Simulation of Helicopter Forward Flight Flow Using Unstructured Overset Grids

The advent of General Purpose Graphics Processing Units (GPGPUs) has spawned a lot of interest in computing high resolution flows in a shorter span of time. In combination with existing parallel programming techniques such as MPI or openMP, one is able to obtain at least one order increase in speed-up for CFD applications on stationary grids. Due to a high throughput/cost ratio, GPUs are increasingly becoming popular among the CFD community. Flow fields on helicopter systems rank among one of the most challenging, and hence are computationally demanding to simulate. Further, presence of multiple bodies moving relative to each other require use of overset grid systems, which in turn, require efficient overset grid assembly methods to support the flow solution. In this context, a computational framework for flow simulation across multiple CPU cores and multiple GPUs has been developed and tested. It is shown that the overall wall-clock time for the simulation can be considerably reduced by using multiple GPU cards. The parallel GPU framework is initially tested using both explicit and implicit schemes for the flow past a sphere. Further, a two-blade hovering rotor test case adopted from literature is used to demonstrate the capability of the code towards simulating rotorcraft flows in a short span of time.

A parallel Navier-Stokes solver based on dynamic overset unstructured grids method is presented to simulate the unsteady turbulent flow field around helicopter in forward flight. The grid method has the advantages of unstructured grid and Chimera grid and is suitable to deal with multiple bodies in relatively moving. Unsteady Navier-Stokes equations are solved on overset unstructured grids by an explicit dual time-stepping, finite volume method. Preconditioning method applied to inner iteration of the dual-time stepping is used to speed up the convergence of numerical simulation. The Spalart-Allmaras one-equation turbulence model is used to evaluate the turbulent viscosity. Parallel computation is based on the dynamic domain decomposition method in overset unstructured grids system at each physical time step. A generic helicopter Robin with a four-blade rotor in forward flight is considered to validate the method presented in this paper. Numerical simulation results show that the parallel dynamic overset unstructured grids method is very efficient for the simulation of helicopter flow field and the results are reliable.

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.

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.

In practical applied aerodynamics, solutions of unsteady time-accurate three-dimensional flows involving multiple bodies in relative motion are frequently required. Typical examples include the store/airframe separation from aircraft or the rotor-fuselage interaction problem in helicopter aerodynamics. Several research works have been previously conducted for the simulation of the complicated unsteady flows involving mutual aerodynamic interference of multiple bodies by solving the Euler or Navier-Stokes equations using either structured or unstructured overset grid methods [1, 2, 3].

Analysis of non-conservative interpolation techniques in overset grid finite-volume methods

Numerical fluid dynamics simulations of multiple bodies in relative motion are being used more extensively as weapons systems become more complex and weapons development cycle times are reduced. To address the need for a flexible, efficient, and user-friendly method for computing these flows, a system for computing moving-body simulations on overset structured and unstructured grids is under development called Fluid Dynamics— Computational Analysis of Dynamically Responsive Environments (FD-CADRE). The system allows existing programs [e.g., computational fluid dynamics (CFD) or grid assembly codes] to be readily inserted into the process. The system is shown to accommodate methods using structured or unstructured grid technologies. The initial FDCADRE system has been developed and has been demonstrated on simple test configurations.

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.

On overset interpolation strategies and conservation on unstructured grids in OpenFOAM

The implementation of two different types of turbulence models for a flow solver using the Chimera overset grid method is examined. Various turbulence model characteristics, such as length scale determination and transition modeling, are found to have a significant impact on the computed pressure distribution for a multielement airfoil case. No inherent problem is found with using either algebraic or one-equation turbulence models with an overset grid scheme, but simulation of turbulence for multiple-body or complex geometry flows is very difficult regardless of the gridding method. For complex geometry flowfields, modification of the Baldwin-Lomax turbulence model is necessary to select the appropriate length scale in wall-bounded regions. The overset grid approach presents no obstacle to use of a one- or two-equation turbulence model. Both Baldwin-Lomax and Baldwin-Barth models have problems providing accurate eddy viscosity levels for complex multiple-body flowfields such as those involving the Space Shuttle.

