Mechanical Engineering - Theses
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ItemThree-dimensional structure and scaling of a canonical turbulent boundary layer( 2021)Wall-bounded turbulent flows are pervasive in nature and are also encountered in many engineering applications; common examples include the flow over an airplane wing, or the atmospheric boundary layer over the Earth's surface, etc. A dominant feature present within these flows is the appearance of recurring eddies or so-called coherent structures that are highly three-dimensional (3-D) in geometry and are statistically significant over a wide range of scales. This has led to the proposal of various coherent structure-based models in the literature, with the attached eddy model (AEM) of wall-turbulence being the most popular amongst them. Their predictive capabilities, however, are still lacking due to the dearth of 3-D information on the coherent motions which they model. The present thesis reports a new and unique set of multi-point hotwire measurements conducted in a frictional Reynolds number, $Re_{\tau}$ $\sim$ $\mathcal{O}$(10$^4$) canonical turbulent boundary layer (TBL) to reconstruct the 3-D statistical picture of these energy-containing motions. The measurements are complemented by performing a similar reconstruction using published direct numerical simulation datasets at $Re_{\tau}$ $\sim$ $\mathcal{O}$(10$^3$), thereby facilitating an examination of the scaling of these structures, in flows spanning over a decade of $Re_{\tau}$. Results of these investigations provide direct empirical support towards the AEM, with the prospect of further enhancing its efficiency by defining the representative eddy geometry based on data-driven estimates. The first part of the thesis focuses on investigating characteristics of the inertially dominated wall-coherent structures (i.e. the ones extending down to the wall), which are responsible for the increased skin-friction in high-$Re_{\tau}$ TBLs. Their geometric characteristics are investigated in the wall-parallel plane by estimating, for the first time, the 2-D cross-spectrum of the streamwise velocity using the synchronous velocity fluctuations measured at a log-region ($z_{o}$) and near-wall ($z_{r}$) location. Constant energy contours of this spectrum, which are representative of the energy distribution across the range of streamwise (${\lambda}_{x}$) and spanwise (${\lambda}_{y}$) wavelengths, are found to follow the ${{\lambda}_{x}}/{z_{o}}$ $\approx$ 7(${{\lambda}_{y}}/{z_{o}}$) relationship in the large-scale range, indicative of geometric self-similarity. This suggests that a self-similar structure conforming to Townsend's attached eddy hypothesis (Townsend 1976) is ingrained in the flow, and can be conceptually modelled using the AEM framework given by Perry \& Chong (1982). The very-large-scale wall-coherent structures (i.e. the superstructures), on the other hand, do not conform to Townsend's attached eddies and are found to have a similar spanwise width as the largest motions in the self-similar hierarchy. This result, which is found via a scale-specific coherence analysis of the velocity fluctuations, also reveals the periodic organization of the superstructures along the spanwise direction. Finally, an analysis of the scale-specific phase of the coherence reveals the streamwise inclination angle of the large wall-coherent motions, which is found to be nominally 45$^{\circ}$. This fulfills the minimum geometric information required to statistically model these energetic wall-coherent motions based on the AEM. The second part of the thesis focuses on investigating the range of energy-containing structures coexisting in the log-region, which contribute significantly to the bulk turbulence production in high-$Re_{\tau}$ wall-bounded flows. Townsend (1961) hypothesized that these structures can be segregated into active and inactive motions, where the active motions are solely responsible for producing the Reynolds shear stress, the key momentum transport term in these flows. While the wall-normal component of velocity is associated exclusively with the active motions, the wall-parallel components of velocity are associated with both active and inactive motions. To test this hypothesis, the present study proposes a methodology to segregate the active and inactive components of the 2-D energy spectrum (${\Phi}_{ii}$, where $i$ denotes the velocity-component) at $z_{o}$, thereby permitting to test the self-similarity characteristics of the former which are central to theoretical models for wall-turbulence. The methodology utilizes the multi-point dataset, in conjunction with a spectral linear stochastic estimation-based procedure, to linearly decompose the total energy at $z_{o}$ (${\Phi}_{ii}$) into contributions predominantly from the active (${\Phi}^{a}_{ii}$) and inactive (${\Phi}^{ia}_{ii}$) motions. This is confirmed by ${\Phi}^{a}_{ii}$ exhibiting wall-scaling for both ${\lambda}_x$ and ${\lambda}_y$. The Reynolds shear stress cospectra, estimated solely from the active contributions, is also found to closely match the one obtained conventionally from the dataset, thereby providing direct empirical support for the concept of active and inactive motions. Both ${\Phi}^{a}_{ii}$ and ${\Phi}^{ia}_{ii}$ contours are found to depict geometric self-similarity in the log-region, suggesting that this entire region can be conceptually modelled using the AEM framework. Inactive contributions from the attached eddies also bring out the pure $k^{-1}$-scaling for the associated 1-D spectra (where $k$ is the streamwise/spanwise wavenumber), lending further empirical support to the AEM.
