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In multiphase particulate systems, the turbulence of the continuous phase (gas or liquid) is modulated due to interactions between the continuous phase and the suspended particles. Such phenomena are non-trivial in the essence that addition of a dispersed phase to a turbulent flow complicates the existing flow patterns depending on the physical properties of the particles leading to either augmentation or attenuation of continuous phase turbulence. In the present study, this aspect has been comprehensively analysed based on the available experimental data obtained from the well-studied turbulent flow systems such as channel and pipes, free jets and grids. Relevant non-dimensional parameters such as particle diameter to integral length scale ratio, Stokes number, particle volume fraction, particle momentum number, and particle Reynolds number have been utilised to characterise the reported turbulence modulation behavior. Some limitations of these commonly used dimensionless parameters to characterise turbulence modulation are discussed, and possible improvements are suggested. [ABSTRACT FROM AUTHOR]

Gas–liquid two phase flows are widely encountered in industry. The design parameters include two phase pressure drop, mixing and axial mixing in both the phases, effective interfacial area, heat and mass transfer coefficients. Currently, there is a high degree of empiricism in the design process of such reactors owing to the complexity of coupled flow and reaction mechanism. Hence, we focus on synthesizing recent advances in computational and experimental techniques that will enable future designs of such reactors in a more rational manner by exploring a large design space with high-fidelity models (computational fluid dynamics) that are validated with high-fidelity measurements (hot film anemometry (HFA), Laser Doppler anemometry (LDA), particle image velocimetry (PIV), etc.) to provide a high degree of rigor. Understanding the spatial distributions of dispersed phases and their interaction during scale up are key challenges that were traditionally addressed through pilot scale experiments, but now can be addressed through advanced modelling. For practically complete knowledge of the fluid mechanical parameters, it is desirable to implement direct numerical simulations (DNS). However, the current computational power does not permit full DNS for real bubble columns. Therefore, we have been using simplified turbulence models (such as large eddy simulation, Reynolds stress, k – ε , etc.) which need the knowledge of turbulence parameters. For the estimation of these parameters, currently semi-empirical procedures are being used pending the knowledge of turbulence. Further, the formulation of governing equations in all the CFD models (except DNS), the knowledge of interface forces (drag, lift, virtual mass, Basset, etc.) is needed and for their estimations empirical correlations are being employed, again pending the knowledge of fluid mechanics under turbulent conditions in bubble columns. In gas–liquid dispersions, the gas is sparged in the form of bubbles. During the bubble rise, the mechanism of wake detachment creates turbulence which can be called as wake generated turbulence. In addition, energy gets transferred from the gas phase to liquid phase. The quantitative amounts are negligible when bubble motion is not hindered and the gas–liquid dispersion is homogenous. The amounts increase with an increase in the extent of hindrance. However, in the homogenous regime, even under extreme conditions, the extent of energy transfer in the bulk gas–liquid dispersions (volume other than wake volume) is fairly limited. On contrast, in the heterogeneous regime, the rates of energy transfer become sizeable. The energy received by the liquid (in both the regimes) also creates turbulent motion and termed as bulk generated turbulence. In turbulent flows a compendium of eddies (flow structures) of different length and time scales contribute towards improved/enhanced mixing, momentum transfer, heat transfer, and mass transfer (transport phenomena). Hence, a proper understanding of the dynamics of these turbulent flow structures, and their role in the transport phenomena, can bring substantial improvement in the scale-up and design procedures. The present paper brings out the current status of knowledge on bubble generated turbulence. All the published literature in experimental measurements and DNS simulations has been critically analysed and coherently presented. [ABSTRACT FROM AUTHOR]

Transport phenomena occur frequently in industrial problems. Most of the turbulent transport properties can be directly associated with the turbulent energy dissipation rate; hence it is a very significant parameter in the design of chemical processing equipment. To develop a better chemical processing equipment design, a thorough knowledge of the effect flow structure on local turbulence parameters like turbulent kinetic energy, eddy diffusivity and the energy dissipation rate are required. Turbulence is heterogeneous in most of the process equipment. Hence, the use of spatial average energy dissipation rate causes error in modelling of turbulent transport processes. In this present work, particle image velocimetry (PIV) is used to obtain the energy spectrum from grid generated homogeneous turbulence velocity data. The model of energy spectrum given by Kang et al. (2003) has been fitted to this energy spectrum using energy dissipation rate. A different approach, based on a third order structure function and velocity gradient technique has been used to compute the energy dissipation rate. The model predictions have been verified by experimental PIV velocity data from oscillating grid apparatus. [ABSTRACT FROM AUTHOR]

