Aerodynamic Pressure Fluctuations Research Articles

Generation of acoustic tones in round jets at a Mach number of 0.9 impinging on a plate with and without a hole

The generation of acoustic tones in four round jets at a Mach number of 0.9 impinging on a plate at a distance $L=6r_0$ from the nozzle exit, where $r_0$ is the nozzle radius, has been investigated by large-eddy simulation. Three plates are perforated by holes of diameters $h=2r_0$ , $3r_0$ and $4.4r_0$ , centred on the jet axis, whereas the fourth plate has no hole, in order to study the effects of the hole on the jet flow and acoustic fields. In all cases, acoustic tones emerge in the jet near field, upstream but also downstream of the plate for the perforated plates. Their frequencies are similar for all jets and close to those predicted for aeroacoustic feedback loops establishing between the nozzle and the plate, involving flow disturbances convected downstream and waves propagating upstream at the ambient speed of sound. Their levels, however, decrease with the hole diameter, by a few dB for $h\leqslant 3r_0$ but approximately by 40 dB for $h=4.4r_0$ . The features of the feedback loops are identified by computing two-dimensional space–time correlations and frequency–wavenumber spectra of the pressure fluctuations in the jet mixing layers. These loops are found to be closed by free-stream upstream-propagating guided jet waves, produced by the impingement of vortical structures on the plate for $h\leqslant 3r_0$ and by the scattering of the jet aerodynamic pressure fluctuations by the hole edges for $h=4.4r_0$ . Finally, an acoustic analogy is used to show that the contributions of the pressure fluctuations on the plate to the radiated noise are dominant.

Identification of aerodynamic pressure fluctuations generated from high-speed trains passing an open section

Identification of aerodynamic pressure fluctuations generated from high-speed trains passing an open section

Особенности применения интенсиметрии для определения мощности акустического излучения панели в поле аэродинамических пульсаций давления

Особенности применения интенсиметрии для определения мощности акустического излучения панели в поле аэродинамических пульсаций давления

Aerodynamic Pressure Fluctuations Associated With Flow-Induced Vibration of the Head Gimbals Assembly Inside a Hard Disk Drive

This paper presents an experimental investigation and numerical simulation on the spectrum-spatial characteristics of aerodynamic pressure fluctuations around the head gimbals assembly (HGA) in a working hard disk drive (HDD). The pressure fluctuations are measured through tiny holes on the HDD top cover above the HGA. The positioning error signal and servo system sensitivity in the working HDD are measured as well to evaluate the slider off-track vibration. Comparisons between the spectra of pressure fluctuations and the slider off-track vibration show that, the pressure fluctuations around the HGA are highly associated with the slider off-track vibration in terms of principal peaks in the spectra, especially at 1.83 kHz and 2.54 kHz. The results also show that the spatial coherence of pressure fluctuation around the HGA remains high at frequencies around the principal peaks in the spectra, which has been further confirmed by the numerical simulation based on a two-dimensional large eddy simulation model. It is concluded that the flow-induced HGA vibrations in an HDD can be detected and evaluated through the pressure fluctuations by a sensor around the HGA.

The Sound of Music and Voices in Space Part 1: Theory

While probes to other planets have carried an impressive array of sensors for imaging and chemical analysis, no probe has ever listened to the soundscape of an alien world. With a small number of exceptions, planetary science missions have been deaf. The most successful acoustic measurements were made by the European Space Agency's 2005 Huygens probe to Titan, but although this probe was spectacularly successful in measuring the atmospheric sound speed and estimating the range to the ground using an acoustic signal that the probe itself emitted, we still have no measurements of sounds generated by alien worlds. Although microphones have been built for Mars, the Mars Polar Lander was lost during descent on 3 December 1999, and the Phoenix probe microphone was not activated (because the Mars Descent Imager system to which it belonged was deactivated for fear of tripping a critical landing system). Instead of measuring acoustic signals that had propagated to the microphone from a distance, aerodynamic pressure fluctuations on the microphone (caused by wind on the surface of Venus in the case of the 1982 Russian Venera 13 and 14 probes, and turbulence during the parachute descent in the case of Huygens) masked the soundscape on these Venus and Titan missions. Given the lack of such data from these earlier missions, some early enthusiasts for acoustics in the space community are now skeptical as to whether it will ever have a useful role. However basing such an assessment on past performance presupposes that the sensor systems have been optimized for the environment in question.

