Step Change In Velocity Research Articles

Transient Flow Uniformity Evolution in Realistic Exhaust Gas Aftertreatment Systems Using 3D-CFD

To precisely control a vehicle powertrain to minimize emissions, accurate and detailed models are needed to capture the spatio-temporal variability of the variables of interest. The aim of this work is to analyze flow and temperature fields in a geometrically realistic — and thus complex — exhaust gas aftertreatment system under transient conditions. The spatio-temporal response of these fields to upstream step changes is predicted using three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) kappa - omega simulations where the catalytic converter is described as a porous medium. A catalytic converter geometry with a 90^{circ }-bend and a partially dead volume is used to demonstrate the effects of time-resolved flow maldistribution on the profiles of velocity and temperature. Two sets of transient simulations in terms of step changes in velocity and temperature are performed. Uniformity indices are used to characterize the distribution and variability of the different catalyst channels under transient conditions. The evolution of the uniformity indices as functions of time and axial distance into the catalyst are calculated at different cross-sectional planes. The results show that the evolution of the temperature uniformity is rate controlling, continuously modulating the otherwise much faster flow uniformity response via the fluid properties. The temperature uniformity time scale is determined by the balance of flow, thermal inertia, and the heat losses from the system. The interplay between pressure drop and heat losses governs the transition to the new steady state in uniformity. These types of transient simulations and analyses can contribute essential information when developing reduced-order engineering models to represent the spatio-temporal variability in exhaust aftertreatment systems, in particular during rapid events such as cold start.

Full Assessment of Micromachine Friction Within the Rate–State Framework: Experiments

This work represents the first full mapping of the microscale kinetic friction phase diagram. A micromachined friction test platform that allows three orders of magnitude variation of normal load, spring constant, and puller velocity is used to achieve this objective. It enables transitions between stick–slip and steady sliding through each of these variables. Experimentally, the sliding behavior of surface micromachined interfaces coated with a tridecafluorotris(dimethylamino)silane (FOTAS, CF3(CF2)5(CH2)2Si(N(CH3)2)3) monolayer is examined. Static aging, a friction decrease then increase with puller velocity, an instantaneous increase in friction in response to a step change in velocity, and stick–slip to steady sliding transitions with decreasing normal load and increasing velocity, respectively, are observed in a wear-free regime. The results form a complete data set that is used in the context of coupled rate–state theory equations to develop and validate model-based predictions for the friction phase diagram in a companion paper (Shroff et al. in Tribol Lett, 2016).

Development of a Material-Testing Machine for Study of Friction: Experimental Analysis of Machine Dynamics and Friction of Rock

Experimental investigation of the coefficient of sliding friction in rock at normal stress and sliding velocity of typical earthquakes (10–100 MPa and 0.01–1 m/s) is necessary to develop velocity-dependent constitutive relations useful for earthquake rupture simulations. The velocity dependence of rock friction is best explored in parametric studies of interfacial friction by imposing step-like changes in velocity and measuring the frictional force, but such experiments are technically challenging. We present a testing-machine incorporating a prototype loading system designed to achieve high accelerations (up to 100 g) in test samples at earthquake conditions. A 3-degree-of-freedom model representing dynamics and vibrations of the machine is developed to assess machine–sample interactions and the capability to achieve step-changes in velocity. In addition, the prototype loading-system is instrumented with several sensors and operated in order to validate model analyses. Several preliminary experiments on test-samples that display different behaviors were conducted with the prototype system to document machine-sample interactions. The results show that for an impulse-type loading condition associated with dramatic weakening of test-samples, undesirable vibrations of the system can be significant. The dynamic model is modified to identify and treat the source of vibrations accurately, and is used to improve the design of the ultimate loading-system to minimize vibrations and best emulate load paths appropriate to earthquake slip. Finally, a set of sliding friction experiments on rock is presented and the weakening micro-mechanism is investigated. The results suggest that flash weakening at asperity contact points can be a dominant weakening mechanism in earthquake slip.

Adaptive Multivariable Super-Twisting Sliding Mode Controller and Disturbance Observer Design for Hypersonic Vehicle

A multivariable super-twisting sliding mode controller and disturbance observer with gain adaptation, chattering reduction, and finite time convergence are proposed for a generic hypersonic vehicle where the boundary of aerodynamic uncertainties exists but is unknown. Firstly, an input-output linearization model is constructed for the purpose of controller design. Then, the sliding manifold is designed based on the homogeneity theory. Furthermore, an integrated adaptive multivariable super-twisting sliding mode controller and disturbance observer are designed in order to achieve the tracking for step changes in velocity and altitude. Finally, some simulation results are provided to verify the effectiveness of the proposed method.

