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cfd_body_drag

Estimate external-flow drag for spheres, cylinders, and bluff bodies without a solver. Screen design candidates in milliseconds before committing to a CFD solve.

Instructions

Analytic EXTERNAL-flow drag screen (NO solver, milliseconds) — the external twin of cfd_pipe_flow, and the banded oracle the wind-tunnel solve cfd_external_flow_submit(model=…) is checked against. Use this FIRST to narrow a design space; escalate to the solve only for the shapes that survive.

Three families:

  • shape='sphere' — Clift–Gauvin over the whole standard drag curve, Cd = 24/Re·(1+0.15·Re^0.687) + 0.42/(1+4.25e4·Re^-1.16). Collapses to the EXACT Stokes 24/Re as Re→0; valid to Re=2e5 (it does not model the drag crisis). Pass diameter_mm + velocity_m_s.

  • shape='cylinder' — Sucker–Brauer crossflow Cd (axis ⟂ flow), Cd ≈ 10 at Re=1, 1.45 at Re=100, 1.2 at Re=1e5. Pass diameter_mm, velocity_m_s, optional length_mm (default 1 m, i.e. drag per unit span; L/D<10 warns about end relief).

  • a tabulated bluff/streamlined shape — 'cube_face_on', 'flat_plate_normal', 'hemisphere_open_back', 'streamlined_body', 'car_modern', … (shape='list' returns the whole table). Needs frontal_area_mm2 (or a model handle, whose silhouette along flow_direction is measured off the live solid) + velocity_m_s; cd overrides the table with a known value.

Fidelity: sphere/cylinder are correlations with band_pct 10/15; the table is band_pct 20 and only valid for Re ≈ 1e4–1e6 on a shape that genuinely matches.

Returns {cd, drag_force_n, frontal_area_m2, dynamic_pressure_pa, velocity_m_s, fidelity, band_pct, escalate_to} plus {reynolds, regime, valid_range_ok, warnings} for sphere/cylinder — or {shapes: {name: cd}} for shape='list'.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
cdNo
fluidNoair-20c
modelNo
shapeNosphere
mu_pa_sNo
length_mmNo
rho_kg_m3No
diameter_mmNo
velocity_m_sNo
flow_directionNo
frontal_area_mm2No
stl_tolerance_mmNo

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observed

TDQS

A4.7/5.0
Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

No annotations exist, so the description carries the full burden and does so generously: no solver, millisecond latency, per-family fidelity bands (10/15/20%), validity ceilings (Re=2e5, table only Re≈1e4–1e6), the drag-crisis limitation, and an L/D<10 warning. This is exactly the behavioral context needed to trust or reject the number.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with the key decision (screen vs. solve), then tightly bulleted families each carrying formulas, required params, and validity. Dense but every clause is functional; slightly long, and the embedded equations are borderline but justified by the fidelity claims.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a 12-parameter tool with no annotations and no output schema, the description does the heavy lifting, including a full return-value breakdown and the sphere/cylinder-vs-list conditional shape of the response. The only remaining gap is the handful of undocumented fluid/property/tolerance parameters.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

With 0% schema coverage the description must compensate, and it largely does: it documents shape (listing concrete family values and shape='list'), diameter_mm, velocity_m_s, length_mm (with default/semantics), frontal_area_mm2, model, cd override, and flow_direction. It omits fluid, mu_pa_s, rho_kg_m3, and stl_tolerance_mm, so a few of the 12 params remain unexplained.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a precise verb+resource+method: 'Analytic EXTERNAL-flow drag screen (NO solver, milliseconds)'. It immediately distinguishes itself from its internal twin cfd_pipe_flow and from the wind-tunnel solve cfd_external_flow_submit, so an agent can classify it without opening the schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicit routing: 'Use this FIRST to narrow a design space; escalate to the solve only for the shapes that survive.' It names the alternative and the exact condition that selects it, leaving nothing to inference.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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