Turbulence Modulation Research Articles

Particle image and tracking velocimetry of solid-liquid turbulence in a horizontal channel flow

The turbulence statistics of the solid and liquid phases in a horizontal turbulent channel flow are experimentally investigated to identify the effect of particle size, flow Reynolds number (Re) and the volumetric concentration of particles. Glass beads with diameters of 285, 340, 530 and 700 µm are used to produce suspensions at particle volumetric concentrations between 0.05 and 0.18%. Measurements are made at Re of 50,000, 75,000, 100,000, and 125,000 based on channel height and average velocity. The instantaneous velocity of the glass beads is obtained using a correlation-based particle tracking velocimetry (PTV) while particle image velocimetry (PIV) using smaller tracers is applied simultaneously to obtain the liquid phase velocity. It is observed that the solid phase velocity is larger than the liquid phase in the near-wall region for the larger particles and higher flow Re. The particle Re number, Rep, defined based on the particle diameter and the slip velocity (mean relative velocity between the liquid and solid phases), and the Stokes number (St) are used to evaluate the turbulence modulation of the liquid phase. Particle Re numbers as large as 225 are calculated for the 700 µm particles at Re = 125,000, which leads to vortex shedding and, consequently, increases turbulence intensity of the liquid phase relative to the unladen flow. The PTV results indicate that the turbulence intensity of the solid phase decreases with increase of St (i.e. larger particles and higher flow Re). The solid phase volumetric concentration has negligible effect on the solid phase turbulence, while the wall-normal turbulence intensity of the liquid phase increases with the volumetric concentration. This observation reveals the two-way coupling of the solid and liquid phases at the range of concentrations investigated in the current study.

Modelling bubble induced turbulence for gas-liquid bubbly flow in a vertical pipe

A two-fluid model in the Eulerian-Eulerian framework has been implemented for the turbulent gas-liquid bubbly upward flow in a vertical pipe. The transport equations, i.e. Reynolds-Averaged Navier-Stokes equations for the two-fluid model, are discretized using the finite volume method. The effect of the disperse gas phase on the liquid phase turbulence, referred to as turbulence modulation, was accounted for through source terms in the transport equations for a low Reynolds number k-ε turbulence closure. The model was used to predict the turbulence kinetic energy and its dissipation rate, mean phasic velocities and volume fraction distribution for a set of test cases selected from the available literature. The focus of the present study was on assessing the model predictions using measurements of the turbulence kinetic energy of the liquid phase. A relatively novel aspect of the present analysis is the use of the budgets of the transport equations for the turbulence kinetic energy and dissipation rate to assess the effect of the turbulence modulation on the liquid turbulence field. Although the present model formulation is shown to adequately predict the effect of bubbles in some flows and perform better than some of the other model formulations, the overall conclusion is that the present approaches for incorporating turbulence modulation are at best partially successful and still need further development.

Turbulence modulation in channel flow of finite-size spheroidal particles

Finite-size particles modulate wall-bounded turbulence, leading, for the case of spherical particles, to increased drag also owing to the formation of a particle wall layer. Here, we study the effect of particle shape on the turbulence in suspensions of spheroidal particles at volume fraction $\unicode[STIX]{x1D719}=10\,\%$ and show how the near-wall particle dynamics deeply changes with the particle aspect ratio and how this affects the global suspension behaviour. Direct numerical simulations are performed using a direct-forcing immersed boundary method to account for the dispersed phase, combined with a soft-sphere collision model and lubrication corrections for short-range particle–particle and particle–wall interactions. The turbulence reduces with the aspect ratio of oblate particles, leading to drag reduction with respect to the single-phase flow for particles with aspect ratio ${\mathcal{A}}{\mathcal{R}}\leqslant 1/3$, when the significant reduction in Reynolds shear stress is more than the compensation by the additional stresses, induced by the solid phase. Oblate particles are found to avoid the region close to the wall, travelling parallel to it with small angular velocities, while preferentially sampling high-speed fluid in the wall region. Prolate particles also tend to orient parallel to the wall and avoid its vicinity. Their reluctance to rotate around the spanwise axis reduces the wall-normal velocity fluctuation of the flow and therefore the turbulence Reynolds stress, similar to oblates; however, they undergo rotations in wall-parallel planes which increase the additional solid stresses due to their relatively larger angular velocities compared to the oblates. These larger additional stresses compensate for the reduction in turbulence activity and lead to a wall drag similar to that of single-phase flows. Spheres on the other hand, form a layer close to the wall with large angular velocities in the spanwise direction, which increases the turbulence activity in addition to exerting the largest solid stresses on the suspension, in comparison to the other studied shapes. Spherical particles therefore increase the wall drag with respect to the single-phase flow.