Flight control for a flexible air-breathing hypersonic vehicle based on quasi-continuous high-order sliding mode

The focus of this paper is on control design and simulation for the longitudinal model of a flexible air-breathing hypersonic vehicle (FAHV). The model of interest includes flexibility effects and intricate couplings between the engine dynamics and flight dynamics. To overcome the analytical intractability of this model, a nominal control-oriented model is constructed for the purpose of feedback control design in the first place. Secondly, the multi-input multi-output (MIMO) quasi-continuous high-order sliding mode (HOSM) controller is proposed to track step changes in velocity and altitude, which is based on full state feedback. The simulation results are presented to verify the effectiveness of the proposed control strategy.

Quasi-continuous high-order sliding mode controller and observer design for flexible hypersonic vehicle

The problem of robust output feedback control and simulation for the longitudinal model of a flexible air-breathing hypersonic vehicle (FAHV) is investigated in this paper. A control-oriented uncertainty model is established for the feedback linearization design, and the multi-input and multi-output (MIMO) quasi-continuous high-order sliding mode (HOSM) controller is formulated to track the responses of the vehicle to a step change in velocity and altitude based on full state feedback. In addition, a high order sliding mode observer is designed to estimate the angle of attack and the flight path angle, which are difficult to measure in a hypersonic flight. Furthermore, the combination of the HOSM controller and observer is proposed for vehicle model with partial state measurements. The simulation results are presented to verify the effectiveness of the proposed control strategy.

Discrete element modelling of a bucket elevator head pulley transition zone

This paper presents the results of discrete element simulations applied to a bucket elevator model with particular reference to the head pulley transition zone. This is the first stage in a larger study to better understand the mechanics of bucket elevator operation with reference to the discharge of particles at the head end. At the head end two issues arise; mechanically, the buckets are bolted to the conveying media (typically a fabric reinforced belt) and at the point of belt to headpulley tangency, the tip of the bucket undergoes a theoretical step change in velocity. This theoretical step change results in a classical under-damped response in the buckets tip velocity. In undergoing this motion, there are stresses that are passed to the carcass of the conveying media; understanding the magnitude of these stresses is one longer term goal of this research allowing a quantitative basis for the existing qualitative design guidelines such as (Handbook for conveyor and elevator belting, Apex Belting Pty Ltd). The discharge of the bulk material from the bucket has been addressed Beverly et al. (Bulk Solids Handling, 1983) but this analysis is dependent on simple, but common, bucket geometry and ignores the initial transition to the headpulley. Ignoring the transition with a low speed discharge elevator is not likely to impact on the predicted discharge pattern, however with high speed discharge elevators, the destabilising effect of the transition is expected to promote premature discharge of bulk material from the bucket. Depending on the design of the elevator casing this early discharge may or may not impact on the overall conveying efficiency.

Study of laminar mixing in kenics static mixer by using Positron Emission Particle Tracking (PEPT)

Positron Emission Particle Tracking is a unique Lagrangian flow visualization technique, which allows measuring experimentally the flow patterns by recording the movements of a radioactive particle. We used PEPT to characterize the mixing performance of a high viscosity Newtonian fluid in a Kenics static mixer (KM), in order to develop methods for quantifying flow and mixing in laminar system. A single radioactive tracer is pass through the mixer several times in order to record several trajectories. By using the data from PEPT, particle trajectories of the various passes have been plotted and superimposed, to obtain an understanding of flow behavior and residence time distribution. We were being able to observe the local RTD along the mixer, and the effect of different entrance position of the tracer on the residence time distribution. The particle tends to move at constant axial velocity through the elements but can undergo a significant step change in velocity in the transition between elements. For the first time the degree of segregation inside the KM has been experimentally quantified using PEPT data. The introduction of a new method to study the segregation along the mixer has opened the way to comparison of these finding to previous numerical works that have been done on KM, to see whether using PEPT gives us more information about the mixing performance in comparison with other experimental techniques

A New Bearing Fault Detection Method in Induction Machines Based on Instantaneous Power Factor

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Fault detection and diagnosis of asynchronous machine has become a central problem in industry over the past decade. A solution to tackle this problem is to use stator current for a condition monitoring, referred to as motor current signature analysis. This paper argues that bearing faults would have a negligible effect on motor currents and instead argues that the more likely reason why the faults can be detected in currents is because they entail a fluctuating resistive torque which acts immediately, in contrast to the radial displacement which takes time to integrate to a perceptible displacement even in response to a step change in velocity. In this context, we propose a new method for detecting bearing defects based on the exploitation of the instantaneous power factor that varies according to torque oscillations. Experimental results show the good performances of the proposed method which will be compared with the instantaneous power method to highlight the feasibility and advantages of this method. </para>

