Pseudo Time Research Articles

High-order non-inertial computation method for helicopter rotor flows in forward flight

Purpose This paper aims to improve the computational efficiency and to achieve high-order accuracy for the computation of helicopter rotor unsteady flows in forward flight during the industrial preliminary design stage. Design/methodology/approach The integral arbitrary Lagrangian–Eulerian form of unsteady compressible Navier–Stokes equations with low Mach number preconditioned pseudo time terms based on non-inertial frame of reference undergoing rotating and translating was derived and discretized in the framework of multi-block structured finite volume grid using three types of spatial reconstruction schemes, i.e. the third-order accurate monotonic upwind scheme for conservation laws, the fifth-order accurate weighted essentially non-oscillatory and the fifth-order accurate weighted compact nonlinear schemes. Findings The results show that the present non-inertial computational method can obtain comparable results with other methods, such as the dynamic overset method, and make sure that the higher-order spatial schemes can significantly improve the tip vortex resolution. Originality/value The computational grid used by the present method remained static during the whole unsteady computation process, with only local deformations induced by blade cyclic pitch and other operating motions, which greatly reduced the complexity of grid motion and enhanced the efficiency and robustness.

Pseudo-spatial coherence resonance in an excitable laser with long delayed feedback.

The effect of noise in an excitable semiconductor laser with feedback is studied in the framework of the spatio-temporal representation of long delayed systems. Propagation, noise-induced creation, and destruction of excitable pulses in the pseudo time are observed. The addition of a variable quantity of noise leads to the occurrence of a phenomenon that we term "pseudo-spatial coherence resonance." A phenomenological model well describes the system and allows for a comparison with the experimental observations. A simple Monte Carlo approach is also introduced and permits to explain the features observed in terms of the key dynamical ingredients of the physical system.

Numerical Investigation of the Droplet Behavior in Cascades Using a Finite Volume Method

This paper describes an Eulerian/Lagrangian two-phase model for wet steam. Two-dimensional inviscid transonic cascade flow is simulated using a cell-vertex finite volume space discretization method on structured triangular mesh. A pseudo time scheme is used to march the solution to steady state. The model provides an approach for including the interaction between the liquid and gas phases for a pure fluid. The analysis consists in the application of the discrete phase model, for modeling the liquid particle flow, and the Eulerian conservation equations to the continuous phase. The investigation permits us to know the influence of parameters such as: particle diameter, flow angle and particle velocity on deposition of drops onto stator blade surfaces. These parameters are analyzed to different inlet flow angles and drop sizes in the turbine. Deposition rate was found to be strongly dependent on increasing inlet flow angle and drop size in the example computed. The purpose of this paper is to present the results of some calculations of deposition which are based on more recent theories or less simplified flow models than previously published work

A high‐order flux reconstruction adaptive mesh refinement method for magnetohydrodynamics on unstructured grids

SummaryWe report our recent development of the high‐order flux reconstruction adaptive mesh refinement (AMR) method for magnetohydrodynamics (MHD). The resulted framework features a shock‐capturing duo of AMR and artificial resistivity (AR), which can robustly capture shocks and rotational and contact discontinuities with a fraction of the cell counts that are usually required. In our previous paper, we have presented a shock‐capturing framework on hydrodynamic problems with artificial diffusivity and AMR. Our AMR approach features a tree‐free, direct‐addressing approach in retrieving data across multiple levels of refinement. In this article, we report an extension to MHD systems that retains the flexibility of using unstructured grids. The challenges due to complex shock structures and divergence‐free constraint of magnetic field are more difficult to deal with than those of hydrodynamic systems. The accuracy of our solver hinges on 2 properties to achieve high‐order accuracy on MHD systems: removing the divergence error thoroughly and resolving discontinuities accurately. A hyperbolic divergence cleaning method with multiple subiterations is used for the first task. This method drives away the divergence error and preserves conservative forms of the governing equations. The subiteration can be accelerated by absorbing a pseudo time step into the wave speed coefficient, therefore enjoys a relaxed CFL condition. The AMR method rallies multiple levels of refined cells around various shock discontinuities, and it coordinates with the AR method to obtain sharp shock profiles. The physically consistent AR method localizes discontinuities and damps the spurious oscillation arising in the curl of the magnetic field. The effectiveness of the AMR and AR combination is demonstrated to be much more powerful than simply adding AR on finer and finer mesh, since the AMR steeply reduces the required amount of AR and confines the added artificial diffusivity and resistivity to a narrower and narrower region. We are able to verify the designed high‐order accuracy in space by using smooth flow test problems on unstructured grids. The efficiency and robustness of this framework are fully demonstrated through a number of two‐dimensional nonsmooth ideal MHD tests. We also successfully demonstrate that the AMR method can help significantly save computational cost for the Orszag‐Tang vortex problem.

