Array Tilt Research Articles

Beetle-Inspired Bidirectional, Asymmetric Interlocking Using Geometry-Tunable Nanohairs

We present bidirectional, asymmetric interlocking behaviors between tilted micro- and nanohair arrays inspired from the actual wing locking device of beetles. The measured shear adhesion force between two identical tilted microhair arrays (1.5 μm radius, 30 μm height) turned out to be higher in the reverse direction than that in the angled direction, suggesting that the directionality of beetle's microtrichia may play a critical role in preventing the elytra from shifting along the middle of insect body. Furthermore, we observed dramatic enhancement of shear adhesion using asymmetric interlocking of various nanohair arrays (tilting angle, δ < 40°). A maximum shear locking force of ∼60 N/cm(2) was measured for the nanohair arrays of 50 nm radius and 1 μm height with a hysteresis as high as ∼3. A simple theoretical model was developed to describe the measured asymmetric adhesion forces and hysteresis, in good agreement with the experimental data.

Accommodation of speckle in object-based phasing

This work addresses the physical basis of the measurement process for object-based phasing of an array of telescopes. In this regard an enhanced least-squares estimator that is capable of differentiating among three families of array aberrations in an object-based phasing system is developed. In a system of this nature the system to be phased illuminates the object of interest and the return radiation is detected. Telescope aberrations, atmospheric aberrations, and speckle-induced aberrations are all reported by the estimator to facilitate correction of telescope and atmospheric aberrations. This is accomplished by proper handling of the unobservable modes and recognizing that the five global aberrations-telescope array piston, atmospheric array piston and tilt, and speckle array piston and tilt-cannot be measured accurately so they need to be projected out of the estimated piston commands. Except for these relatively benign array aberrations, the disturbances for all three families of array aberrations are estimated exactly. An interesting feature of the speckle array aberrations is that a synthetic aperture is developed that is almost twice as large as the array of telescopes under consideration.

A Software Tool for Optimal Sizing of PV Systems in Malaysia

This paper presents a MATLAB based user friendly software tool called as PV.MY for optimal sizing of photovoltaic (PV) systems. The software has the capabilities of predicting the metrological variables such as solar energy, ambient temperature and wind speed using artificial neural network (ANN), optimizes the PV module/ array tilt angle, optimizes the inverter size and calculate optimal capacities of PV array, battery, wind turbine and diesel generator in hybrid PV systems. The ANN based model for metrological prediction uses four meteorological variables, namely, sun shine ratio, day number and location coordinates. As for PV system sizing, iterative methods are used for determining the optimal sizing of three types of PV systems, which are standalone PV system, hybrid PV/wind system and hybrid PV/diesel generator system. The loss of load probability (LLP) technique is used for optimization in which the energy sources capacities are the variables to be optimized considering very low LLP. As for determining the optimal PV panels tilt angle and inverter size, the Liu and Jordan model for solar energy incident on a tilt surface is used in optimizing the monthly tilt angle, while a model for inverter efficiency curve is used in the optimization of inverter size.

Analysis of the near-field-far-field pattern of tilted planar antenna arrays

Based on the principle of planar near-field measurement and theory on the spatial pattern of planar antenna arrays, a comprehensive investigation on the spatial pattern of planar antenna arrays tilting away from the planar near-field scanning plane is performed. An efficient computational algorithm on the near-field-far-field pattern in the plane determined by the rotation axis and normal line of the tilted array is developed. The parametric equations on the intersecting curve locus between the plane and unit directional sphere of the planar near-field scanning are thus derived. Consequently, the spatial pattern of the tilted antenna array based on the near-field data is calculated by fast Fourier transform technique, and at the same time a two-dimensional interpolating scheme to evaluate the pattern in the plane is developed. The capability, efficiency, and feasibility of this new algorithm are demonstrated by the good consistency of the calculated results with the numerical simulations and experimental measurements.

