Transient Wind Conditions Research Articles

Unsteady-state CFD simulations on the impacts of urban geometry on outdoor thermal comfort within idealized building arrays

This study investigated outdoor thermal comfort within 5 × 5 idealized building arrays on five consecutive days by conducting unsteady computational fluid dynamic (CFD) simulations. The dynamic interacting effects of building height topology, building distance and building layout on airflow patterns, spatial distributions and spatially averaged outdoor thermal environment were evaluated by UTCI (Universal Thermal Comfort Index). CitySim Pro was used to simulate the transient solar-induced temperatures on the walls of buildings and grounds inside building arrays, which were then set as thermal boundary conditions in CFD simulations. The results showed that the influence of transient wind conditions on airflow patterns and urban thermal comfort was significant so steady simulations or unsteady simulations of one typical day might not provide a complete overview of the wind-thermal environment. The results also showed that presenting the results by average UTCI or spatial distribution could lead to different conclusions on the impacts of urban geometry on urban thermal comfort, especially the impact of building distance. Increasing the building height always provided positive effects on urban thermal comfort. The spatial distributions of UTCI in three-dimensional building arrays with equal aspect ratio diverse significantly, which was not observed in the pattern of average UTCI.

Ocean Surface Boundary Layer Response to Abruptly Turning Winds

AbstractTurbulence driven by wind and waves controls the transport of heat, momentum, and matter in the ocean surface boundary layer (OSBL). For realistic ocean conditions, winds and waves are often neither aligned nor constant, for example, when winds turn rapidly. Based on a Large Eddy Simulation (LES) method, which captures shear-driven turbulence (ST) and Langmuir turbulence (LT) driven by the Craik-Leibovich vortex force, we investigate the OSBL response to abruptly turning winds. We design idealized LES experiments, whose winds are initially constant to equilibrate OSBL turbulence before abruptly turning 90° either cyclonically or anticyclonically. The transient Stokes drift for LT is estimated from a spectral wave model. The OSBL response includes three successive stages that follow the change in direction. During stage 1, turbulent kinetic energy (TKE) decreases due to reduced TKE production. Stage 2 is characterized by TKE increasing with TKE shear production recovering and exceeding TKE dissipation. Transient TKE levels may exceed their stationary values due to inertial resonance and non-equilibrium turbulence. Turbulence relaxes to its equilibrium state at stage 3, but LT still adjusts due to slowly developing waves. During stages 1 and 2, greatly misaligned wind and waves lead to Eulerian TKE production exceeding Stokes TKE production. A Reynolds stress budget analysis and Reynolds-averaged Navier-Stokes equation models indicate that Stokes production furthermore drives the OSBL response. The Coriolis effects result in asymmetrical OSBL responses to wind turning directions. Our results suggest that transient wind conditions play a key role in understanding realistic OSBL dynamics.

Validation of a non-contact technique for torque measurements in wind turbines using an enhanced transient FSV approach

In-service turbine monitoring is essential for maximizing the wind energy contribution to the global energy budget. Measurement of turbine shaft torque under transient wind conditions is fundamental to develop reliable condition monitoring techniques. Contact based measurements bring their own disadvantages and non-contactless measurements have many potential advantages. However, their performance needs to be validated against standard methods. This paper focuses on the development of an enhanced transient Feature Selective Validation (FSV) techniques to undertake this analysis with an emphasis on transient data processing. The nature of FSV makes it a natural technique to consider for this problem space. Open questions have existed as to how transients should be dealt with in FSV. This paper overcomes the limitations of previous approaches for step-function transient comparison and presents analytical methods to ensure the transient feature itself is considered, irrespective of how much pre- and post- transient data happens to be included.

A 3-DOF forced vibration system for time-domain aeroelastic parameter identification

A novel three-degree-of-freedom (DOF) forced vibration system has been developed for identification of aeroelastic (self-excited) load parameters used in time-domain response analysis of wind-excited flexible structures. This system is capable of forcing sinusoidal motions on a section model of a structure that is used in wind tunnel aeroelastic studies along all three degrees of freedom - along-wind, cross-wind, and torsional - simultaneously or in any combination thereof. It utilizes three linear actuators to force vibrations at a consistent frequency but varying amplitudes between the three. This system was designed to identify all the parameters, namely, aeroelastic- damping and stiffness that appear in self-excited (motion-dependent) load formulation either in time-domain (rational functions) or frequency-domain (flutter derivatives). Relatively large displacements (at low frequencies) can be generated by the system, if required. Results from three experiments, airfoil, streamlined bridge deck and a bluff-shaped bridge deck, are presented to demonstrate the functionality and robustness of the system and its applicability to multiple cross-section types. The system will allow routine identification of aeroelastic parameters through wind tunnel tests that can be used to predict response of flexible structures in extreme and transient wind conditions.

