1 

A model for flowinduced noise of Helmholtz resonatorlike cavities
This paper presents a single prediction model for the noise generated by a turbulent boundry layer flow grazing on the opening of a Helmholtzresonator like cavity. The prediction model is validated by comparison with an experimental study. The measured spectra inside the cavity are correctly predicted by the model.

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2 

Flow induced pulsations caused by corrugated tubes
Corrugated tubes can produce a tonal noise when used for gas transport, for instance in the case of flexible risers. The whistling sound is generated by shear layer instability due to the boundary layer separation at each corrugation. This whistling is examined by investigating the frequency, amplitude and onset of the pulsations generated by 2" artificially corrugated tubes and cable feeds. Special attention is given to the influence of the geometry of the corrugations and to the influence of the boundary conditions of the tubes. Two distinct modes are measured. One high mode with a typical Strouhal number Sr=0.35 and one with a Strouhal number of Sr=0.1. The relative length scale for the corrugations to be used in the Strouhal number is a modified gap width, which is the gap width excluding the downstream edge radius. The exact Strouhal number for a corrugation is furthermore dependent on details of the corrugation, as the convective velocity of the flow disturbances is influenced by details in the geometry such as edge rounding. The amplitude of the generated pulsations scales with the acoustic pressure (ρcU) and will saturate for higher flow rates (p'/ρcU=constant). The saturation level is independent of pressure and tube length and is solely dependent on the corrugation geometry. Larger cavities will generate higher amplitude pulsations. The onset of the whistling is dependent on the tube itself and the system boundaries. Only for very long tubes is the onset insensitive to the system boundaries and will the onset be determined by corrugations. In that case the onset is determined by a critical boundary layer thickness. For smaller tubes, this critical layer thickness is still relevant, but the boundary conditions will have a large effect. A system with a high reflection coefficient will start at lower gas velocities than a system with a low reflection coefficient. Based on the current results the frequency and amplitude of the pulsations can be predicted. Copyright © 2007 by ASME.

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3 

Modeling aerosols and extinction in the marine atmospheric boundary layer
An analysis is presented of aerosol particle size distributions measured over the North Atlantic and extinction coefficients derived from these data. Two empirical models, an aerosol model and an extinction model, are formulated in terms of simple meteorological parameters (wind speed, relative humidity, air temperature and sea temperature). The choice of these parameters is based on considerations of their effects on the aerosol physics in the marine atmosphere. The performance of the models for predictions of the extinction coefficients at (laser) wavelengths in the visible and IR atmospheric windows is assessed. The results are compared with predictions of the Navy Aerosol Model (NAM).

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4 

Vaneless diffuser core flow instability and rotating stall characteristics
Since rotating stall in centrifugal compressors limits the operating range more knowledge on flow dynamics of rotating stall mechanism is required. To study the vaneless diffuser rotating stall a twodimensional flow model is used in which the influence of the wall boundary layers is neglected. At the diffuser inlet rotating jetwake velocity is prescribed and at the outlet constant static pressure is assumed. Under these conditions a twodimensional rotating instability is observed, which is studied in terms of the rotating stall frequencies and pressure fluctuations. These pressure signals can be used to identify characteristics and behavior of rotating stall phenomena.

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5 

Prediction of wallpressure fluctuations using the statistical approach to turbulence induced noise (Satin)
The noise prediction model SATIN (Statistical Approach to Turbulence /nduced Noise) is presented. SATIN assumes axial symmetric turbulence with the length scale ratio λ and the Reynolds stress ratio ξ as two anisotropy parameters. The model is based on Lighthill's acoustic analogy and allows to predict both the farfield noise radiation as well as nearfield wallpressure fluctuations. In this paper, we focus on the latter because they are usually much larger in amplitude than farfield noise and therefore less sensitive to wind tunnel background noise. Experimental investigations of wallpressure fluctuations under a turbulent boundary layer flow were done by TNOTPD. During the experiments, the flow velocity, the boundary layer thickness and the friction coefficient were varied producing different types of turbulent boundary layers. The wallpressure fluctuations were measured with an array of microphones flush mounted in the wall. Predictions and measurements are compared on the basis of single microphone spectra. Input parameters of SATIN are characteristic values of the turbulent boundary layer, i. e. the boundary layer thickness and the friction coefficient or the friction velocity, respectively. These properties were extracted from measurements of the mean velocity distribution. The measured and predicted wallpressure fluctuations show good agreement. © 2001 by J. S. D. Ostertag, J. Golliard and S. Wagner.

