Experiments on VIV Under Frequency Modulation and at Constant Reynolds Numbers

The present paper presents a new mechanical apparatus, specially designed for investigating fundamental aspects of VIV in a Towing Tank. An electrically driven movable spring-leaf system was conceived and constructed to deal with elastic mounted cantilevered cylinders, either rigid or flexible ones. The apparatus has low structural damping and allows the excitation of VIV under continuously modulated spring rigidity, aiming at investigating the effect of natural frequency modulation on the response. The experiments were carried out at IPT Towing Tank, which is 280m long, 6m wide and 4m deep. Both rigid and flexible cantilevered cylinders mounted on the new apparatus were studied. In this paper, some results on the rigid cylinder are presented. A striking feature of such apparatus is the possibility to obtain the whole VIV amplitude and frequency response curves from a single run, under a single towing velocity, therefore at the same Reynolds number. This is possible as the reduced velocity varies along that particular constant velocity run, according to the natural frequency modulation.