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ItemTime-varying secondary flows in turbulent boundary layers over surfaces with spanwise heterogeneity( 2020)The behaviour of turbulent boundary layers over surfaces composed of spanwise-alternating smooth and rough strips is investigated experimentally. The width of the strips S vary such that 0.32 < S/\delta < 6.81, where \delta is the boundary-layer thickness averaged over one spanwise wavelength of the heterogeneity. The experiments are configured to examine the influence of spanwise variation in wall shear stress over a large S/\delta range. Hot-wire anemometry (HWA) and particle image velocimetry (PIV) reveal that the half-wavelength S/\delta governs the diameter and strength of the resulting mean secondary flows. Three possible cases are observed: limiting cases where S/\delta << 1 or S/\delta >> 1 and the secondary flows are either confined near the wall or near the roughness change, respectively, and intermediate cases (S/\delta \approx 1), where the secondary flows fill the entire boundary layer and the outer layer similarity is destroyed. The size and strength of the time-averaged secondary flows are approximately capped by either the boundary-layer thickness \delta or the roughness patch width S. Instantaneously, however, these secondary flows appear very similar to naturally occurring large-scale structures that are spanwise-locked by the roughness transition with a residual meandering tendency about these locations. The efficacy of the roughness to lock the secondary flows in place and the meandering of the secondary flows are a function of S/\delta, most prominent when S/\delta \approx 1. Further analysis of the energy spectrograms and fluctuating flow fields obtained from PIV show that both secondary flows and the naturally occurring large-scale structures formed in turbulence over smooth walls meander in a similar manner and both coexist in the limits where S/\delta << 1 and S/\delta >> 1.
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ItemTowards modelling the downstream development of a turbulent boundary layer following a rough-to-smooth step change( 2020)Turbulent boundary layers are important phenomena, affecting the performance of numerous systems in engineering and nature. At most practical Reynolds numbers, these flows occur over rough surfaces and this surface roughness is often distributed heterogeneously (such as biofouling on a ship hull). Understanding the behaviour of the turbulent boundary layer over this type of roughness is essential for improving the estimation of the drag penalty in full-scale systems. In this work, a turbulent boundary layer over a rough-to-smooth change in the surface condition is examined experimentally. A comprehensive dataset is acquired, which allows us to study the effect of the step height, friction Reynolds number and viscous-scaled equivalent sandgrain roughness height separately. Multiple experimental techniques are employed, including hotwire anemometry and high-magnification particle image velocimetry. The recovering wall-shear stress on the smooth surface is measured directly using oil-film interferometry. Early works of Antonia and Luxton (J. Fluid Mech., 53:737–757, 1972) questioned the reliability of standard smooth-wall methods to measure the wall-shear stress immediately downstream of a rough-to-smooth transition, and subsequent studies show significant disagreement depending on the approach used to determine the wall-shear stress downstream. Here we address this by utilising a collection of experimental databases that have access to both ‘direct’ and ‘indirect’ measures of the wall-shear stress to understand the recovery to equilibrium conditions to the new surface. We present evidence that any estimate of the wall-shear stress from the mean velocity profile in the buffer region or further away from the wall tends to underestimate its magnitude in the near vicinity of the rough-to-smooth transition. This is likely to be partly responsible for the large scatter of recovery lengths to equilibrium conditions reported in the literature. Our results also reveal that the smaller energetic scales in the near-wall region recover to an equilibrium state associated with the new wall conditions within one boundary layer thickness downstream of the transition, while the larger energetic scales exhibit an over-energised state for many boundary layer thicknesses downstream of the transition. Based on these observations, an alternative approach to estimating the wall-shear stress from the premultiplied energy spectrum is proposed. For surfaces with heterogeneous roughness, there can be a change of virtual origin alongside the variation of roughness heights, an extreme case of which is the flow over a forward- or backward-facing step, where no roughness exists but only a height difference is present. In the present study, cases with various step heights between the rough and smooth surfaces behave similarly beyond two boundary layer thicknesses downstream of the roughness transition, and the higher order statistics seem to recover within approximately the same fetch as required for the recovery of the mean velocity profiles, suggesting that the influence of the step height is limited to the vicinity of the roughness transition with no far-field effect. A comparison of the experimental results