Delaying transition further with the aid of a short compliant panel in a Blasius boundary layer flow

Abstract

The cost of fuelling especially for those in the transport industries could be reduced drastically if there is a means of reducing drag force over their vehicles while in motion. One way to overcome this is to use compliant (membrane) surface; a passive control means which has been proved in various theoretical studies as a promising tool in delaying transition further. In this paper, following the earlier work done on flow over rigid wall within a Blasius boundary layer, we account for the current study carried out on the evolution of pulse-initiated disturbance wavepackets over a finite-length compliant panel by direct numerical simulation (DNS) method. For the single-panel case, a finite section of the wall from X = x/δ0 = 450 -762, was replaced by a tensioned membrane on a viscoelastic foundation, whose properties were designed to inhibit the development of compliant-wall modes. Where δ0 is the displacement thickness at the perturbation location. A small amplitude vertical initiating delta pulse was introduced from the wall streamwise location X0 = 349.4 (x0 = 81cm), and study in detail both spatially and spectrally how the wavepackets generated evolve from the initiating point to the breakdown location over a Blasius boundary layer. The simulation results showed that, the upstream intervention by the finite compliant panel effectively delayed the onset of the incipient turbulent spot by a further distance of Δx = 550, when compared with the rigid wall case results that earlier broke down at X = 1420. This represents an approximately 51% increase in the transition distance measured from the point of wavepacket initiation. Spectral study indicated that the relatively short compliant panel was able to effectively weaken the primary 2-D Tollmien-Schlichting (TS) wave mode, thereby extending the linear regime, so that resultant wavepacket after the panel is dominated by two oblique wave modes and this is the effective strategy of transition delay.