On the numerical simulation of non-classical quasi-1D steady nozzle flows: Capturing sonic shocks

The suitability of Roe-type upwind schemes for the computation of steady quasi-1D flows of non-ideal fluids using explicit integration in the pseudo time is discussed. Based on the particular Roe linearization and entropy fix technique applied, several numerical difficulties can arise in modeling shock waves that are sonic either on the upstream or downstream side of the shock. These so-called sonic shocks typically occur away from stationary points of the cross-sectional area distribution, namely, where the geometrical source term does not vanish. The problem of selecting suitable formulations for the accurate simulation of non-classical quasi-1D steady flows is therefore addressed. Numerical experiments indicate a limited influence of the chosen Roe linearization technique, provided the so-called Property U, allowing the exact representation of steady shocks, is satisfied. Nevertheless, application of standard entropy fixes may either predict an incorrect steady-state transonic expansion neighbouring the sonic shock or even fail to attain a discrete steady state. In the latter case, lack of convergence is due to numerical unbalancing of the flux difference and source term integral over the transonic expansion which occurs in the close proximity of sonic shocks approaching their steady-state position. A simple modification to the synchronous splitting technique of [28] is proposed, which is able to produce the desired steady-state balance and allows substantial improvement in the resolution of sonic shocks.

Production data analysis for dual-porosity gas/liquid systems: A density-based approach

Production data analysis impacts assets value and financial decisions significantly. Naturally fractured reservoirs exist widely around the world. A common method to represent naturally fractured reservoirs is dual-porosity model. Intrinsic characteristics of dual-porosity systems and their unique rate-transient profile require models able to capture well behavior and to achieve production data analysis, which is a proven difficulty for dual-porosity gas system and not completely solved. Moreover, past studies on dual-porosity systems assume constant bottom-hole pressure (BHP) or constant rate production, which impose significant constraint to production data analysis.Recently a density-based approach for analyzing production data was proposed for single-porosity system, which has been proven to successfully describe gas well behavior under boundary-dominated flow (BDF). Using depletion-driven parameters, λ and β, this state-of-the-art approach circumvents pseudo time. It was then extended to variable-pressure-drawdown/variable-rate gas systems and proven successful.In this study, we adopt a rigorously derived interporosity flow equation for gas, which captures viscosity-compressibility change of matrix outflow. Based on that, a density-based, rescaled exponential model for variable pressure drawdown/variable rate production was developed for dual-porosity gas system. Also, we explore straight-line analysis for convenient prediction of OGIP and production rate. The density-based model is tested in a variety of scenarios to showcase its validity.Moreover, based on Warren and Root model, a density-based exponential model for variable pressure drawdown/variable rate in dual-porosity liquid system is proposed and fully verified. A straight-line analysis is proposed to enable explicit OOIP prediction and convenient future production calculation. Moreover, a convenient double-exponential decline model under constant BHP is developed for liquid.To summarize our work, we developed rate-transient analysis method for two types of reservoirs-naturally fractured gas reservoir and naturally fractured oil reservoirs-producing at arbitrary type of production scenario in boundary-dominated-flow stage. The proposed method can evaluate reserve in a convenient and accurate manner. Moreover, the method to predict production using the theory is also provided. An explicit equation correlating dimensionless production rate and dimensionless time is also developed for dual-porosity liquid system producing at constant bottomhole pressure in both early transient stage and boundary-dominated-flow stage.

