Overset Unstructured Grid Research Articles

Unsteady aerodynamic characteristics of a morphing tail configuration

Morphing aircraft is an important trend in the future development of next-generation aircraft. This paper focuses on aircraft with a small aspect ratio flying wing configuration that includes morphing tails. The unsteady aerodynamic characteristics of the morphing tail configuration are numerically simulated using the unstructured overset grid technique and the dual-time method, focusing on the effects of the tail deflection angle over time, Mach number, and side slip angle on the unsteady aerodynamic characteristics of the entire aircraft and tails. The second derivative of the tail deflection angle with time should be continuous, and the maximum angular velocity and maximum angular acceleration should be minimized. The hysteresis loop area is closely related to the Mach number. The sideslip angle aggravates the nonlinearity of the unsteady aerodynamic characteristics of the entire aircraft. The unsteady aerodynamic characteristics during tail morphing are affected by static (wing downwash effect and projected area effect) and dynamic (flow field hysteresis effect, additional motion effect, and wall implication effect) characteristics. The enclosed research provides a reference for the aerodynamic and control system designs of morphing tail configurations.

Investigation of Aeroelasticity Effect on Missile Separation from the Internal Bay

There is a strong aerodynamic interference when launching the missile in the embedded mode. During the separation process, the carrier aircraft safety may be threatened due to large slenderness ratio, low structural stiffness, and aeroelasticity effects of the missile. The present study simulates missile separation in the presence of the aeroelasticity effects based on the computational fluid dynamics (CFD), rigid body dynamics (RBD), and computational structure dynamics (CSD) coupling method. A hybrid dynamic grid method consisting of the mixed overset unstructured grid and deformation grid is utilized. In order to verify the accuracy of the coupled numerical method, store separation from a wing and AGARD 445.6 wing flutter are first simulated as two standard test cases. The verification results imply that the present coupled numerical method is reliable and capable in simulation of the aeroelastic effect in missile separation. The influence of aeroelasticity on the separation trajectory of a missile from the internal bay is systematically studied at different states. Numerical results show that aeroelasticity substantially affects the missile angular displacement, while it has a slight impact on the linear displacement of the center of mass. Mach number and flight altitude are two important flight parameters that characterize the aeroelasticity effect on missile separation from the internal bay.

Efficient Parallel Dynamic Overset Unstructured Grid for Flow Simulation with Moving Bodies

The overset grid method for simulation of unsteady flow with moving bodies faces several issues, including low assembly efficiency for a large number of bodies, difficult parallel implementation, and the requirement of manual intervention. To address these issues, in this paper, we develop an efficient, automatic, and robust parallel dynamic overset-unstructured-grid assembly method. It is achieved by parallelizing its two main steps: hole cutting, and identification of interpolation stencils. The hole cutting is simply implemented in parallel by identifying the active zone of each grid with a criterion of the global minimum wall distance. The global minimum wall distance is efficiently calculated on each process with the oriented-bounding-box-based -dimensional (-D) trees of the wall surface grids. For identification of the interpolation stencil, we use the results of the first step, optimally define the interpolation boundary nodes, and determine their candidate donor partitions in each process. This efficiently minimizes the number of query nodes and their donor cell candidates. The donor cell search is efficiently performed in parallel in each candidate donor partition by using the oriented-bounding-box-based -D tree of the field volume grid. Several cases are adopted to test the efficiency and capability of the proposed parallel dynamic overset-unstructured-grid assembly approach.

Enhancement on parallel unstructured overset grid method for complex aerospace engineering applications

In the present study, an efficient overset grid method by means of parallel implicit hole-cutting is proposed for the sake of simulating unsteady flows in aerospace engineering involving multiple bodies in relative movement. In view of the degraded computational efficiency and robustness for conventional overset grid assembly, several innovative techniques are developed within the overset grid assembly process, viz., a bookkeeping alternative digital tree method to speed up the donor-cell searching, a fast parallel advancing front algorithm to accelerate the wall-distance calculation and a message-passing strategy with efficient information communication and lower storage expenditure within distributed computational architecture. The contribution of the developed techniques is evidenced by comparison with the existing alternative ways in terms of computing efficiency. Subsequently, the overset grid method is embedded into an in-house programed URANS solver to examine its capability in predicting the flow field of complex applications such as helicopter, store separation and component deploying. Results show that the developed overset grid methodology is, in practice, able to resolve the aerodynamic characteristics of complex aerospace engineering with a high-fidelity flow topology and accuracy.

