Does Reynolds number alone prove that flow is laminar or turbulent?

No. Reynolds number is a similarity coordinate. Transition also depends on geometry, characteristic length, inlet disturbances, roughness, curvature, pulsation and observation distance/time. The 2300–4000 band is shown only as a conventional screen for steady flow in a long straight circular full pipe.

May I enter ISO VG 46 as exactly 46 cSt?

Not without controlled property data. ISO VG is a viscosity classification, not a certificate that every fluid has exactly the grade number at the actual operating temperature, pressure, ageing and shear condition.

Can standard gas flow be entered as actual pipe flow?

No. The flow, density and viscosity must describe compatible actual conditions at the evaluated state and section. Convert standard or normal volume flow to actual conditions using the applicable method first.

Why was the entry-length result removed?

The former empirical laminar and turbulent correlations were presented as universal and the turbulent expression was applied throughout the transition band. Entry length depends on inlet profile, geometry, definition of fully developed flow and regime, so it requires a separately sourced method.

Documented dimensionless coordinate

Controlled Reynolds Number Worksheet

Calculate Reynolds number from compatible velocity, characteristic length and viscosity. A conventional straight circular-pipe screening coordinate is separated from the general calculation and is never presented as a guaranteed flow-state verdict.

Re = VL / νRe = ρVL / μNo universal transition verdict

Interpretation context

Motion input

Length unit

Characteristic length L (m)

Mean/reference velocity V (m/s)

Velocity unit

Actual volume flow Q (m³/s)

Flow unit

Viscosity input

Kinematic viscosity ν (m²/s)

Kinematic viscosity unit

Dynamic viscosity μ (Pa·s)

Dynamic viscosity unit

Mass density ρ (kg/m³)

Density unit

Property, geometry and operating-state source

I confirm that velocity/actual flow, characteristic length, density and molecular viscosity describe the same operating state; the fluid is single-phase and Newtonian for this calculation; and the selected characteristic length follows the applicable method. For the pipe screen or flow-rate route, the pipe is full and circular, L is internal diameter, and V is bulk mean velocity. I will not treat Re or the conventional band as a friction factor, entry length, pressure drop, heat-transfer coefficient, transition guarantee or safety/compliance decision.

Reynolds-number record

Reynolds number Re

Normalized velocity V

Normalized length L

Normalized kinematic viscosity ν

Derived actual flow Q

Interpretace

Re is a coordinate, not a verdict: transition and correlations depend on geometry, length definition, roughness, curvature, inlet disturbance, pulsation, property variation and observation distance/time. Use the applicable controlled method for the engineering decision.

General Reynolds number

Re = ρVL / μ = VL / ν
ν = μ / ρ

V is the compatible mean or reference velocity in m/s; L is the applicable characteristic length in m; ρ is mass density in kg/m³; μ is molecular dynamic viscosity in Pa·s; and ν is kinematic viscosity in m²/s. Re is dimensionless. Dimensional checks: (m/s)(m)/(m²/s)=1 and (kg/m³)(m/s)(m)/(kg/(m·s))=1.

Circular full-pipe flow route

A = πD² / 4
V̄ = Q/A = 4Q/(πD²)
Re_D = V̄D/ν = 4Q/(πDν)

For this route, Q is actual volume flow at the evaluated state, D is circular internal diameter, and V̄ is bulk mean velocity. Standard/normal gas volume flow cannot be substituted without conversion to compatible actual conditions. A non-circular duct needs the characteristic or hydraulic diameter defined by the applicable method; this worksheet does not invent one.

Použitelnost

The basic relation is used for single-phase Newtonian continuum flow with compatible properties. Non-Newtonian fluids can require a generalized Reynolds number and an apparent-viscosity model. Compressible flow can change state and Re along a passage. Multiphase, rarefied, pulsating, curved, rotating or strongly developing flows require the applicable definition and evaluation location.

Worksheet coordinate	Permitted interpretation	Not established by this result
Re < 2 300	Below a commonly used straight circular-pipe screening boundary.	Guaranteed laminar state in every inlet, length, disturbance, pulsation or geometry.
2,300 ≤ Re ≤ 4,000	Within a conventional transition-screening band; use the applicable correlation/test.	A stable universal “transition zone” with fixed end points.
Re > 4 000	Above a commonly used straight circular-pipe screening boundary.	Fully developed turbulence, roughness-independent friction, mixing, heat transfer or safe design.

Historical and modern experiments show why the wording matters. A Bureau of Standards study calculated about Re=2010 for the onset in Reynolds’s long-pipe data, explicitly called 2000 approximate and showed that it did not apply to short tubes. Modern peer-reviewed pipe-flow work finds that the natural transition point depends on finite disturbances and experimental imperfections. The worksheet therefore reports coordinates, not a binary physical diagnosis.

