The turbulence enhancement by particle wake effect is studied by large eddy simulation （LES） of turbulent gas flows passing a single particle. The predicted time-averaged and root-mean-square fluctuation velocities ...The turbulence enhancement by particle wake effect is studied by large eddy simulation （LES） of turbulent gas flows passing a single particle. The predicted time-averaged and root-mean-square fluctuation velocities behind the particle are in agreement with the Reynolds-averaged Navier-Stokes modeling results and experimental results. A semi-empirical turbulence enhancement model is proposed by the present-authors based on the LES resuits. This model is incorporated into the second-order moment two-phase turbulence model for simulating vertical gas-particle pipe flows and horizontal gas-particle channel flows. The simulation results show that compared with the model not accounting for the particle wake effect, the present model gives simulation results for the gas turbulence modulation in much better agreement with the experimental results.展开更多
he paper focuses on the turbulence modulation problem in gas–particle flow with the use of probability density function（PDF） approach. By means of the PDF method, a general statistical moment turbulence modulation ...he paper focuses on the turbulence modulation problem in gas–particle flow with the use of probability density function（PDF） approach. By means of the PDF method, a general statistical moment turbulence modulation model without considering the trajectory difference between two phases is derived from the Navier–Stokes equations. A new turbulence production term induced by the dispersed-phase is analyzed and considered. Furthermore, the trajectory difference between two media is taken into account. Subsequently, a new k–ε turbulence modulation model in dilute particle-laden flow is successfully set up. Then, the changes to several terms, including the turbulence production, dissipation, and diffusion terms, are well described consequently. The promoted model provides a more probable explanation for the modification of particles on the turbulence. Finally, we applied the model to simulate a gas–particle turbulence flow case in a wall jet, and found that the simulation results agree well with the experimental data.展开更多
A semi-empirical turbulence enhancement model accounting for the particle-wake effect was incorporated into the second-order moment two-phase turbulence model and employed to simulate gas-particle flows in a swirling ...A semi-empirical turbulence enhancement model accounting for the particle-wake effect was incorporated into the second-order moment two-phase turbulence model and employed to simulate gas-particle flows in a swirling sudden-expansion chamber. The simulated results for two-phases mean velocities and fluctuation velocities coincide well with the experiment ones, which demonstrates that this model, in comparison with the turbulence model not accounting for the wake effect, leads to higher calculating accuracy.展开更多
A study is presented to evaluate the capabilities of the standard k–ε turbulence model and the k–ε turbulence model with added source terms in predicting the experimentally measured turbulence modulation due to th...A study is presented to evaluate the capabilities of the standard k–ε turbulence model and the k–ε turbulence model with added source terms in predicting the experimentally measured turbulence modulation due to the presence of particles in horizontal pneumatic conveying, in the context of a CFD–DEM Eulerian–Lagrangian simulation. Experiments were performed using a 6.5-m long, 0.075-m diameter horizontal pipe in conjunction with a laser Doppler anemometry （LDA） system. Spherical glass beads with two sizes, 1.5 and 2 mm, were used. Simulations were performed using the commercial discrete element method software EDEM, coupled with the computational fluid dynamics package FLUENT. Hybrid source terms were added to the conventional k–ε turbulence model to take into account the influence of the dispersed phase on the carrier phase turbulence intensity. The simulation results showed that the turbulence modulation depends strongly on the model parameter Cε3. Both the standard k–ε turbulence model and the k–ε turbulence model with the hybrid source terms could predict the gas phase turbulence intensity trend only generally. A noticeable discrepancy in all cases between simulation and experimental results was observed, particularly for the regions close to the pipe wall. It was also observed that in some cases the addition of the source terms to the k–ε turbulence model did not improve the simulation results when compared with those of the standard k–ε turbulence model. Nonetheless, in the lower part of the pipe where particle loading was greater due to gravitational effects, the model with added source terms performed somewhat better.展开更多
