# Category: Research

## Urban Flow and Dispersion

CFD Large Eddy Simulation을 이용한 실제 도시 스케일의 유동 및 확산 특성에 대한 연구

Turbulent flow and dispersion characteristics over a real urban street canyon are investigated by large-eddy simulation model. Two kinds of sub-grid scale (SGS) model, which are the constant coefficient Smagorinsky model and Vreman model, are assessed to evaluate SGS model performance. Turbulent statistics, particularly turbulent stresses, and wake patterns are compared between the two SGS models for three different wind directions. We found that while the role of the SGS model is small in average sense, local or instantaneous contribution to total stress near the surface or edge of the buildings is not negligible. Yielding smaller eddy viscosity near the walls than the Smagorinsky model, the Vreman model appears to be more appropriate for the simulation of a flow in a complex urban street canyon. In the following investigation of a flow field over a real urban street canyon, we found that depending on wind direction relative to the street canyon, wind fields, turbulence statistics, and dispersion patterns show very different characteristics. Particularly, tall buildings near the street canyon predominantly generate turbulence, leading to homogenization of the mean flow inside the street canyon. Furthermore, the release position of pollutants sensitively determines subsequent dispersion characteristics.

## Immersed Boundary Method for Complex Flows

##### 복잡한 형상 물체 주위에 형성되는 유체현상을 고정격자계를 사용하여 해석할 수 있는 가상경계기법 (Immersed Boundary Method)를 개발하였으며, 그 일례로 보행시 발생되는 난류현상을 예측하였다.

An immersed boundary method for time-dependent, three-dimensional, incompressible flows is presented in this paper. The incompressible Navier-Stokes equations are discretized using a low-diffusion flux splitting method for the inviscid fluxes and second-order central-differences for the viscous components. Higher-order accuracy achieved by using weighted essentially non-oscillatory (WENO) or total variation diminishing (TVD) schemes. An implicit method based on artificial compressibility and dual-time stepping is used for time advancement. The immersed boundary surfaces are defined as clouds of points, which may be structured or unstructured. Immersed-boundary objects are rendered as level sets in the computational domain, and concepts from computational geometry are used to classify points as being outside, near, or inside the immersed boundary. The velocity field near an immersed surface is determined from separate interpolations of the components tangent and normal to the surface. The tangential velocity near the surface is constructed as a power-law function of the local wall normal distance. Appropriate choices of the power law enable the method to approximate the energizing effects of a turbulent boundary layer for higher Reynolds number flows. Four different flow problems (flow past a circular cylinder, a NACA0012 airfoil, a sphere, and a stationary mannequin) are simulated using the present immersed boundary method, and the predictions show good agreement with previous computational and experimental results. Finally, the flow induced by realistic human walking motion is simulated as an example of a problem involving multiple moving immersed objects.

Choi et al. (2007) Journal of Computational Physics

Schematic illustrating classification of cell-centered points for a complex immersed body

Evolution of iso-surfaces of streamwise velocity and coherent vertical structures induced human walking motion at each time instant

## Human-Induced Contaminant Transports in Indoor Environments

##### 실내환경내에 보행자의 출입과정에서 유발되는 오염물질의 이동에 대한 수치 연구를 기반으로 생화학 무기 공격 혹은 독성물질의 누출, 병원 내 전염 통제등을 위한 연구를 하고자 한다.

A large eddy simulation is used to investigate contaminant transport due to complex human and door motions and vent-system activity in room compartments where a contaminated and clean room are connected by a vestibule. Human and door motions are simulated with an immersed boundary procedure. We demonstrate details of contaminant transport due to human- and door-motion induced wake development during a short-duration event involving the movement of a person (or persons) from a contaminated room, through a vestibule, into a clean room. Parametric studies that capture the effects of human walking pattern, door operation, over-pressure level, and vestibule size are systematically conducted. A faster walking speed results in less mass transport from the contaminated room into the clean room. The net effect of increasing the volume of the vestibule is to reduce the contaminant transport. The results show that swinging-door motion is the dominant transport mechanism and that human-induced wake motion enhances compartment-to-compartment transport.

