Measurement of fluid force on a bluff body in a quasy-steady free surface flow

The understanding of the interaction between fluid current and obstacle represent an important issue in the field of fluid mechanics research with relevant impact on both environment and water-work application. In the last years high level and quality of the studies carried on this topics outline the great care concerning the argument which presents a common interest among, open channel flow, hydraulics and maritine constructions. Study of flow forces on submerged structures as well as characterization of large roughness flows or erosion and sediment transport processes are typical examples. Experiments have been carried out to study the dynamic response of a bluff body in a quasi-steady free surface flow in a horizontal short flume. Flow features were obtained by using several flowmeter equipments, current meter and acoustic doppler velocimeter, in order to select different measurement control volumes and, thus, different velocity scale. The obstacle was a sphere placed in the middle cross section of the flume. Measurements were performed in the flume at Reynolds number from 10,000 up to 60,000, based on the sphere diameter. The drag and lift forces were measured directly at the fixed end of the pivot supporting the sphere by the use of a set of strain gauges. Data analyses have been performed in order to derive both the drag and lift coefficients as well as the inertial longitudinal and transverse coefficients related to the time variation of the flow features. Drag and lift coefficients were derived comparing the steady longitudinal and transverse measured force to the time average velocity data, while the inertial coefficient was proposed versus Strouhal and Reynolds numbers. Finally a comparison between forces and velocity spectrum was performed to ensure possible resonance effect.