Calculator Inputs
Enter SI units for a clean numerical example. Use the manual Strouhal option when you have test data.
Formula Used
Reynolds number: Re = ρVD / μ
Vortex shedding frequency: f = St × V / D
Period: T = 1 / f
Angular frequency: ω = 2πf
Wake wavelength: λ = V / f = D / St
Dynamic pressure: q = 0.5ρV²
Projected area: A = D × L
Estimated RMS lift force: F = q × A × CL
Velocity for resonance check: Vcritical = fnD / St
How to Use This Calculator
- Select the body shape that best matches the exposed object.
- Enter characteristic diameter, velocity, density, and viscosity in SI units.
- Use automatic Strouhal estimation or enter your own measured value.
- Add span length and lift coefficient for a screening force estimate.
- Enter natural frequency to check possible lock-in.
- Press the calculate button and review the result above the form.
- Use the CSV or PDF button to save the numerical example.
Example Data Table
| Case | Shape | D (m) | V (m/s) | ρ (kg/m³) | μ (Pa·s) | St | Approx. f (Hz) |
|---|---|---|---|---|---|---|---|
| Small pipe in air | Circular | 0.05 | 8 | 1.225 | 1.81e-5 | 0.20 | 32.00 |
| Stack in wind | Circular | 1.20 | 18 | 1.225 | 1.81e-5 | 0.20 | 3.00 |
| Square sign post | Square | 0.10 | 15 | 1.225 | 1.81e-5 | 0.13 | 19.50 |
| Pipe in water | Circular | 0.20 | 1.5 | 998 | 0.001 | 0.20 | 1.50 |
Numerical Example for Vortex Shedding
What Vortex Shedding Means
Vortex shedding is a repeating flow pattern. It appears behind bluff bodies. A round pipe, mast, cable, or chimney can create it. The flow separates from each side. Swirling vortices then leave the body in alternating order. This creates a pressure pulse. The pulse may shake the structure.
How the Example Starts
A numerical example starts with diameter, flow speed, density, and viscosity. These values give the Reynolds number. Reynolds number shows whether the flow is laminar, transitional, or turbulent. Periodic shedding normally begins above a critical range. For many circular cylinders, a Strouhal number near 0.2 is common. The calculator can estimate it. You may also enter your own value.
Important Output Values
The main result is shedding frequency. It is found by multiplying Strouhal number by velocity, then dividing by diameter. A small diameter or high speed raises frequency. A larger diameter lowers frequency. The period is the time for one shedding cycle. Wavelength shows the streamwise spacing between similar vortex groups.
Engineering Use
Engineers use these numbers for early vibration checks. If the shedding frequency is close to a natural frequency, resonance may occur. This is often called lock-in. The vibration response can grow when damping is low. That risk is higher for slender structures exposed to steady wind or water flow. The tool compares the shedding frequency with a user supplied natural frequency.
Force Screening
The force estimate is a screening value. It uses dynamic pressure, projected area, and an assumed lift coefficient. It is not a replacement for detailed testing. It helps create a first load scale. It also helps compare alternatives, such as changing diameter, speed, or span.
Final Review Tips
Use conservative inputs when data is uncertain. Check the selected shape. Review the Reynolds regime message. Compare several velocities, not only one. Real structures may have roughness, turbulence, end effects, and support flexibility. Those details can shift the shedding response. Still, this numerical example gives a clear starting point for design review, classroom work, and quick field calculations. Record each assumption before sharing results. Small unit mistakes can change frequency greatly. Always confirm diameter direction, fluid properties, and velocity basis. For important projects, compare this estimate with standards, simulations, or measured vibration data from the installation.
FAQs
1. What is vortex shedding frequency?
It is the rate at which alternating vortices leave a body in flowing fluid. The value is usually measured in hertz. It depends on Strouhal number, flow speed, and characteristic diameter.
2. Why is the Strouhal number important?
The Strouhal number links body size, flow speed, and shedding frequency. For many round cylinders, it is near 0.2 across a useful range. Other shapes need different values.
3. What units should I use?
Use meters, seconds, kilograms, pascal-seconds, and hertz. The calculator expects SI units. Mixing inches, feet, or centipoise without conversion will give incorrect results.
4. What does Reynolds number show?
Reynolds number compares inertial and viscous effects in the flow. It helps identify whether shedding is likely, stable, transitional, or strongly affected by turbulence and roughness.
5. What is lock-in?
Lock-in can occur when shedding frequency is close to a structural natural frequency. The structure may respond more strongly. Actual risk also depends on damping, stiffness, flow quality, and geometry.
6. Is the lift force exact?
No. It is a screening estimate based on dynamic pressure, projected area, and lift coefficient. Use testing, detailed simulation, or design standards for important structural decisions.
7. Can I use this for water flow?
Yes. Enter water density and dynamic viscosity. The same core equations apply. Use a Strouhal value suitable for the body shape and Reynolds number range.
8. Why does diameter reduce frequency?
The formula divides by diameter. A larger body creates a wider wake. That usually increases the time between similar vortex releases and lowers shedding frequency.