Roughness elements are various in a mountain area; they include gravel and ground surface vegetation that often result in surface friction drag to resist overland flows. The variation and characteristics of flow resis...Roughness elements are various in a mountain area; they include gravel and ground surface vegetation that often result in surface friction drag to resist overland flows. The variation and characteristics of flow resistance strongly impact the overland flow process and watershed floods. In view of the universal existence of natural vegetation, such as Chlorophytum malayense(CM) or Ophiopogon bodinieri(OB), and the sand-gravel bed of the river channel, it is important to understand the role of different types of roughness elements in flow resistance. This study was performed to investigate and compare through flume experiments the behaviors of overland flow resistance by the reaction of multi-scale configuration of different roughness elements. The result showed that the resistance coefficient gradually reduced versus the increase of flow rate in unit width and tended to be a constant when q = 3.0 l/s.m, Fr = 1.0, and Re = 4000 for slopes of 6 to 10 degrees. The gap of the vegetated rough bed and the gravel rough bed is limited to the same as the gap of the two types of vegetation, CM and OB. It was noted that the vegetation contributed to the increase in form resistance negatively and may lead to the mean resistance on decrease. To classify the flow pattern, the laminar flows were described by DarcyWeisbach's equation. In the study the f-Re equation of vegetated bed was developed with f ?5000 Re.The friction coefficient for laminar flows can be regarded as the critical value for identifying the transformation point of the flow pattern.展开更多
Boundary-layer wind tunnel provides a unique platform to reproduce urban, suburban and rural atmospheric boundary layer （ABL） by using roughness devices such as vortex generators, floor roughness, barrier wails, and...Boundary-layer wind tunnel provides a unique platform to reproduce urban, suburban and rural atmospheric boundary layer （ABL） by using roughness devices such as vortex generators, floor roughness, barrier wails, and slots in the extended test-section floor in the contraction cone. Each passive device impacts wind properties in a certain way. In this study, influence of various passive devices on wind properties has been investigated. Experi- ments using eighteen different configurations of the passive devices have been carried out to simulate urban, sub-urban, and rural climate conditions in a boundary-layer wind tunnel. The effect of each configuration on the wind characteristics is presented. It was found that higher barrier height and more number of roughness elements on the floor, generated higher turbulence and therefore higher model scale factors were obtained. However, in- creased slot width in the extended test-section floor in the contraction cone of the wind tunnel seemed to have a little effect on wind characteristics.展开更多
A comparison between the efficacy of surface boundary structure and presence of nanoparticles on the condensation two-phase flow inside rough nanochannels has been accomplished by applying molecular dynamics procedure...A comparison between the efficacy of surface boundary structure and presence of nanoparticles on the condensation two-phase flow inside rough nanochannels has been accomplished by applying molecular dynamics procedure to evaluate the thermal conductivity and flow characteristics.Simulation is performed in a computational region with two copper walls containing rectangular rough elements under different saturated temperatures.The main properties of liquid–vapor interface including density and the number of liquid atoms,are obtained.It is observed that the density profile is more affected by nanoparticles than the roughness.Also,compared to the condensation of nanofluid in a smooth nanochannel,the rough wall causes a greater drop in the temperature at the early time steps and by development of liquid films,effects of the wall roughness reduce.At the first of the condensation process,adding nanoparticle causes that transferring argon particles to the liquid phase increases with a steeper slope.Furthermore,heat current autocorrelation function(HCACF)for nanofluid condensation flow over considered correlation time is analyzed and following that the thermal conductivity for different saturated conditions is calculated.It has been represented that at lower temperatures the roughness makes more significant influence on the heat transfer of two-phase flow,while at higher temperatures the importance of nanoparticles prevails.展开更多
