Velocity Gradient Method Research Articles

Turbulence in a wall-wake flow downstream of two horizontal cylinders

Experimental investigations of flow past two (one above other) horizontal cylinders have been presented in this paper. The experimental data captured over a rough-bed with double cylinders having three different diameters, were used for the analysis. Firstly, general characteristics such as streamwise velocity, Reynolds shear stress and turbulence intensity profiles have been elucidated at different downstream locations from cylinder position; then some advanced analysis such as length scales, turbulent kinetic energy (TKE) fluxes and budget have been investigated to exhibit their variations. Length scale profiles exhibit higher values in near bed; indicating comparatively larger eddies’ downstream of both cylinders. TKE budget profiles indicate highly negative pressure energy diffusion rate together with a sufficient TKE production rate, stabilized by the amplified TKE diffusion and dissipation rates. To determine one of the most prime behaviors of turbulence characteristics, namely TKE dissipation rate more accurately, the concept of structure function has been employed. Primarily velocity gradient method elucidates TKE dissipation rate, then it has been verified by Kolmogorov’s two-thirds law using second order structure function and Kolmogorov’s − 5/3 law using the method of power spectra.

A Linear Velocity Gradient in the Subsolar Magnetosheath

AbstractTracking the location and motion of the bow shock and magnetopause can reveal many aspects concerning the nature of the solar wind‐magnetosphere interaction. In the absence of missions that image the magnetopause, we propose to detect and track the magnetopause and bow shock motion using multipoint in situ observations of the magnetosheath. Making theoretical assumptions corroborated by numerical simulations, Sibeck et al. (2022, https://doi.org/10.1029/2022JA030704) proposed a method to predict the subsolar magnetopause and the bow shock from the linear variation in magnetosheath plasma velocity expected from global magnetohydrodynamic simulations along the Sun‐Earth line. This paper presents a case study that employs the velocity gradient method using multi‐point THEMIS plasma observations from 21 to 22 June 2007. Using the five THEMIS spacecraft we demonstrate that the radial gradient in plasma velocity remains nearly linear across the subsolar magnetosheath, except when the magnetosheath lies behind the quasi‐parallel bow shock and conditions are very turbulent. The bow shock and magnetopause move outward in response to a gradual decrease in solar wind dynamic pressure. We show that results from the velocity gradient method agree well with predictions of global magnetohydrodynamic and empirical models for this time interval. The velocity gradient method can therefore be used to remotely determine the location of the magnetopause and bow shock, solely based on two‐spacecraft measurements at the subsolar magnetosheath when not downstream of the quasi‐parallel shock.

Massive Molecular Gas Reservoir in a Luminous Submillimeter Galaxy during Cosmic Noon

Abstract We present multiband observations of an extremely dusty star-forming lensed galaxy (HERS1) at z = 2.553. High-resolution maps of HST/WFC3, SMA, and ALMA show a partial Einstein ring with a radius of ∼3″. The deeper HST observations also show the presence of a lensing arc feature associated with a second lens source, identified to be at the same redshift as the bright arc based on a detection of the [N ii] 205 μm emission line with ALMA. A detailed model of the lensing system is constructed using the high-resolution HST/WFC3 image, which allows us to study the source-plane properties and connect rest-frame optical emission with properties of the galaxy as seen in submillimeter and millimeter wavelengths. Corrected for lensing magnification, the spectral energy distribution fitting results yield an intrinsic star formation rate of about 1000 ± 260 M ⊙ yr−1, a stellar mass M * = 4.3 − 1.0 + 2.2 × 10 11 M ⊙ , and a dust temperature T d = 35 − 1 + 2 K. The intrinsic CO emission line (J up = 3, 4, 5, 6, 7, 9) flux densities and CO spectral line energy distribution are derived based on the velocity-dependent magnification factors. We apply a radiative transfer model using the large velocity gradient method with two excitation components to study the gas properties. The low-excitation component has a gas density n H 2 = 10 3.8 ± 0.6 cm−3 and kinetic temperature T k = 18 − 5 + 7 K, and the high-excitation component has n H 2 = 10 3.1 ± 0.4 cm−3 and T k = 480 − 220 + 260 K. Additionally, HERS1 has a gas fraction of about 0.19 ± 0.14 and is expected to last 100 Myr. These properties offer a detailed view of a typical submillimeter galaxy during the peak epoch of star formation activity.

