Velocity Mode Research Articles

Analysis of the information overlap between the PIV and [formula omitted] chemiluminescence signals in turbulent flames using a sparse sensing framework

The purpose of this study is to quantify the information overlap between the OH* chemiluminescence signal and the three-dimensional Particle Image Velocimetry (PIV) signal for a set of bluff-body stabilized, swirling and non-swirling turbulent methane/air flames issued from the Cambridge/Sandia Stratified Swirl Burner. The time-resolved velocity data was collected using high-speed stereoscopic PIV operated at 3 kHz. This sampling rate was sufficiently high to enable accurate tracking of the large-scale spatial and temporal features of the flow. To investigate flame-flow interactions, high-speed (8 kHz) OH* chemiluminescence imaging was applied on the same set of flames. The two datasets are first independently analyzed using multi-scale Proper Orthogonal Decomposition (mPOD) for the purpose of detecting the relevant flow and heat release rate structures. The resulting modes are qualitatively compared to assess if it is possible to identify the same flow structures from the two datasets. Finally, the information overlap is quantified by predicting the velocity field from the chemiluminescence signal using a sparse sensing framework. Sparse sensing is a mathematical technique that leverages the low-dimensional representation of a physical phenomenon to predict the entire state of the system using few measurements. The results show that the velocity and chemiluminescence mPOD modes display broadly similar features, from which is possible to retrieve the same flow instabilities in the corresponding frequency range. Moreover, the prediction of the velocity field obtained using chemiluminescence as input of the sparse sensing model achieves a good level of accuracy, pointing to an important degree of information overlap between the two signals.

A method for evaluating the force factor in oscillating Kibble balances with application to the Planck-Balance

The Planck-Balance is a table-top version of a Kibble balance, developed in a collaboration between the PTB and TU Ilmenau. In contrast to most Kibble balances, where the motion range is of up to several centimeters in the velocity mode, in the Planck-Balance it is below 100 μm. For a reliable determination of the force factor, the voice coil is set into harmonic oscillation, resulting in an induced AC voltage. The amplitude of the first harmonic is then taken to determine the force factor. However, due to the non-linear magnetic field across the motion range, the first harmonic is subject to a bias, and higher harmonics are excited. Those higher harmonics contain the information about the relative magnitude of the bias in the first harmonic, and can be used for a correction. In this paper we present an analytical model and show its application with real measurements. The estimated relative bias amounts to , with a residual relative uncertainty of , for an oscillation amplitude of 20 μm. This method is especially of importance for Kibble balances with short motion range and working with harmonic oscillation. After such a correction is applied, a table-top version of a Kibble balance can be used as a conventional balance calibrated by means of E2 mass standards in the whole E2 class mass range.

Flower-shape resonator-based triple-band metamaterial wave absorbers to find the dispersion relation utilizing one dimensional (1-D) periodic waveguide

This paper proposes a flower-shape resonator-based triple band with a high-performance metamaterial wave absorber to find the dispersion relation utilizing one dimensional (1-D) periodic waveguide. The proposed absorber comprises flower-shaped square split ring resonator-based resonators separated by a Rogers RT5870 (lossy) dielectric layer with a thickness of 1.575 mm and back annealed copper with 0.035 mm thickness, and the size of the unit cell is 8 mm × 8 mm. The outcomes of simulations conducted using the Computer Simulator Technology (CST) Microwave Studio simulator reveal the presence of three absorption peaks at frequencies of 5.032, 7.96, and 13.94 GHz with an absorption of 99.709%, 99.966%, and 99.973%, respectively. As a result, this work presents a substitute concept in terms of phase velocity of mode in limited with periodic arrangements, showing it determines the type with terms of modalities a one-dimensional periodic concave parallel plates wavelength. The suggested phase velocity has been analysed using simulations using the CST Microwave Studio simulator and the CST eigenmode solver. A comparable circuit analysis demonstrates the superior performance of the metamaterial wave absorber, indicating its potential for excellent functionality within the advanced design system (ADS) software. Additionally, the proposed structures, modelled with the High-Frequency Structure Simulator (HFSS), exhibit a close correspondence with the maximum absorbance observed at each resonance peak in CST simulation results. The proposed research demonstrates excellent features such as unity absorption, angle insensitivity, and more, making it highly suitable for dispersion relation as well as C, X, and Ku band applications.

