Structured Overset Grids Research Articles

Shock capturing using discontinuous Galerkin method and overset grids for two-dimensional Euler equations

A new procedure to capture shocks which confines the shock capturing algorithms to overset grids is proposed and demonstrated for steady state flows containing straight and bow shocks using a discontinuous Galerkin solver for two dimensional Euler equations. Preliminary results for capturing moving straight shocks are also presented. Troubled cell data, obtained using a coarse grid, is used to determine the approximate location of the shock. A refined and structured overset grid aligned to the shock is constructed based on this information. The solver is run again with the coarse grid solution as the initial condition using a shock capturing overset grid to give a solution with the shock aligned to the grid line. Solutions obtained with a coarse grid alone, overset grid aligned to the shock, and one not aligned to the shock are used to demonstrate the efficacy of the scheme for problems having analytical solutions. Results for supersonic flow over a ramp, shock reflecting off a flat plate, the supersonic flow over a circular cylinder, and a simple Mach reflection are presented. For the flow over a circular cylinder, the results obtained are validated using an existing analytical method for calculating the shock offset distance. Since there is a manual component in the construction of overset grids, as a first step towards eliminating it, a moving normal shock is also captured accurately using an overset grid at the location of the shock with an arbitrary Lagrangian-Eulerian discontinuous Galerkin method.

Numerical simulations of complex aircraft configurations using structured overset grids with implicit hole-cutting

Abstract A structured overset-grids method with enhanced implicit hole-cutting has been developed to simulate the flow around aircraft configurations, especially high-lift configurations. The current research uses the implicit hole-cutting method to improve the automation of domain connectivity and optimize the overlapping areas. Some enhancements for the original implicit hole-cutting method are proposed to make it more effective. Furthermore, a simple method for calculating the force on the overlapping solid surfaces is also presented. Jameson's second-order finite volume method is utilized to solve the Navier-Stokes equations. A wing-body configuration and two high-lift configurations are numerically tested and the numerical results are compared with the experimental data.

Development of a structured overset Navier–Stokes solver with a moving grid and full multigrid method

An unsteady Reynolds averaged Navier–Stokes solver with a structured overset grid method has been developed. Velocity–pressure coupling is achieved using an artificial compressibility approach, spatial discretization is based on a finite-volume method, and convective fluxes are evaluated by a third-order upwind scheme based on flux-difference splitting. Body motions are considered using the grid deformation technique and grid velocities in the convective term. Viscous fluxes are evaluated via second-order centered differencing. The full multigrid (FMG) method is applied to obtain fast convergence. The cell flag on a coarse grid level is determined using the cell flag on a fine grid level. In the coarse grid-level calculations at the FMG stage, the data are interpolated until the finest grid level is achieved at an overset update interval. Then, the data are updated based on the overset relations at the finest grid level and then transferred to a coarser grid level. For free surface treatments, a single-phase localized level-set method is employed. The computations for flows around a hull form, including an unsteady simulation with regular waves, are demonstrated.

Time-Accurate Coupling of Three-Degree-of-Freedom Parachute System with Navier–Stokes Equations

A parachute with payload is modeled as a three-degree-of-freedom system suitable for time-accurate coupling with the Navier–Stokes flow equations. The coupled equations of motion are formulated following an instantaneous Lagrangian–Eulerian approach, and solved using Newmark’s time-integration method. Flow solutions are computed by solving the Reynolds-averaged Navier–Stokes equations with structured overset grids. A sensitivity analysis is used to check the adequacy of grid and time-step requirements. The time-accurate coupling procedure is validated by comparing the results with a frequency-domain approach. The results are demonstrated for a real parachute system, including a comparison with the results based on the linear theory. Possible instabilities, such as divergence of inclination angle and flutter associated with fluctuating drag force, are predicted. The present work advances the fidelity of analysis procedures beyond those in current use based on loose coupling with the Navier–Stokes equations.

