Turbulent Agitation Research Articles

Suspension of large inertial particles in a turbulent swirling flow

We present experimental observations of the spatial distribution of large inertial particles suspended in a turbulent swirling flow at high Reynolds number. The plastic particles are heavier than the working fluid so that their dynamics results from a competition between gravitation and turbulent agitation. Two suspension regimes are observed. At low rotation rate particles are confined near the bottom for any size or density. At high rotation rate, particles are loosely confined: small particles are nearly homogeneously distributed and large ones found near the top as if gravity was reversed. We discuss these observations using a minimal random walk model accounting for particle inertia.

The Effect of Large Particle Addition on Heat Transfer from Side Wall in Turbulent Agitation Vessel

The Effect of Large Particle Addition on Heat Transfer from Side Wall in Turbulent Agitation Vessel

Budget analysis of a pseudo-single-phase transport model for slurry flows

This study aims at understanding the influence of the various physical mechanisms involved in the transport of solid particles within slurry flows. The present model simulates such mixtures as pseudo-single-phase fluids. The approach is based on a constitutive equation, which accounts for the shear-induced particle migration. Two supplementary terms are added to this equation to account for particle buoyancy and turbulent dispersion, respectively. The present model is validated against experimental data published in the literature, and the influence of each mechanism is discussed in detail for two particle diameters $$d=125$$ and $$440 \, \upmu \mathrm{m}$$ and bulk Reynolds numbers $$\mathrm{Re}$$ ranging between 46,000 and 109,000. The detailed budget analysis of the present transport equation reveals that the terms accounting for the particle transport due to the viscosity variation and the particle collision frequency variation get preponderant in the near-wall regions. This last term is besides at the origin of the repelling phenomenon. The turbulent agitation term counterbalances the sedimentation effect. When the convective flux term is important in the bulk region, the flow tends to be homogeneous. The present model can be used confidently to model two-phase flows under certain multiphase flow regimes while requiring less computational efforts than other more complex two-phase models.

Sediment resuspension due to near-bed turbulent coherent structures in the nearshore

Laboratory and field experiments have suggested that near-bed sediment resuspension is an intermittent process influenced by turbulent coherent structures. This is in contrast to many sediment transport theories that relate sediment transport rates to mean near bed flow parameters. This study presents observations of turbulent bursting events obtained from two contrasting environments dominated by unidirectional and oscillatory flow regimes with mean currents higher than the critical value for sand transport. In agreement with previous laboratory and field experiments the data investigated in this study (both in unidirectional current and oscillatory flow environments) indicated that sediment resuspension was an event based system with sediment flux controlled by ejections and sweeps. Both the magnitude of the Reynolds stress and duration of the events contributed to sediment resuspension. Wavelet analysis confirmed that over time turbulence occurred in slowly evolving clusters that were closely followed by periods of strong resuspension near the bed, evolving from the primary leading scales at low frequencies, and decaying in time after the termination of the turbulent agitation. The results highlight the necessity of considering the instantaneous Reynolds shear stress effects in the existing sediment transport equations which are developed based on a time-averaged flow parameters and mean a critical velocity. Field observations presented in this study highlight the importance of considering turbulence events along with the concept of impulse as an essential parameter to be considered into future modelling of sediment movement under both unidirectional and oscillatory flows.

Biodiesel synthesis from Acacia concinna seed oil: A comprehensive study

ABSTRACTIn the present investigation, biodiesel (BD) production from Acacia concinna nonedible seed oil using physical pretreatment (turbulent agitation method) and transesterification process has been optimized and modeled using neural network Artificial neural network- Adaptive neuro-fuzzy inference system (ANN-ANFIS), Grey relational analysis​ and desirability function approach​ approach considering both quantity (production yield) and quality (fuel properties) responses. Different process parameters were examined for their relative significance on output responses. At optimized process variables, methanol/oil (8.3:1), catalyst KOH (0.95 wt%), and reaction temperature and time (65°C and 37.5 min), augment the yield and calorific value by 17.2 and 5.77% and reduce the viscosity and free fatty acid​ valueby 18.26 and 57.30%, respectively, with global desirability of D= 0.664. The produced BD was characterized by 1H NMR, fatty esters (GC analysis), and fuel properties. The developed model equations for output responses help in accurate prediction of results. A. concinna feedstock proved to be a viable source for biodiesel production.

