Coordinate system and units¶

The axis convention Bramble uses, and the units of every length, speed, angle and result. Geometry is in millimetres. Almost everything else is in SI units, with lengths in metres.

Geometry must be in millimetres

Bramble assumes every uploaded Part is in millimetres and converts it to metres for the simulation. Files aren't scaled on upload, and there's no unit option on the upload form or in the Template Case. A model exported in metres ends up 1000 times too small. See prepare CAD for upload.

To check, open the Part in the model viewer and read Dimensions (mm) in the Model panel.

Axes¶

Bramble uses a right-handed frame, the usual one for automotive CFD.

Axis	Positive direction	Viewer colour
+X	Downstream, towards the rear of the vehicle. At 0° yaw the flow travels in +X	Red
+Y	To the driver's right	Green
+Z	Up	Blue

Seen from above (+Z towards you), with the vehicle facing −X:

                      +Y  (driver's right)
                       ^
                       |
   wind at 0° yaw      |    front          rear
   ------------->      +----[================]------>  +X  (downstream)
                       |
                       |
                      -Y  (driver's left)

Origin. Bramble keeps your CAD coordinates. It doesn't move or re-centre the model.
Left and right. Left-hand points have zero or negative Y and right-hand points have positive Y. In the Kinematic Setup, Reflect to Right fills in the right-hand side by negating Y.
Ground. The ground plane starts at z = 0. With a Kinematic Setup, Bramble fits it through the setup's front and rear reference points at the baseline ride heights.

Yaw and flow angles¶

Angle	Sign convention
Yaw	Positive yaw means the wind comes from the driver's right (the +Y side). At +90° the flow enters through the +Y face of the domain. The geometry isn't rotated; yaw sets the flow direction
Pitch (flow angle)	Positive pitch means the flow comes from below. At +90° it enters through the −Z face
Roll	Set by the kinematic model. Contact support@bramblecfd.com for the convention your model uses
Steer	Set by the kinematic model

Ride heights¶

Ride heights are measured vertically (in Z) at the Kinematic Setup's front and rear reference points, so Front RH and Rear RH together set the pitch attitude.
A Map's ride heights are applied as the difference from the Kinematic Setup's Default Ride Parameters.
The vehicle stays still and the ground moves: a lower ride height moves the ground up.

Units by area¶

Where	Quantity	Unit
Parts	Uploaded geometry	mm
Parts	Transform Part translation and rotation origin	mm
Model viewer	Dimensions (mm), Intersection Point (mm), measurements, clipping Distance (mm)	mm
Template Case	All lengths: domain, refinement and mesh boxes, radiator Inside Point, rotating-zone Origin, movie slice positions, probes	m
Template Case	Star-CCM+ Base Size and Prism Layer Thickness	mm
Template Case	Velocity	m/s
Template Case	Yaw, rotation and other angles	degrees
Template Case	Rotating-zone Omega	Not labelled
Template Case	Reference area	m²
Template Case	Reference density, pressure, temperature, kinematic viscosity	kg/m³, Pa, K, m²/s
Map	Front RH, Rear RH, Corner Radius	m, or mm with Map Units
Map	Velocity	m/s (no unit shown)
Map	Yaw, Roll, Pitch	degrees (no unit shown)
Map	Steer	Depends on the kinematic model (no unit shown)
Kinematic Setup	Kinematic Setup Coordinates	m, or mm with the units toggle
Kinematic Setup	Default Ride Parameters	Ride heights in m, velocity in m/s, angles in degrees, whatever the units toggle shows
Postpro cameras	Focal Point, Distance (m)	m
Postpro cameras	Elevation (°), Azimuth (°)	degrees

For the full list of Template Case settings, see the Template Case file reference.

Unit toggles¶

These toggles only change how values are shown and entered. Bramble always stores the values in metres, and converts back when you save.

Where	Toggle	Applies to
Map	Map Units: m / mm (default m)	Front RH, Rear RH and Corner Radius. The column headers show the unit. Ride heights must be within ±1 m (±1000 mm)
Kinematic Setup	Kinematic setup units are in: m / mm (default m)	Kinematic Setup Coordinates only
Template Case editor	Use (mm) / Use (m)	Length columns. The headers change to "(mm)"
Extra Exports	Use (mm) / Use (m)	Length fields in the export forms

Default Ride Parameters aren't converted

The Kinematic Setup units toggle doesn't apply to Default Ride Parameters. They have no unit label and are saved exactly as typed. Enter ride heights in metres, even when the coordinates are shown in mm, because they're compared with the Map's ride heights, which are stored in metres.

Forces and coefficients¶

Result	Unit or definition
Forces	N. The reference density defaults to 1.225 kg/m³
Force directions	Along the global axes, so they stay vehicle-aligned at any yaw: Fx is drag, Fy is side force and Fz is lift. With +Z up, downforce is negative Fz
Coefficients	Dimensionless: C = F / (½ ρ V² A)
Area coefficients (for example CxA)	m²: CA = F / (½ ρ V²)
V	The freestream velocity in m/s
A (reference area)	The Template Case's reference area in m². If it isn't set, Bramble uses the model's projected frontal area
Reference length	m. Only used for dynamic-motion stability coefficients

Postpro camera views use Azimuth 0° = from the left, 90° = from the rear, 180° = from the right and 270° = from the front. Elevation +90° looks down from above.