Kinetic Energy Budget Analysis Research Articles

Heat transfer enhancement for electro-thermo-convection of FENE-P viscoelastic fluid in a square cavity

This study numerically investigates the heat transfer enhancement for viscoelastic electro-thermo-convection in a two-dimensional differentially heated cavity with injection from below. The flow motion is assumed to be incompressible, which is driven by the Coulomb force and the thermal buoyant force. The polymers are described by the FENE-P model which exhibits typical shear-thinning and elastic properties. Based on the extensibility parameter (L), the cases are divided into several scenarios, corresponding to weak elasticity with strong shear-thinning, moderate elasticity with moderate shear-thinning, and strong elasticity with weak shear-thinning, respectively. We find that the competition between the shear-thinning and elasticity dominates the flow state and heat transport. The shear-thinning effect tends to facilitate heat transfer, while its elastic properties tend to decrease it. In the scenario of weak elasticity with strong shear-thinning (L2=10), the polymer additives significantly improve the heat transfer enhancement (HTE) of the electric field as the polymer viscosity ratio (β) decreases or Weissenberg number (Wi) increases, where the maximum HTE reaches around 92.1%. The amount of HTE first increases rapidly with Wi but then remains almost constant once a critical Wi is exceeded. However, the HTE significantly decreases in the scenario of strong elasticity with weak shear-thinning (300≤L≤1000) since the elasticity dominates over the shear-thinning. These heat transfer performances are then corroborated with the boundary layer and kinetic energy budget analysis.

Wake dynamics of side-by-side hydrokinetic turbines in open channel flows

Lateral placement of hydrokinetic turbines is an interesting topic, as the blockage effect can increase the flow speed and increase the power coefficient (CP) for neighboring turbines. This study investigates wake dynamics in hydrokinetic turbine arrays with single- (1T), double- (2T), and triple-turbine (3T) configurations under various tip speed ratios (λ = 3.5, 5.8, and 7.1) using large eddy simulation coupled with the actuator line (AL) model. Results indicate that CP increases as lateral spacing decreases, which highlights the advantages of tighter lateral placement. The CP of the 3T-S turbine (the side turbine in the 3T configuration) is larger than those of the other configurations, following the trend CP,3T−S>CP,3T−M>CP,2T>CP,1T, which reflects a growing blockage effect with more turbines. Wake dynamics are analyzed using time-averaged and instantaneous methods. In 3T scenarios, blockage enhances turbulence kinetic energy, facilitating faster wake recovery, aided by turbine interference. Mean kinetic energy budget analysis shows that 3T-S wakes recover fastest due to increased turbulent convection. For instantaneous analysis, pre-multiplied power spectral density reveals vertical meandering begins at approximately 3D (D is the rotor diameter) and horizontal meandering starts near 4D, with a dominant frequency of St=0.28. Integral length scales show an initial increase followed by a downstream decrease, with minima marking the onset of wake meandering. Dynamic mode decomposition analysis reveals that high-frequency disturbance amplitudes increase with the number of turbines. At the optimal λ, wake effects dominate over inflow effects.

The Adiabatic Evolution of 3D Annular Vortices with a Double-Eyewall Structure

Tropical cyclones (TCs) can be characterized as a 3D annular structure of elevated potential vorticity (PV). However, strong mature TCs often develop a secondary eyewall, leading to a 3D annular vortex with a double-eyewall structure. Using 2D linear stability analysis, it is shown that three types of barotropic instability (BI) are present for annular vortices with a double-eyewall structure: Type-1 BI across the secondary eyewall, Type-2 BI across the moat of the vortex, and Type-3 BI across the primary eyewall. The overall stability of these vortices (and the type of BI that develops) depends principally upon five vortex parameters: the thickness of the primary eyewall, the thickness of the secondary eyewall, the moat width, the vorticity ratio between the eye and the primary eyewall, and the vorticity ratio between the primary and secondary eyewall. The adiabatic evolution of 3D annular vortices with a double-eyewall structure is examined using a primitive equation model in normalized isobaric coordinates. It is shown that Type-2 BI is the most common type of BI for 3D annular vortices whose vortex parameters mimic TCs with a double-eyewall structure. During the onset of Type-2 BI, eddy kinetic energy budget analysis indicates that barotropic energy conversion from the mean azimuthal flow is the dominant energy source of the eddies, which produces a radial velocity field with a quadrupole structure. Absolute angular momentum budget analysis indicates that Type-2 BI generates azimuthally averaged radial outflow across the moat, and the eddies transport absolute angular momentum radially outward towards the secondary eyewall. The combination of these processes leads to the dissipation of the primary eyewall and the maintenance of the secondary eyewall for the vortex.

