Instantaneous Basis Research Articles

PERFORMANCE EVALUATION OF SWIRL FLOW SOLAR AIR HEATER

The objective of the present study is to evaluate the performance of a new design of solar air heater. In the new design, the air is swirled by using twisted-tape insert in the air passage. Initially, the heat transfer and pressure drop for swirled flow has been tested in order to identify the optimum tape twist ratio which satisfies a maximum heat transfer coefficient, with accepted value of air pressure drop. The proposed air heater was compared with the conventional air heater under the same operating conditions. The results show that the proposed air heater is superior - instantaneous and daily bases- in energy output, efficiency and exit-air temperature.

Application of SEBAL Model for Mapping Evapotranspiration and Estimating Surface Energy Fluxes in South-Central Nebraska

Knowledge of spatiotemporal distribution of evapotranspiration ET on large scales, as quantified by satellite remote sensing techniques, can provide important information on a variety of water resources issues such as evaluating water distributions, water use by different land surfaces, water allocations, water rights, consumptive water use and planning, and better management of ground and surface water resources. The objective of this study was to assess the operational characteristics and performance of the surface energy balance algorithm for land SEBAL model for estimating crop ET ETc and other energy balance components, and mapping spatial distribution and seasonal variation of ETc on a large scale in south-central Nebraska climatic conditions. A total of seven cloud free Landsat Thematic Mapper TM/Enhanced Thematic Mapper ETM satellite images May 19, June 20, July 22, August 7, September 8, September 16, and October 18, 2005 were processed to generate ETc maps and estimate surface energy fluxes. Predictions from the SEBAL model were compared with the Bowen ratio energy balance system BREBS-measured fluxes on an instantaneous and daily basis. The ETc maps generated by the model for seven Landsat overpass days showed a very good progression of ETc with time during the growing season in 2005 as the surface conditions continuously changed. The predictions for some surface energy fluxes were very good. Overall, a very good correlation was found between the BREBS-measured and SEBAL-estimated ETc with a good r 2 of 0.73 and a root-mean-square difference RMSD of 1.04 mm day 1 . The estimated ETc was within 5% of the measured ETc. The model was able to predict growing season from emergence to physiological maturity cumulative daily corn ET reasonable well within 5% of the BREBS-measured values. The model overestimated the surface albedo by about 26% with a RMSD of 0.05. The difference between the measured and predicted albedo was the greatest on May 19, early in the growing season before the full canopy cover. The second largest difference between the two albedo values was on October 18, a day after harvest. The model significantly under predicted soil heat flux with a large RMSD of 80 W m 2 and most of the underestimation occurred in the late growing season. Local calibration of soil heat flux significantly improved the agreement between the measured and predicted values. Furthermore, the sensible heat flux was underestimated between September 20 after physiological maturity and October 18 a day after harvest. While our results showed that SEBAL can be a viable tool for generating ETc maps to assess and quantify spatiotemporal distribution of ET on large scales as well as estimating surface energy fluxes, its operational assessment for estimating sensible heat flux and ETc, especially during the drier periods for different surfaces, needs further development.

An educational tool for controlling of SRM

AbstractThis article introduces an educational tool for a switched reluctance motor (SRM) drive system. It is prepared for undergraduate and graduate level students. Classical PI and Genetic PI controllers are used in SRM drive system. The Genetic PI controller was applied to the speed loop, replacing the classical PI controller. The tool software was implemented using C++ Builder on a PC. It has flexible structure and graphical interface. The students can be easily establishing a thorough understanding of both classical PI and genetic PI controller for a SRM drive system. The education tool allowed the student to interact with the SRM drive system and it is using controllers. Then it is responses on a dynamic and instantaneous basis under different operating conditions. © 2008 Wiley Periodicals, Inc. Comput Appl Eng Educ 16: 268–279, 2008; Published online in Wiley InterScience (www.interscience.wiley.com); DOI 10.1002/cae20148

Fading-Resilient Super-Orthogonal Space-Time Signal Sets: Can Good Constellations Survive in Fading?

