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Geometrical structures and operating performance of volutes used in centrifugal pumps are assessed in cross sections. By a volute, we mean the wetted part surrounding the impeller in a pump that converts the kinetic energy of water into potential energy. It is important to develop improved design principles and approaches for volutes to reduce hydraulic loss and increase the efficiency of pumps. However, when designing volutes, pump designers often neglect a key factor related to pump efficiency, namely, the energy loss due to the friction at the volute sidewall is directly proportional to the wetted perimeter of the cross section. In this paper, we show that the length of the wetted perimeter of the cross section is mathematically proportional to the friction loss in the volute. In addition, we present the design principles and calculation process to minimize the wetted perimeters of two kinds of cross section based on the velocity coefficient method using the same statistical data. The structures and shapes of the new cross sections are completely different from those of traditional volutes. Moreover, two different volutes with different cross sections are numerically investigated using a verified computational fluid dynamics technique. Both the head and the efficiency of the pump with the new volute are higher than those of a conventional pump under all working conditions. This paper provides a new design approach for the energy-saving volutes of centrifugal pumps. [ABSTRACT FROM AUTHOR]

In order to solve the blockage effect and energy dissipation phenomenon caused by cavitation in the low-pressure vortex core region, this paper analyzes the spatial evolution of vorticity intensity and turbulent kinetic energy intensity under different cavitation conditions based on the Wray–Agarwal (WA) model. First, the tip leakage flow characteristics are studied, the evolution of vorticity and vorticity intensity is analyzed, then the distribution of turbulent kinetic energy distribution in the blade tip region is studied, and finally, the vorticity transport characteristics of the tip region are analyzed. It is found that the tip leakage rate is less affected by the vortex cavitation of the tip leakage, and there is a strong interaction between the leakage flow at the tip leading edge and the trailing edge, and the separation vortices and low-speed regions formed in the end-wall region cause blockage of the flow passage. Low pressure causes cavitation to cover most regions of the suction surface, inhibiting the formation and development of the tip leakage vortices. The distribution range of high turbulent kinetic energy region is almost the same as that of high-vorticity region, and there is a positive correlation between the two intensities. Severe cavitation causes the high turbulent kinetic energy region at the outlet of the flow passage to develop in the radial and axial directions of the impeller, which increases the turbulent dissipation and energy loss. The change of vorticity transport intensity caused by cavitation is mainly reflected in the expansion contraction term, and the Coriolis force term plays a dominant role in the vorticity transport process. This paper provides a reference for further improving the performance of mixed-flow pumps. [ABSTRACT FROM AUTHOR]

In porous media, non-wetting phase droplets snapping off in a constricted microchannel are one of the most common phenomena in two-phase flow processes. In this paper, the application range of the classic quasi-static criterion in rectangular cross section microchannels is obtained. For three different droplet breakup phenomena—total breakup, partial breakup, and non-breakup—observed in experiments when a non-wetting phase droplet passes through a microchannel constriction, the breakup is caused by the droplet neck snapping off in a channel constriction. A critical criterion for the dynamic snap-off event in a two-phase flow is proposed considering the effect of viscous dissipation by mechanical analysis, energy dissipation analysis, and many microfluidic experiments. When the droplet front flows out of the constriction, snap-off will occur if the surface energy release exceeds the required energy for viscous dissipation and kinetic energy conversion. The unique partial breakup phenomenon is affected by droplet surfactant distribution and the acceleration effect in the constriction center. This partial breakup phenomenon in experiments is an essential evidence for the non-uniform distribution of surfactants in the droplet surface. The results of this study contribute to understanding pore-scale mass transfer and flow pattern changes within porous media. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on the effects of the Reynolds number (Resj = 300, 600, and 900) and porosity (ϕ = 20%–85%) on synthetic jet vortex rings impinging onto a porous wall. Laser-induced fluorescence and particle image velocimetry are used to acquire flow information qualitatively and quantitatively. When Resj is low (Resj = 300), ϕ plays a key role in determining the formation of transmitted vortex rings downstream. For the first time, a row of individual small-scale vortex rings that form at the lowest porosity (ϕ = 20%) have been observed in the synthetic jet/porous wall interaction. As Resj increases to 900, the triggered Kelvin–Helmholtz instability promotes the vorticity cancellation at a low porosity (ϕ = 30%), and thus contributes to the formation of a transmitted vortex ring. It is concluded that the vorticity cancellation is the dominant factor affecting the generation of a transmitted vortex ring. Time-averaged characteristics indicate that for a low Resj, the incoherence of the vortex ring is mainly due to the viscous effects. However, for a high Resj, it is the transition that leads to a significant enhancement in the turbulent kinetic energy. Measurements of flow macroscopic parameters show that the loss of the momentum flux exhibits a linear relationship with ϕ for all Resj, while the loss of the kinetic energy transport is nonlinearly dependent on ϕ. Incorporating ϕ, this study presents a more comprehensive similarity parameter, ϕln(Resj2 d h * 3 ), to characterize the synthetic jet/porous wall interaction. [ABSTRACT FROM AUTHOR]

