Neutral Atmospheric Flow Research Articles

Study on the wind-farm wake under neutral atmospheric condition

ABSTRACT Wake effect leads to the power loss and fatigue damage increase on wind turbines. Therefore, wind turbine wake is seriously considered as one of the key parameters in the micro-sitting and layout optimization. Interestingly, according to the recent research of observations and simulations, similar to individual wind turbine wake, the wake of large-scale wind farm also exists which has significant impact on surrounding wind farms and the local meteorology. However, few research focused on quantitative study of wind farm wake. As a result, in this research, firstly, the numerical simulation for the neutral atmospheric flow has been carried out and preliminarily verified. By using actuator disc model, the horizontal and vertical velocity profiles behind the wind farm are analyzed. Secondly, the effective wind-farm drag coefficient has been decomposed into two components, which represent the vertical and horizontal momentum flux, respectively. By analyzing the drag coefficient, the momentum transport of wind farm was studied. The average velocity behind wind farm was also calculated, from which it was found that, with the length scale increasing, the wind farm starts to absorb energy from upper atmosphere. Lastly, the velocity decay behind wind farm can also be modeled by the formulation of Jensen wake model.

Numerical Modelling of Neutral Atmospheric Boundary Layer Flow Through Heterogeneous Forest Canopies in Complex Terrain (A Case Study of a Swedish Wind Farm)

This paper exposes the risk of generalization of wind conditions from a single met-mast measurement to be representative of the actual flow field in a wind farm situated in complex terrain. The Large-Eddy Simulation of the neutral atmospheric flow for a mid-western Sweden wind farm is performed. The site-specific complex topography and the forest properties are extracted from the Airborne Laser Scanning data. To emphasize the impact of the local topography and surface roughness on the wind field, the wind turbines are not included in the numerical simulations. The predicted wind speeds are compared with the nacelle-mounted anemometers taken from the wind farm's turbine SCADA data where a sufficient degree of match is observed. The results show that mean wind speed, shear exponent and turbulence intensity vary at each wind turbine location justifying the need for turbine-specific assessment of the wind resource in a wind farm located in forested complex terrain. The inter-turbine difference in wind resource is quantified in terms of the difference in turbine-specific mechanical loads. The results show that not only inter-turbine loads varying significantly in the wind farm, but the turbine loads also differ significantly if a homogeneous assumption is made for the forest.

An improved k –ω turbulence model for the simulations of the wind turbine wakes in a neutral atmospheric boundary layer flow

Correct prediction of the recovery of wind turbine wakes in terms of the wind velocity and turbulence downstream of the turbine is of paramount importance for the accurate simulations of turbine interactions, overall wind farm energy output and the impact to the facilities downstream of the wind farm. Conventional turbulence models often result in an unrealistic recovery of the wind velocity and turbulence downstream of the turbine. In this paper, a modified k – ω turbulence model has been proposed together with conditions for achieving a zero streamwise gradient for all the fluid flow variables in neutral atmospheric flows. The new model has been implemented in the simulation of the wakes of two different wind turbines and the commonly used actuator disk model has been employed to represent the turbine rotors. The model has been tested for different wind speeds and turbulence levels. The comparison of the computational results shows good agreement with the available experimental data, in both near and far wake regions for all the modeled wind turbines. A zero streamwise gradient has been maintained in the far wake region in terms of both wind speed and turbulence quantities.

Investigation and validation of wake model combinations for large wind farm modelling in neutral atmospheric boundary layers

This study is focused on assessing the ability of two refined large wind farm models to describe the disturbance of the neutral atmospheric flow caused by large offshore wind farms. Sensitivity studies of internal boundary layer parameters are carried out. An optimum large wind farm correction is then proposed and combined with two different standard single wake models, the Park and EVM models. The large wind farm wake effect is evaluated and validated against measurements of two offshore wind farms at Horns Rev and Nysted and four standard wake models by computing velocity deficit and normalized power. All large wind farm models proposed were able to capture wake width to some degree and the decrease of power output moving through the wind farm. Despite some uncertainties, this very promising model combinations allows us to take into account the slowdown in large wind farms.

Large-Eddy Simulation Comparison of Neutral Flow Over a Canopy: Sensitivities to Physical and Numerical Conditions, and Similarity to Other Representations

The representation of a neutral atmospheric flow over roughness elements simulating a vegetation canopy is compared between two large-eddy simulation models, wind-tunnel data and recently updated empirical flux-gradient relationships. Special attention is devoted to the dynamics in the roughness sublayer above the canopy layer, where turbulence is most intense. By demonstrating that the flow properties are consistent across these different approaches, confidence in the individual independent representations is bolstered. Systematic sensitivity analyses with the Dutch Atmospheric Large-Eddy Simulation model show that the transition in the one-sided plant-area density from the canopy layer to unobstructed air potentially alters the flow in the canopy and roughness sublayer. Anomalously induced fluctuations can be fully suppressed by spreading the transition over four steps. Finer vertical resolutions only serve to reduce the magnitude of these fluctuations, but do not prevent them. To capture the general dynamics of the flow, a resolution of 10 % of the canopy height is found to suffice, while a finer resolution still improves the representation of the turbulent kinetic energy. Finally, quadrant analyses indicate that momentum transport is dominated by the mean velocity components within each quadrant. Consequently, a mass-flux approach can be applied to represent the momentum flux.

