Using data from direct numerical simulation （DNS） of incompressible and compressible channel flow, we develop a method of sub-ensemble decomposition to investigate the pressure gradient effect on the Karman constant...Using data from direct numerical simulation （DNS） of incompressible and compressible channel flow, we develop a method of sub-ensemble decomposition to investigate the pressure gradient effect on the Karman constant and the additive constant B characterizing the mean velocity profile （MVP）. The sub-ensemble decomposition is defined according to the magnitude of vertical fluctuation velocity, which mimics coherent motions like ejection and sweep. DNS data analysis shows that each sub-ensemble displays a distinct Karman constant, with a variation which mimics effects of pressure gradient. The latter is demonstrated by a relation between sub-ensembles＇ km and Bm similar to empirical data under various pressure gradients. A set of global parameters, k0-pg=0.39 ＆ B0-pg=5.5, are then derived for interpreting two constants observed by Nagib et al.展开更多
A theoretical analysis of velocity profiles in sediment-laden flows is presented by means of Prandtl-Karman mixing length theorem. The study shows that the upward velocity of liquid-phase caused by settling sediment l...A theoretical analysis of velocity profiles in sediment-laden flows is presented by means of Prandtl-Karman mixing length theorem. The study shows that the upward velocity of liquid-phase caused by settling sediment leads to the invalidity of the log-law and Rouse equation. The theoretical analysis takes into account the upward velocity and shows: 1) the mean velocity in sediment-laden flows follows the log-law, but the Karman constant reduces in the main flow region, 2) sediment concentration reduces the mixing length of fluid particles, 3) flow resistance reduces with the presence of sediment concentration, and 4) the sediment concentration profile deviates from the well know Rouse equation. The experimental data agree well with the equations derived on the basis of non-zero wall velocity. It is found that the wall-normal velocity should not be neglected for density gradient flows because it induces more than for pure water flows.展开更多
The quadratic rheology model considers the yield stress,viscous stress,turbulent stress and disperse stress,so it is used in this study to derive the velocity profile of debris flows.The quadratic model with the parab...The quadratic rheology model considers the yield stress,viscous stress,turbulent stress and disperse stress,so it is used in this study to derive the velocity profile of debris flows.The quadratic model with the parabolic eddy viscosity was numerically solved,and an analytical solution was derived for the quadratic model with a constant eddy viscosity.These two solutions were compared with the Arai-Takahashi model that excluded the viscous stress and the yield stress.The three models were tested by using 17 experiment cases of debris flows over rigid beds.The results prove that the quadratic model with parabolic and constant eddy viscosities is applicable to muddy and granular flows,whereas the Arai-Takahashi model tends to overestimate the flow velocity near the water surface if a plug-like layer exists.In addition,the von Karman constant and the zero-velocity elevation in the three models are related to sediment concentration.The von Karman constant decreases first and then increases as the sediment concentration increases.The zero-velocity elevation is below the bed surface,likely due to the invalidity of the non-slip boundary condition for the debris flows over fixed beds.展开更多
基金supported by the National Natural Science Foundation of China (Grant No. 90716008 and 10921202)the National Basic Research Program of China (Grant No. 2009CB724100)
文摘Using data from direct numerical simulation （DNS） of incompressible and compressible channel flow, we develop a method of sub-ensemble decomposition to investigate the pressure gradient effect on the Karman constant and the additive constant B characterizing the mean velocity profile （MVP）. The sub-ensemble decomposition is defined according to the magnitude of vertical fluctuation velocity, which mimics coherent motions like ejection and sweep. DNS data analysis shows that each sub-ensemble displays a distinct Karman constant, with a variation which mimics effects of pressure gradient. The latter is demonstrated by a relation between sub-ensembles＇ km and Bm similar to empirical data under various pressure gradients. A set of global parameters, k0-pg=0.39 ＆ B0-pg=5.5, are then derived for interpreting two constants observed by Nagib et al.
文摘A theoretical analysis of velocity profiles in sediment-laden flows is presented by means of Prandtl-Karman mixing length theorem. The study shows that the upward velocity of liquid-phase caused by settling sediment leads to the invalidity of the log-law and Rouse equation. The theoretical analysis takes into account the upward velocity and shows: 1) the mean velocity in sediment-laden flows follows the log-law, but the Karman constant reduces in the main flow region, 2) sediment concentration reduces the mixing length of fluid particles, 3) flow resistance reduces with the presence of sediment concentration, and 4) the sediment concentration profile deviates from the well know Rouse equation. The experimental data agree well with the equations derived on the basis of non-zero wall velocity. It is found that the wall-normal velocity should not be neglected for density gradient flows because it induces more than for pure water flows.
基金supported by the Second Tibetan Plateau Scientific Expedition and Research Program(No.2019QZKK0902)National Natural Science Foundation of China(No.41601004 and 41702369)Scientific and Technological Project in Henan Province(No.212102310055).
文摘The quadratic rheology model considers the yield stress,viscous stress,turbulent stress and disperse stress,so it is used in this study to derive the velocity profile of debris flows.The quadratic model with the parabolic eddy viscosity was numerically solved,and an analytical solution was derived for the quadratic model with a constant eddy viscosity.These two solutions were compared with the Arai-Takahashi model that excluded the viscous stress and the yield stress.The three models were tested by using 17 experiment cases of debris flows over rigid beds.The results prove that the quadratic model with parabolic and constant eddy viscosities is applicable to muddy and granular flows,whereas the Arai-Takahashi model tends to overestimate the flow velocity near the water surface if a plug-like layer exists.In addition,the von Karman constant and the zero-velocity elevation in the three models are related to sediment concentration.The von Karman constant decreases first and then increases as the sediment concentration increases.The zero-velocity elevation is below the bed surface,likely due to the invalidity of the non-slip boundary condition for the debris flows over fixed beds.