Parametric synthesis of stabilizing neurocular control of a technological module

The article proposes a new approach to combined parametric synthesis (synthesis in real or accelerated time) of neuroregulators of multidimensional and multi-connected physical systems and technological processes based on the application of the velocity gradient method in differential form and the theory of sliding modes.

The abundance of S- and Si-bearing molecules in O-rich circumstellar envelopes of AGB stars.

We aim to determine the abundances of SiO, CS, SiS, SO, and SO2 in a large sample of oxygen-rich asymptotic giant branch (AGB) envelopes covering a wide range of mass loss rates to investigate the potential role that these molecules could play in the formation of dust in these environments. We surveyed a sample of 30 oxygen-rich AGB stars in the λ 2 mm band using the IRAM 30m telescope. We performed excitation and radiative transfer calculations based on the large velocity gradient (LVG) method to model the observed lines of the molecules and to derive their fractional abundances in the observed envelopes. We detected SiO in all 30 targeted envelopes, as well as CS, SiS, SO, and SO2 in 18, 13, 26, and 19 sources, respectively. Remarkably, SiS is not detected in any envelope with a mass loss rate below 10-6 M⊙ yr-1, whereas it is detected in all envelopes with mass loss rates above that threshold. From a comparison with a previous, similar study on C-rich sources, it becomes evident that the fractional abundances of CS and SiS show a marked differentiation between C-rich and O-rich sources, being two orders of magnitude and one order of magnitude more abundant in C-rich sources, respectively, while the fractional abundance of SiO turns out to be insensitive to the C/O ratio. The abundance of SiO in O-rich envelopes behaves similarly to C-rich sources, that is, the denser the envelope the lower its abundance. A similar trend, albeit less clear than for SiO, is observed for SO in O-rich sources. The marked dependence of CS and SiS abundances on the C/O ratio indicates that these two molecules form more efficiently in C- than O-rich envelopes. The decline in the abundance of SiO with increasing envelope density and the tentative one for SO indicate that SiO and possibly SO act as gas-phase precursors of dust in circumstellar envelopes around O-rich AGB stars.

A Godunov-type tensor artificial viscosity for staggered Lagrangian hydrodynamics

This paper describes a tensor artificial viscosity for staggered Lagrangian hydrodynamics. Specifically, two viscous tensors are constructed using a discrete velocity gradient method. Under the condition of a piecewise constant distribution of velocity in each subcell, the Generalized Riemann Invariant relation, in the spirit of the Godunov methods, is applied to determine the coefficients of the viscous tensors. The artificial viscosity is found to be dissipative. Besides, the cylindrical symmetry of the developed artificial viscosity is demonstrated in an equi-angular polar grid. Typical numerical cases with strong shocks show that the tensor artificial viscosity is robust and performs well in different grid types.

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The problem of adaptive control system synthesis using the local criterion is considered. The criterion selection allows you to take into account ship course errors and desired small deviations of rudder. The theoretical basis for solving this problem is the velocity gradient method. To obtain laws (algorithms) for adjusting the regulator parameters, a local quadratic quality criterion that is oriented on ensuring small deviations from the set course and keeping the ship’s heading has been chosen. Adaptive course control systems, obtained on the basis of theoretical research, are modeled in the MATLAB environment. It is shown that the originally obtained adaptation laws can be modified by coarsening due to introducing a negative feedback on the tunable parameter. The results of modeling the ship course control system under the influence of an external disturbance are also presented in the paper. The algorithm developed in the paper for setting the regulator parameters allows controlling the ship movement along the course under wind-wave action with wave amplitude of 6 degrees and the constant wave component of 6 degrees. The results of modeling the ship course control system without setting the regulator and with the setting enabled confirm the effectiveness of the proposed approach.

