Influence Of Velocity Gradient Research Articles

Modes of synchronisation around a near-wall oscillating cylinder in streamwise directions

Two-dimensional direct numerical simulations of a cylinder undergoing forced streamwise oscillations in steady approaching flow are conducted over ranges of oscillation amplitude, oscillation frequency and gap distance between the cylinder and the wall at a Reynolds number of 175. The flow characteristics are found to be strongly affected by the gap distance, compared to those observed around an isolated cylinder (Tang et al., J. Fluid Mech., vol. 832, 2017, pp. 146–169). The synchronisation modes are mapped out in the parameter ranges. The existence of the plane wall leads to an increased chance of occurrence of high-order modes with the denominator being an odd number. Two new flow phenomena, namely the period doubling and transition to quasi-periodic states through cascade of period doubling within the primary synchronisation region, are observed. The interaction of the plane-wall boundary layer with vortices shed from the cylinder and the asymmetry of the flow through the gap and around the top side of the cylinder are identified as the primary physical mechanisms responsible for the observed behaviours. The influence of velocity gradient in the plane-wall boundary layer on the two new phenomena is quantified through a numerical test involving linear shear flow around an isolated cylinder. The period-doubling phenomenon occurs only when the velocity gradient is larger than a critical value. The results obtained through three-dimensional simulations suggest that the synchronisation modes identified through two-dimensional simulations are not significantly affected by the three-dimensionality of the flow over the parameter ranges covered in the present study.

Experimental study of transient friction characteristics and velocity distribution of pulsatile flow in rod bundles

Rod bundles are widely used in various devices and energy engineering including heat exchangers, nuclear fuel assembly and so on. The transient fluctuation of flow rate in the thermal devices is a common phenomenon under accident scenarios or ocean condition, the study of pulsatile flows has significant practical relevance for the design and safety of thermal devices. In order to determine the flow characteristics of pulsatile flows in rod bundles, the experimental investigations for transient friction loss and evolution of flow field of pulsatile flows of a 5 × 5 rod bundle (P/D = 1.326) are performed. The experiments were conducted under the ranges of time-averaged Reynolds number Reta = 2000–7000, pulsatile amplitude Au = 0.2–0.6 and pulsatile period T = 10 s–100 s. A comparison of friction factors under steady flows and pulsatile flows is performed to determine the influence of pulsatile flows on frictional characteristics. The results show that the transient friction factors versus flow rate under pulsatile flow presents an “oval” shape, and the “oval” shape has a variation with pulsatile parameters changing. To further reveal the details of pulsatile flow fields, the evolution of flow pattern in the pulsatile flow of Reta = 5200, Au = 0.6, T = 10 s is visualized in rod bundles by Particle Image Velocimetry (PIV). The velocity distribution in different stages of pulsatile flows are displayed and discussed. The influence of velocity gradient and turbulent shear stress in the whole pulsatile period for frictional loss is analyzed, and the variation of transient friction factors under different pulsatile parameters is revealed. The results describe transient behaviors to contribute a better understanding for transient flows in fuel assemblies.

Numerical modeling and simulation of the shedding mechanism and vortex structures at the development stage of ventilated partial cavitating flows

Abstract Ventilated partial cavitation is a complex multi-phase turbulent flow due to the strong interactions between gas and liquid. In the present work, we specially focus on the numerical modeling and simulation of the shedding mechanism and vortex structures. The Reynolds Averaged Navier–Stokes (RANS) method combined with a filter-based turbulence model (FBM) is proposed to explore the physical mechanism of the ventilated partial cavitating flows. Experimental results of cavity evolution and pressure are utilized to assess the prediction ability of the proposed method. Good agreements are observed between experimental measurements and numerical predictions, including the ventilated cavity growth, break off, shedding and the transient dynamic pressure inside the cavity. Based on the model strategy, the cavity dynamic evolution and shedding mechanism are analyzed. The results indicate that the re-entrant flow gives birth to the gas leakage at the cavity interface and is responsible for the ventilated cavity shedding. In addition, streamline vortex is presented to reveal the ventilated cavity shedding characteristics. Moreover, based on the vorticity transport equation, the influence of velocity gradient, fluid volumetric expansion/contraction, pressure gradient and the viscous dissipation factors on the vortex production in ventilated cavitating flows is examined. The present study can provide important basis to better understand the shedding mechanism and vortex structures on the development stage of ventilated partial cavitating flows.

