Unsteady Surface Pressure Fluctuations Research Articles

Relationship between vortex shedding noise and remotely-sensed surface pressure fluctuations of a structured porous-coated cylinder

Tonal noise suppression of a cylinder placed in uniform flow has been achieved, to some extent, by coating it with a structured porous material as a form of passive flow and noise control. A previously studied structured porous-coated cylinder is investigated in an anechoic wind tunnel to determine the relationship between the far-field vortex shedding noise and the pressure recorded on the outer porous surface. To date, no experimental studies have been conducted on the surface pressure of any type of porous-coated cylinder. Acoustic measurements are obtained using an equispaced microphone arc array and simultaneously unsteady surface pressure fluctuations are obtained around the cylinder mid-span circumference using remote-sensing techniques. By obtaining simultaneous time-dependent signals, more light is shed on the underlying noise-reduction mechanism of the structured porous-coated cylinder. In this paper, strong relationships between surface pressures and acoustic signals are revealed at the vortex shedding frequency. A spatio-temporal relationship between surface pressure and vortex shedding phenomena is also presented that helps explain the role of the structured porous media in passive flow and noise control.

Experimental Analysis of Pressure Shielding Mechanisms in Bioinspired Unidirectional Canopies

Previous studies have demonstrated that treatments such as a canopy or finlets placed within a boundary layer can shield surfaces from unsteady pressure fluctuations without substantially compromising the aerodynamic performance. This paper describes research into fundamental mechanisms of this phenomenon known as pressure shielding. Unidirectional canopy is an idealized surface treatment which consists of a streamwise array of rods cantilevered at the downstream end, inspired from the downy coating on owls’ wings. Experiments show that such a canopy attenuates the surface pressure in two distinct frequency ranges. At low frequencies associated with convective scales much greater than the canopy height, the attenuation spectra show scaling on the Strouhal number based on canopy height. At high frequencies, associated with convective scales of the order of the canopy height or lower, a dissipation-type frequency scaling appears more appropriate. The ratio of streamwise distance over the height is an important parameter at the low-frequency regions of attenuation, while the open-area ratio controls the broadband magnitude of attenuation spectra. Spatial and temporal correlations further shed light on the effects of the canopy in reducing the larger, energetic turbulent structures associated with the wall jet unsteady surface pressure fluctuations.

Airfoil Trailing-Edge Noise Reduction by Application of Finlets

This experimental study investigates the effect of biologically inspired finlets on the reduction of trailing-edge noise of a NACA 0012 airfoil. At a chord-based Reynolds number of 400,000, the far-field noise measurements show that the finlets are capable of reducing the trailing-edge noise effectively from 1000 to 4000 Hz with up to 6 dB reduction, for effective angles of attack from 0 to 8 deg. By correlating the far-field noise to the boundary-layer measurements, an optimal finlet height to boundary-layer thickness ratio to achieve the highest noise reduction is discussed. Static pressure and dynamic pressure fluctuations are examined, particularly within the finlet-treated area, to capture the flow dynamics through the treatments. The unsteady surface pressure fluctuation spectra show strong reduction at frequencies higher than 1000 Hz in the finlet wake, complying with the far-field measurements. Both the auto- and cross-correlation results of the unsteady surface pressure reveal the formation of large-scale turbulence at the entrance and toward the exit of the finlets. Moreover, they suggest that the formation and subsequent mixing of these large-scale structures with the boundary layer can be beneficial to reducing the unsteady surface pressure fluctuations. The findings will be useful to potential optimization of the finlets.

Aeroacoustic analysis of slat tones

Experimental measurements were carried out to assess the aeroacoustic characteristics of a 30P30N high-lift device, with particular attention to slat tonal noise. Three different types of slat modifications, namely slat cove filler, serrated slat cusp, and slat finlets have been experimentally examined. The results are presented for an angle of attack of α = 18 at a free-stream velocity of U = 30 m/s, which corresponds to a chord-based Reynolds number of Re = 7 x 10. The unsteady surface pressure near the slat region and far-field noise were made simultaneously to gain a deeper understanding of the slat noise generation mechanisms. The nature of the low-frequency broadband hump and the slat tones were investigated using higher-order statistical approaches for the baseline 30P30N and modified slat configurations. Continuous wavelet transform of the unsteady surface pressure fluctuations along with secondary wavelet transform of the broadband hump and tones were carried out to analyze the intermittent events induced by the tone generating resonant mechanisms. Stochastic analysis of the wavelet coefficient modulus of the surface pressure fluctuations was also carried out to demonstrate the inherent differences of different tonal frequencies. An understanding into the nature of the noise generated from the slat will help design the new generation of quite high-lift devices.

On the aeroacoustic characterization of a robust trailing-edge serration

This paper presents an experimental study on the aeroacoustics of a flat plate rig with a highly instrumented serrated trailing-edge. The role of near-field flow properties, namely, surface pressure fluctuations and spanwise coherence, in the noise suppression capability of serration is not properly understood. The results from this test rig aim to provide additional insight into the effects of the serration on the hydrodynamic field (flow field) and the scattering of the pressure waves along the trailing-edge. Despite its unconventional size, beamforming results showed a significant reduction of far-field noise over a broadband frequency range. The associated flow field is characterized by mean and spectral analyses of static and dynamic surface pressure measurements as well as hot-wire measurements. The mean pressure coefficient results and the boundary layer velocity profiles over the serrated trailing-edge showed minute differences compared to the baseline straight trailing-edge. However, the frequency-dependent energy content of the unsteady surface pressure fluctuations demonstrates an elevated energy region around the serration edges at low frequencies. Although there is an increase in the energy content of the low frequency pressure fluctuations on the serrated trailing-edge, a significant phase difference of the pressure waves is observed, which may be indicative of destructive interference. The temporal studies regarding the unsteady surface pressure fluctuations corroborate the presence of quasi-periodic large scale structures emanating from the serration edges.

