In this paper, two-dimensional numerical study of dual cylinders with tandem arrangement undergoing sinusoidal oscillation in still water at a relatively low Reynolds number of 200 is conducted with overset grid technique in our solver pimpleDyMFoam-os, which is developed on the basis of open source code OpenFOAM to remain high mesh quality in spite that objects are performing large amplitude motion. Mesh dependent study and model validation are achieved firstly focusing on cases with single cylinder and tandem dual cylinders, where drag coefficients and added mass coefficients are taken into consideration to validate the accuracy of the solver as well as overset grid technique. As for cases with tandem dual cylinders, gap ratios (G/D, where G means gap between two cylinders and D represents diameter of each cylinder) are set as 0.5 and 1.0; Keulegan-Carpenter (KC) numbers are from 5 to 20 with an interval of 2.5, and β numbers are adjusted according to Reynolds Numbers and KC numbers. Drag coefficient and added mass coefficient are obtained with least square method. Moreover, strongly asymmetrical wake about the center line has been clearly observed in some cases with large motion amplitude, hence RMS value of non-dimensional lift force perpendicular to direction of forced motion is calculated to evaluate the degree of wake asymmetry, and its reasonability has been validated to some extent. Moreover, when the gap ratio is small with tandem dual cylinders, the block effect of the gap is strong and the wakes of after each cylinder tend to connect and finally become a union large vortex pair. This vortex pair and the block effect induce wakes to be symmetrical. Thus development of this unique characteristic in time domain and interaction between wakes of each cylinder are deeply analyzed and discussed.
Cylinders in oscillatory flow have been attracting extensive concern due to its significant scientific contribution to people's understanding on mechanism of vortex and its wide spread in engineering field after reasonable simplification since water particle trajectories near seabed are nearly cyclic circular motion. The in-line force on a cylinder in oscillatory flow agrees with Morison Equation, and a linear sum of the drag forces and inertial forces per unit length are just shown as the following Eq. 1.