A layered compression algorithm is presented which delivers spatial scalable encoded bit streams for remote video monitoring system. The complexity of the algorithm is modest and is well suited to real time implementa...A layered compression algorithm is presented which delivers spatial scalable encoded bit streams for remote video monitoring system. The complexity of the algorithm is modest and is well suited to real time implementation. Based on the layered compression algorithm, a codec system model is established. High-speed video compression can be realized with parallel data compression in this codec system. For image reconstruction, a prediction method using two nearest pix points is presented.展开更多
The effect of the face thickness to core height ratio on different multi-layer pyramidal core sandwich columns under in-plane compression is investigated theoretically and numerically. Numerical simulation is in good ...The effect of the face thickness to core height ratio on different multi-layer pyramidal core sandwich columns under in-plane compression is investigated theoretically and numerically. Numerical simulation is in good agreement with theory. Results indicate that one specified face thickness to core height ratio corresponds to one optimum layer number of multi-layer pyramidal core sandwich columns in consideration of engineering application. This result can guide the sandwich structure design.展开更多
Experimental study was conducted for boundarylayers on a sharp 5° half-angle cone of 400mm length at angles of attack. The model was tested in the T-326 hypersonic wind tunnel (ITAM) at freestream Mach number M...Experimental study was conducted for boundarylayers on a sharp 5° half-angle cone of 400mm length at angles of attack. The model was tested in the T-326 hypersonic wind tunnel (ITAM) at freestream Mach number M = 5.95. Mean and fluctuation wall characteristics of the boundary layer are measured at 0°, 2°, 3° and 4° angles of attack for different stagnation pressures. Pulsation measurements are carried out by means of ALTP sensor. Pressure and temperature distributions along the model are obtained, and transition beginning and end locations have been found. Boundary layer stabilization with the increase of angle of attack and the decrease of stagnation pressure is observed. High frequency pulsations inherent to hypersonic boundary layer (second mode) have been detected.展开更多
Discontinuous Galerkin(DG) method is known to have several advantages for flow simulations,in particular,in fiexible accuracy management and adaptability to mesh refinement. In the present work,the DG method is deve...Discontinuous Galerkin(DG) method is known to have several advantages for flow simulations,in particular,in fiexible accuracy management and adaptability to mesh refinement. In the present work,the DG method is developed for numerical simulations of both temporally and spatially developing mixing layers. For the temporally developing mixing layer,both the instantaneous fiow field and time evolution of momentum thickness agree very well with the previous results. Shocklets are observed at higher convective Mach numbers and the vortex paring manner is changed for high compressibility. For the spatially developing mixing layer,large-scale coherent structures and self-similar behavior for mean profiles are investigated. The instantaneous fiow field for a three-dimensional compressible mixing layer is also reported,which shows the development of largescale coherent structures in the streamwise direction. All numerical results suggest that the DG method is effective in performing accurate numerical simulations for compressible shear fiows.展开更多
The propagation of the disturbance waves in a boundary layer at Mach number = 4.5 is studied by direct numerical simulation (DNS), using NND scheme, and different amplitudes A = 0.01, 0.001, 0.000 1 of the disturbance...The propagation of the disturbance waves in a boundary layer at Mach number = 4.5 is studied by direct numerical simulation (DNS), using NND scheme, and different amplitudes A = 0.01, 0.001, 0.000 1 of the disturbance have been assumed. The numerical result shows that there might be shocklets induced in the boundary layer, even when the amplitude of disturbance wave is still small.展开更多
The problem of momentum and heat transfer in a compressible boundary layerbehind a thin expansion wave was solved by the application of the similarity transformation and theshooting technique. Utilizing the analytical...The problem of momentum and heat transfer in a compressible boundary layerbehind a thin expansion wave was solved by the application of the similarity transformation and theshooting technique. Utilizing the analytical expression of a two-point boundary value problem formomentum transfer, the energy boundary layer solution was represented as a function of thedimensionless velocity, and as the parameters of the Prandtl number, the velocity ratio, and thetemperature ratio.展开更多
