Michel Roger

Broadband Self-Noise from Loaded Fan Blades

An experimental investigation and analytical modeling were conducted of the broadband self-noise radiated by an industrial cambered airfoil embedded in an homogeneous flow at low Mach number. The instrumented airfoil is placed at the exit of an open jet anechoic wind tunnel. Sound is measured in the far field at the same time as the statistical properties of the wall pressure fluctuations close to the trailing edge. Three different flows with different statistical behaviors are investigated by changing the angle of attack, namely, the turbulent boundary layer initiated by a leading-edge separation, the nearly separated boundary layer with vortex shedding at the trailing edge, and the laminar boundary layer with Tollmien‐Schlichting waves. The far-field spectrum is related to the spectrum and spanwise correlation length of the wall pressure fluctuations. Simple statistical models based on Howe’s theory and on an extension of the original Amiet’s theory show a good agreement with the experimental results. They provide helpful tools to predict the self-noise from subsonic fans in an industrial context.

Analysis of flow conditions in freejet experiments for studying airfoil self-noise

A new set of mean wall pressure data has been collected on a controlled diffusion airfoil at a chord Reynolds number of 1.2 £ £105 in a freejet anechoic wind tunnel. Comparisons of the experimental data with Reynoldsaveraged Navier‐ Stokes (RANS) simulationsin freeairshow signie cant e owe eld and pressure loading differences, indicatingsubstantialjetinterferenceeffects.Toanalyzetheseeffects,asystematicRANS-basedcomputationale uid dynamicsstudyoftheexperimentale owconditionshasbeencarriedout,whichquantie esthestrongine uenceofthe e nite jet (nozzle) width on the aerodynamic loading and e ow characteristics. When the jet width is not sufe ciently large compared to the frontal wetted area of the airfoil, the airfoil pressure distribution is found to be closer to the distribution on a cascade than that of an isolated proe le. The airfoil lift is signie cantly reduced. Accounting for the actual wind-tunnel setup recovers the wall pressure distribution on the airfoil without further empirical angle-of-attack corrections. These jet interference effects could be responsible for the discrepancies among some earlier experimental and computational studies of airfoil self-noise. They should be accounted for in future noise computations to ensure that the experimental e ow conditions are simulated accurately.

Competing Broadband Noise Mechanisms in Low-Speed Axial Fans

This paper compares two broadband noise mechanisms, the trailing-edge noise or self-noise, and the leading-edge noise or turbulence-ingestion noise, in several blade technologies. Two previously developed analytical models for these broadband contributions are first validated with well-defined measurements on several airfoils embedded in an homogeneous flow at low-Mach number. Each instrumented airfoil is placed at the exit of an open jet anechoic wind tunnel with or without a grid generating turbulence upstream of it. Sound is measured in the far field at the same time as the wall-pressure fluctuations statistics close to the airfoil trailing edge and the inlet velocity fluctuation statistics impacting the airfoil leading edge. The models are then compared in some practical cases representative of airframes, wind turbines, and automotive engine cooling modules. The airfoil models of the two mechanisms are then extended to a full rotating machine in open space. The model predictions of both mechanisms are compared with in-flight helicopter measurements and automotive engine cooling modules measurements. In both instances, the turbulence-ingestion noise is found to be a dominant source over most of the frequency range. The self-noise only becomes a significant contributor at high angles of attack close to flow separation.

Effect of Airfoil Aerodynamic Loading on Trailing Edge Noise Sources

A previous experimental investigation of the broadband self noise radiated by an industrial cambered controlled-diffusion airfoil embedded in an homogeneous flow at low Mach number has been extended to various aerodynamic loadings. The instrumented airfoil is placed at the exit of an open-jet anechoic wind tunnel, with a jet width of about four chord lengths. Sound is measured in the far field at the same time as the statistical properties of the wall-pressure fluctuations close to the trailing edge. A new set of mean wall-pressure data has been collected on this airfoil at a chord Reynolds number of 2.9 x 105, which provides some insight on the Reynolds-number effect. Two previously investigated flow regimes with different statistical behaviors are investigated by changing the angle of attack from 8 to 15 deg. They respectively correspond to the nearly separated boundary layer with vortex shedding at the trailing edge and to the turbulent boundary layer initiated by a leading-edge separation.

Extensions and limitations of analytical airfoil broadband noise models

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 Schwarzschilds 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.

