Overset Grid System Research Articles

Explicit Source 기반의 Actuator Model 사용을 위한 OpenFOAM의 중첩 격자 라이브러리 수정

This study purpose is modifying the overset grid library of OpenFOAM to use an explicit source in the overset grid system. To this end, the functions of the OpenFOAM"s overset grid library that classify the computational volume cells was modified. Actuator Disk Model based on an explicit source was employed to confirm the functionality of the modified functions. The performance analysis at hover flight and the inflow analysis at forward flight were performed using the modified overset library. Since the hovering rotor performance results with single and overset meshes were almost similar, it was confirmed that the modified overset library well performed at hover flight. Through the forward flight rotor analysis, it was confirmed that the rotor inflow analysis results were similarly predicted in the single and overset meshes. Through previous analysis results, it was confirmed that the modified overset library worked properly in the problem of using explicit source.

Enhancement on parallel wall distance calculation methodology for partitioned unstructured grid

AbstractMinimum distance to a solid wall is a primary parameter in turbulence models and overset grid assembly for computational fluid dynamics. In present work, a parallel advancing front method is proposed based on partitioned unstructured grids for the sake of further efficient wall distance computation. In order to overcome the inherent problem of conventional advancing front method in parallel environment, a novel “advancing twice and rippling once” strategy is developed to compute wall distance efficiently and further recover the accuracy. Significantly, the established framework is of modular nature to be easily extended to overset grid system for complex multi‐body configurations. The performance of the developed techniques is evidenced by comparison with the existing alternative ways in terms of computing efficiency and accuracy. Subsequently, further case studies are performed to examine its capability to deal with complex engineering applications such as a full transportation, a wing‐store configuration, a helicopter and a F‐16 fighter with onboard payloads. Results show that the proposed advancing front methodology is in practice able to solve extremely complex geometries with both convex and concave shapes with high efficiency and robustness.

Numerical Investigation of an Unsteady Blade Surface Blowing Method to Reduce Rotor Blade-Vortex Interaction Noise

This paper presents a numerical investigation of unsteady surface blowing using periodic variations of jet velocity with azimuthal angle to reduce helicopter rotor blade-vortex interaction (BVI) noise. The unsteady blowing is modeled as the mass flow outlet boundary condition of time-varying jet velocity on the blade surface grid using computational fluid dynamics. The same high-resolution overset grid system and flow/noise solver are used to perform a detailed flow field simulation and noise prediction for the nonblowing baseline case and the steady/unsteady blowing cases under the rotor BVI condition, and a grid convergence study for the steady and unsteady blowing cases is carried out. The BVI noise reduction and rotor thrust coefficient results of the unsteady blowing method and the previously published steady blowing with constant jet velocity are then compared. The noise reduction level of unsteady blowing is approximately equivalent to that of steady blowing (noise reduction is more than 3 dB). However, the loss in rotor thrust coefficient caused by unsteady blowing (3.3%) is only half of that by steady blowing (6.3%); the air mass cost by unsteady blowing is only 63.7% of that by steady blowing per rotation revolution. The results show that unsteady blowing can effectively reduce BVI noise with lower cost and less thrust loss.

A new method to couple FEM mooring program with CFD to simulate Six-DoF responses of a moored body

The objective of this study is to develop an accurate and efficient coupled program, which incorporates computational fluid dynamics, a six-DoF motion solver, and nonlinear finite-element cable analysis program MOORING3D. The new coupled method aims at studying dynamic responses of moored structures in environments which cannot be modeled by conventional approach. The novelty of the coupled method also lies in that we develop the methodology for an overset grid system of embedding, overlapping, and moving structured grids. Large eddy simulation was used as the turbulence model. The level-set method was applied to capture free surface effects. Two buoys were studied. Wave-induced motions of the first buoy were simulated by both coupled CFD-mooring method and commercial software. The discrepancy was investigated to illustrate advantages of the coupled method in addressing viscous and nonlinear hydrodynamic effects. The wave-current-body interaction of another buoy was simulated with RAOs being compared against experiments and vortex-induced motion being investigated. The conclusions indicated that the coupled CFD-FEM method provides feasible solutions for floating systems exposed to complicated environmental conditions.

