The PSP technique has been used for investigations of periodic and unsteady flows: first, a 65° delta wing was tested in the transonic wind tunnel DNW-TWG in Göttingen. A specially designed roll apparatus enabled roll rates up to 10 Hz. The experiments were carried out at angles-of-attack up to α = 17° at Ma = 0.8. Since the rotation of the delta wing is a periodic motion, the phase-locked unsteady PSP technique can be applied. A typical PSP result of the suction side of the investigated model, using the phase locked unsteady PSP data acquisition, for the steady and unsteady case, for Ma = 0.8 and Re = 3.5 million at an nominal angle of incidence α = 17° and roll angle Φ = -40° is shown in Fig. 1. For both cases well developed leading edge vortices can be seen in the PSP results. In the steady case vortex breakdown occurs on the port side of the wing. This can be seen in the PSP result from the strong pressure gradient. In the unsteady case the vortex breakdown on the port side of the wing is shifted downstream. From the PSP results pressure data can be plotted along different cross sections. In Fig. 2 the Cp value is plotted for the steady and unsteady case mentioned above at x/c = 80%. From this pressure data it is obvious that for the unsteady case no vortex breakdown has occurred on the port side of the wing. That means that only the phase locked unsteady PSP measurement technique reveals the complex structure of the flow field very clearly.
In a second wind tunnel campaign in the DNW-TWG in collaboration with the DLR Institute of Aeroelasticity, a 2D-wing-profile (NLR7301) model, which is pitch oscillating at up to 30 Hz, was investigated. The experiments were performed at angles-of-attack α = 1.12° ± 0.6° at Ma = 0.72. Three typical PSP results using unsteady PSP data acquisition, for the 30 Hz pitch oscillating case of the investigated model on the suction side, for Ma = 0.72 at angles of attack (AoA) 0.588, 1.828, 1.564° are shown in Figure 3. In this figure flow is coming from the left for all three different angles-of-attack and the pressure coefficient is shown in the area beginning from the centerline of the model in direction to the wind tunnel side wall. Independent of the angle of incidence the non-2-dimensional pressure distribution is clearly visible especially nearby the wind tunnel side wall, caused by the interaction of the flow around the 2D-wing-profil model and the wind tunnel boundary layer. In addition for all different AoA a low pressure area is measured which is caused by a vortex generated at the leading edge of the model close to the wind tunnel side wall. For all different AoA significant movement of the shock system is also found. For α = 0.588° and α = 1.828° two separate shock positions are found. The first shock is moving downstream into the direction of the second shock whereas the second shock is not moving downstream. The results for the different phase positions for the pitch oscillation show significant differences in the position of the shocks. For α = 1.564° only a single shock is measured by means of unsteady PSP. Thus, the response time of the pressure-sensitive paint formulation is sufficiently small to resolve the pressure fluctuations arising from the pitch oscillation of the 2D-wing-profile model in the flow.
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.