Investigation on the turbulent stress anisotropy of axisymmetric turbulence under rapid rotation

박주엽,정명균 대한기계학회 2009 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.23 No.3

The root cause of different oscillatory behavior of turbulent stress anisotropy under rapid rotation of initially axisymmetric turbulence is theoretically investigated. For this, based on the velocity spectral tensor of axisymmetric turbulence, the rapid part of the pressure-strain is determined and the equation of the turbulent stress anisotropy is solved for initial conditions generated by axisymmetric expansion and contraction of isotropic turbulence. As is well known, the damping of turbulent stress anisotropy under rapid rotation is observed for both initial conditions, and this feature is attributed to the linear rapid rotation effect on turbulence. On the other hand, the oscillatory development of turbulent stress anisotropy can be seen conspicuously only for the initial turbulence generated by axisymmetric expansion. This selective oscillatory feature is found to be strongly related to the total strain that is applied to the isotropic turbulence to generate the initial axisymmetric turbulence. And, through an asymptotic approach, it is also found that the material frame-indifference principle of two-dimensional turbulence is the underlying physics in this different oscillatory behavior.

Effects of inflow turbulence and slope on turbulent boundary layers over two-dimensional hills

Tong Wang,Shuyang Cao,Yaojun Ge 한국풍공학회 2014 Wind and Structures, An International Journal (WAS Vol.19 No.2

The characteristics of turbulent boundary layers over hilly terrain depend strongly on the hill slope and upstream condition, especially inflow turbulence. Numerical simulations are carried out to investigate the neutrally stratified turbulent boundary layer over two-dimensional hills. Two kinds of hill shape, a steep one with stable separation and a low one without stable separation, and two kinds of inflow condition, laminar and turbulent, are considered. An auxiliary simulation, based on the local differential quadrature method and the recycling technique, is performed to simulate the inflow turbulence to be imposed at the inlet boundary of the simulation with turbulent inflow, which preserves very well in the computational domain. A large separation bubble is established on the leeside of the steep hill with laminar inflow, while the reattachment point moves upstream under turbulent inflow condition. There is stable separation on the lee side of the low hill with laminar inflow, while not with turbulent inflow. Besides increase of turbulence intensity, inflow turbulence can efficiently enhance the speedup around hills.So in practice, it is unreasonable to study wind flow over hilly terrain without considering inflow turbulence.

대류경계층에서 난류흐름에 관한 수치해석적 연구

박종혁,이우범,이정전 國立 麗水大學校 環境問題硏究所 1998 環境硏究論文集 Vol.1 No.-

As turbulent flow include a complex physical process in the convective boundary layer(CBL), it is difficult to explain the physical phenomena exactly. Therefore, it is possible to explain the model in a simple and limited way. For the explanation of the CBL turbulence, enormous amounts of data are required concerning in CBL turbulence. A method to study the structure and characteristics of CBL is numerical simulation of important parts of the flow field i.e. through large eddy simulation(LES). In this study, turbulence in the CBL uniformly heated from below and topped by a layer of uniformly stratified fluid is investigated for flows by numerical simulation(LES). LES uses a finite-difference method to integrate the three-dimensional grid-volume-averaged Navier-Stokes equation for a Boussinesq fluid. In LES, the contribution of the large-carrying structures to momentum, heat and energy transfer are computed exactly, and only the effect of the smallest scales of turbulence is modeled. The purpose of this study is to analyze the velocity and temperature distributions in CBL turbulence in order to study the dispersion of atmospheric pollutant by LES.

Effective feedback algorithm for application of MI-simulation to turbulent orifice flow

Mitsuhiro Nakao,Kenji Kawashima,Toshiharu Kagawa 제어로봇시스템학회 2009 제어로봇시스템학회 국제학술대회 논문집 Vol.2009 No.8

Measurement-integrated simulation (MI-simulation) is a numerical simulation in which experimental results are fed back to the simulation. The calculated values become closer to the experimental values. In this paper, we propose a new feedback method based on the conservation of momentum for MI-simulation built-in standard k-εmodel to turbulent orifice flow. The axial velocities in simulation are compensated by using proportional controllerusing the estimated pressure distribution based on the conservation of momentum. The signals are fed back to the axial velocity control volumes in the cross section downstream of the orifice. The effectiveness of the method was confirmed compared with the experimental results at downstream of orifice.

안정 성층인 채널유동에서 난류의 감쇠

김병구(Byung-Gu Kim),이창훈(Changhoon Lee) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11

Suppression of turbulence in stably stratified flow is investigated in this study using large eddy simulation. Stably stratified turbulent flow owing to negative buoyancy or adverse density gradient is important phenomenon that is frequently observed in geophysical flow as well as in engineering flow. Severe suppression of turbulence or relaminarization was reported from many experiments but it was not reproduced by using numerical simulation typically performed for not only low Richardson number but also low Reynolds number flow. In this study, near-wall behavior of suppression of turbulence in stably stratified channel flow was investigated in both wide range of Richardson number and Reynolds number. To accomplish sufficiently high Reynolds number flow and to diminish a numerical error, large eddy simulation employing spectral method is conducted.