A Numerical Ventilation Problem on Fridsma Hull Form Using an Overset Grid System

Numerical ventilation problem (NPV) is a problem that often occurs during the numerical simulations process. This problem mostly occurs when the vessel has a high Froude Number, causing inaccurate ship resistance predictions. Numerical ventilation can be considered as one of the major sources of error in numerical simulations that require further analysis. The present paper aims to validate a numerical resistance prediction using Fridsma’s hull form. An overset grid system is implemented in this study to solve using the Volume of Fluid method. The method for improving the accuracy of ship resistance prediction are refinement method, visualization method, and phase replacement method. In this paper, the RANS equation is used to describe the turbulence model using k-ε. On most occasions, the Volume of Fluid model uses multiphase Euler flow, assuming air and water as a phase. This research concludes that mesh refinement method is able to solve numerical ventilation problem with such a good result.

Rotor Airload and Acoustics Prediction Based on CFD/CSD Coupling Method

An advanced airload and noise prediction method based on computational fluid dynamics/computational structural dynamics (CFD/CSD) coupling for helicopter rotor has been developed in this paper. In the present method, Navier-Stokes equation is applied as the governing equation, and a moving overset grid system is generated in order to account for the blade motions in rotation, flapping and pitching. The blade structural analysis is based on 14-DOF Euler beam model, and the finite element discretization is conducted on Hamilton's variational principle and moderate deflection theory. Aerodynamic noise is calculated by Farassat 1A formula derived from FW-H equation. Using the developed method, numerical example of UH-60A is performed for aeroelastic loads calculation in a low-speed forward flight, and the calculated results are compared with both those from isolated CFD method and available experimental data. Then, rotor noise is emphatically calculated by CFD/CSD coupling method and compared with the isolated CFD method. The results show that the aerodynamic loads calculated from CFD/CSD method are more satisfactory than those from isolated CFD method, and the exclusion of blade structural deformation in rotor noise calculation may cause inaccurate results in low-speed forward flight state.

Enhancement of Navier–Stokes solver based on an improved volume-of-fluid method for complex interfacial-flow simulations

In this study, an extended validation of an improved volume-of-fluid (VOF)-based method for 3D highly nonlinear, complex, and breaking free-surface problems in engineering is presented. The VOF interface-tracking scheme is implemented with a Navier–Stokes solver in a curvilinear coordinate system. The numerical procedure is advanced by implementing the scheme on moving Chimera (overset) grids for application in complex engineering problems. The overset grids are used to facilitate the flow simulations of complex geometries and arbitrary motions of objects and improve computational productivity by taking advantage of their flexibility. To solve the six degrees of freedom (6DOF) motions of a rigid body simultaneously, the nonlinear 6DOF motion equations in a vector form system are strongly combined with a flow solver. A collection of problems is carefully selected such that a large density ratio and complex cases under a wide variety of Froude numbers are presented with the aim of demonstrating the capabilities of the new enhanced method on a complicated moving overset grid system. The method is validated for a wide range of parameters. The results of different free-surface problems are presented. The numerical results are compared with numerical alternatives and experimental measurements, and accurate approximations and good agreement are obtained for complex flows.

‘The Equator–Pole grid system’: an overset grid system for the sphere, with an optimal uniformity property

This article proposes a new grid system for the sphere, which consists of three orthogonal and almost uniform grids. The basic one is a latitude-longitude grid covering an annular band around the equator. For the rest of the sphere this grid is complemented by two grids covering the polar regions, and based on suitably modified stereographic coordinates. The rectangular structure of the grids makes highly efficient implementation on massively parallel computer systems possible. Numerical experiments with the new grid system are carried out for two advection examples, namely smooth deformational flow and rotation of the Cosine bell, and for the test problems 2, 3, and 6 from Williamson et al., concerning the non-linear shallow water equations. For problem 6, the Rossby–Haurwitz wave, we study conservation properties for mass and energy. The computational results compare favourably with results for other grids. Our focus is the definition of the grid system together with the connection between grids by overlapping and the use of centred interpolation formulas. The method of centred finite differences is used for the spatial discretisation of the differential equations, because it is well-known and easy to implement.

