Paul Hayden

Large-eddy simulation of dispersion: comparison between elevated and ground-level sources

Large-eddy simulation (LES) is used to calculate the concentration fluctuations of passive plumes from an elevated source (ES) and a ground-level source (GLS) in a turbulent boundary layer over a rough wall. The mean concentration, relative fluctuations and spectra are found to be in good agreement with the wind-tunnel measurements for both ES and GLS. In particular, the calculated relative fluctuation level for GLS is quite satisfactory, suggesting that the LES is reliable and the calculated instantaneous data can be used for further post-processing. Animations are shown of the meandering of the plumes, which is one of the main features to the numerical simulations. Extreme value theory (EVT), in the form of the generalized Pareto distribution (GPD), is applied to model the upper tail of the probability density function of the concentration time series collected at many typical locations for GLS and ES from both LES and experiments. The relative maxima (defined as maximum concentration normalized by the local mean concentration) and return levels estimated from the numerical data are in good agreement with those from the experimental data. The relative maxima can be larger than 50. The success of the comparisons suggests that we can achieve significant insight into the physics of dispersion in turbulent flows by combining LES and EVT.

Large-Eddy simulation of turbulent flow over a rough surface

A family of wall models is proposed that exhibits moresatisfactory performance than previousmodels for the large-eddy simulation (LES) of the turbulentboundary layer over a rough surface.The time and horizontally averaged statistics such asmean vertical profiles of windvelocity, Reynolds stress, turbulent intensities, turbulentkinetic energy and alsospectra are compared with wind-tunnel experimental data.The purpose of the present study is to obtain simulatedturbulent flows that are comparable with wind-tunnelmeasurements for use as the wind environment for thenumerical prediction by LES of source dispersion in theneutral atmospheric boundary layer.

Measurements and Computations of Flow in an Urban Street System

We present results from laboratory and computational experiments on the turbulent flow over an array of rectangular blocks modelling a typical, asymmetric urban canopy at various orientations to the approach flow. The work forms part of a larger study on dispersion within such arrays (project DIPLOS) and concentrates on the nature of the mean flow and turbulence fields within the canopy region, recognising that unless the flow field is adequately represented in computational models there is no reason to expect realistic simulations of the nature of the dispersion of pollutants emitted within the canopy. Comparisons between the experimental data and those obtained from both large-eddy simulation (LES) and direct numerical simulation (DNS) are shown and it is concluded that careful use of LES can produce generally excellent agreement with laboratory and DNS results, lending further confidence in the use of LES for such situations. Various crucial issues are discussed and advice offered to both experimentalists and those seeking to compute canopy flows with turbulence resolving models.

Vortex shedding behind tapered obstacles in neutral & stratified flow

Results of laboratory and numerical experiments on both homogeneous and density-stratified flow over single, bluff obstacles of various shapes are presented. The obstacle height is in most cases of the same order as the base diameter and the major controlling (flow) parameter is the Froude number, defined here as Fh=U/Nh, where U is the (uniform) upstream velocity, h the obstacle height and N is the buoyancy frequency. Attention is concentrated, firstly, on the case of homogeneous flows over rather weakly tapered obstacles and, secondly, for bodies whose height is similar to their base width, on the case Fh=0.1, representing stratification sufficiently strong that lee-wave motions do not play a significant role in the flow dynamics. For right-circular cones it is shown that the sectional contributions to the total fluctuating side force (lift) show significant phase variations up the height of the obstacle, which are not always reflected in the developed vortex street further downstream. For some obstacle shapes, the vortex lines linking the von Karman eddies at different heights can be significantly tilted, particularly in the upper part of the wake. Vortex convection speeds do not appear generally to vary greatly with height and, as found in previous work, the shedding frequency remains constant with height, despite the strong variation of cross-stream obstacle width. By comparison with the homogeneous results, it is suggested that the stratification enhances the shedding instability, which would otherwise be very weak for squat obstacles, but does not annihilate the ability of the flow at one level to influence that at another.

Is the Actuator Disc Concept Sufficient to Model the Far-Wake of a Wind Turbine?

In order to study the wind turbine wake and its eventual interactions with neighbouring wind turbines, several numerical and physical modelling approaches are used. Some model the wind turbine with the simplest model, that is the actuator disc concept, adding a drag source (i.e. pressure loss) within the surface swept by the blades (numerical [2], physical [1]). Some use the Blade Element Momentum Theory, which takes into account the blade rotation effect on the wake and the aerodynamic features of the blades [3]. Some use Large Eddy Simulation to compute the unsteady flow around the entire rotor [5]. In a wind resource assessment context, the latter one is not practical enough to be used since the computation times are extremely long.

Evaluation of CFD model predictions of local dispersion from an area source on a complex industrial site

The CFD model Fluidyn-Panache was configured to model atmospheric transport from an area source. Modelled flow and turbulence were evaluated by comparison with on-site meteorological measurements, whilst atmospheric dispersion was compared with wind tunnel measurements. The results showed that higher rates of vertical and lateral dispersion were modelled than were determined in the wind tunnel, though modelled and measured ground-level centreline concentration data were within a factor of two. Uncertainties in wind tunnel and numerical modelling were highest close to the source. Consideration of fine-scale features was only necessary for receptors in the immediate near-field.

