Turbulent Convection Research Articles

Turbulent convection in rotating slender cells

Turbulent convection in the interiors of the Sun and the Earth occurs at high Rayleigh numbers $Ra$ , low Prandtl numbers $Pr$ , and different levels of rotation rates. To understand the combined effects better, we study rotating turbulent convection for $Pr = 0.021$ (for which some laboratory data corresponding to liquid metals are available), and varying Rossby numbers $Ro$ , using direct numerical simulations in a slender cylinder of aspect ratio 0.1; this confinement allows us to attain high enough Rayleigh numbers. We are motivated by the earlier finding in the absence of rotation that heat transport at high enough $Ra$ is similar between confined and extended domains. We make comparisons with higher aspect ratio data where possible. We study the effects of rotation on the global transport of heat and momentum as well as flow structures (a) for increasing rotation at a few fixed values of $Ra$ , and (b) for increasing $Ra$ (up to $10^{10}$ ) at the fixed, low Ekman number $1.45 \times 10^{-6}$ . We compare the results with those from unity $Pr$ simulations for the same range of $Ra$ and $Ro$ , and with the non-rotating case over the same range of $Ra$ and low $Pr$ . We find that the effects of rotation diminish with increasing $Ra$ . These results and comparison studies suggest that for high enough $Ra$ , rotation alters convective flows in a similar manner for small and large aspect ratios, so useful insights on the effects of high thermal forcing on convection can be obtained by considering slender domains.

Turbulent convection in emulsions: the Rayleigh–Bénard configuration

This study explores heat and turbulent modulation in three-dimensional multiphase Rayleigh–Bénard convection using direct numerical simulations. Two immiscible fluids with identical reference density undergo systematic variations in dispersed-phase volume fractions, $0.0 \leq \varPhi \leq 0.5$ , and ratios of dynamic viscosity, $\lambda _{\mu }$ , and thermal diffusivity, $\lambda _{\alpha }$ , within the range $[0.1\unicode{x2013}10]$ . The Rayleigh, Prandtl, Weber and Froude numbers are held constant at $10^8$ , $4$ , $6000$ and $1$ , respectively. Initially, when both fluids share the same properties, a 10 % Nusselt number increase is observed at the highest volume fractions. In this case, despite a reduction in turbulent kinetic energy, droplets enhance energy transfer to smaller scales, smaller than those of single-phase flow, promoting local mixing. By varying viscosity ratios, while maintaining a constant Rayleigh number based on the average mixture properties, the global heat transfer rises by approximately 25 % at $\varPhi =0.2$ and $\lambda _{\mu }=10$ . This is attributed to increased small-scale mixing and turbulence in the less viscous carrier phase. In addition, a dispersed phase with higher thermal diffusivity results in a 50 % reduction in the Nusselt number compared with the single-phase counterpart, owing to faster heat conduction and reduced droplet presence near walls. The study also addresses droplet-size distributions, confirming two distinct ranges dominated by coalescence and breakup with different scaling laws.

Evolution of amplitude and longitude phase of tachocline Rossby waves diffusing to the photosphere

ABSTRACT Physics of magnetohydrodynamic (MHD) Rossby waves in the tachocline-layer were studied by Dikpati et al., using a fluid-particle-trajectory approach along with solving vorticity and induction equations. By extending that model to include a hydrodynamic turbulent convection zone (CZ), we examine how MHD Rossby waves generated in the tachocline might diffuse upward through the CZ to solar surface. We find that pure hydrodynamic Rossby wave amplitudes decline with height due to viscous diffusion at a rate that is independent of viscosity and increases with longitude wavenumber. Fast MHD Rossby waves amplitude declines faster with height for increasing toroidal field, due to their longitude-phase shifting with height, which increases dissipation of kinetic energy in the wave velocities. Slow MHD Rossby waves decline even faster with height because their longitude-phase shifts more rapidly with height, due to their slow phase speed. We conclude that low wavenumber HD and fast MHD Rossby waves, originating in the tachocline, might be detected at the photosphere, but slow MHD Rossby waves should be virtually impossible to detect. We infer from fluid particle trajectories that wave amplitudes declining with height and longitude phase shifting with height associated with decline, implies a powerful mechanism for tangling of magnetic fields, distinct from convective turbulence effects. This could cause a sustained or dissipative local dynamo action triggered by Rossby waves.

