A numerical simulation is performed to find out a key vortical structure in the laminar-turbulent transition. A low-speed streak is generated inside a laminar boundary layer using an isolated cuboid roughness, aimed a...A numerical simulation is performed to find out a key vortical structure in the laminar-turbulent transition. A low-speed streak is generated inside a laminar boundary layer using an isolated cuboid roughness, aimed at providing an environment unstable to outer disturbances. Then, a short duration jet is issued into the boundary layer. When the jet velocity is low, some vortices appear in the boundary layer, but the transition of the boundary layer does not take place. However, when the jet velocity exceeds a certain threshold, two vortices newly appear above the elongated legs of a V-shaped vortex and only one of them is stretched and survives. After that, vortices are generated one after another around the survived one. By comparing the decayed and the survived vortices, it is found that the difference in their heights is the key characteristic which leads to the transition.展开更多
Accurate prediction of the onset of turbulent fluidization still remains elusive owing to the dependence of the transition velocity on several factors including measurement methods and interpretation of results. In th...Accurate prediction of the onset of turbulent fluidization still remains elusive owing to the dependence of the transition velocity on several factors including measurement methods and interpretation of results. In this work, numerical simulations using the two fluid model (TFM) are performed in an attempt to predict the regime change reported by Gopalan etal.(2016) in a small scale pseudo-2D gas-solid fluidized bed containing Geldart D particles. Various time and frequency domain analyses were applied on predicted absolute and differential pressure time series data to reveal the bed dynamics. Numerical predictions of the transition velocity, Uc are in reasonably good agreement with experimental results from the small scale challenge problem. The literature correlations completely fail to predict the transition velocity for the system considered in this work. This work thus provides a different approach for validating the CFD model against experimental measurements.展开更多
Axis-symmetric spheroids, such as rod-like and disk-like particles, have been found to orient preferentially in near-wall turbulence by both experiment and numerical simulation. In current work we examined the orienta...Axis-symmetric spheroids, such as rod-like and disk-like particles, have been found to orient preferentially in near-wall turbulence by both experiment and numerical simulation. In current work we examined the orientation of inertialess spheroids in a turbulent channel flow at medium friction Reynolds number Reτ=100 given based on the half of channel height. Both elongated prolate spheroid and flat oblate spheroid are considered and further compared with the reference case of spherical particle. The statistical results show that in near wall region the prolate spheroids tend to align in the streamwise direction while the oblate spheroids prefer to orient in the wallnormal direction, which are consistent with earlier observation in low Reynolds number (Reτ=180)wall turbulence. Around the channel center we found that the orientation of spheroids is not fully isotropic, even though the fluid vorticity are almost isotropic. The mechanism that gives rise to such particle orientations in wall-turbulence has been found to be related to fluid Lagrangian stretching and compression (Zhao and Andersson 2016). Therefore, we computed the left Cauchy-Green strain tensor along Lagrangian trajectories of tracer spheroids in current flow field and analyzed the fluid Lagrangian stretching and compression. The results indicated that, similar to the earlier observations, the directions of the Lagrangian stretching and compression in near-wall region are in the streamwise and wall-normal directions, respectively. Furthermore, cross over the channel the prolate spheroids aligned with the direction of Lagrangian stretching but oblate spheroids oriented with the direction of Lagrangian compression. The weak anisotropy of orientations of fluid Lagrangian stretching and compression observed at the channel center could be the reason for the aforementioned modest anisotropic orientation of spheroids in channel central region.展开更多
This paper attempts to develop a scaling procedure to measure structural vibration caused simultaneously by wall pressure fluctuations and the thermal load of hypersonic flow by a wind tunnel test. However, simulating...This paper attempts to develop a scaling procedure to measure structural vibration caused simultaneously by wall pressure fluctuations and the thermal load of hypersonic flow by a wind tunnel test. However, simulating the effect of thermal load is difficult with a scaled model in a wind tunnel due to the nonlinear effect of thermal load on a structure. In this work, the temperature variation of a structure is proposed to indicate the nonlinear effect of the thermal load,which provides a means to simulate both the thermal load and wall pressure fluctuations of a hypersonic Turbulent Boundary Layer(TBL) in a wind tunnel test. To validate the scaling procedure,both numerical computations and measurements are performed in this work. Theoretical results show that the scaling procedure can also be adapted to the buckling temperature of a structure even though the scaling procedure is derived from a reference temperature below the critical temperature of the structure. For the measurement, wall pressure fluctuations and thermal environment are simulated by creating hypersonic flow in a wind tunnel. Some encouraging results demonstrate the effectiveness of the scaling procedure for assessing structural vibration generated by hypersonic flow. The scaling procedure developed in this study will provide theoretical support to develop a new measurement technology to evaluate vibration of aircraft due to hypersonic flow.展开更多
