Sudheer N. Nayani

Specialized CFD Grid Generation Methods for Near-Field Sonic Boom Prediction

Ongoing interest in analysis and design of low sonic boom supersonic transports re- quires accurate and ecient Computational Fluid Dynamics (CFD) tools. Specialized grid generation techniques are employed to predict near- eld acoustic signatures of these con- gurations. A fundamental examination of grid properties is performed including grid alignment with ow characteristics and element type. The issues a ecting the robustness of cylindrical surface extrusion are illustrated. This study will compare three methods in the extrusion family of grid generation methods that produce grids aligned with the freestream Mach angle. These methods are applied to con gurations from the First AIAA Sonic Boom Prediction Workshop.

Evaluation of Grid Modification Methods for On- and Off-Track Sonic Boom Analysis

Grid modification methods have been under development at NASA to enable better predictions of low boom pressure signatures from supersonic aircraft. As part of this effort, two new codes, Stretched and Sheared Grid - Modified (SSG) and Boom Grid (BG), have been developed in the past year. The CFD results from these codes have been compared with ones from the earlier grid modification codes Stretched and Sheared Grid (SSGRID) and Mach Cone Aligned Prism (MCAP) and also with the available experimental results. NASA’s unstructured grid suite of software TetrUSS and the automatic sourcing code AUTOSRC were used for base grid generation and flow solutions. The BG method has been evaluated on three wind tunnel models. Pressure signatures have been obtained up to two body lengths below a Gulfstream aircraft wind tunnel model. Good agreement with the wind tunnel results have been obtained for both on-track and off-track (up to 53 degrees) cases. On-track pressure signatures up to ten body lengths below a Straight Line Segmented Leading Edge (SLSLE) wind tunnel model have been extracted. Good agreement with the wind tunnel results have been obtained. Pressure signatures have been obtained at 1.5 body lengths below a Lockheed Martin aircraft wind tunnel model. Good agreement with the wind tunnel results have been obtained for both on-track and off-track (up to 40 degrees) cases. Grid sensitivity studies have been carried out to investigate any grid size related issues. Methods have been evaluated for fully turbulent, mixed laminar/turbulent and fully laminar flow conditions. Nomenclature

USM3D Predictions of Supersonic Nozzle Flow

This study focused on the NASA Tetrahedral Unstructured Software System CFD code (USM3D) capability to predict supersonic plume flow. Previous studies, published in 2004 and 2009, investigated USM3Ds results versus historical experimental data. This current study continued that comparison however focusing on the use of the volume souring to capture the shear layers and internal shock structure of the plume. This study was conducted using two benchmark axisymmetric supersonic jet experimental data sets. The study showed that with the use of volume sourcing, USM3D was able to capture and model a jet plumes shear layer and internal shock structure.

USM3D Analysis of Low Boom Configuration (Invited)

In the past few years considerable improvement was made in NASA’s in house boom prediction capability. As part of this improved capability, the USM3D Navier-Stokes flow solver, when combined with a suitable unstructured grid, went from accurately predicting boom signatures at 1 body length to 10 body lengths. Since that time, the research emphasis has shifted from analysis to the design of supersonic configurations with boom signature mitigation In order to design an aircraft, the techniques for accurately predicting boom and drag need to be determined. This paper compares CFD results with the wind tunnel experimental results conducted on a Gulfstream reduced boom and drag configuration. Two different wind-tunnel models were designed and tested for drag and boom data. The goal of this study was to assess USM3D capability for predicting both boom and drag characteristics. Overall, USM3D coupled with a grid that was sheared and stretched was able to reasonably predict boom signature. The computational drag polar matched the experimental results for a lift coefficient above 0.1 despite some mismatch in the predicted lift-curve slope.

Preliminary Test Results for Stability and Control Characteristics of a Generic T-tail Transport Airplane at High Angle of Attack

Tests of a generic T-tail transport airplane, in flaps-up configuration, were conducted using two wind tunnels, a water tunnel, and computational fluid dynamics. Static force and moment testing, forced oscillation testing and dye flow visualization test techniques were used. The purpose of the testing was to obtain stability and control characteristics for development of a research flight simulator aerodynamic database. The purpose of that database was for assessment of aerodynamic model fidelity requirements to train airline pilots to recognize and recover from full stall conditions. Preliminary results, at initial stall conditions, include: an unstable stall pitch break, and near-neutral roll damping. Preliminary results, at deep stall conditions, include: a potential static longitudinal trim condition at approximately 35 degrees angle of attack, large aerodynamic asymmetries, and localized unstable dynamic stability.