Numerical Investigation of Turbulent Convection Flow in a Rectangular Closed Cavity

Natural turbulent convection in closed cavities has many practical applications in the field of engineeringsuch as the design of electronic computer chips, atomic installation and industrial cooling among others. Inparticular, it enables in achieving a desired micro-climate and efficient ventilation in a building. Recent studiesshow that turbulent flow is affected by variations in Rayleigh numbers, aspect ratio, and heater positionamong others. Temperature is kept constant in all these studies hence inadequate literature on the effectsof temperature on a turbulent flow. In this study, aspect ratio and Rayleigh numbers are kept constant at2 and 1012 respectively and natural turbulent convection flow in a closed rectangular cavity is investigatednumerically as the operating temperature is varied from 285.5K to 293K. The rectangular cavity’s lower wallwas heated and cooling done at the top face wall while the rest of the vertical walls were kept in adiabaticcondition. Material properties such as density of the fluid kept on changing at any given temperature. Thethermal profile data generated influenced the nature of the turbulent flow. The non-linear averaged continuity,momentum, and energy equation terms were modeled by the SST k − ω model to generate streamlines,isotherms, and velocity magnitude for a different operating temperature and presented graphically. The finitedifference method and FLUENT were used to solve two SST k − ω model equations, vortices, and energy withboundary conditions. It was discovered that, as the operating temperature increased turbulence decreaseddue to a decrease in the velocity of the elements and vortices became more parallel and smaller.

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.

Turbulent mixed convection in vertical and horizontal channels

Turbulent shear flows driven by a combination of a pressure gradient and buoyancy forcing are investigated using direct numerical simulations. Specifically, we consider the set-up of a differentially heated vertical channel subject to a Poiseuille-like horizontal pressure gradient. We explore the response of the system to its three control parameters: the Grashof number $Gr$ , the Prandtl number $Pr$ , and the Reynolds number $Re$ of the pressure-driven flow. From these input parameters, the relative strength of buoyancy driving to the pressure gradient can be quantified by the Richardson number $Ri=Gr/Re^2$ . We compare the response of the mixed vertical convection configuration to that of mixed Rayleigh–Bénard convection, and find a nearly identical behaviour, including an increase in wall friction at higher $Gr$ , and a drop in the heat flux relative to natural convection for $Ri=O(1)$ . This closely matched response is despite vastly different flow structures in the systems. No large-scale organisation is visible in visualisations of mixed vertical convection – an observation that is confirmed quantitatively by spectral analysis. This analysis, combined with a statistical description of the wall heat flux, highlights how moderate shear suppresses the growth of small-scale plumes and reduces the likelihood of extreme events in the local wall heat flux. Vice versa, starting from a pure shear flow, the addition of thermal driving enhances the drag due to the emission of thermal plumes.

ЧИСЛЕННАЯ МОДЕЛЬ СВОБОДНОЙ КОНВЕКЦИИ В ПРОСТРАНСТВЕ ЭКРАННО-ВАКУУМНОЙ ИЗОЛЯЦИИ ПРИ ТЕПЛОВОМ ВОЗДЕЙСТВИИ ПОЖАРА

The objective of the study is to find the patterns of the conjugate heat transfer process in a closed space of a coaxial cylinder’s pair under the influence of an external source of thermal radiation within the framework of the free convection model. It is assumed that the priority type of heat transfer in the gas filling the coaxial space is turbulent free convection, and thermal conductivity in the external thermal insulation of the pipeline. A model of the free convection process in a coaxial gas cavity in the presence of local heating in the form of a system of differential equations is created. Comparative results of a full-scale and numerical experiment on measuring the effective coefficient of thermal conductivity of screen-vacuum insulation are presented.

Limiting regimes of turbulent horizontal convection. Part 1. Intermediate and low Prandtl numbers

