Large eddy simulations of solitons colliding with intrusions

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

Mixing in lock-release gravity currents propagating up a slope

Lock-exchange gravity currents propagating up a slope are investigated by large eddy simulations, focusing on the entrainment and mixing processes occurring between the dense current and the ambient fluid. Relevant parameters, such as the aspect ratio of the initial volume of dense fluid in the lock R, the angle between the bottom boundary and the horizontal direction \u3b8 and the depth aspect ratio \u3c6, are varied. The numerical results are compared with laboratory experiments and a good agreement is found. Entrainment and mixing in a lock-release gravity current are studied using different entrainment parameters and an energy budget method. The entrainment is found to depend on both Froude, Fr, and Reynolds, Re, numbers. In addition, the dependence of both entrainment and mixing on the parameters varied is discussed. The entrainment decreases with increasing steepness of the bottom and R. Irreversible mixing is not affected by the varied parameters during the slumping phase, while during the successive phases of motion, it is found to decrease with the increase of \u3b8 and R. Low entrainment and mixing occur for \u3c6 <

Long term methylphenidate exposure and growth in children and adolescents with ADHD. A systematic review and meta-analysis

BACKGROUND: Methylphenidate (MPH) is an efficacious treatment for ADHD but concerns have been raised about potential adverse effects of extended treatment on growth.OBJECTIVES: To systematically review the literature, up to December 2018, conducting a meta-analysis of association of long-term (&gt; six months) MPH exposure with height, weight and timing of puberty.RESULTS: Eighteen studies (ADHD n = 4868) were included in the meta-analysis. MPH was associated with consistent statistically significant pre-post difference for both height (SMD = 0.27, 95% CI 0.16-0.38, p &lt; 0.0001) and weight (SMD = 0.33, 95% CI 0.22-0.44, p &lt; 0.0001) Z scores, with prominent impact on weight during the first 12 months and on height within the first 24-30 months. No significant effects of dose, formulation, age and drug-naïve condition as clinical moderators were found. Data on timing of puberty are currently limited.CONCLUSIONS: Long-term treatment with MPH can result in reduction in height and weight. However, effect sizes are small with possible minimal clinical impact. Long-term prospective studies may help to clarify the underlying biological drivers and specific mediators and moderators.</p

Bed shear stress estimation for gravity currents performed in laboratory

Gravity currents are caused by density differences between two fluids which may be due to temperature, dissolved substances or the presence of particles in suspension. In this study saline currents, in which the higher density is produced by dissolved salt, are reproduced in laboratory with the aim to characterize the bed shear stress. Saline currents can in fact be responsible for high erosion rates and the bed shear stress is a quantification of this erosive capacity. The dynamics of buoyancy driven flows are complex and the effect of the initial density gravity current on the bed shear stress is not explored yet. The results herein showed confirm the importance of detailed velocity profile measurements for the determination of the friction velocity which is a key parameter for the currents propagation and for characterizing the momentum and mass exchanges between the current and the bed. The spatial evolution of the bed shear stress caused by the passage of a gravity current is here estimated using the logarithmic velocity profile method for, as a first attempt, a value of the von Kármán constant of k 0.405. The use of this constant is then verified and discussed

Mixing induced in a dense current flowing down a sloping bottom in a rotating fluid

A density driven current was generated in the laboratory by releasing dense fluid over a sloping bottom in a rotating freshwater system. Over a wide range of parameter values, the following four flow types were found: laminar, wave, turbulent and eddy regime. The amount of mixing between the dense and the ambient fluids was measured and its dependence on the Froude number and on the distance downslope was determined for increasing values of the Reynolds number. Mixing increased significantly when passing from the laminar to the wave regime; i.e. with increasing Froude number. We believe that mixing between the dense salty water and the lighter fresh water was caused by breaking waves. We quantified the amount of mixing observed and estimated the value of the entrainment velocity at the interface between the dense fluid and the fresh overlying fluid. The results have been compared with previous laboratory experiments which presented the classic turbulent entrainment behavior and observational estimates of the Mediterranean and Denmark Strait overflow

Scour due to a horizontal turbulent jet: numerical and experimental investigation

In this paper both numerical and experimental investigations of local scour downstream of a sill followed by a rigid apron are presented. Nine laboratory experiments were carried out in clear water scour conditions, with different values of discharge. At the end of each run, velocity measurements both on the apron and on the scour hole were performed by ultrasonic Doppler velocimetry. A mathematical-numerical model was developed, simulating local scour downstream of a sill followed by an apron. The model uses information related both to the measured velocity fields and to the physical and mechanical properties of the sand constituting the mobile bed. The mathematical structure of the model consists of a second order partial differential parabolic equation whose unknown is the shape of the mobile bed. The numerical integration of this nonlinear equation, with suitable boundary conditions, is in agreement with the measured scour profiles at the end of the run. Upon comparing experimental and numerical data, a similar temporal evolution of the maximum scour depth is observed