Shanti Bhushan

Computational ship hydrodynamics: Nowadays and way forward

Computational fluid dynamics for ship hydrodynamics has made monumental progress over the last ten years, which is reaching the milestone of providing first-generation simulation-based design tools with vast capabilities for model- and full-scale simulations and optimization. This is due to the enabling technologies such as free surface tracking/capturing, turbulence modeling, 6 degree of freedom (DoF) motion prediction, dynamic overset grids, local/adaptive grid refinement, high performance computing, environmental modeling and optimization methods. Herein, various modeling, numerical methods, and high performance computing approaches for computational ship hydrodynamics are evaluated thereby providing a vision for the development of the next-generation high-fidelity simulation tools. Verification and validation procedures and their applications, including resistance and propulsion, seakeeping, maneuvering, and stability and capsizing, are reviewed. Issues, opportunities, and challenges for advancements in higher-fidelity two-phase flow are addressed. Fundamental studies for two-phase flows are also discussed. Conclusions and future directions are also provided.

A Numerical Study to Investigate the Relationship between Moisture Convergence Patterns and Orography in Central Mexico

Abstract This study examines small-scale orographic effects on atmospheric moisture convergence at the ridge–valley scale in the Grande de Santiago River basin in central Mexico during a major monsoon storm on 13–14 August 1999. The simulation was performed using a coupled land–cloud resolving model on three nested grids (at 12-, 3-, and 1-km resolutions). The specific objective is to investigate the physical mechanisms that explain the regional space–time organization of orographic precipitation and cloudiness identified in the region from satellite data. The overarching goals of the research were 1) to characterize the effects of landform and topography-flow geometry relationships on the spatial distribution of precipitation and clouds, especially with regard to the role of mountain winds and lateral drainage flows, and 2) to assess the influence of land–atmosphere interactions (specifically latent and sensible heat fluxes) on moisture convergence patterns during monsoon storms. The model results indica...

Recent progress in CFD for naval architecture and ocean engineering

An overview is provided of CFDShip-Iowa modeling, numerical methods and high performance computing (HPC), including both current V4.5 and V5.5 and next generation V6. Examples for naval architecture highlight capability and needs. High fidelity V6 simulations for ocean engineering and fundamental physics describe increased resolution for analysis of physics of fluids. Uncertainty quantification research is overviewed as the first step towards development stochastic optimization.

A dynamic hybrid Reynolds-averaged Navier Stokes–Large eddy simulation modeling framework

A dynamic framework for hybrid Reynolds-averaged Navier-Stokes (RANS)—large eddy simulation (LES) modeling is proposed, wherein the RANS-to-LES transition parameter is adjusted to maintain continuity in turbulence production. The model is applied for temporally developing plane channel flow at varying Reynolds numbers with different initial turbulence intensity and grid resolution. On sufficiently fine grids, dynamic hybrid RANS-LES model (DHRL) yields similar results to LES simulations. On coarse grids, DHRL activates RANS mode in the log layer, thus improving the mean flow predictions. The model framework addresses grid sensitivity issues observed in other hybrid RANS-LES approaches and may be used with any desired combination of LES and RANS basis models.

Vortical Structures and Instability Analysis for Athena Wetted Transom Flow with Full-Scale Validation

Vortical structures and associated instabilities of appended Athena wetted transom flow in full-scale conditions are studied using DES to explain the source of dominant transom flow frequency, including verification and validation using full-scale experimental data. The results are also compared with model-scale bare and appended hull predictions and experiments. The grid used for the validation is sufficiently fine as it resolves 70% and 91% of the experimental inertial subrange and turbulent kinetic energy values, respectively. The model-scale bare and appended hull resistance predictions compare within 2.5%D and 5.4%D of the experimental data D, respectively. The full-scale appended hull resistance predictions compare within 4.2%D of the extrapolated data using the ITTC line. The averaged comparison error of the full-scale transom wave elevation mean, RMS and dominant frequency predictions and the experimental data is 8.1%D, and the predictions are validated at an averaged 11.2%D interval. The transom wave elevation unsteadiness is attributed to the Karman-like transom vortex shedding as both show the same dominant frequency. The Karman-like instability shows St = 0.148 for the bare hull and St = 0.103 ± 4.4% for model- and full-scale appended hull. The appended hull simulations also predict: horseshoe vortices at the juncture of rudder-hull with St = 0.146 ± 3.9% and strut-hull with St = 0.053 ± 2%; shear layer instability at the strut-hull intersection with St = 0.0067 ± 3%; and unsteady sinkage and trim induced by transom vortex shedding with St = 2.19. The instabilities do not show significant variation on scale, propeller or motions. The bare hull simulation also predicts flapping-like instability in the wake with St = 0.144.

Investigation of turbulence model and numerical scheme combinations for practical finite-volume large eddy simulations

Large eddy simulation (LES) is known to suffer from two primary error sources – the subgrid stress (SGS) model and the numerical discretization scheme. These cannot be separately quantified for finite-volume numerical simulations, but an appropriate combination can yield ‘engineering accurate’ prediction of turbulence dynamics. This paper seeks to evaluate combinations of commonly used second-order numerical schemes and Smagorinsky-type SGS models for practical LES. Error assessments are performed for isotropic decaying turbulence using both pseudospectral and finite-volume solvers, followed by validation for a complex turbulent flow of engineering interest. Error assessment using pseudospectral techniques is performed to isolate finite-differencing and modeling errors by explicitly adding numerical derivative error terms to the simulations. Error assessment using the finite-volume method focuses on identification of optimal model-discretization combinations for the best LES predictions using application solvers. The pseudospectral and finite-volume approaches show consistent predicted behavior of the interplay between numerical and modeling errors for different model-discretization combinations. Of those studied here, the two most optimal combinations are identified as: (1) standard or dynamic Smagorinsky SGS model with a bounded central difference scheme; and (2) Monotonic Integrated LES (MILES) with either a second-order upwind or QUICK scheme. These combinations were applied for an axisymmetric jet flow at Re ∼ 105, using an ‘engineering quality’ mesh, where the MILES model with either an upwind or QUICK scheme showed the best predictive capability.