Optimization of a PV/wind micro-grid for rural housing electrification using a hybrid iterative/genetic algorithm: Case study of Kuala Terengganu, Malaysia

This paper presents an optimization for hybrid PV/wind system based on loss of load probability (LLP) and system cost. This paper is divided into three main parts: optimization of PV array, wind turbine and battery, optimum PV array tilt angle and optimization of the inverter size. Firstly, the optimization performed is aimed to select the optimal capacities of the PV array and wind turbine that give minimum system cost. However, the optimization was done using a hybrid iterative/genetic algorithm. The iterative part of the proposed algorithm is applied to generate a set of possible configurations for the proposed system, while the genetic algorithm is applied to find the optimum configuration. Secondly, a model for optimizing the tilt angle of the PV array is presented based on the Liu and Jordan model. Finally, an iterative method for optimizing the inverter size in the proposed system is presented. The results showed that the optimum sizes of the PV array (CA), wind turbine (Cs) and storage battery (CB) are 1.1411, 0.7159, and 0.550, respectively, while the optimum inverter's size (Rs) is 1.37. Lastly, by applying the calculated monthly optimum tilt angle, the collected yield of the PV array is increased by 7.96%.

A 50 MW very large-scale photovoltaic power plant for Al-Kufra, Libya: energetic, economic and environmental impact analysis

Libya has a growing demand for electricity and presently generates almost all of its electrical energy using fossil-fuelled generation plant. An opportunity exists to use the naturally high solar radiation resource that occurs in the south of the country to meet this demand with a renewable energy source. This paper describes the design of a 50 MW photovoltaic (PV) power plant which has been modelled on the conditions pertaining to Al-Kufra. The general energy situation within Libya is described, along with the solar conditions at the proposed location of the power plant. An HIT type PV module has been selected and modelled. The effectiveness of the use of a cooling jacket on the modules has been evaluated. The results show an average increase in efficiency of 0.6%; however, this is not considered to be a justifiable expense. The optimum tilt angle and array layout have been evaluated for the proposed site. The projected energy output has been determined to be 114 GWh per annum with a payback time of 2.7 years and a reduction of CO 2 pollution by 76 thousand tonnes per year. It is recommended that very large-scale PV plants of this type are installed within Libya for the sake of benign environmental impact and diversification of the electrical generation mix. Copyright , Oxford University Press.

Modeling Fraunhofer diffractive characteristics for modulation transfer function analysis of tilted ring metallic mesh

In order to analyze the Fraunhofer diffractive characteristics and modulation transfer function (MTF) of a tilted ring metallic mesh, an optical intensity distribution model of Fraunhofer diffraction is built using Huygens–Fresnel diffraction theory and the diffraction integral is carried out directly in the tilted mesh plane. The diffraction characteristics of the tilted ring metallic mesh are in good agreement with experimental results, which proves the correctness of the model established. MTF of an optical system with metallic mesh is calculated based on the model established and Fourier transform. Analysis shows that the degradation of MTF caused by diffraction of a ring mesh is much less than that of a square mesh whether they are vertical or tilted to the optical axis. Therefore, ring mesh can provide higher imaging quality than square mesh when they are used as high-pass filters in optical windows. A tilted array diffraction modulating factor is abstracted and believed useful in the analysis of diffractive characteristics of tilted square mesh and ring mesh, and it can be extended to Fraunhofer diffractive characteristics analysis of other tilted diffraction arrays.

Analysis of the vertical directionality of ambient noise: SPICE04 experiment measurements and model simulations.

The SPICE04 experiment provided a unique opportunity to observe the ambient noise in the North Pacific [Worcester et al., J. Acoust. Soc. Am. 120(5), p. 3020]. Receptions on two 700-m segments of a deep vertical line array (DVLA) facilitate measurements of the vertical directionality of the noise above and below the surface conjugate depth. Several authors have proposed models for deep ocean noise, e.g., the spatial harmonic model developed by Cox [J. Acoust. Soc. Am. 54(5), pp. 1289–1301] and the normal mode model developed by Kuperman and Ingenito [J. Acoust. Soc. Am. 67(6), 1988–1996] and extended by Perkins et al. [J. Acoust. Soc. Am. 93(2), pp. 739–752]. The simulated data provided by these models can be used to assess the effects of array tilt on estimates of vertical directionality. In this talk, we first analyze the impact of tilt for the DVLA using the array element positions measured during the experiment. Then we compare the SPICE04 noise estimates for the two 700-m array segments and the relevant model output. [Work sponsored by ONR.]

