Arne Henning

Simultaneous Multiplane PIV and Microphone Array Measurements on a Rod-Airfoil Configuration

The influence of the elastic rotor blade deformation and the aerodynamic interference from the fuselage on the rotor aerodynamics as well as rotor noise characteristics was studied. A BO105 Main Rotor /Tail Rotor/Fuselage (FUS) configuration is chosen for the numerical simulations. A coupling of unsteady free wake 3-D panel method (UPM) and Airbus Helicopters’ (formerly: Eurocopter) rotor code HOST was conducted to account for the effect of elastic blade deformation as well as compressibility correction. The effect of fuselage is simulated by using two fuselage models in UPM, (1) potential theory in form of a panelized fuselage and (2) an analytic fuselage influence formulation derived from isolated fuselage simulation based on (1).Measurement techniques based on microphone-arrays are well-known and common practice on scaled models in wind tunnels with closed test section. Usually, full-scale Reynolds numbers are not achieved. To increase the Reynolds number measurements are performed in cryogenic and/or pressurized wind tunnels. At the DLR Institute of Aerodynamics and Flow Technology the microphone array measurement technique was further developed to perform measurements in a cryogenic wind tunnel at temperatures down to 100 K. In order to use a microphone array in a cryogenic environment, coming to grips with cold hardiness and ensuring long term stability of the array fairing and the electronic devices, especially the microphones, are the primary challenge. In a first step measurements of the radiated noise from a single rod configuration have been conducted in the cryogenic wind tunnel DNW-KKK using a prototype microphone array designed for cryogenic environment. Measurements were carried out with a wide range of operational flow parameters. The measured sound radiation results showed a very good agreement with theory and a Reynolds number dependency of the measured and predicted sound power was shown.

Systematic Comparison of Microphone Array Measurements in Open and Closed Wind Tunnels

A systematic comparison between microphone array measurements in closed and open test sections of wind tunnels is presented. The study focuses on the beamforming accuracy in both wind tunnel types with respect to source position, relative and absolute levels of beamforming results and integrated spectra using two difierent sound sources. At flrst, an in-∞ow calibration sound source incorporated in an electromechanic driver and with known characteristics is used. This kind of source allows to investigate the in∞uence of the difierent ∞ow conditions on the sound propagation and their in∞uence on beamforming results. In the second part of the test a plate with various cavities serves as an aeroacoustic sound source. In both wind tunnels the same phased microphone array is used for the measurements, whereby the aim is to keep the experimental setup and conditions as identical as possible in both cases in order to be able to examine the test section-dependent efiects. The beamforming results of the two difierent sources, obtained from measurements in both test sections, show a high qualitative and quantitative agreement provided that the SNR is su‐ciently high enough. The wind tunnel dependent deviations of the integrated source spectra do not exceed 4.7 dB.

A Stereo PIV Investigation of a Vortex Breakdown Above a Delta Wing by Analysis of the Vorticity Field.

The vortex breakdown above a generic delta wing was experimentally investigated in order to contribute to the better understanding of this flow phenomenon. The velocity field inside the breakdown region was measured with a three component Particle Image Velocimetry (PIV) system with a high spatial resolution and accuracy, such that the vortical structure could be analysed based on the vorticity field. One research goal was to get more information about the physical mechanism which lead to the dominant vortex breakdown phenomenon. Another important goal of the performed measurement campaign was to investigate the influence of the lower sweep angle on the flow structures. Additionally we compared our PIV and force measurement data with results of numerical simulations and the therewith identified structure of the vortex breakdown in the front part of the delta wing. We were able to confirm the occurrence of a bubble type vortex breakdown predicted by the numerical simulations.

Investigation of the unsteady flow development over a pitching airfoil by means of TR-PIV

The flow over an OA209 airfoil subjected to a sinusoidal pitching motion under dynamic stall conditions is investigated experimentally by means of time resolved particle image velocimetry (TR-PIV) and surface pressure measurements. Dynamic stall is distinguished by the formation and convection of large scale coherent structures and a delay in massive flow separation. A vortex detection scheme based on an identification function derived directly from the velocity fields is adopted to identify vortex cores. The combination of global time resolved imaging and an automated vortex identification algorithm allows for the investigation of the spatial and temporal evolution of vortical structures within a single oscillation. Furthermore, the mechanisms associated with the dynamic stall delay are considered.

Analysis of simultaneous measurement of acoustic pressure in the far-field and density gradient in the near-field in a cold jet

The purpose of the investigation presented here is to study the degree of correlation between the density gradient in the near eld and the aeroacoustic sound generation in the far field of a cold jet. The variations of the density gradient in the flow are visualized and recorded by the Background Oriented Schlieren (BOS) technique. In this method high power LEDs are used as illumination source. Simultaneously the pressure fluctuations in the acoustic far-field are recorded by a set of microphones. Measurements on a round nozzle and a chevron nozzle are conducted and the obtained results are compared. Large structures emitting noise are localized, being in the case of the chevron nozzle further upstream due to the higher level of entrainment of the jet with the surrounded ambient fluid. In addition, it is demonstrated that the noise produced by these structures is radiated mainly downstream, having the highest correlations with the pressure fluctuations recorded with the microphone located at an angle Theta = 26°respect to the jet-axis.

Cross-correlation analysis of synchronized PIV and microphone measurements of an oscillating airfoil

The present study focuses on the correlation between the flow structures evolving during the dynamic stall processes of a two-dimensional NACA64-618 airfoil, which performs a sinusoidal movement about its quarter chord axis, and their aeroacoustic response in the far field. Experiments are conducted in an anechoic wind tunnel at a Reynolds number of

Investigation of laminar detachment by means of simultaneous microphone and surface hot wire measurements

8 \times 10^{5}

Eulerian and Lagrangian views of a turbulent boundary layer flow using time-resolved tomographic PIV

8×105 based on the chord length and include simultaneous velocity field measurements in the vicinity of the airfoil and microphone measurements in the acoustic far field. A causality correlation method based on phase locked snapshots of the velocity field allows for the identification of specific structures at different phases of the dynamic stall life cycle that contribute to the sound generation process. The sound emission during the stall development and flow reattachment phases is attributed to coherent structures evolving downstream of the trailing edge. When the flow is fully stalled, the region that contributes to the sound emission increases. The position of the sound emitting coherent structures also fluctuates stronger between oscillation cycles during full stall.Graphical abstract