• Journal of Korean Society of Coastal and Ocean Engineers
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• v.32 no.1
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• pp.69-84
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• 2020

Natural shoreline repeats its re-treatment and advance in response to the endlessly varying sea-conditions, and once severely eroded under stormy weather conditions, natural beaches are gradually recovered via a boundary layer streaming when swells are prevailing after storms cease. Our understanding of the boundary layer streaming over surf-zone often falls short despite its great engineering value, and here it should be noted that the most sediments available along the shore are supplied over the surf-zone. In this rationale, numerical simulation was implemented to investigate the hydraulic characteristics of boundary layer streaming over the surf zone in this study. In doing so, comprehensive numerical models made of Spatially filtered Navier-Stokes Eq., LES (Large Eddy Simulation), Dynamic Smagorinsky turbulence closure were used, and the effects of turbulence closure such as Dynamic Smagorinsky in LES and k-ε on the numerically simulated flow field were also investigated. Numerical results show that due to the intrinsic limits of k-ε turbulence model, numerically simulated flow velocity near the bottom based on k-ε model and wall function are over-predicted than the one using Dynamic Smagorinsky in LES. It is also shown that flow velocities near the bottom are faster than the one above the bottom which are relatively free from the presence of the bottom, complying the typical boundary layer streaming by Longuet-Higgins (1957), the spatial scope where boundary layer streaming are occurring is extended well into the surf zone as incoming waves are getting longer. These tendencies are plausible considering that it is the bottom friction that triggers a boundary layer streaming, and longer waves start to feel the bottom much faster than shorter waves.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.7
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• pp.1691-1701
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• 1995

Two aspects of Large-Eddy Simulation(LES) are investigated in order to improve its performance. The first one is on how to determine the model coefficient in conjunction with a dynamic subgrid-scale model, and the second one is on a wall-layer model(WLM) which allows one to skip near-wall regions to save a large number of grid points otherwise required. Especially, a WLM suitable for a separated flow is considered. Firstly, an averaging technique to calculate the model coefficient of dynamic subgrid-scale modeling(DSGSM) is introduced. The technique is based on the concept of local averaging, and useful to stabilize numerical solution in conjunction with LES of complex turbulent flows using DSGSM. It is relatively simple to implement, and takes very low overhead in CPU time. It is also able to detect the region of negative model coefficient where the "backscattering" of turbulence energy occurs. Secondly, a wall-layer model based on a local turbulence intensity is considered. It locally determines wall-shear stresses depending on the local flow situations including separation, and yields better predictions in separated regions than the conventional WLM. The two techniques are tested for a turbulent obstacle flow, and show the direction of further improvements.rovements.

• Nuclear Engineering and Technology
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• v.55 no.9
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• pp.3367-3382
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• 2023

Smoothed Particle Hydrodynamics (SPH) is a Lagrangian computational fluid dynamics method that has been widely used in the analysis of physical phenomena characterized by large deformation or multi-phase flow analysis, including free surface. Despite the recent implementation of eddy-viscosity models in SPH methodology, sophisticated turbulent analysis using Lagrangian methodology has been limited due to the lack of computational performance and numerical consistency. In this study, we implement the standard and dynamic Smagorinsky model and dynamic Vreman model as sub-particle scale models based on a weakly compressible SPH solver. The large eddy simulation method is numerically identical to the spatial discretization method of smoothed particle dynamics, enabling the intuitive implementation of the turbulence model. Furthermore, there is no additional filtering process required for physical variables since the sub-grid scale filtering is inherently processed in the kernel interpolation. We simulate lid-driven flow under transition and turbulent conditions as a benchmark. The simulation results show that the dynamic Vreman model produces consistent results with experimental and numerical research regarding Reynolds averaged physical quantities and flow structure. Spectral analysis also confirms that it is possible to analyze turbulent eddies with a smaller length scale using the dynamic Vreman model with the same particle size.

• Journal of Ocean Engineering and Technology
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• v.18 no.2
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• pp.1-9
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• 2004

As the first step to investigate the nonlinear interactions between turbulence and marine structures inside a viscous NWT, a LES technique was applied to solve the turbulent channel flow for =150. The employed turbulence models included 4 types: the Smagorinsky model, the Dynamic SGS model, the Structure Function model, and the Generalized Normal Stress model. The simulated data in time-series for the LESs were averaged in both time and space, and statistical analyses were performed. The results of the LESs were compared with those of a DNS, developed in the present study and two spectral methods by Yoon et al.(2003) and Kim et a1.(1987). Based on this research, the accuracy of LESs has been found to be still related to the number of grids for fine grid size).