In this correspondence, first-tier indirect (direct) discernible constellation expansions are defined for generalized orthogonal designs. The expanded signal constellation, leading to so-called super-orthogonal codes, allows the achievement of coding gains in addition to diversity gains enabled by orthogonal designs. Conditions that allow the shape of an expanded multidimensional constellation to be preserved at the channel output, on an instantaneous basis, are derived; this property, referred to as fading resilience, is first discussed in the context of quasi-static fading, then generalized to coordinate interleaved transmissions for rapid fading. In both cases - and despite of piecing together (in the latter case) a constellation point as a result of coordinate deinterleaving - the channel alters neither the relative distances nor the angles between signal points in a fading-resilient signal constellation. It is shown that nonorthogonal MIMO constellations having a geometric structure that is resilient to fading do exist, and their further study is warranted.

Convective heat transfer characteristics from combined mechanical and supply pulsed radial reattaching jets

The heat transfer characteristics of combined mechanically and supply pulsated radial reattaching jets, CPRJR, were documented as a function of nozzle exit angle (0° and 20°), non-dimensional gap height (0.05 and 0.13), non-dimensional flow guide height (0.8 and 1.16), mechanical pulsation rate (5 and 10 Hz), mechanical to supply pulsation rate (1:1, 1:2, 1:3, and 1:4), phase angle (0° and 180°), and Reynolds number (1683 and 2366). Air was forced through a supply pulsation mechanism and then through a pulsated nozzle diverter apparatus. The air impinged on a heated plate where instantaneous heat flux and surface temperature measurements were collected and analyzed on instantaneous, ensemble-averaged, and area-averaged bases. CPRJR heat transfer can be characterized as a frequency interference of mechanical and supply pulsation effects. Significant improvement, up to 72%, over the corresponding mechanical pulsation case was found to occur for a supply-to-mechanical ratio of 3:1 with a phase angle of 0°. In general, the increased heat transfer rates were associated with the an increase in surface-directed momentum caused by increased cycle “on” mass flow rate and corresponding entrainment.

Voltage Regulation With STATCOMs: Modeling, Control and Results

This paper presents system modeling and control design for fast load voltage regulation using static compensators (STATCOMs). The modeling strategy gives a clear representation of load voltage magnitude and STATCOM reactive current on an instantaneous basis. The particular coordinate transformation employed here also facilitates extraction of linearized system dynamics in conjunction with circuit simulators. It is rigorously shown that the control problem of load voltage regulation using reactive current is nonminimum phase. Linear and nonlinear controllers for the regulation problem are designed and compared via simulation results. Internal dynamics of the STATCOM are modeled using the same strategy. Lyapunov based adaptive controllers are designed for controlling the STATCOM reactive current while maintaining its dc bus voltage. Simulation results of the controlled STATCOM integrated with the load bus voltage controller are presented to show efficacy of the modeling and control design.

Compensation heat-pulse measurements of sap flow for estimating transpiration in young lemon trees

Potted two-year-old lemon trees [Citrus limon (L.) Burm. f.], cv. Verna grafted on sour orange (C. aurantium L.) rootstock, growing in greenhouse, were subjected to drought for 33 d. Control plants were daily irrigated at field capacity. Values of sap flow (SF) were compared with transpiration (E) rates measured gravimetrically. The results underlined the robustness and high sensitivity of the compensation heat-pulse technique for estimating transpiration on a wide range of SF. Good direct correlations between E and SF rates on an instantaneous and daily basis were obtained in both treatments. On a daily basis, a common calibration curve can be used for both irrigation treatments. On an instantaneous basis, changes in SF were matches by similar changes in E in both treatments, although the relationships between these parameters presented different intercepts in each treatment. Sap flow rates were influenced by weather conditions in trees growing in non-limiting soil water conditions. This makes it possible to evaluate the significance of any sap flow measurement in relation to the reference value calculated for the vapour pressure deficit at the time the measurement was taken.

