An Experimental Investigation of the 30P30N Multi-Element High-Lift Airfoil (original) (raw)
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An Experimntal Investigation of the 30P30N Multi-Element High-Lift Airfoil
2014
High-lift devices often generate an unsteady flow field producing both broadband and tonal noise which radiates from the aircraft. In particular, the leading edge slat is often a dominant contributor to the noise signature. An experimental study of a simplified unswept high-lift configuration, the 30P30N, has been conducted to understand and identify the various flow-induced noise sources around the slat. Closed-wall wind tunnel tests are performed in the Florida State Aeroacoustic Tunnel (FSAT) to characterize the slat cove flow field using a combination of surface and off-body measurements. Mean surface pressures compare well with numerical predictions for the free-air configuration. Consistent with previous measurements and computations for 2D high-lift configurations, the frequency spectra of unsteady surface pressures on the slat surface display several narrowband peaks that decrease in strength as the angle of attack is increased. At positive angles of attack, there are four prominent peaks. The three higher frequency peaks correspond, approximately, to a harmonic sequence related to a feedback resonance involving unstable disturbances in the slat cove shear layer. The Strouhal numbers associated with these three peaks are nearly insensitive to the range of flow speeds (41-58 m/s) and the angles of attack tested (3-8.5 • ). The first narrow-band peak has an order of magnitude lower frequency than the remaining peaks and displays noticeable sensitivity to the angle of attack. Stereoscopic particle image velocimetry (SPIV) measurements provide supplementary information about the shear layer characteristics and turbulence statistics that may be used for validating numerical simulations.
The generation of discrete acoustic tones in the compressible flow around an aerofoil is addressed in this work by means of nonlinear numerical simulations and global stability analyses. The nonlinear simulations confirm the appearance of discrete tones in the acoustic spectrum, and for the chosen flow case, the global stability analyses of the meanflow dynamics reveal that the linearized operator is stable. However, the flow response to incoming disturbances exhibits important transient growth effects that culminate into the onset of aeroacoustic feedback loops, involving instability processes on the suctionand pressure-surface boundary-layers together with their cross interaction by acoustic radiation at the trailing edge. The features of the aeroacoustic feedback loops and the appearance of discrete tones are then related to the features of the least stable modes in the global spectrum: the spatial structure of the direct modes display the coupled dynamics of hydrodynamic instabilities on the suction surface and the near wake. Finally, different families of global modes will be identified and the dynamics that they represent will be discussed. despite four decades of research efforts and studies, an encompassing and widely accepted description of the physical mechanisms underlying the rise of discrete tones is still wanting. Based on the above-mentioned investigations, we shall give a brief résumé of the principal features and our current understanding of tonal-noise on aerofoils; for a chronological presentation we refer the reader to .
Airfoil noise measurements at various angles of attack and low Reynolds number
2009
ABSTRACT Airfoils produce tonal noise when operated at low-to-moderate Reynolds number. It is particularly annoying to the human ear and is problematic for the design of fans, compressors, helicopter rotors and unmanned air vehicles. Despite recent advances in the understanding of this phenomenon, there are still many unresolved aspects regarding the aerodynamic source generation mechanism.
Experimental investigation of aerofoil tonal noise generation
Journal of Fluid Mechanics, 2014
The present study investigates the mechanisms associated with tonal noise emission from a NACA 0012 aerofoil at moderate incidence ($0^{\circ },1^{\circ },2^{\circ }$ and 4circ4^{\circ }4circ angle of attack) and with Reynolds numbers ranging from 100 000 to 270 000. Simultaneous time-resolved particle image velocimetry (PIV) of the aeroacoustic source region near the trailing edge and acoustic measurements in the far field are performed in order to establish the correspondence between the flow structure and acoustic emissions. Results of these experiments are presented and analysed in view of past research for a number of selected cases. Characteristics of the acoustic emission and principal features of the average flow field agree with data presented in previous studies on the topic. Time-resolved analysis shows that downstream convecting vortical structures, resulting from growing shear layer instabilities, coherently pass the trailing edge at a frequency equal to that of the dominant to...