with a direct numerical simulation of an open-channel flow shows a qualitatively similar effect on the mean velocity profile, and the change in the virtual origin \Delta d is found to be a useful length scale in normalising the recovery length. Dimensional analysis shows that the upstream condition of a turbulent boundary layer over a rough-to-smooth change in the surface condition can be fully described by two non-dimensional numbers: Re_{\tau 0} (friction Reynolds number) and k_{s0}^+ (viscous-scaled equivalent sandgrain roughness height). Therefore, to investigate the dependence of the flow on each parameter separately, we acquire a unique and comprehensive dataset which takes two cuts in the parameter space: one at a fixed k_{s0}^+ with varying Re_{\tau 0}, and the other at a fixed Re_{\tau 0} with varying k_{s0}^+. With the aid of these data, a blending model of the mean velocity profile is developed. The modelled mean velocity profile approaches an equilibrium smooth-wall velocity profile in the near-wall region, and it asymptotes to the upstream rough-wall profile above the IBL (internal boundary layer). The blending model is then incorporated into the original Elliott’s model (Trans. Am. Geophys. Union, 39:1048–1054, 1958) together with further refinements. The refined Elliott’s model leads to a better agreement with the experimental data compared to the original one, permitting an improved prediction of the evolution of a developing turbulent boundary layer downstream of a step change in surface roughness.
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ItemCharacteristics of energetic motions in turbulent boundary layers( 2019)In this dissertation, we present the first measurements of two-dimensional (2-D) energy spectra of the streamwise velocity component (u) in high Reynolds number turbulent boundary layers. The measurements in the logarithmic region of turbulent boundary layers give new evidence supporting the self-similarity arguments that are based on Townsend’s (1976) attached eddy hypothesis. The 2-D spectrum is found to be able to isolate the range of self-similar scales from the broadband turbulence, which is not possible with the measurement of a 1-D energy spectrum alone. High Reynolds number flows are characterized by large separation of scales. Therefore, to obtain converged 2-D statistics while resolving the broad spectrum of length and time scales, a novel experimental technique is required. To this end, we devise a technique employing multiple hot-wire probes to measure the 2-D energy spectra of u. Taylor’s frozen turbulence hypothesis is used to convert temporal-spanwise information into a 2-D spatial spectrum which shows the contribution of streamwise (λx) and spanwise (λy) length scales to the streamwise variance at a given wall height (z). The validation of the measurement technique is performed at low Reynolds number by comparing against the direct numerical simulation (DNS) data of Sillero et al. (2014). Based on these comparisons, a correction is introduced to account for the spatial resolution associated with the initial separation of the hot-wires. The proposed measurement technique is used to measure the 2-D spectra in the logarithmic region for friction Reynolds numbers ranging from 2400 to 26000. At low Reynolds numbers, the shape of the 2-D spectra at a constant energy level shows λy/z ∼ (λx/z)1/2 behaviour at large scales, which is in agreement with the existing literature. However, at high Reynolds numbers, it is observed that the square-root relationship tends towards a linear relationship (λy ∼ λx) as required for self-similarity and predicted by the attached eddy hypothesis. Finally, we present a model for the logarithmic region of turbulent boundary layers, which is based on the attached eddy framework and driven by the scaling of experimental 2-D spectra of u. The conventional attached eddy model (AEM), which comprises self-similar wall-attached eddies (Type A) alone, represent the large scale motions at high Reynolds numbers reasonably well. However, the scales that are not represented by the conventional AEM are observed to carry a significant proportion of the total kinetic energy. Therefore, in the present study we propose an extended AEM, where in addition to Type A eddies, we also incorporate Type CA and Type SS eddies. These represent the self-similar but wall-detached low-Reynolds number features and the non-self-similar wall-attached superstructures, respectively. The extended AEM is observed to predict a greater range of energetic length scales and capture the low- and high-Reynolds number scaling trends in the 2-D spectra of all three velocity components.
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ItemThe turbulent boundary layer studied using novel numerical frameworks( 2018)Numerical simulation of turbulent boundary layers is a challenging task that has prompted the development of different numerical setups. To complement existing techniques, the temporal boundary layer is investigated and found to be a good model for the spatially developing turbulent boundary layer. The remaining temporal development is subsequently eliminated giving a pared-down model yielding the statistically stationary, fully developed boundary layer. A practical use of the temporal boundary layer setup is demonstrated by the addition of free-stream disturbances.