Gentamicin ototoxicity: clinical features and the effect on the human vestibulo-ocular reflex

Conclusions. Gentamicin ototoxicity presents with gait imbalance and oscillopsia, but only rarely with hearing loss and vertigo. Sinusoidal rotational stimuli with high accelerations such as the bedside head-thrust test or rotational step changes in velocity are useful to diagnose bilateral vestibulopathy. Objective. To describe the salient clinical features and vestibular testing results in gentamicin ototoxicity. Patients and methods. A retrospective review of the quantitative vestibular function testing results for patients presenting to the UCLA Neurotology Clinic with gentamicin ototoxicity over the past 10 years (n=35). Results. All patients presented with imbalance and 33 out of 35 had oscillopsia. Three patients reported a noticeable change in hearing and five reported vertigo. Of the 35 patients, 15 were in renal failure at the time of gentamicin administration. Patients with pre-existing peripheral neuropathy compensated poorly. Sinusoidal rotational testing demonstrated profoundly decreased gain and increased phase lead over the entire frequency range, with a subset of patients having relatively preserved gain at the intermediate frequencies (0.8–1.6 Hz) and low acceleration (<30°/s). There was little or no response to high acceleration step changes in velocity. The time constant measured both by sinusoidal and step responses was ultra-low. All patients tested had a positive head-thrust test bilaterally.

Unsteady free-surface waves due to a submerged body moving in a viscous fluid

Unsteady viscous free-surface waves generated by a three-dimensional submerged body moving in an incompressible fluid of infinite depth are investigated analytically. It is assumed that the body experiences a Heaviside step change in velocity at the initial instant. Two categories of the velocity change, (i) from zero to a constant and (ii) from a constant to zero, will be analyzed. The flow is assumed to be laminar and the submerged body is mathematically represented by an Oseenlet. The Green functions for the unbounded unsteady Oseen flows are derived. The solutions in closed integral form for the wave profiles are given. By employing Lighthill's two-stage scheme, the asymptotic representations of free-surface waves in the far wake for large Reynolds numbers are derived. It is shown that the effects of viscosity and submergence depth on the free-surface wave profiles are respectively expressed by the exponential decay factors. Furthermore, the unsteady wave system due to the suddenly starting body consists of two families of steady-state waves and two families of nonstationary waves, which are confined within a finite region. As time increases, the waves move away from the body and the finite region extends to an infinite V-shaped region. It is found that the nonstationary waves are the transient response to the suddenly started motion of the body. The waves due to a suddenly stopping body consist of a transient component only, which vanish as time approaches infinity.

Simulation of transient friction of a cylinder between two planes

The need for good friction models for transient motions has increased as a consequence of the increased use of mechatronics and control engineering principles in precision mechanics. The machine elements in such equipment often involve rolling and sliding contacts. Most studies of friction in rolling and sliding contacts running under dry or boundary lubricated conditions have examined steady-state conditions. This paper describes simulations of the motion of a cylinder between two planes, first with a step change in velocity and then with an oscillating motion of the upper plane. The motion of the cylinder is determined by the friction in the contacts and the inertia. The friction in the rolling and sliding contacts is simulated with a brush model. The surfaces are assumed to be ideally smooth. For the step change in velocity, there is initially a period of complete sliding in the upper contact. During the sliding period, the friction force is the maximum possible, but it decreases as the complete sliding ends. The simulations show heavily damped oscillations, with frequencies corresponding to the natural translatory and torsional frequencies of the system. For the oscillating motions the sliding increases with the frequency of the motion, as expected.

Friction of simulated fault gouge for a wide range of velocities and normal stresses

