Essam F. Sheta

Solid Brick Analogy For Automatic Grid Deformation For Fluid-Structure Interaction

Modern aerospace vehicles that operate at high angles of attack and perform aggressive maneuvers are subject to several dynamic loads problems such as buffet and flutter. Accurate model-based simulation of these problems requires careful attention to fluid-grid movement because it directly affects the energy transformation between the fluid and structure modes. A novel structural-based “solid brick analogy” for dynamic grid movement and remeshing for unsteady computational aeroelasticity is presented. The methodology is applicable to all types of structured and unstructured grids. In the solid brick analogy, the structural Navier equation is applied to the deforming portion of the CFD mesh, and solved by finite element or modal analysis. To preserve the grid smoothness and orthogonality, the nonlinear, large deformation with small strain rate theory is used. The geometrically nonlinear “brick analogy” can sustain shear deformation since the equilibrium is set up for a solid element instead of truss. The efficacy of the methodology is demonstrated and assessed for CFD grid movement of several aeroelastic problems. In addition, a high-fidelity multidisciplinary unsteady simulation of vertical tail buffeting of fighter aircraft model is presented. The results indicate the ability of the methodology to preserve grid quality, sustain shear, and reduce grid distortion.

Efficient Adaptive Cartesian Vorticity Transport Solver for Vortex-Dominated Flows

An efficient solver for the velocity―vorticity form of the Navier―Stokes equations on adaptive Cartesian grids is presented. The excessive numerical dissipation common to most grid-based Navier―Stokes solvers is avoided by solving the fluid dynamic equations in vorticity conservation form. Additionally, an adaptive Cartesian solver is employed to efficiently capture and preserve vorticity on-the-fly as the flow develops. For practical purposes, this solver would be used in the wake region and coupled with a full Navier―Stokes solver in the near-body region, thus allowing vorticity to be accurately generated and then convected in the wake region with minimal dissipation. The adaptive Cartesian framework allows for the rapid evaluation of the velocity field using a fast-summation technique based on the Cartesian Treecode method. The implementations of both the solution algorithm and velocity calculation are described in detail. Results are presented for vortex convection applications that show good agreement with the analytical solution, and the accuracy of the scheme is verified numerically using a series of increasingly fine grids. Additionally, fully three-dimensional flow in the presence of a vortex ring is investigated and results are shown to be in very close agreement with published data.

An Efficient Adaptive Cartesian Vorticity Transport Solver for Rotorcraft Flowfield Analysis

An efficient solver for the velocity-vorticity form of the Navier-Stokes equations on adaptive Cartesian grids is presented. The excessive numerical dissipation common to most grid-based Navier-Stokes solvers is avoided by solving the fluid dynamic equations in vorticity conservation form. Additionally, an adaptive Cartesian solver is employed to efficiently capture and preserve vorticity on-the-fly as the flow develops. For practical purposes, this solver would be utilized in the wake region and coupled with a full Navier-Stokes solver in the near-body region, thus allowing vorticity to be accurately generated and then convected in the wake region with minimal dissipation. The adaptive Cartesian framework allows for the rapid evaluation of the velocity field using a fast summation technique based on the Cartesian treecode method. The implementations of both the solution algorithm and velocity calculation are described in detail. Results are presented for vortex convection which show good agreement with the analytical solution, and the accuracy of the scheme is verified numerically using a series of increasingly fine grids. Additionally, fully 3D flow in the presence of a vortex ring is investigated and results are shown to be in very close agreement with published data.

Novel Volume of Solid Technology for Nonlinear Aeroelastic Stability Analysis

Morphing technology enables aerospace vehicles to achieve a broader range of operational modes. Computational aeroelastic and design analysis tools of these vehicles must be able to handle arbitrarily large deformations and shape changes. The objective of this study is to improve aircraft performance, expand its flight envelope, replace conventional control surfaces, reduce drag, and reduce vibration/flutter by morphing technology. This paper presents a revolutionary approach to solve aeroelastic instabilities of air vehicles undergoing large deformation and arbitrarily large shape change. The technology was implemented in a CFD code and demonstrated for several solid-fluid interaction and morphing telescope wing problems. This innovative technology casts structure/body-dynamics equations in an Eulerian reference frame, so that the dependent variables for the fluid and solids are continuous and coupled. The volume of solid tracks the evolution of solid-fluid interface, while implicit formulation embeds the interaction forces into the solution procedure. The method is devoid of troublesome moving mesh or tedious grid regeneration issues. The developed technology will provide the lacking design and analysis capability in conceptualizing how vehicles should safely change shape during flight.

A coupled overset vorticity transport and compressible Euler solver for vortex-dominated flows

An efficient solver for the incompressible vorticity transport equations on adaptive Cartesian grids is coupled to an unstructured spectral volume solver for the conservative form of the compressible Euler equations using overset grid assembly and interpolation provided by SUGGAR and DiRTlib, respectively. Vortical flow structures originate at solid surfaces in near-body regions employing the Euler solver and are transported into the wake region that employs an Eulerian Vorticity Transport (EVT) solver. The excessive numerical dissipation common to most grid-based Navier-Stokes solvers is avoided in the wake region by solving the fluid dynamic equations in vorticity conservation form. In addition, the adaptive Cartesian mesh utilized in the wake region allows for efficient transport and preservation of vortical structures by means of localized adaptive mesh refinement and coarsening. Two different approaches for evaluating the EVT velocity field, one involving a fast summation technique based on the Cartesian Treecode method, and the other utilizing a multigrid Poisson approach, are presented and compared. The implementation of both the solution algorithm and overset coupling methodology is described in detail, and results for several different test cases are presented which demonstrate the effectiveness of the hybrid EVT-Euler simulation capability for accurately transporting vortical structures between a near-body compressible Euler solver and an off-body EVT solver.

Development and Performance Assessment of Hypersonic Inflatable Aerodynamic Decelerator

Inflatable aerodynamic architectures are one of the candidate enabling technologies for landing large payloads on celestial bodies with atmospheres for both human and robotic missions. The primary challenge of deployable space inflatable design is to demonstrate robust predictability, rigidity, and scalability of the inflatable’s integrated structures. A unique inflatable architecture is tested and analyzed as proposed basis for Inflatable Aerodynamic Decelerator (IAD) applications. This Ultra-High Performance Vessel (UHPV) inflatable technology is a flexible-deployable softgoods pressure vessel comprising an impervious barrier structure enveloped by an integrated array of meridional tendons. A multidisciplinary computational tool was developed and improved to characterize very thin inflatable structures under severe structural conditions. The nonlinear aero-structural interaction tool was verified and validated against experimental data to simulate the behavior of very thin materials under typical loading conditions. Details of the UHPV design and its performance assessment are described. The simulation results in terms of unsteady load fluctuation, inflation pressures, and fabric deflections are presented at various angles of attack.