This paper is devoted to application of the Reduced-Order Model(ROM)based on Volterra series to prediction of lift and drag forces due to airfoil periodic translation in transonic flow region.When there is large ampli...This paper is devoted to application of the Reduced-Order Model(ROM)based on Volterra series to prediction of lift and drag forces due to airfoil periodic translation in transonic flow region.When there is large amplitude oscillation of the relative Mach number,as appeared in helicopter rotor movement in forward flight,the conventional Volterra ROM is found to be unsatisfactory.To cover such applications,a matched Volterra ROM,inspired from previous multistep nonlinear indicial response method based on Duhamel integration,is thus considered,in which the step motions are defined inside a number of equal intervals with both positive and negative step motions to match the airfoil forward and backward movement,and the kernel functions are constructed independently at each interval.It shows that,at least for the translation movement considered,this matched Volterra ROM greatly improves the accuracy of prediction.Moreover,the matched Volterra ROM,with the total number of step motions and thus the computational cost close to those of the conventional Volterra ROM method,has the additional advantage that the same set of kernels can match various translation motions with different starting conditions so the kernels can be predesigned without knowing the specific motion of airfoil.展开更多
In a magnetohydrodynamic(MHD)driven fluid cell,a plane non-parallel flow in a square domain satisfying a free-slip boundary condition is examined.The energy dissipation of the flow is controlled by the viscosity and l...In a magnetohydrodynamic(MHD)driven fluid cell,a plane non-parallel flow in a square domain satisfying a free-slip boundary condition is examined.The energy dissipation of the flow is controlled by the viscosity and linear friction.The latter arises from the influence of the Hartmann bottom boundary layer in a three-dimensional(3D)MHD experiment in a square bottomed cell.The basic flow in this fluid system is a square eddy flow exhibiting a network of N~2 vortices rotating alternately in clockwise and anticlockwise directions.When N is odd,the instability of the flow gives rise to secondary steady-state flows and secondary time-periodic flows,exhibiting similar characteristics to those observed when N=3.For this reason,this study focuses on the instability of the square eddy flow of nine vortices.It is shown that there exist eight bi-critical values corresponding to the existence of eight neutral eigenfunction spaces.Especially,there exist non-real neutral eigenfunctions,which produce secondary time-periodic flows exhibiting vortices merging in an oscillatory manner.This Hopf bifurcation phenomenon has not been observed in earlier investigations.展开更多
A numerical investigation has been carried out to examine turbulent flow and heat transfer characteristics in a three-dimensional ribbed square channels. Fluent 6.3 CFD code has been used. The governing equations are ...A numerical investigation has been carried out to examine turbulent flow and heat transfer characteristics in a three-dimensional ribbed square channels. Fluent 6.3 CFD code has been used. The governing equations are discretized by the second order upwind differencing scheme, decoupling with the SIMPLE (semi-implicit method for pressure linked equations) algorithm and are solved using a finite volume approach. The fluid flow and heat transfer characteristics are presented for the Reynolds numbers based on the channel hydraulic diameter ranging from 104 to 4 ′ 104. The effects of rib shape and orientation on heat transfer and pressure drop in the channel are investigated for six different rib configurations. Rib arrays of 45° inclined and 45° V-shaped are mounted in inline and staggered arrangements on the lower and upper walls of the channel. In addition, the performance of these ribs is also compared with the 90° transverse ribs.展开更多
The stall margin of compressor could be improved effectively by rotor tip injection,and the periodic injection is commonly used in the research.The purpose of this work is to investigate the influence of injection fre...The stall margin of compressor could be improved effectively by rotor tip injection,and the periodic injection is commonly used in the research.The purpose of this work is to investigate the influence of injection frequency on the rotor stall margin.An unsteady CFD code was employed to simulate the flow field of the rotor with injections of different frequencies.Comparing the stall margin of the rotor with injections of different frequencies,it is shown that there is an optimal injection frequency,around which the rotor stability enhancement is the largest.When the injection frequency is away form the optimal frequency,the improvement in stable flow range decreases correspondingly.For the rotor in this paper,the optimal frequency was 1.5 times the frequency of tip leakage vortex(for short,TLV) fluctuation.Time-averaged loading distribution at 98.5% span indicates that the loading of the rotor near the leading edge is decreased through injection with the optimal frequency,and therefore,the stall could be delayed.展开更多
We show existence of time-periodic supersonic solutions in a finite interval, after certain start-up time depending on the length of the interval, to the one space-dimensional isentropic compressible Euler equations, ...We show existence of time-periodic supersonic solutions in a finite interval, after certain start-up time depending on the length of the interval, to the one space-dimensional isentropic compressible Euler equations, subjected to periodic boundary conditions. Both classical solutions and weak entropy solutions, as well as high-frequency limiting behavior are considered. The proofs depend on the theory of Cauchy problems of genuinely nonlinear hyperbolic systems of conservation laws.展开更多