Modulation of turbulence scales passing through the rotor of a wind turbine

ABSTRACTThe modulation of boundary-layer turbulence across scales by passage through the rotor of a model wind turbine is assessed experimentally in a wind tunnel. Synchronous measurements of the flow were performed using hotwire anemometers located directly upwind and at various locations downwind of the turbine along the horizontal axis of symmetry. The synchronous data allowed for the distinct quantification of added and suppressed turbulence by considering the temporal correlation between upwind and downwind time series. The suppressed turbulence exhibited larger characteristic time scales than the added turbulence which decreased slightly with distance, while the integral time scale of the added turbulence increased substantially with downwind distance. The intensity of the suppressed turbulence did not change substantially with downwind distance, suggesting that the turbine had a much stronger effect on the dampening of the turbulence than simple natural evolution. The cross spectra between upwind and downwind velocity measurements suggest a dispersion relation for different time scales. In the near wake, lower-frequency components appear to be advected at velocity lower than the local wake velocity, and this advection velocity asymptotically approaches the local velocity at high frequency. This trend diminished in magnitude with downwind distance.

Interface-resolved direct numerical simulations of the interactions between neutrally buoyant spheroidal particles and turbulent channel flows

A parallel direct-forcing fictitious domain method is employed to perform interface-resolved numerical simulations of the interactions between neutrally buoyant finite-size spheroidal particles and turbulent channel flows. The effects of the aspect ratio of the spheroidal particles on the turbulence modulation and the rotation of the particles are investigated at the friction Reynolds number of 180, with the ratio of the particle equivalent diameter to the channel width being 0.1, the particle volume fraction ranging from 0.79% to 14.16%, and the particle aspect ratio ranging from 1/3 to 8. Our results show that the flow friction decreases as the prolate particles become more slender or the oblate particles become flatter and is smaller than that of the single-phase flow for the aspect ratio being 1/3 and 8 at the particle volume fraction of 2.36%. Both effects of the low particle volume fraction in the near-wall region and the relatively small Reynolds stress are important to the occurrence of the drag-reduction by the non-spherical particles, and a lower flow drag for the oblate particles compared to the prolate particles at comparable aspect ratios (e.g., 1/3 vs 4) is mainly caused by a lower Reynolds stress contribution. The prolate particles preferentially align their symmetry axes with the streamwise direction, and the oblate particles preferentially align their symmetry axes with the wall-normal direction. However, the most probable orientation of the major axes of both prolate and oblate particles in the near-wall region is not exactly the streamwise direction but has a positive inclination angle with the streamwise direction. Generally the prolate particles have higher spinning velocities and lower tumbling velocities in the entire channel region, compared to the oblate particles.

Numerical Investigation on the Influence of Bubble Velocity, Gravity Level and Bubble Coalescence on Liquid Turbulence Modulation in Horizontal Channel Bubbly Flow

Numerical Investigation on the Influence of Bubble Velocity, Gravity Level and Bubble Coalescence on Liquid Turbulence Modulation in Horizontal Channel Bubbly Flow

