Direct numerical simulation is carried out for a spatially evolving supersonic turbulent boundary layer at freestream Mach number 6. To overcome numerical instability, the seventh-order WENO scheme is used for the con...Direct numerical simulation is carried out for a spatially evolving supersonic turbulent boundary layer at freestream Mach number 6. To overcome numerical instability, the seventh-order WENO scheme is used for the convection terms of Navier-Stokes equations, and fine mesh is adopted to minimize numerical dissipation. Compressibility effects on the near-wall turbulent kinetic energy budget are studied. The cross-stream extended self-similarity and scaling exponents including the near-wall region are studied. In high Mach number flows, the coherence vortex structures are arranged to be smoother and streamwised, and the hair-pin vortices are less likely tO OCCUr.展开更多
The critical transition Reynolds number is the lowest value at which the turbulent flow can hold in real flows.The determination of the critical transition Reynolds number not only is a scientific problem,but also is ...The critical transition Reynolds number is the lowest value at which the turbulent flow can hold in real flows.The determination of the critical transition Reynolds number not only is a scientific problem,but also is important for some engineering problems.However,there is no available theoretical method to search the critical value.For the hypersonic boundary layer with significant importance for engineering problems,there is no available experimental method to search the critical value so far.Consequently,it is imperative to take numerical method to search it.In this paper,direct numerical simulations(DNS)method is employed to determine the critical transition Reynolds number for the incompressible flat-plate boundary layer.Firstly,under the assumption of parallel flow,the temporal mode DNS is performed to determine the critical value as Re_(xpcr)=43767,which is quite close to the numerical results of other people.Secondly,under the condition of nonparallel flow,the spatial mode DNS is performed to determine the critical transition Reynolds number as Re_(xcr)=3×10^(5),which is well consistent with the experimental results.In principle,the proposed method in this paper can be extended to the supersonic/hypersonic boundary layer,and that problem will be discussed in the subsequent papers.展开更多
A new turbulent constitutive relation was directly derived from Boussinesqs hypothesis and mixing length theory,and then implemented in the standard k-ε model.The performance of this constitutive relation was valid...A new turbulent constitutive relation was directly derived from Boussinesqs hypothesis and mixing length theory,and then implemented in the standard k-ε model.The performance of this constitutive relation was validated in zero pressure gradient flat-plate boundary layer flow,fully-developed turbulent channel flow and separated flow in a plane asymmetric diffuser.The investigation demonstrated that,this new constitutive relation gave very accurate results in the former two basic cases and provided significant improvement in prediction of separated and reattachment points in the plane asymmetric diffuser.Separation and reattachment points at x/H =7.5and 29 were calculated accurately in comparison to experimental results,and the static pressure coefficient of 0.82 was very close to large eddy simulation calculation.These results are very encouraging but further verification and extensive application of the new constitutive relation to other two-equation eddy viscosity model are needed.展开更多
The early stage of a boundary layer bypass transition induced by the direct impingement of a circular cylinder wake is experimentally investigated in water tunnel,with the primary interest in both the evolution of coh...The early stage of a boundary layer bypass transition induced by the direct impingement of a circular cylinder wake is experimentally investigated in water tunnel,with the primary interest in both the evolution of coherent structures and their effects on the disturbance growth inside the boundary layer.It is found that spanwise vortices with small scale first form in the near-wall region around the leading-edge,which are either the residual of the wake rollers cut by the leading-edge or the high-order structures induced by the wake rollers.The formation of these spanwise vortices leads to the first rapid disturbance growth inside the boundary layer.On the other hand,streamwise vortices,which result from the impingement of longitudinal braids onto the leading-edge,are also observed inside the boundary layer.They lead to the three dimensional destabilization and the subsequent dispersion of spanwise vortices,and soon become the most dominant coherent structures inside the transitional boundary layer.It is suggested that the formation and evolution of these streamwise vortices contribute to the secondary disturbance growth stage and thus promote the completion of the transition process.The difference between the present transition scenario triggered by direct wake impingement and that by indirect wake-vortex inducement is further discussed.展开更多
Almost 80-90%of energy is wasted as heat(provides no value)in a photovoltaic(PV)panel.An integrated photovoltaic-thermal(PVT)system can utilize this energy and produce electricity simultaneously.In this research,throu...Almost 80-90%of energy is wasted as heat(provides no value)in a photovoltaic(PV)panel.An integrated photovoltaic-thermal(PVT)system can utilize this energy and produce electricity simultaneously.In this research,through energy and exergy analysis,a novel design and methodology of a PVT system are studied and validated.Unlike the common methods,here the collector is located outside the PV panel and connected with pipes.Water passes over the top of the panel and then is forced to the collector by a pump.The effects of different water-mass flow rates on the PV panel and collector,individual and overall efficiency,mass loss,exergetic efficiency are examined experimentally.Results show that the overall efficiency of the system is around five times higher than the individual PV-panel efficiency.The forced circulation of water dropped the panel temperature and increased the panel efficiency by 0.8-1%and exergy by 0.6-1%,where the overall energy efficiency was~81%.展开更多
