Velocity Disturbances Research Articles

Linear instability of concentric annular flow: Effect of Prandtl number and gap between cylinders

The stability of non-isothermal Poiseuille flow in a coaxial annular domain is studied using normal mode analysis. The stably stratified flow (i.e., when buoyant force is in the direction of forced flow) is induced due to external pressure gradient and maintenance of linear variation of the temperature of the inner cylinder. We have emphasized the impact of the gap between cylinders in terms of curvature parameter (C) and Prandtl number (Pr) on the instability of the flow under axisymmetric as well as non-axisymmetric disturbances. In general, it has been found that the flow of low Pr fluids under axisymmetric disturbance is more stable in comparison with the flow under non-axisymmetric disturbance. When the gap between cylinders is relatively small (i.e. C is large), the flow under non-axisymmetric is most stable for the fluids having high Pr. It is known that the Newtonian pipe flow is linearly stable for all values of Reynolds number (Re). However, in the present study, we have seen the instability of the pipe flow with a thin rod placed at the center even for a very small value of heat source intensity (in terms of Rayleigh number) at Re=2000. Here the disturbance velocity is concentrated in the vicinity of the inner wall and instability is due to transfer of kinetic energy from the basic flow through Reynold’s stress. Depending on the value of controlling parameters, three types of instability: thermal-shear, interactive, thermal-buoyant have been observed. In general, for a fixed value of Pr, the type of instability is not affected by C and Re. The non-isothermal Poiseuille flow of low Pr (including mercury and gases) is governed by thermal-shear instability and the same of high Pr (including liquid and oil) is governed by thermal-buoyant instability.

Multiparameter identification for SPMSMs using NLMS adaptive filters and extended sliding‐mode observer

The authors propose a parameter identification method for sequential identification of electrical and mechanical parameters of surface-mounted permanent magnet synchronous motors (SPMSMs). Two normalised least mean square (NLMS) adaptive filters (AFs) are designed for identifying the electrical parameters, where the first AF identifies the stator inductance and the second AF identifies the stator resistance and rotor flux linkage. The NLMS AFs achieve faster transient responses than recursive least squares (RLS) AFs owing to lower computing load and smaller memory size. Regarding mechanical parameters, an extended sliding-mode mechanical parameter observer (ESMMPO) is employed to estimate the system disturbance and angular velocity, from which the rotational inertia, viscous damping coefficient, and load torque are identified. The rotor flux linkage identified from the second NLMS AF is used for estimating the real-time system disturbance of the ESMMPO, which enables the identification of mechanical parameters with higher accuracy. The proposed method effectively integrates the NLMS AFs and ESMMPO into a single framework to identify both electrical and mechanical parameters of the SPMSMs. The experimental results of the proposed method are compared with those of RLS AFs and the conventional ESMMPO, which demonstrates the faster response and less steady-state parameter errors of the proposed method.

Fixed-time Output Feedback Consensus of Second-order Multi-agent Systems with Settling Time Estimation

This study discusses fixed-time consensus problem of second-order multi-agent systems with unmeasur-able velocity and uncertain disturbance. The proposed control scheme includes two parts: one part is a fixed-time convergent state observer to estimate the unknown velocity while the other part is a fixed-time consensus algorithm based on integral sliding mode. Mathematical proof is given and some stability conditions are derived. Moreover, the settling time depends on the parameters of state observer and consensus algorithm, which can be theoretically estimated offline regardless of initial states. Finally, the proposed control scheme is employed to coordinated control of single-link robotic manipulators and the simulation examples verify the efficiency of the results.

