Browsing by Subject "Marine propeller"

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    Large Eddy Simulation of complex flow over submerged bodies
    (2018-02) Kumar, Praveen
    Predicting the complex flow over a submerged marine vessel in maneuver has two major challenges: the hull boundary layer and the flow due to the propeller. Large eddy simulation (LES) using the dynamic Smagorinsky model (DSM) (Germano \textit{et al.} 1991, Lilly 1992) and discrete kinetic energy conserving numerical method of Mahesh \textit{et al.} (2004) has successfully predicted complex flows in the past. This dissertation discusses four advancements towards reliably using LES to predict and understand the complex flows encountered during maneuvers of submerged marine vessels: (1) understanding skin-friction in axisymmetric boundary layers evolving under pressure gradients, (2) simulating attached flow over axisymmetric hulls and wake evolution, (3) assessing the dependence of the stern flow and axisymmetric wake on hull boundary layer characteristics, and (4) simulating flow through a propeller at design operating condition. Axisymmetric boundary layers are studied using integral analysis of the governing equations for axial flow over a circular cylinder. The analysis includes the effect of pressure gradient and focuses on the effect of transverse curvature on boundary layer parameters such as shape factor ($H$) and skin-friction coefficient ($C_f$), defined as $H = \delta^*/\theta$ and $C_f = \tau_w/(0.5\rho U_e^2)$ respectively, where $\delta^*$ is displacement thickness, $\theta$ is momentum thickness, $\tau_w$ is the shear stress at the wall, $\rho$ is density and $U_e$ is the streamwise velocity at the edge of the boundary layer. Useful relations are obtained relating the mean wall-normal velocity at the edge of the boundary layer ($V_e$) and $C_f$ to the boundary layer and pressure gradient parameters. The analytical relations reduce to established results for planar boundary layers in the limit of infinite radius of curvature. The relations are used to obtain $C_f$ which shows good agreement with the data reported in the literature. The analytical results are used to discuss different flow regimes of axisymmetric boundary layers in the presence of pressure gradients. Wall-resolved LES is used to simulate flow over an axisymmetric body of revolution at a Reynolds number, $Re=1.1 \times 10^6$, based on freestream velocity and the length of the body. The geometry used in the present work is an idealized submarine hull (DARPA SUBOFF without appendages) at zero angle of pitch and yaw. The computational domain is chosen to avoid confinement effects and capture the wake up to fifteen diameters downstream of the body. The unstructured computational grid is designed to capture the fine near-wall structures as well as the wake. LES results show good agreement with the available experimental data. The axisymmetric turbulent boundary layer has higher skin-friction and higher radial decay of turbulence away from the wall, compared to a planar turbulent boundary layer under similar conditions. The mean streamwise velocity exhibits self-similarity, but the turbulent intensities are not self-similar over the length of the simulated wake, consistent with previous studies reported in the literature. The axisymmetric wake transitions from high-$Re$ to low-$Re$ equilibrium self-similar solutions, as theoretically proposed and observed for axisymmetric wakes in the past. The recycle-rescale method of \citet{Lund} is first implemented for unstructured grids and massively parallel platforms and then extended to spatially developing thin axisymmetric turbulent boundary layers. LES of flow over the stern portion of the hull is performed with a prescribed turbulent inflow at a momentum thickness $\theta/a=0.078$ and a momentum thickness-based Reynolds number $Re_{\theta}=2000$, where $a$ is the radius of curvature, to understand the dependence of the flow field in the stern region and the wake, on hull boundary layer characteristics. Additional simulations are performed to study the effect of $Re_\theta$ and $\theta/a$ at the inflow. The turbulent inflows needed for the simulations are generated from auxiliary simulations employing the recycle-rescale methodology. Results are compared to past studies, and used to describe the effect of incoming TBL on the overall flow field. The pressure coefficient on the body is largely insensitive to the incoming boundary layer characteristics, except in the vicinity of flow separation, where it is more sensitive to $\theta/a$. Skin-friction on the other hand, is very sensitive to the boundary layer characteristics. The boundary layer characteristics determine the location of flow separation and hence, the flow field in the stern region and the wake. The wake of the body is more sensitive to $Re_{\theta}$ compared to $\theta/a$. The wake of a five-bladed marine propeller at design operating condition is studied using LES. The mean loads and phase-averaged flow field show good agreement with experiments. Phase-averaged and azimuthal-averaged flow fields are analyzed in detail to examine the mechanisms of wake instability. The propeller wake consisting of tip and hub vortices undergoes streamtube contraction, which is followed by the onset of instabilities as evident from the oscillations of the tip vortices. Simulation results reveal a mutual induction mechanism of instability where instead of the tip vortices interacting among themselves, they interact with the smaller vortices generated by the roll-up of the blade trailing edge wake in the near wake. It is argued that although the mutual-inductance mode is the dominant mode of instability in propellers, the actual mechanism depends on the propeller geometry and the operating conditions. The axial evolution of the propeller wake from near to far field is discussed. Once the propeller wake becomes unstable, the coherent vortical structures break up and evolve into the far wake composed of a fluid mass swirling around an oscillating hub vortex. The hub vortex remains coherent over the length of the computational domain.
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    Large Eddy simulation of high Reynolds number complex flows
    (2012-09) Verma, Aman
    Marine configurations are subject to a variety of complex hydrodynamic phenomena affecting the overall performance of the vessel. The turbulent flow affects the hydrodynamic drag, propulsor performance and structural integrity, control-surface effectiveness, and acoustic signature of the marine vessel. Due to advances in massively parallel computers and numerical techniques, an unsteady numerical simulation methodology such as Large Eddy Simulation (LES) is well suited to study such complex turbulent flows whose Reynolds numbers (Re) are typically on the order of 10^6. LES also promises increased accuracy over RANS based methods in predicting unsteady phenomena such as cavitation and noise production. This dissertation develops the capability to enable LES of high Re flows in complex geometries (e.g. a marine vessel) on unstructured grids and provide physical insight into the turbulent flow. LES is performed to investigate the geometry induced separated flow past a marine propeller attached to a hull, in an off-design condition called crashback. LES shows good quantitative agreement with experiments and provides a physical mechanism to explain the increase in side-force on the propeller blades below an advance ratio of J=-0.7. Fundamental developments in the dynamic subgrid-scale model for LES are pursued to improve the LES predictions, especially for complex flows on unstructured grids. A dynamic procedure is proposed to estimate a Lagrangian time scale based on a surrogate correlation without any adjustable parameter. The proposed model is applied to turbulent channel, cylinder and marine propeller flows and predicts improved results over other model variants due to a physically consistent Lagrangian time scale. A wall model is proposed for application to LES of high Reynolds number wall-bounded flows. The wall model is formulated as the minimization of a generalized constraint in the dynamic model for LES and applied to LES of turbulent channel flow at various Reynolds numbers upto Re_tau=10000 and coarse grid resolutions to obtain significant improvement.