Michael F. Barad

Space-Time Accuracy Assessment of CFD Simulations for the Launch Environment

Time-accurate high-fidelity Computational Fluid Dynamics (CFD) simulations of the launch environment are an important part of the successful launch of new and existing space vehicles. The capability to accurately predict certain aspects of the launch environment, such as ignition overpressure (IOP) waves and launch acoustics, is paramount to mission success. Implicit dual-time stepping methods represent one approach to provide accurate computational results in a timely manner. Two simplified test cases related to the launch environment are examined. The first test case models the IOP waves generated from a 2D planar jet located above a 45-degree flat plate, while the second case investigates launch acoustic noise generated from the jet of a rocket impinging on an axisymmetric flame trench and mobile launcher. Sensitivity analysis has been performed and a verification procedure was applied to investigate the necessary spatial and temporal resolution requirements for CFD simulations of the launch environment using an implicit dual-time method.

Open Rotor Computational Aeroacoustic Analysis with an Immersed Boundary Method

Reliable noise prediction capabilities are essential to enable novel fuel efficient open rotor designs that can meet the community and cabin noise standards. Toward this end, immersed boundary methods have reached a level of maturity so that they are being frequently employed for specific real world applications within NASA. This paper demonstrates that our higher-order immersed boundary method provides the ability for aeroacoustic analysis of wake-dominated flow fields generated by highly complex geometries. This is the first of a kind aeroacoustic simulation of an open rotor propulsion system employing an immersed boundary method. In addition to discussing the peculiarities of applying the immersed boundary method to this moving boundary problem, we will provide a detailed aeroacoustic analysis of the noise generation mechanisms encountered in the open rotor flow. The simulation data is compared to available experimental data and other computational results employing more conventional CFD methods. The noise generation mechanisms are analyzed employing spectral analysis, proper orthogonal decomposition and the causality method.

An Immersed Boundary Method for Solving the Compressible Navier-Stokes Equations with Fluid Structure Interaction

An immersed boundary method for the compressible Navier-Stokes equation and the additional infrastructure that is needed to solve moving boundary problems and fully coupled fluid-structure interaction is described. All the methods described in this paper were implemented in NASA’s LAVA solver framework. The underlying immersed boundary method is based on the locally stabilized immersed boundary method that was previously introduced by the authors. In the present paper this method is extended to account for all aspects that are involved for fluid structure interaction simulations, such as fast geometry queries and stencil computations, the treatment of freshly cleared cells, and the coupling of the computational fluid dynamics solver with a linear structural finite element method. The current approach is validated for moving boundary problems with prescribed body motion and fully coupled fluid structure interaction problems in 2D and 3D. As part of the validation procedure, results from the second AIAA aeroelastic prediction workshop are also presented. The current paper is regarded as a proof of concept study, while more advanced methods for fluid structure interaction are currently being investigated, such as geometric and material nonlinearities, and advanced coupling approaches.

A Comparison of Higher-Order Shock Capturing Schemes Within the LAVA CFD Solver

The efficiency of large-eddy type simulations can be greatly increased by employing higher-order accurate numerical schemes which provide superior accuracy for a given cost. For unsteady turbulent flow simulations including shocks, contacts, and/or material discontinuities, various numerical higher-order shock capturing schemes are available in the literature. The desired numerical scheme should be free of spurious numerical oscillations across discontinuities and obtain higher-order accuracy in smooth flow regions in an efficient manner. Sufficient robustness is absolutely necessary for effectively utilizing these numerical methods in engineering and science applications. Three types of numerical higher-order schemes are discussed in this paper, i.e., central finite-difference schemes with explicit artificial dissipation, a compact centered finite-difference scheme with localized artificial diffusivity and weighted essentially non-oscillatory schemes in explicit and compact finite difference forms. Variations of these numerical schemes were implemented and tested in the Launch Ascent and Vehicle Aerodynamics (LAVA) solver, using a block-structured Cartesian mesh. The current paper provides a detailed discussion and comparison of these numerical schemes. The variety of test cases ranges from 1D shock problems to homogeneous isotropic turbulence at a turbulent Mach number of 0.5 where shocklets form.

Aerodynamic Database Generation for SRB Separation from a Heavy Lift Launch Vehicle

An engineering approach is presented for database generation of aerodynamic force and moment coefficients on Solid Rocket Boosters (SRBs) during separation from a Heavy Lift Launch Vehicle. The approach balances accuracy and affordability by generating a steadystate database of solutions using the inviscid flow solver Cart3D with adjoint based adaptive mesh refinement. The procedure also includes point checking the database using the viscous Reynolds Averaged Navier-Stokes solver OVERFLOW. The simulation matrix consists of 3 degrees of freedom by planar translation and rotation of the SRBs from their original attached positions. Good agreement with viscous multi-species simulations has confirmed that the single-species inviscid assumptions are valid for this plume impingement flow. The database provides axial forces, side forces, and yaw moments on the SRBs during separation from the core. This data is to be used for vehicle and trajectory planning to avoid re-contacting the core.

