Modeling the unsteady lift and drag on a finite-length circular cylinder in cross-flow

Semi-empirical models for unsteady lift and drag are developed to predict t he spectral features of the unsteady forces on a finite-length, right circu lar cylinder in cross-flow. In general, the models consist of two parts; th e spatial variation of r.m.s wall pressure on the cylinder, and the correla tion lengths which describe the spatial extent of the correlation of the un steady wall pressures. Experiments were conducted in a low noise wind tunne l as a function of cylinder diameter Reynolds number (19200 < Re < 32000) a nd the Strouhal number (0.05 < St < 3.33), to measure the statistics of the unsteady wall pressures on a model cylinder. These results are incorporate d into the theoretical models, and predictions of the spectral characterist ics of the lift and drag are made. The r.m.s. wall pressures on the cylindr ical surface are found to have the largest amplitude near the cylinder end- cap, and on the rearward portion of the cylinder body. The high levels in t hese locations are attributed to the separated flow region over the end-cap . The circumferential and axial length-scales decrease exponentially with S trouhal number. Both length-scales exhibit maxima near the Strouhal sheddin g frequency of St = 0.21. The axial length-scales are found to depend on th e measurement reference location due to the three-dimensional flow and sepa rated flow region near the end-cap. The unsteady lift and drag predictions using the models developed in this work agree well with previously measured unsteady force data measured on inertial hydrophones exposed to flow. The broadband unsteady lift is found to be greater than the broadband unsteady drag by nominally 3 dB. (C) 2000 academic Press.