Variable Reflection Coefficient Research Articles

Electromagnetic fields of lightning return stroke to wind turbines with discontinuous impedance model

This study introduces an electric field calculation model for wind turbine return strokes, incorporating impedance discontinuity within the turbine, a feature not considered in traditional tall tower models. The model introduces a reflection coefficient, ρx, at the connection between the blades and the tower. Disregarding the generation of upward leaders, this paper calculates the electric field of subsequent return strokes for different distances from the wind turbine and for different model parameters. The results demonstrate that changes in the reflection coefficient ρx affect oscillation amplitude and also slightly influence the pulse width, with a negative ρx leading to smaller and more frequent pulses. Contrasts with measured waveforms indicate that models with a constant reflection coefficient are inaccurate. Adopting variable reflection coefficient values yields a more accurate match. This approach offers a new perspective for electromagnetic field calculations in lightning-struck structures, emphasizing the significance of impedance dynamics of return stroke channel.

Designing variable reflection coefficient for upstream and downstream terminations to study their effect on flame thermoacoustics

In this paper, the design, construction and results of experiments performed on a generic combustion system are presented. The setup is supplemented by various weakly frequency-dependent variable reflection coefficient (RC) devices as upstream and downstream acoustic terminations. The main objective of building such terminations is to provide a method to study burner/flame stability when it is placed between various acoustic configurations (RC: 0.1-0.9) and to determine the figure of merit of a burner based on the evaluation of its map of (in-)stability. Furthermore, burner design parameters such as the burner perforation pattern (holes diameter, pitch, perforation area, etc.) which will provide combustion stability for the widest range of burner's acoustic embedding conditions are identified. The experimental setup comprises of an upstream acoustic termination, a telescopic tube with adjustable length is placed after the upstream termination followed by the burner and the quartz tube. On the top of the quartz tube, the replaceable downstream terminations are installed. Nine downstream terminations are constructed by stacking plates of 0.25 mm thickness separated by spacers ranging from 0.1 to 1 mm thickness. Particularly, for the burners tested in this setup, the smallest hole diameter burner (with the largest perforation area) results in the largest stable region on the stability map in the parameter space. An increase in the flow velocity leads to an increase in the frequency of instability and makes a stable system tend to become unstable, while an increase in the equivalence ratio contributes to stabilizing system instability

The Reynolds wave shear stress in partially reflected waves

Wind–waves play a relevant role in the downward flux of mass, momentum and heat, and in their balances. Thus, they are quite significant in small- and large-scale processes like global warming, especially in continental shelf. There is still no consensus on the effectiveness of different mechanisms in enhancing, e.g., the carbon dioxide (CO2) uptake in the continental shelves with respect to the open ocean. To our best knowledge, the role of partial reflections of short and long waves in the budget has been largely neglected without a specific justification, even though reflection in water waves is a recurring phenomenon. Presumably, this is a consequence of the difficulties in performing experiments with variable reflection coefficient and phase shift of the waves approaching coastal structures. We report experiments on the vertical momentum transfer in partially reflecting waves propagating on a flat horizontal bottom. We measured the free surface level and the fluid velocity, and estimated the Reynolds wave–generated shear stress. A set of experiments using paddle waves or a paddle plus wind waves and a reflection coefficient ranging from 0.10−0.75, with a phase shift of π/4−π rad, confirms the theoretical model. The results indicate that reflection must be carefully considered in the correct interpretation of data, particularly when considering momentum transfer in the vertical. The new contributions in the paper are (i) the adoption of a generation–absorption system that controls the reflection coefficient and phase shift between incident and reflected waves; (ii) the quantification of the phase shift influence on the vertical momentum transfer and its horizontal variability; (iii) the consequences for locally–generated wind–waves and wind generated surface currents. To our knowledge, these are the first experiments where the characteristics of the reflected waves have been imposed, allowing a thoroughly analysis of their effects on the flow field.