Physical interpretation of probability density functions of bubble-induced agitation

A stochastic model is presented for the probability density function (p.d.f.) of the liquid velocity fluctuations generated by high-Reynolds-number rising bubbles. It considers three elementary sources of fluctuations: the potential flow disturbance around each bubble; the average bubble wakes, which are assumed to decay exponentially; and the turbulent agitation resulting from the flow instability, which is assumed to be isotropic, homogeneously distributed all over the flow and statistically independent of the two others. The model reproduces well and explains the characteristics of the experimental p.d.f.s: exponential tails, asymmetry of vertical fluctuations and evolution with the gas volume fraction. The model involves twoa prioriunknown parameters: the volume of the wake and the velocity scale of the turbulent agitation. Because some parts of the probability functions depend only on a single contribution, these two parameters can be uniquely and independently determined from experimental p.d.f.s. This defines an objective method to separate the various kinds of fluctuations and allows one to determine the contribution of each of them to the total agitation.

Physical simulation of an active pollutant dispersion in a trapezoidal channel

Organic, thermal, and chemical pollutions that are injected either on purpose or accidentally in the river hydrosystems are transported under the effect of an average fluid motion by convection, and disseminated in the river hydrosystems by turbulent agitation. These two processes control the pollution in a natural watercourse. The goal of this paper is to study the evolution of an active pollutant dispersion, in this case phenol, in time and space, inside a trapezoidal channel. After presenting the experimental apparatus, designed and manufactured in our laboratory, several types of tests are carried out by injecting the selected pollutant inside the channel, with different concentrations of the phenol solutions. Treatments and analyses of the different tests were conducted, highlighting the evolution of the phenol concentration in time and space, with profiles in the lateral and longitudinal flow directions. A qualitative understanding of the specific phenomenal pollutant movement is described in detail, showing experimental results in agreement with general theories describing the phenomenal pollutants movement in prismatic experimental channels studied previously.

Chaotic advection and heat transfer in two similar 2-D periodic flows and in their corresponding 3-D periodic flows

Chaotic advection can effectively enhance the heat transfer rate between a boundary and fluids with high Prandtl number. These fluids are usually highly viscous and thus turbulent agitation is not a viable solution since the energy required to mix the fluid would be prohibitive. Here, we analyze previously obtained results on chaotic advection and heat transfer in two similar 2-D periodic flows and on their corresponding 3-D periodic flows when an axial velocity component is superposed. The two flows studied are the flow between eccentric rotating cylinders and the flow between confocal ellipses. For both of these flows the analysis is simplified because the Stokes equations can be solved analytically to obtain a closed form solution. For both 2-D periodic flows, we show that chaotic heat transfer is enhanced by the displacement of the saddle point location during one period. Furthermore, the enhancement by chaotic advection in the elliptical geometry is approximately double that obtained in the cylindrical geometry because there are two saddle points instead of one. We also explain why, for high eccentricity ratios, there is no heat transfer enhancement in the cylindrical geometry. When an axial velocity component is added to both of these flows so that they become 3-D, previous work has shown that there is an optimum modulation frequency for which chaotic advection and heat transfer enhancement is a maximum. Here we show that the optimum modulation frequency can be derived from results without an axial flow. We also explain by physical arguments other previously unanswered questions in the published data.

A new set-up for PIV measurements in rotating turbulent duct flows

A novel set-up designed for flow visualizations and turbulence measurements in rotating high-aspect-ratio ducts has been described. The 8 m long duct was placed across a 13 m diameter turntable and the set-up enabled stereoscopic PIV-measurements over a wide range of rotation numbers Ro ≡ 2 Ω H / U ≤ 0.80 . Measurements of all three components of the instantaneous velocity vector were made in a plane spanning the channel from the pressure to the suction side. Together with the mean flow characteristics, the important turbulence statistics, such as all components of the Reynolds stress tensor and the spanwise component of the vorticity field, were obtained and compared to DNS results. The field measurements provided more details about the flow than earlier experimental studies of turbulent flows in rotating ducts. The overall observation was that the Coriolis force due to the imposed system rotation damped the turbulence level along the suction side whereas an augmentation of the turbulent agitation was found at the pressure side of the rotating channel. These findings are fully consistent with data from earlier DNS studies. The same set-up can readily be used to explore the influence of rotation on internal flows with separation bubbles provoked by massive obstructions, for instance turbulent flows over a backward-facing step or in ducts with transverse ribs of various shapes.

