SUMMARYNumerical simulations have been performed for flow past two equal-sized square cylinders in tandem arrangement subjected to incoming planar shear flow. Effect of L/d ratio and the shear parameter has been studied. The range of L/d ratio (ratio of center-to-center distance (L) to cylinder width (d)) is varied from 2 to 7 and the non-dimensional shear parameter (K ) is varied from 0.0 to 0.4 in steps of 0.1. For all the cases the Reynolds number (Re) based on centerline velocity and cylinder width is fixed at 100. The results are compared with that of isolated square cylinder with uniform flow. Strouhal number decreases with increasing shear parameter. There are more than one shedding frequency at high shear parameters and L/d ratios. The mean drag coefficient is decreased with shear parameter and lesser than that of the single cylinder. The root mean square (RMS) value of both lift and drag coefficients is higher for the downstream cylinder for all values of shear parameter. With increasing L/d ratio, for both lift and drag, the RMS value increases and then decreases for upstream cylinder, whereas it continuously increases for the downstream cylinder. The stagnation point is moved towards the top leading edge with increasing shear. The critical L/d ratio, which is defined as the distance between two cylinders, beyond which the vortex shedding from the upstream cylinder occurs, decreases with increasing shear parameter.

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A computational fluid dynamics approach to study drag reduction of axisymmetric underwater bodies by air jet injection in the boundary layer is presented. The well-known 'mixture' model is used to capture the multiphase flow and the SST k-ω (shear stress transport) turbulence closure model has been used in the computations. Well-studied Afterbody1 (Huang et al., 1978) which has a tapered and smooth stern profile is considered. A companion shape of Afterbody1, which has a blunt stern profile, is also studied. The numerical study is carried out with different air jet velocity to body velocity ratios, various angles of air jet and various angles of attack of the body. Effects of these parameters on drag reduction are reported. The effect of tapered vs. blunt aft shape of Afterbody1 has been found to have significant effect on drag reduction performance.

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Non-canonical wakes of two-dimensional elliptic cylinders are analysed numerically for their near- and far-wake characteristics. The governing equations are solved using an immersed boundary method based projection scheme. The wakes are then classified into three distinct types according to diverse flow and thermal properties. An unexpected mean temperature evolution along the centreline of the wake is observed for certain wake states. In order to explain this unusual variation, novel heat transport models are constructed based on the vortex dynamics. These models are derived by considering vorticity is acted by flow, which has shear and swirl. Mechanisms of the primary vortex street breakdown and formation of the secondary vortex street are also proposed based on these models. A new phenomenon namely ‘dual near-wall instantaneous recirculation’ is observed, and its appearance is found to be a function of length of the primary von Kármán vortex street. The same phenomenon is also found to be responsible for the secondary peak in the Nusselt number variation along the circumference of the cylinder. Despite varied differences between the wake types, it is observed that the transitions occur through a supercritical Hopf bifurcation in all of them, at least in the von Kármán region of the wake. Low-frequency unsteadiness observed in the far wakes is examined through a signal decomposition method. Our results show that the secondary low frequency is resulting from the transition region which has a negative instability slope. Finally, onset of the primary vortex street breakdown and its scale in terms of Reynolds number is computed.

SUMMARYThe transition of square cylinder wake flow from two-dimensional (2-D) to three-dimensional (3-D) when inflow is subjected to linear shear is examined numerically. The value of the non-dimensional shear parameter (K ) considered in this study are 0.0, 0.1, and 0.2. The range of Reynolds number (Re) defined based on the centerline velocity and cylinder width is from Re = 150 to 700. The transition of the wake flow from 2-D laminar to 3-D is marked by streamwise vortical structures. Unlike in uniform flow, in shear flow the transition is characterized by single mode of spanwise wavelength. The critical Reynolds number (Re crit ), at which the transition from 2-D to 3-D occurs, is less in case of shear flow. The magnitude of the mean lift coefficient increases with increasing shear parameter on the positive side. The strength of the Karman vortices on the top side is higher and on the bottom side is lower when compared with the same in the uniform flow.

SUMMARYIn uence of ÿnite di erence schemes and subgrid-stress models on the large eddy simulation calculation of turbulent ow around a blu body of square cylinder at a laboratory Reynolds number, has been examined. It is found that the type and the order of accuracy of ÿnite-di erence schemes and the subgrid-stress model for satisfactory results are dependent on each other, and the grid resolution and the Reynolds number. Using computational grids manageable by workstation-level computers, with which the near-wall region of the separating boundary layer cannot be resolved, central-di erence schemes of realistic orders of accuracy, either fully conservative or non-conservative, su er stability problems. The upwind-biased schemes of third order and the Smagorinsky eddy-viscosity subgrid model can give reasonable results resolving much of the energy-containing turbulent eddies in the boundary layers and in the wake and representing the subgrid stresses in most parts of the ow. Noticeable improvements can be obtained by either using higher order di erence schemes, increasing the grid resolution and=or by implementing a dynamic subgrid stress model, but each at a cost of increased computational time. For further improvements, the very small-scale eddies near the upstream corners and in the laminar sublayers need to be resolved but would require a substantially larger number of grid points that are out of the range of easily accessible computers.

This paper addresses the Computational Fluid Dynamics Approach (CFD) to simulate the flow over underwater axisymmetric bodies at higher angle of attacks.  Three Dimensional (3D) flow simulation is carried out over MAYA Autonomous Underwater Vehicle (AUV) at a Reynolds number (Re) of 2.09×106. These 3D flows are complex due to cross flow interaction with hull which produces nonlinearity in the flow. Cross flow interaction between pressure side and suction side is studied in the presence of angle of attack. For the present study standard k-ε model, non-linear k-ε model models of turbulence are used for solving the Reynolds Averaged Navier-Stokes Equation (RANS). The non-linear k-ε turbulence model is validated against DARPA Suboff axisymmetric hull and its applicability for flow simulation over underwater axisymmetric hull is examined. The non-linear k-ε model performs well in 3D complex turbulent flows with flow separation and flow reattachment.  The effect of angle of attack over flow structure, force coefficients and wall related flow variables are discussed in detail. Keywords: Computational Fluid Dynamics (CFD); Autonomous Underwater Vehicle (AUV); Reynolds averaged Navier-Stokes Equation (RANS); non-linear k-ε turbulence modeldoi: http://dx.doi.org/10.3329/jname.v8i2.6984   Journal of Naval Architecture and Marine Engineering 8(2011) 149-163