Mechanical Engineering - Theses
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ItemTurbulent flow over spatially varying roughness( 2023-03)Large-scale heterogeneity in surface roughness, such as uneven biofouling on ships, can lead to phenomena such as internal boundary layers (IBLs) and mean secondary flows that increase drag. Contemporary studies in the field often lack near-wall measurements either due to limitations in experimental methods or prohibitive computational costs for direct numerical simulation (DNS). As a result, contemporary studies on heterogeneous roughness model the near-wall flow using the equilibrium log-law without fully accounting for its validity in the presence of spatial heterogeneity. This thesis considers heterogeneities idealised as alternating strips of roughness and uses DNS in conjunction with an immersed-boundary method to resolve the complex surface geometries. This thesis explores two main themes: the equilibrium log-law and flow phenomena associated with heterogeneous roughness. For the former, the equilibrium log-law is found to yield accurate predictions of the wall shear stress when applied to spanwise-heterogeneous roughness. Where velocity measurements are not available, predictions of the skin-friction coefficient can be made in the asymptotic limits of small and large wavelengths of spanwise heterogeneity. The latter theme is explored by perturbing the spanwise heterogeneity through introducing a moving roughness geometry and through introducing an oblique angle to the heterogeneity. The conventional intuition of mean secondary flows as counter-rotating vortex pairs is found to not generalise outside of spanwise-heterogeneous roughness. Finally, IBLs are identified in all obliquely heterogeneous cases where the roughness is not aligned with the flow.
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ItemPredicting turbulent heat transfer over rough surfaces( 2023-04)When fluids such as air or water flow over solid surfaces, a transfer of heat between the fluid and solid interface occurs. This heat transfer underpins numerous industrial and natural systems, making routine predictions for heat transfer of paramount interest. In practice this is no easy feat. The fluid flow is often turbulent and the underlying surface is often rough, both of which must be accounted for in accurate predictions. Unfortunately, current state-of-the-art models for this purpose are insufficient, as these models are usually restricted to empirical fits, which admit a diverse set of behaviours in their predictions. To rectify this current ambiguity in rough-wall heat transfer, we have systematically conducted high-fidelity direct numerical simulations (DNSs), aiming to unveil the physical mechanisms governing rough-wall heat transfer. In these comprehensive datasets, we cover a wide array of roughness regimes (controlled by the roughness Reynolds number k+), whilst simultaneously varying the working fluid (dictated by the Prandtl number Pr ). A further dataset is also considered where we vary both k+ and the roughness topography, which we control through the frontal solidity, defined as the frontal projected area divided by the total plan area. These simulations are conducted using the minimal channel framework (MacDonald et al., 2017), which can accurately resolve the near-wall roughness sublayer flow at affordable cost, thus enabling the comprehensive parameter sweep. We examine the disagreements which currently linger regarding the prediction of fully rough (high-k+ ) heat transfer. We focus on the fully rough phenomenologies. Although we find the mean heat transfer favours the scaling of Brutsaert (1975), the Prandtl–Blasius boundary-layer ideas associated with the Reynolds–Chilton–Colburn analogy of Owen & Thomson (1963); Yaglom & Kader (1974) can remain an apt description of the flow locally in regions exposed to high shear. Sheltered regions, meanwhile, violate this behaviour and are instead dominated by reversed flow, where no clear correlation between heat and momentum transfer is evident. The overall picture of fully rough heat transfer is then not encapsulated by one singular mechanism or phenomenology, but rather an ensemble of different behaviours locally. Building on this intuition of distinct heat transfer behaviours locally, we develop a predictive model which idealises the total heat transfer as being comprised of two competing heat transfer mechanisms: exposed heat transfer, which follows a Reynolds–Analogy-like scaling and sheltered heat transfer, which is spatially uniform. The summation of these contributions, each weighted by their individual area fractions yields the total rough-wall heat transfer, and is ultimately dependent on the roughness topography controlled by the frontal solidity. We show that a simple ray-tracing model parameterised solely by a sheltering angle is capable of capturing this sheltered and exposed area partition. Finally, we provide a view to transitionally rough, low-k + heat transfer. Here, we have demonstrated that the virtual-origin framework of Luchini (1996) can provide a valid avenue for the prediction of transitionally rough heat transfer.
