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A CFD based procedure is described to identify the optimum inclination angle of a small-scale roof-top solar chimney for maximum ventilation performance. The absorber wall of the chimney under consideration is 500-mm long and the air gap width is 40 mm. Firstly, CFD simulations are performed on a two-dimensional solar chimney model with inclination angles varying from 30° to 90° relative to the horizontal plane under different heat fluxes. Subsequently, a mathematical procedure using the CFD data to estimate the ventilation performance of the solar chimney at different inclination angles under real climate conditions is described. The procedure accounts for the effect of the inclination angle on receivable solar irradiance and is applied to three Australian cities, corresponding to three different latitudes. It is found that the optimum inclination angle varies from 45° to 60°, depending on the latitude and season of operation.

Subject to daytime heating and nighttime cooling, a horizontal temperature gradient is generated in nearshore waters owing to the gradually deepening topography offshore. The resulting natural convection has significant biological and environmental implications. The present investigation is concerned with natural convection in a reservoir model induced by a periodically varying surface temperature. A hybrid of semi-analytical approach coupled with scaling analysis and numerical simulation is adopted to characterize the periodic flow. The present scaling focuses on the long period scenario for which a quasi-steady state is reached within a thermal forcing cycle. This is the case in natural water bodies under typical field conditions. Compared to the previously reported constant heating or cooling cases, the present scaling analysis is more complex and the developed scales depend on the period of the thermal forcing. Two critical functions of the Rayleigh number have been derived to identify the distinctness and stability of the thermal boundary layer. Distinct subregions are identified, and the scales of flow velocity and phase lag are derived for different subregions. These scaling results are verified by numerical simulations. Numerical results show that the flow response varies significantly with the length of period. For short periods, the large scale flow never reverses from cooling-induced flow. For the long period scenario, flow reversal occurs, with much more intense flow during the cooling phase than during the heating phase. The phase lag of flow response decreases with increasing period length. Finally, the present scaling results are applied to field situations, which agree well with field observations.

Dynamic wetting plays an important role in the physics of multiphase flow, and has a significant influence on many industrial and geotechnical applications. In this work, a modified smoothed particle hydrodynamics (SPH) model is employed to simulate surface tension, contact angle and dynamic wetting effects at meso-scale. The wetting and dewetting phenomena are simulated in a capillary tube, where the liquid particles are raised or withdrawn by a shifting substrate. The SPH model is modified by introducing a newly developed viscous force formulation at the liquid–solid interface to reproduce the rate-dependent behaviour of the moving contact line. Dynamic contact angle simulations with the interfacial viscous force are conducted to verify the effectiveness and accuracy of this new formulation. In addition, the influence of interfacial viscous forces with different magnitude on the contact angle dynamics is examined by empirical power-law correlations; the derived constants suggest that the dynamic contact angle changes monotonically with the interfacial viscous force. The simulation results are consistent with experimental observations and theoretical predictions, implying that the interfacial viscous force can be associated with the slip length of flow and the microscopic surface roughness. This work demonstrates that the modified SPH model can successfully account for the rate-dependent effects of a moving contact line, and can be used for realistic multiphase flow simulations under dynamic conditions.

The K-type transition of a natural convection boundary layer of Ra = 9.8 × 109 is studied by PIV (Particle Image Velocimetry) measurements. To excite the transition, Tollmien-Schlitchting (TS) and oblique waves of the same frequency are introduced into the upstream boundary layer in the form of velocity perturbations. It is found that resonant interactions between the characteristic frequency of the natural convection boundary layer and the superimposed TS and oblique waves are present, which trigger the K-type transition of the boundary layer. The typical aligned Λ-shaped flow structures characterising the K-type transition present in the transition of Blasius boundary layers are also observed in the present natural convection boundary layers. They occur when the primary instability grows to a certain extent. The appearance of the spanwise mode characterised by the aligned Λ-shaped flow structures is found to be an ‘abrupt’ process, although the development of the three-dimensionality in the natural convection boundary layer is found to be a ‘gradual’ process. A peak in the typical profile of the Root-Mean-Square (RMS) of the amplitude of streamwise velocity is also noted around the transition point, beyond which distinct three-dimensional Λ-shaped flow structures are observed. The transition point has been determined consistently using different approaches. A Bicoherence analysis suggests that the interaction between the external excitation frequency and the characteristic frequency of the boundary layer is responsible for the production of new harmonic frequencies and resonance groups. The PIV measurements have been extended to a range of perturbation frequencies for the boundary layer, and the dependence of the K-type transition on the perturbation frequency is discussed.

Natural convection induced by absorption of solar radiation in the near shore of lakes and reservoirs with a sloping bed is investigated experimentally utilising an open top triangular tank and a light source to simulate solar radiation. The present experimental investigation aims to examine the flow quantitatively for experimental parameters up to Rayleigh numbers one order of magnitude greater than that reported by Lei and Patterson (2002). Concurrent shadowgraph/Particle Image Velocimetry (PIV) technique is used to characterise the flow quantitatively for a number of experimental cases. Three temporal flow stages, initiation, transitional and quasi steady are identified and three spatial sub-regions, conductive, stable convection and unstable convective, are observed, consistent with previous scaling and numerical results. Scales for the onset of instability, velocities along the slope and the movement of the plumes are also verified by the experiments.

