Kenneth J. Plotkin

An efficient method for incorporating computational fluid dynamics into sonic boom prediction

A method has been developed for utilizing Computational Fluid Dynamics (CFD) flow solutions as a starting point for sonic boom propagation calculations. An existing CFD code was shown to predict near-field flow with adequate resolution for sonic boom analysis. However, within the flowfield domain for which this CFD calculation is practical, there can be significant unresolved diffraction effects. Neglecting these effects can underpredict boom at the ground. A matching methodology has therefore been developed. based on an acoustic multipole formulation. The multipole formulation allows a transformation from near-field flow to the h a l far-field azimuthal pattern. An example of the application of this methodology to a wing-body configuration is presented.

Lift-Off Acoustics Predictions for the Ares I Launch Pad

A model has been prepared to predict noise on the mobile launcher and tower for the Ares I launch vehicle. The model is based on classic semi-empirical methods developed during the Apollo program. It has been updated with more recent data and algorithms. It has been implemented in two software modules. The first performs the noise calculations itself, yielding octave band and PSD spectra at specific points on the launcher or on a grid covering a region of interest. These results are written as simple ASCII tables. The second is a display program that graphically presents the results.

Solution of the Lossy Nonlinear Tricomi Equation Applied to Sonic Boom Focusing

Mathematical modeling of sonic boom focusing theory has been augmented with new terms representing the influence of atmospheric loss mechanisms. The new terms account for atmospheric absorption and dispersion in the vicinity of the caustic. An additional term has been derived to include the effect of wind in the direction tangent to the caustic. A code was developed to numerically implement the newly derived lossy nonlinear Tricomi equation. A numerical validation verified the calculation of diffraction effects due to linear focusing at a fold caustic. A numerical check was also performed to verify the calculation of the nonlinear, absorption, and dispersion coupling using an analytical solution of lossy nonlinear propagation through a homogeneous medium. The numerical results showed good agreement with the analytical solutions. Comparisons were also made between predictions from the lossy nonlinear Tricomi equation code and focus-boom measurements from the Superboom Caustic Analysis and Measurement Progr...

Ground Measurements of a Shaped Sonic Boom

Minimization of sonic boom shock waves by shaping has long been recognized as the primary technology for reducing sonic boom loudness, but had never been demonstrated in actual atmospheric flight. The Shaped Sonic Boom Demonstrator is an F-5E aircraft whose forward fuselage has been modified according to shaping theory to generate a flat-top boom while in steady flight at Mach 1.4 and an altitude of 30,000 feet above ground level. Three demonstration flights were conducted in August 2003, under rather adverse (hot) weather conditions. These were followed by a larger series of flights in January 2004, under good (normal to cool) conditions. The persistence of shaping through the atmosphere was clearly demonstrated. This paper presents representative ground measurement results.

Flight Demonstration Of Low Overpressure N-Wave Sonic Booms And Evanescent Waves

The recent flight demonstration of shaped sonic booms shows the potential for quiet overland supersonic flight, which could revolutionize air transport. To successfully design quiet supersonic aircraft, the upper limit of an acceptable noise level must be determined through quantitative recording and subjective human response measurements. Past efforts have concentrated on the use of sonic boom simulators to assess human response, but simulators often cannot reproduce a realistic sonic boom sound. Until now, molecular relaxation effects on low overpressure rise time had never been compared with flight data. Supersonic flight slower than the cutoff Mach number, which generates evanescent waves, also prevents loud sonic booms from impacting the ground. The loudness of these evanescent waves can be computed, but flight measurement validation is needed. A novel flight demonstration technique that generates low overpressure N‐waves using conventional military aircraft is outlined, in addition to initial quanti...

Measured Effects of Turbulence on the Loudness and Waveforms of Conventional and Shaped Minimized Sonic Booms

Turbulence has two distinctive effects on sonic booms: there is distortion in the form of random perturbations that appear behind the shock waves, and shock rise times are increased randomly. A first scattering theory by S.C. Crow in the late 1960s quantified the random distortions, and Crows theory was shown to agree with available flight test data. A variety of theories for the shock thickness have been presented, all supporting the role of turbulence in increasing rise time above that of a basic molecular-relaxation structure. The net effect of these phenomena on the loudness of shaped minimized booms is of significant interest. Initial analysis suggests that there would be no change to average loudness, but this had not been experimentally investigated. The January 2004 flight test of the Shaped Sonic Boom Demonstrator (SSBD), together with a reference unmodified F-5E, included a 12500- foot linear ground sensor array with 28 digitally recorded sensor sites. This data set provides an opportunity to re-test Crows theory for the post-shock perturbations, and to examine the net effect of turbulence on the loudness of shaped sonic booms.