Improvement of Backdrivability of a Force-Controlled EHA by Introducing Bypass Flow Control

The dynamic response of an electro-hydrostatic actuator (EHA) is limited due to rotational inertia of its servomotor and pump. Thus, when an external disturbance with a high frequency is input to the force-controlled EHA, the pump speed may not be fast enough to attenuate the force control errors. In order to expedite the oil transfer between the cylinder chambers, a bypass valve was employed in this study. The bypass valve connecting both the cylinder chambers can then decompress the high-pressure chamber quickly by bypassing the oil to the low-pressure chamber to compensate for force control errors. To control the bypass flow without affecting the system stability, while the rotational velocity of pump was continuously varied by the force controller, a sliding mode control technique was applied. The sliding mode controller showed satisfactory control performance with respect to stability and robustness, in spite of highly nonlinear properties in the proposed system. Experimentally, where sinusoidal or stepwise velocity disturbance was exerted on the proposed EHA system with the force reference values set at 0 N or 200 N, the force control errors could be reduced markedly, compared to a conventional force-controlled EHA without bypass flow control. The performance improvement was more obvious when the sinusoidal excitation frequency increased from 1 to 3 Hz.

A novel varistructure grey forecasting model with speed adaptation and its application

Traditional grey forecasting models cannot achieve satisfactory predictive accuracy on approximate non-homogeneous exponential sequence with acceleration, velocity and constant perturbation terms. Hence, it is hard to forecast the developing trend of non-homogeneous exponential data sequences widespread in the real world. It is for this reason that an extended intelligent grey forecasting model with variable speed and adaptive structure (VSSGM for short) is provided in this paper. The proposed model possesses three advantages, namely, strong adaptability to perturbation sequence, self-changing structure, and outstanding accuracy in simulation and prediction. One prominent property is that it can simulate/forecast any given inhomogeneous sequence with acceleration, velocity and constant disturbances, which are the most common perturbations widespread in nature and society. And the proposed VSSGM model can be reducible to DGM(1,1), NDGM, SAIGM and VCGM models in theory. Therefore, the proposed model outperforms the traditional grey models. In order to verify the effectiveness, universality, practicality and feasibility of the model, we conducted an experiment to analyze the precision of the model and employ it to simulate China’s industrial added value from 2006 to 2015, then utilize it to forecast the industrial added values from 2019 to 2022. Comparing with the real statistic data, it shows that the proposed model has better prediction performance than traditional grey models.

Vortex dynamics and scalar transport in the wake of a flat-plate controlled by a vibrating trailing-edge flap

We investigate the onset and development of vortical flow disturbances introduced into the wake of a horizontally fixed flat-plate by means of the controlled motion of a trailing edge flap. The vibrating mechanics of the flap allows for the introduction of both impulsive and harmonic weak amplitude velocity disturbances which are propagated downstream into the wake flow of the flat-plate. Quantitative experimental and numerical predictions of both steady and unsteady wake flow velocity resulting from different flapping frequencies are made at low Reynolds numbers (Re < 104). Frequency response tests of the wake confirmed the existence of two dominant frequencies where the wake flow organises with a particular arrangement of downstream moving vortex structures. Numerical predictions of steady (unforced) and forced wake velocity profiles and kinetic energy profiles are in good agreement with the experimental results. In order to understand practical implications of the dominant vortex structures in scalar transport, we have extended the numerical part of the study solving for the concentration equation of a passive scalar being injected in particular regions of the physical domain. A spatial correlation between the trajectory of vortex structures and the scalar concentration downstream the wake is observed. Moreover, the onset of tip vortex structures produced during the forcing cycle seems to be responsible of a local increase of scalar concentration near the span wise flap ends.

A Study of the Performance of a Wind-Lens Turbine in Steady Wind with Velocity Disturbance

A Study of the Performance of a Wind-Lens Turbine in Steady Wind with Velocity Disturbance