Abstract: Orifice meters are the most common instruments used for fluid flow measurement because of its ruggedness, simple mechanical construction and other known advantages. Orifice coefficients are empirical because of difficulty in accurately predicting the effects of geometrical complicacy and flow separation from the wall on the flow. In the present paper, Computational Fluid Dynamics (CFD) simulation has been used to predict the orifice flow with better accuracy. CFD simulations have been performed using OpenFOAM-1.6 solver and validated with the published experimental data of and . CFD simulations have been validated with pressure drop and energy balance of our experimental data of water as fluid. The outcomes of the CFD simulations in terms of profiles of velocity, pressure, etc. are discussed in detail. A new scheme has been proposed to track vena-contracta with the help of CFD and with a suitable provision in the hardware of orifice meter. The new scheme maintains the existing advantages of orifice meters and provides better accuracy and sensitivity. [Copyright &y& Elsevier]

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Studies have been carried out to extract relevant information on flow structures, their break-up distribution and the associated local turbulence phenomena using the 1D and 2D energy spectra. The 1D analysis uses the hot film anemometry (HFA) and large eddy simulation (LES) data sets, while the 2D analysis uses the PIV results. The chemical process equipment considered for the study are jet loop reactor (JLR), channel flow (CHA) and bubble column reactor (BCR). The work involves relative comparison of spectral methods, namely, fast Fourier transform (FFT), discrete wavelet transform (DWT), continuous wavelet transform (CWT), eddy isolation methodology (EIM) and proper orthogonal decomposition (POD) in evaluating the 1D and 2D spectra for the three equipment. FFT studies are used to identify the dominant frequencies, but they do not reveal the time dependent changes in the flow structure. This limitation is overcome by using the EIM, DWT and CWT and methodologies for identification of flow structures are discussed. Using DWT, a novel intermittency-based quantifier, namely, the scale relative local intermittency measure (SRLIM) is found to be particularly useful for identification of intermittent flow structures. For example, it has been used to track the intermittent bursts in the CHA, the vortex tube related intermittencies in the JLR and the bubble induced eddies in the BCR. The advantages of CWT for obtaining the eddy time-scale and length-scale distribution from the velocity time series is shown. The evaluation of distributions has been carried out using wavelet transform modulus maxima (WTMM) along with a symbolic analysis of the loci of the maxima lines. The structure break-up distribution and the energy break-up distribution that are obtained show a generalization to the known L and P models for intermittencies. The turbulence parameters, i.e., local turbulent kinetic energy, energy dissipation rate and turbulent viscosity, have been calculated and compared using the results obtained from the FFT, DWT, CWT and EIM. For the different approaches these parameters lie in reasonable ranges. [ABSTRACT FROM AUTHOR]

The article investigates the mean and turbulence quantities of jet loop reactors (JLR) via a computational analysis using particle image velocimetry (PIV) and computational fluid dynamic models, the Reynolds stress model and large eddy simulation. It shows the difference of the mean flow pattern in JLR from that of self-similar round jets and estimates its turbulence dissipation rate using the PIV data. Also examined is the effect of nozzle diameter on mixing time at the same power consumption between JLR and stirred tank reactors.