Activity budgets and habitat use of the Green Pygmy-goose (Nettapus pulchellus) on dry-season refuges in Kakadu National Park, Northern Territory

We studied behavioural activity budgets and habitat use of Green Pygmy-geese (Nettapus pulchellus) in Kakadu National Park during the dry season in 2000, in order to assess the resource requirements and conservation needs of the species. Observations were made in three wetlands, using focal-interval sampling on an instantaneous basis. No differences were found in behaviour or habitat use between the sexes among the three wetland sites. Green Pygmy-geese were largely restricted to floating vegetation. Individuals spent most of each day feeding, with shorter periods spent preening or swimming. During the dry season, wetlands contract to some 5% of the area covered during the wet, probably increasing competition among the birds for food and habitat and leaving them vulnerable to threatening processes such as wetland draining. We suggest that a program of survey and monitoring be implemented at selected sites, and that the impacts of introduced herbivores be assessed and appropriately managed.

A detailed study on the effects of noise on speech reception

The effect of adding continuous noise to a speech signal was studied by comparing, for a series of quarter octave bands, the band output for the original speech and for the speech-plus-noise. Three separate effects were identified. (a) Average envelope-modulation reduction: the original intensity-envelope is, on average, raised by the mean noise intensity, resulting in a reduction of the original modulation index. (b) Random instantaneous envelope fluctuations: on an instantaneous basis, the speech-plus-noise envelope shows random variations, caused by the stochastic nature of the noise, and by the instantaneous changes in the phase relation between the speech and the noise. (c) Perturbations of the carrier phase: in the band output carrier signal the addition of the noise causes random phase changes. By applying signal processing techniques, we were able to either include or exclude each of these three effects separately. The results of intelligibility measurements indicated the following order of importance of the three different effects: (1) the average envelope-modulation reduction, (2) the perturbation of the carrier phase, and (3) the random envelope fluctuations. The results will be discussed in the light of modeling and enhancing (noise suppression schemes) speech reception in noise.

Adiabatic theorem and continuous coupled-mode theory for efficient taper transitions in photonic crystals.

We prove that an adiabatic theorem generally holds for slow tapers in photonic crystals and other strongly grated waveguides with arbitrary index modulation, exactly as in conventional waveguides. This provides a guaranteed pathway to efficient and broad-bandwidth couplers with, e.g., uniform waveguides. We show that adiabatic transmission can only occur, however, if the operating mode is propagating (nonevanescent) and guided at every point in the taper. Moreover, we demonstrate how straightforward taper designs in photonic crystals can violate these conditions, but that adiabaticity is restored by simple design principles involving only the independent band structures of the intermediate gratings. For these and other analyses, we develop a generalization of the standard coupled-mode theory to handle arbitrary nonuniform gratings via an instantaneous Bloch-mode basis, yielding a continuous set of differential equations for the basis coefficients. We show how one can thereby compute semianalytical reflection and transmission through crystal tapers of almost any length, using only a single pair of modes in the unit cells of uniform gratings. Unlike other numerical methods, our technique becomes more accurate as the taper becomes more gradual, with no significant increase in the computation time or memory. We also include numerical examples comparing to a well-established scattering-matrix method in two dimensions.