In aerodynamics, the laminar or turbulent regime of a boundary layer has a strong influence on friction or heat transfer. In practical applications, it is sometimes necessary to trip the transition to turbulent, and a...In aerodynamics, the laminar or turbulent regime of a boundary layer has a strong influence on friction or heat transfer. In practical applications, it is sometimes necessary to trip the transition to turbulent, and a common way is by use of a roughness element （e.g. a step） on the wall. The present paper is concerned with the numerical im- plementation of such a trip in large-eddy simulations. The study is carried out on a flat-plate boundary layer con- figuration, with Reynolds number Rex=l.3x 106. First, this work brings the opportunity to introduce a practical methodology to assess convergence in large-eddy simulations. Second, concerning the trip implementation, a volume source term is proposed and is shown to yield a smoother and faster transition than a grid step. Moreover, it is easier to implement and more adaptable. Finally, two subgrid-scale models are tested： the WALE model of Nic0ud and Ducros （Flow Turbul. Combust., vol. 62, 1999） and the shear-improved Smagorinsky model of Ldv^que et al. （J. Fluid Mech., vol. 570, 2007）. Both models allow transition, but the former appears to yield a faster transition and a better prediction of friction in the turbulent regime.展开更多
A combined experimental and numerical investigation of the heat transfer and flow characteristics of the roughened target plate has been conducted.All the data are compared with the flat plate.Three novel streamlined ...A combined experimental and numerical investigation of the heat transfer and flow characteristics of the roughened target plate has been conducted.All the data are compared with the flat plate.Three novel streamlined roughness elements are proposed:similar round protuberance,similar trapezoidal straight rib,and similar trapezoidal curved rib.The experiments are carried out in a perspex model using the transient thermochromic liquid crystal method.The effect of jet Reynolds number,rib height,and rib shape on the Nusselt number and flow discharge coefficient has been investigated.Higher ribs provide higher heat transfer enhancement.The curved ribs provide better heat transfer performance.Within the experimental scope,combined straight rib plate and combined curved rib plate increase the area averaged Nusselt number by 11.5%and 13.8%respectively.The experiment is complemented by a numerical part,which can provide flow field analysis and the Nusselt number on the surface of the small size roughness element.The numerical results show the protuberance can shorten the nozzle to plate distance and make the shifting point move forward.The ribs have a guidance effect on crossflow and reduce the transverse interference to the downstream jet.The transferred heat flux caused by the side surface of the roughness element is very obvious.The heat flux contributed by the side surface of the protuberance and ribs can reach 26%and 10%respectively.展开更多
基金support from the authorities of the National Natural Science Foundation of China (Grant No. 41171016)Sichuan Province Science and technology support program (Grant No. 2014SZ0163)the Open Foundation of State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University (Grant No. SKHL1309 and SKHL1418)
文摘Roughness elements are various in a mountain area; they include gravel and ground surface vegetation that often result in surface friction drag to resist overland flows. The variation and characteristics of flow resistance strongly impact the overland flow process and watershed floods. In view of the universal existence of natural vegetation, such as Chlorophytum malayense(CM) or Ophiopogon bodinieri(OB), and the sand-gravel bed of the river channel, it is important to understand the role of different types of roughness elements in flow resistance. This study was performed to investigate and compare through flume experiments the behaviors of overland flow resistance by the reaction of multi-scale configuration of different roughness elements. The result showed that the resistance coefficient gradually reduced versus the increase of flow rate in unit width and tended to be a constant when q = 3.0 l/s.m, Fr = 1.0, and Re = 4000 for slopes of 6 to 10 degrees. The gap of the vegetated rough bed and the gravel rough bed is limited to the same as the gap of the two types of vegetation, CM and OB. It was noted that the vegetation contributed to the increase in form resistance negatively and may lead to the mean resistance on decrease. To classify the flow pattern, the laminar flows were described by DarcyWeisbach's equation. In the study the f-Re equation of vegetated bed was developed with f ?5000 Re.The friction coefficient for laminar flows can be regarded as the critical value for identifying the transformation point of the flow pattern.