The laser-ion acceleration from the ultra-short and ultra-intense laser-matter interactions attracts more and more interest nowadays. When a laser pulse interacts with a target, relativistic electrons are generated in...The laser-ion acceleration from the ultra-short and ultra-intense laser-matter interactions attracts more and more interest nowadays. When a laser pulse interacts with a target, relativistic electrons are generated in a period of few femtoseconds and driven away by the ponderomotive force, then a huge charge-separation field forms. In general cases, the ion acceleration is determined by this charge-separation field and the scale length of the plasma density. A general time-dependent solution is obtained to describe laser-plasma isothermal expansions into a vacuum, which is the fundamental theory of the laser-ion acceleration. It is adequate for non-quasi-neutral plasmas and different types of the scale length of the density gradient. The previous solutions are some special cases of our general solution. It is found that there exist both a compression layer of the ion velocity distribution and a potential well for sorue initial conditions. However, many unaccounted idiographic solutions, which may be used to reveal new mechanisms of ion acceleration, may be deduced from our general solutions.展开更多
This paper used the Floquet's three _dimensional linear stability theory in the analysis of two_dimensional compress ible boundary layer, a set of stability equations is constructed, the effect of three dimensio...This paper used the Floquet's three _dimensional linear stability theory in the analysis of two_dimensional compress ible boundary layer, a set of stability equations is constructed, the effect of three dimensional linear small perturbation on the two_dimensional compressible boundary layer transition is studied, and the effect of coming flow Ma number on growth and development of the subharmonics is calculated. It can be seen from t he calculations, the effect caused by the interaction of two_dimensional and thr ee_dimensional perturbation waves on the development of two_dimensional compress ible laminar boundary layer.展开更多
The dynamics of the compressed electron layer(CEL) are investigated when a linearly polarized(LP) laser pulse irradiates a plasma target. The turbulent motion of the CEL is investigated by a simple model, which is...The dynamics of the compressed electron layer(CEL) are investigated when a linearly polarized(LP) laser pulse irradiates a plasma target. The turbulent motion of the CEL is investigated by a simple model, which is verified by particlein-cell(PIC) simulations. It is found that the compressed layer disperses in a few cycles of the laser duration, because the CEL comes back with a large velocity in the opposite direction of the laser incident. A larger wavelength laser can be used to tailor the proton beam by reducing the turbulence of the CEL in the region of the LP laser acceleration.展开更多
Semi-periodic structures namely inclined wavy structures (IWS) are experimentally observed in compressible mixing layers at two convective Mach numbers (Mc = 0.11 and 0.47). Flow structures are visualized by the l...Semi-periodic structures namely inclined wavy structures (IWS) are experimentally observed in compressible mixing layers at two convective Mach numbers (Mc = 0.11 and 0.47). Flow structures are visualized by the laserinduced planar laser Mie scattering (PLMS) technique. Two methods are developed to investigate the spatial distribu- tion and geometry of IWS: (1) the dominant mode extrac- tion (DME) method, to extract the dominant modes of IWS from the streamwise gray-level fluctuation, and (2) the phase tracking (PT) method, to identify the shape of IWS. The re- sults suggest that pressure perturbations account for the for- marion of IWS in the initial mixing region and the joint effect of dilatation and coherent vortices enhances IWS in the well- developed region. The large transverse (cross-flow) scale of the IWS and their relation to coherent vortices (CV) indicate that the disturbance originated from CV in the mixing center propagates far into the free streams. The DME and the PT method are shown to be the effective tools to study the geometrical features of wavy structures in compressible shear flows.展开更多
A recent formula for the lift force on a low speed wing of circular arc cross-section [<span style="font-family:Verdana;"><span style="font-family:Verdana;"><b><span style=&quo...A recent formula for the lift force on a low speed wing of circular arc cross-section [<span style="font-family:Verdana;"><span style="font-family:Verdana;"><b><span style="font-family:Verdana;"><a href="#ref1">1</a></span></b></span></span><span><span></span></span><span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">] is adapted to the upward pressure force on the crests of a surface gravity wave propagating in the wind. In both cases</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">,</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"> the main feature is the utilization of the air’s compressibility. At and near a wave crest</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">,</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"> it is predicted that the air density is increased over the ambient value and that the air density decreases inversely as the square of the upward distance from the radius of curvature of the crest. As a consequence</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">,</span></span></span><span><span><span style="font-family:;" "=""><span style="font-family:Verdana;"> the air pressure also decreases upward inversely as the square of the same distance. Therefore, an upward pressure force on each crest occurs which presumably will make the crests grow. Growth rates are largest for small </span><span style="font-family:Verdana;">wavelengths and large mean slopes of the wave surface. Contrary winds should produce </span><span style="font-family:Verdana;">wave growth (not damping) as well as no wind at all.</span></span></span></span>展开更多