Comparison of OpenFOAM and EllipSys3D for neutral atmospheric flow over complex terrain

Abstract. The flow solvers OpenFOAM and EllipSys3D are compared in the case of neutral atmospheric flow over terrain using the test cases of Askervein and Bolund hills. Both solvers are run using the steady-state Reynolds-averaged Navier–Stokes k–ϵ turbulence model. One of the main modeling differences between the two solvers is the wall-function approach. The OpenFOAM v.1.7.1 uses a Nikuradse's sand roughness model, while EllipSys3D uses a model based on the atmospheric roughness length. It is found that Nikuradse's model introduces an error dependent on the near-wall cell height. To mitigate this error the near-wall cells should be at least 10 times larger than the surface roughness. It is nonetheless possible to obtain very similar results between EllipSys3D and OpenFOAM v.1.7.1. The more recent OpenFOAM v.2.2.1, which includes the atmospheric roughness length wall-function approach, has also been tested and compared to the results of OpenFOAM v.1.7.1 and EllipSys3D. The numerical results obtained using the same wall-modeling approach in both EllipSys3D and OpenFOAM v.2.1.1 proved to be almost identical. Two meshing strategies are investigated using HypGrid and SnappyHexMesh. The performance of OpenFOAM on SnappyHexMesh-based low-aspect-ratio unstructured meshes is found to be almost an order of magnitude faster than on HypGrid-based structured and high-aspect-ratio meshes. However, proper control of boundary layer resolution is found to be very difficult when the SnappyHexMesh tool is utilized for grid generation purposes. The OpenFOAM is generally found to be 2–6 times slower than EllipSys3D in achieving numerical results of the same order of accuracy on similar or identical computational meshes, when utilization of EllipSys3D default grid sequencing procedures is included.

One‐way mesoscale–microscale coupling for the simulation of atmospheric flows over complex terrain

AbstractThe microscale model WINDIE, initially developed for the simulation of neutral atmospheric flows over complex topography, is here extended to the study of stratified atmospheric flows with Coriolis effects, with particular focus on its application to wind farm development projects. The code now uses E–l and E– ϵ–l turbulence models, which have shown to be more adequate than the standard E– ϵ model for the simulation of atmospheric flows. The validation tasks include 1D atmospheric boundary layers from the first two cases produced by the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study: a stably stratified boundary layer and a diurnal‐cycle over land, respectively. To test the applicability of the new code to real situations, a series of simulations were performed of the time‐varying atmospheric flow (a 3‐month period between February and May 2012) over a moderately complex topography in the Portuguese mainland, using the Weather Research and Forecasting with Advanced Research WRF (WRF‐ARW) mesoscale code on a 3 km mesh to produce time‐varying boundary conditions for the microscale code, in a dynamic coupling fashion. Comparisons with sonic anemometer measurements at the hill top and with WRF‐ARW results from a finer horizontal resolution mesh ( Δx,y = 1 ∕ 3 km) showed that the code can adequately simulate real atmospheric flows over complex topography. Copyright © 2014 John Wiley & Sons, Ltd.

The response of plasma density to breaking inertial gravity wave in the lower regions of ionosphere

We present a three-dimensional numerical study for the E and lower F region ionosphere coupled with the neutral atmosphere dynamics. This model is developed based on a previous ionospheric model that examines the transport patterns of plasma density given a prescribed neutral atmospheric flow. Inclusion of neutral dynamics in the model allows us to examine the charge-neutral interactions over the full evolution cycle of an inertial gravity wave when the background flow spins up from rest, saturates and eventually breaks. Using Lagrangian analyses, we show the mixing patterns of the ionospheric responses and the formation of ionospheric layers. The corresponding plasma density in this flow develops complex wave structures and small-scale patches during the gravity wave breaking event.

