Browsing by Author "Zhao, .X."

Filter results by typing the first few letters
Now showing 1 - 2 of 2
  • No Thumbnail Available
    Item
    Delaying transition further with the aid of a short compliant panel in a Blasius boundary layer flow
    (The Chinese Society of Theoretical and Applied Mechanics (CSTAM), Fluid Mechanics Division Conference, Guilin, China., 2012-11-12) Bori Ige; Yeo, .K. S.; Dou, .H-S.; Zhao, .X.
    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.
  • No Thumbnail Available
    Item
    Delaying transition in a Blasius boundary layer with finite compliant panels
    (Fourth International Symposium on Bifurcations and Instabilities in Fluid Dynamics (BIFD),, 2011-07-18) Bori Ige; Zhao, .X.; Yeo, .K. S.
    Compliant surfaces have been shown to be a promising passive control measure for controlling and delaying boundary layer transition in various theoretical studies [1-2]. In this paper, we report on a recent study we have done on the evolution of pulse-initiated disturbance wavepackets over one or more finite-length compliant panels. The broadband nature of a wavepacket offers a central advantage in permitting natural selection of most dominant waves to operate through the sum of its growth processes. This may be helpful in identifying the critical waves and key processes that are involved at the various stages in natural transition. The initiation, evolution and final breakdown of wavepackets into the incipient turbulent spots in a Blasius boundary layer was modelled by Direct Numerical Simulation (DNS) briefly described in [3]. The comparative evolution and transition performance of three cases are discussed here, namely the rigid-wall case, a single-panel wall and a two-panel wall. In all cases, a fixed vertical-directed delta pulse of small amplitude was initiated at the point x /  349.4, where   2.3182103m is the displacement thickness of the boundary layer at the initiation point. The evolution and breakdown of the wavepacket in a Blasius boundary layer on a rigid wall has already been reported in [3]. For the single-panel case, a finite section of the wall from x /  450 to 762 was replaced by a tensioned membrane on a viscoelastic foundation, whose properties were designed to inhibit the development of compliant-wall modes. The simulation results showed that, the upstream intervention by the finite compliant panel effectively delayed the onset of the incipient turbulent spot by a distance of about 100 cm ( x /  430). This represents an approximately 30% increase in the transition distance measured from the point of wavepacket initiation. Spectral study indicated that the relatively short membrane panel was able to effectively attenuate the primary 2-D Tollmien-Schlichting (TS) wave mode so that resultant wavepacket after the panel was dominated by a pair of oblique waves. Subharmonic secondary instabilities [4-5], which are responsible for nonlinear disturbance wave amplification on a rigid wall, were thus inhibited by the absence or near absence of the 2-D TS wave mode. Staggered Λ-structures and streamwise streaky structures similar to those found in the rigid wall case were observed for the single-panel case, but much further downstream. A second tensioned membrane panel of the same length was added at x /  1359-1658 to form the two-panel case. The last stage of the present simulation shows the wavepacket arriving the location x /  2000 in a perfectly laminar form ( max | u | /U 0.05   ) – this already represents an increase in transition distance of about 50% over the corresponding rigid-wall case. The eventual breakdown location will be further downstream as the wavepacket has not displayed the usual structural features that signify imminent breakdown. This study has shown the efficacy of short compliant panel(s) in controlling and delaying transition.