Surface Pressure Spectra Research Articles

Using the Boundary Element Method for airfoil trailing edge noise prediction

This study investigates two aspects of the prediction of trailing edge noise from an airfoil using the Boundary Element Method to solve the scattering phenomenon. The first one consists in defining the surface pressure spectrum near the trailing edge as a source for the turbulent boundary layer-trailing edge interaction, using either an empirical model or one-dimensional turbulent boundary layer theory. The second aspect focuses on the capability of the Boundary Element Method to handle general boundary conditions. It is applied to predict the noise emitted by a porous trailing edge. This work is conducted in the context of aerodynamic noise from wind turbine blades, but has a wider range of engineering applications.

Aeroacoustics of a Square Finite-Wall-Mounted Cylinder in Pressure-Gradient Flows

An experimental investigation of the wake flow structures, surface pressure fluctuations, and noise production of a square finite-wall-mounted cylinder with an aspect ratio of 2.4 is presented. The cylinder was immersed in flows with favorable, near-zero, and adverse pressure gradients at a Reynolds number of 48,000, based on cylinder width. Far-field noise, unsteady surface pressure, and particle image velocimetry measurements were taken simultaneously using the open-jet pressure-gradient test rig in the UNSW Anechoic Wind Tunnel. Favorable and adverse pressure gradients were found to enhance and suppress the cylinder tonal noise, respectively. A favorable pressure gradient reduced the size of the recirculation region in the wake, which intensified the free-end downwash and suppressed the junction upwash. The intensities of the cylinder surface pressure fluctuations were slightly increased at the primary and secondary shedding Strouhal numbers (based on cylinder width) of approximately 0.1 and 0.2. Conversely, the recirculation region expanded under an adverse pressure gradient, with the downwash weakened and the upwash enhanced. The peaks in the surface pressure spectra were noticeably attenuated at the primary shedding Strouhal number and completely suppressed at the secondary Strouhal number. Wake flow structures correlated with the surface pressure fluctuations and far-field noise were identified to understand the enhancement or suppression mechanisms of the surface pressure fluctuations and the associated shedding regimes.

Wall-Modeled Large-Eddy Simulation of Transonic Buffet over NASA-CRM Using FFVHC-ACE

This study investigates the capability of wall-modeled large-eddy simulation (WMLES) in predicting the transonic buffet phenomenon over three-dimensional aircraft configurations at high Reynolds numbers. The WMLES is conducted using the Cartesian-grid-based flow solver FrontFlow/Violet Hierarchical Cartesian for Aeronautics Based on Compressible-Flow Equations (FFVHC-ACE). To extend the capability of FFVHC-ACE to transonic flow simulations with shock waves, a hybrid kinetic energy and entropy preserving/upwind scheme is implemented in FFVHC-ACE. To validate the employed simulation framework, the flow around the NASA Common Research Model in the near-cruise condition is simulated. In this case, the surface pressure distributions and force coefficients are reasonably predicted and show the trend of grid convergence. Furthermore, the WMLES at the buffet conditions reproduces the wavy structures of the shock wave (i.e., buffet cells) propagating outboard. The obtained propagation speed of the buffet cells and surface pressure spectra show reasonable agreement with the wind tunnel experiments.

In Situ Observations at the Air‐Sea Interface by Expendable Air‐Deployed Drifters Under Hurricane Michael (2018)

AbstractAn array of surface drifters deployed ahead of Hurricane Michael measured the surface temperature, pressure, directional wind and wave spectra, and surface currents one day before it made landfall as a Category 5 Hurricane. The drifters, 25–50 km apart, spanned two counter‐rotating ocean eddies as Hurricane Michael rapidly intensified. The drifters measured the shift of wave energy between frequency bands in each quadrant of the storm, the response of upper ocean currents, and the resulting cold wake following Michael's passage. Wave energy was greatest in the front quadrants and rapidly decreased in the left‐rear quadrant, where wind and wave energy were misaligned, and components of the wave field were aligned with currents. Hurricane Michael's wave field agreed with previous studies of nondirectional wave spectra across multiple tropical cyclones but had some unique characteristics. The analysis demonstrates how co‐located surface wind and wave observations can complement existing airborne and satellite observations.