In this paper, we present an overview of concepts and data concerning inverse cascades of excitation towards scales larger than the forcing scale in a variety of contexts, from two-dimensional fluids and wave turbulence to geophysical flows in the presence of rotation and stratification. We briefly discuss the role of anisotropy in the occurrence and properties of such cascades. We then show that the cascade of some invariant, for example, the total energy, may be transferred through nonlinear interactions to both the small scales and the large scales, with in each case a constant flux. This is in contrast to the classical picture, and we illustrate such a dual cascade in the context of atmospheric and oceanic observations, direct numerical simulations, and modeling. We also show that this dual cascade of total energy can in fact be decomposed in some cases into separate cascades of the kinetic and potential energies, provided the Froude and Rossby numbers are small enough. In all cases, the potential energy flux remains small, of the order of 10% or less relative to the kinetic energy flux. Finally, we demonstrate that, in the small-scale inertial range, approximate equipartition between potential and kinetic modes is obtained, leading to an energy ratio close to one, with strong departure at large scales due to the dominant kinetic energy inverse cascade and piling-up at the lowest spatial frequency and at small scales due to unbalanced dissipation processes, even though the Prandtl number is equal to one. [ABSTRACT FROM AUTHOR]

In this paper, we provide an analysis of turbulent mixing in stably stratified wall-bounded flows to highlight a number of important issues such as prediction of the turbulent viscosity, the turbulent diffusivity, and the irreversible flux Richardson number. By invoking the equilibrium assumption between the production rate of the turbulent kinetic energy (P), the dissipation rate of the turbulent kinetic energy (ϵ), and the turbulent potential energy dissipation rate (ϵPE) as P â ϵ + ϵPE and also assuming equilibrium between the buoyancy flux (B) and ϵPE as â'B â ϵPE, we first propose that the irreversible flux Richardson number (R*f=ϵPE/(ϵ+ϵPE)) can be approximated with the flux Richardson number (Rf = â' B/P), especially for low gradient Richardson numbers. Second, we propose that the turbulent viscosity ŷtâ(1â'Râ--f)â'1ϵ/S², where S is the mean shear rate. We then extend our analysis to propose appropriate velocity and length scales. Tests using the direct numerical simulation (DNS) data of turbulent channel flow of García-Villalba and del Álamo ["Turbulence modification by stable stratification in channel flow," Phys. Fluids 23, 045104 (2011)] are performed to evaluate our propositions. The comparisons between the exact turbulent viscosity, length and velocity scales, and the proposed formulations are excellent. Also, the agreement between Rf and Râ--f is reasonable for the bulk of the flow depth with differences of about 20% on average, except at depths very close to the flow boundaries. Finally, by invoking the equilibrium assumption between the buoyancy flux (B) and the dissipation rate of the turbulent potential energy (ϵPE) as â'B â ϵPE, we infer the turbulent diffusivity as ŷ°t â ϵPE /N², where N is the buoyancy frequency. The comparison of the proposed turbulent diffusivity with the exact turbulent diffusivity computed from DNS data is good especially close to the wall but the agreement deteriorates far away from the wall, indicating the breakdown of assuming equilibrium as â'B â ϵPE which is attributed to the presence of linear internal wave motions in this far-wall region. [ABSTRACT FROM AUTHOR]

