Stefan Pröbsting

Vortex shedding noise from a beveled trailing edge

Coherent vortex shedding from blunt and beveled trailing edges generates tonal noise, which is usually undesired. To obtain a better understanding of the noise generation under such conditions, the flow field around a beveled trailing edge was characterized for Reynolds numbers based on the bluntness ranging from 2.5 × 104 to 5.1 × 104. Flow field statistics were obtained by means of planar high-speed two-component and stereoscopic particle image velocimetry measurements. The development of the shear layers and vortex roll-up is described in the present study. Related length scales, the vortex formation length, and wake thickness parameter were derived from the measurements. Noise emission due to vortex shedding was predicted from an analytic solution, derived from diffraction theory and the reversed Sears’ problem, and compared to acoustic phased array measurements. This approach has previously been shown to provide accurate results for sharply truncated edges, but questions with regard to the applicability with different trailing edge geometries remained open. The prediction required the auto-spectral density, correlation length, and convective velocity of the upwash velocity component in the vortex formation region. Direct application with data obtained from particle image velocimetry measurements showed an overestimation of about 20 dB when compared to the acoustic measurements. The results thus showed that the prediction of vortex shedding noise based on the simplified wake model and diffraction theory is not generally applicable.

PIV Investigation of the Flow Past Solid and Slitted Sawtooth Serrated Trailing Edges

Turbulent boundary layer trailing edge noise (TBL-TE) is an important source of aeroacoustic noise in many aerodynamic applications. Along with conventional solid sawtooth serrated geometries, new alternative slitted sawtooth design solutions seem to further improve the reduction of TBL-TE noise. In this study, particle image velocimetry (PIV) is used to investigate the flow past solid and slitted sawtooth serrations, installed at the trailing edge of a NACA 0018 airfoil at several angles of attack. Acoustic measurements obtained from a microphone array, in combination with a conventional beamforming algorithm, are employed to investigate the noise reduction obtained from the new serration designs. At zero angle of attack, the PIV measurements show similar mean flow and turbulence statistics between serrated and slitted serrations, despite notable differences in noise reduction. At nonzero incidence, the slitted serrations show less perturbations in the mean and turbulence flow fields with respect to the solid serrations. Despite this, the noise reduction trend seen between the two serration geometries remains similar.

Flow Field Around a Serrated Trailing Edge at Incidence

The flow close to the surface and in the wake of sawtooth trailing edge serrations is investigated using particle image velocimetry and Reynolds-averaged Navier-Stokes simulations. This is done in order to establish changes it undergoes when the serrations are placed at different degrees of incidence, which is prescribed through variations in the serration flap angle, and the angle of attack of the airfoil on which the serrations are installed. What has been found is that the flow close to the surface is particularly sensitive to the flap angle and shows various degrees of deflection that develop into streamwise vortices in the wake. Because of this level of complexity in the flow, the degree of incidence should be considered an important factor when trying to predict the sound emitted by serrated trailing edges using the established analytical solution.

Tomographic-PIV investigation of the flow over serrated trailing-edges

The three-dimensional characterization of the flow field over the suction side of a NACA 0018 airfoil with and without trailing-edge serrations is performed by means of time-resolved tomographic particle image velocimetry. The acoustic reduction efficiency of the investigated serrations is assessed by means of beamforming measurements. The mean flow characterization shows that at the location of the straight trailing-edge the flow is weakly perturbed by the presence of the serrations. The three-dimensional flow field is characterized by counter-rotating streamwise oriented vortices developing from the root of the serrations. They generate a funneling motion that alters the characteristics of the turbulent flow approaching the edge of the serrations.

Tomographic PIV for Beveled Trailing Edge Aeroacoustics

This study investigates the flow field around beveled trailing, defined by the ratio of radius of curvature R to plate thickness T and the enclosed 25° trailing edge angle, which parametrically resembles a variety of trailing edge flows ranging from sharp to blunt. In particular, for aeroacoustically generated trailing edge noise and vibro-structural problems the characteristics of the unsteady surface pressure near the trailing edge are of interest under these different conditions. Complementary to unsteady surface pressure sensors, highspeed tomographic Particle Image Velocimetry (PIV) can provide time-resolved and volumetric information on the flow field. Recent advances of multi-frame correlation methods and the introduction of pressure reconstruction methods based on PIV data allow for estimation of quantities related to the unsteady pressure, which otherwise requires complex instrumentation of the model. The present contribution focuses on the case R/T=10. Acoustic phased array microphone measurements have been performed and the sound pressure level has been determined by integration of the source maps over the relevant source region. Visualizations of the high-speed tomographic PIV measurements reveal the structure of the adverse pressure gradient, separating flow close to the trailing edge. PIV based reconstructed and measured unsteady surface pressure spectra show agreement over an extended range of frequencies. Estimates of the spanwise coherence length of velocity fluctuations are obtained, showing a decrease of the coherence length in the lower part of the separating shear layer. In contrast to a zero pressure gradient boundary layer, the reconstructed pressure spectra show a decrease of unsteady surface pressure convection velocities with increasing frequency for the beveled edge.