Modeling the Surface Pressure Spectrum on Rough Walls in Pressure Gradients

Abstract Models for surface pressure spectra beneath rough wall boundary layers are assessed, with particular emphasis on prediction from steady, Reynolds-averaged Navier–Stokes (RANS) data. RANS roughness boundary conditions are shown to have qualitatively good trends between roughness function and roughness Reynolds number, but model-to-model discrepancies remain and the universality of an equivalent sandgrain roughness height for turbulence models is doubtful. Existing empirical models for the surface pressure spectrum show good agreement in some spectral regions and a newly proposed model shows good matching across the spectrum in a variety of pressure gradient conditions. Adjustments are made to existing TNO analytical models to incorporate rough wall effects, including changes to the velocity spectrum model and the inclusion of a wall-shift, shown to be independent of local Reynolds number, pressure gradient, or turbulence model. The mathematical character of the rough wall spectrum has been revealed, but challenges remain to implement both flow and spectral modeling without a priori knowledge of the flow.

Modeling the Surface Pressure Spectrum Beneath Turbulent Boundary Layers in Pressure Gradients

A wide variety of models and methods for the prediction of the surface pressure spectrum beneath turbulent boundary layers is presented and assessed. A thorough review is made of the current state of the art in empirical and analytical pressure spectrum models; and predictions of zero, adverse, favorable, and nonequilibrium pressure gradient boundary layers are examined using a steady Reynolds-averaged Navier–Stokes (RANS) prediction of a subset of a pressure gradient boundary-layer benchmark flow case. The existing empirical models show either an inability to adapt to pressure gradient conditions or an oversensitivity to model inputs, producing nonphysical results under certain flow conditions. New empirical models are created using a gene expression programming machine-learning algorithm based on both experimental and RANS inputs. The various input options for analytical Toegepast Natuurwetenschappelijk Onderzoek (TNO) modeling are presented and assessed, and recommendations for best practices are made. The developed models show improvement in both accuracy and robustness over existing models.

Increase of Unsteady Pressure at a Trailing Edge due to Inflow Turbulence

This paper presents new insights into the penetration of the freestream turbulence in a turbulent boundary layer. Experiments evidence an increase in the velocity fluctuations and integral length scale across the boundary layer in addition to an increase of the surface pressure fluctuations close to the airfoil trailing edge. This paper proposes a new empirical model to predict the integral length scale inside the boundary layer at the trailing edge of an airfoil subjected to inflow turbulence. The potential increase of the airfoil trailing noise is discussed. The flow mechanism studied in this paper is relevant to the noise prediction of many applications, including vertical-axis wind turbines subjected to high-inflow turbulence winds, generally observed in urban zones. Experiments performed on a NACA 0012 airfoil subjected to inflow turbulence of 10 and 20% intensity measured the boundary-layer turbulence and the surface pressure fluctuations close to the airfoil trailing edge. Measurements showed that the high-inflow turbulence penetrates the turbulent boundary layer reaching the inner sublayer. A freestream turbulence of 10% increases the velocity fluctuations inside the boundary layer caused by the larger turbulent structures, whereas an inflow of 20% turbulence intensity causes the increase of the velocity fluctuations caused by all turbulent scales. Furthermore, the freestream turbulence significantly increases the surface pressure fluctuations at the trailing edge. For an inflow with 10% turbulence intensity, the surface pressure spectrum increases by more than 6 dB in the entire frequency range. The increase is above 10 dB if the inflow turbulence intensity is 20%.

Steady and unsteady aerodynamic loading of a NACA 16-616 aerofoil in a uniform flow

AbstractThis paper investigates the hydrodynamic near-field of a NACA 16-616 aerofoil over a range of angles-of-attack, encompassing the pre-stall, stall and post-stall flow regimes. In both the static pressure and the pressure fluctuation results, it is shown that each flow regime is easily distinguished, and it is further shown that each regime has different spectral behaviour and boundary layer characteristics. It is found that the NACA 16-616 aerofoil stalls by an abrupt leading-edge mechanism, characterised by a sudden change in the static pressure and unsteady surface pressure spectra between $16^\circ $ and $17^\circ $ angles-of-attack, but of more interest is that there is a secondary yet significant trailing-edge flow separation mechanism occurring upstream of the trailing-edge and moving further upstream as the angle-of-attack increases in the pre-stall regime. A comparison is made between the spectra and coherence of the unsteady surface pressure of the NACA 16-616 aerofoil and those of the classic NACA 0012 aerofoil and shows that such a secondary mechanism has a significant impact for large pre-stall angles-of-attack on the unsteady surface pressure. This will have a significant impact on the radiated far-field sound, distinguishing the NACA 16-616 aerofoil from aerofoils that do not have this secondary mechanism. The existence and extent of this secondary trailing-edge separation mechanism is further shown by the hot-wire anemometry boundary layer velocity results that indicate separation within the pre-stall regime.