Growth of organized flow coherent motions within a single-stream shear layer: 4D-PTV measurements

This study investigates the evolution of a single-stream shear layer (SSSL) originating from a wall boundary layer past a backward-facing step. Utilizing a time-resolved 3D-Particle Tracking Velocimetry (4D-PTV) technique, we track the trajectories of fluorescent particles to gain insight into the flow characteristics of the SSSL. A compact water tunnel facility (Reτ=1240\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{Re}_\ au =1\\,240$$\\end{document}) is fabricated to obtain an SSSL with a perpendicular slow entrainment stream past the separation edge. A hybrid interpolation approach that combines ensemble binning and Gaussian weighting is implemented to derive minimally filtered mean and instantaneous lower- and higher-order flow field parameters. Spanwise-dominant coherent motion accompanied by finer flow scales is observed to grow due to flow entrainment through “nibbling” actions of small-scale vortices, “engulfing” by large-scale vortices, and vortex pairing events. Furthermore, the non-zero-speed stream edge grows relatively faster than the zero-speed stream edge, showing a strong asymmetry in mixing composition across a mixing layer. The SSSL reaches self-similarity at a streamwise distance of ≈55θ0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx 55\\,\ heta _{0}$$\\end{document}, where θ0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ heta _0$$\\end{document} is the initial momentum thickness from the separation edge, i.e., considerably shorter than reported in previous studies. A literature comparison of growth rate parameters raises intriguing questions regarding a potential inclusive growth scaling unifying the free shear layers. A turbulent kinetic energy (TKE) budget analysis reveals a negative production region immediately downstream of the separation edge attributed to a large positive streamwise gradient of streamwise velocity. In the self-similar region, the phase-averaged flow mapping demonstrates a larger concentration of turbulence production rate around the outer edges of spanwise vortices, specifically at the intersection of braids and vortices. Furthermore, a spatial separation exists in the regions of peak production and dissipation rates within the vortex core region favoring dissipation. The braids exhibit a larger concentration of turbulence diffusion rates, indicating their function as a conduit for exchanging turbulence between neighboring coherent motions.

Instability and transition control by steady local blowing/suction in a hypersonic boundary layer

The efficacy of steady large-amplitude blowing/suction on instability and transition control for a hypersonic flat plate boundary layer with Mach number 5.86 is investigated systematically. The influence of the blowing/suction flux and amplitude on instability is examined through direct numerical simulation and resolvent analysis. When a relatively small flux is used, the two-dimensional instability critical frequency that distinguishes the promotion/suppression mode effect closely aligns with the synchronisation frequency. For the oblique wave, as the spanwise wavenumber increases, the suppression effects would become weaker and the mode suppression bandwidth diminishes/increases in general in the blowing/suction control. Increasing the blowing/suction flux can effectively broaden the frequency bandwidth of disturbance suppression. The influence of amplitude on disturbance suppression is weak in a scenario of constant flux. To gain a deeper insight into disturbance suppression mechanism, momentum potential theory (MPT) and kinetic energy budget analysis are further employed in analysing disturbance evolution with and without control. When the disturbance is suppressed, the blowing induces the transport of certain acoustic components along the compression wave out of the boundary layer, whereas the suction does not. The velocity fluctuations are derived from the momentum fluctuations of the MPT. Compared with the momentum fluctuations, the evolutions indicated by each component's velocity fluctuations greatly facilitate the investigations of the acoustic nature of the second mode. The rapid variation of disturbance amplitude near the blowing is caused by the oscillations of the acoustic component and phase speed differences between vortical and thermal components. Kinetic energy budget analysis is performed to address the non-parallel effect of the boundary layer introduced by blowing/suction, which tends to suppress disturbances near the blowing. Moreover, viscous effects leading to energy dissipation are identified to be stronger in regions where the boundary layer is rapidly thickening. Finally, it is demonstrated that a flat plate boundary layer transition triggered by a random disturbance can be delayed by a blowing/suction combination control. The resolvent analysis further demonstrates that disturbances with frequencies that dominate the early transition stage are dampened in the controlled base flow.

