Jeroen van Beeck

Numerical and experimental modelling of pollutant dispersion in a street canyon

The pollutant dispersion in a two-dimensional street canyon is studied in this project. The principal parameter investigated is the height of the downstream building. The pollutant source is situated in the middle of the street. The investigation is performed in two ways. Experiments have been carried out in the L-2B wind tunnel at von Karman Institute and numerical simulations have been done with the CFD software Fluent 5.2. The concentration measurements have been performed by means of light scattering technique and the velocity field has been measured with particle image velocimetry. In the numerical simulations, a preliminary study about the backward-facing step has been performed in order to select the best turbulence model in Fluent for these complex flows characterized by separation, stagnation, recirculation, reattachment, etc. The best model appeared to be the realizable k–e model with the two-layer zonal approach to the wall, which predicts the reattachment length after the step with <1% error in comparison with the value obtained from direct numerical simulation by Le, Moin and Kim (Direct numerical simulation of turbulent flow over a backward-facing step, Report No. TF-58, 1996). This model has been applied in the street canyon simulations. Comparison with the experimental results has been made. Besides the height of the downstream building, the influence of a third building situated upstream of the street canyon in the flow and dispersion inside the street has been investigated.

Bridging the Transition from Mesoscale to Microscale Turbulence in Numerical Weather Prediction Models

With a focus towards developing multiscale capabilities in numerical weather prediction models, the specific problem of the transition from the mesoscale to the microscale is investigated. For that purpose, idealized one-way nested mesoscale to large-eddy simulation (LES) experiments were carried out using the Weather Research and Forecasting model framework. It is demonstrated that switching from one-dimensional turbulent diffusion in the mesoscale model to three-dimensional LES mixing does not necessarily result in an instantaneous development of turbulence in the LES domain. On the contrary, very large fetches are needed for the natural transition to turbulence to occur. The computational burden imposed by these long fetches necessitates the development of methods to accelerate the generation of turbulence on a nested LES domain forced by a smooth mesoscale inflow. To that end, four new methods based upon finite amplitude perturbations of the potential temperature field along the LES inflow boundaries are developed, and investigated under convective conditions. Each method accelerated the development of turbulence within the LES domain, with two of the methods resulting in a rapid generation of production and inertial range energy content associated to microscales that is consistent with non-nested simulations using periodic boundary conditions. The cell perturbation approach, the simplest and most efficient of the best performing methods, was investigated further under neutral and stable conditions. Successful results were obtained in all the regimes, where satisfactory agreement of mean velocity, variances and turbulent fluxes, as well as velocity and temperature spectra, was achieved with reference non-nested simulations. In contrast, the non-perturbed LES solution exhibited important energy deficits associated to a delayed establishment of fully-developed turbulence. The cell perturbation method has negligible computational cost, significantly accelerates the generation of realistic turbulence, and requires minimal parameter tuning, with the necessary information relatable to mean inflow conditions provided by the mesoscale solution.

Measurement of the turbulent mass flux with PTV in a street canyon

Abstract Simultaneous measurement of velocity and concentration provides a tool for studying the turbulent diffusion. This quantity until now was mainly modeled and not measured and it plays an essential role in turbulent diffusion of pollutant. We present a new technique to measure the turbulent mass flux vector, u′c′ , based on the same PIV/PTV images, employed for both instantaneous velocity and concentration measurements. Then we applied the new technique to check the correctness of the model widely used for the turbulent mass flux in RANS simulations, in a street canyon type flow. We found places where the turbulent diffusion is opposite to the gradient in mean concentration.

Nesting Turbulence in an Offshore Convective Boundary Layer Using Large-Eddy Simulations

The applicability of the one-way nesting technique for numerical simulations of the heterogeneous atmospheric boundary layer using the large-eddy simulation (LES) framework of the Weather Research and Forecasting model is investigated. The focus of this study is on LES of offshore convective boundary layers. Simulations were carried out using two subgrid-scale models (linear and non-linear) with two different closures [diagnostic and prognostic subgrid-scale turbulent kinetic energy (TKE) equations]. We found that the non-linear backscatter and anisotropy model with a prognostic subgrid-scale TKE equation is capable of providing similar results when performing one-way nested LES to a stand-alone domain having the same grid resolution but using periodic lateral boundary conditions. A good agreement is obtained in terms of velocity shear and turbulent fluxes, while velocity variances are overestimated. A streamwise fetch of 14 km is needed following each domain transition in order for the solution to reach quasi-stationary results and for the velocity spectra to generate proper energy content at high wavelengths, however, a pile-up of energy is observed at the low-wavelength portion of the spectrum on the first nested domain. The inclusion of a second nest with higher resolution allows the solution to reach effective grid spacing well within the Kolmogorov inertial subrange of turbulence and develop an appropriate energy cascade that eliminates most of the pile-up of energy at low wavelengths. Consequently, the overestimation of velocity variances is substantially reduced and a considerably better agreement with respect to the stand-alone domain results is achieved.