Settling of finite-size particles in turbulence at different volume fractions

We study the settling of finite-size rigid spheres in quiescent fluid and in sustained homogeneous isotropic turbulence (HIT) by direct numerical simulations using an immersed boundary method to account for the dispersed solid phase. We consider semi-dilute and dense suspensions of rigid spheres with solid volume fractions $$\phi =0.5{-}10\%$$ , solid-to-fluid density ratio $$R=1.02$$ , and Galileo number (i.e., the ratio between buoyancy and viscous forces) $$Ga=145$$ . In HIT, the nominal Reynolds number based on the Taylor microscale is $$Re_{\lambda } \simeq 90$$ , and the ratio between the particle diameter and the nominal Kolmogorov scale is $$(2a)/\eta \simeq 12$$ (being a the particle radius). We find that in HIT the mean settling speed is less than that in quiescent fluid for all $$\phi $$ . For $$\phi =0.5\%$$ , the mean settling speed in HIT is $$8\%$$ less than in quiescent fluid. However, by increasing the volume fraction the difference in the mean settling speed between quiescent fluid and HIT cases reduces, being only $$1.7\%$$ for $$\phi =10\%$$ . Indeed, while at low $$\phi $$ the settling speed is strongly altered by the interaction with turbulence, at large $$\phi $$ this is mainly determined by the (strong) hindering effect. This is similar in quiescent fluid and in HIT, leading to similar mean settling speeds. On the contrary, particle angular velocities are always found to increase with $$\phi $$ . These are enhanced by the interaction with turbulence, especially at low $$\phi $$ . In HIT, the correlations of particle lateral velocity fluctuations oscillate around zero before decorrelating completely. The time period of the oscillation seems proportional to the ratio between the integral lengthscale of turbulence and the particle characteristic terminal velocity. Regarding the mean square particle displacement, we find that it is strongly enhanced by turbulence in the direction perpendicular to gravity, even at the largest $$\phi $$ . Finally, we investigate the collision statistics for all cases and find the interesting result that the collision frequency is larger in quiescent fluid than in HIT for $$\phi =0.5{-}1\%$$ . This is due to frequent drafting–kissing–tumbling events in quiescent fluid. The collision frequency becomes instead larger in HIT than in still fluid for $$\phi =5{-}10\%$$ , due to the larger relative approaching velocities in HIT, and to the less intense drafting–kissing–tumbling events in quiescent fluid. The collision frequency also appears to be almost proportional to the estimate for small inertial particles uniformly distributed in space, though much smaller. Concerning the turbulence modulation, we find that the mean energy dissipation increases almost linearly with $$\phi $$ , leading to a large reduction of $$Re_{\lambda }$$ .

Assessment of numerical methods for fully resolved simulations of particle-laden turbulent flows

During the last decade, many approaches for resolved-particle simulation (RPS) have been developed for numerical studies of finite-size particle-laden turbulent flows. In this paper, three RPS approaches are compared for a particle-laden decaying turbulence case. These methods are, the Volume-of-Fluid Lagrangian method, based on the viscosity penalty method (VoF-Lag); a direct forcing Immersed Boundary Method, based on a regularized delta function approach for the fluid/solid coupling (IBM); and the Bounce Back scheme developed for Lattice Boltzmann method (LBM-BB). The physics and the numerical performances of the methods are analyzed. Modulation of turbulence is observed for all the methods, with a faster decay of turbulent kinetic energy compared to the single-phase case. Lagrangian particle statistics, such as the velocity probability density function and the velocity autocorrelation function, show minor differences among the three methods. However, major differences between the codes are observed in the evolution of the particle kinetic energy. These differences are related to the treatment of the initial condition when the particles are inserted in an initially single-phase turbulence. The averaged particle/fluid slip velocity is also analyzed, showing similar behavior as compared to the results referred in the literature. The computational performances of the different methods differ significantly. The VoF-Lag method appears to be computationally most expensive. Indeed, this method is not adapted to turbulent cases. The IBM and LBM-BB implementations show very good scaling.