Množství	Conversion used
Délka	1 ft = 0.3048 m exactly; 1 in = 0.0254 m exactly
Rychlost	1 ft/s = 0.3048 m/s exactly; 1 mile/h = 0.44704 m/s exactly
Volume flow	1 m³/h = 1/3600 m³/s; 1 L/min = 1/60000 m³/s; 1 U.S. gal = 0.003785411784 m³ exactly
Viscosity	1 cSt = 1 mm²/s = 10⁻⁶ m²/s; 1 cP = 1 mPa·s = 10⁻³ Pa·s
Mass density	1 lb/ft³ = 0.45359237/0.3048³ = 16.01846337396014 kg/m³

No fluid presets: viscosity and density change with composition, temperature, pressure, ageing and measurement method. ISO 3448 is a viscosity-classification standard; an ISO VG grade label is not an exact property value for the actual operating state. Use a current product data sheet, laboratory result or authoritative property model.

Public equation sources

NASA Glenn’s official Reynoldsovo číslo derivation gives Re=ρVL/μ, ν=μ/ρ and Re=VL/ν and identifies Re as dimensionless. A NIST/NBS technical study of pipe corrosion likewise uses pipe diameter and average velocity in Re=ρVD/μ and notes that viscosity and density depend on water temperature.

Published numerical check

NASA’s official soccer-ball example publishes ρ=2.37×10⁻³ slug/ft³, V=40 mile/h, diameter 0.75 ft and μ=3.61×10⁻⁷ slug/(ft·s), reporting Re≈3.0×10⁵. The exact arithmetic from those rounded inputs is 288,864.2659, which rounds to the published one-significant-digit value. It validates the general characteristic-length equation only; it is intentionally not assigned a circular-pipe screening label.

Transition evidence

The public NIST/NBS paper Determination of Absolute Viscosity by Short-Tube Viscosimeters, Technologic Paper 100 records the approximate/geometry-dependent limitation of a critical value. Avila et al., The Onset of Turbulence in Pipe Flow, Science 333 (2011), experimentally showed the subcritical, disturbance- and lifetime-dependent nature of sustained pipe turbulence. These sources do not justify a universal binary threshold for every flow.

Standards boundary

ISO 80000-11:2019, Edition 2, Quantities and units—Part 11: Characteristic numbers, is Published, confirmed in 2025, and has published Amendment 1:2025. The exact licensed entry is not reproduced or claimed as the source of this implementation: NEEDS_LICENSED_SOURCE for clause-level conformity.

ISO 3448:1992, Edition 2, Industrial liquid lubricants—ISO viscosity classification, is Published and confirmed in 2023; Technical Corrigendum 1:1993 is also Published. Its classification table is not copied and no VG number is substituted for measured viscosity.

NIST SP 811 Chapter 8.8 gives Pa·s and m²/s as the SI units for dynamic and kinematic viscosity and supplies the stoke/poise relations used for explicit conversions.

Former output or claim	Confirmed problem and correction
Laminar below 2,300; transition 2,300–4,000; turbulent above 4,000 as a definitive state	Those are conventional straight circular-pipe screening coordinates, not universal physics. Geometry, characteristic length and disturbances were missing. The output now screens only a confirmed pipe context and emits no guaranteed state.
Entry length 0.06ReD or 4.4Re^1/6D for every result	Empirical entrance correlations need a defined inlet, regime and “fully developed” criterion. The old code applied the turbulent expression throughout transition. The output was removed pending a separately controlled method.
ISO VG 32/46/68 values treated as exact cSt presets	An ISO grade is a classification, not the exact viscosity of an arbitrary product at its actual state. All presets and the unsourced property table were removed.
Oil viscosity falls 50% per 20°C; hydraulic oil flow is almost always laminar	Both were unsupported universalizations. Viscosity-temperature behavior is product-specific and hydraulic-flow Re depends on velocity, diameter and viscosity. These claims were removed.
Critical velocity at exactly Re=2,300	This gave false precision to a conventional coordinate and implied one universal transition. It was removed.
Rounded gpm/ft conversions, defaults, auto-calculation, partial parsing and persisted URL/local state	The replacement uses exact-definition constants, starts blank, requires provenance and confirmation, strictly parses complete numbers, calculates only on submit and writes results with textContent.

No. It is a dimensionless similarity coordinate. The pipe band is a conventional screen with explicitly limited geometry, not a guaranteed state.

Use molecular dynamic or kinematic viscosity at the same temperature, pressure, composition and condition as the velocity/flow and density. Do not use a grade name as a measured value.

Not in this worksheet. Converting Q to V requires cross-sectional area and the applicable characteristic-length definition. The provided flow route is restricted to a circular full pipe.

No. Those require roughness, geometry, development and correlation-specific conditions. The former universal outputs were removed.