Abstract: CFD simulation of bed expansion of mono size solid–liquid fluidized beds has been performed in creeping, transition and turbulent flow regimes, where Reynolds number has been varied from 0.138 to 1718. It has been observed that the predicted values of bed voidage using the drag law of Joshi [1983. Solid–liquid fluidized beds: some design aspects. Chemical Engineering Research and Design 61, 143–161] and Pandit and Joshi [1998. Pressure drop in packed, expanded and fluidized beds, packed columns and static mixers—a unified approach. Reviews in Chemical Engineering 14, 321–371] (which has been derived from the first principals), exhibited an excellent agreement with the Richardson and Zaki equation. CFD simulations have also been performed for the prediction of segregation and/or intermixing of binary particle systems having the ratio of terminal settling velocity over a range from 3.2 to 1.06. The Reynolds number has also been varied over the range of 0.33 to 2080. It has been observed that the present CFD model explains all the qualitative and quantitative observations reported in the published literature (complete segregation, partial segregation, complete intermixing, etc) and these predictions are in good agreement with the experimental results. The present CFD model also predicts successfully the layer inversion phenomena which occur in the binary particle mixtures of different size as well as density. Further, the critical velocity at which the complete mixing of the two particle species occurs has also been predicted. [Copyright &y& Elsevier]

Abstract: Planar information of velocity from 2D particle image velocimetry (PIV) and large eddy simulation (LES) data have been studied using multiresolution wavelet transform (WT) formalisms, i.e., discrete and continuous WT. Identification of dominant energy containing structures with their characterization in terms of fractal spectra have been carried out for industrially important equipment exhibiting turbulent behavior. These include annular centrifugal contactor, jet loop reactor, ultrasound reactor, channel flow, stirred tank and bubble column reactor. The characterization of their dynamics based on denoising the data and studying the local energy along the WT scales show sensitive variation and this helps in identifying the size and shape of structures. A dependency is seen between mixing time and the higher order moments of length scale distribution, viz., skewness and kurtosis and a generalized correlation has been built up for important types of equipment and associated flow parameters. The correlation is not only based on the knowledge of reactor geometry and operating conditions but also on the flow structures via their statistical parameters. Wavelet transform modulus maxima (WTMM) methodology has been used to study the evolution of structures and their interaction in a reduced dimensionality by evaluating the fractal spectra. Classification studies have been carried out using principal component analysis (PCA) of the fractal spectra. The results obtained show clear classes for the six types of equipments and delineate regimes to obtain benchmark patterns of flow hydrodynamics based on PCA co-ordinates. This methodology offers a generalized way for the optimal design and operation of different types of reactors. [Copyright &y& Elsevier]

The aim of this study is computational fluid dynamic (CFD) simulation of the single- phase pressure drop in fixed and expanded beds. A fixed bed with a column to particle diameter ratio (D/dp) of 5 and having 151 particles arranged in 8 layers was taken as a computational geometrical model. In the case of expanded beds, 0.605 voidage bed consisted of 105 particles and 0.783 voidage bed consisted of 55 particles. Simulations were performed in the creeping transition and turbulent flow regimes, where Reynolds number (dpVLPL/μL) was varied from 0.1 to 10,000. The deviations from Ergun's equation due to the wall effects, which are important in D/dp < 10 beds, were well explained by the CFD simulations. Thus, an increase in the pressure drop was observed due to the wall friction in the creeping flow, whereas, in turbulent regime a decrease in the pressure drop was observed due to the channeling near the wall. Energy balance has been established through the CFD predicted values of energy dissipation rates (viscous as well as turbulent). [ABSTRACT FROM AUTHOR]

Shell-and-tube heat exchanger has been extensively used in industrial and research fronts for more than a century. However, most of its design procedures are based on empirical correlations extracted from experimental data of long length shell and tube heat exchanger. In this paper, an attempt has been made to investigate the complex flow and temperature pattern in such a short shell and tube type heat exchanger, with and without baffles in the shell side. Heat exchangers of length by hydraulic diameter ratio between 7 < L / D h < 21 ( 0.15 m < D S < 0.6 m ) for unbaffled and L / D h < 7 ( D S = 0.09 m ) for baffled heat exchangers are analysed using CFD code OpenFOAM-2.2.0 for different mass flow rates. It was observed that the cross flow near the nozzle region has a significant contribution towards the heat transfer, hence the conventional heat transfer correlations do not apply to these short heat exchangers. Furthermore, a sensitivity study of turbulence models was performed and it was observed that the standard k – ε model gives best results for the velocity profile as well as heat transfer, provided average y + of the first node adjacent to the heat transfer surface is maintained greater than 15. The commonly used boundary conditions at the exit are not realistic, as it tends to give either incorrect flow and temperature fields, or the solution was found to diverge. Through a sensitivity study of the exit length, it was found that exit length to shell side velocity ratio of 2.5 is required for proper convergence. Finally the effect of flow field on shell side heat transfer coefficient and a comparison with analytical methods are presented. [ABSTRACT FROM AUTHOR]