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ItemMeasurements and analysis of turbulent boundary layers subjected to streamwise pressure gradients( 2021)This thesis details the changes in structure between an adverse pressure gradient turbulent boundary layer (APG TBL), zero pressure gradient (ZPG) TBL, and channel flow by means of the mean momentum balance (MMB). In order to understand the effects of a pressure gradient on a turbulent boundary layer, aspects of the physical flow are studied via mean statistics, turbulence measurements, and spectra analysis. This study uses new experimental measurements that are conducted along an APG ramp as well as measurements downstream of the ramp insert to study the flow as it relaxes towards equilibrium. In the present experimental set-up the boundary layer is under modest APG conditions, where the Clauser pressure-gradient parameter $\beta$ is $\leq 1.8$. Well-resolved hot-wire measurements are obtained at the Flow Physics Facility (FPF) at the University of New Hampshire. Comparisons are made with ZPG TBL experimental data at similar Reynolds number and computational data at lower Reynolds number. Present measurements are also compared to existing APG TBL lower Reynolds number experimental and computational data sets. Finally, it is shown how these findings relate to an analytical transformation. The primary takeaways from the MMB analysis presented herein are $(i)$ distance-from-the-wall scaling can result from an assumption of self-similar mean dynamics, and does not require primacy of a single velocity scale, and $(ii)$ distance-from-the-wall scaling does not necessarily imply a logarithmic mean velocity profile; a power-law velocity scale hierarchy along with self-similar mean dynamics simultaneously produces distance-from-the-wall scaling \textit{and} a power law mean velocity profile. The choice to refer to the (potentially) self-similar subdomain as the `inertial sublayer' in the present study (rather than the `log' layer) is therefore deliberate.
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ItemEffect of solidity on momentum and heat transfer of rough-wall turbulent flows( 2020)A major area of interest in engineering is the skin-friction drag and convective heat transfer of surfaces in turbulent flow, such as turbine blades, marine vehicles, and airplanes. While these surfaces may appear smooth, they almost always have some form of roughness, for example, pitting on the surface of a turbine blade, barnacles on the hull of a marine vehicle, or rivets on the wings of an airplane. For a given roughness and flow speed, the Moody diagram can be used to find the frictional drag or pressure drop. A similar diagram can be constructed to find heat transfer. Although widely used, the biggest limitation of the Moody diagram is that the Nikuradse equivalent sand grain roughness has to be known for the rough surface in question. Another limitation of the Moody diagram is in predicting skin friction for transitionally rough surfaces, owing to the unrepresentative Colebrook fit. Also, while the effects of varying key roughness topographical parameters on momentum transfer have been studied extensively, relatively little is known on heat transfer. Over the years, researchers have used computational and experimental methods to investigate the flows over a number of roughness types. This thesis expands on the computational works on sinusoidal roughness by systematically investigating the effect of varying roughness solidity on both momentum and heat transfer in turbulent air flow, and the underlying flow physics that give rise to the observed behaviour. Rough-wall flows transfer more momentum and heat when compared to smooth-wall flows, and it is found that an increase in solidity for a matched equivalent sand-grain roughness height causes a greater increase in heat transfer than the increase in momentum transfer due to increased wetted area and increased recirculation region that facilitates mixing.