Influence of velocity gradient and rapid mixing time on flocs formed by polysilica iron (PSI) and polyaluminum chloride (PACl)

ABSTRACT The aim of this investigation was to evaluate the effect of different velocity gradient and rapid mixing time on coagulation and floc properties, using polysilica iron (PSI) and polyaluminum chloride (PACl) with different rapid mixing speeds and times. The growth, breakage, and re-growth of flocs were monitored by a particle-size analyzer during the rapid mixing, in order to determine the underlying mechanisms. For PSI and PACl, three different zones were found to depending on the G value in the coagulation process. At low G values (G 546 s−1 for PSI flocs and G > 390 s−1 for PACl flocs), breakup dominated over floc formation. Broken flocs did not fully re-grow after breakage, probably as a result of a change...

The influence of velocity gradient on properties and filterability of suspension formed during water treatment

This paper deals with the influence of the global velocity gradient G (in the range of 28–307s−1) on properties of suspension (size and structure) formed during water treatment. Furthermore, it describes the influence of these properties on separation using depth filtration (filtration velocities of 3 and 6mh−1). The methods of image and fractal analysis were used to determine the aggregate size and structure, respectively. The experiments were conducted in a pilot plant (a mixing tank and a rapid gravity sand filter) with ferric sulphate used as coagulant. The experiments confirmed that with an increasing velocity gradient the aggregate size decreased; the aggregate size distribution narrowed (suspension was more homogenous); the fractal dimension D2 increased (aggregates were more compact) and the fractal dimension Dpf diminished (aggregates were more regular). The smallest and most compact aggregates (d∼60μm and D2=1.9) formed at G>200s−1 displayed the best filterability with the time of filtration up to 78h. On the contrary, the filtration of large and porous aggregates formed at G∼30s−1 lasted only about 30h.

Influence of Velocity Gradient on Optimisation of the Aggregation Process and Properties of Formed Aggregates

Influence of Velocity Gradient on Optimisation of the Aggregation Process and Properties of Formed AggregatesThe follow up research into the IHDS process was carried out with a Couette device. The outcome of this study provides a comprehensive understanding of the effect that both the agitation intensity and the agitation time have on the kinetics and the mechanism of the aggregation process. The results obtained confirm the very favourable influence of high agitation intensity for the formation of more compact and dense aggregates than those formed by the accustomed flocculation conditions with low agitation intensity. This research also proved that the agitation intensity and time are the inherent means profoundly influencing the properties of the resultant aggregates such as their size, shape, density and homogeneity. Further, it was confirmed that the aggregation process passes through a minimum. Furthermore, it was verified that the aggregation process takes place in four consecutive phases, namely a) the phase of formation, b) the phase of compaction, c) the phase of a steady (equilibrium) state and d) most probably the phase of inner restructuring. The pattern of the aggregates development in these phases remains the same irrespective of the magnitude of the velocity gradient applied but the time at which these phases are completed is velocity gradient dependent. Last but not least this study proved that the dimensionless productCa=G T= const. has no general validity.

On the accuracy of simultaneously measuring velocity component statistics in turbulent wall flows with arrays of three or four hot-wire sensors

A highly resolved turbulent channel flow direct numerical simulation (DNS) with Reτ = 200 has been used to investigate the ability of probes made up of arrays of three or four hot-wire sensors to simultaneously and accurately measure statistics of all three velocity components in turbulent wall flows. Various virtual sensor arrangements have been tested in order to study the effects of position, number of sensors and spatial resolution on the measurements. First, the effective cooling velocity was determined for each sensor of an idealized probe, where the influence of the velocity component tangential to the sensors and flow blockage by the presence of the probe are neglected. Then, simulating the response of the virtual probes to obtain the effective velocities cooling the sensors, velocity component statistics have been calculated neglecting the velocity gradients over the probe sensing area. A strong influence of both mean and fluctuation velocity gradients on measurement accuracy was found. A new three-sensor array configuration designed to minimize the influence of the velocity gradients is proposed, and its accuracy is compared to two-sensor X- and V-array configurations.