Hybrid Ambient Backscatter Communication Systems With Harvest-Then-Transmit Protocols

In this paper, we investigate and analyze a hybrid ambient backscatter (AmBack) scheme with harvest-then-transmit protocols. In this scheme, the whole transmission is divided into three phases (time slots): energy harvesting (EH), AmBack, and data transmission (DT). Depending on the type of the reflection coefficient (RC) for AmBack, two hybrid schemes are considered: variable RC (VRC) and fixed RC (FRC). In the VRC scheme, the RC changes with the channel state information, and the harvested energy in the first phase is totally used for DT. On the other hand, in the FRC scheme, the RC is fixed to be one, and the harvested energy in the first phase is in part used to cover the tag circuit operation in the second phase and in part for DT in the third phase. The resulting optimization problem is convex for the VRC scheme, but non-convex for the FRC scheme. However, by utilizing the maximum principle in convex optimization, we are able to derive closed-form expressions for optimal solutions for both schemes. In order to get more insights, we also analyze the upper bound performance of both schemes. Simulation results demonstrate that the AmBack/EH-AmBack scheme can always achieve the optimal performance.

Performance optimization of the diode-pumped solid-state laser for space applications

Subject of Research. Thermophysical and optical techniques of parameter regulation for diode pumped solid-state laser are studied as applied to space laser communication and laser ranging lines. Methods. The investigations are carried out on the base of the original design of diode pumped solid-state laser module that includes the following: Nd:YAG slab element, diode pumped by 400W QCW produced by NORTHROP GRUMMAN; two-pass unstable resonator with rotation of the laser beam aperture about its axis through 1800; the output mirror of the resonator with a variable reflection coefficient; hyperthermal conductive plates for thermal stabilization of the laser diode generation modes. The presence of thermal conductive plates excludes conventional running water systems applied as cooling systems for solid-state laser components. The diodes temperature stabilization is achieved by applying the algorithm of pulse-width modulation of power of auxiliary electric heaters. To compensate for non-stationary thermal distortions of the slab refractive index, the laser resonator scheme comprises a prism reflector with an apex angle of 1200. Narrow sides of the prism are covered with reflective coating, and its wide side is sprayed with antireflection coating. The beam aperture is turned around its axis through 1800 because of triple reflection of the beam inside the prism. The turning procedure leads to compensating for the output beam phase distortions in view of symmetric character of the aberrations of slab refractive index. To suppress parasitic oscillations inside the slab, dielectric coatings of wide sides of the slab are used. Main Results. We have demonstrated theoretically and experimentally that the usage of hyperthermal conductive plates together with the algorithm of pulse-width modulation provides stabilizing of the diode substrate temperature accurate within ± 0.1 °С and smoothing the temperature distribution along the plate surface accurate within 1 °С. Optical schematic diagram of the laser resonator keeps the laser beam divergence not exceeding a diffraction limit more than twice under a light pump power of 100 W. We have also shown that to increase the lasing efficiency, slab multilayer dielectric coatings made of SiO2 и ZrO2 should be used. Practical Relevance. We have proposed original design of the diode pumped solid-state laser module optimizing the generation and pump modes of solid-state lasers by the temperature stabilization technique for laser diode array and by compensation of the slab aberrations. The techniques are also applicable under space conditions; that is an important factor at developing the space-based lasers.

Quantum interference of photons in simple networks

A theoretical investigation of quantum interference of photonic multistates in simple devices like beam splitters, Mach---Zehnder interferometers and double-loop devices are presented. Variable transmission and reflection coefficients as well as variable phase shifts are included in order to calculate quantum states and mean photon numbers at the outputs. Various input states like Fock states and coherent states and a combination of both are considered as well as squeezed states. Two methods are applied: The direct matrix method and the method of unitary representation. Remarkable results appear in a double-loop interferometer where for special phase shifts equal mean photon numbers in the three output ports are obtained provided certain input states are given. A computerized simulation of general networks using various input Fock states is presented. Multistate devices will be used in future linear quantum computation and quantum information processing schemes.

Virtual reflector representation theorem (acoustic medium)

The virtual reflector method simulates new seismic signals by processing traces recorded by a plurality of sources and receivers. The approach is based on the crossconvolution of the recorded signals and makes it possible to obtain the Green's function of virtual reflected signals as if in the position of the receivers (or sources) there were a reflector, even if said reflector is not present. This letter presents the virtual reflector theory based on the Kirchhoff integral representation theorem for wave propagation in an acoustic medium with and without boundary and a generalization to variable reflection coefficients for scattered wavefields.