For improving the in-cylinder flow characteristics of intake air/charge and for strengthening the turbulence intensity, specific intake port geometries have shown significant potential in compression ignition engines. In this experimental study, effects of intake port geometries on air-flow characteristics were investigated using tomographic particle imaging velocimetry (TPIV). Experiments were performed using three experimental conditions, namely, swirl port open (SPO), tangential port open (TPO), and both port open (BPO) configurations in a single cylinder optical research engine. Flow investigations were carried out in a volumetric section located in the middle of the intake and exhaust valves. Particle imaging velocimetry (PIV) images were captured using two high speed cameras at a crank angle resolution of 2° in the intake and compression strokes. The captured PIV images were then pre-processed and post-processed to obtain the final air-flow-field. Effects of these two intake ports on flow-field are presented for air velocity, vorticity, average absolute velocity, and turbulent kinetic energy. Analysis of these flow-fields suggests the dominating nature of the swirl port over the tangential port for the BPO configuration and higher rate of flow energy dissipation for the TPO configuration compared to the SPO and BPO configurations. These findings of TPIV investigations were experimentally verified by combustion and particulate characteristics of the test engine in thermal cylinder head configuration. Combustion results showed that the SPO configuration resulted in superior combustion amongst all three port configurations. Particulate characteristics showed that the TPO configuration resulted in higher particulate compared to other port configurations. [ABSTRACT FROM AUTHOR]

In this study, we investigate the energy conversion and dissipation mechanisms of spreading droplets on a solid surface at a low Weber number regime, which neither conventional energy-balance-based theories nor empirical scaling laws can completely explain. The energetic analysis presented in this study shows that on a hydrophilic surface, the actual primary energy source driving the spreading process is the initial surface energy not the initial kinetic energy. The conventional energy-balance-based approaches are found to be valid only for the spreading process on a hydrophobic surface. Particular attention is also paid to the roles of the capillary waves. The capillary waves are found to play significant roles in all of the important flow physics, that is, the interfacial structure, the oscillatory motions and the rapid collapse of the liquid film, the onset of the viscous regime, and the energy loss mechanism. It is also shown that the energy dissipation caused by the capillary-wave-induced phenomena can be estimated to be 25%–35% and 55%–65% of the total energy loss for a hydrophilic and a hydrophobic surface, respectively, at the low Weber number regime. [ABSTRACT FROM AUTHOR]

The sweeping jet actuator (SJA), a type of fluid oscillator, has attracted considerable attention in the areas of enhanced heat transfer and flow control. In this study, an unsteady Reynolds-averaged Navier–Stokes numerical investigation (k–ω baseline) was conducted to evaluate the flow mechanisms and heat transfer characteristics of a sweeping jet (SJ) on a confined concave surface. A conventional oscillating jet with a unit aspect ratio was used to generate the SJ. The heat transfer at the target surface and the flow details inside the flow field are discussed in detail to analyze the mechanism of the effect of SJ on impinging cooling. First, an appropriate numerical model was selected based on a comparison with a previous experiment. Subsequently, the time-averaged and time-resolved flow field and heat transfer results were investigated at three target distances for three different Reynolds numbers. The unsteady time-averaged results show that the SJ exhibits better heat transfer performance at higher Reynolds numbers and narrower target spacings than circle jet, which improves by 13% at Re = 30 000, H/D = 1. Subsequently, the turbulent kinetic energy and velocity loss theories were combined to compare the time-averaged and time-resolved flow field details of the two jets. The heat transfer characteristics of the target surface and the flow details inside the flow field in time and space were effectively correlated. Finally, the topology and three-dimensional (3D) vortex structure inside the confined channel were remodeled to better understand the unsteady sweeping process and provide theoretical support for subsequent applications. [ABSTRACT FROM AUTHOR]

