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  • 1
    Electronic Resource
    Electronic Resource
    The effect of surface roughness on flow structures in a neutrally stratified planetary boundary layer flow (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 9 (1997), S. 3235-3249 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effects of surface roughness on the structures of a neutrally stratified planetary boundary layer flow are investigated by the large-eddy simulation technique. Our numerical model, which assumes horizontal periodicity, shows that the growth of an internal boundary layer (IBL) in response to an abrupt change of surface roughness (either smooth-to-rough transition or rough-to-smooth transition) obeys the 4/5th power of the time, similar to that along the downwind fetch. A sudden increase or decrease in the surface shear stress during the transition is also observed. A quadrant analysis shows that during the transition, ejections and sweeps are altered significantly. Flow visualization further illustrates that the distribution density and the strength of coherent vortical structures and ejection eddies increase substantially during the smooth-to-rough transition. Conversely, these parameters decrease in the rough-to-smooth transition. The mean velocity profile has an inflection point at the IBL top, but the coherent vortical motions and ejection eddies affected by the change of the roughness are inside the IBL, suggesting that this inflection point is more static than dynamic. We also compare the quasi-steady coherent flow structures of different surface roughness values after the transition period. Streak spacing appears to increase with increasing surface roughness. Ejection eddies and vortical structures increase in scale as well as in strength as the surface roughness increases. The correlation between drag coefficient and flow structures in boundary layer flows is discussed. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869439
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  • 2
    Electronic Resource
    Electronic Resource
    Coherent structures and dynamics in a neutrally stratified planetary boundary layer flow (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 8 (1996), S. 2626-2639 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Coherent structures and the dynamics of a neutrally stratified planetary boundary layer flow are studied through a large eddy simulation, which includes surface roughness, Coriolis force, and a capping inversion. Quadrant analysis and flow visualization show that low-speed negative momentum flux (ejection) is the dominant feature throughout most of the boundary layer. The initiation of vortical structures is observed to be associated with vorticity sheets and pressure maxima, which are formed dynamically when low-speed negative momentum flux collides with either high-speed negative momentum flux (sweep) or the mean flow. Four dimensional conditional averages are used to study the statistical behavior of ejections and sweeps. The shape, strength, lifetime, and origin of the conditionally sampled structures at three different heights are discussed. Near the surface, sweeps are observed to induce ejections when colliding with the surface. The evolution of sweep-induced ejections near the wall is discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869048
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  • 3
    Electronic Resource
    Electronic Resource
    Local pressure-transport structure in a convective atmospheric boundary layer (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 12 (2000), S. 1112-1128 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Local pressure-transport structure in a convective atmospheric boundary layer is studied through large-eddy simulation and a conditional sampling technique. Two cases are simulated: A free-convection boundary layer and a sheared convective boundary layer with −zi/L(approximate)17, where zi is the boundary layer height and L is the Monin–Obukhov length. Results show that pressure-transport flux tends to increase turbulent kinetic energy in the lower part of the sheared convective boundary layer. Furthermore, the root-mean-square resolved pressure fluctuation and the resolved negative pressure fluctuation due to −u1,2ru2,1r become much stronger in the sheared case. Flow visualization demonstrates that strong pressure transport is physically correlated with vortical structure embedded within large-scale updrafts. A conditional sampling technique is applied to study statistical characteristics of resolved fields surrounding strong pressure transport events. The conditional field reveals a boundary-layer-scale roll circulation with a large-scale thermal located at its center and characterized by a negative pressure minimum. Conditional pressure transport is a gain in the lower part of the pressure minimum and a loss in the upper part. The conditional vorticity lines converge to four distinct regions relative to the thermal: Large-scale horseshoe-shaped vorticity lines are wrapped around the thermal; small-scale arch-shaped vorticity lines drag behind the thermal; helical vorticity lines originate in the thermal core; and converging vorticity lines are found above the neck of the large-scale horseshoe-shaped vorticity lines. These regions roughly coincide with conditional negative momentum fluxes. We thus conclude that local pressure-transport structures are spatially associated with localized low pressure regions and strong vertical vorticity fluctuations, being embedded within thermals and advected along with large-scale convective rolls. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870365
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  • 4
    Electronic Resource
    Electronic Resource
    Near-grid-scale energy transfer and coherent structures in the convective planetary boundary layer (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 11 (1999), S. 3482-3494 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Coherent structures associated with forward- and backward-scatter energy transfer in the convective planetary boundary layer are studied using large-eddy simulation. A box filter is adopted for calculation of the resolved near-grid-scale stress tensor (Lij), strain-rate tensor (Sij) and dissipation rate (LijSij). Results of conditional sampling at two different heights are presented. The conditional events are resolved dissipation rate and vertical velocity fluctuation. The latter is to distinguish the forward and backward scatter associated with upward and downward motions, respectively. Near the surface, the forward-scatter event with positive vertical velocity fluctuation is physically associated with large-scale elongated updrafts. Therefore, the contribution of spanwise component L22S22 to the resolved dissipation rate is most significant. In the outer layer, the forward-scatter event with positive vertical velocity fluctuation occurs at the top of rising updrafts, consistent with negative S33 and the dominance of L33S33. In like manner, the forward scatter with negative vertical velocity fluctuation is found ahead of downdraft motions where S33 is negative and L33S33 dominates. Near the surface, the off-diagonal component L13S13 becomes dominant as the result of downdraft motions to the surface. For the backward scatter, the vertical diagonal component L33S33 is strongest except near the surface in regions of downward motions where L13S13 tends to be most intense. The strong L33S33 event is embedded within updrafts and is characterized by minimum pressure. Flow visualization suggests that the backward scatter occurs on the upwash side of the vortex where updrafts pass through. It is argued that the diameters of vortices can be effectively increased by nearby updrafts, representing energy transfer from small- to large-scale structures. The backward scatter associated with downward motions near the surface appears to generate small-scale counter-rotating motions, resembling sweeps. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870206
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