A guided reading route through three useful distinctions in the literature: surface characterisation, spatial organisation, and hydrodynamic representation.
Editorial perspective
A reading route, not a taxonomy.
BUR Atlas uses these three distinctions as editorial lenses for a starting route through the literature. They overlap, individual studies may span more than one, and they are not presented as a complete or field-wide classification. The selected sources below illustrate why each distinction is useful; they are not a canon or ranking.
01 / Surface characterisation
Start with what was actually measured.
Guiding question
What physical surface quantity is being described, and under what measurement conditions?
A roughness value is meaningful only in relation to the surface or profile that was assessed and the way it was measured. Height statistics such as Ra and Rq compress part of that description into a scalar, while spectra retain information about spatial scales.
This lens keeps geometric or statistical surface descriptors separate from hydraulic quantities used to represent flow response. A paper may connect the two, but they answer different questions and should not be treated as interchangeable by default.
Original editorial schematicConceptual · not data
A visual reading aid for this lens; it is not reproduced from a published figure and does not represent a specific experiment or hull.
What to look for
Whether the measurement is profile-based or areal, and what region of the surface was assessed.
Instrument, resolution, filtering, sampling and the reported descriptor.
Which spatial scales are retained or removed by the measurement and processing.
Whether a measured descriptor is later converted, calibrated or inferred into a hydraulic roughness quantity.
Evidence notes
Ra and Rq summarise height deviations in an assessed profile. Their interpretation depends on how that profile was obtained and processed.
How are different surface conditions distributed, spaced, oriented or clustered over the assessed area?
Two surfaces can share a similar overall roughness level while arranging that roughness differently in space. Coverage, patch size, spacing, orientation, clustering and gradients therefore belong to the description when the surface is heterogeneous.
The hydrodynamic importance of a particular arrangement depends on geometry and flow conditions. Evidence from idealised roughness patterns is useful for identifying mechanisms, but it should not be transferred to a ship hull without checking what is actually comparable.
Original editorial schematicConceptual · not data
A visual reading aid for this lens; it is not reproduced from a published figure and does not represent a specific experiment or hull.
What to look for
The location and extent of distinct surface conditions, not only an area-averaged value.
Patch size, spacing, orientation, clustering, gradients or other organisation at relevant length scales.
Whether the distribution is measured, prescribed, idealised or reconstructed.
The geometry and flow conditions under which a spatial effect is reported.
Evidence notes
In a laboratory rough-wall boundary layer, changing the spanwise spacing of streamwise roughness strips was associated with changes in large-scale secondary flow. BUR Atlas uses this as evidence that arrangement can matter for those experimental surfaces, not as a universal hull rule.
Work on patchy or heterogeneous surfaces makes the spatial distribution part of the representation problem because a single homogeneous value must stand in for a non-uniform surface.
Hull-specific CFD studies provide a complementary example: when the overall roughness magnitude is held fixed, changing its prescribed spatial distribution can still alter resistance and boundary-layer development. These results are numerical examples for the studied hulls and conditions, not a general rule for every vessel.
Included as a hull-specific numerical example in which several spatial roughness distributions are compared while the arithmetic mean roughness is held constant.
How is a measured or prescribed surface translated into a quantity or model that represents its hydrodynamic effect?
Flow calculations and drag correlations rarely operate on a complete surface map without some form of representation. Hydraulic roughness scales, roughness functions, wall-model inputs and equivalent homogeneous values are examples of ways a complex surface can be reduced for a particular flow problem.
That reduction is not neutral. The chosen quantity, averaging rule, calibration and flow regime influence what the representation means and where it can reasonably be applied. When a heterogeneous surface is replaced by one value, the method used to construct that value becomes part of the modelling assumption.
Original editorial schematicConceptual · not data
A visual reading aid for this lens; it is not reproduced from a published figure and does not represent a specific experiment or hull.
What to look for
The flow-relevant roughness quantity or boundary condition actually used by the model or correlation.
How that quantity is obtained from measured, prescribed or previously calibrated surface information.
Any equivalent-homogeneous assumption, averaging rule or homogenisation step.
The flow regime, calibration range and other conditions attached to the representation.
Evidence notes
For heterogeneous surfaces, an equivalent homogeneous roughness is not simply a neutral average; the way it is constructed is part of the modelling problem.
A hull-specific study further examines the interpretation of a single globally applied equivalent roughness when the underlying hull roughness varies spatially, keeping the distinction between the physical distribution and its procedural representation explicit.
Reviews of hydraulic roughness and rough-surface drag show that relating surface topography to a flow-response parameter is a separate step from measuring the surface itself.