In this study, a Particle Image Velocimetry (PIV) measurement is carried out to investigate the flow past a highly confined circular cylinder at a fixed blockage ratio of 60%. The Reynolds number (Re) in terms of the cylinder diameter and the mean incoming flow velocity is varied from 318 to 1431. It is observed that the characteristic frequency of the separated shear layers is amplified through a so-called frequency-filtering process. The instability of the separated shear layers causes two different vortex shedding modes including alternating and symmetric shedding modes, which are observed at Re = 927–1242. An intermittent switch between the two shedding modes is also observed. The theory of mixing layers is applied with modification to predict the characteristics of the separated shear layers, and the prediction shows a good agreement with the experimental data. It is found that the vortex roll-up and merging locations can be estimated by calculating the energy contents of individual frequency modes.

Natural convection arising over vertical and horizontal heated flat surfaces is one of the most ubiquitous flows at a range of spatiotemporal scales. Despite significant developments over more than a century contributing to our fundamental understanding of heat transfer in natural convection boundary layers, certain “hidden” characteristics of these flows have received far less attention. Here, we review scattered progress on less visited fundamental topics that have strong implications to heat and mass transfer control. These topics include the instability characteristics, laminar-to-turbulent transition, and spatial flow structures of vertical natural convection boundary layers and large-scale plumes, dome, and circulating flows over discretely and entirely heated horizontal surfaces. Based on the summarized advancements in fundamental research, we elaborate on the selection of perturbations and provide an outlook on the development of perturbation generators and methods of altering large-scale flow structures as a potential means for heat and mass transfer control where natural convection is dominant.

A three-dimensional liquid bridge is considered in this study to numerically investigate the effects of an external non-uniform rotating magnetic field (RMF) on the thermocapillary flow in semiconductor melt under microgravity. Simulations are carried out to examine the convection and instability features of the thermocapillary flow over a range of Marangoni numbers (Ma = 15–50) under a non-uniform RMF. The present results show that applying an external non-uniform RMF enhances the maximum tangential velocity and depresses the maximum axial velocity. As a consequence, an approximately axisymmetric flow is maintained in the melt under the effect of the non-uniform RMF, which is beneficial for growing high quality crystal. Further investigation of the thermocapillary flow subject to different non-uniform RMFs (corresponding to Taylor numbers Ta = 3.8 × 10 2 –1.86 × 10 4 and Rotating Reynolds number Re ω = 2.2 × 10 4 ) reveals that the thermocapillary convection may undergo a transition from the approximately axisymmetric steady flow to a periodically oscillatory flow for Ma above a critical value. The critical Ma generally increases with the intensity of the non-uniform RMF.

The K-type and H-type transitions of a natural convection boundary layer of a fluid of Prandtl number 7 adjacent to an isothermally heated vertical surface are investigated by means of three-dimensional direct numerical simulation (DNS). These two types of transitions refer to different flow features at the transitional stage from laminar to turbulence caused by two different types of perturbations. To excite the K-type transition, superimposed Tollmien–Schlichting (TS) and oblique waves of the same frequency are introduced into the boundary layer. It is found that a three-layer longitudinal vortex structure is present in the boundary layer undergoing the K-type transition. The typical aligned $\wedge$-shaped vortices characterizing the K-type transition are observed for the first time in pure natural convection boundary layers. For exciting the H-type transition, superimposed TS and oblique waves of different frequencies, with the frequency of the oblique waves being half of the frequency of the TS waves, are introduced into the boundary layer. Unlike the three-layer longitudinal vortex structure observed in the K-type transition, a double-layer longitudinal vortex structure is observed in the boundary layer undergoing the H-type transition. The successively staggered $\wedge$-shaped vortices characterizing the H-type transition are also observed in the downstream boundary layer. The staggered pattern of $\wedge$-shaped vortices is considered to be caused by temporal modulation of the TS and oblique waves. Interestingly the flow structures of both the K-type and H-type transitions observed in the natural convection boundary layer are qualitatively similar to those observed in Blasius boundary layers. However, an analysis of turbulence energy production suggests that the turbulence energy production by buoyancy rather than Reynolds stresses dominates the K-type and H-type transitions. In contrast, the turbulence energy production by Reynolds stresses is the only factor contributing to the transition in Blasius boundary layers.

This investigation is motivated by interest in the mixing caused by the absorption of solar radiation in the near-shore regions of lakes. In the daytime, incoming solar radiation is absorbed by the water following Beer’s law. In the near-shore region where the water depth is less than the penetration depth of solar radiation, the residual radiation reaches the bottom and is absorbed and re-emitted back into the water body as a bottom heat flux, resulting in an adverse temperature gradient which may become unstable. In this laboratory-scale experimental study, this situation is modelled with a water-filled, open top tank with a black bottom subjected to radiative heating from a halogen theatre spotlight. Concurrent shadowgraph and Particle Image Velocimetry (PIV) are utilised for simultaneous visualisation of the temperature field and flow measurements. Initially, there is no motion and two distinct thermal boundary layers simultaneously grow from the water surface and from the bottom boundary. The bottom boundary layer becomes unstable at a certain thickness, and rising plumes are formed. The plumes transfer heat and cause circulation and vertical mixing in the water body. Subsequently the plumes disappear as the bottom boundary layer is dismantled, the bottom boundary layer is re-established, and the plumes re-formed. This paper will focus on characterising the first and the second stages. Accordingly, the onset of convection, plume rise height, and plume rise velocity are investigated through analysis of the PIV and shadowgraph images and compared with previously published scaling.