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6 

Orifice impedance under grazing flow measured with a single microphone method
The effect of onesided grazing mean flow on the acoustical impedance of rectangular orifices is measured at low Mach number and low Helmholtz number by means of a single microphone method. The results are fairly consistent with previous experimental results obtained by means of a twomicrophone impedance tube. Furthermore no significant influence of the aperture aspect ratio and aperture wall thickness on the nondimensional scaled impedance is found, at least for the qualitative trend. Comparison with an existing theoretical model shows reasonable agreement for the resistance, provided that the experimental results are tentatively corrected for boundary layer and induced flow effects. For the reactance no agreement is found.

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7 

Influence of turbulent grazing flow on the impedance of an opening
In this paper, the impedance of a rectangular opening submitted to a turbulent grazing flow is investigated experimentally. The opening is located in a flat plate where a turbulent boundary layer flow develops. The impedance is measured with a twomicrophone measuring pipe installed below the opening. The opening has a rectangular shape of large aspect ratio. With the long side oriented perpendicular to the flow. The results, the contribution due to the flow of the acoustic resistance and length correction, are nondimensionalized and compared with equivalent values computed with a vortexsheet model.

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8 

Twodimensional rotating stall analysis in a wide vaneless diffuser
We report a numerical study on the vaneless diffuser core flow instability in centrifugal compressors. The analysis is performed for the purpose of better understanding of the rotating stall flow mechanism in radial vaneless diffusers. Since the analysis is restricted to the twodimensional core flow, the effect of the wall boundary layers is neglected. A commercial code with the standard incompressible viscous flow solver is applied to model the vaneless diffuser core flow in the plane parallel to the diffuser walls. At the diffuser inlet, rotating jetwake velocity pattern is prescribed and at the diffuser outlet constant static pressure is assumed. Under these circumstances, twodimensional rotating flow instability similar to rotating stall is found to exist. Performed parameter analysis reveals that this instability is strongly influenced by the diffuser geometry and the inlet and outlet flow conditions.

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9 

Extension and application of a scaling technique for duplication of inflight aerodynamic heat flux in ground test facilities
To enable direct experimental duplication of the inflight heat flux distribution on supersonic and hypersonic vehicles, an aerodynamic heating scaling technique has been developed. The scaling technique is based on the analytical equations for convective heat transfer for laminar and turbulent boundary layers that follow from applying the Reynolds analogy. The method was developed starting from an elementary isothermal cold flat plate at zero angle of attack and subsequent introduction of wall temperature effects, geometrical scaling effects and angle of attack effects. The present paper extends the scaling technique by introducing correction factors enabling full duplication of the laminar and turbulent boundary layer heat transfer downstream of a shock wave generated by a flat plate at angle of attack. A similar extension is developed to enable full duplication of the laminar and turbulent boundary layer heat transfer downstream of a Prandt Meyer expansion. The scaling technique is applied to a simplified vehicle configuration consisting of two convex surfaces. The results clearly show that scaled test conditions can be derived at which the inflight distribution of heat flux on either one of the two surfaces can be fully duplicated. After implementation of high temperature effects this scaling technique may allow for experimental verification of the heat flux distribution on hypersonic vehicles at supersonic test conditions. In this way, test facilities that are far less expensive to operate can provide valuable information during the initial design phase of a hypersonic vehicle.

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