Lagrange Regularisation Approach to Compare Nested Data Sets and Determine Objectively Financial Bubbles' Inceptions

Inspired by the question of identifying the start time τ of financial bubbles, we address the calibration of time series in which the inception of the latest regime of interest is unknown. By taking into account the tendency of a given model to overfit data, we introduce the Lagrange regularisation of the normalised sum of the squared residuals, χ2np(Φ), to endogenously detect the optimal fitting window size := w∗ ∈ [τ : t2] that should be used for calibration purposes for a fixed pseudo present time t2. The performance of the Lagrange regularisation of χnp(Φ) defined as χ2λ(Φ) is exemplified on a simple Linear Regression problem with a change point and compared against the Residual Sum of Squares (RSS) := χ2 (Φ) and RSS/(N-p):= χ2np (Φ), where N is the sample size and p is the number of degrees of freedom. Applied to synthetic models of financial bubbles with a well-defined transition regime and to a number of financial time series (US S&P500, Brazil IBovespa and China SSEC Indices), the Lagrange regularisation of χ2λ(Φ) is found to provide well-defined reasonable determinations of the starting times for major bubbles such as the bubbles ending with the 1987 Black-Monday, the 2008 Sub-prime crisis and minor speculative bubbles on other Indexes, without any further exogenous information. It thus allows one to endogenise the determination of the beginning time of bubbles, a problem that had not received previously a systematic objective solution.

Life prediction for a vacuum fluorescent display based on two improved models using the three-parameter Weibull right approximation method.

To obtain precise life information for vacuum fluorescent displays (VFDs), luminance degradation data for VFDs were collected from a group of normal life tests. Instead of exponential function, the three-parameter Weibull right approximation method (TPWRAM) was applied to describe the luminance degradation path of optoelectronic products, and two improved models were established. One of these models calculated the average life by fitting average luminance degradation data, and the other model obtained VFD life by combining the approximation method with luminance degradation test data from each individual sample. The results indicated that the test design under normal working stress was appropriate, and the selection of censored test data was simple. The two models improved by TPWRAM both revealed the luminance decaying law for VFD, and the pseudo failure time was accurately extrapolated. It was further confirmed by comparing relative error that using the second model gave a more accurate prediction of VFD life. The improved models in this study can provide technical references for researchers and manufacturers in aspects of life prediction methodology for its development.

High-resolution simulation and parametric research on helicopter rotor vortex flowfield with TAMI control in hover

In order to investigate the effect of active jet control on the rotor vortex flowfield in hover, a high-accuracy numerical method is presented in this paper, and a novel computer code is developed to calculate rotor flowfield with tip air mass injection. In the present computational fluid dynamics method, the Navier–Stokes equation is selected as the governing equation. For the sake of reducing numerical diffusion in calculation, the upwind Roe scheme with a fifth-order WENO scheme is employed to spatial discretization. A dual-time method is utilized in time marching and the high-efficiency implicit LU-SGS scheme is applied to every pseudo time step. The Baldwin–Lomax turbulence model is employed for eddy viscosity modeling. A surface boundary condition, which may effectively simulate the effect of tip air mass injection, is introduced into the current method. The overset grid method, which can veritably simulate the rotor blade motion, is adopted to exchange the flowfield information between the rotor blade and background grids. By the established method, the rotor flowfield with jet control of different jet angles is simulated. Furthermore, parametric analyses of jet velocity are carried out at different tip March numbers and collective pitches. It is demonstrated that tip air mass injection can make the tip vortex rolled up and generated in advance compared with the clean rotor, which can change the position and strength of the vortex core greatly. It is also shown that the rotor aerodynamic characteristics, such as blade surface pressure and radial lift distributions, are more sensitive to tip air mass injection control at the condition with a lower Mach number or a lower collective pitch.