Numerical simulation of propeller jet field based on Star-ccm+

During the ship’s voyage, the propeller jet affects the movement of silt at the bottom of the bed. To research the influence of the bottom boundary on the propeller jet field, this paper takes the standard propeller DTRC4119 propeller as the research object and uses the CFD software Star-ccm+ to carry out a numerical simulation of the propeller jet under uniform flow. The flow velocity distribution of the jet under four operating conditions is mainly analyzed, including the axial velocity, tangential velocity and radial velocity of the jet. The results show that the distance between the propeller and the boundary does not affect the magnitude and distribution of the velocity on the initial plane but affects the shape of the axial velocity and the velocity on the central axis in the development zone; The closer to the bottom boundary, the greater the disturbance of tangential velocity and radial velocity, the peak value of tangential velocity will be affected and changed from central symmetry of velocity to unanimous trend earlier. The radial velocity contributes less to the overall velocity and can be ignored

An iterative method for solving nonlinear equations of real gas pipeline transport

The study is devoted to the development of a numerical method to solve the equations of pipeline transport of compressible gas, taking into account the forces of resistance, gravity, local and convective components of the gas inertia force in the isothermal mode of factors. The power of the unknowns in the momentum conservation equation is reduced with the introduction of the auxiliary function of natural logarithm of the reduced density. Autonomous equations of linear combinations of the hydrodynamic flow rate and a newly introduced function were compiled with the participation of the propagation velocity of small pressure disturbances. In this paper, an iterative method is developed to solve equations for the implementation of the input condition for pressure and output condition for mass flow; it allows determining the unknown boundary and internal values of gas-dynamic parameters. The results of a computational experiment on the route change in flow indices at various slopes of the gas pipeline route are analyzed.

Управление течением в гиперзвуковом пограничном слое

A method of flow control in a two-dimensional hypersonic boundary layer under strong viscous-inviscid interaction is considered. The effect of blocking the process of transferring disturbances upstream due to a decrease in the propagation velocity of disturbances with a decrease in the temperature factor of the surface area is shown.

THE STUDY OF DISTURBANCE PROPAGATION IN FLOWS WITH REVERSED STREAMLINES

The processes of disturbance propagation in laminar boundary layers at high external flow velocities are studied. For the first time, solutions to the Faulkner−Skan equation for compressible flows are obtained, in addition to those found by Stewartson. The dependence of the propagation velocity of disturbances upstream on the pressure gradient parameter is determined for various values of the temperature factor.

Mechanism of the Occurrence of Rotordynamic Fluid Forces on a Closed Type Centrifugal Impeller in Whirling Motion

Rotordynamic fluid forces and a mechanism of their occurrence were examined in experiments and computations for a closed type centrifugal impeller in whirling motion. The rotordynamic fluid force on a front shroud was a main component of the rotordynamic fluid force. In a clearance between the front shroud and a casing, velocity disturbances were caused by an eccentricity of the impeller and a squeeze effect due to the whirling motion of the impeller. Appling the Bernoulli equation to disturbed swirling flow in the clearance in a relative coordinate fixed on the whirling motion, the pressure disturbance which generated the rotordynamic fluid force could be reasonably explained.

Resolving the Interstellar Medium in Ultraluminous Infrared QSO Hosts with ALMA

Abstract We present ALMA observations of the CO(1−0) line and 3 mm continuum emission in eight ultraluminous infrared (IR) quasi-stellar objects (QSOs) at z = 0.06–0.19. All eight IR QSO hosts are clearly resolved in their CO molecular gas emission with a median source size of 3.2 kpc, and seven out of eight sources are detected in 3 mm continuum, which is found to be more centrally concentrated with respect to molecular gas with sizes of 0.4−1.0 kpc. Our observations reveal a diversity of CO morphology and kinematics for the IR QSO systems, which can be roughly classified into three categories: rotating gas disk with ordered velocity gradient, compact CO peak with disturbed velocity, and multiple CO distinct sources undergoing a merger between a luminous QSO and a companion galaxy separated by a few kpc. The molecular gas in three of the IR QSO hosts is found to be rotation-dominated with a ratio of the maximum rotation velocity to the local velocity dispersion of V rot/σ = 4–6. Basic estimates of the dynamical masses within the CO-emitting regions give masses between 7.4 × 109 and 6.9 × 1010 M ⊙. We find an increasing trend between black hole mass accretion rate and star formation rate (SFR) over 3 orders of magnitude in far-IR luminosity/SFR, in line with the correlation between QSO bolometric luminosity and star formation activity, indicative of a likely direct connection between active galactic nuclei and star formation activity over galaxy evolution timescales.