A new compact active turbulence generator is developed, tested, and characterized, which extends the capabilities of such generators used in turbulent premixed combustion research. The generator is composed of two blades that resemble the shape of two bow-ties. Hot-wire anemometry and high-speed imaging are performed to characterize the non-reacting flow produced by the generator and the blades dynamics, respectively. Two mean bulk flow velocities of 5.0 and 7.0 m/s are examined. For comparison purposes, in addition to the developed generator, tests are also performed for a free jet as well as one and two perforated plates. The results show that the centerline root mean square velocity fluctuations can become as large as 1.8 m/s. For the newly developed device, the power-law decay of the one-dimensional kinetic energy is −1.0 and −1.3 for the mean bulk flow velocities of 5.0 and 7.0 m/s, respectively. The normalized energy dissipation rate is relatively small for the newly developed device, while the energy dissipation rate is relatively large. The spectral analysis of the velocity data does not show dominant frequencies equal to the blades rotation frequencies, and the one dimensional kinetic energy and dissipation spectra follow −5/3 and 1/3 power-law relations, respectively. It is shown that the small eddies produced by the newly developed device dissipate the turbulent kinetic energy faster than those corresponding to the rest of the tested turbulence generators. [ABSTRACT FROM AUTHOR]

We numerically study the impact of a liquid drop onto no-slip rigid substrates with different wettabilities using a diffuse interface method, aiming to obtain a universal model for the maximal spreading of the impacting drop at moderate Weber numbers. We find that the wettability plays an important role in the maximal spreading and that the ratio of the surface energy to the initial kinetic energy of the drop at the maximal spreading, η, follows η ∼ W e − 1 / 2 at high fixed Reynolds numbers, where We is the Weber number. Taking account of the wettability effect, we obtain a scaling law at high Reynolds numbers from an analysis of energy transformation. This scaling law is compatible with the one derived from the momentum balance at the high impact velocity by Clanet et al. ["Maximal deformation of an impacting drop," J. Fluid Mech. 517, 199–208 (2004)]. Moreover, we attribute it to the presence of a viscous–capillary regime, in which the viscous dissipation of the kinetic energy from the substrate is as significant as the kinetic energy transformed into the surface energy. Accordingly, we identify a new impact parameter, which makes all the numerical results of maximum drop deformation (from the viscous regime to the viscous–capillary regime with Reynolds number up to 104) collapse onto a single curve. Finally, we propose a universal model, the predictions of which are shown to agree well with numerical results for a wide range of Weber and Reynolds numbers. [ABSTRACT FROM AUTHOR]

The splash phenomenon is being increasingly explored with the use of modern measurement tools, including the high-speed cameras. Recording images at a rate of several thousand frames per second facilitates parameterization and description of the dynamics of splash phases. This paper describes the impact of a single drop of a liquid falling on the surface of the same liquid. Three single-phase liquid systems, i.e., water, petrol, and diesel fuel, were examined. The falling drops were characterized by different kinetic energy values depending on the height of the fall, which ranged from 0.1 to 7.0 m. Four forms, i.e., waves, crowns, semi-closed domes, and domes, were distinguished depending on the drop energy. The analysis of the recorded images facilitated determination of the static and dynamic parameters of each form, e.g., the maximum height of each splash form, the width of the splash form at its maximum height, and the rate of growth of the splash form. We, Re, Fr, and K numbers were determined for all analyzed liquid systems. On the basis of the obtained values of dimensionless numbers, the areas of occurrence of characteristic splash forms were separated. [ABSTRACT FROM AUTHOR]