Разработка искусственной нейронной сети для управления характеристиками амортизационных стоек шасси самолетов

During their calendar life, passenger and transport aircraft run more than 200 thousand kilometers on the runways, which cause a significant part of the damage, both in the landing gears and in other units of the airframe. To reduce aircraft overloads at the stages of takeoff and landing (run-up and run on the runway) and taxiing, shock-absorbing struts with variable elastic-damping characteristics are used. Due to the fact that the parameters of the runway irregularities are in a wide range of values, it is necessary to use an adaptive system for controlling the stiffness coefficients and damping of the shock absorber strut, designed using an artificial neural network. The paper considered a network containing three layers. Using such a model, it is possible to implement an adaptive control circuit adjusting the elastic-damping parameters of the aircraft shock absorber struts to specific runway conditions (length and height of the irregularity, specific hardness of the runway). The velocity gradient method was used to train the artificial neural network. Half the square of the mismatch signal was used as the target criterion to be minimized. The calculated studies of the run up and run of the Il-114 aircraft on a dirt runway showed the possibility of reducing vertical overloads by up to 15% when equipped with a system controlling elastic-damping characteristics with a neural network. The comparison was carried out with an aircraft equipped with a “classical” (non-adaptive) system for controlling landing gear parameters.

Study of CS, SiO, and SiS abundances in carbon star envelopes: Assessing their role as gas-phase precursors of dust.

We aim to determine the abundances of CS, SiO, and SiS in a large sample of carbon star envelopes covering a wide range of mass loss rates to investigate the potential role that these molecules could play in the formation of dust in the surroundings of the central AGB star. We surveyed a sample of 25 carbon-rich AGB stars in the λ 2 mm band, more concretely in the J = 3 - 2 line of CS and SiO, and in the J = 7 - 6 and J = 8 - 7 lines of SiS, using the IRAM 30 m telescope. We performed excitation and radiative transfer calculations based on the large velocity gradient (LVG) method to model the observed lines of the molecules and to derive their fractional abundances in the observed envelopes. We also assessed the effect of infrared pumping in the excitation of the molecules. We detected CS in all 25 targeted envelopes, SiO in 24 of them, and SiS in 17 sources. Remarkably, SiS is not detected in any envelope with a mass loss rate below 10-6 M⊙ yr-1 while it is detected in all envelopes with mass loss rates above that threshold. We found that CS and SiS have similar abundances in carbon star envelopes, while SiO is present with a lower abundance. We also found a strong correlation in which the denser the envelope, the less abundant are CS and SiO. The trend is however only tentatively seen for SiS in the range of high mass loss rates. Furthermore, we found a relation in which the integrated flux of the MgS dust feature at 30 μm increases as the fractional abundance of CS decreases. The decline in the fractional abundance of CS with increasing density could be due to gas-phase chemistry in the inner envelope or to adsorption onto dust grains. The latter possibility is favored by a correlation between the CS fractional abundance and the 30 μm feature, which suggests that CS is efficiently incorporated onto MgS dust around C-rich AGB stars. In the case of SiO, the observed abundance depletion with increasing density is most likely caused by an efficient incorporation onto dust grains. We conclude that CS, SiO (very likely), and SiS (tentatively) are good candidates to act as gas-phase precursors of dust in C-rich AGB envelopes.

Three regimes of CO emission in galaxy mergers

The conversion factor αCO from the observable CO(1-0) luminosity to the mass of molecular gas is known to vary between isolated galaxies and some mergers, but the underlying reasons are not clearly understood. Thus, the value(s) of αCO that are to be adopted remain highly uncertain. To provide better constraints, we applied the large velocity gradient method to a series of hydrodynamical simulations of galaxies and derived the evolution of αCO. We report significant variations of αCO, and identify three distinct regimes: disk galaxies, starbursts, and post-burst phases. We show that estimating the star formation rate over 20Myr smoothes out some of these differences, but still maintains a distinction between disks and starbursts. We find a tighter correlation of αCO with the gas depletion time than with star formation rate, but deviations are induced by the transitions to and from the starburst episodes. We conclude that αCO fluctuates because of both feedback energy and velocity dispersion. Identifying the phase of an interaction by classical means (e.g., morphology or luminosity) could then help to select the relevant conversion factor that is to be used and to obtain more accurate estimates of the molecular masses of galaxies.

