Parallelized Large-eddy Simulation Model Research Articles

Large-eddy simulation of reactive pollutant exchange at the top of a street canyon

The exchange of reactive pollutants (NO, NO2, and O3) at the top (roof level) of a street canyon are investigated using the parallelized large-eddy simulation model (PALM). The transport equations of NO, NO2, and O3 with simple photochemical reactions are combined within the LES model for this study. NO and NO2 are emitted from an area source located near the canyon floor, and O3 is included within the ambient air and inflow. A clockwise-rotating vortex appears in the street canyon and transports NO, NO2, and O3. NO and NO2 are transported along the ground and leeward wall and escape from the canyon at the roof level. O3 enters the canyon at the roof level and is transported along the windward wall. The mean O3 production rate is generally negative with large magnitudes at and near the roof level and near the windward wall. The chemical reactions reduce the mean NO and O3 concentrations in the canyon by 31% and 84%, respectively, and increase the mean NO2 concentration in the canyon by 318%. The exchange of reactive pollutants at the roof level is significantly affected by small-scale eddies at the roof level and low- or high-speed streaks above the canyon. Air in the canyon with high NO and NO2 concentrations escapes from the canyon when low-speed air parcel appears due to small-scale eddies at the roof level or low-speed streak above the canyon. In contrast, air outside the canyon with a high O3 concentration enters the canyon when high-speed air parcel appears because of small-scale eddies at the roof level or high-speed streak above the canyon. The time-lagged correlation analysis reveals that NO, NO2, and O3 concentrations near the ground are affected by low- or high-speed streaks above the canyon but not significantly affected by small-scale eddies at the roof level.

A constrained model predictive wind farm controller providing active power control: an LES study

The objective of active power control in wind farms is to provide ancillary grid services. Improving this is vital for a smooth wind energy penetration in the energy market. One of these services is to track a power reference signal with a wind farm by dynamically de- and uprating the turbines. In this paper we present a computationally efficient model predictive controller (MPC) for computing optimal control signals for each time step. It is applied in the PArallelized Large-eddy simulation Model (PALM), which is considered as the real wind farm in this paper. By taking measurements from the PALM, we show that the closed-loop controller can provide power reference tracking while minimizing variations in the axial forces by solving a constrained optimization problem at each time step. A six turbine simulation case study is presented in which the controller operates with optimised turbine yaw settings. We show that with these optimized yaw settings, it is possible to track a power signal that temporarily exceeds the power harvested when operating under averaged greedy control turbine settings. Additionally, variations in the turbine’s force signals are studied under different controller settings.

Large-eddy simulation study of wind farm active power control with a coordinated load distribution

In this paper, an active power control (APC) approach for wind farms is studied, for a case in which the wind turbines interact with each other aerodynamically through their wakes. We demonstrate that the structural loadings on the individual wind turbines can be coordinated to expand their lifetimes, while the wind farm production tracks a power reference signal. We propose an additional feedback control loop in order to adjust the distribution of the regulated power demands among the wind turbines, exploiting the non-unique solutions of APC for wind farms. The axial induction factor of each wind turbine is considered as a control input to influence the overall wind farm performance. The applicability of the controller is tested with a wind farm example consisting of 2×3 turbines with partial wake overlaps and detailed interactions with the atmospheric boundary layer, simulated with the PArallelized Large-eddy simulation Model (PALM). The results demonstrate the effectiveness of the proposed approach and point to some future studies that may improve and extend the performance.

