Vortex shedding and transverse (lift) forces Flashcards
What physical phenomenon causes an unsteady lift force on a cylinder at high Reynolds number?
Flow separation and vortex shedding from alternating sides of the cylinder.
Vortex shedding occurs due to _______ shear in separated flow layers.
strong
What is a Kármán vortex street?
A repeating pattern of swirling vortices formed by alternating vortex shedding behind a cylinder.
True or False: In a Kármán vortex street, vortices are shed randomly from each side of the body.
False – they are shed alternately and periodically
According to streamline curvature, smaller radius of curvature produces ______ pressure.
lower
How does streamline curvature affect lift force direction?
Pressure is lower on the side with tighter (smaller radius) curvature, pushing the body toward the higher-pressure side.
What is galloping in the context of vortex-induced vibration?
A resonant instability when the natural frequency of the structure matches the vortex shedding frequency.
What is the Strouhal number St?
St = (fₛ·D) / U
- fₛ = vortex shedding frequency
- D = diameter of the cylinder
- U = flow velocity
True or False: The Strouhal number depends only on Reynolds number.
False – it depends on both geometry and Reynolds number.
Name two environments where vortex shedding is significant.
The ocean and the atmosphere.
For Re < 5 × 10⁵, the Strouhal number St ≈ ___
0.2
Match the solution with the issue:
- Dynamic excitation from shedding
- Need to reduce lift without increasing drag
- Avoid matching structure frequency to shedding frequency
- Use helical strakes to disrupt vortex shedding
Helical strakes are used on ________ and ________ to suppress vortex shedding.
chimneys, spars