Turbulent channel flow past a moving array of spheres

We have performed a particle-resolved direct numerical simulation of a turbulent channel flow past a moving dilute array of spherical particles. The flow shares important features with dilute vertical gas solid flow at high Stokes number, such as significant attenuation of the turbulence kinetic energy (TKE) at low particle volume fraction. The flow has been simulated by means of an overset grid method, using spherical grids around each particle overset on a background non-uniform Cartesian grid. The main focus of the present paper is on the TKE budget, which is analysed both in the fixed channel frame of reference and in the moving particle frame of reference. The overall (domain-integrated) TKE and turbulence production due to mean shear are reduced compared to unladen flow. In the fixed frame, the interfacial term, which represents production due to relative (slip) velocity, accounts for approximately 40 % of the total turbulence production in the channel. As a consequence, the total turbulence production and the overall turbulence dissipation rate remain approximately the same as in the unladen flow. However, a comparison with laminar flow past the same particle configuration reveals that significant parts of various fixed-frame statistics are due to non-turbulent structures, spatial variations that are steady in the moving particle frame. In order to obtain a clearer picture of the modification of the true turbulence and in order to reveal the rich three-dimensional (3-D) statistical structure of turbulence interacting with particles, time averaging in the moving frame of reference of the particle is used to extract the fluctuations entirely due to true turbulence. In the moving frame, the turbulence production is positive near the sides and in the wake, but negative in a region near the front of the particle. The turbulence dissipation rate and even more the dissipation rate of the 3-D mean flow attain very large values on a large part of the particle surface, up to approximately 400 and 4000 times the local turbulence dissipation rate of the unladen flow, respectively. Very close to the particle, viscous diffusion is the dominant transport term, but somewhat further away, in particular near the front and sides of the particle, pressure diffusion and also convection provide large and positive transport contributions to the moving-frame budget. A radial analysis shows that the regions around the particles draw energy from the regions further away via the surprising dominance of the pressure diffusion flux over a large range of radii. Spectra show that (very) far away from the particles all scales of the (true) turbulence are reduced. Near the particles enhancement of small scale turbulence is observed, for the streamwise component of the velocity fluctuation more than for the other components. The most important reason for turbulence reduction and anisotropy increase appears to be particle-induced non-uniformity of the mean driving force of the flow.

Modulation of Mean Wind and Turbulence in the Atmospheric Boundary Layer by Baroclinicity

Abstract This paper investigates the effects of baroclinic pressure gradients on mean flow and turbulence in the diabatic atmospheric boundary layer (ABL). Large-eddy simulations are conducted where the direction of the baroclinicity, its strength, and the surface buoyancy flux are systematically varied to examine their interacting effects. The thermal wind vector, which represents the vertical change in the geostrophic wind vector resulting from horizontal temperature gradients, significantly influences the velocity profiles, the Ekman turning, and the strength and location of the low-level jet (LLJ). For instance, cold advection and positive (negative) geostrophic shear increased (decreased) friction velocity and changed the LLJ elevation. Given the baroclinicity strength and direction under neutral conditions, a simple reduced model is proposed and validated here to predict the general trends of baroclinic mean winds. The baroclinic effects on turbulence intensity and structure are even more intricate, with turbulent kinetic energy (TKE) profiles displaying an increase of TKE magnitude with height for some cases. When a fixed destabilizing surface heat flux is added, a positive geostrophic shear favors streamwise aligned roll-type structures, which are typical of neutral ABLs. Conversely, a negative geostrophic shear promotes cell-type structures, which are typical of strongly unstable ABLs. Furthermore, baroclinicity increases shear in the outer ABL and tends to make the outer flow more neutral by decreasing the Richardson flux number. These findings are consequential for meteorological measurements and the wind-energy industry, among others: baroclinicity alters the mean wind profiles, the TKE, coherent structures, and the stability of the ABL, and its effects need to be considered.

Buoyancy effects on large-scale motions in convective atmospheric boundary layers: implications for modulation of near-wall processes

