A methodology for digitizing and processing calcareous biofouling typically found on US Navy ship hulls has been developed. Panels that were immersed in seawater and allowed to grow biofouling were captured using 3-D laser scanning. The advantage of these digital replicas over real biofouled rough surfaces are many-fold: the surfaces can be manipulated to meet channel flow and large eddy simulation (LES) viscous size constraints; 3-D printing can then be used to build scaled rough surfaces that can be used in the fully developed turbulent channel flow; complex statistical and geometric parameters that encapsulate drag-producing physics can be computed; subregions of the surfaces can be tiled together to create composite surfaces that can span various parameter spaces. This paper describes, in detail, the digitizing, surface preparation, and 3-D printing methodologies. In addition, it describes the surface characterization software. Data from nine scanned surfaces, with biofouling from coastal Florida and Pearl Harbor, Hawaii are shown with preliminary correlations between pierside data and more complex geometric parameters. The work described herein is part of a larger project to develop a fast and accurate ReynoldsAveraged Navier Stokes (RANS) computational fluid dynamics (CFD) method to predict the drag penalty of fouled ships based on data obtained from pierside underwater surveys.

A high-accuracy large eddy simulation (LES) is applied to ows over a sphere and an underwater vehicle. Both objects have relatively weak separated stern ows that may depend upon the accurate resolution of turbulence structures in the attached ow regions upstream of the separation point. For the sphere we compute ows over a range of Reynolds numbers from subto super-critical (Re = 1 10 to Re = 1:14 10, respectively) for which we obtain decent agreement for the separation location, pressure distributions and integrated forces. Long time series data shows evidence of low-frequency shedding phenomena. We perform LES on the Advanced SEAL Delivery System (ASDS), an underwater vehicle with a rounded-rectangular cross section and stern slope that promotes weak ow separations. We compute the fully resolved ow over the ASDS for length-based Reynolds numbers 128 10, 256 10 and 512 10. We show that the mean ow elds over the attached ow region are reasonable in that the boundary layer pro les, shape factors and skin friction agree with other examples of developing turbulent boundary layer ows. The instantaneous ow elds exhibit near-wall turbulence structures with the correct length scales and dynamics as compared with the wall-bounded turbulent ow literature. The separation point moves aft and the extent of the separation region decreases markedly as the Reynolds number increases. Nomenclature As Surface area of sphere, m CD Integrated drag coe cient, Fx=(0:5 U oAs) Cf Local drag coe cient, w=(0:5 U o ) Cp Pressure coe cient, p=(0:5 U o ) D Sphere diameter, m Fx;y;z Force in streamwise, spanwise and vertical directions, N h Height of ASDS parallel middle body normalized by ‘gu H Boundary layer shape factor ‘gu Length of grid unit for ASDS, 5:08 cm p Pressure, N=m r;R Radii of ring-torus topology as described in the sphere results section. ReL Reynolds number for ASDS based on total length and free stream velocity. Rex Reynolds number for ASDS based on distance from bow and free stream velocity. Re Reynolds number for ASDS based on displacement thickness and free stream velocity. Corresponding author. Email: peter.chang@navy.mil, Computational Hydromechanics Division, Naval Surface Warfare Center | Carderock Division (NSWCCD), West Bethesda, MD 20817. Senior Member. yComputational Hydromechanics Division, NSWCCD, West Bethesda, MD 20817, non-member. zComputational Hydromechanics Division, NSWCCD, West Bethesda, MD 20817, non-member. xComputational Hydromechanics Division, NSWCCD, Member. {Professor, Dept. of Aerospace and Mechanical Engineering, University of Minnesota, Minneapolis, MN, 55455, Associate Fellow This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.2011

Despite efforts to maintain streamlined shapes for minimal resistance and noise, US Navy marine vehicles oftentimes must have bluff body geometries or must operate in off-design modes. These situations produce separated flows that are unsteady. At large scales the flow unsteadiness may be the cause of structural concerns while the entire range of fluid dynamic scales, from large to small, may be the cause of unwanted hydro-acoustic radiated noise. Large-eddy simulations (LES) are a class of fluid flow solvers that resolve the energy-containing scales of attached and separated flows, potentially providing the velocity field forcing functions for structural and hydro-acoustics analyses. LES have strict temporal and spatial grid spacing requirements and in order to resolve the energy-containing turbulence scales at practical flow velocities, the problem sizes and run times can be quite impressive. The objective of this Challenge Project is to apply LES to several problems of urgent US Navy need that have heretofore been considered too large to solve in a timely fashion. The first of these problems occurs when a marine vehicle, translating forward, must stop suddenly. This entails reversing the rotation of the propeller in a maneuver called crashback. This generates the largest forces that a propeller will undergo in its lifetime, and therefore, prediction of the forces and determination of procedures to reduce the forces, are of utmost importance. Using LES we have been able to uncover and verify the physics of force generation. With this Challenge Project, methodologies are being developed for one-way coupling between fluid flow and structural calculations that entail 300,000+ hours of CPU time. Another problem that is being tackled with the help of the Challenge grant is flow about the Advanced SEAL Delivery System (ASDS). This is a challenging geometry as it has both complex geometry and is a high-Reynolds number attached flow. Thus, the grid quality and size are important issues to overcome. In this paper, we document simulations of bare-hull structured grids with 4 and 20 million cells. Results of our wall-resolved simulation show that the attached boundary layers behave as expected and therefore, give us confidence that we can correctly predict the occurrence and strength of stern flow separations.