HYBRID LATTICE BOLTZMANN AND FINITE VOLUME METHODS FOR FLUID FLOW PROBLEMS

Two hybrid methods that combine the lattice Boltzmann method (LBM) and finite volume method (FVM) are proposed in this paper. The D2Q9 model is used in LBM, while the SIMPLE algorithm with QUICK scheme is chosen for FVM. The hybrid methods are fulfilled by dividing the computational domain into the LBM and FVM zones, with a public message-passing zone in between, and only macroscopic velocities are needed to be passed between the two separated zones. A nonequilibrium extrapolation scheme and the finite difference velocity gradient method are used to pass information between the SIMPLE and LBM zones. The proposed methods are used to solve problems with the liddriven flow, and the results obtained using pure LBM, pure SIMPLE, and the hybrid methods with different schemes agreed with each other very well in different Reynolds numbers.

Decentralized adaptive control of synchronization of dynamic system networks at bounded disturbances

The problem is considered for adaptive synchronization in the output of the network of interconnected nonlinear dynamic systems with bounded disturbances. The structure of a controller and the adaptation algorithm are found with the aid of the velocity gradient method and the passification method. The sufficient conditions of synchronization and the upper bound for the convergence set relative to the leading subsystem are given. The conditions of the accessibility of synchronization are also obtained for a certain class of monotone nonlipschitz nonlinearities. The results are illustrated by an example of synchronization of the network of interconnected Chua chains with disturbances.

Comparison of SAR and optical data in deriving glacier velocity with feature tracking

Feature tracking is an efficient way to derive glacier velocity. It is based on a cross-correlation algorithm that seeks offsets of the maximal correlation windows on repeated satellite images. In this paper we demonstrate that different window sizes lead to different velocities. The averaged velocity gradient (AVG) method is proposed to improve window sizes in feature tracking and to obtain the most suitable flow field. The AVG method measures velocity variation between adjacent windows on the whole glacier in the image. Different window sizes lead to different AVG values, and the best-size window corresponds to the value where the AVG changes from abrupt to gradual. Using improved feature tracking, two flow fields of the same glacier are acquired with Advanced Land Observing Satellite (ALOS) optical and synthetic aperture radar (SAR) data, respectively. The advantages, application conditions, accuracy and disadvantages of the two kinds of data using the feature tracking method are discussed.

A particle velocity gradient beam forming system.

The topic of this paper is the determination of the acoustic source distribution in the far field with a small, three dimensional (3-D) system consisting closely spaced sound pressure sensitive microphones and particle velocity sensitive Microflown. Sound pressure sensors do have a zero order directionality (that is, no directionality). Particle velocity vector sensors have a first order directionality (this is a cosine shape directionality). With two closely spaced zero order sensors, a first order system can be created. Disadvantages are the low sensitivity for low frequencies and a limited high-frequency response. With two closely spaced first order sensors, a second order system is created. The directionality is a squared cosine shape. With this higher directivity it is possible to create a very small 3-D beam forming system with a reasonable resolution which is the topic of this paper. A second order system can be made with accelerometers or pressure sensors; however, the low-frequency response is very poor. The Microflown has a very high sensitivity at low frequencies so the velocity gradient signal is good. In this paper the velocity gradient method is presented, and a 3-D velocity gradient system is demonstrated.