Lidar-based closed-loop wake redirection in high-fidelity simulation

This work presents the next step in realizing lidar-based closed-loop wake redirection control. Lidar-based closed-loop wake redirection aims at repositioning the wake at a desired position by yawing the wind turbine. The actual wake deflection is derived from lidar measurements and used in a closed-loop control scheme. Compared to an open-loop setting in which temporal changes are not taken into account, lidar-based closed-loop wake redirection can react on temporal disturbances. This yields a more robust control solution due to the employed closed-loop control framework. In this work, for the first time, the concept is implemented in an LES environment namely the PArallelized Large-eddy simulation Model (PALM) code. In PALM lidar measurements are simulated using a lidar model which are processed to estimate the wake position. A controller is synthesized by the usage of a the reduced order wind farm model WindFarmSimulator (WFSim). High-fidelity simulation results illustrate the controller’s ability to adapt to a temporal changing crosswind disturbance in a turbulent simulation scenario. Consequently, it increases the power output of the two-turbine scenario compared to the open-loop approach.

A Model Predictive Wind Farm Controller with Linear Parameter-Varying Models

In this paper, we present an implementation of a model predictive controller (MPC) for wind farm power tracking problem. The controller is evaluated in the high-fidelity PAral-lelized Large-eddy simulation Model (PALM). By taking measurements from PALM, we show that the closed-loop MPC can provide power reference tracking while reducing force variations on a farm level by solving a constrained optimization problem at each time step. A six turbine wind farm case study is presented in which the controller operates with yawed turbines that increases the potential power that can be harvested with the wind farm, and we show that it is possible to track a reference power signal that temporarily exceeds the power harvested when operating under the so-called greedy control settings.

Large‐eddy simulation of vortex streets and pollutant dispersion behind high‐rise buildings

Understanding turbulent flow and pollutant dispersion in urban areas is one of the important problems in urban meteorology and environmental fluid mechanics. In this study, we examine turbulent flow and pollutant dispersion in a densely built‐up area in Seoul, South Korea, using the parallelized large‐eddy simulation model (PALM). In particular, we focus on vortex streets and associated pollutant dispersion behind high‐rise buildings. The turbulence recycling method is used to produce inflow profiles. Vortices are generated near the high‐rise buildings and propagate downstream forming vortex streets behind the high‐rise buildings. To investigate characteristics of the vortex streets, spectral and correlation analyses are performed. The spectral analysis reveals that vortices have a non‐dimensional vortex shedding frequency of 0.1–0.2, and this periodicity is weakened due to the influence of other buildings. The correlation analysis shows that vortices appear frequently in regions of negative pressure perturbation. The vertical turbulent momentum fluxes induced by ejections and sweeps largely contribute to the total vertical turbulent momentum flux downstream of the high‐rise buildings. Especially, ejections in the wake region are stronger compared to other regions because ejections are induced by vortices near the top of the high‐rise buildings. It is found that pollutant dispersion is interrupted by both low‐rise and high‐rise buildings. Strong updraughts behind the high‐rise buildings transport pollutant upward and increase the mean pollutant concentration at upper levels. Vortices forming the vortex streets play a role in pollutant mixing in such a way that the vortices eject air of high pollutant concentration from the wake region behind the high‐rise buildings and entrain air of low pollutant concentration into the wake region. The mixing by vortices is verified by the correlation between vorticity and pollutant concentration.

Large-eddy simulations of ventilation for thermal comfort — A parametric study of generic urban configurations with perpendicular approaching winds

This study investigates ventilation performance in parametric urban scenarios using a large-eddy simulation (LES) model called the Parallelized LES Model (PALM). With various combinations of planning parameters, air flows and pedestrian-level velocity ratios in a total of 48 scenarios are investigated. Major findings and recommendations are: First, ground coverage ratio (λp) is the most important factor for good ventilation. Second, in cases of homogeneous building heights, a power regression between velocity ratios and aspect ratios of parallel street canyons can be derived, which suggests that good understanding of local microclimate, especially prevailing wind directions in summer, is needed in urban planning. Third, the effects of building height differentials on urban ventilation are connected to urban density. In low-density scenarios, inhomogeneous building heights give worse ventilation performance compared to homogeneous cases. In high-density scenarios, inhomogeneous building heights result in better ventilation performance than homogeneous cases. Inhomogeneous building heights generate more vertical momentum fluxes in street canyons and have a negative (positive) effect on velocity ratios of low-density (high-density) parametric urban fabrics. The application of this point is that homogeneous building heights are recommended when low density is present, and inhomogeneous building heights may be better in cases of high density.