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ItemBoundary layer and bulk dynamics in vertical natural convection( 2017)Results from direct numerical simulations of vertical natural convection at Rayleigh numbers 10^5–10^9 and Prandtl number 0.709 are found to support a generalised applicability of the Grossmann–Lohse (GL) theory, which was originally developed for Rayleigh–Bénard convection. In accordance with the GL theory, we show that the normalised mean boundary- layer thicknesses of the velocity and temperature fields obey the laminar-like Prandtl– Blasius–Pohlhausen scaling, corresponding to the “classical” state. Away from the walls, the dissipation of the turbulent fluctuations, which can be interpreted as the “bulk” or “background” dissipation of the GL theory, is found to obey the Kolmogorov– Obukhov–Corrsin scaling for fully developed turbulence. The present results suggest that, similar to Rayleigh–Bénard convection, a pure power-law relationship between the Nusselt, Rayleigh and Prandtl numbers is not the best description for vertical natural convection and existing empirical relationships should be recalibrated to better reflect the underlying physics. On closer scrutiny of the boundary layers, we find evidence that the boundary layers are undergoing a transition from the classical state to the “ultimate” shear-dominated state. In particular, we observe near-wall higher-shear patches that occupy increasingly larger fractions of the wall-areas. These higher-shear patches exhibit turbulent features, for instance (i) the patches appear streaky, reminiscent of the characteristic near- wall streaks in canonical wall-bounded turbulence, (ii) the local mean temperature profile yields a logarithmic variation, in agreement with the logarithmic law of the wall for mean temperature, and (iii) the local Nusselt number follows an effective Rayleigh number power-law scaling exponent of 0.37, consistent with the logarithmically corrected 1/2 power-law scaling predicted for ultimate thermal convection. We reason that both turbulent and laminar-like boundary layers coexist in the transitional regime of vertical natural convection, consistent with the findings reported for Rayleigh–Bénard convection and Taylor–Couette flows. When the walls are instead removed and boundary layers eliminated, the new setup mimics turbulent bulk-dominated thermal convection. We refer to this new setup as homogeneous vertical natural convection. A direct application of scaling arguments to the governing equations of this new setup yields the asymptotic 1/2 power-law scaling relations for the Nusselt and Reynolds numbers, in accordance to previous theoretical predictions of turbulent bulk-dominated thermal convection. Results from direct numerical simulation of the new setup further supports the predicted 1/2 power- law relations. When employing bulk quantities for the wall-bounded setup, we too find the aforementioned 1/2 power-law scaling. This extended result suggests that the 1/2 power-law scaling relation may even be present at lower Rayleigh numbers provided the appropriate quantities in the turbulent bulk flow are employed for the definitions of the Rayleigh, Reynolds and Nusselt numbers. Lastly, we perform a straightforward assessment of the mixing efficiency in vertical natural convection. The value is predicted and found to be approximately 0.5, which suggests that the dissipation rate of kinetic energy is directly proportional to the rate at which gravitational potential energy is readily available for conversion.
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ItemEvolution of canonical turbulent boundary layers( 2017)An experimental investigation of evolving turbulent boundary layers in a tow tank facility is performed in this study. The main aim here is to investigate the development of a canonical turbulent boundary layer from the trip to a high Reynolds number state. A review of the literature reveals that while the dynamic characteristics of the near-wall turbulent structures are reasonably well understood, the origin, evolution and dynamics of large-scale coherent structures in the logarithmic and outer regions of turbulent boundary layers remain largely unresolved. Unveiling the dynamics of large-scale coherent motions is essential to understanding the complicated physics behind wall-bounded turbulent flows. Therefore, an experimental set-up with a long flat plate mounted on a traversing carriage has been carefully designed and constructed. The spatially developing turbulent boundary layers formed on the bottom surface of the towed plate are investigated using a stationary PIV system, affording a unique frame of reference. Large-field-of-view high-resolution PIV and high-speed PIV techniques are employed to measure the instantaneous streamwise-wall-normal plane of the evolving turbulent boundary layer. The measurement system including the traversing carriage is fully automated such that a large number of passes can be performed to obtain converged statistics. We start our study by validating the flow statistics and canonical-state of the turbulent boundary layers. The statistics are compared with results from DNS datasets at matched Reynolds numbers. In the process of validating the PIV obtained flow statistics, we propose a robust technique to estimate small-scale missing energy due to the spatial resolution issues. The estimation tool is based on arguments that (i) the inner-scaled small-scale turbulence energy is invariant with Reynolds number and that (ii) the spatially under-resolved measurement is sufficient to capture the large-scale information, which is Reynolds number dependent. This tool can be used to diagnose whether PIV statistics are beset by some wider issue. Having validated the measurements in this way, the evolution of mean flow parameters of the developing boundary layers is compared with the predicted evolution of mean