Static Distortion Research Articles

A Method for Quantifying Static Shift Distortions Using a Magnetic Field of Controlled Source (CSAMT)

A Method for Quantifying Static Shift Distortions Using a Magnetic Field of Controlled Source (CSAMT)

Influence of vaned diffuser on the potential effect of volute and stall inception of centrifugal compressor

In the automotive turbochargers centrifugal compressor, the volute has a strong potential effect, leading to circumferential nonuniformity of the impeller flow field and compressor stall. In this study, full-annulus unsteady simulations for centrifugal compressors with vaned/vaneless diffusers are carried out. The influence of the diffuser vane on the potential effect of the volute and stall behavior of a centrifugal compressor is studied in detail. Based on the distribution of the casing static pressure, the formation mechanism of the circumferential distribution of static pressure and the reverse propagation process of pressure waves caused by pressure distortion are revealed. The results of this study show that the diffuser vanes can weaken the potential effect of the volute on the impeller flow field by reducing the degree of static pressure distortion. The number of static pressure peaks in the circumferential direction is related to the number of vanes/blades. The diffuser vanes can change the circumferential position of the stall inception, but cannot eliminate the “locking effect” of the volute tongue on the circumferential position of the stall. In other words, the circumferential position of the stall inception is still determined by the volute tongue for a centrifugal compressor with a vaned diffuser. Although the degree of circumferential static pressure distortion inside the impeller is reduced, the stable operating range of the compressor cannot be significantly widened by adjusting the stagger angle of the diffuser vane when the stall first occurs at the impeller inlet.

Simultaneous Motion and Distortion Correction Using Dual-Echo Diffusion-Weighted MRI.

Geometric distortions resulting from large pose changes reduce the accuracy of motion measurements and interfere with the ability to generate artifact-free information. Our goal is to develop an algorithm and pulse sequence to enable motion-compensated, geometric distortion compensated diffusion-weighted MRI, and to evaluate its efficacy in correcting for the field inhomogeneity and position changes, induced by large and frequent head motions. Dual echo planar imaging (EPI) with a blip-reversed phase encoding distortion correction technique was evaluated in five volunteers in two separate experiments and compared with static field map distortion correction. In the first experiment, dual-echo EPI images were acquired in two head positions designed to induce a large field inhomogeneity change. A field map and a distortion-free structural image were acquired at each position to assess the ability of dual-echo EPI to generate reliable field maps and enable geometric distortion correction in both positions. In the second experiment, volunteers were asked to move to multiple random positions during a diffusion scan. Images were reconstructed using the dual-echo correction and a slice-to-volume registration (SVR) registration algorithm. The accuracy of SVR motion estimates was compared to externally measured ground truth motion parameters. Our results show that dual-echo EPI can produce slice-level field maps with comparable quality to field maps generated by the reference gold standard method. We also show that slice-level distortion correction improves the accuracy of SVR algorithms as slices acquired at different orientations have different levels of distortion, which can create errors in the registration process. Dual-echo acquisitions with blip-reversed phase encoding can be used to generate slice-level distortion-free images, which is critical for motion-robust slice to volume registration. The distortion corrected images not only result in better motion estimates, but they also enable a more accurate final diffusion image reconstruction.

Transition-Metal Monofluorophosphate Ba2M2(PO3F)F6 (M = Mn, Co, and Ni): Varied One-Dimensional Transition-Metal Chains and Antiferromagnetism.