Using radar-sounding data to identify the distribution and sources of subglacial water: application to Dome C, East Antarctica

AbstractBasal radar reflectivity is the most important measurement for the detection of subglacial water. However, dielectric loss in the overlying ice column complicates the determination of basal reflectivity. Dielectric attenuation is a function of ice temperature and impurity concentration. Temperature distribution is a function of climate history, basal heat flow and vertical strain rate, all of which can be partially inferred from the structure of dated internal layers. Using 11 dated layers, isotope records from Dome C, East Antarctica, and a model of the spatial variation of geothermal flux, we calculate the vertical strain rate and accumulation-rate history, allowing identification of areas where the basal melt rate exceeds 1.5 mm a−1. The accumulation-rate history and vertical strain rates are then used as inputs for a transient temperature model. The model outputs for the present-day temperature distribution are then combined with depth-dependent ionic concentrations to model dielectric loss and infer basal reflectivity. The resulting reflection coefficients are consistent (∼−5 dB) across a variety of subglacial water bodies. We also identify a high reflectivity >−15 dB in Concordia Trench and along suspected subglacial water-flow routes in Vincennes Basin. Highland areas tend to have highly variable reflection coefficients near −30 dB, consistent with an ice–bedrock interface. This combined model also identifies three areas of enhanced basal melting along Concordia Ridge, Concordia Subglacial Lake and Vincennes Basin. Melt at Concordia Subglacial Lake exceeds 5 mm a−1. The inferred basal melt at these locations is not possible without enhanced geothermal flux. We demonstrate how radar-sounding data can provide both input and verification for a self-consistent model of vertical strain, vertical temperature distribution and meltwater distribution.

A 1D arterial blood flow model incorporating ventricular pressure, aortic valve and regional coronary flow using the locally conservative Galerkin (LCG) method

AbstractThere is an important interaction between the pumping performance of the ventricle, arterial haemodynamics and coronary blood flow. While previous non‐linear 1D models have focused only on one of these components, the model presented in this study includes coronary and systemic arterial circulations, as well as ventricular pressure and an aortic valve that opens and closes ‘independently’ and based on local haemodynamics. The systemic circulation is modelled as a branching network of elastic tapering vessels. The terminal element applied at the extremities of the network is a single tapering vessel which is shown to adequately represent the input characteristics of the downstream vasculature. The coronary model consists of left and right coronary arteries which both branch into two ‘equivalent’ vessels that account for the lumped characteristics of subendocardial and subepicardial flows. As contracting heart muscle causes significant compression of the subendocardial vessels, a time‐varying external pressure proportional to ventricular pressure is applied to the distal part of the equivalent subendocardial vessel. The aortic valve is modelled using a variable reflection coefficient with respect to backward‐running aortic waves, and a variable transmission coefficient with respect to forward‐running ventricular waves. A realistic ventricular pressure is the input to the system; however, an afterload‐corrected ventricular pressure is calculated and results in pressure gradients between the ventricle and aorta that are similar to those observed in vivo. The 1D equations of fluid flow are solved using the locally conservative Galerkin method, which provides explicit element‐wise conservation, and can naturally incorporate vessel branching. Each component of the model is verified using a number of tests to ensure accuracy and reveal the underlying processes that give rise to complex pressure and flow waveforms. The complete model is then implemented, and simulations are performed with input parameters representing ‘at rest’ and exercise states for a normal adult. The resulting waveforms contain all of the important features seen in vivo, and standard measures of haemodynamic state are found to be normal. In addition, one or several characteristics of some common diseases are imposed on the model and are found to produce haemodynamic changes that agree with experimental observations in the published literature. Using a patient‐specific carotid bifurcation geometry, 1D velocity waveforms are also compared with waveforms obtained from a three‐dimensional model. The 1D and 3D results show good agreement. Copyright © 2008 John Wiley & Sons, Ltd.