文摘Boundary-layer wind tunnel provides a unique platform to reproduce urban, suburban and rural atmospheric boundary layer （ABL） by using roughness devices such as vortex generators, floor roughness, barrier wails, and slots in the extended test-section floor in the contraction cone. Each passive device impacts wind properties in a certain way. In this study, influence of various passive devices on wind properties has been investigated. Experi- ments using eighteen different configurations of the passive devices have been carried out to simulate urban, sub-urban, and rural climate conditions in a boundary-layer wind tunnel. The effect of each configuration on the wind characteristics is presented. It was found that higher barrier height and more number of roughness elements on the floor, generated higher turbulence and therefore higher model scale factors were obtained. However, in- creased slot width in the extended test-section floor in the contraction cone of the wind tunnel seemed to have a little effect on wind characteristics.
文摘A comparison between the efficacy of surface boundary structure and presence of nanoparticles on the condensation two-phase flow inside rough nanochannels has been accomplished by applying molecular dynamics procedure to evaluate the thermal conductivity and flow characteristics.Simulation is performed in a computational region with two copper walls containing rectangular rough elements under different saturated temperatures.The main properties of liquid–vapor interface including density and the number of liquid atoms,are obtained.It is observed that the density profile is more affected by nanoparticles than the roughness.Also,compared to the condensation of nanofluid in a smooth nanochannel,the rough wall causes a greater drop in the temperature at the early time steps and by development of liquid films,effects of the wall roughness reduce.At the first of the condensation process,adding nanoparticle causes that transferring argon particles to the liquid phase increases with a steeper slope.Furthermore,heat current autocorrelation function(HCACF)for nanofluid condensation flow over considered correlation time is analyzed and following that the thermal conductivity for different saturated conditions is calculated.It has been represented that at lower temperatures the roughness makes more significant influence on the heat transfer of two-phase flow,while at higher temperatures the importance of nanoparticles prevails.
文摘In aerodynamics, the laminar or turbulent regime of a boundary layer has a strong influence on friction or heat transfer. In practical applications, it is sometimes necessary to trip the transition to turbulent, and a common way is by use of a roughness element （e.g. a step） on the wall. The present paper is concerned with the numerical im- plementation of such a trip in large-eddy simulations. The study is carried out on a flat-plate boundary layer con- figuration, with Reynolds number Rex=l.3x 106. First, this work brings the opportunity to introduce a practical methodology to assess convergence in large-eddy simulations. Second, concerning the trip implementation, a volume source term is proposed and is shown to yield a smoother and faster transition than a grid step. Moreover, it is easier to implement and more adaptable. Finally, two subgrid-scale models are tested： the WALE model of Nic0ud and Ducros （Flow Turbul. Combust., vol. 62, 1999） and the shear-improved Smagorinsky model of Ldv^que et al. （J. Fluid Mech., vol. 570, 2007）. Both models allow transition, but the former appears to yield a faster transition and a better prediction of friction in the turbulent regime.
基金supported by the National Science and Technology Major Project(Grant No.2017-Ⅲ-0001-0025)。
文摘A combined experimental and numerical investigation of the heat transfer and flow characteristics of the roughened target plate has been conducted.All the data are compared with the flat plate.Three novel streamlined roughness elements are proposed:similar round protuberance,similar trapezoidal straight rib,and similar trapezoidal curved rib.The experiments are carried out in a perspex model using the transient thermochromic liquid crystal method.The effect of jet Reynolds number,rib height,and rib shape on the Nusselt number and flow discharge coefficient has been investigated.Higher ribs provide higher heat transfer enhancement.The curved ribs provide better heat transfer performance.Within the experimental scope,combined straight rib plate and combined curved rib plate increase the area averaged Nusselt number by 11.5%and 13.8%respectively.The experiment is complemented by a numerical part,which can provide flow field analysis and the Nusselt number on the surface of the small size roughness element.The numerical results show the protuberance can shorten the nozzle to plate distance and make the shifting point move forward.The ribs have a guidance effect on crossflow and reduce the transverse interference to the downstream jet.The transferred heat flux caused by the side surface of the roughness element is very obvious.The heat flux contributed by the side surface of the protuberance and ribs can reach 26%and 10%respectively.