LES prediction of wall-pressure fluctuations and noise of a low-speed airfoil

This paper discusses the prediction of wall-pressure fluctuations and noise of a low-speed flow past a thin cambered airfoil using large-eddy simulation (LES). The results are compared with experimental measurements made in an open-jet anechoic wind-tunnel at Ecole Centrale de Lyon. To account for the effect of the jet on airfoil loading, a Reynolds-averaged Navier-Stokes calculation is first conducted in the full wind-tunnel configuration, and the mean velocities from this calculation are used to define the boundary conditions for the LES in a smaller domain within the potential core of the jet. The LES flow field is characterized by an attached laminar boundary layer on the pressure side of the airfoil and a transitional and turbulent boundary layer on the suction side, in agreement with experimental observations. An analysis of the unsteady surface pressure field shows reasonable agreement with the experiment in terms of frequency spectra and spanwise coherence in the trailing-edge region. In the nose region, characterized by unsteady separation and transition to turbulence, the wall-pressure fluctuations are highly sensitive to small perturbations and thus diffcult to predict or measure with certainty. The LES, in combination with the Ffowcs Williams and Hall solution to the Lighthill equation, also predicts well the radiated trailing-edge noise. A finite-chord correction is derived and applied to the noise prediction, which is shown to improve the overall agreement with the experimental sound spectrum.

Prediction of Rod-Airfoil Interaction Noise Using the Ffowcs-Williams-Hawkings Analogy

SoundgeneratedatlowMachnumberbyanairfoilin thewakeofarodisinvestigatednumerically.TheGaussian spanwise loss of coherence of the vortex shedding is shown to have a signie cant ine uence on the broadband noise. Spanwise effects are successfully introduced into a time-domain formulation of the Ffowcs-Williams ‐Hawkings analogy, which is applied to aerodynamicdata computed on various contoursaround the sourceregion. Itisshown that a careful choice of these contours is required. The e owe eld is obtained from a two-dimensional Reynolds averaged Navier ‐Stokes calculation. Computed far-e eld spectra compare very well to measurements obtained in an accompanying experiment. Nomenclature Cp = pressure coefe cient c = airfoil chord d = rod diameter f = frequency g = integration surface, 0 k = turbulent kinetic energy Lg = Gaussian correlation length l = span length Mi = Mach number of g D0 Moi = observer Mach number vector Mref = reference Mach number M1 = ine ow Mach number

AERODYNAMIC NOISE OF A TWO-DIMENSIONAL WING WITH HIGH-LIFT DEVICES.

This paper concerns the experimental characterization of the aerodynamic noise of a two-dimensional 1/11 scale wing, supplied with a leading-edge slat and a trailing-edge flap. Three configurations are depicted, takeoff, cruise and landing. The experimental results are analyzed for the noisiest configuration, that is landing. Two noise sources, localized in the slat-wing slot and the wing-flap slot, are displayed. Sound is preferentially radiated from the wing pressure side. Moreover radiation from the wing-flap slot and from the flap trailing-edge are shown to interfer in far field. In parallel to the experimental study, an attempt is made to represent the wingflap noise radiation with the help of existing analytical theories.

Rotating Blade Trailing-Edge Noise: Experimental Validation of Analytical Model

This paper deals with the experimental validation of an analytical trailing-edge noise model dedicated to low-speed fans operating in free field. The model is intrinsically related to the aerodynamics of the blades and should lead to a useful fast-running tool to be included in a blade-design process in an industrial context. The investigations are made on a two-bladed low-speed axial fan without shroud, installed inside an anechoic room. The blades are instrumented with two sets of embedded small-size microphones (2.5 mm diam), and the wall-pressure signals are acquired via a slip ring mounted on the fan axis. The chord-based Reynolds number is about 200,000, and the tip Mach number about 0.07. The data base is completed by far-field measurements made with a single microphone on a moving support. The analytical model is based on a previously published extension of Amiets trailing-edge noise theory. A blade is split into several strips in the spanwise direction, and the model is applied to each strip. For this the input data are interpolated from the measurements performed with the aforementioned sets of microphones. The trailing-edge noise model is more reliable for observer positions within ±30° from the fan-rotation plane.

Broadband Noise Reduction With Trailing Edge Brushes

Airfoil broadband trailing edge noise is reduced by modification of the trailing edge geometry. A brush made of a single row of flexible polypropylene fibers is integrated in the trailing edge of a cambered airfoil. Far field acoustic measurements show a noise reduction potential reaching 3 dB on a wide frequency range. Due to high curvature of the incident flow, a secondary acoustic source partly masks the trailing edge noise reduction. Hot wire correlation measurements in the very near wake of the airfoil show that longitudinal as well as transversal length scales are affected by the brush. Span wise coherence length of boundary layer eddies falls off by 25 % in the presence of a brush in the adequate frequency range, possibly explaining a 1.3 dB contribution to the noise reduction mechanism. Boundary layer turbulence exhibits a preferred coherence length l y v on a wide frequency range. l y v /d ≈ 2, is considered a proper brush design law, d being the diameter of the brush.