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The dynamics of lock-release Intrusive Gravity Currents (IGCs) generating Internal Solitary Waves (ISWs) are investigated by three-dimensional large eddy simulations. We set the numerical, laboratory-scale domain in order to release a uniform fluid in multi-layer, stratified ambient, exciting pycnocline displacements. By adopting different initial settings, we analyzed the influence of the ambient stratification on both IGCs and ISWs features. We present the main flow dynamics and the time evolution of IGC and ISW front and trough positions, respectively. During the simulations, the ISW is allowed to reach the vertical wall at the end of the domain, and it undergoes reflection. We then analyzed the interaction between the IGC and the reflected ISW: the wave is observed to accelerate as it is pushed upwards by the intrusion, which, in turns, flows below the ISW, decelerating. By analyzing instantaneous velocity fields and flow rates, we found that during this interaction, the ISW increases its celerity in response of the reduced area available for its propagation, partially occupied by the intrusion, and because the velocity field in the IGC interface surroundings acts to facilitate the ISW passage.

Free-surface effects induced by internal solitary waves (ISWs) propagating through a background shear current are theoretically investigated in a shallow water framework. Following the Hamiltonian approach, we implement a mathematical formulation valid for a free surface, two-layer stratified system in the presence of mobile layers. Associated to the undisturbed condition characterized by the background fluid at rest, solitons with both positive and negative polarities are considered. To reproduce the typical oceanic conditions, we focus on theoretical predictions for a density ratio set to 0.99. Although under Boussinesq conditions a rigid-lid at the top of the theoretical domain is considered a good approximation, our analysis shows that main ISWs features may change when the fluid system, forced by shear currents, is modeled with a free surface. Signs assumed by three dimensionless quantities, i.e., the deformation parameter, the background velocity, and the undisturbed amplitude, allowed us to uniquely predict how the undisturbed solitons modify their interfacial profiles and change their celerity, in response to the background forcing. Our results show that waves celerity predicted by the rigid-lid model can be lower than the one resulting from the Hamiltonian formulation, although this never occurs in the absence of mobile layers. Theoretical predictions reveal that shearing current can induce deviations from the standard ISWs free-surface profile. We discuss the role of nonhydrostatic pressures in inducing free-surface short disturbances, in the form of undulated or multihumped profiles. The typical phase-opposition between interfacial and surface displacements can be lost as the free-surface displacements assume multihumped configurations.

Nonlinear internal solitary waves (ISWs) propagating through a two-layer stratified system, in the presence of a shear background current, are theoretically investigated. We implement a new version of the Miyata-Choi-Camassa model with mobile layers (MCC-ML), by considering an asymptotic, uniform velocity distribution for each layer. To investigate the typical geophysical flow conditions observed in the coastal oceans, we focused on theoretical predictions for a density ratio between the two layers set to 0.99. A rigid-lid at the top of the theoretical domain is considered since it represents a good approximation under the Boussinesq condition. By varying the ratio of the undisturbed layer thickness from 0.1 to 10, we considered ISWs with both positive and negative polarities, when the background fluid is at rest. For increasing velocity differences between the two layers, ISWs tend to broaden (steepen) when the background velocities assume the same (opposite) direction of those induced by the wave. We show that the polarity conversion can be easily predicted since it directly depends on both stratification features and ambient velocities. The shear current affects also the wave celerity: for increasing background shear, upstream-propagating solitons reach a critical condition for which the wave celerity is equal to zero. We found that this occurrence is associated with a well-defined value of the wave amplitude. For even larger background shears, the waves are observed to change their direction of propagation. By linear analysis, we finally obtained the limiting background shear current for which the MCC-ML model does not provide any solution.

Public hospitals that are directly managed by government perform poorly in many developing countries. Approaches to improving them through internal managerial reforms have failed, and effective alternatives are much needed. Policymakers are considering reforms through public-private partnerships (PPPs)--a promising but so far unevaluated approach. We present results of a successful reform in Sao Paulo, Brazil. The PPP model gave facility managers latitude to manage human resources--a factor critical to success. Given the prevalence of direct management of public hospitals in developing countries, the Sao Paulo experience has implications for policymakers seeking to improve hospital performance.

We present experimental results on the formation of streamwise asymmetric, migrating bedforms resulting from the transition of successive surface solitary waves (SSW) interacting with an initially flat sandy bottom. We analyze seven cases, with differing ratios between SSW height and water depth. For each experiment, we generated 400 SSWs having the same features, investigating their effects on the bed, in terms of near-bed velocity and erosional patterns. Our work proves that SSWs can generate asymmetric bedforms, similar in shape to dunes. The triggering process occurs in the region where the reverse flow induced by bottom boundary layer separation produces local erosion. The action of successive SSWs over the newly formed discontinuities gives rise to bedforms. Differently from steady flows, dune load decreases for larger wave-induced bottom shear stress.