A number of recent studies have demonstrated the existence of so-called large- and very-large-scale motions (LSM, VLSM) that occur in the logarithmic region of inertia-dominated wall-bounded turbulent flows. These regions exhibit significant streamwise coherence, and have been shown to modulate the amplitude and frequency of small-scale inner-layer fluctuations in smooth-wall turbulent boundary layers. In contrast, the extent to which analogous modulation occurs in inertia-dominated flows subjected to convective thermal stratification (low Richardson number) and Coriolis forcing (low Rossby number), has not been considered. And yet, these parameter values encompass a wide range of important environmental flows. In this article, we present evidence of amplitude modulation (AM) phenomena in the unstably stratified (i.e. convective) atmospheric boundary layer, and link changes in AM to changes in the topology of coherent structures with increasing instability. We perform a suite of large eddy simulations spanning weakly ($-z_{i}/L=3.1$) to highly convective ($-z_{i}/L=1082$) conditions (where $-z_{i}/L$ is the bulk stability parameter formed from the boundary-layer depth $z_{i}$ and the Obukhov length $L$) to investigate how AM is affected by buoyancy. Results demonstrate that as unstable stratification increases, the inclination angle of surface layer structures (as determined from the two-point correlation of streamwise velocity) increases from $\unicode[STIX]{x1D6FE}\approx 15^{\circ }$ for weakly convective conditions to nearly vertical for highly convective conditions. As $-z_{i}/L$ increases, LSMs in the streamwise velocity field transition from long, linear updrafts (or horizontal convective rolls) to open cellular patterns, analogous to turbulent Rayleigh–Bénard convection. These changes in the instantaneous velocity field are accompanied by a shift in the outer peak in the streamwise and vertical velocity spectra to smaller dimensionless wavelengths until the energy is concentrated at a single peak. The decoupling procedure proposed by Mathis et al. (J. Fluid Mech., vol. 628, 2009a, pp. 311–337) is used to investigate the extent to which amplitude modulation of small-scale turbulence occurs due to large-scale streamwise and vertical velocity fluctuations. As the spatial attributes of flow structures change from streamwise to vertically dominated, modulation by the large-scale streamwise velocity decreases monotonically. However, the modulating influence of the large-scale vertical velocity remains significant across the stability range considered. We report, finally, that amplitude modulation correlations are insensitive to the computational mesh resolution for flows forced by shear, buoyancy and Coriolis accelerations.

Finite size analysis of a double crossover in transitional wall turbulence

This article presents the finite size analysis of two consecutive crossovers leading laminar-turbulent bands to uniform wall turbulence in transitional plane Couette flow. Direct numerical simulations and low order modeling simulations of the flow are performed. The kinetic energy E of the turbulent flow and the order parameter M, a measure of the spatially organised modulation of turbulence, are sampled. These two quantities are processed in view of analytical results from the phenomenology of phase transitions. The first crossover concerns the loss of spatial organisation of turbulence in the flow. In the band phase, the order parameter M decreases continuously with the Reynolds number R toward a small value, while its response function displays a maximum at the crossover. The increase of the maximum of the response function with domain size is consistent with a polynomial law , . A first critical Reynolds number Rc,1 can be defined as the value at which the maximum of the response function is reached. In the uniform phase, the order parameter M and its variance σ decrease toward zero following mean field scalings as R is increased. The kinetic energy E is an affine function of R except in a small range where a sharp increase is detected, which corresponds to the second crossover. A second critical Reynolds number Rc,2 can be defined as the center of this sharp increase range. In this range, spatial and temporal coexistence of the uniform turbulence phase and laminar-turbulent bands phase is observed. This sharp increase is concomitant with a maximum of the response function of the kinetic energy. The finite size analysis reveals that the jump does not steepen and that the maximum of response function of E saturates as size is increased. The first crossover is formally identical to a critical phenomenon in condensed matter. The second crossover is in agreement with a first order phase transition smeared by finite noise. The analytical analysis of this phenomenon assuming a non interacting gas of fronts between domains of the two phases provides a scaling of the response function consistent with that of E. In our context, this amounts to the statistics of the grain boundaries between domains of banded turbulence and uniform turbulence. We eventually discuss how this formalism could explain a breakdown of the orientation order of the bands, with a grain boundary between domains of different orientations. This breakdown may occur in extremely large size domains and could affect the order parameter and its response function.

Theoretical investigations of infrared optical wave modulation transfer function models in anisotropic marine turbulence.

The previously derived marine turbulence modulation transfer function (MTF) adopts the classical assumption of isotropic and Kolmogorov turbulence. However, more theoretical research shows that marine turbulence demonstrates anisotropic non-Kolmogorov properties. In this study, new long-exposure plane and spherical wave MTF models under anisotropic non-Kolmogorov marine turbulence will be investigated. Comparative analyses are performed for the new models and the existing ones that have been developed under the isotropic marine turbulence and the anisotropic terrestrial turbulence cases. It turns out that the anisotropic marine turbulence influences the electro-optical imaging system less than the isotropic marine turbulence does. Also, under the same turbulence strength condition, due to the humidity fluctuations of marine atmosphere, the anisotropic marine turbulence will reduce more severely the quality of electro-optical imaging systems than the anisotropic terrestrial turbulence. Investigations conducted in this paper are beneficial to understand better the electro-optical imaging mechanism in marine atmospheric turbulence media.

