Mechanical Engineering - Theses
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ItemThe influence of realistic roughness on turbulent boundary-layers( 2023-10)Up to 90% of the drag that affects engineering systems with wall-bounded turbulent flows results from the turbulence induced skin-friction. In nearly all practical scenarios, this drag is exacerbated due to the presence of surface roughness. Therefore the accurate prediction of this drag penalty is highly desirable. However, current models are limited as they neglect some common aspects of realistic roughness - viz. anisotropy, multi-scaled nature, and temporal variability. This thesis presents three experimental campaigns that address these shortcomings. The effects of anisotropy of roughness are examined by varying ESy while keeping ESx fixed, where ES is the effective slope (in streamwise, x, and spanwise, y, directions). Four cases are investigated – two isotropic cases with ESx = ESy = 0.24 and 0.13, and two anisotropic cases with ESx = 0.24 and ESy = 0.13 and vice-versa. The skin-friction curves, obtained with a drag balance, suggest that anisotropic cases (ESy/ESx < 1) exhibit a higher drag penalty than the isotropic cases (ESy/ESx = 1) at matched ESx. The Hama roughness function obtained from the mean velocity profiles, is seen to increase by 6 to 8% when ESy/ESx is reduced by a factor of nearly 2 across the Reynolds number range of the measurements. For all cases, there is evidence for Townsend’s (1976) outer-layer similarity hypothesis. However, the wall-normal extents of the enhanced dispersive velocities are seen to increase as ESy is decreased and ESx is held constant. This extent is reasonably captured by 0.5Ly, where Ly is the mean spanwise wavelength, as suggested by Chan et al. (2018). Consequently, models for drag prediction should be adapted to also consider anisotropy in the form of ESy. The importance of ESx for drag prediction is evident, yet for practically occurring multi-scale roughness, it can remain unbounded if all the scales of the topography are not resolved. Consequently, two questions are asked: (i) At what flow-defined length scale do the small-scale features affect the drag? (ii) Can ESx reliably predict the drag of two surfaces with matched ESx but different scale composition? To answer these questions, drag balance measurements are conducted with a set of multi-scaled rough surfaces, where only the contribution of the small scales of the topography is varied. The relative increase in the drag penalty is seen to scale with the viscous scaled height of the small-scales, and is appreciable only when their heights exceed 2–3 times the viscous length scale. However, even when the small-scale height is O(10) viscous units, the additional drag penalty seen is much lower (nearly 28%) than another case with matched ESx, but where the ESx results from larger scale features alone. These findings confirm that for multi-scaled surfaces, one should consider which scales contribute to their measure of ESx to avoid incorrect predictions of the drag penalty. Finally, the response of a TBL to a well-defined oscillating rough surface is examined. For the dimensionless frequencies of the roughness studied, the mean flow is seen to oscillate between the static smooth and rough limits. A quasi-steady state is seen at low frequencies, which diminishes as the dimensionless frequency increases. A transition front is identified from the phase-averaged statistics, which suggests that the response of the boundary-layer is due to the growth of internal boundary-layers (IBLs). The response is modelled based on an argument of effective fetch, which relates the phase of the oscillation to a static step change in roughness at an effective upstream position, which in turn permits existing models for IBL growth to be used. This argument is confirmed by comparison with static streamwise heterogeneous data. The modelled front with this argument shows a good match to the front identified from the experiments along with the broad changes in the inclination of the front with increasing dimensionless frequency. Thus, a case can be made that turbulent boundary-layers over oscillating roughness can be modelled as growing internal layers.
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ItemParticle-induced modulation and dispersion of inertial particles in turbulent wall flows( 2023)Turbulent wall flows laden with dispersed particles represent a significant canonical problem in modern turbulence research. The modelling of fluids laden with a dispersed phase has received substantial attention by fluid mechanicians and indeed physicists---from Einstein's effective eddy viscosity for dilute suspensions of small rigid spheres to sonoluminescence where collapsing bubbles excite such strong energy that light emission occurs, part of Lohse's wide investigation into bubble puzzles. This thesis drives further physical understanding of particle-fluid flows by simulations performed on state-of-the-art heterogeneous computing architectures. So, the generated data illuminates mathematical modelling from the twentieth century of some particles in turbulence scenarios. Restricting attention firstly to the underlying fluid, chapter 3 in this thesis devotes attention to the spatial requirements for accurate numerical calculation of the turbulent fluid. Then, the migration of inertial solid dense spherical particles in wall-bounded turbulence is studied, which represents a scenario where a dilute loading of particles is present. The findings support turbophoretic drift modelling of and `roll-off' extension of that relates particle accumulation at walls with the viscous Stokes number, validating and contextualising these models. Thereafter, the modification of turbulence by the presence of a higher particle loading reveals similarity between two canonical flows, pipes and channels. It was found that near to the wall, there is remarkable agreement in the modification of turbulent scales because heavy inertial sub-Kolmogorov particles deplete turbulent structures. Finally, Reynolds number effects are studied in classical Taylor dispersion of solutes. When characterisation of the particles is based off the small-scale motions, it is found that increasing Reynolds number leads to scale separation whereby inertial effects of particles are diminished. There is also a dramatic reduction in skewness of particle dispersion as Reynolds number increases due to enhanced radial mixing.