Sea surface salinity measurements in the historical database

We have examined historical distributions of sea surface salinity (SSS) observations in a data set consisting of a combination of the World Ocean Database 1998 (WOD98) and a thermosalinograph and bucket salinity database collected from volunteer observing ships. It is well known that SSS in much of the world's ocean is measured infrequently or not at all. We find that 27% of one‐degree squares in the world ocean (open and coastal, excluding the Arctic Ocean) had no observations of SSS in the historical database, and 70% had 10 or fewer. Systematic sampling of SSS (more than 10,000 observations per year globally) did not start until after 1960. Most SSS observations in the WOD98 are concentrated in the North Sea and coast of northern Europe, the east and west coasts of North America, and around Japan. About 28% of SSS measurements are in coastal waters. We plotted frequency histograms of SSS for some selected well‐sampled one‐degree squares in the North Atlantic and tropical Pacific. We found most frequency histograms to be non‐Gaussian. The main departure from normal distribution is due to anomalous low‐salinity measurements creating a negative skewness. This conclusion is verified as a global phenomenon by examining statistics of mean‐median SSS difference within one‐degree squares. This quantity is found to be predominantly negative over the global ocean. These anomalous low‐salinity values may be due to rainfall events, but there are other plausible physical mechanisms, like frontal movement and eddy activity. There were also areas where the distributions were bimodal due to the presence of meandering fronts with little cross‐frontal mixing. Examples are shown where the non‐Gaussian nature of the distributions in the areas examined is both a short‐term and a long‐term phenomenon. That is, the distributions are skewed on a nearly instantaneous (∼1 month) basis and averaged over long time periods (1 + years). This has important implications for climatologies because the differences between mean and modal SSS, for the analyzed one‐degree squares, is of order 0.1. Furthermore, the implication for validation studies for remote sensing missions is that the studies must make enough measurements of SSS to determine the extent to which the probability density is not Gaussian.

A comparison study of the auroral lower thermospheric neutral winds derived by the EISCAT UHF radar and the Tromsø medium frequency radar

A comparative study of the neutral wind in the polar upper mesosphere/lower thermosphere is conducted using two radars, the European Incoherent Scatter (EISCAT) UHF radar (ν = 931 MHz) and the Tromsø MF radar (ν = 2.8 MHz) colocated in northern Scandinavia. Comparisons of winds are made in case studies as well as on a statistical basis. On the basis of simultaneous observations obtained by the two radars, comparisons over a height range from ∼90 to ∼100 km are made for about 20 days between February and October 1999. In these comparisons we directly compare temporal wind variations. On the other hand, mean winds as well as diurnal and semidiurnal components are derived using 1‐month‐averaged wind data obtained by the medium frequency (MF) radar for 1999. They are compared with those derived from 56 days of EISCAT data studied by Nozawa and Brekke [1999a, 1999b]. In the case studies, we have also utilized altitude profiles of electron density obtained by the EISCAT radar at and above 62 km height to determine the total reflection height as well as to estimate the effect of group retardation for 2.8 MHz radio wave. It can be seen that the effect of particle precipitation sometimes penetrates into the D region (down to ∼84 km). Generally, wind velocities derived by EISCAT with the monostatic method exhibit significant scatter with time below 100 km, while the number of the MF radar wind data sharply decreases above 90 km except for summer. These facts make the comparison difficult, but no significant departures between different radar winds are identified on an instantaneous basis. In the statistical studies, generally both altitude profiles are well connected, or continuous, but significant disagreements are observed for the mean wind and the tidal components for summer. On the basis of these comparisons, we raise strong concerns regarding the use of summer wind data above 91 km obtained by MF radar located at high latitude under high solar activity conditions. The wind values during the winter and equinox observed by the two radars are complementary. Thus here is a good opportunity to utilize wind data from the two different radars through a year, which then enables us to cover a wide height range (approximately 70–120 km). This wide range gives us good opportunities to examine the mesosphere/lower thermosphere coupling processes based on observational data.

Response of turbulent boundary layers to multiple strain rates

Many practical applications, such as in blade cascades and turbomachinery, involve inhomogeneous turbulent shear flows subjected simultaneously to multiple strains. In principle, the applied strain can be combined to yield an effective strain. However, no simple stress–strain relation is capable of establishing turbulent stress or energy balance in the mean or on an instantaneous basis. In the current investigation, a turbulent boundary layer is examined in the presence of convex curvatures of different strengths combined with streamwise (favourable and adverse) pressure gradients, with various values of pressure gradient ratio, (∂P/∂s)/(∂P/∂n). Measurements of the mean and turbulent parameters and flux Richardson number show appreciable changes, mainly in the outer portion of the boundary layer (y+ > 100). The turbulent burst frequency, particularly at the location of application of the additional strain rate, also changes relative to its value with wall curvature alone.Three primary observations from these experiments are as follows: (i) in all cases, the mean velocity profile and all of the measured Reynolds stresses collapse in the near-wall region using standard inner scaling; (ii) the applied strains combine nonlinearly, with one of the strains dominating the local flow during its development; (iii) the ratio of the radial to axial pressure gradient magnitude influences both classical turbulence correlations and mean flow, as well as the physical production cycle of turbulence; and (iv) application rate of newly introduced strain rates is at least as important as their magnitudes.