Lattice Boltzmann and Navier-Stokes Cartesian CFD Approaches for Airframe Noise Predictions

Airframe noise is the noise that is generated by non-propulsive components of an aircraft. It can be divided into noise sources from: wings, including tail surfaces and fuselage; high lift devices, including leading edge slats, flap side edges, and brackets; and undercarriage, which includes wheels, axles, legs/struts, fairings, brake cables, pipes, wheel wells, and doors.1 This noise is a nuisance in the vicinity of both major and minor airports throughout the world, and is a major focus of manufacturers designing, retrofitting, and operating current and future aircraft. There is increasing evidence that airframe and engine noise are comparable in the approach to landing phase. Thus e↵orts to reduce aircraft noise further should necessarily target ways to reduce airframe noise in addition to jet noise, turbofan noise and core noise. Computational prediction of airframe noise is based on lower-fidelity empirical approaches, or increasingly, based on computational fluid dynamics (CFD) which is often coupled with an acoustic analogy. The focus of this work is on high-fidelity CFD approaches, and specifically on accurate methods with a fast turn-around time. More complete overviews of airframe noise prediction approaches is available in the literature.2,3 CFD has achieved global acceptance as a mature discipline that complements traditional wind tunnel and flight testing. In a modern, fast-paced design environment where decisions are tightly supported/driven by extensive simulation data, increasing pressure is being exerted on CFD practitioners to improve geometric fidelity and reduce turnaround times. This trend represents a paradigm shift that favors e cient and versatile CFD frameworks in place of specialized legacy CFD solvers typically optimized for a limited set of applications. The emphasis on CFD simulation turnaround time highlights several bottlenecks in the simulation process, most significant of which is the preparation of the computational geometry and its volumetric meshing. Several di↵erent meshing and numerical discretization strategies such as structured Cartesian4–11 and unstructured12,13 have emerged as alternatives to the classical structured curvilinear14,15 approach (see Kiris et al16 for examples). Structured curvilinear grid generation is extremely time-consuming and labor⇤Computational Aerosciences Branch, NAS Division, MS N258-2, AIAA Senior Member

Verification and Validation Studies for the LAVA CFD Solver

The verification and validation of the Launch Ascent and Vehicle Aerodynamics (LAVA) computational fluid dynamics (CFD) solver is presented. A modern strategy for verification and validation is described incorporating verification tests, validation benchmarks, continuous integration and version control methods for automated testing in a collaborative development environment. The purpose of the approach is to integrate the verification and validation process into the development of the solver and improve productivity. This paper uses the Method of Manufactured Solutions (MMS) for the verification of 2D Euler equations, 3D Navier-Stokes equations as well as turbulence models. A method for systematic refinement of unstructured grids is also presented. Verification using inviscid vortex propagation and flow over a flat plate is highlighted. Simulation results using laminar and turbulent flow past a NACA 0012 airfoil and ONERA M6 wing are validated against experimental and numerical data.

Computational Prediction of Pressure and Thermal Environments in the Flame Trench With Launch Vehicles

One of the key objectives for the development of the 21st Century Space Launch Com- plex is to provide the exibility needed to support evolving launch vehicles and spacecrafts with enhanced range capacity. The launch complex needs to support various proprietary and commercial vehicles with widely di erent needs. The design of a multi-purpose main ame de ector supporting many di erent launch vehicles becomes a very challenging task when considering that even small geometric changes may have a strong impact on the pressure and thermal environment. The physical and geometric complexity encountered at the launch site require the use of state-of-the-art Computational Fluid Dynamics (CFD) tools to predict the pressure and thermal environments. Due to harsh conditions encountered in the launch environment, currently available CFD methods which are frequently employed for aerodynamic and ther- mal load predictions in aerospace applications, reach their limits of validity. This paper provides an in-depth discussion on the computational and physical challenges encountered when attempting to provide a detailed description of the ow eld in the launch environ- ment. Several modeling aspects, such as viscous versus inviscid calculations, single-species versus multiple-species ow models, and calorically perfect gas versus thermally perfect gas, are discussed. The Space Shuttle and the Falcon Heavy launch vehicles are used to study di erent engine and geometric con gurations. Finally, we provide a discussion on traditional analytical tools which have been used to provide estimates on the expected pressure and thermal loads.