The renormalization group (RNG) turbulent model is used to investigate the fluid resonance in a moonpool formed by two identical rectangular hulls under synchronous heaving excitation (e.g., a catamaran or dual pontoo...The renormalization group (RNG) turbulent model is used to investigate the fluid resonance in a moonpool formed by two identical rectangular hulls under synchronous heaving excitation (e.g., a catamaran or dual pontoon). The numerical model is validated against the available experimental data, and accurate numerical solutions are obtained. The present study focuses on the amplitude of the moving hulls and the edge configuration of the moonpool entrance, as well as their influences on the piston-modal resonant wave in the moonpool. The dependence of the resonant wave amplitude in the moonpool on the heaving amplitude, the characteristic moonpool dimensions and the local velocity magnitude is derived based on a theoretical analysis, and the results are in good agreement with the RNG turbulent solutions. Five different edge profiles are considered, including two convex edges, two concave edges (both with various dimensions), and a sharp edge. Numerical examinations show that the edge configuration has a significant influence on the piston-modal resonant responses, a larger opening size leading to a higher resonant frequency and a larger resonant wave amplitude in the moonpool. Various flow patterns of the piston-modal resonance in the vicinity of the moonpool entrance are also identified, mainly depending on the edge profile. More intensive turbulent and vortical flows give rise to more significant dissipation, accounting for the smaller relative wave amplitude in the moonpool. With the increase of the heaving amplitude, the relative piston-modal resonant amplitude is decreased in an approximate power function. Within the scope of this work, the numerical investigations show that the piston-modal resonant frequency is hardly affected by the heaving amplitude.展开更多
In this paper,the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with the direct numerical simulation(DNS)methods for different Reynolds...In this paper,the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with the direct numerical simulation(DNS)methods for different Reynolds numbers.A formulation is derived to express the relation between the drag and the Reynolds shear stress.With the application of optimal electromagnetic force,the in-depth relations among characteristic structures in the flow field,mean Reynolds shear stress,and the effect of drag reduction for different Reynolds numbers are discussed.The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers.The regular quasi-streamwise vortex structures,which appear in the flow field,have the same period with that of the electromagnetic force.These structures suppress the random velocity fluctuations,which leads to the absolute value of mean Reynolds shear stress decreasing and the distribution of that moving away from the wall.Moreover,the wave number of optimal electromagnetic force increases,and the scale of the regular quasi-streamwise vortex structures decreases as the Reynolds number increases.Therefore,the rate of drag reduction decreases with the increase in the Reynolds number since the scale of the regular quasi-streamwise vortex structures decreases.展开更多
Based on the Fourier–Chebyshev spectral method,the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with direct numerical simulation (DNS...Based on the Fourier–Chebyshev spectral method,the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with direct numerical simulation (DNS) methods for different Reynolds numbers.A formula is derived to express the relation between fluctuating velocities and the friction drag coefficient.With the application of electromagnetic force,the in-depth relations among the fluctuating velocities near the wall,Reynolds stress,and the effect of drag reduction for different Reynolds numbers are discussed.The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers.The fluctuating velocities along the streamwise and normal directions are suppressed significantly,while the fluctuating velocity along the spanwise direction is enhanced dramatically due to the spanwise electromagnetic force.However,the values of Reynolds stress depend on the fluctuating velocities along the streamwise and normal directions rather than that along the spanwise direction.Therefore,the significant effect of drag reduction is obtained.Moreover,the maximum drag reduction is weakened due to the decay of control effect for fluctuating velocities as the Reynolds number increases.展开更多
Flow transition from laminar to turbulent mode (and vice versa)—that is, the initiation of turbulence—is one of the most important research subjects in the history of engineering. Even for pipe flow, predicting the ...Flow transition from laminar to turbulent mode (and vice versa)—that is, the initiation of turbulence—is one of the most important research subjects in the history of engineering. Even for pipe flow, predicting the onset of turbulence requires sophisticated instrumentation and/or direct numerical simulation, based on observing the instantaneous flow structure formation and evolution. In this work, a local Reynolds number equivalence c (ratio of local inertia effect to viscous effect) is seen to conform to the Universal Law of the Wall, where c = 1 represents a quantitative balance between the abovementioned two effects. This coincides with the wall layer thickness (y+= 1, where y+ is the dimensionless distance from the wall surface defined in the Universal Law of the Wall). It is found that the characteristic of how the local derivative of c against the local velocity changes with increasing velocity determines the onset of turbulence. For pipe flow, c - 25, and for plate flow, c - 151.5. These findings suggest that a certain combination of c and velocity (nonlinearity) can qualify the source of turbulence (i.e., generate turbulent energy). Similarly, a re-evaluation of the previous findings reveals that only the geometrically narrow domain can act locally as the source of turbulence, with the rest of the flow field largely being left for transporting and dissipating. This understanding will have an impact on the future large-scale modeling of turbulence.展开更多
