Vortex dynamics under pulsatile flow in axisymmetric constricted tubes

An improved understanding of how vortices develop and propagate under pulsatile flow can shed important light on the mixing and transport processes including the transition to turbulent regime occurring in such systems. For example, the characterization of pulsatile flows in obstructed artery models serves to encourage research into flow-induced phenomena associated with changes in morphology, blood viscosity, wall elasticity and flow rate. In this work, an axisymmetric rigid model was used to study the behaviour of the flow pattern with varying constriction degree ($d_0$), mean Reynolds number ($\bar{Re}$) and Womersley number ($\alpha$). Velocity fields were acquired experimentally using Digital Particle Image Velocimetry and generated numerically. For the acquisition of data, $\bar{Re}$ was varied from 385 to 2044, $d_0$ was 1.0 cm and 1.6 cm, and $\alpha$ was varied from 17 to 33 in the experiments and from 24 to 50 in the numerical simulations. Results for the considered Reynolds number, showed that the flow pattern consisted of two main structures: a central jet around the tube axis and a recirculation zone adjacent to the inner wall of the tube, where vortices shed. Using the vorticity fields, the trajectory of vortices was tracked and their displacement over their lifetime calculated. The analysis led to a scaling law equation for the maximum vortex displacement as a function of a dimensionless variable dependent on the system parameters Re and $\alpha$

Lagrangian mixing of pulsatile flows in constricted tubes

In this work several lagrangian methods were used to analyze the mixing processes in an experimental model of a constricted artery under a pulsatile flow. Upstream Reynolds number $Re$ was changed between 1187 and 1999, while the pulsatile period $T$ was kept fixed at 0.96s. Velocity fields were acquired using Digital Particle Image Velocimetry (DPIV) for a region of interest (ROI) located downstream of the constriction. The flow is composed of a central jet and a recirculation region near the wall where vortex forms and sheds. To study the mixing processes, finite time Lyapunov exponents (FTLE) fields and concentration maps were computed. Two lagrangian coherent structures (LCS) responsible for mixing and transporting fluid were found from FTLE ridges. A first LCS delimits the trailing edge of the vortex, separating the flow that enters the ROI between successive periods. A second LCS delimits the leading edge of the vortex. This LCS concentrates the highest particle agglomeration, as verified by the concentration maps. Moreover, from the particle residence time maps (RT) the probability for a fluid particle of leaving the ROI before one cycle was measured. As $Re$ increases, the probability of leaving the ROI increases from 0.6 to 0.95. Final position maps $r{_f}$ were introduced to evaluate the flow mixing between different subregions of the ROI. These maps allowed us to compute an exchange index between subregions, $\bar{\mathrm{EI}}$, which shows the main region responsible for the mixing increase with $Re$. Finally by integrating the results of the different lagrangian methods (FTLE, Concentration maps, RT and $r_f$ maps), a comprehensive description of the mixing and transport of the flow was provided

Elucidating coherent structures, transport barriers and entrainment in turbulent fountains in stratified media

We analyse the flow organization of turbulent fountains in stratified media under different conditions, using three-dimensional finite-time Lyapunov exponents. The dominant Lagrangian coherent structures responsible for the transport barriers in three different configurations suggest a self-similarity behaviour. After proposing a criterion for delimiting the boundary surface of the uprising fountain, we quantify the entrainment and re-entrainment rates under fully developed flow conditions using the proper coefficients. Finally, our analysis was applied to the Selective Inverted Sink, a technological application of turbulent fountains, identifying turbulence as the primary mechanism favouring the device's efficiency

Wave Farms Integration in a 100% renewable isolated small power system -frequency stability and grid compliance analysis

The increase in the penetration of variable renewable energies (RE) is having a negative impact on the frequency of electrical grids due to effects such as the reduction of inertia when RE is connected by power electronics systems. In addition, the deterioration of the electrical frequency causes that the frequency containment mechanisms to act more frequently, which increases the wear and tear on the conventional generation plants that supply them. Within variable renewable energies is wave energy, which adds to the variability (or lack of manageability) of other renewable energies (e.g. wind, solar) an oscillatory component that translates into an oscillating power injected in the electrical grid. This power oscillation produces oscillations in electrical frequency that can take an electrical system outside of normal operating limits. The effect of frequency oscillations is magnified in weak electrical grids such as the insular system in which the study of this article is proposed, the island of El Hierro. A specific tool is used to evaluate the electrical frequency of the system, also evaluating the wear and tear of conventional generation plants. The limits in the penetration of the wave generation are analyzed for this specific power system