Transition due to isolated roughness in a swept wing boundary layer

Abstract

The present work is dedicated to the investigation of the effect of an isolated roughness element on a swept wing boundary layer. In particular, the flow modifications incurred by a single cylindrical element applied on a swept wing model are measured, towards describing the nature of the perturbations introduced in the flow field, their development in the near and far wake region, as well as their eventual breakdown. The measurements are performed using infrared thermography, to achieve a general overview of the element wake origin and spatial spreading. Local quantitative characterization of the stationary and unsteady disturbances evolving in the flow are instead acquired through hot wire anemometry. When present in an undisturbed laminar boundary layer, isolated roughness elements are found to introduce flow disturbances which lead to the formation of a turbulent wedge. As it develops downstream, the wedge undergoes rapid spanwise expansion, affecting the adjacent laminar flow regions. The wedge origin and development is mostly associated with the instabilities introduced by the shedding process initiated in the roughness element wake, comparably to the dominant flow features characterizing the transition of two-dimensional boundary layers conditioned by an isolated roughness element. Nonetheless, the presence of the crossflow velocity component in the boundary layer baseflow notably affects the overall flow development, introducing an asymmetric evolution of the main flow features.

The Near-Wake of Super-Critical Discrete Roughness Elements on Swept Wings

“…The 𝑢 ′ spatial organization and evolution, shows that the initiated high-fluctuation region develops along a 6 • inboard tilted trajectory. This is in agreement with previous investigations [12,37], and can be related to the suppression of the flow structures counter-rotating with the baseflow crossflow velocity component.…”

“…As growing further downstream, the high-speed wake appears to expand along the spanwise and wall-normal direction, as also visible in figure 3(c, f). This behaviour strongly resembles the flow topology and evolution observed for isolated DRE of super-critical amplitude both in 2D and 3D boundary layers [12,15,36,37].…”

“…In fact, both in the near-wake and in the far-wake region the amplitude peak is reached at a wall-normal distance comparable with the element height. This y-location corresponds to the maximum fluctuations locii typically identified in the wake of isolated DREs [15,37,39]. Such correspondence confirms that the disturbances introduced in the element's wake relate to the enhanced shears and unsteadiness pertaining to the recirculation region developing aft of the element, rather than being related to modal instability mechanisms (as further discussed in §IV).…”

“…More specifically, the near-wake is characterized by the development of a high-fluctuation locii in between the low-speed region and the high-speed streaks ( [15,36]). This correspondence suggests that the disturbances 0.155 0.16 0.165 0.17 initiated in the near-wake strongly correlate to the wall-normal and spanwise shear layers developing in the recirculating flow region [13,37]. The formation and initial growth of the high-speed wake, is accompanied by an increase of the corresponding fluctuations level.…”

“…at x/c≃0.158). This process drives the increase of the overall fluctuations level, which is indicative of the growth of the unsteady wake disturbances [16,37]. The time-averaged nature of the 3D-PTV measurements presented up to this point, does not allow for more detailed investigation of the role of the unsteady disturbances in the transitional process.…”