Experimental studies of flow noise around a surfacing device

The wall pressure fluctuations in turbulent boundary layers play an important role in acoustic measurements carried out in moving media. Results of measuring the frequency spectra of wall pressure fluctuations around a surfacing device are presented. The spatial resolution achieved in measuring the wall pressure fluctuations is investigated. It is demonstrated that the results of hydrodynamic flow noise measurements strongly depend on the aperture size of the measuring acoustic transducer and its orientation in the turbulent boundary layer. The pseudosound pressure fluctuation spectra observed in a series of experiments with surfacing devices show that the resolution of the pressure receivers operating in the turbulent boundary layers considerably varies. On the basis of systematic measurements of wall pressure fluctuations by miniature and distributed receivers at high Reynolds numbers, the effect of the geometric dimensions of a pressure receiver on its resolution in the flow noise measurements is studied. An experimental method is proposed for estimating the receiver-induced distortions.     

Experimental simulation of hydrodynamic flow noises in an autonomous marine laboratory

The spectral characteristics of turbulent pressure pulsations arising from the flow around an autonomous marine laboratory have been measured. The autonomous marine laboratory is an upgraded Dolphin buoyant device, equipped with modern digital facilities for autonomous recording, which extends the frequency range of the signals under study up to 50–60 kHz. The power spectra and cross spectra of turbulent pressure pulsations have been measured at different points of the measuring section by point pressure transducers 1.3 or 20 mm in diameter at a speed of the autonomous marine laboratory of 8–22 m/s. The measurements have revealed unexpected behavioral features of the spectra (their inflection points) at high frequencies. An analysis is performed to explain these features. In particular, it is shown that the correction function based on the more complex cross spectrum model (in comparison with the Corcos cross spectrum model) developed by Smol’yakov in 2006 predicts a monotonic (without inflection points) decrease in the spectral levels at high frequencies for reconstructed (corrected) spectra.     

Wind noise measured at the ground surface

Measurements of the wind noise measured at the ground surface outdoors are analyzed using the mirror flow model of anisotropic turbulence by Kraichnan [J. Acoust. Soc. Am. 28(3), 378-390 (1956)]. Predictions of the resulting behavior of the turbulence spectrum with height are developed, as well as predictions of the turbulence-shear interaction pressure at the surface for different wind velocity profiles and microphone mounting geometries are developed. The theoretical results of the behavior of the velocity spectra with height are compared to measurements to demonstrate the applicability of the mirror flow model to outdoor turbulence. The use of a logarithmic wind velocity profile for analysis is tested using meteorological models for wind velocity profiles under different stability conditions. Next, calculations of the turbulence-shear interaction pressure are compared to flush microphone measurements at the surface and microphone measurements with a foam covering flush with the surface. The measurements underneath the thin layers of foam agree closely with the predictions, indicating that the turbulence-shear interaction pressure is the dominant source of wind noise at the surface. The flush microphones measurements are intermittently larger than the predictions which may indicate other contributions not accounted for by the turbulence-shear interaction pressure.     

Low frequency wind noise contributions in measurement microphones

In a previous paper [R. Raspet, et al., J. Acoust. Soc. Am. 119, 834-843 (2006)], a method was introduced to predict upper and lower bounds for wind noise measured in spherical wind-screens from the measured incident velocity spectra. That paper was restricted in that the predictions were only valid within the inertial range of the incident turbulence, and the data were from a measurement not specifically designed to test the predictions. This paper extends the previous predictions into the source region of the atmospheric wind turbulence, and compares the predictions to measurements made with a large range of wind-screen sizes. Predictions for the turbulence-turbulence interaction pressure spectrum as well as the stagnation pressure fluctuation spectrum are calculated from a form fit to the velocity fluctuation spectrum. While the predictions for turbulence-turbulence interaction agree well with measurements made within large (1.0 m) wind-screens, and the stagnation pressure predictions agree well with unscreened gridded microphone measurements, the mean shear-turbulence interaction spectra do not consistently appear in measurements.     

