【正文】 fi represents the Reynolds stresses for which a turbulence model is required for closure. To numerically solve the rapidly varying flow over an ogee crest, it is important that the free surface is accurately tracked. In FLOW3D, free surface is defined in terms of the volume of fluid (VOF) function which represents the volume of fraction occupied by the fluid. A twoequation renormalized group theory models (RNG model) was used for turbulence closure. The RNG model is known to describe more accurately low intensity turbulence flows and flow having strong shear regions (Yakhot et al., 1992). The flow region is subdivided into a mesh of fixed rectangular cells. With each cell there are associated local average values of all dependent variables. All variables are located at the centers of the cells except for velocities, which are located at cell faces (staggered grid arrangement). Curved obstacles, wall boundaries, or other geometric features are embedded in 11 the mesh by defining the fractional face areas and fractional volumes of the cells that are open to flow. 4. Conclusions In this study, flow characteristics such as flowrate, water surfaces, crest pressures on the ogeespillway, and vertical distributions of velocity and pressure in consideration of model scale and surface roughness effects are investigated in detail by using mercial CFD model, FLOW3D which is widely verified and used in the field of spillway flow analysis. To investigate the scaling and roughness effects, six cases are adopted. Namely, numerical modeling on the hydraulically smooth (PR00), k = mm (PR05), and k = mm (PR30) for the investigation of roughness effects and prototype (PR05), 1/ 50 model (M50), 1/00 model (M100), 1/200 model (M200) for the investigation of scale effects are carried out. In the modeling of the scaled model, grid resolution, surface roughness,and upstream boundary conditions were adjusted as the geometric similarity to exclude a generation of different numerical error. The important simulation results prise the following: 1) The discharge flowrate decreases slightly as surface roughness height and the length scale of the model to the prototype increase. The water surface fluctuation is negligible and some crest pressure variation occurs with a change of surface roughness and model scale. Numerical errors due to the surface roughness are insignificant if we just use a general roughness height of construction materials and the scale effects of the model are appeared in within an acceptable error range if the length scale ratio is less than 100 or 200. 2) The modeling results show that increasing of the length scale ratio give rise to similar phenomena due to increasing of the surface roughness. If hm is chosen as a reference point, the velocity of the prototype is larger than that of the scaled model below the reference point but the velocity of the prototype is smaller than that of the scaled model above the reference point. The roughness and scale effects are more severe below the reference point. 3) The pressures on the spillway crest are somewhat different with a change of the surface roughness and model scale. But the vertical pressure distributions are almost same to each other regardless of the surface roughness and model scale. 4) Maximum velocity at any section is slightly decreasing as the surface roughness and the length scale ratio increase. The vertical location on which maximum velocity occurs is located on lower position as the upstream water head increase and the location is almost linearly increasing with the distance from the spillway 12 front. KSCE Journal of Civil Engineering Vol. 9, No. 2 / March 2020 pp. 161~169 。 Ai is fra