Laminar boundary layer (BL), under adverse pressure gradient, can separate. The separated shear layer reattaches to form a laminar separation bubble. Such bubbles are usually observed on gas turbine blades, on low Rey...Laminar boundary layer (BL), under adverse pressure gradient, can separate. The separated shear layer reattaches to form a laminar separation bubble. Such bubbles are usually observed on gas turbine blades, on low Reynolds number wings and close to the leading edges of airfoils. Presence of bubbles has a weakening effect on the performance of a fluid device. The understanding of the prevailing mechanism of the separation bubble and ways to control it are essential for the efficient design of these devices. This is due to the significance of drag reduction in these various aerodynamic devices, such as gas turbines, re-entry space vehicles and airfoils. This study introduces a two-dimensional mathematical formulation of bubble formation after flow separation. The laminar BL equations with appropriate boundary conditions are dimensionalized using the Falkner-Skan transformation. Additionally, using the Keller-box method, the nonlinear system of partial differential equations (PDEs) is numerically solved. This study presents preliminary numerical results of bubble formation in low Mach numbers. These results reveal that after separation, a laminar bubble is formed in all studied cases, for Mach numbers, M = 0.2, 0.33 and 1.0. The flow after separation reverses close to the wall and finally reattaches downstream, in a new location. As the Mach number increases, this effect is more intense. After reattachment, the BL is again established in a lower energy level and the velocity field is substantially reduced, for all cases.展开更多
The method of nonlinear parabolized stability equations(PSE) is applied in the simulation of vortex structures in compressible mixing layer.The spatially-evolving unstable waves,which dominate the vortex structure,a...The method of nonlinear parabolized stability equations(PSE) is applied in the simulation of vortex structures in compressible mixing layer.The spatially-evolving unstable waves,which dominate the vortex structure,are investigated through spatial marching method.The instantaneous flow field is obtained by adding the harmonic waves to basic flow.The results show that T-S waves do not keep growing exponentially as the linear evolution,the energy transfer to high order harmonic modes,and that finally all harmonic modes get saturated due to nonlinear interaction.The mean flow distortion induced by the nonlinear interaction between the harmonic modes and their conjugate harmonic ones,makes great change of the average flow and increases the thickness of mixing layer. PSE methods can well capture the two- and three-dimensional large scale nonlinear vortex structures in mixing layers such as vortex roll-up,vortex pairing,and A vortex.展开更多
文摘A layered compression algorithm is presented which delivers spatial scalable encoded bit streams for remote video monitoring system. The complexity of the algorithm is modest and is well suited to real time implementation. Based on the layered compression algorithm, a codec system model is established. High-speed video compression can be realized with parallel data compression in this codec system. For image reconstruction, a prediction method using two nearest pix points is presented.
基金supported by the National Natural Science Foundation of China under Grant No. 11432004
文摘The effect of the face thickness to core height ratio on different multi-layer pyramidal core sandwich columns under in-plane compression is investigated theoretically and numerically. Numerical simulation is in good agreement with theory. Results indicate that one specified face thickness to core height ratio corresponds to one optimum layer number of multi-layer pyramidal core sandwich columns in consideration of engineering application. This result can guide the sandwich structure design.
基金the Russian Foundation for Basic Research (Grant 08-01-91956-NNIO)ADTP RNP 2.1.1/3963Program RAS (project 11/9)
文摘Experimental study was conducted for boundarylayers on a sharp 5° half-angle cone of 400mm length at angles of attack. The model was tested in the T-326 hypersonic wind tunnel (ITAM) at freestream Mach number M = 5.95. Mean and fluctuation wall characteristics of the boundary layer are measured at 0°, 2°, 3° and 4° angles of attack for different stagnation pressures. Pulsation measurements are carried out by means of ALTP sensor. Pressure and temperature distributions along the model are obtained, and transition beginning and end locations have been found. Boundary layer stabilization with the increase of angle of attack and the decrease of stagnation pressure is observed. High frequency pulsations inherent to hypersonic boundary layer (second mode) have been detected.