Internal Gravity Waves in a Saturated Moist Neutral Atmosphere

AbstractThis work is motivated by an unusual feature associated with the start-up of a moist nearly neutral atmospheric flow over a mountain ridge that was previously observed in a full-physics numerical model. In that study, the upstream propagation of a wave of subsidence precluded the establishment of upward-displaced and saturated flow that might be expected upstream of the topography. This phenomenon was hypothesized to be a consequence of the peculiar property of saturated moist neutral flow: an upward air parcel displacement produces zero buoyancy, while a downward displacement desaturates the air parcel and produces a positive buoyancy anomaly. In the present study, this hypothesis is confirmed within numerical solutions to a reduced system of equations that incorporates the saturated-atmosphere property in a particularly simple manner. The relatively uncomplicated nature of these solutions motivates the numerical solution of a further simplified initial-value problem for both nonhydrostatic and hydrostatic flow. Exact analytic solutions are developed for the latter hydrostatic case, which explains the upstream-propagating wave of subsidence as a shock phenomenon.

Validation of a CFD model of wind turbine wakes with terrain effects

Wind turbine wakes and the neutral atmospheric wind flow over complex terrain are investigated in this paper using the Computational Fluid Dynamics software Fluent. An actuator disc model based on the Blade Element Theory is implemented for the simulation of the rotor effects. Assuming a 3-D, steady-state flow, the Reynolds-averaged Navier–Stokes equations are solved, along with the Reynolds Stress Model to account for the anisotropy of atmospheric turbulence. The approach is initially validated with widely documented wake measurements over flat terrain. Additionally, the model of a neutral atmospheric flow over a real hill is validated with full-scale observations. Ultimately, a coastal complex terrain wind farm is examined and results are validated with SCADA measurements and compared with simulations using the wind modelling software WAsP.

Vertical cross-spectral phases in neutral atmospheric flow

The cross-spectral phases between velocity components at two heights are analyzed from observations at the Høvsøre test site and from the field experiments under the Cooperative Atmosphere-Surface Exchange Study in 1999. These phases represent the degree to which turbulence sensed at one height leads (or lags) in time the turbulence sensed at the other height. The phase angle of the cross-wind component is observed to be significantly greater than the phase for the along-wind component, which in turn is greater than the phase for the vertical component. The cross-wind and along-wind phases increase with stream-wise wavenumber and vertical separation distance, but there is no significant change in the phase angle of vertical velocity, which remains close to zero. The phases are also calculated using a rapid distortion theory model and large-eddy simulation. The results from the models show similar order in phasing, but the slopes of the phase curves are slightly different from the observations, especially for low wavenumbers.

Effect of Turbulence Models and Spatial Resolution on Resolved Velocity Structure and Momentum Fluxes in Large-Eddy Simulations of Neutral Boundary Layer Flow

Abstract This paper demonstrates the importance of high-quality subfilter-scale turbulence models in large-eddy simulations by evaluating the resolved-scale flow features that result from various closure models. The Advanced Regional Prediction System (ARPS) model was used to simulate neutral flow over a 1.2-km square, flat, rough surface with seven subfilter turbulence models [Smagorinsky, turbulent kinetic energy (TKE)-1.5, and five dynamic reconstruction combinations]. These turbulence models were previously compared with similarity theory. Here, the differences are evaluated using mean velocity statistics and the spatial structure of the flow field. Streamwise velocity averages generally differ among models by less than 0.5 m s−1, but those differences are often significant at a 95% confidence level. Flow features vary considerably among models. As measured by spatial correlation, resolved flow features grow larger and less elongated with height for a given model and resolution. The largest differences are between dynamic models that allow energy backscatter from small to large scales and the simple eddy-viscosity closures. At low altitudes, the linear extent of Smagorinsky and TKE-1.5 structures exceeds those of dynamic models, but the relationship reverses at higher altitudes. Ejection, sweep, and upward momentum flux features differ among models and from observed neutral atmospheric flows, especially for Smagorinsky and TKE-1.5 coarse-grid simulations. Near-surface isopleths separating upward fluxes from downward are shortest for the Smagorinsky and TKE-1.5 coarse-grid simulations, indicating less convoluted turbulent interfaces; at higher altitudes they are longest. Large-eddy simulation (LES) is a powerful simulation tool, but choices of grid resolution and subfilter model can affect results significantly. Physically realistic dynamic mixed models, such as those presented here, are essential when using LES to study atmospheric processes such as transport and dispersion—in particular at coarse resolutions.

