Experimental Study of Instability Amplitude and Pressure Distribution Effects on Critical Step Heights in Crossflow-Dominated Transition

Kurzfassung

Accurate tolerance criteria for two-dimensional surface irregularities such as steps and gaps are important for future laminar wing designs. Research into step height tolerances is more advanced for two-dimensional than for three-dimensional boundary layers. At DLR, the SPECTRA (Swept flat PlatE Crossflow TRAnsition) configuration is used to study crossflow-dominated laminar-turbulent transition. Recently, the configuration has been refined concerning its experimental capabilities. A new motorized hot-wire positioning system was developed and a global transition-detection system based on temperature-sensitive paint (TSP) was integrated. The configuration can now be used for investigations in the DNW-NWB wind tunnel in addition to the DLR-1MG wind tunnel. In Fig. 1, a cross-section of the SPECTRA configuration is shown with an exaggerated forward-facing step at the chordwise location xc = 150 mm. At that location, the flat plate is split into two segments. The nose segment can be shifted vertically relative to the other segment to produce forward- or backward-facing steps. The newest addition to SPECTRA is a system for motorized adjustment of this vertical shift and therefore the step height, inspired by the system used by Duncan et al.. Eppink investigated forward-facing steps and demonstrated an effect of the amplitude of stationary crossflow instabilities (CFI) on the criticality of a fixed step height. To study this effect in more detail, the CFI amplitude was varied systematically in parallel to a systematic variation of forward-facing step heights in SPECTRA experiments in the 1MG wind tunnel. The initial CFI amplitude and therefore the amplitude at the step location was varied by changing the diameter and chordwise location of discrete roughness elements (DRE) in a spanwise-equidistant array with constant spanwise spacing. The amplitude of the excited CFI was quantified with hot-wire measurements in the linear regime of their development for each roughness configuration. LST analysis was used to derive the CFI amplitude at the average chordwise excitation location. For each test case, the spanwise-averaged transition location was derived from TSP measurements. With increasing step height, the transition location eventually moves rapidly upstream towards the step location. This movement marks the "critical" step height and step Reynolds number. In Fig. 2, the critical step Reynolds number is shown to decrease approximately linearly with the initial CFI amplitude a0. For a given setup, the latter determines the CFI amplitude at the step location, which is most likely crucial for the onset of step criticality. In addition, it is not yet fully understood how the properties of the pressure distribution affect the critical step height in crossflow-dominated laminar-turbulent transition. In SPECTRA investigations in DNW-NWB, the criticality of forward-facing steps was investigated under the influence of six different pressure distributions, created by a variation of the angle of attack of the displacement body. The experimental results (shown in Fig. 3) have been interpreted together with results from a stability analysis of the respective base flows without a step. In the presentation and the full paper, a connection between the results from the variation of the pressure distribution and the isolated variation of the CFI amplitude will be demonstrated.