This study investigates a solenoidal linear forcing scheme with reduced oscillation of a turbulent Mach number MT for direct numerical simulations (DNS) of statistically steady, homogeneous isotropic turbulence. A conventional linear forcing scheme results in a large temporal oscillation of MT, where the maximum MT reaches about 1.1 times the time-averaged MT. Therefore, strong shocklets are generated when MT becomes large although such strong shocklets hardly appear when MT is close to the time-averaged value. DNS with the proposed forcing scheme confirms that the temporal oscillation of MT is effectively reduced by adjusting a forcing coefficient with a ratio between velocity variance and its steady state value prescribed as a parameter. The time-dependent forcing coefficient results in the variation of the power input to kinetic energy. Therefore, the temporal oscillation of the Reynolds number for this forcing scheme is as large as that for the conventional linear forcing. The ratio between the solenoidal and dilatational kinetic energy dissipation rates increases with MT, and the MT dependence is consistent between the present solenoidal linear forcing and the low-wavenumber solenoidal forcing in wavenumber space. The skewness and flatness of the velocity derivative become large compared with incompressible turbulence when MT exceeds 0.6. Both average and root-mean-squared fluctuation of the shock Mach number of shocklets increase with MT. The most typical thickness of shocklets decreases with MT and asymptotically approaches about 1.5 times the Kolmogorov scale. The shocklet thickness normalized by the Kolmogorov scale hardly depends on the Reynolds number. [ABSTRACT FROM AUTHOR]

Decay characteristics of turbulent kinetic energy and enstrophy in grid turbulence have been investigated in the far downstream region (x / M ∼ 10 3 : x is the downstream distance from the grid, M is the mesh size of the grid) through wind tunnel experiments using hot-wire anemometry, with the lowest turbulent Reynolds number R e λ ≈ 5. The non-dimensional dissipation rate C ε increases rapidly toward the final stage of the transition period of decay and the profile agrees well with previous direct numerical simulation [W. D. McComb et al., "Taylor's (1935) dissipation surrogate reinterpreted," Phys. Fluids 22, 061704 (2010)] and theoretical estimation [D. Lohse, "Crossover from high to low Reynolds number turbulence," Phys. Rev. Lett. 73, 3223 (1994)] at very low R e λ in decaying and stationary isotropic turbulence. The present result of C ε is an update on the experimental data in grid turbulence toward a very low R e λ , where measurements have been absent. The energy spectrum in the dissipation range at very low R e λ deviates from a universal form observed at high Reynolds numbers. The decay rate of enstrophy is proportional to S + 2 G / R e λ (S is the skewness of the longitudinal velocity derivative and G is the destruction coefficient). It is shown that G and S + 2 G / R e λ increase rapidly with decreasing R e λ at very low R e λ , indicating that the effect of enstrophy destruction is dominant in the final stage of the transition period of decay. The profiles of S + 2 G / R e λ against R e λ is well fitted by a power-law function even in the final stage of the transition period of decay. [ABSTRACT FROM AUTHOR]

Here, we present some interesting new results on the linear modal stability of the Couette–Poiseuille flow (CPF), by numerically solving the Orr–Sommerfeld equation within the temporal framework. We provide a mechanistic explanation to the well-known result that the upper plate movement causes progressive stabilization of the CPF. The temporal energy budget reveals appearance of a region of negative energy production with plate movement, which is located near the plate that has a higher relative velocity in the direction of the bulk flow. This has a negative contribution to the integral production term, which, along with the expectation that energy dissipation is nominally constant (to leading order) at a given Reynolds number (Re), reduces the growth rate of kinetic energy, thereby causing flow stabilization with an increase in plate speed. Next, we compare the linear stability portrait of the CPF for positive and negative plate speeds and find that the upper and lower neutral branches in the frequency-Re plane cross over each other (to form a "loop") for a certain range of negative plate speeds. Interestingly, this behavior is not seen for positive plate speeds. At high Re and small plate speeds (positive/negative), we observe that the CPF supports an additional unstable mode manifested in the neutral curves as a bifurcation of the unstable region into primary and secondary regions. The secondary unstable mode is found to be sufficiently distinct in character from the primary mode. These results contribute to the contemporary efforts to better understand the stability of the CPF. [ABSTRACT FROM AUTHOR]

