W. Nathan Alexander

The Relationship Between Roughness Noise and the Near- Field Pressure Spectrum

Extensive measurements have been made of the roughness noise and wall pressure fluctuations produced by a wall jet boundary layer. Short sandpaper roughness fetches, with grit sizes from 220 to 20 were immersed in a 15 mm thick boundary layer and studied as a function of flow speed. A three to one range of boundary layer edge velocities produced a wide range of flows from hydrodynamically smooth through the low end of the fully rough regime. Roughness noise levels are typically 60 to 70 dB below the level of wall pressure fluctuations, and the spectra have quite different forms. Nevertheless, for all the rough surfaces and conditions tested, the roughness noise spectrum Φ(ω) varies closely as the product of the single-point wall pressure spectrum Gpp(ω), the frequency ω squared, and the mean-square roughness height hrms 2 . The only exception to this conclusion occurs for the largest roughnesses (40 grit or greater) at high frequencies, where the ratio of the roughness noise to wall pressure spectral values falls below the ω 2 curve.

Rotor Inflow Noise Caused by a Boundary Layer: Inflow Measurements and Noise Predictions

Detailed measurements of the four-dimensional space time correlation function of a thick turbulent boundary layer have been made and used as source terms for the prediction of sound radiated from a 10-bladed rotor partially immersed in this flow. This simple configuration recreates in a canonical setting the important physics produced when a rotor ingests inhomogeneous anisotropic turbulence, without many of the complications present in most practical applications. The correlation measurements show substantial turbulence anisotropy in the body of the boundary layer which is characterized by streamwise elongated structures inclined relative to the wall. Translated to the rotor frame this anisotropy leads substantial asymmetry in blade-to-blade upwash coherence. Predictions of radiated sound spectra and directivity are performed for a range of conditions. Sound predictions reveal spectra dominated by ‘haystacking’ - distinct peaks at multiples of the blade passing frequency produced by multiple cutting of the same turbulent structures.

Predictive Limits of Acoustic Diffraction Theory for Rough Wall Flows

Rough walls can produce noise through local effects such as vortex shedding or through diffraction of the convected turbulent flowfield. It is known that diffraction dominates for hydrodynamically smooth surfaces and self-noise dominates for obstructions many times the size of the boundary layer, but as of yet, the transition between regimes has not been thoroughly investigated. In this paper, the limits of acoustic diffraction theory for rough wall flows are examined by analyzing the unsteady drag inferred from measurement of the far-field noise produced by fetches of discrete roughness elements ranging in size from h/δ=5 to 6% and from 14 to 18%. Measured unsteady drag spectra are compared to theoretically predicted spectra from diffraction theory. It is shown that the effectiveness of diffraction theory is not solely dependent on the relative turbulence and roughness scales, but on the ratio of roughness height to boundary-layer thickness as well. It is also shown that the unsteady drag spectra produce...

Aerodynamic Noise from Sparse Surface Roughness

The noise produced by fetches of 3mm hemispherical and cubic roughness in a turbulent boundary layer was studied. The wall pressure was recorded in and around 42 element fetches of the roughness. Far field measurements were made using a linear phased microphone array. The microphone array data was analyzed using a conventional beamforming method as well as a least-squares analysis technique which can separate individual source strengths in the multi-element fetches. The cubic elements produce much stronger far field pressure fluctuations than the hemispherical elements. This radiated noise is produced by the inhomogeneous wall pressure fluctuations on the surface of the roughness elements. Examination of different roughness noise directivity models confirms the presence of separate spanwise and streamwise dipoles at each roughness element location. The relative strengths of the sources for a single 3mm cubic element vary with frequency and therefore produce a directivity pattern that varies with frequency. Normalized estimated mean square unsteady drag spectra for cubic and hemispherical elements have a similar shape and magnitude to those computed using LES by Yang & Wang (2011). For wall jet nozzle velocities of 50 and 60m/s, a single cubic element’s estimated spanwise dipole is significantly weaker than the streamwise dipole at lower nondimensional frequencies. Above fh/U=2, estimated streamwise strengths are similar to the spanwise dipoles. Source strength estimations of the multi-element fetches show that the addition of trailing cuboidal elements enhance the streamwise noise produced by upstream elements. Also, the estimated spanwise dipole strength of cubic elements was enhanced by the presence of spanwise adjacent elements.

Wall-Mounted Finite Airfoil-Noise Production and Prediction

This paper presents the results of an experimental investigation of the flow-induced sound produced by a smooth, wall-mounted finite length airfoil with a flat-ended tip and a tripped turbulent boundary layer. Acoustic measurements have been taken in the Stability Wind Tunnel at Virginia Polytechnic Institute and State University with a microphone array at a range of Reynolds numbers (ReC=7.9×105−1.6×106, based on chord), angles of attack (α=0–12  deg), and for a variety of airfoil aspect ratios (airfoil length-to-chord ratio of ). Spectral data show the dominant noise sources are airfoil trailing-edge noise and tip-vortex-formation noise. Acoustic data are also used to evaluate semi-empirical prediction of wall-mounted finite airfoil trailing edge and tip noise with the so-called Brooks, Pope, and Marcolini model. The prediction method employs the Brooks, Pope, and Marcolini trailing-edge-noise model modified to incorporate spanwise variations in flow properties in combination with the Brooks, Pope, and ...

Directivity of Noise from Discrete Elements in a Turbulent Boundary Layer

Measurements were taken in the Virginia Tech Anechoic Wall Jet Facility to study the directivity of roughness noise produced by single 8mm and 3mm cubes and varying size grid patterns of 3mm cubes. Boundary layer thickness to roughness height ratios varied from 2-5.3 over the examined conditions. An arrangement of 8 far field microphones creating two arcs overtop and around the side of the roughness were used in combination with a linear microphone array of 32 sensors which was resting on the flat plate of the wall jet. Single microphone data, beamformed source images, and a least-squares method were used to characterize the noise produced by the rough surfaces. The least-squares method was used to estimate the source strengths of both streamwise and spanwise dipoles originating from the cubic roughness by minimizing the error between the measured spectra and model spectra. This method was validated for frequencies above 10kHz by comparison with single microphone measurements. The results show that the strength of the streamwise dipole emanating from a single element was found to be equal to or stronger than the spanwise dipole. Finally, the lead row of a multi-row fetch of cubic roughness elements produced the strongest spanwise dipole and the strength of the spanwise dipole decreased with each succeeding row.

Turbulence Ingestion Noise from an Open Rotor with Different Inflows

Measurements have been performed on a scaled version of a Sevik rotor ingesting both a planar turbulent wake and a turbulent boundary layer flow. In both cases, detailed measurements were made of the inflow turbulence, including three-component turbulence profiles and the full cross-sectional 4-dimensional space-time correlation function. Far-field sound measurements were also made of the turbulence ingestion noise for a comprehensive range of rotor advance ratios varying from zero to high thrust, for rotor yaw angles out of the plane of the wake from −15 to 15°, and for a range of wake strike positions on the rotor disk. Probes mounted on two of the rotor blades were used to measure upwash fluctuations seen in the rotating frame, as well as blade-to-blade coherence spectra. Comparisons have been made with predictions of the far-field sound levels based on the measured inflow turbulence for both configurations and good results were obtained in all cases.