Question 1

What is the von K�rm�n vortex street?

Accepted Answer

The von K�rm�n vortex street (named after Theodore von K�rm�n, 1911) is a repeating pattern of swirling vortices shed alternately from the top and bottom of a bluff body (such as a cylinder) in a flowing fluid. Above a critical Reynolds number (~47 for a circular cylinder), the symmetric wake becomes unstable and vortices are shed periodically, creating two parallel rows of counter-rotating vortices downstream. The shedding frequency is characterised by the Strouhal number St = f�D/U, which equals approximately 0.2 for a circular cylinder over a wide range of Reynolds numbers (200�200,000).

Question 2

What is the Lattice Boltzmann Method (LBM)?

Accepted Answer

The Lattice Boltzmann Method (LBM) is a computational fluid dynamics method that simulates fluid flow by tracking the evolution of particle distribution functions on a discrete lattice rather than directly solving the Navier-Stokes equations. In the D2Q9 variant (2D, 9 velocities), each grid cell stores 9 distribution functions f_i corresponding to particles resting or moving in 8 directions. The BGK collision operator relaxes f toward equilibrium: f_i ? f_i + (f_i_eq - f_i)/t, where t is the relaxation time related to kinematic viscosity by ? = cs�(t - �). LBM naturally parallelises, handles complex geometries via simple bounce-back boundary conditions, and efficiently visualises vorticity and flow features.

Question 3

Why does a splitter plate suppress vortex shedding?

Accepted Answer

Vortex shedding arises because the wake behind a bluff body is unstable: vorticity from the top and bottom shear layers interacts and triggers alternating roll-up. A splitter plate � a thin plate aligned with the flow and attached to the rear of the cylinder � separates the two shear layers and prevents the interaction that triggers roll-up. This delays or completely suppresses vortex shedding, reducing drag by 30�50% and eliminating the alternating lift force (vortex-induced vibration). The same principle is used in bridge deck design, chimney stabilisers, and marine riser control.

Question 4

How does Reynolds number control vortex shedding?

Accepted Answer

Re = ?UD/� = UD/? (ratio of inertial to viscous forces). For a circular cylinder: Re < 5: creeping flow, no separation; Re 5�47: steady symmetric recirculation bubbles in the wake; Re ~47: Hopf bifurcation � onset of periodic shedding (Strouhal instability); Re 47�200: laminar vortex shedding, St � 0.1�0.2; Re 200�400: 3D instabilities develop; Re > 1000: turbulent wake. The LBM simulator models the 2D incompressible regime Re 50�400, covering the transition from steady wake to fully developed vortex street.

🧪 Vortex Shedding – von Kármán Vortex Street

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