Boundary-layer instability noise on aerofoils

1999 ◽  
Vol 382 ◽  
pp. 27-61 ◽  
Author(s):  
EMMA C. NASH ◽  
MARTIN V. LOWSON ◽  
ALAN McALPINE
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Separation ◽  
Noise Generation ◽  
Tonal Noise ◽  
Instability Waves ◽  
S Waves ◽  
Pressure Surface ◽  
Boundary Layer Instability ◽  
Moderate Reynolds Number ◽  
Tollmien Schlichting

An experimental and theoretical investigation has been carried out to understand the tonal noise generation mechanism on aerofoils at moderate Reynolds number. Experiments were conducted on a NACA0012 aerofoil section in a low-turbulence closed working section wind tunnel. Narrow band acoustic tones were observed up to 40 dB above background noise. The ladder structure of these tones was eliminated by modifying the tunnel to approximate to anechoic conditions. High-resolution flow velocity measurements have been made with a three-component laser-Doppler anemometer (LDA) which have revealed the presence of strongly amplified boundary-layer instabilities in a region of separated shear flow just upstream of the pressure surface trailing edge, which match the frequency of the acoustic tones. Flow visualization experiments have shown these instabilities to roll up to form a regular Kármán-type vortex street.A new mechanism for tonal noise generation has been proposed, based on the growth of Tollmien–Schlichting (T–S) instability waves strongly amplified by inflectional profiles in the separating laminar shear layer on the pressure surface of the aerofoil. The growth of fixed frequency, spatially growing boundary-layer instability waves propagating over the aerofoil pressure surface has been calculated using experimentally obtained boundary-layer characteristics. The effect of boundary-layer separation has been incorporated into the model. Frequency selection and prediction of T–S waves are in remarkably good agreement with experimental data.

scholarly journals On the receptivity of aerofoil tonal noise: an adjoint analysis

2017 ◽  
Vol 812 ◽  
pp. 771-791 ◽  
Author(s):  
Miguel Fosas de Pando ◽  
Peter J. Schmid ◽  
Denis Sipp
Keyword(s):  
Boundary Layer ◽  
Acoustic Radiation ◽  
Acoustic Scattering ◽  
Surface Boundary ◽  
Frequency Range ◽  
Tonal Noise ◽  
Surface Boundary Layer ◽  
Instability Waves ◽  
Pressure Surface ◽  
Global Modes

For moderate-to-high Reynolds numbers, aerofoils are known to produce substantial levels of acoustic radiation, known as tonal noise, which arises from a complex interplay between laminar boundary-layer instabilities, trailing-edge acoustic scattering and upstream receptivity of the boundary layers on both aerofoil surfaces. The resulting acoustic spectrum is commonly characterised by distinct equally spaced peaks encompassing the frequency range of convectively amplified instability waves in the pressure-surface boundary layer. In this work, we assess the receptivity and sensitivity of the flow by means of global stability theory and adjoint methods which are discussed in light of the spatial structure of the adjoint global modes, as well as the wavemaker region. It is found that for the frequency range corresponding to acoustic tones the direct global modes capture the growth of instability waves on the suction surface and the near wake together with acoustic radiation into the far field. Conversely, it is shown that the corresponding adjoint global modes, which capture the most receptive region in the flow to external perturbations, have compact spatial support in the pressure surface boundary layer, upstream of the separated flow region. Furthermore, we find that the relative spatial amplitude of the adjoint modes is higher for those modes whose real frequencies correspond to the acoustic peaks. Finally, analysis of the wavemaker region points at the pressure surface near 30 % of the chord as the preferred zone for the placement of actuators for flow control of tonal noise.

Acoustic receptivity and evolution of two-dimensional and oblique disturbances in a Blasius boundary layer

2001 ◽  
Vol 432 ◽  
pp. 69-90 ◽  
Author(s):  
RUDOLPH A. KING ◽  
KENNETH S. BREUER
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Acoustic Wave ◽  
Three Dimensional ◽  
Two Dimensional ◽  
Surface Waviness ◽  
S Wave ◽  
Boundary Layer Instability ◽  
Blasius Boundary Layer ◽  
Tollmien Schlichting

