Fast PSP on an Air Force 1/15th Scale Model of an Advanced Tactical Fighter (U.S. Air Force Photo)
Conventional polymer-based paint formulations have response times on the order of ~1 second, making them unsuitable for evaluating unsteady aerodynamic phenomena. The temporal-response characteristics of PSP are primarily governed by the thickness of the paint formulation and the diffusion coefficient of the binder material. The response time due to diffusion (τdiff) increases with the paint thickness (h) squared and decreases with increasing diffusion coefficient (Dm).
Some fast responding paints have focused on decreasing the thickness of the paint in order to improve the temporal response characteristics. This approach sacrifices luminescent output from the paint, and thus the signal-to-noise ratio, for a faster response time and a lower signal-to-noise ratio. Porous binders have been developed with the goal of enhancing the oxygen diffusion within the paint layer and, thus, improving the temporal response without reducing the signal-to-noise ratio.
ISSI’s Porous, Fast PSP is an example of this type of PSP structure, exhibiting good pressure sensitivity (more than 0.6%/kPa), strong signal and excellent temporal response of 20-kHz. The paint may be applied to clean, dry, materials such as Aluminum, Glass, Plastic, or Steel, and no base coat or heat treating is required. Porous, Fast PSP is recommended for applications that require a bright paint with frequency response of up to 20-kHz.
Fast PSP Tests
Fast PSP Under UV Excitation on a Space Launch Vehicle Model (Left), Processed Unsteady Pressure Fluctuations from Buffeting at 6.10 Hz (Right), (Credit: D. Hart / NASA Ames Research Center)
Fast PSP in a Subsonic Cavity Under UV Excitation (Left), Standing Pressure Waves in a Subsonic Cavity (Right), (Courtesy: US Air Force)
Fast PSP in a Turret Model Under UV Excitation (Left), Instantaneous Pressure Field on Turret (Right), (Courtesy of Stanislav Gordeyev, AME Dept, University of Notre Dame)
TURBOFIB™ PRESSURE SENSITIVE PAINT
(Product ID: TF-XXX)
TurboFIB™ pressure sensitive paint (PSP) is a single-luminophore, single application PSP formulated to be applied with paint spraying equipment. TurboFIB™ provides many of the advantages of UNIFIB PSP, but with faster response (1 kHz) and reduced temperature sensitivity. This paint formulation may be excited effectively from 380-nm to 550-nm, however, 400-nm radiation from LM2X-DM-400 LED modules is recommended. The paint may be applied to most surfaces, though it is recommended that plastics and rapid prototyping resins first be coated with SCR-XXX (Screen layer) or FB-XXX (FIB basecoat) to protect the surface from solvent damage. The calibration of TurboFIB™ (shown below) is very stable and repeatable thus TurboFIB™ is recommended for advanced/professional PSP users who seek high quality data. Paint is sold in quantities of 200-ml, 400-ml and 750-ml. Custom quantities are available upon request. Surface coverage is approximately 1 m2 per liter.
|Pressure sensitivity||0.8% per kPa|
|Pressure range||0-kPa to 200-kPa|
|Temperature sensitivity||0.4% per °C|
|Temperature range||0°C to 50°C|
|Response time||<1 ms|
|Excitation||380-nm to 550-nm (400-nm ideal)|
|Emission||620-nm to 750-nm|
|Photo-degradation rate||1% per hour (Excitation)|
Available quantities: 200-, 400-, 750-ml
Spectral Response (PSP Emission Scaled)
TEMPERATURE SENSITIVE PAINT
Boundary layer separation using TSP
Image courtesy of DLR Traditional measurement techniques for acquiring surface temperature distributions on models have utilized embedded arrays of thermocouples and RTD’s. This requires significant construction and setup time while producing data with limited spatial resolution. An alternative approach is to use temperature sensitive paint (TSP) to measure surface temperature. The advantages of temperature sensitive paint include non-intrusive measurements and high spatial resolution when compared to conventional measurement techniques. Image based temperature measurements using TSP are accomplished by coating the model surface with the paint and illuminating the surface with light of the appropriate wavelength. The luminescence from the surface is recorded using a CCD camera through a long-pass filter to separate the luminescent signal from the excitation light. The luminescence from the TSP is a function of the local temperature, and therefore, each pixel on the camera acts as a thermocouple.
A typical TSP consists of the luminescent molecule and an oxygen impermeable binder. The basis of the temperature sensitive paint method is the sensitivity of the luminescent molecules to their thermal environment. The luminescent molecule is placed in an excited state by absorption of a photon. The excited molecule deactivates through the emission of a photon. A rise in temperature of the luminescent molecule will increase the probability that the molecule will return to the ground state by a radiation-less process, this is known as thermal quenching. The temperature of the painted surface can be measured by detecting the fluorescence intensity of the luminescent paint.
TSPs are used to measure surface temperature distributions to estimate heat transfer rates over a surface and to capture boundary layer transition from laminar to turbulent.