Electrophoresis in dilute polymer solutions

We analyse the electrophoresis of a weakly charged particle with a thin double layer in a dilute polymer solution. The particle velocity in polymer solutions modelled with different constitutive equations is calculated using a regular perturbation in the polymer concentration and the generalized reciprocal theorem. The analysis shows that the polymer is strongly stretched in two regions, the birefringent strand and the high-shear region inside the double layer. The electrophoretic velocity of the particle always decreases with the addition of polymers due to both increased viscosity and fluid elasticity. At a small Weissenberg number ( ), which is the product of the polymer relaxation time and the shear rate, the polymers inside the double layer contribute to most of the velocity reduction by increasing the fluid viscosity. With increasing , viscoelasticity decreases and shear thinning increases the particle velocity. Polymer elasticity alters the fluid velocity disturbance outside the double layer from that of a neutral squirmer to a puller-type squirmer. At high , the strong extensional stress inside the birefringent strand downstream of the particle dominates the velocity reduction. The scaling of the birefringent strand is used to estimate the particle velocity.

FEM for Blood-Based SWCNTs Flow Through a Circular Cylinder in a Porous Medium with Electromagnetic Radiation**Supported by Deanship of Scientific Research, Northern Border University under Grant No. SCI-2018-3-9-F-7614, KSA

This work aims to study magnetohydrodynamic flow through a circular cylinder in a horizontal position of SWCNTs in blood as a base fluid in the existence of non-linear thermal radiation and heat source/sink. Three kinds of nanoparticles shapes are considered. The study is employed the finite element technique to explore and enhance the influences of essential parameters on temperature profiles and is debated the heat transport within blood injects with SWCNTs and exposes to electromagnetic radiation. The treatment with thermal analysis and heat transfer rate being a better substitute more than surgery and chemotherapy for cancer therapy. Utilizing of nanoparticles thermal features is a mounting area of nanomedicine field because of the probable for purposeful demolition of cancer cells. This remedy is relied on many parameters, including nanofluid thermal conductivity, nanoparticles volume fraction, thermal radiation and power and heat source. The numerical solutions for flow and heat transfer features are assessed for diverse governing parameters values. The obtained results are substantiated against the relevant numerical results in the published researches. Results show that both flow velocity and temperature increase for larger values of thermal radiation, heat source and SWCNTs volume fraction with lamina and cylinder shapes. Also, spherical shape of SWCNTs occurs high disturbances in velocity and temperature distribution in the case of cooled cylinder.

On the New Nonlinear Properties of the Nonlinear Heat Conductivity Problem

In this article, we discuss one problem of nonlinear thermal conductivity with double nonlinearity; an exact analytical solution has been found for it, the analysis of which allows revealing a number of characteristic features of thermal processes in nonlinear media. The following nonlinear effects have been established: the inertial effect, the finite propagation velocity of thermal disturbances, the spatial localization of heat, and the effect of the finite time of the existence of a thermal structure in an absorption medium.

Properties of the Ionosphere during an Extreme Storm

Based on an analysis of the F2-layer critical frequency (foF2) at Irkutsk station during the extreme storm of March 13–15, 1989, it is found that geomagnetic activity index aa is a more adequate indicator than ap for foF2 during the maximum of an extreme storm. This is related to the existence of the upper limit of changes in ap (ap = 400) and this fact imposes some limitations on using the ap index for extreme events. In the initial period of the recovery phase of the considered storm, a rapid decrease of geomagnetic activity occurred. A strong deviation of the experimental values of foF2 from the values calculated by the semi-empirical model, which considers the dependences of the thermospheric temperature and composition on geomagnetic activity by the NRLMSISE-00 atmospheric model, was observed during that period. It is assumed that this could be related to the effect of disturbance of thermospheric wind velocity due to the rapid decrease of geomagnetic activity. This assumption is rather qualitative and requires a special study.

