Conference programme

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Thursday, 19 November 2015
09:00 - 10:30 Advanced modeling of offshore and stratified flow
Resource assessment  
Onshore      Offshore    


Room: Montparnasse

In this session on advanced flow modelling focus will be on the following issues: modelling flow under stable atmospheric conditions, tools for offshore wind farm planning and the benefits of high-resolution modelling. The stable flows will be modelled with CFD codes in three different ways, applied to on-shore and coastal environments.

Learning objectives

Delegates will be able to explain:

  • why flow under stable conditions is different from neutral flow
  • the main additions to the CFD codes in order for the models to improve results under stable conditions
  • the main parts of a tool for offshore wind farm planning and how they work together
  • how increasing the resolution of models for resource assessment improves the value of the output  
Lead Session Chair:
Lars Landberg, DNV-GL, Denmark
Li Ru Meteodyn Company, France
Co-authors:
LI Ru (1) F BULLIDO GARCIA Maria (1) DELAUNAY Didier (1)
(1) Meteodyn, Nantes, France

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2015 and are published here unedited

Ms. LI Ru has obtained Ph.D. degree in Paris-Est University in 2012 in the domain of computational fluid dynamics and the Heat Transfer. During her study, she has published five research papers about heat radiations and convection flows. She is currently a Research and Development Engineer in Meteodyn company. Her research is focused on developing the advanced numerical method and thermal stability model in ABL for the software Meteodyn WT.

Abstract

New turbulence model for the Stable Boundary Layer with application to CFD in wind resource assessment

Introduction

In wind resource assessment, the impact of the atmospheric thermal stability attracts more and more attention. The first reason is the increase of wind turbines height. The second reason is that the practice of WRA engineers is evolving towards a regular use of time series extrapolations (measurements or mesoscale data) or at least more detailed statistical analysis. Using time series instead of global statistics leads to considering more stable situations.

Approach

In CFD models, the turbulent fluxes are linked to the gradients of the mean variables via the concept of turbulent viscosity, considered as the product of a turbulent wind speed scale, and a turbulent length scale. Up to now, this approach failed to reproduce strong stability cases because of the underlying hypothesis of the Monin-Obukhov Similarity Theory (MOST). We propose a new multi-layer approach for the turbulent viscosity modeling by taking benefit of the k-L model which offers a very good adaptation to real atmospheric flows.

Main body of abstract

Above the dynamic layer, dominated by mechanical effects, 3 layers are considered:
1. The MOST layer where the gradient Richardson number is inferior to a value of about 0.10: the model of Yamada and Arritt is used introducing modifications in the mixing length formulation in order to take into account a limiting buoyancy length scale.
2. A transitional layer, where the fluxes decrease with height, modeled according to a “z-less scaling” or a local MOST theory.
3. The outside layer, where the turbulent fluxes depend on large scale turbulence generated at the meso-scale level.
This model is implemented in the CFD software Meteodyn WT. The model calibration has been made with high quality measurements at the Cabauw tower (the Netherlands), located in a flat and low roughness terrain. Wind and temperature measurements were available from 10 m to 200 m height. A Stability Index was defined considering temperature and wind speed measurements at 10 m and 80 m height. The model parameters depending on stability are: the Obukhov length inside the MOST layer, the height of the MOST layer, the height of the local MOST theory layer. These parameters values are given in the final paper.
The validation of the model has been made with data obtained at Rödeser Berg (Germany) on a hilly forested site. Mean wind speeds are available between 10 m and 200 m height. The stability was obtained calculating the Obukhov length deduced from heat and momentum fluxes measured with a 3D ultrasonic anemometer at 40 m height.
It is shown that the implementation of this model in a CFD software (here Meteodyn WT) allows to reproduce mean wind speed and TKE profiles even in cases where the Stable Boundary layer lays below the height of 200 m or under (until 75 m).


Conclusion

We propose a new turbulence model for implementation in CFD codes so that strong stability cases can be considered. Applying a k-L multi-layer model is a way to reproduce the behavior of the Stable Boundary Layer, especially when the height of the SBL falls under the wind turbine hub height.


Learning objectives
The first learning objective is to attract the attention of WRA engineers about the consequences on CFD modelling of using either global statistics or time series as data input.
The second learning objective is to demonstrate that there are some available solutions for studying strong stability cases. The new model described in this abstract will be soon available in Meteodyn WT software.