An OpenFOAM based turbulence combustion solver with flamelet generated manifolds (FGMs) is presented in this paper. A series of flamelets, representative for turbulent flames, are calculated first by a one-dimensional...An OpenFOAM based turbulence combustion solver with flamelet generated manifolds (FGMs) is presented in this paper. A series of flamelets, representative for turbulent flames, are calculated first by a one-dimensional (1D) detailed chemistry solver with the consideration of both transport and stretch/curvature contributions. The flame structure is then parameterized as a function of multiple reaction control variables. A manifold, which collects the 1D flame properties, is built from the 1D flame solutions. The control variables of the mixture fraction and the progress variable are solved from the corresponding transport equations. During the calculation, the scalar variables, e.g., temperature and species concentration, are retrieved from the manifolds by interpolation. A transport equation for NO is solved to improve its prediction accuracy. To verify the ability to deal with the enthalpy loss effect, the temperature retrieved directly from the manifolds is compared with the temperature solved from a transport equation of absolute enthalpy. The resulting FGM-computational fluid dynamics (CFD) coupled code has three significant features, i.e., accurate NO prediction, the ability to treat the heat loss effect and the adoption at the turbulence level, and high quality prediction within practical industrial configurations. The proposed method is validated against the Sandia flame D, and good agreement with the experimental data is obtained.展开更多
A new combined Fermi,betatron,and turbulent electron acceleration mechanism is proposed in interaction of magnetic islands during turbulent magnetic reconnection evolution in explosive astrophysical phenomena at large...A new combined Fermi,betatron,and turbulent electron acceleration mechanism is proposed in interaction of magnetic islands during turbulent magnetic reconnection evolution in explosive astrophysical phenomena at large temporal-spatial scale(LTSTMR),the ratio of observed current sheets thickness to electron characteristic length,electron Larmor radius for low-βand electron inertial length for high-β,is on the order of 10^10–10^11;the ratio of observed evolution time to electron gyroperiod is on the order of 10^7–10^9).The original combined acceleration model is known to be one of greatest importance in the interaction of magnetic islands;it assumes that the continuous kinetic-dynamic temporal-spatial scale evolution occurs as two separate independent processes.In this paper,we reconsider the combined acceleration mechanism by introducing a kinetic-dynamic-hydro full-coupled model instead of the original micro-kinetic or macro-dynamic model.We investigate different acceleration mechanisms in the vicinity of neutral points in magnetic islands evolution,from the stage of shrink and breakup into smaller islands(kinetic scale),to the stage of coalescence and growth into larger islands(dynamic scale),to the stages of constant and quasi-constant(contracting-expanding)islands(hydro scale).As a result,we give for the first time the acceleration efficiencies of different types of acceleration mechanisms in magnetic islands’interactions in solar atmosphere LTSTMR activities(pico-,10^–2–10^5m;nano-,10^5–10^6m;micro-,10^6–10^7m;macro-,10^7–10^8m;large-,10^8–10^9m).展开更多
Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify t...Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify the interaction of the riblet surfaces with the coherent, energy-bearing eddy structures in the near-wall region. Results provide statistical evidence that the averaged organization of the streamwise vortices in the drag-reducing case is lifted above the riblet tip, while in the drag-increasing case the streamwise vortices are embedded further into the riblet cove. In the spanwise direction, the cores of the streamwise vortices over the riblet surfaces are shown to be closer to each other than those for flow over the smooth wall, and wider riblet spacing leads to more reduction on their spanwise distances. In the cases with riblets the streamwise vortices have longer streamwise lengths, but their inclination angles do not change much.展开更多
Accurate wind and turbulence information are essential to wind energy research and utilization, among which wind shear and turbulence intensity/scale have seldom been investigated. In this paper, the observational dat...Accurate wind and turbulence information are essential to wind energy research and utilization, among which wind shear and turbulence intensity/scale have seldom been investigated. In this paper, the observational data from the100-m high wind towers in Xilinhot in Inner Mongolia(2009–10;grassland region), Huanghua in Hebei Province(2009–10;coastal flat region), and Xingzi County in Jiangxi Province(2010–11;mountain–lake region) are used to study the variations in near surface winds and turbulence characteristics related to the development of local wind energy over different underlying surfaces. The results indicate that(1) the percentage of the observed wind shear exponents exceeding 0.3 for the grassland region is 6%, while the percentage is 13% for the coastal flat region and 10%for the mountain–lake region. In other words, if the wind speed at 10 m is 10 ms–1, the percentage of the wind speed at 100 m exceeding 20 ms–1 for the grassland region is 6%, while the percentage is 13% for the coastal flat region and 10% for the mountain–lake region.(2) In terms of the turbulent intensity in the zonal, meridional, and vertical directions(Iu, Iv, and Iw, respectively), the frequencies of Iv/Iu < 0.8 in the grassland, coastal flat, and mountain–lake regions are 23%–29%, 32%–38%, and 30%–37%, respectively. Additionally, the frequencies of Iw/Iu < 0.5 in the grassland, coastal flat, and mountain–lake regions are 45%–75%, 52%–70%, and 43%–53%, respectively. The frequencies of Iv/Iu < 0.8 and Iw/Iu < 0.5 in each region mean that Iu is large and the air flow is unstable and fluctuating,which will damage the wind turbines. Therefore, these conditions do not meet the wind turbine design requirements,which must be considered separately.(3) At 50-and 70-m heights, the value of the turbulence scale parameter Λ in the grassland region is greater than that in the coastal flat region, and the latter is greater than that in the mountain–lake region. Therefore, under the same conditions, some paramet展开更多