Turbulence enhancement by particle wake effect is studied by numerical simulation of gas turbulent flows passing over particle under various particle sizes, inlet gas velocities, gas viscosity, gas density and the dis...Turbulence enhancement by particle wake effect is studied by numerical simulation of gas turbulent flows passing over particle under various particle sizes, inlet gas velocities, gas viscosity, gas density and the distance of particles. By performing dimension analysis and using the form of gas-particle interaction source term for reference, a new semi-empirical turbulence enhancement model by the particle-wake effect is proposed. The turbulence model is then incorporated into second-order moment model for simulating gas-particle flows in a horizontal channel with different wall roughness and a sudden-expansion chamber. The results show that this model is with higher calculating accuracy than another two turbulence models in comparison with the experimental results.展开更多
Interaction between turbulence and particles is investigated in a channel flow. The fluid motion is calculated using direct numerical simulation(DNS) with a lattice Boltzmann(LB) method, and particles are tracked in a...Interaction between turbulence and particles is investigated in a channel flow. The fluid motion is calculated using direct numerical simulation(DNS) with a lattice Boltzmann(LB) method, and particles are tracked in a Lagrangian framework through the action of force imposed by the fluid. The particle diameter is smaller than the Kolmogorov length scale, and the point force is used to represent the feedback force of particles on the turbulence. The effects of particles on the turbulence and skin friction coefficient are examined with different particle inertias and mass loadings. Inertial particles suppress intensities of the spanwise and wall-normal components of velocity, and the Reynolds shear stress. It is also found that, relative to the reference particle-free flow,the overall mean skin-friction coefficient is reduced by particles. Changes of near wall turbulent structures such as longer and more regular streamwise low-speed streaks and less ejections and sweeps are the manifestation of drag reduction.展开更多
The presence of solid particles or water droplets in continuous fluid flow can either induce turbulence attenuation or amplification. The modification of the state of the turbulence depends on the characteristics of t...The presence of solid particles or water droplets in continuous fluid flow can either induce turbulence attenuation or amplification. The modification of the state of the turbulence depends on the characteristics of the particles, such as volume fraction, mean diameter, mass density, or carrier phase flow properties. In this brief review, the main physical concepts related to the most important physical aspects of turbulence modulation are summarized. Different criteria used to distinguish the enhancement or the attenuation effects of the particles on the carrier phase flows are recalled. For the interest of large-scale industrial applications, several theoretical,experimental and empirical approaches are discussed, which provides an interesting framework for the study of the effect of particles on turbulence behavior modification.展开更多
A review of the main mechanisms influencing turbulent modulation in the presence of spherical and non-spherical particles is presented. The review demonstrates the need for more numerical and experimental work with hi...A review of the main mechanisms influencing turbulent modulation in the presence of spherical and non-spherical particles is presented. The review demonstrates the need for more numerical and experimental work with higher accuracy than obtained so far and the need to resolve the flow near the surface of particles with the aim to re-evaluate the quantitative effect of different parameters on turbulent modulation. The review reveals that non-spherical particles have more adverse effect on turbulence as compared to spherical ones, for the same ambient conditions.展开更多
Based on the space diversity reception,the binary phase-shift keying(BPSK) modulated free space optical(FSO) system over Málaga(M) fading channels is investigated in detail.Under independently and identically dis...Based on the space diversity reception,the binary phase-shift keying(BPSK) modulated free space optical(FSO) system over Málaga(M) fading channels is investigated in detail.Under independently and identically distributed and independently and non-identically distributed dual branches,the analytical average bit error rate(ABER) expressions in terms of H-Fox function for maximal ratio combining(MRC) and equal gain combining(EGC) diversity techniques are derived,respectively,by transforming the modified Bessel function of the second kind into the integral form of Meijer Gfunction.Monte Carlo(MC) simulation is also provided to verify the accuracy of the presented models.展开更多
