Mechanical Engineering - Theses
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ItemHighly ordered surface roughness effects on turbulent boundary layers( 2015)The effects of highly ordered riblet type surface roughness with convergingdiverging/ herringbone pattern in zero pressure gradient (ZPG) turbulent boundary layers are investigated experimentally. The study is based on a novel investigation by Koeltzsch et al. (2002), where a new class of riblet type surface roughness with converging-diverging/herringbone riblet pattern is applied inside the surface of fully-developed turbulent pipe-flow. Their experimental results show that the unique pattern generates a largescale azimuthal variation in the mean velocity and turbulence intensity. Inspired by these results we intend to extend their study and investigate it in more detail, particularly in zero pressure gradient (ZPG) turbulent boundary layers. The general results from the present study show that the converging region forms a common-flow-up that transfers the highly turbulent and slower nearwall fluid away from the surface, resulting in a low local mean velocity and high local turbulence intensity. The low momentum over the converging region results in a thicker turbulent boundary layer. Over the diverging region the opposite situation occurs, the diverging pattern forms a common-flowdown that forces the faster and less turbulent fluid that originally resides further from the wall to move towards the surface, resulting in a high local mean velocity and low local turbulence intensity. The high local mean velocity over the diverging region results in a thinner turbulent boundary layer. For certain cases, the spanwise variation in boundary layer thickness between the converging and diverging region is almost double. Such large and aggressive variation is uncommon considering that the height of the riblets is ≈ 1% of the boundary layer thickness. The combination of the common-flow-up and common-flow-down regions forms a large scale counter rotating vortices that dominate the entire boundary layer. The resulting magnitude of maximum spanwise and wall-normal velocity components of the counter-rotating vortices are ≈ 2 − 3% of U∞, which is comparable to the lower end strength of typical vortex generators for turbulent flow. Our study reveals that the strength of the spanwise variation depends on several parameters, namely : yaw angle (α), viscous-scaled riblet height (h+), streamwise fetch/development length over the rough surface (Fx), and relaxation distance/development length over the smooth surface (rs). The riblet pattern may offer a unique technique to generate counter-rotating roll-modes within turbulent boundary layers and act as a low-profile flow control mechanism. Analysis of the pre-multiplied energy spectra suggests that the converging and diverging pattern has redistributed the large-scale turbulent features. Over the converging region the large-scales are found to be very dominant in the logarithmic region, which closely resembles the recently discovered ‘superstructures’ by Hutchins and Marusic (2007a,b). The highly ordered surface roughness pattern seems to preferentially arrange and lock the largest scales over the converging region. Further examination of the three-dimensional conditional structures strengthen this view. The conditionally averaged large-scale low-speed feature over the converging region is wider in size and has a stronger magnitude than the equivalent feature over the diverging region. We also look into amplitude modulation over the converging and diverging pattern. Recent reports by Hutchins and Marusic (2007b); Mathis et al. (2009a,b); Marusic et al. (2010b); Chung and McKeon (2010a); Ganapathisubramani et al. (2012) reveal that large-scale structures in turbulent boundary layers modulate the amplitude and frequency of the small-scale energy. The amplitude and frequency magnitude of the near-wall small-scale structures are found to be reduced when low-speed large-scale features are detected. Our analyses show that over the converging region, the reduction in the conditioned small-scale variance/small-scale energy is stronger than over the diverging region. This finding further strengthens our previous observation that the highly ordered riblet pattern preferentially arranges and locks the largest scales over the converging region. Finally, we look into the turbulent and non-turbulent interface (TNTI) of the converging and diverging pattern and found that the interface location experiencing spanwise variation in the same manner as the boundary layer thickness. Furthermore, there are not many differences in the TNTI properties (i.e. interface position and width) between the smooth wall case, converging region, and diverging region when they are scaled with their respective boundary layer thickness.