基金supported by the National Natural Science Foundation of China (90716008,10572004 and 10921202)MOST 973 Project (2009CB724100) and CSSA
文摘Discontinuous Galerkin(DG) method is known to have several advantages for flow simulations,in particular,in fiexible accuracy management and adaptability to mesh refinement. In the present work,the DG method is developed for numerical simulations of both temporally and spatially developing mixing layers. For the temporally developing mixing layer,both the instantaneous fiow field and time evolution of momentum thickness agree very well with the previous results. Shocklets are observed at higher convective Mach numbers and the vortex paring manner is changed for high compressibility. For the spatially developing mixing layer,large-scale coherent structures and self-similar behavior for mean profiles are investigated. The instantaneous fiow field for a three-dimensional compressible mixing layer is also reported,which shows the development of largescale coherent structures in the streamwise direction. All numerical results suggest that the DG method is effective in performing accurate numerical simulations for compressible shear fiows.
文摘The propagation of the disturbance waves in a boundary layer at Mach number = 4.5 is studied by direct numerical simulation (DNS), using NND scheme, and different amplitudes A = 0.01, 0.001, 0.000 1 of the disturbance have been assumed. The numerical result shows that there might be shocklets induced in the boundary layer, even when the amplitude of disturbance wave is still small.
基金This work was supported by the "Cross-Century Talents Projects of the Educational Ministry of China"the "Projects of Investigations of Post Graduate School, University of Science and Technology Beijing".
文摘The problem of momentum and heat transfer in a compressible boundary layerbehind a thin expansion wave was solved by the application of the similarity transformation and theshooting technique. Utilizing the analytical expression of a two-point boundary value problem formomentum transfer, the energy boundary layer solution was represented as a function of thedimensionless velocity, and as the parameters of the Prandtl number, the velocity ratio, and thetemperature ratio.
基金supported by the Key Project of Chinese National Programs for Fundamental Research (973 Program) (No.2006CB806004)National Natural Science Foundation of China (No.10834008)
文摘The laser-ion acceleration from the ultra-short and ultra-intense laser-matter interactions attracts more and more interest nowadays. When a laser pulse interacts with a target, relativistic electrons are generated in a period of few femtoseconds and driven away by the ponderomotive force, then a huge charge-separation field forms. In general cases, the ion acceleration is determined by this charge-separation field and the scale length of the plasma density. A general time-dependent solution is obtained to describe laser-plasma isothermal expansions into a vacuum, which is the fundamental theory of the laser-ion acceleration. It is adequate for non-quasi-neutral plasmas and different types of the scale length of the density gradient. The previous solutions are some special cases of our general solution. It is found that there exist both a compression layer of the ion velocity distribution and a potential well for sorue initial conditions. However, many unaccounted idiographic solutions, which may be used to reveal new mechanisms of ion acceleration, may be deduced from our general solutions.
文摘This paper used the Floquet's three _dimensional linear stability theory in the analysis of two_dimensional compress ible boundary layer, a set of stability equations is constructed, the effect of three dimensional linear small perturbation on the two_dimensional compressible boundary layer transition is studied, and the effect of coming flow Ma number on growth and development of the subharmonics is calculated. It can be seen from t he calculations, the effect caused by the interaction of two_dimensional and thr ee_dimensional perturbation waves on the development of two_dimensional compress ible laminar boundary layer.
基金Project supported by the Shanghai Provincial Special Foundation for Outstanding Young Teachers in University,China(Grant No.yyy10043)
文摘The dynamics of the compressed electron layer(CEL) are investigated when a linearly polarized(LP) laser pulse irradiates a plasma target. The turbulent motion of the CEL is investigated by a simple model, which is verified by particlein-cell(PIC) simulations. It is found that the compressed layer disperses in a few cycles of the laser duration, because the CEL comes back with a large velocity in the opposite direction of the laser incident. A larger wavelength laser can be used to tailor the proton beam by reducing the turbulence of the CEL in the region of the LP laser acceleration.