Modelling Turbine Loads during an Extreme Coherent Gust using Large Eddy Simulation

A group of wind turbines operating in extreme transient wind conditions has been simulated using LES and an actuator model. An extreme wind event is introduced into the simulation domain using transient boundary conditions. The event is based on the extreme coherent gust (ECG) structure from the International Wind Turbine Design Standard IEC61400-1:2005 which consists of a simultaneous gust and wind direction change. Details of the implementation are discussed with regard to adapting the analytical functions described in the standard. A recently developed actuator sector method is used to represent the wind turbines in the simulation. The actuator method is coupled to the aero-elastic wind turbine simulation code FAST to allow dynamic control of the wind turbines based on the ambient flow conditions. Standard baseline controllers are used to regulate generator torque, actuate blade pitch angle and control yaw direction. A span-wise periodic array of turbines operating in a steady atmospheric boundary layer is simulated before the introduction of the ECG structure. The convection of the wind event is analysed, along with the subsequent response of the wind turbines and loading during the wind event is quantified. The simulations demonstrate a methodology for modelling groups of turbines operating in transient wind conditions that can be used to study turbine loads or test new turbine control strategies.

Dispersion of carbon dioxide from vertical vent and horizontal releases—A numerical study

Numerical simulations of far-field carbon dioxide dispersion were conducted for a vertical vent release and a horizontal release from a shock tube. These scenarios had also been studied experimentally at field scale commissioned by National Grid. This work and the experiments both form part of the National Grid dense phase CO2 pipeLine TRANSportation (COOLTRANS) research programme. All tests involved releases of dense phase CO2 into an atmospheric flow. The dispersing plumes were subjected to transient wind conditions where both the direction and magnitude of the wind fluctuated with time. As part of the COOLTRANS research programme, the far-field dispersion simulations started from source terms derived from the near-field simulations conducted by the University of Leeds and outflow simulations conducted by University College London. The numerical model used for the far-field simulations is based on OPENFOAM, which is an object-oriented open source computational fluid dynamics toolbox. A dedicated solver CO2FOAM has been developed within the framework of OPENFOAM for simulating dispersion from dense phase CO2 releases. This has included the implementation of the homogeneous equilibrium method for fully compressible two-phase flow, treatment of the transient atmospheric boundary conditions and the time-varying inlet boundary conditions. The experimental measurements were supplied to the authors after the predictions were completed and submitted to National Grid. Hence, the validation reported here is indeed “blind.” While further fine tuning of the model and validation is still underway, the relatively good agreement between the predictions and measurements in the present study has demonstrated the potential of CO2FOAM as an effective predictive tool for far-field CO2 dispersion in the context of pipeline transportation for carbon capture and storage.

Eddies and filaments of the Western Adriatic Current near Cape Gargano: Analysis and prediction