In this paper, we document large eddy simulations (LES) performed on a sphere and the Advanced Swimmer Delivery System (ASDS), a small blunt-ended submarine. The objective of this work is to develop a methodology for being able to compute the occurrence and strength of stern separation in order to accurately predict drag, maneuvering and structural loads, and acoustic signatures. We are using an energy-conserving large eddy simulations (LES) code called MPCUGLES that runs with hybrid structured-unstructured meshes. We document LES of flow over a sphere at Reynolds numbers 10,000 to 1.14 million with excellent comparison to experimental data. We also document a preliminary effort for LES of flow over ASDS. Even the simplest of these computations is very CPU-intensive necessitating the large amounts of CPU time available through the HPCMP Challenge Project C3U.

This study investigates the topographic deformation due to the erosion of a sand bed impinged by a moving submerged turbulent round jet in a large-scale laboratory. The test conditions represent the case of discharges beneath a vessel while operating in water with a limited clearance such as a shallow navigation channel. The jet moves horizontally and discharges water vertically downward towards the bed. The distance between the jet nozzle and the bed equals six times the jet diameter so the jet flow is in the potential core region. The speed of the jet horizontal motion was varied to examine its effect on the scour profile. The characteristic lengths of the scour profile in the asymptotic state were determined by modifying the empirical formulas in Aderibigbe and Rajaratnam [1996. Erosion of loose beds by submerged circular impinging vertical turbulent jets. Journal of Hydraulic Research 34(1), 19–33]. The maximum scour depth, the scour hole radius, and the ridge height were found to be a function of the ratio of the jet exit to jet translation velocities and were modeled using a hyperbolic function. Empirical equations describing the scour profile were developed and the scour profile was found to be self-similar when normalized by appropriate length scales.

Propeller crashback, an emergency maneuver undertaken when a forward-moving vessel needs to stop quickly, involves reversing the propeller rotation and running the propeller in reverse into the oncoming flow. This maneuver generates low frequency, high amplitude forces on the blades which impart pitch and yaw moments on the vessel. The origins of these forces are uncertain and there are no tools based on first principles with which propeller designers can predict the maximum loadings during crashback. In order to better understand the origins of these phenomena and develop a loading prediction tool a large eddy simulation (LES) methodology using an unstructured, finite volume, incompressible LES code with a 2nd-order accurate central difference (CD) flux reconstruction methodology, and a dynamic sub-grid scale model is being applied to crashback. In this paper, we compare results of this code with experimental data and a commercially-available unstructured finite volume LES code with upwind (UW) flux reconstruction without sub-grid scale model. Crashback simulations have been performed on the 0.3048 m diameter, zero-skew angle, 5-bladed Propeller 4381operating at J=-0.5. We compare the mean, RMS higher order statistical moments for the integrated forces and moments to experimental data. Comparison to the experimental power spectral density (PSD) functions shows that both codes correctly predict the low-frequency blade loading and the blade rate energy concentrations. However, the CD code predicts a wider range of turbulence scales around the blade rate peak that may be important for loading dynamics. The CD LES data reveals that the flow has a bi-modal behavior that switches between vortex ring (VR) and axial jet (AJ) modes which are associated with minimum and maximum loadings, respectively.