Highly Efficient Wall-Distance-Based Parallel Unstructured Overset Grid Assembly

The overset grid assembly (OGA) method is an effective computational fluid dynamics technique for the simulation of flows with complex geometries or relative moving parts. Using OGA, the effort for mesh generation is significantly reduced by generating the grid for different parts and then assembling them to form the computational grid. New methods have been developed that are generally divided into implicit and explicit methods in which a donor cell search for the defined query points is often time-consuming. Besides, the parallelization of OGA is a complex task that is not currently supported by many software platforms. In this paper, a new OGA approach is developed based on the wall distance and a user-defined reference length. The proposed approach is relatively more convenient to implement and more robust among the existing methods. The number of query points defined by this method is also significantly reduced, resulting in a higher speed of performing the overall donor search process. Validity tests are conducted, and the results confirm the effectiveness of the proposed method.

Research on shock-vortex interaction of fairings based on dynamic unstructured overset grid

The fairing is used to protect the aircraft from aerodynamic heat and force at high-speed, while there exists strong shock-vortex interaction during fairing separation. The unsteady shock-vortex interaction is numerically studied based on dynamic unstructured overset grid in this paper, where the N-S equations and 6-DOF equations are couple solved. Due to the blocking effect of fairings and shocks, there is a strong vortex inside the fairings. With the separation of fairings, the vortex is strongly compressed by the shock of fairing. When the shock is very close to fairing, the vortex is forced into the Mach cone of aircraft and enlarges the Mach cone. There exists a strong shock-vortex interaction of shock-vortex compressing, transforming of vortex boundary and vortex-shock compressing during fairing separation.

A High-Accuracy and Conservative Unstructured Overset Grid data transfer Method for Numerical Simulation of Turbomachinery

The overset grid method is getting more and more popular in the numerical simulation of turbomachinery, especially in the simulation of wind turbines. Reasonable interpolation between overset grid interfaces is one of the bases to ensure correct calculation of flow fields, non-conservative interpolation is insufficient to guarantee the mass fluxes in the numerical simulation of turbomachinery. This paper develops a conservative interpolation method for hybrid overset mesh based on supermesh technology and centre-based finite volume method. A method to build the local supermesh during overset grid interface by the cell-cut algorithm is presented, and flow field variables could be interpolated conservatively during overset meshes with second-order accuracy via the local supermesh. Numerical results show that the proposed method is strictly conservative for second-order distributed flow variables. Compared with trilinear interpolation method and reverse distance-weighted interpolation method, the method in this paper decreases the numerical errors and the variables are interpolated more accurately. Engineering cases show that the method in this paper is robust and practical for engineering applications.

A numerical method for multi-body separation with collisions

Multi-body separation with collisions is ubiquitous in aerospace engineering. In this paper, a numerical method for this problem is developed. Coupling with the equations for six-degree-of-freedom rigid-body motion, the unsteady compressible Reynolds-Averaged Navier-Stokes equations are solved. The dynamic overset unstructured grid based on the criteria of wall distance is developed to handle the relative motion of multiple bodies. The collision among rigid bodies is modeled by using an impulse-based method. Efficient algorithms for collision detection and contact determination utilizing hole cutting information of overset unstructured grid are proposed. In the simulation of six-degree-of-freedom rigid-body motion, a collision event can be robustly detected and the collision time can be accurately determined, without needing additional data structures. Test cases of store separation and ball collision are used to validate the accuracy of the dynamic solvers and the collision algorithms. An application to the release of a large number of projectiles from a carrier assesses the capability of the present method in simulating the problems of complex multi-body separation with collisions.

Explicit assembly algorithm of unstructured overset grid

Explicit assembly algorithm of unstructured overset grid

On overset interpolation strategies and conservation on unstructured grids in OpenFOAM

In this paper, the effects of interpolation methodology on the accuracy of computed solutions are discussed in the context of unstructured overset grids. An in-house implementation of the overset method for OpenFOAM, OPERA (Chandar, 2015; Chandar et al., 2015) has been used. Several verification and validation studies reported previously used a simple interpolation technique such as inverse distance weighting and this was found to be adequate for prediction of forces. For closed problems however, the force history is plagued with unphysical oscillations. Also, a quantitative assessment of interpolation errors is lacking. The current work aims to understand the effects of interpolation error on conservation and how this error can be minimized by enforcing flux correction and/or better interpolation techniques. Apart from enforcing conservation, the pressure equation needs to be adjusted as well in an implicit manner to guarantee the transfer of fluxes correctly between overlapping regions. Importance of conservation and higher order interpolation are also discussed in the context of multiphase flows.