Wave number-frequency spectrum of turbulent pressure fluctuations: Methods of measurement and results

The methods proposed earlier for measuring the wave number-frequency spectrum of wall pressure fluctuations beneath a turbulent boundary layer are considered: the spatial filtering of the pressure field components by special-purpose transducers (wave filters) and the digital processing of signals obtained from an array of transducers. It is shown that, for the wave filters, transducers with a rectangular shape of sensitive surface rather than those with a circular one are necessary. Results of measuring the wave number-frequency spectrum of turbulent pressure fluctuations in a low-noise wind tunnel are presented. The measurements are performed with the use of four wave filters consisting of rectangular transducers with a constant sensitivity distribution over their surfaces. The mathematical model of the wave number-frequency spectrum proposed earlier by the authors is compared with the measurement data reported by Abraham and Keith. The model is used for processing the results of measurements in the wind tunnel. The measured spectra are compared with the data obtained by Martin and Leehey.     

Turbulent pressure fluctuations in a long cylinder’s boundary layer in the longitudinal flow around it

Frequency spectra of the turbulent pressure’s pulsations measurements have been performed in the low-noise wind tunnel on the extended cylinder’s surface during the longitudinal flow around it. The measurement results for frequency spectra caused by the integral contribution of all the pressure field’s modes as well as only the zero (ring) mode are presented. The obtained results are compared with other known experimental data. The longitudinal cross-spectra of the pressure field for the zero mode have been measured. With the help of Fourier transformation for the cross-spectra the analytical expressions for dimensional and dimensionless wavenumber-frequency spectrum of the zero mode have been obtained. The frequency-wave’s spectrum features considering large wavenumbers are discussed.     

Improved prediction of the turbulence-shear contribution to wind noise pressure spectra

In previous research [Raspet et al., J. Acoust. Soc. Am. 123(3), 1260-1269 (2008)], predictions of the low frequency turbulence-turbulence and turbulence-mean shear interaction pressure spectra measured by a large wind screen were developed and compared to the spectra measured using large spherical wind screens in the flow. The predictions and measurements agreed well except at very low frequencies where the turbulence-mean shear contribution dominated the turbulence-turbulence interaction pressure. In this region the predicted turbulence-mean shear interaction pressure did not show consistent agreement with microphone measurements. The predicted levels were often much larger than the measured results. This paper applies methods developed to predict the turbulence-shear interaction pressure measured at the ground [Yu et al., J. Acoust. Soc. Am. 129(2), 622-632 (2011)] to improve the prediction of the turbulence-shear interaction pressure above the ground surface by incorporating a realistic wind velocity profile and realistic turbulence anisotropy. The revised prediction of the turbulence-shear interaction pressure spectra compares favorably with wind-screen microphone measurements in large wind screens at low frequency.     

The application of single-pulse CARS for temperature measurements in a turbulent stagnation flame

N₂ Q-branch CARS spectra have been recorded and evaluated for temperature determination in a turbulent, premixed CH₄/air stagnation flame with a burner of 40 mm diameter and 22 kW thermal load. Temperature histograms on the flame axis at different distances from the stagnation plate have been measured. Problems of practical applicability are addressed, including those arising from the limited spatial resolution of the BOXCARS geometry, from an insufficient dynamic range of the diode array detector, and from a memory effect of the detector in the case of measurements in highly turbulent flame areas with strong intermittency. Some information is given on the computerized acquisition and on the evaluation of the large amounts of data that are necessary for extensive investigations in large combustion systems.     

Experimental and numerical investigation of flow separation in a supersonic nozzle

The flow in a two-dimensional symmetric convergent-divergent supersonic nozzle with and without a deflector at the exit plane was tested experimentally and numerically. Combined optoelectronic devices were used to demonstrate the possibilities of the shadow and schlieren methods and holographic interferometry for the flow visualization especially in the separation region. A series of experiments was performed in a trisonic wind tunnel (VTI, Beograd). The experimental results obtained using optical methods are compared with the pressure measurements and numerical calculations. Numerical results were obtained using a code for solving the average Navier-Stokes equations. Five structured meshes with different resolutions and two turbulent models were employed. The coupled influence of the mesh resolution and the order of accuracy of the used numerical scheme is analyzed.     