Effect of serrated trailing edge on boundary layer instability noise

Wings operating at low and moderate Reynolds number such as the ones of UAVs or the blades of small wind turbines and of compressor fans, can be the source of an aeroacoustics phenomenon called laminar boundary layer instability noise. This paper presents an investigation with time-resolved PIV and far field microphones of the dynamical flow phenomena and the acoustic emissions of a NACA 0018 airfoil, with and without serrated trailing edges, operating in a transitional regime (Reynolds number≈ 3.3− 4.7× 10). The measurements are done in an open jet wind tunnel facility with low noise and turbulence intensity. The planar PIV measurements are done on the pressure side of the airfoil in two different planes, parallel and orthogonal to the wing span at the trailing edge region, to capture the flow evolution along the airfoil chord as well as its spanwise arrangement. The results presented here show the effectiveness of the serrated geometries to reduce the noise emitted by this type of flows. Recently published literature outcomes concerning the streamwise evolution of the flow with and without serrations are confirmed. Moreover new insights related to the spanwise coherence of the traveling vortical structures are presented leading to a better understanding of the hydrodynamic and acoustic mechanisms involved in this aeroacoustics phenomenon.

On the use of Particle Image Velocimetry to predict trailing edge noise

The feasibility of aeroacoustic noise predictions based on Particle Image Velocimetry (PIV) measurements is studied. For this purpose, experiments are conducted on a sharp trailing edge (TE) flow developed along a flat plate at free stream velocity of 15m/s. The acoustic emissions were characterized in the NLR Small Anechoic Wind Tunnel (KAT) by means of microphone measurements. The result is used for benchmarking the PIV based noise predictions. PIV measurements were carried in a low-speed wind tunnel of TU Delft with similar properties to that of the KAT facility. Planar PIV measurements, performed at high spatial resolution, characterize the turbulent properties of the boundary layer travelling across the trailing edge. Time resolved Tomographic PIV measurements are obtained over a volume of 19.7mm x 9.1mm x 33.4mm at an acquisition rate of 12kHz. Two different approaches are compared for the prediction of TE noise based on PIV data. The first follows diffraction theory and requires the deduction of the instantaneous surface pressure fluctuations from PIV data. The second one relies on the integral solution of the Lighthill equation obtained through a tailored Green’s function for a semi-infinite half plane. This allows to calculate the scattered noise directly from the velocity data. The predictions based on diffraction theory agree well with the measured far field spectrum up to 4kHz. In contrast, the Green’s function approach yields a significant and systematic overprediction of approximately 12dB.

Investigation of Tonal Noise Generation on an Airfoil with Time-Resolved PIV

Tonal noise generated by airfoils at low to moderate Reynolds numbers has been focus of research since decades. Several explanations of the observed phenomena relate the noise spectrum to the instability of the laminar boundary layer, which has lead to the term laminar boundary layer instability noise. The particular features of boundary layer instability noise have been reported in literature, but remain controversial to date. In the past, studies have been performed relying on hot-wire, Laser Doppler Velocimetry (LDV), phase-locked Particle Image Velocimetry (PIV), and acoustic measurements, as well as numerical simulations and theoretical approaches. In recent years, high-speed PIV has become accepted as a tool for the investigation of aeroacoustic problems since it can provide spatio-temporal information on the structure of the source field. In the present study, far-field microphone measurements are conducted to map the noise emissions of a NACA 0012 airfoil over a range of angles of attack and velocities. The principal features observed in the noise maps agree with findings of previous experiments. For the investigation of the flow structure and source field at the trailing edge, high-speed PIV in conjunction with simultaneous microphone measurements has been performed. The information obtained using this recent measurement technique provides detail on the relation between the acoustic spectrum and the dynamic characteristics of the aeroacoustic source field.

Data reduction from time-resolved tomographic PIV for unsteady pressure evaluation

With the development of high-speed PIV and tomographic PIV, a number of novel measurement approaches are emerging. One of those is “pressure-by-PIV” and enables the spatio-temporal pressure fluctuations generated by airflow to be measured in the fluid region as well over the surface of solid objects. This experimental method is based on the application of three-dimensional time-resolved velocity data and invokes the Navier-Stokes equations to evaluate the instantaneous pressure gradient field from the numerical evaluation of fluid acceleration, in the assumption of incompressible flow. A number of experiments conducted first by thin-volume tomographic PIV and later by fully volumetric tomographic PIV at measurement rates up to 10 kHz demonstrate the applicability of the PIV-based pressure evaluation to determine broadband pressure fluctuations in a fully developed turbulent boundary layer at outer velocity of 10m/s.