Empirical Model for Low-Speed Rough-Wall Turbulent Boundary Layer Pressure Spectra

An empirical model for estimating surface pressure spectra in high Reynolds number rough-wall turbulent boundary layers in the low-speed regime is presented. The model is based on the low-frequency mean velocity defect scaling [1] and the high-frequency shear friction velocity scaling [2]. This model satisfactorily predicts surface pressure spectra measured over a wide range of roughness configurations in high Reynolds number flows free of transitional effects. These data also provide insights into the characteristics of the turbulent scales which are on the order of the roughness size and comprise the mid-frequency region. The slope of this region is highly dependent on roughness element density and element-relative measurement location. This contradicts the premise of an overlap region with slope . An evanescent pressure decay model provides evidence that the slope changes are due to a roughness-dependent pressure decay within the interstitial flow and suggests that a traditional mid-frequency similarity law may not be possible.

Surface pressure spectrum variation with Mach number on a CD airfoil

The surface pressure spectrum characteristics are studied on a Controlled Diffusion airfoil at varying Mach numbers and a fixed Reynolds number. Compressible large eddy simulation datasets have been used for this analysis. The study focusses on the noise generation mechanism and the source locations responsible for it. A mid-high frequency hump is observed for the higher Mach number cases in the pressure spectrum and the amplitude of this observed hump increases with Mach number. To investigate the cause of this hump, different hydrodynamic and acoustic pressure modes traveling at specific speeds are obtained by a wavenumber–frequency decomposition. The acoustic component originating from the separation bubble region near the leading edge is found to be responsible for the hump in the pressure spectrum. The interaction of hydrodynamic and acoustic waves also plays a significant role in the appearance of the hump in the spectrum. There is also an increase in the amplitude of the high frequency region in the acoustic spectrum as we go towards the trailing edge for higher Mach number which can be attributed to an increase in turbulent fluctuation amplitude in the leading edge region. Furthermore, stability analysis has been performed on one-dimensional mean boundary-layer profiles. The higher reverse velocity in the separation bubble with increasing Mach number seems to be causing higher growth rates of instabilities, ultimately resulting in the significant surface spectrum variations. The directivity in the farfield also confirms a significant contribution of the hump in the spectrum in the overall sound pressure level.

Experimental Investigation of Near-Field Aeroacoustic Characteristics of a Pre- and Post-Stall NACA 65-410 Airfoil

The present study reports an experimental investigation of the near-field aeroacoustic characteristics of a cambered NACA 65-410 airfoil. The experiments were performed in an aeroacoustic wind tunnel facility at a moderate chord-based Reynolds number of 4.2×105. Detailed static and dynamic surface pressure information was recorded to reveal the pre- and post-stall dynamics. By varying the angle of attack from 7° to 15°, a stall behavior similar to thin airfoil stall is observed from the evolution of the static and dynamic surface pressure spectra at different chordwise locations. Moreover, while the surface pressure spectra show a monotonic decrease with constant gradients after entering full stall, the pre-stall behavior shows a different trend and partly resembles the attached and stalled flows. Furthermore, the unsteady surface pressure spectra are compared to those obtained from a NACA 0012 airfoil, showing notable differences in the near-field stall dynamics. A scaling formula proposed by a previous study is applied to the present pressure spectra and found to scale well except for measurements close to the leading edge. The present near-field measurements and the streamwise and spanwise length scales can be useful input to the modeling of airfoil separation and stall noise for cambered airfoils.

