Dynamic boundary conditions (DBC) for solid surfaces are standard in the weakly compressible smoothed particle hydrodynamics (SPH) code DualSPHysics. A stationary solid is simply represented by fixed particles with pressure from the equation of state. Boundaries are easy to set up and computations are relatively stable and efficient, providing robust numerical simulation for complex geometries. However, a small unphysical gap between the fluid and solid boundaries can form, decreasing the accuracy of pressures measured on the boundary. A method is presented where the density of solid particles is obtained from ghost positions within the fluid domain by linear extrapolation. With this approach, the gap between fluid and boundary is reduced and pressures in still water converge to hydrostatic, including the case of a bed with a sharp corner. The violent free-surface cases of a sloshing tank and dam break impact on an obstacle show pressures measured directly on solid surfaces in close agreement with experiments. The complex 3-D flow in a fish pass, with baffles to divert the flow, is simulated showing close agreement with measured water levels with weirs open and gates closed, but less close with gates open and weirs closed. This indicates the method is suitable for rapidly varying free-surface flows, but development for complex turbulent flows is necessary. The code with the modified dynamic boundary condition (mDBC) is available in DualSPHysics to run on CPUs or GPUs.

New analytical models are introduced to describe the motion of a Herschel–Bulkley fluid slumping under gravity in a narrow fracture and in a porous medium. A useful self-similar solution can be derived for a fluid injection rate that scales as time $t$ ; an expansion technique is adopted for a generic injection rate that is power law in time. Experiments in a Hele-Shaw cell and in a narrow channel filled with glass ballotini confirm the theoretical model within the experimental uncertainty.

A series of measurements in a flume with a particle-tracking system in three dimensions applied to breaking waves is used to analyse the structure of turbulence with a full set of variables that usually are available only in numerical simulations. After extracting turbulence, in addition to the standard analysis aiming to quantify the fluxes, i.e., the time-average and the phase-average levels of turbulence and vorticity (details are given in two former papers), a more in-depth description of the structure of turbulence Reynolds stress tensor is given, focussing on the invariants evolution in time and in the vertical. A relation between the components of the Reynolds stress tensor and of the dissipation tensor is depicted. This relation is finalised to possible models of turbulence in breaking waves.

The wide use of wind tunnels, as a tool to measure the flow properties and the flow effects on different structures/ecosystems, makes it necessary to guarantee the correct functioning of the facility and to carry out a continuous monitoring. The aim of this work is (i) to check the homogeneity and quality of the air flow in an open-circuit boundary layer wind tunnel, (ii) to provide data and ideas that could help other researchers to improve similar tunnels, and (iii) to compare some of the results with the behavior of a classic closed-circuit wind tunnel. Experiments are carried out working with a constant reference velocity, using a hot cross wire anemometry system to obtain high resolution measurements, in the entrance and test sections for different longitudinal and cross planes. The results concern the characteristics of the mean and turbulent flow. In this manuscript, we report the analysis of the turbulence production, the Reynolds stresses, the vertical velocity skewness, the vorticity, and the spectral properties, and a quadrant decomposition is also performed. Finally, comparing the results with respect to a closed-circuit wind tunnel, it is shown that the turbulence intensity is generally higher and the mean flow is more homogeneous in the present open-circuit wind tunnel. Moreover, the Reynolds number is similar in both tunnels, which indicates that both tunnels are mechanically similar.