Efficient solution algorithms for multiaxial probabilistic elasto‐plastic constitutive simulations of soils

SummaryA Fokker‐Planck‐Kolmogorov (FPK) equation approach has recently been developed to probabilistically solve any elastic‐plastic constitutive equation with uncertain material parameters by transforming the nonlinear, stochastic constitutive rate equation into a linear, deterministic partial differential equation (PDE) and thereby simplifying the numerical solution process. For an uniaxial problem, conventional numerical techniques, such as the finite difference or finite element methods, may be used to solve the resulting univariate FPK PDE. However, for a multiaxial problem, an efficient algorithm is necessary for tractability of the numerical solution of the multivariate FPK PDE. In this paper, computationally efficient algorithms, based on a Fourier spectral approach, are presented for solving FPK PDEs in (stress) space and (pseudo) time, having space‐independent but time‐dependent coefficients and both space‐ and time‐dependent coefficients, that commonly arise in probabilistic elasto‐plasticity. The algorithms are illustrated by probabilistically simulating 2 common laboratory constitutive experiments in geotechnical engineering, namely, the unconfined compression test and the unconsolidated undrained triaxial compression test.

Forecasting the volatility of Nikkei 225 futures

This article proposes an indirect method for forecasting the volatility of futures returns, based on the relationship between futures and the underlying asset for the returns and time‐varying volatility. The paper considers the stochastic volatility model with asymmetry and long memory, using high frequency data of the underlying asset, for forecasting its volatility. The empirical results for Nikkei 225 futures indicate that the adjusted R2 supports the appropriateness of the indirect method, and that the new method based on stochastic volatility models with asymmetry and long memory outperforms the forecasting model based on the direct method using the pseudo long time series.

Electrocardiogram characteristics of patients with ventricular arrhythmia originating from the distal great cardiac vein

Objective: To explore the electrocardiographic characteristics of patients with idiopathic ventricular arrhythmias (VAs) originating from different portions of distal great cardiac veins (DGCV). Methods: The study included 49 patients underwent successful RFCA of premature ventricular complex(PVCs)/ventricular tachycardia(VT) from different portions of the DGCV in our department from July 2009 to March 2016. The surface 12-lead electrocardiogram (ECG) and intraventricular ablation mapping features were analyzed. Patients were divided into four groups according to the mapping and ablation results: DGCV1(10 patients), DGCV2 (13 patients), proximalanterior interventricular vein (PAIV, 17 patients)and extend distal great cardiac vein (EDGCV, 9 patients). We analyzed the similarities and differences between surface 12-lead ECG of patients with PVCs/VT from different portions of DGCV, and compared with random chosen 290 patients with PVCs/VT from ventricular outflow tract and adjacent structure. Results: A positive R wave in inferior leads, a negative QS morphology in lead aVL and aVR were found among all groups. The different characteristics of surface 12-lead ECG of VAs originating from DGCV were as follows: (1)EDGCV patients demonstrated a positive R or r wave on lead Ⅰ(6/9) while a negative rS or qr wave was evidenced in other three groups (39/40). (2)A positive R pattern on lead V(1), V(5)-V(6) (11/13) was presented in patients of DGCV2 group; R (without S or s) wave on V(1) (9/10), RS or Rs wave on V(5)-V(6) were found in DGCV1 group; RS or rS wave was seen on lead V(1), R(without S)wave in lead V(5)-V(6) (25/26) were found in EDGCV and PAIV group and the amplification of R wave in EDGCV was higher than V(1) of PAIV group.(3)Precordial lead transition zone was in front of V(1) for DGCV1 and DGCV2 groups (23/23), within V(1)-V(3) for EDGCV group, but on V(2) or within V(2)-V(3) for PAIV group.(4)Patients of DGCV1 and DGCV2 demonstrated a longer Pseudo delta wave time(PdW), intrinsicoid deflection time (IDT), significantly larger maximum deflection index (MDI) than those in PAIV and EDGCV groups (P<0.001). (5)The different characteristics of surface 12-lead ECG between VAs originating from DGCV and those from ventricular outflow tract and adjacent structure were as follows: ① The ECG features were similar between PVIA and LCC group, both demonstrated a rs wave on the lead Ⅰ, rS wave on V(1)-V(2) and R wave on V(5)-V(6); ②The ECG features were similar betweenEDGCV and RCC group, both presented with R or r wave on the lead Ⅰ, the QRS wave of precordial leads was similar as PAIV and LCC groups; ③A R wave on the lead V(1), V(5)-V(6) was found in group DGCV2, and ILCC; ④Similar to the group Endo-MAA, patients in DGCV1 group also demonstrated a R wave on the lead V(1) and a Rs wave on V(5)-V(6). Conclusion: A positive R wave in inferior leads, a negative QS morphology in lead aVL and aVR are seen in all patients, but different electrocardiographic characteristics of PVC/VT originating from the different portions of the DGCV are presented on lead Ⅰ and V(1)-V(6).

