Transient stratification force on particles crossing a density interface

Abstract

We perform a series of experiments to measure Lagrangian trajectories of settling and rising particles as they traverse a density interface of thickness ℎ using an index-matched water-saltethanol solution. The experiments confirm the substantial deceleration that particles experience as a result of the additional force exerted on the particle due to the sudden change in density. This stratification force is calculated from the measurement data for all particle trajectories. In absence of suitable parameterisations in the literature, a simple phenomenological model is developed which relies on parameterisations of the effective wake volume and recovery time scale. The model accurately predicts the particle trajectories obtained in our experiments and those of Srdić-Mitrović, Mohamed and Fernando (1999). Furthermore, the model demonstrates that the problem depends on four key parameters, namely the entrance Reynolds number 1 , entrance Froude number , particle to fluid density ratio ∕ , and relative interface thickness ℎ∕ .

Dynamics of a reactive spherical particle falling in a linearly stratified fluid

“…For all radii, however, a jump of velocity occurs when the spheres reach the bottom of the stratified layer. These observations agree with the recent study of Verso et al [14], which relates velocity changes to the crossing of a relatively sharp density interface. In our experiment, the transition is always sharp at the bottom of the stratified layer (see Fig.…”

“…Experimental studies have reported a drag increase for inert particles falling through a sharp density gradient [9][10][11][12][13][14]. In a linearly stratified fluid, Yick et al [15] showed that the drag coefficient can be enhanced by a factor 3 compared to the one in a homogeneous fluid for small Reynolds number (Re = 2aU ν with U the falling velocity, a the sphere radius and ν the fluid kinematic viscosity).…”

Critical density triplets for the arrestment of a sphere falling in a sharply stratified fluid

“…The second parameter is the relative thickness of the fluid density transition layer h 2a which is characterized by the ratio of the layer thickness h to the particle diameter 2a. Several studies have explored gravitational particle settling in sharply stratified fluids [24,27] but reported no bounce phenomenon. In those studies the parameter, h 2a 1, takes value 30 and 10 respectively, whereas in the work of Abaid et al this parameter was much smaller, taking value less than 2.…”