Measurement of tonal-noise characteristics and periodic flow structure around NACA0018 airfoil
Experiments in Fluids, 2006
The characteristics of tonal noise and the variations of flow structure around NACA0018 airfoil in a uniform flow are studied by means of simultaneous measurement of noise and velocity field by particle-image velocimetry to understand the generation mechanism of tonal noise. Measurements are made on the noise characteristics, the phase-averaged velocity field with respect to the noise signal, and the cross-correlation contour of velocity fluctuations and noise signal. These experimental results indicate that the tonal noise is generated from the periodic vortex structure on the pressure surface of the airfoil near the trailing edge of the airfoil. It is found that the vortex structure is highly correlated with the noise signal, which indicates the presence of noise-source distribution on the pressure surface. The vorticity distribution on the pressure surface breaks down near the trailing edge of the airfoil and forms a staggered vortex street in the wake of the airfoil.
Extensions and limitations of analytical airfoil broadband noise models
International Journal of Aeroacoustics, 2010
The present paper is a state-of-the-art of a special class of analytical models to predict the broadband noise generated by thin airfoils in a flow, either clean or disturbed. Three generating mechanisms are addressed, namely the noise from the impingement of upstream turbulence called turbulence-interaction noise, the noise due to the scattering of boundary-layer turbulence as sound at the trailing edge for an attached flow called trailing-edge noise, and the noise generated due to the formation of a coherent vortex shedding in the near wake of a thick trailing edge, called vortex-shedding noise. Different analytical models previously proposed for each mechanism are reviewed, as declinations of the same basic approach inherited from the pioneer work performed by Amiet in the seventies and based on an extensive use of Schwarzschild's technique. This choice is only an alternative to other models available in the literature and is made here for the sake of a unified approach. Issues dealing with the input data and related to the practical applications to fan noise predictions are rapidly outlined. The validity of the models is ckeched against dedicated experiments with thin airfoils and the limitations as the real configurations depart from the model assumptions are pointed out.
On the acoustics of a circulation control airfoil
Journal of Sound and Vibration, 2017
A two-dimensional elliptical circulation control airfoil model is studied in the Florida State Aeroacoustic Tunnel. Far-field acoustics are obtained via a 55 microphone phased array. Single microphone spectra are also obtained, and it is shown that background noise is significant. In order to circumvent this problem, beamforming is employed. The primary sources of background noise are from the tunnel collector and jet/sidewall interaction. The deconvolution approach to mapping acoustic sources (DAMAS) is employed to remove the effects of the array point spread function. Spectra are acquired by integrating the DAMAS result over the source region. The resulting DAMAS spectral levels are significantly below single microphone levels. Although the DAMAS levels are reduced from those of a single microphone or delay and sum beamforming (DAS), they are still above those of a NACA 0012, estimated using NAFNoise, at the same geometric and free-stream conditions. A scaling analysis is performed on the processed array data. With a constant free-stream velocity and a varying jet velocity the data scale as jet Mach number to the 6th power. If the momentum coefficient is held constant and the free-stream velocity is varied the data scale as free-stream Mach number to the 7th power. 1. Introduction 8 Traditionally, lift is proportional to the square of the forward velocity of a body. This means that 9 vehicles moving at very low speeds, such as submarines or aircraft during approach, have drastically reduced 10 maneuverability, making them more difficult to control. Circulation control (CC) provides a mechanism for 11 increased lift and maneuverability even at low forward speeds. On air-and hydro-foils, CC generally provides 12 these advantages by issuing a high speed jet from a spanwise slot, tangential to a curved trailing edge. The 13 jet entrains low-momentum fluid, and the flow stays attached longer due to the Coanda effect. This delays 14 separation and increases circulation by shifting the stagnation point, which results in augmented lift [1]. 15 Due to this lift increase, CC could allow for reduced mechanical complexity on aircraft and underwater 16 vehicles in the event that it replaces current mechanical systems that change the angle of attack. The fluid 17 dynamic mechanisms and benefits of CC have been studied extensively and are well understood [2]. Though 18 the advantages gained from CC are significant, it comes with drawbacks [3]. 19 One of the issues presented by CC is the production of significant levels of undesirable acoustic noise. This 20 additional noise is detrimental to its implementation both in air and underwater. In air, strict requirements 21 are placed on acoustic levels for the areas surrounding airports. Underwater, CC noise reduces stealth 22 capabilities and sonar system reliability and therefore lifespan of the vehicle. Compared to the copious 23 number of studies on CC fluid dynamics, there is a relative scarcity of work regarding CC acoustics. 24 The first study on CC acoustics was performed by Williams and Cheeseman [4] who suggested ten 25 primary sources of rotor-craft CC noise. Later Salikuddin et al. [5] performed experiments varying jet 26 speed and slot height, finding that noise levels increased as either parameter increased. Howe [6] then