It is a very difficult task to develop a method of reducing turbulent boundary layer drag.However,in recent years,plasma flow control technology has demonstrated huge potential in friction drag reduction.To further in...It is a very difficult task to develop a method of reducing turbulent boundary layer drag.However,in recent years,plasma flow control technology has demonstrated huge potential in friction drag reduction.To further investigate this issue,a smooth plate model was designed as a testing object arranged with a bidirectional dielectric-barrier-discharge(DBD)plasma actuator.In addition,measurement of skin friction drag was achieved by applying hot wire anemometry to obtain the velocity distribution of the turbulent boundary layer.A method of quantifying the friction drag effect was adopted based on the Spalding formula fitted with the experiment data.When plasma actuation was conducted,a velocity defect occurred at the two measuring positions,compared with the no plasma control condition;this means that the DBD plasma actuation could reduce the drag successfully in the downstream of the actuator.Moreover,drag reduction caused by backward actuation was slightly more efficient than that caused by forward actuation.With an increasing distance from plasma actuation,the drag-reduction effect could become weaker.Experimental results also show that the improvement of drag-reduction efficiency using a DBD plasma actuator can achieve about 8.78%in the local region of the experimental flat model.展开更多
A direct-forcing fictitious domain(DFFD)method is used to perform fully resolved numerical simulations of turbulent channel flows laden with large neutrally buoyant particles.The effects of the particles on the turbul...A direct-forcing fictitious domain(DFFD)method is used to perform fully resolved numerical simulations of turbulent channel flows laden with large neutrally buoyant particles.The effects of the particles on the turbulence(including the mean velocity,the root mean square(RMS)of the velocity fluctuation,the probability density function(PDF)of the velocity,and the vortex structures)at a friction Reynolds number of 395 are investigated.The results show that the drag-reduction effect caused by finite-size spherical particles at low particle volumes is negligibly small.The particle effects on the RMS velocities at Reτ=395 are significantly smaller than those at Reτ=180,despite qualitatively the same effects,i.e.,the presence of particles decreases the maximum streamwise RMS velocity near the wall via weakening the large-scale streamwise vortices,and increases the transverse and spanwise RMS velocities in the vicinity of the wall by inducing smaller-scale vortices.The effects of the particles on the PDFs of the fluid fluctuating velocities normalized with the RMS velocities are small,regardless of the particle size,the particle volume fraction,and the Reynolds number.展开更多
In recent years, the numbers of patents have been devoted to the development of rough pipes. The technique theoretical settlement determine of factor of hydraulic resistance for round pipes with rough walls is develop...In recent years, the numbers of patents have been devoted to the development of rough pipes. The technique theoretical settlement determine of factor of hydraulic resistance for round pipes with rough walls is developed on the basis of a principle of a superposition of complete viscosity in turbulent a layer mainly distinguished from the exist-ing theories. The received results of account for the extended range of determining parameters much distinguished from appropriate given for round pipes with turbulizers, specify a level и intensification of heat exchange.展开更多
The direct numerical simulation (DNS) is carried out for the incompressible viscous turbulent flows over an anisotropic porous wall. Effects of the anisotropic porous wall on turbulence modifications as well as on the...The direct numerical simulation (DNS) is carried out for the incompressible viscous turbulent flows over an anisotropic porous wall. Effects of the anisotropic porous wall on turbulence modifications as well as on the turbulent drag reduction are investigated. The simulation is carried out at a friction Reynolds number of 180, which is based on the averaged friction velocity at the interface between the porous medium and the clear fluid domain. The depth of the porous layer ranges from 0.9 to 54 viscous units. The permeability in the spanwise direction is set to be lower than the other directions in the present simulation. The maximum drag reduction obtained is about 15.3% which occurs for a depth of 9 viscous units. The increasing of drag is addressed when the depth of the porous layer is more than 25 wall units. The thinner porous layer restricts the spanwise extension of the streamwise vortices which suppresses the bursting events near the wall. However, for the thicker porous layer, the wall-normal fluctuations are enhanced due to the weakening of the wall-blocking effect which can trigger strong turbulent structures near the wall.展开更多
文摘A numerical simulation is performed to find out a key vortical structure in the laminar-turbulent transition. A low-speed streak is generated inside a laminar boundary layer using an isolated cuboid roughness, aimed at providing an environment unstable to outer disturbances. Then, a short duration jet is issued into the boundary layer. When the jet velocity is low, some vortices appear in the boundary layer, but the transition of the boundary layer does not take place. However, when the jet velocity exceeds a certain threshold, two vortices newly appear above the elongated legs of a V-shaped vortex and only one of them is stretched and survives. After that, vortices are generated one after another around the survived one. By comparing the decayed and the survived vortices, it is found that the difference in their heights is the key characteristic which leads to the transition.
