A joint research project was initiated for the development of economic and safe concepts for future space missions, which is entitled:
Technological foundations for the design of thermally and mechanically highly loaded components of future space transportation systems. The project is funded by the
Deutsche Forschungsgemeinschaft (DFG). Within this project, the
Institute of Fluid Mechanics and Aerodynamics of the
Bundeswehr University in
Munich (UniBwM) is fundamentally interested in the characterization of the dynamic loads caused by the fluid-mechanical interaction between the flow and the structure of the nozzle shortly after take-off
\(({\rm M}_{\infty} = 0.3)\) and at transonic conditions
\(({\rm M}_{\infty} = 0.7)\). During these conditions, large-scale vortex structures detach from the base of the spacecraft and interact with its main exhaust nozzle. This leads to high mechanical loads and, moreover, raises critical safety aspects (Gülhan
2008).
Detailed flow investigations based on PIV measurements are outlined in Bitter et al. (
2011) and Scharnowski and Kähler (
2011), which indicate the topology and dynamics of the wake. These results are in good agreement with the numerical investigations by Statnikov et al. (
2012). However, the numerical prediction of a mode-like structure on the base of the configuration could not be validated experimentally so far. This is one scope of this article. In addition, the results are analyzed to answer the following scientific questions: (1) How is the dynamic behavior of the coherent wake structures characterized? (2) Are the dynamic phenomena dominated by a certain frequency? (3) Does the boundary layer/wake interaction result in a coherent mode pattern on the base? (4) Is this mode pattern somehow time-dependent? The identification of convecting structures, especially the stream-wise convection behind a 2d or axisymmetric backward-facing step, was extensively studied in the literature for a large variety of flow properties, see Eaton (
1980), Lee and Sung (
2001,
2002), Spazzini et al. (
2001) and Mabey (
1972). To answer the questions mentioned above, the time-resolved pressure-sensitive paint measurement technique is required. The physical working principle of the pressure-sensitive paint (PSP) measurement technique is based on the detection of fluorescence intensities. Oxygen-reactant molecules (e.g. porphyrin or pyrene) are excited to an unstable higher energy state by short-wave radiation (near UV). The molecules can emit long-wave radiation during their recurrence into the stable ground state. This radiation can be registered with a photoelectric cell. The intensity of the delivered radiation is dependent on the oxygen concentration in the closer surroundings of the excited molecule. The excited molecule will likely collide with an oxygen molecule and lose its energy due to this interaction, if the
O
2-concentration is high. This incident is known as oxygen-quenching. Quenched molecules relapse back into the ground state without radiation. The
O
2-concentration is directly proportional to the oxygen partial pressure and, therefore, to the static pressure of the fluid, according to
Henry’s law. The behavior of a luminophor quenched by oxygen molecules can be described by the
Stern-Volmer relation (
1):
$$ I_{0}/I = 1+c_{{\rm q}}(T)\cdot p_{{\rm O}_2} $$
(1)
where
I
0 is the luminescence intensity at vacuum conditions,
I the detected luminescence intensity,
c
q(
T) the temperature-dependent quenching constant and
p
O
2 the oxygen partial pressure. The
Stern-Volmer relation is slightly modified for its use in PSP applications to relation (
2):
$$ p/p_{{\rm ref}} = c_1(T)+c_2(T)(I_{{\rm ref}}/I)+ \cdots +c_n(T)(I_{{\rm ref}}/I)^n $$
(2)
for the determination of unknown static surface pressures
p. Relation (
2) is commonly used up to a second-order formulation. It contains temperature-dependent calibration coefficients
c
n
(
T), which are luminophore-specific parameters and which can be determined in a static calibration chamber. Furthermore, it contains the measured luminescence intensity
I
ref and static pressure
p
ref at a reference state (known as
wind-off condition) and the measured luminescence intensity
I under flow conditions (known as
wind-on condition). Hence, the static pressure
p can be calculated by determining the paint characteristic coefficients and the intensity ratio
I
ref/
I. More thorough explanations of this measurement technology can be found in Liu and Sullivan (
2005).
The worldwide application of PSP to assess fluid-mechanical problems in wind tunnels at larger research facilities like NASA, DLR, JAXA, ONERA dates back to the mid-1990s. As reported by Airaghi (
2006), Crafton et al. (
1999), Nakakita (
2007), Woodmansee and Dutton (
1998), Yang et al. (
2012), this technique is increasingly used at university departments. The
Institute of Fluid Mechanics and Aerodynamics began its installation of a PSP-system several years ago, see Bitter et al. (
2009). An overview of recent approaches for the optical determination of unsteady surface pressure distributions with instationary pressure-sensitive paint (iPSP) can be found in Gregory et al. (
2008).
Currently, time-varying processes with characteristic frequencies up to 20 kHz can be resolved precisely with this technique, according to Gregory et al. (
2007). To achieve short-duration response times within a few microseconds, the anchoring of the PSP-molecules, directly at the surface of the pursued object, is desired to preserve the shortest possible diffusion time between the oxygen molecules and the PSP dye. An anchoring of the molecules in a porous sponge-like surface structure is desired in order to ensure that the molecules stick to the surface and do not get removed immediately by high shear forces under transonic test conditions. Two ways of creating a porous surface structure have been established for unsteady PSP measurements over the years. On the one hand, a porous surface structure is created by anodizing the model surface by an electro-chemical catalysis process. This technique was first developed by Asai et al. (
1998) and was further improved to resolve flow phenomena with large dynamic ranges, see Gregory et al. (
2007), Kameda et al. (
2005), Mérienne et al. (
2004), Sakaue et al. (
2006), Singh et al. (
2011). A second approach is the application of an emulsion to the model surface, which becomes a highly porous structure after drying, see Asai et al. (
2001), Juliano et al. (
2012), Klein et al. (
2010). In both cases, the active polymer is later anchored directly into the porous structure either by dipping or by spraying.