Hydrodynamic assessment of planing hulls using overset grids

In conjunction with high performance computers, recent developments in computational science paved the path to more accurate representation of body motions inside fluids. Small motions inside the flow can be computationally approximated by using rigid body motion but it is incapable of accurately predicting the large motions of a planing vessel. The implementation of overset grid has made it possible to better approximate the complex fluid-structure interaction problem of the planing regime. The focus of this study was to evaluate the opportunity of using an overset grid system to numerically solve the flow around a planing hull and to understand the planing regime with this invaluable tool. It was shown in this study that the overset grid better captures the large motions of the planing hull at high Froude numbers. Then, the results obtained by overset grid were used to calculate the resistance components of a planing hull in a wide Froude number range. The resistance components were discussed with respect to values generated by Savitsky approach. Using the benefits that the computational science brings, the flow was visualized to explain some underlying physics relevant to the planing regime.

A new coupled level set and volume-of-fluid method to capture free surface on an overset grid system

We present a new coupled level set and volume-of-fluid (CLSVOF) method for free surface flow simulations on an overset grid system. The coupled method takes advantages of the strengths of the level set (LS) method and the volume-of-fluid (VOF) method, and is superior to either single method. The novelty of the present method lies in that we develop the methodology for an overset grid system of embedding, overlapping and moving structured grids. The new methodology accurately captures interface and greatly preserves mass on an overset grid system by demonstrating the 3D sphere advection test. The method is coupled to a well validated Reynolds-Averaged Navier–Stokes incompressible flow solver. The method is validated with the dam-breaking flow interacting with a 3D obstacle (square structure/circular cylinder) by comparing the numerical results with available experimental and numerical studies. The water impact of a sphere case is further performed to demonstrate the capabilities of the new method on a complicated moving overset grid system.

CFD prediction of vortex induced vibrations and fatigue assessment for deepwater marine risers

Using 3D computational fluid dynamics techniques in recent years have shed significant light on the Vortex Induced Vibrations (VIV) encountered by deep-water marine risers. The fatigue damage accumulated due to these vibrations has posed a great concern to the offshore industry. This paper aims to present an algorithm to predict the crossflow and inline fatigue damage for very long (L/D > 103) marine risers using a Finite-Analytical Navier-Stokes (FANS) technique coupled with a tensioned beam motion solver and rainflow counting fatigue module. Large Eddy Simulation (LES) method has been used to simulate the turbulence in the flow. An overset grid system is employed to mesh the riser geometry and the wake field around the riser. Risers from NDP (2003) and Miami (2006) experiments are used for simulation with uniform, linearly sheared and non-uniform (non-linearly sheared) current profiles. The simulation results including inline and crossflow motion, modal decomposition, spectral densities and fatigue damage rate are compared to the experimental data and useful conclusions are drawn.

Computational Evaluation of Flow Control for Enhanced Control Authority of a Vertical Tail

A computational method for predicting the aerodynamic performance of a vertical tail due to active flow control has been developed. Enhanced directional control using flow-control technology can potentially result in reduced fuel consumption and improved airplane performance. The numerical method is based on the unsteady Reynolds-averaged Navier–Stokes formulation for overset grid systems. The flow control is modeled for an array of synthetic jet actuators mounted at the trailing edge of the stabilizer. Results of the validation for various flow conditions, actuation parameters, and port layouts are used to establish the accuracy of the numerical tool. The side-force increments on the tail due to flow control at various sideslip conditions are presented. Sensitivities to flow-control layouts, especially with respect to spanwise flow development, are also investigated. A practical approach to facilitate preliminary evaluations of active flow control with quick computational turnaround is proposed. It is shown that constant actuation can be used as proxy for predicting the effects of the unsteady actuation from synthetic jets. Its confirmation in the context of the current application has been invaluable for developing practical flow-control implementations. Aspects of integration are presented, and guidelines for preferred modes of actuation are put forth.