A hybrid polarization division multiplexed-differential phase shift keying-multipulse pulse position modulation(PDM-DPSK-MPPM) scheme for multi-hop free-space optical(FSO) communication is investigated. The analytical...A hybrid polarization division multiplexed-differential phase shift keying-multipulse pulse position modulation(PDM-DPSK-MPPM) scheme for multi-hop free-space optical(FSO) communication is investigated. The analytical bit error rate(BER) of the proposed system in Gamma-Gamma turbulence channels is derived and verified using computer simulation. The results show that both multi-hop and hybrid modulation schemes are efficient techniques to improve the performance of FSO links. Compared with the traditional binary phase shift keying(BPSK) and MPPM, the hybrid scheme can improve the bandwidth-utilization efficiency and reliability of the system. Compared with the coherent demodulation of PDM-QPSK-MPPM, the system complexity is reduced at the cost of the degradation of BER performance, which can improve the practicality of hybrid modulation technology in FSO system.展开更多
Analytical formulas for a class of tunable random electromagnetic beams propagating in a turbulent atmosphere through a complex optical system are derived with the help of a tensor method. One finds that the far field...Analytical formulas for a class of tunable random electromagnetic beams propagating in a turbulent atmosphere through a complex optical system are derived with the help of a tensor method. One finds that the far field intensity distribution is tunable by modulating the source correlation structure function. The on-axis spectral degree of polarization monotonically increases to the same value for different values of order M in free space while it returns to the initial value after propagating a sufficient distance in turbulence. Furthermore, it is revealed that the state of polarization is closely determined by the initial correlation structure rather than by the turbulence parameters.展开更多
We perform a series of experiments to study the velocity fluctuations generated by the flow past fixed spheres.Planar particle image velocimetry(PIV)is carried out to characterize the properties of the liquid fluctuat...We perform a series of experiments to study the velocity fluctuations generated by the flow past fixed spheres.Planar particle image velocimetry(PIV)is carried out to characterize the properties of the liquid fluctuation.The fluctuation induced by the spheres can be decomposed into the temporal fluctuation and the spatial fluctuation,which represent the contribution of flow instability and spatial inhomogeneity,respectively.In particularly,we focus on the contributions of temporal fluctuations and spatial fluctuations between low and high Reynolds number.At low Reynolds number(20<Re≤200),the total fluctuation mostly comes from the spatial fluctuation and increases as the area of velocity deficit in the wake of the sphere increases.The temporal fluctuation cannot be neglected at larger Reynolds number(200<Re≤700),and the total velocity fluctuation is induced by both flow instability and spatial inhomogeneity.Furthermore,the energy distribution in the flow direction and the span direction also changes drastically as the Reynolds number increases.The wavenumber spectra of fluctuations show a−3 slope at large scale,−5/3 slope at small scale.The starting scale of the−3 slope decreases with the increase of the Reynolds number.Specifically,it is consistent with the integral length scaleΛfor Re>200,but not for Re<200.The ending scale is aroundλ=0.5d and decreases slightly as Re increases.展开更多
基金Supported by the Major Project of National Natural Science Foundation of China (No.10632070) the Postdoctoral ScienceFoundation (No.2004036239).
文摘The turbulence enhancement by particle wake effect is studied by large eddy simulation （LES） of turbulent gas flows passing a single particle. The predicted time-averaged and root-mean-square fluctuation velocities behind the particle are in agreement with the Reynolds-averaged Navier-Stokes modeling results and experimental results. A semi-empirical turbulence enhancement model is proposed by the present-authors based on the LES resuits. This model is incorporated into the second-order moment two-phase turbulence model for simulating vertical gas-particle pipe flows and horizontal gas-particle channel flows. The simulation results show that compared with the model not accounting for the particle wake effect, the present model gives simulation results for the gas turbulence modulation in much better agreement with the experimental results.
基金Project supported by the National Natural Science Foundation of China(Grant No.51176044)
文摘he paper focuses on the turbulence modulation problem in gas–particle flow with the use of probability density function（PDF） approach. By means of the PDF method, a general statistical moment turbulence modulation model without considering the trajectory difference between two phases is derived from the Navier–Stokes equations. A new turbulence production term induced by the dispersed-phase is analyzed and considered. Furthermore, the trajectory difference between two media is taken into account. Subsequently, a new k–ε turbulence modulation model in dilute particle-laden flow is successfully set up. Then, the changes to several terms, including the turbulence production, dissipation, and diffusion terms, are well described consequently. The promoted model provides a more probable explanation for the modification of particles on the turbulence. Finally, we applied the model to simulate a gas–particle turbulence flow case in a wall jet, and found that the simulation results agree well with the experimental data.
