轴对称湍流射流流场的数值模拟

借助商业软件 CFX4.4,对 Laval 喷管外的射流流场进行数值模拟,分析讨论了不同滞止压力、环境温度对湍流射流轴中心线上流场的影响,对比以往模拟结果和实际现场氧枪操作曲线验证了是k-ε湍流模型在描述湍流射流流场方面的适用性.     

环境温度对超音速氧气射流特性影响的数值模拟

通过数值模拟的方法研究了不同环境温度条件下超音速氧气射流的特性,并与前人的实验结果进行了对比分析.研究结果表明:与低温环境条件相比,高温环境条件下超音速氧气射流的速度衰减受到抑制,射流核心段长度得到延长;不同环境温度条件下,氧气射流的温度随着氧气射流的扩散不断升高,最终趋于环境温度;射流的压力分布趋势与射流速度分布趋势一致.数值模拟得到的射流速度、温度和压力结果与实测值吻合度较高.      

不同湍流模型在中间包钢液流动数值模拟中的适用性研究

摘要：连铸中间包内部结构复杂,钢液流动状态多样,详尽准确的钢液流场信息是中间包控制和优化的前提。数值模拟方法已广泛应用于中间包内钢液流场研究,钢液流场的精确数值模拟离不开合适的湍流模型及相应的边界条件。基于CFD开源代码包OpenFOAM v8,分别应用标准k-ε模型、RNG k-ε模型、SST k-ω模型3种湍流模型,对称面边界、自由滑移边界2种液面边界条件,对中间包流场进行了数值模拟。通过与文献中实验结果比较,发现采用SST k-ω模型可以成功预测RTD曲线的“双峰”,且响应时间、峰值时间与实验结果较为接近;应用SST k-ω模型时,将液面边界类型由自由滑移改为对称面,获得的示踪剂响应时间的误差由93.89%降低至8.35%以下,峰值时间的误差由100.78%降低至12.32%左右。因此,SST k-ω模型、对称面液面边界可以较好地描述中间包内钢液流动过程。     

单孔氧枪喷头射流流场的仿真研究

借助商业软件CFX4．4对单孔氧枪Laval喷管内及喷管外的射流场进行数值模拟，分析了不同滞止压力下喷管外的轴对称射流流场的分布情况，并验证了k-ε湍流模型的适用性。结果表明，300K环境温度下，射流的径向横截面面积受轴向至喷口的距离影响较大，而在不同滞止压力下（0．596—0．996MPa）射流沿纵向扩展变化不大；在相同滞止压力下，随环境温度增加，射流径向扩展明显加大。关键词单孔氧枪Laval喷管轴对称湍流射流数值模拟     

组合桨液相搅拌槽内流动特性的实验研究及数值模拟

采用粒子图像测速技术(PIV),对直径为0.19 m的三层组合桨(HEDT+2WH)搅拌槽(直径为0.48 m)内的流场进行了实验研究,并利用标准k-ε模型对相应的流动特性进行了数值模拟.实验结果表明:通过改变层间距、顶层桨的浸没深度及上两层桨的操作方式可以得到4种不同流型,每种流型内循环结构的数目各不相同;上两层桨下压式操作时,流场的循环结构最少,只有两个;高速区和高能量区的分布相同,都位于各个桨叶的射流区内,且底桨射流区内的速度值和湍流动能值都大于上两层桨.模拟结果表明:标准k-ε模型对流场的预测较为准确,但对于有5个循环结构的流型模拟误差较大;湍流动能分布型式的模拟值与PⅣ实验结果吻合较好,但数值偏低,表明标准k-ε模型在预测复杂流型时需要改进;功率准数的模拟值与实验值基本一致.     

狭缝阵列射流流动数值模拟及试验研究

采用Realizable k-ε湍流模型对狭缝阵列射流状态进行数值模拟,并通过试验研究狭缝阵列射流规律,将计算结果与试验结果进行比较分析。研究结果表明:吸风口将对狭缝射流速度方向产生偏移影响,射流距离超过100 mm后速度衰减显著增大,狭缝阵列平均射流速度计算值与试验值偏差较小。     

湍流模型对结晶器内流场数值模拟结果的影响

以Fluent 6.3为计算平台,采用数值模拟的方法研究了5种常用湍流模型对电磁制动下连铸板坯结晶器内钢液流动计算结果的影响,并与水模型和实测结果进行了对比.结果表明:标准κ-ε和低雷诺数κ-ε模型的计算结果与实际流场较为接近,而重整化群κ-ε和剪切应力输运κ-ω模型与实际流场相差较远.不同湍流模型计算得到的弯月面形状大致相同,但标准κ-ε模型与实测结果更为接近.Spalan-Allmaras 模型所需内存最小,花费时间最短;剪切应力输运κ-ω模型花费时间最长,其他模型所需内存大致相当.     

