Large-scale secondary motions are known to occur in turbulent flows over surfaces with spanwise roughness heterogeneity.Numerical studies often use adjacent high-and low-roughness longitudinal strips to investigate th...Large-scale secondary motions are known to occur in turbulent flows over surfaces with spanwise roughness heterogeneity.Numerical studies often use adjacent high-and low-roughness longitudinal strips to investigate these secondary rolls in boundary layers without any thermal stratification.In the present study,the effect of unstable thermal stratification on secondary rolls in a very high-Reynolds-number turbulent flow with spanwise-heterogeneous roughness is investigated by means of large-eddy simulation.The strength of the unstable stratification is systematically changed from L/h=−20 to L/h=−1,where L and h are Monin-Obukhov length and boundary-layer height,respectively.This range covers the transition from neutral stratification to unstable stratification.The results show that the positive buoyancy associated with the unstable thermal stratification acts against the roughness-induced secondary rolls.In the case of unstable stratification,secondary rolls are completely canceled out by buoyancy and replaced by new stronger convection-induced rolls rotating in opposite directions.展开更多
Based on conventional high-altitude aerial and ground data, regional intensive rainfall data, Doppler weather radar and satellite cloud data, the occurrence and development process of a regional rainstorm in South Sha...Based on conventional high-altitude aerial and ground data, regional intensive rainfall data, Doppler weather radar and satellite cloud data, the occurrence and development process of a regional rainstorm in South Shaanxi during July 17 -18, 2010 was discussed deeply. The results showed that the occurrence and development of the mesoscale weather system resulted in the heavy storm directly besides favorable large-scale background, and the the occurrence and development of the system had strong local and sudden characters. In addition, heavily concentrated wa- ter vapor in the troposphere at low and middle altitudes as well as existence of unstable conditions of convection might be favorable to the develop- ment of the system, while dynamic action of terrain, ground weak cold front and disturbance of the low-level jet stream might trigger the occurrence and development of the system.展开更多
基金P.F.thanks the Aarhus University Research Foundation(AUFF)for the financial support.M.A.acknowledges the financial support from the Aarhus University Centre for Digitalisation,Big Data and Data Analytics(DIGIT).
文摘Large-scale secondary motions are known to occur in turbulent flows over surfaces with spanwise roughness heterogeneity.Numerical studies often use adjacent high-and low-roughness longitudinal strips to investigate these secondary rolls in boundary layers without any thermal stratification.In the present study,the effect of unstable thermal stratification on secondary rolls in a very high-Reynolds-number turbulent flow with spanwise-heterogeneous roughness is investigated by means of large-eddy simulation.The strength of the unstable stratification is systematically changed from L/h=−20 to L/h=−1,where L and h are Monin-Obukhov length and boundary-layer height,respectively.This range covers the transition from neutral stratification to unstable stratification.The results show that the positive buoyancy associated with the unstable thermal stratification acts against the roughness-induced secondary rolls.In the case of unstable stratification,secondary rolls are completely canceled out by buoyancy and replaced by new stronger convection-induced rolls rotating in opposite directions.
文摘Based on conventional high-altitude aerial and ground data, regional intensive rainfall data, Doppler weather radar and satellite cloud data, the occurrence and development process of a regional rainstorm in South Shaanxi during July 17 -18, 2010 was discussed deeply. The results showed that the occurrence and development of the mesoscale weather system resulted in the heavy storm directly besides favorable large-scale background, and the the occurrence and development of the system had strong local and sudden characters. In addition, heavily concentrated wa- ter vapor in the troposphere at low and middle altitudes as well as existence of unstable conditions of convection might be favorable to the develop- ment of the system, while dynamic action of terrain, ground weak cold front and disturbance of the low-level jet stream might trigger the occurrence and development of the system.
