Ohnesorge Number Calculator

The Ohnesorge number is widely used in droplet and jet physics because it compresses four measurable properties into one scale-aware indicator. It is especially helpful when comparing fluids of different viscosities or when the same fluid is used at different nozzle diameters or film thicknesses. By converting inputs to SI internally, this calculator supports consistent reporting across experiments, simulations, and production settings, and quality control checks.

1) What the Ohnesorge number represents

Oh compares viscous resistance to capillary–inertial effects in free-surface flows. It helps predict whether a droplet, jet, or filament will oscillate and break up quickly or remain strongly damped by viscosity at the selected length scale.

2) Variables and units used here

This calculator uses dynamic viscosity μ, density ρ, surface tension σ, and characteristic length L. Inputs can be entered in practical lab units and are converted to SI before evaluating Oh = μ / √(ρ σ L).

3) Choosing the characteristic length L

Pick L to match the feature controlling capillary motion. In inkjet and sprays, L is often nozzle or droplet diameter. In coatings, L can be film thickness. Changing L changes Oh because capillary timescale grows with √L.

4) Typical fluid property ranges

At ~20–25°C, water has μ≈1 mPa·s and σ≈72 mN/m. Light oils commonly fall near 10–100 mPa·s with σ≈20–35 mN/m. High-viscosity liquids or polymer solutions can reach 0.1–10 Pa·s.

5) Interpreting Oh in practice

A common guideline is Oh<0.1 for weak viscous damping, 0.1–1 for mixed behavior, and Oh>1 for strong viscous control. These bands are not universal, but they are useful for comparing formulation changes or temperature effects.

6) Relationship to Reynolds and Weber

When velocity and length definitions are consistent, Oh connects to other groups by Oh = √We / Re. This is valuable for atomization: We sets capillary breakup driving, Re sets inertia-to-viscosity balance, and Oh summarizes damping directly.

7) Sprays, jets, and droplet formation

Increasing viscosity raises Oh and usually increases breakup length and reduces fine droplet production. Lowering surface tension lowers Oh’s denominator and can move a system toward faster pinch-off. Engineers often tune surfactants and temperature to target a desired regime.

8) Reporting and experimental workflow

Measure μ, ρ, and σ at the experiment temperature and record how L was defined (nozzle diameter, droplet size, or film thickness). Export the SI-converted values with Oh to keep lab notes, CFD inputs, and design reviews consistent.

1) Is the Ohnesorge number dimensionless?

Yes. Units of μ cancel with √(ρσL), so Oh has no units and can be compared across fluids, sizes, and operating conditions.

2) What should I use for the characteristic length L?

Use the feature that controls capillary motion: nozzle diameter, droplet diameter, jet diameter, or film thickness. State your definition of L when sharing results.

3) Can I use kinematic viscosity instead of dynamic viscosity?

This tool expects dynamic viscosity. If you have ν, convert using μ = ρν with consistent units and temperature, then enter μ and ρ.

4) Why do my results change with temperature?

Viscosity and surface tension vary with temperature. Because Oh depends on both, warming or cooling can shift regimes, affecting breakup length, oscillations, and pinch-off behavior.

5) What does Oh < 0.1 usually imply?

It typically indicates weak viscous damping. Surface tension and inertia dominate, so interfaces can deform and break up faster, especially when the Weber number is high.

6) What does Oh > 1 usually imply?

It often indicates strong viscous control. Capillary thinning and oscillations slow down, and clean pinch-off can become difficult without higher driving pressure or altered surface tension.

7) Should I report Oh by itself?

Oh is helpful, but report Reynolds and Weber numbers too, plus temperature and L definition. That combination better captures inertia, capillarity, and viscosity for reproducibility.