Joung-ho Lee - Academia.edu (original) (raw)

Papers by Joung-ho Lee

Research paper thumbnail of Structures in turbulent boundary layers subjected to adverse pressure gradients

Journal of Fluid Mechanics, 2009

The effects of adverse pressure gradients on turbulent structures were investigated by carrying o... more The effects of adverse pressure gradients on turbulent structures were investigated by carrying out direct numerical simulations of turbulent boundary layers subjected to adverse and zero pressure gradients. The equilibrium adverse pressure gradient flows were established by using a power law free-stream distribution U ∞ ∼ x m . Twopoint correlations of velocity fluctuations were used to show that the spanwise spacing between near-wall streaks is affected significantly by a strong adverse pressure gradient. Low-momentum regions are dominant in the middle of the boundary layer as well as in the log layer. Linear stochastic estimation was used to provide evidence for the presence of low-momentum regions and to determine their statistical properties. The mean width of such large-scale structures is closely associated with the size of the hairpin-like vortices in the outer layer. The conditionally averaged flow fields around events producing Reynolds stress show that hairpin-like vortices are the structures associated with the production of outer turbulence. The shapes of the vortices beyond the log layer were found to be similar when their length scales were normalized according to the boundary layer thickness. Estimates of the conditionally averaged velocity fields associated with the spanwise vortical motion were obtained by using linear stochastic estimation. These results confirm that the outer region of the adverse pressure gradient boundary layer is populated with streamwise-aligned vortex organizations, which are similar to those of the vortex packet model proposed by Adrian, Meinhart & Tomkins (J. Fluid Mech., vol. 422, 2000, pp. 1-54). The adverse pressure gradient augments the inclination angles of the packets and the mean streamwise spacing of the vortex heads in the packets.

Research paper thumbnail of Effects of an adverse pressure gradient on a turbulent boundary layer

International Journal of Heat and Fluid Flow, 2008

Direct numerical simulations were performed to investigate the physics of turbulent boundary laye... more Direct numerical simulations were performed to investigate the physics of turbulent boundary layer flows subjected to adverse pressure gradient. A fully implicit fractional step method was employed to simulate the flows. To avoid generating an inflow with adverse pressure gradient, the sufficient streamwise length was placed from the inlet to the sudden change of free-stream velocity. The spatially-developing characteristics of turbulent boundary layer with adverse pressure gradient were examined. The present results were in good agreement with previous experimental ones. The visualization results showed that the adverse pressure gradient weakens the vortical structures. This causes a reduction in turbulence intensity near the wall. NOMENCLATURE p C = Pressure coefficient, normalized by the inlet free-stream velocity, 0 U G = Shape factor, = ( ) 2 0

Research paper thumbnail of Coherent structures in turbulent boundary layers with adverse pressure gradients

Journal of Turbulence, 2010

The coherent structures in turbulent boundary layers (TBLs) subjected to adverse pressure gradien... more The coherent structures in turbulent boundary layers (TBLs) subjected to adverse pressure gradients (APGs) were investigated by analyzing a database of direct numerical simulations. The equilibrium adverse pressure gradient flows were established by using a power law free-stream distribution. The population trends of the spanwise vortices show that the outer regions of the APG TBLs are densely populated with hairpin-like vortices. These vortical structures induce low-momentum regions in the middle of the boundary layers, which result in an outer peak in the Reynolds shear stress. The 3-D features of the hairpin packets were deduced from their spatial characteristics in the spanwise-wall-normal plane. The conditionally averaged velocity fields show that there are counter-rotating v − w swirling motions that represent cross-sectional evidences of the packets. Moreover, two-point correlations and linear stochastic estimations were used to provide statistical information about the hairpin packet motions in the crossstream planes of the APG TBLs.

Research paper thumbnail of Response of Spatially Developing Turbulent Boundary Layer to Spanwise Oscillating Electromagnetic Force

Transactions of The Korean Society of Mechanical Engineers B, 2005

Direct numerical simulations were performed to investigate the physics of a spatially-developing ... more Direct numerical simulations were performed to investigate the physics of a spatially-developing turbulent boundary layer flow subjected to spanwise oscillating electromagnetic force. The electromagnetic force beneath the flat plate was locally given with a finite length. A fully implicit fractional step method was employed to simulate the flow. The mean flow properties and the Reynolds stresses were obtained to analyze the nearwall turbulent structure. The skin friction and turbulent kinetic energy can be reduced by the electromagnetic forces. In the absence of the electromagnetic force, the flow eventually relaxes back to a two-dimensional equilibrium boundary layer.

Research paper thumbnail of Response of a spatially developing turbulent boundary layer to a spanwise oscillating electromagnetic force

Journal of Turbulence, 2005

Direct numerical simulations were performed to investigate the physics of a spatially-developing ... more Direct numerical simulations were performed to investigate the physics of a spatially-developing turbulent boundary layer flow subjected to spanwise oscillating electromagnetic force. The electromagnetic force beneath the flat plate was locally given with a finite length. A fully implicit fractional step method was employed to simulate the flow. The mean flow properties and the Reynolds stresses were obtained to analyze the nearwall turbulent structure. The skin friction and turbulent kinetic energy can be reduced by the electromagnetic forces. In the absence of the electromagnetic force, the flow eventually relaxes back to a two-dimensional equilibrium boundary layer.

