Laminar buffet and flow control

Abstract

An experimental investigation of the transonic flow past the laminar OALT25 airfoil has been conducted to analyze the impact of laminar flow upon the shock wave dynamics and the existence of a laminar buffet like phenomenon. Tests have been carried out at freestream Mach numbers varying in the range of 0.7–0.8, angle of attack from 0.5° to 4°, and with two tripping configurations at the upper surface of the wing. The (airfoil) chord based Reynolds number is about three million. Results obtained from pressure taps and sensors measurements, as well as Schlieren visualizations of the flow reveal the presence of a laminar buffet phenomenon in sharp contrast with the turbulent phenomenon, as it features a freestream- and chord-based normalized frequency of about unity while turbulent buffet occurs for a frequency close to 0.07 (Jacquin et al., AIAA J 2009; 47). A low-frequency mode, at a frequency of about 0.05 is also present in the laminar situation, notably lower than the high-frequency component. The latter exhibits strong oscillations of the shock foot and vertical wavelike deformations of the shock wave and the former moves the shock back and forth over a small portion of chord, quite similar to the turbulent phenomenon. The mean flow past the laminar wing is characterized by a laminar separation bubble under the shock foot, which likely contributes much to the novel dynamics revealed by the present experiments. Two control strategies of the unsteady shock wave are implemented, one consisting of three-dimensional bumps and one consisting of steady jets blowing transversely to the freestream. It is found that bumps provide a significant reduction of the buffet intensity in the laminar situation. The jets are able to completely remove the flow unsteadiness in both laminar and turbulent conditions.

Wide domain simulations of flow over an unswept laminar wing section undergoing transonic buffet

“…Analysing ONERA's OALT25 profile using large-eddy simulations, [29] reported a sharp spectral peak at significantly higher frequencies (St = 1.2 instead of St = 0.06) and more confined shock motion compared to a case with tripped boundary layers. Experimental investigations on the same airfoil also reported this sharp peak at St = 1.2, but with a less dominant broadband phenomenon at St ≈ 0.05 [30]. Analyses of Dassault Aviation's laminar-flow V2C profile showed a quite different trend, as Strouhal numbers were reported in the range of St = 0.12 − 0.16 for laminar as well as for tripped turbulent cases, albeit different from typical Strouhal numbers mentioned above.…”

“…This airfoil geometry was the subject of experimental [31,32] as well as numerical studies [33][34][35][36] and is also considered for our investigations. It is likely that, over some parameter ranges and airfoil geometries, different types of buffet exist (or can even coexist) ( [16,30,37]) and it is still unclear how they relate to each other and to turbulent cases [38].…”

Unsteady PSP Measurements of the Shock Dynamics on a Transonic Laminar Airfoil

“…These were obtained with the model painted with black paint to improve infrared thermography (used in this previous work to check the transition of the boundary layer). However in a more recent work [12] measurements of the same laminar buffet dynamics was achieved with the same model without paint. In this case the frequency shifts to about 880Hz that is = 0.85, as shown in Figure 3.…”

Uncertainty Quantification Applied to the Propagation of a Transonic Wind Tunnel Inflow Inhomogeneities

“…The flow turns turbulent whenever one of these two criteria activates. The AHD criterion is determined by the N factor of the freestream which is set to 5.5 [29].…”