Spatio-temporal instabilities in viscoelastic channel flows: The centre mode

We study in this work the spatio-temporal characteristics of the centre-mode linear instability in viscoelastic channel flows of Oldroyd-B and FENE-P fluids. The linear complex Ginzburg–Landau equation derived using an amplitude expansion method is adopted to determine whether the flow is convectively unstable or absolutely unstable. Comparison of the obtained results with those from the conventional saddle-point searching method shows a favourable agreement. This demonstrates the good predictability of the linear complex Ginzburg–Landau equation which is more suitable for a parametric study in a large space than the conventional method. We found that the centre-mode instability in the viscoelastic channel flow is of a convective nature, i.e., the trailing edge of the wavepacket travels downstream. This could be attributed to the relatively high phase speed, low spreading rate and low growth rate of the instability, collectively preventing a disturbance from travelling upstream. Our results show that stronger polymer elasticity (a greater elastic number or larger polymer concentration) marginally affects the trailing-edge velocity of the centre mode. Decreasing the maximum polymer extensibility in the FENE-P model reduces the spreading rate of the wavepacket and makes the centre-mode disturbance more convective. Theoretical derivations at asymptotically high Reynolds numbers confirm the convective nature of the instabilities from numerical observations. Our results may be conducive to a better understanding of the spatio-temporal instability in viscoelastic experiments.

Comparison of fiber interferometric sensor with a commercial interferometer for a Kibble balance velocity calibration

This article presents a fiber interferometric sensor (FIS) for measuring the velocity amplitude of an oscillatory vibrating object, with a focus on velocity mode measurement in applications using the Kibble balance principle. The sensor uses the range-resolved interferometry method to measure the displacement of the moving object and employs a multi-harmonic sine-fit algorithm to estimate the displacement amplitude and frequency, thereby determining the velocity amplitude. This article provides a comprehensive explanation of the experimental setup and the measurement techniques employed, as well as a detailed analysis of the uncertainty budget, with the performance validation of the FIS benchmarked against a commercial interferometer within a Kibble balance setup. The velocity amplitude of a coil of the Kibble balance, oscillating with an approx. amplitude of 20 μm and a frequency of 0.25 Hz, was measured using the sensor and found to be 31.282 31 μm s−1 with a relative deviation of −1.9 ppm compared to a commercial interferometer. The high performance of the FIS, especially with regard to non-linearity errors, and the small size of the measuring head enable universality of integration into a wide variety of measurement systems, also including the use as general-purpose vibration and displacement sensor.

Entropic equilibrium for the lattice Boltzmann method: Hydrodynamics and numerical properties.

The entropic lattice Boltzmann framework proposed the construction of the equilibrium by taking into consideration minimization of a discrete entropy functional. The effect of this entropic equilibrium on properties of the resulting solver has been the topic of discussions in the literature. Here we present a rigorous analysis of the hydrodynamics and numerics of the entropic equilibrium. We demonstrate that the entropic equilibrium features unconditional linear stability, in contrast to the conventional polynomial equilibrium. We reveal the mechanisms through which unconditional linear stability is maintained, most notable of which are adaptive propagation velocity of normal modes and the positive-definite nature of the dissipation rates of hydrodynamic eigenmodes. We further present a simple local correction to considerably reduce the deviations in the effective bulk viscosity.

Exact Intermittent Solutions in a Turbulence Multi-Branch Shell Model

Reproducing complex phenomena with simple models marks our understanding of the phenomena themselves, and this is what Jack Herring’s work demonstrated multiple times. In that spirit, this work studies a turbulence shell model consisting of a hierarchy of structures of different scales ℓn such that each structure transfers its energy to two substructures of scale ℓn+1=ℓn/λ. For this model, we construct exact inertial range solutions that display intermittency, i.e., absence of self-similarity. Using a large ensemble of these solutions, we investigate how the probability distributions of the velocity modes change with scale. It is demonstrated that, while velocity amplitudes are not scale-invariant, their ratios are. Furthermore, using large deviation theory, we show how the probability distributions of the velocity modes can be re-scaled to collapse in a scale-independent form. Finally, we discuss the implications the present results have for real turbulent flows.