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ItemFlame Wall Interactions for Flames Diluted by Hot Combustion Products( 2020)Flames diluted by combustion products can reduce emissions such as Carbon Monoxide (CO) and Oxides of Nitrogen (NOx) in industrial applications. In applications such as gas turbines, these flames are confined in a combustor and can interact with relatively cold walls. This interaction can quench the flame, producing incomplete combustion products. In this study, Flame-Wall Interaction (FWI) for methane/air flames diluted by hot combustion products was investigated using Direct Numerical Simulation (DNS). One-Dimensional (1D) Head-On Quenching (HOQ) was first simulated to examine operating parameter effects on CO emissions from transient quenching processes. Average CO within the quenching region was used to evaluate these effects, and the species transport budget was used to investigate the dominant terms. At higher dilution levels, the peak average near-wall CO decreases, and the rate of near-wall CO reduction also decreases. At higher wall temperatures, the peak average near-wall CO and its reduction rate increases. The near-wall CO may be modelled under some conditions using only the integrated diffusion term. Then, a two-Dimensional (2D) laminar V-flame was simulated in both steady and forced conditions. The changes in peak near-wall CO due to varying dilution level and wall temperature show similar trends to the 1D results. The exhaust CO is linked directly to the oxidation residence time, which is determined by the flame length. Due to the role of the flame length, the contribution of near-wall CO to the exhaust CO increases as dilution level or the wall temperature is reduced. Premixed flames can extinguish inside the cold-wall thermal boundary layer, which can leave high near-wall CO. This results in disproportionate levels of CO mass flux in the near-wall regions. The near-wall CO features large variations when the local Damkohler number is greater than 0.1. Analysis of the CO transport budget shows that unlike 1D simulation, both convection and diffusion dominate the CO transport in the near-wall region, except for the case with autoignition at the wall. Finally, a three-Dimensional (3D) turbulent V-flame in a channel was simulated with hot and cold walls. A main reaction zone in the central region supported by periodic bulk ignition events changes the position of volumetric reaction zones where CO is formed. Consistent with the 2D results, a lower wall temperature leads to a longer flame, thereby having more contribution to the exhaust CO. Near-wall turbulence-flame interaction creates wrinkled and streaky flame surfaces, and localizes the near-wall CO distribution. The high mean of CO mass fraction locates in the free-stream where the free-stream autoignition happens, while the high RMS of CO mass fraction is present closer to the wall. 1D flame solutions might be sufficient for modelling CO in the free-stream region and some parts of the near-wall region but not closer to be adjacent to the wall. Turbulent mixing and diffusion contribute to this deviation. These results set a benchmark for future near-wall CO modelling.
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ItemSensor and actuator selection for feedback control of fluid flows( 2019)The present thesis regards linear estimation and control for two fluid flows, with a particular focus on the placement of sensors and actuators. In the first part of the thesis, we study the complex Ginzburg-Landau equation, a simple model for spatially developing flows such as jets, wakes and cavities. (This equation can be seen as a low-dimensional substitute for the Navier-Stokes equations.) The specific focus is on the extent to which estimation and control are (i) fundamentally difficult and (ii) limited by having only a single sensor and a single actuator. To answer these questions, we study three problems. First, we consider the optimal estimation problem in which a single sensor is used to estimate the entire flow field (without any control). Second, we consider the full information control problem in which the whole flow field is known, but only a single actuator is available for control. Third, we consider the overall input-output control problem in which only a single sensor is available for measurements; and only a single actuator is available for control. By considering the optimal sensor placement, optimal actuator placement or both while varying the stability of the system, fundamental placement trade-offs are made clear. We discuss implications for effective feedback control with a single sensor and a single actuator and compare the results to previous placement studies. In the second part of this thesis, we look at an incompressible turbulent channel flow at a friction Reynolds number of Re$_\tau = 2000$. A linear Navier-Stokes operator is formed about the turbulent mean and augmented with an eddy viscosity. Velocity perturbations are then generated by stochastically forcing the linear Navier- Stokes operator. The objective is to estimate and control these perturbations. The estimation and control problems perform best for the largest scales that (i) are high in energy when stochastically forced, (ii) exhibit large transient growth and (iii) are coherent over large wall-normal distances. We determine the locations of sensors and actuators for which estimation and control are most effective by looking at two arrangements: (i) placing them at the wall; and (ii) placing them some distance off the wall. Finally, it is shown that a control arrangement with a well-placed sensor and actuator performs comparably to either measuring the flow everywhere (while actuating it at a single wall height) or actuating it everywhere (while measuring it at a single wall height). In this way, we gain insight (at low computational cost) into how specific scales of turbulence are most effectively estimated and controlled.
