# 8.1.5. Boundary Conditions#

Boundary conditions are of crucial importance in CFD as they, in addition to the initial condition, ensure that the problem is well posed, which means that the solution of the system of partial differential equations (PDEs) is unique and consistent. The numerical boundary conditions must ensure that the interface between the domain interior and exterior resembles the physical phenomena, without adversely affecting the solution stability. The following section provides guidance for specifying boundary conditions in Flow360.

## 8.1.5.1. Expressions as Inputs#

For a number of boundary conditions such as NoSlipWall, IsothermalWall and Freestream, a mathematical expression can be used to define the velocity field (Velocity). Expressions define a parameter based on component-wise independent variables [x,y,z]. The result will be evaluated and returned as the value of the parameter. The allowed operators to define an expression can be found here.

## 8.1.5.2. Specifying Turbulence Quantities#

Turbulence conditions can be specified with some boundary condition types. They can also be specified as the initial condition for the simulation. The types of turbulence quantities that can be specified depends on the turbulence model being used. Valid combinations of turbulence property specifications are listed in the following table.

Valid combination

Description

Default (no specification)

The default turbulence depends on the turbulence model. For SA model without transition model this is equivalent to set modifiedTurbulentViscosityRatio = 3.0 (or effectively turbulentViscosityRatio = 0.210438). For SA model with transition model, modifiedTurbulentViscosityRatio = 0.1 (or effectively turbulentViscosityRatio = 2.794e-7). For SST model the default turbulence is turbulentViscosityRatio = 0.01 with default specificDissipationRate = $$MachRef/L_{box}$$ where $$L_{box} \triangleq exp\left(\displaystyle\sum_{i=1}^{3}log(x_{i,max}-x_{i,min}\right)$$. $$x_{i,max},x_{i,min}$$ is the bounding box dimension for wall boundaries.

turbulentViscosityRatio alone

This applies to both SST and SA model. For SST model this is effectively an override of the above default turbulentViscosityRatio value while keeping the default specificDissipationRate. For SA model the turbulentViscosityRatio will be converted to the modifiedTurbulentViscosityRatio.

turbulentKineticEnergy (or turbulentIntensity) alone

For SST model only. specificDissipationRate will be set to the default value.

turbulentLengthScale alone

For SST model only. specificDissipationRate will be set to the default value.

modifiedTurbulentViscosity

For SA model only.

modifiedTurbulentViscosityRatio

For SA model only.

turbulentKineticEnergy (or turbulentIntensity) with specificDissipationRate

For SST model only.

turbulentKineticEnergy (or turbulentIntensity) with turbulentViscosityRatio

For SST model only.

turbulentKineticEnergy (or turbulentIntensity) with turbulentLengthScale

For SST model only.

specificDissipationRate with turbulentViscosityRatio

For SST model only.

specificDissipationRate with turbulentLengthScale

For SST model only.

turbulentViscosityRatio with turbulentLengthScale

For SST model only.