The Tahe oilfield,located in the southwest of the Akekule nosing structure,northern Tarim basin,was the most prolific oilfield targeting at the Ordovician carbonate reservoirs in China.The reservoir space was dominant...The Tahe oilfield,located in the southwest of the Akekule nosing structure,northern Tarim basin,was the most prolific oilfield targeting at the Ordovician carbonate reservoirs in China.The reservoir space was dominant with fracture-cave systems commonly induced by tectonics and karstification.Although hydrocarbon production had proceeded for two decades in the Tahe oilfiled,the control of oil and gas accumulations was still doubtful.In this work,the periodic fluid flow induced by cyclic tectonic stresses was proposed as the mechanism of hydrocarbon migration in the fracture-cave systems of carbonate reservoirs.The fracture networks formed conduits for fluid flow,and the fluid pressure in caves transmitted from stress field provided the driving force.The constitutive equations were established among stresses,fracture densities and flow velocities.Four quasi-3D geological models were constructed to simulate the flow velocities on the Ordovician surface of Akekule nosing structure in the critical tectonic stages.The simulated results supplied indicative information on oil and gas migration and accumulation in the tectonic stages.Combining with the oil and gas charge history,a conceptual model was built to reveal the multi-stage oil and gas charge and accumulation in the Ordovician of Akekule nosing structure.展开更多
Let X be a compact metric space, F : X ×R→ X be a continuous flow and x ∈ X a proper quasi-weakly almost periodic point, that is, x is quasi-weakly almost periodic but not weakly almost periodic. The aim of thi...Let X be a compact metric space, F : X ×R→ X be a continuous flow and x ∈ X a proper quasi-weakly almost periodic point, that is, x is quasi-weakly almost periodic but not weakly almost periodic. The aim of this paper is to investigate whether there exists an invariant measure generated by the orbit of x such that the support of this measure coincides with the minimal center of attraction of x? In order to solve the problem, two continuous flows are constructed. In one continuous flow,there exist a proper quasi-weakly almost periodic point and an invariant measure generated by its orbit such that the support of this measure coincides with its minimal center of attraction; and in the other,there is a proper quasi-weakly almost periodic point such that the support of any invariant measure generated by its orbit is properly contained in its minimal center of attraction. So the mentioned problem is sufficiently answered in the paper.展开更多
文摘This paper is devoted to application of the Reduced-Order Model(ROM)based on Volterra series to prediction of lift and drag forces due to airfoil periodic translation in transonic flow region.When there is large amplitude oscillation of the relative Mach number,as appeared in helicopter rotor movement in forward flight,the conventional Volterra ROM is found to be unsatisfactory.To cover such applications,a matched Volterra ROM,inspired from previous multistep nonlinear indicial response method based on Duhamel integration,is thus considered,in which the step motions are defined inside a number of equal intervals with both positive and negative step motions to match the airfoil forward and backward movement,and the kernel functions are constructed independently at each interval.It shows that,at least for the translation movement considered,this matched Volterra ROM greatly improves the accuracy of prediction.Moreover,the matched Volterra ROM,with the total number of step motions and thus the computational cost close to those of the conventional Volterra ROM method,has the additional advantage that the same set of kernels can match various translation motions with different starting conditions so the kernels can be predesigned without knowing the specific motion of airfoil.
基金Project supported by the National Natural Science Foundation of China(No.11571240)the Shenzhen Natural Science Fund of China(the Stable Support Plan Program No.20220805175116001)。
文摘In a magnetohydrodynamic(MHD)driven fluid cell,a plane non-parallel flow in a square domain satisfying a free-slip boundary condition is examined.The energy dissipation of the flow is controlled by the viscosity and linear friction.The latter arises from the influence of the Hartmann bottom boundary layer in a three-dimensional(3D)MHD experiment in a square bottomed cell.The basic flow in this fluid system is a square eddy flow exhibiting a network of N~2 vortices rotating alternately in clockwise and anticlockwise directions.When N is odd,the instability of the flow gives rise to secondary steady-state flows and secondary time-periodic flows,exhibiting similar characteristics to those observed when N=3.For this reason,this study focuses on the instability of the square eddy flow of nine vortices.It is shown that there exist eight bi-critical values corresponding to the existence of eight neutral eigenfunction spaces.Especially,there exist non-real neutral eigenfunctions,which produce secondary time-periodic flows exhibiting vortices merging in an oscillatory manner.This Hopf bifurcation phenomenon has not been observed in earlier investigations.
