Velocity Cross-spectra Research Articles

On the propagation of gravity waves in the lower solar atmosphere in different magnetic configurations

Gravity waves are generated by turbulent subsurface convection overshooting or penetrating locally into a stably stratified medium. While propagating energy upwards, their characteristic negative phase shift over height is a well-recognized observational signature. Since their first detailed observational detection and estimates of energy content, a number of studies have explored their propagation characteristics and interaction with magnetic fields and other waves modes in the solar atmosphere. Here, we present a study of the atmospheric gravity wave dispersion diagrams utilizing intensity observations that cover photospheric to chromospheric heights over different magnetic configurations of quiet-Sun (magnetic network regions), a plage, and a sunspot as well as velocity observations within the photospheric layer over a quiet and a sunspot region. In order to investigate the propagation characteristics, we construct two–height intensity - intensity and velocity- velocity cross-spectra and study phase and coherence signals in the wavenumber - frequency dispersion diagrams and their association with background magnetic fields. We find signatures of association between magnetic fields and much reduced coherence and phase shifts over height from intensity-intensity and velocity-velocity phase and coherence diagrams, both indicating suppression/scattering of gravity waves by the magnetic fields. Our results are consistent with the earlier numerical simulations, which indicate that gravity waves are suppressed or scattered and reflected back into the lower solar atmosphere in the presence of magnetic fields.

Proper Orthogonal Decomposition Method for the Prediction of Fan Broadband Interaction Noise

The turbulent wake generated by a rotor interacting with outlet guide vanes (OGVs) is one of the dominant broadband noise sources in a turbofan engine. An accurate representation of the rotor wake turbulence is therefore important for the reliable prediction of the rotor–OGV interaction noise. This paper presents a turbulence synthesis method based on a spectral proper orthogonal decomposition (POD) representation of the turbulent wake that aims to reproduce the desired velocity cross-spectrum along the OGV leading edges where noise is emitted due to turbulence–OGV interaction. The method is first developed in the frequency domain based on a superposition of vortical modes with the appropriate amplitudes. Fourier modes and POD modes are proposed to represent the two-point velocity spectrum. The POD modes will be shown to be highly efficient in reconstructing the flowfield near the tip region where a large-scale coherent structure is present. An extension of the POD synthetic turbulence method to the time domain is also presented by means of a white noise filtering technique to allow the generation of time-varying velocity signals with the desired cross-spectral characteristics. The results show that both the one- and two-point statistics can be closely reproduced. The proposed frequency-domain POD synthetic turbulence method for fan broadband noise prediction for a realistic fan–OGV configuration is illustrated by the use of a frequency-domain linearized Navier–Stokes solver to predict the sound power radiation due to each vortical mode. Overall sound power levels at a number of discrete frequencies are predicted and compared against measured noise data. Agreement is found to be within the uncertainty of the noise sound power measurement.

Pressure and velocity cross-spectrum of normal modes in low-frequency acoustic vector field of shallow water and its application

The pressure and horizontal particle velocity combined descriptions in the very low frequency acoustic field of shallow water integrated with the concept of effective depth of Pekeris waveguide is proposed, especially the active component of the pressure and horizontal particle velocity cross-spectrum, also called horizontal complex cross acoustic intensity, when only two normal modes are trapped in the waveguide. Both the approximate theoretic analysis and the numerical results show that the sign of the horizontal complex cross acoustic intensity active component is independent of the range when vertically deployed receiving dual sensors are placed in appropriate depths, the sum of which is equal to the waveguide effective depth, so it can be used to tell whether the sound source is near the surface or underwater; while the range rate is expected to be measured by utilizing the sign distribution characteristic of the reactive component. The further robustness analysis of the depth classification algorithm shows that the existence of shear waves in semi infinite basement and the change of acoustic velocity profiles have few effects on the application of this method, and the seabed attenuation will limit the detection range, but the algorithm still has a good robustness in the valid detection range.

Research on the double vector hydrophones’ location for underwater low frequency source depth identification

The double vertically arranged vector hydrophones’ pressure and horizontal velocity cross spectrum in Pekeris waveguide is derived, and the sign distribution of its active component is analyzed. The sign distribution varies with the horizontal ranges and source depths regularly, the signs change in a certain depth and the depth is defined as critical depth. By locating the vector hydrophones properly, a critical depth which is independent of horizontal range can be obtained, and this characteristic can be used for discriminating the source depth. The method of forecasting the vector hydrophones’ locating depths according to the requirements of the critical depth is studied, and the forecast accuracy is validated by the simulation results. A reasonable set of the critical depth is conducive to discriminate the source depth accurately and effectively, which has extensive application prospects.

On the phase velocities of the motions in an offset attaching planar jet

An experimental investigation was performed to characterize the phase velocity of different frequency fluctuations in a turbulent offset attaching jet with a Reynolds number of 44 000 and initial offset distance of H j . The velocities were determined from the change in the phase of the cross-spectra of the fluctuating pressure along the wall and the cross-spectra of the fluctuating wall pressure and fluctuating velocity throughout the flow. The results showed there was upstream propagation of the wall pressure fluctuations near the separation point associated with the flapping motion, but the upstream propagation apparent in the correlation coefficient of the pressure was due to higher frequency fluctuations associated with the recirculating flow. The phase velocities determined from the pressure–velocity cross-spectra for the higher frequency motions showed less-frequency dependence and less acceleration than those determined from the pressure cross-spectra. There was also evidence of two velocities for the shear layer motions and a velocity for the lower frequency wall jet motions not apparent in the results from the pressure cross-spectra. Thus, the phase velocities determined from pressure cross-spectra do not fully reflect the velocities of the motions in the flow.

General motors sulfate dispersion experiment

The mean wind field and the wind fluctuations observed near a controlled, heavily traveled level roadway were investigated. The mean wind field was studied in terms of velocity defects relative to the ambient mean wind. The wind fluctuations were studied in terms of energy spectra, velocity cross-spectra, excesses of velocity variances and cross-covariances over their ambient values. Two-point cross-spectra and correlations were also used to study the propagation of wind fluctuations. The influence of traffic on the ambient wind field extended vertically up to at least 4.5m above ground, and horizontally to at least 30 m downwind of the road. The vertical extension was more gradual at high cross-road winds than at low cross-road winds. The wind tended to move up as it approached the road and down as it left the road, except when the cross-road wind was low, where the downward motion was reduced or reversed. This is likely due to the buoyancy effect of the heated exhaust. The traffic effects propagated outward at about 1 m s-1. Thus at cross-road winds of less than 1 ms-1, the influence of traffic could be observed upwind of the road. When intense shear existed between the local wind field and an opposing traffic direction, large turbulent energy production resulted, generating large eddies, in additions to the high frequency (> 0.1 Hz) eddies generated by vehicular motions in the absence of intense shear. The large eddies also increased the mixing volume, so that when the ambient wind opposed the traffic direction on the upwind lanes, the pollutant concentration upwind of the road could be higher than that downwind. While the cross-covariance excesses generally agreed with the gradient transport assumption, disagreements also existed, which may not be surprising in view of the participation of large eddies in the momentum transport.