Tracking control of nonlinear systems actuated by saturated oscillatory force generator

This research has been devoted to the construction of a permanent-stable algorithm for tracking control of a nonlinear 1-degree of freedom (DOF) mechanical system affected by a second-order vibratory constrained actuator. The dynamics of the actuator is involved in governing equation of the mechanical system, and using the two-step Adams-Bashforth and Adams-Moulton methods, the resultant continuous-time model is formulated in the discrete-time domain. The discrete-time model of the whole system is then employed in a discrete-time power reaching law-based controller architecture. Using the reaching law strategy and considering the saturation boundaries of the actuation system, various available bounds of position and velocity control signals can be determined with respect to the reaching law coefficients. Then, special optimization algorithms are employed to determine the optimal value of position and velocity power reaching law coefficients as well as the optimally modified trajectory references. The position and velocity control commands resulting from the concept of optimal trajectory reference, along with the optimal reaching law coefficient, ensure the permanent-stable tracking control of both position and velocity modes regarding the actuation constraint. In the next step, another concept, namely the optimal combined position-velocity controller, is taken into account, which is a Gaussian-weighted average of the position and velocity control commands to form the main control command of the proposed controller. Consequently, the designed controller will be a permanent-stable control approach that never violates the actuation constraint. Finally, numerical simulations are conducted to evaluate the performance of the proposed method for two types of desired inputs, including piecewise-step and harmonic trajectories. The results suggest the proposed controller efficiency in satisfying pre-determined characteristics.

Impact of toroidal rotation on the resistive ballooning modes in ASDEX Upgrade tokamak

In this work, we investigate the behavior of instabilities appearing between type-I edge localized modes (ELMs), with increasing neutral beam injection (NBI) power concomitant increase in toroidal rotation, and compare it to the modeling result of the linear magneto-hydrodynamic (MHD) code CASTOR3D. An injection of one NBI beam, increasing toroidal rotation, results in the mode slowing down from 12 kHz to 7 kHz, and its associated radial displacement decreases from 5 mm to 3.5 mm. In addition, modes shift radially outwards towards higher q, decreasing their poloidal mode numbers. The mode velocity is measured to be close to the E × B velocity with significant uncertainties. Through a set of CASTOR3D simulations with varying profiles, resistivity has been identified as the primary contributor to the growth rates. Only a small stabilizing effect due to toroidal rotation has been observed. While experimental results show a decrease of mode frequency with rotation, the opposite trend is observed in modeling. Reasons for discrepancies between modeling and experiment are discussed. Nevertheless, a main contributor to the mode frequency has been identified to be rotation velocity. CASTOR3D classifies modes as resistive ballooning modes as they do not appear unstable in ideal MHD.

Extended spectral proper orthogonal decomposition for analysis of correlated surrounding flow structures and wind load components of a building

Proper orthogonal decomposition (POD) has been used in numerous studies in wind engineering to extract key features of a building's surrounding flow field and surface pressure, the connections between which, however, remain difficult to quantify. This study combined the extended POD with spectral POD (SPOD) method into a new method called extended SPOD (ESPOD) to correlate flow structures with surface pressure. SPOD wind force spectra were defined to quantify how much each pair of velocity and pressure modes contribute to the wind force on a building. The method was validated by a case study on a typical isolated high-rise building, in which periodic coherent structures were extracted to reveal the main mechanisms of the wind forces, including the influences from approaching turbulence, wake vortices, and conical vortices. Phase synchronization, which is utilized in ESPOD, is an effective criterion for distinguishing the multiple physical mechanisms at the same frequency. Additional information provided by the correlated velocity mode helps interpret the physical meanings of the relatively less informative pressure modes. Finally, compared to velocity-based approaches, the pressure-based approach can capture the wind force fluctuations more completely, and the velocity modes are not distorted too much by the non-optimal decomposition.

