泵叶轮数值研发理论的若干关键问题

随着流体机械理论、计算机技术和数控加工技术的发展,泵叶轮的研发技术正在从传统的"相似设计-试制-试验-改进"的低效率反复手工过程向全过程的数值优化设计过渡,但在数值研发叶轮的过程中,由于受到水力设计理论、高精度高分辨率叶轮流场计算、叶轮造型与流场结构之间的影响规律及叶轮优化设计方法等诸多因素的制约,要想真正实现高性能泵叶轮完全数值研发的目标,还需要逐一深入探讨这些关键因素,为将来最终解决其中的基础理论问题提供必要指导。

Numerical Investigations on the Aerodynamic Performance of Wind Turbine： Downwind Versus Upwind Configuration

Although the upwind configuration is more popular in the field of wind energy, the downwind one is a promising type for the offshore wind energy due to its special advantages. Different configurations have different aerodynamic performance and it is important to predict the performance of both downwind and upwind configurations accurately for designing and developing more reliable wind turbines. In this paper, a numerical investigation on the aerodynamic performance of National Renewable Energy Laboratory （NREL） phase V1 wind turbine in downwind and upwind configurations is presented. The open source toolbox OpenFOAM coupled with arbitrary mesh interface （AMI） method is applied to tackle rotating problems of wind turbines. Two 3D numerical models of NREL phase VI wind turbine with downwind and upwind configurations under four typical working conditions of incoming wind velocities are set up for the study of different unsteady characteristics of the downwind and upwind configurations, respectively. Numerical results of wake vortex structure, time histories of thrust, pressure distribution on the blade and limiting streamlines which can be used to identify points of separation in a 3D flow are presented. It can be concluded that thrust reduction due to blade-tower interaction is small for upwind wind turbines but relatively large for downwind wind turbines and attention should be paid to the vibration at a certain frequency induced by the cyclic reduction for both configurations. The results and conclusions are helpful to analyze the different aerodynamic performance of wind turbines between downwind and upwind configurations, providing useful references for practical design of wind turbine.

A Numerical Study of the Effects of Coastal Geometry in the Bohai Sea on Storm Surges Induced by Cold-Air Outbreaks

Strom surges are not only determined by the atmospheric forcing,but also influenced by the coastal geometry and bathymetry.The Bohai Sea,as one of China’s marginal seas,is seriously harmed by storm surges,especially those caused by cold-air outbreaks.As the coastline of the Bohai Sea has changed evidently these years,storm surges may have new characteristics due to the changes in the local geometry.This paper aims to find out these new characteristics by primarily investigating the influence of the changes in the local geometry on storm surges with numerical methods.20 scenarios were constructed based on the track and inten-sity of the cold-air outbreaks to describe the actual situation.By analyzing the model results of the control scenarios,it is found that the main changes of the maximum surge elevation occur in the Bohai Bay and the Laizhou Bay.At the top of the Bohai Bay,the maximum surge elevation is obviously decreased,while in the Laizhou Bay,it is enhanced by the growing Yellow River Delta.This,however,does not suggest that the storm surges in the Laizhou Bay become more serious.A comparison of the risk assessment of storm surges in the Tanggu,Huanghua and Yangjiaogou regions shows that the risk of storm surges in these coastal areas is lightened by the evolvement of the coastal geometry.Particularly near Yangjiaogou,though the maximum surge elevation becomes higher to subject more areas to risk,the risk is still reduced by the evolvement of the Yellow River Delta.