Sound generated in the vicinity of the trailing edge of an upper surface blown flap

Far field noise data indicated that for practical upper surface blown flap configurations, the noise radiated below the flap is dominated by the noise generated in the vicinity of the trailing edge. The sound field caused by turbulent mixing in the trailing edge wake is investigated experimentally and theoretically. Hot wire measurements were made downstream of the trailing edge to determine the gross turbulent mixing characteristics of the flow. This information is used as input to a theoretical analysis of the sound field. Favorable agreement is found between predicted and measured far field noise directivity at various frequencies and noise power spectra in various directions.     

Turbulent statistical characteristics associated to the north wind phenomenon in southern Brazil with application to turbulent diffusion

A parameterization for the transport processes in a shear driven planetary boundary layer (PBL) has been established employing turbulent statistical quantities measured during the north wind phenomenon in southern Brazil. Therefore, observed one-dimensional turbulent energy spectra are compared with a spectral model based on the Kolmogorov arguments. The good agreement obtained from this comparison leads to well defined formulations for the turbulent velocity variance, local decorrelation time scale and eddy diffusivity. Furthermore, for vertical regions in which the wind shear forcing is relevant, the eddy diffusivity derived from the north wind data presents a similar profile to those obtained from the non-extensive statistical mechanics theory. Finally, a validation for the present parameterization has been accomplished, using a Lagrangian stochastic dispersion model. The Prairie Grass data set, which presents high mean wind speed, is simulated. The analysis developed in this study shows that the turbulence parameterization constructed from wind data for north wind flow cases is able to describe the diffusion in a high wind speed, shear-dominated PBL.     

Compressible turbulent flame speeds of highly turbulent standing flames

In this paper we present the first measurement of turbulent burning velocities of a highly turbulent compressible standing flame induced by shock-driven turbulence in a Turbulent Shock Tube. High-speed schlieren, chemiluminescence, PIV, and dynamic pressure measurements are made to quantify flame–turbulence interaction for high levels of turbulence at elevated temperatures and pressure. Distributions of turbulent velocities, vorticity and turbulent strain are provided for regions ahead and behind the standing flame. The turbulent flame speed is directly measured for the high-Mach standing turbulent flame. From measurements of the flame turbulent speed and turbulent Mach number, transition into a non-linear compressibility regime at turbulent Mach numbers above 0.4 is confirmed, and a possible mechanism for flame generated turbulence and deflagration-to-detonation transition is established.     

Density Fluctuation Spectrum of Solar Wind Turbulence between Ion and Electron Scales

We present a measurement of the spectral index of density fluctuations between ion and electron scales in solar wind turbulence using the EFI instrument on the ARTEMIS spacecraft. The mean spectral index at 1?AU was found to be -2.75±0.06, steeper than predictions for pure whistler or kinetic Alfvén wave turbulence but consistent with previous magnetic field measurements. The steep spectra are also consistent with expectations of increased intermittency or damping of some of the turbulent energy over this range of scales. Neither the spectral index nor the flattening of the density spectra before ion scales were found to depend on the proximity to the pressure anisotropy instability thresholds, suggesting that they are features inherent to the turbulent cascade.     

A new model for the cross spectrum and wavenumber-frequency spectrum of turbulent pressure fluctuations in a boundary layer

A model is developed for the statistical characteristics of the turbulent pressure field in a boundary layer. It is shown that the coherence scales are limited at low frequencies by the finiteness of the boundary layer thickness and at high frequencies by the effect of viscous forces acting in the flow. A relationship between the behavior of the coherence scales and that of the power spectra is demonstrated. The model is characterized by convertibility, which allows changes from cross spectra to wavenumber-frequency spectra and back via Fourier transformations. Calculations performed using the proposed model agree well with experimental results.     