The Parallelized Large-Eddy Simulation Model (PALM) version 4.0 for atmospheric and oceanic flows: model formulation, recent developments, and future perspectives

Abstract. In this paper we present the current version of the Parallelized Large-Eddy Simulation Model (PALM) whose core has been developed at the Institute of Meteorology and Climatology at Leibniz Universität Hannover (Germany). PALM is a Fortran 95-based code with some Fortran 2003 extensions and has been applied for the simulation of a variety of atmospheric and oceanic boundary layers for more than 15 years. PALM is optimized for use on massively parallel computer architectures and was recently ported to general-purpose graphics processing units. In the present paper we give a detailed description of the current version of the model and its features, such as an embedded Lagrangian cloud model and the possibility to use Cartesian topography. Moreover, we discuss recent model developments and future perspectives for LES applications.

Impacts of Mesoscale Wind on Turbulent Flow and Ventilation in a Densely Built-up Urban Area

AbstractTurbulent flow in a densely built-up area of Seoul, South Korea, for 0900–1500 LST 31 May 2008 is simulated using the parallelized large-eddy simulation model (PALM) coupled to a mesoscale model (Weather Research and Forecasting Model). Time-varying inflow that is composed of mesoscale wind and turbulent signals induces different mean flows and turbulence structures depending on time. Sweeps induced by upper flow are distinct for 0900–0910 LST, and strong ejections and weaker sweeps are dominant for 1450–1500 LST at height z = 200 m. To investigate pedestrian wind environment and ventilation, mean wind velocity and turbulent kinetic energy at 2.5 m above streets are analyzed. The reference mean wind speed at z = 600 m continuously increases after 1010 LST. The pedestrian mean streamwise velocity tends to decrease after 1100 LST, although the pedestrian mean wind speed tends to slowly increase. Whereas the temporal velocity variations related to mesoscale wind are distinct in a street canyon and an intersection, the variations induced by mesoscale wind disappear in a dense building area, indicating strong decoupling from mesoscale wind. The velocity ratio of the pedestrian mean wind speed to the reference mean wind speed, representing a measure of ventilation in urban areas, is high on broad streets and at intersections and is low in dense building areas. Vortices in street canyons and winding flows around tall buildings seem to induce high velocity ratio there. The velocity ratio is shown to be linearly proportional to the pedestrian mean streamwise velocity.

An Improved Three-Dimensional Simulation of the Diurnally Varying Street-Canyon Flow

The impact of diurnal variations of the heat fluxes from building and ground surfaces on the fluid flow and air temperature distribution in street canyons is numerically investigated using the PArallelized Large-eddy Simulation Model (PALM). Simulations are performed for a 3 by 5 array of buildings with canyon aspect ratio of one for two clear summer days that differ in atmospheric instability. A detailed building energy model with a three-dimensional raster-type geometry—Temperature of Urban Facets Indoor-Outdoor Building Energy Simulator (TUF-IOBES)—provides urban surface heat fluxes as thermal boundary conditions for PALM. In vertical cross-sections at the centre of the spanwise canyon the mechanical forcing and the horizontal streamwise thermal forcing at roof level outweigh the thermal forces from the heated surfaces inside the canyon in defining the general flow pattern throughout the day. This results in a dominant canyon vortex with a persistent speed, centered at a constant height. Compared to neutral simulations, non-uniform heating of the urban canyon surfaces significantly modifies the pressure field and turbulence statistics in street canyons. Strong horizontal pressure gradients were detected in streamwise and spanwise canyons throughout the day, and which motivate larger turbulent velocity fluctuations in the horizontal directions rather than in the vertical direction. Canyon-averaged turbulent kinetic energy in all non-neutral simulations exhibits a diurnal cycle following the insolation on the ground in both spanwise and streamwise canyons, and it is larger when the canopy bottom surface is paved with darker materials and the ground surface temperature is higher as a result. Compared to uniformly distributed thermal forcing on urban surfaces, the present analysis shows that realistic non-uniform thermal forcing can result in complex local airflow patterns, as evident, for example, from the location of the vortices in horizontal planes in the spanwise canyon. This study shows the importance of three-dimensional simulations with detailed thermal boundary conditions to explore the heat and mass transport in an urban area.

