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Linear proportional–integral control for skin-friction reduction in a turbulent channel flow
Kim, Euiyoung,Choi, Haecheon Cambridge University Press 2017 Journal of fluid mechanics Vol.814 No.-
<P>In the present study, we apply a proportional (P)–integral (I) feedback control to a turbulent channel flow for skin-friction reduction. The instantaneous wall-normal velocity at a sensing plane above the wall is measured as a sensing parameter, and blowing/suction is provided at the wall based on the PI control. The performance of PI controls is estimated by the change in the skin friction while varying the sensing plane location$y_{s}$and the proportional and integral feedback gains ($\unicode[STIX]{x1D6FC}$and$\unicode[STIX]{x1D6FD}$respectively). The opposition control proposed by Choi<I>et al.</I> (<I>J. Fluid Mech.</I>, vol. 262, 1994, pp. 75–110) corresponds to a P control with$\unicode[STIX]{x1D6FC}=1$. When the sensing plane is located close to the wall ($y_{s}^{+}\lesssim 10$), PI controls result in greater skin-friction reductions than corresponding P controls. The root-mean-square (r.m.s.) sensing velocity fluctuations, considered as the control error, approach zero with successful PI controls, but do not with P controls. Successful PI controls reduce the strength of near-wall coherent structures and the r.m.s. velocity fluctuations above the wall apart from those near the wall due to the control input. The frequency spectra of the sensing velocity show that the I component of PI controls significantly reduces the energy at low frequencies, much more than P controls do. Proportional–integral controls are also applied to a linearized flow model having transient growth of disturbances. The performance of PI controls for a linearized flow model is very similar to that for a turbulent channel flow, i.e. the low-frequency components of disturbances are significantly reduced by the I component of PI controls, and the transient energy growth is suppressed more than by P controls.</P>
Aerodynamics of Heavy Vehicles
Choi, Haecheon,Lee, Jungil,Park, Hyungmin Annual Reviews 2014 Annual review of fluid mechanics Vol.46 No.-
<P>We present an overview of the aerodynamics of heavy vehicles, such as tractor-trailers, high-speed trains, and buses. We introduce three-dimensional flow structures around simplified model vehicles and heavy vehicles and discuss the flow-control devices used for drag reduction. Finally, we suggest important unsteady flow structures to investigate for the enhancement of aerodynamic performance and future directions for experimental and numerical approaches.</P>
A Dynamic Globalization Model for Large Eddy Simulation of Complex Turbulent Flow
Haecheon Choi(최해천),Noma Park(박노마),Jinseok Kim(김진석) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11
A dynamic subgrid-scale model is proposed for large eddy simulation of turbulent flows in complex geometry. The eddy viscosity model by Vreman [Phys. Fluids, 16, 3670 (2004)] is considered as a base model. A priori tests with the original Vreman model show that it predicts the correct profile of subgrid-scale dissipation in turbulent channel flow but the optimal model coefficient is far from universal. Dynamic procedures of determining the model coefficient are proposed based on the 'global equilibrium' between the subgrid-scale dissipation and viscous dissipation. An important feature of the proposed procedures is that the model coefficient determined is globally constant in space but varies only in time. Large eddy simulations with the present dynamic model are conducted for forced isotropic turbulence, turbulent channel flow and flow over a sphere, showing excellent agreements with previous results.
Haecheon CHOI,Jungil LEE 한국산업응용수학회 2011 한국산업응용수학회 학술대회 논문집 Vol.6 No.2
In Choi & Moin (2011), it was shown that the number of grid points (N) required for solving turbulent boundary layer flow using wall-modeled large eddy simulation (WMLES) is proportional to ReLx, but a wall-resolving LES requires N ∼ Re<SUP>13/7</SUP>Lx , where Lx is the flat-plate length in the streamwise direction. This grid-point requirement indicates the importance of WMLES for high Reynolds number flow. In this study, we provide the mean wall shear stress as a boundary condition for WMLES without any further modeling near the wall. The motivation of using this wall boundary condition for WMLES is such that in the framework of finite volume method, an accurate information of mean wall shear stress is the most important in the momentum transport near the wall, even if the first grid used in WMLES locates far away from the wall. For turbulent channel flow, the mean wall shear stress is balanced with the mean pressure gradient and thus is provided a priori during the simulation. DNS and LES with this boundary condition show that the results agree very well with those with no-slip boundary condition. WMLES up to Reτ = 2 × 10? (based on the wall shear velocity and boundary layer thickness) with current boundary condition predicts the log law very well.
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최진(Jin Choi),전우평(Woo-Pyung Jeon),최해천(Haecheon Choi) 한국유체기계학회 2006 유체기계 연구개발 발표회 논문집 Vol.- No.-
In this paper, we present a detailed mechanism of drag reduction by dimples and roughness on a sphere by measuring the streamwise velocity above the dimpled and roughened surfaces, respectively. Dimples cause local flow separation and trigger the shear layer instability along the separating shear layer, resulting in generation of large turbulence intensity. With this increased turbulence, the flow reattaches to the sphere surface with high momentum near the wall and overcomes strong adverse pressure gradient formed in the rear sphere surface. As a result, dimples delay main separation and reduce drag significantly. The present study suggests that generation of a separation bubble, i.e. a closed-loop streamline consisting of separation and reattachment, on a body surface is an important flow-control strategy for drag reduction on a bluff body such as the sphere and cylinder. In the case of roughened sphere, the boundary layer flow is directly triggered by roughness and changes to a turbulent flow. Due to this change, the drag significantly decreases. As the Reynolds number further increases, transition to turbulence occurs earlier on the sphere surface. Because of faster growth of turbulent boundary layer by roughness, earlier transition thickens the boundary layer, resulting in earlier separation and drag increase with increasing Reynolds number.