A transient heat balance model (THBM) based on energy conservation is developed for predicting the thermal performance of a semi-transparent water wall system. In order to validate the THBM against field measurements, real climate conditions in Sydney, Australia are adopted and the time variations of both internal and external convective and radiative heat transfer coefficients are calculated using empirical correlations from the literature. An alternative concrete wall model is also developed for further validation of the THBM. Good agreements between the THBM simulation and field data are achieved for both the water wall and concrete wall systems. The validated THBM is then adopted in a sensitivity analysis in order to assess the thermal performance of the semi-transparent water wall system under different configurations. It is found that the daily maximum water and air temperatures decrease significantly with the decrease of the transmissivity of the semi-transparent panels, whereas an increase of the attenuation coefficient of solar radiation in water has an insignificant impact on the water and air temperatures. The present results also indicated that increasing the thickness of the water column can reduce the fluctuations of the water temperature and the diurnal peak air temperature. From the practical application point of view, reducing the transmissivity of the external panel provides the most effective way to mitigate over-heating in the semi-transparent water wall system.

The thermal performance of an opaque water wall system is numerically investigated using the shear-stress transport (SST) k–ω turbulence model and the Discrete Ordinates radiation model for typical winter and summer climate conditions in Sydney, Australia. With a periodic sol-air temperature specified on the outside Perspex panel, the energy performance of the water wall system is examined over a range of water wall thicknesses. The computational fluid dynamics (CFD) model is compared with an experimentally validated transient heat balance model (THBM), and a fair agreement between the CFD model and the THBM results is achieved. The present numerical results indicate that the performance of the opaque water wall system is improved by increasing the thickness of the water column under the winter climate condition in Sydney. It is also found that less supplementary energy is required in winter than that in summer in order to maintain a comfortable interior temperature. Further, a comparison between the present water wall system and a conventional concrete wall system shows that the water wall system performs significantly better than the concrete wall system of the same thickness in the winter climate of Sydney, whereas both the water wall and concrete wall systems have a similar performance in terms of energy savings in the summer climate of Sydney.

The ‘natural transition’ of a natural convection boundary layer adjacent to an isothermally heated vertical surface is investigated by means of three-dimensional direct numerical simulation (DNS). In order to trigger ‘natural transition’ numerically, spatially and temporally random perturbations are introduced into the upstream boundary layer. The propagation of the random perturbations in the streamwise direction is observed. It is found that there exist two competing wavenumbers of spanwise vortical structures, one large and the other small. The large wavenumber dominates in the upstream boundary layer, whereas the small wavenumber dominates in the downstream boundary layer. The streamwise evolution of the mean (time-averaged) streamwise vorticity observed at planes perpendicular to the heated surface in general reveals two- and three-layer longitudinal roll structures. Nonlinear processes in a transitioning natural convection boundary layer are also analysed using Bicoherence method. The transition route and mechanism are discussed based on the power spectra and Bicoherence spectra of the temperature time series obtained in the boundary layer. A spectrum filling process during the ‘natural transition’ to turbulence is also observed, which is qualitatively similar to that observed in Blasius boundary layers.

The buoyancy-induced air flow through a two-dimensional vertical ventilation channel is calculated. One of the channel walls is heated uniformly, and the other wall is adiabatic. A thin baffle is placed on the heated wall to manipulate the air flow through the channel. Numerical results are obtained for baffles of different lengths and placed at various heights along the heated wall. It is found that the baffle is effective in weakening a reverse flow at the exit of the channel, and significant enhancement of ventilation performance may be achieved with the presence of the baffle.

This paper presents a numerical procedure for modelling the thermal performance of ventilated hollow core slabs (VHCS). A turbulence model suitable for this purpose is identified first by considering a smooth horizontal pipe subjected to turbulent mixed convention conditions typical of VHCSs. Comparison of the fully-developed dimensionless velocity (u+) and temperature (T+) profiles as well as the Nusselt numbers (Nu) predicted by five different turbulence models against empirical expressions available in the literature shows that the Standard and Realisable k-ε models provide the best overall predictions of u+, T+ and Nu. Since the Standard k-ε model gives slightly better estimates of the Nu values, it is adopted to model the thermal performance of a VHCS geometry for which experimental thermal responses are reported in the literature. The numerical predictions of local temperatures within the VHCS agree well with the experimental measurements, and hence the Standard k-ε model is recommended here for the modeling of VHCSs.

Antifouling and drag reduction are two critical issues for the shipping industry. Previous studies have revealed that biomimetic shell surfaces have distinct antifouling features. In this study, ten simplified surface microstructures with the same cross-section area are designed based on the antifouling scale of the shell surface morphology. Turbulent flow over the riblet structures is simulated using a finite volume based CFD solver with the Shear Stress Transport k-ω model to examine the effects of the riblet structures on skin friction with mean flow velocities ranging from 1 to 7 m/s. The corresponding dimensionless square root of the groove cross-section area is l g + ≈4.5–25.6. Considering the predicted drag performance and issues concerning riblet tip rounding and fabrication, an optimal geometry with both antifouling and drag reduction features has been identified. The present study confirms that the riblets lift off streamwise vortices, pushing the high turbulent kinetic energy region away from the surface, and impede the spanwise movement of streamwise vortices, leading to skin-friction reduction. Further, the investigation of the effect of the riblet interval on the drag behaviour indicates the existence of a balance point between the wetted perimeter and the total area of the riblet surface for minimizing drag.