EVALUATION OF THE OPTIMUM TILT OF A PV ARRAY USING MAXIMUM GLOBAL INSOLATION TECHNIQUE

The photovoltaic (PV) is considered to be the most promising energy source in many applications, due to its safety and high reliability. But the initial high cost of the PV arrays and the large areas necessitate their optimum use. One of the solutions that optimizes the array performance is to what extent the array tilt should be adjusted such that the incident insolation on the array becomes as large as possible. In this paper, a new technique, which maximizes the incident solar insolation at solar noon on the tilted PV array, is proposed and developed. The monthly mean values of the solar insolation incident on a horizontal surface is calculated using a statistical methods. Also, a comparison between the proposed technique and the most common one is studied. The results indicate the superiority of the proposed technique. Copyright © 2004 John Wiley & Sons, Ltd.

An extraction method of seabed reflection loss based on towed tilted line array

Seabed acoustic parameters are very important to underwater acoustic field prediction in shallow water. The existing inversion methods generally uses a vertical line array to receive the signal and its platform is stationary, which can not be operated for a large area with high resolution. The methods for extracting seabed reflection losses based on three kinds of line arrays were studied. A fast inversion scheme for the seabed acoustic parameters was presented, which uses a towed tilted line array and three acoustic sources to cover a large scope of grazing angles. The error analysis was made for this method. The simulation results show that different arrays require different experimental configurations to cover the same scope of grazing angles. The vertical line array must operate with several acoustic sources and require a stationary platform of the array. The towed horizontal line array needs only one acoustic source but requires a large-aperture line array. The towed tilted line array combines the merits of the vertical line array and the horizontal line array, which can inverse the seabed acoustic parameters in a moving and high resolution way and requires only a line array of smaller aperture.

Properties of one-dimensional tilted carbon nanorod arrays synthesized by the catalyst-assisted oblique angle deposition technique

One-dimensional tilted carbon nanorod arrays were synthesized on molybdenum substrates by using the catalyst-assisted oblique angle deposition technique. The structures of the one-dimensional tilted carbon nanorods evolve with substrate temperatures, but otherwise identical growth conditions. The crystallographic structures, chemical compositions and bond structures of the tilted carbon nanorods were investigated by using x-ray diffraction, energy dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy and Raman scattering spectroscopy. The cross-sectional SEM image showed that multilayered tilted carbon nanorods were also obtained. The electron field emission (FE) behaviours of the obtained one-dimensional carbon tilted nanorod arrays were greatly improved with the increase in substrate temperature. Meanwhile, the sample with multiple layers of carbon tilted nanorods exhibits better FE behaviours than those with single layer.

Bathymetric Sidescan Sonar Bottom Estimation Accuracy: Tilt Angles and Waveforms

This paper presents a detailed analysis of bottom estimation performance of bathymetric sidescan sonar. Both general and closed-form expressions for the cross correlations of the received backscatter across a receive array are determined for a square pulse, match-filtered square pulse, and a chirp Gaussian pulse. The closed-form expressions clearly show the contributions that the various error mechanisms make to the correlation. Through geometry, simulation, and the Cramer-Rao lower bound (CRLB), it is demonstrated that when the array is tilted, a double-angle region results, which requires two angles to be estimated rather than just one, if the location of the bottom is to be determined correctly. Estimating two angles requires an array with at least three elements as opposed to two elements typically employed in simple relative-phase bathymetric sidescan sonar. It is shown through simulation that multiple angle estimates made with a simple linear prediction algorithm attain estimates near the CRLB and that the CRLB can be used to establish confidence limits. The bottom estimation performance related to a match-filtered Gaussian pulse and Gaussian chirp pulse are compared with that obtained with a match-filtered square pulse. Little difference in performance is found except when the angle estimation accuracy is thermal noise limited and then the chirp pulse performs better because of the higher resultant <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">snr</i> . A simple approximation procedure for predicting estimation performance is developed along with conditions for its application. Examples of bottom estimation performance are given for different array sizes, tilt angles, frequencies, and water types.

On some possibilities and properties of matched-field geoacoustic inversion in shallow water

The aim of this paper is to estimate possibilities of matched-field geoacoustic inversion (MFI) in shallow water, and to recommend optimal arrangements of signal source and hydrophone array in variety of particular environments. We assumed ocean to be range independent with bottom consisting of homogeneous liquid layers. Sound fields were calculated as superposition of normal modes and continuous spectrum for tonal point source and vertical line array. MFI based on Bartlett processor was used. Possibilities of MFI were characterized by MFI penetration depth, sensitivity to various bottom parameters and non-uniqueness of inverted data. These characteristics were analysed as functions of frequency and the source depth and range for different values of sound attenuation in sediments and for various sound-speed profiles. To estimate possibilities of MFI in real ocean conditions, influence of array tilt and inadequacy of geoacoustic model were analysed. The influence of continuous spectrum was also discussed. Among the major results, optimal source ranges corresponding to the maximum penetration depth of MFI were calculated, and high influence of attenuation in sediments on possibilities of MFI was revealed.

