Investigation and parameterization of transition shielding in roughness-disturbed boundary layer with direct numerical simulations

Abstract

Roughness-induced transition control is a key technology for aircraft design, and associated research is useful in practical applications as well as for understanding the mechanism of the roughness-induced transition. One practical approach involves the “shielding effect,” whereby the roughness-induced transition is shielded by smaller pockets of surrounding roughness. In this paper, we investigate the shielding effect of a two-dimensional downstream strip in the boundary layer disturbed by discrete smooth-edged roughness and focus on the shielding strip height kss and the distance between the two areas of roughness xss. Our results indicate that downstream shielding delays the transition by weakening the strongest streamwise vortices in the middle of the wake, thus inhibiting the “lift-up” effect that induces growth in the disturbance. The main mechanisms for reducing the streamwise vorticity are (a) enhanced dissipation of the streamwise vorticity and (b) conversion of streamwise vorticity into more stable spanwise vorticity. The strip suppresses the separation zone behind the roughness, thus affecting the receptivity process and reducing the initial disturbance. However, the strip strengthens other streamwise vortices in the wake. When kss exceeds a critical value, vortices closer to the wall will induce stronger lift-up than those in the middle of the wake, resulting in an earlier transition. Analysis of xss shows a simpler trend, whereby the onset position of the transition moves upstream as xss increases. This is because the shielding strip weakens the streamwise vortices earlier and the separation zone becomes smaller as the strip moves closer to the discrete roughness patch.

Delaying transition induced by a strip of distributed roughness using additional fine grit roughness

“…This demonstrates that a smooth strip can also function as a secondary strip to delay boundary layer transition caused by naturally occurring roughness. It is pertinent to note here that transition delay using a smooth strip placed downstream has been demonstrated numerically by Lu et al [22] and Suryanarayanan et al [2] for boundary layer transition caused by isolated and distributed roughness respectively. This technique of using downstream roughness of limited length can have several applications in practical engineering scenarios like wind turbine blades, aircraft wings, and gas turbine blades where the secondary roughness strip can be pre-applied in scenarios where we expect roughness accumulation: for example, an aircraft/UAV about to fly in icing conditions, or a wind turbine blade exposed to dust or insects.…”

“…The reduction in streamwise vorticity, which is expected to play a key role in distributed roughness-induced transition (figure 9, see also [28]) results in transition delay. Note that it is also possible for the secondary roughness downstream to cause dissipation of streamwise vorticity generated by the primary roughness (as discussed in [22], [21], [2]) Previous numerical studies by Suryanarayanan et al [21] demonstrated delaying transition of an isolated roughness element using a distributed roughness or flat strip placed upstream. The transition delay is due to lifting of the boundary layer by the upward strip, thereby reducing the effective Re 𝑘 of the isolated roughness element.…”

“…Moreover, using numerical simulations, they showed that transition delay is due to a reduction in streamwise vorticity generated from the discrete isolated roughness element by the distributed roughness placed downstream. Lu et al [22] reported simulations on delaying transition caused by isolated roughness by placing a two-dimensional strip downstream, wherein they demonstrated that a downstream roughness strip dissipates streamwise vorticity, delaying transition. They also reported that placing the two-dimensional strip immediately downstream of the discrete roughness element causes maximum transition delay and moving the two-dimensional strip further downstream has smaller effect on transition delay.…”

Direct numerical simulation of roughness-induced transition controlled by two-dimensional wall blowing

“…2009; Lu et al. 2020 a , b ). For the spatial discretization of the convective terms, the WENO-SYMBO method (Martin et al.…”

“…Lu et al. (2020 a , b ) analysed the effect of upstream and downstream-positioned control roughness separately and argued that their roles in transition control are different. However, as a passive strategy, it is unable to adjust the control effect.…”