Drop a cylinder into a flowing stream and the fluid can't stay attached to its back side. Instead, alternating vortices peel off one side, then the other, forming a Kármán vortex street. The shedding frequency is almost perfectly proportional to flow velocity over a wide range — which is the entire basis of the vortex flow meter.

The governing relationship is the Strouhal number:

• St = f · d / v
• f = shedding frequency (Hz), d = bluff body width (m), v = fluid velocity (m/s)
• For a properly shaped bluff body (typically trapezoidal or T-shaped), St ≈ 0.27 and stays nearly constant from Reynolds number ~10,000 up to ~7,000,000

That constancy is the magic trick. Frequency depends only on velocity and a fixed geometric dimension — not on fluid density, viscosity, temperature, or pressure. The meter becomes a digital pulse generator: count pulses, get volumetric flow.

How the pulses get detected: the vortices create alternating pressure differentials across the bluff body. A piezoelectric crystal, capacitive sensor, or thermistor mounted in or just downstream of the shedder picks up the oscillation. No moving parts in the flow path — just a stationary obstruction and a sensor.

Real-world example: a 4-inch (102 mm) vortex meter measuring steam in a process plant has a shedder bar ~25 mm wide. At 15 m/s steam velocity, f = 0.27 × 15 / 0.025 ≈ 162 Hz. Counting pulses for one second gives you the average velocity directly. Multiply by cross-sectional area (0.0081 m²) and you have 0.122 m³/s of volumetric flow — and since steam density is known from pressure/temperature, you also get mass flow.

Where vortex meters dominate:

• Steam metering (their killer app — they tolerate high temperature, no moving parts to seize)
• Compressed air and industrial gases
• Clean liquids like water, solvents, and process chemicals
• Cryogenic services

Where they fail:

• Low flow — below ~Re 10,000, vortex shedding becomes erratic; meters have a hard low-end cutoff (turndown typically 20:1 to 30:1)
• Pulsating flow — reciprocating compressors upstream wreck the signal
• Dirty or slurried fluids — particulates erode the shedder edge, drifting the K-factor
• Two-phase flow — bubbles or droplets disrupt clean shedding

Installation rule of thumb: 15–20 pipe diameters of straight run upstream, 5 downstream. Vortex meters are exceptionally sensitive to swirl from elbows and valves — worse than orifice plates. Flow conditioners can shorten the upstream requirement but add pressure drop.