An isotropic turbulent flow field was synthesized and advected to a targeted turbulence intensity and wave number spectrum using a large eddy simulation (LES). The turbulence was synthesized at the inflow boundary of the computational domain using the Random Flow Generation (RFG) technique. In this particular implementation of the RFG method, the turbulent velocity fluctuations are synthesized from samples of a Gaussian velocity distribution that is deconstructed into its Fourier harmonics. The synthesized turbulence was then advected downstream to a spatial location where a target turbulence intensity and wave number spectrum were specified. Controllable parameters at the inflow boundary were inflow turbulence intensity and length scale. These two parameters, along with the size and spatial resolution of the computational domain, and the advection time step, were varied until the target turbulence intensities and spectrum were achieved. In the LES technique, the filtered, incompressible Navier-Stokes equations were discretized using a second order, bounded central difference scheme. Time advancement was implicit and second order accurate using a fractional step method. The sub-grid scale turbulence was accounted for with a dynamic Smagorinsky model. The final turbulence intensity and wave number spectrum are reasonably close to the targeted values.

ATTACHED, wall-bounded flows impose computational requirements on LES that increase drastically with Reynolds number. For that reason, even simple geometries, such as airfoils at small angles of attack, with spanwise uniform section shape, challenge the bounds of LES as chord-based Reynolds numbers increase much above 1 million. Of particular concern is the ability of LES to predict the occurrence, and strength of, weak vortex shedding from the airfoil trailing edge (by weak vortex shedding we mean that the acoustic vortex shedding signature may rise only a few decibels above that for the broadband turbulent boundary layer acoustic sources). Correct prediction of weak vortex shedding may depend on accurately predicting the flow over the entire airfoil that includes the attached, turbulent upstream flow, adverse pressure gradient and separated flow regions and finally, the turbulent wake. This paper compares results of two full-LES and two LES with wall-stress model for the flow about a modified NACA 0016 airfoil with a 41° trailing edge apex angle and a slightly convex pressure side. Comparisons of vortex shedding, as measured by the power spectral density (PSD) of wall pressure fluctuations (WPF) on the pressure side of the TE and the PSD of the vertical velocity fluctuations in the wake are made. The results indicate that vortex shedding predictions are dependent upon the stream-wise and spanwise grid resolution. In order to reduce the large computational times required for simulating the high-Reynolds number flows with fully-resolved LES, a wall-stress model that solves the turbulent boundary layer equations in the near-wall region is applied. Compared with the fully-resolved LES, the LES with wall-stress simulations require about 20 percent the number of grid points and require about 10 percent of the computational time. However, the LES with wall stress model results under-predict the vortex shedding peak in the wake and are not able to predict the vortex shedding signature in TE wall pressure spectra. These results indicate that near-wall turbulence structures need to be resolved in order to correctly predict the occurence and strength of vortex shedding.

A detailed study of the compressible, turbulent flow around a circulation control airfoil has been conducted using a large eddy simulation (LES). The circulation control airfoil investigated is the NCCR 1510-7067N, which has a circular arc shaped trailing edge. The Reynolds number for the problem is 5.45x10, based on chord, with a free stream Mach number of 0.12. The non-dimensional blowing rate (Cμ) for the Coanda jet is 0.093. The filtered Navier-Stokes equations are discretized using a fifth-order spatially accurate, upwind scheme with no limiting. The sub-grid scale turbulence model combines a transport equation for the sub-grid scale turbulent kinetic energy with a turbulent length scale, based on local grid dimensions, to form a expression for the sub-grid scale eddy viscosity. The resulting mean surface pressure distribution compares fairly well to experimentally measured values for the airfoil. Resulting mean velocity and Reynolds stress profiles at selected locations around the airfoil are examined.

The flows about 2-D and 3-D bluff trailing edge circulation control (CC) airfoils are computed using steady Reynolds Averaged Navier-Stokes (RANS) methods. The 2-D foil is the NCCR 1510-7067 elliptical CC airfoil with circular and logarithmic spiral trailing edge geometries. The free stream Reynolds number, based on chord, is 5.45 × 10 5 , with a free stream Mach number of 0.12. For the circular trailing edge the slot height, blowing rate and angle of attack are varied, while for the logarithmic spiral only the blowing rate is varied. The 3-D foil is a semi-span wing with an elliptical cross section. It is run with a chord-based Reynolds number of 2 × 10 6 and two blowing rates. The 2-D flows are computed using the compressible, segregated solver, Fluent. 2-D results show that the full-Reynolds stress turbulence model (FRSM) predicts the correct jet detachment behavior for the circular trailing edge although the integrated lift forces are consistently underpredicted. The coanda jet detachment point for the logarithmic spiral trailing edge is predicted correctly for a lower blowing rate, but as blowing rate increases, the jet does not detach until it has wrapped around to the pressure side. We show additional 2-D results using mesh refinement via grid adaption and isotropic eddy viscosity turbulence models. The 3-D simulations use the incompressible segregated Fluent solver applying the k −ω SST turbulence model. Results show a slight attachment of the the coanda jet on the pressure side, but the results are generally encouraging.