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NASA NTRSDate not supplied

Empirical Noise Modeling of Internally Mixed Exhaust Systems

There appear to be no non-proprietary methods to predict the noise of internally mixed exhaust systems, and no guidance for how to adapt known jet noise models for these configurations. This paper surveys literature and historical databases acquired at NASA Glenn's Aero-Acoustic Propulsion Lab to give such guidance. The core premise is that an exhaust system with well-designed mixer produces noise that is to first approximation the same as a fully-mixed jet flow. Refinement of what is meant by a “fully mixed jet” can lead to more accurate prediction of the main jet noise. Additional noise is often generated within the nozzle, typically at high frequencies, whose source mechanism(s) are not obvious. However, a noise prediction method can be established that captures some aspects of the excess noise and provide an estimate of the total jet noise. Explorations of source mechanisms associated with the internal mixer have led to a new noise model which includes the effects of having an external plug nozzle, a feature desirable for near-term supersonic aircraft. Statistical analysis of historical data is provided to estimate the uncertainty in using this method given the variations found that cannot be directly computed without detailed mixer geometry.

Noise Prediction↗

NASA NTRSDate not supplied

Empirical Noise Modeling of Internally Mixed Exhaust Systems

There appears to be no non-proprietary methods to predict the noise of internally mixed exhaust systems, and no guidance for how to adapt known jet noise models for these configurations. This paper surveys literature and historical databases acquired at NASA Glenn's Aero-Acoustic Propulsion Lab to give such guidance. The core premise is that an exhaust system with well-designed mixer produces noise that is to first approximation the same as a fully-mixed jet flow. Refinement of what is meant by a “fully mixed jet” can lead to more accurate prediction of the main jet noise. Additional noise is often generated within the nozzle, typically at high frequencies, whose source mechanism(s) are not obvious. However, a noise prediction method can be established that captures some aspects of the excess noise and provide an estimate of the total jet noise. Explorations of source mechanisms associated with the internal mixer have led to a new noise model which includes the effects of having an external plug nozzle, a feature desirable for near-term supersonic aircraft. Statistical analysis of historical data is provided to estimate the uncertainty in using this method given the variations found that cannot be directly computed without detailed mixer geometry.

Noise Prediction↗

NASA NTRS2009

FPGA-Based Networked Phasemeter for a Heterodyne Interferometer

A document discusses a component of a laser metrology system designed to measure displacements along the line of sight with precision on the order of a tenth the diameter of an atom. This component, the phasemeter, measures the relative phase of two electrical signals and transfers that information to a computer. Because the metrology system measures the differences between two optical paths, the phasemeter has two inputs, called measure and reference. The reference signal is nominally a perfect square wave with a 50- percent duty cycle (though only rising edges are used). As the metrology system detects motion, the difference between the reference and measure signal phases is proportional to the displacement of the motion. The phasemeter, therefore, counts the elapsed time between rising edges in the two signals, and converts the time into an estimate of phase delay. The hardware consists of a circuit board that plugs into a COTS (commercial, off-the- shelf) Spartan-III FPGA (field-programmable gate array) evaluation board. It has two BNC inputs, (reference and measure), a CMOS logic chip to buffer the inputs, and an Ethernet jack for transmitting reduced-data to a PC. Two extra BNC connectors can be attached for future expandability, such as external synchronization. Each phasemeter handles one metrology channel. A bank of six phasemeters (and two zero-crossing detector cards) with an Ethernet switch can monitor the rigid body motion of an object. This device is smaller and cheaper than existing zero-crossing phasemeters. Also, because it uses Ethernet for communication with a computer, instead of a VME bridge, it is much easier to use. The phasemeter is a key part of the Precision Deployable Apertures and Structures strategic R&D effort to design large, deployable, segmented space telescopes.

Rao, Shanti↗

NASA NTRSDate not supplied

Study of a Plug Nozzle for Supersonic Aircraft Concepts

Nozzles with external plugs are candidates for the exhaust system of supersonic aircraft concepts in an ongoing research effort at NASA. As part of that effort a fundamental study is being conducted at NASA Glenn Research Center (GRC). Experimental and limited numerical simulation results from this study, for a plug nozzle with a convergent nozzle (cowl),have been reported in recent publications[1,2]. Conventional method-of-characteristics (MoC) design for the plug requires that the flow is choked at the nozzle exit. This is why a sharp convergence of the cowl is invoked in such a design in most previous studies [3-5], leading to the earlier choice of the convergent cowl. Such a nozzle, however, is not suitable for supersonic flights because of large boat tail drag suffered by its outer surface. The boat tail drag consideration dictates that the cowl be more or less cylindrical in shape. While various other shapes have been studied in the past [6], essentially a cylindrical shape is adopted in the larger research program at NASA. The cylindrical shape has certain advantages. With the plug crown located somewhat inside the nozzle, the flow experiences convergence and divergence as with a C-D nozzle. By translating the plug, different throat-to-exit area ratio could be achieved to obtain fully expanded condition at different values of NPR. This offers a relatively easy active control strategy for the nozzle geometry throughout the flight regime from landing and takeoff (LTO) to cruise conditions. A plug nozzle model mimicking the geometry being considered in the NASA program, i.e., having a cylindrical exit, have been fabricated recently for continued fundamental studies. Some of the plugs used with the earlier convergent cowl [1] are readily adopted to this model. In addition, a few other plugs have been obtained following a numerical optimization study [7]. The various configurations are to be investigated for thrust, noise and flow fields, with complementing numerical simulations. Thrust data for a variety of configurations as well as limited noise data are in hand. Further explorations including schlieren visualization are to be conducted in the near future. The proposed paper is to summarize all these results. In the following, a few key results obtained so far are described.