Time-Accurate Flow Simulations Using a Finite-Volume Based Lattice Boltzmann Flow Solver with Dual Time Stepping Scheme

In this study, the Lattice Boltzmann Method (LBM) is implemented through a finite-volume approach to perform 2D, incompressible, and time-accurate fluid flow analyses on structured grids. Compared to the standard LBM (the so-called stream and collide scheme), the finite-volume approach followed in this study necessitates more computational effort, but the major limitations of the former on grid uniformity and Courant–Friedrichs–Lewy (CFL) number that is to be one are removed. Even though these improvements pave the way for the possibility of solving more practical fluid flow problems with the LBM, time-accurate simulations are still restricted due to the stability criteria dictated by high-aspect ratio grid cells that are usually required for adequate resolution of boundary layers and the stiffness due to the nature of the equation that are being solved. To overcome this limitation, a Dual Time Stepping (DTS) scheme, which iterates the solution in pseudo time using an Implicit-Explicit (IMEX) Runge–Kutta method while advancing the solution in physical time with an explicit scheme (backward difference formula), is developed and implemented. The accuracy of the resulting flow solver is evaluated using benchmark flow problems and overall second-order accuracy is demonstrated.

A constitutive analysis of the extensional flows of nearly monodisperse polyisoprene melts

Here two particular formulations [M.H. Wagner, S. Kheirandish, O. Hassager, Journal of Rheology 49 (6) (2005) 1317–1327; H.K. Rasmussen, Q. Huang, Rheologica Acta 53 (3) (2014) 199–208; G. Marrucci, G. Ianniruberto, Macromolecules 37 (10) (2004) 3934–3942] of the ‘interchain pressure’ [39], incorporated into the molecular stress function method [M.H. Wagner, S. Kheirandish, O. Hassager, Journal of Rheology 49 (6) (2005) 1317–1327], are used to assess the extensional [J.K. Nielsen, O. Hassager, H.K Rasmussen, G.H. McKinley, Journal of Rheology 53 (6) (2009) 1327–1346; G. Liu, H. Sun, S. Rangou, K. Ntetsikas, A. Avgeropoulos, S.-Q. Wang, Journal of Rheology 57 (1) (2013) 89–104] and shear viscosities [D. Auhl, J. Ramirez, A.E. Likhtman, P. Chambon, C. Fernyhough, Journal of Rheology 52 (3) (2008) 801–835] of narrow molecular weight distributed (NMMD) polyisoprene melts. These two formulations are expected to represent the highest [M.H. Wagner, S. Kheirandish, O. Hassager, Journal of Rheology 49 (6) (2005) 1317–1327, G. Marrucci, G. Ianniruberto, Macromolecules 37 (10) (2004) 3934–3942] and lowest level [H.K. Rasmussen, Q. Huang, Rheologica Acta 53 (3) (2014) 199–208] of the ‘interchain pressure’. The needed Rouse times are here defined as τR/τmax ∝ (M/Me)−1.4 with a proportional factor of 1.4, achieved based on the viscosity measurement. τmax is the maximal relaxation time, M the molecular weight and the entanglement molecular weight Me=(4/5)ρRT/GN0 [M. Doi M, S.F. Edwards, The Theory of Polymer Dynamics; Clarendon Press: Oxford (1986)]. ρ is the density, R the gas constant, T the temperature and GN0 the plateau modulus. The method by [M.H. Wagner, S. Kheirandish, O. Hassager, Journal of Rheology 49 (6) (2005) 1317–1327, G. Marrucci, G. Ianniruberto, Macromolecules 37 (10) (2004) 3934–3942] predicts start-up of extensional viscosities significantly below the measured value. The formulations by [H.K. Rasmussen, Q. Huang, Rheologica Acta 53 (3) (2014) 199–208] seem to be in agreement with both the start-up of extension as well as the shear flow of all NMMD polyisoprenes. Potential non-isothermal effects were addressed computationally using the pseudo time principle, assuming the most critical case of adiabatic heating.