Influence of velocity gradient on aluminum and iron floc property for NOM removal from low organic matter surfacewater by coagulation

Prehydrolyzed coagulants (polyaluminum chloride, denoted as PAC, and polyferric chloride, denoted as PFC) were used to study the effect of different velocity gradient values ( G ranging from 4.4 s −1 to 28.3 s −1) on the flocculation performance and floc properties from the low organic matter (OM) surfacewater. For different coagulants there were three distinct zones observed in terms of the magnitude of G. At low G values ( G < 7.6 s −1), mean particle diameter increased with G, showing that aggregation dominated over breakup. The flocs were loosely compacted with poor settling ability and high strength and recovery factors. At intermediate G values (7.6 s −1 < G < 15.1 s −1 for PFC; 7.6 s −1 < G < 11.2 s −1 for PAC), flocculation rates were maximized, resulting in large flocs with low strength and recovery factors. The residual turbidity was lower and the flocs were more compact. At higher G values ( G > 15.1 s −1 for PFC; G > 11.2 s −1 for PAC), the dominant effect of breakup was shown through reduced maximum floc size with increasing G values. The strength and recovery factors increased and the residual turbidity also increased due to the small flocs. In addition, the PFC-NOM flocs properties were more influenced by the velocity gradients. In terms of NOM removal, the differences of the residual UV 254 and DOC in the range of G studied were slight. The lowest SUVA value was achieved at G 7.6 s −1 and 15.2 s −1 respectively for PFC and PAC.

Influence of velocity gradient in a hydraulic flocculator on NOM removal by aerated spiral-wound ultrafiltration membranes (ASWUF)

A hydraulic coagulation-flocculation processes combined with aerated spiral-wound ultrafiltration membranes (ASWUF) was designed with the objective of improving natural organic matter (NOM) removal by ASWUF in the treatment of water for human consumption. The pilot-scale experimental system had capacity for treating 0.9 m(3)/h. Dosage of Cl(3)Fe as coagulant and hydraulic retention time (HRT) were calculated to generate microflocculation and different velocity gradients (G=27, 47, 87 and 104 s(-1)) were applied in the hydraulic flocculator. Operating alone, the ASWUF system achieved an NOM removal performance of 39% without problems of membrane clogging, although there was a significant correlation between effluent and influent quality. Application of microflocculation achieved considerable improvement in NOM removal, but values of G< or =87 s(-1) resulted in rapid clogging of the membrane due to flocs disintegration in the aerated membrane tank. Particle analysis revealed that the reduction of the velocity gradient had the effect of inclining the particle size distribution towards larger sizes, affecting both NOM removal capacity and membrane clogging. For G=104 s(-1) an NOM removal yield of 90% was reached, while transmembrane pressure (TMP) was stabilised as a result of the control of membrane clogging.

Simultaneous measurement of absorption coefficient, thermal diffusivity, and flow velocity in a gas jet with pulsed photothermal deflection spectroscopy

Simultaneous measurement of the absolute optical absorption coefficient or species concentration, thermal diffusivity, and flow velocity in a gaseous medium has been demonstrated by using pulsed photothermal deflection spectroscopy (PTDS) and a multiparameter fitting procedure. The spatial distribution of the three parameters in a laminar flow has been simultaneously determined and the influence of velocity gradient on the measurement has been discussed. Saturation behavior of the PTDS signals was observed and found to influence significantly the fitted results. The saturation influences most significantly the absorption coefficient determination, somewhat less significantly the thermal diffusivity determination, and only negligibly the flow velocity determination. For an absolute and accurate measurement of the absorption coefficient or species concentration, the saturation-induced error has to be corrected by using a model taking into account the effects of the saturation. The influence of saturation on the thermal diffusivity determination can be significantly accounted for by using a simple model in which the pump beam radius is set as a free parameter in the multiparameter fitting procedure. Both multipulse and single-pulse measurements are performed. For multipulse measurement, the estimated measurement uncertainties are ±2% for the flow velocity, and ±5% for the thermal diffusivity and absorption coefficient, while for single-pulse measurement, the estimated uncertainties are ±2% and ±10%, respectively.