Four transition-metal monofluorophosphates, with a chemical formula of Ba2M2(PO3F)F6 (M = Mn, Co, Ni, and Cu), have been synthesized hydrothermally using phosphoric and fluorophosphoric acids. The structures of Ba2M2(PO3F)F6 were intensively investigated by single-crystal and powder X-ray diffraction. Their networks exhibited three-dimensional frameworks formed by cis-MO2F4 octahedra and PO3F tetrahedra. Ba2M2(PO3F)F6 (M = Mn, Co, and Cu) are isostructural, crystallizing in the monoclinic space group P21/c, while Ba2Ni2(PO3F)F6 adopted an unpredetermined structure, crystallizing in the monoclinic space group P21/n. The different arrangements of the same cis-MO2F4 moieties and PO3F groups within one-dimensional chains lead to different frameworks in this late-first-row transition-metal series. By a comparison of normalized bond lengths with their normal counterparts, the more elongated octahedral cis-MO2F4 units in Ba2Cu2(PO3F)F6 reveal the presence of static Jahn-Teller distortion, uncovering the puzzling reason for the exceptional structure of Ba2Ni2(PO3F)F6 in the series. Ba2M2(PO3F)F6 (M = Mn, Co, and Ni) forms spin chains of M2+ made of coupled dimers or tetramers, showing dominant antiferromagnetic behavior.

Multi-scale Calibration Board Based Accurate Calibration of a Scanning Electron Microscope

We present a new calibration method based on multi-scale calibration board for scanning electronic microscopy (SEM) at low magnifications. The implementation of the calibration mainly depends on automatic center-point features detection from the calibration board image instead of corner detection. The center-point feature is the centroid of each squares, which is obtained via morphological processing. We use the calibration model consisting of projective model and static distortion model. It is proved the point detection proposed in this paper is more accurate, efficient and robust, compared with corner point detection in the traditional calibration process. Then, reprojection error is used to evaluate the accuracy of the calibration results. At last, the method is efficient and reliable with accuracy, which is testified by experiment.

Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline \u03b1-MgAgSb

Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material, in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe, and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion combined with phonon-anharmonic-induced ultralow lattice thermal conductivity in α-MgAgSb. Since the transverse acoustic phonons are almost fully scattered by the compound’s intrinsic distorted rocksalt sublattice, the heat is mainly transported by the longitudinal acoustic phonons. The ultralow thermal conductivity in α-MgAgSb is attributed to its atomic dynamics being altered by the structure distortion, which presents a possible microscopic route to enhance the performance of similar thermoelectric materials.

Intelligent Recognition of Fatigue and Sleepiness Based on InceptionV3-LSTM via Multi-Feature Fusion

Fatigue is a common state of mankind characterized by a reduction in the level of consciousness and alertness. Therefore, the recognition of fatigue and sleepiness has become indispensable in many alertness-dependent situations, such as when driving vehicles on public roads, performing demanding tasks in the workplace, or monitoring intensive care unit patients. This study proposes a method based on novel multi-feature fusion to detect fatigue and sleepiness by using traditional image processing and heart rate variability (HRV). The proposed method performs initial feature extraction using InceptionV3 (a convolutional neural network (CNN)), following which the second decision is made by a long short-term memory network (LSTM) using the features collected by InceptionV3 to process the sequence of video data for recognition. The LSTM provides coherent and precise sequence recognition that avoids static distortions. Then, the final decision is made by the blood volume pulse vector (PBV) method after the features are fused. Because fatigue recognition is usually employed to monitor driver fatigue, we verified the feasibility of our method by testing its ability to successfully recognize driver fatigue. Following the experiments, we compared the different steps in the proposed method with those in existing methods. We selected four other methods to perform the comparison tests and used the same videos for training networks. In comparison with state-of-the-art methods, our method in its entirety achieved an average increase of 5% in terms of both accuracy and stability.

Magnetotelluric Apparent Resistivity Tensors for Improved Interpretations and 3‐D Inversions

AbstractThe complex magnetotelluric (MT) apparent resistivity tensor can be decomposed into two real tensors, the apparent resistivity and the resistivity phase tensors, which represent relationships between the observed electric field at a point on the Earth's surface and an associated apparent current density. We explain the differences between these tensors and conventional estimates of apparent resistivity and phase for simple resistivity environments and demonstrate, using canonical models in 1‐D and 2‐D environments, that both tensors are more sensitive to vertical and horizontal resistivity gradients than their conventional counterparts. The properties of the new tensors are explained using electromagnetic induction theory and the effects of associated charges at resistivity boundaries. We introduce a new way to plot tensor ellipses, which brings significant improvements to the interpretation of MT data, using appropriate visualization software. The apparent resistivity tensor gives information about the magnitude and direction of apparent resistivity subsurface structures and has a strong response to vertical resistivity contrasts. The resistivity phase tensor is highly sensitive to vertical boundaries and the associated fields in the TM mode. It is also free from static distortions under the same conditions implied for the conventional phase tensor. These findings have prompted a study in the potential of the new tensors for 3‐D inversions. The results from a 3‐D inversion of a canonical oblique conductor straddling two quarter spaces show distinct improvements in resolving the boundaries of the conductor and open a promising field for future studies.