Choi & Edwards (2008) Indoor Air ; Choi & Edwards (2011) Indoor Air

Snapshots of vorticity iso-surfaces colored by SF6 mass concentration for person walking event; Contour plots of concentration in (b) vertical plane view; at (c) z=1.0 and (d) z=1.5m in horizontal plane view.

## Particle Deposition in Human Lungs

##### 인체내 입자점착에 대한 수학모형을 개발하고, 객체, 입자, 환경특성을 고려한 흡수선량 (dosimetry)을 예측하여 대기오염에 따른 인체유해성 평가를 하고자 한다.

A dynamic single-path mathematical model was developed that is capable of analyzing detailed deposition patterns of inhaled particles in human lungs. Weibel’s symmetric lung morphology was adopted as the basic lung structure, and detailed transport processes were evaluated numerically using the fully implicit procedure. Deposition efficiencies by specific mechanisms were individually examined for accuracy and new empirical formulas were incorporated whenever appropriate. Deposition in the alveolar region was divided into deposition fractions in the alveolar duct and alveoli considering active transport processes between the two regions. The deposition fractions were obtained for each airway generation, serial lung volumetric compartments, and conventional three-compartment anatomic lung regions. In addition, the surface dose and cumulative deposition with time were analyzed. The results showed excellent agreement with available experimental data. The present model provides an improvement from the previously reported models and can be used as a tool in assessing internal dose of inhaled particles under various inhalation conditions.

Choi & Kim (2007) Inhalation Toxicology

Schematic diagram of one-dimensional trumpet morphology of human lungs.

Schematic diagram of particle transport and deposition processes in the alveolar region: (a) inspiration and (b) expiration.

Total deposition fraction vs. particle diameter in the normal lung at FRC = 3000 ml for three different breathing patterns.

## Hybrid LES/RANS Methods for Compressible Turbulent Boundary Layer

난류경계층 유동해석을 효율적으로 모사하기 위해, Reynolds-averaged Navier-Stokes Simulation기법과 Large Eddy Simulation기법의 혼합형태의 새로운 수치모형을 개발하고자 한다

A new hybrid large-eddy simulation / Reynolds-averaged Navier-Stokes simulation (LES/RANS) method is presented in this work. In this approach, the resolved turbulence kinetic energy, ensemble-averaged modeled turbulence kinetic energy and turbulence frequency, and time-resolved turbulence frequency are used to form an estimate of an outer-layer turbulence length scale that is nearly Reynolds-number independent. The ratio of this outer-layer scale with an inner-layer length scale (proportional to the wall distance) is used to construct a blending function that facilitates the shift between an unsteady RANS formulation near solid surfaces and a LES formulation away from the wall. The new model is tested through simulations of compressible flat-plate boundary layers over a wide range of Reynolds numbers and Mach 2.85 flow over a smooth compression ramp. The results show that the new model provides results for mean and second-moment statistics that are in good agreement with experiment and are comparable to those obtained using an earlier model which required a case-by-case calibration of a model constant.

Edwards et al. (2008) AIAA Journal; Choi et al. (2008) AIAA Journal; Gieseking et al. (2010) AIAA Journal

Mean velocity profies for the present hybrid LES/RANS simulation

The effect of Reynolds number on skin-friction coefficient

Vortical structures visualized using an iso-surface of swirl strength

## Motivation

본 연구는 비압축성 유체를 수치적으로 해석하기 위한 새로운 수치적 방법에 대한 내용이다. 유동에 대한 지배방정식인 Navier-Stokes equation의 continuity 조건을 만족시키기 위해 압력에 대한 Poisson equation을 풀어야 하는데, 기존의 implicit solver는 많은 계산비용이 요구된다. 이를 해결하기 위해 explicit한 방법을 모색하였다.

