查看更多>>摘要：The presence of particles and the shock waves generated by the cavitation bubbles can significantly affect the safety and the performance of hydrodynamic machineries.In the present paper,the shock waves generated by cavitation bubble collapsing near the particle are numerically investigated based on the OpenFOAM together with the numerical schlieren for the shock wave identifications.The numerical results reveal that the stand-off distance is one of the paramount factors affecting the interactions between the particle and the shock waves.Several different kinds of shock waves(e.g.,bubble-inception,jet formation,particle reflected and jet-split shock waves)are observed during the bubble collapsing near the particle.For stand-off distance smaller than 0.5 or larger than 1.1,the maximum pressure at particle surface generated by the bubble growth can surpass those of the collapse stage.
查看更多>>摘要：The load rejection transient process of bulb turbine units is critical to safety of hydropower stations,and determining appropriate closing laws of guide vanes(GVs)and runner blades(RBs)for this process is of significance.In this study,we proposed a procedure to optimize the co-closing law of GVs and RBs by using computational fluid dynamics(CFD),combined with the design of experiment(DOE)method,approximation model,and genetic optimization algorithm.The sensitivity of closing law parameters on the histories of head,speed,and thrust was analyzed,and a two-stage GVs'closing law associating with a linear RBs'closing law was proposed.The results show that GVs dominate the transient characteristics by controlling the change of discharge.Speeding GVs'first-stage closing speed while shortening first-stage closing time can not only significantly reduce the maximum rotational speed but also suppress the water hammer pressure;slowing GVs'second-stage closing speed is conducive to controlling the maximum reverse axial force.RBs directly affect the runner force.Slowing RBs'closing speed can further reduce the rotational speed and the maximum reverse axial force.The safety margin of each control parameter,flow patterns,and pressure pulsations of a practical hydropower station were all improved after optimization,demonstrating the effectiveness of this method.
查看更多>>摘要：The wake induced vibration(WIV)of a one-and two-degree-of-freedom(1DOF,2DOF)downstream wave-cone cylinder(WCC)behind a stationary equal-size upstream wave-cone cylinder in the staggered arrangement is numerically investigated at subcritical Reynolds number of 3 900 by using shear stressed transfer(SST)k-ω turbulence model.The streamwise pitch ratios(P/Dm)vary from 4 to 6 with a fixed incident angle α=8°.Experimental measurements were also performed for the validation of the present numerical models.It is found that the largest vibration amplitude in crossflow direction occurred at P/Dm=4,Ur=8 with small difference of streamwise vibration at P/Dm=4,6.Different from single wavy-cone cylinder(SWCC),the downstream flexible one of a pair staggered WCCs got larger vibration amplitude during phase switching stage instead of in-phase stage.The upstream wake will suppress the triple frequency of main frequency in the power spectra density(PSD)functions of Cl but stimulate the double one of that.An intriguing vibration mechanism happened in all 2DOF cases where the trajectory of downstream WCC is a significant ellipse rather than a figure of 8.The transformation of phase switching and the variation of the main frequency of drag coefficient(Cd)can be explained by the vortex-shedding modes of downstream WCC.
查看更多>>摘要：Accurate modeling for highly non-linear coupling of a damaged ship with liquid sloshing in waves is still of considerable interest within the computational fluid dynamics(CFD)and AI framework.This paper describes a data-driven Stacking algorithm for fast prediction of roll motion response amplitudes in beam waves by constructing a hydrodynamics model of a damaged ship based on the dynamic overlapping grid CFD technology.The general idea is to optimize various parameters varying with four types of classical base models like multi-layer perception,support vector regression,random forest,and hist gradient boosting regression.This offers several attractive properties in terms of accuracy and efficiency by choosing the standard DTMB 5415 model with double damaged compartments for validation.It is clearly demonstrated that the predicted response amplitude operator(RAO)in the regular beam waves agrees well with the experimental data available,which verifies the accuracy of the established damaged ship hydrodynamics model.Given high-quality CFD samples,therefore,implementation of the designed Stacking algorithm with its optimal combination can predict the damaged ship roll motion amplitudes effectively and accurately(e.g.,the coefficient of determination 0.9926,the average absolute error 0.0955 and CPU 3s),by comparison of four types of typical base models and their various forms.Importantly,the established Stacking algorithm provides one potential that can break through problems involving the time-consuming and low efficiency for large-scale lengthy CFD simulations.
查看更多>>摘要：The objective is to study the vortical structural behaviors of a transient pitching hydrofoil and their effects on the hydrodynamic performance.The pitching motion of the hydrofoil is set to pitch up with an almost constant rate from 5° to 15° and then back to 5°,with the Reynolds number 4.4×105 and the frequency 2 Hz.The results show that the main coherent structures around the pitching hydrofoil include small-scale laminar separation bubble(LSB),large-scale second vortex(SV)and trailing edge vortex(TEV)which are all vortical.The relationship between the vortical structure and the lift is investigated with the finite-domain impulse theory.It indicates that the major part of the lift is contributed by the LSB,whereas the shedding and the formation of the SV and TEV cause the fluctuation of the lift.The proper orthogonal decomposition(POD)method is applied to capture the most energetic modes,revealing that the LSB mode occupies a large amount of energy in the flow field.The dynamic mode decomposition(DMD)method accurately extracts the dominant frequency and modal characteristics,with the first mode corresponding to the mean flow,the second mode corresponding to the LSB structure and the third and fourth modes corresponding to the vortex shedding.
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