The Strouhal number is a dimensionless constant that links how fast vortices shed off a bluff body to how fast the fluid is flowing past it. Name a flow, drop an obstruction in it, and downstream you get an alternating train of swirls - a von Karman vortex street - that peels off at a frequency proportional to velocity. The Strouhal number is the proportionality itself, and its near-constant value over a wide range of conditions is the single physical fact that lets a vortex flow meter turn a frequency into a flow rate. Understanding it explains both why the meter is linear and why it quits at low flow.
Strouhal Number in one line: The Strouhal number (St) is a dimensionless ratio relating vortex-shedding frequency, bluff-body width, and flow velocity as St = f times d divided by v. Because it stays nearly constant across a broad Reynolds-number range, shedding frequency tracks velocity directly, which is the working principle of the vortex flow meter.
Write the Strouhal number as St equals f times d over v, where f is the shedding frequency, d is the characteristic width of the bluff body, and v is the flow velocity. Rearranged, f equals St times v over d. Since St and d are both fixed once the meter is built, frequency becomes a straight-line function of velocity. Count the vortices per second and you have velocity; multiply by pipe area and you have volumetric flow. The whole measurement rests on that one rearrangement.
What makes this useful rather than merely tidy is that the Strouhal number does not wander much as flow changes. Over the turbulent range where a vortex meter is designed to work, St for a well-shaped bluff body sits close to a constant value and barely moves as velocity climbs. That plateau is what gives the meter its excellent linearity: double the flow and you very nearly double the frequency, with no calibration curve to interpolate through. The meter reads a raw frequency and scales it, and the physics keeps the scaling honest.
The number is dimensionless on purpose. Because it is a ratio of a time scale set by the shedding to a time scale set by the flow, it collapses geometry, speed, and fluid together into a single figure that behaves the same in water, oil, steam, or gas. That universality is why the same bluff-body design works across services that look nothing alike, provided the flow stays inside the range where St holds steady.
The number a technician actually configures is not the Strouhal value but the meter K-factor, expressed as pulses per unit volume. The two are directly related. Since frequency is proportional to velocity through the Strouhal number, and velocity times pipe area gives volumetric flow, the pulses the meter emits per gallon or per cubic meter fall out of St, the bluff-body width, and the bore. The manufacturer folds all of that geometry into a single K-factor stamped on the meter, so the field never sees the Strouhal number directly.
This is why a vortex meter is treated as a near-linear device with a fixed K-factor rather than something needing a lookup table. As long as the Strouhal number is riding its constant plateau, one K-factor covers the entire usable span, and the flow computer simply divides the pulse rate by that factor. It is the same reasoning behind a turbine meter's K-factor, but here the pulses come from vortices rather than blade passes, and the constancy comes from fluid mechanics rather than a spinning rotor.
Because the K-factor is baked into the geometry, anything that changes the effective bluff-body width or the flow area shifts it. A meter installed in the wrong bore, a buildup of scale or wax narrowing the passage, or a damaged shedder bar will all move the real K-factor away from the stamped one and bias the reading. The Strouhal relationship still holds - it is the assumed geometry underneath it that has quietly changed.
The constant-Strouhal plateau has a floor. As velocity falls, the Reynolds number drops, and below a certain point the vortex street stops forming as a clean, countable train. The Strouhal number is no longer constant there, shedding becomes weak or intermittent, and the meter can no longer resolve a reliable frequency from the noise. Rather than report a wandering value, a vortex meter simply drops its output to zero below a low-flow cutoff. This is the well-known low-flow dropout, and it is a physical limit of the shedding, not a fault in the electronics.
For a monitoring system that limitation matters because a vortex meter reading exactly zero is ambiguous. It may mean genuine no-flow, or it may mean the line is trickling below cutoff while product still moves. A cloud SCADA platform such as Merobix historizes the vortex frequency and the derived flow together, so an engineer reviewing a trend can see whether the meter fell off a cliff into cutoff or truly stopped. Trending the raw pulse quality alongside the flow makes the difference visible from a dashboard instead of a site visit.
Knowing the Strouhal-driven turndown of a given meter also shapes how alarms are set. There is little point alarming on a low-flow value that sits underneath the meter's own cutoff, since the instrument cannot report it anyway. Sizing the meter so that normal operation stays well up on the constant-Strouhal plateau, and setting SCADA thresholds above the dropout point, keeps the data trustworthy and the alarms meaningful across remote and unmanned oil and gas sites.
Because shedding frequency equals the Strouhal number times velocity divided by bluff-body width, and the Strouhal number stays nearly constant over the meter's working range. With that constant and the geometry both fixed, frequency becomes a straight-line function of velocity. The meter can therefore convert a counted frequency into flow with a single scaling factor and no calibration curve.
The K-factor, in pulses per unit volume, is derived from the Strouhal number combined with the bluff-body width and the pipe bore. Because frequency tracks velocity through the Strouhal number, the pulses emitted per gallon or cubic meter follow directly from that geometry. Manufacturers roll it all into one stamped K-factor so the field configures a single number rather than the underlying Strouhal value.
At low velocity the Reynolds number falls and the vortex street stops forming as a clean, countable train, so the Strouhal number no longer holds constant. The shedding becomes weak or intermittent and the meter cannot resolve a reliable frequency. Rather than report a wandering number, the meter drops its output to zero below a low-flow cutoff, which is the low-flow dropout.
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