High-frequency visualization of flexible structures using an event-triggered camera: multiple flapping membranes

Optical measurements of fluid-structure interaction (FSI) usually require high-speed imaging techniques. Traditional high-speed cameras are expensive and cannot record data continuously for long periods. An inexpensive and high-frequency measurement method that utilizes an event-triggered camera and a strobe laser is proposed in this paper. In this method, the k-means clustering technique was first employed to determine the precise time window in which to reconstruct frames from event data. Then, a Kalman filtering algorithm was used to extract the dynamic deformation process of the flexible structures from the noisy event data. Wind tunnel flutter tests were conducted using various sets of parallel membranes to examine the proposed method. Under the illumination of a 30 mW strobe laser, membrane flapping was recorded at a resolution of 1280 x 720 pixels with a frame rate of 10 000 fps. With no camera memory storage limitations, the proposed method could continuously record the membrane flapping, thereby making it easy to capture the intermittent flutter phenomenon at the flutter boundary. Two flapping modes, i.e. symmetric mode and in-phase mode, were observed in the flutter of two membranes. Similar symmetric mode and out-of-phase mode were also observed in the flutter of three membranes. Membranes collisions and mode transitions were found in a certain wind speed range. For the first time, the collision processes between membranes were recorded during the flutter of multiple membranes. These collisions were found to be associated with the transitions between different flapping modes.