Janos Agocs

Application of Stereo PIV to the VFE-2 65° Delta Wing Configuration at Sub- and Transonic Speeds

The present paper describes the application of Particle Image Velocimetry (PIV) to the flow around a delta wing in a pressurized transonic wind tunnel. These investigations are the second part of International Vortex Flow Experiment 2 (VFE-2) measurements carried out at DLR Gottingen on a NASA wind tunnel model. The first part comprised surface pressure measurements by means of Pressure Sensitive Paint. These results were used to select the test cases for PIV. Flow fields for the Mach numbers of 0.4 and 0.8 and for the Reynolds numbers of 2 and 3 million at four incidences of 11, 13, 20 and 25 degree were captured within different planes perpendicular to the model axis. Delta wings with sharp as well as with rounded leading edges were investigated. Details of the stereoscopic PIV system such as the overall arrangement, the image evaluation and the flow seeding technique will be described. Techniques to overcome problems caused by density changes within the wind tunnel are described too, such as deflections of the laser light beam and camera viewing. Laser light flare on the model surface is reduced by a specially developed fluorescent paint which allows for the detection of small flow structures very close to the model surface.

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.

Vortex investigation over a rolling delta wing model in transonic flow by stereo PIV measurements

An investigation of the vortex behaviour over a delta wing model under different angles of incidence and roll has been performed using Particle Image Velocimetry (PIV) under transonic conditions at DLR Gottingen, Germany. These tests are important for the understanding of the leading edge vortex behaviour which is responsible for enhanced lift forces and CFD validation. The investigation used a stereo PIV arrangement, with both cameras in forward-scatter mode. For high-quality stereo PIV measurements of the flow around such models under transonic conditions, many details had to be optimised, especially as the light sheets were positioned in the cross-plane to the freestream over the model. Parameters like light sheet thickness and displacement with a certain overlap had to be considered carefully. It was also necessary to observe the optical system by means of additional cameras, as the starting and stopping of the tunnel affected the light sheet. Fortunately, readjustment of its position whilst the wind tunnel was running could be done by remote control and optical tools outside the tunnel. This had to be done in connection with a special layout for the trigger sequence, as lengths of wires or optical fibres for the transmission of TTL pulses became relevant for delays in the order of microseconds. It was expected that strong centrifugal and axial accelerations would be present and tiny particles had to be used for seeding. It could be established that the vortex structure is dependent strongly on the direction of change of the roll angle. A very impressive phenomenon was the so-called busting of vortices, which was accompanied by rapidly-changing pressure distributions.

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.

Multi-window PIV for high-lift measurements

Experiments using multi-window, particle image velocimetry (PIV) were performed in the low-speed wind tunnel of Airbus Bremen, Germany, over a two-dimensional, slat/wing/flap model in a high-lift configuration. For this, a system for acquiring PIV data from multiple cameras simultaneously was devised and installed underneath the LSWT. The PIV experiments were performed at the mid-span of the model, for incidences of 12/spl deg/, 17.5/spl deg/ & 19/spl deg/. Nearly 5000 PIV images were obtained during the tests, the images capturing the flowfields from the slat wake, slat/wing gap, flow mixing over the main wing, wing/flap-gap and wing wake. The analysis of the results reveals that such a technique is a viable and valuable tool for industrial wind tunnel tests, with high quality data showing the unsteady nature of the slat wake and slat gap flow with distance downstream and unsteady flow separation over the flap.

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.

Evaluation of Large-Scale Wing Vortex Wakesfrom Multi-Camera PIV Measurements in Free-Flight Laboratory

Multiple-vortex systems of aircraft wakes have been investigated experimentally in a unique large-scale laboratory facility, the free-flight B20 catapult bench, ONERA Lille. 2D/2C PIV measurements have been performed in a translating reference frame, which provided time-resolved crossvelocity observations of the vortex systems in a Lagrangian frame normal to the wake axis. A PIV setup using a moving multiple-camera array and a variable double-frame time delay has been employed successfully. The large-scale quasi-2D structures of the wake-vortex system have been identified using the QW criterion based on the 2D velocity gradient tensor ∇H u, thus illustrating the temporal development of unequal-strength corotating vortex pairs in aircraft wakes for nondimensional times tU0∕b≲45.