Wake flow and angular oscillation behavior of two cylinders in tandem arrangement forming a van der Pol oscillator

Data result from careful measurements with hot wires and a high-speed camera in an aerodynamic channel with a rectangular cross-section of 0.146 m x 0.193 m (height × width), 2.29 m long, to study the wake behavior and the oscillating pattern of two cylinders in tandem arrangement assembled on a free-to-oscillate table. The cylinders were made from commercial PVC (PolyVinyl Chloride) with 25.1 mm diameter and pitch-to-diameter ratio p/d = 1.26. One of the cylinders is aligned with the table axis, while the second one is placed eccentrically. The table is coupled on the upper and lower channel walls using low wear bearings. The blockage ratio is 13% when the set is perfectly aligned with the flow. The airflow, from a 0.75 kW centrifugal blower, passes through a diffuser, two honeycombs, and two screens, to reduce the flow turbulence intensity to about 1. Upstream of the test section, a Pitot tube measures the free stream velocity, used for the calculation of the Reynolds number. The Reynolds numbers (Re) of the experiments were 8.47 × 10³, 12.7 × 104, and 16.6 × 104, based on the cylinder diameter and a flow velocity of respectively 5, 7.5, and 10 m/s. The axial component of the wake flow velocity behind the cylinder set and its fluctuations is measured with a DANTEC StreamLine constant hot-wire anemometry, with a DANTEC 55P11 single wire probe. The sampling frequency was 1 kHz, low pass filter at 300 Hz, and time acquisition of 32.768 s. Data acquisition was performed by a USB 16-bit NI USB-9162 A/D board. The oscillation of the cylinder set was studied using a high-speed Phantom V411 – Ametek camera with a 50 mm AF-S VR Micro-Nikkor lens, with a resolution of 640 × 480 pixels and 1000 frames per second, corresponding to the 1 kHz sampling frequency of the hot-wire signal. The camera was triggered and synchronized with the CTA via an Ethernet cable. A mirror inclined 45° was mounted at the top of the channel to track the position of a pointer, which indicated the angle between the cylinders and the main flow. The free software Tracker© (https://physlets.org/tracker/) was used to analyze the digital movies to generate time series of the angular displacement of the cylinders. The mean error for this analysis is 0.0239 rad. The experimental procedure was as follows: data acquisition and video recording started while the air blower was turned on. After the setup achieved an apparent stabilization, a disturbance device was inserted from downstream the cylinders