Reinhard Geisler

The complex aerodynamic footprint of desert locusts revealed by large-volume tomographic particle image velocimetry

Particle image velocimetry has been the preferred experimental technique with which to study the aerodynamics of animal flight for over a decade. In that time, hardware has become more accessible and the software has progressed from the acquisition of planes through the flow field to the reconstruction of small volumetric measurements. Until now, it has not been possible to capture large volumes that incorporate the full wavelength of the aerodynamic track left behind during a complete wingbeat cycle. Here, we use a unique apparatus to acquire the first instantaneous wake volume of a flying animals entire wingbeat. We confirm the presence of wake deformation behind desert locusts and quantify the effect of that deformation on estimates of aerodynamic force and the efficiency of lift generation. We present previously undescribed vortex wake phenomena, including entrainment around the wing-tip vortices of a set of secondary vortices borne of Kelvin–Helmholtz instability in the shear layer behind the flapping wings.

Experimental and Numerical Investigation of a Contra Rotating Open-Rotor Flowfield

Contrarotating open rotor propulsion systems have seen renewed interest as a possible economic and environmentally friendly powerplant for future transport aircraft. While the potential efficiency benefits are well accepted, concerns persist regarding the probable rotor-to-rotor interaction-driven noise penalty this type of engine would have in comparison to modern ducted turbofan engines. This paper presents results of a collaborative experimental and numerical study to quantify and study in-depth the complex flowfield of a generic contrarotating open rotor model at wind-tunnel scale. The model has 10 front blades and 8 aft blades, with blade design similar to modern propellers for high-disk loadings. The comparison of flow visualization results obtained through the use of modern stereoscopic particle image velocimetry and unsteady Reynolds-averaged Navier–Stokes simulations helps to improve understanding of the interactions of front-rotor-blade wakes and tip vortices with the aft rotor, which is an important aspect to guide the design of future efficient and quiet contrarotating open rotor engines. The generally good match between the experimental and numerical slipstream results gives confidence in the utility for their analysis capabilities in this field.

High-Speed PIV Applied to Wake of NASA CRM Model in ETW Under High Re-Number Stall Conditions for Sub- and Transonic Speeds

Particle Image Velocimetry (PIV) using high-speed camera and laser has been applied to turbulent flow regions in the wake of a stalled aircraft wing. The measurements took place on the Common Research Model (CRM) of NASA in the pressurized cryogenic European Transsonic Wind tunnel (ETW) within the framework of the EU project ESWIRP. The employed high-speed PIV setup is described and preliminary results are presented comprising sub- (M = 0.17) and transsonic (M = 0.85) stall conditions at flight Reynolds numbers of 7 and 30 million, respectively.

A fast double shutter system for CCD image sensors

A new fast shutter system for CCD sensor-based cameras is presented. This system is capable of providing for the first time short exposure times for two consecutive frames recorded in fast succession with high spatial resolutions. In addition, a third frame can be recorded with a long exposure time, this then corresponding to the well-known ‘double shutter’. Measurement techniques such as particle image velocimetry (PIV) will benefit from the new fast shutter when being used in bright environments. Using the optional third frame, advanced evaluation schemes can be implemented to measure acceleration, increase the dynamic range of the velocity measurements or reduce the number of false matches (ghost particles) in tomographic PIV or particle tracking. It will be shown that for double-frame applications this new shutter timing outperforms even the latest commercially available single-chip high-speed cameras.

Flow Field Measurements by PIV at High Reynolds Numbers

Stereo PIV measurements have been applied to the wake flow on a high-lift aircraft configuration in the cryogenic pressurized transonic wind tunnel ETW at realistic Mach and Reynolds numbers. A cryo PIV system, developed specially for the ETW to enable flow field measurements at gas temperatures down to 110 K and gas pressures of up to 450 kPa, has been adapted to a large measurement field extending over the complete span width of the half wing model. The specific seeding technique using ice particles and the optical systems are described as well as the automation the PIV measurements linked with the wind tunnel control system. First results of the vortical wing wake flow obtained at a Reynolds number of 17 million will be presented.

