Airborne Background Oriented Schlieren Technique

aerospace
Airborne Background Oriented Schlieren Technique (TOP2-271)
Patent Only, No Software Available For License.
Overview
NASA has developed a novel method to render visible the density changes in air that cause a refractive index change by an airborne vehicle. These density changes include shock waves, vortices, engine exhaust, and wakes. The determination of location and strength of shockwaves and vortices is fundamental to understanding the flow around an aircraft. These features are strong enough to affect the environment that the vehicle inhabits: for example, they can cause drag and/or produce undesirable noise. The researcher must be able to predict and mitigate the effects of these flow features. This invention is a robust visualization technique that will permit the measurement of the strengths and positions of shock waves caused by supersonic vehicles. The technique is applicable to all flight regimes, however, and can be used for visualizing tip vortices, engine exhaust plumes.

The Technology
This invention is an imaging method that requires very simple optics on an airborne vehicle, a camera with an appropriate lens, and an area on the ground that provides visual texture. The complexity with this method is in the image processing and not as much with the hardware or positioning, making Background Oriented Schlieren (BOS) an attractive candidate for obtaining high spatial resolution imaging of shock waves and vortices in flight. First, images are obtained of a visually textured background pattern from an appropriate altitude. Next, a series of images are collected of a vehicle in flight below the observer vehicle and over the same spot on the ground that serves as a background pattern. Shock waves are deduced from distortions of the background pattern resulting from the change in refractive index due to density gradients. The invention requires special software to create the schlieren images. The schlieren image is a contour plot of a two-dimensional data array of measured distortions, in pixel units. The results are used by researchers to help understand the flow phenomenon and compare to computational models. The BOS method also yields measured deflection distances, which can be used to determine the strength of a given density gradient. The system design and flight planning were based on the camera characteristics, airplane coordination, and airspace limitations.
Left: Schlieren image dramatically displays the shock wave of a supersonic jet flying over the Mojave Desert. Averaging multiple frames produce a low-noise picture of the shock waves. 
Right: Horizontal gradient reveals tip vortices from the same image set.
Benefits
  • Capture more density gradients details at higher magnifications than other methods
  • Gradients can be mapped in any orientation
  • The software registers the data image to the reference image which makes the technique robust in off-nominal conditions
  • Tracks the movement of the target aircraft to permit averaging and direct velocity measurement
  • Requires very simple optics and cameras on an inexpensive air platform

Applications
  • Aerospace Industry
  • Vortices tracking around airport
  • Validate design models of future prototype and demonstrator low-boom aircraft
Technology Details

aerospace
TOP2-271
ARC-17673-2
10,169,847
Similar Results
Source is Free NASA Image library
Projected Background-Oriented Schlieren Imaging
The Projected BOS imaging system developed at the NASA Langley Research Center provides a significant advancement over other BOS flow visualization techniques. Specifically, the present BOS imaging method removes the need for a physically patterned retroreflective background within the flow of interest and is therefore insensitive to the changing conditions due to the flow. For example, in a wind tunnel used for aerodynamics testing, there are vibrations and temperature changes that can affect the entire tunnel and anything inside it. Any patterned background within the wind tunnel will be subject to these changing conditions and those effects must be accounted for in the post-processing of the BOS image. This post-processing is not necessary in the Projected BOS process here. In the Projected BOS system, a pattern is projected onto a retroreflective background across the flow of interest (Figure 1). The imaged pattern in this configuration can be made physically (a pattern on a transparent slide) or can be digitally produced on an LCD screen. In this projection scheme, a reference image can be taken at the same time as the signal image, facilitating real-time BOS imaging and the pattern to be changed or optimized during the measurements. Thus far, the Projected BOS imaging technology has been proven to work by visualizing the air flow out of a compressed air canister taken with this new system (Figure 2).
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