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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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ItemExperimental investigation of velocity and vorticity in turbulent wall flows( 2019)This thesis details the results of a research effort both to acquire sufficiently-resolved velocity and vorticity vector time-series in turbulent wall-bounded flows, as well as to use the acquired data to juxtapose two canonical wall-bounded flows: the zero-pressure-gradient boundary layer and the fully-developed pipe. Towards these ends, a novel configuration of measurement sensors has been designed, evaluated, and deployed in three of the largest canonical wall-flow facilities in existence: the Flow Physics Facility (FPF) at the University of New Hampshire, the High Reynolds Number Boundary Layer Wind Tunnel (HRNBLWT) at the University of Melbourne, and the Centre for International Cooperation in Long Pipe Experiments (CICLoPE) at the University of Bologna. The datasets presented herein are the first to contain simultaneously-acquired velocity and vorticity statistics in both pipe and boundary layer flows under matched conditions. The capacity of the measurement probe to resolve the velocity and vorticity vectors under idealised conditions is evaluated via `synthetic experiments', whereby the response of the probe to a simulated turbulent flow is modeled, and the resulting aggregate statistics compared to those of the known input. The synthetic experimental results are then compared to statistics obtained from physical experiments, and show close agreement for most quantities despite differences in Reynolds number. Disagreement between the physical and synthetic experimental results in several quantities is used to diagnose a limitation of the present sensor system. An awareness of the measurement capabilities (and limitations) afforded by the comparison between the synthetic and physical experiments pervades the ensuing analysis and discussion of additional physical experimental results. Normalised statistical moments (up to the kurtosis) of velocity and vorticity are presented for both pipe and boundary layer cases. The two flows are shown to exhibit virtually no differences from one another wallward of the wake region aside from the transverse velocity component variances. The boundary layer wake is characterised by higher turbulence enstrophy and turbulence kinetic energy (TKE) than the pipe, despite containing both turbulent and non-turbulent states. Although there is no `free-stream' in the fully-developed pipe, a significant time-fraction of the flow can be described as `quasi-irrotational' near the centreline. This results in a departure of the velocity and vorticity kurtosis (and, when not identically zero, skewness) from Gaussian behaviour in the outer region of the pipe, as is known to occur in the boundary layer (owing to turbulent/non-turbulent intermittency). Spectral properties of the velocity and vorticity signals acquired in both flows are examined, both for their own content as well as to compare the two flows. Despite very little difference in the observed streamwise turbulence kinetic energy, the contributions to the total differ considerably by scale between the two flows. The pipe is observed to contain more streamwise TKE than the boundary layer in scales longer than 10 times the outer length scale $\delta$, while the opposite is true for scales shorter than $10\delta$. This `crossover' scale also applies to the spectra of the transverse velocity components and Reynolds shear stress. The measured enstrophy spectrum is shown to be approximately invariant under Kolmogorov normalisation for wall-distances greater than about 200 viscous-lengths. Finally, local isotropy and axisymmetry in the velocity components are evaluated. Local isotropy is observed to be satisfied over a range of scales for which the characteristic timescale of inertial energy transfer is expected to be small relative to the timescale of the mean strain rate. The relationship between the transverse velocities and the streamwise velocity, however, does not appear to approach isotropy along the pipe centreline, suggesting that the bounding wall also plays a role in imposing a sense of direction on the turbulence. Indeed, the aforementioned timescale criterion is shown to identify a cutoff scale that increases proportionally with wall-distance, confounding a conclusive statement regarding the primary source of anisotropy away from the centreline.