Detachment Mechanisms in a Liquid Fluidised Bed

This paper provides a cross discipline critical review of the various efforts undertaken to assess the detachment mechanisms in a liquid fluidised bed. Points of agreement as well as those of divergence are stressed out, and attempts are made to clarify the discrepancies frequently observed. An experimental work aiming at the removal of clay particles from the surface of fine salt grains is presented, and its results are treated in the light of the preceded discussion. The influence of velocity gradients, shear stress and particle attrition on detachment are discussed. A realistic model is provided for the prediction of attrition in particle beds during fluidisation.

Influences of velocity gradient on hot-wire anemometry with an X-wire probe

The influence of velocity gradients on hot-wire anemometry with an X-wire probe were investigated. Analysis showed a strong possibility that the values obtained by the usual method of measurements with an X-wire probe involve serious experimental errors caused by the velocity gradients. In order to eliminate these errors, the authors devised an entirely new method of measurement on the basis of the analysis. In this method, two X-wire probes, one of which is the mirror image of the other, are used for measuring a quality at a location. Then an ultimate value of the quantity at the location is given as an arithmetic mean of the values obtained from the two probes. The validity of the analysis was confirmed by measurements of a turbulent flow in a square duct with the present method. The results showed that the present method is very effective for the measurement of quantities such as the secondary flow velocities and the turbulent shear stresses, quantities which are very sensitive to the influences of velocity gradients.

Influence of velocity gradient and wall proximity on the readings of a pitot tube in measurement of surface friction and velocity distribution in a turbulent boundary layer

The authors present results of an experimental investigation of the correlations and readings of a Pitot tube in measuring velocity in a turbulent boundary layer. A new calibration relation is proposed for measuring surface friction.

4861. Influence of velocity gradients on measurements of velocity and streamwise vorticity with hot-wire X-array probes: P Vukoslavĉevic and J M Wallace, Rev Sci Instrum, 52 (6), 1981, 869-879

4861. Influence of velocity gradients on measurements of velocity and streamwise vorticity with hot-wire X-array probes: P Vukoslavĉevic and J M Wallace, Rev Sci Instrum, 52 (6), 1981, 869-879

Influence of strain on flame propagation in laminar flow

The influence of velocity gradients, in a laminar approach flow, on the local burning velocity was investigated, to supplement and verify recent theoretical work. Different velocity profiles were obtained at the exit from a circular burner, using pipes of different lengths between the exit and a settling chamber, and these profiles were measured using hot wire anemometry. The burning velocity and the velocity component parallel to the flame front were obtained by means of the “angle method” using direct photographs of the flame. The results, presented as burning velocity versus velocity gradient along the flame front, show that the local laminar burning velocity decreases as the rate of “stretching” of the flame is increased.

Influence of velocity gradients on measurements of velocity and streamwise vorticity with hot-wire X-array probes

An analysis and measurement of the effects of the streamwise velocity gradients, ∂U/∂y and ∂U/∂z, on the velocity components, U, v, and w, and the streamwise vorticity component, ωx, measured in turbulent flow with a pair of orthogonal hot-wire X arrays, is presented. It is shown that these gradients, which can have the same order of magnitude instantaneously as the mean shear stress at the wall, cause extremely large errors in the measured instantaneous cross-stream velocity and streamwise vorticity components.

Hydrodynamically induced crystallization of isotactic polystyrene

This report provides a brief preliminary account of the solution crystallization of isotactic polystyrene under the influence of velocity gradients. During crystallizations from 1,3,5-trimethylbenzene and cyclohexanol solutions, the polymer appears in a number of morphological forms which are composed of microfibrillar units. In isotactic polystyrene produced by heterogeneous Ziegler–Natta catalysis, similar microfibrillar units are observed. It is suggested, therefore, that these two apparently different processes have a common underlying mechanism in the formation of the microfibrils. The possible mechanisms of fibril formation are discussed.