DPW-6: Drag Analyses and Increments Using Different Geometries of the Common Research Model Airliner

This paper describes the CFD studies carried out at ONERA in the framework of the Sixth AIAA Drag Prediction Workshop. Different configurations of the well-known Common Research Model have been used to perform drag analyses and increment assessments. The structured overset grids provided by The Boeing Company to the DPW community have been preprocessed with the in-house software Cassiopée: CGNS conversion, cell blanking, and overlapping issues have been handled before running these grids with the RANS solver elsA and the far-field code ffd72. Contrary to the original CRM configuration used in DPW-5, the wing shape is now based on wind-tunnel aeroelastic measurements: through a grid convergence process, a four-count drag increment between new and former wing–body geometries has been quantified, which is very consistent with previous ONERA results. Then, a CRM configuration including a throughflow nacelle–pylon installation has been computed: the corresponding drag increment of about 22 counts is in very close agreement with the NASA experimental data. Finally, horizontal and vertical tails have been added to the wing–body–nacelle–pylon geometry so that the aerodynamic performance of such a complete cruise configuration can be assessed. Numerical considerations involving the SA-QCR2000 turbulence model are highlighted.

OVERSET-RANS COMPUTATIONS OF TWO SURFACE SHIPS MOVING IN VISCOUS FLUIDS

A structured overset grid approach coupled with Reynolds-Averaged Navier-Stokes (overset-RANS) method is presented to provide an accurate resolution of two surface ships moving with opposite velocity in viscous fluids. The RANS equations with shear stress transport (SST) k - ω model are employed to treat the viscous turbulent flows. The fully nonlinear boundary condition at the free surface is satisfied at each time step and the evolution of the free surface is achieved by using the level set method. A structured overset grid approach is used to allow flexibility in grid generation, local mesh refinement, as well as the simulation of moving objects while maintaining good grid quality. The presented overset-RANS method is demonstrated by two surface Wigley ship hulls moving with opposite velocity in still water. The simulating results illustrate the feasibility of the presented method to compute the complex viscous free surface flows interacting with many moving ships in still water or in waves.

URANS simulations for a high-speed transom stern ship with breaking waves

An exploratory study of high-speed surface ship flows is performed to identify modelling and numerical issues, to test the predictive capability of an unsteady RANS method for such flows, to explain flow features observed experimentally, and to document results obtained in conjunction with the 2005 ONR Wave Breaking Workshop. Simulations are performed for a high-speed transom stern ship (R/V Athena I) at three speeds Froude number (Fr) = 0.25, 0.43 and 0.62 with the URANS code CFDSHIP-IOWA, which utilizes a single-phase level set method for free surface modelling. The two largest Fr are considered to be high-speed cases and exhibit strong breaking plunging bow waves. Structured overset grids are used for local refinement of the unsteady transom flow at medium speed and for small scale breaking bow and transom waves at high-speeds. All simulations are performed in a time accurate manner and an examination of time histories of resistance and free surface contours is used to assess the degree to which the solutions reach a steady state. The medium speed simulation shows a classical steady Kelvin wave pattern without breaking and a wetted naturally unsteady transom flow with shedding of vortices from the transom corner. At higher speeds, the solutions reach an essentially steady state and display intense bow wave breaking with repeated reconnection of the plunging breaker with the free surface, resulting in multiple free surface scars. The high-speed simulations also show a dry transom and an inboard breaking wave, followed by outboard breaking waves downstream. In comparison to an earlier dataset, resistance is well predicted over the three speeds. The free surface predictions are compared with recent measurements at the two lowest speeds and show good agreement for both non-breaking and breaking waves.