Thermal convection of viscoelastic fluids in concentric rotating cylinders: Elastic turbulence and kinetic energy budget analysis

Thermal convection of viscoelastic fluids in concentric rotating cylinders: Elastic turbulence and kinetic energy budget analysis

A numerical study of rainfall effects on wind turbine wakes

Rainfall has significant impacts on the operations of wind turbines due to erosion of turbine blades and alterations in aerodynamic performance of wind turbines. However, little is understood in the influences of rainfall on wind turbine wakes. In this study, large-eddy simulation (LES) coupled with actuator-disk model with rotation (ADM-R) is used to investigate impacts of rainfall on wind turbine wakes. A double Euler method is employed in ADM-R to simulate the rainfall injection. The results of the model indicate that rainfall reduces wind speed in the sweep area and increases wind speed in the outer region of the top tip level by 2.1 %. Furthermore, rainfall reduces turbulence intensity in the near wake and outer wake region of the top tip (up to 2.0 %), with the influence extending up to 10 d (diameters of the wind turbine) in the downstream. These changes have a positive correlation with the rainfall intensity and an inverse correlation with the wind speed. The mean kinetic energy (MKE) and turbulent kinetic energy (TKE) budget analysis reveals that (1) the turbulent radial transport variation of MKE is the primary reason for the rainfall-induced wind speed change; (2) the change in shear production of TKE is responsible for the rainfall-induced turbulent intensity change; (3) the rainfall-induced Reynolds stress −u′w′‾ is the dominant factor of this dynamics.

Barotropic and Nonlinear Decay of the Wintertime Baroclinic Wave Packets over the North Pacific: Energetics Analysis

Abstract Based on daily data from the Japanese 55-year Reanalysis (JRA-55) covering the winters (NDJFM) from 1958 to 2018, this study examines the growth and decay mechanisms of the baroclinic wave packet (BWP) inferred from regression analysis over the North Pacific. Day-to-day kinetic energy (KE) and available potential energy (APE) budget analysis suggest that BWP is driven mainly by baroclinic energy conversion (CPB), barotropic energy conversion (CKB), and the nonlinear term (CKE). CPB acts as a predominant APE source for BWP. Part of CPB acts to overcome the APE loss caused by transient eddy flux and most of it acts as a dominant KE source to drive BWP throughout its lifespan. CKB acts as a KE source before day −1, and as a major KE sink to damp BWP afterward, in which the north–south gradient of the climatological meridional flow plays a key role. Similarly, CKE acts as a KE source before day 0 and as a major KE sink afterward. The damping effect of CKE comes mainly from the scale interaction through the advection of high-frequency meridional momentum by the low-frequency zonal flow. It turns out that the vertical geopotential flux divergence also plays an active role in the dynamical coupling of different vertical BWP parts. There is persistent geopotential flux transfer from the middle-tropospheric layer into the lower- and upper-tropospheric layers, which serves as a major KE source to drive the BWP anomalies for the two layers and a major KE sink for the middle-tropospheric layer where the baroclinic energy conversion is the strongest. Significance Statement Previous studies indicate that baroclinic waves tend to organize into wave packets and are driven by the baroclinic instability. This study finds that the baroclinic waves decay mainly through the barotropic energy conversion and nonlinear processes. The results also indicate that the vertical geopotential flux divergence plays an active role in the dynamical coupling of the BWP anomalies in different layers of the troposphere. It is therefore very important to improve the representations of the climatological-mean flow, wave–mean flow interaction, and wave–wave interaction between high- and low-frequency waves in the midlatitudes, as well as the process of vertical transfer of the geopotential flux in numerical models for a better prediction of weather and climate variability in the extratropic regions.