SCAMP: Rapid Focused Sonic Boom Waypoint Flight Planning Methods, Execution, and Results

Successful execution of the flight phase of the Superboom Caustic Analysis and Measurement Project (SCAMP) required accurate placement of focused sonic booms on an array of prepositioned ground sensors. While the array was spread over a 10,000-ft-long area, this is a relatively small region when considering the speed of a supersonic aircraft and sonic boom ray path variability due to shifting atmospheric conditions and aircraft trajectories. Another requirement of the project was to determine the proper position for a microphone-equipped motorized glider to intercept the sonic boom caustic, adding critical timing to the constraints. Variability in several inputs to these calculations caused some shifts of the focus away from the optimal location. Reports of the sonic booms heard by persons positioned amongst the array were used to shift the focus closer to the optimal location for subsequent passes. This paper describes the methods and computations used to place the focused sonic boom on the SCAMP array and gives recommendations for their accurate placement by future quiet supersonic aircraft. For the SCAMP flights, 67% of the foci were placed on the ground array with measured positions within a few thousand feet of computed positions. Among those foci with large caustic elevation angles, 96% of foci were placed on the array, and measured positions were within a few hundred feet of computed positions. The motorized glider captured sonic booms on 59% of the passes when the instrumentation was operating properly.

SCAMP: Superboom Caustic Analysis and Measurement Project Overview

The objectives of the Superboom Caustic Analysis and Measurement Project (SCAMP) are to validate, via flight test measurements, models for sonic boom signatures in and around focal zones, and to apply these models to predict focus booms for low-boom aircraft designs. Focus due to acceleration to supersonic speeds is an unavoidable maneuver, and it must be quantified in order to assess the acceptability of overland supersonic flight. This paper provides an overview of the SCAMP project, including activities related to experimental design, planning, execution and analysis as well as the efforts in developing and improving the focused boom computational models and their application to shaped low boom aircraft operations. The flight test gathered an extensive empirical database containing flight parameters from 13 dedicated SCAMP F-18B flights which created 70 sonic boom events captured on a 10,000 ft long, 125 ft spacing, 81-microphone, high-fidelity ground-based acoustic array. Additional instrumentation included airborne microphones on a sailplane and blimp, surface and upper air atmospheric sensors, seismometers and cameras. Computational Fluid Dynamics simulations were generated for as-flown F-18B maneuvers in order to quantify the aircraft pressure flowfield and then utilized to assess, improve and validate three different focus boom models. These validated models were then applied to shaped sonic boom configurations in order to gain a better understanding of the propagation of shaped signatures through focal zones. The SCAMP team, led by Wyle and NASA, includes partners from the Boeing Company, Pennsylvania State

Acoustic Beamforming: Mapping Sources of Truck Noise

This report documents the use of the acoustic beamforming technique to pinpoint and measure noise levels from heavy truck traffic. The system uses an elliptical array of more than 70 microphones and data acquisition software to measure noise levels from a variety of noise sources on large trucks—including the engine, tires, mufflers, and exhaust pipes. The results validate the feasibility of beamforming technology, offer new insight into the distribution of truck noise sources, and provide valuable input to the design and testing of quieter pavements and noise barrier systems. This report will be of interest to anyone concerned with understanding and mitigating highway noise levels.

Prediction and measurement of a weak sonic boom from an entry vehicle

There is a current interest in measuring low‐amplitude sonic booms in the atmosphere. This interest is related to verifying the predicted loudness and structure of the shock waves. The reentry of the Stardust comet sample probe on 15 January 2006 provided an opportunity for such a measurement. PCBoom4 was used, together with Stardust’s projected reentry trajectory, to predict the sonic boom footprint. A theory for the sonic boom of a drag‐dominated blunt hypersonic vehicle had been developed by Tiegermann [Ph.D. thesis, Cornell University, 1975]. This theory provides an effective F‐function based on energy dissipation, using a blast wave analogy and numeric solution. The inputs to that model are the vehicle drag and flight conditions. Tiegermann’s model was implemented in PCBoom4. Drag was obtained by computing deceleration from the expected entry trajectory and multiplying by vehicle weight. A boom with peak overpressure of 0.0524 psf, arriving at 10:00:28.465 UTC, was predicted for a planned measurement...