One of the most important steps in solving fluid flow problems using CFD is selecting and setting the correct boundary conditions that accurately represent what is happening in real-life. Boundary conditions provide information to the solver on the behavior of the fluid at openings and surfaces in the computational domain (or as the name suggests, at the boundaries). In this blog post, we briefly review some of the commonly applied boundary conditions in CFD.
Common Types of Boundary Conditions
1. Inlet Boundary Condition
An Inlet boundary condition specifies how the fluid enters the computational domain. The computational domain can have multiple inlets of various types and combinations. An inlet boundary condition can be specified using either an inlet velocity, inlet mass flow rate, or inlet pressure.
- Velocity Inlet: The user prescribes the velocity of the fluid entering the computational domain. This is the most common type of inlet condition for incompressible flows. Either a uniform velocity profile or a varying velocity profile both in space and/or time can be specified. An example of time varying inlet boundary condition is for simulating blood flow through blood vessels, where a time-varying velocity inlet profile can be prescribed to imitate the behavior of a heart pumping out blood. An example of a spatially varying velocity inlet profile is an atmospheric boundary layer profile to simulate wind turbine flows or flows around city buildings.
- Mass Flow Inlet: The user prescribes a mass flow rate of the fluid entering the domain. This is usually used for compressible flows.
- Pressure Inlet: The user prescribes the pressure at the inlet and the solver automatically calculates the velocity based on the prescribed pressure value. This can be used for both compressible and incompressible flows.
2. Outlet Boundary Condition
Outlet boundary conditions specify how the fluid leaves the domain. The most common types are as follows.
- Pressure Outlet: The user prescribes the static pressure at the outlet. The solver automatically calculates the other flow properties depending on the pressure value. This can be suitable for both incompressible and compressible flows.
- Outflow: This condition is used when the details of the flow velocity and pressure leaving the domain are not known a priori. This assumes that all flow variables have fully developed at the outlet. Here, the normal gradient for all flow variables except for pressure is set to 0. Care should be taken when using this boundary condition to ensure flow is fully developed close to the outflow boundary, or a non-physical solution will be predicted.
- Mass Flow Outlet: This is similar to the mass flow inlet, where the user prescribes the mass flow rate leaving the domain. This is often applied in compressible flow simulations.
3. Wall Boundary Condition
The wall boundaries define how the fluid interacts with solid surfaces in the computational domain. The common wall boundaries are as follows.
- No-Slip Condition: The no-slip boundary condition is important to capture viscous effects of the fluid on the wall and to account for boundary layers over the wall surface. This condition specifies the tangential fluid velocity to be zero on the wall surface (or same as the wall velocity in case of a moving wall), and the normal fluid velocity to be zero. For example, in case of flow over a stationary flat plate where the no-slip condition is applied, all components of fluid velocity on all computational nodes on the flat plate will be set to zero. This is crucial to accurately estimate shear stresses on the surface, ultimately leading to accurate force prediction on the surfaces. For example, drag force over a wing or a car.
- Slip Condition: In this condition, the fluid is allowed to ‘slip’ over the wall. This essentially means that the walls are treated as frictionless, and hence no boundary layer will develop over the wall. In slip boundaries, only the normal velocity component is set to zero. This can be used when viscous forces are not important.
- Moving Wall: Certain problems involve a moving wall, such as rotating machinery or turbomachinery. Here, a moving wall boundary can be defined with a certain velocity. The moving wall can be prescribed to have either a no-slip condition (most common) or a slip condition.
4. Symmetry Boundary Condition
- Symmetry boundary condition: If the CFD computational domain has a plane of symmetry, the symmetry boundary condition can be applied on the plane of symmetry. This helps cut down on computational costs, since only half of the domain needs to be accounted for. This is similar to the slip boundary condition, where zero normal velocity and zero gradients of all other flow variables is assumed across the symmetry boundary.
- Axisymmetric boundary condition: This is used in problems that have an axis of symmetry. Here, the boundary condition is symmetric about the axis, meaning that all flow variables will have the same value at a particular radius and axial location along all rotational angles.
5. Periodic Boundary Condition
Periodic boundary condition is used when the flow pattern periodically repeats in space. For example, consider a flow in a long channel. The boundaries other than the no-slip walls can be set to periodic as the flow patterns are repeating. Other examples are flow over an infinitely long cylinder, or a flow over a wing section (airfoil with a finite span) where periodicity exists in the spanwise direction.
6. Far-Field Boundary Condition
A far-field boundary condition is used to represent flow conditions far away from the disturbance source. Here the far-field conditions, such as velocity, pressure, temperature, Mach number, etc. are specified. This boundary should be physically located far away from any disturbance source in your CFD domain. This is commonly used in external aerodynamics simulations.
Boundary conditions for sample CFD problems
Given below are some examples of typical boundary conditions that are applied to some common flow problems.
1. Flow through a duct
2. Flow over a flat plate
3. Flow through a long channel
Final Thoughts
To obtain a true representation of the fluid domain that you are simulating using CFD, it is crucial to understand the physics of the boundaries of your computational domain and properly apply boundary conditions to accurately get reliable CFD results. In this blog post, we briefly reviewed the commonly used boundary conditions in CFD. In the future we will explore some more advanced boundary conditions, such as a fan boundary condition, porous media, free surface, etc. In addition, we will dive deeper into different options and values that can be set for a turbulent inflow boundary condition, such as incoming flow turbulent intensity and turbulence length scale.
If you have questions about CFD boundary conditions, or anything else CFD related for that matter, don’t hesitate to get in touch with our expert team!