A comparative study of fractional step method in its quasi-implicit, semi-implicit and fully-explicit forms for incompressible flows

Purpose – The purpose of this paper is to describe and analyse a class of finite element fractional step methods for solving the incompressible Navier-Stokes equations. The objective is not to reproduce the extensive contributions on the subject, but to report on long-term experience with and provide a unified overview of a particular approach: the characteristic-based split method. Three procedures, the semi-implicit, quasi-implicit and fully explicit, are studied and compared. Design/methodology/approach – This work provides a thorough assessment of the accuracy and efficiency of these schemes, both for a first and second order pressure split. Findings – In transient problems, the quasi-implicit form significantly outperforms the fully explicit approach. The second order (pressure) fractional step method displays significant convergence and accuracy benefits when the quasi-implicit projection method is employed. The fully explicit method, utilising artificial compressibility and a pseudo time stepping procedure, requires no second order fractional split to achieve second order or higher accuracy. While the fully explicit form is efficient for steady state problems, due to its ability to handle local time stepping, the quasi-implicit is the best choice for transient flow calculations with time independent boundary conditions. The semi-implicit form, with its stability restrictions, is the least favoured of all the three forms for incompressible flow calculations. Originality/value – A comprehensive comparison between three versions of the CBS method is provided for the first time.

A high-order solver for unsteady incompressible Navier–Stokes equations using the flux reconstruction method on unstructured grids with implicit dual time stepping

We report development of a high-order compact flux reconstruction method for solving unsteady incompressible flow on unstructured grids with implicit dual time stepping. The method falls under the class of methods now referred to as flux reconstruction/correction procedure via reconstruction. The governing equations employ Chorin's classic artificial compressibility formulation with dual time stepping to solve unsteady flow problems. An implicit non-linear lower–upper symmetric Gauss–Seidel scheme with backward Euler discretization is used to efficiently march the solution in pseudo time, while a second-order backward Euler discretization is used to march in physical time. We verify and validate implementation of the high-order method coupled with our implicit time stepping scheme using both steady and unsteady incompressible flow problems. The current implicit time stepping scheme is proven effective in satisfying the divergence-free constraint on the velocity field in the artificial compressibility formulation within the context of the high-order flux reconstruction method. This compact high-order method is very suitable for parallel computing and can easily be extended to moving and deforming grids.

The speed graph method: pseudo time optimal navigation among obstacles subject to uniform braking safety constraints

This paper considers the synthesis of pseudo time optimal paths for a mobile robot navigating among obstacles subject to uniform braking safety constraints. The classical Brachistochrone problem studies the time optimal path of a particle moving in an obstacle free environment subject to a constant force field. By encoding the mobile robot's braking safety constraint as a force field surrounding each obstacle, the paper generalizes the Brachistochrone problem into safe time optimal navigation of a mobile robot in environments populated by polygonal obstacles. Convexity of the safe travel time functional, a path dependent function, allows efficient construction of a speed graph for the environment. The speed graph consists of safe time optimal arcs computed as convex optimization problems in $$O(n^3 \log (1/\epsilon ))$$O(n3log(1/∈)) total time, where n is the number of obstacle features in the environment and $$\epsilon $$∈ is the desired solution accuracy. Once the speed graph is constructed for a given environment, pseudo time optimal paths between any start and target robot positions can be computed along the speed graph in $$O(n^2\log n)$$O(n2logn) time. The results are illustrated with examples and described as a readily implementable procedure.

