influence on the amount of drag reduction. The influence is not straightforward since stronger oscillations can lead to a higher drag under certain conditions and to a lower drag under dierent conditions. It is shown that the oscillating riblets are able to induce a similar lateral motion as an oscillating wall.

Exploratory Experiments on Machined Riblets for 2-D Compressor Blades

During the last decades, riblets have shown a potential for viscous drag reduction. Several investigations and measurements of skin-friction in the boundary layer over flat plates and on turbomachinery type blades with ideal riblet geometry have been reported in the literature. The purpose of the present study is to investigate whether laser machined and ground riblet-like structures could be successfully employed on conventional 2-D (NACA) compressor blades in order to assess the potential of industrial machining processes for the creation of the riblet effect. Perfectly trapezoid riblets were designed specifically for the flow parameters in the wind tunnel. Parameters describing the geometry and the deviation from ideal riblets are developed. Riblet machining by high precision material ablation has the potential of achieving micro-machining quality. In comparison to ns-laser processing using either Q-switched solid-state lasers or excimer lasers, the results for high precision material ablation show the enormous potential of ps-laser radiation and achieve the required quality, free of thermally induced defects and, consequently, with high reproducibility. For grinding riblets, geometrically defined microprofiles must firstly be generated via a profile dressing process and then ground onto the work piece surface. A precise adjustment of the grinding wheel system (grit, bonding) and the dressing/grinding conditions is necessary, in order to satisfy the opposing requirements at both dressing and grinding. The blade specimens were geometrically measured with a confocal microscope as well as secondary electron microscope using a specially developed riblet-oriented analysis. For verifying the measurement results, an Atomic Force Microscope was used. The specimens, i.e. flat plates and compressor blades, are aerodynamically tested in a cascade wind tunnel and properly scaled model surfaces were tested in an oil channel in order to quantify skin-friction reduction. Wake measurements of a cascade with NACA-profiles which have the resulting riblet-like structured surface show that the laser shaped as well as ground riblets reduce skin-friction almost as well as the ideal ones, which means a skin friction reduction of up to 7%.Copyright

Influence of Wave-Like Riblets on Turbulent Friction Drag

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.

Wavy riblets for turbulent drag reduction

A combined numerical and experimental study is carried out to investigate the capability of sinusoidal riblet patterns to reduce turbulent drag. In contrast to conventional straight riblets, sinusoidal riblets are able to induce a small amount of lateral motion in the ow. Therefore, they are thought to emulate the eect of lateral wall oscillations and hence to achieve a higher reduction of turbulent drag than straight riblets. This parameter study shows that lateral velocity oscillations are successfully induced. Positive eects of small geometric amplitudes are weak and could not be determined outside the uncertainty range of the measurements or simulation statistics. Larger amplitudes lessen the overall drag reduction which is mainly due to increasing pressure drag.

Experimental Investigation of Oscillating Riblets for Turbulent Drag Reduction

Direct shear stress measurements were carried out on a riblet test plate with rectangular riblet grooves in a fully turbulent oil channel flow. The band-shaped riblets were tilted in spanwise direction by means of an electric drive up to an amplitude of 30° and a frequency of 4 Hz. Those oscillating riblets were intended to combine the drag reducing effect of conventional, stationary riblets with that of a laterally oscillating wall. Shear stress measurements on oscillating riblets with a cross-section optimal for stationary riblets yielded improvements in the drag-reducing capability of order of the measurement accuracy. For elongated riblets, a measurable increase in the drag reduction was obtained at certain values of the dimensionless oscillation period. Those results are in qualitative congruence to results from DNS simulations previously published.However, the maximum drag reduction of stationary riblets could not be increased by oscillating riblets. By 3D-PIV measurements, the successful generation of an additional spanwise velocity component in the near-wall region was proven.

Artificial shark skin on its way to technical application

The present paper gives an overview of the riblet research with the main emphasis on the european research in this field, which was motivated by the observation of the structure of the scales of fast swimming sharks. The drag reducing effect of this structure, which consists of fine streamwise aligned ribs, was shown experimentally and theoretically. Furthermore, an explanation how riblets do reduce the turbulent skin friction is given. The results of a parameter optimization of the riblet geometry is shown. Several aspects of the riblet film application on an long range aircraft are considered. The main focus lays here on the effects of an angle of yaw on the riblet performance.

Investigation of the wear properties of a riblet paint structure on an Airbus A300-600ST Beluga

While the application of drag reducing riblets on aircraft in the past was done by an adhesive film, a new promising technique was developed by the Fraunhofer Institute IFAM. The riblets are imprinted very precisely into the paint on the aircraft. To investigate the durability of such a riblet structure, free flight tests on an Airbus A300-600ST Beluga were performed. According to a specified time schedule, samples from the aircraft were taken. This riblet samples were prepared to measure the aerodynamic properties in an appropriate test facility, the DLR oil channel. The preparation of the test samples consists of several steps like moulding, casting, three dimensional surface measurements and a rapid prototyping technique. The sample preparation chain is evaluated by means of a common numerical method. Results of the experimental investigation show the influence of the wear during the flight tests on the riblet surface.