Structural and stoichiometric modifications in ultrathin epitaxial BaBiO3 films

${\mathrm{BaBiO}}_{3}$ (BBO) is well known as the parent material for the high-${T}_{c}$ superconducting compounds ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{BiO}}_{3}$ and ${\mathrm{BaPb}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}{\mathrm{O}}_{3}$. In its pristine state, BBO is a charge-ordered (CO) insulator, resulting from a static breathing distortion of the ${\mathrm{BiO}}_{6}$ octahedra with alternating long and short bond lengths. Recently, it has been reported that the CO state is suppressed for BBO films grown on ${\mathrm{SrTiO}}_{3}$ (STO) below a thickness of approximately 4 nm, possibly resulting in a metallic phase. While we do confirm structural modifications in our BBO/Nb:STO samples in this thickness range by Raman spectroscopy and electron diffraction, in situ photoemission evidences that these changes are accompanied by a Bi deficit and that the films remain insulating. We hence conclude that, in line with previous findings for the BBO/STO interface, the thickness-controlled suppression of the CO state is not purely driven by the two-dimensional confinement but rather originates from modifications of the composition and structure inherent to the epitaxial growth of BBO on ${\mathrm{SrTiO}}_{3}$(001).

Unconventional orbital ordering and emergent dimensional reduction in fulleride superconductors

Nonlocal order parameters in space-time are proposed to characterize the unconventional orbital-selective conducting state in fulleride superconductors, called the Jahn-Teller metal. In previous works, it has been argued that this state can be interpreted as a spontaneous orbital-selective Mott state, in which the electrons in two of the three ${t}_{1u}$ molecular orbitals are localized, while those in the third one are metallic. Here, based on the realistic band structure for fullerides, we provide a systematic study of nonlocal order parameters and characterize the Jahn-Teller metal, for which there exists no one-body local order parameter in contrast to conventional orderings. It is shown that the Mottness, or integer filling nature for each orbital due to strong correlation effects, is a relevant feature of the present orbital order. The local orbital moment thus vanishes and the static distortion associated with a conventional orbital moment is absent. Transport characteristics are also investigated, and it is found that the dimensionality is effectively reduced from three to two at low energies, while the cubic nature is recovered at high energies. This accounts for the high upper critical field observed in the superconducting state of the fcc fullerides inside the Jahn-Teller metal regime.

The impact of geometric distortions in multiconjugate adaptive optics astrometric observations with future extremely large telescopes

Astrometry is one of the main science case which drives the requirements of the next multiconjugate adaptive optics (MCAO) systems for future extremely large telescopes. The small diffraction limited point-spread function (PSF) and the high Signal-to-Noise Ratio (SNR) of these instruments, promise astrometric precision at the level of micro-arcseconds. However, optical distortions have to be as low as possible to achieve the high demanding astrometry requirements. In addition to static distortions, the opto-mechanical instabilities cause astrometric errors that can be major contributors to the astrometry error budget. The present article describes the analysis, at design level, of the effects of opto-mechanical instabilities when coupled with optical surface irregularities due to the manufacturing process. We analyse the notable example of the Multi-conjugate Adaptive Optics RelaY (MAORY) for the extremely large telescope (ELT). Ray-tracing simulations combined with a Monte Carlo approach are used to estimate the geometrical structure and magnitude of field distortion resulting from the optical design. We consider the effects of distortion on the MCAO correction showing that it is possible achieve the micro-arcseconds astrometric precision once corresponding accuracy is obtained by both optical design and manufacturing. We predict that for single-epoch observations, an astrometric error below 50$\mu$as can be achieved for exposure times up to 2 min, provided about 100 stars are available to remove fifth-order distortions. Such performance could be reproducible for multi-epoch observations despite the time-variable distortion induced by instrument instabilities.