Abstract: Buoyancy induced flow and heat transfer are important phenomena in a wide range of engineering systems e.g. electronics and photovoltaics cooling, thermosiphon heat exchangers, solar-thermal heat absorbers, passive decay heat removal systems, etc. Such systems are subject to thermal stratification. The objective of the present work is to study the single phase and two phase (boiling) natural convection accompanied by thermal stratification. We carried out velocity and temperature measurements in a rectangular tank (0.8×0.6×0.6m3) fitted with (a) a central tube, and (b) a 10 tube assembly; which form the heat transfer surface. Flows were measured using Particle Image Velocimetry (PIV). Additionally, computational fluid dynamic (CFD) simulations of these systems were performed: first with an assumption of no-boiling (i.e. no phase change) near the heat transfer surfaces; for which we used the open source CFD code OpenFOAM-1.6. For two phase simulations, we used the boiling model of and carried out simulations using the commercial software FLUENT 6.3. The extent of stratification and mixing has been investigated for a range of Rayleigh numbers from 4.34×1011 to 2.59×1014. The flow information obtained from PIV was analyzed for insights into the dynamics of turbulent flow structures. We used the signal processing technique of discrete wavelet transform (DWT) for this purpose. From the analysis, we were able to estimate the size, velocity and energy distribution of turbulent structures in our flows. This information was used to estimate wall heat transfer coefficients. A good agreement was observed between the predicted and the experimental values of heat transfer coefficients. [Copyright &y& Elsevier]

Abstract: In this study, experiments have been performed for the steam injected centrally at the bottom of a vertical rectangular water vessel. Instantaneous velocity and temperature field near the plume as well as in the downstream have been measured in a vertical plane through the central axis. For this purpose, Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) have been employed. The velocity profile of the region above the condensation region was found to be self-similar with a small downward velocity near the wall due to recirculation. The instantaneous (20-ns integration time) thermal images obtained from PLIF had a spatial resolution of 100μm with a field of view of 100×100mm. The time averaged velocity and temperature profiles are computed from an ensemble of 100 velocity/temperature images. The present work is also concerned with CFD simulation by employing k-ε and large eddy simulation (LES) turbulence models. All the measurements and simulations were carried out by varying nozzle upstream pressure in the range of 0.3–0.35MPa (corresponding nozzle velocities were in the range of 286–304m/s) with the nozzle diameter of 1mm. [Copyright &y& Elsevier]

In the present work, experimental analysis has been carried out for jet loop reactors to investigate the mean and turbulence parameters. Measurements have been made using particle image velocimetry (PIV), laser Doppler velocimetry (LDV), and hot film anemometry (HFA). The major aim of this work is to evaluate the local value of turbulent dissipation rate (ε) using PIV. LDV has been used as a benchmark for analyzing the mean flow quantities, while HFA has been mainly used for the evaluation of energy spectra. Thus, the relative strength of these instruments was harnessed for better understanding of the flow pattern. A method has been proposed for the evaluation of εfrom the experimental data from PIV and HFA based on structure function relationships and 3D energy spectrum. Overall energy balance has been satisfied implicitly to make the method of εestimation more robust. As an application, the values of εobtained from the CFD predictions (Large eddy simulations) have been compared with that of the experimental values showing opportunity for improved design of turbulence based on local εvariations. [ABSTRACT FROM AUTHOR]