We report the existence of two new limiting turbulent regimes in horizontal convection (HC) using direct numerical simulations at intermediate to low Prandtl numbers. In our simulations, the flow is driven by a step-wise buoyancy profile imposed at the surface, with free-slip, no-flux conditions along all other boundaries, except along the spanwise direction, where periodicity is assumed. The flow is shown to transition to turbulence in the plume and the core, modifying the rate of heat and momentum transport. These transitions set a sequence of scaling laws that combine theoretical arguments from Shishkina, Grossmann and Lohse (SGL) and Hughes, Griffiths, Mullarney and Peterson (HGMP). The parameter range extends through Rayleigh numbers in the range [ $6.4\times 10^5, 1.92\times 10^{15}$ ] and Prandtl numbers in the range [ $2\times 10^{-3},2$ ]. At low Prandtl numbers and intermediate Rayleigh numbers, a core-driven regime is shown to follow a Nusselt-number scaling with $Ra^{1/6}Pr^{7/24}$ . For Rayleigh numbers larger than $10^{14}$ , the Nusselt number scales with $Ra^{0.225}Pr^{0.417}$ . For these particular regimes, the Reynolds number is found to scale as $Ra^{2/5}Pr^{-3/5}$ for the low-Prandtl-number regime and $Ra^{1/3}Pr^{1}$ for Rayleigh numbers larger than $10^{14}$ . These results embed the HGMP model in the SGL theory and extend the known regime diagram of HC at high Rayleigh numbers. In particular, we show that HC and Rayleigh–Bénard share similar turbulent characteristics at low Prandtl numbers, where HC is shown to be ruled by its core dynamics and turbulent boundary layers. This new scenario confirms that fully turbulent HC enhances the transport of heat and momentum with respect to previously reported regimes at high Rayleigh numbers. This work provides new insights into the applicability of HC for geophysical flows such as overturning circulations found in the atmosphere, the oceans, and flows near the Earth's inner core.

Limiting regimes of turbulent horizontal convection. Part 2. Large Prandtl numbers

Horizontal convection at large Rayleigh and Prandtl numbers is studied experimentally in a regime up to seven orders of magnitude larger in terms of Rayleigh numbers than previously achieved. To reach Rayleigh numbers up to $10^{17}$ , the horizontal density gradient is generated using differential solutal convection by a differential input of salt and fresh water controlled by diffusion in a novel experiment in which the zero-net mass flux of water is ensured through permeable membranes. This set-up allows us to accurately measure the Nusselt number in solutal convection by carefully controlling the amount of salt water exchanged through the membranes. Combined measurements of density and velocity across more than five orders of magnitude in Rayleigh numbers show that the flow transitions from the Beardsley & Festa (J. Phys. Oceanogr., vol. 2, issue 4, 1972, pp. 444–455), Shishkina & Wagner (Phys. Rev. Lett., vol. 116, issue 2, 2016, 024302) regime to the Chiu-Webster et al. (J. Fluid Mech., vol. 611, 2008, pp. 395–426) regime and frames the present results within the scope of Shishkina et al. (Geophys. Res. Lett., vol. 43, issue 3, 2016, pp. 1219–1225), and the theory of Part 1 (Passaggia & Scotti, vol. 997, 2024, J. Fluid Mech., A5). In particular, we show that, even for large Prandtl numbers, the circulation eventually clusters underneath the forcing horizontal boundary, leaving a stratified core without motion. Finally, the previous regime diagrams (Hughes & Griffiths, Annu. Rev. Fluid Mech., vol. 40, 2008, pp. 185–208; Shishkina et al., Geophys. Res. Lett., vol. 43, issue 3, 2016, pp. 1219–1225) are extended by combining the present results at high Prandtl numbers, the results at low Prandtl numbers of Part 1, together with previous results from the literature. This work sets a new picture of the transition landscape of horizontal convection over six orders of magnitude in Prandtl number and sixteen orders of magnitude in Rayleigh number.

