The results of an experiment on the response of model stacks free to oscillate under the influence of aerodynamic forces in a compressed air tunnel are presented. Reynolds number had a considerable influence on the response in both subcritical and supercritical smooth flow. For both models with polished and with slightly roughened surfaces, large peak amplitudes of oscillation found at subcritical Reynolds numbers decreased as the critical regime was approached. For the models with roughened surfaces, further increases of Reynolds number produced continuously higher peak amplitudes and the reduced velocity giving the peak amplitudes decreased from approximately 6.5 to 5. For the polished models no such similar peaks were found in the supercritical regime up to R = 2 X 106, the maximum available, but peak amplitudes occurred at reduced velocities of 10 and 13. For all Reynolds numbers the amplitude of oscillation had a considerable effect on the characteristics of the vortex shedding forces on the stacks. Notationcylinder diameter non-dimensional aerodynamic damping coefficient, 2m8JpD2= K,/pNDa, where K,, is the multiplier of velocity R to give phase component of aerodynamic force length of model equivalent mass per unit length frequency of oscillation of stack (Hz) frequency of shedding of vortex pairs from the sides of a cylinder (Hz) Reynolds number VD/v wind velocity reduced velocity (V/ND) displacement natural logarithm of the ratio of the amplitudes of successive cycles of oscillation (logarithmic decrement), S, refers to structural damping, S, refers to aerodynamic damping (usually negative) normalized displacement, x / D kinematic viscosity of air density of air

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