3D flow initial conditions

In addition to the 3D flow initial conditions defined in the Solution dialog box, which sets velocity, pressure, temperature, humidity, and tracer fluids for the whole model, you can also define values for elements related to specific geometry by creating an Initial Conditions constraint. See Initial Conditions for more information.

Initial Conditions constraints only have an effect with the Automatic or Uniform options. They are ignored when used in conjunction with the From Results in Other Directory option.

• AutomaticDefault option.For a new analysis, Automatic sets uniform values for the 3D flow variables. Initial velocities are set to 0. Ambient Conditions are used for the initial pressure, fluid temperature, humidity and other tracer fluid values. Initial condition values of turbulence quantities are computed using the mixing length turbulence model for the five first iterations of the solve.For a restart, Automatic uses the files flow.bfi and flow.fli that exist in the specified Restart Directory to initialize the fluid variables. The other solution files existing in the specified Restart Directory are used for the restart process.

• UniformSpecifies values in order to speed convergence for a steady state analysis by estimating final results, or to define fluid conditions at time zero for a transient analysis.The X Velocity, Y Velocity, and Z Velocity values defined here apply uniformly to all 3D fluid elements.Pressure and Fluid Temperature specify uniform initial values for all the 3D fluid elements. Use the Uniform option when known conditions in the flow domain differ from the ambient environment.Turbulence Characteristics list lets you select the Turbulence Characteristics modeling object to specify initial turbulence based on available data derived from experiment or standard references, or let the software compute them automatically. These values are applied uniformly to all 3D fluid elements. For more information, see Turbulence Characteristics.Humidity lets you select the method to define the initial humidity. Relative lets you define the initial humidity by specifying Relative Humidity (%) as a percentage of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a given temperature. Specific lets you define the initial humidity by specifying the Water Vapor to Dry Air Mass Ratio. These values are applied uniformly to all 3D fluid elements.The Tracer Fluid list lets you define the initial mass fractions for the tracer fluids. You can select None or Specify Fractions. If you select Specify Fractions, select the tracer fluid from the Tracer Fluid List, and in the Tracer Fluid Mass Fractions box, define their corresponding initial mass fractions in the flow volume. See Model a tracer fluid for more information.The Mixture list lets you define the initial mass fractions for the mixture. You can select None or Specify Fractions. If you select Specify Fractions, define the corresponding initial mass fractions in the flow volume. See Homogeneous Gas Mixture for more information.Water Content lets you define initial water content on all of the top/bottom thermal meshes. Using the Top Surface Water Content or Bottom Surface Water Content lists, you can define the initial water content in terms of the following options: Film Thickness lets you define the thickness of the water present on all surfaces.Total Mass lets you define the mass of the water present on all the surfaces.Surface Density lets you enter the total mass per unit area present on all surfaces.

• From Results In Other DirectorySpecifies the directory containing the solver files flow.bfi and flow.fli. The solver uses the flow results in these files to initialize the calculation field. The analysis that generated these files and the current analysis can have different meshes as the software uses interpolation to map the flow field on the new mesh. The solve-time units must be the same for both analyses. You specify these units on the Solution Units page in the Solution dialog box.You can choose any time step for the initial boundary condition distribution that is available in the specified flow solver files. It does not need to be the start time for the current solution.Use this option to start an analysis from a previous solution without doing a restart. In this way, you can add, modify or remove flow boundary conditions at the start time of the new analysis, in effect defining time dependent flow boundary conditions. This enables you to remove or add boundary conditions and still start from a previous solution.For example, on the second analysis, you can replace a fan with a vent or a flow surface. In another case, you may want to change a Flow Surface into a Flow Boundary Condition or a wall, and so on.If you do a restart and use this option, the flow.bfi and flow.fli files from the specified directory are used to initialize the fluid variables. If these two files are in the current run directory, they are ignored. The other solution files must exist in the run directory for restart to work.Note: If you run a particle tracking simulation with your flow simulation from results in another directory, the particle density distribution will be initialized to zero everywhere in the flow domain.

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