Experimental measurement of wall shear stress in strongly disrupted flows

Mean and fluctuating wall shear stress is measured in strongly disrupted cases generated by various low-porosity wall-mounted single- and multi-scale fences. These grids generate a highly turbulent wake which interacts with the wall-bounded flow modifying the wall shear stress properties. Measurement methods are validated first against a naturally growing zero pressure gradient turbulent boundary layer showing accuracies of 1% and 4% for extrapolation and direct measurement of the mean shear stress respectively. Uncertainty associated with the root mean square level of the fluctuations is better than 2% making it possible to measure small variations originating from the different fences. Additionally, probability density functions and spectra are also measured providing further insight into the flow physics. Measurement of shear stress in the disrupted cases (grid+TBL) suggest that the flow characteristics and turbulence mechanisms remain unaltered far from the grid even in the most disrupted cases. However, a different root mean square level of the fluctuations is found for different grids. Study of the probability density functions seem to imply that there are different degrees of interaction between the inner and outer regions of the flow.     

Measured wavenumber: frequency spectrum associated with acoustic and aerodynamic wall pressure fluctuations

Direct measurements of the wavenumber-frequency spectrum of wall pressure fluctuations beneath a turbulent plane channel flow have been performed in an anechoic wind tunnel. A rotative array has been designed that allows the measurement of a complete map, 63×63 measuring points, of cross-power spectral densities over a large area. An original post-processing has been developed to separate the acoustic and the aerodynamic exciting loadings by transforming space-frequency data into wavenumber-frequency spectra. The acoustic part has also been estimated from a simple Corcos-like model including the contribution of a diffuse sound field. The measured acoustic contribution to the surface pressure fluctuations is 5% of the measured aerodynamic surface pressure fluctuations for a velocity and boundary layer thickness relevant for automotive interior noise applications. This shows that for aerodynamically induced car interior noise, both contributions to the surface pressure fluctuations on car windows have to be taken into account.     

On the vertical large eddy breakup device capability to decrease the turbulent drag

The possibility of decreasing the turbulent friction with the use of streamwise-aligned vertical large eddy breakup devices installed normal to the surface of a flat plate in an incompressible equilibrium turbulent boundary layer with a nominally gradientless flow past this plate is studied experimentally. The Reynolds number based on the boundary-layer momentum thickness is 1099 in the cross section where these vertical devices are mounted. It is shown that elements of this geometry are effective tools for modification of the turbulent boundary layer from the viewpoint of both the gain in friction drag and the suppression of turbulent fluctuations of velocity and, hence, can be considered as one of the most important factors of controlling the structure of the near-wall turbulence in the flow past a flat surface.     

层/湍流等离子体射流波动特性实验研究

本文应用电压传感器、光电倍增管及水冷皮托管,对产生射入空气中的纯氩层流和湍流等离子体射流的弧电压波动、发生器出口处的射流光强波动以及沿射流轴线的滞止压力波动进行了测量。结果显示层流等离子体射流各参数的波动幅度远小于湍流射流的对应值;弧电压的波动幅度随气流量的变化明显,但随电流的变化很小;弧电压的波动幅度与其平均值之比随电流增加呈下降的趋势。     

On the sensitivity of sound power radiated by aircraft panels to turbulent boundary layer parameters

The objective of the present study is to investigate and quantify how sensitive the response of an aircraft panel is to the change of the turbulent flow parameters. Several empirical models currently exist that provide the turbulent boundary layer wall pressure cross spectrum. These wall pressure cross spectrum models are usually dependent on four parameters: the reference power spectrum, the flow convective velocity, and the coherence lengths in streamwise and spanwise directions. All the proposed models provide different predictions for the wall pressure cross spectrum. Also, real flow conditions over aircraft do not conform to the ideal behavior of the turbulent boundary layer pressure predicted by the models. In this context, the questions that this work aims to explore are “What is the impact of different wall pressure estimates in the radiated sound power?” and “What is the effect of the range of possible flow conditions on the radiated sound power?”. For that objective, data from flight tests and estimates provided by the empirical models are used to predict radiated sound power, and the results are compared. A sensitivity analysis is performed and the relative contribution of each boundary layer parameter to the radiated sound power is obtained.