Retroreflective Temperature- and Pressure-Sensitive Paints
Materials and Coatings
Retroreflective Temperature- and Pressure-Sensitive Paints (LAR-TOPS-402)
Simultaneous measurements in a single system, streamlining aerodynamic testing.
Overview
Pressure-sensitive paints (PSP) and temperature-sensitive paints (TSP) are widely used for surface measurement and flow visualization, yet their application prevents simultaneous schlieren-based visualization as the model on which they're applied obstructs schlieren line-of-sight and has separate optical access and alignment requirements. By making PSP/TSP surfaces exhibit retroreflective properties, methods such as self-aligned focusing schieren (SAFS), background-oriented schlieren (BOS), and shadowgraph could leverage the surface to provide a simultaneous off-body flow visualization capability. However, achieving desired accuracy, optical clarity, and retroreflective properties within conventional PSP systems depends on coating composition, surface preparation, and optical properties. This restricts flexibility in obtaining simultaneous on-body surface measurements and off-body flow visualization.
To address these limitations, researchers at NASA Langley Research Center developed Retroreflective-Enhanced PSP/TSPs for Simultaneous Schlieren-Based Measurements. This solution enables simultaneous acquisition of surface measurements and schlieren imagery from a single optical access point across wider aerodynamic testing scenarios. The retroreflective-enhanced PSP/TSP enhances combined surface and flow field measurements with improved optical efficiency, making it a versatile tool for comprehensive aerodynamic testing and flow visualization.
The Technology
The retroreflective-enhanced system combines PSP/TSP with specially treated glass microspheres to enable simultaneous surface and flow field measurements. The process involves a multi-layer coating system including primer, epoxy base coat, and acrylic polymer/ceramic binder, with microspheres applied while the binder retains adhesive properties. The glass microspheres may be uncoated, half-coated with aluminum, or pre-processed to be coated in another chemical. The system leverages dual optical characteristics: the underlying PSP/TSP responds to pressure and temperature changes through luminescence intensity variations at specific wavelengths, while embedded microspheres provide retroreflective properties enabling focused SAFS, shadowgraph, or BOS visualization techniques. This configuration allows simultaneous capture of on-body surface measurements and off-body flow field disturbances. The invention enables measurements from a single viewing orientation rather than requiring orthogonal optical access points. While specific excitation lighting, wavelength filtering, and camera positioning are still necessary, the system significantly streamlines experimental setup compared to traditional separate approaches.
While initially developed for aerodynamic testing and flow visualization research, this invention supports optical measurement and surface analysis applications. By enabling simultaneous measurements from a single optical access point, the retroreflective-enhanced PSP/TSP offers a streamlined solution for systems where optical access limitations are critical. The system is a TRL 6, having undergone successful validation in wind tunnel testing, and is available for patent licensing.
Benefits
- Simplified Optical Setup: Reduces requirements for multiple optical access points by enabling simultaneous surface and flow field measurements from a single position.
- Surface Compatibility: Microspheres can be applied to existing PSP/TSP coated surfaces without requiring fundamental changes to surface preparation methods.
- Enhanced Versatility: Provides both on-body surface pressure/temperature data and off-body flow visualization (schlieren-based measurements).
- Coating Uniformity: Powder gun application methods deliver consistent microsphere distribution, resulting in better signal contrast and measurement quality.
Applications
- Aerospace Testing: Comprehensive aerodynamic analysis of aircraft components, spacecraft surfaces, and propulsion systems.
- Automotive: Vehicle aerodynamics development and optimization.
- Turbomachinery Research: Analyzing blade surface conditions and flow patterns in gas turbines, compressors, and wind turbine systems.
- Civil Engineering: Bridge and building aerodynamics studies and wind loading and structural response assessments.
Technology Details
Materials and Coatings
LAR-TOPS-402
LAR-20558-1
LAR-19882-1
LAR-19947-1
LAR-20041-1
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Similar Results
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).
Assembly for Simplified Hi-Res Flow Visualization
NASAs single grid, self-aligned focusing schlieren optical assembly is attached to a commercial-off-the-shelf camera. It directs light from the light source through a condenser lens and linear polarizer towards a polarizing beam-splitter where the linear, vertically-polarized component of light is reflected onto the optical axis of the instrument. The light passes through a Ronchi ruling grid, a polarizing prism, and a quarter-wave plate prior to projection from the assembly as right-circularly polarized light. The grid-patterned light (having passed through the Ronchi grid) is directed past the density object onto a retroreflective background that serves as the source grid. Upon reflection off the retroreflective background, the polarization state of light is mirrored. It passes the density object a second time and is then reimaged by the system. Upon encountering the polarizing prism the second time, the light is refracted resulting in a slight offset. This refracted light passes through the Ronchi ruling grid, now serving as the cutoff grid, for a second time before being imaged by the camera.
