CFX学习案例：烟囱排入大气流场计算
本案例将学习以下内容：
LIBRARY: &replace ADDITIONAL VARIABLE: smoke Option = Definition Tensor Type = SCALAR Units = [kg m^-3] Variable Type = Volumetric END END
首先对稳态数值计算进行设置，计算域的设置包含附加变量的相关设置：
标签 |
设置 |
值 |
Basic Settings |
Location and Type> Location |
B1.P3 |
Fluid and Particle Definitions |
Fluid 1 |
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0 [atm] |
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Heat Transfer > Option |
None |
| Additional Variable Models > Additional Variable | smoke | |
| Additional Variable Models > Additional Variable > smoke | (Selected) | |
| Additional Variable Models > Additional Variable >smoke > Kinematic Diffusivity | (Selected) | |
| Additional Variable Models> Additional Variable > smoke > Kinematic Diffusivity > Kinematic Diffusivity | 1.0E-5[m^2 s^-1](代表烟雾在空气中运动扩散速率) |
空气进口边界条件的创建及设置：Insert > Boundary >Set Name to Wind
标签 |
设置 |
值 |
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Boundary Type |
Inlet |
Location |
Wind | |
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Mass and Momentum > Option |
Cart. Vel. Components |
Mass and Momentum > U |
1 [m s^-1] |
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Mass and Momentum > V |
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Mass and Momentum > W |
0 [m s^-1] |
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| Turbulence > Option | Intensity and Length Scale | |
| Turbulence > Fractional Intensity | 0.05 | |
| Turbulence > Eddy Length Scale | 0.25 [m] | |
| Additional Variables > smoke > Option | Value | |
| Additional Variables > smoke > Add. Var. Value | 0 [kg m^-3] |
大气开放边界条件的创建及设置：Insert > Boundary >Set Name to Atmosphere。
标签 |
设置 |
值 |
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Boundary Type |
Opening |
Location |
Atmosphere | |
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Mass and Momentum > Option |
Opening Pres. and Dirn |
| Mass and Momentum > Relative Pressure | 0 [Pa] |
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Flow Direction > Option |
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| Turbulence > Fractional Intensity | 0.05 | |
| Turbulence > Eddy Length Scale | 0.25 [m] | |
| Additional Variables > smoke > Option | Value | |
| Additional Variables > smoke > Add. Var. Value | 0 [kg m^-3] |
烟雾进口边界条件的创建及设置：Insert > Boundary >Set Name to Vent。
标签 |
设置 |
值 |
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Boundary Type |
Inlet |
Location |
Vent | |
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Mass And Momentum > Normal Speed |
0.01 [m s^-1] |
| Turbulence > Option | Turbulence > Option | Intensity and Eddy Viscosity Ratio |
| Turbulence > Fractional Intensity | 0.05 | |
| Additional Variables > smoke > Option | Value | |
| Additional Variables > smoke > Add. Var. Value | 0 [kg m^-3] |
初始值按照默认给定即可。
求解控制的设置：
标签 |
设置 |
值 |
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Advection Scheme > Option |
High Resolution |
Convergence Control > Max. Iterations |
75 |
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Convergence Control > Fluid Timescale Control >Timescale Control |
Auto Timescale |
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Convergence Control > Fluid Timescale Control > Timescale Factor |
1.0 |
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Convergence Criteria > Residual Type |
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Convergence Criteria > Residual Target |
通过上述步骤完成了稳态的计算设置，接下来将稳态计算的设置复制一个，进行修改完成对瞬态计算的设置。
瞬态计算总时间为30 s，时间步长设置为4*0.25, 2*0.5, 2*1, 13*2 s，这样设置是给定了变化的时间步长，即第一秒的时间步长为0.25 s，第二秒的时间步长为0.5 s，以此类推。为方便更改，先创建表达式ExpFunction对烟雾进口（Vent）烟雾的流量进行定义。
LIBRARY: CEL: &replace EXPRESSIONS: ExpFunction = FinalConcentration*abs(1-exp(-t/TimeConstant)) FinalConcentration = 1 [kg m^-3] TimeConstant = 3 [s] END END END
在烟雾进口（Vent）边界条件中，将流速更改为0.2 [m s^-1]，并将添加的附加变量smoke进行更改，Additional Variables > smoke > Add. Var. Value > ExpFunction。
在求解设置中，将最大迭代次数设置为3，即Convergence Control > Max. Coeff. Loops to 3。
在输出控制中，在瞬态结果中设置两个瞬态结果输出，在输出变量中给定Pressure,Velocity,smoke，第一个瞬态结果输出（Transient Results 1）中给定时间列表（Time List）为1, 2, 3 s，第二个瞬态结果输出（Transient Results 2）中给定时间间隔（Time Interva）为4 s。
基于以上设置完成了对稳态与瞬态计算的设置，接下来对计算的配置（Configuration ）进行设置。分别创建两个计算配置，即Simulation Control > Configurations and select Insert > Configuration > Set Name to Steady State/Transient。在Steady State中，稳态布局对应稳态计算的设置部分，即Flow Analysis选择Steady State Analysis，Activation Conditions > Activation Condition 1 > Option选择Start of Simulation；在Transient中，瞬态布局对应瞬态计算的设置部分，并设置接着稳态计算结束后计算，即Flow Analysis选择Transient Analysis，Activation Conditions > Activation Condition 1 > Option选择End of Simulation，Activation Conditions > Activation Condition 1 > Configuration Name选择Steady State，并在Initial Values中选择稳态计算的结果作为初始值。设置完成后输出计算文件(CircVent.mdef)并开始计算即可。
计算结果：