Temperature distribution over the absorber plate of a parallel flow flat-plate solar collector is numerically analyzed. The governing differential equations with boundary conditions are solved numerically using fluent...Temperature distribution over the absorber plate of a parallel flow flat-plate solar collector is numerically analyzed. The governing differential equations with boundary conditions are solved numerically using fluent software. Effects of the inlet mass flux, inlet temperature and tube spacing on velocity and temperature distributions are discussed. Numerical results show that the distributions of velocity and temperature of fluid is unsymmetrical inside pipe.展开更多
In this paper,the visualization of the thermo-hydrodynamic behavior in flat-plate pulsating heat pipe(FP-PHP)with HFE-347 is experimentally investigated.The FP-PHP is vertically placed with filling rate of 20%to 70%an...In this paper,the visualization of the thermo-hydrodynamic behavior in flat-plate pulsating heat pipe(FP-PHP)with HFE-347 is experimentally investigated.The FP-PHP is vertically placed with filling rate of 20%to 70%and heating power of 20 W to 140 W.A high-speed camera is used to record the two-phase flow in the FP-PHP.Four flow pattern types and four flow directions are observed.The flow directions of the two-phase flow inside the FP-PHP with medium filling rate(40%–60%)are the most complex,and the FP-PHP with high filling rate(70%)is most likely to form a directional circulating flow.At high heating power(100 W to 140 W),the flow patterns in FP-PHP with medium(40%–60%)and high filling rate(70%)are dominated by mixed flow.The wall temperature fluctuates greatly at moderate heating power(60 W to 80 W)owing to the uncertainty of the flow direction.The temperature distribution of the FP-PHP is highly affected by the heat transfer intensity of the working fluid under different flow states,so that the state of fluid flow and the thermal performance of FP-PHP can be evaluated through the infrared thermal image of the FP-PHP.展开更多
Ships which have large structures above water surface,such as pure car carriers(PCCs) and container vessels,have large speed reduction by wind pressure.In the present study,the running speed of a large PCC with two or...Ships which have large structures above water surface,such as pure car carriers(PCCs) and container vessels,have large speed reduction by wind pressure.In the present study,the running speed of a large PCC with two or more sails for using wind power is simulated.The simulated results demonstrate that the ship can keep a constant service speed even in winds of 20m/s except head and bow winds.This sail system can shorten annual average navigation time by about 4 hours per voyage.展开更多
基金Supported by the National Natural Science Foundation of China under Grant Nos 90205025, 19872069, 170176033, and 10502052, and the Informatization Construction of Knowledge Innovation Projects of Chinese Academy of Sciences under Grant No INF 105-SCE. The authors would like to thank the State Key Laboratory of Scientific and Engineering Computing （LSEC） and Supercomputing Centre of Chinese Academy of Sciences （SCCAS） for providing computer time. The authors would like to thank Professor Zhou Heng and Professor Luo Jisheng of Tianjin University for helpful discussion.
文摘Direct numerical simulation is carried out for a spatially evolving supersonic turbulent boundary layer at freestream Mach number 6. To overcome numerical instability, the seventh-order WENO scheme is used for the convection terms of Navier-Stokes equations, and fine mesh is adopted to minimize numerical dissipation. Compressibility effects on the near-wall turbulent kinetic energy budget are studied. The cross-stream extended self-similarity and scaling exponents including the near-wall region are studied. In high Mach number flows, the coherence vortex structures are arranged to be smoother and streamwised, and the hair-pin vortices are less likely tO OCCUr.
基金supported by grants from the National Key Research and Development Program of China(Grant No.2016YFA0401200)the National Natural Science Foundation of China(Grant Nos.12072230,11672204,91952301,and 11732011).
文摘The critical transition Reynolds number is the lowest value at which the turbulent flow can hold in real flows.The determination of the critical transition Reynolds number not only is a scientific problem,but also is important for some engineering problems.However,there is no available theoretical method to search the critical value.For the hypersonic boundary layer with significant importance for engineering problems,there is no available experimental method to search the critical value so far.Consequently,it is imperative to take numerical method to search it.In this paper,direct numerical simulations(DNS)method is employed to determine the critical transition Reynolds number for the incompressible flat-plate boundary layer.Firstly,under the assumption of parallel flow,the temporal mode DNS is performed to determine the critical value as Re_(xpcr)=43767,which is quite close to the numerical results of other people.Secondly,under the condition of nonparallel flow,the spatial mode DNS is performed to determine the critical transition Reynolds number as Re_(xcr)=3×10^(5),which is well consistent with the experimental results.In principle,the proposed method in this paper can be extended to the supersonic/hypersonic boundary layer,and that problem will be discussed in the subsequent papers.
