Near Field-based Analysis of Large Scale Antenna Arrays With Reduced Number of Field Samples

2018 ◽  
Author(s):  
Huapeng Zhao ◽  
Xinzhi Li ◽  
Sihong Tao
Keyword(s):  
Antenna Arrays ◽  
Large Scale ◽  
Near Field ◽  
Field Samples

A proposed wavy shield for suppression of supersonic jet noise utilizing reflections

2021 ◽  
Vol 20 (1-2) ◽  
pp. 4-34
Author(s):  
Reda R Mankbadi ◽  
Saman Salehian
Keyword(s):  
Large Scale ◽  
Flat Surface ◽  
Near Field ◽  
Supersonic Jet ◽  
Jet Noise ◽  
Dominant Frequency ◽  
Wavy Surface ◽  
Initial Region ◽  
Reflected Waves ◽  
Free Jet

In this work we propose replacing the conventional flat-surface airframe that shields the engine by a wavy surface. The basic principle is to design a wavy pattern to reflect the incoming near-field flow and acoustic perturbations into waves of a particular dominant frequency. The reflected waves will then excite the corresponding frequency of the large-scale structure in the initial region of the jet’s shear layer. By designing the frequency of the reflected waves to be the harmonic of the fundamental frequency that corresponds to the radiated peak noise, the two frequency-modes interact nonlinearly. With the appropriate phase difference, the harmonic dampens the fundamental as it extracts energy from it to amplify. The outcome is a reduction in the peak noise. To evaluate this concept, we conducted Detached Eddy Simulations for a rectangular supersonic jet with and without the wavy shield and verified our numerical results with experimental data for a free jet, as well as, for a jet with an adjacent flat surface. Results show that the proposed wavy surface reduces the jet noise as compared to that of the corresponding flat surface by as much as 4 dB.

scholarly journals Instability wave models for the near-field fluctuations of turbulent jets

2011 ◽  
Vol 689 ◽  
pp. 97-128 ◽  
Author(s):  
K. Gudmundsson ◽  
Tim Colonius
Keyword(s):  
Flow Field ◽  
Large Scale ◽  
Microphone Array ◽  
Near Field ◽  
Turbulent Jets ◽  
Instability Wave ◽  
Mean Flow ◽  
Velocity Fluctuations ◽  
The Mean ◽  
Scale Structures

AbstractPrevious work has shown that aspects of the evolution of large-scale structures, particularly in forced and transitional mixing layers and jets, can be described by linear and nonlinear stability theories. However, questions persist as to the choice of the basic (steady) flow field to perturb, and the extent to which disturbances in natural (unforced), initially turbulent jets may be modelled with the theory. For unforced jets, identification is made difficult by the lack of a phase reference that would permit a portion of the signal associated with the instability wave to be isolated from other, uncorrelated fluctuations. In this paper, we investigate the extent to which pressure and velocity fluctuations in subsonic, turbulent round jets can be described aslinearperturbations to the mean flow field. The disturbances are expanded about the experimentally measured jet mean flow field, and evolved using linear parabolized stability equations (PSE) that account, in an approximate way, for the weakly non-parallel jet mean flow field. We utilize data from an extensive microphone array that measures pressure fluctuations just outside the jet shear layer to show that, up to an unknown initial disturbance spectrum, the phase, wavelength, and amplitude envelope of convecting wavepackets agree well with PSE solutions at frequencies and azimuthal wavenumbers that can be accurately measured with the array. We next apply the proper orthogonal decomposition to near-field velocity fluctuations measured with particle image velocimetry, and show that the structure of the most energetic modes is also similar to eigenfunctions from the linear theory. Importantly, the amplitudes of the modes inferred from the velocity fluctuations are in reasonable agreement with those identified from the microphone array. The results therefore suggest that, to predict, with reasonable accuracy, the evolution of the largest-scale structures that comprise the most energetic portion of the turbulent spectrum of natural jets, nonlinear effects need only be indirectly accounted for by considering perturbations to the mean turbulent flow field, while neglecting any non-zero frequency disturbance interactions.