Acoustic influence on aggregation and agglomeration of crystals in reaction crystallization of cerium carbonate

Sonication and turbulent agitation were used to investigate the mechanism of crystal aggregation and agglomeration during the reaction crystallization of cerium carbonate. Aggregation and agglomeration occurred simultaneously during the crystallization, thus the product suspension included both crystal aggregates and agglomerates. As crystal aggregation involves the physical adhesion of crystals, the particle size changed sensitively according to the sonication energy and agitation speed during the crystallization. In contrast, since crystal agglomeration is based on molecular growth following crystal aggregation, the agglomerates were strong enough to resist the acoustic impact of the sonication and turbulent shear of the agitation during the crystallization. Thus, the crystal aggregates in the product suspension were completely re-dispersed by an acoustic after-treatment of 30 W for 20 min. Acoustic after-treatment was also used to determine the agglomerate size and estimate the degree of aggregation of the product suspension. As a result, sonication above 60 W was found to be adequate to prevent any aggregation or agglomeration during the crystallization, regardless of the agitation conditions.

Influence of hydraulic conditions over dunes on the distribution of the benthic macroinvertebrates in a large sand bed river

This study aims to relate the flow structure over mobile dunes recorded on the channel bed of the Paraná River (Argentina) with the spatial distribution of the benthic macroinvertebrates. The following main conclusions have been obtained: (1) the dunes found in the active channel could be considered as hydraulic biotopes at a mesohabitat scale: the bed forms in the thalweg region are subjected to higher shear stresses with low benthic densities; (2) differences in benthic densities were also recorded at within‐dunes microhabitat scales: the largest densities were found in the dune troughs where small bed shear stresses occur and minimum densities on the low stoss side of dunes where turbulent agitation near the bottom strongly disturb the bed particles; (3) superimposed dunes on larger dunes may be considered as another microhabitat of still smaller dimensions. Summarizing, multiscale approaches are needed if a comprehensive understanding linking hydrodynamics and morphodynamics processes with benthic ecology is intended.

Underwater Electrical Discharge with a Large Surface of Radiation

Breakdown and formation of surface electric discharge in a zone of turbulent agitation of water jet transverse to the axis of an electrode system are studied. The jet boundary is actually a two-phase medium favorable for electron breeding consisting of cavitation microbubbles up to 107 cm-3 in volume. These features differentiate the considered discharge from a low-voltage breakdown in a stationary liquid. Abnormal properties of electric discharge in a submerged water stream take place both at the initiation and at the stationary stage. The threshold voltages are reduced by several times at the stream velocities of about 30 m/s, simultaneously the duration of the prebreakdown stage is reduced by one to two orders of magnitude. At the arc stage of the discharge, there is a developed emitting surface with plasma shell thickness being about 1 mm. The change of initial water conductivity by four orders of magnitude does not lead to large changes in breakdown dynamics.