Simulation of the Two-Phase Flow Hydrodynamics in an IRDE Reactor

Many industrial processes deal with gas bubbles, e.g., the chlor-alkali processes or a side reaction in metal deposition reactions. It is therefore very important to describe the influence of gas bubbles on the fluid flow in a quantitative way. In the present paper, the two-phase flow is both experimentally characterized and numerically modeled in a reactor with a rotating flow field such as the inverted rotating disk electrode (IRDE). Polarization curves of the hydrogen evolution in 0.1 M Na 2 SO 4 at pH 2.5 are recorded at different rotation speeds. The bubble dispersion and size distribution of the hydrogen bubbles are determined by laser marked shadowgraphy and interferometric laser imaging for droplet sizing. Concerning the numerical investigations, in the first step the single-phase flow solution in the vicinity of the IRDE is compared to the analytical solution of the flow field, as proposed by Cochran [Proc. Cambridge Philos. Soc., 30, 365 (1934)]. In the following step, an Eulerian-Lagrangian two-phase flow model is used to track the bubbles. Two-way momentum coupling effects between bubbles and electrolyte flow are taken into account. The calculated two-phase flow field compares well against the experimental data of the two-phase flow field obtained from the optical imaging techniques.

Feasibility of using glory and speckle patterns for sizing spherical and irregular particles

A backscattered laser light technique for sizing spherical and irregular particles is investigated in this paper. Two different interference patterns (glory and speckle), appearing in the backscatter region when a single droplet is illuminated with a laser light source, were recorded by a CCD camera. A theoretical model, based on a geometrical optics approximation, has been first developed to retrieve particle size from the analysis of these patterns and then applied to liquid and frozen water droplets with sizes ranging from 1 to 2 mm. The satisfactory agreement obtained between the theoretical model and the experimental data encourage further development of this technique.

Experimental and Numerical Study of Convective Heat Transfer in an Array of Slot Jets

The paper describes a study of convective heat transfer in a multiple-jet systems composed of straight and inclined slot nozzles. The application concerned is the fast cooling of moving strip. The experimental approach involves the application of infrared thermography associated with the steady-state heated foil technique. Three-dimensional numerical simulations performed with the code FLUENT compare agreeably with the IR data. The study aims to determine the effect on the average heat transfer coefficient of the slot Reynolds number up to the value of 100000, the nozzle spacing normalised by the slot hydraulic diameter in the range 6 ≤ W/S ≤ 18, the normalised nozzle emergence length, E/S, from 5 to 17 and the normalised nozzle to strip standoff distance Z/S from 3 to 10. The geometrical arrangements tested include perpendicular (90°) and tilted (60°) nozzles. A thermal entrainment phenomenon is found for cooling system of small width. A corrective factor is derived to account for this effect. The experimental findings are compared with existing correlation; deviations, which are observed at high values of the Reynolds number may reach 25%. The numerical simulation emphasises the benefit to use H2 /N2 gas mixture to enhance significantly the cooling rate.Copyright

Development of wake meandering detection algorithms and their application to large eddy simulations of an isolated wind turbine and a wind farm

We investigate several algorithms to detect the centerline of a wind turbine wake. First, we apply these methods during Large Eddy Simulations of an isolated wind turbine subject to a uniform (TI = 0%) and a synthetic turbulent inflow (TI = 10%). The simulations are performed using a vortex-particle mesh method with the blades modeled using immersed lifting lines. The most robust algorithm is then applied to investigate the wakes positions inside a wind farm. At wind farm scale, the simulations are performed with a fourth-order finite difference code inside which the wind turbines are accounted for through advanced actuator disks.

Experimental Study on the Wake Meandering Within a Scale Model Wind Farm Subject to a Wind-Tunnel Flow Simulating an Atmospheric Boundary Layer

The phenomenon of meandering of the wind-turbine wake comprises the motion of the wake as a whole in both horizontal and vertical directions as it is advected downstream. The oscillatory motion of the wake is a crucial factor in wind farms, because it increases the fatigue loads, and, in particular, the yaw loads on downstream turbines. To address this phenomenon, experimental investigations are carried out in a wind-tunnel flow simulating an atmospheric boundary layer with the Coriolis effect neglected. A