Behavioral patterns of turbulent kinetic energy budget and quadrant analysis for flows over bedform under the influence of downward seepage

Seepage is one of the important factors involved in natural flow conditions, contributing to changes in flow turbulence patterns and morphological changes due to the transport of sediments. This transport of sediment particles influences the development of fluvial bedforms in any river channel. However, previous research on fluvial dynamics has not considered the influence of seepage on the flow field over the fluvial bedforms. The present experimental research aims to explore the behavioral patterns of turbulent kinetic energy, the turbulent kinetic energy (TKE) budget, and quadrant analysis for flows over two-dimensional dune shaped bedforms in the absence and presence of downward seepage. Results from the study illustrate that at the measurement locations on the initial and lee side sections of a dune, the TKE increases with the introduction of downward seepage, leading to an increase in turbulence production near the bed-surface region. The flow energy under both no seepage and seepage conditions contributes mainly to the turbulent production. Turbulence diffusion and dissipation rates have been found to decline in the near-bed region of the initial and lee side sections of the dune. However, turbulent production has been found to be significantly higher in the presence of downward seepage than under the no seepage condition. Similarly, turbulent kinetic energy flux increases in the streamwise direction, while it reduces in the vertical direction at initial sections and lee side sections of the dune under seepage conditions. However, at the middle sections and crest portion of the dune, opposite behavioral patterns are observed for all the aforementioned turbulent entities. Quadrant analysis reveals that the sweep and ejection event increases while inward and outward interaction reduces in the near bed zone. Although the contribution of both sweep and ejection events increases in the presence of downward seepage, sweep events have clear dominance in the near bed region, suggesting the possibility of a higher rate and amount of sediment transportation than under the no seepage condition.

Effect of rotating magnetic field on the stability of thermocapillary flow in a gallium arsenide liquid bridge between unequal ends

In this study, we investigated the impact of a rotating magnetic field on the stability of a thermocapillary flow in a gallium arsenide liquid bridge (Prandtl number Pr = 0.068) situated between two unequal disks, considering two different scenarios with radius ratios of Γr = 0.98 and Γr = 0.60 for the upper heated disk. By utilizing linear stability analysis based on the Legendre spectral element method, we first identified the critical parameters of the onset of flow instability, including critical Marangoni number (Mac), dimensionless oscillation frequency (fc), and azimuthal wavenumber (m). Then, we employed kinetic energy budget analysis to uncover the underlying instability mechanism. For radius ratio Γr = 0.98, three transitions between axisymmetric steady flow and three-dimensional oscillatory flow in the narrow range of Taylor number Ta (8700≤Ta ≤ 9500) are observed; these transitions arise due to the interplay between the flow induced by rotating magnetic field and thermocapillary flow. For the Γr = 0.60 scenario, the rotating magnetic field is observed to significantly enhance the flow stability. Additionally, our analysis identifies four instability types dominated by the hydrodynamic mechanism. In the meantime, the thermocapillary mechanism also contributes to flow instability in the specific region of Taylor number Ta (1250≤Ta ≤ 8000) for radius ratio Γr = 0.98.

Mechanisms for generating and connecting the Yanai-mode and Rossby-mode tropical instability waves in the equatorial Pacific

It is generally believed that both barotropic and baroclinic instabilities may account for the tropical instability wave (TIW) variabilities in the equatorial Pacific Ocean. However, the coupling mechanisms between the two different meridional TIW modes, i.e., the Yanai-mode at the equator and the Rossby-mode around 5°N, and the associated vertical connections, are yet to be clarified. This is done here using a recently developed multiscale decomposition tool, multiscale window transform (MWT), and the MWT-based theory of canonical transfer and TIW-scale eddy kinetic energy (EKE) budget analysis, based on the (1/12.5)∘ HYCOM reanalysis data. In the subsurface layer (30–200 m), barotropic instability is the primary energy source for the Yanai-mode TIWs, while baroclinic instability dominates the EKE generation of the Rossby-mode TIWs. In contrast, the two TIW modes in the surface layer (0–30 m) are largely modulated by nonlocal mechanisms through pressure work, which functions to transport EKE upward from the subsurface to the surface layer, and southward from the Rossby-mode region to the Yanai-mode region. The pressure fluxes substantially couple the two TIW modes in both the meridional and vertical directions, leading to significant coherence between EKE in different vertical layers and regions of the TIWs. In addition, the prevailing vertical pressure flux identified north of the equator explains why the EKE signal is more vertically coherent in the Rossby-mode region than the Yanai-mode region. This nonlocal energy pathway, as well as the well-known instability processes, undergoes strong seasonal variations that determine the seasonal cycles of the TIWs in the equatorial Pacific Ocean.

