Ramp Tests: A Novel Approach to VIV Model Testing of Flexible Cylinders Using Continuously Varying Towing Speeds

In this paper we present a small portion of the results from the 2011 SHELL tests at the MARINTEK basin. The tests involved towing densely instrumented flexible cylinders at Reynolds numbers up to 150,000 in order to study the Vortex-Induced Vibration (VIV) response in uniform current profiles. This paper presents the experimental results collected from a series of tests where the towing speed was continuously varied while the cylinder and carriage traversed the basin. As the cylinder is accelerated (or decelerated) the incident current speed is continuously changing which means that multiple modes can be excited consecutively in a single tow through the basin. These varying towing speed tests are collectively referred to as ‘ramp tests’. The response data collected in these ‘ramp tests’ are presented in terms of CF response amplitudes and strains and are carefully compared with the response data collected during conventional steady towing speed tests. The data shows that when the acceleration of the carriage is kept below a critical value the ‘ramp tests’ are then able to provide VIV response information which is equivalent to that obtained from many constant speed tests. One ramp test provides the equivalent response data of (up to) 10 constant speed tests in a single run. This paper also introduces a dimensionless parameter γ, which determines if the proposed acceleration for a ramp test is within acceptable limits and may be used as a substitute for many constant speed tests. The parameter can be used to determine the appropriate acceleration or deceleration rate in order to ensure that a ramp test will yield suitable VIV response data. The parameter also allows one to know quickly whether or not fully developed VIV is possible in a given set of unsteady flow conditions.