• Journal of Korean Society of Coastal and Ocean Engineers
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• v.22 no.2
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• pp.73-78
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• 2010

Wave energy is dissipated mainly by friction on the seabed until the waves reach the surf zone. Many researchers have investigated the mechanism of wave friction and the bottom shear stress induced by wave motion at a certain point is now well estimated by introducing the wave friction factor related to the near bed velocity given by linear wave theory. The variation of wave energy or wave height over a long distance can be, however, estimated by an iteration process when the propagation of waves is strongly influenced by bed friction. In the present study simple semi-theoretical equation has been developed to compute the variation of wave height for the condition of wave propagation on a constant beach slope. The ratio of wave height is determined by the product of shoalng factor and wave height friction factor (frictional wave energy dissipation factor). The wave height estimated by the new equation is compared with the wave height estimated by the solution of numerical integration for the condition that the waves propagate on a constant slope.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.12
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• pp.2349-2357
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• 1992

Experimental were performed to investigate the turbulent boundary layer over the flat plate when the surface roughness undergoes a step change from rough to smooth under zeoro pressure gradient. well sthear stress was measured by the Computational Preston Tube Method(CPM). The inner layer near the wall adapts rapidly to a new surface condition but the outer flow far from the wall rather slowly. After a sudden change of roughness, the values of wall shear stress discontinuously reduces and then slowly approaches to the value in the equilibrium boundary layer at the down stream. The variation of the von Karman constant indirectly measured by CPM method shows that the flow near the wall at the downstream is highly non-equilibrium state.

• Journal of computational fluids engineering
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• v.18 no.1
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• pp.91-97
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• 2013

In the present study, the effects of contaminant accumulation and surface roughness on the aerodynamic performance of wind turbine blade sections were numerically investigated by using a flow solver based on unstructured meshes. The turbulent flow over the rough surface was modeled by a modified ${\kappa}-{\omega}$ SST turbulence model. The calculations were made for the NREL S809 airfoil with varying contaminant sizes and positions at several angles of attack. It was found that as the contaminant size increases, the degradation of the airfoil performance becomes more significant, and this trend is further amplified near the stall condition. When the contaminant is located at the upper surface near the leading edge, the loss in the aerodynamic performance of the blade section becomes more critical. It was also found that the surface roughness leads to a significant reduction of lift, in addition to increased drag.

• Journal of the Korean Society of Marine Environment & Safety
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• v.23 no.1
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• pp.104-111
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• 2017

This research focuses on simulation and visualization of flow field characteristics inside a centrifugal pump. The 3D numerical analysis was carried out by using a numerical CFD tool, addressing a Reynolds Average Navier-Stock code with a standard k-${\varepsilon}$ two-equation turbulence model. The simulation accounts for friction head loss due to rough walls at suction, impeller, discharge areas and volumetric head loss at impeller wear ring. A comparison of performance curves between simulation and experimentation is included, and it reveals a same trend of those results with a small difference of maximum 5 %. At best efficiency point, velocity vectors are smooth but it changes significantly under off-design point, a strong recirculation appears at the outlet of impeller passages near tongue area. A relatively uniform preassure distribution was observed around the impeller in despite of the tongue. Within the volute, because of its geometry, spiral vortexes formed, proving that the flow field in this region was relatively turbulent and unsteady.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.6B
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• pp.627-634
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• 2008

In a numerical simulation of open channel turbulent flows using RANS (Reynolds averaged Navier-Stokes) equations model equipped with VOF (Volume of Fluid) scheme, the determination of wall roughness for wall function was studied. The roughness constant, based on the law-of-the-wall for flow on rough walls, obtained by experimental works for pipe flows is employed in general wall functions. However, this constant of wall function is the function of Froude number in open channel flows. Thus, the wall roughness should be determined by taking into account the effect of Froude number. In addition, the wall roughness should be corresponding to Manning's roughness coefficient widely used for open channels. In this study, the relation between wall roughness height as an input condition and Manning's roughness coefficient was investigated, and an equation for effective wall roughness height considering the characteristics of numerical models was proposed as a function of Manning's roughness coefficient.

• Journal of Advanced Marine Engineering and Technology
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• v.34 no.5
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• pp.638-644
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• 2010

Numerical predictions of a fully developed turbulent flow through a square duct ($30mm{\times}30mm$) with twisted tape inserts and with twisted tape plus interrupted ribs are respectively conducted to investigate regionally averaged heat transfer and flow patterns. A rib height-to-channel hydraulic diameter(e/$D_h$) of 0.067 and a lengthto-hydraulic diameter(L/$D_h$) of 30 are considered at Reynolds number ranging 8,900 to 29,000. The interrupted ribs are axially arranged on the bottom wall. The twisted tape is 0.1 mm thick carbon steel sheet with diameter of 28 mm, length of 900 mm, and 2.5 turns. Each wall of the square channel is composed of isolated aluminum sections. Two heating conditions are investigated for test channels with twisted tape inserts and rib turbulators: (1) electric heat uniformly applied to four side walls of the square duct, and (2) electric heat uniformly applied to two opposite walls of the square channel. The results show that uneven surface heating enhances the heat transfer coefficient over uniform heating conditions, and significant improvements can be achieved with twisted tape inserts plus interrupted ribs.