Fluctuations under turbulent flows: Enhanced methods for separation of propagative wavenumbers from wall pressure dataset

This work is a part of a more general study on automobile interior noise due to acoustic and aerodynamic wall-pressure fluctuations. Using experimental data of wall-pressure fluctuations measured with a microphone array beneath several kinds of flows, a wave-number analysis based on recently developed signal processing methods -- the spatial Empirical Mode Decomposition (sEMD) and the Ensemble Empirical Mode Decomposition (E-EMD) -- is carried out, in order to separate acoustic and aerodynamic pressure fluctuations. In opposition with an existing classical method previously used, based on the spatial correlogram, these methods do not require a stationary uniform flow. A turbulent boundary layer on a flat plate and a detached/reattached flow downstream three different forward-facing steps are tested, with flow velocities from 0 to 40 m.s-1, with or without a well-controlled artificial acoustic source. The sEMD method is first used as a wavenumber filter, and is shown to improve the detection of acoustic fluctuations of about 5 dB on classical wavenumber (k,f) representations. The E-EMD method is developed in order to decompose the (x,t) representation to make possible the separation of the acoustic and aerodynamic components, regardless of the stationarity or the uniformity of the flow.

Tillage and wind effects on soil CO 2 concentrations in muck soils

Rising atmospheric carbon dioxide (CO 2) concentrations from agricultural activities prompted the need to quantify greenhouse gas emissions to better understand carbon (C) cycling and its role in environmental quality. The specific objective of this work was to determine the effect of no-tillage, deep plowing and wind speeds on the soil CO 2 concentration in muck (organic) soils of the Florida Everglades. Miniature infrared gas analyzers were installed at 30 cm and recorded every 15 min in muck soil plowed with the Harrell Switch Plow (HSP) to 41 cm and in soil Not Tilled (NT), i.e., not plowed in last 9 months. The soil CO 2 concentration exhibited temporal dynamics independent of barometric pressure fluctuations. Loosening the soil resulted in a very rapid decline in CO 2 concentration as a result of “wind-induced” gas exchange from the soil surface. Higher wind speeds during mid-day resulted in a more rapid loss of CO 2 from the HSP than from the NT plots. The subtle trend in the NT plots was similar, but lower in magnitude. Tillage-induced change in soil air porosity enabled wind speed to affect the gas exchange and soil CO 2 concentration at 30 cm, literally drawing the CO 2 out of the soil resulting in a rapid decline in the CO 2 concentration, indicating more rapid soil carbon loss with tillage. At the end of the study, CO 2 concentrations in the NT plots averaged about 3.3% while that in the plowed plots was about 1.4%. Wind and associated aerodynamic pressure fluctuations affect gas exchange from soils, especially tilled muck soils with low bulk densities and high soil air porosity following tillage.

Back-scattering correction and further extensions of Amiet's trailing-edge noise model. Part 1: theory

A previously published analytical formulation aimed at predicting broadband trailing-edge noise of subsonic airfoils is extended here to account for all the effects due to a limited chord length, and to infer the far-field radiation off the mid-span plane. Three-dimensional gusts are used to simulate the incident aerodynamic wall pressure that is scattered as acoustic waves. A leading-edge back-scattering correction is derived, based on the solution of an equivalent Schwarzschild problem, and added to the original formula. The full solution is found to agree very well with other analytical results based on a vanishing Mach number Green's function tailored to a finite-chord flat plate and sources close to the trailing edge. Furthermore, it is valid for any subsonic ambient mean flow velocity. The back-scattering correction is shown to have a significant effect at lower reduced frequencies, for which the airfoil chord is acoustically compact, and at the transition between supercritical and subcritical gusts. It may be important for small-size airfoils, such as automotive fan blades and similar technologies. The final far-field noise formula can be used to predict trailing-edge noise in an arbitrary configuration, provided that a minimum statistical description of the aerodynamic pressure fluctuations on the airfoil surface close to the trailing edge is available.