A Large-Eddy Simulation Study of Bottom-Heating Effects on Scalar Dispersion in and above a Cubical Building Array

AbstractThermal effects on scalar dispersion in and above a cubical building array are numerically investigated using the parallelized large-eddy simulation model (PALM). Two cases (no heating and bottom heating) are simulated, and scalar dispersion patterns in the two cases are compared. In the no-heating case, scalar ejections in the low-speed flow structures play an important role in transporting scalar upward above the building array. In the bottom-heating case, streamwise elongated and isolated scalar ejections appear below upper low-speed and upper high-speed regions above the building array. In both cases, bottom-emitted scalar flux is balanced by streamwise scalar advection and vertical turbulent scalar flux at the rooftop height. The vertical turbulent scalar flux at the rooftop height is mainly composed of scalar ejections and scalar sweeps that are related to low- and high-speed flow structures, respectively. Furthermore, the low- and high-speed flow structures at the rooftop height induce spanwise converging and spanwise diverging flow in the building array in both the no-heating and bottom-heating cases. Thus, the mean scalar concentration in the building array is high below the low-speed flow structures (above the building array) in both cases. Dominant scalar dispersion patterns in the building array are found to be spanwise scalar transport events that are composed of negative scalar concentration perturbation and spanwise flow therein. In the bottom-heating case, a large-scale secondary circular flow develops, causing stronger spanwise scalar dispersion patterns in the building array.

A Large-Eddy Simulation Study of Thermal Effects on Turbulence Coherent Structures in and above a Building Array

AbstractThermal effects on turbulent flow in and above a cubical building array are numerically investigated using the parallelized large-eddy simulation model (PALM). Two cases (no heating and bottom heating) are simulated and are compared with each other, focusing on thermal effects on turbulence coherent structures. In the no-heating case, the streaky or streamwise-elongated structures of low-speed regions appear above the building array and ejections in the low-speed regions play an important role in transporting momentum downward. In the bottom-heating case, plume-shaped structures appear with streamwise-elongated structures and the magnitude of vertical turbulent momentum flux averaged over the low-speed regions increases. Elliptical structures of negative streamwise velocity perturbation and vortical structures similar to hairpin vortices appear above the building array in the conditionally averaged fields in both cases, and the coherent structures expand more vertically when the bottom is heated. At or just above the rooftop height, high-speed streaks are distinct and sweeps induced by the streaks or shear instability are important for momentum transport in both cases. In the bottom-heating case, the magnitude of vertical turbulent momentum flux at the tops of cavity spaces increases, partly owing to the strengthened ejections. Below the rooftop height, the high-speed streaks occasionally enter intersection spaces and induce spanwise diverging flow there in both cases. When the bottom is heated, intensified updrafts induce more organized secondary circular flow and the spanwise flow in the building array is strengthened by the secondary flow.

High resolution urban large-eddy simulation studies from street canyon to neighbourhood scale