Both small- and large-scale experimental set ups have been evaluated and shown to be capable of fields-of-view of 10 and 300 millimeters respectively. Observed depths of field were found to be comparable to existing systems. Light sources, polarizing prisms, retroreflective materials and lenses can be customized to suit a particular experiment. For example, with a high speed camera and laser light source, the system has collected flow images at a rate of 1MHz.
Digital Projection Focusing Schlieren System
NASAs digital projection focusing schlieren system is attached to a commercial-off-the-shelf camera. For focusing schlieren measurements, it directs light from the light source through a condenser lens and linear polarizer towards a beam-splitter where linear, vertically-polarized component of light is reflected onto the optical axis of the instrument. The light passes through the patterned LCD element, a polarizing prism, and a quarter-wave plate prior to projection from the assembly as left- or right-circularly polarized light. The grid-patterned light (having passed through the LCD element) is directed past the density object onto a retroreflective background (RBG) that serves as the source grid. Upon reflection off the RBG, the polarization state of light is mirrored. It passes the density object a second time and is then reimaged by the system. Upon encountering the polarizing prism the second time, the light is slightly offset. This refracted light passes through the LCD element, now serving as the cutoff grid, for a second time before being imaged by the camera.
The LCD element can be programmed to display a variety of grid patterns to enable sensitivity to different density gradients. the color properties of the LCD can be leveraged in combination with multiple colored light sources to enable simultaneous multi-color, multi-technique data collection.
Reflection-Reducing Imaging System for Machine Vision Applications
NASAs imaging system is comprised of a small CMOS camera fitted with a C-mount lens affixed to a 3D-printed mount. Light from the high-intensity LED is passed through a lens that both diffuses and collimates the LED output, and this light is coupled onto the cameras optical axis using a 50:50 beam-splitting prism.
Use of the collimating/diffusing lens to condition the LED output provides for an illumination source that is of similar diameter to the cameras imaging lens. This is the feature that reduces or eliminates shadows that would otherwise be projected onto the subject plane as a result of refractive index variations in the imaged volume. By coupling the light from the LED unit onto the cameras optical axis, reflections from windows which are often present in wind tunnel facilities to allow for direct views of a test section can be minimized or eliminated when the camera is placed at a small angle of incidence relative to the windows surface. This effect is demonstrated in the image on the bottom left of the page.
Eight imaging systems were fabricated and used for capturing background oriented schlieren (BOS) measurements of flow from a heat gun in the 11-by-11-foot test section of the NASA Ames Unitary Plan Wind Tunnel (see test setup on right). Two additional camera systems (not pictured) captured photogrammetry measurements.
Calculation of Unsteady Aerodynamic Loads Using Fast-Response Pressure-Sensitive Paint (PSP)
Traditionally, unsteady pressure transducers have been the instrumentation of choice for investigating unsteady flow phenomena which can be time-consuming and expensive. The ability to measure and compute these flows has been a long-term challenge for aerospace vehicle designers and manufacturers. Results using only the pressure transducers are prone to inaccuracies, providing overly conservative load predictions in some cases and underestimating load predictions in other areas depending on the flow characteristics. NASA Ames has developed a new state-of-the-art method for measuring fluctuating aerodynamic-induced pressures on wind tunnel models using unsteady Pressure Sensitive Paint (uPSP). The technology couples recent advances in high-speed cameras, high-powered energy sources, and fast response pressure-sensitive paint. The unsteady pressure-sensitive paint (uPSP) technique has emerged as a powerful tool to measure flow, enabling time-resolved measurements of unsteady pressure fluctuations within a dense grid of spatial points on a wind tunnel model. The invention includes details surrounding uPSP processing. This technique enables time-resolved measurements of unsteady pressure fluctuations within a dense grid of spatial points representing the wind tunnel model. Since uPSP is applied by a spray gun, it is continuously distributed. With this approach, if the model geometry can be painted, viewed from a camera, and excited by a lamp source, uPSP data can be collected. Unsteady PSP (uPSP) has the ability to determine more accurate integrated unsteady loads.