Collaborative Evaluation of CFD-to-ROM Dynamic Modeling

This work presents and discusses findings from a NATO STO collaborative research on the reduced order aerodynamic modeling of a NACA 0012 airfoil and a generic swept wing UCAV. The linear and nonlinear reduced order models are created based on the superposition integrals of the step response with the derivative of its corresponding input signal. Step responses are calculated using CFD and a grid motion approach that allows separating the effects of angle of attack and sideslip angle from angular rates. This approach was previously tested using Cobalt flow solver, however to demonstrate its generalization capability, four different flow solvers are used in this study: Cobalt and Kestrel at United States Air Force Academy (USAFA), USM3D at NASA Langley Research Center (LaRC), and ENSOLV at the Netherlands National Aerospace Laboratories (NLR). Step changes in the angle of attack and pitch rate are obtained using these CFD codes. For the UCAV configuration, the lateral step responses to sideslip angle, roll and yaw rates are also calculated. The step predictions of the codes are compared with each other. Aerodynamic models are then created from these step responses and are used to predict responses to arbitrary motions (inputs). The model predictions are compared with CFD (full-order) and available experiments. The results demonstrate that step functions can be easily calculated by CFD codes. Overall, the angle-of-attack and pitch rate responses are very similar for each solver particulary at small angles of attack. Discrepancies at higher angles are probably due to differences in grids and solver numerical algorithms. The step responses show an initial jump as the grid begins to move. The initial jumps become smaller with increasing Mach number. All responses will then asymptotically reach a steady-state value. The results show that significantly fewer time steps are required to reach the steady-state solutions for the UCAV geometry than two-dimensional airfoil. Finally, the model predictions match the CFD data of different motions, all generated within the range of data used for model generation, very well.

Numerical Examination of Shock Generator Geometries and Nozzle Plume Effects on Pressure Signature

The NASA Advanced Air Vehicles Program, Commercial Supersonics Technology Project seeks to advance tools and techniques to make over-land supersonic flight feasible. In this study, preliminary computational results are presented for future tests in the NASA Ames 9’ x 7’ supersonic wind tunnel to be conducted in early 2016. Shock-plume interactions and their effect on pressure signature are examined for six model geometries. Near-field pressure signatures are assessed using the CFD code USM3D to model the proposed test geometries in free-air. Additionally, results obtained using the commercial grid generation software Pointwise R

Computational and Experimental Study of Supersonic Nozzle Flow and Shock Interactions

This study focused on the capability of NASA Tetrahedral Unstructured Software System’s CFD code USM3D capability to predict the interaction between a shock and supersonic plume flow. Previous studies, published in 2004, 2009 and 2013, investigated USM3D’s supersonic plume flow results versus historical experimental data. This current study builds on that research by utilizing the best practices from the early papers for properly capturing the plume flow and then adding a wedge acting as a shock generator. This computational study is in conjunction with experimental tests conducted at the Glenn Research Center 1’x1’ Supersonic Wind Tunnel. The comparison of the computational and experimental data shows good agreement for location and strength of the shocks although there are vertical shifts between the data sets that may be do to the measurement technique.

Unstructured Grids for Sonic Boom Analysis and Design

An evaluation of two methods for improving the process for generating unstructured CFD grids for sonic boom analysis and design has been conducted. The process involves two steps: the generation of an inner core grid using a conventional unstructured grid generator such as VGRID, followed by the extrusion of a sheared and stretched collar grid through the outer boundary of the core grid. The first method evaluated, known as COB, automatically creates a cylindrical outer boundary definition for use in VGRID that makes the extrusion process more robust. The second method, BG, generates the collar grid by extrusion in a very efficient manner. Parametric studies have been carried out and new options evaluated for each of these codes with the goal of establishing guidelines for best practices for maintaining boom signature accuracy with as small a grid as possible. In addition, a preliminary investigation examining the use of the CDISC design method for reducing sonic boom utilizing these grids was conducted, with initial results confirming the feasibility of a new remote design approach.

Computational and Experimental Study of Plume and Shock Interaction Effects on Sonic Boom in the NASA Ames 9x7 Supersonic Wind Tunnel [STUB]

A wind tunnel test was performed in the NASA Ames 9x7 Supersonic Wind Tunnel focusing on the shock waves traveling through and interacting with an exhaust nozzle plume. This experimental study was conducted to develop and validate the CFD capability required to accurately include nozzle flow with impinging shock effects on near field and ground‐propagated sonic boom signatures. The model was made to be generic, and included a simple nozzle shape, two different aft decks, and a few generic horizontal tails. High pressure air was pumped through a nozzle at various nozzle pressure ratios (NPR) to represent the engine plume in flight. The three different aft body representations each created a different shock wave signature that passed through the plume. An aft deck configuration, where part of the aircraft shields the nozzle plume, was also tested. Retroreflective Background-Oriented Schlieren (RBOS) was used to obtain schlieren images of the flow field around the model and behind the model. This study compares wind tunnel data and numerical simulations conducted by the NASA Tetrahedral Unstructured Software System CFD code, USM3D. * Aerospace Engineer, Configuration Aerodynamics Branch, Mail Stop 499, AIAA Senior Member. † Aerospace Engineer, Configuration Aerodynamics Branch, Mail Stop 499, AIAA Associate Fellow. ‡ Aerospace Engineer, Configuration Aerodynamics Branch, Mail Stop 499, AIAA Senior Member. § Graduate Student, College of Engineering and Mathematical Sciences, AIAA Student Member. ** Senior Scientist, CFD Group, 107 Research Drive, AIAA Associate Fellow. †† Aerospace Engineer, Experimental Aero‐Physics Branch, Moffett Field, AIAA Senior Member. https://ntrs.nasa.gov/search.jsp?R=20180006298 2020-03-07T07:19:33+00:00Z