An experimental investigation was conducted to examine acoustic receptivity and subsequent boundary-layer instability evolution for a Blasius boundary layer formed on a flat plate in the presence of two-dimensional and oblique (three-dimensional) surface waviness. The effect of the non-localized surface roughness geometry and acoustic wave amplitude on the receptivity process was explored. The surface roughness had a well-defined wavenumber spectrum with fundamental wavenumber kw. A planar downstream-travelling acoustic wave was created to temporally excite the flow near the resonance frequency of an unstable eigenmode corresponding to kts = kw. The range of acoustic forcing levels, ε, and roughness heights, Δh, examined resulted in a linear dependence of receptivity coefficients; however, the larger values of the forcing combination εΔh resulted in subsequent nonlinear development of the Tollmien–Schlichting (T–S) wave. This study provides the first experimental evidence of a marked increase in the receptivity coefficient with increasing obliqueness of the surface waviness in excellent agreement with theory. Detuning of the two-dimensional and oblique disturbances was investigated by varying the streamwise wall-roughness wavenumber αw and measuring the T–S response. For the configuration where laminar-to-turbulent breakdown occurred, the breakdown process was found to be dominated by energy at the fundamental and harmonic frequencies, indicative of K-type breakdown.

Resonant flow in small cavities submerged in a boundary layer

1988 ◽  
Vol 420 (1859) ◽  
pp. 219-245 ◽  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Resonant Cavity ◽  
Cavity Flow ◽  
Flow Speed ◽  
Small Scale ◽  
S Waves ◽  
Grooved Surface ◽  
Tollmien Schlichting

The viscosity-dominated unsteady flow in a row of small transverse square cavities lying submerged in a turbulent boundary layer is first considered. Experiments performed primarily with one size of cavities show that the cavity flow can be excited by freestream disturbances in a narrow frequency band that is independent of the flow speed. The turbulent boundary layer in which the cavities are submerged remains transparent to the disturbances. The cavity flow resonates when the depths of the cavity and the Stokes layer are nearly the same, that is when 2π fk 2 / v = 1, where f is the frequency of the resonant cavity flow, k is the cavity height and v is the kinematic viscosity of the fluid. An associated laminar boundary-layer excitation experiment shows that the instability process over the grooved surface also involves the amplification of Tollmien–Schlichting (T–S) waves in much the same manner as in a smooth-wall Blasius profile but the grooves enhance receptivity. A theory is given proposing that the resonant groove flow in the low Reynolds number turbulent boundary layer is driven by highly amplified matched T–S waves. The possible relevance of the observed coupling between the large-scale freestream disturbances and the small-scale cavity flows to the turbulence production mechanism in a smooth flat-plate turbulent boundary layer is also discussed.

A local scattering approach for the effects of abrupt changes on boundary-layer instability and transition: a finite-Reynolds-number formulation for isolated distortions

2017 ◽  
Vol 822 ◽  
pp. 444-483 ◽  
Author(s):  
Zhangfeng Huang ◽  
Xuesong Wu
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Eigenvalue Problem ◽  
Boundary Conditions ◽  
Abrupt Change ◽  
Base Flow ◽  
Wall Suction ◽  
S Waves ◽  
Boundary Layer Instability ◽  
Abrupt Changes

We investigate the influence of abrupt changes on boundary-layer instability and transition. Such changes can take different forms including a local porous wall, suction/injection and surface roughness as well as junctions between rigid and porous walls. They may modify the boundary conditions and/or the mean flow, and their effects on transition have usually been assessed by performing stability analysis for the modified base flow and/or boundary conditions. However, such a conventional local linear stability theory (LST) becomes invalid if the change occurs over a relatively short scale comparable with, or even shorter than, the characteristic wavelength of the instability. In this case, the influence on transition is through scattering with the abrupt change acting as a local scatter, that is, an instability mode propagating through the region of abrupt change is scattered by the strong streamwise inhomogeneity to acquire a different amplitude. A local scattering approach (LSA) should be formulated instead, in which a transmission coefficient, defined as the ratio of the amplitude of the instability wave after the scatter to that before, is introduced to characterize the effect on instability and transition. In the present study, we present a finite-Reynolds-number formulation of LSA for isolated changes including a rigid plate interspersed by a local porous panel and a wall suction through a narrow slot. When the weak non-parallelism of the unperturbed base flow is ignored, the local scattering problem can be cast as an eigenvalue problem, in which the transmission coefficient appears as the eigenvalue. We also improved the method to take into account the non-parallelism of the unperturbed base flow, where it is found that the weak non-parallelism has a rather minor effect. The general formulation is specialized to two-dimensional Tollmien–Schlichting (T–S) waves. The resulting eigenvalue problem is solved, and full direct numerical simulations (DNS) are performed to verify some of the predictions by LSA. A parametric study indicates that conventional LST is valid only when the change is sufficiently gradual, and becomes either inaccurate or invalid when the scale of the local distortion is short. A local porous panel enhances T–S waves, while a local suction with a moderate mass flux significantly inhibits T–S waves. In the latter case, a comprehensive comparison is made between the theoretical predictions and experimental data, and a satisfactory quantitative agreement was observed.