Experimental study on the wind turbine wake meandering with the help of a non-rotating simplified model and of a rotating model

In order to study the wind turbine wake and its eventual interactions with neighbouring wind turbines, several numerical and physical modelling approaches are used. Some model the wind turbine with the simplest model, that is the actuator disc concept, adding a drag source (i.e. pressure loss) within the surface swept by the blades (numerical, physical,). Some use the Blade Element Momentum Theory, which takes into account the blade rotation effect on the wake and the aerodynamic features of the blades,. Some use Large Eddy Simulation to compute the unsteady flow around the entire rotor. In a wind resource assessment context, the latter one is not mature enough to be used since the computation times are extremely long. The second one has more acceptable computation time but the first one is still the most attractive to model the far wake, according to its simplicity of implementation and short computation time. On the other hand, the issue is that it is difficult to assess the errors induced by the absence of blades and associated rotation momentum on the wake development. Furthermore, the level of turbulence intensity encountered in the atmospheric incoming flow plays a role on this issue : First, the higher the turbulence intensity is, the faster the spectral signature of the blades disappears in the wake and the faster the azimuthal velocity induced by the rotational momentum is overwhelmed in ambiant velocity fluctuations. These intuitive remarks need to be quantified. In this context, a previous study of the present authors compared the wake properties of a model of a 3-blade rotating wind turbine (D = 416mm, TSR = 5.8, CT = 0.50)(Fig. 2) and of a porous disc made of metallic mesh (Fig. 3), generating the same velocity deficit as the wind turbine (Fig. 3). Both models were tested in an atmospheric boundary layer (ABL) wind tunnel. A special care had been supplied to reproduce in the wind tunnel, at a smaller geometric scale, the flow and turbulence properties of an ABL (called later on ”imodelled ABL”?). The spectral content of the turbulence and so, the turbulence length scales of the ABL are then reproduced. The modelled boundary layer had neutral stability conditions and the turbulence intensity at hub height was 13%. The 3D flow properties were measured from x = 0.5D to 3D downstream of the wind turbine. The mean flow and the spectral content were studied and the conclusion was that the additional rotation momentum and the blade signature in flow are no longer visible at the beginning of the far wake, expected at x = 3D). On the other hand, this statement is valid for the tested freestream turbulence intensity (13% at hub height) and cannot be extrapolated to any freestream conditions. One still can expect that the weaker the freestream turbulence intensity is, the farther the tip vortex signature and the additional rotation momentum remain visible within the wake. To complete this results, an experimental study about the unsteady properties of the wake downstream of the rotating wind turbine and of the porous disc located in a modelled ABL has been undertaken. Indeed, it is established that the large turbulence length scales present in the ABL can be responsible of the wake meandering process ). This phenomenom will be studied in the wind tunnel through space-time correlations of velocity signals measured at locations diametrically opposed in the wake, and compared for the two type

Scalar fluxes near a tall building in an aligned array of rectangular buildings

Scalar dispersion from ground-level sources in arrays of buildings is investigated using wind-tunnel measurements and large-eddy simulation (LES). An array of uniform-height buildings of equal dimensions and an array with an additional single tall building (wind tunnel) or a periodically repeated tall building (LES) are considered. The buildings in the array are aligned and form long streets. The sensitivity of the dispersion pattern to small changes in wind direction is demonstrated. Vertical scalar fluxes are decomposed into the advective and turbulent parts and the influences of wind direction and of the presence of the tall building on the scalar flux components are evaluated. In the uniform-height array turbulent scalar fluxes are dominant, whereas the tall building produces an increase of the magnitude of advective scalar fluxes that yields the largest component. The presence of the tall building causes either an increase or a decrease to the total vertical scalar flux depending on the position of the source with respect to the tall building. The results of the simulations can be used to develop parametrizations for street-canyon dispersion models and enhance their capabilities in areas with tall buildings.

A Wind-Tunnel Artificially-Thickened Simulated Weakly Unstable Atmospheric Boundary Layer

A wind-tunnel simulation of an atmospheric boundary layer, artificially thickened as is often used in neutral flow wind-loading studies, has been investigated for weakly unstable stratification, including the effect of an overlying inversion. Rather than using a uniform inlet temperature profile, the inlet profile was adjusted iteratively by using measured downstream profiles. It was found that three cycles are sufficient for there to be no significant further change in profiles of temperature and other quantities. Development to nearly horizontally-homogeneous flow took a longer distance than in the neutral case because the simulated layer was deeper and therefore the length scales larger. Comparisons show first-order and second-order moments quantities are substantially larger than given by ‘standard forms’ in the mixed layer but are close in the surface layer. Modified functions, obtained by matching one to the other, are suggested that amount to an interpolation in the mixed layer between the strongly unstable and the weakly unstable cases.

Mean and turbulent mass flux measurements in an idealised street network

Pollutant mass fluxes are rarely measured in the laboratory, especially their turbulent component. They play a major role in the dispersion of gases in urban areas and modern mathematical models often attempt some sort of parametrisation. An experimental technique to measure mean and turbulent fluxes in an idealised urban array was developed and applied to improve our understanding of how the fluxes are distributed in a dense street canyon network. As expected, horizontal advective scalar fluxes were found to be dominant compared with the turbulent components. This is an important result because it reduces the complexity in developing parametrisations for street network models. On the other hand, vertical mean and turbulent fluxes appear to be approximately of the same order of magnitude. Building height variability does not appear to affect the exchange process significantly, while the presence of isolated taller buildings upwind of the area of interest does. One of the most interesting results, again, is the fact that even very simple and regular geometries lead to complex advective patterns at intersections: parametrisations derived from measurements in simpler geometries are unlikely to capture the full complexity of a real urban area.