jets↗

NASA NTRS2021

Plug20 Test Report

This report documents a test of aircraft exhaust nozzle designs applicable to commercial supersonic aircraft as envisioned in the 2025-2035 time frame. The test is primarily intended to provide data about the noise such aircraft would produce during landing and takeoff (LTO) operation, specifically the jet noise component of this noise. The tests were conducted at the NASA Glenn Research Center’s Aero-Acoustic Propulsion Laboratory in March 2020. The tests used the High-Flow Jet Exhaust Rig in the Nozzle Acoustic Test Rig which simulates the flow from a dual-stream turbofan engine in a flight stream. Seven exhaust configurations were tested with various combinations of internal mixer and external plug, and covered engine cycles having nozzle pressures from 1.5 to 2.3 at a flight speed of Mach 0.3. A few unheated flow conditions were also tested to tie results to other baseline test results. Data acquired and included in this report are 1) far-field acoustic spectra, 2) phased array measurements of noise source distributions, and 3) background-oriented schlieren of the flow near the nozzles. These data, along with the nozzle geometry in CAD format, are given as Appendices and are available online.

Jet noise↗

NASA NTRS2002

A Facility and Architecture for Autonomy Research

Autonomy is a key enabling factor in the advancement of the remote robotic exploration. There is currently a large gap between autonomy software at the research level and software that is ready for insertion into near-term space missions. The Mission Simulation Facility (MST) will bridge this gap by providing a simulation framework and suite of simulation tools to support research in autonomy for remote exploration. This system will allow developers of autonomy software to test their models in a high-fidelity simulation and evaluate their system's performance against a set of integrated, standardized simulations. The Mission Simulation ToolKit (MST) uses a distributed architecture with a communication layer that is built on top of the standardized High Level Architecture (HLA). This architecture enables the use of existing high fidelity models, allows mixing simulation components from various computing platforms and enforces the use of a standardized high-level interface among components. The components needed to achieve a realistic simulation can be grouped into four categories: environment generation (terrain, environmental features), robotic platform behavior (robot dynamics), instrument models (camera/spectrometer/etc.), and data analysis. The MST will provide basic components in these areas but allows users to plug-in easily any refined model by means of a communication protocol. Finally, a description file defines the robot and environment parameters for easy configuration and ensures that all the simulation models share the same information.

Pisanich, Greg↗

NASA NTRSDate not supplied

Flow Fields of Internally Mixed Exhaust Systems With External Plug For Supersonic Transport Applications

Commercial supersonic vehicles of the future will likely use engines with lower bypass ratios, where the design of the internal mixer will have a strong impact on the noise produced by the jet plume. Their exhaust systems may also feature external plugs to improve boattail angle for cruise performance at supersonic speeds. Currently there are no publicly available empirical noise models for such nozzle systems, and insight into their flow fields will help in creating these models. For those attempting to make large eddy simulations and other higher fidelity methods be their main prediction tool, the internally mixed exhaust system is also a good test case when going beyond simple single-stream jets. The turbulent flow statistics of several configurations previously tested for noise and shocks have been measured for flow conditions that can be used in development of empirical models and validating scale-resolving prediction tools. These measurements are presented and briefly analyzed for insights into the noise impacts from the flow impacts observed.

Turbulence, Jets, Particle Image velocimetry↗

NASA NTRSDate not supplied

Flow Fields of Internally Mixed Exhaust Systems With External Plug For Supersonic Transport Applications

Fluid Mechanics and Thermodynamics↗

NASA NTRS2023

Plug20 Test Report: Particle Image Velocimetry (PIV) Measurements

This report provides turbulent flow statistics made using particle image velocimetry (PIV) for several internally mixed exhaust systems with external plugs (Plug20 designs). The tests were conducted at the NASA Glenn Research Center’s Aero-Acoustic Propulsion Laboratory (AAPL) in May and June 2022. The corresponding acoustic data for the configurations and flows presented in this report can be found in an earlier Technical Memorandum, NASA/TM-20210010291, “Plug20 Test Report.” The Plug20 nozzle family was designed to represent commercial supersonic vehicles in the 2025 to 2035 timeframe with a bypass ratio around 4, having an internal mixer, with and without an external plug to improve nacelle boattail drag and shock. The flow conditions measured represent engine cycles for commercial supersonic aircraft at takeoff, having pressure ratios of 2.0 and heated fan and core streams, measured both with and without a flight stream surrounding the plume. A few unheated flow conditions were also tested to tie results to other baseline test results. The PIV measurements in both streamwise and cross-stream planes are described, along with brief analysis to visually catalog the contents and confirm their validity. In this analysis, the streamwise PIV is found to match benchmark datasets within its experimental error. However, the cross-stream turbulent quantities measured by two systems do not match each other or the streamwise measurements in amplitude—caution is required when using the cross-stream data. Data acquired and included in this report are (1) velocity statistics of the plumes in both a streamwise and several cross-stream planes, and (2) the hot nozzle geometry. Tabular forms of the data and computer-aided design (CAD) files of the hot geometry are presented as supplemental files described in Appendix A (TM-20230006779SUPPL1.zip) and Appendix B (TM-20230006779SUPPL2.zip).

Turbulence↗