The minimal seed for transition to convective turbulence in heated pipe flow

It is well known that buoyancy suppresses, and can even laminarise, turbulence in upward heated pipe flow. Heat transfer seriously deteriorates in this case. A new direct numerical simulation model is established to simulate flow-dependent heat transfer in an upward heated pipe. The model shows good agreement with experimental results. Three flow states are simulated for different values of the buoyancy parameter $C$ : shear turbulence, laminarisation and convective turbulence. The latter two regimes correspond to the heat transfer deterioration regime and the heat transfer recovery regime, respectively (Jackson & Li 2002; Bae et al., Phys. Fluids, vol. 17, issue 10, 2005; Zhang et al., Appl. Energy, vol. 269, 2020, 114962). We confirm that convective turbulence is driven by a linear instability (Su & Chung, J. Fluid Mech., vol. 422, 2000, pp. 141–166) and that the deteriorated heat transfer within convective turbulence is related to a lack of rolls near the wall, which leads to weak mixing between the flow near the wall and the centre of the pipe. Having surveyed the fundamental properties of the system, we perform a nonlinear non-modal stability analysis, which seeks the minimal perturbation that triggers a transition from the laminar state. Given the differences between shear and convective turbulence, we aim to determine how the nonlinear optimal (NLOP) changes as the buoyancy parameter $C$ increases. We find that at first, the NLOP becomes thinner and closer to the wall. Most importantly, the critical initial energy $E_0$ required to trigger turbulence keeps increasing, implying that attempts to trigger it artificially may not be an efficient means to improve heat transfer at larger $C$ . At $C=6$ , a new type of NLOP is discovered, capable of triggering convective turbulence from lower energy, but over a longer time. It is active only in the centre of the pipe. We next compare the transition processes, from linear instability and by the nonlinear non-modal excitation. At $C=4$ , linear instability leads to a state that approaches a travelling wave solution or periodic solutions, while the minimal seed triggers shear turbulence before decaying to convective turbulence. Deeper into the parameter space for convective turbulence, at $C=6$ , the new nonlinear optimal triggers convective turbulence directly. Detailed analysis of the periodic solution at $C=4$ reveals three stages: growth of the unstable eigenfunction, the formation of streaks, and the decay of the streaks. The stages of the cycle correspond to changes in the linear instability of the turbulent mean velocity profile. Unlike the self-sustaining process for classical shear flows, where the streak is disrupted via instability, here, decay of the streak is more closely linked to suppression of the linear instability of the mean flow, and hence suppression of the rolls. Flow visualisations at $C$ up to $10$ also show similar processes, suggesting that the convective turbulence in the heat transfer recovery regime is sustained by these three typical processes.

Stochastic excitation of waves in magnetic stars

Context. Stellar oscillations are key to unravelling stellar properties, such as their mass, radius, and age. This in turn enables us to date and characterize their exoplanetary systems. The amplitudes of acoustic (p-) modes in solar-like stars are intrinsically linked to their convective turbulent excitation source, which in turn is influenced by magnetism. In the observations of the Sun and stars, the mode amplitudes are modulated following their magnetic activity cycles: the higher the magnetic field, the lower the mode amplitudes. When the magnetic field is strong, it can even inhibit acoustic modes, which are not detected in most of the solar-like stars that are strongly magnetically active. Magnetic fields are known to freeze convection when they stronger than a critical value: the so-called on-off approach is used in the literature. Aims. We investigate the impact of magnetic fields on the stochastic excitation of acoustic modes. Methods. First, we generalise the forced-wave equation formalism, including the effects of magnetic fields. Second, we assess how convection is affected by magnetic fields using results from the magnetic mixing-length theory. Results. We provide the source terms of the stochastic excitation, including a new magnetic source term and the Reynolds stresses. We derive scaling laws for the mode amplitudes that take both the driving and the damping into account. These scalings are based on the inverse Alfvén dimensionless parameter: The damping increases with the magnetic field and reaches a saturation threshold when the magnetic field is strong. The driving of the modes diminishes when the magnetic field becomes stronger and the turbulent convection is weaker. Conculsions. As expected from the observations, we find that a stronger magnetic field diminishes the resulting mode amplitudes. The evaluation of the inverse Alfvén number in stellar models provides a means for estimating the expected amplitudes of acoustic modes in magnetically active solar-type stars.

Thermal management and power generation characteristics in a bifurcating channel by using a piezo-embedded inclined elastic fin assembly during turbulent forced convection of ternary nanofluid

Numerous engineering systems use piezoelectric energy harvesters (PE-EHs), which have several benefits including low cost, simplicity, better power density, and ease of installation. They can be used in different applications for energy harvesting and different external sources such as wind and vortex induced vibrations due to fluid–structure interaction can be utilized. This study uses a unique elastic fin PE/EH assembly for power generation, thermal management, and flow control in a bifurcating channel. Performance of the system is improved by using ternary nanofluid in the channel cooling system. Galerkin weighted residual FEM with ALE is used as the solution method. The convective heat transfer performance and power generation by the EH device are investigated in relation to the varying following parameters: Reynolds number (Re between 10000 and 30000), PE-EH inclination (γ between 0 and 60), fin horizontal location (xf between −H and H), and nanoparticle loading in the base fluid (ϕ between 0 and 0.03). The vortex size and distribution near the junction are significantly influenced by the fin inclination and horizontal location of the fin assembly within the bifurcating channel. When varying other parameters of interest, using nanofluid at the highest loading results in significant deflection of the fin assembly and power generation within the PE-EH. Enhancement factors (EFs) for power generation becomes 15.6 and 39.8 when cases of lowest and highest Re are compared with pure fluid and nanofluid while they are 2.93 and 3.38 for thermal performance improvements. Fin inclination of γ=30 is found as the optimum inclination for achieving the highest power from the assembly. Higher inclination of elastic fin/PE-EH assembly results in cooling performance deterioration while average Nu reduces by a factor of 2.94 by varying inclination from γ=30 to γ=60 with nanofluid. For power generation, effects of using nanofluid with varying inclination is significant and EF becomes 252 from γ=0 to γ=30. There are opposing tendencies for the device’s power generation and cooling performance enhancement when the fin’s horizontal placement is changed. EF for average Nu is 7.25 for varying fin location. As the loading of nanoparticle inside the base fluid increases, the average Nu and generated power exhibit non-linear rising characteristics. Polynomial type correlations are provided for the average Nu and power from the device by varying elastic fin assembly inclination and nanoparticle solid volume fraction. Potential of using hybrid nanofluid in PE-EH embedded thermo-fluid system is shown. The design, development, and optimization of self-sufficient power production systems that may be used to various thermo-fluid systems, such as electronic cooling and thermal control in a range of heat transfer devices, may benefit from the findings.