During earthquake rupture, faults slip at velocities of cm/s to m/s. Fault friction at these velocities strongly influences dynamic rupture but is at present poorly constrained. We study friction of simulated fault gouge as a function of normal stress (σn = 25 to 70 MPa) and load point velocity (V = 0.001 to 10 mm/s). Layers of granular quartz (3 mm thick) are sheared between rough surfaces in a direct shear apparatus at ambient conditions. For a constant σn, we impose regular step changes in V throughout 20 mm net slip and monitor the frictional response. A striking observation at high velocity is a dramatic reduction in the instantaneous change in frictional strength for a step change in velocity (friction direct effect) with accumulated slip. Gouge layers dilate for a step increase in velocity, and the amount of dilation decreases with slip and is systematically greater at higher velocity. The steady state friction velocity dependence (a‐b) evolves from strengthening to weakening with slip but is not significantly influenced by V or σn. Measurements of dilation imply that an additional mechanism, such as grain rolling, operates at high velocity and that the active shear zone narrows with slip. Data from slow (μm/s) and fast (mm/s) tests indicate a similar displacement dependent textural evolution and comparable comminution rates. Our experiments produce a distinct shear localization fabric and velocity weakening behavior despite limited net displacements and negligible shear heating. Under these conditions we find no evidence for the strong velocity weakening or low friction values predicted by some theoretical models of dynamic rupture. Thus certain mechanisms for strong frictional weakening, such as grain rolling, can likely be ruled out for the conditions of our study.

Effects of temperature on friction: Constitutive equations and experiments with quartz gouge

A state‐variable constitutive relation that can describe the dependence of friction on temperature at or near steady state conditions is presented. The relation is derived from existing state‐variable relations used to describe velocity dependence and the assumption that the micromechanisms of friction are thermally activated and follow an Arrhenius relationship. The relation adequately describes the transient and steady state friction behavior displayed in sliding experiments on 1.5‐mm‐thick layers of fine‐grained (&lt;100 μm diameter) quartz gouge. Gouge layers were sheared up to 10 mm between rough steel surfaces at a constant effective confining pressure of 20 MPa and under room‐dry or water‐saturated conditions in a servo‐controlled triaxial apparatus. In each experiment, velocity was stepped between 4, 0.4, and 0.04 μm/s at constant temperature, and temperature was stepped between 24°, 57°, and 82°C at a constant velocity of 0.04 μm/s. Coefficient of friction was calculated from measurements of sample strength using corrections for all apparatus effects including the temperature and velocity dependent strength of the metal sleeves used to isolate the sample and of the graphite‐lubricated interface that allowed lateral slip of the sample halves. Experiments indicate that (1) an abrupt increase in temperature induces a transient friction response similar to that induced by a step decrease in velocity, (2) the transient friction response is relatively symmetric for steps up and steps down in temperature, and (3) the characteristic slip distance for friction to evolve to steady state after step changes in temperature is the same as after step changes in velocity. The apparent activation energy determined for wet quartz gouge at the test conditions is 89±23 kJ/mol. This value is consistent with models of friction involving failure of contact junctions by subcritical crack growth and time dependent strengthening of contact junctions by either increasing the true area of contact through subcritical cracking or by increasing the quality of contact junctions through healing.

Velocity‐depth ambiguity of reflection traveltimes

The conversion of time horizons to depth is fundamental to exploration geophysics. The interval velocity used in the conversion is often estimated from the stacking velocity, assuming that each layer’s interval velocity is homogeneous. However, even for one laterally inhomogeneous layer above a flat reflector the stacking velocity can swing violently about its average and conventional methods of velocity estimation fail. I show that violent swings in the stacking velocity are a symptom of a long‐wavelength ambiguity between the burial depth to an interface and interval velocity. Lateral variations in seismic velocity with a spatial wavelength of about 2.7 D, where D is the depth to the reflecting horizon, cannot be unambiguously resolved from traveltime measurements. The spatial wavelength of this ambiguous component varies from 2.57 D, for very small source‐receiver separations, to 2.86 D for source‐receiver separations equal to D. Spectral components of the stacking velocity at wavelengths shorter than this ambiguous value are amplified in size and reversed in polarity relative to the interval velocity. A practical inverse filter that corrects for these distortions produces an interval velocity that is almost totally lacking in low‐frequency components, giving a very distorted picture of the interval velocity. Since the wavelength of total ambiguity changes with offset, a complete description of the velocity and depth fields can, in theory, be extracted from a combination of multiple‐offset traveltime measurements. However, the wavelength of total ambiguity is such a weak function of source‐receiver separation that multiple offset processing, in practice, does little to resolve the ambiguity. In fact, the Rayleigh resolution limit implies that three or more offset measurements are more effective than two only if the seismic‐line length is at least 20 D. In a series of numerical experiments with the line set to 100 D and a spatial noise level of .01% in each channel I used a two‐channel Wiener filter to successfully extract the full‐band response for a simultaneous step change in velocity and in depth. The method fails for lines shorter than 20 D because of the transients that arise when the data are shorter than the filter. Stability was achieved by increasing the noise level to 1% in the design of the Wiener filter, but low spatial frequencies were lost and the estimated velocity‐depth model was distorted. If the results of this single flat‐layer analysis apply to practical situations, the velocity‐depth ambiguity may continue to plague exploration seismologists for some time to come.