Investigation of Slotted Tip Parameters on the Aerodynamic Characteristic of Helicopter Rotor

Numerical simulations are conducted to investigate the effect of helicopter rotor tip vortex alleviation using a slotted tip, and effects of parameters involving location, number, aperture of the slot on tip vortex structure and aerodynamic performance are analyzed in detail. The flow field of hovering helicopter rotor is simulated by solving Navier-Stokes equations in the rotating coordinate system. Finite volume method combined with overset unstructured grids algorithm are adopted. The calculation of inviscid fluxes uses second-order central scheme with artificial dissipation, and turbulence model utilizes Spalart-Allmaras model. Simulation results show that the location of the slot has little effect on tip vortex alleviation. The larger the number and aperture of the slot, the better the effect of slotted tips weakening tip vortex. Lift coefficient of helicopter rotor with a slotted tip increases slightly compared with that without slots, while power coefficient increases drastically.

Numerical analysis of a high-order unstructured overset grid method for compressible LES of turbomachinery

Large-Eddy Simulation (LES) is recognized as a promising method for high-fidelity flow predictions in turbomachinery applications. The presented approach consists of the coupling of several instances of the same LES unstructured solver through an overset grid method. A high-order interpolation, implemented within this coupling method, is introduced and evaluated on several test cases. It is shown to be third order accurate, to preserve the accuracy of various second and third order convective schemes and to ensure the continuity of diffusive fluxes and subgrid scale tensors even in detrimental interface configurations. In this analysis, three types of spurious waves generated at the interface are identified. They are significantly reduced by the high-order interpolation at the interface. The latter having the same cost as the original lower order method, the high-order overset grid method appears as a promising alternative to be used in all the applications.

Deformable Overset Grid for Multibody Unsteady Flow Simulation

A deformable overset grid method is proposed to simulate the unsteady aerodynamic problems with multiple flexible moving bodies. This method uses an unstructured overset grid coupled with local mesh deformation to achieve both robustness and efficiency. The overset grid hierarchically organizes the subgrids into clusters and layers, allowing for overlapping/embedding of different type meshes, in which the mesh quality and resolution can be independently controlled. At each time step, mesh deformation is locally applied to the subgrids associated with deforming bodies by an improved Delaunay graph mapping method that uses a very coarse Delaunay mesh as the background graph. The graph is moved and deformed by the spring analogy method according to the specified motion, and then the computational meshes are relocated by a simple one-to-one mapping. An efficient implicit hole-cutting and intergrid boundary definition procedure is implemented fully automatically for both cell-centered and cell-vertex schemes based on the wall distance and an alternative digital tree data search algorithm. This method is successfully applied to several complex multibody unsteady aerodynamic simulations, and the results demonstrate the robustness and efficiency of the proposed method for complex unsteady flow problems, particularly for those involving simultaneous large relative motion and self-deformation.

A dynamic mesh strategy applied to the simulation of flapping wings

SummaryA robust and efficient dynamic grid strategy based on an overset grid coupled with mesh deformation technique is proposed for simulating unsteady flow of flapping wings undergoing large geometrical displacement. The dynamic grid method was implemented using a hierarchical unstructured overset grid locally coupled with a fast radial basis function (RBF)‐based mapping approach. The hierarchically organized overset grid allows transferring the grid resolution for multiple blocks and overlapping/embedding the meshes. The RBF‐based mapping approach is particularly highlighted in this paper in view of its considerable computational efficiency compared with conventional RBF evaluation. The performance of the proposed dynamic mesh strategy is demonstrated by three typical unsteady cases, including a rotating rectangular block in a fixed domain, a relative movement between self‐propelled fishes and the X‐wing type flapping‐wing micro air vehicle DelFly, which displays the clap‐and‐fling wing‐interaction phenomenon on both sides of the fuselage. Results show that the proposed method can be applied to the simulation of flapping wings with satisfactory efficiency and robustness. Copyright © 2016 John Wiley & Sons, Ltd.

Numerical prediction of unsteady aerodynamics for a ducted fan micro air vehicle

In this paper, the unsteady aerodynamics of a ducted fan micro air vehicle is investigated using an unstructured overset grid technique and momentum source method. The in-house programmed compressible Navier–Stoke solver is preconditioned for low Mach number flow regime, and a dual time-stepping strategy is employed to guarantee the computing accuracy and efficiency. Momentum source items are added in the Navier–Stoke solver to replace the contra-rotating propellers in numerical simulation which simplify the inherently unsteady flow into a quasi-steady one. The developed method was verified and validated as a reliable tool for predicting the unsteady aerodynamic performance in low Reynolds flow regime. The effects of reduced frequency, flight velocity and propeller speed on the aerodynamic performance of the ducted fan micro air vehicle are evaluated in this paper. Results show that the hysteresis effect of aerodynamic coefficient increases as induced frequency, freestream velocity and propeller speed increases.