A numerical study of rock burst development and strain energy release

We consider rock burst to be a dynamic disaster similar to earthquakes,rapid land sliding,or coal mine gas dynamic disasters.Multi-scale mechanical principles imply the same mechanism of damage evolution proceeds the catastrophe.Damage may occur at various scales from a meso-scopic scale to a macroscopic,or engineering scale.Rock burst is a catastrophe at the scale of the engineering structure,such as a tunnel cross section or the work face of a long wall mine.It results from dynamic fracture of the structure where microscopic damage nucleates,expands,and finally propagates into a macroscopic sized fracture band.Rock burst must,therefore,undergo a relatively long development,or gestation,time before its final appearance.In this paper,a study of rock burst within a deeply buried tunnel by numerical methods is described.The results show that during rock burst gestation the distributed microscopic damage in the rock surrounding the tunnel localizes,intersects,and then evolves into a set of concentrated ''V'' shaped damage bands.These concentrated damage bands propagate in the direction of maximum shear as shearing slide bands take shape.Rock burst happens within the wedge separated by the shear bands from the native tunnel rock.An analysis of the wedge fracture shows that the unloading effects result in rock burst and rapid release of the strain energy.The implications for rock burst prediction in tunnels are that:(1) rock burst develops in the upper arch corners of in the tunnel cross section prior to developing in other zones,so good attention must be paid there;(2) all monitoring,prevention,and treatment of rock burst should be done during the gestation phase;(3) the shear bands contain abundant information concerning the physics and mechanics of the process and they are the foundation of physical and mechanical monitoring of acoustic emission,micro seismic events,stress,and the like.Thus a special study of the shearing mechanism is required.

Numerical Study of the Evaporation Effects on the Microdroplet Dynamics in Digital Microfluidic Biochips

In this paper, an electrohydrodynamic approach is used to model and study dynamics of evaporating microdroplets in digital microfluidic systems. A numerical eleetrohydrodynamic approach is used to calculate the driving force and shear force （due to the walls）. Effects of contact line pinning is considered by adding a three-phase contact line force, and also considering dynamic contact angle which modifies the mierodroplet boundary conditions. Since air is used as the filler fluid, the drag force is neglected. Although energy equation is not solved （constant temperature assumption）, effects of the evaporation is considered from two aspects： It is shown that an additional force is needed to balance the dynamic equation of the mierodroplet motion. Also, at each time step the microdroplet interface has to be deformed due to the change in the microdroplet radius. Important findings of the proposed model includes the transient velocity and displacement of the microdroplet as well as the driving and opposing forces acting on the microdroplet as functions of time. It is shown that mass loss due to evaporation tends to accelerate the droplet; whereas the competitive effect of the reduced driving force decelerates the droplet at the end of motion. The modeling results indicate that evaporation plays a crucial role in microdroplet motion by changing the force balance and the microdroplet boundary condition.

Numerical Study of Thermoelectric Generation within a Reciprocal Flow Porous Media Burner

A numerical study based on direct thermal to electric energy conversion was performed in a reciprocal flow porous media burner embedded with two layers of thermoelements. The burner lean combustibility limit was sought in order to maximize global efficiency of thermal to electrical energy conversion by minimizing fuel consumption. Once the pairs of operational variables, composition and filtrational velocity of gas inlet mixture were found, the optimal length and placement of thermoelectric elements within the reactor high thermal gradients were sought to maximize the electric current, thermoelements and system overall efficiency. A two temperature-resistance model for finite time thermodynamics was developed for the thermoelectric elements energy fluxes. Results indicate a distribution of current and efficiencies that presents a maximum at different themoelements length. Maximum values for current and system efficiency obtained were 44.3 m A and 2.5%, respectively.

Numerical study on the characteristics of natural supercavitation by planar symmetric wedge-shaped cavitators for rotational supercavitating evaporator

With the application of supercavitation effect, a novel device named rotational supercavitating evaporator(RSCE) was recently designed for desalination. In order to improve the blade shape of rotational cavitator in RSCE for performance optimization and then design three-dimensional blades, numerical simulations are conducted on the supercavitating flows(with cavitation number ranging from 0.055 to 0.315) around two-dimensional planar symmetric wedge-shaped cavitators with different wedge angles varied from 10 to 180 degrees. Proper numerical method for simulating supercavitating flows around planar symmetric cavitator is established, and assessment of k-ε-v2 -f turbulence model in simulating cavitating flows is conducted. It shows that the size of computational domain would affect the simulation result. Empirical formulae for supercavity dimensions about cavitation number at different wedge angles are obtained, which are of significant importance in the subsequent design of three-dimensional blade. The characteristics of resistance at different wedge angles are discussed, which, together with the characteristics of supercavity dimensions, play important roles in the optimal design of RSCE.