Aeroacoustic investigation of an oscillating airfoil in the pre- and post-stall regime

The present study describes an experimental investigation of the aerodynamic and aeroacoustic characteristics of a sinusoidally oscillated NACA 0012 airfoil. The experiments were conducted in an aeroacoustic wind tunnel with a uniquely designed Kevlar-walled test section. Prior to experiments, these Kevlar walls were calibrated carefully and shown to provide reliable and accurate aerodynamic and aeroacoustic measurements. Two different regimes of interest, namely the pre- and post-stall angle of attack regimes, have been examined for lift curve polars, far-field noise spectra and unsteady surface pressure spectra. Interestingly, when the lift curve polar hysteresis is small at pre-stall angles of attack, the unsteady surface pressure spectra of the oscillating airfoil can be predicted with satisfactory accuracy using a position-based weighted averaging approach from its static counterparts. On the other hand, such a method becomes invalid at post-stall angles due to the presence of a significant dynamic stall hysteresis. Instead, an increase in the mean surface pressure and far-field noise spectra is observed at dynamic stall conditions. Furthermore, a short-time Fourier transform analysis reveals that the increase of the surface pressure spectra is a direct result of the periodic production and convection of dynamic stall vortices.

Turbulent flow interaction with a circular cylinder

This paper presents a comprehensive experimental study on the unsteady pressure exerted on the surface of a round cylinder in smooth and turbulent flows. A highly instrumented cylinder with several static pressure taps and dynamic pressure transducers at different spanwise and peripheral locations was used, enabling extensive dynamic surface pressure, coherence, and turbulence length-scale analysis. The effects of the free-stream turbulence and turbulent length scale are investigated by placing the turbulent-generating grids within the wind tunnel duct. For both the laminar and turbulent incident flows, the surface pressure results show the emergence of the fundamental, first and second harmonics at most peripheral angles, while at the cylinder base, the surface pressure spectra are dominated by the first harmonic. It has also been observed that an increase in the level of the turbulence intensity results in an increase in the energy level of unsteady pressure acting on the cylinder. An increase in the length scale of the incoming flow structures is shown to result in an increase in the energy level of the tonal frequencies and the broadband content of the surface pressure spectra. The spanwise coherence results have also shown that an increase in the length scale of the flow structures increases the spanwise correlation length of the flow structures at the vortex shedding frequency at the stagnation point, while at the cylinder base, the spanwise correlation length decreases at the vortex shedding frequency.

Cross-spectral analysis of PSP images for estimation of surface pressure spectra corrupted by the shot noise

We proposed applying the cross-spectrum analysis to the unsteady pressure-sensitive paint (PSP) images to eliminate the electronic shot noise component from the pressure fluctuation spectrum. The data of surface pressure fluctuation behind a square cylinder which measured by means of the polymer/ceramic PSP with a frequency response of approximately 5 kHz which is sufficiently high for studying the Karman vortex shedding was used for the validation of this analysis method. The cross spectrum is compared with the auto-spectrum of PSP images and that of the Kulite pressure transducer that corresponds to the reference signal. The cross spectrum can drastically reduce the electronic shot noise component in the pressure fluctuation spectrum. The second peak of vortex shedding which is not observed in the result of auto-spectrum can be observed thanks to the noise reduction. The convergence of calculation is faster as the size of the spatial area for calculating cross spectrum increases.

Direct noise computation of a generic vehicle model using a finite volume method

This paper demonstrates the applicability of a finite volume method for the direct noise computation of road vehicles. For this purpose, a generic vehicle model consisting of major aeroacoustic components of a production car is considered. Such components are the A-pillar and the side mirror. The flow past the vehicle model is calculated using a compressible flow solver for low Mach number flows, accompanied with an approach based on detached eddy simulation for turbulence modelling. Two variants of the vehicle model are calculated, namely with and without side mirror. Validation of aerodynamic results is performed by comparing simulated surface pressure spectra on the side window to wind tunnel measurements. Thereafter, acoustic waves generation and propagation are examined. The propagation of acoustic waves from the sources to the windows surface is calculated directly using the compressible solver, as well as indirectly using a surface integral method. The results of both methods are then used as an input for a vibroacoustic model and surface acceleration spectra are compared between both approaches and wind tunnel measurements. A general assessment of both numerical approaches is given and the aeroacoustic impact of the A-pillar as well as the side mirror regarding their contribution to the overall acoustic level is presented.