Computational Study of the Wake and Contaminant Transport of a Walking Human

The unsteady aerodynamic wake of a human is studied using a time-accurate computational fluid dynamics simulation. Transport of a scalar contaminant, which originates on the body, is also considered. An existing Reynolds-averaged Navier-Stokes solver is modified to include energy, scalar-transport, and thermal buoyancy effects. Structured overset grids are used to discretize the geometry and the flow field. Results indicate two distinct wake regions: an unsteady bluff-body wake behind the torso which is characterized by a mean recirculation zone, and a region of unsteady vortex shedding behind the legs which is dominated by a “jet” of air formed between the legs. A significant downwash occurs behind the body which has the effect of laterally spreading the lower portions of the wake. The magnitude of the scalar contaminant is shown to decay exponentially within the wake and is found to be highly dependent upon the source location.

Aerodynamic Analysis of a Spinning Missile with Dithering Canards

A generic spinning missile with dithering canards is used to demonstrate the utility of an overset structured grid approach for simulating the aerodynamics of rolling airframe missile systems. A grid convergence study andassessment of viscous effects shows that a medium-resolution viscous grid with 8 million grid points provides a good compromise between solution accuracy and throughput. Viscous effects should be included in detailed studies to capture the interaction between the inboard canard vortex and the fuselage and tail boundary layers. The computed results agree well with experimental data. A database of cases with variation of angle of attack and the strength of control authority of a pitch-up maneuver is computed to evaluate missile performance. The case throughput rates are sufficient to contemplate population of aerodynamic databases with Navier-Stokes computations. Overall, the structured overset grid approach enables accurate and efficient simulation of rolling airframe missile configurations that involve relative motion between system components.

Aeroelastic Analysis of Helicopter Rotor Blades on Deformable Chimera Grids

Aeroelastic Reynolds-averaged Navier‐Stokes and Euler computations are presented for an articulated model rotor in hover and forward flight. Comparative rigid-blade simulations are carried out to assess the effects of blade dynamics and elasticity on the numerical results. The INROT flow solver operates on deformable structured overset grids and can be tightly coupled with a finite element model of the rotor blade structure (DYNROT) based on Timoshenko beam theory. The order of time accuracy of fluid and structure modules is maintained in the overall analysis by an appropriate staggered coupling scheme. At the investigated thrust setting, global hover performance values computed by the coupled fully turbulent Navier‐Stokes analysis agree fairly well with available experimental data. In forward flight, the aeroelastic results are in much better agreement with the measurements than those obtained from rigid-blade simulations with prescribed articulation. Apart from superior rotor power predicition, the local pitching moment coefficients computed by the viscous analysis are found to correlate better with wind-tunnel data than the corresponding Euler output.

Computation of Viscous Flow for a Boeing 777 Aircraft in Landing Configuration

A series of Navier-Stokes simulations of a complete Boeing 777-200 aircraft configured for landing is obtained using a structured overset grid process and the OVERFLOW CFD code. At approach conditions, the computed forces for the 777 computation are within 1.5% of experimental data for lift, and within 4% for drag. The computed lift is lower than the experiment at maximum-lift conditions, but shows closer agreement at post-stall conditions. The effect of sealing a span-wise gap between leading edge elements, and adding a chine onto the nacelle is computed at a high angle of attack. These additions make a significant difference in the flow over the wing near these elements. Detailed comparisons between computed and experimental surface pressures are shown. Good agreement is demonstrated at lower angles of attack, including a prediction of separated flow on the outboard flap.

Two-dimensional unsteady Navier-Stokes solution method with moving overset grids

A simple numerical algorithm to localize Intergrid boundary points and to interpolate unsteady solution variables across two-dimensional, structured overset grids is presented. Overset grids are allowed to move in time relative to each other. Intergrid boundary points are localized in a triangular stencil on the donor grid by a directional search algorithm. The final parameters of the search algorithm give the interpolation weights at the intergrid boundary point. Numerical results are presented for steady and unsteady viscous flow solutions over an airfoil undergoing a sinusoidal flapping motion. Computed flowfields demonstrate the accuracy of the method, and excellent agreement is obtained against the single grid solutions. The method is independent of numerical solution algorithms, and it may easily be implemented on any two-dimensional, single-block flow solver to make it a multiblock, zonal solver with arbitrarily overset/overlapping computational grids.