基金the National Natural Science Foundation of China (91434205)the National Science Fund for Distinguished Young (21525627)+1 种基金the Natural Science Foundation of Zhejiang Province (LR14B060001)the Specialized Research Fund for the Doctoral Program of Higher Education of China (20130101110063).
文摘Accurate prediction of the onset of turbulent fluidization still remains elusive owing to the dependence of the transition velocity on several factors including measurement methods and interpretation of results. In this work, numerical simulations using the two fluid model (TFM) are performed in an attempt to predict the regime change reported by Gopalan etal.(2016) in a small scale pseudo-2D gas-solid fluidized bed containing Geldart D particles. Various time and frequency domain analyses were applied on predicted absolute and differential pressure time series data to reveal the bed dynamics. Numerical predictions of the transition velocity, Uc are in reasonably good agreement with experimental results from the small scale challenge problem. The literature correlations completely fail to predict the transition velocity for the system considered in this work. This work thus provides a different approach for validating the CFD model against experimental measurements.
基金the National Natural Science Foundation of China (91752205, 11702158 and 11490551)the Programme for Supercomputing(NN2649K).
文摘Axis-symmetric spheroids, such as rod-like and disk-like particles, have been found to orient preferentially in near-wall turbulence by both experiment and numerical simulation. In current work we examined the orientation of inertialess spheroids in a turbulent channel flow at medium friction Reynolds number Reτ=100 given based on the half of channel height. Both elongated prolate spheroid and flat oblate spheroid are considered and further compared with the reference case of spherical particle. The statistical results show that in near wall region the prolate spheroids tend to align in the streamwise direction while the oblate spheroids prefer to orient in the wallnormal direction, which are consistent with earlier observation in low Reynolds number (Reτ=180)wall turbulence. Around the channel center we found that the orientation of spheroids is not fully isotropic, even though the fluid vorticity are almost isotropic. The mechanism that gives rise to such particle orientations in wall-turbulence has been found to be related to fluid Lagrangian stretching and compression (Zhao and Andersson 2016). Therefore, we computed the left Cauchy-Green strain tensor along Lagrangian trajectories of tracer spheroids in current flow field and analyzed the fluid Lagrangian stretching and compression. The results indicated that, similar to the earlier observations, the directions of the Lagrangian stretching and compression in near-wall region are in the streamwise and wall-normal directions, respectively. Furthermore, cross over the channel the prolate spheroids aligned with the direction of Lagrangian stretching but oblate spheroids oriented with the direction of Lagrangian compression. The weak anisotropy of orientations of fluid Lagrangian stretching and compression observed at the channel center could be the reason for the aforementioned modest anisotropic orientation of spheroids in channel central region.
基金the Equipment Priority Research Field Foundation of China(No.6140246030216ZK01001).
文摘This paper attempts to develop a scaling procedure to measure structural vibration caused simultaneously by wall pressure fluctuations and the thermal load of hypersonic flow by a wind tunnel test. However, simulating the effect of thermal load is difficult with a scaled model in a wind tunnel due to the nonlinear effect of thermal load on a structure. In this work, the temperature variation of a structure is proposed to indicate the nonlinear effect of the thermal load,which provides a means to simulate both the thermal load and wall pressure fluctuations of a hypersonic Turbulent Boundary Layer(TBL) in a wind tunnel test. To validate the scaling procedure,both numerical computations and measurements are performed in this work. Theoretical results show that the scaling procedure can also be adapted to the buckling temperature of a structure even though the scaling procedure is derived from a reference temperature below the critical temperature of the structure. For the measurement, wall pressure fluctuations and thermal environment are simulated by creating hypersonic flow in a wind tunnel. Some encouraging results demonstrate the effectiveness of the scaling procedure for assessing structural vibration generated by hypersonic flow. The scaling procedure developed in this study will provide theoretical support to develop a new measurement technology to evaluate vibration of aircraft due to hypersonic flow.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 51490673, 51679035)This work was supported by the Pre-research field Fund Project of the Central Military Commission of China (Grant No. 61402070201), the Fundamental Research Funds for the Central Universities (Grant No. DUT18LK09, DUT2017TB05). The authors grate-fully acknowledge the Supercomputer Center of Dalian University of Technology for providing computing resources.
