BPS5234A Chap.7 Boundary Conditions and Solver Settings
Boundary Conditions and Solver Settings
The bridge between reality and the computer
The course calls boundary conditions and numerical settings the bridge between physical reality and numerical prediction, and names boundary conditions as the dominant source of uncertainty in building CFD.
A well-posed problem identifies every external boundary, uses condition types consistent with the expected flow direction, sets values that represent real operating conditions, and avoids constraining the solution artificially.
Which condition goes where
Velocity inlets suit known supplies such as air-conditioning diffusers and imposed wind profiles; mass flow inlets suit compressible or mixed-flow problems.
The pressure outlet is the most widely used outlet in building CFD, and the outflow condition is generally not recommended. Walls are no-slip, with fixed temperature, fixed heat flux or coupled conduction, and their roughness must be consistent with the turbulence model and inlet profile in outdoor work.
A symmetry plane lets nothing flow through it and nothing change across it; periodic boundaries suit repeating layouts.
The approaching wind
Outdoor studies prescribe a logarithmic wind profile set by a reference speed, a reference height and a roughness length, together with turbulence quantities.
If the ground roughness in the wall function disagrees with the profile, the wind changes before it reaches the buildings.
Solving and stopping
Low-speed building flows use pressure-based solvers, segregated for robustness or coupled for speed. First-order upwind schemes start a solution; second-order schemes are recommended for final results.
A run has converged when residuals have fallen a thousandfold, the domain budgets close, and the monitored quantities hold still.
What this chapter covers
- 01
What makes a CFD problem well posed
- 02
Inlet and outlet types and when each misleads
- 03
Walls, symmetry planes and periodic boundaries
- 04
Logarithmic wind profiles and ground roughness
- 05
Pressure-based solvers and scheme order
- 06
Convergence criteria and the working-example settings
Worked example · free
Is the mass balance closed?
- 2Compute the imbalance as a percentage of the inflow.
- 1Compare it with the global balance criterion.
- 1State what to do before reporting.
Key terms
- Velocity Inlet
- A boundary that prescribes the speed and direction of incoming flow, used where the supply or wind is known.
- Pressure Outlet
- A boundary that fixes static pressure and lets the flow decide how much leaves, the most common outlet in building CFD.
- Symmetry Plane
- A boundary across which nothing flows and nothing varies, used to stand in for a wide surrounding environment.
- Logarithmic Wind Profile
- A mean wind speed that grows with the logarithm of height, set by a reference speed and a roughness length.
- Second-Order Upwind
- A discretisation scheme whose error falls with the square of cell size, recommended for final results.
- Segregated Solver
- A pressure-based solver that solves the equations one after another, robust and memory-efficient for indoor flows.
Boundary Conditions and Solver Settings FAQ
Why is a pressure outlet preferred to a velocity condition at an exhaust?
A velocity at an exhaust forces how much air leaves before the simulation has decided, which constrains the solution. A pressure outlet fixes only the pressure and allows the computed flow field to determine the outflow, which is usually the realistic situation.
Why do the urban labs use symmetry on the top and sides of the domain?
Nothing crosses a symmetry plane and nothing changes across it, which approximates a wide city without walls. They limit artificial shear and reflection provided the domain is large enough that the planes do not squeeze the flow round the buildings.
Why start with a first-order scheme if the final result must be second order?
First-order upwind is stable and robust, which helps a solution get started from a rough initial field. Its error is larger and behaves like extra diffusion, so the course reserves it for initialisation and asks for second-order results to be reported.
Why does the urban array lab switch gravity off?
The study models a neutral atmospheric boundary layer around unheated buildings, where temperature differences play no role. Without heating there is no buoyancy to represent, so gravity can be removed without losing physics.
What turbulence quantities does a wind inlet need?
Besides the mean profile, the inlet needs a description of turbulence, given in the urban lab as a 10 per cent intensity and a 20 m length scale. Those values, the wall roughness and the turbulence model must be consistent with one another.
Assessment move
For your allocated example, copy its inlet, outlet, wall, top and side conditions into a table and add one column headed why. If you cannot fill a cell in a sentence, that condition is the one to read about, because the report will need its justification.