Discrete Adjoint-Based Design for Unsteady Turbulent Flows on Dynamic Overset Unstructured Grids

A discrete adjoint-based design methodology for unsteady turbulent flows on three-dimensional dynamic overset unstructured grids is formulated, implemented, and verified. The methodology supports both compressible and incompressible flows and is amenable to massively parallel computing environments. The approach provides a general framework for performing highly efficient and discretely consistent sensitivity analysis for problems involving arbitrary combinations of overset unstructured grids that may be static, undergoing rigid or deforming motions, or any combination thereof. General parent–child motions are also accommodated, and the accuracy of the implementation is established using an independent verification based on a complex-variable approach. The methodology is used to demonstrate aerodynamic optimizations of a wind-turbine geometry, a biologically inspired flapping wing, and a complex helicopter configuration subject to trimming constraints. The objective function for each problem is successfully reduced, and all specified constraints are satisfied.

A fast and reliable overset unstructured grids approach

A cell-centred overset unstructured grids approach is developed. In this approach, the intergrid boundary is initially established based on the wall distance from the cell centre, and is then optimized. To accelerate the intergrid-boundary definition much more, a neighbor-to-neighbor donor search algorithm based on advancing-front method is modified with the help of minimum cuboid boxes. To simplify the communications between different grid cell types and to obtain second-order spatial accuracy, a new interpolation method is constructed based on linear reconstruction, which employs only one layer of fringe cells along the intergrid boundary. For unsteady flows with relative motion, the intergrid boundary can be redefined fast and automatically. Several numerical results show that the present dynamic overset unstructured grids approach is accurate and reliable.

A Computational Study on the Aerodynamic Influence of a Propeller on an MAV by Unstructured Overset Grid Technique and Low Mach Number Preconditioning

The influence of a propeller on the aerodynamic performance of an MAV is investigated using an unstructured overset grid technique. The flow regime of a fixed-wing MAV powered by a propeller contains both incompressible regions due to the low flight speed, as well as compressible flow areas near the propeller-tip region. In order to simulate all speed flow efficiently, a dual-time preconditioning method is employed in the present study. The methodology in this paper is verified as providing a reliable numerical simulation tool for all flow regimes, in the additional presence of moving boundaries, which is treated with an overset grid approach.

Numerical simulation of rotor-airframe aerodynamic interaction based on unstructured dynamic overset grids

A method of unstructured dynamic overset grids is developed for the numerical simulation of helicopter unsteady rotorairframe aerodynamic interaction. For the effective treatment of the relative motion between the rotor and the airframe, the domain of flowfield is divided into two overset subzones, namely, a rotational subzone containing the blades and a stationary subzone containing the airframe. The overset part of two subzones is used to convect the flow variables of the two zones. The Taylor series expansion is used to obtain a second-order spatial accuracy, and dual-time stepping is adopted to improve the solution accuracy. Mesh deformation from the blade motion in forward flight is treated by using a spring analogy. Validation is made by numerically simulating the flows around a wind tunnel configuration and comparing the predicted time-averaged and instantaneous inflow and airframe surface pressure distributions with the experimental data. It shows that the present method is efficient and robust for the prediction of complicated unsteady rotor-airframe aerodynamic interaction phenomena. unstructured grid, overset grids, rotor-airframe interaction, spring analogy, relative motion

Numerical Study of Propeller Slipstream Based on Unstructured Dynamic Overset Grids

The propeller slipstream is numerically simulated by solving three-dimensional unsteady Euler equations. The method of unstructured overset grids is applied to the simulation of relative motion between the propeller and airframe. The rotationalmotion of the propeller is simulated by establishing a rotational subzone, and the airframe is contained in the stationary subzone. During the unsteady simulation with the rotational subzone rotating, there is always an overlapping area between the rotational and stationary subzones, and the overlapping area is used to convect the flow variables of the two subzones. The governing equations are solved by using the dual-time method and standard explicit four-stage Runge-Kutta method. The unsteady flowfields of different working conditions are calculated, and the influence of propeller slipstream is analyzed. Some calculating results are compared with experimental data, and they have a good agreement with each other. It is demonstrated that the present method is efficient and robust for the simulation of an unsteady propeller slipstream flow.