Mathieu Pourquie

Changes in Shear Stress-Related Gene Expression After Experimentally Altered Venous Return in the Chicken Embryo

Hemodynamics play an important role in cardiovascular development, and changes in blood flow can cause congenital heart malformations. The endothelium and endocardium are subjected to mechanical forces, of which fluid shear stress is correlated to blood flow velocity. The shear stress responsive genes lung Krüppel-like factor (KLF2), endothelin-1 (ET-1), and endothelial nitric oxide synthase (NOS-3) display specific expression patterns in vivo during chicken cardiovascular development. Nonoverlapping patterns of these genes were demonstrated in the endocardium at structural lumen constrictions that are subjected to high blood flow velocities. Previously, we described in chicken embryos a dynamic flow model (the venous clip) in which the venous return to the heart is altered and cardiac blood flow patterns are disturbed, causing the formation of congenital cardiac malformations. In the present study we test the hypothesis that disturbed blood flow can induce altered gene expression. In situ hybridizations indeed show a change in gene expression after venous clip. The level of expression of ET-1 in the heart is locally decreased, whereas KLF2 and NOS-3 are both upregulated. We conclude that venous obstruction results in altered expression patterns of KLF2, ET-1, and NOS-3, suggestive for increased cardiac shear stress.

NUMERICAL STUDY OF TRANSIENT AND STEADY LAMINAR BUOYANCY-DRIVEN FLOWS AND HEAT TRANSFER IN A SQUARE OPEN CAVITY

Numerical simulations are presented for transient and steady laminar buoyancy-driven flows and heat transfer in a square cavity open on one side. Computations were performed within the domain of the cavity. Vorticity transport and energy equations were solved using the alternating direction implicit scheme, and a successive overrelaxation method was employed to obtain solutions for the streamfunction. A range of values were considered for Gr and Pr. The results indicate that natural convection in the cavity does not depend on the computational domain or on the boundary conditions at the open side, which influence only a small region nearby. Heat transfer from the cavity is calculated, and flow and transport characteristics are discussed

Large eddy simulation of an Ahmed reference model

This paper presents the first large eddy simulation (LES) of the Ahmed reference model. LES allows a much greater depth of analysis than most other turbulent simulation methods. Here the richness of information provided by LES is used to show a wide range of flow results such as: time averaged mean profiles, time-dependent traces, flow spectra and a number of two-dimensional and three-dimensional animations of the flowfield. The body geometry is analysed at a given slant angle of 28° using the PRICELES/TRIO_U code platform. This angle is chosen as it is close to the critical angle at which the flow changes between two different regimes. A non-structured tetrahedral grid is used which enables accurate modelling of the body geometry while avoiding the need for a prohibitively expensive mesh. The drag estimation and flow field visualizations indicate that the flow is in the regime corresponding to angles above the critical angle, although the geometry is just below the critical angle. The simulation revealed...

A numerical study of steady and unsteady cavitation on a 2d hydrofoil

The steady and unsteady cavitation phenomena on a 2D NACA0015 hydrofoil predicted by the multiphase RANS code FLUENT are studied in this paper. Besides a numerical sensitivity study of the non-cavitating condition, the present investigation focuses on two cavitation numbers: σ = 1.6 (steady cavitating flow) and σ = 1.0 (with dynamic shedding). With a modified SST k − ω turbulence model, a periodic shedding is revealed: the main sheet cavity breaks up by the re-entrant jet and a cloudy cavity forms and is convected with the downstream flow. Finally, the experience with FLUENT has been used to discuss the general ability of multiphase RANS codes to predict the cavitation erosion risk.

Numerical Study on the Flow Physics of a T-Shaped Micro Mixer

A T-shaped micro mixer has two characteristic flow regimes that dependent on the Reynolds number and the geometry in which the mixing of the fluids entering in the two channels is determined by diffusion or by convective transport. In one regime the flow in the T-shaped micro mixer is plane-symmetric with respect to one symmetry plane of the T mixer and mixing of fluids from the two inlet channels is determined by diffusion. This regime is referred to as the diffusion regime in the remainder of this paper. In the other regime the flow is symmetric with respect to the mixing channel’s centerline, and the mixing of the fluids from the two inlet channels is primarily determined by convection. The aim of this work is to study numerically the flow topology in the transition from the flow regime of diffusive to convective mixing. Therefore a systematic study was performed to evaluate the influence of the discretization scheme, the spatial resolution and the choice of channel inlet lengths on the flow topology. The systematic investigation showed that an improper choice of spatial resolution as well as an insufficient channel inlet length can lead to a complete elimination of the Reynolds number dependent onset of convective mixing in the flow. The T-shaped micro mixer flow is represented by vortex core regions which are defined by a λ 2-criterion. They show that the secondary vortical structures in the flow consist of two Dean vortex pairs, both in the flow regime of diffusive and convective mixing. The convective regime distinguishes itself from the diffusive one by an unequal swirling strength of the Dean vortices which leads to the characteristic co-rotating vortices in the mixing channel. We show that stationary flow states exist in a narrow Reynolds number range that link the flow regime of diffusive and convective mixing. From our investigation we conclude that the transition from a diffusion to a convection dominated T-mixer flow is continuous.