Remote sensing and in situ observations, and model simulations made during the Dynamics of the Adriatic in Real Time (DART) project in the central Adriatic Sea during 2006 reveal intense mesoscale eddy activity in the West Adriatic Current (WAC) system, that flows along the Italian coast. We investigate the origin, scale and rate of evolution of these features, and consider their potential contributions to across-shelf exchange, as a basis for planning future operational coastal prediction and monitoring programs. Predictions from a regional Adriatic Sea implementation of the Navy Coastal Ocean Model (NCOM) are assessed using observations from satellites, current meter moorings, and ship hydrographic transects. Near Cape Gargano in summer, mesoscale eddies and filaments observed in remote sensing imagery and in situ data exhibit horizontal and vertical scales of about 30 km and 25 m, respectively. The observed eddies develop along the WAC front under transient wind conditions, while a quasi-permanent anti-cyclonic circulation appears downstream of the cape. NCOM model predictions show that well-organized, multiple-eddy features appear, grow in unison, then dissipate along the coast west of Cape Gargano in response to transient along-shelf wind forcing, while east of it individual eddies are generated by interaction of the WAC main stream with the bathymetry surrounding the cape. The eddies evolve over time scales of a few days to a week in response to forcing transients. Moderate to strong northwest winds compress the WAC coastward and suppress the instabilities, while relaxing northwest winds or shifts to southeast winds expand the WAC and enhance instability growth. Together with entrained filaments of coastal water, the eddies translate, stretch and rotate in response to WAC horizontal advection and shear. The qualitative character and evolution of the modeled features agree with the observations, and model statistics reveal patterns of variability on monthly and seasonal time scales that are consistent with the appearance of individual eddies in both the model results and observations. In the model, patterns of upwelling and downwelling associated with the WAC main stream and eddies may be linked to dynamical characteristics of the WAC flow and specific bathymetric features. Under wintertime conditions, theoretical arguments predict an upwelling response as flow divergence changes absolute vorticity where the WAC rounds the cape, while Ekman pumping driven by surface winds or bottom stress and associated secondary circulation could explain upwelling and downwelling patches appearing around the rims of individual eddies. The upwelling dynamics are complicated by seasonal stratification effects and bathymetric features, and more work is needed to elucidate these processes. Based on their frequent occurrence, large size compared to the shelf width, high intensity and current shear, and secondary circulations, the eddies and filaments could contribute significantly to across-shelf exchange and coastal/deep-ocean mixing, with important implications for coastal monitoring and prediction. Their generation and evolution present special challenges for coastal prediction, but our results demonstrate that combined use of remote sensing imagery and numerical model simulations can yield valuable insight into such processes, and thereby help plan future monitoring and prediction efforts.

The Use of Three-Dimensional Numerical Air Pollution Models In Planning Plant Location Design And Operation

Abstract The costs associated with plant location, construction, design and operation are, to a considerable extent, influenced by air quality requirements. Standard analytical methods for predicting the environmental impact of plant emissions do not consider:the effect of terrain in modifying the wind field.transient winds orthe cumulative effects of multiple sources, except in an additive manner. These factors can greatly affect pollutant concentrations and need to be considered in plant planning to avoid future costly modifications and for efficient plant operation. This paper discusses the use of a comprehensive mathematical model that describes wind flow over or around topographical features, simulates transient wind conditions and simultaneously solves for the pollutant concentrations caused by multiple sources. The applications described in the paper include determination of plant location, plant design and operation in areas of complicated topography, and the determination of monitor locations. Also discussed is the use of the model in conjunction with monitors Lo provide complete areal and vertical coverage of pollutant concentrations. INTRODUCTION During Recent Years, environmental impact statements have become one of the primary considerations involved in the utilization of natural resource. Today a company is expected to show that its operations whether for new or existing plants - are not causing emissions which harm the environment on either an instantaneous or cumulative basis. A comprehensive mathematical model which defines the ambient concentrations from given emission levels can help provide these necessary impact statements. Ambient monitoring and mathematical modelling are a tremendous complementary team. Ambient monitoring cannot provide the impact statement of process changes of a new plant except after the fact. On the other hand, mathematical modelling may not have the desired precision unless controlled by matching measured concentration and/or wind velocity data. This may sound contradictory when considering a new plant - but it is not. One of the first requirements for a good environmental impact statement is the definition of background concentration level before the new plant or an operational change is introduced. Through history-matching measured data, a numerical model which includes all important aspects of the problem can accurately predict either the additional impact due to new plants and increased throughput or the reduced impact due to abatement equipment being installed. Problem Areas Perhaps the mathematical models most widely used in providing impact statements are the Gaussian plume models (1,2,3). Unfortunately, these models have a number of restrictions which limit their use particularly limiting are the assumptions of how terrain can influence the wind and the restriction to steady-state, uniform wind velocity. The two assumptions most often used with the plume models concerning terrain influence on wind are:the wind streamlines remain horizontal and intersect terrain orthe wind always flows parallel to the ground surface. An application problem contained in the paper evaluates these assumptions as they compare with the three-dimensional numerical model results. Several recent presentations have described methods which can include topographic effects(4, 5. 6) or large urban buildings(7,8).

Correlation of 630-cps Shallow-Water Ambient Noise with Wind Speed and Waveheight

The sound-pressure spectrum level derived from a 13-octave filler centered at 630 cps was chosen to study the mutual associations among ambient noise, wind speed, and waveheight in Narragansett Bay, Rhode Island. Data were divided into three groups corresponding to relatively steady, increasing, or decreasing wind speeds. The results of correlation analyses indicate that waveheight is as significantly related to the sound-pressure spectrum level at 630 cps in this locate as is wind speed, provided that the wind speed is relatively constant. Wind speed is the more significant factor of the two under transient wind conditions.