150t钢包自由表面旋涡的数值模拟与湍流模型的选择

钢包在非稳态浇注过程中易形成旋涡,导致钢渣及空气的大量卷入,严重危害了钢的质量.为了更好的研究旋涡,采用ANSYS CFX软件,分别采用k-ε和RNG k-ε2种湍流模型对钢包非稳态浇注过程中自由表面旋涡进行数值模拟,得到自由表面从表面旋转到旋涡贯通水口的演化过程,将两者的计算结果与相关文献以及Burgers涡模型进行相互对比,得到:两者计算的旋涡演化过程、旋涡产生临界高度无差别;两者的速度场与相关文献均一致,但RNG k-ε模型更真实的表现了旋涡的剪切流动;将两者的切向速度分布与Burgers涡对比,得到RNG k-ε湍流模型与理论模型更加吻合.综上:用RNG k-ε湍流模型计算自由表面旋涡更加正确合理.     

150 t钢包自由表面旋涡的数值模拟与湍流模型的选择

钢包在非稳态浇注过程中易形成旋涡,导致钢渣及空气的大量卷入,严重危害了钢的质量.为了更好的研究旋涡,采用ANSYS CFX软件,分别采用k-ε和RNG k-ε 2种湍流模型对钢包非稳态浇注过程中自由表面旋涡进行数值模拟,得到自由表面从表面旋转到旋涡贯通水口的演化过程,将两者的计算结果与相关文献以及Burgers涡模型进行相互对比,得到:两者计算的旋涡演化过程、旋涡产生临界高度无差别;两者的速度场与相关文献均一致,但RNG k-ε模型更真实的表现了旋涡的剪切流动;将两者的切向速度分布与Burgers涡对比,得到RNG k-ε湍流模型与理论模型更加吻合.综上:用RNG k-ε湍流模型计算自由表面旋涡更加正确合理.      

单孔射流冲击流动与换热过程的数值模拟

应用三种RNG k-ε湍流模型对单孔气体射流冲击流动与换热过程进行了数值仿真计算,与实验结果的比较分析表明;RNG k-e系列模型能够对射流冲击流动和换热过程进行较为准确的预测.在此基础上,进一步对局部Nu数、平均Nu数分布的分析表明:喷孔直径D对冲击点处Nu的数值大小无明显影响;射流冲击高度HI/D对Nu数的分布规律...     

Study on applicability of different turbulence models in simulation of molten steel flow in tundish

The internal structure of continuous casting tundish is complex, and the flow state of molten steel is diverse. Detailed and accurate information of molten steel flow field is the premise of tundish control and optimization. Numerical simulation method has been widely used in the study of molten steel flow field in tundish. The accurate numerical simulation of molten steel flow field is inseparable from the appropriate turbulence model and the corresponding boundary conditions. Based on the CFD open source code OpenFOAM v8, three different types of turbulence models (standard k-ε model, RNG k-ε model, and SST k-ω model) were applied to simulate the molten steel flow in the tundish. Additionally, two boundary conditions of symmetry plane and the free slip were also applied. Comparing the simulation results to the experimental data, it shows the RTD curve obtained by the simulation with the SST k-ω model can successfully predict the "double peaks" that appeared in the experiment. Besides, the response time and peak value time are closest to that of experimental results. For SST k-ω, when changing the type of liquid surface boundary from free slip to symmetrical plane, the error of tracer response time obtained is reduced from 93.89% to less than 8.35%, and the error of peak time is reduced from 100.78% to about 12.32%. It can be concluded that the SST k-ω model and the symmetry boundary are applicable for the simulation of molten steel flow in the tundish.     

Numerical Simulation on Flow Control for Drag Reduction of Revolution Body Using Dimpled Surface

Numerical simulation on the flow fields near the dimpled and the smooth revolution bodies are performed and compared by using SST k-ω turbulence model, to explain the reasons of friction and base drag reductions on the bionic dim-pled surface and the control behaviors of dimpled surface to boundary layer near wall of the revolution body. The simulation results show that the dimpled surface reduces the skin friction drag through reducing the velocity gradient and turbulent in-tensity , and reduces the base drag through weakening the pumping action on the flow behind the revolution body caused by the external flow; the low speed rotating vortexes in the dimples segregate the external flow and the revolution body; and the low speed rotating vortexes forming in the bottom of dimples can produce negative skin friction.     

Numerical Solution of Fully Developed Flow with Vegetative Resistance

This paper presents a numerical solution of the Reynolds averaged Navier-Stokes and the near-wall k- (turbulent kinetic energy) and ω- (specific dissipation or dissipation per unit kinetic energy) transport equations, which are modified to include vegetative drag terms. For similar treatment of the model coefficients, the use of the near-wall k-ω model produces similar results to previous models that employed the standard k-ε models with wall functions. The study shows that reasonable predictions of streamwise velocity and Reynolds stress profiles can be achieved by adopting universal values for all model coefficients, but the calculated energy gradient can have significant error. The study also indicates that predictions of streamwise turbulence intensity are significantly improved by adopting the universal values of Cfk = 0.05 and Cfω = 0.16 rather than the theoretically based values, Cfk = 1.0 and Cfω = β/αβ?Cfk.     