Turbulent energy dissipation rate modification due to particle insertion: Effect of Reynolds number

A hierarchical spatiotemporal dynamical system model (tree model), modified for the insertion of dispersed phase particle, is utilized to study the turbulence modulation caused by the presence of particle in high Reynolds numbers. The focus in this study is to see how the energy dissipation rate is escalated in the vicinity of particle surface. In the present study, shell model simulations are performed for two different Reynolds numbers, i.e. Re∼1/ν, as 5 × 104 and 106, corresponding to two particle sizes of 256 and 64 times of the Kolmogorov length scale, respectively. The results are compared to the corresponding unladen cases, as well as the lower Reynolds number of 104. The scaling properties of the modified tree model are assessed through the velocity structure functions, which reveals the existence of intermittency.

Spatial modulations of kinetic energy in the roughness sublayer

High-Reynolds-number experiments are conducted in the roughness sublayer of a turbulent boundary layer developing over a cubical canopy. Stereoscopic particle image velocimetry is performed in a wall-parallel plane to evidence a high degree of spatial modulation of the small-scale turbulence around the footprint of large-scale motions, despite the suppression of the inner layer by the high roughness elements. Both Fourier and wavelets analyses show that the near-wall cycle observed in smooth-wall-bounded flows is severely disrupted by the canopy, whose wake in the roughness sublayer generates a new range of scales, closer to that of the outer-layer large-scale motions. This restricts significantly scale separation, hence a diagnostic method is developed to divide carefully and rationally the fluctuating velocity fields into large- and small-scale components. Our analysis across all turbulent kinetic energy terms sheds light on the spatial imprint of the modulation mechanism, revealing a very different signature on each velocity component. The roughness sublayer shows a preferential arrangement of the modulated scales similar to what is observed in the outer layer of smooth-wall-bounded flows – small-scale turbulence is enhanced near the front of high momentum regions and damped at the front of low momentum regions. More importantly, accessing spanwise correlations reveals that modulation intensifies the most along the flanks of the large-scale motions.

Highly scalable DNS solver for turbulent bubble-laden channel flow

We present an efficient and highly scalable solver for direct numerical simulation (DNS) of dispersed gas-liquid flow, containing a large number of deformable bubbles. We apply this to O(104) bubbles in a turbulent flow. This was accomplished by a well-considered combination of state-of-the-art numerical methods, as well as fast and scalable numerical algorithms that have their origin in single-phase and two-phase flows. The features key-elements of the algorithm of curvature computation, bubble collision and variable-coefficient Poisson equation solver are presented. Resolution requirements for an accurate advection of a rising bubble have been established by comparison with the (Hysing, 2009) benchmark. The Generalized Height Function (GHF) method, was adopted for the curvature computations. We observed agreement with theoretical convergence rates, as well as a significant reduction of spurious velocities when employing GHF, compared to a finite difference approach. The main interaction mechanisms between bubbles and walls of the domain were analyzed, showing second order convergence of the underlying numerical methods.A detailed analysis of the parallel performance of the Navier–Stokes (NS) solver and the solver for the gas volume fraction was carried out. The analysis revealed close to linear scaling up to ≈ 18000 cores on a computational grid with 1 billion cells for the NS solver, and an ideal scaling of the gas volume fraction solver up to O(103) bubbles. Beyond that, acceptable overhead for up to O(104) bubbles was found.A simulation of a downflow configuration of a turbulent channel loaded with a total of 10000 bubbles illustrates the computational capabilities. First and second order statistics of the velocity field were computed as well as the profiles of the average gas volume fraction field in the statistically steady state. In the case considered, a 47% increase of the wall shear stress was observed, brought about by turbulence modification arising from the embedded bubbles. The interaction between turbulence and bubbles at high volume fraction resulted in a strong attenuation of the rms of the velocity fluctuations in a wide region of the core of the channel.

Hysteresis Relation between Turbulence and Temperature Modulation during the Heat Pulse Propagation into a Magnetic Island in DIII-D.

