Nowadays, Autonomous Underwater Vehicles(AUVs) are frequently used for exploring the oceans. The hydrodynamics of AUVs moving in the vicinity of the water surface are significantly different at higher depths. In this ...Nowadays, Autonomous Underwater Vehicles(AUVs) are frequently used for exploring the oceans. The hydrodynamics of AUVs moving in the vicinity of the water surface are significantly different at higher depths. In this paper, the hydrodynamic coefficients of an AUV in non-dimensional depths of 0.75, 1, 1.5, 2, and 4D are obtained for movement close to the free-surface. Reynolds Averaged Navier Stokes Equations(RANS) are discretized using the finite volume approach and the water-surface effects modeled using the Volume of Fraction(VOF) method. As the operating speeds of AUVs are usually low, the boundary layer over them is not fully laminar or fully turbulent, so the effect of boundary layer transition from laminar to turbulent flow was considered in the simulations. Two different turbulence/transition models were used: 1) a full-turbulence model, the k-ε model, and 2) a turbulence/transition model, Menter's Transition-SST model. The results show that the Menter's Transition-SST model has a better consistency with experimental results. In addition, the wave-making effects of these bodies are studied at different immersion depths in the sea-surface vicinity or at finite depths. It is observed that the relevant pitch moments and lift coefficients are non-zero for these axi-symmetric bodies when they move close to the sea-surface. This is not expected for greater depths.展开更多
Since 1970s,several experimental works revealed that the cavitation sheet inception does not occur at the minimum pressure location but further downstream at the location of a laminar/turbulent transition.Most of the ...Since 1970s,several experimental works revealed that the cavitation sheet inception does not occur at the minimum pressure location but further downstream at the location of a laminar/turbulent transition.Most of the cavitation models use the saturation vapour pressure as a threshold to initiate the production of vapour and therefore,are not able to capture such flows.In this paper,three modifications of the Schnerr and Sauer cavitation model are proposed and coupled with an algebraic laminar/turbulent transition model.Application to a NACA 16012 profile shows the ability of the modifications to move the cavitation inception at the right location compared with the experiment.One of them,based on the multiplication of the evaporation term by the square of the turbulent intensity seems promising.展开更多
In this work,we studied the bluntness effect on the hypersonic boundary-layer transition over a slender cone at Mach 6 with interchangeable tips in a noisy Ludwieg tube tunnel before the so-called“transition reversal...In this work,we studied the bluntness effect on the hypersonic boundary-layer transition over a slender cone at Mach 6 with interchangeable tips in a noisy Ludwieg tube tunnel before the so-called“transition reversal”phenomenon occurs.The evolution of instability waves is characterized using surface flush-mounted pressure sensors deployed along the streamwise direction within unit Reynolds number from 4E+6/m≤Reunit≤10E+6/m,and the bluntness of the cone nose ranges from 0.1 mm to 5 mm.Power spectral density(PSD)of pressure fluctuation indicates that small nose bluntness(ReR≤2000)has little influence on the evolution of instability waves along the hypersonic boundary-layer,whereas with a moderate nose size(2000≤ReR≤5000),the hypersonic boundary layer transition is delayed monotonically as the nose radius increases before the boundary-layer turns into fully laminar without instability waves.The delaying effect can be attributed to the increased entropy-layer swallowing distance with a large tip radius.Instability wave characterization reveals that the second mode instability wave plays a dominant role before the transition reversal happens.The quadratic phase locking of second mode instabilities can be identified by bispectral analysis,and it attenuates as the nose tip radius increases.展开更多
To investigate the convective heat transfer of nanofluids, experiments were performed using silver-water nanofluids under laminar, transition and turbulent flow regimes in a horizontal 4.3 mm inner-diameter tube-in-tu...To investigate the convective heat transfer of nanofluids, experiments were performed using silver-water nanofluids under laminar, transition and turbulent flow regimes in a horizontal 4.3 mm inner-diameter tube-in-tube counter-current heat transfer test section. The volume concentration of the nanoparticles varied from 0.3% to 0.9% in steps of 0.3%, and the effects of thermo-physical properties, inlet temperature, volume concentration, and mass flow rate on heat transfer coefficient were investigated. Experiments showed that the suspended nanoparticles remarkably increased the convective heat transfer coefficient, by as much as 28.7% and 69.3% for 0.3% and 0.9% of silver content, respectively. Based on the experimental results a correlation was developed to predict the Nusselt number of the silver-water nanofluid, with ＋10% agreement between experiments and prediction.展开更多