基金China Academy of Aeronautics Propulsion Innovation Fund
文摘A new turbulent constitutive relation was directly derived from Boussinesqs hypothesis and mixing length theory,and then implemented in the standard k-ε model.The performance of this constitutive relation was validated in zero pressure gradient flat-plate boundary layer flow,fully-developed turbulent channel flow and separated flow in a plane asymmetric diffuser.The investigation demonstrated that,this new constitutive relation gave very accurate results in the former two basic cases and provided significant improvement in prediction of separated and reattachment points in the plane asymmetric diffuser.Separation and reattachment points at x/H =7.5and 29 were calculated accurately in comparison to experimental results,and the static pressure coefficient of 0.82 was very close to large eddy simulation calculation.These results are very encouraging but further verification and extensive application of the new constitutive relation to other two-equation eddy viscosity model are needed.
基金supported by the National Natural Science Foundation of China (Grant No. 10832001)
文摘The early stage of a boundary layer bypass transition induced by the direct impingement of a circular cylinder wake is experimentally investigated in water tunnel,with the primary interest in both the evolution of coherent structures and their effects on the disturbance growth inside the boundary layer.It is found that spanwise vortices with small scale first form in the near-wall region around the leading-edge,which are either the residual of the wake rollers cut by the leading-edge or the high-order structures induced by the wake rollers.The formation of these spanwise vortices leads to the first rapid disturbance growth inside the boundary layer.On the other hand,streamwise vortices,which result from the impingement of longitudinal braids onto the leading-edge,are also observed inside the boundary layer.They lead to the three dimensional destabilization and the subsequent dispersion of spanwise vortices,and soon become the most dominant coherent structures inside the transitional boundary layer.It is suggested that the formation and evolution of these streamwise vortices contribute to the secondary disturbance growth stage and thus promote the completion of the transition process.The difference between the present transition scenario triggered by direct wake impingement and that by indirect wake-vortex inducement is further discussed.
文摘Almost 80-90%of energy is wasted as heat(provides no value)in a photovoltaic(PV)panel.An integrated photovoltaic-thermal(PVT)system can utilize this energy and produce electricity simultaneously.In this research,through energy and exergy analysis,a novel design and methodology of a PVT system are studied and validated.Unlike the common methods,here the collector is located outside the PV panel and connected with pipes.Water passes over the top of the panel and then is forced to the collector by a pump.The effects of different water-mass flow rates on the PV panel and collector,individual and overall efficiency,mass loss,exergetic efficiency are examined experimentally.Results show that the overall efficiency of the system is around five times higher than the individual PV-panel efficiency.The forced circulation of water dropped the panel temperature and increased the panel efficiency by 0.8-1%and exergy by 0.6-1%,where the overall energy efficiency was~81%.
文摘Temperature distribution over the absorber plate of a parallel flow flat-plate solar collector is numerically analyzed. The governing differential equations with boundary conditions are solved numerically using fluent software. Effects of the inlet mass flux, inlet temperature and tube spacing on velocity and temperature distributions are discussed. Numerical results show that the distributions of velocity and temperature of fluid is unsymmetrical inside pipe.
基金financial support provided by National Natural Science Foundation of China(Project No.51506033)Guangxi Natural Science Foundation(Grant No.2017JJA160108)Guangxi Colleges and Universities Program of Innovative Research Team and Outstanding Talent。
文摘In this paper,the visualization of the thermo-hydrodynamic behavior in flat-plate pulsating heat pipe(FP-PHP)with HFE-347 is experimentally investigated.The FP-PHP is vertically placed with filling rate of 20%to 70%and heating power of 20 W to 140 W.A high-speed camera is used to record the two-phase flow in the FP-PHP.Four flow pattern types and four flow directions are observed.The flow directions of the two-phase flow inside the FP-PHP with medium filling rate(40%–60%)are the most complex,and the FP-PHP with high filling rate(70%)is most likely to form a directional circulating flow.At high heating power(100 W to 140 W),the flow patterns in FP-PHP with medium(40%–60%)and high filling rate(70%)are dominated by mixed flow.The wall temperature fluctuates greatly at moderate heating power(60 W to 80 W)owing to the uncertainty of the flow direction.The temperature distribution of the FP-PHP is highly affected by the heat transfer intensity of the working fluid under different flow states,so that the state of fluid flow and the thermal performance of FP-PHP can be evaluated through the infrared thermal image of the FP-PHP.
文摘Ships which have large structures above water surface,such as pure car carriers(PCCs) and container vessels,have large speed reduction by wind pressure.In the present study,the running speed of a large PCC with two or more sails for using wind power is simulated.The simulated results demonstrate that the ship can keep a constant service speed even in winds of 20m/s except head and bow winds.This sail system can shorten annual average navigation time by about 4 hours per voyage.
