Passive Laminar Flow Control at Low Turbulence Levels

Abstract

S PANWISE arrays of discrete roughness elements (DREs) are used in wind tunnel and flight experiments to excite stationary crossflow vortices (a recent example is [1]). Because flight turbulence intensities are low, the stationary crossflow mode dominates the traveling mode in that environment and leaves surface roughness as the important initiator of stationary crossflow vortices [2]. A precursor to using arrays of DREs, the experiments by Radeztsky et al. [3] examined the role of isolated roughness elements on steady crossflow mode-packet initiation and growth. Small circular roughness elements (height k of 6 μm and diameter d less than half of the most unstable crossflow mode wavelength) applied near the neutral stability point effectively initiated crossflow vortices in a swept-wing boundary layer. As a result, the average transition location across the span x∕cjtr was advanced upstream by as much as 0.30c. Reibert et al. [4] used arrays of DREs to excite uniform series of stationary crossflow vortices. In addition to nonlinear saturation of the disturbance amplitudes, several important observations regarding theDRE arrayweremade.When the roughness spacing λkwas that of the most unstable or critical crossflow wavelength (λk λcr 12 mm in those experiments), the strong growth of disturbances modulatedwith λcr and λcr∕2was observed. Increasing the roughness spacing to λk 36 mm (3λcr) produced a crossflow disturbance spectrally composed of λk and its first eight harmonics (18–4 mm). No subharmonics were observed in the wavelength spectrum. Saric et al. [5] recognized the importance of the absent subharmonics and used a DRE array with λk 8 mm to excite a stationary crossflow disturbance that was eventually less unstable than the naturally arising λcr disturbance. In doing so, the growth of λcr was suppressed. The remarkable effect of these so-called control DREs in this experiment was to extend laminar flow past the pressure minimum. The potential for passive control of the crossflow instability stimulated a series of wind-tunnel and flight experiments examining this prospect. To extend the results of Saric et al. [5] to higherRec, the flight experiments of Carpenter et al. [6] investigated the effectiveness of DREs in a 30 deg swept-wing boundary layer at Rec 7.5 × 10. Applique DREs and variable-height DREs spaced at λcr (4.5 mm, in this case) were used. Although measurements from a spanwise array of hot-film sensors confirmed that the roughness did excite disturbances of the expected wavelength, x∕cjtr was unchanged below a critical roughness height. Continuing these experiments, Saric et al. [7] delayed the transition using 24-μm-high DREs spaced at λk 2.25 mm (half of the most unstable wavelength) with a painted leading edge; the extent of laminar flow was increased to 0.60c. In recent wind-tunnel experiments, Hunt and Saric [8] demonstrated that the amplitude of disturbances created by the DREs increases linearlywith k. In these experiments, DREs spaced at λcr 12 mm moved the transition forward via stationary crossflow mode growth. However, the expected control using DREs with λk 6 mm was not achieved. Reduced freestream turbulence in the Klebanoff–Saric wind tunnel (KSWT) might be responsible for rendering the control DREs ineffective in these experiments. This possibility prompted Downs [9] to examine the effect of moderate freestream turbulence on crossflow instability using one option for control roughness with λk 6 mm. Although increasing the turbulence intensity Tu was shown to destabilize the boundary layer in the baseline and critical roughness configurations, little change to x∕cjtr was observed with control roughness at Rec 2.8 × 10. The role of higher freestream turbulence was also examined through experimentation by Muller and Bippes [10]. Although control was achieved at the Arizona State University (ASU) unsteady wind tunnel (UWT) by Saric et al. [5], it has proven difficult to realize both in flight (successful control was shown only once by Saric et al. [7]) and at the Texas AM the levels measured in flight and at the KSWT are lower than at the ASU UWT. The objective of this work is to determine if laminar flow control can be achieved through the use of DREs at Tu < 0.04%. To answer this question, the transition locations behind DRE arrays of various wavelengths and heights are measured and compared to baseline roughness cases. These experiments are conducted in the KSWT, which has freestream turbulence intensity (vortical component) of approximately 0.02% [11] (u 0 rms values quoted in [11] are the complete signal; this value is computed using a sound/turbulence separation technique). Previous wind-tunnel experiments in which DRE transition control was effective were conducted in a freestream turbulence intensity of approximately 0.04% [12].