Acute Simvastatin Treatment Enhances the Efficacy of Rho Kinase Inhibitor Mediated Vasodilatation in Rat Caudal Arteries

these components currently exists. The aim of this study was to develop and verify a one-dimensional model of systemic and coronary arterial circulations incorporating ventricular pressure as the input via an aortic valve. The systemic circulation was modelled as a network of branching elastic vessels. The coronary circulation consisted of left and right coronary arteries which both branched into two ‘equivalent’ vessels to account for the lumped characteristics of subendocardial and subepicardial flows. A time-varying external pressure was applied to the subendocardial vessels to simulate myocardial compression. The aortic valve was modelled as a variable reflection coefficient for backward-running aortic waves, and a variable transmission coefficient for forwardrunning ventricular waves. To verify the model, simulations were performed with input parameters representing ‘at rest’ and exercise states, as well as two valve disease states (stenosis and incompetence). In accord with previous clinical and experimental observations, the model predicted increased cardiac output and CBF with exercise, but reduced output and CBF with both stenosis and incompetence. In addition, incompetence was associated with negative aortic flow throughout diastole, while stenosis produced high ventriculo-aortic pressure gradients. Conclusion: A newmodel incorporating systemic and coronary arterial circulations, ventricular pressure and aortic (p< 0.05). Simvastatin retained 80% of its vasodilator properties in the absence of an endothelium or in the presence of L-NAME. H-1152 relaxed U46619 and U46619/simvistatin constricted vessels in a concentrationdependent manner. The potency of H-1152 was increased in the presence of simvastatin. Conclusion: In the context of U46619 contraction, acute treatment with simvastatin increases vasorelaxation in both a nitric oxide-dependent and smooth muscledependent manner. Simvastatin treatment also increases the efficacy of direct Rho kinase inhibition on vasodilation, suggesting a therapeutic value of combined statin and low dose Rho kinase inhibitor therapy. doi:10.1016/j.hlc.2008.05.593

On water availability in saline soils

We present a formulation for the effect of the osmotic pressure of the soil solution on the availability of soil water for plant uptake in the extreme case that the reflection coefficient of root-cell walls is always unity. We also present a new equation to fit the water retention curve, which allows for an inflection point and is solidly anchored at both the wet end (saturated water content) and the dry end (water content at 150 m head, the permanent wilting point). By differentiating the fitting-equation one finds the differential water capacity, which is subsequently multiplied by a weighting function to account for the impediment caused by soluble salts. The weighted differential water capacity is then integrated over the entire range of the matric head from zero to infinity. This produces the integral water capacity and constitutes the total amount of water the soil can hold and release to a hypothetical plant that behaves like a perfect osmometer. We illustrate the approach using data found in the literature for a wide range of soil textures. In this paper the lower boundary of water availability in the presence of soluble salts is defined and calculated as would be registered by a perfect osmometer (reflection coefficient of unity). The upper boundary of water availability is found by setting the weighting coefficient at unity at all times, which implies a reflection coefficient of zero, and in turn that the salts in the soil solution have no influence on the availability of water (as would be registered by a tensiometer). The upper and the lower boundaries constitute the envelope within which the actual availability of water to real plants occurs, and implies a variable reflection coefficient plus the occurrence of active plant osmo-regulation. This establishes a framework within which water availability to real plants experiencing real osmo-matric conditions can be evaluated.

Depth profiling the optical absorption and thermal reflection coefficient via an analysis based on the method of images (abstract)

The problem of depth profiling optical absorption in a thermally depth variable solid is a problem of direct interest for the analysis of complex structured materials. In this work, we introduce a new algorithm to solve this problem in a planar layered sample which is impulse irradiated. The sample is comprised of “N” model layers of thickness Δx, of constant diffusivity α, where the conductivity varies depth wise with each layer. This derivation extends to the general case of a depth variable thermal reflection coefficient with depth variable optical source density. In such a sample, at finite time, t, past excitation, thermal energy can only significantly penetrate NL model layers NL≈4αt[−ln(ε)]/2Δx, where ε is a small error (ε⩽10−6) and a double transit through each layer is assumed. The depth profile of optical absorption in each layer, i, is approximated by δ(x−iΔx), weighted by the optical source density Si. The temperature at x=0− just inside a front medium contacting the sample is given by T(x=0,t)= ∑ i=12NLSi⋅GR(x,x0=iΔx,t)]x=0, where GR(x,x0,t) represents an effective Green’s function for optical absorption at the depth x0=iΔx in the sample. The method of images1 gives GR(x,x0=iΔx,t) in the following form: [GR(x,0Δx,t)GR(x,2Δx,t)…GR(x,2NLΔx,t)]=[A10 A12 A14 A16 …..A1,2NL0 A32 A34 A36 …..A3,2NL….0……A2NL−1,2NL][G(x−0Δx,t)G(x−2Δx,t)……G(x−2NLΔx,t)]. The G(x−nΔx,t) are shifted image fields obtained from the infinite domain Green’s function for one-dimensional heat conduction. They account for thermal wave reflection/transmission over the path length nΔx from the source (at interface i) to the surface (x=0). The Ain are lumped coefficients giving the efficiency of heat transmission from the ith source to the surface for each path order n. They are determined by a mapping procedure that identifies all propagation paths of each order, n, and computes the individual and lumped reflection coefficients. Equation (2) is written for sources placed at odd ordered model interfaces. A similar upper triangular matrix results for the placement of sources at the even ordered interfaces. Recovery of the optical absorption profile proceeds by inversion of Eq. (1) for known GR(x,iΔx,t). Determination of the kernel requires a solution of the related type II inverse problem.2,3 An evaluation of this procedure and its conditioning will be presented.

