查看更多>>摘要:A laboratory study was conducted using particle image velocimetry(PIV)to measure flow velocity distributions over two-dimensional smooth and rough fixed dunes.It comprised 28 tests,each yielding 146 velocity profiles over one complete dune length.Two kinds of double-averaged velocity profiles were computed,one based on all the 146 lines of data(called global average),and the others from only some of them(called partial average).The results show that the global average velocity distribution is generally close to the partial average profile derived from evenly-distributed three or five lines along one dune length.Furthermore,the global average velocity profile can also be reasonably approximated using a single profile,measured at the representative line in this paper.The representative line is found to locate near the reattachment point.This result would be helpful to simplify measurements of general velocity distribution for a flow over dunes.The paper also applies the concept of representative line to the description of distributions of turbulence characteristics.
查看更多>>摘要:In this paper,the dynamic characteristics of the cylindrical bubbles under triple-frequency acoustic excitation are investigated theoretically.The analytical solution of the primary-superharmonic-subharmonic(PRI-SUPER-SUB)simultaneous resonance is obtained through the multi-scale method.Based on the analysis of the frequency response,the influencing mechanisms of the primary parameters(e.g.,the total amplitude,amplitude ratio,liquid viscosity,polytropic exponent,and bubble equilibrium radius)on the resonance are investigated quantitatively.The main conclusions include:(1)The solution for the simultaneous resonance of the cylindrical bubble exhibits jumping and hysteresis phenomena in the vicinity of the resonance frequency.(2)As the total amplitude,amplitude ratio,and equilibrium radius increase,the response amplitude of the PRI-SUPER-SUB simultaneous resonance increases,while the influence of the viscosity is the opposite.(3)The regions dominated by the instability of the simultaneous resonance is significantly affected by the system parameters.
查看更多>>摘要:The local distributions of both the temperature and pressure have a great influence on the rheological characteristics of the drilling fluid,thereby affecting its flow law in a wellbore.Along these lines,in this work,the rheology of water-based drilling fluid samples under high-temperature(30℃-210℃)and high-pressure(34.5 MPa-172.4 MPa)(HTHP)conditions was systematically analyzed.The constitutive model of the variation of the apparent viscosity of the drilling fluid with the temperature and pressure was successfully established.The analysis revealed that,among the Bingham model,the Power law model,the Herschel-Bulkley(H-B)model,and the Casson model,the H-B model can accurately describe the rheology of the drilling fluid under HTHP conditions.Therefore,the H-B model was used to perform numerical simulations of the flow law of the water-based drilling fluid in the wellbore.The simulation results demonstrated that the drilling fluid viscosity decreased as the depth of the wellbore increased,and was mainly influenced by the temperature.The maximum viscosity inside the drill pipe was mainly concentrated in the middle region,and that of the fluid when flowing in the annulus was mainly concentrated on the side near the outer wall of the annulus.This work provides valuable insights for setting the key parameters of the drilling fluid and wellbore cleaning in the drilling operation of a lx 104 m deep well.
查看更多>>摘要:Flow around a ship that advances at a constant speed V in calm water of uniform finite depth D is considered within the practical,realistic and commonly-used framework of the Green-function and boundary-integral method in conjunction with potential-flow theory.This framework entails accurate and efficient numerical evaluation of flows due to singularities(sources,dipoles)distributed over flat or curved panels of diverse geometries(quadrilaterals,triangles)that are employed to approximate the ship hull surface.This basic core element of the Green-function and boundary-integral method is considered for steady ship waves in the subcritical flow regime gD/V2>1 and the supercritical flow regime gD/V2<1,where g is the acceleration of gravity.The special case of deep water is also considered.An analytical representation of flows due to general distributions of singularities over hull-surface panels is given.This flow-representation adopts the Fourier-Kochin method,which prioritizes spatial integration over the panel followed by Fourier integration,in contrast to the conventional method in which the Green function(defined via a Fourier integration)is initially evaluated and subsequently integrated over the panel.The mathematical and numerical complexities associated with the numerical evaluation and subsequent panel integration of the Green function for steady ship waves in finite water depth are then circumvented in the Fourier-Kochin method.A major advantage of this method is that panel integration merely amounts to integration of an exponential-trigonometric function,a straightforward task that can be accurately and efficiently performed.The analytical flow-representation proposed in the study offers a smooth decomposition of free-surface effects into waves,defined by a regular single Fourier integral,and a non-oscillatory local flow,characterized by a double Fourier integral featuring a smooth integrand that primarily dominates within a compact region near the origin of the Fourier plane.Illustrative numerical applications to the flow potentials and velocities associated with a typical distribution of sources over a panel show that the flow-representation given in the study yields a practical method well suited for accurate and efficient numerical evaluation.
Nicolas HanousekBikash RanabhatAaron EnglishReza Ahmadian...
991-1007页
查看更多>>摘要:With the global demand for more electricity,and for that electricity to be produced using low-carbon generation,a turbine was designed to extract energy from underutilised flows.The mechanism by which the turbine operates makes it highly demanding to represent using mesh-based numerical schemes,resulting in a need to investigate alternative methods.The Smoothed Particle Hydrodynamics(SPH)software,DualSPHysics,utilising the Chrono solid body solver,was used to represent the turbine as a free body in a 2-D environment allowing for evaluation of the free-spin velocity to be assessed.The aim of this being to ascertain the applicability of SPH to the modelling of vertical axis turbines with multiple moving parts,and also develop an understanding of the design itself.The model was found to compare favourably with lab results,showing that a vertical axis turbine may be represented in this fashion.The resilience of the device,a design driver and previously untested mode,was assessed by considering post-damage scenarios.From this,future flume study and parallel numerical modelling can guide this or other vertical axis turbines towards improved performance.