A New Method for Designing Vortical Aerators

The oxygen saturation of water for various uses — water treatment for communal and industrial needs, waste effluent treatment, biological self-cleaning in reservoirs and in streams, etc. — is an important step in water improvement technology. For that purpose, various methods and variously designed mechanical and pneumatic aerators are used in practice. A high-efficiency vortical aerator has been designed at the MGSU [1]. Operation of this aerator is based on the interaction of two coaxial circular flows rotating in opposite directions (Fig. 1). The flow swirling is effected by means of tangential inlet ducts 1. The inner circular flow develops a flow discontinuity cavity in the central part, conventionally called the air-vapor core 2. Because of the centrifugal effect, the air-vapor core pressure is lower than the atmospheric pressure, owing to which the atmospheric air is automatically drawn in the air-vapor core through air inlet pipe 3. In mixing chamber 4, the circular water flows and the central air flow converge. Owing to the intense turbulent agitation, the convergent flows in the mixing chamber mingle in an axial flow saturated with a vast number of uniformly distributed minute air bubbles, which creates a large air–water contact surface. Owing to this, the process of saturation of water with dissolved oxygen intensifies appreciably under vortical aerator conditions. The familiar methods for analysis of vortical aerators [5] fail to reflect in full measure the complicated hydraulic events involved that can be accounted for only using adequate experimental data. Viewed from a hydraulic standpoint, one can draw an analogy with the swirling flows in vortical aerators and hydraulic turbines. In both cases, the flow channel represents a kind of hydraulic resistance which is difficult (if possible at all) to take account of theoretically. For this reason, in a hydraulic-turbine analysis a common procedure is to determine experimentally the characteristics of a model hydraulic turbine and then to scale them up to life-size analogs. In our case, one may also assume that the fluid flows in geometrically similar vortical aerators differing in absolute dimensions are mechanically similar. If so, one can use the same principle of analysis as that for hydraulic turbines. An analogy between swirling spillways (equipped with vortical gates) and hydraulic turbines was earlier noted in [3].

Effect of Blade Shape on the Performance of Six-Bladed Disk Turbine Impellers

Different modifications of the Rushton turbine were studied in a dual-impeller agitated tank (T = 0.4 m), to find the effect of blade form on power draw, turbulent dispersion, gas handling capacity, mixing, gas holdup, and mass-transfer rate performance under turbulent agitation in an air−water system. Blade streamlining was found to lead to a lower ungassed power number, a higher gas flow number before flooding, and increased insensitivity of impeller power dissipation to the gassing rate. This is consistent with the formation of smaller trailing vortices and ventilated cavities behind the blade. At the same power input and superficial gas velocity, however, the different impellers provided the same mixing time t0.05, gas holdup εG, and specific mass-transfer coefficient KLa. Each of these variables correlates with the specific power input PG/VL, clearly suggesting that a better performance may be expected after retrofitting of Rushton turbines with streamlined impellers.

Modeling Thermodynamic Ice–Ocean Interactions at the Base of an Ice Shelf

Abstract Models of ocean circulation beneath ice shelves are driven primarily by the heat and freshwater fluxes that are associated with phase changes at the ice–ocean boundary. Their behavior is therefore closely linked to the mathematical description of the interaction between ice and ocean that is included in the code. An hierarchy of formulations that could be used to describe this interaction is presented. The main difference between them is the treatment of turbulent transfer within the oceanic boundary layer. The computed response to various levels of thermal driving and turbulent agitation in the mixed layer is discussed, as is the effect of various treatments of the conductive heat flux into the ice shelf. The performance of the different formulations that have been used in models of sub-ice-shelf circulation is assessed in comparison with observations of the turbulent heat flux beneath sea ice. Formulations that include an explicit parameterization of the oceanic boundary layer give results that...

Solids Concentration Measurements of Floating Particles Suspended in a Stirred Vessel Using Sample Withdrawal Techniques

Axial and radial concentration profiles formed during suspension of floating particles in liquid under turbulent agitation are presented in this paper. Monomodal suspensions of polyethylene in tap water stirred by a pitched blade turbine have been studied. The local solids concentration from a slurry mixing tank have been measured by the withdrawal of a sample from the vessel. The dependence of the distribution of the monosized suspension on the sample tube design and sampling technique has been investigated. Three different probes have been tested. The obtained results have to be related to the direction of the local fluid velocity at the point of sampling in the mixing tank and the orientation of the suction section of the sampling tube. The effects of the particle size, bulk solids concentration, and impeller speed on the solids concentration profiles were also examined in detail. The solids concentration profiles in the mixing tank are described by means of the one-dimensional dispersion model. The applied model includes two parameters, Peb, modified Peclet number, based on solids upward velocity due to the buoyance force effect, and q representing the ratio of Peclet numbers for liquid and solid phases. These parameters were found to be a function of the hydrodynamic conditions in the mixing tank.