Snap-off usually occurs during two-phase fluid displacement in a constricted capillary, where the nonwetting phase fluid is cut into blobs or ganglia due to surface tension. Snap-off has been intensely recognized as a predominant pore-scale mechanism that may be responsible for the breakup and trapping of the nonwetting phase in complex geophysical structures. Herein, we investigated the dynamics of snap-off in a constricted pore and throat structure with a square cross-section using the volume of fluid method. Despite the geometric constraint dictated by Roof, a new judging diagram for the occurrence of snap-off was proposed as a function of Ca number and viscosity ratio. Our prediction from the numerical simulation is consistent with the analytical solution derived from the balance of capillary and hydrodynamic pressure. Furthermore, the associated transient energy balance was thoroughly studied, considering the alteration of the surface energy, kinetic energy, total input energy, and viscous dissipation during the period of snap-off. The results indicated that snap-off is always characterized by a sharp decline in the surface energy, which resulted in a surge in the kinetic energy and viscous dissipation. In addition, we observed a sharp surge in the viscous dissipation rate curve associated with such energy change, which is attributed to the redistribution of the velocity field. The sudden surge unanimously decreased while increasing the Ca number or viscosity ratio. Meanwhile, the position at which snap-off took place was shifted downstream of the throat, explaining the condition of the snap-off had become much more difficult. [ABSTRACT FROM AUTHOR]

The present study investigates the influence of particle additives on the transfer, conversion, and dissipation of kinetic energy (KE) of a turbulent gas–solid channel flow. We derived the equations of KE, mean-flow KE, and turbulent KE (TKE) of the particle-laden flow and further performed two-way coupled direct numerical simulations of channel flow laden with four-million particles with Stokes number St = 30 (corresponding to a mass loading ratio of around one) with an Eulerian–Lagrangian approach. We found that, in the unladen flow, more than half of the input energy is directly dissipated in the mean flow, whereas the rest is converted to maintain the turbulence. By contrast, in the laden flow, both mean dissipation and energy supply are comparable with the unladen flow. However, the turbulence production is greatly reduced in the particle-laden flow. Another sink term due to the presence of the particle–fluid interactions corresponds to the rest loss of the total energy supply. The results reveal the particle-induced redistribution of mean KE, which is transferred from the mean flow to particles in the channel core, whereas the flow gains energy from particles in the near-wall region. In total, there is a loss of the mean-flow energy due to the presence of the inertial particles. Regarding TKE balance, the particles, gaining energy from the mean flow, transfer the energy to the fluid across the channel, which contributes around one third of the TKE source. The present results provide a general picture of KE balance of a particle-laden channel flow. [ABSTRACT FROM AUTHOR]

The present paper examines the effect of Reynolds number on turbulence properties in the transition region of a circular jet issuing from a smoothly contracting nozzle. Hot-wire measurements were performed for this investigation through varying the jet-exit Reynolds number Red (≡ Ujd/ν, where Uj, d, and ν are the jet-exit mean velocity, nozzle diameter, and kinematic viscosity) approximately from Red ≈ 4 × 103 to Red ≈ 2 × 104. Results reveal that the rates of the mean flow decay and spread vary with Reynolds number for Red < 104 and tend to become Reynolds-number independent at Red ≥ 104. Even more importantly, the small-scale turbulence properties, e.g., the mean rate of dissipation of kinetic energy (&eh;), the Kolmogorov and Taylor microscales, are found to vary in different forms over the Red ranges of Red > 104 and Red < 104. Namely, the critical Reynolds number appears to occur at Red,cr ≈ 104 across which the jet turbulence behaves distinctly. Two turbulence regimes are therefore identified: (i) developing or partially developed turbulence at Red < Red,cr and (ii) fully developed turbulence at Red ≥ Red,cr. It is suggested that the energy dissipation rate (DR) can be expressed as &eh;∼νUc2/R2 in regime (i) and &eh;∼Uc3/R in regime (ii), where Uc and R are the centerline (or maximum) mean velocity and half-radius at which the mean velocity is 0.5Uc. In addition, the critical Reynolds number appears to vary from flow to flow. [ABSTRACT FROM AUTHOR]