The impact of observational sampling on time series of global 0–700 m ocean average temperature: a case study

ABSTRACTThe limited historical observational sampling of the ocean gives rise to uncertainty in time series of global ocean temperature anomalies calculated from those observations. Without knowledge of the true global state of the oceans, it is difficult to characterize the errors caused by these sampling issues. One way to quantify them is to use climate model data. Pseudo observational time series can be constructed from the model data using knowledge of where observations occurred. Comparison of these with time series constructed from the full model fields yields information about how observational sampling impacts time series of the temperature change in the modelled world. This can then be related back to the time series generated from the real observations. In this study, climate model data were used to investigate sampling errors in 0–700 m global average ocean temperature anomaly time series calculated using a straightforward gridding approach. The sampling had two impacts. First, sampling causes issues with constructing a climatology that is representative of the long‐term average state of the ocean. Climatology errors were shown to have the potential to cause systematically changing errors in anomaly time series. Second, some regions of the ocean were poorly observed prior to improvements brought about by the Argo project. This was found to cause spurious variability, both year to year and over multi‐year time scales. The latter had similar magnitude to the actual multi‐year variability seen in the model data but was smaller than the model's long‐term temperature change. The features of these errors depend on the ocean state and therefore varied between climate model runs. More sophisticated methods used to calculate ocean temperature time series are expected to be less impacted by sampling. Nevertheless, sampling errors will still occur and therefore this type of study is recommended even for those techniques.

Self-Localization of Ad-Hoc Arrays Using Time Difference of Arrivals

We investigate the problem of sensor and source joint localization using time-difference of arrivals (TDOAs) of an ad-hoc array. A major challenge is that the TDOAs contain unknown time offsets between asynchronous sensors. To address this problem, we propose a low-rank approximation method that does not need any prior knowledge of sensor and source locations or timing information. At first, we construct a pseudo time of arrival (TOA) matrix by introducing two sets of unknown timing parameters (source onset times and device capture times) into the current TDOA matrix. Then we propose a Gauss-Newton low-rank approximation algorithm to jointly identify the two sets of unknown timing parameters, exploiting the low-rank property embedded in the pseudo TOA matrix. We derive the boundaries of the timing parameters to reduce the initialization space and employ a multi-initialization scheme. Finally, we use the estimated timing parameters to correct the pseudo TOA matrix, which is further applied to sensor and source localization. Experimental results show that the proposed approach outperforms state-of-the-art algorithms.

An improved modeling for life prediction of high-power white LED based on Weibull right approximation method

Aiming at precisely predicting the life of the high-power white light LED (HPWLED), a three-parameter Weibull function and the right approximation method were employed to establish the luminance degradation model. The lumen maintenance data collected according to the IES LM-80-08 lumen maintenance test standard were fitted with and without error corrections, and the pseudo failure time of each HPWLED sample was extrapolated. The statistical analysis on the failure time was achieved by using Weibull distribution, normal distribution, lognormal distribution and Akaike Information Criterion (AIC). Then the life information was acquired. The results indicate that Weibull right approximation luminance degradation model (WRALDM) accurately reflects the variation of the lumen law with time. The failure time is accurately obtained. The best life distributions before and after the error correction to the lumen maintenance data are identified, based on AIC, as Weibull distribution and lognormal distribution, respectively. It is further confirmed by comparing the widths of life confidence interval and the life provided by the IES TM-21-11 method that the HPWLED life using WRALDM has a better accuracy. The optimized model provides researchers and manufacturers with significant guidelines for the further development of life prediction methodology.