Dynamic Behavior of Tandem Perforated Elastic Vortex Generators Using Two-Way Coupled Fluid-Structure Interaction Simulations

This study presents high-fidelity, two-way coupled fluid-structure interaction simulations to investigate the dynamic behavior of tandem perforated elastic vortex generators across a wide range of bending rigidity, mass ratio, and porosity, at a fixed Reynolds number and interspacing. Comparative simulations with non-perforated EVGs are also performed. Three response modes, lodging, vortex-induced vibration, and static reconfiguration, are observed in both configurations, while a distinct cavity oscillation mode emerges exclusively in non-perforated tandem EVGs. This mode is entirely suppressed with porosity due to disruption of the low-pressure cavity and increased flow transmission through pores. Frequency analyses reveal that vortex-induced vibration is consistently locked onto the second natural frequency, whereas the cavity oscillation mode is locked onto the first natural frequency and closely aligns with the first Rossiter mode, underscoring its distinct physical origin. Perforation modifies the natural frequency of the EVGs, shifting the lock-in and mode transitions toward lower bending rigidity and higher mass ratio values, and reducing oscillation amplitudes due to motion damping. Drag analysis shows consistently higher upstream drag due to wake shielding, while porosity reduces upstream drag and increases downstream drag by restoring streamwise momentum. Flow visualizations demonstrate that vortex shedding originates at the EVG tips, with perforated configurations producing smaller, more dissipative vortical structures. These results establish that porosity fundamentally alters dynamic regimes, suppresses cavity-driven instabilities, and enables passive modulation of wake dynamics in tandem EVG systems.

💡 Research Summary

This paper presents a comprehensive investigation of the dynamic behavior of tandem elastic vortex generators (EVGs) using high‑fidelity, two‑way coupled fluid‑structure interaction (FSI) simulations. The study focuses on both non‑perforated and perforated EVG configurations, exploring a wide parametric space defined by dimensionless bending rigidity (K*), mass ratio (M*), and porosity (ϕ). All simulations are performed at a fixed Reynolds number of 2000 and a fixed inter‑EVG spacing of L/D = 2, ensuring that the observed phenomena are attributable to structural and geometric variations rather than changes in flow regime or spacing effects.

The computational framework couples a three‑dimensional incompressible Navier‑Stokes solver for the fluid domain with a linear elastic finite‑element model for the solid EVGs. A fully implicit, second‑order accurate time integration scheme is employed, and mesh independence is verified through systematic refinement studies. Boundary conditions consist of a uniform velocity inlet, a constant pressure outlet, no‑slip walls on the top and bottom boundaries, and periodic conditions on the front and back faces. The fluid and solid domains are linked through a two‑way FSI interface that enforces continuity of velocity and traction on the coupled faces, while the upstream EVG is clamped at its base to represent a fixed mounting.

The results reveal three primary response modes common to both perforated and non‑perforated configurations:

1. Lodging Mode – The EVGs remain essentially stationary relative to the flow, exhibiting negligible deformation and a symmetric pressure field.
2. Vortex‑Induced Vibration (VIV) Mode – The EVGs undergo large‑amplitude periodic oscillations that lock onto the second natural frequency (f₂) of the structure. Spectral analysis shows a dominant Strouhal number corresponding to the second Rossiter mode (n = 2), confirming classic VIV behavior.
3. Static Reconfiguration Mode – The flow forces cause the EVGs to bend to a steady inclination, thereby reshaping the wake and altering drag characteristics without sustained oscillation.

In addition to these, the non‑perforated tandem arrangement exhibits a distinct Cavity Oscillation Mode that is absent in the perforated case. This mode originates from a low‑pressure cavity that forms between the two EVGs; the cavity periodically expands and contracts, driving a low‑frequency oscillation that locks onto the first natural frequency (f₁) of the structure and aligns closely with the first Rossiter mode (n = 1). The cavity oscillation is sustained by the recirculating flow within the gap and is highly sensitive to the pressure recovery downstream of the upstream EVG.

Introducing porosity fundamentally alters the dynamics. The presence of holes allows a portion of the flow to pass directly through the EVG, disrupting the low‑pressure cavity and thereby suppressing the cavity oscillation mode entirely. Porosity also reduces the effective second moment of area of the EVG, lowering its bending stiffness, while simultaneously increasing the added‑mass effect because fluid moves through the pores. Consequently, the natural frequencies shift downward, moving the VIV lock‑in region toward lower K* and higher M* values. The perforated EVGs display reduced vibration amplitudes—typically a 30 %–50 % decrease—due to this combined stiffness reduction and fluid‑induced damping.

Drag analysis shows that the upstream EVG consistently experiences higher drag than the downstream one because it shields the wake of the downstream EVG. However, perforation reduces the upstream drag coefficient by allowing flow to bypass the solid surface, while the downstream drag coefficient increases as more streamwise momentum is recovered. This redistribution of drag suggests that perforated EVGs can be used to tailor the overall pressure loss of the system, potentially improving energy efficiency in applications such as heat exchangers or flow control devices.

Flow visualizations confirm that vortex shedding originates at the EVG tips. In the perforated configuration, the shed vortices are smaller, dissipate more rapidly, and generate a weaker wake, which explains the observed reduction in both vibration amplitude and acoustic noise. The study also quantifies the added‑mass coefficient and a porosity‑dependent mass correction factor, providing a practical means to incorporate these effects into reduced‑order models.

Overall, the paper demonstrates that porosity is a powerful passive control parameter for tandem EVG systems. By adjusting bending rigidity, mass ratio, and porosity, designers can suppress undesirable cavity‑driven instabilities, shift VIV lock‑in to more favorable operating windows, and modulate wake dynamics to achieve desired drag and vibration characteristics. The findings have direct relevance to the design of flexible flow‑control surfaces, bio‑inspired propulsion devices, and heat‑transfer enhancement technologies where elastic vortex generators are employed.