The ‘preferred mode’ of the axisymmetric jet

1981 ◽  
Vol 110 ◽  
pp. 39-71 ◽  
Author(s):  
A. K. M. F. Hussain ◽  
K. B. M. Q. Zaman
Keyword(s):  
Reynolds Number ◽  
Reynolds Stress ◽  
Coherent Structure ◽  
Large Scale ◽  
Near Field ◽  
Saddle Points ◽  
Initial Boundary ◽  
Phase Average ◽  
Background Turbulence ◽  
Time Average

The ‘preferred mode’ of an incompressible axisymmetric free jet has been organized through controlled perturbation, and spatial distributions of time-average as well as phase-average flow properties in the near field are documented. The excitation produces noticeable changes in the time-average measures of the jet, although these changes are less dramatic than those for the excitation producing stable vortex pairing. For different stages in the evolution of the preferred-mode coherent structure, the phase-average vorticity, coherent Reynolds stress, and incoherent turbulence intensities and Reynolds stress have been educed through phase-locked hot-wire measurements, over the spatial extent of the structure and without invoking the Taylor hypothesis. For a particular stage of the evolution (i.e. when the structure is centred at x/D ≃ 3) the distributions of these quantities have been compared for both initially laminar and fully turbulent exit boundary layers, and for four jet Reynolds numbers. The relative merits of the coherent structure streamline and pseudo-stream-function patterns, as compared with phase-average velocity contours, for structure boundary identification have been discussed. The structure shape and size agree closely with those inferred from the average streamline pattern of the natural structure educed by Yule (1978).These data as well as τ-spectra show that even excitation at the preferred mode cannot sustain the initially organized large-scale coherent structure beyond eight diameters from the jet exit. The background turbulence is organized by the coherent motions in such a way that the maximum rate of decrease of the coherent vorticity occurs at the structure centres which are the saddle points of the background-turbulence Reynolds-stress distributions. The structure centres are also the locations of peak phase-average turbulence intensities. The evolving shape of the structure as it travels downstream helps explain the transverse variations of the wavelength and convection velocity across the mixing layer. The coherent structure characteristics are found to be independent of whether the initial boundary layer is laminar or turbulent, but depend somewhat on the jet Reynolds number. With increasing Reynolds number, the structure decreases in the streamwise length and increases in the radial width and becomes relatively more energetic, and more efficient in the production of coherent Reynolds stress.

Predictions of residual carrying-capacities for fire and near-field blast-damaged reactive powder concrete-filled steel tube columns

2018 ◽  
Vol 9 (4) ◽  
pp. 525-553 ◽  
Author(s):  
Wanxiang Chen ◽  
Zixin Zhou ◽  
Huihui Zou ◽  
Zhikun Guo
Keyword(s):  
Carrying Capacity ◽  
Large Scale ◽  
Blast Resistance ◽  
Near Field ◽  
Theoretical Method ◽  
Steel Tube ◽  
Reactive Powder Concrete ◽  
Concrete Filled Steel Tube ◽  
Damage Indices ◽  
Reactive Powder

An approximate approach is developed to estimate the residual carrying-capacities of fire and near-field blast-damaged reactive powder concrete-filled steel tube columns. The single-degree-of-freedom model is employed to calculate the initial deflections of fire-damaged reactive powder concrete-filled steel tube columns subjected to axial and blast-induced transverse loads, and then a modified formula including double coefficient is further proposed to predict the ultimate resistance. Then, a series of blast-resistance and load carrying-capacity tests on six large-scale reactive powder concrete-filled steel tube columns are conducted to validate the suitability of theoretical method presented in this article. Blast tests demonstrate that the blast-resistances of reactive powder concrete-filled steel tube columns are more sensitive to fire durations than to scale distances. In addition, it is indicated that ISO-834 standard fire exposures cause significant degradations of material properties and have remarkable effects on the residual carrying-capacities of reactive powder concrete-filled steel tube columns. No local bucking and burst could be observed in the residual carrying-capacity tests; also, there are no visible hinge-like deformations in the mid-span area, and the excellent fire-resistances and blast-resistances of reactive powder concrete-filled steel tube columns are experimentally verified. Analytical results show that the predicted axial load capacities of six reactive powder concrete-filled steel tube columns are in good agreement with experimental data. All damage indices of the test specimens are within 0.8, meaning only minor to severe damage is done to the reactive powder concrete-filled steel tube column during fire and blast attacks, which is consistent with the test results.

Convex Optimization of Pencil Beams Through Large-Scale 4-D Antenna Arrays

2018 ◽  
Vol 66 (7) ◽  
pp. 3453-3462 ◽  
Author(s):  
Feng Yang ◽  
Shiwen Yang ◽  
Yikai Chen ◽  
Shiwei Qu ◽  
Jun Hu
Keyword(s):  
Convex Optimization ◽  
Antenna Arrays ◽  
Large Scale
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