Post Workshop Computations and Analysis for KVLCC2 and 5415

The Workshop submissions for the local flow predictions for straight ahead KVLCC2 and 5415 were on large disparate grids ranging from 0.6M to 300M, which made it difficult to draw concrete conclusions regarding the most reliable turbulence model, appropriate numerical method and grid resolution requirements. In this chapter, additional analysis including grid verification study is performed on intermediate grids to shed more light on these issues. Second order TVD or bounded central difference schemes are found to be sufficient for URANS, whereas fourth or higher order schemes are required for hybrid RANS/LES (HRLES). Resistance predictions show grid uncertainties £ 2.2 % for URANS on 50M grid and HRLES on 300M grid, which suggests that these grids are approaching asymptotic range. URANS with anisotropic turbulence model perform better than URANS with isotropic turbulence model. Grid with 3M points are found to be sufficient for resistance predictions, however, grids with up to 10s M points are required for local flow predictions. Adaptive grid refinement is helpful in generating optimal grids; however available grid refinement technique based on the Hessian of pressure, fails to refine the grid further downstream along the hull. HRLES simulations are promising in providing the details of the flow topology. However, they show limitations such as grid induced separation for bluff body KVLCC2 and inability to trigger turbulence for slender body 5415. Implementation of improved delayed DES and/or physics based RANS/LES transition is required to address these limitations. Grid resolution of 300M shows resolved turbulence levels of > 95 % for bluff body, thus such grids seem sufficiently fine for HRLES. The free-surface predictions do not show significant dependence on boundary layer predictions, and accurate prediction for 5415 at Fr = 0.28 is obtained using just 2M grid points. The free-surface reduces pressure gradients on the sonar dome, causing weaker vortical structures than single phase. Flow over 5415 shows three primary vortices, and all of them originate from the sonar dome surface. Onset analysis shows that all the three vortices have open-type separation, and separate from the surface due to cross flow. Further investigation of the cause of differences in KVLCC2 CFD submissions and experimental data suggests that it could be due to differences in the sharpness of the stern.

Wall-Layer Modeling for Cartesian Grid Solver Using an Overset Boundary Layer Orthogonal Curvilinear Grid

Wall-layer modeling for a Cartesian grid solver is developed by coupling an orthogonal curvilinear grid solver using overset grid interpolation and coupled pressure Poisson solver. A thin wall-layer grid is considered sufficient to resolve the boundary layer. Initial validation is performed for laminar flows over a circular cylinder, where the wall-layer model shows up to 10% improvement in the surface pressure and vorticity prediction for Re = 40, and 3% improvement in the lift and drag coefficient amplitude for Re = 200 compared to the IBM Cartesian grid solver on the same background grid. Next, validation is performed for the turbulent flow over a circular cylinder at Re = 3900 using LES. The results show very good agreement for the mean circulation bubble, drag coefficients, unsteady vortex shedding Strouhal number, and mean and turbulent velocity profiles when compared with experimental data and benchmark LES simulations. Initial results for LES of interface piercing circular cylinder at Re = 2.7×10 4 and Fr = 0.8 are presented. The results are encouraging, but suggests that the current grid is coarse in the normal direction and finer grids are required.

A note on spectral energy transfer for multiscale eddy viscosity models in large-eddy simulations

The multiscale approach to large-eddy simulation has shown potential to yield improved results over conventional eddy viscosity implementations with a relatively small increase in computational expense. Several variants of the modeled subgrid stress have been proposed, but no conclusions have been drawn regarding which formulations are the most accurate or physically appropriate. This Brief Communication presents a case for choosing a particular form for the subgrid stress tensor, based on relatively simple energy transfer concepts and inertial subrange scaling laws. It is proposed that two requirements should be satisfied. First, that the eddy viscosity depend only on the portion of the resolved energy spectrum near the filter cutoff, and second, that the modeled stress tensor permit energy transfer from the low wave-number modes to the subgrid scales. The result is a “small-large” multiscale variant for the subgrid stress. Results are presented for isotropic decaying turbulence and a transitioning plana...

Numerical simulations of flow pattern and particle trajectories in feline aorta for hypertrophic cardiomyopathy heart conditions

ABSTRACT Numerical simulations of pulsatile flow in a feline aorta for hypertrophic cardiomyopathy (HCM) heart conditions are performed to predict flow details and to evaluate possible thrombus trajectory patterns. The study demonstrates that average flow rate boundary conditions (QBC) at artery outlets act as a resistance-type boundary condition for pulsatile flow. For simulations when the exact artery outflows are not known, specification of estimated values from physiological conditions is a plausible approach. This boundary condition is further improved using an iterative method (I-QBC) to accurately satisfy outflow conditions when expected outflow is known. The approach is validated against experimental data for the prediction of iliac artery flow and wall stresses in a human abdominal aorta. The feline aorta simulations including Lagrangian particle transport are performed on grids with up to 11M cells for a generalized feline aorta. It is found that LES on larger grids performs significantly better than URANS for the prediction of vortical structures. Simulations for both healthy and HCM conditions show similar flow patterns in the upper abdominal aorta. However, the HCM condition shows the presence of large recirculation regions in the thoracic aorta resulting in 50% lower flow through the iliac arteries and increased entrapment of fluid-borne particles near the trifurcation region compared to the healthy condition.