Spectral analysis of turbulence kinetic and internal energy budgets in hypersonic turbulent boundary layers

The spectral transport equations of turbulence kinetic and internal energy (TKE and TIE) are rigorously derived in the compressible turbulent flows, exhibiting a full structural similarity in terms of two-point correlations of velocity and a sound-speed-like variable. The scale dependence of TKE and TIE flow across space and scales, and among components is revealed comprehensively in a similar manner, in hypersonic turbulent boundary layers. Special attention is paid to the effect of wall cooling on the energy evolution, which is observed to exist throughout the boundary layer primarily by means of modifying the small-scale motions.

Distinct Influences of Two Boreal Summer Intraseasonal Oscillation Modes on Synoptic-Scale Eddies over the Western North Pacific

Abstract The boreal summer intraseasonal oscillation (BSISO), which consists of a 10–30-day component and a 30–90-day component, is vigorous over the East Asian and the western North Pacific (WNP) monsoon regions where synoptic-scale eddies are also active. In this study, we systematically compare and diagnose the modulating effects of the 10–30- and 30–90-day BSISO modes on the development and maintenance of the WNP synoptic-scale eddies in terms of eddy energetics. During the developing phase of the synoptic eddies, the eddies tend to grow faster under the background conditions associated with the 10–30-day BSISO mode, compared with that associated with the 30–90-day mode, indicating the importance of the 10–30-day mode in eddy development. In contrast, the 30–90-day BSISO mode shows a larger contribution to the maintenance of the synoptic eddy intensity after the eddies reach their maximum amplitude. On the basis of the diagnoses of the eddy kinetic energy (EKE) budget equation, we find that the positive EKE advection induced by the 10–30-day flow is the leading process resulting in the increased EKE for eddy development. However, the 30–90-day circulation anomalies provide long-lasting favorable conditions (relative to those of the 10–30-day anomalies) to maintain the eddy intensity by generating a positive barotropic energy conversion from 30- to 90-day kinetic energy to EKE. These results not only advance our understanding of multiscale interactions over the WNP but also help toward better monitoring and prediction of synoptic-scale eddies, including tropical cyclones, using the background BSISO information. Significance Statement The western North Pacific is a region with active but complex multiscale interactions among synoptic-scale disturbances, intraseasonal oscillation, and low-frequency background monsoonal activity. The relative effects of 10–30- and 30–90-day intraseasonal variability on the synoptic-scale disturbances were quantitatively examined based on the eddy kinetic energy budget analysis. The results show that the 10–30-day (30–90-day) mode of intraseasonal oscillations plays a key role in the growth (maintenance) of synoptic-scale disturbances through the advection (barotropic energy conversion) process. The results suggest the implication of monitoring and predicting the activity of synoptic-scale disturbances using the information of two different intraseasonal modes.

On the dynamics of buoyant jets in a linearly stratified ambient

We report mean flow and turbulence characteristics of a buoyant jet evolving in a linearly stratified ambient with stratification strength N=0.4 s−1. The velocity and density fields are captured experimentally using simultaneous particle image velocimetry and planar laser-induced fluorescence technique. We report our findings by strategically choosing four axial locations such that it covers different flow regimes; namely, momentum-dominated region, buoyancy-dominated region, neutral buoyant layer, and plume cap region. The results at these axial locations are presented as a function of the radial co-ordinate to provide a whole field picture of the flow dynamics. From the mean axial velocity and density fields, it is seen that the velocity and the scalar (density) widths are of the same magnitude in the momentum-dominated region but show significant difference in the buoyancy-dominated region and beyond. It is also seen that the axial velocity for the buoyant jet is consistently higher than pure jet at different axial locations due to buoyancy-aided momentum. With the help of turbulent kinetic energy (TKE) budget analysis, it is seen that the shear production (P) and TKE dissipation (ϵ) for a buoyant jet are higher compared to the case of pure jet at different axial locations, cementing the role of buoyancy and stratification on the flow dynamics. Further, it is observed that the buoyancy flux (B) aids and destroys TKE intermittently in the radial direction, and it is at least O(102) smaller than P, ϵ, and the mean flow buoyancy flux (F). Finally, the relative strength of the turbulent transport of momentum to that of scalar in the radial direction is quantified using the turbulent Prandtl number, Prt. It is seen that Prt ≈1 upto the neutral buoyant layer and ≈ 0.6 in the plume cap region. The current set of results obtained from experiments are first of its kind and elucidates various aspects of the flow which hitherto remained unknown and will also prove to be useful in testing numerical simulations for buoyancy-driven flows.

