Three-Dimensional Flow Field Investigations of Flapping Wing Aerodynamics

Three-dimensional unsteady flow fields of a flapping, low-aspect-ratio wing have been investigated by means of highly resolved tomographic particle image velocimetry (Tomo-PIV) measurements and computational fluid dynamics (CFD). Furthermore, force measurements have been carried out. Tomo-PIV was applied to the flow above a flat plate wing during the downstroke. High spatial resolution and large volume thickness could be achieved by using sensitive sCMOS cameras and a traversing setup. The CFD calculations covered the complete period of motion. The analysis of the vortex-dominated flow fields provides a deeper understanding of vortex interaction and three-dimensionality of low Reynolds number (Re = 18;000 and Re = 36;000) flows. Two different Strouhal numbers (St = 0.06 and St = 0.13) are considered and their effects on the development of a leading edge and tip vortex are discussed. The PIV results show instantaneous flow fields after a leading edge separation that are dominated by small-scale turbulent vortex structures. The presented CFD approach is able to predict these vortices by using highly resolved meshes. Coarser grids compare well with the phase-averaged experimental flow fields, which feature multiple large-scale leading edge vortices developing during the downstroke. Turbulent effects decrease for the lower Reynolds number. Force and moment hystereses as well as large-scale leading edge vortice circulation, calculated from the PIV results, increase with increasing Strouhal number. Vortex breakdown of the wing tip vortex can be observed during the downstroke in the experimental data

3D Flow Field Investigations on Flapping Wing Aerodynamics

Three-dimensional, unsteady flow fields of a flapping low aspect ratio wing are investigated by means of tomographic PIV (Tomo-PIV) measurements and computational fluid dynamics (CFD). Furthermore force measurements have been done. Tomo-PIV was applied to the flow above the flat-plate-wing during the down-stroke. A high spatial resolution and a large volume thickness could be achieved by using sensitive sCMOS cameras and a traversable set-up. The CFD calculations cover the complete period of motion. Analyzing the vortex dominated flow fields provides a deeper understanding of vortex interaction and three-dimensionality of low Reynolds number (Re = 18,000 and Re = 36,000) flows. Two different Strouhal numbers (St = 0.06 and St = 0.13) are considered and their effects on the development of a leading edge and tip vortex are discussed

Experimental and numerical investigations of pitch-plunging wing aerodynamics at low Reynolds number

In the present paper, the first results of a broad study of static and moving flat plate wings for Micro Areal Vehicles (MAVs) are presented. Wind tunnel experiments with force measurements and Particle Image Velocimetry (PIV) were performed in a specially designed wind tunnel environment for low Reynolds number investigations. Corresponding numerical flow simulations were also carried out. Effects of the wing shape (elliptical and rectangular), the Reynolds number (18000, 36000 and 108000) and the kinematics using different motion forms of pure plunge and combined pitch/plunge with a reduced frequency of k=0.2 are discussed

Ermittlung der Massenkräfte periodisch bewegter Tragflügelmodelle unter Anwendung von optischer Messtechnik

Im Rahmen eines DFG-Projektes zur instationären Aerodynamik von Tragflügeln bei kleinen Reynolds-Zahlen werden Messungen der aerodynamischen Kräfte von im Windkanal periodisch bewegten Tragflächen verschiedener Geometrien und Bewegungsparameter durchgeführt. Die Massenträgheitskräfte, welche durch die Schlagbewegung hervorgerufen werden, sind mit 80 − 90 % der Gesamtkräfte deutlich größer als die aerodynamischen Kräfte der kleinen Tragflügelmodelle. Somit ist eine Kompensation der mittelsWindkanalwaage gemessenen Kräfte erfahrungsgemäß mit größeren Unsicherheiten behaftet. Diese Arbeit soll einen neuen Ansatz zur Bestimmung der Massenträgheitskräfte während eines Windkanalversuchs aufzeigen. Eine optische Positionserfassungs-Messtechnik wird zur Rekonstruktion der 3D-Verschiebungen des Tragflügels verwendet. Diese werden weiterhin zum Berechnen der Tragflächen-Beschleunigungen herangezogen. Mit der bekannten Masse, der Schwerpunktlage sowie dem Massenträgheitsmoment des Modells, können nun die exakten Massenkräfte und -momente des Tragflügels bestimmt werden

Three-Dimensional Flow Field Investigations on Pitching Low Aspect Ratio Wings at Low Reynolds Numbers

Three-dimensional, unsteady flow fields of a pitching low aspect ratio wing were measured by tomographic PIV [6] in air. The analysis of the vortex dominated flow field provides a deeper understanding of vortex interaction and three-dimensionality of the low Reynolds number (Re ≈ 10,000) flow. In order to recover the complete flow field the measurement set-up was designed to be traversable. A high spatial resolution and a large volume thickness could be achieved by a set of high sensitive sCMOS cameras. A specific feature of this campaign was to locate the measurement domain directly above the flat-plate-wing surface. Evaluation of the measurement data is performed by DLR in-house software. A selection of measurement results of this highly complex flow is presented in this paper

Simultaneous Measurements of Unsteady Aerodynamic Loads, Flow Velocity Fields, Position and Wing Deformations of MAVs in Plunging Motion

A new wind tunnel environment for low Reynolds number testing of Micro Air Vehicles (MAV) is introduced, providing a test rig for plunge and pitch motions as well as an especially designed 6-component balance to obtain dynamic force data. In this study a rigid as well as a flexible version of a typical MAV wing is investi-gated with respect to flexibility effects. Optical measurements techniques are adapted to measure simultaneously the instantaneous model position, orientation, wing deformations and flow fields