Dynamic stall development

Dynamic stall on an oscillating airfoil was investigated by a combination of surface pressure measurements and time-resolved particle image velocimetry. Following up on previous work on the onset of dynamic stall (Mulleners and Raffel in Exp Fluids 52(3):779-793, 2012), we combined time-resolved imaging with an extensive coherent structure analysis to study various aspects of stall development. The formation of the primary dynamic stall vortex was identified as the growth of a recirculation region and the ensuing instability of the associated shear layer. The stall development can be subdivided into two stages of primary and secondary instability with the latter being the effective vortex formation stage. The characteristic time scales associated with the primary instability stage revealed an overall decrease in dynamic stall delay with increasing effective unsteadiness of the pitching airfoil. The vortex formation stage was found to be largely unaffected by variations of the airfoil’s dynamics

Flying Qualities of Otto Lilienthal's Large Biplane

The paper describes full-scale balance tests, which have proven structural integrity and longitudinal static stability of an authentic replica of Otto Lilienthal's Large Biplane glider. In addition to that, the replica, built after patent drawings, photographs, and textual descriptions by the Otto-Lilienthal-Museum, was towed using a rope winch and finally flown downhill without rope or guide wires. The tests allow for the assessment of the aircraft's controllability and stability not only during steady flight, but also during takeoff, landing, and on the occasion of wind gusts. A brief description of the design parameters and applied methods is given along with force and moment data gained from balance measurements as well as the evaluation of a limited set of data acquired during flight, including speed, duration, and distances flown

Review of measurement techniques for unsteady helicopter rotor flows

The helicopter group at the DLR in Göttingen has been actively involved in the development of measurement techniques for unsteady flows, particularly as they apply to the problems found in unsteady rotor blade aerodynamics. This includes the development and validation of new techniques for the detection of dynamically moving boundary layer transition, and for the detection of dynamic stall and other transient flow separation events. These new techniques include pressure sensor analysis, differential infrared thermography, local infrared thermography and the automated analysis of hot-film data. Particle image velocimetry (PIV) and background oriented schlieren (BOS) have been used for the analysis of the unsteady off-body flow, and synchronised PIVBOS-pressure measurements have allowed direct comparisons between different methods. The Lagrangian volumetric PIV variant, shake-the-box, has been used to analyse secondary vortex structures in the vortex wake. This review article will give an overview of the advances in that group, as well as placing their activities in the context of international advances in these areas

Simulation of powder flow behavior in an artificial feed frame using an Euler‐Euler model

The Eulerian approach is an alternative numerical method to the traditionally used discreet particle techniques for modeling powder flow, avoiding limitations on particle number and diameter. The feasibility of an Euler-Euler simulation in a pharmaceutical application was investigated. In two- and three-dimensional flow simulations, computational fluid dynamics models and parameters were determined and verified based on comparison with experiments. Residence time distributions were calculated to show the applicability of the Eulerian model with two granular phases under the constraint of a continuous setup. Finally, this model was implemented to improve the process understanding of the powder flow in an artificial feed frame of a rotary tablet press

Analytical and experimental investigations of dual‐plane particle image velocimetry

In its classical form particle image velocimetry (PIV) extracts two components of the flow velocity vector by measuring the displacement of tracer particles within a double‐pulsed laser light sheet. The method described in this paper is based on the additional recording of a third exposure of the tracer particles in a parallel light sheet, which is slightly displaced with respect to the first one. The particle images resulting from these three exposures are stored on separate frames. The locations of the correlation peaks, as obtained by cross‐correlation methods, are used to determine the projections of the velocity vectors onto the plane between the two light sheets. The amplitudes of these peaks are used to obtain information about the velocity component perpendicular to the light‐sheet planes. The mathematical background of this method is described. Numerical simulations show the influence of the main parameters (e.g., light‐sheet thickness, light‐sheet displacement and out‐of‐plane flow component) on the resolution and reliability of the new method. A new recording procedure and its results will be shown to demonstrate the ease of operation when applying this technique to liquid flows

Infrared thermography for boundary layer transition measurements

Infrared thermography has been successfully adopted in the field of flow diagnostics over the last decades. Detecting the laminar{turbulent boundary layer transition through variations in the convective heat transfer is one of the primary applications due to its impact on the aerodynamic performance. Recent developments in fast{ response infrared cameras allow unsteady measurement of fast{moving surfaces and moving transition positions, which must consider the thermal responsiveness of the surface material. Experimental results on moving boundary layer transition positions are highly valuable in the design and optimization of airfoils or rotor blades in unsteady applications, for example regarding helicopter main rotors in forward flight. This review article summarizes recent developments in steady and unsteady infrared thermography, particularly focusing on the development of differential infrared thermography (DIT). The new methods have also led to advances in the analysis of unmoving boundary layer transition for static airfoil test cases which were previously difficult to analyze using single{image methods

Flight testing Otto Lilienthal's experimental monoplane

Free flights and tethered flights were performed using an authentic replica of Otto Lilienthal's experimental monoplane, the "Vorflügelapparat", achieving flight distances of more than 100m. The flight stability, controllability and performance of the design has been verified through balance measurements using a 1:5 scaled wind tunnel test

Über die Flugeigenschaften von Otto Lilienthals großem Doppeldecker

Der Artikel beschreibt aerodynamische Messungen, die die strukturelle und die statische Längsstabilität eines authentischen Nachbaus von Otto Lilienthals großem Doppeldecker nachweisen. Darüber hinaus wurde der nach Patentzeichnungen, Fotos und Textbeschreibungen vom Otto-Lilienthal-Museum gebaute Nachbau des historischen Gleiters mit einer Seilwinde geschleppt und schließlich, ganz ohne Seile, bergab geflogen. Die Tests ermöglichen die Beurteilung der Steuerbarkeit und Stabilität des Flugzeugs nicht nur im stationären Flug, sondern auch bei Start, Landung und bei Windböen. Eine kurze Beschreibung der Konstruktionsparameter und der angewandten Methoden wird ebenso beschrieben wie die Auswertungen Kraft- und Momentdaten, die aus Waagenmessungen gewonnen wurden. Dargestellt wird darüber hinaus die Auswertung eines begrenzten Satzes von Daten, die während des Fluges gewonnen wurden, einschließlich Geschwindigkeit, Dauer und geflogene Distanzen