Experimental investigation of the flow field topology for several cargo drop configurations

Various cargo drop configurations have been analyzed using stereoscopic Particle Image Velocimetry (SPIV). The PIV measurement planes were located at several positions in the wake of a generic transport aircraft model with an open ramp. The stream- and spanwise positioning of the wind tunnel model was facilitated by means of positioning mechanism of the wind tunnel facility. The first measured configuration consisted of only the wake of the transport aircraft. The cargo configurations consisted of a model of a cargo container, including a rigid parachute and a cargo with an airdrop support platform. A fixed haltering mechanism was used to keep the cargo at a fixed position behind the wind tunnel model for the PIV measurements. The resulting velocity fields (obtained at several chord wise positions) are used to discuss the flow topologies for these airdrop configurations.

Experimental and numerical investigation of a counter rotating open rotor flow field

Stereoscopic Particle Image Velocimetry (SPIV) and unsteady Reynolds-averaged Navier Stokes (uRANS) has been applied for the flow field investigation behind a counter rotating open rotor (CROR) model. Of particular interest is the validation of numerical codes as well as a dedicated recording of the phase delays to determine the phase positions of both propellers at the experimental investigation of the flow field.

Extensive characterisation of a high Reynolds number decelerating boundary layer using advanced optical metrology

ABSTRACT Over the last years, the observation of large-scale structures in turbulent boundary layer flows has stimulated intense experimental and numerical investigations. Nevertheless, partly due to the lack of comprehensive experimental data at sufficiently high Reynolds number, our understanding of turbulence near walls, especially in decelerating situations, is still quite limited. The aim of the present contribution is to combine the equipment and skills of several teams to perform a detailed characterisation of a large-scale turbulent boundary layer under adverse pressure gradient. Extensive particle image velocimetry (PIV) measurements are performed, including a set-up with 16 sCMOS cameras allowing the characterisation of the boundary layer on 3.5 m, stereo PIV and high resolution near wall measurements. In this paper, detailed statistics are presented and discussed, boundary conditions are carefully characterised, making this experiment a challenging test case for numerical simulation.

Tracking the Nacelle Vortex Above Aircraft Wing in the ETW at Real Mach- and Reynolds Numbers by Means of PIV

Stereo PIV measurements have been performed in multiple planes of the flow above the wing of a high-lift aircraft configuration in order to investigate the performance of strake vortex generators attached to the nacelle with regard to the evolution of flow separations on the main wing at high angles of attack. The measurements have been carried out in the cryogenic pressurized transonic wind tunnel facility of ETW that allows to perform the measurements at real flight Mach and Reynolds numbers by using total gas temperatures and pressures down to 110 K and up to 450 kPa, respectively. Because of this extreme wind tunnel conditions a specific cryo-PIV measurement system has been used. To provide insights into the streamwise development of the vortical flow a light-sheet, oriented perpendicular to the direction of free stream velocity, has been positioned to different chord stations of the wing. This paper focuses on a description of the employed PIV setup in the ETW showing first results for a standard configuration obtained at flight Reynolds numbers.

Temperature and velocity measurements in a fluid layer using background-oriented schlieren and PIV methods

In this paper, we discuss temperature and velocity measurements inside a thin fluid layer using background-oriented schlieren (BOS) and particle image velocimetry (PIV) methods. BOS is a suitable technique for quantitative temperature measurements, but so far it has been used in fully transparent systems only. Introducing a reflective surface inside the measurement geometry which is optically accessible from only one viewing direction, we measure the refractive index change of a flow provided by two elliptical jets by visualizing displacements on a background target. Relation between the refractive index change and the temperature gradients is used to compute 2D temperature fields. Measurements are carried out for various temperature differences between the jets for both steady and dynamic flow. The simultaneous implementation of BOS and PIV techniques provides instantaneous, two-dimensional temperature gradients and velocity vectors inside the thin fluid layer.