Optimization of Interplanetary Trajectory for Direct Fusion Drive Spacecraft

The Direct Fusion Drive (DFD) technology, which is being developed at present, will allow fast and affordable interplanetary travel. This is a result of the very high specific impulse and the low specific mass of DFD thrusters which outperform more conventional Nuclear Electric Propulsion (NEP), with which it shares the ability of providing a low (albeit higher than in the case of NEP) continuous thrust. It is well known that, to optimize the payload fraction, the thruster should operate in Variable Exhaust Velocity (VEV) mode and that the lower is the specific mass, the higher should be the maximum specific impulse the thruster can produce. A low thrust interplanetary travel, from the orbit around the starting planet to the orbit around the destination planet, can be considered as made of three parts: a first planetocentric phase, a second heliocentric phase and finally a third planetocentric phase; in all of them the trajectory is a sort of a spiral, but while in the first and third the spacecraft makes several (or even a large number) turns about the two planets, the second consists of a fraction of a turn about the Sun. In the first and last one the optimal specific impulse is not much variable and should remain quite low, while in the second one it must go through large variations, reaching a very high value at roughly midway between the planets. To show the potentialities of DFD, three typical fast missions are studied: to the Moon, to Mars and to Titan, showing that this propulsion device will allow humans to reach practically the whole solar system in a reasonable time. Keywords: Interplanetary travel, Human Mars Exploration, Direct Fusion Drive, Trajectory Optimization, Specific Impulse Optimization

Optimization of Interplanetary Trajectory for Direct Fusion Drive Spacecraft

The Direct Fusion Drive (DFD) technology, which is being developed at present, will allow fast and affordable interplanetary travel. This is a result of the very high specific impulse and the low specific mass of DFD thrusters which outperform more conventional Nuclear Electric Propulsion (NEP), with which it shares the ability of providing a low (albeit higher than in the case of NEP) continuous thrust. It is well known that, to optimize the payload fraction, the thruster should operate in Variable Exhaust Velocity (VEV) mode and that the lower is the specific mass, the higher should be the maximum specific impulse the thruster can produce. A low thrust interplanetary travel, from the orbit around the starting planet to the orbit around the destination planet, can be considered as made of three parts: a first planetocentric phase, a second heliocentric phase and finally a third planetocentric phase; in all of them the trajectory is a sort of a spiral, but while in the first and third the spacecraft makes several (or even a large number) turns about the two planets, the second consists of a fraction of a turn about the Sun. In the first and last one the optimal specific impulse is not much variable and should remain quite low, while in the second one it must go through large variations, reaching a very high value at roughly midway between the planets. To show the potentialities of DFD, three typical fast missions are studied: to the Moon, to Mars and to Titan, showing that this propulsion device will allow humans to reach practically the whole solar system in a reasonable time. Keywords: Interplanetary travel, Human Mars Exploration, Direct Fusion Drive, Trajectory Optimization, Specific Impulse Optimization

Rossby Waves and Zonal Flux Anomalies in Hadley and Ferrell Cell Analogs of the General Atmospheric Circulation: Model and Experiments

The circulation of the form of axysimmetrically flow with induced anomalies (disturbances) in closed rotating annular channel with conical bottom is generated by sources-sinks and MHD-method. The reducing of external forcing affects the changes of the anticyclones translation speed, but not the cyclones. The great part of moving anticyclones can disappear or stoped or new quasistationary cyclones may realized, whereas there are no change in the flow pattern in the sector where external anomalies take a place. But the variations in averaged parameters of the flow vortex are distinguished for the entire channel area and for their part. These anomalies physically are the weakness of subtropical Hadley cell with diminishing of passats in some sector of subequatorial atmospheric circulation and also the weakness the westerlies in the middle latitudes. The superposition of blocked and transported vortices are investigated with the help of simple analytical resonance interaction model for transient (with the maximum of velocity) modes in the shear flow. The amplitude of stationary background and the famous Sverdrup relation of the free surface streamfunction in the west oceanic shore intensification are affected in the same way from the beta-effect.