Local air circulation is influenced by the drag forces exerted by urban vegetation. In this study, tree canopy parameterization in computational fluid dynamics (CFD) with source term modifications is conducted at the tree scale, which builds upon the models previously used in the literature. A simplified three-dimensional model representing tree canopies is resolved and validated against available wind tunnel experiment. The prediction of turbulence kinetic energy (TKE) is shown to be highly sensitive to the model coefficients in the source term of turbulence dissipation rate (TDR). No optimal set of the model coefficients in the TDR source term can be identified. However, C e 4 > C e5 is found to better reproduce the turbulent quantities behind the canopy than C e4 = C e5 . The reduction of C e 5 value from 1.5 to 0.4 yields a higher TDR source term level by one order of magnitude. The prediction of the mean velocity is primarily controlled by the form drag coefficient, which is heavily correlated to all source terms and their components. Results indicate the feasibility of using a more straightforward modification method in the vegetated microclimate simulations by only adding momentum source terms.

Turbulent natural convection with and without radiation transfer in two-dimensional (2D) and three-dimensional (3D) air-filled differentially heated cavities is numerically investigated using various RANS (Reynolds Averaged Navier–Stokes) turbulence models and the Discrete Ordinates radiation model. Five different two-equation eddy-viscosity models including the standard k–e model, the renormalization group (RNG) k–e model, the realisable k–e model, the standard k–ω model and the shear-stress transport (SST) k–ω model are selected for comparison. Qualitative and quantitative data are presented to demonstrate the effects of three-dimensionality, radiation transfer and the thermal boundary conditions on the horizontal surfaces on the numerical solution of the convective flow in the cavity. The present numerical results are compared against published experimental and direct numerical simulation data. It is found that the predicted thermal stratification in the interior of the cavity is improved when the simulation is extended from 2D to 3D and when the effect of radiation transfer is accounted for. The discrepancy with regard to the interior stratification between the experiment and numerical simulation is mainly caused by the negligence of radiation transfer. The thermal boundary conditions on the horizontal surfaces also have a significant impact on the numerical solution, especially when the radiation transfer is not accounted for. Further, the present results show that all the RANS models are capable of capturing the main features of the flow and the overall performance of these turbulence models in terms of predicting time-averaged quantities is acceptable. It is found that the variation of the numerical results obtained with the three k–e models is very small, whereas the discrepancy between the two k–ω models is significant. The SST k–ω model has the best overall performance and the standard k–ω model has the worst overall performance.

This paper considers linear and oscillatory behaviours of the natural convection boundary layer induced by the absorption of incoming radiation in response to constant and time-varying (ramp) thermal forcing. The considered configuration is relevant to the flow in littoral regions of lakes and reservoirs in response to the daytime thermal forcing due to solar radiation. The absorption of the incoming solar radiation is depth-dependent, following Beer’s law, whilst the residual radiation at the bottom bathymetry is absorbed and released back to the water body as a boundary flux. Therefore, a potentially unstable thermal boundary layer forms adjacent to the bottom bathymetry and the associated thermoconvective instability induces vertical mixing in the form of rising plumes (i.e. intermittent convection). The linear theory is applied to investigate the incipient evolution of the instability, and direct comparisons of linear and direct stability analyses are performed. The fastest growing mode and the corresponding temporal growth rate obtained via a quasi-static linear stability analysis are shown to be in excellent agreement with the results of the direct stability analysis based on numerical simulations of the flow. Further, the time and frequency scales of the unstable bottom thermal boundary layer are derived via a local Rayleigh number analysis. It is then demonstrated that the boundary layer response to the time-varying thermal forcing is in equilibrium with the forcing intensity at each instant of time.

The transient stability of penetrative convection induced by internal heating coupled with a Robin (or third kind) boundary condition is investigated by adopting different transient approaches. A Robin boundary condition introduces a heat exchange coefficient as an additional parameter. In the system, a destabilising boundary flux is determined by the temperature increase of the fluid adjacent to the boundary, which is in turn controlled by the internal heat source, thus the boundary flux increases gradually with time. The study shows that the stability properties of the flow are independent of the Prandtl number and the transient approach being used. Further, the results of the present study demonstrate the effect of varying the heat exchange coefficient on the critical conditions and the formation of primary convection rolls. Interestingly, the horizontal and vertical length scales of the convection rolls are found to be insensitive to the value of the heat exchange coefficient, whilst the onset time of the penetrative convection is strongly dependent on this parameter.

This paper reports a numerical investigation of the buoyancy induced turbulent air flow in an inclined passive wall solar chimney (IPWSC) attached to a room (ventilated space) for ventilation applications over a range of controlling parameters. The standard k − ɛ turbulence model has been employed to model air turbulence in the solar chimney system. With an isoflux heating condition specified on the absorber wall, the ventilation performance of the IPWSC design has been examined over the Rayleigh number range of 1.36 × 10 13 ≤ Ra ≤ 1.36 × 10 16 and inclination angles from 0 to 6. The present numerical result shows that the turbulent kinetic energy and turbulent intensity in the solar chimney decrease with the increase of the inclination of the passive wall. The effectiveness of the IPWSC design for enhancing the thermally driven ventilation has been confirmed. Additional simulations have been carried out with a standalone solar chimney model. The result shows that the standalone model over-predicts the mass flow rate compared to that predicted by the attached model. The average difference in the predicted mass flow rates between the standalone model and the attached model with inclination angles from 0 to 6 at a representative Rayleigh number of Ra = 1.36 × 10 14 is around 10%.