Assessment of Cavitation Erosion With a URANS Method

An assessment of the cavitation erosion risk by using a contemporary unsteady Reynolds-averaged Navier–Stokes (URANS) method in conjunction with a newly developed postprocessing procedure is made for an NACA0015 hydrofoil and an NACA0018-45 hydrofoil, without the necessity to compute the details of the actual collapses. This procedure is developed from detailed investigations on the flow over a hydrofoil. It is observed that the large-scale structures and typical unsteady dynamics predicted by the URANS method with the modified shear stress transport (SST) k-ω turbulence model are in fair agreement with the experimental observations. An erosion intensity function for the assessment of the risk of cavitation erosion on the surface of hydrofoils by using unsteady RANS simulations as input is proposed, based on the mean value of the time derivative of the local pressure that exceeds a certain threshold. A good correlation is found between the locations with a computed high erosion risk and the damage area observed from paint tests.

Simple Wake Models for Tidal Turbines in Farm Arrangement

From wind turbines it is known that the wake, induced by a turbine, has a negative impact on the energy production of downstream devices. Basically, the wake is a zone with reduced velocity behind a turbine. Further downstream, the velocity recovers gradually by turbulent mixing with the ambient flow. In order to optimize the design of a tidal farm, the aim of this paper is to find simple relations that can be used to predict the energy output of a given farm configuration. The energy output of a turbine depends on its inflow velocity. Therefore, the strategy is to find a model that is able to predict the velocity field in the tidal farm. Such ‘wake models’ exist already for wind turbines and thruster-thruster interaction. In this research, the applicability of these wake models to tidal turbines is investigated by comparing their results to reference data of tidal turbines. Only limited measurement data for tidal turbines are available; therefore a CFD model of a tidal turbine is used to generate the reference data. The velocity in the wake is simulated for different conditions with the CFD model. The CFD model is validated with the available data in the literature. The velocity in the wake for a single turbine is predicted accurately for different initial conditions. Modeling of the turbulence showed some discrepancies in the far wake, consequently the wake of turbines in farm configurations is predicted less accurate. Three wake models, selected from the literature, are compared to the CFD simulations of the wake behind a single turbine. The wind turbine wake model of Jensen performed best; the velocity in the wake is calculated accurate for different situations. Mutual interaction of wakes will occur inside tidal farms. Several methods from wind turbines theory are used to estimate the velocity in interaction situations. Three basic situations of wake interaction are distinguished: tandem operation, wake interference and overlapping inflow. The interaction methods are tested with CFD reference data for each situation separately. Most methods compared reasonably well; the most suitable interaction methods are selected. A small tidal farm case study is performed to test the combination of wake model and interaction methods. The flow in the cluster of 5 turbines is predicted satisfactorily by the wake model for different inflow velocities. All results indicate that the principle of applying wind turbine wake models to tidal turbine has good potential. However the number of test cases conducted in the thesis is limited and the incorrect turbulence modeling of the CFD model caused some uncertainties for multiple turbine situation.© 2010 ASME

Simulation of Reacting Spray in a Multi-Point Lean Direct Injection Combustor

This paper investigates the reacting spray phenomena in a multi-point lean direct injection (MPLDI) combustor to characterize the effects of highly swirling air flows on spray combustion. The Reynolds-averaged Navier Stokes (RANS) code is applied to simulate the turbulent, reacting, and swirling flow associated with the combustor. For the liquid spray modeling, several spray sub-models are used. Properties of both the gas and liquid phases are analyzed. The reacting flow simulations show short flames emanating from the individual injectors, uniformly low temperature distribution inside the combustor, and a uniform temperature profile at the chamber exit. With an increase in air flow velocity, the flow field becomes highly strained at the injector exits where the fuel and air streams mix and at the interfaces of the neighboring swirlers, allowing the mixing process to speed up. Overall, the computational results are able to capture and explain some of the fundamental features of the MPLDI combustor, such as the fuel-air mixing, drop size distribution, drop vaporization, and spray combustion process.

DIAGONAL CARTESIAN METHOD ON STAGGERED GRIDS FOR A DNS IN A TUBE BUNDLE

Flow in a staggered tube bundle is investigated in this paper. The governing equations are discretized on a staggered grid with a Diagonal Cartesian Method allowing a special treatment of the non-Cartesian boundaries. This approach is computationally less expensive than a method based on the use of body-fitted coordinates. The principle of the Diagonal Cartesian Method is described, with a special attention close to a non-Cartesian boundary. Simulations show good agreement with the experimental data of Simonin and Barcouda.

Numerical Study of Non-Reacting and Reacting Flow Characteristics in a Lean Direct Injection Combustor

The Lean Direct Injection (LDI) combustion concept has been of active interest due to its potential for low emissions under a wide range of operational conditions. This might allow the LDI concept to become the next generation gas-turbine combustion scheme for aviation engines. Nevertheless, the underlying unsteady phenomena, which are responsible for low emissions, have not been widely investigated. This paper reports a numerical study on the characteristics of the non-reacting and reacting flow field in a single-element LDI combustor. The solution for the non-reacting flow captures the essential aerodynamic flow characteristics of the LDI combustor, such as the reverse flow regions and the complex swirling flow structures inside the swirlers and in the neighborhood of the combustion chamber inlet, with reasonable accuracy. A spray model is introduced to simulate the reacting flow field. The reaction of the spray greatly influences the gas-phase velocity distribution. The heat release effect due to combustion results in a significantly stronger and compact reverse flow zone as compared to that of the non-reacting case. The inflow spray is specified by the Kelvin-Helmholtz breakup model, which is implemented in the Reynolds-Averaged Navier Stokes (RANS) code. The results show a strong influence of the high swirling flow field on liquid droplet breakup and flow mixing process, which in turn could explain the low-emission behavior of the LDI combustion concept.Copyright