Quickstart

CFDPre exposes a single high-level function, yhgrcalc, which works for two flow regimes selected with the flow_type argument. This page shows both in code; the Theory & Methodology page explains why each number comes out the way it does.

1. Internal flow (pipe / duct)

This is the default (flow_type="internal"). You provide a mass flow rate and a hydraulic diameter, and CFDPre derives the bulk velocity and Reynolds number for you.

from cfdpre import yhgrcalc

result = yhgrcalc(
    fluid="Air",
    temperature_c=50,        # degrees Celsius
    pressure_bar=10,         # bar (absolute)
    massflow_kgpersec=2.5,   # kg/s
    hydraulicdia_mm=125,     # mm
    target_yplus=1,          # wall-resolved
    num_layers=8,
)

for key in ("reynolds number", "first layer height [yh] [m]",
            "Growth Ratio", "Final Layer Thickness [m]"):
    print(f"{key:32s}: {result[key]}")

About the growth ratio in this example

With target_yplus=1 and only 8 layers at this high Reynolds number (~1.29 × 10⁶), the growth ratio comes out around 3.66 — well above the ~1.3 maximum recommended for mesh quality. CFDPre raises a UserWarning to tell you so. In practice you would either use many more layers (20–40+) or restrict the prism stack to part of the radius (see below).

Restricting the prism stack thickness

Filling the entire pipe radius with prism layers is rarely what you actually mesh — usually you grow prisms in the near-wall region and let a coarser core mesh take over. Use bl_thickness_fraction (a fraction of the radius) or bl_thickness_mm (an absolute thickness) to control this:

# Span only 30 % of the pipe radius with the prism stack
yhgrcalc("Air", 50, 10, 2.5, 125, 1, 8, bl_thickness_fraction=0.3)   # GR ~ 3.04

# Or specify an absolute total stack thickness of 10 mm
yhgrcalc("Air", 50, 10, 2.5, 125, 1, 20, bl_thickness_mm=10)          # GR ~ 1.39

Rough walls

The turbulent internal correlation (Haaland) accepts an absolute wall roughness:

yhgrcalc("Air", 50, 10, 2.5, 125, 1, 8, roughness_mm=0.5)

2. External flow (flat plate / aerodynamic surface)

For external flows, set flow_type="external". Here hydraulicdia_mm is reinterpreted as the characteristic length along the flow (e.g. chord or plate length), and you must supply the free-stream velocity with flow_velocity_mpersec — deriving a velocity from mass flow and a pipe-area assumption is meaningless for external flow.

from cfdpre import yhgrcalc

result = yhgrcalc(
    fluid="Air",
    temperature_c=25,
    pressure_bar=1.01325,
    massflow_kgpersec=2.5,        # ignored for external flow
    hydraulicdia_mm=1000,         # characteristic length (1 m chord) in mm
    target_yplus=1,
    num_layers=15,
    flow_type="external",
    flow_velocity_mpersec=30,     # required for external flow
)

print(result["Growth Ratio"])                          # ~ 1.49
print(result["boundary layer thickness [delta99] [m]"]) # ~ 0.0210 m

You can still override the correlation-based boundary-layer thickness with bl_thickness_mm if you want to target a specific stack height.

Reading the result

yhgrcalc returns a plain dict. The keys most relevant to meshing are:

Key	Meaning
first layer height [yh] [m]	First (wall-adjacent) layer thickness
Growth Ratio	Geometric expansion ratio between layers
Final Layer Thickness [m]	Thickness of the outermost layer
boundary layer thickness [delta99] [m]	Total thickness the stack spans
reynolds number	Reynolds number used for the correlations
skin friction coefficient [cf]	Fanning skin friction coefficient

The full set of keys (including all intermediate fluid properties) is listed in the API Reference.