Research paper thumbnail of Structures in turbulent boundary layers subjected to adverse pressure gradients

Journal of Fluid Mechanics, 2009

The effects of adverse pressure gradients on turbulent structures were investigated by carrying o... more The effects of adverse pressure gradients on turbulent structures were investigated by carrying out direct numerical simulations of turbulent boundary layers subjected to adverse and zero pressure gradients. The equilibrium adverse pressure gradient flows were established by using a power law free-stream distribution U ∞ ∼ x m . Twopoint correlations of velocity fluctuations were used to show that the spanwise spacing between near-wall streaks is affected significantly by a strong adverse pressure gradient. Low-momentum regions are dominant in the middle of the boundary layer as well as in the log layer. Linear stochastic estimation was used to provide evidence for the presence of low-momentum regions and to determine their statistical properties. The mean width of such large-scale structures is closely associated with the size of the hairpin-like vortices in the outer layer. The conditionally averaged flow fields around events producing Reynolds stress show that hairpin-like vortices are the structures associated with the production of outer turbulence. The shapes of the vortices beyond the log layer were found to be similar when their length scales were normalized according to the boundary layer thickness. Estimates of the conditionally averaged velocity fields associated with the spanwise vortical motion were obtained by using linear stochastic estimation. These results confirm that the outer region of the adverse pressure gradient boundary layer is populated with streamwise-aligned vortex organizations, which are similar to those of the vortex packet model proposed by Adrian, Meinhart & Tomkins (J. Fluid Mech., vol. 422, 2000, pp. 1-54). The adverse pressure gradient augments the inclination angles of the packets and the mean streamwise spacing of the vortex heads in the packets.

Research paper thumbnail of Effects of an adverse pressure gradient on a turbulent boundary layer

International Journal of Heat and Fluid Flow, 2008

Direct numerical simulations were performed to investigate the physics of turbulent boundary laye... more Direct numerical simulations were performed to investigate the physics of turbulent boundary layer flows subjected to adverse pressure gradient. A fully implicit fractional step method was employed to simulate the flows. To avoid generating an inflow with adverse pressure gradient, the sufficient streamwise length was placed from the inlet to the sudden change of free-stream velocity. The spatially-developing characteristics of turbulent boundary layer with adverse pressure gradient were examined. The present results were in good agreement with previous experimental ones. The visualization results showed that the adverse pressure gradient weakens the vortical structures. This causes a reduction in turbulence intensity near the wall. NOMENCLATURE p C = Pressure coefficient, normalized by the inlet free-stream velocity, 0 U G = Shape factor, = ( ) 2 0

Research paper thumbnail of Coherent structures in turbulent boundary layers with adverse pressure gradients

Journal of Turbulence, 2010

The coherent structures in turbulent boundary layers (TBLs) subjected to adverse pressure gradien... more The coherent structures in turbulent boundary layers (TBLs) subjected to adverse pressure gradients (APGs) were investigated by analyzing a database of direct numerical simulations. The equilibrium adverse pressure gradient flows were established by using a power law free-stream distribution. The population trends of the spanwise vortices show that the outer regions of the APG TBLs are densely populated with hairpin-like vortices. These vortical structures induce low-momentum regions in the middle of the boundary layers, which result in an outer peak in the Reynolds shear stress. The 3-D features of the hairpin packets were deduced from their spatial characteristics in the spanwise-wall-normal plane. The conditionally averaged velocity fields show that there are counter-rotating v − w swirling motions that represent cross-sectional evidences of the packets. Moreover, two-point correlations and linear stochastic estimations were used to provide statistical information about the hairpin packet motions in the crossstream planes of the APG TBLs.

Research paper thumbnail of Response of Spatially Developing Turbulent Boundary Layer to Spanwise Oscillating Electromagnetic Force

Transactions of The Korean Society of Mechanical Engineers B, 2005

Direct numerical simulations were performed to investigate the physics of a spatially-developing ... more Direct numerical simulations were performed to investigate the physics of a spatially-developing turbulent boundary layer flow subjected to spanwise oscillating electromagnetic force. The electromagnetic force beneath the flat plate was locally given with a finite length. A fully implicit fractional step method was employed to simulate the flow. The mean flow properties and the Reynolds stresses were obtained to analyze the nearwall turbulent structure. The skin friction and turbulent kinetic energy can be reduced by the electromagnetic forces. In the absence of the electromagnetic force, the flow eventually relaxes back to a two-dimensional equilibrium boundary layer.

Research paper thumbnail of Response of a spatially developing turbulent boundary layer to a spanwise oscillating electromagnetic force

Journal of Turbulence, 2005

Direct numerical simulations were performed to investigate the physics of a spatially-developing ... more Direct numerical simulations were performed to investigate the physics of a spatially-developing turbulent boundary layer flow subjected to spanwise oscillating electromagnetic force. The electromagnetic force beneath the flat plate was locally given with a finite length. A fully implicit fractional step method was employed to simulate the flow. The mean flow properties and the Reynolds stresses were obtained to analyze the nearwall turbulent structure. The skin friction and turbulent kinetic energy can be reduced by the electromagnetic forces. In the absence of the electromagnetic force, the flow eventually relaxes back to a two-dimensional equilibrium boundary layer.