Numerical study on the hydrodynamic forces on a ship berthing to quay by taking free-surface effect into account

In the present study, numerical simulation of the berthing maneuver of a ship in the prescribed translational motion is performed. The transient viscous flow and hydrodynamic forces on the hull are calculated by solving the unsteady Reynolds-Averaged Navier–Stokes equations in overset grid system, and the free surface is captured using volume-of-fluid (VOF) approach. The present numerical results have been compared with previous computational results by Toda and available experimental data respectively. Since the effects of the quaywall and free surface are taken into consideration in the present study, it is found that the agreement is significantly better than that resulting from Toda’s 3D CFD based approach. Then the effects of various standoff distances between the ship and quaywall on the lateral forces are investigated. Meanwhile, the detailed transient flow features around the berthing ship are obtained, which are helpful to understand the interactional effects between the ship and quaywall. The present results may provide helpful guidance for vessels’ safe maneuvering in berthing motion and the design of fender system in the quay.

CFD-based study on the prediction of wave-induced surge force

The prediction of wave-induced surge force acting on a ship running in following seas is a key issue for the accurate estimation of surf-riding and broaching phenomena. It is pointed out that the Froude–Krylov calculation generally overestimates the amplitude of wave-induced surge force. Therefore a simple correction formula to the Froude–Krylov force was proposed in the second-generation intact stability criteria under discussion at the International Maritime Organization. In this paper, a RANS solver with an overset grid system is used to obtain more accurate prediction of wave surging force. Then CFD-based studies are performed, to evaluate the current correction formula, using a series of hull forms with variety of hull form parameters. Finally several CFD simulations are conducted to reveal essential elements which improve the prediction accuracy of the wave-induced surge force.

Yin–Yang–Zhong grid: An overset grid system for a sphere

For numerical simulations inside a sphere, an overset grid system, Yin-Yang-Zhong grid, is proposed. The Yin-Yang-Zhong grid is an extension of the Yin-Yang grid, which is widely used in various simulations in spherical shell geometry. The Yin-Yang grid is itself an overset grid system with two component grids, and a new component grid called Zhong is placed at the center of the Yin-Yang grid. The Zhong grid component is constructed on Cartesian coordinates. Parallelization is intrinsically embedded in the Yin-Yang-Zhong grid system because the Zhong grid points are defined with cuboid blocks that are decomposed domains for parallelization. The computational efficiency approaches the optimum as the process number increases. Quantitative test simulations are performed for a diffusion problem in a sphere with the Yin-Yang-Zhong grid. Correct decay rates are obtained by the simulations. Two other tests in magnetohydrodynamics (MHD) in a sphere are also performed. One is an MHD dynamo simulation, and the other is an MHD relaxation simulation in a sphere.

Numerical simulation of wave slamming on 3D offshore platform deck using a coupled Level-Set and Volume-of-Fluid method for overset grid system

The numerical simulation of wave slamming on a 3D platform deck was investigated using a coupled Level-Set and Volume-of-Fluid (CLSVOF) method for overset grid system incorporated into the Finite-Analytic Navier-Stokes (FANS) method. The predicted slamming impact forces were compared with the corresponding experimental data. The comparisons showed that the CLSVOF method is capable of accurately predicting the slamming impact and capturing the violent free surface flow including wave slamming, wave inundation and wave recession. Moreover, the capability of the present CLSVOF method for overset grid system is a prominent feature to handle the prediction of wave slamming on offshore structure.