Equivalent elastic modulus measurement of cross-ply composite plates using Lamb waves

This paper proposes a simple and effective Lamb wave-based measurement method for the elastic moduli of cross-ply composite plates, considered equivalent to a single-layer orthotropic plate with nine independent elastic constants. The effects of theses constants on the phase velocities of the fundamental S0 and A0 modes are investigated. The phase velocity of the S0 mode depends only on the equivalent tensile elastic modulus and in-plane Poisson’s ratio in the low-dispersive frequency-thickness product regime, whereas the phase velocity of the A0 mode depends almost only on the equivalent shear elastic modulus. Moreover, the in-plane Poisson’s ratio of cross-ply laminates changes sufficiently small to neglect its effect on the phase velocities of the S0 mode. Therefore, the tensile and shear elastic moduli of the plates can be estimated using the S0 and A0 modes, and the mapping relations between the phase velocities of these modes and the elastic moduli are established. The proposed approach has been numerically and experimentally tested on cross-ply carbon fiber reinforced laminates, and validated by theoretical values and conventional tensile tests.

Patient-specific hemodynamic feature of central venous disease intervened by stent: A numerical study.

Central venous disease (CVD) with stenosis or occlusion is a severe and prevalent complication for chronic hemodialysis (HD) patients, resulting in dialysis access dysfunction. Percutaneous transluminal angioplasty with stent placement (PTS) has become one of the first-line treatments for CVD. In clinical practice, the extra stents would be used if the curative efficacy of a single stent were unsatisfactory. Aiming to evaluate the therapeutic effect of different PTS schemes, computational fluid dynamics (CFD) simulations on four patients were performed to compare the hemodynamic characteristics of real-life HD patients after stent placement. The three-dimensional central vein's models of each patient were built using computational tomography angiography (CTA) images, and idealized models were constructed as contrast. Two inlet velocity modes were imposed to imitate the blood flow rate of healthy and HD patients. The hemodynamic parameters for different patients were investigated, including wall shear stress (WSS), velocity, and helicity. The results showed that the implantation of double stents is able to improve flexibility. When subjected to external force, the double stents have better radial stiffness. This paper evaluated the therapeutic efficacy of stent placement and provided a theoretical basis for CVD intervention in hemodialysis patients.

Defect identification in variable thickness waveguides using SH0 guided wave

Guided waves, especially Lamb waves, propagating in plate-like structures with constant thicknesses have been investigated intensively in the past decades in the context of structural health monitoring (SHM) and non-destructive testing (NDT). However, structures with variable thicknesses are also very common in practice, such as ship hulls whose thickness varies for satisfying various load conditions, and defect identification for such structures is also of importance. In this paper, we compare the propagation characteristics of both Lamb modes and SH0 mode in variable thickness plates and propose to use the SH0 mode for defect identification in such waveguides because of its non-dispersive property. First, the non-dispersive property of the SH0 mode in waveguides with variable thickness is demonstrated in theory. Then, a wave prediction method is adopted to illustrate the differences between the Lamb modes and the SH0 mode when propagating in waveguides with constant or variable thickness. Finally, experiments are conducted on a variable thickness plate to verify the non-dispersive property of the SH0 mode and further compare the results with those of the Lamb modes. Recently developed thickness-poled omnidirectional shear horizontal piezoelectric transducers (OSH-PTs) are used to produce pure SH0 waves. It is observed that the velocity of the Lamb modes may change significantly and the group velocity of the S0 mode even decreases by 55.1% during propagation, which hampers Lamb modes from defect identification in variable thickness plates. By contrast, the SH0 mode is non-dispersive thus promising for damage detection, and its effectiveness for defect localization in such structures is validated experimentally.