flow parameters for canonical turbulent boundary layers proposed in the literature. This assessment determines whether the turbulent boundary layers considered in this study can be classified as being in the canonical state. Good agreement has been observed between the experimental results and the predicted evolution. The unique aspect of the current experiment is that we can observe a time-resolved view of an evolving structure within turbulent boundary layers. Any large-scale coherent motions that have a convection velocity close to the freestream will remain nominally stationary within the field of view, whereas for the conventional frame of reference (i.e. wind tunnels or water channels), coherent structures convect away from the field of view with local flow velocities. Therefore, by obtaining a temporally-resolved view of developing boundary layers in this unique frame of reference, the evolution of large-scale coherent motions has been analysed. It is shown that there is a mismatch in the convection velocities between low- and high-speed regions at a matched wall-height. This convection velocity difference allows the low- and high-speed regions to interact, causing the formation of a strong internal shear layer. Both instantaneous and conditionally averaged velocity fluctuations are examined to support these findings. It is also shown that a local shear layer instability develops as the shear layer evolves. The development of this local shear layer instability leads to a shear layer roll-up process which perturbs low- and high-speed regions, causing the regions to migrate to different wall heights (ejections and sweep like events). Finally, all the dynamic properties and interactions of large-scale coherent motions discovered in this study are incorporated to construct a conceptual model. This model describes the role of shear layers and the effect of dynamic interactions of large-scale coherent motions in the outer layer of turbulent boundary layers. The model is derived with the intention of providing a better understanding of large-scale coherent motions and potentially the dynamics described in the model can be incorporated into existing kinematic models to advance future efforts for a complete model for wall-bounded turbulent flows.
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ItemMulti-component velocity measurements in turbulent boundary layers( 2015)An experimental investigation of high Reynolds number (Re) turbulent boundary layers is undertaken in this study. The primary focus here is to measure the spanwise and wall-normal velocity components, in addition to the streamwise velocity. This study has been undertaken in an attempt to address the lack of spanwise and wall-normal velocity measurements at high Re, identified in the existing literature. For this purpose, we have utilised a custom dual hot-wire probe that is spatially compact, to reduce the volume occupied by the sensing elements. Measurements at high Re are particularly challenging due to the increased scale separation between the smallest and largest energetic scales. To overcome the challenges of resolving these range of scales, experiments are conducted using a specialised wind tunnel, located at the University of Melbourne, whose 27m length allows a thick boundary layer to be developed. Since Re is equal to the ratio between the largest and smallest scales in the flow, a thicker boundary layer (the largest scale in the flow) equates to a larger permissible physical dimension for sensors to capture the smallest scale, for a fixed Re. We start our study by investigating the effects of finite sensor dimensions on the measured turbulence statistics. In this work, the effects of finite sensor dimensions are simulated numerically using a box filtering process on a three-dimensional velocity field obtained through direct numerical simulation. Two typical dual hot-wire probe configurations, namely V- and X-probes are considered. The simulated results show that X-probes are better suited at measuring the turbulence statistics in a wall-bounded flow compared to V-probes. This is attributed to the wire separation in X-probes, which can be physically configured to be closer than in V-probes. Furthermore, the simulation results suggest that the deviation in the turbulence statistics obtained is a function of utilised sensor dimensions, scaled with viscous units. Therefore, care is taken to match the spatial resolution of the sensor used during the experiments at multiple Re, to avoid contamination from the spatial resolution effects to the Re trends identified. The measured broadband turbulent stresses and cross power spectrogram in the logarithmic region are compared against scaling laws derived using the attached eddy hypothesis. A logarithmic relationship between the streamwise and spanwise turbulence intensities with distance from the wall is observed as predicted by the attached eddy hypothesis. Furthermore, the spanwise, wall-normal and Reynolds shear stress spectrogram obtained are consistent with the notion that the logarithmic region in the wall-bounded flow can be considered to be a collection of self-similar eddies that scale with the wall height; an underlying assumption in the attached eddy hypothesis.
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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.
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ItemLow-Reynolds-number turbulent boundary layers( 1988-12)This thesis documents an extensive experimental investigation into low-Reynolds-number turbulent boundary layers flowing over a smooth flat surface in nominally zero pressure gradients. The way in which these layers are affected by variations in R(theta), i.e. the Reynolds number based on the boundary-layer momentum thickness, type of tripping device used and variations in freestream velocity, each considered independently, are investigated.