## Details

The Poisson equation for pressure arising from nonzero divergence of the nonlinear term in the integration of the Navier-Stokes equations requires a lot of computational cost except for cases with periodic domain. In order to mitigate this cost, we propose a new project algorithm which is fully explicit, thus not requiring iterations. The projection operator, , which projects any vector field with divergence into the divergence-free subspace in the Fourier space, when the distance from the point in question. This allows truncation so that the resulting local distribution of the projection operator, through convolution, can be used to obtain projected nonlinear terms which have relatively small divergence. This approximate’ projection scheme was then applied to direct numerical simulation of isotropic turbulence to investigate effectiveness and efficiency of the scheme in reducing divergence and correct projection of the nonlinear terms through the statistical properties of the turbulent flow.

#### Test results in isotropic turbulence

Used number of coefficients:
dt=0.001
Maximum iteration number: 3000(1sec)
Record: 600(0.2sec)
                 Correlation                                                Energy spectrum `

## Numerical simulation of particle-laden turbulent channel flow

수치해석 방법을 이용하여 Particle-laden turbulent channel flow의 특성을 연구한다.

• Motivation

The study on the behavior of inertial particles suspended in wall-bounded turbulence has a wide range of applications, from industrial devices to environmental systems, and is expected to resolve most of the practical problems involving walls (e.g., transport of contaminants in urban area and deposition of fine dust in a human respiratory system). From an engineering perspective, it is very important to accurately predict the particle motion and understand particle dynamics in wall-bounded turbulence.

• Lagrangian particle tracking
• Particles near the wall moving in the x-direction

Particles are concentrated in the low speed streaks (blue regions in the right figure).

• Y-Z plane views of particle distribution

Particles are transferred towards the wall, interacting with turbulence.

## Geometric nature of particle trajectories

입자의 궤적을 통해 알수있는 곡률(curvature) 및 비틀림률(torsion)의 성질을 조사함으로써, 난류의 회전구조 및 헬리시티와의 상관성을 조사하였다.

The geometric nature of particle trajectory is investigated using direct numerical simulation of isotropic turbulence. Probability density functions and autocorrelations along a fluid particle trajectory associated with geometric quantities such as curvature and torsion of the Lagrangian trajectory are provided. We proposed the ratio of torsion to curvature as parameter to identify the particle trajectory and it is found to play crucial role in understanding the geometric shape of particle trajectory. The relationship between Lagrangian helicity and the ratio of torsion to curvature is investigated where Lagrangian helicity is defined as dot product of velocity and vorticity at the point of fluid particle. We found that probability density functions of torsion and torsion normalized by curvature clearly show well-established slope in log–log plots. Lagrangian helicity is intermittently distributed and high Lagrangian helicity is always found, where high acceleration is observed. Regarding the relationship between coherent structure and acceleration, coherent structure can be understood in terms of Lagrangian helicity, curvature, and torsion. Geometric characteristics for solid particles are also investigated and its results are varied according to Stokes number.

## Acceleration model in Turbulent flows

난류에서 나타나는 가속도의 간헐성을 회전에 의해 나타나는 가속도와 소산에 의해 나타나는 가속도로 분리함으로써 해석하고자 하였다.

Compare the Lamb vortex model and real turbulent flow

It is well known that fluid acceleration in turbulence is highly intermittent. Source of the intermittency was found to be closely related to the rotational motion of coherent vortical structures. From the Poisson equation for pressure, acceleration, which is mostly the negative of pressure gradient, can be expressed as a sum of acceleration-like below.
$$\frac{\partial u}{\partial t} + u\cdot\nabla u = -\frac{1}{\rho} \nabla P + \nu \nabla^{2}u = a^{I} + a^{S}$$
$$a=-(\nabla^2)^{-1} \nabla \Omega + (\nabla^2)^{-1} \frac{\nabla \epsilon}{2\nu} =a^{\Omega} + a^{\epsilon}$$

They are acceleration due to rotational motion of eddy and acceleration due to irrotational strain field, respectively. We investigated the statistical characteristics of those accelerations by using direct numerical simulation of isotropic turbulence. Flatness of acceleration is of order of 10 but flatness of new defined acceleration terms are 3~5 which represents less intermittency in the range of moderate Reynolds number. Numerical and experimental results do not show clear slope since the Reynolds number is relatively low, but an asymptotic behavior is observed.