A new extremely flexible finite volume framework is presented. It is based on updating cell average values of the dependent variables. It is embedded in a unified-grid framework that unifies the treatment of structured and unstructured grids, single and multi-block grids, patched-aligned, patched-non-aligned and overset grids, and various cell shapes, including hexahedra, triangular prisms and tetrahedra in three dimensions, quadrilaterals and triangles in two dimensions and the linear element in one dimension. A novel discretization approach has been developed to deal with unified-grid topologies. It includes a piecewise-linear multi-dimensional non-oscillatory reconstruction procedure that is based on synergistic utilization of polynomials at cell vertices. Least-squares reconstruction is used when necessary. The concept of generalized neighborhoods is introduced to account for cell neighborhoods that include cells that are not logically connected through common nodes, faces, etc. This helps in the automatic treatment of all types of multi-block meshes. To go with such a general discretization procedure, new implicit relaxation procedures are introduced that help achieve fast convergence to steady state solutions. The framework has been implemented in a new CFD code (CFD++) using a finite volume formulation. The paper presents the methodology with the help of several annotated examples taken from various compressible and incompressible flows. Copyright © 1999 John Wiley & Sons, Ltd.

Accurate prediction of complex rotor-airframe integration is essential for optimum design and accurate analysis of rotorcraft. Over the years, a broad array of computational methodologies has been employed to pre dict the rotor-airframe interaction. In the present study three leading computational approaches are developed and employed in the study of rotor -airframe integration problems. These approaches include an overset grid -based, unstructured tetrahedral -based, and unstructured Cartesian grid - based methods. The computational results are compared with experimental data on a generic rotor - airframe configuration tested at Georgia Tech and on the Robin helicopter geometry tested at NASA Langley. The agreemen t with experimentally measured surface pressure is found to depend strongly on the assumed rotor load distribution . These studies are valuable in helping develop and document the capability of these approaches to provide accurate predictions of rotor -airframe interaction. Introduction The flowfield around a single rotor helicopter, whether in forward flight or hover, is difficult to model due to the presence of strong vorticity which lingers in the vicinity of the rotor. In modern helicopter designs, the rotor blades are placed very close to the fuselage for compact configurations and to reduce the drag associated with the mast. In such configurations, the vortex system generated by the rotor blades interacts with the body surface in a complex unsteady manner. Rotors and airframes designed without taking into account the rotor -airframe aerodynamic interactions tend to perform significantly below their predicted capabilities. 1 Over the years, a broad array of computational methods have been employed to predict the rotor -airframe interaction. Several reviews of these methods are available. 2,3,4 These methods include modeling of individual rotors and tip vortices and modeling the rotor as a single actuator disk.

*† This paper deals with the use of the adjoint equation for aerodynamic shape optimization of complex configurations with overset grids methods. The use of overset grid eases the grid generation process, but the nontrivial task of ensuring communication between the overlapping grids needs careful attention. This need is effectively addressed by using a practically useful technique known as Implicit Hole Cutting (IHC) method first introduced by Lee and Baeder 1 and further demonstrated by the authors 2 in a previous study. The method depends on the simplicity of the cell selection process based on the main criterion of cell size, and all grid points including hole interior points and hole fringe points are treated indiscriminately in the flow computation. The simplicity of the IHC method is exploited for the adjoint equation solver. Similar to the flow solver, the adjoint equations are solved on the conventional point-matched grids and overlapped grids within a multi-block framework. Parallel computing with MPI is also used to improve the overall efficiency of the design optimization process. The method is successfully applied to several two and threedimensional shape optimization cases for both external and internal flows problems.

Should off-pump coronary artery bypass surgery be abandoned: A potential solution

In this paper, we develop a series of robust, arbitrary-location nonlinear interpolations for inter-grid boundaries in curvilinear structured overset grids for problems with strong discontinuities and complex shock reflections. In order to be capable of robustly capturing multi-discontinuities within one discretization stencil, we extend the multi-resolution weighted essentially non-oscillatory (WENO) schemes to the overset framework by developing the unequal-size nonlinear interpolations on nested sub-stencils. In addition, we also extend the freestream preserving discretization to the overset framework. The proposed methods finally offer several advantages: first, the degradation of accuracy near discontinuities can progressively reduce to first order, which ensures correct behavior in problems with multiple discontinuities in a local stencil, whereas the original WENO method only allows one discontinuity in a local stencil; second, both the linear weights and the smoothness indicators remain independent of the interpolation location, making the implementation simple and straightforward. Finally, a series of numerical test cases is carried out with strong and complex discontinuities, demonstrating the robustness of the developed method at inter-grid boundaries. Moreover, the accuracy and free-stream preservation of the multi-resolution strategy on curvilinear overset grids are also verified, showing the desired second-, fifth-, and seventh-order of accuracy and a machine zero order of freestream preservation error. Numerical experiments indicate that the nonlinear interpolations developed in this paper provide a suitable choice for structured curvilinear overset grid methods with strong and complex discontinuities.