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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'The model chain: First steps towards tomorrow's technology' taking place on Thursday, 13 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Tilman Koblitz DTU, Denmark
Co-authors:
Tilman Koblitz (1) F P Andreas Bechmann (1) Niels Sørensen (1) Andrey Sogachev (1)
(1) DTU, Roskilde, Denmark

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Abstract

Atmospheric stability and complex terrain: comparing measurements and CFD

Introduction

For wind resource assessment, the wind industry is increasingly relying on Computational Fluid Dynamics (CFD) models that focus primarily on modeling the airflow in a neutrally stratified surface layer. So far, physical processes that are crucial to the atmospheric boundary layer (ABL), for example the Coriolis force, buoyancy forces and heat transport, are mostly ignored in state of the art CFD models. In order to decrease the uncertainty of wind resource assessment, especially in complex terrain, the effect of thermal stratification on the ABL should be included in such models.

Approach

The starting point for the present study is the existing in-house CFD code EllipSys3D. This general purpose CFD solver has been initially developed for flow over terrain and is used for a wide range of wind energy applications. To model the flow within the ABL more appropriately, the finite-volume code is modified.

Main body of abstract

The present study considers the simulation of the diurnal cycle in the ABL. The focus is on flow over complex terrain, subjected to temporally varying surface temperatures. The aim of this work is to study the combined effects of temperature and complex terrain on the resulting wind field, and to validate it’s representation in the model. To model the ABL more appropriately the effect of the Coriolis forcing and buoyancy are included in the CFD code EllipSys3D. Therefore an equation for the energy in terms of the potential temperature is solved in addition to the RANS equations. To close the given set of equations a modified version of the k-ε turbulence model is used: in contrast to the standard formulation we use a limiter on the resulting length-scale, and additional buoyancy terms. With these modifications the model is capable of representing non-neutral conditions
The resulting surface winds, temperature stratifications and TKE values are compared against observations taken during a field experiment in 2010 close to the village of Benakanahalli in India. Five 80m masts were erected near and on a long almost two-dimensional 120m natural ridge equipped with sonic anemometers and temperature sensors. The motivation of this campaign was to study the combined effects of complex terrain and atmospheric stability.
Comparison against observations raises the issue of initial and boundary conditions of numerical experiments, because perfect test cases do not occur in reality.


Conclusion

The combined effects of atmospheric stability and complex terrain are analyzed based on observations from a field experiment in India and compared against simulations.
The advantage of the presented RANS model framework is its general applicability. All implementations in the model are tuning free, and except for general site specific simulation parameters, no additional model coefficients need to be specified before the simulation. The developed ABL model significantly improves predictions when compared to the neutral model. In summary the results show that the implemented modifications are applicable and reproduce the main flow characteristics for non-neutral flow over flat and complex terrain.


Learning objectives
- Address the importance of atmospheric stability in wind resource assessment
- Show how atmospheric stability can be included in CFD models
- Analyze and compare combined effects of atmospheric stability and complex terrain on measurements and models