文摘Nowadays, Autonomous Underwater Vehicles(AUVs) are frequently used for exploring the oceans. The hydrodynamics of AUVs moving in the vicinity of the water surface are significantly different at higher depths. In this paper, the hydrodynamic coefficients of an AUV in non-dimensional depths of 0.75, 1, 1.5, 2, and 4D are obtained for movement close to the free-surface. Reynolds Averaged Navier Stokes Equations(RANS) are discretized using the finite volume approach and the water-surface effects modeled using the Volume of Fraction(VOF) method. As the operating speeds of AUVs are usually low, the boundary layer over them is not fully laminar or fully turbulent, so the effect of boundary layer transition from laminar to turbulent flow was considered in the simulations. Two different turbulence/transition models were used: 1) a full-turbulence model, the k-ε model, and 2) a turbulence/transition model, Menter's Transition-SST model. The results show that the Menter's Transition-SST model has a better consistency with experimental results. In addition, the wave-making effects of these bodies are studied at different immersion depths in the sea-surface vicinity or at finite depths. It is observed that the relevant pitch moments and lift coefficients are non-zero for these axi-symmetric bodies when they move close to the sea-surface. This is not expected for greater depths.
文摘Since 1970s,several experimental works revealed that the cavitation sheet inception does not occur at the minimum pressure location but further downstream at the location of a laminar/turbulent transition.Most of the cavitation models use the saturation vapour pressure as a threshold to initiate the production of vapour and therefore,are not able to capture such flows.In this paper,three modifications of the Schnerr and Sauer cavitation model are proposed and coupled with an algebraic laminar/turbulent transition model.Application to a NACA 16012 profile shows the ability of the modifications to move the cavitation inception at the right location compared with the experiment.One of them,based on the multiplication of the evaporation term by the square of the turbulent intensity seems promising.
基金National Natural Science Foundation of China(Grant No.92052301)。
文摘In this work,we studied the bluntness effect on the hypersonic boundary-layer transition over a slender cone at Mach 6 with interchangeable tips in a noisy Ludwieg tube tunnel before the so-called“transition reversal”phenomenon occurs.The evolution of instability waves is characterized using surface flush-mounted pressure sensors deployed along the streamwise direction within unit Reynolds number from 4E+6/m≤Reunit≤10E+6/m,and the bluntness of the cone nose ranges from 0.1 mm to 5 mm.Power spectral density(PSD)of pressure fluctuation indicates that small nose bluntness(ReR≤2000)has little influence on the evolution of instability waves along the hypersonic boundary-layer,whereas with a moderate nose size(2000≤ReR≤5000),the hypersonic boundary layer transition is delayed monotonically as the nose radius increases before the boundary-layer turns into fully laminar without instability waves.The delaying effect can be attributed to the increased entropy-layer swallowing distance with a large tip radius.Instability wave characterization reveals that the second mode instability wave plays a dominant role before the transition reversal happens.The quadratic phase locking of second mode instabilities can be identified by bispectral analysis,and it attenuates as the nose tip radius increases.
基金King Mongkut’s University of Technology Thonburi, the Thailand Research Fund, the Office of the Higher Education Commissionthe National Research University Project
文摘To investigate the convective heat transfer of nanofluids, experiments were performed using silver-water nanofluids under laminar, transition and turbulent flow regimes in a horizontal 4.3 mm inner-diameter tube-in-tube counter-current heat transfer test section. The volume concentration of the nanoparticles varied from 0.3% to 0.9% in steps of 0.3%, and the effects of thermo-physical properties, inlet temperature, volume concentration, and mass flow rate on heat transfer coefficient were investigated. Experiments showed that the suspended nanoparticles remarkably increased the convective heat transfer coefficient, by as much as 28.7% and 69.3% for 0.3% and 0.9% of silver content, respectively. Based on the experimental results a correlation was developed to predict the Nusselt number of the silver-water nanofluid, with ＋10% agreement between experiments and prediction.
