Direct Numerical Simulation of turbulent flows over crops with application to ventilation in greenhouses

MSc graduation project of Maximo Leon Ganem
Location: Lab. for Aero & Hydrodynamics
Supervisors: W.-P. Breugem & M.J.B.M. Pourquie
Defense: August 24 2011, 2pm

Project description:

The climate in greenhouses is determined by complex and mutually interacting processes such as the ventilation of air and the transport of heat and moisture. These processes determine the micro-climate within the crop, but they are themselves also influenced by the crop. With increasing computer power, there is a growing interest in the application of commercial Computational Fluid Dynamics (CFD) packages to simulation of the climate in greenhouses (Norton et al., 2007). The knowledge gained from such simulations can be used to improve the design and optimize the performance of greenhouses.

The aim of this project is to gain insight in the influence of the crop on the turbulent flow of air in greenhouses with a focus on the ventilation in the vicinity of the crop cover. To simplify the problem the influence of heat and moisture on the flow is neglected. Furthermore, a simplified flow geometry is considered in which the air flows through a plane channel with a solid top wall and a bottom wall covered with a uniform crop layer. Direct Numerical Simulation (DNS) is used to resolve the turbulent flow both in space and time. In the DNS the crop layer is modeled as a porous medium and characterized by the porosity and the leaf area density. The Volume-Averaged Navier-Stokes (VANS) equations are used to solve the macroscopic flow inside the crop layer (Breugem et al., 2006). The VANS equations require a closure model for the drag force exerted by the crop on the macroscopic flow. Several closures are available in literature, which contain drag coefficients that have been determined from wind tunnel experiments of air flow through crops such as tomato, sweet pepper, aubergine and bean (Moline-Aiz et al., 2006).

In this project a literature study will be made of available closure models for the drag force exerted on the macroscopic flow through crops. Next, a suitable closure model will be employed in a DNS study of turbulent channel flow over several different crops. Statistical methods will be used to study the influence of the crop on the dynamics and structure of the turbulent flow. In a later stage of the project, the results of the DNS can be used to test popular turbulent-viscosity models used in commercial CFD codes for simulation of the climate in greenhouses. Furthermore, the DNS can be extended with a heat transport equation to study turbulent diffusion of heat and moisture across the crop cover.

References:
W.-P. Breugem, B.J. Boersma and R.E. Uittenbogaard. The influence of wall permeability on turbulent channel flow. Journal of Fluid Mechanics 562, 35-72, 2006.
F.D. Molina-Aiz, D.L. Valera, A.J. Àlvarez and A. Madueño. A wind tunnel study of airflow through horticultural crops: determination of the drag coefficient. Biosystems Engineering 93(4), 447-457, 2006.
T. Norton, D.-W. Sun, J. Grant, R. Fallon and V. Dodd. Applications of Computational Fluid Dynamics (CFD) in the modelling and design of ventilation systems in the agricultural industry: A review. Bioresource Technology 98, 2386-2414, 2007.

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