Role of Turbulence Model in Prediction of Pump-Bay Vortices

A numerical model was developed by the writers to simulate the flow in a water-pump intake bay. Among the flow features of interest are subsurface vortices attached to the walls of the channel, and free-surface vortices. Such vortices are observed in laboratory models. This technical note is concerned with the role of the turbulence model and influence of wall roughness in the prediction of location, size and strength of different types of vortices, and the level of swirl in the intake pipe. Calculations made with the k-ε and the k-ω models were found to predict vortices of similar shape and size, but their locations and strength depended on the turbulence model and treatment of the near-wall flow. Weaker vortices were predicted when wall roughness was taken into account, raising the possibility of using roughness as a vortex suppression device.     

狭缝射流冲击柱状凸形表面流动换热特性

采用数值方法研究了狭缝射流冲击柱状凸形表面的流动换热特性,通过四种湍流模型计算结果与实验数据对比,确定了湍流模型适用性.以压力梯度分布为依据,重点分析了狭缝射流沿柱状凸形表面的流动结构和边界层分离特点及柱状凸形表面的强化换热特性.结果表明:RNG k-ε和Realizable k-ε模型具有预测适应性;狭缝射流冲击至柱状凸形表面,气体沿表面运动,速度降低,并在流动下游发生边界层分离;量纲一的逆压梯度随量纲一的曲率半径(D/B)的减小而增大,使得边界层分离更早出现;驻点区域换热Nu随量纲一的曲率半径(D/B)的减小而获得增强,但流动进入下游后,D/B对换热基本无影响;压力梯度是影响狭缝射流冲击柱状凸形表面换热分布的重要因素.      

Mean Flow and Turbulence Structure of Open-Channel Flow through Non-Emergent Vegetation

The ability of turbulence models, based on two equation closure schemes (the k-ε and the k-ω formulations) to compute the mean flow and turbulence structure in open channels with rigid, nonemergent vegetation is analyzed. The procedure, developed by Raupach and Shaw (1982), for atmospheric flows over plant canopies is used to transform the 3D problem into a more tractable 1D framework by averaging the conservation laws over space and time. With this methodology, form∕drag related terms arise as a consequence of the averaging procedure, and do not need to be introduced artificially in the governing equations. This approach resolves the apparent ambiguity in previously reported values of the drag-related weighting coefficients in the equations for the turbulent kinetic energy and dissipation rates. The working hypothesis for the numerical models is that the flux gradient approximation applies to spatial∕temporal averaged conservation laws, so that the eddy viscosity concept can be used. Numerical results are compared against experimental observations, including mean velocities, turbulence intensities, Reynolds stresses, and different terms in the turbulent kinetic energy budget. The models are used to further estimate vegetation-induced flow resistance. In agreement with field observations, Manning's coefficient is almost uniform for some critical plant density and then increases linearly.     

Validation of a Large-Eddy Simulation Model to Simulate Flow in Pump Intakes of Realistic Geometry

This paper describes efforts toward developing a reliable numerical model to predict pump intake flow and associated vortices. Numerical prediction of these flows characterized by the formation of unsteady (meandering) intermittent vortices and presence of massive separation is very challenging. Successful prediction of these phenomena and their effects on the mean flow fields requires numerical methods and turbulence models that can accurately capture the dynamics of the main coherent structures in these flows. In the present work, large-eddy simulation (LES) in conjunction with an accurate nondissipative nonhydrostatic Navier-Stokes massively parallel solver is used to predict the flow and vortical structures in a pressurized pump intake of complex geometry. The LES model is validated using particle image velocimetry data recently collected on a laboratory model of a realistic geometry pump intake. To better put in perspective the predictive performance of the LES model, results from steady simulations employing the shear stress transport (SST) Reynolds-averaged-Navier-Stokes (RANS) model are presented and compared with LES. It is shown that even if SST can fairly successfully capture the mean velocity distribution and mean vortical structures in some regions, overall LES can more accurately predict the mean flow and turbulence statistics compared to the steady SST model.     

超音速氧枪射流特性的数值模拟

利用GAMBIT建立了轴向计算长度2 200 mm和径向计算长度800 mm的超音速氧枪的数学模型,并采用FLUENT软件对氧枪射流特性进行数值仿真研究。分析了单孔氧枪超音速射流特性,以及操作压力(0.6~1.0 MPa)和环境温度(298~1 873 K)对流动特性的影响。结果表明,入口滞止压力在设计压力±25%内对射流轴向衰减及径向扩展影响不大,其与射流的超音速区长度呈二次曲线关系变化,随环境温度升高,射流轴向衰减变缓慢,核心区长度增加,超音速区长度和环境温度呈线性关系,环境温度对射流径向影响很小。