Gradient operators for feature extraction from omnidirectional panoramic images

We propose an extension of the Laplacian operator and the class of differential corner detectors for omnidirectional panoramic images covering 360° views. It solves two problems of nonuniformity of spatial resolution and spherical polar coordinates singularity. For the first problem, we derive a Laplacian operator and a class of differential corner detectors that can handle spatial inhomogeneities in omnidirectional panoramic images using differential schemes. To solve the second problem, we use an overset grid system on the sphere called the Yin–Yang grid. This is realized based on the property that each of the modified Laplacian operator and corner detector is invariant with respect to coordinate transformation between the two spherical polar coordinate systems associated with the Yin–Yang grid system.

Summary of the Fourth AIAA Computational Fluid Dynamics Drag Prediction Workshop

Results from the Fourth AIAA Drag Prediction Workshop are summarized. The workshop focused on the prediction of both absolute and differential drag levels for wing–body and wing–body/horizontal-tail configurations of the NASA Common Research Model, which is representative of transonic transport aircraft. Numerical calculations are performed using industry-relevant test cases that include lift-specific flight conditions, trimmed drag polars, downwash variations, drag rises, and Reynolds-number effects. Drag, lift, and pitching moment predictions from numerous Reynolds-averaged Navier–Stokes computational fluid dynamics methods are presented. Solutions are performed on structured, unstructured, and hybrid grid systems. The structured-grid sets include point-matched multiblock meshes and overset grid systems. The unstructured and hybrid grid sets comprise tetrahedral, pyramid, prismatic, and hexahedral elements. Effort is made to provide a high-quality and parametrically consistent family of grids for each grid type about each configuration under study. The wing–body/horizontal families comprise coarse, medium, and fine grids; an optional extrafine grid augments several of the grid families. These mesh sequences are used to determine asymptotic grid-convergence characteristics of the solution sets and to estimate grid-converged absolute drag levels of the wing–body/horizontal configuration using Richardson extrapolation.

Aerodynamic and acoustic analysis of an industrial fan

The efforts to predict noise radiation for an industrial fan using direct computational fluid dynamics (CFD) simulation is presented in this paper. Industry has been using CFD tool to guide fan design in terms of efficiency prediction and improvement. However, the use of CFD tool for aero-dynamic noise prediction is very limited in the past, partly due to the fact that research in aero-acoustics field was not practical for industry application. With the most recent technologies in CFD field and increasing computational power, the industry application of aero-acoustics becomes much more promising. It is demonstrated here that fan tonal noise and broadband noise at low frequencies can be directly predicted using an Overset grid system and high order finite difference schemes with acceptable fidelity.

A HYBRID SOLAR WIND MODEL OF THE CESE+HLL METHOD WITH A YIN-YANG OVERSET GRID AND AN AMR GRID

A hybrid three-dimensional (3D) MHD model for solar wind study is proposed in the present paper with combined grid systems and solvers. The computational domain from the Sun to Earth space is decomposed into the near-Sun and off-Sun domains, which are respectively constructed with a Yin-Yang overset grid system and a Cartesian adaptive mesh refinement (AMR) grid system and coupled with a domain connection interface in the overlapping region between the near-Sun and off-Sun domains. The space-time conservation element and solution element method is used in the near-Sun domain, while the Harten-Lax-Leer method is employed in the off-Sun domain. The Yin-Yang overset grid can avoid well-known singularity and polar grid convergence problems and its body-fitting property helps achieve high-quality resolution near the solar surface. The block structured AMR Cartesian grid can automatically capture far-field plasma flow features, such as heliospheric current sheets and shock waves, and at the same time, it can save significant computational resources compared to the uniformly structured Cartesian grid. A numerical study of the solar wind structure for Carrington rotation 2069 shows that the newly developed hybrid MHD solar wind model successfully produces many realistic features of the background solar wind, in both the solar corona and interplanetary space, by comparisons with multiple solar and interplanetary observations.