A laboratory experiment was conducted to study the propagation of short duration (25 μs) and high amplitude (1000 Pa) acoustic N-waves in turbulent flow. Turbulent flows with a root-mean-square value of the fluctuating velocity up to 4 m/s were generated using a bidimensional nozzle (140 × 1600 mm2). Energy spectra of velocity fluctuations were measured and found in good agreement with the modified von Karman spectrum for fully developed turbulence. Spherical N-waves were generated by an electric spark source. Distorted waves were measured by four 3 mm diameter microphones placed beyond the turbulent jet. The presence of turbulence resulted in random focusing of the pulse; more than a threefold increase of peak pressures was occasionally observed. Statistics of the acoustic field parameters were evaluated as functions of the propagation distance and the level of turbulence fluctuations. It is shown that random inhomogeneities decrease the mean peak positive pressure up to 30% at 2 m from the source, double the mean rise time, and cause the arrival time about 0.3% earlier than that for corresponding conditions in still air. Probability distributions of the pressure amplitude possess autosimilarity properties with respect to the level of turbulence fluctuations.

https://acoustique.ec-lyon.fr/publi/averiyanov_jasa11b.pdf

Random focusing of nonlinear acoustic N-waves in fully developed turbulence: laboratory scale experiment.

NASA has chartered teams to study commercial transports that can overcome significant performance and environmental challenges for the benefit of the general public. The key technical objective of this effort was to generate promising supersonic concepts for the 2030-2035 timeframe and to develop plans for maturing the technologies required to make those concepts a reality. The N+3 program is aligned with NASA's Supersonic Project and is focused on providing alternative system-level solutions capable of overcoming the efficiency, environmental, and performance barriers to practical supersonic flight. 1.0 Summary Lockheed Martin Aeronautics Company (LM Aero), working in conjunction with seven industry and academia sub-contracting teammates, executed an 18 month program responsive to the NASA sponsored N+3 NRA: ―Advanced Concept Studies for Supersonic Commercial Transports Entering Service in the 2030-2035 Period.‖ ‗N+3' denotes three generations beyond the current commercial transport fleet. The N+3 program is aligned with NASA's Supersonic Project and is focused on providing alternative system-level solutions capable of overcoming the environmental, efficiency and performance barriers to practical supersonic flight. The environmental goals focus on low sonic boom, airport noise and cruise emissions. The program considered promising concepts and enabling technologies in an extensively integrated analysis process required particularly to achieve low sonic boom with efficiency. The reason for investigating alternative system-level solutions has to do with the projected status of our air transportation system. In addition to FAA regulations, the Next Generation (NextGen) Air Traffic System (ATS) congestion levels are a concern as they are expected to increase by a factor of 2 to 3 in the 2030 timeframe. Understanding how supersonic aircraft affect future congestion levels requires a system of systems analysis that integrates vehicle design, operating environment, and economic interaction into a single process. LM Aero worked with a sister company, Transportation Security & Solutions (TSS), and Purdue University to assess the value that a supersonic transport concept vehicle brings to the NextGen ATS. A fast time modeling and simulation study done by TSS revealed that commercial supersonic vehicles will not impact future airport capacity. However, supersonic air vehicles in the 2030 timeframe will exert additional demand for airport operations. Purdue University simulated numerous future Civil Air Transport System scenarios, allocating N+3 vehicles to maximize system-wide productivity while also computing fleet-wide emissions and direct operating costs. These results showed that the total value of time saved by passengers on N+3 supersonic transports exceed the added operating costs incurred by the aircraft. These system-level scenarios showed that supersonic transport is a viable solution for increased productivity and promotes the renewed viability of supersonic travel. Our extended team contributed to a preferred supersonic configuration and developed plans for maturing the identified, enabling technologies required to meet the N+3 performance and environmental goals. Working in conjunction with GE Global Research Center (GRC), John Hansman from MIT, Helen Reed & Bill Saric from Texas A&M, Wyle Laboratories, Purdue, and Penn State - an initial low-boom, supersonic configuration was used to assess potential airframe and propulsion technologies that were projected to meet or exceed the future supersonic boom, noise, emissions, cruise speed, range, payload, and fuel efficiency goals. Multi-Disciplinary Analysis and Optimization (MDAO) showed it was possible to achieve the N+3 boom goal with an ―inverted-V‖, engine-under wing configuration. Further sizing and quantified analysis proved that using revolutionary technologies enabled this configuration to achieve the range, payload, and cruise speed goals.

Advanced Concept Studies for Supersonic Commercial Transports Entering Service in 2030-35 (N+3)

The nonlinear propagation of spark-generated N-waves through thermal turbulence is experimentally studied at the laboratory scale under well-controlled conditions. A grid of electrical resistors was used to generate the turbulent field, well described by a modified von Karman model. A spark source was used to generate high-amplitude (∼1500 Pa) and short duration (∼50 μs) N-waves. Thousands of waveforms were acquired at distances from 250 to 1750 mm from the source (∼15 to 100 wavelengths). The mean values and the probability densities of the peak pressure, the deviation angle, and the rise time of the pressure wave were obtained as functions of propagation distance through turbulence. The peak pressure distributions were described using a generalized gamma distribution, whose coefficients depend on the propagation distance. A line array of microphones was used to analyze the effect of turbulence on the propagation direction. The angle of deviation induced by turbulence was found to be smaller than 15°, wh...

