Strouhal Number Calculator
Find the vortex shedding frequency f = St*V/L for flow past a cylinder or bluff body, and the corresponding shedding period.
🌀 What is the Strouhal Number Calculator?
This Strouhal number calculator finds the vortex shedding frequency f=St*V/L for flow past a cylinder or bluff body, along with the shedding period T=1/f. Enter a flow velocity, a characteristic length, and the Strouhal number (0.2 by default), and it returns the predicted frequency and period instantly.
The Strouhal number is a dimensionless quantity that describes oscillating flow. When fluid flows past a bluff body such as a cylinder, chimney, cable, or bridge deck, vortices peel off alternately from each side, a phenomenon called vortex shedding. This calculator is used by structural and mechanical engineers to predict the frequency of that shedding, for example estimating how fast vortices shed behind a chimney, power line cable, antenna mast, or slender bridge pier in wind.
A common misconception is that the Strouhal number changes much with wind speed or fluid type. For a circular cylinder over a wide subcritical Reynolds number range (roughly 300 to 2x10^5), St stays close to 0.2, which is why so many practical estimates simply plug in St=0.2 rather than measuring it directly.
This calculator is useful for civil and mechanical engineers checking vortex-induced vibration risk, for physics and fluid dynamics students studying periodic flow phenomena, and for anyone estimating the audible hum or vibration frequency caused by wind blowing past a wire, mast, or pipe.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 - Chimney in a steady wind
Example 2 - Power line cable in high wind
Example 3 - Slender antenna mast
❓ Frequently Asked Questions
🔗 Related Calculators
What is the Strouhal number?
The Strouhal number, St, is a dimensionless quantity that describes oscillating flow, most commonly the vortex shedding frequency behind a cylinder or bluff body. It is defined as St=fL/V, where f is the shedding frequency, L is a characteristic length, and V is the flow velocity.
What is the formula for the Strouhal number?
St = fL/V. Rearranged to solve for the practically useful quantity, the vortex shedding frequency is f = St*V/L, where St is the Strouhal number, V is flow velocity, and L is the characteristic length (such as a cylinder's diameter).
What Strouhal number should I use for a circular cylinder?
St is approximately 0.2 for a circular cylinder across a wide subcritical Reynolds number range, roughly 300 to 2x10^5. This calculator defaults to St=0.2, but you can edit it if your geometry or Reynolds number regime calls for a different value.
How do I find the vortex shedding frequency of a chimney or cable in wind?
Enter the wind velocity V, the diameter L of the chimney, cable, or mast, and the Strouhal number St (0.2 is typical for a cylinder). The calculator returns f=St*V/L, the frequency at which vortices shed and the structure experiences a periodic side force.
What is vortex shedding?
Vortex shedding is the periodic separation of vortices from alternating sides of a bluff body as fluid flows past it, creating an oscillating side force on the body. It is the phenomenon behind the audible hum of wires in wind and the wagging motion of a flag.
Why does the shedding period T matter?
The shedding period T=1/f is the time between successive vortices shedding from one side of the body. Comparing T (or its inverse f) to a structure's natural vibration period helps engineers check whether vortex shedding could excite a resonant, potentially damaging, vibration.
What is vortex-induced vibration?
Vortex-induced vibration (VIV) occurs when the vortex shedding frequency f approaches a structure's natural frequency, causing the oscillating side force to resonate and amplify the structure's motion. Tall chimneys, suspension cables, and slender masts are commonly checked for this risk.
What characteristic length should I use for L?
Use the dimension of the body facing the flow that determines the wake width, most commonly a cylinder's diameter. For non-circular bluff bodies, use the frontal width perpendicular to the flow direction.
Does the Strouhal number depend on the fluid?
Not directly. St depends primarily on the body's shape and the Reynolds number regime, not on the specific fluid (air, water, and so on). The fluid's properties (density, viscosity) matter only indirectly, through their effect on the Reynolds number.
Is the Strouhal number the same as the Reynolds number?
No. Reynolds number compares inertial to viscous forces and predicts whether a flow is laminar or turbulent, while Strouhal number describes the frequency of periodic, oscillating flow such as vortex shedding. Both numbers are often reported together for a given flow regime.