The separation of plumes from an adiabatic horizontal thin fin attached to a sidewall of a differentially heated cavity at quasi-steady stage is experimentally and numerically studied at three Rayleigh numbers (0.92 × 10 9 , 1.84 × 10 9 and 3.68 × 10 9 ) and over a range of fin positions. Regular plume separation is observed over the present range of parameters during the quasi-steady stage. Both experimental and numerical results reveal that the plume separation frequency increases with the Rayleigh number and decreases with the fin height measured from the leading edge. A higher Rayleigh number leads to a more unstable flow above the horizontal thin fin which in turn leads to a higher plume separation frequency. The decrease of the plume separation frequency with the increasing fin height is mainly due to the reduction of the adverse temperature gradient in the unstable layer above the thin fin as a result of the cavity-wide temperature stratification. It is further revealed that the heat transfer through the sidewall is improved by the presence of the thin fin. An optimum fin height for maximum heat transfer enhancement has been identified for the case with a Rayleigh number of 0.92 × 10 9 . For the other two higher Rayleigh number cases considered in this study, the heat transfer through the sidewall monotonically decreases with the fin height.

The present study considers the early stage of the flow development in a water-filled, three-dimensional parallelepiped cavity subject to solar radiation. The thermal structure consists of an upper stable stratification due to internal heating, provided by the direct absorption of radiation, and a bottom potentially unstable boundary layer due to the absorption and re-emission of the residual radiation by the bottom boundary. The growth of the competing thermal layers and the stability properties of the bottom thermal boundary layer are investigated by means of a scaling analysis and a direct stability analysis based on three-dimensional numerical simulation. The scaling relations are benchmarked against the numerical simulation, and the effects of the normalised water depth, the Rayleigh number and the box aspect ratio on the thermal instability are studied.

This study focuses on the marginal stability of the natural convection flow induced by the absorption of radiation. Non-uniform internal heating associated with the absorption of radiation following Beer’s law and the absorption and re-emission of residual radiation by the bottom boundary wall lead to a nonlinear base temperature profile that evolves with time. A horizontal throughflow and lateral confinement are incorporated in the analysis, and two forms of convective instability, i.e. longitudinal and transverse rolls, are investigated. It is shown that the formation of longitudinal rolls, with their axes parallel to the throughflow direction, is not affected by the presence of the throughflow. For transverse rolls, with their axes perpendicular to the throughflow direction, however, the throughflow is shown to have a stabilising effect, with the critical Rayleigh number increasing with the increasing throughflow Reynolds number. It follows that longitudinal rolls are the preferred form of instability in laterally unconfined domains for any nonzero Reynolds number and in confined domains if the Reynolds number is greater than a critical value. The value of the critical Reynolds number, which separates the preferred mode of instability, is found to rapidly reduce with increasing lateral extent and approaches zero for a normalised lateral extent greater than 2.0. The lateral confinement is also shown to have a stabilising effect for both longitudinal and transverse rolls, with the critical Rayleigh number increasing with the reducing lateral extent. It is shown that the lateral confinement only substantially affects the wave number of the rolls with their axes parallel to the confining walls but not the rolls with their axes perpendicular to the confining walls.

This study considers the natural convection flow in a water body subjected to heating by solar radiation. The investigation into this type of natural convection flow has been motivated by the fact that it is known to play a crucial role in the daytime heat and mass transfer in shallow regions of natural water reservoirs and lakes, with a resultant impact on biological activity. An analytical solution for temperature in such an internally heated system shows that the temperature stratification consists of an upper stable stratification and a lower unstable stratification. One of the important consequences of such a nonlinear temperature stratification is the limitation of the mixing driven by rising thermal plumes with the penetration length scale of the plumes determining the lower mixed layer thickness. A theoretical analysis conducted in the present study suggests that in relatively deep waters, the lower mixed layer thickness is equal to the attenuation length of the radiation, which has important implications for water quality, including the transport of pollutants and nutrients in the water body. Scalings are also obtained for the quasi-steady boundary layer. The theoretical analysis is validated against numerical simulations.

Previous studies have revealed that heat transfer through a convective thermal boundary layer can be significantly enhanced by perturbing the thermal boundary layer to advance linear to nonlinear transition. It has also been demonstrated that the enhancement of heat transfer is mostly achieved in the nonlinear regime. In this study, the transition of the thermal boundary layer adjacent to an isothermally heated vertical surface is revisited by means of Bicoherence analysis, which is a statistical approach for identifying and quantifying quadratic wave interactions. The streamwise evolution of Bicoherence spectra suggests that the thermal boundary layer can be classified into three regimes: a linear flow regime, a transitional flow regime and a nonlinear flow regime. The positions of the transition from the transitional to nonlinear regimes in the thermal boundary layer at various Rayleigh numbers, perturbation frequencies and perturbation amplitudes are determined using Bicoherence analysis. It is found that in the nonlinear flow regime, the number of resonance groups fluctuates, which indicates the occurrence of coupling and decoupling of harmonics in the boundary layer. This process may be the mechanism responsible for the resonance induced enhancement of heat transfer.

Ongoing investigations into solar chimney development have resulted in constantly evolving new designs. In this study, experiments are carried out with an inclined passive wall solar chimney (IPWSC) model with a uniform heat flux on the active (absorptive) wall. The effectiveness of this design has been examined for the heat flux range of 100 W/m2–500 W/m2 with a fixed base air gap width of 0.1 m and inclination angles of the passive wall in the range of 0–6 degrees. The experimental results show that the inclination angle of the passive wall has no significant effect on the temperature distribution across the air gap width and along the chimney height. On the other hand, the averaged air flow velocity across the air gap width is strongly affected by the inclination angle. The experimental results also show that the IPWSC with 0.7 m absorber height and 0.1 m air gap width at an inclination angle of 6° and input heat flux of 500 W/m2 can produce sufficient ventilation for a 27 m3 room based on ASHREA standards. Further, the present experimental results show that the IPWSC design can significantly improve the ventilation performance of a solar chimney in comparison to the conventional chimney design with vertical passive wall configuration. The experimental results are supported by flow visualization experiments and are consistent with scaling predictions.

