Search

Purpose: Hypolimnetic hypoxia has become increasingly prevalent in stratified water bodies in recent decades due to climate change. One primary sink of dissolved oxygen (DO) is sediment oxygen uptake ( J O 2 ). On the water side of the sediment–water interface (SWI), J O 2 is controlled by a diffusive boundary layer (DBL), a millimeter-scale layer where molecular diffusion is the primary transport mechanism. In previous studies, the DBL was determined by visual inspection, which is subjective and time-consuming. Material and methods: In this study, a computational procedure is proposed to determine the SWI and DBL objectively and automatically. The procedure was evaluated for more than 300 DO profiles in the sediment of three eutrophic water bodies spanning gradients of depth and surface area. Synthetic DO profiles were modeled based on sediment characteristics estimated by laboratory experiments. The procedure was further verified adopting the synthetic profiles. Results and discussion: The procedure, which was evaluated for both measured and synthetic DO profiles, determined the SWI and DBL well for both steady and non-steady state DO profiles. A negative relationship between DBL thickness and aeration rates was observed, which agrees with existing literatures. Conclusions: The procedure is recommended for future studies involving characterizing DBL to improve efficiency and consistency. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this study, the instability mechanisms in the conjugate thermal boundary layers (TBLs) adjacent to a partition in a differentially heated dual-chamber cavity are investigated numerically. The two chambers of the cavity are filled with air (Pr = 0.71) and water (Pr = 8.58), and the partition is assumed to have a zero thickness. The effects of the aspect ratios of both chambers (Aair and Awater) and the overall temperature difference (ΔT) on the interactions between the conjugate air- and water-side TBLs are extensively investigated. It is found that Aair has a more significant impact on the instabilities of the TBLs than Awater. For a relatively small Aair (e.g., 10/3) and a relatively large Aair (e.g., 10), the water-side TBL resonates with the air-side TBL at the frequencies of the corner flow instabilities in the air chamber. For medium Aair (e.g., 4, 5, and 20/3), the air-side TBL becomes weakly turbulent, causing the water-side TBL to become chaotic. Furthermore, if ΔT is reduced, the air-side TBL becomes quasi-periodic, limiting the turbulence growth on the water side. A stability map illustrating the major instability mechanisms controlling the interactions between the conjugate TBLs is presented on the (Aair, Ra) domain for Awater = 5. [ABSTRACT FROM AUTHOR]

Flow over circular cylinders covered with circumferential riblets of triangular shape with spanwise coverage ratios from 0 (bare cylinder) to 1 (fully covered) is investigated by large eddy simulation at three subcritical Reynolds numbers (2000, 3000 and 3900 respectively). The bare cylinder is considered as a reference case and for validation purpose. Detailed flow structures and boundary layer characteristics are described. For the fully covered cylinder, it is found that flows on the two sides of each riblet interact, forming a structure with stronger turbulent fluctuations and leading to a significantly shorter recirculation length compared to the other cases. The turbulent fluctuations are attenuated to various degrees for the partly covered cylinders. Two semicircular-shaped flow zones with distinct spanwise flows are observed around the riblets adjacent to the bare sections. Among the three partly covered cases under consideration (with spanwise coverage ratios of 0.25, 0.5 and 0.75 respectively), the half-covered cylinder has the largest semicircular-shaped flow zones, associated with the lowest turbulence in the flow field. A zigzag flow separation line is observed on all sections covered with riblets. The variations of the flow behaviours result in different characteristics of the hydrodynamic forces acting on the various cylinders. It is revealed that the lift fluctuation is the lowest for the half-covered cylinder and the highest for the fully covered cylinder. An examination of the sectional hydrodynamic forces further reveals that the sectional drag is higher at the riblet tip than that in the valley. • Spanwise coverage ratio of riblets has significant effects on the vortical structures around the cylinders. • The longest recirculation length is observed behind the half-covered cylinder, whereas the shortest recirculation length is observed behind the fully covered cylinder. • The half-covered cylinder has the lowest spanwise averaged fluctuating lift among all the cases. • A zigzag-shaped flow separation line is observed on the cylinders covered with riblets.. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

The instability characteristics and resonance of a natural convection boundary layer adjacent to an isothermally heated vertical surface are investigated using direct stability analyses. The detailed streamwise evolution of the boundary-layer frequencies is visualized via the power spectra of the temperature time series in the thermal boundary layer. It is found that the entire thermal boundary layer may be divided into three distinct regions according to the frequency profile, which include an upstream low-frequency region, a transitional region (with both low- and high-frequency bands) and a downstream high-frequency region. The high-frequency band in the downstream region determines the resonance characteristics of the thermal boundary layer, which can be triggered by a single-mode perturbation at frequencies within the high-frequency band. The single-mode perturbation experiments further reveal that the maximum resonance of the thermal boundary layer is triggered by a perturbation at the characteristic frequency of the boundary layer. For the boundary-layer flow at $\mathit{Ra}= 3. 6\times 1{0}^{10} $ and $\mathit{Pr}= 7$, a net heat transfer enhancement of up to 44 % is achieved by triggering resonance of the boundary layer. This significant enhancement of heat transfer is due to the resonance-induced advancement of the laminar–turbulent transition, which is found to be dependent on the perturbation frequency and amplitude. Evidence from different perspectives revealing the same position of the transition are provided and discussed. The outcomes of this investigation demonstrate the prospect of a resonance-based approach for enhancing heat transfer. [ABSTRACT FROM PUBLISHER]

Highlights: [•] The transition to unsteady coupled thermal boundary layers is investigated. [•] The critical Rayleigh number for the transition is obtained. [•] Features of unsteady coupled thermal boundary layers are described. [•] Dependence on the Rayleigh number of frequency and speed of waves is quantified. [Copyright &y& Elsevier]

Abstract: A scaling analysis is performed for the transient boundary layer established adjacent to an inclined flat plate following a ramp cooling boundary condition. The imposed wall temperature decreases linearly up to a specific value over a specific time. It is revealed that if the ramp time is sufficiently large then the boundary layer reaches quasi-steady mode before the growth of the temperature is finished. However, if the ramp time is shorter then the steady state of the boundary layer may be reached after the growth of the temperature is completed. In this case, the ultimate steady state is the same as if the start up had been instantaneous. Note that the cold boundary layer adjacent to the plate is potentially unstable to Rayleigh–Bénard instability if the Rayleigh number exceeds a certain critical value for this cooling case. The onset of instability may set in at different stages of the boundary layer development. A proper identification of the time when the instability may set in is discussed. A numerical verification of the time for the onset of instability is presented in this study. Different flow regimes based on the stability of the boundary layer have also been discussed with numerical results. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The transition to a periodic flow induced by a thin fin on the sidewall of a differentially heated cavity is numerically investigated. The numerical results are compared with a previously reported experiment. It is demonstrated that the transient flow obtained numerically shows features consistent with the experimental flow. Based on the present numerical results, the temporal development and spatial structures of the thermal flow around the fin are described, and the separation of the thermal flow above the fin is discussed. It is found that the presence of the fin changes the flow regime and results in the transition of the thermal flow to a periodic flow. The present numerical results also indicate that the unstable temperature configuration above the fin results in intermittent plumes at the leeward side of the fin, which in turn induce strong oscillations of the downstream boundary layer flow. It is demonstrated that the oscillations of the boundary layer flow significantly enhance the heat transfer through the finned sidewall (by up to 23%). [Copyright &y& Elsevier]

Abstract: 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 (), 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. [Copyright &y& Elsevier]