Development of overturning circulation in sloping waterbodies due to surface cooling

Cooling the surface of freshwater bodies, whose temperatures are above the temperature of maximum density, can generate differential cooling between shallow and deep regions. When surface cooling occurs over a long enough period, the thermally induced cross-shore pressure gradient may drive an overturning circulation, a phenomenon called ‘thermal siphon’. However, the conditions under which this process begins are not yet fully characterised. Here, we examine the development of thermal siphons driven by a uniform loss of heat at the air–water interface in sloping, stratified basins. For a two-dimensional framework, we derive theoretical time and velocity scales associated with the transition from Rayleigh–Bénard type convection to a horizontal overturning circulation across the shallower sloping basin. This transition is characterised by a three-way horizontal momentum balance, in which the cross-shore pressure gradient balances the inertial terms before reaching a quasi-steady regime. We performed numerical and field experiments to test and show the robustness of the analytical scaling, describe the convective regimes and quantify the cross-shore transport induced by thermal siphons. Our results are relevant for understanding the nearshore fluid dynamics induced by nighttime or seasonal surface cooling in lakes and reservoirs.

Triggering Vortex Shedding for the Free Stream Flow of Nanofluids Around Bluff Objects

The free stream flow around a bluff object shows steady symmetric nature in the low Reynolds number laminar regime. However, when the Reynolds number increases to a critical value, the flow shows unsteadiness with alternate shedding of vortices. We show here numerically that the vortex shedding could be initiated for flow of a nanofluid over a bluff object even the Reynolds number is lying in the steady regime (10 ≤ Re ≤ 30 ). Cu-H2O and Ag-H2O nanofluids are used and the volume fractions of Cu and Ag nanoparticles are gradually increased. At some critical values of the volume fractions, the flow shows unsteadiness with vortex shedding. The critical solid volume fraction is estimated from the convective stability analysis following the extended Landau model. The shedding phenomenon is established through contour plots, phase diagrams and analysis of the time signals of lift coefficient. The critical volume fractions for the two different nanofluids for transition of steady to unsteady flow over circular and square shaped bluff objects are observed to decrease with increasing Reynolds number.

A Steady Regime of Volume and Heat Transports in the Eastern Arctic Ocean in the Early 21st Century

Mooring observations in the eastern Eurasian Basin of the Arctic Ocean showed that mean 2013–2018 along-slope volume and heat (calculated relative to the freezing temperature) transports in the upper 800 m were 4.8 ± 0.1 Sv (1 Sv = 106 m3/s) and 34.8 ± 0.6 TW, respectively. Volume and heat transports within the Atlantic Water (AW) layer (∼150–800 m) in 2013–2018 lacked significant temporal shifts at annual and longer time scales: averaged over the two periods of mooring deployment in 2013–2015 and 2015–2018, volume transports were 3.1 ± 0.1 Sv, while AW heat transports were 31.3 ± 1.0 TW and 34.8 ± 0.8 TW. Moreover, the reconstructed AW volume transports over longer, 2003–2018, period of time showed strong interannual variations but lacked a statistically significant trend. However, we found a weak positive trend of 0.08 ± 0.07 Sv/year in the barotropic AW volume transport estimated using dynamic ocean topography (DOT) measurements in 2003–2014 – the longest period spanned by the DOT dataset. Vertical coherence of 2013–2018 transports in the halocline (70–140 m) and AW (∼150–800 m) layers was high, suggesting the essential role of the barotropic forcing in constraining along-slope transports. Quantitative estimates of transports and their variability discussed in this study help identify the role of atlantification in critical changes of the eastern Arctic Ocean.

Horizontal convection in a rectangular enclosure driven by a linear temperature profile

The horizontal convection in a square enclosure driven by a linear temperature profile along the bottom boundary is investigated numerically by using a finite difference method. The Prandtl number is fixed at 4.38, and the Rayleigh number Ra ranges from 107 to 1011. The convective flow is steady at a relatively low Rayleigh number, and no thermal plume is observed, whereas it transits to be unsteady when the Rayleigh number increases beyond the critical value. The scaling law for the Nusselt number Nu changes from Rossby’s scaling Nu ∼ Ra1/5 in a steady regime to Nu ∼ Ra1/4 in an unsteady regime, which agrees well with the theoretically predicted results. Accordingly, the Reynolds number Re scaling varies from Re ∼ Ra3/11 to Re ∼ Ra2/5. The investigation on the mean flows shows that the thermal and kinetic boundary layer thickness and the mean temperature in the bulk zone decrease with the increasing Ra. The intensity of fluctuating velocity increases with the increasing Ra.

Development of Attached Cavitation at Very Low Reynolds Numbers from Partial to Super-Cavitation

The present study focuses on the inception, the growth, and the potential unsteady dynamics of attached vapor cavities into laminar separation bubbles. A viscous silicon oil has been used in a Venturi geometry to explore the flow for Reynolds numbers ranging from Re=800 to Re=2000. Special care has been taken to extract the maximum amount of dissolved air. At the lowest Reynolds numbers the cavities are steady and grow regularly with decreasing ambient pressure. A transition takes place between Re=1200 and Re=1400 for which different dynamical regimes are identified: a steady regime for tiny cavities, a periodical regime of attached cavity shrinking characterized by a very small Strouhal number for cavities of intermediate sizes, the bursting of aperiodical cavitational vortices which further lower the pressure, and finally steady super-cavitating sheets observed at the lowest of pressures. The growth of the cavity with the decrease of the cavitation number also becomes steeper. This scenario is then well established and similar for Reynolds numbers between Re=1400 and Re=2000.