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.32 no.1
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• pp.55-68
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• 2020

In order to investigate the hydraulic characteristics of a boundary layer streaming over the beach cusps appeared in swells prevailing mild seas, we numerically simulated the shoaling process of Edge waves over the beach cusp. Synchronous Edge waves known to sustain the beach cusps could successfully be duplicated by generating two obliquely colliding Edge waves in front of beach cusps. The amplitude A_B and length L_B of Beach Cusp were elected to be 1.25 m and 18 m, respectively based on the measured data along the Mang-Bang beach. Numerical results show that boundary layer streaming was formed at every phase of shoaling process without exception, and the maximum boundary layer streaming was observed to occur at the crest of sand bar. In RUN 1 where the shortest waves were deployed, the maximum boundary layer streaming was observed to be around 0.32 m/s, which far exceeds the amplitude of free stream by two times. It is also noted that the maximum boundary layer streaming mentioned above greatly differs from the analytical solution by Longuet-Higgins (1957) based on wave Reynolds stress. In doing so, we also identify the recovery procedure of natural beaches in swells prevailing mild seas, which can be summarized such as: as the infra-gravity waves formed in swells by the resonance wave-wave interaction arrives near the breaking line, the sediments ascending near the free surface by the Phase II waves orbital motion were carried toward the pinnacle of foreshore by the shoreward flow commenced at the steep front of breaking waves, and were deposited near the pinnacle of foreshore due to the infiltration.

• Journal of Advanced Marine Engineering and Technology
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• v.30 no.3
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• pp.369-375
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• 2006

Large eddy simulation has been applied to simulate turbulent flow around a cubic obstacle mounted on a channel surface for a Reynolds number of 40000(based on the incoming bulk velocity and the obstacle height) using a Smagorinsky model and a Lagrangian dynamic model. In order to develop the LES to the practical engineering application, the effect of upwind scheme, turbulent sub-grid scale model were investigated. The computed velocities. turbulence quantifies, separation and reattachment length were evaluated by compared with the previous experimental results.

• Journal of computational fluids engineering
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• v.13 no.3
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• pp.28-34
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• 2008

In engineering application of large eddy simulation, there are still questions as follows grid dependency on numerical results, the effect of upwind scheme against a calculation instability, appropriate boundary conditions dealing with turbulence fluctuation and the performance of SGS models. In this study, in order to develop the LES to the engineering application, large eddy simulation was carried out to investigate the effect of upwind scheme, turbulent subgrid model and the grid dependancy of the flow around a wall-mounted cube in a channel at Re=40,000 based on cubic height and bulk mean velocity. The computed velocities, turbulence quantities, separation and reattachment length were evaluated compared with the experimental results of R. Matinuzzi and C. Tropea.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.32 no.9
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• pp.720-727
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• 2008

Time delay effects on near-wall turbulent structures are investigated by performing a large-eddy simulation of a transient turbulent flow in a pipe. To elucidate the time delay effects on the near-wall turbulence, we selected the dimensionless acceleration parameter which was used in the previous study. Various turbulent statistics revealed the distinctive features of the delay. It was shown that the dynamic Smagorinsky model is valid to capture the alterations of the turbulence physics well. A dimensionless time for the responses of the flow quantities was introduced to give the detailed information on the delay of the nearwall turbulence. The conditionally-averaged flow fields associated with Reynolds shear stress producing events show that sweep and ejections are closely related to the delays of the turbulence production and the turbulence propagation toward the pipe center. The present study suggested that the enhanced anisotropy of the turbulence in the initial and transient stages would be a challenging problem to standard turbulence models.

• Journal of Mechanical Science and Technology
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• v.15 no.12
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• pp.1835-1844
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• 2001

The turbulent flow with wake, reattachment and recirculation is a very important problem that is related to vehicle dynamics and aerodynamics. The Smagorinsky Model (SM), the Dynamics Subgrid Scale Model (DSM), and the Lagrangian Dynamic Subgrid Scale Model (LDSM) are used to predict the three-dimensional flow field around a bluff body model. The Reynolds number used is 45,000 based on the bulk velocity and the height of the bluff body. The fully developed turbulent flow, which is generated by the driver part, is used for the inlet boundary condition. The Convective boundary condition is imposed on the outlet boundary condition, and the Spalding wall function is used for the wall boundary condition. We compare the results of each model with the results of the PIV measurement. First of all, the LES predicts flow behavior better than the k-$\xi$ turbulence model. When ew compare various LES models, the DSM and the LDSM agree with the PIV experimental data better than the SM in the complex flow, with the separation and the reattachment at the upper front part of th bluff body. But in the rear part of the bluff body, the SM agrees with the PIV experimental results better than them. In this case, the SM predicts overall flow behavior better than the DSM nd the LDSM.

• Wind and Structures
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• v.39 no.4
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• pp.287-304
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• 2024

An accurate assessment of aerodynamic effects on structures is essential for a reliable wind design for high-rise buildings. Turbulence model is a key ingredient of computational fluid dynamics (CFD) in calculating the wind flow fields. This paper aims to identify the properties of representative RANS and LES models particularly for wind load determination. The models investigated are the realizable k-ε model for RANS and the dynamic Smagorinsky model for LES. In this study, their application aspects are discussed to provide enhanced reproducibility and reliability. The airflow around a building at Reynolds number 76,000 is simulated and the numerical results are also compared with wind tunnel experiment data. The wind design loads, such as story shear forces and overturning or torsional moments, are calculated based on the numerical results. Both RANS and LES models accurately capture surface pressure profiles, while LES results demonstrate proper energy decay in the power spectra. The numerical results highlight the effects of aspect ratio of building and the attack angle on the wind loads. This information would be of great help in designing tall buildings resilient to wind environments using CFD models.