A review of experiments on stationary bluff-body wakes

Experimental studies dealing with the wake of isolated stationary bluff-bodies are reviewed. After briefly recalling the pioneering works in this domain, the paper focuses on recent research conducted with the latest experimental methods and techniques. The review encompasses a range of topics, including, the effects of bluff-body geometry (non-circular cross sections and nonuniformity in spanwise direction), steady and unsteady (periodic and non-periodic) inflow conditions; surface proximity (rigid wall, confinement and water free surface) and non-Newtonian fluids. Focus is brought to the flow physics of the wakes, including especially the complex threedimensional and oscillatory behaviours induced by the periodic vortex shedding phenomenon. The paper aims to offer a critical and systematic review of new knowledge and findings on the subject area, as well as emerging? and the most frequently adopted experimental techniques. The review also helps identifying knowledge gaps in the literature that need to be addressed in future investigations

The effect of contact angles and capillary dimensions on the burst frequency of super hydrophilic and hydrophilic centrifugal microfluidic platforms, a CFD study.

This paper employs the volume of fluid (VOF) method to numerically investigate the effect of the width, height, and contact angles on burst frequencies of super hydrophilic and hydrophilic capillary valves in centrifugal microfluidic systems. Existing experimental results in the literature have been used to validate the implementation of the numerical method. The performance of capillary valves in the rectangular and the circular microfluidic structures on super hydrophilic centrifugal microfluidic platforms is studied. The numerical results are also compared with the existing theoretical models and the differences are discussed. Our experimental and computed results show a minimum burst frequency occurring at square capillaries and this result is useful for designing and developing more sophisticated networks of capillary valves. It also predicts that in super hydrophilic microfluidics, the fluid leaks consistently from the capillary valve at low pressures which can disrupt the biomedical procedures in centrifugal microfluidic platforms

Fluid to fluid scaling of heat transfer with supercritical pressure fluids: recent considerations and future perspectives

The contribution reports on the ongoing work at the University of Pisa about fluid to fluid scaling with supercritical pressure fluids. The first studies in this regard started in 2006 when relevant non dimensional quantities were introduced for analysing flow instabilities, supposing the existence of similarities between the exceeding of the pseudo-critical temperature in supercritical fluids and the boiling threshold in subcritical ones. Since the beginning interesting features where noticed, e.g. the fact that the non-dimensional density trend ( ) seems to be a function of the dimensionless enthalpy ( ) only, regardless of the supercritical pressure and, in general, of the fluid. After several attempts, a methodology for performing fluid-to-fluid scaling with supercritical fluids was proposed in the frame of the latest work Pucciarelli and Ambrosini (2016) where the proposed technique was validated via RANS calculations. The very feature of the cited methodology is the abandonment of the search of geometrical correspondence in terms of x/D in favour of dimensionless bulk enthalpy thus taking into account the different behaviours and development lengths for the thermal field of the various fluids. The present paper reports further analyses performed adopting more reliable simulation techniques such as LES and DNS. Two operating conditions adopting carbon dioxide were selected as reference case and reproduced with supercritical water: several attempts were performed for the LES case while for the DNS application previous analyses were performed using RANS techniques in order to be sure to obtain a good scaling with the first shot. The calculations returned promising results, for the DNS application good coherence was obtained while some discrepancies were observed in the LES case; nevertheless the main purpose of the present work is try and evaluate the plausibility of the adopted strategy and no perfect matching is requested at this stage

CFD models comparative study on nanofluids subcooled flow boiling in a vertical pipe

The effects of subcooled flow boiling of nanofluids (Al2O3/water and Cu/water) through a vertical heated pipe are investigated numerically considering a number real flow factors such as uneven particle distributions and particle deposition on the wall surface. The results from the Eulerian–Eulerian two-phase model (liquid/nanofluid–gas) and those of the Eulerian–Lagrangian three-phase model (liquid–gas particles) are compared in detail. For the heat transfer coefficient prediction, the latter model gives about 6% error, whereas the Eulerian–Eulerian model gives about 12% error when it is compared with Chen’s correlation. The uneven distribution of nanoparticles concentrations, i.e., lower near the tube wall and higher at the pipe center, is well predicted by the Eulerian–Lagrangian model. On top that, for the first time, the changes on heating surface wettability induced by nanoparticles deposition is included via user define functions

A comparative study of turbulence in ramp-up and ramp-down unsteady Flows

DNS of a turbulent channel flow subjected to a step change in pressure gradient are performed to facilitate a direct comparison between ramp-up and ramp-down flows. Strong differences are found between behaviours of turbulence in the two flows. The wall shear stress in the ramp-up flow first overshoots, and then strongly undershoots the quasi-steady value in the initial stage of the excursion, before approaching the quasi-steady value. In a strongly decelerating flow, the wall shear stress tends to first undershoot but then overshoot the quasi-steady value. 'Slow' response of turbulence as well as flow inertia is responsible for these behaviours. In the ramp-up flow, the response of turbulence is similar to that observed in uniformly accelerating flows from previous studies, exhibiting a three-stage development. However, the transition between the various stages is more gradual and the responding stage is much longer and slower in the flows considered here. It has been shown that the delay in the near wall region is longer than that in the buffer layer confirming that turbulence response first occurs at the location of peak turbulence production. In a strongly decelerating flow, the response of turbulence exhibits a two-stage development. In both ramp-up and ramp down flows, the energy distribution in the three components of turbulent kinetic energy deviates from that of the steady flow. In a ramp-up flow, more energy is in upsilon(1) and less in u(2) and u(3,) whereas the trend is reversed in a ramp-down flow. This is a reflection of the redistribution of turbulence from u(1) to u(2) and u(3.