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Direct numerical simulations (DNS) of the thermally-driven flow of air inside differentially heated cavities (DHC) are presented. The variation of air properties is taken into account using the low-Mach approximation, to obtain solutions outside the limits of the Oberbeck–Boussinesq (OB) approximation. The effects of the cavity’s aspect-ratio A and large temperature differences ΔT were investigated over the range 1 ≤ A ≤ 8 and ΔT ≤ 720 K. The Rayleigh and Prandtl numbers were set at R a = 10 9 and P r = 0.7 , considering a reference temperature of T 0 = 600 K. Time-averaged and r.m.s. statistics of the temperature field along with other derived quantities are presented and discussed. It is found that the time-averaged temperature fields vary significantly between non-Oberbeck–Boussinesq (NOB) and OB cases for larger aspect-ratios. This is also true for the instantaneous temperature and temperature r.m.s. fields which demonstrate significant qualitative differences. In addition, as the temperature difference increases and the property variations become more intense, the boundary layers at the vertical walls become turbulent at lower positions. The relocation of the laminar-turbulent transition point is found to be strongly dependent on the cavity’s aspect-ratio. An explanation of this phenomenon is given, based on the disruption of the vertical boundary layers by horizontal streams originating from the downstream part of the vertical walls.

Buoyancy-driven flows inside enclosures are in the center of problems related to heat transfer because they can provide a significant insight into the physical mechanisms of heat transfer. Typical examples of such flows include Rayleigh–Bénard convection, differentially heated cavities and partially divided enclosures. In the present study, the buoyancy-driven flow inside an asymmetrically heated closed cavity is investigated and proposed as a benchmark case for future studies to assess the accuracy of simulations and to help in the validation of coarsened turbulence models. Additionally, from an application perspective such a configuration is highly relevant, e.g., in view of its similarity with passive solar systems such as ventilated building facades (Puangsombuta et al, J Fluid Mech 42(6):2218–2226, 2007, [1]) and Trombe walls (Zamora and Kaiser, Heat Mass Transf 45(11):1393–1407, 2009, [2]). 299 305

In the present work, the forces exerted on spherical sand particles by oscillatory boundary layer flows at Re = 25 , 000 and Re = 55 , 000 are examined by means of Large Eddy Simulations. The particles move within the carrier fluid under the influence of the Stokes Drag, Gravity, Added Mass, Pressure Drag and Basset forces. The model has been validated for the cases of a particle accelerating within a stagnant fluid under the influence of gravity and for a particle within an oscillatory flow. The Basset force is computed in an efficient way, by calculating the Basset integral only for a specific time in the particle’s immediate history, called “window time”. In order to define the window time, existing literature approaches have been implemented. The rest of the particle’s history is calculated by an efficient second order methodology suitable for a large number of particles moving within the carrier fluid. The importance of each of the hydrodynamic forces has been examined when the particles are released far from the horizontal bed and when they are released in the boundary layer region, in the absence of gravity. The results indicate that Stokes Drag, Basset force and Pressure Drag are equally important. The contribution from the Added Mass force was found to be orders of magnitude smaller and the acceleration components in the other directions are also negligible. Furthermore, in the boundary layer region, a near-wall zone of negative mean Stokes Drag and positive mean Basset force has been identified. Moving away from the wall but still in the boundary layer region, another zone of positive mean Stokes Drag and negative mean Basset have been identified.

Kolokythas, G.A.; Grigoriadis, D.G.E., and Dimas, A.A., 2018. Dynamic friction coefficient in formulas of bed-load transport induced by waves over vortex orbital ripples. Empirical formulas for the computation of bed sediment transport induced by wave propagation over sand ripples often depend on, among other parameters, the dynamic friction coefficient, μd, of the sand grain motion. Here, a new approach is presented for the determination of μd based on the physics of the interaction between flow and sediment transport. Specifically, the correlation of μd to the wave, ripple, and sand grain characteristics was obtained using coupled numerical simulations of the flow over the rippled bed, the bed-load transport, and the bed morphology evolution. Flow results were obtained by two different numerical methods, one for wave boundary layer flow and one for purely oscillatory flow, both based on the solution of the Navier-Stokes equations. The bed-load transport rate, which drives the bed morphology evo...

Understanding the buoyancy-driven flow in gas gap enclosures can provide significant physical insight into a variety of technological applications ranging from cooling of electronic equipment to energy related components and systems. 499 504

Predicting regional deposition patterns of inhaled aerosols is important for the design and optimization of pharmaceutical formulations. © 2018, Springer International Publishing AG. 24 351 357 351-357