/pdf/laboratory-scale-experiment-to-study-nonlinear-n-wave-b5wrkx9z8e.pdf

Laboratory-scale experiment to study nonlinear N-wave distortion by thermal turbulence.

An extensive sonic boom propagation database with low- to normal-intensity booms (overpressures of 0.08 lbf/sq ft to 2.20 lbf/sq ft) was collected for propagation code validation, and initial results and flight research techniques are presented. Several arrays of microphones were used, including a 10 m tall tower to measure shock wave directionality and the effect of height above ground on acoustic level. A sailplane was employed to measure sonic booms above and within the atmospheric turbulent boundary layer, and the sailplane was positioned to intercept the shock waves between the supersonic airplane and the ground sensors. Sailplane and ground-level sonic boom recordings were used to generate atmospheric turbulence filter functions showing excellent agreement with ground measurements. The sonic boom prediction software PCBoom4 was employed as a preflight planning tool using preflight weather data. The measured data of shock wave directionality, arrival time, and overpressure gave excellent agreement with the PCBoom4-calculated results using the measured aircraft and atmospheric data as inputs. C-weighted acoustic levels generally decreased with increasing height above the ground. A-weighted and perceived levels usually were at a minimum for a height where the elevated microphone pressure rise time history was the straightest, which is a result of incident and ground-reflected shock waves interacting.

/pdf/initial-results-from-the-variable-intensity-sonic-boom-4vur2m3vew.pdf

Initial Results from the Variable Intensity Sonic Boom Propagation Database

The N+3 Final Report documents the work and progress made by Lockheed Martin Aeronautics in response to the NASA sponsored program "N+3 NRA Advanced Concept Studies for Supersonic Commercial Transports Entering Service in the 2030 to 2035 Period." The key technical objective of this effort was to generate promising supersonic concepts for the 2030 to 2035 timeframe and to develop plans for maturing the technologies required to make those concepts a reality. The N+3 program is aligned with NASA's Supersonic Project and is focused on providing alternative system-level solutions capable of overcoming the efficiency, environmental, and performance barriers to practical supersonic flight

Final Report for the Advanced Concept Studies for Supersonic Commercial Transports Entering Service in the 2030 to 2035 Period, N+3 Supersonic Program

Atmospheric turbulence is known to alter the pressure waveform created by supersonic flight. It is challenging to predict how a particular sonic boom waveform will be affected by a particular atmospheric realization without performing intense atmospheric modeling calculations. This paper attempts to approximate the effects of turbulence as a linear FIR filter. Such a filter is estimated from real sonic boom data measured on the ground and at altitude.

Modeling atmospheric turbulence as a filter for sonic boom propagation

The Nonlinear Progressive wave Equation (NPE) is modified to include a turbulence representation based upon temperature fluctuations. The turbulence is composed of the sum of 600 Fourier modes based on a modified von Karmspectrum with an outer length scale of 100 m and an inner length scale of .001 m. Variations in temperature induce variations in the local speed of sound. The NPE propagates for several seconds with a propagation time step of 1e-5 s. The resultant 2D waveform is then used to generate filter functions which approximate the eects of propagation through the turbulence. The filter functions can then be used to represent turbulent propagation by convolving the filter with any waveform, assuming it has sucient high frequency content.

Sonic boom post processing to include atmospheric turbulence effects

It is challenging to develop a physics‐based model for propagation through atmospheric turbulence A number of models are currently available but are typically not based on experimental results A fresh approach might be to construct some type of “black box” filter function for producing typical distorted sonic boom waveforms on the ground The input to the filter could be arbitrary, non‐distorted sonic boom waveforms above the turbulence As an initial model, a single input‐output system was assumed Ground‐recorded waveforms were used as outputs of the system The transfer function between the two signals was estimated using Welch’s average periodogram method Data obtained from the 2004 Shaped Sonic Boom Experiment (SSBE) was analyzed and corresponding transfer functions were obtained With such transfer functions established, FIR filters were determined and convolved with an appropriate input waveform Such FIR filters allow for the production of “typical” atmospherically distorted waveforms given arbitrary input waveforms

FIR Models for Sonic Boom Propagation Through Turbulence

Efforts have been underway to develop filter functions suitable for adding turbulent atmospheric effects to theoretical low‐boom waveforms. Filter functions have been created based on a measured input‐output relationship. The input is a relatively clean sonic boom waveform measured at altitude by a glider, and the outputs are turbulized’ waveforms measured on the ground. One input waveform and multiple output waveforms are used to represent multiple realizations of the atmosphere. Work presented in 2005 [Locey and Sparrow, Innovations in Nonlinear Acoustics, 17th International Symposium on Nonlinear Acoustics (American Institute of Physics, Melville, NY, 2006)] yielded an initial set of filter functions using one particular algorithm and data collected during the Shaped Sonic Boom Experiment (SSBE) in January of 2004. In this talk new results will be presented based on high‐fidelity measurements made at NASA Dryden Flight Research Center in June of 2006 [T. Gabrielson et al., Proc. Internoise (2006)]. Tim...

Lance L. Locey

Papers

Initial Results from the Variable Intensity Sonic Boom Propagation Database

Modeling atmospheric turbulence as a filter for sonic boom propagation

Sonic boom post processing to include atmospheric turbulence effects

FIR Models for Sonic Boom Propagation Through Turbulence

Atmospheric turbulence filter functions derived from high‐fidelity measurements