Regimes of tonal noise on an airfoil at moderate Reynolds number

2015 ◽  
Vol 780 ◽  
pp. 407-438 ◽  
Author(s):  
S. Pröbsting ◽  
F. Scarano ◽  
S. C. Morris
Keyword(s):  
Reynolds Number ◽  
Angle Of Attack ◽  
Suction Side ◽  
Small Scale ◽  
Noise Generation ◽  
Tonal Noise ◽  
Flow Topology ◽  
Trailing Edge ◽  
Moderate Reynolds Number ◽  
Two Sides

Tonal noise generated by airfoils at low to moderate Reynolds number is relevant for applications in, for example, small-scale wind turbines, fans and unmanned aerial vehicles. Coherent and convected vortical structures scattering at the trailing edge from the pressure or suction sides of the airfoil have been identified to be responsible for such tonal noise generation. Controversy remains on the respective significance of pressure- and suction-side events, along with their interaction for tonal noise generation. The present study surveys the regimes of tonal noise generation for low to moderate chord-based Reynolds number between $\mathit{Re}_{c}=0.3\times 10^{5}$ and $2.3\times 10^{5}$ and effective angle of attack between $0^{\circ }$ and $6.3^{\circ }$ for the NACA 0012 airfoil profile. Extensive acoustic measurements with smooth surface and with transition to turbulence forced by boundary layer tripping are presented. Results show that, at non-zero angle of attack, tonal noise generation is dominated by suction-side events at low Reynolds number and by pressure-side events at high Reynolds number. At smaller angle of attack, interaction between events on the two sides becomes increasingly important. Particle image velocimetry measurements complete the information on the flow field structure in the source region around the trailing edge. The influences of both angle of attack and Reynolds number on tonal noise generation are explained by changes in the mean flow topology, namely the presence and location of reverse flow regions on the two sides. Data gathered from experimental and numerical studies in the literature are reviewed and interpreted in view of the different regimes.

scholarly journals On the Asymptotic Approach to Thermosolutal Convection in Heated Slow Reactive Boundary Layer Flows

2008 ◽  
Vol 2008 ◽  
pp. 1-15
Author(s):  
Stanford Shateyi ◽  
Precious Sibanda ◽  
Sandile S. Motsa
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Flow ◽  
Chemical Species ◽  
Reynolds Numbers ◽  
Thermosolutal Convection ◽  
Boundary Layer Flows ◽  
Instability Waves ◽  
Tollmien Schlichting ◽  
Layer Flow ◽  
Negative Buoyancy

The study sought to investigate thermosolutal convection and stability of two dimensional disturbances imposed on a heated boundary layer flow over a semi-infinite horizontal plate composed of a chemical species using a self-consistent asymptotic method. The chemical species reacts as it diffuses into the nearby fluid causing density stratification and inducing a buoyancy force. The existence of significant temperature gradients near the plate surface results in additional buoyancy and decrease in viscosity. We derive the linear neutral results by analyzing asymptotically the multideck structure of the perturbed flow in the limit of large Reynolds numbers. The study shows that for small Damkohler numbers, increasing buoyancy has a destabilizing effect on the upper branch Tollmien-Schlichting (TS) instability waves. Similarly, increasing the Damkohler numbers (which corresponds to increasing the reaction rate) has a destabilizing effect on the TS wave modes. However, for small Damkohler numbers, negative buoyancy stabilizes the boundary layer flow.

On the Generation of Instability Tollmien-Schlichting Waves by Free-Stream Turbulence

2017 ◽  
Vol 9 (2) ◽  
pp. 429-438
Author(s):  
Luyu Shen ◽  
Changgen Lu
Keyword(s):  
Boundary Layer ◽  
Acoustic Waves ◽  
Free Stream ◽  
Leading Edge ◽  
S Wave ◽  
S Waves ◽  
Free Stream Turbulence ◽  
Temporal Mode ◽  
Tollmien Schlichting ◽  
Experimental Researches

AbstractThe beginning of the transition from the laminar to a turbulent flow is usually the generation of instability Tollmien-Schlichting (T-S) waves in the boundary layer. Previously, most numerical and experimental researches focused on generating instability T-S waves through the external disturbances such as acoustic waves and vortical disturbances interacting with wall roughness or at the leading-edge of flatplate, whereas only a few paid attention to the excitation of the T-S waves directly by free-stream turbulence (FST). In this study, the generating mechanism of the temporal mode T-S waves under free-stream turbulence is investigated by using direct numerical simulation (DNS) and fast Fourier transform. Wave packets superposed by a group of stability, neutral and instability T-S waves are discovered in the boundary layer. In addition, the relation between the amplitude of the imposed free-stream turbulence and the amplitude of the excited T-S wave is also obtained.

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