Reynolds-averaged Navier–Stokes modelling in transonic S-ducts with passive flow control

S-duct diffusers are used in aircraft with embedded engines to route ambient air to the fan face. Sizing and stealth considerations drive a need for high curvature ducts, but the curvature causes complex secondary flows that lead to total pressure distortion and swirl velocities at the engine face. These must be controlled for stable engine operation. In this paper, tubercles, a novel bio-inspired passive flow control method, are analysed numerically in a duct with transonic flow. The results are compared to experimental data obtained as part of a campaign at the Royal Military College, Canada to investigate the effects of S-duct geometry and novel passive flow control devices on the performance of transonic S-ducts. The performance of Reynolds-averaged Navier–Stokes turbulence models in the S-ducts is assessed – Menter's shear stress transport model predicts excessive losses due to the overactivity of its stress limiter. The realisable k–ɛ model gives a significant improvement in the prediction of static pressure distributions, but losses and distortion characteristics are predicted poorly due to the model's inability to resolve the effects of unsteadiness in separated regions. Large tubercle geometries are found to trigger earlier separation in the centre of the duct by concentrating low momentum fluid in valleys, but they also act as boundary layer fences away from the duct centre. Smaller geometries are found to generate vortices that re-energise the boundary layer, delaying flow separation. Methods are recommended for future computational analyses of S-ducts and new designs of tubercles.

Recovering the original physical signal from the statically distorted one by limited frequency range

Abstract Is it possible to recover the original physical signal s(t) (t denotes time) distorted by the physical converter with the static nonlinear conversion function z = f(s), which is monotonic in some range s ∈ [ s min , s max ] ? Answer is yes, if the output signal f(s(t)) has an unlimited spectrum range. Then s(t) = f−1(z(t)), where f−1(z(t)) denotes the inverse function that returns only one real root s ( t ) ∈ [ s min , s max ] corresponding to z(t) (other roots are not in our interest). Is it possible to recover the original signal s(t) ∈ [ s min , s max ] , if the spectrum range of the output signal f(s(t)) is limited to the spectrum range of the original signal s(t)? It is known that nonlinear distortion of the signal produces a plenty of high harmonics. For the first look, the initial information about the original signal is spread in these multiple harmonics, and, if we want to extract this information, we should use whole signal only. This article shows that the initial information does not disappear by filtering out the higher harmonics, and how to recover the original signal from the statically distorted one, when the frequency range of the output signal is limited to the spectrum range of the original signal s(t). In other words, if orthogonal frequency components of s(t) are considered as independent states of the information space, the information space dimension or the number of the independent states does not change in the process of the static nonlinear distortion. Usually, conventional measurement methods use linear properties of the physical converters. However, ideal linear converters are not available, and linear methods are strongly influenced either by nonlinear distortions or the low signal-to-noise ratio, if the level of the signal is artificially decreased to diminish the nonlinear distortions. Nonlinear converters are the way to solve this dilemma. They permit to significantly increase the level of the signal, and to diminish noise simultaneously. The result of this research is a new wide spectrum measurement method of s(t), when converter is nonlinear, and the frequency range of the output signal f(s) is limited to the frequency range of the original signal s(t). The original signal s(t) is expressed as Fourier series. The spectral components of the converter output f(s(t)) are determined by using Fourier transform, and limited to the spectral range of the original signal s(t). The obtained system of nonlinear equations is solved using the least squares technique that permits to find s(t) in the presence of noise. This method allows one to eliminate the influence of nonlinear distortions, and consequently to eliminate harmonic distortion and intermodulation effects. This method enables the use of the converters that are normally not suitable due to the high nonlinearity.

Exploratory study of geometric distortion correction of prostate diffusion-weighted imaging using B0 map acquisition.