基金supported by National Nature Science Foundation of China(90716008,10572004,and 11172006)by MOST 973 Project(2009CB724100)
文摘Semi-periodic structures namely inclined wavy structures (IWS) are experimentally observed in compressible mixing layers at two convective Mach numbers (Mc = 0.11 and 0.47). Flow structures are visualized by the laserinduced planar laser Mie scattering (PLMS) technique. Two methods are developed to investigate the spatial distribu- tion and geometry of IWS: (1) the dominant mode extrac- tion (DME) method, to extract the dominant modes of IWS from the streamwise gray-level fluctuation, and (2) the phase tracking (PT) method, to identify the shape of IWS. The re- sults suggest that pressure perturbations account for the for- marion of IWS in the initial mixing region and the joint effect of dilatation and coherent vortices enhances IWS in the well- developed region. The large transverse (cross-flow) scale of the IWS and their relation to coherent vortices (CV) indicate that the disturbance originated from CV in the mixing center propagates far into the free streams. The DME and the PT method are shown to be the effective tools to study the geometrical features of wavy structures in compressible shear flows.
文摘A recent formula for the lift force on a low speed wing of circular arc cross-section [<span style="font-family:Verdana;"><span style="font-family:Verdana;"><b><span style="font-family:Verdana;"><a href="#ref1">1</a></span></b></span></span><span><span></span></span><span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">] is adapted to the upward pressure force on the crests of a surface gravity wave propagating in the wind. In both cases</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">,</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"> the main feature is the utilization of the air’s compressibility. At and near a wave crest</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">,</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"> it is predicted that the air density is increased over the ambient value and that the air density decreases inversely as the square of the upward distance from the radius of curvature of the crest. As a consequence</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">,</span></span></span><span><span><span style="font-family:;" "=""><span style="font-family:Verdana;"> the air pressure also decreases upward inversely as the square of the same distance. Therefore, an upward pressure force on each crest occurs which presumably will make the crests grow. Growth rates are largest for small </span><span style="font-family:Verdana;">wavelengths and large mean slopes of the wave surface. Contrary winds should produce </span><span style="font-family:Verdana;">wave growth (not damping) as well as no wind at all.</span></span></span></span>
文摘Laminar boundary layer (BL), under adverse pressure gradient, can separate. The separated shear layer reattaches to form a laminar separation bubble. Such bubbles are usually observed on gas turbine blades, on low Reynolds number wings and close to the leading edges of airfoils. Presence of bubbles has a weakening effect on the performance of a fluid device. The understanding of the prevailing mechanism of the separation bubble and ways to control it are essential for the efficient design of these devices. This is due to the significance of drag reduction in these various aerodynamic devices, such as gas turbines, re-entry space vehicles and airfoils. This study introduces a two-dimensional mathematical formulation of bubble formation after flow separation. The laminar BL equations with appropriate boundary conditions are dimensionalized using the Falkner-Skan transformation. Additionally, using the Keller-box method, the nonlinear system of partial differential equations (PDEs) is numerically solved. This study presents preliminary numerical results of bubble formation in low Mach numbers. These results reveal that after separation, a laminar bubble is formed in all studied cases, for Mach numbers, M = 0.2, 0.33 and 1.0. The flow after separation reverses close to the wall and finally reattaches downstream, in a new location. As the Mach number increases, this effect is more intense. After reattachment, the BL is again established in a lower energy level and the velocity field is substantially reduced, for all cases.
基金supported by the National Natural Sci- ence Foundation of China(11102198)
文摘The method of nonlinear parabolized stability equations(PSE) is applied in the simulation of vortex structures in compressible mixing layer.The spatially-evolving unstable waves,which dominate the vortex structure,are investigated through spatial marching method.The instantaneous flow field is obtained by adding the harmonic waves to basic flow.The results show that T-S waves do not keep growing exponentially as the linear evolution,the energy transfer to high order harmonic modes,and that finally all harmonic modes get saturated due to nonlinear interaction.The mean flow distortion induced by the nonlinear interaction between the harmonic modes and their conjugate harmonic ones,makes great change of the average flow and increases the thickness of mixing layer. PSE methods can well capture the two- and three-dimensional large scale nonlinear vortex structures in mixing layers such as vortex roll-up,vortex pairing,and A vortex.