文摘The renormalization group (RNG) turbulent model is used to investigate the fluid resonance in a moonpool formed by two identical rectangular hulls under synchronous heaving excitation (e.g., a catamaran or dual pontoon). The numerical model is validated against the available experimental data, and accurate numerical solutions are obtained. The present study focuses on the amplitude of the moving hulls and the edge configuration of the moonpool entrance, as well as their influences on the piston-modal resonant wave in the moonpool. The dependence of the resonant wave amplitude in the moonpool on the heaving amplitude, the characteristic moonpool dimensions and the local velocity magnitude is derived based on a theoretical analysis, and the results are in good agreement with the RNG turbulent solutions. Five different edge profiles are considered, including two convex edges, two concave edges (both with various dimensions), and a sharp edge. Numerical examinations show that the edge configuration has a significant influence on the piston-modal resonant responses, a larger opening size leading to a higher resonant frequency and a larger resonant wave amplitude in the moonpool. Various flow patterns of the piston-modal resonance in the vicinity of the moonpool entrance are also identified, mainly depending on the edge profile. More intensive turbulent and vortical flows give rise to more significant dissipation, accounting for the smaller relative wave amplitude in the moonpool. With the increase of the heaving amplitude, the relative piston-modal resonant amplitude is decreased in an approximate power function. Within the scope of this work, the numerical investigations show that the piston-modal resonant frequency is hardly affected by the heaving amplitude.
基金the National Natural Science Foundation of China(No.11672135)the Foundation for the Author of National Excellent Doctoral Dissertation of China(No.201461).
文摘In this paper,the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with the direct numerical simulation(DNS)methods for different Reynolds numbers.A formulation is derived to express the relation between the drag and the Reynolds shear stress.With the application of optimal electromagnetic force,the in-depth relations among characteristic structures in the flow field,mean Reynolds shear stress,and the effect of drag reduction for different Reynolds numbers are discussed.The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers.The regular quasi-streamwise vortex structures,which appear in the flow field,have the same period with that of the electromagnetic force.These structures suppress the random velocity fluctuations,which leads to the absolute value of mean Reynolds shear stress decreasing and the distribution of that moving away from the wall.Moreover,the wave number of optimal electromagnetic force increases,and the scale of the regular quasi-streamwise vortex structures decreases as the Reynolds number increases.Therefore,the rate of drag reduction decreases with the increase in the Reynolds number since the scale of the regular quasi-streamwise vortex structures decreases.
基金the National Natural Science Foundation of China (Grant No.11672135)a Foundation for the Author of National Excellent Doctoral Dissertation of China (Grant No.201461).
文摘Based on the Fourier–Chebyshev spectral method,the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with direct numerical simulation (DNS) methods for different Reynolds numbers.A formula is derived to express the relation between fluctuating velocities and the friction drag coefficient.With the application of electromagnetic force,the in-depth relations among the fluctuating velocities near the wall,Reynolds stress,and the effect of drag reduction for different Reynolds numbers are discussed.The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers.The fluctuating velocities along the streamwise and normal directions are suppressed significantly,while the fluctuating velocity along the spanwise direction is enhanced dramatically due to the spanwise electromagnetic force.However,the values of Reynolds stress depend on the fluctuating velocities along the streamwise and normal directions rather than that along the spanwise direction.Therefore,the significant effect of drag reduction is obtained.Moreover,the maximum drag reduction is weakened due to the decay of control effect for fluctuating velocities as the Reynolds number increases.
文摘Flow transition from laminar to turbulent mode (and vice versa)—that is, the initiation of turbulence—is one of the most important research subjects in the history of engineering. Even for pipe flow, predicting the onset of turbulence requires sophisticated instrumentation and/or direct numerical simulation, based on observing the instantaneous flow structure formation and evolution. In this work, a local Reynolds number equivalence c (ratio of local inertia effect to viscous effect) is seen to conform to the Universal Law of the Wall, where c = 1 represents a quantitative balance between the abovementioned two effects. This coincides with the wall layer thickness (y+= 1, where y+ is the dimensionless distance from the wall surface defined in the Universal Law of the Wall). It is found that the characteristic of how the local derivative of c against the local velocity changes with increasing velocity determines the onset of turbulence. For pipe flow, c - 25, and for plate flow, c - 151.5. These findings suggest that a certain combination of c and velocity (nonlinearity) can qualify the source of turbulence (i.e., generate turbulent energy). Similarly, a re-evaluation of the previous findings reveals that only the geometrically narrow domain can act locally as the source of turbulence, with the rest of the flow field largely being left for transporting and dissipating. This understanding will have an impact on the future large-scale modeling of turbulence.
