Robert E. Harris

Development and Validation of a Fluid-Structure Interaction Capability in the Loci/CHEM Solver

There are numerous phenomena encountered in modern fluid dynamic design in which traditional CFD analysis techniques do not provide sufficient physical insight due to the presence of strong interdisciplinary coupling effects. Tightly-coupled multidisciplinary analyses are often required in such situations to provide a complete understanding of the underlying physics. Numerous such applications are encountered in spacecraft propulsion analyses, including: liquid interaction with flexible fuel tank shells; fluid-induced vibration of turbine and inducer blades; liquid damping devices; and others. As a step toward bridging the gap and providing accurate predictive capabilities for multidisciplinary spacecraft propulsion analysis, a nonlinear finite-element structural solver is employed to enable tightly-coupled fluid-structure interaction simulations in the highly-scalable Loci/CHEM production flow solver. The multidisciplinary computing environment (MDICE) is utilized to provide physics-based interpolation mechanisms and socketbased communication between the flow and structural solvers. This approach allows each solver to independently manage memory and parallelism, and is thus wellsuited for use in heterogeneous computing environments. Implementation of the fluid-structure coupling procedure is described in detail, and validation studies are presented which demonstrate the effectiveness of the capability for aeroelastic predictions. Results for several well-known test cases are presented and comparisons are made with published experimental data to establish the accuracy of the developed methods.

Hybrid Discontinuous Galerkin and Finite Volume Method for Launch Environment Acoustics Prediction

Launch vehicles experience extreme acoustic loads during liftoff driven by the interaction of rocket plumes and plume-generated acoustic waves with ground structures. Currently employed predictive capabilities to model the complex turbulent plume physics are too dissipative to accurately resolve the propagation of acoustic waves throughout the launch environment. Higher fidelity liftoff acoustic analysis tools to design mitigation measures are critically needed to optimize launch pads for the Space Launch System and commercial launch vehicles. To this end, a new coupled two-field simulation capability has been developed to enable accurate prediction of liftoff acoustic physics. Established unstructured computational fluid dynamics algorithms are used for simulation of acoustic generation physics and a high-order-accurate discontinuous Galerkin nonlinear Euler solver is employed to accurately propagate acoustic waves across large distances. An innovative hybrid computational fluid dynamics/computational ae...

Coupled Fluid-Structure Interaction Analysis of Solid Rocket Motor with Flexible Inhibitors

A capability to couple NASA production CFD code, Loci/CHEM, with CFDRC’s structural finite element code, CoBi, has been developed. This paper summarizes the efforts in applying the installed coupling software to demonstrate/investigate fluid-structure interaction (FSI) between pressure wave and flexible inhibitor inside reusable solid rocket motor (RSRM). First a unified governing equation for both fluid and structure is presented, then an Eulerian-Lagrangian framework is described to satisfy the interfacial continuity requirements. The features of fluid solver, Loci/CHEM and structural solver, CoBi, are discussed before the coupling methodology of the solvers is described. The simulation uses production level CFD LES turbulence model with a grid resolution of 80 million cells. The flexible inhibitor is modeled with full 3D shell elements. Verifications against analytical solutions of structural model under steady uniform pressure condition and under dynamic condition of modal analysis show excellent agreements in terms of displacement distribution and eigen modal frequencies. The preliminary coupled result shows that due to acoustic coupling, the dynamics of one of the more flexible inhibitors shift from its first modal frequency to the first acoustic frequency of the solid rocket motor.

Coupled Overset Unstructured Discontinuous Galerkin Method for Launch Environment Acoustics Prediction

A novel approach for the accurate prediction of launch environment acoustic physics is presented. Launch vehicles experience extreme acoustic loads during liftoff, driven by the interaction of rocket plumes and plume-generated acoustic waves with ground structures. In this work, a well-established hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation unstructured mesh solver is used to efficiently model the complex turbulent plume physics, and a high-order accurate discontinuous Galerkin solver is used to accurately propagate acoustic waves across large distances throughout the launch environment. The two solvers operate on separate overlapping meshes, and an innovative overset coupling approach is used to transmit the plume-generated acoustics to the far field in a one-way manner in which the turbulent plume prediction is unaffected by the outer acoustic propagation physics. The framework upon which the solvers are developed is described along with details outlining the overset domain connectivity...

Solution-Adaptive Method for Prediction of Aerodynamic Interaction in Multiple-Body High-Speed Air-Delivered Systems

An unstructured solution-adaptive method for prediction of aerodynamic interaction in multiple-body high-speed air-delivered systems is presented. Development of this capability is motivated by the need to efficiently characterize systems of high-speed unguided projectiles that are subject to a complex flow environment in which inter-body collisions, large interference pressures, and strong moving shock systems are prevalent. These unique physics are primarily encountered in the strong interaction phase early in the projectile deployment, and have a first-order effect on the resulting projectile dispersion pattern. To accurately resolve these highly localized physics, time-accurate 6-DoF/CFD simulations with adaptive mesh refinement are performed to efficiently resolve the moving projectiles as well as the flowfield and shock interference between the projectiles. The CFD analysis and adaptive mesh refinement method are described in detail along with validation studies to demonstrate the validity of the developed technologies for transonic wing and supersonic high-angle-of-attack missile aerodynamics predictions. The capability is then demonstrated for prediction of shock and aerodynamic interference effects for multiple projectile darts flying in close proximity.