In this study the natural convection flow adjacent to a finned sidewall of a differentially heated cavity is experimentally investigated using the shadowgraph technique. Two Perspex fins of different lengths are placed horizontally at different heights along each of the cavity sidewalls. It is revealed from the shadowgraph observation of the heated sidewall that similar horizontal hot intrusions form underneath the fins and the cavity ceiling at the early stage. In the transitional stage, the thermal flows bypassing the fins are entrained towards the heated sidewall and a cavity-scale interior temperature stratification is gradually established. In the quasi-steady stage, regular oscillations are observed above the lower fin. The oscillation in the horizontal thermal flow above the lower fin results in strong mixing in the thermal boundary layer between the two fins and that downstream of the upper fin. As a consequence, heat transfer through the downstream thermal boundary layer is greatly enhanced. A corresponding numerical simulation is also conducted and a good agreement between the simulation and the experiment is achieved. It is further revealed numerically that the convective flow across the cavity is significantly enhanced due to the presence of the fins. The heat transfer is improved by up to 17.1% over the investigated Rayleigh number range. The dependency of the time averaged Nusselt number at the finned sidewall on the Rayleigh number has been quantified.

The present investigation is concerned with the transient flow response of a reservoir model to periodic thermal forcing at water surface. A previously reported concurrent PIT/PIV experiment is numerically reproduced using a Computational Fluid Dynamics (CFD) code coupled with a Discrete Phase Model (DPM). Both qualitative and quantitative agreements are achieved between the present numerical simulation and the experiment. A parametric study is also carried out using the numerical model to determine the dependence of the flow response and its stability properties on the Grashof number.

The coupled thermal boundary layers adjacent to a vertical partition placed in the middle of a differentially heated cavity are experimentally investigated using the shadowgraph technique over the range of Rayleigh numbers from 2.6 × 109 to 1011. The cavity is filled with water and a temperature difference between the two sidewalls of the partitioned cavity is imposed. The experimental visualisation shows that the coupled thermal boundary layers are unsteady in the fully developed stage if the Rayleigh number is sufficiently high (e.g. Ra > 1010). The properties of the instability in the coupled thermal boundary layers adjacent to the partition are described and the dependence of these properties on the Rayleigh number is quantified.

This study is concerned with particle transport in a reservoir model subject to diurnal temperature variation at the water surface. A Computational Fluid Dynamics (CFD) code coupled with a Discrete Phase Model (DPM) is adopted to examine the particle transport in the reservoir. The particle deposition and dispersion as well as the concentration of particles in various regions of the water body are examined. The present study demonstrates the importance of buoyancy-driven flows for particle deposition and dispersion in reservoirs.

The flow behavior due to natural convection of air (with a Prandtl number less than 1) inside a solar chimney with an imposed heat flux on a vertical absorber wall is investigated by a scaling analysis and a corresponding numerical simulation. Three distinct flow regimes are identified, one with a distinct thermal boundary layer and the other two without a distinct thermal boundary layer, depending on the Rayleigh number. The two regimes without a distinct thermal boundary layer are further classified into low and medium Rayleigh number sub-regimes respectively. These sub-regimes are characterized by conduction dominance in which the thermal boundary layer grows to encompass the entire width of the channel before convection becomes important. Flow development in each of these flow regimes and sub-regimes is characterized through transient scaling, and scaling correlations are developed to describe the temperature, flow velocity and mass flow rate, which characterize the ventilation performance of the solar chimney. The scaling arguments are validated by the corresponding numerical data.

Nearshore natural convection induced by differential heating across shore has significant biological and environmental implications. The present investigation is concerned with convection in a wedge-shaped reservoir sidearm induced by isothermal heating at the water surface which increases linearly over a specified time period from an initial value to a final steady value (i.e. ramped heating), extending our previous report on convection induced by constant isothermal heating. A semi-analytical approach coupled with scaling analysis and numerical simulation is adopted to resolve the problem. It is revealed that the flow domain is composed of two distinct subregions, a conductive region nearshore which finally becomes isothermal and stationary, and a convective region offshore which eventually maintains a steady flow velocity. Different scenarios are revealed through the comparison between the ramp duration P and a characteristic time t c of the induced flow. For the conductive region, the characteristic time is the time for the thermal boundary layer formed at the surface to reach the full local depth. For the convective region, it is the steady state time when convection balances conduction. For a short ramp ( P t c ), no steady state is reached within the ramping stage. For a long ramp ( P > t c ), a quasi-steady state is reached for both subregions before the ramp finishes. For the conductive region, the quasi-steady state is characterized by a steady flow velocity and the same local temperature growth rate as the water surface. For the convective region, the quasi-steady state is characterized by a change in the growth rate of the velocity. Major time and velocity scales governing the flow development in both subregions are proposed by a semi-analytical solution coupled with scaling and verified by the numerical simulation. Implication of the present study for field situations is also discussed.

