Pressure Gradient and Nonadiabatic Surface Effects on Boundary Layer Transition

Abstract

The influence of the streamwise pressure gradient and a nonadiabatic surface on boundary layer transition was experimentally investigated at the Cryogenic Ludwieg-Tube Göttingen, Germany. Boundary layer transition was detected nonintrusively by means of the temperature-sensitive paint technique. The wind-tunnel model was designed to achieve a quasi-uniform streamwise pressure gradient over a large portion of the model chord length. This allowed the effects on boundary layer transition of the streamwise pressure gradient and wall temperature ratio to be decoupled. The model was tested at high Reynolds numbers and at a high subsonic Mach number. Favorable, almostzero, and adverse streamwise pressure gradients were considered; and various temperature differences between the flow and the model surface were implemented. Stronger flow acceleration and lower wall temperature ratios led to an increase of the transition Reynolds number. Larger increases in the transition Reynolds number were obtained at more pronounced flow acceleration for the same reduction in the wall temperature ratio. The measured pressure distributions served as input for boundary layer stability computations, performed according to compressible, linear, local stability theory under the (quasi-) parallel-flow assumption. Amplification factors of the Tollmien-Schlichting waves were shown to be reduced by stronger favorable pressure gradients and lower wall temperature ratios, which was in agreement with the observed variation in the transition Reynolds number.

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

“…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).…”

“…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.…”

“…On the lower surface of the original PaLASTra model an abrupt contour change was imposed at x∕c = 80% ( Fig. 1) to fix boundary layer separation (Costantini et al 2016a). However, the large separation region, originating downstream of the original PaLASTra model, leads to strong pressure fluctuations (i.e., acoustic noise) inside the test section; these are likely to increase the initial amplitude of the disturbances within the laminar boundary layer (through the receptivity process) and thus reduce transition Reynolds numbers (Costantini et al 2016a).…”

Boundary-Layer Detection at Cryogenic Conditions Using Temperature Sensitive Paint Coupled with a Carbon Nanotube Heating Layer

“…Recent studies by Constantini et al [29,30] have shown that care must be taken when interpreting transition results obtained using TSP and applying a temperature step. Their work has shown that when the surface temperature (T w ) is greater than the adiabatic–wall temperature (T aw ), then the transition location can vary depending on the T w /T aw ratio.…”

The New Mode of Instability in Viscous High-Speed Boundary Layer Flows

“…Compared with zero pressure gradient, favorable pressure gradient (hereafter referred to as FPG) significantly stabilizes the boundary layer in both incompressible and compressible flows (the first mode as well as Mack's second mode). This is supported by a number of studies, e.g., with direct numerical simulation [5,10,18], linear stability theory [4,6,7] and very recent experiments [23,25]. Hence, in the review by Reed et al [9], the instability of boundary layer with FPG is described as "very weak, if it exists at all".…”