弯曲管段内流动的大涡模拟

本文应用湍流大涡模拟方法对三维空间的弯管流进行了数值模拟。计算中使用了隐含流线迎风耗散作用的具有三阶部分展开的 Taylor_ Galerkin离散格式 ,导出了经 Gauss滤波后的基本方程组的有限元列式。数值计算结果与实验结果符合较好 。

泥浆立管系统振动仿真分析

泥浆立管在工作时会产生较大振动,分析了引起振动的主要原因。采用有限元方法计算了立管系统的固有频率,并使用CFD方法分析了泥浆自身脉动激励现象。通过对比计算,确定了大涡湍流模拟方法适用于流体脉动现象仿真。计算结果表明:泥浆自身脉动激励不是立管振动的主要因素,同时提出了有效遏制立管振动的方法和建议。

脊状表面降噪特性研究

采用大涡模拟湍流模型(LES)仿真计算V型脊状面的湍流边界层流噪声,研究了V型脊状结构高宽比对降噪特性的影响。计算结果表明,大高宽比脊状结构降噪效果更加突出,其高频区的声压密度及峰值处的频率较其他两种尺寸都有明显减少,其中声压级最大减小量5dB。通过比较其上部区域流场的涡量及湍流脉动强度,发现如果脊峰更加"尖削",则脊状表面抑制涡发展、"稳定"湍流脉动的能力增强,而这就是其获得更好降噪效果的主要原因之一。

NUMERICAL SIMULATION AND ANALYSIS OF HORSESHOE-SHAPED VORTEX IN NEAR-WALL REGION OF TURBULENT BOUNDARY LAYER

The low-Reynolds-number full developed turbulent flow in channels is simulated using large eddy simulation（LES）method with the preconditioned algorithm and the dynamic subgrid-scale model,with a given disturbance in inlet boundary,after a short development section.The inlet Reynolds number based on momentum thickness is 670.The computed results show good agreement with direct numerical simulation（DNS）,which include root mean square fluctuated velocity distribution and average velocity distribution.It is also found that the staggered phenomenon of the coherent structures is caused by sub-harmonic.The results clearly show the formation and evolution of horseshoe vortex in the turbulent boundary layer,including horseshoe vortex structure with a pair of streamwise vortexes and one-side leg of horseshoe vortex.Based on the results,the development of the horseshoe-shaped coherent structures is analyzed in turbulent boundary layer.

Large eddy simulation of the rotation effect on the ocean turbulence kinetic energy budget in the surface mixed layer

A non-hydrostatic, Boussinesq, and three-dimensional large eddy simulation(LES) model was used to study the impact of the Earth's rotation on turbulence and the redistribution of energy in turbulence kinetic energy(TKE) budget. A set of numerical simulations was conducted,(1) with and without rotation,(2) at different latitudes(10°N, 30°N, 45°N, 60°N, and 80°N),(3) with wave breaking and with Langmuir circulation, and(4) under different wind speeds(5, 10, 20, and 30 m/s). The results show that eddy viscosity decreases when rotation is included, indicating that rotation weakens the turbulence strength. The TKE budget become tight with rotation and the effects of rotation grow with latitude. However, rotation become less important under Langmuir circulation since the transport term is strong in the vertical direction. Finally, simulations were conducted based on field data from the Boundary Layer and Air-Sea Transfer Low Wind(CBLAST-Low) experiment. The results, although more complex, are consistent with the results obtained from earlier simulations using ideal numerical conditions.

Large eddy simulation of a particle—laden turbulent plane jet

Gas solid two-phase turbulent plane jet is applied to many natural s it uations and in engineering systems. To predict the particle dispersion in the ga s jet is of great importance in industrial applications and in the designing of engineering systems. A large eddy simulation of the two-phase plane jet was con d ucted to investigate the particle dispersion patterns. The particles with Stokes numbers equal to 0 0028, 0 3, 2 5, 28 (corresponding to particle diameter 1 μm , 10 μm, 30 μm, 100 μm, respectively) in \%Re\%=11 300 gas flow were studied. The simulation results of gas phase motion agreed well with previous experimental re sults. And the simulation results of the solid particles motion showed that part icles with different Stokes number have different spatial dispersion; and that p articles with intermediate Stokes number have the largest dispersion ratio.

Modeling Abrasion Forces in a Pneumatically Powered Grinding Tool Using Compressible Large Eddy Simulation

This paper describes the design of a new kind of miniature abrading sphere, which is magnetically mounted inside a spherical gap and set in rotation pneumatically with air. Large eddy simulation is performed in conjunction with the compressible Smagorinsky model. Minimal temperature variation allows for the assumption of adiabatic walls. Fluid-solid interaction is modeled using the law of the wall for compressible turbulent flow. A parametric study is done to determine optimal geometric layout while taking physical restrictions into account. The resulting optimal configuration is then examined in detail in order to determine demands to be met by the computerized control of the magnetic bearing as well as to quantify the force available to the abrasion process. Finally, a mathematical relation is given that determines available abrasion force depending on standard volumetric flow rate and rotation frequency. The findings presented here provide a basis for further development of smaller versions of the tool.