Control of separated flow transition over a highly loaded compressor blade via dynamic surface deformation

The laminar separation and transition process seriously deteriorate the aerodynamic performance of a compressor blade at a low Reynolds number (Re). To suppress the laminar separation without reattachment over a highly loaded compressor blade at Re=3.5 × 105, an active flow control strategy based on dynamic surface deformation (DSD) is numerically investigated by large eddy simulations (LESs). The control mechanism of DSD is analyzed, and the vortex dynamics in the transition process are resolved. Furthermore, the loss mechanism is clarified based on the turbulent kinetic energy (TKE) budget analysis. For the uncontrolled case, the rolling-up and breakdown of large-scale hairpin vortices accelerate the generation of turbulent fluctuations, which distinctly extend the region of high-level TKE production term and thus cause large total pressure loss. As the DSD is activated at the oscillation frequency f0 (f0 is the vortex shedding frequency in separated shear layers), periodic pressure fluctuation waves are induced with the appearance of small-scale vortices. The convection of these vortices enlarges the local vorticity and makes the boundary layer velocity profile fuller, which successfully eliminates the large-scale laminar separation on the compressor blade. As such, the region of high-level TKE production term shrinks distinctly due to the weakened vortex dynamics, and the total pressure loss is reduced by 78.9%. With increasing the oscillation frequency from f0 to 2f0 or 4f0, the loss is slightly enlarged due to the rapid growth of turbulent boundary layers, but it is still lower than that of the uncontrolled case. The results demonstrate that DSD is feasible in controlling large-scale laminar separation, and there exists an optimal oscillation frequency.

Direct numerical simulation of supersonic bump flow with shock impingement

Direct numerical simulations are carried out to identify the effects of shock impingement on the behavior of bump flow at freestream Mach number of 2.25. Two cosine-shaped bump cases, with and without an impinging oblique shock at an angle of 33.2°, are compared. The shock impingement exhibits a remarkable influence on the pattern of the shock system and on the size of the separation region. A spectral analysis finds that low-frequency unsteadiness is significantly enhanced by the impingement interaction, and the proper orthogonal decomposition highlights the low-frequency breathing motion of the separation bubble, which is accurately reconstructed using only the first ten low-order modes. Downstream of the bump, both the Reynolds stress components and the turbulence kinetic energy exhibit a general amplification, with the peaks reoccurring at outer wall-normal locations. A turbulent kinetic energy budget analysis shows the greatly increased production in the outer layer which is balanced by turbulent transport and dissipation. An anisotropy-invariant map analysis identifies enhanced isotropic turbulence in the vicinity of the bump, which is qualitatively modified into a two-component axisymmetric state around the reattachment point. In addition, the mean skin friction decomposition suggests that the shock impingement has little influence on the predominant contribution of turbulence kinetic energy production, apart from the spatial growth dominance at the bump summit in the absence of the impinging shock. Interestingly, a scale-decomposed analysis quantitatively demonstrates that the contributions of small-scale structures are attenuated, but those of large-scale ones are relatively increased, with a contribution of more than 80% with shock impingement.

Physical structure and evolution of a cyclonic eddy in the Northern South China sea

The spatial-temporal evolution of a cyclonic eddy (CE) in the northern South China Sea (SCS) was analyzed based on observational data from underwater glider and satellite altimetry and outputs from Hybrid Coordinate Ocean Model (HYCOM). The CE was found to originate in the northeastern SCS and propagate along the continental slope, passing by the Xisha Islands. It eventually vanished offshore of the western Vietnam. Detailed subsurface temperature and salinity structures of CE were observed by underwater glider around the western Xisha Islands. The eddy experienced three evolution stages: continuously increase during its growth stage, gradually decrease when moving by the Xisha Islands, and rapidly weakening and merging with another cyclonic eddy to the west of Vietnam. During the first stage, the generation and growth of this CE were revealed to be due to the barotropic (BT) and baroclinic (BC) instabilities, with the deformation of the eddy being conducive to obtaining energy via the instabilities. During the second stage, i.e., after the CE reached the Xisha Islands, the eddy released (drained) energy to (from) the mean flow with negative (positive) BT and BC values in its northern (eastern) part. Overall, the volume-integrated BT and BC over the eddy area contributed to the eddy energy increase. For the decay of this CE near the Xisha Islands, the eddy kinetic energy budget analysis results indicated that it was mainly due to the interior eddy viscous dissipation.