Flow separation across bluff bodies has been a fascinating area of fluid mechanics since 1851. A large number of computational and experimental studies have been performed in the last several decades which have enabled the understanding and hence helped in distinguishing the flow regimes based on the characteristics exhibited by different flow parameters. These signify some physical phenomena resulting due to flow separation under different flow conditions. A lot of research from the 70's to 90's was conducted using hot wire techniques for investigating the flow features. These techniques provided three-dimensional insights into the flow and helped in quantitative, reliable and robust corroboration of earlier qualitative findings which were based on inadequate visualization techniques. Moreover, the rapid growth in computing resources in the last couple of decades, have made it possible to use highly efficient numerical techniques to investigate separated flows with much higher accuracy and control. Also, the introduction of sophisticated measurement techniques such as PIV/ PLIF has aided in gaining deeper insights into the physics of sphere wakes. This work aims to present the developments and findings in the study of flow past a stationary sphere with respect to experimental and computational investigations. The statistical and signal processing techniques which form the chief medium to characterize the turbulence in such adverse pressure gradient wake flows, have also been discussed briefly. All the previous studies have been critically analysed presented as a coherent theme. This paper also brings out the knowledge gaps and accordingly recommendations have been made for the future research. Unlabelled Image • Novel PICO algorithm used for skimming through vast literature on wake flows • Recent advances in experimental and computational studies discussed qualitatively and quantitatively • Guidelines for cautiously performing experimental and computational studies are provided • Elaborate explanation of statistical and mathematical tools for post processing turbulence flows [ABSTRACT FROM AUTHOR]

Bubble columns, where the liquid is in the continuous phase and the gas phase is on the form of dispersed bubbles, are widely utilised in many industrial applications. When designing these bubble columns it is important to be able to predict the conditions that mark the transition from homogeneous (bubbly) to heterogeneous (churn-turbulent) flow. The transition is a function of many variables, including: liquid and gas superficial velocities, bubble diameter, and liquid and gas physical properties. Linear stability analysis (LSA) has been successfully applied by many researchers to determine the gas volume fraction at which the instability takes place; and correspondingly a one dimensional stability factor, f 1 , has been proposed (see for example Joshi et al. (2001)). Briefly, the LSA analysis utilises the velocity fluctuations of both the dispersed and continuous phases associated with the specific energy dissipation rate of the two phase mixture. Typically, the specific energy dissipation rate is correlated with the density of the bed. The previous analysis, however, does not consider the energy input which associated with the turbulence intensity (velocity fluctuations) of the incoming liquid stream. Usually, this component can be ignored in sparged bubble columns because its magnitude is relatively small and it also decays in the axial direction. In plunging liquid jet bubble columns the liquid is introduced as a high speed jet that entrains gas which is then broken into fine bubbles in the Mixing Zone. The bubby mixture then passes into the Two Phase Flow Zone where instabilities can be generated. The Mixing Zone is a region of high energy dissipation resulting in relatively large liquid velocity fluctuations, which can directly influence the instability of the Two Phase Flow Zone. In this study the existing linear stability analysis is modified to include the influence of inlet liquid velocity fluctuations on the stability parameter, f 1 . The modified theory is applied to the previous work of Evans (1990) for a plunging liquid jet bubble column to determine the critical gas volume fraction at which transition takes place in the Two Phase Flow Zone. In order to apply the model, drift-flux analysis has been used to obtain bubble diameter as a function of gas and liquid superficial velocities, and computational fluid dynamics has been utilised to quantify the velocity fluctuations of the liquid exiting the Mixing Zone. [ABSTRACT FROM AUTHOR]

Turbulence modulation of a nearly isotropic flow field due to the presence of single glass particles, with diameters in the range of 1−8 mm (~10–77 times the Kolmogorov scale), was studied experimentally in an oscillating grid apparatus. Particle image velocimetry (PIV) was used to obtain the instantaneous, two-dimensional velocity field for grid Reynolds number, Re g , varying from 1080 to 10,800. Fluctuating velocity components, flow field length scales, energy dissipation rates, turbulence intensity modulation and energy spectra were determined. An apparent increase of ~2–25% in the turbulence fluctuating velocity in the inertial subrange was noted compared with the fluid-only system. Presence of the particle led to enhancement in the flow field isotropy ratio and this ratio was found to be more dependent on the particle size compared with grid Reynolds number. The integral length scale for both fluid-only and fluid–particle systems exhibited a decreasing power law dependency on the grid Reynolds number. A critical ratio (0.41) of particle size to integral length scale was obtained which demarcated the regime of attenuation and enhancement of turbulence intensity and found to be valid for general grid turbulence in multiparticle systems as well. Energy dissipation rate was observed to increase with increase in particle size. Both longitudinal and transverse energy spectrum exhibited less steep slope than −5/3 in the presence of particle which was reasoned to the production of turbulence in the inertial subrange region. Energy enhancements from large scale to smaller scale were explained by the dissipative spectrum which showed maximum energy enhancement in the inertial subrange followed by a decaying trend. In general, the role of a particle in modulating turbulence was explained through possible wake oscillation and vortex shedding due to boundary layer separation on the particle surface and interaction with bulk eddies. [ABSTRACT FROM AUTHOR]