A model of radiation interception and use by a maize–bean intercrop canopy

A modelling study on intercropping was carried out to understand the radiation interception and use. The radiation transmission models on both instantaneous and daily bases were described and validated. For testing the model, photosynthetically active radiation was measured above and beneath a maize–bean intercrop canopy in both north–south (NS) and east–west (EW) rows at Bloemfontein, Free State, South Africa. Both models accurately predicted radiation transmitted through the intercrop canopy throughout the vegetative stage. For the instantaneous model, two methods were compared, namely, the geometrical method versus the statistical method. The geometrical method was equal in radiation transmitted per unit row to the statistical method. The daily amount of radiation intercepted and used by each component crop was estimated. Concerning the radiation utilisation, the intercropping was equivalent in growth efficiency of maize to the sole cropping whereas beans had greater radiation use efficiency in intercropping than in sole cropping, which might explain the intercrop yield advantage.

Cloud radiative forcing at the top of the atmosphere during FIRE ACE derived from AVHRR data

Cloud radiative forcing at the top of the atmosphere is derived from narrowband visible and infrared radiances from NOAA‐12 and NOAA‐14 advanced very high resolution radiometer (AVHRR) data taken over the Arctic Ocean during the First ISCCP Regional Experiment Arctic Cloud Experiment (FIRE ACE) during spring and summer 1998. Shortwave and longwave fluxes at the top of the atmosphere (TOA) were computed using narrowband‐to‐broadband conversion formulae based on coincident Earth Radiation Budget Experiment (ERBE) broadband and AVHRR narrowband radiances. The NOAA‐12/NOAA‐14 broadband data were validated using model calculations and coincident broadband flux radiometer data from the Surface Heat Budget of the Arctic Ocean experiment and from aircraft data. The AVHRR TOA albedos agreed with the surface‐ and aircraft‐based albedos to within one standard deviation of ±0.029 on an instantaneous basis. Mean differences ranged from −0.012 to 0.023 depending on the radiometer and platform. AVHRR‐derived longwave fluxes differed from the model calculations using aircraft‐ and surface‐based fluxes by −0.2 to −0.3 W m−2, on average, when the atmospheric profiles were adjusted to force agreement between the observed and the calculated downwelling fluxes. The standard deviations of the differences were less than 2%. Mean total TOA albedo for the domain between 72°N and 80°N and between 150°W and 180°W changed from 0.695 in May to 0.510 during July, while the longwave flux increased from 217 to 228 W m−2. Net radiation increased from −89 to −2 W m−2 for the same period. Net cloud forcing varied from −15 W m−2 in May to −31 W m−2 during July, while longwave cloud forcing was nearly constant at ∼8 W m−2. Shortwave cloud forcing dominated the cloud effect, ranging from −22 W m−2 during May to −40 W m−2 in July. The mean albedos and fluxes are consistent with previous measurements from the ERBE, except during May when the albedo and longwave flux were greater than the maximum ERBE values. The cloud‐forcing results, while similar to some earlier estimates, are the most accurate values hitherto obtained for regions in the Arctic. When no significant melting was present, the clear‐sky longwave flux showed a diurnal variation similar to that over land under clear skies. These data should be valuable for understanding the Arctic energy budget and for constraining models of atmosphere and ocean processes in the Arctic.

PARTICLE CREATION IN AN OSCILLATING SPHERICAL CAVITY

We study the creation of massless scalar particles from the quantum vacuum due to the dynamical Casimir effect by spherical shell with oscillating radius. In the case of a small amplitude of the oscillation, to solve the infinite set of coupled differential equations for the instantaneous basis expansion coefficients we use the method based on the time-dependent perturbation theory of the quantum mechanics. To the first order of the amplitude we derive the expressions for the number of the created particles for both parametric resonance and non-resonance cases.