Counter-rotating Taylor-Couette flows with radial temperature gradient

In the present work, counter-rotating turbulent Taylor-Couette flows with radial temperature gradient have been studied as an extension of previously studied simple-rotating turbulent Taylor-Couette flows. The rotation axis is orthogonal to the gravity vector. Direct Numerical Simulations (DNS) have been carried out for Reynolds number ranging from 1000 to 5000 and Richardson number varying from 0 to 0.4. The effect of variation of Reynolds number and Richardson number on the flow statistics, flow dynamics, and near-wall coherent structures are studied. For neutrally buoyant cases, near-wall coherent structures exist near the inner and the outer wall, with the core being almost vortex-free. With an increase in Richardson number, more dense and finer vortical structures spread out in the core in half the circumferential domain only. This behavior is due to the spatial stable or unstable stratification in the azimuthal direction, leading to the generation of turbulence in the lower half of the domain and mitigation of turbulence in the upper half of the domain. Further, the turbulent kinetic energy (TKE) budget analysis reveals an increase in turbulence on heating the outer cylinder wall due to an increase in the production term. Heating the outer cylinder wall leads to a slight decrease in skin friction for the inner cylinder.

Experimental properties of continuously forced, shear-driven, stratified turbulence. Part 2. Energetics, anisotropy, parameterisation

In this Part 2 we study further experimental properties of two-layer exchange flows in a stratified inclined duct, which are turbulent, strongly stratified, shear-driven and continuously forced. We analyse the same state-of-the-art data sets using the same ‘core’ shear-layer methodology as in Part 1 (Lefauve & Linden, J. Fluid Mech., vol. 937, 2022, A34), but we focus here on turbulent energetics and mixing statistics. The detailed analysis of kinetic and scalar energy budgets reveals the specificity and scalings of ‘SID turbulence’, while energy spectra provide insight into the current strengths and limitations of our experimental data. The anisotropy of the flow at different scales characterises the turbulent kinetic energy production and dissipation mechanisms of Holmboe waves and overturning turbulence. We then assess standard mixing parameterisation models relying on uniform eddy diffusivities, mixing lengths, flux parameters, buoyancy Reynolds numbers or turbulent Froude numbers, and we compare our representative values with the stratified mixing literature. The dependence of these measures of mixing on controllable flow parameters is also elucidated, providing asymptotic estimates that may be extrapolated to more strongly turbulent flows, quantified by the product of the tilt angle of the duct and the Reynolds number. These insights may serve as benchmark for the future generation of experimental data with superior spatio-temporal resolution required to probe increasingly vigorous turbulence.

Dynamics of a single bubble rising in a quiescent medium

In the present work, an experimental analysis was performed to characterise the flow field around a single bubble of different diameters ∼ 2.77–3.53 mm) rising in a quiescent medium aiming to determine the effect of bubble size on kinetic energy distribution. The velocity field was measured using a non-intrusive particle image velocimetry (PIV) technique and kinetic energy spectrum was determined in both transverse and longitudinal directions applying a Fast Fourier Transformation (FFT). Both small- and large-scale motions of the flow field were identified and separated using a discreate wavelet transformation (DWT) method. It was found that the energy spectrum of the large-scale motions depended on the bubble size while the small-scale energy spectrum was nearly independent of it. The slopes of the energy spectrum were found to be close to -5/3 and -3 for the large- and small-scale regimes, respectively and the transition of slope was observed to occur at the wavenumber corresponding to the bubble diameter. Using the measured velocity field data, a turbulence kinetic energy (TKE) budget analysis was performed involving five components namely kinetic energy production, turbulent transport, pressure diffusion, viscous diffusion, and energy dissipation. Overall, it was observed that in the vicinity of bubble surface, turbulence production term was not entirely balanced by the dissipation term; and turbulent transport and pressure diffusion term also had significant contributions.