Urban turbulence characteristics are investigated at street canyon and neighbourhood scale. Three high resolution urban large-eddy simulation (LES) studies are performed using the urban version of the parallelized LES model PALM. Validation shows that the urban PALM version is in line with experimental and previous LES results, i.e. superior to the faster/cheaper conventional Reynolds-averaged (RANS) models. Two studies focus on quasi-2D urban street canyons driven by perpendicular flow. First is a parametric study of turbulence characteristics and flow dynamics within the canyon. The main results are: (1) Integral vertical turbulence profiles in deep canyons scale with canyon width. This is relevant for urban canopy parameterizations in larger-scale meteorological models. (2) A new concept of a “cavity shear layer” complements classical free shear layer concepts. (3) For the first time in urban LES Kelvin–Helmholtz instabilities are identified at the top of the urban street canyon. This is relevant for modelling urban dispersion, because the street canyon circulation is more intermittent than suggested by previous RANS results. Second, an Eulerian dispersion case study shows that differences in canyon flow dynamics are reflected in canyon dispersion characteristics compared to a previous RANS study. Third is a neighbourhood scale urban LES feasibility study: a passive tracer Lagrangian dispersion animation of Shinjuku, downtown Tokyo reveals turbulent flow features, upstream flow and intermittency. The main implications are always to use 3D models for turbulence simulations even in quasi-2D geometries, and not to underestimate the intermittency of turbulent flow. Standard deviations of velocity components within the canyon should not be treated as constant for perpendicular ambient wind but may be parameterized conveniently based on a vertical scaling with canyon width in deep canyons.

Large Eddy Simulation of Thermally Induced Oscillations in the Convective Boundary Layer

Mesoscale circulations induced by differential boundary layer heating due to surface inhomogeneities on scales of 5 km and more can significantly change the average properties and the structure of the convective boundary layer (CBL) as well as trigger off temporal oscillations. The results of one of the first numerical case studies using large eddy simulation (LES) on the mesoscale suggest that mesoscale circulations exhibit a considerably larger average kinetic energy than convection under homogeneous conditions. This affects turbulent transport processes and should be accounted for in larger-scale models even if their turbulence parameterizations rely on homogeneous control runs of high-resolution models. This case study uses the Hannover parallelized large eddy simulation model (PALM) with prescribed 1D sinusoidal surface heat flux variations on wavelengths from 2.5 to 40 km. The resulting mesoscale circulations are analyzed by means of domain-averaged cross sections, time averaged and normalized with the boundary layer height, as well as power spectra and domain-averaged time series. The simulated mesoscale circulations were periodic. Vertical profiles and time series demonstrate that the onset of the mesoscale circulation triggers off a temporal boundary layer oscillation, whose period and amplitude depend on the surface heat flux perturbation wavelength and amplitude and on the background wind component perpendicular to the surface inhomogeneity orientation. Spectral analysis shows that the mesoscale circulations damp convection equally in all directions. A hypothesis of the oscillation mechanism is briefly discussed.

LARGE-EDDY SIMULATION OF THERMALLY INDUCED OSCILLATIONS IN THE CONVECTIVE BOUNDARY LAYER

Abstract Mesoscale circulations induced by differential boundary layer heating due to surface inhomogeneities on scales of 5 km and more can significantly change the average properties and the structure of the convective boundary layer (CBL) as well as trigger off temporal oscillations. The results of one of the first numerical case studies using large eddy simulation (LES) on the mesoscale suggest that mesoscale circulations exhibit a considerably larger average kinetic energy than convection under homogeneous conditions. This affects turbulent transport processes and should be accounted for in larger-scale models even if their turbulence parameterizations rely on homogeneous control runs of high-resolution models. This case study uses the Hannover parallelized large eddy simulation model (PALM) with prescribed 1D sinusoidal surface heat flux variations on wavelengths from 2.5 to 40 km. The resulting mesoscale circulations are analyzed by means of domain-averaged cross sections, time averaged and normali...

PALM - A large-eddy simulation model performing on massively parallel computers

An existing code of a large-eddy simulation (LES) model for the study of turbulent processes in the atmospheric and oceanic boundary layer has been completely recoded for use on massively parallel systems with distributed memory. Parallelization is achieved by two-dimensional domain decomposition and communication is realized by the message passing interface (MPI). Periodic boundary conditions, which are used in both horizontal directions, helped to minimize the parallelization effort. The performance of the new PArallelized LES Model (PALM) is excellent on SGI/Cray-T3E systems and an almost linear speed-up is achieved up to very large numbers of processors. Parallelization strategy and model performance is discussed and validation experiments as well as future applications are presented.