基金the China Postdoctoral Science Foundation (Grant No.2004036239).
文摘A semi-empirical turbulence enhancement model accounting for the particle-wake effect was incorporated into the second-order moment two-phase turbulence model and employed to simulate gas-particle flows in a swirling sudden-expansion chamber. The simulated results for two-phases mean velocities and fluctuation velocities coincide well with the experiment ones, which demonstrates that this model, in comparison with the turbulence model not accounting for the wake effect, leads to higher calculating accuracy.
文摘A study is presented to evaluate the capabilities of the standard k–ε turbulence model and the k–ε turbulence model with added source terms in predicting the experimentally measured turbulence modulation due to the presence of particles in horizontal pneumatic conveying, in the context of a CFD–DEM Eulerian–Lagrangian simulation. Experiments were performed using a 6.5-m long, 0.075-m diameter horizontal pipe in conjunction with a laser Doppler anemometry （LDA） system. Spherical glass beads with two sizes, 1.5 and 2 mm, were used. Simulations were performed using the commercial discrete element method software EDEM, coupled with the computational fluid dynamics package FLUENT. Hybrid source terms were added to the conventional k–ε turbulence model to take into account the influence of the dispersed phase on the carrier phase turbulence intensity. The simulation results showed that the turbulence modulation depends strongly on the model parameter Cε3. Both the standard k–ε turbulence model and the k–ε turbulence model with the hybrid source terms could predict the gas phase turbulence intensity trend only generally. A noticeable discrepancy in all cases between simulation and experimental results was observed, particularly for the regions close to the pipe wall. It was also observed that in some cases the addition of the source terms to the k–ε turbulence model did not improve the simulation results when compared with those of the standard k–ε turbulence model. Nonetheless, in the lower part of the pipe where particle loading was greater due to gravitational effects, the model with added source terms performed somewhat better.
基金the National Natural Science Foundation of China(No.50736006)the Aero-Science Fund(No.2009ZB56004)the Jiangxi Provincial Natural Science Foundation(Nos.2009GZC0100 and 2008GZW0016)
文摘Turbulence enhancement by particle wake effect is studied by numerical simulation of gas turbulent flows passing over particle under various particle sizes, inlet gas velocities, gas viscosity, gas density and the distance of particles. By performing dimension analysis and using the form of gas-particle interaction source term for reference, a new semi-empirical turbulence enhancement model by the particle-wake effect is proposed. The turbulence model is then incorporated into second-order moment model for simulating gas-particle flows in a horizontal channel with different wall roughness and a sudden-expansion chamber. The results show that this model is with higher calculating accuracy than another two turbulence models in comparison with the experimental results.
基金Project supported by the National Natural Science Foundation of China(Nos.11572183 and 11272198)
文摘Interaction between turbulence and particles is investigated in a channel flow. The fluid motion is calculated using direct numerical simulation(DNS) with a lattice Boltzmann(LB) method, and particles are tracked in a Lagrangian framework through the action of force imposed by the fluid. The particle diameter is smaller than the Kolmogorov length scale, and the point force is used to represent the feedback force of particles on the turbulence. The effects of particles on the turbulence and skin friction coefficient are examined with different particle inertias and mass loadings. Inertial particles suppress intensities of the spanwise and wall-normal components of velocity, and the Reynolds shear stress. It is also found that, relative to the reference particle-free flow,the overall mean skin-friction coefficient is reduced by particles. Changes of near wall turbulent structures such as longer and more regular streamwise low-speed streaks and less ejections and sweeps are the manifestation of drag reduction.
基金the financial support from Electricitéde France(EDF)within the framework of the Generation II&III nuclear reactor research program。
文摘The presence of solid particles or water droplets in continuous fluid flow can either induce turbulence attenuation or amplification. The modification of the state of the turbulence depends on the characteristics of the particles, such as volume fraction, mean diameter, mass density, or carrier phase flow properties. In this brief review, the main physical concepts related to the most important physical aspects of turbulence modulation are summarized. Different criteria used to distinguish the enhancement or the attenuation effects of the particles on the carrier phase flows are recalled. For the interest of large-scale industrial applications, several theoretical,experimental and empirical approaches are discussed, which provides an interesting framework for the study of the effect of particles on turbulence behavior modification.