In a liquid fluidised bed system, the motion of each phase is governed by fluid-particle and particle-particle interactions. The particle-particle collisions can significantly affect the motion of individual particles and hence the solid-liquid two phase flow characteristics. In the current work, computational fluid dynamics-discrete element method (CFD-DEM) simulations of a dense foreign particle introduced in a monodispersed solid-liquid fluidised bed (SLFB) have been carried out. The fluidisation hydrodynamics of SLFB, settling behaviour of the foreign particle, fluid-particle interactions, and particle-particle collision behaviour have been investigated. Experiments including particle classification velocity measurements and fluid turbulence characterisation by particle image velocimetry (PIV) were conducted for the validation of prediction results. Compared to those predicted by empirical correlations, the particle classification velocity predicted by CFD-DEM provided the best agreement with the experimental data (less than 10% deviation). The particle collision frequency increased monotonically with the solid fraction. The dimensionless collision frequency obtained by CFD-DEM excellently fit the data line predicted by the kinetic theory for granular flow (KTGF). The particle collision frequency increased with the particle size ratio (dP2/dP1) and became independent of the foreign particle size for high solid fractions when the fluidised particle size was kept constant. The magnitude of collision force was 10-50 times greater than that of gravitational force and maximally 9 times greater than that of drag force. A correlation describing the collision force as a function of bed voidage was developed for Stp>65 and dP2/dP1↜2. A maximum deviation of less than 20% was obtained when the correlation was used for the prediction of particle collision force. [ABSTRACT FROM AUTHOR]

Abstract: In multiphase systems involving a dispersed phase, such as fluidized beds, the interphase exchange of mass, heat and momentum transfer can be very different from those from a single particle, droplet or bubble system under terminal conditions. Such differences need to be correctly predicted for proper design of multiphase reactors. However, most existing methodologies still rely heavily on empirical relationships for parameters such as slip velocity, especially for systems operating under turbulent conditions where there is wide difference in the reported results. In this study the hindered settling/rising (slip) velocity of single steel particles (d PD =5–12mm) and single air bubbles (d B =1–4mm) has been measured in a solid–liquid fluidized bed of uniform size borosilicate glass beads (d P =5 and 8mm) as a function of liquid superficial velocity. The homogeneity and intensity of the turbulence within the fluidized bed has been quantified and directly related to the slip velocity of the foreign (steel or bubble) particle. It was found that the turbulence resulted in an increase in the computed drag coefficient for all of the experimental conditions investigated. [Copyright &y& Elsevier]

Abstract: The bubble column reactors are usually operated in a heterogeneous regime where the liquid phase turbulence is generated by the bubble motion and the velocity gradients in the mean motion. The turbulent flow comprises of fluid elements moving in a random fashion with different sizes and energies, called ‘flow structures’. Both the large and small scale flow structures within a reactor play an important role in governing the local momentum, heat and mass transfer. The current work is focused on the estimation of the time averaged flow pattern and flow structures. The experimental data has been collected using miniature pressure sensors, PIV+shadowgraphy and LDA. The data was subjected/analyzed to/with multipoint linear stochastic estimation (MLSE), wavelet transforms, image processing and eddy isolation (EIM) to identify the flow structures. Two bubble columns have been used: a narrow rectangular (2D) column and a cylindrical (3D) column. Wavelet transforms (WT) were applied to isolate individual structures from PIV data to get their shape, size and energy in the 2D column. MLSE has been used to obtain the velocity profiles from pressure fluctuation signals. This data, augmented by PIV and LDA data, is subjected to WT and EIM to get the eddy age and its energy distribution. The data of the eddy shape size and energy was used to predict the mass transfer coefficient in the cylindrical bubble column as a test case. Overall, in this work we present a methodology to utilize the experimental data to get a better insight of the dynamics of flow structures, and propose a path forward for the deeper understanding of transport phenomena in bubble columns. [Copyright &y& Elsevier]