We present a study of a large-scale energy spectrum and integral invariants in temporally developing grid turbulence at mesh Reynolds numbers of ReM = 10 000 and 20 000 by employing direct numerical simulations (DNSs) in a periodic box. The simulations are initialized with a velocity field that approximates the wakes induced by the bars of conventional square grids. The turbulence statistics obtained in the temporal DNS agree well with those of the previous experiments in both the production and decay regions. The temporally developing grid turbulence also has a so-called non-equilibrium region, which is consistent with its spatially developing counterpart, where the normalized dissipation rate of turbulence kinetic energy (TKE), Cε, increases as the turbulence decays. The decay exponent n of TKE is n = 1.22 at ReM = 20 000 and n = 1.35 at ReM = 10 000, which are close to the values for the Saffman turbulence [i.e., 6/5 for ReM = 20 000 and 6(1 + p)/5 ≈ 1.36 for ReM = 10 000 with p ≈ 0.13 obtained by Cε ∼ tp at large t]. The longitudinal integral length scale and the TKE dissipation rate also exhibit temporal evolutions consistent with the Saffman turbulence for both ReM. The Saffman integral directly evaluated in the grid turbulence tends to be time-independent after the turbulence evolves for about 200 times of characteristic time scale defined by mesh size divided by the mean velocity of a fluid passing the grid. A direct evaluation of the TKE spectrum E(k) shows that E(k) ≈ Lk2/4π2 is valid for a finite range of low wavenumbers. [ABSTRACT FROM AUTHOR]

Since the seminal article "Inertial ranges in two-dimensional turbulence" by Kraichnan in 1967, our understanding of the dynamics and transport properties of two-dimensional turbulence is largely built on the assumption of homogeneity and isotropy of statistically steady or decaying turbulence. In the last two decades, more attention has been paid to the presence of lateral walls, either with stress-free or no-slip boundary conditions, and also considering a variety of geometries such as square, rectangular, or circular domains. The impact of confining boundaries on the dynamics of two-dimensional turbulence is important. This is in sharp contrast with three-dimensional turbulence, where homogeneity and isotropy are locally restored due to the cascade process in the inertial range. The impact of confining boundaries is therefore limited in three-dimensional turbulence. The presence of an inverse energy cascade in two-dimensional turbulence, however, will continuously generate large-scale energy-containing eddies, and their vigorous interaction with, in particular, no-slip walls generates large amounts of vorticity and contributes significantly to the dissipation of kinetic energy of the flow. The dissipation is even strongly enhanced compared with the unbounded case. In this review, we will focus on one of the elementary structures observed in two-dimensional turbulent flows: the dipolar vortex. With its self-induced velocity, it propagates easily through the domain and is hence likely to interact with domain boundaries. Standard vortex generation mechanisms allow to create well-defined dipoles and to investigate the collision of such structures with rigid domain boundaries in detail. Relevant aspects of the collision process concern the dynamics and stability of the generated boundary layers, the vorticity, and vorticity gradients contained in these boundary layers, and the dissipation of kinetic energy when dipoles collide with walls. Some of these aspects will be discussed in this review. Moreover, we are interested in the Reynolds-number dependence of these processes, including exploration of the vanishing viscosity limit. [ABSTRACT FROM AUTHOR]

Direct numerical simulations of lean methane/air flames are carried out to study the effects of premixed combustion on turbulence. The equivalence ratio of the flame is 0.5 and non-dimensional turbulence intensities (urms/SL) are between 2 and 25. The mixture pressure is 20 bars and temperature is 810 K to simulate approximate conditions in lean-burn natural gas engines. The Karlovitz number (Ka) varies from 1.1 to 49.4, and the Damköhler number (Da) varies from 0.26 to 3.2 corresponding to turbulent premixed combustion in the thin reaction zone (TRZ) regime. It is found that turbulence kinetic energy (TKE) and its dissipation rate decrease monotonically across the flame brush while the integral length scale increases monotonically for flames in the TRZ regime. The transport equation of TKE is then examined, and the scaling of the terms in the equation is discussed. It is found that the sink term which represents molecular diffusion and viscous dissipation is the dominant term in the TKE balance and it scales with the square of Ka. The relative importance of the other terms with respect to the dissipation term is studied. With increasing Ka, the other terms in the TKE balance become less important compared to the dissipation term. [ABSTRACT FROM AUTHOR]