Acoustic and flow characteristics of an airfoil fitted with morphed trailing edges

An experimental study of a simple NACA 0012 airfoil fitted with two different flap profiles was carried out to characterize their aerodynamic and aeroacoustic performance. The airfoil with a flap deflection angle of β = 10° was tested for a wide range of angles of attack at a chord-based Reynolds number of Rec=2.6×105. The aerodynamic lift and drag measurements show improved lift-to-drag performance for the morphed flap airfoil compared to the hinged flap airfoil at low angles of attack. Surface flow visualization and boundary layer measurements on the suction surface of the flap show delayed separation for the morphed flap airfoil. Higher-order moments of the wall pressure fluctuations were also used to observe the flow separation over the flap. Additionally, Particle Image Velocimetry was also used to study the flow over the flap and at the airfoil wake. Flow measurements showed that the downstream wake development could be significantly influenced by the flap camber. The mean velocity contours at the wake showed increased wake velocity deficit and turbulent kinetic energy for the morphed flap airfoil. The turbulent kinetic energy results displayed a characteristic double peak behavior which was also the dominant characteristics of the streamwise Reynolds shear stress component. Near-field unsteady surface pressure fluctuations and far-field noise measurements show reduced point spectra and noise levels for morphed flap configuration at low angles of attack but considerably increased noise levels at high angles of attack compared to hinged flap configuration.

On the use of leading-edge serrations for noise control in a tandem airfoil configuration

Passive noise control for a tandem NACA 65-710 airfoil configuration is experimentally investigated by applying leading-edge serrations on the rear airfoil. With a sliding side-plate mechanism that allows the rear airfoil to move in the vertical direction relative to the front airfoil, the position of maximum turbulence interaction noise is first identified from the far-field noise measurements. Subsequently, detailed static surface pressure distribution and unsteady surface pressure fluctuations are acquired to shed more light on the physical phenomenon and underlying noise-reduction mechanism of the leading-edge serrations. The far-field noise measurements confirm that a notable turbulence interaction noise reduction can be achieved from 600 Hz < f < 3000 Hz, agreeing well with the previous literature on the effectiveness of the leading-edge serrations. The near-field hydrodynamic analyses obtained using remote-sensing techniques of the fluctuating pressure fields over the airfoil show that a significant reduction in the surface pressure fluctuation levels up to 20 dB/Hz can be observed at the serrated-tip plane of the rear serrated airfoil close to the leading-edge regions, over the range of frequencies investigated. Although reduction can also be observed on the serrated-root plane, the magnitude is much less significant. The present results suggest that the modification of the unsteady loading on the rear airfoil by the leading-edge serrations plays a crucial role in the reduction of turbulence interaction noise in the tandem airfoil configuration, which may find practical application for noise reduction in aerodynamic systems involving rows of airfoils, such as contra-rotating open rotors and outlet guide vanes.

Wall pressure sources near an airfoil trailing edge under turbulent boundary layers

In the present work the wall pressure sources under turbulent boundary layers developing on the surface of an airfoil have been investigated in the region near the trailing edge. A trip wire placed on the surface of the airfoil close to the leading edge ensured a turbulent boundary layer for the two Reynolds numbers investigated. When the angle of attack was different than zero, for the lower Reynolds number, a laminar/transitional boundary layer was observed on the airfoil pressure surface due to relaminarisation of the flow. This study aimed at improving the understanding of the relationship between the vortical velocity field and its surface pressure signature which is a matter of relevance for the so-called airfoil self-noise and for the vibration phenomena observed in ship hulls. A NACA0012 airfoil of 30cm chord and aspect ratio of 1 placed at the exit of an open-circuit blower type wind tunnel was used in the investigation. Two different flow conditions corresponding to Reynolds numbers based on the chord of the airfoil of Rec=2×105 and 4×105 and three different angles of attack have been analysed. Simultaneous measurements of the unsteady surface pressure fluctuations and of the velocity field in the boundary layer and wake of the airfoil were performed, allowing the calculation of cross-correlations and cross-spectra between the two components of the velocity and the surface pressure. This has permitted the analysis of the surface pressure generating flow structures and revealed information about the sources of wall pressure fluctuations, which are closely related to those of the radiated noise.

Passive control of surface pressure fluctuations in reattaching flows

AbstractExperiments assessing the effectiveness of passive control involving a porous surface and cavity underneath for reducing the level of surface pressure fluctuations in reattaching flows are reported. Measurements are made on a generic axisymmetric body, representative of a launch vehicle configuration, in the Mach number range of 0·8 to 1·2. The measurements consist of distributions of mean surface pressure and unsteady surface pressure fluctuations in the boat-tail separated region. The passive control significantly reduces the peak surface pressure fluctuations in the reattachment zone, even in the absence of a strong shock wave. The spectra of pressure fluctuations reveal that with passive control the energy is appreciably reduced over a wide range of frequencies.