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ItemAnalysis of resolvent method for turbulence inflow generation( 2018)High-fidelity simulations of turbulent flows aim to accurately reproduce the statistical and structural properties of real-life turbulence. Such simulations rely on accurate inflow boundary conditions. A novel turbulent inflow generation method for high-fidelity DNS/LES has been developed which utilizes reduced order modelling (ROM) and evolutionary algorithms. The core idea behind this method is the classical view of turbulence which represents it as a collection of coherent structures. A low-rank approximation approach known as the resolvent analysis, developed by McKeon and Sharma [Journal of Fluid Mechanics, Vol. 658, 336-382 (2010)], is used to represent the governing equations as a linear input-output system. The non-linearities in the governing Navier-Stokes equations are the driving force behind the ow. This forcing of the linear system produces a response which represent the velocity perturbations. The resolvent analysis is performed at different wavenumber-frequency combinations which are selected in a manner to represent a variety of energetic coherent structures like the near-wall longitudinal streaks, hairpin vortices, Large Scale Motions (LSMs) and Very Large Scale Motions (VLSMs). A Singular Value Decomposition (SVD) of the linear operator in the input-output system is performed to generate a set of orthonormal basis functions for the forcing and response fields. A major advantage of the resolvent analysis is its reduced dependence on external data. It requires only the mean statistical quantities as an input which are readily available for various ow problems or can be easily obtained from cost-effective RANS simulations. The amplitudes of the selected modes were linearly scaled such that the turbulence kinetic energy (TKE) of the resolvent modes is equal to the target TKE. This technique successfully resulted in a fully developed turbulent field, although with a large development length. A further improvement of this method is obtained by optimizing the amplitude of each resolvent mode, which represents the energy content of the associated coherent structure. A genetic algorithm approach has been used to optimize the resolvent modes to represent the target Reynolds stress profiles. This modified process results in a significantly improved development length.
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ItemStructure of mean dynamics and spanwise vorticity in turbulent boundary layers( 2016)The overall aim of this research is to develop a refined scaling analysis for the two-dimensional mean momentum balance (MMB) for the zero-pressure gradient turbulent boundary layer (TBL), and to describe the mean and fluctuating spanwise vorticity structure in a context consistent with the mean dynamical scaling. Towards this aim, a custom multi-element hot-wire sensing array was used to measure spanwise vorticity fluctuations and Reynolds shear stress time series in turbulent boundary layers. Smooth wall boundary layer profiles, with very good spatial and temporal resolution, were acquired over a K\'arm\'an number range of $2,400-16,400$ at the Melbourne Wind Tunnel at the University of Melbourne, and in the University of New Hampshire's Flow Physics Facility (FPF). These data represent the most highly resolved measurements over this Reynolds number range available to date. For canonical boundary layers the mean inertia, which is a function of the wall-normal distance, appears instead of the constant mean pressure gradient force in the MMB for pipes and channels. The constancy of the pressure gradient has led to theoretical treatments for pipes/channels, that are more precise than for the TBL. Elements of these analyses include the logarithmic behaviour of the mean velocity, specification of the Reynolds shear stress peak location, the square-root Reynolds number scaling for the log layer onset, and a well-defined layer structure based on the balance of terms in the MMB. The present analyses evidence that similarly well-founded results also hold for turbulent boundary layers. This follows from transforming the mean inertia term in the MMB into a form that resembles that in pipes/channels, and is constant across the outer inertial region of the TBL. The present measurements support these analytical developments. Congruent with the self-similar structure admitted by the transformed mean governing equations, a description of the mean and fluctuating vorticity structure in turbulent boundary layers is presented. Due to the decreasing mean value of vorticity with increasing wall distance and Reynolds number, the fluctuating enstrophy approaches that of the instantaneous enstrophy on the inertial outer domain. Furthermore, this underlies an emerging self-similarity between the mean and standard deviation of vorticity over the same subdomain where the mean velocity profile is logarithmic. The accompanying data analysis includes a detailed description of the spanwise vorticity statistical structure. For flat plate boundary layers the spanwise vorticity is the only component that fluctuates about a non-zero mean, and hence was selected for this study. Empirical support for the proposed similarity structure is also provided.