文摘A review of the main mechanisms influencing turbulent modulation in the presence of spherical and non-spherical particles is presented. The review demonstrates the need for more numerical and experimental work with higher accuracy than obtained so far and the need to resolve the flow near the surface of particles with the aim to re-evaluate the quantitative effect of different parameters on turbulent modulation. The review reveals that non-spherical particles have more adverse effect on turbulence as compared to spherical ones, for the same ambient conditions.
基金supported by the National Natural Science Foundation of China(No.61474090)the Nature Science Basic Research Plan in Shaanxi Province of China(No.2014JM8340)the China Postdoctoral Science Special Foundation(No.201104659)
文摘Based on the space diversity reception,the binary phase-shift keying(BPSK) modulated free space optical(FSO) system over Málaga(M) fading channels is investigated in detail.Under independently and identically distributed and independently and non-identically distributed dual branches,the analytical average bit error rate(ABER) expressions in terms of H-Fox function for maximal ratio combining(MRC) and equal gain combining(EGC) diversity techniques are derived,respectively,by transforming the modified Bessel function of the second kind into the integral form of Meijer Gfunction.Monte Carlo(MC) simulation is also provided to verify the accuracy of the presented models.
基金supported by the National Natural Science Foundation of China(No.61373124)
文摘A hybrid polarization division multiplexed-differential phase shift keying-multipulse pulse position modulation(PDM-DPSK-MPPM) scheme for multi-hop free-space optical(FSO) communication is investigated. The analytical bit error rate(BER) of the proposed system in Gamma-Gamma turbulence channels is derived and verified using computer simulation. The results show that both multi-hop and hybrid modulation schemes are efficient techniques to improve the performance of FSO links. Compared with the traditional binary phase shift keying(BPSK) and MPPM, the hybrid scheme can improve the bandwidth-utilization efficiency and reliability of the system. Compared with the coherent demodulation of PDM-QPSK-MPPM, the system complexity is reduced at the cost of the degradation of BER performance, which can improve the practicality of hybrid modulation technology in FSO system.
基金supported by the National Natural Science Foundation of China (No. 11504172)the Natural Science Foundation of Jiangsu Province (No. BK20150763)the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions
文摘Analytical formulas for a class of tunable random electromagnetic beams propagating in a turbulent atmosphere through a complex optical system are derived with the help of a tensor method. One finds that the far field intensity distribution is tunable by modulating the source correlation structure function. The on-axis spectral degree of polarization monotonically increases to the same value for different values of order M in free space while it returns to the initial value after propagating a sufficient distance in turbulence. Furthermore, it is revealed that the state of polarization is closely determined by the initial correlation structure rather than by the turbulence parameters.
基金supported by the State Key Program of National Natural Science of China(Grant No.91852204)the National Natural Science Foundation of China(Grant No.11772298).
文摘We perform a series of experiments to study the velocity fluctuations generated by the flow past fixed spheres.Planar particle image velocimetry(PIV)is carried out to characterize the properties of the liquid fluctuation.The fluctuation induced by the spheres can be decomposed into the temporal fluctuation and the spatial fluctuation,which represent the contribution of flow instability and spatial inhomogeneity,respectively.In particularly,we focus on the contributions of temporal fluctuations and spatial fluctuations between low and high Reynolds number.At low Reynolds number(20<Re≤200),the total fluctuation mostly comes from the spatial fluctuation and increases as the area of velocity deficit in the wake of the sphere increases.The temporal fluctuation cannot be neglected at larger Reynolds number(200<Re≤700),and the total velocity fluctuation is induced by both flow instability and spatial inhomogeneity.Furthermore,the energy distribution in the flow direction and the span direction also changes drastically as the Reynolds number increases.The wavenumber spectra of fluctuations show a−3 slope at large scale,−5/3 slope at small scale.The starting scale of the−3 slope decreases with the increase of the Reynolds number.Specifically,it is consistent with the integral length scaleΛfor Re>200,but not for Re<200.The ending scale is aroundλ=0.5d and decreases slightly as Re increases.