Acoustic modeshape inversion using deep water ambient noise measurements

Assuming that ambient noise can be represented by a sum of uncorrelated acoustic modes, the eigenvectors of the noise covariance matrix for a vertical line array should correspond to the sampled modeshapes. In previous work several authors have investigated using an eigendecomposition of the noise covariance to estimate the mode functions in shallow water, e.g., Wolf et al. [Proc. of the 1993 IEEE Oceans Conf.,vol. I, pp. 99‐104], Hursky et al. [J. Acoust. Soc. Am., 109(4), pp. 1355‐1366], and Nielsen and Westwood [J. Acoust. Soc. Am., 111(2), pp. 748‐756]. While the same approach should work for deep water scenarios, only few deep water experiments have deployed arrays with sufficient aperture to resolve the modes, e.g., the work D'Spain et al. [Pure appl. geophys., Vol. 158, pp. 475‐512]. This paper explores the problem of inverting for the acoustic modes of a deep water waveguide using ambient noise measurements. In particular the paper focuses on important signal processing issues, including data snapshot requirements, and the effects of array tilt. Data from a deep water propagation experiment will be used to quantify how well the empirical modes match the true modes derived from measured environmental data. [Work supported by an ONR Young Investigator Award.]

Robustness and constraints of ambient noise inversion

One of the most dominant sources of error in the estimation of sonar performance in shallow water is the geoacoustic description of the sea floor. As reviewed in this paper, various investigators have studied the possible use of ambient noise to infer some key parameters such as the critical angle, geoacoustic properties, or bottom loss. A simple measurement approach to infer the bottom loss from ambient noise measurement on a vertical line array (VLA) is very attractive from environmental and operational perspectives. This paper presents a sensitivity study conducted with simulations and measurements that demonstrates mitigating factors to maximize the accuracy of estimated bottom loss. This paper quantifies the robustness and operational constraints of this measurement approach using an ambient noise model that accounts for wind, shipping, and thermal noise. Also demonstrated are the effects of unaccounted water absorption, array tilt, nearby ship interference, flow noise, calibration error, and array deformation on sonar performance estimation. VLA measurements collected during the Asian Seas International Acoustics Experiment in May-June 2001 were also processed to validate the approach via comparisons with measured bottom loss and transmission loss.

Laminar free convection underneath a downward facing inclined hot fin array

A combined theoretical, experimental and numerical study was conducted to investigate the problem of laminar free convection underneath a hot isothermal and inclined fin array. The influence of inclination on the location where the flow stagnates, and splits, was examined. Heat transfer rates were calculated for different fin array geometries and temperatures. The results show that for small inclination angles the cooling rate is essentially constant. Beyond a certain angle, the tilting of the fin array enhances substantially the heat transfer rate. Sensitivity analyses indicate that the heat transfer coefficient increases at higher fin temperatures and larger fin spacing, but is of a lesser sensitivity to fin height changes. Additionally, it was discovered that the array optimal fin spacing do not depend on the inclination angle. In the theoretical part, a semi empirical model was developed for the heat transfer coefficient of horizontal and slightly inclined arrays that have large fin spacing. In effect it constitutes the necessary modeling addition to the previously developed model for moderately and tightly spaced fins of slightly tilted arrays. Together, they provide analytical expressions for the heat transfer coefficient of slightly inclined arrays, for any fin spacing.