Evaluation of prostate MRI relies on diffusion-weighted imaging (DWI), commonly distorted by susceptibility artifacts, thereby creating a need for approaches to correct such distortion. To compare geometric distortion on prostate MRI between standard DWI and a geometric distortion correction method for DWI described as static distortion correction DWI (SDC DWI). Retrospective case study. Thirty patients (ages 31-81 years) undergoing prostate MRI. Geometric distortions from echo planar imaging were corrected with the SDC DWI protocol, which first acquires a B0 -field map to estimate geometric distortions. Contours of the prostate were placed on axial T2 -weighted imaging (T2 WI) as an anatomic standard. Pixel shifts and apparent diffusion coefficient (ADC) values were compared between prostate contours applied to the SDC DWI and standard DWI sequences. Detailed characterization of the impact of SDC DWI was performed in three representative patients. One-way analysis of variance (ANOVA) test, Spearman correlation test, and Bland-Altman plots were calculated. There was significantly greater overlap of the SDC DWI prostate region of interest (ROI) with T2 WI than standard DWI with T2 WI (10.56 cm2 ± 3.14, P < 0.05). R2 of ADC values from standard DWI vs. SDC DWI in the 30 patients ranged from 0.02-0.94 (mean 0.60). A patient without susceptibility artifact demonstrated minimal pixel shift ranging from 0.6-1.3 mm and high correlation of ADC values (R2 = 0.89) between SDC DWI and standard DWI. A patient with rectal gas showed greater pixel shift (range: -2.5 to -0.5 mm) and less ADC value correlation (R2 = 0.69). A patient with a pelvic phlebolith adjacent to the prostate showed an even greater pixel shift (range: 10-16 mm) and decreased ADC correlation (R2 = 0.21). SDC DWI appears to correct for susceptibility-related pixel shifts in the prostate compared with standard DWI, which may have value for assessing prostate lesions obscured by geometric warping. Level of Evidence 4 Technical Efficacy Stage 1 J. Magn. Reson. Imaging 2019;50:1614-1619.

3-D magnetotelluric inversion of 2-D profiles: Application to land and marine data for shallow crustal investigation

The 3-D magnetotelluric (MT) inversion has a great potential for providing useful information on the geological background needed in petroleum and mining exploration. We developed a simple and robust full tensor 3D MT data inversion scheme that was tested on several real data sets. The technique allows to provide a detailed 3-D image of complicated geological contexts such as rift basins beneath thick basaltic screen covers. However, for practical reasons, 2-D profiles are still carried out to investigate geological questions. In such cases, tensor decomposition helps to obtain a local or regional strike which enable 2-D inversion but static distortion and effects of local and regional 3-D structures may affect the data. Ad hoc static shift techniques and heavy smoothing techniques are often used to obtain a reasonable solution but they may induce possible bias in the result. Here, we propose a 3-D inversion approach to interpret 2-D data without any decomposition technique or static correction required. The 3-D inversion uses the full MT tensor information in order to recover structures on either side of the 2-D profile. We present results from a synthetic test and applications to land and marine real data sets. Comparison between a standard 2-D approach and our local 3-D inversion provides clues about the bias induced by 2-D inversion of 3-D structures.

Strong-correlated behavior of 4f electrons and 4f5d hybridization in PrO2

Bringing oxygen atoms from infinite, passing equilibrium until short enough distances, we aim to reveal the 4f5d electron bonding property and its relevance to the peculiar physical properties within PrO2 based on both accounting for electron Coulomb repulsion and spin-orbit coupling effects in combination with Wannier function methods. The microscopic mechanism of static Janh-Teller distortions and the physical insight into the dynamic Jahn-Teller effects are clarified. Peculiarly, the magnetic coupling is suggested to be via 4f-5d-O2p-5d-4f pathway in PrO2, and the coupling between spin and orbital ordering of 4f electrons is for the first time disclosed. The 5d orbitals, hybridized with 4f electrons, are found to play important roles in these processes.