Solar wind‐magnetosphere‐ionosphere coupling: Neutral atmosphere effects on signal propagation

Waves that propagate between the outer magnetospheric regions (in particular the magnetopause or the magnetotail) and the ionosphere and thereby mediate the dynamic processes of the magnetosphere‐ionosphere‐thermosphere system are affected at the lower end of their paths by the interaction of the plasma with the neutral atmosphere. We use three‐fluid equations (electrons, ions, and neutral particles) to study wave propagation and derive the general dispersion relation, under the simplifying assumptions of incompressible parallel propagation in a locally uniform system. Included are the effects of ion‐neutral, electron‐neutral, and ion‐electron collisions on the electric current and on the plasma flow, as well as the effect of plasma‐neutral collisions on the neutral atmospheric flow. At low frequencies, near and below the ion‐neutral collision frequency, the properties of propagating perturbations are modified as a result of the interaction with the neutral atmosphere. Because the wavelengths may be too long for the local‐uniformity assumption to apply, the quantitative statement of the modifications may not be reliable, but their qualitative properties are clear. Wave speed becomes substantially slower than the Alfvén speed because of inertia loading by neutrals. Waves are significantly damped because of plasma‐neutral friction. Left‐handed and right‐handed waves have different dispersion properties because ion motion is inhibited much more than electron motion by collisions with neutrals. At still lower frequencies, below the neutral‐ion collision frequency, plasma and neutrals move together, leading to wave speed equal to the Alfvén speed based on the total (plasma plus neutral) mass density and to disappearance of damping and of differences between left‐handed and right‐handed dispersions. These results indicate that the coupling of ionosphere and neutral atmosphere is better described as a (frictional) neutral‐drag process rather than as Ohmic dissipation, in agreement with the conclusions of another direct study of energy equations in the ionosphere‐atmosphere system.

A Fine-Scale k−ε Model for Atmospheric Flow over Heterogeneous Landscapes

A multi-purpose model for small-scale atmospheric flows over heterogeneous landscapes is being developed. The aim of this research is to build a tool able to predict the dynamical (wind, turbulence) and diffusive (gases, particles) fields over landscapes characterised by heterogeneous plant cover. In its present stage of development the model is based on the numerical integration of neutral atmospheric flow equations, using an energy-dissipation closure scheme and over a domain that may include vegetation layers. Three validation cases of the model are presented: (i) response of the airflow to a change in surface roughness; (ii) airflow within and above a horizontally homogeneous plant canopy; (iii) airflow over two complex forest-to-clearing and clearing-to-forest transitions. All simulations provide results in good agreement with the experimental data, except for turbulent kinetic energy just after a clearing-to-forest transition. This result is not surprising for a statistical k−e model in a flow region characterised by strong distorsion and intermittent turbulence. However the overall good performance of the model is promising for environmental research at fine scales over heterogeneous landscapes.

Large‐eddy simulation of neutral atmospheric boundary layer flow over heterogeneous surfaces: Blending height and effective surface roughness

A new generation large‐eddy simulation (LES), based on a Lagrangian scale‐dependent dynamic subgrid model, is applied to neutral atmospheric flow over heterogeneous land surfaces. This LES is faithful to the physics of the interaction of the lower atmosphere and the land surface based on classical validation tests of the simulated mean wind profile and the atmospheric turbulence. Simulations of the atmospheric boundary layer (ABL) over heterogeneous land surfaces with a range of characteristic lengths and surface roughness values are performed, each simulated surface consisting of equal‐size stripes of different roughness. The simulated mean wind profiles are analyzed to identify the height of the blending layer and used to develop a relationship between blending layer height and characteristic surface length scales. For hydrologic and atmospheric applications where the regional‐scale surface roughness needs to be known, the analysis is extended to derive an effective surface roughness knowing local surface patch roughness values.

Pressure fluctuations on a square building model in boundary-layer flows

Spatio-temporal measurements of a fluctuating pressure field acting on the side faces of a square prism of finite height in boundary-layer flows are presented for 0° angle of attack. Two typical neutral atmospheric flow conditions were simulated in the wind tunnel to represent open country and urban flow environments. The fluctuating pressure field data allowed computations of the mean and r.m.s. (root mean square) pressure coefficients, power spectral density, autocorrelations, co-spectra, cross-correlations, orthogonal eigenfunction expansions and statistical dependence. Increased levels of turbulence in the incident flow have a marked influence on the fluctuating pressure field, through modifications which take place in the structure of the separated shear layers. The periodic vortex-shedding process is vitiated in the presence of high levels of turbulence intensity in the incident flow, resulting in redistribution of the energy associated with pressure fluctuations over a wider frequency range.

Mean force and moment coefficients for buildings in turbulent boundary layers

Mean force and moment coefficients for a series of flat-roofed rectangular buildings are presented for a range of wind directions. The coefficients were determined by integrating mean pressures measured on model structures immersed in thick turbulent boundary layers simulating four typical neutral atmospheric flow conditions (power-law exponents of 0.12, 0.27, 0.34, and 0.38). A series of thirteen different building geometries were studied. The ratios of adjacent sides were 1, 0.5, 0.25, and the aspect ratio, the ratio of the height to the smaller side, ranged from 1 to 8. The effects of side ratio, aspect ratio, incident turbulence intensity, longitudinal integral scale, and the ratio of building height to boundary-layer thickness are discussed.