Trajectory Design and Maneuver Strategy for the MESSENGER Mission to Mercury

Destined to become the first spacecraft to orbit the planet Mercury, the MESSENGER spacecraft was launched on 3 August 2004. The 6.6-year ballistic trajectory to Mercury will utilize six gravity-assist flybys of Earth (one), Venus (two), and Mercury (three). With three trajectory correction maneuvers completed by mid-December 2005, many more maneuvers will be necessary during the journey to Mercury and the subsequent 1-year duration Mercury orbit phase. The spacecraft's design and operational capability will enable real-time monitoring of every course-correction maneuver. A complex mission plan will provide multiple opportunities to obtain observational data that will help fulfill the mission's scientific objectives. Soon after entering Mercury orbit in mid-March 2011, the initial primary science orbit will have an 80-deg orbit inclination relative to Mercury's equator, 200-km periapsis altitude, 60°N subspacecraft periapsis latitude, and a 12-h orbit period. With science goals requiring infrequent orbit-phase trajectory adjustments, pairs of orbit-correction maneuvers occur at about the same time every Mercury year, or every 88 days. For the first time, the spacecraft's orbit design at Mercury accounts for the best available Mercury gravity model, small solar pressure perturbations due to changes in the solar array tilt angle, and an improved strategy for performing orbit correction maneuvers.

Robustness and constraints of high-frequency ambient noise inversion for sonar performance estimation

Perhaps the most dominant source of error in sonar performance estimation in shallow waters is the geoacoustic description of the seafloor. Various investigators have studied the possible use of ambient noise to infer some key parameters such as the critical angle, geoacoustic properties, or bottom loss. Most operational sonar performance models and tactical decision aids require the bottom loss as a function of grazing angles and frequency to describe the reflected acoustic energy off the ocean floor. Harrison and Simons [Harrison &amp; Simons, ‘‘Geoacoustics inversion of ambient noise: A simple method,’’ J. Acoust. Soc. Am. 112, 1377 (2002)] described a simple measurement approach to infer the bottom loss from ambient noise measurement on a vertical line array. This approach is well suited as an environmental measurement approach during naval operations. The robustness and operational constraints of this measurement approach are quantified using a high-frequency ambient noise model that accounts for wind, shipping, and thermal noise [Ferat &amp; Arvelo, ‘‘Mid-Frequency Ambient Noise Directivity And Array Response in Range-Dependent Environments,’’ High-frequency Ocean Acoustics Conference Proceedings, 2004)]. The effects of array tilt, nearby shipping noise, water absorption, thermal noise, surface-noise dipole pattern, and array beam response on sonar performance estimation are demonstrated. Mitigating factors to maximize estimated bottom loss accuracy will also be outlined.

Air-deployable sonobuoy array concepts and their performance against high-frequency noise anisotropy

High-frequency sonobuoy arrays with vertical aperture were developed and tested to exploit the noise anisotropy for enhanced array gain [Ferat and Arvelo, J. Acoust. Soc. Am. 114, 2462–2463 (2003)]. Afterwards, an inexpensive and conformal piezoelectric wire material was employed in the design and construction of a volumetric array [Arvelo et al., J. Acoust. Soc. Am. 116, 2650 (2004)]. However, detection performance of air-deployed sonobuoys is severely affected by noise interference through the beamformers sidelobes. An improved sonobuoy system design with in-buoy processing for sidelobe suppression is introduced and its detection performance in anisotropic wind noise with nearby shipping interference is evaluated and compared against previous designs in selected environmental conditions. The high-frequency array gain for increased vertical aperture will be compared against the gain for increased horizontal aperture to explore coherence limitations. The effects of array tilt, as well as interelement amplitude, phase, and topology errors, on the performance deterioration of the new design will also be addressed. [This effort is supported by the Maritime Sensing (MS) Program of the Office of Naval Research (ONR Code 321).]

Structural characterization of nanoporous Pd films grown via ballistic deposition

Nanoporous Pd films were vapor-deposited onto an oxidized Mo(1 0 0) substrate at 22 K and various dosing angles. Temperature programmed desorption (TPD) of N 2 was used to determine the surface area of the Pd films immediately after deposition at 22 K, and following annealing. The surface area of Pd films was found to depend dramatically on the Pd dosing angle and annealing temperature. Pd films grown at 22 K with a 85° deposition angle exhibit the highest surface area of 120 m 2/g. Ex situ SEM imaging reveals that these films consist of a tilted array of nanocrystalline filaments. The annealing studies show that the films densify upon annealing and lose approximately 50% of their surface area by 300 K and are almost completely dense by 500 K. Pd deposition at elevated temperatures (<300 K) produces denser Pd films compared to those grown at 22 K. Growth above 300 K leads to dewetting of Pd from the substrate.