Turbulence generated by an oscillating grid in a two-layer stably stratified system is a classical flow utilised to study various aspects of turbulence in presence of stratification without mean shear.This flow evolves in a quasi-steady state, in which the layer thickness and density difference evolves in a quasi-steady manner due to the large separation of timescales between the turbulence and the setup. We present an extension of the classical setup that enables full steady state conditions and in which the entrainment velocity can be prescribed separately from the Richardson number.We develop a theoretical box-model and show that the model is in good agreement with the experiments. The model allows to predict the transient response of the system for a variety of initial conditions and the imposed steady state. The new setup is necessary to obtain the steady position of the density interface, for example when using advanced optical techniques to measure the small-scale features of turbulence near the interface.

Non-intrusive quantitative fluid density measurement methods are essential in the stratified flow experiments. Digital imaging leads to synthetic schlieren methods in which the variations of the index of refraction are reconstructed computationally. In this study, an extension to one of these methods, called background oriented schlieren, is proposed. The extension enables an accurate reconstruction of the density field in stratified liquid experiments. Typically, the experiments are performed by the light source, background pattern, and the camera positioned on the opposite sides of a transparent vessel. The multimedia imaging through air-glass-water-glass-air leads to an additional aberration that destroys the reconstruction. A two-step calibration and image remapping transform are the key components that correct the images through the stratified media and provide a non-intrusive full-field density measurements of transparent liquids.

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∕ .

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We present a laboratory experiment of the growth of a turbulent patch in a stably stratified fluid, due to a localized source of turbulence, generated by an oscillating grid. Synchronized and overlapping particle image velocimetry and planar laser induced fluorescence measurements have been conducted capturing the evolution of the patch through its initial growth until it reached a maximum size, followed by its collapse. The simultaneous measurements of density and velocity fields allow for a direct quantification of the degree of mixing within the patch, the propagation speed of the turbulent/non-turbulent interface and its thickness. The velocity measurements indicate significant non-equilibrium effects inside the patch which are not consistent with the classical used grid-action model. A local analysis of the turbulent/non-turbulent interface provides direct measurements of the entrainment velocity w e as compared to the local vertical velocity and turbulent intensity at the proximity of the interface. It is found that the entrainment rate E is constrained in the range of 0 ÷ 0.1 and that the local, gradient Richardson number at the interface is O(100).Finally, we show that the mean flow is responsible for the patch collapse.