Quick calculators for engineers, grouped by topic. Results are initial estimates; check them against the model or measured data.
Mesh, boundary-condition and time-step parameters for a CFD model, in set-up order.
Flow regime, friction factor and pressure drop in pipes and ducts.
The Reynolds number compares the inertia of the flow with the viscosity of the fluid. Low Re: smooth flow with layers sliding past each other (laminar). High Re: strongly mixing flow (turbulent).
Re tells you whether a turbulence model is needed. It is also the input to most other estimates: friction factor, y+ and inlet turbulence.
Choose the cross-section and enter the bulk velocity and dimensions. Non-circular sections are converted to a hydraulic diameter Dh. Drag the velocity slider to see the flow change from laminar to turbulent.
Check the incompressible assumption and choose a suitable density model.
The Mach number is the ratio of the flow velocity to the speed of sound in the fluid. Ma = 1 means the flow moves as fast as sound.
At low Ma the gas density hardly changes and can be treated as constant, which keeps the model simple and fast. At high Ma the gas is noticeably compressed; ignoring this gives wrong pressure, temperature and flow rate.
Enter the largest velocity in the domain, usually at a nozzle, a narrow gap or a blade tip. Below 0.3: constant density; from 0.3 upwards: the ideal-gas model.
Near-wall cell height for a target y+, estimated from a skin-friction correlation.
Next to a solid surface (a pipe wall, a fan blade, an equipment casing) the flow slows to zero within a very thin layer. y+ measures the distance from the wall to the centre of the first mesh cell on the scale of that layer: the smaller y+, the closer the cell is to the wall.
Turbulence models treat the near-wall region in one of two ways: resolve it down to the wall (y+ ≈ 1) or estimate it with wall functions (y+ from 30 to 300). If the first cell falls in the wrong range, friction, heat transfer and separation are all predicted wrongly.
Choose y+ ≈ 1 when heat transfer or separation must be accurate; choose 30–100 when pressure drop and flow rate are the main results. Enter Δy₁ as the first layer height in your software.
Total prism layer thickness, outermost layer and transition to the core mesh.
Prism layers are thin cells stacked parallel to the surface, growing thicker away from the wall. They capture the steep velocity change inside the boundary layer, where an ordinary mesh is too coarse.
Layers that are too thin do not cover the boundary layer. Layers that grow too fast, or a last layer very different from the core cells, increase errors and make the solution harder to converge.
Take Δy₁ and δ from the y+ calculator, then adjust the growth ratio r and the number of layers N until the total thickness covers the boundary layer and the last layer is close to the core cell size. The remarks below show whether the set-up is acceptable.
Inlet k, ε and ω from the turbulence intensity and length scale.
In turbulent flow the velocity always fluctuates around its mean. The k-ε and k-ω models describe these fluctuations with k (their energy) and ε or ω (the rate at which that energy dissipates).
At an inlet the software needs to know how turbulent the incoming flow already is. Values that are too high or too low distort the mixing, heat transfer and losses downstream of the inlet.
Estimate the turbulence intensity I, the size of the fluctuations relative to the mean velocity, from the real conditions; for pipe flow, press "Use" for a suggested value. Enter k, ε and ω as the inlet boundary condition.
Time step for transient simulation and the number of steps per flow-through of the domain.
The Courant number (CFL) tells you how many mesh cells the flow crosses in one time step. CFL = 1 means exactly one cell per step.
With too large a time step the flow skips several cells per step: the solution loses detail or diverges. With too small a step the run takes longer without a better result.
Enter the largest velocity and the smallest cell in the region of interest, and choose a CFL suited to the solution method. The number of steps per flow-through helps estimate the total run time.
Physical properties of air, water and gas mixtures against temperature, as input data for simulation and design calculations.
Density, viscosity, specific heat and thermal conductivity of dry air from −50 to 1 500 °C.
Properties of flue gas, exhaust gas and fuel gas from their N₂, O₂, CO₂, H₂O, Ar, CH₄, CO and H₂ content, with the dew point.
Properties of liquid water from 0 to 100 °C, with saturation pressure and surface tension.
Technical documents — a guide to applying CFD in HVAC design and a simulation problem checklist, sent by email on request.
See documentsSend a description of the problem and the intended parameters; CFDWAYS engineers will comment on the mesh, boundary conditions and physics models.
