Hydrodynamic force on a small squirmer moving with a time-dependent velocity at small Reynolds numbers

We calculate the hydrodynamic force on a small spherical, unsteady squirmer moving with a time-dependent velocity in a fluid at rest, taking into account convective and unsteady fluid-inertia effects in perturbation theory. Our results generalise those of Lovalenti and Brady (1993) from passive to active spherical particles. We find that convective inertia changes the history contribution to the hydrodynamic force, as it does for passive particles. We determine how the hydrodynamic force depends on the swimming gait of the unsteady squirmer. Since swimming breaks the spherical symmetry of the problem, the force is not completely determined by the outer solution of the asymptotic-matching problem, as it is for passive spheres. There are additional contributions brought by the inhomogeneous solution of the inner problem. We also compute the disturbance flow, illustrating convective and unsteady fluid-inertia effects for a sudden start of the centre-of-mass motion, and for swimming with a periodic gait. We discuss the implications of our findings for small motile organisms in a marine environment.Comment: 16 pages, 4 figure

Experimental study of fire containment using water mist curtains in a reduced-scale deck of a ro-ro ship

Experiments have been conducted to evaluate the containment of smoke and heat using water mist curtains in a model setup of a ro-ro ship's cargo deck with a scale of 1:13, providing practical insights into the application of such fire protection systems in the cargo deck as well as valuable data for future numerical simulations. In this regard, the requirements of the international convention of Safety of Life at Sea (SOLAS) are studied for the side openings of so-called ‘open decks’ in comparison with ‘closed decks’, especially to examine the feasibility of using water mist curtains for creating isolated subdivisions in the ro-ro space as a fire management strategy. The water mist curtains are created with one or two rows of water mist nozzles at pressures ranging from 3 to 8 bar, while the source of smoke and heat is a liquid pool fire, and inert cargo items are used in some experiments. Correspondingly, the interaction between the water mist curtain(s) and the fire is evaluated in terms of its heat release rate, and the containment effect is quantified via measurements of smoke flow through the deck and through the windows, concentrations of gaseous species, as well as gas temperatures at various key locations. The study shows that water mist curtains have a strong effect on fire dynamics and smoke propagation, but containment is dependent on the configuration of side openings and the location of fire, among other important factors

Inertial drag on a sphere settling in a stratified fluid

We compute the drag force on a sphere settling slowly in a quiescent, linearly stratified fluid. Stratification can significantly enhance the drag experienced by the settling particle. The magnitude of this effect depends on whether fluid-density transport around the settling particle is due to diffusion, to advection by the disturbance flow caused by the particle, or due to both. It therefore matters how efficiently the fluid disturbance is convected away from the particle by fluid-inertial terms. When these terms dominate, the Oseen drag force must be recovered. We compute by perturbation theory how the Oseen drag is modified by diffusion and stratification. Our results are in good agreement with recent direct-numerical simulation studies of the problem at small Reynolds numbers and large (but not too large) Froude numbers.Comment: 10 pages, 1 figur

Unsteady and inertial dynamics of a small active particle in a fluid

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Non-Boussinesq experiments on natural ventilation in a 2D semi-confined enclosure

International audienceThis paper presents an experimental study which aimed to quantify the transition between displacement and blocked regimes in a box naturally ventilated through a lower and an upper opening. It complements and extends the results obtained by Paranthoën & Gonzalez (IJHFF, 2010) with heated air in a similar box. The ventilation was here driven by a buoyant air/helium mixture which was continuously released through a slot at the bottom of the box. The slot and the openings were the same width as the box, so that the experiments can be considered as quasi-2D. The Richardson number Ri of the buoyant source (determined by the ratio between buoyancy and momentum) varied over two orders of magnitude (between 0.1 and 10) in our experiments. An intermediate regime defined by the occurrence of bidirectional flow at the base opening, with incoming fresh air and outgoing buoyant fluid, was observed. This intermediate regime can be predicted by a simple correlation based on the Richardson number of the source and the dimensions of opening areas. The full blocked regime (with no more incoming fresh air) was only obtained for large flow rates and/or weakly buoyant releases corresponding to Richardson numbers lower than about 2−3. We found a good agreement with the classical relation published by Woods, Caulfield and Phillips (JFM, 2003) provided that the density differences involved were not too high

Influence of source conditions and heat losses on the upwind back-layering flow in a longitudinally ventilated tunnel

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Aerodynamics of buoyant releases within a longitudinally ventilated tunnel

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Numerical study of the combustion regimes in naturally-vented compartment fires

Numerical simulations are performed to investigate the classical compartment fire problem involving a single door opening. Three different configurations were considered, namely a full-scale (ISO 9705), an intermediate and a small-scale enclosure with various opening heights and widths. A large number of numerical simulations was carried out using the CFD code Fire Dynamics Simulator (FDS)(version 6.7.0). Based on the variation of the average temperature inside the compartment, three combustion regimes were identified namely well-ventilated, transitional and under-ventilated regime. This variation also allowed us to identify the boundaries between regimes. Furthermore, by adopting a non-dimensional representation of the fire heat release rate inside the compartment as a function of the Global Equivalence Ratio (GER), a clear demarcation between these combustion regimes was obtained. A linear correlation has been established between the maximum heat release rate inside the compartment and the ventilation factor. The latter is expressed as Q̇inmax=850AH. A linear relation between the maximum air flow rate ṁin and the ventilation factor AH was found, i.e. ṁinmax=0.46AH where A is the door surface area and H its height

Use of a water mist for smoke confinement and radiation shielding in case of fire during tunnel construction

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