Major sources of error in PSP data are due to illumination variations and temperature changes during the data collection period while the wind tunnel is running. Blow-down type wind tunnels will change temperature during the course of a run while closed-circuit or continuous wind tunnels are more stable with temperature. Errors in pressure measurements taken at low wind speeds are largely the result of temperature gradients on the model surface. These temperature gradients can be the result of model construction, tunnel operation, or fluid dynamics. A rapid prototype model, for example, is constructed using an internal metal structure and a polymer resin. The thermal signature of the internal structure is apparent when the surface of the model is subjected to a heat flux. The model is exposed to a heat flux due to changes in tunnel Mach number. This condition is most apparent during tunnel startup. Temperature errors can be minimized by using temperature controlled tunnels and constructing the model from materials with high thermal conductivity like aluminum or stainless steel. Model construction and tunnel operation are key considerations for effective lows-peed PSP measurements. Illumination variations can be caused by non-stable output from LEDs and by vibrations of the model or camera relative to one another. To the camera, these changes are interpreted as pressure changes.
A way of dealing with the temperature errors is to add a second component to the PSP. This is known as Binary PSP. What Binary PSP adds that single-component PSP lacks is the ability to correct for temperature and illumination induced errors. Binary PSP works by acquiring data from both the oxygen sensitive component and the second component, known as the reference molecule. The emission from the reference and oxygen sensitive molecules are spectrally independent. A color camera is used to separate the signals from the reference and oxygen sensitive components of the Binary PSP. From this, two images are acquired (one of the reference and one of the oxygen sensitive component). Taking the ratio of the oxygen sensitive image over the reference image eliminates dependence on temperature. This is due to the reference and oxygen sensitive molecules having the same sensitivity to temperature. By dropping the temperature dependence, an ideal PSP is created where the only dependence is on the pressure change.
Emission from Binary PSP (Left), Color Separation on a Color CCD Camera Sensor (Right)
Where temperature changes during the data acquisition are not a problem, single-component PSPs are still used. Higher resolution images can be obtained using single-component PSPs as they use a monochrome camera and don't reduce the resolution like a color camera does when it separates colors.
BINARY UNICOAT PRESSURE SENSITIVE PAINT
(Product ID: BUNC-12)
UniCoat is a single-luminophore pressure sensitive paint (PSP) packaged in an aerosol can for ease of application. UniCoat has slightly higher temperature sensitivity and slightly lower pressure sensitivity than UniFIB PSP, but is a simple shake and spray application at a lower cost than other products and is ideal for academic purposes. This single coat application paint may be applied directly to most materials. UniCoat is an effective quantitative PSP well suited for isothermal environments (large metal models and temperature controlled tunnels) or where strong pressure variations are present (transonic and supersonic flows). UniCoat is recommended for introductory PSP users who seek an inexpensive paint for the purpose of developing their PSP capabilities.
|Pressure sensitivity||0.5% per kPa|
|Pressure range||1-kPa to 200-kPa|
|Temperature sensitivity||0.9% per °C|
|Temperature range||-10°C to 60°C|
|Response time||750 ms|
|Excitation||380-nm to 520-nm (400-nm ideal)|
|Emission||500-nm to 750-nm|
|Photo-degradation rate||1% per hour (Excitation)|
Available Quantities: 12-oz aerosol spray can
Spectral Response (PSP Emission Scaled)
(Product ID: BF-XXX)
BinaryFIB™ pressure sensitive paint (PSP) is a dual-luminophore, single application PSP formulated to be applied with paint spraying equipment. The binary paint approach involves acquiring data from two distinct luminescent dyes and using these signals to compensate for errors caused by model displacement and deformation as well as temperature. One dye is pressure and temperature sensitive and the other dye is temperature sensitive only. The ratio of the signals from the two dyes allows the temperature sensitive signal to be isolated from the pressure sensitive signal. The temperature sensitivity of the paint can be minimized over a wide range of temperatures and pressures as shown in the calibration below. The paint may be applied to most materials, however a white base coat such as SCR-XXX (Screen layer) or FB-XXX (FIB basecoat) is recommended. Models constructed of materials that may be attacked by solvents such as plastic or rapid prototyping resin should be coated with a screen layer or FIB basecoat. The calibration of BinaryFIB™ is very stable, repeatable, and exhibits very little temperature sensitivity. BinaryFIB™ is recommended for advanced/professional PSP users who seek high quality data in low-speed environments or where temperature gradients are larger and have a greater impact on the signal-to noise ratio.
|Pressure sensitivity||0.6% per kPa|
|Pressure range||0-kPa to 200-kPa|
|Temperature sensitivity||0.03% per °C|
|Temperature range||0°C to 50°C|
|Response time||300 ms|
|Excitation||380-nm to 420-nm (400-nm ideal)|
|Emission||500-nm to 720-nm|
|Photo-degradation rate||1% per hour (Excitation)|
Available quantities: 200-, 400-, 750-ml