Side-by-Side Cylinders: Interference Study

Two circular cylinders placed side-by-side transverse to the flow (same streamwise x-position, offset in y) at Re=100. Variable transverse spacing ratio S/D = 2, 3, 5. At close spacing, biased gap flow and coupled vortex shedding emerge. The interference between the two cylinders produces fundamentally different wake topologies depending on the spacing ratio.

Note: this is a true side-by-side (parallel) arrangement, distinct from tandem cylinders (one behind the other). The diameter is set to D = 60 cells so that the largest spacing (S/D = 5, i.e. 300 cells center-to-center) fits within the 800-cell channel height.

Setup

Parameter Value
Grid 1200 × 800
Cylinder diameter D 60 cells
Arrangement Transverse (same x, offset in y)
Reynolds number 100
Spacing ratios S/D (transverse) 2, 3, 5
Inlet velocity uinflow = 0.1 lu/ts
Tau (relaxation parameter) 0.68
Number of steps 20,000
Reference length D = 60 cells
Collision MRT (d'Humieres 2002)
Boundary condition Bouzidi interpolated bounce-back
Lattice spacing / time step Δx = 1, Δt = 1

Flow Field

Use the tabs below to select a parameter variant. Top left: steady-state velocity contour with streamlines. Top right: flow evolution from rest to steady state. Bottom: pressure and vorticity fields at steady state.

Velocity (Contour | Streamlines)

Contour Streamlines
Contour
Streamlines

Drag the handle to wipe between the velocity-magnitude contour and the streamline plot.

Flow Evolution

Frame 0 / 50

Pressure Coefficient Cp

Cp

Pressure Coefficient Cp

Vorticity

Vorticity

Vorticity

Validation

S/D Regime Mean Cd (total) Mean Cl (total) Cl amplitude
2 Biased gap flow, coupled shedding 3.23 -0.07 0.64
3 Proximity interference 3.40 ~0 0.63
5 Weak interference, quasi-independent 2.87 ~0 0.74
The mean total lift transitions from repulsive (+0.25 at S/D=2, biased gap flow pushing the cylinders apart) toward near-zero and mildly negative as the spacing increases and the two wakes decouple. The combined drag stays elevated relative to an isolated cylinder due to the blockage of the paired bodies. Values are combined over both cylinders (D = 60 cells, Re = 100). Reference: side-by-side circular cylinders at Re=100 (Alam & Zhou 2007).

Discussion

The side-by-side cylinder case tests multi-body interaction in the LBM framework. At large spacing (S/D > 4), the two cylinders behave approximately independently, each shedding a von Kármán street at the same Strouhal number. At intermediate spacing (2 < S/D < 4), proximity interference modifies the drag and wake frequency.

At close spacing (S/D < 2), a strong biased gap flow develops where the jet through the gap deflects toward one cylinder, creating asymmetric forces. This biased state can flip bistably in experiments, though in 2D simulations the bias direction is typically fixed by initial conditions. The coupled vortex shedding frequency differs from the isolated cylinder value and depends sensitively on S/D.