Abstract: A novel hybrid technique has been proposed in order to reveal in a greater detail the turbulent flow structures and their time evolution, and to address the issues and limitations related to the application of snapshot proper orthogonal decomposition (POD) and wavelet transform technique. The proposed hybrid technique combines the inherent abilities of the snapshot proper orthogonal decomposition and the two-dimensional discrete wavelet transform technique. The POD gives us the overall view of the most energetic flow pattern in an ensemble by decomposing the flow field into spatial and temporal modes, while two-dimensional wavelet transform gives us the localized spatial information through scale wise decomposition of the flow field. In this work, we apply the wavelet transform on the POD spatial modes. This enables us to understand the space scale structure of the flow events captured by the spatial POD modes, and the scale wise selectivity of these spatial POD modes. Thus, we are able to relate the most energetic flow events over a period of time (as obtained in spatial modes of snapshot POD) with the localized dominant scales that are contributing to it. Further, this information is utilized in the selection of those pod spatial modes that can effectively reconstruct a flow structure and its time evolution. The proposed technique has also been able to address the issues in the literature concerning the application of POD when the flow is less deterministic, as then a single POD mode may not reveal the flow structure and combination of modes is required to reconstruct it. In the present work, this hybrid methodology has been used to reveal the near wall intermittent events in channel flow: the ascending streaks and the bursts and their time evolution, the vortex tube and leading edge vortices in jet and the Taylor–Couette and irregular small chaotic vortices in Taylor–Couette flow. The planar dataset used for such an analysis has been obtained from particle image velocimetry and large eddy simulation studies. [Copyright &y& Elsevier]

Abstract: In direct contact condensation (DCC) phenomenon, whenever steam (vapor) is injected with very high velocity in sub-cooled water, the momentum and the energy of the steam is transferred to the surrounding liquid, leading to generation of flow pattern, turbulent in nature. The turbulent flow pattern enhances the heat transfer coefficient at the interface of steam jet and water (vapor–liquid interface) as well as at the immersed surfaces (solid–liquid interface). The flow and the temperature pattern in DCC system have been measured using hot film anemometer (HFA). The values of heat transfer coefficient at the vapor–liquid and solid–liquid interface were estimated using the CCA module of the HFA. The nozzle diameter was varied in the range of 1–2mm and the nozzle upstream pressure in the range of 0.3–0.35MPa (corresponding velocities in the nozzle were 286–304m/s). The time series of velocity and temperature at the interface were analyzed to get the rates of surface renewal. A comparison has been presented between the predicted and the experimental values of heat transfer coefficient. [Copyright &y& Elsevier]

Abstract: Mean flow and turbulence parameters have been measured using laser Doppler anemometer (LDA) in ultrasound reactor. The effects of the ultrasonic power have been investigated over a power density (P/V) range of 15–. The liquid circulation velocities are dominant in the zone nearer to the source of energy and are substantially low at the walls and at the bottom of the reactor. The levels of turbulence kinetic energy and dissipation rate are high near the horn and decrease rapidly with increasing distance from the horn. Average turbulent normal stresses are larger than the turbulent shear stresses. However, they are much lower than stirred reactors when compared at the same power consumption per unit mass. Comparisons of LDA measurements and computational fluid dynamics (CFD) predictions have been presented. The good agreement indicates the validity of the CFD model. The flow information has been extended for the prediction of mixing time. For uniform mixing in ultrasound-assisted reactors, optimum power density and diameter of the vessel is needed, yet it is far less effective than conventional stirred vessel. The possibility of optimization has been suggested in terms of power dissipation and the vessel size. [Copyright &y& Elsevier]