Large eddy simulation and proper orthogonal decomposition analysis of turbulent flows in a direct injection spark ignition engine:Cyclic variation and effect of valve lift

Large eddy simulation(LES)is used to calculate the in-cylinder turbulent flow field in a direct injection spark ignition(DISI)engine.The computations are carried out for three different maximum valve lifts(MVL)and throughout 100 consecutive engine cycles.The simulated results as well as corresponding particle image velocimetry(PIV)measurement database are analyzed by the proper orthogonal decomposition(POD)method.Through a new developed POD quadruple decomposition the instantaneous in-cylinder flow fields are decomposed into four parts,named mean field,coherent field,transition field and turbulent field,respectively.Then the in-cylinder turbulent flow characteristics and cycle-to-cycle variations(CCV)are studied separately upon the four part flow fields.Results indicate that each part exhibits its specific characteristics and has close connection with others.The mean part contains more than 50%of the total kinetic energy and the energy cascade phenomenon occurs among the four part fields;the coherent field part possesses the highest CCV level which dominates CCV of the bulk flow.In addition,it is observed that a change in MVL affects significantly the in-cylinder flow behavior including CCV,especially for the coherent part.Furthermore,the POD analysis demonstrates that at least 25 sample cycles for the mean velocity and 50 sample cycles for the RMS velocity are necessary for obtaining converged and correct results in CCV.

Large eddy simulation of unconfined turbulent swirling flow

Large eddy simulations(LES) were performed to study the non-reacting flow fields of a Cambridge swirl burner. The dynamic Smagorinsky eddy viscosity model is used as the sub-grid scale turbulence model. Comparisons of experimental data show that the LES results are capable of predicting mean and root-mean-square velocity profiles. The LES results show that the annular swirling flow has a minor impact on the formation of the bluff-body recirculation zone. The vortex structures near the shear layers, visualized by the iso-surface of Q-criterion, display ring structures in non-swirling flow and helical structures in swirling flow near the burner exit. Spectral analysis was employed to predict the occurrence of flow oscillations induced by vortex shedding and precessing vortex core(PVC). In order to extract accurately the unsteady large-scale structures in swirling flow, a three-dimensional proper orthogonal decomposition(POD) method was developed to reconstruct turbulent fluctuating velocity fields. POD analysis reveals that flow fields contain co-existing helical and toroidal shaped coherent structures. The helical structure associated with the PVC is the most energetic dynamic flow structure. The latter toroidal structure associated with vortex shedding has lower energy content which indicates that it is a secondary structure.

Large eddy simulation of a 3-D spatially developing turbulent round jet

A three-dimensional large eddy simulation （LES） of a spatially developing round jet is carried out in cylindrical coordinates using a dynamic subgrid model with strong inflow instability. Evolutions of large-scale vortex structures represented by tangential vortices are obtained and compared with flow visualization. Also presented are three-dimensional spatial evolutions of coherent structure, which are of quasi two-dimensional Kelvin-Helmholtz instability and vortex rings as well as breaking up of the vortex rings with fully three-dimensional characteristics. Predicted results of mean velocity and turbulent intensity agree well with experiments. They are also compared with the results predicted by LES using standard Smagorinsky model and show good self-similarity. Turbulence spectrum of the predicted velocity shows the -5/3 decay for higher wave number, as expected for turbulent round jet flows. In addition, fl-test and y-test are carded out for the instantaneous velocity, showing that the present LES method can successfully predict the hierarchical structure of round jet.

Calibration of a new very large eddy simulation(VLES)methodology for turbulent flow simulation

Following the idea of Speziale＇s Very Large Eddy Simulation （VLES） method, a new unified hybrid simulation approach was proposed which can change seamlessly from RANS （Reynolds-Averaged Navier-Stokes） to LES （Large Eddy Simulation） method depending on the numerical resolution. The model constants were calibrated in accordance with other hybrid methods. Besides being able to approach the two limits of RANS and LES, the new model also provides a proper VLES mode between the two limits, and thus can be used for a wide range of mesh resolutions. Also RANS simulation can be recovered near the wall which is similar to the Detached Eddy Simulation （DES） concept. This new methodology was implemented into Wilcox＇s κ- ω model and applications were conducted for fully developed turbulent channel flow at ReT = 395 and turbulent flow past a square cylinder at Re = 22000. Results were compared with LES predictions and other studies. The new method is found to be quite efficient in resolving large flow structures, and can predict satisfactory results on relative coarse mesh.

A Variational Finite Element Model for Large-Eddy Simulations of Turbulent Flows

The authors introduce a new Large Eddy Simulation model in a channel, based on the projection on finite element spaces as filtering operation in its variationM form, for a given triangulation （Th）h〉0. The eddy viscosity is expressed in terms of the friction velocity in the boundary layer due to the wall, and is of a standard sub grid-model form outside the boundary layer. The mixing length scale is locally equal to the grid size. The computational domain is the channel without the linear sub-layer of the boundary layer. The no-slip boundary condition （or BC for short） is replaced by a Navier （BC） at the computational wall. Considering the steady state case, the authors show that the variational finite element model they have introduced, has a solution （Vh,Ph）h〉O that converges to a solution of the steady state Navier-Stokes equation with Navier BC.

Large Eddy Simulation of Micro-Particles Transport with Different Mass Flow Rate in Turbulent Planar Jet Flow

The motion of micro-particles with different mass flow rate in the planer turbulent jet flow has been simulated, using LES method to obtain the flow vorticity evolution and Lagrangian method to track micro-particles. The re- suits showed that when the flow rate is small, the particles more likely to present in the vortex periphery, the dis- tribution pattern is similar to the flow pattern. When the flow rate is high, some particles will escape from the mo- tion region to the original static region, so that in the jet region, particles are relatively evenly distributed. When the flow field is full developed, the particles average concentration in the y direction affected by the mass flow rate relative slightly, the normalized mean particles concentrations at different flow rate were similar to Gaussian shape.