Identification of flow states around three staggered square cylinders at two symmetrical arrangements by a numerical investigation

The fluid flow over three staggered square cylinders at two symmetrical arrangements has been numerically investigated in this study. The numerical calculations are carried out for several values of the Reynolds number (Re) ranging from 1 to 180. The results are presented in the form of vorticity contours and temporal histories of drag and lift coefficients. Furthermore, the physical parameters, namely, the average drag and lift coefficients and Strouhal number are presented as a function of Re. Two different states of flow are found in this work by systematically varying Re: steady and unsteady states. The transition to unsteady state regime is exhibited via Hopf bifurcation first in the second configuration followed consequently by the first one with critical Reynolds number of Re[Formula: see text] and Re[Formula: see text], respectively. It is observed that the bifurcation point of the steady regime to the unsteady one is very much influenced by the change in the geometry of the obstacle. The unsteady periodic wake is characterized by the Strouhal number, which varies with the Reynolds number and the obstacle geometry. Hence, the values of vortex shedding frequencies are estimated for both the considered configurations. Computations obtained also reveal that the spacing in the wake leads to reducing the pressure and enhancing the fluid flow velocity for both arrangements by monotonically strengthening the Reynolds number value. Furthermore, the drag and lift coefficients are determined, which allowed determining; the best configuration in terms of both lift and drag. It is observed that the drag force is dependent on the obstacle geometry and strengthens while lowering the Reynolds number. On the other hand, an opposite trend of the lift drag evolutions is observed for both configurations and considerably affected by the arrangements shape.

Relationship between rainfall and water table in a coastal aquifer: the case study of Castelporziano presidential estate

This study is focused on the analysis of seasonal and annual variability in groundwater levels of the coastal aquifer of Castelporziano Presidential Estate, a protected area of 59 Km2 located in the periphery of Rome. A comparison with the local trends of rainfall at “Castello” gauging station at different time scales (monthly, seasonal and annual) has been carried out. The results highlight differences between the coastal area and eastern and northern sector of the Estate. Indeed, the seasonal effect due to local meteoric recharge is direct and regular during the year in the coastal area in respect to the eastern and northern sectors of the Estate. Moreover, annual steady regime and multi-year trend of groundwater levels suggest the contribution from the adjacent volcanic aquifer of Albani Hills. In the latter case, the regional circulation of groundwater is affected by the effects of intense withdrawals. The maintenance of the monitoring network will allow to define the flow paths of the groundwater that characterize the coastal aquifer of Castelporziano.

Study of an inclined interface of contact using lattice Boltzmann method

The lattice Boltzmann method and the particle image model are adopted to study a heat transfer problem with thermal contact resistance. In this paper, a new study involving an inclined interface of contact between two media is introduced in order to evaluate a 2-D heat transfer in the steady regime. A case of study and numerical results are provided to support this configuration. The obtained results show the effect of the thermal contact resistance on the heat transfer, as well as the temperature distribution on the two contacting media.

Experimental investigation of flapping mechanism of the black-headed gull in forward flight

Many developments in aerospace science have originated from nature. One of these developments has been obtained through inspirations from flying locomotion. The aim of this study is to simulate the flapping mechanism of the black-headed gull in forward flight. The wing of the black-headed gull is characterized entirely by complex dihedral, dividing the wing into two distinct parts. Hence, a flapping mechanism with different bending deflection angles is constructed and compared with a primitive flapping mechanism. Firstly, parametric studies are conducted to assess the role of flapping frequency, velocity and bending deflection angle on the lift, thrust and power loading of the membrane flexible wing at 10 ° angle of attack. Secondly, dimensional analysis is used to establish the similarity between the real gull and the constructed mechanism. Superiority of the bending deflection mechanism is concluded in forward flight against simple flapping wing in terms of aerodynamic forces as well as power loading parameter. It is found that although the aerodynamic coefficients decrease with increase in advance ratio, the best power loading of the black-headed gull is obtained between advance ratio of 2 and 3, in the gull's aerodynamically quasi-steady regime.

Sliding instability of draining fluid films

The aim of this paper is to show that the spontaneous sliding of drops forming from an interfacial instability on the surface of a wall-bounded fluid film is caused by a symmetry-breaking secondary instability. As an example, we consider a water film suspended from a ceiling that drains into drops due to the Rayleigh–Taylor instability. Loss of symmetry is observed after the film has attained a quasi-steady state, following the buckling of the thin residual film separating two drops, whereby two extremely thin secondary troughs are generated. Instability emanates from these secondary troughs, which are very sensitive to surface curvature perturbations because drainage there is dominated by capillary pressure gradients. We have performed two types of linear stability analysis. Firstly, applying the frozen-time approximation to the quasi-steady base state and assuming exponential temporal growth, we have identified a single, asymmetric, unstable eigenmode, constituting a concerted sliding motion of the large drops and secondary troughs. Secondly, applying transient stability analysis to the time-dependent base state, we have found that the latter is unstable at all times after the residual film has buckled, and that localized pulses at the secondary troughs are most effective in triggering the aforementioned sliding eigenmode. The onset of sliding is controlled by the level of ambient noise, but, in the range studied, always occurs in the quasi-steady regime of the base state. The sliding instability is also observed in a very thin gas film underneath a liquid layer, which we have checked for physical properties encountered underneath Leidenfrost drops. In contrast, adding Marangoni stresses to the problem substantially modifies the draining mechanism and can suppress the sliding instability.