Suspension of solid particles in turbine agitated contactors

In this present study in liquid-solid contactors using 6-blade (flat) turbine agitators, an attempt has been made to correlate the system parameters useful for scale-up under turbulent agitation conditions, where the power number is constant. A new correlation involving modified Froude number along with the agitation characteristics, is proposed for the estimation of minimum speed required to maintain complete suspension.

Drop size distributions in high holdup fraction dispersion systems: effect of the degree of hydrolysis of PVA stabilizer

Although turbulence-stabilized dispersions are of considerable industrial importance, their behaviour is generally not well understood, especially for the high dispersed phase volume fractions encountered in suspension polymerization reactors. The dispersion is formed by two dynamic processes, namely, drop breakage and coalescence. Breakage mainly occurs in regions of high shear stress near the impeller or as a result of turbulent velocity and pressure fluctuations along the surface of a drop. Droplet coalescence is prevented by the combined action of turbulent agitation and the addition of protective colloids such as poly(vinyl alcohol) (PVA). Depending on the combination of the degree of agitation and the concentration and type of surface active agent, the average droplet size can exhibit a U-shaped variation with respect to the impeller speed or the degree of hydrolysis of the PVA stabilizer. This U-type behaviour has been confirmed both experimentally and theoretically and has been attributed to a manifestation of the balance between breakage and coalescence rates of the monomer drops. Both processes are related to the interfacial tension and its variation with time as a result of the varying polymer concentration and conformation at the monomer/water interface. In the present study, a series of experiments were carried out with a model system of 50% n-butyl chloride in water in the presence of PVA with varying degrees of hydrolysis at different agitation rates. A generalized numerical algorithm incorporating the most comprehensive models describing the breakage and coalescence processes has been used for simulating the steady-state drop size distributions in a high holdup liquid—liquid dispersion system in order to elucidate the breakage and coalescence mechanisms as a function of the droplet interfacial characteristics. It is shown that the model simulations are in satisfactory agreement with measurements of the drop size distributions over the whole range of the experimental conditions investigated.

Quasiparticles in macroscopically agitated magnetized plasma

In order to explain observations of either laboratory or space plasmas, it is possible to allow for the occurrence of fluid-like turbulent motion in the plasma, which can be produced by large-scale and low-frequency electromagnetic fields, generated either by some external factors or internally, for example due to magnetohydrodynamic turbulence.In this paper the effect of fluid-like motion on the low-frequency fluctuations in the magnetized plasma is studied. The spectral distributions of fluctuations are derived and explained in terms of macro-particles originating from plasma turbulent agitation.

Direct simulations of low-Reynolds-number turbulent flow in a rotating channel

Direct numerical simulations of fully developed pressure-driven turbulent flow in a rotating channel have been performed. The unsteady Navier–Stokes equations were written for flow in a constantly rotating frame of reference and solved numerically by means of a finite-difference technique on a 128 × 128 × 128 computational mesh. The Reynolds number, based on the bulk mean velocity Um and the channel half-width h, was about 2900, while the rotation number Ro = 2|Ω|h/Um varied from 0 to 0.5. Without system rotation, results of the simulation were in good agreement with the accurate reference simulation of Kim, Moin & Moser (1987) and available experimental data. The simulated flow fields subject to rotation revealed fascinating effects exerted by the Coriolis force on channel flow turbulence. With weak rotation (Ro = 0.01) the turbulence statistics across the channel varied only slightly compared with the nonrotating case, and opposite effects were observed near the pressure and suction sides of the channel. With increasing rotation the augmentation and damping of the turbulence along the pressure and suction sides, respectively, became more significant, resulting in highly asymmetric profiles of mean velocity and turbulent Reynolds stresses. In accordance with the experimental observations of Johnston, Halleen & Lezius (1972), the mean velocity profile exhibited an appreciable region with slope 2Ω. At Ro = 0.50 the Reynolds stresses vanished in the vicinity of the stabilized side, and the nearly complete suppression of the turbulent agitation was confirmed by marker particle trackings and two-point velocity correlations. Rotational-induced Taylor-Görtler-like counter-rotating streamwise vortices have been identified, and the simulations suggest that the vortices are shifted slightly towards the pressure side with increasing rotation rates, and the number of vortex pairs therefore tend to increase with Ro.