Mixed state variable friction laws: Some implications for experiments and a stability analysis

Friction laws that depend on both the rate and past history (or state) of a sliding surface have been previously used to describe the sliding friction of rocks and to model some earthquake features. It has been proposed that these laws require an overall velocity weakening for stick‐slip instability to begin. This proposal is based on the assumption that the stress evolution after step changes in slip rate is monotonic. Recent experiments show, however, an irregularly present ‘mixed state variable’ effect in which it appears that a short range state variable is velocity weakening, and a long range state variable velocity strengthening. The stress evolution is clearly not monotonic after imposed step changes in velocity in these cases. It is shown here that instabilities can occur in the presence of overall velocity strengthening. Indeed, mixed state variable laws can explain some paradoxical laboratory observations in a natural way: Slow experiments may be too short to see the effects of the long state variable. Fast experiments may be too fast to resolve the effects of the short state variable. Thus similar experiments can show both velocity strengthening and velocity weakening depending on the total slip length increment measured.

Influence of air velocity on the habit of ice crystal growth from the vapor

The influence of air velocity on ice crystal growth from the vapor has beeninvestigated over the temperature range 0 to -35°C at supersaturations ranging from 2 to 40% over ice in a laboratory chamber in which it is possible to vary each of these parameters independently. Crystal growth was most sensitive to changes in the air velocity at temperatures near -4 and -15°C, where, near water saturation, introduction of only a 5 cm s -1 air velocity induced skeletal transitions — columns to needles near -4°C and plates to dendrites near -15°C. The influence of air velocity on these transitions was found by applying step changes of velocity (increasing or decreasing) while observing crystal growth into the air stream from a fiber support. After conditions were changed, a period of time (as long as 100 s) elapsed before the linear growth rate assumed a new constant value. Under certain conditions, deviations of growth from along the α-axis (near -15°C) or c-axis (near -4°C) resulted from changes of velocity. The influence of air velocity on crystal growth is compared and contrasted with that induced by changing the thermal conductivity and vapor diffusivity of the carrier gas. The results are interpreted in terms of diffusion through a local boundary layer and interaction with different nucleation and growth kinetic processes on the prism and basal surfaces.

Electromagnetic flowmeters for fast reactors

This paper is in four parts. Section 1 explains the theory of the induced-voltage electromagnetic flowmeter and then considers various types which have been used. For the primary circuit of fast reactors both flow-through type and probe type have been proposed, although obtaining magnets which operate satisfactorily at high temperatures has been a problem. In the secondary circuit the high magnetic Reynolds numbers cause the field to be swept out of the magnet gap and this has led to the use of the long saddle-coil flowmeter. In Section 2 flux-distortion flowmeters are described. These have been proposed mainly for monitoring the primary circuit flow and again both flow-through and probe types have been tested. Sections 3 and 4 continue the discussion of the flux-distortion flowmeter by introducing two methods of analysing its performance. The first is a finite difference method which solves the non-linear problem by using a time marching method. It is shown that a linear approximation is adequate for the likely levels of flow encountered in the fast reactor and consequently two linearised solutions are used. The first method is a finite difference one and allows the instantaneous response of a step change in velocity to be observed as well as the effect of bubbles. In Part 4 the second linearized method uses current rings to divide up the conducting material. By considering the interaction of all the rings, it is possible to obtain the current distribution and hence the magnetic field. In conclusion it is suggested that further development would be useful of the devices which are most suited to the liquid metal fast breeder reactor.

Unsteady creeping motion of a sphere at small values of knudsen number

A continuum-slip analysis of transient particle motion at small values of Knudsen number is presented. The equation of motion of a spherical particle is derived. The force resulting from a step change in velocity and the velocity resulting from a step change in force are determined. The results are compared with the known responses in the zero Knudsen number limit.

An improvement of the continuous-ionization method for linearity at low gas-velocities

A modified continuous-ionization method for gas flow-rate measurement is described. It is shown that an extended radioactive source, located in the pipe immediately upstream of the collecting electrode, gives a linear collector current vs. velocity response for small gas velocities. Linearity within 1% can be achieved up to a definite velocity, and extension of the linearity to higher velocities is possible by adding a second source upstream of the first. The effects of the gas composition, pressure and temperature are discussed. It is shown that transients exist in the collector current when the step changes in velocity are applied outside the linear range.