Stability of space-periodic flow with separation of a laminar boundary layer

An unstable laminar flow over a wavy region of a plate placed parallel to the oncoming air flow is studied. The results have been obtained by the hot-wire method in a low-turbulent wind tunnel at low subsonic velocities. Flow near a wall with transverse undulation, whose amplitude is sufficiently large and can contribute to the formation of local regions of boundary layer separation, is considered. The external harmonic (acoustic) forcing of the periodic flow leads to the suppression of perturbations that dominate in the spectrum of velocity disturbances near the model surface. The results of this work may be useful in developing methods for controlling spatially inhomogeneous flows.

Induced drag in two dimensions in ideal fluids

In this paper we suggest a model for how a significant part of the drag forces on two-dimensional objects can be derived using the circulation that is naturally maintained around the objects. We assume incompressible and inviscid potential flow and that the circulation is already generated. The resulting velocity field complements the one that is known to generate Prandtl’s induced drag in three dimensions. We demonstrate how fluid particles in a velocity field are attracted towards an object, and that this, due to conservation of momentum, results not only in lift, but also in drag forces. The magnitude of a disturbance velocity can be derived from the circulation of bound and shed vortices accompanying the object and parameters taken from the von Kármán vortex street description. Another part of the drag is generated by vortices that emerge behind blunt bodies when fluid particles do not follow the surface of the objects. We obtain a mathematical description of the resistance of several types of blunt bodies and rotating cylinders. The model involves no parameters that are derived from empirical data. Still, this inviscid approach corresponds well with experimental data in viscous flow and is close to a mathematical empirical description of rotating cylinders by W. G. Bickley.

Analysis of muscle tone and strength and cerebral blood flow in kickboxers

Purpose: the article deals with providing a characteristic of muscle and tone status as well as precerebral and cerebral hemodynamics in kickboxers. Materials: 102 athletes with the qualification from the Second-class Athlete to the Master of Sport voluntary participated in the study. The average age of athletes at the moment of the study was 20.90±0.50. The average sports experience of participants was equal to 4.15±2.77. The control group consisted of apparently healthy young males of the same age but not involved in sports activities (n=35). The indicators of muscle tone and strength and cerebral blood flow were studied with the help of Doppler ultrasound of the main arteries of the head, electroneuromyography, and transcranial Doppler. Results: the article provides the results of the study on the effect of muscle and tone disturbances in the spinal motion segment on the functional status of precerebral and cerebral blood flow. The disturbances in nerve conduction velocity in the symmetrical groups of trunk muscles are considered as the reason for muscle imbalance in 50% of athletes. The analysis of blood flow indicators in kickboxers with muscle imbalance revealed a typical increase in the tone of precerebral vessels and changes in the gradients of blood flow velocity in various segments of the carotid and vertebral arteries. The decrease of cerebral blood flow in athletes, in particular, the venous outflow of a dystonic nature, is the result of spasmodic and ischemic Doppler patterns against a decreased indicator of arteriovenous balance. Mild traumatic brain injuries should also be taken into account when speaking about the development of cerebrovascular dysfunction as a result of kickboxers’ sparring activities and competitions. Conclusions: Differentiated disturbances in sensory and motor conductivity result in the disturbance of afferentation and muscle imbalance intensity. Angiospasm and vascular ischemia determine the increase in the linear blood flow velocity in the carotid system and the decrease in the vessels of the vertebrobasilar system against the increase in resistivity indicators in the carotid basins and vertebrobasilar system in kickboxers. Correlation analysis revealed visceral motor correlations, which proved the influence of the functional status of the regional muscle system on cerebral hemodynamics. The analysis of tone and strength muscle characteristics and cerebral blood flow in kickboxers indicates the necessity of correcting muscle and tone asymmetry of the paravertebral area.