This paper investigates the dynamics of lock-release gravity currents propagating upslope by laboratory experiments and shallow-water simulations. Both the interface between the dense and the ambient fluid and the instantaneous velocity field were measured by image analysis. Different runs were carried out by varying the initial density of the lock fluid and the bed upslope. As a gravity current moves upslope, the dense layer becomes thinner, and an accumulation region of dense fluid in the initial part of the tank occurs. The current speed decreases as the bed upslope increases, and for the highest up sloping angles, the gravity current stops before reaching the end of the tank. A new two-layer shallow-water model is developed and benchmarked against laboratory experiments. The present model accounts for the mixing between the two layers, the free surface, and the space-time variations of the density. The effect of the horizontal density gradient in the simulation of gravity currents is investigated by comparing the numerical results of both the present model and the model proposed by Adduce et al. [“Gravity currents produced by lock-exchange: Experiments and simulations with a two layer shallow-water model with entrainment,” J. Hydraul. Eng. 138, 111-121 (2012)] with laboratory measurements. The comparison shows that the present model reproduces both the current shape and the front position better than the Adduce et al. model, in particular, for gravity currents flowing up a slope. For these currents, the presence of a backflow near the lock is shown by the analysis of the streamwise depth-averaged velocity predicted by the present model and the velocity measured by particle image velocimetry as well.

The dynamics of unconfined gravity currents generated by a lock-exchange with different lock widths is considered. The gravity currents were generated by density differences caused by a salinity gradient and were realized both numerically and experimentally. Several runs were carried out in order to study the role played by the lock width in the current dynamics. Numerical simulations were performed with a new single-layer, 2D, shallow-water model, which accounts for the dilution of the gravity current due to the entrained ambient fluid. By the comparison between experimental and numerical results, the relevant role played by the entrainment in the propagation velocity of the front emerges, mainly for the cases with a narrow lock. The effect of the lock width on the current dynamics is also highlighted: the lock chocks the flow causing a localized loss of mechanical energy and a consequent deceleration of the flow. Results show that the smaller is the lock, the larger is the loss of energy.

Droplets impinging on solid surfaces, as well as the countless varieties of the resulting possible dynamics, are found frequently in both natural and industrial environments. Among such dynamics, the contact time with the surface, the amount of deformation and the occurrence of breaking induced by geometrical singularities of the solid surface are key aspects in a wide range of applications. We report the results of an extensive experimental activity investigating the capability of liquid droplets to jump over a gap while sliding/rolling over a hydrophobic solid plane. These drops impact on the downstream sharp edge of the gap and undergo the amount of deformation that allows them to climb the edge. We ascribe this unique behaviour to the transformation of rotational momentum into linear momentum. Such conversion can take place only if the right amount of deformation occurs upon impact. Indeed, within the explored range of Weber number $\left (0.5\lesssim {We}\lesssim 40 \right )$ , we show the existence of a sub-range for which the drops show a significantly higher probability of jumping over the gap if compared to solid spheres, whose behaviour is predicted accurately by a purely ballistic and elastic impact model. We formulate a minimal energy balance in order to show that such peculiar drops are indeed the ones featuring high rotational energy. The results of this study also contribute to shed light on the debate about the amount of rotational speed characterizing liquid droplets running over hydrophobic surfaces.

Time resolved PIV encompassing moving and/or deformable objects interfering with the light source requires the employment of dynamic masking (DM). A few DM techniques have been recently developed, mainly in microfluidics and multiphase flows fields. Most of them require ad-hoc design of the experimental setup, and may spoil the accuracy of the resulting PIV analysis. A new DM technique is here presented which envisages, along with a dedicated masking algorithm, the employment of fluorescent coating to allow for accurate tracking of the object. We show results from measurements obtained through a validated PIV setup demonstrating the need to include a DM step even for objects featuring limited displacements. We compare the proposed algorithm with both a no-masking and a static masking solution. In the framework of developing low cost, flexible and accurate PIV setups, the proposed algorithm is made available through a freeware application able to generate masks to be used by an existing, freeware PIV analysis package. Graphic