The hysteresis relation between turbulence and temperature modulation during the heat pulse propagation into a magnetic island is studied for the first time in toroidal plasmas. Lissajous curves of the density fluctuation (n[over ˜]/n) and the electron temperature (T_{e}) modulation show that the (n[over ˜]/n) propagation is faster than the heat pulse propagation near the O point of the magnetic island. This faster n[over ˜]/n propagation is experimental evidence of the turbulence spreading from the X point to the O point of the magnetic island.

Buoyancy induced turbulence modulation in pipe flow at supercritical pressure under cooling conditions

A numerical investigation of cooling of a fluid at supercritical pressure has been performed by means of direct numerical simulations. The simulations were conducted with a uniform heat flux imposed at the wall at an inlet bulk Reynolds number of 5400. The aim of this work is to understand the role of buoyancy in modulating the turbulence in a flow where properties are spatially varying. Heat transfer deterioration followed by recovery is observed in the downward flow while enhancement occurs in the upward flow as compared to forced convection. The decomposition of the skin friction factor and the Nusselt number was performed. The major effects on the skin friction factor were brought by the non-uniform body force due to the gravity. The turbulent parts equally influence the Nusselt number as well as the skin friction factor in supercritical flows. Quadrant analysis and its weighted joint probability density function were analyzed to understand the role of sweep (Q4) and ejection (Q2) events. During the heat transfer deterioration, sweep and ejection events are decreased greatly, triggering the reduction in turbulence. The recovery in turbulence is brought by the Q1 and Q3 (also known as outward and inward interaction) events, contrary to the conventional belief about turbulence generation. The turbulence anisotropy of the Reynolds stress tensor is investigated showing that the turbulence structure becomes rod-like during the deteriorated heat transfer regime in the downward flow and disc-like for the upward flow.

Validation of Lagrangian Two-Way Coupled Point-Particle Models in Large-Eddy Simulations

The accuracy of point-particle models with two-way coupling for particles of Kolmogorov-length-scale size is assessed. Turbulent kinetic energy budgets are analyzed in physical and in spectral space. It is shown that the force projection of the two-way coupling consistently models the direct transfer of kinetic energy on the particle surfaces and the enhanced viscous dissipation in the vicinity of the particles. Direct and large-eddy simulations of particle-laden flows in isotropic decaying turbulence are conducted and compared with direct-particle fluid simulations, where the particle-fluid interaction is fully resolved. An analysis in spectral space shows that turbulence modulation by particles mainly occurs at larger scales, although the momentum transfer takes place at the smallest scales. Therefore, the turbulent kinetic energy cascade of the single phase dominates in particle-laden flows. It is shown that point-particle models do not interfere with subgrid scale models, which usually act on the smallest scale. Consequently, point-particle models predict sufficiently accurate the turbulence modulation in direct numerical simulations and even when a subgrid scale model is used. The resolution of the LES does not affect the accuracy of the point-particle model, when the subgrid kinetic energy is negligible.

Turbulence modifications in a turbulent boundary layer over a rough wall with spanwise-alternating roughness strips

Turbulence modifications over a rough wall with spanwise-varying roughness are investigated at a moderate Reynolds number Reτ ≈ 2000 (or Reθ ≈ 6400), using particle image velocimetry (PIV) and hotwire anemometry. The rough wall is comprised of spanwise-alternating longitudinal sandpaper strips of two different roughness heights. The ratio of high- and low-roughness heights is 8, and the ratio of high- and low-roughness strip width is 0.5. PIV measurements are conducted in a wall-parallel plane located in the logarithmic region, while hotwire measurements are made throughout the entire boundary layer in a cross-stream plane. In a time-average sense, large-scale counter-rotating roll-modes are observed in the cross-stream plane over the rough wall, with downwash and upwash common-flows displayed over the high- and low-roughness strips, respectively. Meanwhile, elevated and reduced streamwise velocities occur over the high- and low-roughness strips, respectively. Significant modifications in the distributions of mean vorticities and Reynolds stresses are observed, exhibiting features of spatial preference. Furthermore, spatial correlations and conditional average analyses are performed to examine the alterations of turbulence structures over the rough wall, revealing that the time-invariant structures observed are resultant from the time-average process of instantaneous turbulent events that occur mostly and preferentially in space.