— QINGDAO, China — Researchers at the 14th National Conference on Fluid Mechanics are turning to synchronized optical measurement systems to solve the complex riddles of Fluid-Structure Interaction (FSI), moving beyond traditional single-field analysis to a unified view of how structures and flows influence each other.
The conference, held recently in Qingdao, featured a dedicated session on “Multi-field Fluid-Structure Interaction Mechanics.” Discussions ranged from cross-media coupling and flow-induced vibration to the dynamics of flexible structures in complex environments. A central theme emerged: the shift from measuring macroscopic outcomes, such as average load, to understanding the non-steady mechanisms that drive structural response and wake evolution.
The Challenge of Time Scales
In the realm of FSI, particularly in Vortex-Induced Vibration (VIV), galloping, and flexible wing dynamics, the interaction between fluid and structure is characterized by non-steadiness, spatial non-uniformity, and distinct time scales. Traditionally, researchers relied on pressure sensors, accelerometers, and strain gauges. While effective for high-frequency structural data, these contact sensors fail to capture full-field spatial deformation.
Conversely, standalone flow measurement techniques like Particle Image Velocimetry (PIV) can visualize vortex shedding but cannot simultaneously determine the structural response state during transient events. This disconnect has long hindered the validation of theoretical models and Computational Fluid Dynamics/Fluid-Structure Interaction (CFD/FSI) simulations.
Bridging the Gap: PIV and DIC Synchronization
To address this, experimentalists are increasingly adopting the synchronization of Particle Image Velocimetry (PIV) and Digital Image Correlation (DIC) . This approach integrates the measurement of flow fields and structural fields under a unified timeline.
PIV uses laser sheet illumination to track tracer particles, yielding quantitative data on velocity vectors, vorticity, and three-dimensional vortex structures. DIC utilizes high-speed cameras to track random speckles on a structure’s surface, recovering displacement, strain, and vibration modes. By triggering both systems simultaneously and calibrating their spatial coordinates, researchers can establish a direct correspondence between flow structure evolution and structural response.
Case Study: Marine Riser VIV
A detailed presentation at the conference demonstrated this methodology using a Revealer Tomographic PIV (3D3C PIV) and 3D DIC synchronous measurement system . The study focused on a flexible marine riser—a critical component in offshore engineering prone to VIV.
VIV occurs when periodic vortex shedding induces alternating fluid forces, causing the structure to vibrate. This motion, in turn, alters the boundary layer separation and wake structure, creating a classic two-way coupling problem.
Figure: Experimental setup for marine riser VIV analysis using synchronous PIV-DIC technology. (Image: Revealer)
Experimental Setup
The experiment involved a 1.5-meter long, 50mm diameter flexible riser placed in a circulating water channel with a flow velocity of 0.2 to 0.5 m/s. The setup employed a dual-system approach:
• Fluid Side (3D3C PIV): Four G2100M dual-frame cameras captured the three-dimensional velocity field and vortex structures near the riser.
• Structure Side (3D DIC): Two G536 Pro high-speed cameras formed a binocular system to capture 3D dynamic displacement of the riser surface.
Unified Time Baseline for Heterogeneous Sampling
A critical innovation in the experiment was the management of differing sampling frequencies. The structural vibration frequency was below 50 Hz, necessitating a 500 Hz sampling rate via DIC to avoid aliasing. The vortex shedding process, occurring over longer time scales, was captured at 10 Hz via PIV.
By using a common external trigger, the team established a unified time baseline. This allowed them to map the n-th frame of the PIV flow field to the exact corresponding frame of the structural vibration data.
Results: Frequency Locking and Vortex Dynamics
The 3D DIC results showed the riser vibrated primarily in the Z-direction with a main frequency near 2 Hz, exhibiting a first-order lateral vibration mode. Displacements at three axial locations ranged from 3.7 mm to 4.1 mm.
Simultaneously, the 3D3C PIV data reconstructed the wake’s three-dimensional vortex structure using the Q-criterion. In the weak vibration stage, regular alternating vortex shedding was observed. As vibration intensified, co-rotating vortices merged into larger coherent structures.
When analyzed on a unified time axis, the data revealed that both vortex shedding frequency and structural vibration frequency locked onto approximately 2 Hz. This synchronization confirmed the frequency locking phenomenon inherent in VIV. Researchers were able to directly observe how alternating vortex shedding generated periodic excitation, and how the structural motion subsequently modified the local flow separation conditions.
Implications for Future Research
The conference observations underscore a broader trend in fluid mechanics: the integration of complementary optical techniques. As FSI research extends to deep-sea structures, cross-medium vehicles, and flow-solid-acoustic systems, the limitations of separating fluid and structural measurements are becoming untenable.
Revealer PIV-DIC synchronous measurement does not seek to replace traditional force and pressure sensors. Instead, it supplements them with essential full-field spatial data and the critical time-series correspondence needed to validate numerical models. For ocean engineering, wind engineering, and aerospace applications, this synchronized approach provides a robust experimental benchmark for understanding complex, non-steady coupling phenomena.
Contact Info:
Name: Harrison Shawn
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Organization: HF Agile Device Co., Ltd.
Website: http://www.revealerhighspeed.com
Release ID: 89200417
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