Application of Pressure Sensitive Paint: Determination of unsteady surface pressures
Application of Pressure Sensitive Paint: Determination of unsteady surface pressures
Fig. 1: Steady and unsteady PSP surface
Steady and unsteady PSP surface pressure coeffficent distribution on the surface on the suction side of the wing for Ma = 0.8 and Re = 3.5 million for αNOMINAL = 17° and roll angle Φ = -40° of the delta-wing model
SUnsteady and steady surface pressure distributions of the suction side of the wing at cross section x/c = 0.8 for Ma = 0.8 and Re = 3.5 million for αNOMINAL = 17° and roll angle Φ = -40° of the delta-wing model
ig. 3: Cp pressure distribution on the surface of the suction side of the NLR7301 model
Cp pressure distribution on the surface of the suction side of the NLR7301 model for Ma = 0.72, pitch oscillating frequency 30 Hz, for three different AoA
Becoming interested by the pressure data measured with PSP for steady cases the aerodynamicists working in the high-speed regime have been asking for a similar technique for investigations of unsteady flow phenomena, such as aeroelastic investigations, turbo machinery, and helicopter rotors. This is the reason why DLR started an internal "Unsteady PSP" project to develop the required hard- and software for industrial unsteady PSP measurements with a time resolution of better than 1 millisecond and with a sampling rate of up to 100 Hz. Application of the PSP technique to unsteady aerodynamic flows requires special measurement instrumentation. The main challenges to obtain PSP results with sufficient accuracy are the development of hardware components, model illumination, and camera observation as well as the pressure-sensitive paint itself. First of all, a dedicated fast paint has to be designed as pressure sensor in order to reduce the response time of the paint with respect to pressure changes. DLR has developed high porosity paints in which the luminescent dyes are incorporated in a porous polymer. The dynamic calibration of the unsteady paint consists of measuring the phase- and amplitude response of the paint under pressure variations. A dynamic calibration system for measuring the transfer function of the paint has been designed and manufactured. Then, the instrumentation has to be able to acquire images with high framing rates and at low light intensities. Moreover, the software should be able to manage all the necessary synchronization between the PSP system and the wind tunnel in a short time.
The transient PSP system was used for the first time in measurements in the DNW-TWG transonic wind tunnel in Göttingen. Initially for investigation on a 65° delta wing model rolling around its own longitudinal axis at 10 Hz. These measurements were carried out at a nominal angle of attack α = 17° and an inflow Mach number of Ma = 0.8. As this is a periodic process, it was possible to work with phase-locked image acquisition. A typical PSP result of this measurement and a pressure curve at 80% wing depth are shown in Figs. 1 and 2.
In a second measurement campaign in the DNW-TWG, measurements were carried out to investigate 30 Hz rigid-body torsional vibrations on the NLR7301 profile model. These measurements were carried out at an angle of attack α = 1.12° ± 0.6° and an inflow Mach number of Ma = 0.72. Three typical results of this transient PSP measurement are shown in Fig. 3. In this figure, the flow comes from the left for all the results shown and the pressure coefficient distribution is shown from the center of the model to the wind tunnel wall. Regardless of the angle of attack, the three-dimensionality of the pressure distribution and two separate shock fronts are clearly visible.
With the newly developed unsteady pressure-sensitive paint formulation the dynamic behaviour of the complete surface flow becomes visible. By using this newly developed unsteady PSP technique the local pressure can be measured in real time. For industrial wind tunnel applications this work extends PSP's useful range to dynamic systems where oscillating pressure changes of the order of 1000 Pa have to be measured at rates of up to 100 Hz.
