Experimental study of bump effects on boundary-layer transition in compressible high Reynolds number flow

Abstract

The influence of surface bumps on boundary-layer transition was experimentally investigated in the present work. The experiments were conducted in a (quasi-) two-dimensional flow at low to high subsonic Mach numbers and chord Reynolds numbers up to 10 million in the low-turbulence Cryogenic Ludwieg-Tube Göttingen. Various streamwise pressure gradients relevant for natural laminar flow surfaces were examined. Quasi-two-dimensional bumps, with a sinusoidal shape in the streamwise direction, fixed length and three different heights, were installed on a two-dimensional flat-plate model. The model was equipped with temperature-sensitive paint for non-intrusive transition detection and with pressure taps for the measurement of the surface pressure distributions. Boundary-layer transition was shown to occur at a more upstream location with increasing bump height-to-length ratio. This was mainly due to the local adverse pressure gradient on the downstream side of the bump, which was particularly pronounced in the case of the bump with the largest height-to-length ratio, thereby inducing boundary-layer separation (as verified via oil-film visualizations). In the case of the bump with the smallest height-to-length ratio, bumpinduced transition was found to be dependent on global pressure gradient, Mach number and Reynolds number; however, the influence of these parameters on transition induced by bumps with larger height-tolength ratios was significantly reduced. The sensitivity of boundary-layer transition to the effect of the bumps was shown to be more pronounced with stronger global flow acceleration and at smaller Mach numbers.

Unit Reynolds number, Mach number and pressure gradient effects on laminar–turbulent transition in two-dimensional boundary layers

“…In the leading edge region the average roughness and mean roughness depth were even further reduced to R a = 0.027 μm and R z = 0.20 μm . The step at the model part junction at x∕c = 35% was less than 0.5 μm (Costantini 2016).…”

“…Downstream of the leading edge region with x∕c > 20% , the pressure gradient is essentially uniform on a large portion of the upper surface. Only around x∕c = 35% the pressure distribution shows some slight variations from an ideally smooth one, due to a model part junction (Costantini et al 2015b(Costantini et al , 2016aCostantini 2016).…”

“…In the current study the charge temperature was kept constant at about 283 K, while the charge pressure of the wind tunnel was varied to adjust the unit Reynolds number. 2 The Mach number of the flow was varied by adjusting the cross section of the sonic throat downstream of the test section (Koch 2004), while a variation of the shape factor was achieved by varying the angle-of-attack of the model (Costantini 2016;Risius et al 2018). Limitations to the test envelope were due to: (a) the maximal charge pressure of DNW-KRG, which limited the maximal unit Reynolds number, (b) boundary-layer separation downstream of a pressure minimum in the leading-edge area at high angles-of-attack, (c) blockage effects of the model inside the test section at large negative angles-of-attack, which limited the range of examinable pressure gradients (shape factors), and (d) the chord length of the model, which limited the detectable transition location.…”

“…The wind tunnel model used for the present study was the two-dimensional PaLASTra model (Costantini et al 2015b(Costantini et al , 2016aCostantini 2016). On the lower surface of the original PaLASTra model an abrupt contour change was imposed at x∕c = 80% ( Fig.…”

“…Furthermore, a new camera setup was developed; it was used during the later entries of the PaLASTra model and increased the spatial resolution, temporal resolution and contrast of the result images even further. The transition detection in the current study was conducted by the maximum gradient technique, which has been described by Costantini (2016) and Costantini et al (2016a). However, it was not only conducted at ten spanwise locations, as in earlier studies (Costantini et al 2015b(Costantini et al , 2016aCostantini 2016), but in this work it was extended to almost the complete span.…”