Turbulent Pressure Spectra for Separated Flow Conditions

<div class="section abstract"><div class="htmlview paragraph">The magnitude of the turbulent pressure spectrum in fluid flow over an obstruction is usually much larger than in attached flow over a smooth surface. External features on a vehicle, such as windshield wipers, side mirrors and pillars which cause flow separation, are a major source of wind noise. The modeling of the pressure spectrum in separated flow is important for designing quiet vehicles. In this study wind tunnel tests have been performed with different shaped obstructions to measure and correlate the surface pressure spectra with flow parameters such as the pressure coefficient and separation size. The model by Chase for attached turbulent boundary layer pressures is generalized to apply to separated flow conditions.</div></div>

Trailing-edge flow manipulation using streamwise finlets

The use of streamwise finlets as a passive flow and aerodynamic noise-control technique is considered in this paper. A comprehensive experimental investigation is undertaken using a long flat plate, and results are presented for the boundary layer and surface pressure measurements for a variety of surface treatments. The pressure–velocity coherence results are also presented to gain a better understanding of the effects of the finlets on the boundary layer structures. The results show that the flow behaviour downstream of the finlets is strongly dependent on the finlet spacing. The use of finlets with coarse spacing leads to a reduction in pressure spectrum at mid- to high frequencies and an increase in spanwise length scale in the trailing-edge region due to flow channelling effects. For the finely distributed finlets, the flow is observed to behave similarly to that of a permeable backward-facing step, with significant suppression of the high-frequency pressure fluctuations but an elevation at low frequencies. Furthermore, the convection velocity is observed to reduce downstream of all finlet treatments. The trailing-edge surface pressure spectrum results have shown that, in order to obtain maximum unsteady pressure reduction, the finlet spacing should be of the order of the thickness of the inner layer of the boundary layer. A thorough study is provided for understanding of the underlying physics of both categories of finlets and their implications for controlling the flow and noise generation mechanism near the trailing edge.

Numerical investigation of noise reduction mechanisms in a bio-inspired airfoil

This paper presents a numerical analysis of an airfoil geometry inspired by the down coat of the night owl. The objective is to understand the mechanisms of airfoil trailing edge noise reduction that has been observed with such designs in previous experiments. The bioinspired geometry consists of an array of finlet “fences” that are applied near the trailing edge of the baseline (NACA 0012) airfoil. Wall-resolved large eddy simulations are performed over the baseline and the bioinspired airfoil geometries and the aeroacoustic performance of the two geometries are contrasted. Both models are simulated at chord-based Reynolds number Rec = 5 × 105, flow Mach number, M∞ = 0.2, and angle of attack, α = 0°. Unsteady surface pressure spectra near the airfoil trailing edge show large reductions at high frequencies but an increase in low frequencies with the bioinspired airfoil, consistent with previous measurements. Farfield noise spectra comparisons between the baseline and the bioinspired airfoil show reductions of up to 10 dB with the fences. The simulations reveal that the fences lift the turbulence eddies away from the airfoil trailing (scattering) edge hence reducing the scattering efficiency. These findings suggest that one of the mechanisms of noise reduction is the increased source-scattering edge separation distance. Two-point correlations show that the fences reduce the spanwise coherence at low frequencies for separation distances greater than a fence pitch. Reduction in spanwise coherence is another potential mechanism of farfield noise reduction at low frequencies.

Direct and integral noise computation of two square cylinders in tandem arrangement

The present paper presents a comparison between two different approaches for calculating far-field noise of two square cylinders in tandem arrangement. Experimental data show that a flow past such an arrangement results in aeolian tones as well as broadband noise. The turbulent flow past the cylinders is computed using a compressible solver for low-Mach number flows. In order to validate the direct computation of acoustic waves, the complete span including endplates is calculated. Wind tunnel measurements show two different but stable flow states past the cylinders: a quiet state, where no vortex shedding is present in the wake of the upstream cylinder, and a loud state, where separated shear layers flow into the gap between the cylinders intermittently. Different initialisations of the velocity field are used in order to capture both flow states. Mean flow quantities as well as surface pressure spectra are compared to wind tunnel measurements. Far-field noise is calculated directly using the compressible solver, as well as indirectly using a hybrid method based on a Ffowcs Williams Hawkings formulation for stationary rigid surfaces. Far-field spectra and directivity patterns are compared to wind tunnel measurements. Results of both methods show good agreement with experimental data.

Unsteady characteristics of a slat-cove flow field

The slat flow field of a multi-element airfoil is experimentally studied. Mean turbulence statistics and phase-locked data link shear layer structures to narrowband peaks in the surface pressure spectra. Most notably, a bulk cove oscillation related to shear layer flapping is found.