Numerical investigation of an advanced aircraft model during pitching motion at high incidence

An unsteady Reynolds averaged Navier–Stokes(URANS) method combined with a rigid dynamic mesh technique was developed to simulate unsteady flows around complex configurations during pitching motion. First, a test case with the NACA0012 airfoil was selected to validate the numerical methods and our in-house codes. Then, we evaluated the unsteady flows around an advanced aircraft model during harmonic pitching motion at high incidence. The effects of pitching motion on the hysteresis of aerodynamic force, the evolution of the leading-edge vortex, and the distribution of pressure on the model's surface were analyzed in detail. The roles of several significant parameters such as the reduced frequency and pitching amplitude were revealed. Several conclusions were found: pitching motion would delay the initiation of the leading-edge vortex, strengthen the vorticity, postpone the occurrence of vortex breakdown, and weaken the massively separated flows, thus causing additional aerodynamic force. Two categories of critical reduced frequency have been found, which divide the influence of reduced frequency on aerodynamic force into three stages, called the linear increasing range, slowly increasing range, and constant range. The first-order phase lag between the aerodynamic force and the incidence is a constant that is independent of the amplitude when the reduced frequency is sufficiently high. A scaled maximum value of C_L is proposed; it depends only on the reduced frequency(instead of the amplitude), and increases linearly when the reduced frequency is sufficiently low.

Numerical Investigation on Detonation Wave through U-bend

Pulse detonation engine (PDE) is expected for a next-generation propulsion system. PDE is a promising engine that can generates power and thrust by using intermittent detonation. Promotion of deflagration to detonation transition (below DDT) is a key issue to realize this system. PDE has experimentally been investigated, and it was confirmed that detonation tubes with U-shaped bends are useful for fast DDT. However, the mechanism of DDT promotion due to U-bends has not been well clarified. In the present study, the influence of a U-bend on detona-tion wave propagation is researched with computational fluid dynamics (CFD). The numerical results show that detonation wave disappears once near the U-bend inlet and restarts after passing through it. In addition, it was found that the use of the U-bend with small channel width and curvature radius can induce fast DDT.

Numerical investigation on the geodynamical mechanism of the first major shock of 2006 Pingtung M_w7.0 earthquake

A strong Mw7.0 earthquake struck Pingtung offshore of Talwan on December 26, 2006. It consisted of two major events with an 8-minute interval. The first major shock occurred at 12：26 UTC. Focal mechanism results from Harvard, USGS, and BATS all indicated that the first major shock was a normal fault earthquake and the second one was dominated by strike-slip offsets. The location of the epicenter varied greatly in depth in different analyses. The latest results showed that the focal depth of the first shock was most probably around 40-44 km, placing the epicenter in the lithospheric mantle. However, this is not a location where earthquakes usually occur. To explore the geodynamical mechanism of this event, we carded out 2D finite element method （FEM） numerical experiments. Our primary results indicate that the geodynamical background, as well as the formation of Pingtung earthquake, is a consequence of the collision between Luzon arc and Chinese continental margin. Although Taiwan Island is in the shadow of NW-SE trending compressive collision zone, the existence of ductile lower crust leads to the decoupling between upper crust and lithospheric mantle. As lithospheric mantle subducts to the depth of around 250 km, the upper part of the bending subduction slab puts itself in an extensional state. The extensional stress from bending induced the occurrence of this normal fault earthquake at the critical point.