Wind field simulation over complex terrain under different inflow wind directions

Wenfeng Huang,Xibin Zhang 한국풍공학회 2019 Wind and Structures, An International Journal (WAS Vol.28 No.4

Accurate numerical simulation of wind field over complex terrain is an important prerequisite for wind resource assessment. In this study, numerical simulation of wind field over complex terrain was further carried out by taking the complex terrain around Siu Ho Wan station in Hong Kong as an example. By artificially expanding the original digital model data, Gambit and ICEM CFD software were used to create high-precision complex terrain model with high-quality meshing. The equilibrium atmospheric boundary layer simulation based on RANS turbulence model was carried out in a flat terrain domain, and the approximate inflow boundary conditions for the wind field simulation over complex terrain were established. Based on this, numerical simulations of wind field over complex terrain under different inflow wind directions were carried out. The numerical results were compared with the wind tunnel test and field measurement data for land and sea fetches. The results show that the numerical results are in good agreement with the wind tunnel data and the field measurement data which can verify the accuracy and reliability of the numerical simulation. The near ground wind field over complex terrain is complex and affected obviously by the terrain, and the wind field characteristics should be fully understood by numerical simulation when carrying out engineering application on it.

Development of semi-Lagrangian gyrokinetic code for full-f turbulence simulation in general tokamak geometry

Kwon, Jae-Min,Yi, Dokkyun,Piao, Xiangfan,Kim, Philsu Elsevier 2015 Journal of computational physics Vol.283 No.-

<P><B>Abstract</B></P> <P>In this work, we report the development of a new gyrokinetic code for full-f simulation of electrostatic turbulence in general tokamak geometry. Backward semi-Lagrangian scheme is employed for noise-free simulation of the gyrokinetic Vlasov equation with finite Larmor radius effects. Grid systems and numerical interpolations are implemented to deal with arbitrary equilibrium information given in a GEQDSK format file. In particular, we introduce an adaptive interpolation technique for fluctuating quantities, which have elongated structures along equilibrium magnetic fields. This field-aligned interpolation can reduce the required number of grid points to represent the fluctuating quantities. Also, it is shown that the new interpolation allows us to choose bigger time sizes with better simulation accuracy. Several benchmark simulation results are presented for comparison with previously known cases. It is demonstrated that the new code can reproduce the well known results of zonal flow and linear ITG instabilities in concentric circular equilibrium. It is also shown that the code can capture the effects of plasma shaping on the zonal flow and ITG instabilities, and the stabilization effects of the shaping result in significant up-shifts of the threshold of ion temperature gradient for ITG in nonlinear simulation.</P>

Effects of inflow turbulence and slope on turbulent boundary layer over two-dimensional hills

Wang, Tong,Cao, Shuyang,Ge, Yaojun Techno-Press 2014 Wind and Structures, An International Journal (WAS Vol.19 No.2

The characteristics of turbulent boundary layers over hilly terrain depend strongly on the hill slope and upstream condition, especially inflow turbulence. Numerical simulations are carried out to investigate the neutrally stratified turbulent boundary layer over two-dimensional hills. Two kinds of hill shape, a steep one with stable separation and a low one without stable separation, two kinds of inflow condition, laminar turbulent, are considered. An auxiliary simulation, based on the local differential quadrature method and recycling technique, is performed to simulate the inflow turbulence be imposed at inlet boundary of the turbulent inflow, which preserves very well in the computational domain. A large separation bubble is established on the leeside of the steep hill with laminar inflow, while reattachment point moves upstream under turbulent inflow condition. There is stable separation on the side of low hill with laminar inflow, whilw not turbulent inflow. Besides increase of turbulence intensity, inflow can efficiently enhance the speedup around hills. So in practice, it is unreasonable to study wind flow over hilly terrain without considering inflow turbulence.

Investigation of Cavitation Models for Steady and Unsteady Cavitating Flow Simulation

Tran, Tan Dung,Nennemann, Bernd,Vu, Thi Cong,Guibault, Francois Korean Society for Fluid machinery 2015 International journal of fluid machinery and syste Vol.8 No.4

The objective of this paper is to evaluate the applicability of mass transfer cavitation models and determine appropriate numerical parameters for cavitating flow simulations. CFD simulations were performed for a NACA66 hydrofoil at cavitation numbers of 1.49 and 1.00, corresponding to steady sheet and unsteady sheet/cloud cavitating regimes using the Kubota and Merkle cavitation models. The Merkle model was implemented into CFX by User Fortran code. The Merkle cavitation model is found to give some improvements for cavitating flow simulation results for these cases. Turbulence modeling is also found to have an important contribution to the prediction quality of the simulations. The relationship between the turbulence viscosity modification, in order to take into account the local compressibility at the vapor/liquid interfaces, and the predicted numerical results is clarified. The limitations of current cavitating flow simulation techniques are discussed throughout the paper.

Investigation of Cavitation Models for Steady and Unsteady Cavitating Flow Simulation

Tan Dung Tran,Bernd Nennemann,Thi Cong Vu,François Guibault 한국유체기계학회 2015 International journal of fluid machinery and syste Vol.8 No.4

The objective of this paper is to evaluate the applicability of mass transfer cavitation models and determine appropriate numerical parameters for cavitating flow simulations. CFD simulations were performed for a NACA66 hydrofoil at cavitation numbers of 1.49 and 1.00, corresponding to steady sheet and unsteady sheet/cloud cavitating regimes using the Kubota and Merkle cavitation models. The Merkle model was implemented into CFX by User Fortran code. The Merkle cavitation model is found to give some improvements for cavitating flow simulation results for these cases. Turbulence modeling is also found to have an important contribution to the prediction quality of the simulations. The relationship between the turbulence viscosity modification, in order to take into account the local compressibility at the vapor/liquid interfaces, and the predicted numerical results is clarified. The limitations of current cavitating flow simulation techniques are discussed throughout the paper.