文摘A numerical investigation has been carried out to examine turbulent flow and heat transfer characteristics in a three-dimensional ribbed square channels. Fluent 6.3 CFD code has been used. The governing equations are discretized by the second order upwind differencing scheme, decoupling with the SIMPLE (semi-implicit method for pressure linked equations) algorithm and are solved using a finite volume approach. The fluid flow and heat transfer characteristics are presented for the Reynolds numbers based on the channel hydraulic diameter ranging from 104 to 4 ′ 104. The effects of rib shape and orientation on heat transfer and pressure drop in the channel are investigated for six different rib configurations. Rib arrays of 45° inclined and 45° V-shaped are mounted in inline and staggered arrangements on the lower and upper walls of the channel. In addition, the performance of these ribs is also compared with the 90° transverse ribs.
文摘The stall margin of compressor could be improved effectively by rotor tip injection,and the periodic injection is commonly used in the research.The purpose of this work is to investigate the influence of injection frequency on the rotor stall margin.An unsteady CFD code was employed to simulate the flow field of the rotor with injections of different frequencies.Comparing the stall margin of the rotor with injections of different frequencies,it is shown that there is an optimal injection frequency,around which the rotor stability enhancement is the largest.When the injection frequency is away form the optimal frequency,the improvement in stable flow range decreases correspondingly.For the rotor in this paper,the optimal frequency was 1.5 times the frequency of tip leakage vortex(for short,TLV) fluctuation.Time-averaged loading distribution at 98.5% span indicates that the loading of the rotor near the leading edge is decreased through injection with the optimal frequency,and therefore,the stall could be delayed.
基金supported by the National Natural Science Foundation of China(11371141 and 11871218)Science and Technology Commission of Shanghai Municipality(STCSM)under Grant No.18dz2271000
文摘We show existence of time-periodic supersonic solutions in a finite interval, after certain start-up time depending on the length of the interval, to the one space-dimensional isentropic compressible Euler equations, subjected to periodic boundary conditions. Both classical solutions and weak entropy solutions, as well as high-frequency limiting behavior are considered. The proofs depend on the theory of Cauchy problems of genuinely nonlinear hyperbolic systems of conservation laws.
基金This work was supported by the National Science and Technology Major Project of China(Grant No.2016ZX05033-001)Hebei GEO University(Grant No.BQ2018033).
文摘The Tahe oilfield,located in the southwest of the Akekule nosing structure,northern Tarim basin,was the most prolific oilfield targeting at the Ordovician carbonate reservoirs in China.The reservoir space was dominant with fracture-cave systems commonly induced by tectonics and karstification.Although hydrocarbon production had proceeded for two decades in the Tahe oilfiled,the control of oil and gas accumulations was still doubtful.In this work,the periodic fluid flow induced by cyclic tectonic stresses was proposed as the mechanism of hydrocarbon migration in the fracture-cave systems of carbonate reservoirs.The fracture networks formed conduits for fluid flow,and the fluid pressure in caves transmitted from stress field provided the driving force.The constitutive equations were established among stresses,fracture densities and flow velocities.Four quasi-3D geological models were constructed to simulate the flow velocities on the Ordovician surface of Akekule nosing structure in the critical tectonic stages.The simulated results supplied indicative information on oil and gas migration and accumulation in the tectonic stages.Combining with the oil and gas charge history,a conceptual model was built to reveal the multi-stage oil and gas charge and accumulation in the Ordovician of Akekule nosing structure.
基金Supported by the National Natural Science Foundation of China(Grant No.11661054)
文摘Let X be a compact metric space, F : X ×R→ X be a continuous flow and x ∈ X a proper quasi-weakly almost periodic point, that is, x is quasi-weakly almost periodic but not weakly almost periodic. The aim of this paper is to investigate whether there exists an invariant measure generated by the orbit of x such that the support of this measure coincides with the minimal center of attraction of x? In order to solve the problem, two continuous flows are constructed. In one continuous flow,there exist a proper quasi-weakly almost periodic point and an invariant measure generated by its orbit such that the support of this measure coincides with its minimal center of attraction; and in the other,there is a proper quasi-weakly almost periodic point such that the support of any invariant measure generated by its orbit is properly contained in its minimal center of attraction. So the mentioned problem is sufficiently answered in the paper.
