Experimental data of the space ratio influence on the excitation frequencies of one and two cylinder free to vibrate in tandem arrangement

Data result from careful measurements with accelerometers in an aerodynamic channel to study the influence of the space ratio on the flow-induced vibration of two cylinders in tandem arrangement with one or both cylinders free to vibrate.
The aerodynamic channel is rectangular (0.193 X 0.146 m), with acrylic walls. A centrifugal blower of 0.75 kW impels the air through a diffuser, two honeycombs, and two screens. The turbulence intensity is 1% of the reference velocity, measured with a Pitot tube upstream of the test section.
The Reynolds number ranged from 7100 to 24000, based on one cylinder diameter, and the reference velocity.
The cylinders are smooth with 25mm diameter and L/D-ratios of 1.26, 1.4, 1.6, and 3.52. The cylinders free-to-vibrate in the transversal direction, are attached to two blades allowing the variation of length, to change the stiffness, as in Neumeister et al. (2021). The blockage ratio is 13.01 %.
Nomenclature SC indicates a single cylinder (as reference). For tandem cylinders, FL means that the first cylinder is free to vibrate, for SL, the second cylinder, and BV both cylinders are free to vibrate.
The cylinder acceleration is measured with accelerometers ADXL 335, data acquisition frequency, 1 kHz, low pass filter at 0.3 kHz. A 16bits A/D board NI USB-9162 was employed for the data acquisition.
The mass-damping parameters were equal for the Cases 01 to 09: the mass ratio m*=608, the damping ratio ζ = 0.03, and the natural frequencies, fn1 = 7.8Hz and fn2 = 21.5 Hz.
Cases 10 and 11 considered acceleration and flow velocity for m* = 539, ζ = 0.004 and the natural frequencies, fn1 = 8.8Hz and fn2 = 23.4 Hz.
The cases with both cylinders had three configurations. First BV-configuration, Case 12: the first cylinder with a lower natural frequency than the second one. The parameters for the first cylinder were m* = 502, the damping ratio ζ = 0.003 and the natural frequencies are fn1 = 13.7Hz and fn2 = 36.2 Hz. For the second cylinder, m* = 547, ζ = 0.006 and the natural frequencies are fn1 = 24.4 Hz and fn2 = 71 Hz. Second BV-configuration, Case 13: the first cylinder with a higher natural frequency than the second. The parameters for the first cylinder were m* = 502, ζ = 0.002 and the natural frequencies are fn1 = 29.3 Hz and fn2 = 58.6 Hz. For the second cylinder, m* = 547, ζ = 0.009, and are fn1 = 10.7 Hz and fn2 = 30.3 Hz. Third BV-configuration, Case 14: both cylinders with the natural frequency in the same range. The parameters for the first cylinder were m* = 502, ζ = 0.008 and the natural frequencies fn1 = 10.8 Hz and fn2 = 22.5 Hz. For the second cylinder, m* = 547, ζ = 0.007 and the natural frequencies are fn1 = 10.7 Hz and fn2 = 31.2 Hz.

Reference
Neumeister, R. F., Petry, A. P., and Möller, S. V. 2021a). Experimental Flow-Induced Vibration Analysis of the Crossflow Past a Single Cylinder and Pairs of Cylinders in Tandem and Side-by-Side. Journal of Pressure Vessel Technology, v. 143(3), p. 031402.