Development of 3D CFD models and observation system design for wind environment assessment over a clear-cut in mountainous region

Abstract

Installation of meteorological observation system in mountain region is important to aid effective monitoring of mountainous meteorology, predictions of mountain disasters such as fires and landslides, and the dispersion of pollutants or airborne viruses. International organizations, which include the World Meteorological Organization (WMO) and the Canadian Forest Sservice (CFS) recommend that weather observation system in mountain area should be located in clear-cut area where forest cover has been cleared or felled, and the distance from the system to any obstacle should be more than 10 times greater than maximum height of any nearby trees. In addition, the height of an anemometer to monitor wind environment in mountain region should be at least 10 m higher than the tallest trees, when the construction of clear-cut was limited. In addition, there is a method to install the meteorological observation system on the flat land in the Republic of Korea, but there are no installation standards of a mountain meteorological observation system. In case of the installation methods of international standards, excessively wide clear-cuts are difficult to find and tall observation masts cost more and may not be as safe as 10 m mast. Therefore, research to determine an appropriate installation methods for the meteorological observation system in mountain region in the Republic of Korea should be conducted. Because of limitations of a field experiment such as difficulty to acquire results due to unstable and unpredictable environmental conditions, high time- and labor costs, and a great deal of capital for a variety of experimental conditions, many researchers have used computational fluid dynamics (CFD). In this thesis, as a first step, comprehensive literature reviews on Meso-scale numerical weather prediction modelling, Micro-scale numerical weather prediction modelling and Air resistance of trees were intimately conducted to build the foundation and to suggest the appropriateness of the study In Chapter 3, a 3D micro-scale CFD model was developed in order to simulate the wind environment in mountain region using ANSYS, one of the generally used commercial CFD software, and proposed methods to apply the air resistance of trees to the CFD model. Proposed methods of applying air resistance of trees using commercial CFD software were a method of designing canopy regions of trees that is distributed in mountain terrain while designing the CFD model and a method of using a UDF (user defined function) in a computation process of the CFD model without dividing canopy regions of trees. Because the method of dividing the canopy regions of the trees was distorted about the boundary of forest regions in process of designing the CFD model, it was determined that it is appropriate to apply the air resistance coefficient of the trees to meshes where the canopy regions are located using the UDF. In Chapter 4, a 3D micro-scale open-source CFD model was also developed in order to simulate the wind environment in mountain region using OpenFOAM, one of the generally used open-source CFD software, and proposed a method to apply the air resistance of trees to the open-source CFD model. First of all, a 3D mountain topography model was designed without dividing domains for the forest regions distributed in the target area. The air resistance coefficients of the trees were applied to each group of meshes corresponding to the forest region of the designed open-source CFD model using developed code. The CFD simulation models developed in Chapter 3 and 4 were validated using the wind environment data monitored in the field and the reliability of the CFD models were secured. In Chapter 5, appropriate installation methods of meteorological observation system in mountain region according to the type of trees, the air-resistance of trees, the tree height, the size of the clear-cut, etc. were proposed based on analysis of wind environment in the clear-cut. In addition, conversion factors for estimating the reference wind speed which is not influenced by the surrounding trees, was derived in order to compensate the cases of not satisfying the appropriate height of the meteorological observation system and the size of the clear-cut in mountain region.