Bistability in the sunspot cycle

A direct dynamical test of the sunspot cycle is carried out to indicate that a stochastically forced nonlinear oscillator characterizes its dynamics. The sunspot series is then decomposed into its eigen time-delay coordinates. The relevant analysis reveals that the sunspot series exhibits bistability, with the possibility of modeling the solar cycle as a stochastically and periodically forced bistable oscillator, accounting for poloidal and toroidal modes of the solar magnetic field. Such a representation enables us to conjecture stochastic resonance as the key mechanism in amplifying the planetary influence on the Sun, and that extreme events, due to turbulent convection noise inside the Sun, dictate crucial phases of the sunspot cycle, such as the Maunder minimum.

Schlieren Imaging of Air-coupled Ultrasound

Schlieren imaging uses changes in the refractive index of a medium to permit visualisation of pressure changes in the medium. Schlieren images have been used to observe turbulent convection flow off heat sources, shock waves from bullets, jets and rockets. For a while, in nondestrructive testing it was a popular option to see beam shapes associated with immersion ultrasonic probes as well as pulse-interactions in immersion test conditions. From the perspective of sound transmission, air can be considered another form of a fluid. Although schlieren imaging has been used to illustrate shockwaves in air, the challenge of imaging ultrasound sound in air is significant, primarily due to the extremely high attenuation. This article provides a description of the techniques used to image ultrasound in air using several air-coupled ultrasonic transducers.

Effects of corrugation on convective heat transfer and power generation in a wavy walled triangular cavity equipped with inclined elastic fin and piezo-energy harvester assembly

Piezoelectric energy harvesters are one of the energy conversion technologies that can be used to convert a variety of external sources, including wind, fluid motion, and vibrations, into electrical energy. This work suggests a novel wall corrugation design in addition to an elastic fin piezo assembly arrangement for power generation and thermal management in a triangular cavity during turbulent forced convection of air. Using the finite element method with ALE, the analysis is conducted for different values of fin inclination (γ between 0 and 60), corrugation amplitude (Af between 0.01H and 0.06H), corrugation frequency (Nf between 1 and 20) and fin distance from the mid-point of the inclined wall in wall direction. The average Nusselt number (Nu) and generated power (PW) show a non-linear increasing trend with increasing fin tilt. Average Nu increment factor becomes 5.17 with the highest fin inclination while the power increments become 11.6 and 4.46 when rising inclination from 30 to 45 and from 45 to 60. The generated power rises with higher corrugation amplitude and wave number. However, increasing the wave number results in thermal performance degradation. Increment of average Nu becomes 1.17 with highest corrugation amplitude while it declines by a factor of 10.3 at the highest wave number. Power enhancement factors of 5.73 and 2.32 are obtained by increasing the corrugation amplitude from Af = 0.0714H to Af = 0.1786H and from Af = 0.1786H to Af = 0.2143H. When fin positions sf = 0.7143H and sf = -0.7143H are compared with the case of fin location at sf = 0, the power increment factors become 12.27 and 4.76, respectively. For the power produced by the piezo device, a polynomial type correlation is suggested by adjusting the parameters of the corrugated wall.

