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CFD visualization helping researchers take the boom out of supersonic flight

Finding a solution may help lift FAA restrictions on supersonic flight in U.S. and open lucrative new markets for supersonic business travel

Back in the 1970s, many corporate leaders envisioned a day when they might start their morning with a breakfast board meeting in New York, meet personally with city and state regulatory officials in Seattle that afternoon, and close their day with a business dinner for key customers in Los Angeles. What made the vision conceivable was the advent of the Concorde Supersonic Transport (SST) – a beautiful and inspiring feat of aeronautical engineering, technology and design. Many were willing to pay a premium for the mobility, convenience and speed it offered.But commercially it never came to be. The Concorde had one weak spot – with its speed and ability to break the sound barrier, it generated concussive sonic booms with the potential for damaging property and the environment as it passed over the country at supersonic velocities.

First flown in 1969, the Concorde went into commercial service in 1976. But by then, the Federal Aviation Administration (FAA) had banned supersonic flight over land by civil aircraft.1 By that time, also, various Congressional and municipal bans had been placed on Concorde landings in the U.S.

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Simulating off-body pressures to mitigate sonic booms.

Although the bans on landing were eventually lifted or relaxed and international Concorde flights were allowed to land at airports such as JFK and Washington Dulles International, the business case for building more supersonic jets was dead. The Concorde flew another 27 years over limited routes, but the expensive and aging fleet was retired in 2003. Lacking a justifiable economic ROI, no plans were made for a second generation SST.

Today, however, NASA and a variety of private aerospace companies are working to bring the vision of supersonic flight back to life using computational fluid dynamics (CFD) and visualization techniques, combined with powerful computers not imaginable in the 1970s and not even available until the last decade. These methods are helping researchers find ways of reshaping the shock wave that creates a sonic boom to minimize the noise and eliminate the risk for damage on the ground.
Their first goal is to get the FAA to lift its ban on overland supersonic flight, which could crack open a lucrative market catering to senior business executives, world leaders or heads of state, and wealthy individuals willing to pay for the prestige and the convenience of getting from one end of the country to the other in as little as two hours.

Rendering a Sonic Boom Harmless

To achieve this goal, forward-thinking aeronautic engineering teams are using CFD methods and visualization software processes to reshape the body of the jet itself. By reforming the body, they hope to restructure the pressure waves and decrease the noise to a level where the sonic boom can barely be heard from the ground.

Off-body pressures

Test case images showing the changes in off-body pressures surrounding a wing before (left) and after (right) the shape is altered. Each image displays three views of the data. Top: pressures on the surface of the wing. Middle: graph of the off-body pressure below the wing. Bottom: off-body pressures surrounding the wing.

(Images by Tecplot, Inc. Test case data provided by Optimal Solutions Software, LLC.) View larger image.

This approach is backed by organizations like MIT’s Partnership for AIR Transportation Noise & and Emissions Reduction which has publicly stated “Recent research on shaped sonic booms has indicated low boom designs are possible and result in significantly less objectionable signatures than classic booms of the 1960s to 1980s. Several recent studies have investigated designs with initial overpressures of no more than 0.3 lb/ft2, in contrast to Concorde’s typical 2 lb/ft2 N-wave signature — a dramatic reduction in noise levels. Due to this technological progress and resulting potential commercial and military application for the United States, supersonic aircraft operation and sonic boom signatures should be investigated for low boom designs.”

The Mechanics of a Sonic Boom

When a supersonic plane moves through the air, pressure waves are created in front of and behind its body, similar to the waves created around a boat as it slices the water. This is not an issue at lesser speeds, but as a plane reaches the speed of sound, the pressure waves compress to the point where they cannot avoid each other and merge into a single shock wave called a Mach cone.

The Mach cone travels with the plane, and its trail extends to the earth, where it is experienced as a sonic boom as it passes over the land. By reshaping that shock wave, researchers hope the resulting sonic boom will virtually disappear by the time it reaches the ground.

CFD Visualization, Study and Design

What researchers are trying to accomplish today would not have been possible even a few years ago. While engineers have been using CFD successfully in research for decades, the number of simulation runs and massive amounts of data required to make significant headway on this particular problem requires powerful computers and software that were not available until recently. It’s not just volume and speed, it’s the accuracy offered by today’s technology that researchers need.

Wind tunnel testing, for example, poses as many issues as it solves. In addition to being expensive and time-consuming, researchers must attach the object being tested in the tunnel to something that will bear its weight and aerodynamic forces. Under supersonic conditions, the attachment itself can alter the wave and skew the results to a significant degree.
But by using CFD methodology, researchers can efficiently simulate the off-body pressures of hypothetical configurations. They look at how pressure waves behave as they interact with a particular body shape, and then see how that behavior changes when the body’s configuration or shape changes.

Each simulation involves three basic steps using three powerful scientific analysis tools.

The researchers start with a CFD solver like Cart3D to compute raw flow field data and calculate the lift, drag, pressure drop, heat transfer, and related variables generated by the interaction between the pressure wave and the shape. The solution is then passed to a CFD visualization and analysis tool, such as Tecplot 360, which extracts the data, performs more computations, and converts the data to visuals in the form of plots, images or animations.

In one research sequence, for example, after engineers examine the plots and imagery, they can determine how and where the shape of the body can be altered to reduce the pressure waves. The analyzed data is then passed from Tecplot 360 to shape-deformation software – such as Sculptor™ from Optimal Solutions Software, LLC. Sculptor allows researchers to change the shape of the body and adjust the corresponding and complex CFD data so that it remains accurate. The process is repeated until a pre-determined “cost function” or result is satisfied, then the researchers move on to test the next hypothetical configuration.

Even with the large amounts of data, complex variables, and powerful computing requirements, it can take engineers 10 minutes or less to complete one turnaround, or complete test. This kind of rapid turnaround, in and of itself, is a breakthrough.

In addition to the speed, complex analysis capabilities, and accurate results that Cart3D, Tecplot 360, and Sculptor offer, the ability to integrate all three tools for an efficient workflow has been crucial to reducing the time spent running simulations.

The Shape of Things to Come

Beyond the business opportunities presented by the prospect of boomless supersonic flight and the possibility of a supersonic jet, there’s the excitement of imagining what such a “reshaped” jet will look like. While it’s too early to say for sure, researchers anticipate the supersonic jet may look vastly different from planes that are in existence today.

Though form will likely follow function, the present concern is persuading the FAA to lift its SST-over-land ban, which will make supersonic travel part of the business landscape once again, in a way that significantly reduces or eliminates the problems of the sonic boom.

And that will be music to everybody’s ears.

1Code of Federal Regulations (CFR) 91.817
2NASA Funds Exploratory Studies For Quieting Sonic Boom
3Project 8: Sonic Boom Mitigation