Simultaneous and inherent correction of B0 and eddy-current induced distortions in high-resolution diffusion MRI using reversed polarity gradients and multiplexed sensitivity encoding (RPG-MUSE)

In diffusion MRI (dMRI), static magnetic field (B0) inhomogeneity and time varying gradient eddy currents induce spatial distortions in reconstructed images. These distortions are exacerbated when high spatial resolutions are used, and many field-mapping based correction techniques often only acquire maps of static B0 distortion, which are not adequate for correcting eddy current induced image distortions. This report presents a novel technique, termed RPG-MUSE, for achieving distortion-free high-resolution diffusion MRI by integrating reversed polarity gradients (RPG) into the multi-shot echo planar imaging acquisition scheme used in multiplexed sensitivity encoding (MUSE). By alternating the phase encoding direction between shots in both baseline and diffusion-weighted acquisitions, maps of both static B0 and eddy current induced field inhomogeneities can be inherently derived, without the need for additional data acquisition. Through both 2D and 3D encoded dMRI acquisitions, it is shown that an RPG-MUSE reconstruction can simultaneously achieve high spatial resolution, high spatial fidelity, and subsequently, high accuracy in diffusion metrics.

Distorter Characterisation Using Mutual Inductance in Electromagnetic Tracking.

Electromagnetic tracking (EMT) is playing an increasingly important role in surgical navigation, medical robotics and virtual reality development as a positional and orientation reference. Though EMT is not restricted by line-of-sight requirements, measurement errors caused by magnetic distortions in the environment remain the technology’s principal shortcoming. The characterisation, reduction and compensation of these errors is a broadly researched topic, with many developed techniques relying on auxiliary tracking hardware including redundant sensor arrays, optical and inertial tracking systems. This paper describes a novel method of detecting static magnetic distortions using only the magnetic field transmitting array. An existing transmitter design is modified to enable simultaneous transmission and reception of the generated magnetic field. A mutual inductance model is developed for this transmitter design in which deviations from control measurements indicate the location, magnitude and material of the field distorter to an approximate degree. While not directly compensating for errors, this work enables users of EMT systems to optimise placement of the magnetic transmitter by characterising a distorter’s effect within the tracking volume without the use of additional hardware. The discrimination capabilities of this method may also allow researchers to apply material-specific compensation techniques to minimise position error in the clinical setting.

Frequency Response of Laser Gyroscopes in a Wide Range of Rotation Velocities

The laser gyro frequency response is studied by means of numerical simulation using a phase equation. The calculation results are compared to the results of experimental measurements on a precision dynamic test-bench. Quantitative relationship between the threshold value of static lock-in zone and frequency response distortions is established. Numerical methods helped to obtain and experimentally confirm the quantitative dependence of frequency response distortion on the amplitude and frequency of meander-shaped dither. It is shown that the dither shape significantly affects the frequency response form. Furthermore, it is demonstrated that frequency-response distortions depend on conditions of its measurement, in particular, on the time. The conducted research allows dither parameters to be optimized with a view to increase the accuracy of measurements using laser gyroscopes.

Effect of Inlet Distortion Features on Transonic Fan Rotor Stall

The effect of inlet distortion from curved intake ducts on jet engine fan stability is an important consideration for next-generation passenger aircraft such as the boundary layer ingestion (BLI) “silent aircraft.” Highly complex inlet flows which occur can significantly affect fan stability. Future aircraft designs are likely to feature more severe inlet distortion, pressing the need to understand the important factors influencing design. This paper presents the findings from a large computational fluid dynamics (CFD) investigation into which aspects of inlet distortion cause the most significant reductions in stall margin and, therefore, which flow patterns should be targeted by mitigating technology. The study considers the following aspects of distortion commonly observed in intakes: steady vortical distortion due to secondary flow, unsteady vortical distortion due to vortex shedding and mixing, static pressure distortion due to curved streamlines, and low momentum endwall flow due to thickened boundary layers or separation. Unsteady CFD was used to determine the stall points of a multipassage transonic rotor geometry with each of the inlet distortion patterns applied. Interesting new evidence is provided, which suggests that low momentum flow in the tip region, rather than distortion in the main body of the flow, leads to damaging instability.