Machine learning algorithms on predicting the turbulent mixed convection flow in a driven-cavity with two horizontal cylinders

Turbulent mixed convection flow and heat transfer properties in a driven cavity with two circular cylinders arranged one above another are analyzed numerically with the incorporation of a finite element scheme, based on the Galerkin method of weighted residuals. A comparison of streamlines, isotherms, and local, average Nusselt number is provided to illustrate how the Richardson number Ri, Reynolds number Re and the eddy viscosity ε affect the transport phenomena inside the cavity. The local and average Nusselt number have been evaluated and it is found that strong convection at the top wall causes the average Nusselt number to progressively rise with respect to Ri and ε, while steady or slightly varying profiles are seen with regard to Re. In order to predict the thermal distribution inside the cavity due to these hot cylinders, Artificial Neural Network (ANN) and Gaussian Process Regression (GPR) have been developed with these model parameters as input and average Nusselt Number as target values. Several plots have been depicted to come to a conclusion that both models have been trained very effectively with the minimal observed Mean Square Error (MSE) values of 0.0026018 and 0.0026428 for ANN and GPR, respectively. In support to these findings, the coefficient of determination R2 suggests that ANN (R2 = 0.89) outperforms GPR (R2 = 0.87) in testing accuracy, hence it is recommended for prediction task.

Scaling the Diurnal Mixing/Mixed Layer Depth in the Tropical Ocean: A Case Study in the South China Sea

AbstractThe diurnal cycling of the surface mixing/mixed layer (ML) depth, air‐sea heat flux, and vertical profiles of the temperature and turbulent kinetic energy dissipation rate in the tropical central South China Sea was observed in summer (June 2017) and winter (January 2018). In the daytime, solar heating warmed and stabilized the ML, and the thickness of the ML can be well characterized by the Zilitinkevich scale as noted in previous studies. By contrast, in the nighttime the ML was deepened by convective turbulence generated by surface cooling. Guided by these observations, we have derived a simple scaling for the nighttime deepening of the ML by simplifying the classic Kraus‐Turner type model. We show that the variation of the ML depth can be scaled by a function of the wind speed, air‐sea heat flux and the temporal variation of the sea surface temperature, all of which are observable variables at the sea surface. It is found that the scaling works well in reproducing observed variations of the ML depth from hydrographic data. As such, this study advances our understanding of the response of the upper ocean to atmospheric forcing and provides a simple way for predicting the ML depth with solely surface observations.

Mean temperature profiles in turbulent internal flows

We derive explicit formulas for the mean profiles of temperature (modeled as a passive scalar) in forced turbulent convection, as a function of the Reynolds and Prandtl numbers. The derivation leverages on the observed universality of the inner-layer thermal eddy diffusivity with respect to Reynolds and Prandtl number variations and across different flows, and on universality of the passive scalar defect in the core flow. Matching of the inner- and outer-layer expression yields a smooth compound mean temperature profile. We find excellent agreement of the analytical profile with data from direct numerical simulations of pipe and channel flows under various thermal forcing conditions, and over a wide range of Reynolds and Prandtl numbers.

Analysis of natural convection in a representative cavity of a room considering oscillatory boundary conditions: An experimental and numerical approach

Natural convection heat transfer in cavities commonly occurs in various engineering problems. Specifically, the problem of a differentially heated cavity with oscillatory boundary conditions arises in the design of energy-saving strategies or energy evaluations within the building sector. This work addressed numerically and experimentally the natural convection in a full-scale room subjected to diurnal heating and nocturnal cooling. The experiment was instrumented with temperature sensors and an air velocity sensor, inspired by works reported in the literature. The numerical model involved solving the equations governing natural convection, considering turbulent bi-dimensional and three-dimensional flow in a transient state. This research allowed the selection of an appropriate turbulence model and evaluated the performance of the two-dimensional and three-dimensional approaches in simulations. Results showed that the k-omega SST model performed best in predicting velocity, while the standard k-omega model performed best in predicting temperature for the two-dimensional simulation, and the RNG k-epsilon model performed best for the three-dimensional simulation. Additionally, the three-dimensional simulation more accurately reproduced the chaotic fluctuations observed in experimental velocity data, demonstrating up to 30 % better estimation during periods of high velocities. The study identified the reversal of convective cells in the cavity, with an approximate duration of 2 h and 40 min in a real case. This study contributes to improving the understanding of turbulent natural convection in a differentially heated cavity with oscillatory boundary conditions. These findings have significant implications for various environmental and engineering applications, including the design of thermally comfortable buildings.

Effect of radius ratio on the sheared annular centrifugal turbulent convection

Effect of radius ratio on the sheared annular centrifugal turbulent convection