In this study we investigate the transition from a steady flow to an unsteady flow induced by an adiabatic fin on the sidewall of a differentially heated cavity using numerical simulation. A range of Rayleigh numbers (10 6 –10 9 ) and various fin lengths with Prandtl number 6.63 are considered. It is found that the flow adjacent to the finned sidewall is unsteady for the cases with Rayleigh numbers higher than a critical value for a given fin length. Such an unsteady flow results from a fluid layer above the fin with an adverse temperature gradient and a through flow, which is a similar mechanism to a Rayleigh–Benard–Poiseuille (RBP) flow, though the configuration is such that it cannot be exactly a RBP flow. The transition from a steady flow to an unsteady flow is sensitive to the fin length, and the critical values of the transition for the different fin lengths are obtained. Moreover, the features of the unsteady flow induced by the fin are characterized and the mechanisms responsible for the unsteadiness are discussed.

The present study numerically investigates the unsteady natural convection in a triangular domain induced by ramped iso-flux cooling at the water surface. The simulations indicate that, although the ramped cooling does not alter the final state of the flow for a given steady state flux, it may change the developing path towards the final state as well as slow down the development of the flow. The ramped cooling delays or completely suppresses the onset of a Rayleigh-Benard type (RBT) instability during the developing process. As a consequence, a transitional stage characterized by the appearance and disappearance of the RBT instability at intermediate Rayleigh numbers does not occur if the ramp time is significantly long. Both qualitative and quantitative information is presented to demonstrate the effects of the ramp time on the transient behaviour of the flow at different Rayleigh numbers. A time scale for the onset time of the RBT instability under ramped cooling is also proposed and verified by the numerical results. The local instantaneous Rayleigh number at different parameters is calculated and its time history confirms the observed transient flow behaviours.

This paper reports a numerical investigation of flow reversal effects on the buoyancy induced air flow in a solar chimney for ventilation applications over a range of controlling parameters. The reverse flow phenomenon is quantitatively examined by calculating its penetration depth. In order to suppress the reverse flow and enhance ventilation performance, a configuration with an inclined passive wall is proposed as a new design; the effectiveness of this design has been examined over the Rayleigh number range of 10 9 ⩽ Ra ⩽ 10 11 . It is identified that there exists an optimum inclination angle corresponding to an optimum aspect ratio at which the mass flow rate is the maximum for a given Rayleigh number. The numerical results are supported by a scaling argument and a flow visualization experiment.

The present investigation is concerned with natural convection in a wedge-shaped domain induced by constant isothermal heating at the water surface. Complementary to the study of daytime heating by solar radiation relevant to nearshore regions of lakes and reservoirs previously reported by the same authors, this study focuses on sensible heating imposed by the atmosphere when it is warmer than the water body. A semi-analytical approach coupled with scaling analysis and numerical simulation is adopted to resolve the problem. Two flow regimes are identified depending on the comparison between the Rayleigh number and the inverse of the square of the bottom slope. For the lower Rayleigh number regime, the entire flow domain eventually becomes isothermal and stationary. For the higher Rayleigh number regime, the flow domain is composed of two distinct subregions, a conductive subregion near the shore and a convective subregion offshore. Within the conductive subregion, the maximum local flow velocity occurs when the thermal boundary layer reaches the local bottom, and the subregion eventually becomes isothermal and stationary. In the offshore convective subregion, a steady state is reached with a distinct thermal boundary layer below the surface and a steady flow velocity. The dividing position between the two subregions and the major time and velocity scales governing the flow development in both subregions are proposed by the scaling analysis and validated by corresponding numerical simulation.

We perform detailed numerical simulations of natural convection in a reservoir model induced by surface heating. The transient behaviour of the flow, including streamlines, isotherms, surface velocity profiles, volumetric flow rates, heat transfer rates, and temperature averaged over the local water depth are presented at different Rayleigh numbers. At an instantaneous time within the sloping bottom region, the surface velocity initially increases and then decreases with an increase of the offshore distance. Over a certain range of Rayleigh numbers, the location of the peak surface velocity may initially move in the offshore direction and then retract with the elapse of time. The point at which the rates of horizontal conduction and convection balance also retracts towards the tip region with the elapse of time. Some insight into understanding the mechanisms that drive the flow is provided according to the detailed transient behaviour of the flow.

The unsteady natural convection flow adjacent to the finned sidewall of a differentially heated cavity is numerically investigated through comparisons between the cases with a conducting fin and an adiabatic fin. The results show that the flow and temperature structures in the transition to a periodic flow induced by a conducting fin are considerably different from those by an adiabatic fin. Based on the present numerical results, the temporal development and spatial structures of the flow adjacent to the finned sidewall are described, and instabilities are characterized. It is found that the conducting fin improves the transient convective flows in the cavity and enhances heat transfer across the cavity (by up to 52% in comparison with the case without a fin).

This study is concerned with transient and steady state heat transfer by natural convection in a differentially heated cavity. The purpose is to evaluate a passive approach for enhancing heat transfer through the cavity. In this study, the effects of three different corner geometries (including sharp, round and straight corners) and adiabatic extensions of various dimensions on natural convection heat transfer are investigated numerically. The numerical results show marginal variations of the heat transfer rates among the three different corner shapes and a strong dependence of heat transfer enhancement on the Rayleigh number. For a given Rayleigh number, the enhancement of heat transfer by adiabatic extensions is limited.

The heat transfer through the attics of buildings under realistic thermal forcing has been considered in this study. A periodic temperature boundary condition is applied on the sloping walls of the attic to show the basic flow features in the attic space over diurnal cycles. The numerical results reveal that, during the daytime heating stage, the flow in the attic space is stratified; whereas at the night-time cooling stage, the flow becomes unstable. A symmetrical solution is seen for relatively low Rayleigh numbers. However, as the Ra gradually increases, a transition occurs at a critical value of Ra. Above this critical value, an asymmetrical solution exhibiting a pitchfork bifurcation arises at the night-time. It is also found that the calculated heat transfer rate at the night-time cooling stage is much higher than that during the daytime heating stage.