Rigid Body Collision Modeling for Multiple-Body Proximate Flight Simulation in Loci/CHEM

Currently, there exists a lack of confidence in the computational simulation of multiple body high-speed air delivered systems. Of particular interest is the ability to accurately predict the dispersion pattern of these systems under various deployment configurations. Classical engineering-level methods may not be able to predict these patterns with adequate confidence due primarily to accuracy errors attributable to reduced order modeling. In the current work, a new collision modeling capability has been developed to enable multiple-body proximate-flight simulation in the Loci/CHEM framework. This approach is well suited for simulation of a large number of projectiles, and maintains high-fidelity aerodynamics with six degrees of freedom modeling, and collision response. The Loci/CHEM architecture provides automatic parallelism and has previously demonstrated extreme scalability on thousands of computer nodes. The proposed simulation system is intended to capture the strong interaction phase, which occurs early in the projectile deployment, with subsequent transfer of projectile positions and flight states to the more economical engineering-level methods. Collisions between rigid bodies are modeled using an impulse-based approach with either an iterative propagation method or a simultaneous method. The latter is shown to be more accurate and robust for cases involving multiple simultaneous collisions as it eliminates the need to sort and resolve the collisions sequentially. The implementation of both the collision detection methodology and impact mechanics are described in detail with validation studies which are presented to demonstrate the efficiency and accuracy of the method. The studies chronologically detail the findings for simulating simple impacts and collisions between multiple bodies with aerodynamic interference effects.

Coupled Overset Unstructured Discontinuous Galerkin Method for Jet Noise Prediction

Launch vehicles experience extreme acoustic loads during lift-off, driven by the interaction of exhaust plumes and plume-generated acoustic waves with ground structures. Prediction of the noise associated with launch vehicle acoustic environments poses several unique challenges when compared to traditional jet noise modeling techniques. In particular, acoustic and Mach waves propagating from the exhaust plume are in the vicinity of obstructions and are generally in the nonlinear regime, which renders the classical acoustic analogy and linearized approaches invalid. In this work, a novel approach for the accurate prediction of launch vehicle lift-off acoustic environments is presented. A well-established hybrid RANS/LES unstructured mesh solver with low-dissipation schemes is utilized to efficiently and accurately predict the complex exhaust plume physics, and a high-order accurate discontinuous Galerkin solver is used to accurately propagate acoustic waves across large distances throughout the launch environment. The two solvers operate on separate overlapping meshes and an innovative overset coupling approach is used to transmit the plume-generated acoustics to the farfield in a one-way manner in which the exhaust plume prediction is unaffected by the outer acoustic propagation physics. The framework upon which the solvers are developed is described along with details outlining the overset grid assembly and interpolation methods. Results are presented which demonstrate the accuracy of the capability for aeroacoustic predictions, and the merits of the approach are evaluated for transonic jet noise prediction for a two-dimensional free jet. The method is shown to be effective and accurate in terms of predicted sound pressure level spectra when compared to direct noise computation.

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.

Towards a Predictive Capability for Multiple-Body Proximate-Flight in High-Speed Air-Delivered Systems

Multiple-body high-speed air-delivered systems are subjected to a complex flow environment characterized by inter-body collisions, large interference pressures, and strong moving shock systems. These unique physics are prevalent in the strong interaction phase early in the projectile deployment, and have a first-order effect on the resulting projectile dispersion pattern. Since classical engineering-level simulation methods cannot predict these patterns with adequate confidence, highfidelity methods that include all relevant physics are necessary to accurately predict the projectile trajectories at the conclusion of the interference-dominated flight phase. Newly developed capabilities in the Loci/CHEM framework have enabled multiple-body proximate flight simulations including high-fidelity aerodynamics with six degrees of freedom modeling and collision response that are well-suited for simulation of large numbers of projectiles. Unlike many approaches for moving body simulation, the approach presented here utilizes unstructured overset grids with fully automatic grid assembly and no ancillary preprocessing requirements beyond those necessary for static body simulation. The solution methodology and collision modeling capabilities are described in detail. Results are presented for supersonic air-delivered systems with dozens of projectiles in close-proximity released from a carrier vehicle under several different deployment configurations. The results demonstrate the effectiveness of the developed capabilities for simulation of high-speed proximate-flight among many projectiles including strong aerodynamic interference effects and frequent inter-body collisions.