Study on Time-frequency Imaging of Ultrasonic Detection with Phase Shifted Fiber Bragg Grating Sensing

Abstract The influence of the wavelength difference between the laser source and the phase-shifted fiber Bragg grating (PS-FBG) on the intensity of the power demodulation system based on an adjustable laser source was studied experimentally, and the optimum of the output laser wavelength was determined. Then, the research on time-frequency imaging damage identification based on smooth pseudo-Wigner-Ville distribution was carried out. The Time of Flight of the acoustic wave signal was calculated and time compensation was made according to the Wigner-Ville distribution and the Lamb wave dispersion curve. The ultrasonic waves before and after damage were measured with spatially arranged PS-FBGs. The difference signals were processed in a window, and then the time-frequency energy of the normalized difference signal was imaged to assess the damage detection and location. Although the mode and group velocity of ultrasound measured by each fiber grating were different, the accurate location and identification of artificial damage in an aluminum alloy plate was realized by using only three PS-FBGs and a smooth Wigner time-frequency imaging method.

Propagation characteristics of rotating detonation with high-temperature hydrogen gas

The rotating detonation characteristics of high-temperature hydrogen-rich gas were studied. Hydrogen-rich gas was generated by the pre-combustion of hydrogen, and a rotating detonation experiment of hydrogen-rich gas and air was subsequently performed. The auto-initiation of high-temperature hydrogen-rich gas was observed in the experiment, and the influence of pre-detonation tube ignition on the steady propagation of the detonation wave was analyzed. The results show that high-temperature hydrogen-rich gas and air have the ability to spontaneously form rotating detonation waves. The operation of the pre-detonation tube has a significant influence on the propagation mode and propagation velocity of the continuous rotating detonation wave after auto-initiation. The rotational detonation wave formed by the auto-initiation of hydrogen-rich gas and air has a short instability in the propagation process. The propagation velocity of the detonation wave before and after the unstable state is 1345.4 m/s and 1425.3 m/s, respectively, the unstable state is 1345.4 m/s and 1425.3 m/s, respectively.

Effect of Lode angle and projectile diameter on the ballistic resistance of 6061-T651 aluminum alloy plates struck by hemispherical nosed projectile

Aluminum alloy has been gradually applied to the field of armor protection because of its excellent material properties. The research on its ballistic resistance has important theoretical and engineering application value. In order to study the ballistic resistance of 6061-T651 aluminum alloy plate, the impact tests of 6061-T651 aluminum alloy plate against hemispherical nosed projectile with a diameter of 12.66 mm were carried out with a light gas gun system and the ballistic limit velocity and failure mode of the target plate were obtained. Subsequently, the numerical simulations corresponding to the tests were carried out by using the Lode dependent and independent fracture criteria, and the effect mechanism of the Lode angle on the numerical simulation results were analyzed. Finally, the effect of projectile diameter on the ballistic resistance of the target plate was analyzed by the expansion of numerical simulation. The research results show that the numerical simulation will be quite different from the test results if the effect of the Lode angle on the material fracture strain is not considered. As the diameter of the projectile increases, the armor-piercing efficiency of the hemispherical nosed projectile is improved since the energy dissipation mechanism of the target plate gradually changes from ductile hole enlargement to shear plugging.

Love‐like wave dispersion in a highly non‐homogeneous viscoelastic orthotropic layer under the effect of non‐local elasticity

This paper examines the effects of non‐local elasticity on the propagation of Love‐like waves in a Voigt‐type viscoelastic layer over an elastic half‐space. The model's geometry is considered orthotropic, highly non‐homogeneous, and initially stressed media in the context of non‐local continuum mechanics. The non‐homogeneity functions are regarded as binomial functions with a positive real exponent. Two different non‐local parameters have been defined by considering different internal characteristic lengths for the layer and half‐space. Modified Bessel functions of the first and second kinds are used to determine displacement functions. Asymptotic representations of derivatives of modified Bessel's functions have been derived to get the compact form of the dispersion equation. A comparison of phase velocity equations with classical Love wave equations has demonstrated the validity of our model. It is found that different modes of Love‐like waves are dispersive and highly affected by the attenuation coefficient. Additionally, non‐local parameters significantly influence the limitation of phase velocity modes of Love‐like waves. The phase and damped velocity versus non‐local parameters have been depicted with the effect of different parameters in the model. It has been found that the non‐local parameters decrease the particle displacement amplitude in elastic half‐space and rapidly eliminate waves with depth.