The natural convection thermal boundary-layer adjacent to an inclined flat plate subject to sudden heating and a temperature boundary condition which follows a ramp function up until a specified time and then remains constant is investigated. The development of the flow from start-up to a steady state has been described based on scaling analyses and verified by numerical simulations. Different flow regimes based on the Rayleigh number are discussed with numerical results for both boundary conditions. For ramp heating, the boundary-layer flow depends on the comparison of the time at which the ramp heating is completed and the time at which the boundary layer completes its growth. If the ramp time is long compared with the steady-state time, the layer reaches a quasi-steady mode in which the growth of the layer is governed solely by the thermal balance between convection and conduction. On the other hand, if the ramp is completed before the layer becomes steady; the subsequent growth is governed by the balance between buoyancy and inertia, as for the case of instantaneous heating.

A fundamental study of the fluid dynamics inside an attic shaped triangular enclosure with cold upper walls and adiabatic horizontal bottom wall is reported in this study. The transient behaviour of the attic fluid which is relevant to our daily life is examined based on a scaling analysis. The transient phenomenon begins with the instantaneous cooling and the cooling with linear decreases of temperature up to some specific time (ramp time) and then maintain constant of the upper sloped walls. It is shown that both inclined walls develop a thermal boundary layer whose thicknesses increase towards steady state or quasi-steady values. A proper identification of the time scales, the velocity and the thickness relevant to the flow that develops inside the cavity makes it possible to predict theoretically the basic flow features that will survive once the thermal flow in the enclosure reaches a steady state. A time scale for the cooling-down of the whole cavity together with the heat transfer scales through the inclined walls has also been obtained through scaling analysis. All scales are verified by the numerical simulations.

In this study, a discussion of the fluid dynamics in the attic space is reported, focusing on its transient response to sudden and linear changes of temperature along the two inclined walls. The transient behaviour of an attic space is relevant to our daily life. The instantaneous and non-instantaneous (ramp) heating boundary condition is applied on the sloping walls of the attic space. A theoretical understanding of the transient behaviour of the flow in the enclosure is performed through scaling analysis. A proper identification of the timescales, the velocity and the thickness relevant to the flow that develops inside the cavity makes it possible to predict theoretically the basic flow features that will survive once the thermal flow in the enclosure reaches a steady state. A time scale for the heating-up of the whole cavity together with the heat transfer scales through the inclined walls has also been obtained through scaling analysis. All scales are verified by the numerical simulations.

Temperature measurements of the thermal flows in a differentially heated cavity with and without a fin on the sidewall were performed. The oscillatory behaviours in the transition from an initially isothermal state to an eventually stratified interior fluid ambient are characterized. It is revealed that, for the case without a fin, the amplitude of the oscillations in the thermal boundary layer increases as the stratification of the interior fluid increases. For the case with a fin, the temperature measurements show that the oscillations are induced by an unstable fluid layer above the fin. The spectral analysis reveals that the frequency properties of the oscillations are different between the cases with and without a fin.

Natural convection in calm near-shore waters induced by daytime heating or nighttime cooling plays a significant role in cross-shore exchanges with significant biological and environmental implications. Having previously reported an improved scaling analysis on the daytime radiation-induced natural convection, the authors present in this paper a detailed scaling analysis quantifying the flow properties at varying offshore distances induced by nighttime surface cooling. Two critical functions of offshore distance have been derived to identify the distinctness and the stability of the thermal boundary layer. Two flow scenarios are possible depending on the bottom slope. For the relatively large slope scenario, three flow regimes are possible, which are discussed in detail. For each flow regime, all the possible distinctive subregions are identified. Two different sets of scaling incorporating the offshore-distance dependency have been derived for the conduction-dominated region and stable-convection-dominated region respectively. It is found that the scaling for flow in the stable-convection-dominated region also applies to the time-averaged mean flow in the unstable region. The present scaling results are verified by numerical simulations.

This work incorporates scaling analysis to characterise unsteady boundary-layer development for thermo-magnetic convection of paramagnetic fluids with Prandtl numbers (Pr ) greater than one. Under consideration is a square cavity with a quiescent isothermal, Newtonian fluid placed in a micro-gravity condition (g≈0g≈0), and under a uniform vertical gradient magnetic field. A distinct magnetic convection boundary layer is produced by the sudden imposition of a higher temperature on the left-hand side vertical sidewall due to the effect of the magnetic body force generated on the paramagnetic fluid. This magnetic force is proportional to the magnetic susceptibility and the gradient of the square of the magnetic induction. According to Curie’s law, the magnetic susceptibility of a paramagnetic fluid is inversely proportional to the absolute temperature. Thermal convection of a paramagnetic fluid can therefore take place even in zero-gravity environments as a direct consequence of temperature differences occurring within the fluid placed within a magnetic field gradient. Scaling predictions presented here are verified by numerical simulations It is shown that the transient flow behaviour of the resulting boundary layer can be described by three stages: a start-up stage, a transitional stage and a steady state. Special attention in this work is paid to the dependency of the flow development upon the Prandtl number, varied over the range of 5–100, thus representing various fluids. Also, the effect of the magnetic momentum parameter, m, and the quantity γRa, upon the flow development obtained in numerical simulations confirms the accuracy of new scaling predictions for paramagnetic fluids.