A comparison of common-midpoint, single-shot, and plane-wave depth migration

A comparison of common-midpoint (CMP), single-shot, and plane-wave migration was made for simple two-dimensional structures such as a syncline and a horizontal reflector with a laterally variable reflection coefficient by using synthetic seismograms. The seismograms were calculated employing the finite-difference technique. CMP sections were simulated by 18-fold stacking and plane-wave sections by slant stacking. By applying a finite-difference scheme, the synthetic wave field was continued downward. The usual imaging condition of CMP migration was extended in order to carry out migration of single-shot and plane-wave sections. The reflection coefficient was reconstructed by comparing the migrated wave field with the incident wave field at the reflector. The results are: (1) all three migration techniques succeeded in reconstructing the reflector position; (2) as a consequence of the finite aperture of the geophone spread, only segments of the reflector could be reconstructed by single-shot and planewave migration; (3) for single-shot and plane-wave migration the reflection coefficient could be obtained; and (4) CMP migration may lead to incorrect conclusions regarding the reflection coefficient.

A comparison of common‐midpoint, single‐shot, and plane‐wave depth migration

A comparison of common‐midpoint (CMP), single‐shot, and plane‐wave migration was made for simple two‐dimensional structures such as a syncline and a horizontal reflector with a laterally variable reflection coefficient by using synthetic seismograms. The seismograms were calculated employing the finite‐difference technique. CMP sections were simulated by 18-fold stacking and plane‐wave sections by slant stacking. By applying a finite‐difference scheme, the synthetic wave field was continued downward. The usual imaging condition of CMP migration was extended in order to carry out migration of single‐shot and plane‐wave sections. The reflection coefficient was reconstructed by comparing the migrated wave field with the incident wave field at the reflector. The results are: (1) all three migration techniques succeeded in reconstructing the reflector position; (2) as a consequence of the finite aperture of the geophone spread, only segments of the reflector could be reconstructed by single‐shot and plane‐wave migration; (3) for single‐shot and plane‐wave migration the reflection coefficient could be obtained; and (4) CMP migration may lead to incorrect conclusions regarding the reflection coefficient.

Wavefront reversal of pulsed CO2laser radiation by a surface

The feasibility of using mirrors with a variable reflection coefficient for wavefront reversal was demonstrated at the wavelength of 10.6 μ. The reversed wave was generated by reflection from a mirror formed as a result of profiled evaporation of a selenium film on a copper substrate. The coefficient of reflection into the reversed wave reached ~45% (of the total energy). The results of theoretical calculations were found to be in satisfactory agreement with the experimental data.

Determination of the coefficient of diffuse scattering for a medium of variable reflection coefficient

The single-scattering approximation is applied to give the coefficient of diffuse scattering for a set of plane-parallel layers of variable reflection coefficient.

Plasma studies survey of experiments - II: Mainly probe studies

Subjects dealt with include the classification of probes; general remarks on metal (Langmuir probes, and the construction of probes for volume exploration; the Langmuir-Druyvesteyn method for determining isotropic distributions, its theory, the problem of variable reflection coefficients, and Boyd's development of techniques; exploration of oscillating plasmas; effects of negative ions on plasma fields and probe currents, and on the stability of discharges.