Stéphane Fohanno

Analysis of the effect of swimmer's head position on swimming performance using computational fluid dynamics.

The aim of this numerical work is to analyze the effect of the position of the swimmers head on the hydrodynamic performances in swimming. In this initial study, the problem was modeled as 2D and in steady hydrodynamic state. The geometry is generated by the CAD software CATIA and the numerical simulation is carried out by the use of the CFD Fluent code. The standard k-epsilon turbulence model is used with a specific wall law. Three positions of the head were studied, for a range of Reynolds numbers about 10(6). The obtained numerical results revealed that the position of the head had a noticeable effect on the hydrodynamic performances, strongly modifying the wake around the swimmer. The analysis of these results made it possible to propose an optimal position of the head of a swimmer in underwater swimming.

Turbulence model choice for the calculation of drag forces when using the CFD method

The aim of this work is to specify which model of turbulence is the most adapted in order to predict the drag forces that a swimmer encounters during his movement in the fluid environment. For this, a Computational Fluid Dynamics (CFD) analysis has been undertaken with a commercial CFD code (Fluent). The problem was modelled as 3D and in steady hydrodynamic state. The 3D geometry of the swimmer was created by means of a complete laser scanning of the swimmers body contour. Two turbulence models were tested, namely the standard k-epsilon model with a specific treatment of the fluid flow area near the swimmers body contour, and the standard k-omega model. The comparison of numerical results with experimental measurements of drag forces shows that the standard k-omega model accurately predicts the drag forces while the standard k-epsilon model underestimates their values. The standard k-omega model also enabled to capture the vortex structures developing at the swimmers back and buttocks in underwater swimming; the same vortices had been visualized by flow visualization experiments carried out at the INSEP (National Institute for Sport and Physical Education in Paris) with the French national swimming team.

Visualizations of the flow around a swimmer in turbulent regime

Graphical abstract

Influence of a postural change of the swimmer's head in hydrodynamic performances using 3D CFD

This study deals with recent researches undertaken by the authors in the field of hydrodynamics of human swimming. The aim of this numerical study was to investigate the flow around the entire swimmers body. The results presented in this article focus on the combination of a 3D computational fluid dynamics code and the use of the k–ω turbulence model, in the range of Reynolds numbers representative of a swimming level varying from national to international competition. Emphasis is placed on the influence of a postural change of the swimmers head in hydrodynamic performances, which is directly related to the reduction of overall drag. These results confirm and complete those, less accurate, of a preliminary 2D study recently published by the authors and allow the authors to optimise the swimmers head position in underwater swimming.

Transient mixed convection flow of nanofluids in a vertical tube

Purpose – In the present work, a theoretical model based on the full Navier-Stokes and energy equations for transient mixed convection in a vertical tube is extended to nanofluids with nanoparticle volume fraction up to 5 percent to ensure a Newtonian fluid behaviour. The paper aims to discuss these issues. Design/methodology/approach – The nanofluids considered, alumina/water and CuO/water, flow inside a vertical tube of circular cross-section, which is subjected to convective heat exchange at the outer surface. The transient regime is caused by a sudden change of nanofluid temperature at the tube inlet. The range of the Richardson number (1.6=Ri=2.5) investigated in this study corresponds to classic cases of mixed convection flow. Findings – Results have shown a significant reduction in the size of the recirculation zone near the wall when the particle volume fraction increases. This may be attributed to the viscosity increase with the volume fraction. Moreover, the flow structure clearly changes when t...

Heat and Mass Transfer in External Boundary Layer Flows Using Nanofluids

The application of additives to base liquids in the sole aim to increase the heat transfer coefficient is considered as an interesting mean for thermal systems. Nanofluids, prepared by dispersing nanometer-sized solid particles in a base-fluid (liquid), have been extensively studied for more than a decade due to the observation of an interesting increase in thermal conductivity compared to that of the base-fluid (Xuan & Roetzel, 2000; Xuan & Li, 2000). Initially, research works devoted to nanofluids were mainly focussed on the way to increase the thermal conductivity by modifying the particle volume fraction, the particle size/shape or the base-fluid (Murshed et al., 2005; Wang & Mujumdar, 2007). Using nanofluids strongly influences the boundary layer thickness by modifying the viscosity of the resulting mixture leading to variations in the mass transfer in the vicinity of walls in external boundary-layer flows. Then, research works on convective heat transfer, with nanofluids as working fluids, have been carried out in order to test their potential for applications related to industrial heat exchangers. It is now well known that in forced convection (Maiga et al. 2005) as well as in mixed convection, using nanofluids can produce a considerable enhancement of the heat transfer coefficient that increases with the increasing nanoparticle volume fraction. As concerns natural convection, the fewer results published in the literature (Khanafer et al. 2003; Polidori et al., 2007; Popa et al., 2010; Putra et al. 2003) lead to more mixed conclusions. For example, recent works by Polidori et al. (2007) and Popa et al. (2010) have led to numerical results showing that the use of Newtonian nanofluids for the purpose of heat transfer enhancement in natural convection was not obvious, as such enhancement is dependent not only on nanofluids effective thermal conductivities but on their viscosities as well. This means that an exact determination of the heat transfer parameters is not warranted as long as the question of the choice of an adequate and realistic effective viscosity model is not resolved (Polidori et al. 2007, Keblinski et al. 2008). It is worth mentioning that this viewpoint is also confirmed in a recent work (Ben Mansour et al., 2007) for forced convection, in which the authors indicated that the assessment of the heat transfer enhancement potential of a nanofluid is difficult and closely dependent on the way the nanofluid properties are modelled. Therefore, the aim of this paper is to present theoretical models fully describing the natural and forced convective heat and mass transfer regimes for nanofluids flowing in semi-infinite geometries, i.e. external boundary layer flows along

Visualizations of Instabilities in Free Convection Plumes

Flow visualization techniques1,2,3 were employed to investigate vortex instabilities in free convection plumes (Fig. 1) as well as in the interaction of opposite buoyancy-induced flows (Figs 2,3). Visualizations were carried out by means of laser tomography combined with the electrolytic precipitation method2 (Figs 1,3) and the dye fluorescent technique4 (Figs 3(a,b)). The opposite flows consist in an ascending boundary layer developing along a uniformly and periodically heated vertical plate and a descending plume generated by a constantly cooled horizontal cylinder placed near the wall4 (Fig. 2). The electrolytic method and the dye technique were used simultaneously for part of the investigation of the opposite buoyancy-induced flows (Figs 3(a,b)) in order to distinguish the boundary-layer development (electrolytic method) from that of the plume (dye technique). Figure 1 highlights the usual development of a mushroom-like vortex above a heated horizontal cylinder while Figure 3 shows some singular structures developing at the plume/boundary layer interface during the interaction of the opposite flows. The wall heating results in the formation of a mushroom-type vortex (not shown here), that is transported along an axis oblique with respect to the wall. Then, the wall thermal relaxation phase leads to an interfacial instability in the form of a KelvinHelmholtz instability (Fig. 3a). This instability becomes the source of three-dimensional small-scale structures resulting in a large three-dimensional spiral motion (Fig. 3b). Finally, the flow is reattaching at the wall in the form of multiple counter-rotating vortices (Fig. 3c) originating from the opposition of the descending plume and the ascending boundary layer.