PARTICLE CREATION BY MOVING SPHERICAL SHELL IN THE DYNAMICAL CASIMIR EFFECT

The creation of massless scalar particles from the quantum vacuum by spherical shell with time varying radius is studied. In the general case of motion the equations are derived for the instantaneous basis expansion coefficients. The examples are considered when the mean number of particles can be explicitly evaluated in the adiabatic approximation.

Evaluation of oxides of nitrogen emissions for the purpose of their transient regulation from a direct injection diesel engine

The concept of defining a regulatory standard for the maximum allowable emissions of oxides of nitrogen (NOx) from a heavy-duty diesel engine on an instantaneous basis is presented. The significance of this concept from a regulatory point of view is the possibility to realise a steady brake specific NOx emissions result independent of the test schedule used. The emissions of oxides of nitrogen from a state-of-the-art direct injection diesel engine have been examined on an integral as well as on an instantaneous basis over the Federal Test Procedure as well as over several other arbitrary transient cycles generated for this study. Three candidate standards of specific NOx emissions have been evaluated on a real-time, continuous basis. These include brake power specific, fuel mass specific, and carbon dioxide mass specific NOx emissions. Retaining the stock engine control module, the carbon dioxide specific emissions of NOx have been shown to be the most uniform, varying only by about 30% of its mean value regardless of the test schedule or engine operation. The instantaneous fuel specific NOx emissions are shown to be relatively less invariant and the least steady are the brake power specific emissions with a coefficient of variation of up to 200%. Advancing injection timing has been shown to have a wide range of authority over the specific emissions of oxides of nitrogen regardless of the units used, when operating at full load in the vicinity of peak torque speeds. The carbon dioxide specific NOx emissions have shown a linear dependence on the power specific emissions, independent of the examined operating conditions. The trade-off between better brake thermal efficiency, lower exhaust gas temperature at advanced timing and lower NOx emissions has also been shown to be independent of the units of the specific standard used.

Polarization beam smoothing for inertial confinement fusion

For both direct and indirect drive approaches to inertial confinement fusion (ICF) it is imperative to obtain the best possible drive beam uniformity. The approach chosen for the National Ignition Facility uses a random-phase plate to generate a speckle pattern with a precisely controlled envelope on target. A number of temporal smoothing techniques can then be employed to utilize bandwidth to rapidly change the speckle pattern, and thus average out the small-scale speckle structure. One technique which generally can supplement other smoothing methods is polarization smoothing (PS): the illumination of the target with two distinct and orthogonally polarized speckle patterns. Since these two polarizations do not interfere, the intensity patterns add incoherently, and the rms nonuniformity can be reduced by a factor of √. A number of PS schemes are described and compared on the basis of the aggregate rms and the spatial spectrum of the focused illumination distribution. The √ rms nonuniformity reduction of PS is present on an instantaneous basis and is, therefore, of particular interest for the suppression of laser plasma instabilities, which have a very rapid response time. When combining PS and temporal methods, such as smoothing by spectral dispersion (SSD), PS can reduce the rms of the temporally smoothed illumination by an additional factor of √. However, it has generally been thought that in order to achieve this reduction of √, the increased divergence of the beam from PS must exceed the divergence of SSD. It is also shown here that, over the time scales of interest to direct or indirect drive ICF, under some conditions PS can reduce the smoothed illumination rms by nearly √ even when the PS divergence is much smaller than that of SSD.

Stochastic resonance in the semiadiabatic limit

Periodically driven Markovian processes are treated in the semiadiabatic limit. In this limit all but one characteristic times of the undriven system are fast compared to the driving frequency. The proposed theory is based on the Floquet representation of the conditional probability and makes use of the instantaneous bases of eigenfunctions of the corresponding forward and backward master operators. The methodology is exemplified with the standard Brownian motion model of stochastic resonance. Quantitative agreement is reached with known results in the fully adiabatic limit and in the regime of linear response theory.