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Python
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CFD Body Drag

cfd_body_drag
Read-only

Estimate external body drag force and drag coefficient in milliseconds using analytic correlations for spheres, cylinders, and bluff shapes. Narrow design candidates before running full CFD.

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

A4.9/5.0
Behavior5/5

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

Annotations only declare readOnlyHint=true and openWorldHint=false; the description carries the behavioral burden and does so richly. It discloses that this is an analytic, non-solver screening tool returning banded fidelity, gives per-family validity ranges (e.g. sphere valid to Re=2e5 and does not model drag crisis), and warns about L/D<10 end relief for cylinders. No contradiction with annotations.

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

Conciseness5/5

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

Long but every sentence earns its place: the first block gives purpose and routing, bullets organize the three parameter families, and the final block states the exact return payload. Formulas, defaults, validity limits, and warnings are all packed in without fluff.

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

Completeness5/5

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

For a 12-parameter, no-output-schema analysis tool, the description is nearly complete. It explains the three invocation modes, which parameters go together, what results come back for each mode, fidelity bands, and escalation guidance. Only low-level property overrides and mesh tolerance are left implicit, which is acceptable given the level of detail already provided.

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?

Schema description coverage is 0%, so the description must compensate, and it does substantially: diameter_mm + velocity_m_s for sphere/cylinder, optional length_mm with default 1 m, frontal_area_mm2 or model handle with flow_direction, and cd as an override. It leaves a few parameters unexplained (fluid, mu_pa_s, rho_kg_m3, stl_tolerance_mm), so it is not quite a full substitute for per-schema documentation.

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?

Opens with a precise verb-resource pair: 'Analytic EXTERNAL-flow drag screen (NO solver, milliseconds)', explicitly distinguishing it from cfd_pipe_flow and cfd_external_flow_submit. The three shape families make the tool's scope unmistakable.

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?

Gives explicit routing: 'Use this FIRST to narrow a design space; escalate to the solve only for the shapes that survive.' It names cfd_external_flow_submit as the checked-against oracle and cfd_pipe_flow as the internal-flow twin, so an agent knows exactly when to choose this tool versus alternatives.

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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