We present a semi-analytical model for predicting the breaking location of internal solitary waves (ISWs) over a sloping seabed. Our conceptual model is based on laboratory experiments, performed in a wave tank, that reproduce the ISW breaking mechanisms and show how the steepening of the trailing edge leads to verticalization of the wave profile during the shoaling phase. We derive the location of ISWs breaking, that is, the wave verticalization point, through two-layer, interfacial theoretical models and conservation of wave mass. We apply our model to the case of tidally forced ISWs that are generated in the Strait of Messina (Central Mediterranean Sea), where northward traveling ISWs are expected to refract and break over the frontal slope of Capo Vaticano. Our application is then assessed through numerical investigations, which allow to consider realistic field conditions in terms of water column stratification and geometrical setting. Our results, and the expected ISW-induced bed shear stress, suggest a link between the predicted breaking locations and the occurrence of sediment resuspension over that specific portion of the slope.

We investigate the interaction of strongly non linear internal solitary waves (ISWs) with boundaries having different slopes by means of high-resolution 3D large Eddy Simulations (LES). Releasing a volume of fresh water into a stratified ambient fluid, three different breaking mechanisms are produced: plunging, collapsing and surging breakers. The different shoaling dynamics affect the ISW evolution over the sloping boundary, inducing different effects on the bottom. In order to investigate the effects of the ISW breaking on the inclined surface, we calculate the bed shear stress and estimate the local flux of sediments entrained from the bed. We analyze the relationship between the breaking criteria and the related effects on the sloping surface. Although plunging breakers are expected to induce significant effects within the fluid, causing larger amount of mixing and fluid entrainment, the effects on the bottom are totally opposite. The collapsing breaker mechanism, indeed, generates boundary layer separation, which in turn induces whirling instabilities. Results show that the ISW interaction with the inclined surface occurs in its close proximity for collapsing breaker mechanism, which explains why the largest bed shear stresses and sediment re-suspension are predicted in the simulation where a collapsing breaker mechanism is observed.

We experimentally investigate the generation of internal solitary waves (ISWs or waves) by intrusions (IGCs), and their interaction. We estimate the induced mixing in terms of increase of the initial pycnocline thickness. The release of a uniform fluid of intermediate density into a two-layer stratified ambient fluid leads to the formation of an IGC, which propagates downstream. The intrusion squeezes the stratified fluid it interacts with, generating a train of ISWs of decreasing size and celerity. The intrusion, dissipating, slows down, allowing the waves to gradually separate from it. The uniform sloping boundary placed downstream induce ISWs partial breaking and upstream reflection. We vary the boundary slope and its position, to induce different waves breaking mechanisms for different intrusion dissipation state. Interacting with the reflected ISWs, the IGC deepens following the pycnocline displacement, while the waves increase their celerity, since they propagate trough a narrower vertical cross section.

We investigated theoretically and experimentally internal solitary waves (ISWs) in a two-layer fluid system with a top free surface. Laboratory experiments are performed by lock-release, under Boussinesq and non-Boussinesq conditions. Experimental results are compared with those obtained by the analytical solution of the Korteweg–de Vries (KdV) weakly nonlinear equation and by the strongly nonlinear Miyata-Choi-Camassa (MCC) model. We analyze the initial conditions which allow to find soliton solutions for both rigid-lid (-RL) and free-surface (-FS) boundary conditions. For the MCC-FS model, we employ a new mathematical procedure to derive the ISW-induced free surface displacement. The density structure strongly affects the elevation of the free surface predicted by the MCC-FS model. The free surface maximum displacement increases mostly with the density difference, assuming non-negligible values also for smaller interfacial amplitudes. Larger displacements occur for thinner upper layer thickness. The MCC-FS model gives the best prediction in terms of both internal waves geometric/kinematic features and surface displacements. The existence of a free surface allows the ISW to transfer part of its energy to the free surface: the wave celerity assumes lower values with respect to ISW speed resulting from the MCC-RL model. For ISWs with a very large amplitude, this behavior tends to fade, and the MCC-RL and the MCC-FS model predict approximately the same celerity and interfacial geometric features. For small-amplitude waves also, the predictions of the KdV-RL equation are consistent with experimental results. Thus, ISWs with an intermediate amplitude should be modeled taking into account a free top surface as the boundary condition.

Internal solitary waves (ISWs) propagating in a two layer stratified fluid system are studied by laboratory experiments through the standard lock-release method. We investigated the generation, propagation, and breaking phases of large amplitude internal solitary waves of depression, propagating horizontally in a wave tank. ISWs main features depend on the geometrical parameters that define the initial experimental setting. Relations between ISWs geometric and kinematic features and the initial setting parameters are analyzed and empirical relations are developed. The approach of the ISWs towards a uniform slope is investigated. Depending on both wave properties and slopes values, different physical processes characterize the shoaling phase, causing different breaker type. Following a qualitatively analysis of the different breaking mechanisms, collapsing-plunging breakers show the larger contribution in terms of mixing. The characteristics of the breaking events affecting internal wave packets propagating towards the North of the Messina Strait (Mediterranean Sea) are discussed. Additional laboratory experiments are performed to investigate how the pycnocline thickness is affected by the breaking of plunging ISWs. The consequent increase of the pycnocline thickness results to be nonlinearly related with the Iribarren number.