PREDICTION OF EROSION EFFECT DUE TO CAVITATION ON AL-MOSUL POWER PLANT TURBINE

In the field of hydraulic power plant, the cavitation is responsible of sever erosion which requires periodic unit shutdowns for inspection and repairs. Al-Mosul hydroelectric power plant is chosen as model in this study. Computer programs are developed by using the velocity gradient method to analyze the flow in the runner blades of the turbine (Francis Turbine) to calculate the available cavitation and compared with the critical cavitation. The erosion of the runner material (erosion rate, weight of lost material and mean depth of erosion) is also calculated to limit the operation hours of the power plant. The presented work shows that the cavitation appears on the underside of the turbine (Francis Turbine) blades in the trailing edge at distance of 82% from the leading edge due to decrease in pressure, flow separation and interference zone. This causes erosion depth of about 4 mm for the first four years of operation which represents about 17% of the blade thickness of the trailing edge. It is found that the operation hours of Al-Mosul power plant should not exceed 24000 operation hour. A good agreement is found between the prototype data obtained from the computer program analysis and experimental visualization shown in the literature and theoretical solution.

Near-bed hydrodynamic measurements above boulders in shallow torrential streams: Implications for stream biota

The near-bed hydraulic environment inhabited by torrential stream fauna was characterized by recording velocity profiles, near-bed velocities, and local wall shear stresses over the upper surface of boulders (or stones) in a mountain stream located in eastern British Columbia. Velocity profiles regularly deviated from a semi-logarithmic shape, and were often found to be "wedge-shaped", due likely to near-bed acceleration resulting from flow constriction over the leading edge of the stones. Local bed shear stress (τw) measured using a Preston-static tube (PST) was generally lowest over the leading edge of the boulders, and increased over the upper surface, reaching a maximum near the rear of the stone (upstream of the point of flow separation). Local bed shear stresses measured by the PST were similar to those estimated from the law of the wall (i.e., velocity gradient method based on estimating τw from the slope of the regression of U on ln(z)) only at locations where velocity profiles were log-normally distributed. Where velocity profiles were wedge-shaped, the law of the wall underestimated τw compared to the direct measurements using the PST. The possible influences of torrential streambed geometry and relative submergence on near-bed flow parameters and the ecology of stream fauna are discussed. Key words: benthic, velocity profile, shear stress, Preston-static tube, law of the wall, rough turbulent flow.

The Water Vapor Abundance in Circumstellar Envelopes

The maser emission of the para-H2O 313 → 220 line at 183 GHz in O-rich evolved stars has been modeled to account for the empirical characteristics of this line reported by González-Alfonso et al. Likewise, efforts have been made to derive water vapor abundance in these sources. The Sobolev or large velocity gradient (LVG) method has been employed to study the intensity of this line as a function of source properties and physical conditions (i.e., mass-loss rate, p-H2O abundance, velocity field, kinetic temperature profile, stellar luminosity, and the set of collisional rates adopted in the calculations). It has been found that the intensity of the 313 → 220 line is sensitive to the mass-loss rate, the p-H2O abundance, and the terminal velocity of the envelope, but it is rather insensitive to the rest of the parameters in stars with high mass-loss rates ( > 10-6 M☉ yr-1). The models reproduce the main spectral characteristics of the emission at 183 GHz in the latter sources. A global fit to the data proves that the observational luminosities can be explained by assuming an H2O abundance relative to H2 [x(H2O)] of 1-2 × 10-4. Detailed fitting to the line profile in five selected objects yields a similar value for x(H2O). The validity of the LVG approach has been verified by modeling the maser emission at 183 GHz through a nonlocal radiative transfer code. The model calculations with both methods lead to similar results. The pumping of the first bending mode of water vapor through absorption of photons emitted by the dust and the star has been also simulated. This effect is found to be important in the pumping of the H2O rotational levels. Hence, in order to recover LVG results, the water abundance must be increased by a factor of ≈2 for stars with high mass-loss rates. Consequently, x(H2O) has been estimated to be 3 × 10-4 within a factor ≃2. With this value for x(H2O), the expected near- and far-infrared spectra of the circumstellar envelopes of O-rich stars for several mass-loss rates have also been computed. Hence, it is possible to predict that, in some stars, the ro-vibrational lines of the 6 μm water vapor band with wavelengths longer that 6.3 μm—the P-branch—can be observed in emission, rather than in absorption.