A numerical study on matching relationships of gravity waves in nonlinear interactions

Applying a fully nonlinear numerical scheme with second-order temporal and spatial precision,nonlinear interactions of gravity waves are simulated and the matching relationships of the wavelengths and frequencies of the interacting waves are discussed.In resonant interactions,the wavelengths of the excited wave are in good agreement with the values derived from sum or difference resonant conditions,and the frequencies of the three waves also satisfy the matching condition.Since the interacting waves obey the resonant conditions,resonant interactions have a reversible feature that for a resonant wave triad,any two waves are selected to be the initial perturbations,and the third wave can then be excited through sum or difference resonant interaction.The numerical results for nonresonant triads show that in nonresonant interactions,the wave vectors tend to approximately match in a single direction,generally in the horizontal direction.The frequency of the excited wave is close to the matching value,and the degree of mismatching of frequencies may depend on the combined effect of both the wavenumber and frequency mismatches that should benefit energy exchange to the greatest extent.The matching and mismatching relationships in nonresonant interactions differ from the results of weak interaction theory that the wave vectors are required to satisfy the resonant matching condition but the frequencies are permitted to mismatch and oscillate with amplitude of half the mismatching frequency.Nonresonant excitation has an irreversible characteristic,which is different from what is found for the resonant interaction.For specified initial primary and secondary waves,it is difficult to predict the values of the mismatching wavenumber and frequency for the excited wave owing to the complexity.

Numerical Investigation of the Flow inside the Combustion Chamber of a Plant Oil Stove

Recently a low cost cooking device for developing and emerging countries was developed at KIT in cooperation with the company Bosch und Siemens Hausger^ite GmbH. After constructing an innovative basic design further development was required. Numerical investigations were conducted in order to investigate the flow inside the combustion chamber of the stove under variation of different geometrical parameters. Beyond the performance improvement a further reason of the investigations was to rate the effects of manufacturing tolerance problems. In this paper the numerical investigation of a plant oil stove by means of RANS simulation will be presented. In or- der to reduce the computational costs different model reduction steps were necessary. The simulation results of the basic configuration compare very well with experimental measurements and problematic behaviors of the ac- tual stove design could be explained by the investigation.

Numerical Investigation of High Pressure and High Reynolds Diffusion Flame Using Large Eddy Simulation

Today,with nonstop improvement in computational power,Large-Eddy Simulation(LES) is a high demanding research tool for predicting engineering flows.Such flows on high pressure condition like diesel engines is extensively employed in ground and marine transportation,oblige the designer to control and predict toxic pollutants,while maintaining or improving their high thermal efficiency.This becomes one of the main challenging issues in decades.In the present work,numerical investigation of diffusion flame dynamics is performed in the near-field of high-Reynolds jet flow on high pressure condition encountered in diesel engine applications.This work discusses the implementation of Partially Stirred Reactor(PaSR) combustion model by the approaches of large eddy simulation(LES).The simulation results show that LES,in comparison with Reynolds-Averaged Navier-Stokes(RANS) simulation predicts and captures transient phenomena very well.These phenomena such as unsteadiness and curvature are inherent in the near-field of high Reynolds diffusion flame.The outcomes of this research are compared and validated by other researchers' results.Detailed comparisons of the statistics show good agreement with the corresponding experiments.

Simplified Numerical Study of Evaporation Processes Inside Vertical Tubes

The paper presents a simplified numerical model of evaporation processes inside vertical tubes.In this model only the temperature fields in the fluid domain(the liquid or two-phase mixture) and solid domain(a tube wall) are determined.Therefore its performance and efficiency is high.The analytical formulas,which allow calculating the pressure drop and the distribution of heat transfer coefficient along the tube length,are used in this model.The energy equation for the fluid domain is solved with the Control Volume Method and for the solid domain with the Finite Element Method in order to determine the temperature field for the fluid and solid domains.