基金the Strategic Priority Research Program of the Chinese Academy of Sciences (No.XDA 21060102)Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development of China (No. y809jh1001).
文摘An OpenFOAM based turbulence combustion solver with flamelet generated manifolds (FGMs) is presented in this paper. A series of flamelets, representative for turbulent flames, are calculated first by a one-dimensional (1D) detailed chemistry solver with the consideration of both transport and stretch/curvature contributions. The flame structure is then parameterized as a function of multiple reaction control variables. A manifold, which collects the 1D flame properties, is built from the 1D flame solutions. The control variables of the mixture fraction and the progress variable are solved from the corresponding transport equations. During the calculation, the scalar variables, e.g., temperature and species concentration, are retrieved from the manifolds by interpolation. A transport equation for NO is solved to improve its prediction accuracy. To verify the ability to deal with the enthalpy loss effect, the temperature retrieved directly from the manifolds is compared with the temperature solved from a transport equation of absolute enthalpy. The resulting FGM-computational fluid dynamics (CFD) coupled code has three significant features, i.e., accurate NO prediction, the ability to treat the heat loss effect and the adoption at the turbulence level, and high quality prediction within practical industrial configurations. The proposed method is validated against the Sandia flame D, and good agreement with the experimental data is obtained.
基金supported by the strategic priority research program of CAS (XDA17040507, XDA15010900)the national basic research program of China (2013CBA01503)+5 种基金the key program of NSFC (11333007)joint funds of NSFC(U1631130)frontier science key programs of CAS (QYZDJ-SSWSLH012)the program for innovation team of Yunnan Provincethe program for Guangdong introducing Innovative and entrepreneurial teams (2016ZT06D211)the special program for applied research on super computation of the NSFC-Guangdong joint fund (second phase) under No.U1501501
文摘A new combined Fermi,betatron,and turbulent electron acceleration mechanism is proposed in interaction of magnetic islands during turbulent magnetic reconnection evolution in explosive astrophysical phenomena at large temporal-spatial scale(LTSTMR),the ratio of observed current sheets thickness to electron characteristic length,electron Larmor radius for low-βand electron inertial length for high-β,is on the order of 10^10–10^11;the ratio of observed evolution time to electron gyroperiod is on the order of 10^7–10^9).The original combined acceleration model is known to be one of greatest importance in the interaction of magnetic islands;it assumes that the continuous kinetic-dynamic temporal-spatial scale evolution occurs as two separate independent processes.In this paper,we reconsider the combined acceleration mechanism by introducing a kinetic-dynamic-hydro full-coupled model instead of the original micro-kinetic or macro-dynamic model.We investigate different acceleration mechanisms in the vicinity of neutral points in magnetic islands evolution,from the stage of shrink and breakup into smaller islands(kinetic scale),to the stage of coalescence and growth into larger islands(dynamic scale),to the stages of constant and quasi-constant(contracting-expanding)islands(hydro scale).As a result,we give for the first time the acceleration efficiencies of different types of acceleration mechanisms in magnetic islands’interactions in solar atmosphere LTSTMR activities(pico-,10^–2–10^5m;nano-,10^5–10^6m;micro-,10^6–10^7m;macro-,10^7–10^8m;large-,10^8–10^9m).
基金the National Basic Research Program of China (973 program)(Grants 2014CB744802 and 2014CB744804)the National Natural Science Foundation of China (Grants 11772194 and 91441205).
文摘Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify the interaction of the riblet surfaces with the coherent, energy-bearing eddy structures in the near-wall region. Results provide statistical evidence that the averaged organization of the streamwise vortices in the drag-reducing case is lifted above the riblet tip, while in the drag-increasing case the streamwise vortices are embedded further into the riblet cove. In the spanwise direction, the cores of the streamwise vortices over the riblet surfaces are shown to be closer to each other than those for flow over the smooth wall, and wider riblet spacing leads to more reduction on their spanwise distances. In the cases with riblets the streamwise vortices have longer streamwise lengths, but their inclination angles do not change much.
基金Supported by the China Meteorological Administration Special Public Welfare Research Fund (GYHY20120626).