Highly purified human immunodeficiency virus type 1 reveals a virtual absence of Vif in virions.

The vif gene of human immunodeficiency virus type 1 (HIV-1) is essential for the productive infection of primary blood-derived lymphocytes, macrophages, and certain human T-cell lines. It has been shown that Vif is associated with HIV-1 virions purified by sucrose density-equilibrium gradient analysis. However, the specificity of Vif incorporation into virions has not been determined. Moreover, recent studies have demonstrated that standard HIV-1 particle preparations created with sucrose density-equilibrium gradients are contaminated with cell-derived microvesicles. Here we demonstrate, as previously reported, that Vif cosediments with HIV-1 particles in sucrose density-equilibrium gradient analysis. However, we also found that, when Vif was expressed in the absence of all other HIV-1-encoded gene products and then isolated by sucrose density-equilibrium gradient centrifugation from extracellular supernatants, its sedimentation pattern was largely unaltered, suggesting that Vif can be secreted from cells. Using a newly developed OptiPrep velocity gradient method, we were able to physically separate most of the extracellular Vif from the HIV-1 virions without disrupting the infectivity of the virus. By titrating serial dilutions of purified Vif and Gag against the viral peak fraction in the OptiPrep gradient, we demonstrate that <1.0 Vif molecule per virion was present. This study shows that Vif is not significantly present in HIV-1 virions, a finding which is consistent with the idea that Vif functions predominantly in the virus-producing cells during virus assembly. The OptiPrep velocity gradient technique described here could be an easy and rapid way to purify HIV and other enveloped viruses from microvesicles and/or cell debris.

Crevasse patterns and the strain-rate tensor: a high-resolution comparison

AbstractValues of the strain-rate tensor represented at a 20 m length scale are found to explain the pattern and orientation of crevasses in a 0.13 km2 reach of Worthington Glacier, Alaska, U.S.A. The flow field of the reach is constructed from surveyed displacements of 110 markers spaced 20-30 m apart. A velocity gradient method is then used to calculate values of the principal strain-rate axes at the nodes of a 20 m x 20 m orthogonal grid. Crevasses in the study reach are of two types, splaying and transverse, and are everywhere normal to the trajectories of greatest (most tensile) principal strain rate. Splaying crevasses exist where the longitudinal strain rate (x) is ≤ 0 and transverse crevasses are present under longitudinally extending flow (i.e. x &gt; 0). The orientation of crevasses changes in the down-glacier direction, but the calculated rotation by the flow field does not account for this change in orientation. Observations suggest that individual crevasses represent local values of the regional flow field and are transient on the time-scale of 1-2 years; they are not persistent features that are translated and rotated by flow. Crevasse patterns are thus found to be a useful tool for mapping the strain-rate tensor in this reach of a temperate valley glacier.

Crevasse patterns and the strain-rate tensor: a high-resolution comparison

AbstractValues of the strain-rate tensor represented at a 20 m length scale are found to explain the pattern and orientation of crevasses in a 0.13 km2reach of Worthington Glacier, Alaska, U.S.A. The flow field of the reach is constructed from surveyed displacements of 110 markers spaced 20-30 m apart. A velocity gradient method is then used to calculate values of the principal strain-rate axes at the nodes of a 20 m x 20 m orthogonal grid. Crevasses in the study reach are of two types, splaying and transverse, and are everywhere normal to the trajectories of greatest (most tensile) principal strain rate. Splaying crevasses exist where the longitudinal strain rate (x) is ≤ 0 and transverse crevasses are present under longitudinally extending flow (i.e.x&amp;gt; 0). The orientation of crevasses changes in the down-glacier direction, but the calculated rotation by the flow field does not account for this change in orientation. Observations suggest that individual crevasses represent local values of the regional flow field and are transient on the time-scale of 1-2 years; they are not persistent features that are translated and rotated by flow. Crevasse patterns are thus found to be a useful tool for mapping the strain-rate tensor in this reach of a temperate valley glacier.