文摘Accurate wind and turbulence information are essential to wind energy research and utilization, among which wind shear and turbulence intensity/scale have seldom been investigated. In this paper, the observational data from the100-m high wind towers in Xilinhot in Inner Mongolia(2009–10;grassland region), Huanghua in Hebei Province(2009–10;coastal flat region), and Xingzi County in Jiangxi Province(2010–11;mountain–lake region) are used to study the variations in near surface winds and turbulence characteristics related to the development of local wind energy over different underlying surfaces. The results indicate that(1) the percentage of the observed wind shear exponents exceeding 0.3 for the grassland region is 6%, while the percentage is 13% for the coastal flat region and 10%for the mountain–lake region. In other words, if the wind speed at 10 m is 10 ms–1, the percentage of the wind speed at 100 m exceeding 20 ms–1 for the grassland region is 6%, while the percentage is 13% for the coastal flat region and 10% for the mountain–lake region.(2) In terms of the turbulent intensity in the zonal, meridional, and vertical directions(Iu, Iv, and Iw, respectively), the frequencies of Iv/Iu < 0.8 in the grassland, coastal flat, and mountain–lake regions are 23%–29%, 32%–38%, and 30%–37%, respectively. Additionally, the frequencies of Iw/Iu < 0.5 in the grassland, coastal flat, and mountain–lake regions are 45%–75%, 52%–70%, and 43%–53%, respectively. The frequencies of Iv/Iu < 0.8 and Iw/Iu < 0.5 in each region mean that Iu is large and the air flow is unstable and fluctuating,which will damage the wind turbines. Therefore, these conditions do not meet the wind turbine design requirements,which must be considered separately.(3) At 50-and 70-m heights, the value of the turbulence scale parameter Λ in the grassland region is greater than that in the coastal flat region, and the latter is greater than that in the mountain–lake region. Therefore, under the same conditions, some paramet
文摘It is a very difficult task to develop a method of reducing turbulent boundary layer drag.However,in recent years,plasma flow control technology has demonstrated huge potential in friction drag reduction.To further investigate this issue,a smooth plate model was designed as a testing object arranged with a bidirectional dielectric-barrier-discharge(DBD)plasma actuator.In addition,measurement of skin friction drag was achieved by applying hot wire anemometry to obtain the velocity distribution of the turbulent boundary layer.A method of quantifying the friction drag effect was adopted based on the Spalding formula fitted with the experiment data.When plasma actuation was conducted,a velocity defect occurred at the two measuring positions,compared with the no plasma control condition;this means that the DBD plasma actuation could reduce the drag successfully in the downstream of the actuator.Moreover,drag reduction caused by backward actuation was slightly more efficient than that caused by forward actuation.With an increasing distance from plasma actuation,the drag-reduction effect could become weaker.Experimental results also show that the improvement of drag-reduction efficiency using a DBD plasma actuator can achieve about 8.78%in the local region of the experimental flat model.
基金the National Natural Science Foundation of China(Nos.91752117,11632016,and 11372275)the Natural Science Foundation of Zhejiang Province of China(No.LY17A020005).
文摘A direct-forcing fictitious domain(DFFD)method is used to perform fully resolved numerical simulations of turbulent channel flows laden with large neutrally buoyant particles.The effects of the particles on the turbulence(including the mean velocity,the root mean square(RMS)of the velocity fluctuation,the probability density function(PDF)of the velocity,and the vortex structures)at a friction Reynolds number of 395 are investigated.The results show that the drag-reduction effect caused by finite-size spherical particles at low particle volumes is negligibly small.The particle effects on the RMS velocities at Reτ=395 are significantly smaller than those at Reτ=180,despite qualitatively the same effects,i.e.,the presence of particles decreases the maximum streamwise RMS velocity near the wall via weakening the large-scale streamwise vortices,and increases the transverse and spanwise RMS velocities in the vicinity of the wall by inducing smaller-scale vortices.The effects of the particles on the PDFs of the fluid fluctuating velocities normalized with the RMS velocities are small,regardless of the particle size,the particle volume fraction,and the Reynolds number.
文摘In recent years, the numbers of patents have been devoted to the development of rough pipes. The technique theoretical settlement determine of factor of hydraulic resistance for round pipes with rough walls is developed on the basis of a principle of a superposition of complete viscosity in turbulent a layer mainly distinguished from the exist-ing theories. The received results of account for the extended range of determining parameters much distinguished from appropriate given for round pipes with turbulizers, specify a level и intensification of heat exchange.
基金the National Natural Science Foundation of China (Nos. 11572183, 91852111, and 11825204)the Program of Shanghai Municipal Education Commission (No. 2019-01-07-00- 09-E00018).
文摘The direct numerical simulation (DNS) is carried out for the incompressible viscous turbulent flows over an anisotropic porous wall. Effects of the anisotropic porous wall on turbulence modifications as well as on the turbulent drag reduction are investigated. The simulation is carried out at a friction Reynolds number of 180, which is based on the averaged friction velocity at the interface between the porous medium and the clear fluid domain. The depth of the porous layer ranges from 0.9 to 54 viscous units. The permeability in the spanwise direction is set to be lower than the other directions in the present simulation. The maximum drag reduction obtained is about 15.3% which occurs for a depth of 9 viscous units. The increasing of drag is addressed when the depth of the porous layer is more than 25 wall units. The thinner porous layer restricts the spanwise extension of the streamwise vortices which suppresses the bursting events near the wall. However, for the thicker porous layer, the wall-normal fluctuations are enhanced due to the weakening of the wall-blocking effect which can trigger strong turbulent structures near the wall.