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ItemSound Generation by Hydrogen/Methane Premixed Flames( 2022)Hydrogen produced by renewable energy is a promising alternative fuel that can be used to decarbonise the electricity generation sector. Mixing hydrogen with natural gas can smoothly transition current energy generators towards producing less greenhouse gas emissions while mitigating the challenges faced by burning pure hydrogen. However, there are challenges in the usage of hydrogen because it has very different flame characteristics compared to conventional fuels used in gas turbines such as natural gas. One of the major challenges faced by combustor designers is thermoacoustic instability. This instability occurs when the acoustics of the combustor couple with the flame and generate very loud high frequency pressure oscillations that can damage the combustor and reduce the lifespan of equipment. Hydrogen addition to the fuel can initiate thermoacoustic instability, hence it is important to understand the impact that hydrogen addition has. Thermoacoustic instabilities are closely linked to sound generation by flames. In addition, many combustors produce loud noise which are subject to stringent regulations. Predicting the sound generated by flames is key to designing safer and quieter combustors. However, there is little information in the literature on the effect of hydrogen addition on the sound generated by flames. This thesis will utilise high fidelity simulations, specifically Direct Numerical Simulations (DNS), to simulate turbulent premixed jet flames with different hydrogen/methane fuel content. Firstly, planar one-dimensional (1D) annihilation events are simulated for different hydrogen/methane mixtures and the pressure waves generated by these events are compared. It is found that the flames fuelled with hydrogen/methane mixtures undergo a three-stage annihilation event, with the first stage generating long-wavelength pressure waves. This first stage is not present in pure hydrogen or pure methane fuelled annihilation events. These simulations are used to develop a reduced chemical mechanism with low computational cost, enabling three-dimensional (3D) DNS of the turbulent premixed jet flames. The 3D DNSs are then performed and the sound generated by the flames is compared for different hydrogen/methane fuel mixtures. The dependence of the sound on the flame dynamics and the flame stretch, displacement speed and curvature is characterised. The Sound Pressure Level (SPL) of the sound waves generated by annihilation events is found to increase with hydrogen content in the fuel. This effect is due to the increase in laminar flame speed with hydrogen addition. In annihilation events, heat release rate fluctuations are the dominant source of noise. However, there has not yet been a full comparison of all noise sources in a turbulent premixed jet flame. In this work, an acoustic analogy is used to find the contribution of all physical mechanisms by which sound is generated in the flame. It is found that the heat release rate fluctuations and turbulence are the principal sources of noise for all cases. To retrieve the pressure fluctuations from turbulent eddies, the superposition of three different sources, specifically the (a) Lighthill stress tensor source, (b) entropy inhomogeneity source, and the (c) surface contribution, must be used. These three sources are found to be intrinsically linked with each other. In addition, the energy transport by mass diffusion is found to be an important source of sound when hydrogen and methane are equally mixed in volume, and unimportant when one fuel is dominant.
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ItemNo Preview AvailablePulsatile effects on turbulence dynamics in pipe flow( 2021)An increasing concern in medical industry on human diseases in artery flows (e, g,. Cardiovascular Disease (CVD)) has risen in the relevant research fields. The turbulent activities and pulsating forcing conditions of the blood flow in aorta increase dramatically the complexity of flow physics. In addition to that, pulsatile turbulent flows are well-encountered in a wide range of engineering applications and physical systems including environmental flows over ocean and reciprocating flows in internal combustion engine. Therefore, increased understanding of pulsatile forcing turbulent flow is of immense technological importance and is beneficial towards a future fluid mechanics field. The objective of this study is to use direct numerical simulations (DNSs) method to conduct a series of numerical experiments on pulsatile turbulent pipe flows and to complement the fundamental understanding of pulsatile forcing effects on turbulence dynamics. To this end, the investigation covers a range of friction Reynolds numbers and forcing conditions and is conducted via the following three topics: i) the turbulence dynamics in single-mode pulsatile forcing pipe flow; ii) the Reynolds-number effect on single-mode forcing conditions; iii) the turbulence dynamics at dual-mode pulsatile forcing condition. The study of single-mode pulsatile pipe flow ranged the Reynolds number at both low (180) and moderate-high (360, 540) friction Reynolds numbers and the forcing frequencies over high (type IV) and very-high (type V) forcing regimes at a fixed pulsatile amplitude of 0.64. In this topic, a new physics-informed method of forcing classification was achieved by directly comparing the applied frequencies with the instantaneous Reynolds shear stress co-spectra, – a rarely reported quantity in most experimental and DNS studies – in the frequency domain and thus a new forcing type – the upper-limit of the very-high frequency regime – was determined as ultra-high frequency (type VI). At the ultra-high forcing type, turbulent activities were fully decoupled from the pulsating flow field. The roadmap of our forcing classification method exhibited a good robustness where the turbulence dynamics presented consistent responses to the high, very-high and ultra-high forcing conditions across these three computational Reynolds numbers. In dual-mode forcing pulsatile flows, the forcing classification acquired from RSS frequency co-spectra gave a good outline. The simulations were conducted by combining either two of the three aforementioned forcing types (types IV, V and VI), and this work complemented the possible ambiguity of the dual-mode forcing effects in past classifications. To be specific, at the forcing combination of types IV and V, observable interactions between the two forcing modes were found and the phase-averaged turbulence statistics at each forcing mode showed difference with the corresponding single-mode values. At the forcing combination of types IV and VI, the first-mode phase-averaged turbulence statistics were no longer affected by the second forcing and showed good agreement with the corresponding single-mode values. In the meantime, the second-mode phase-averaged statistics became phase-independent but showed slight difference with the corresponding single-mode values. At the forcing combination of types V and VI, the two forcing modes were fully decoupled with each other and both modes showed good agreement with the corresponding single-mode results. In addition, my work extended the previous studies by complementing a detailed analysis of a series of time- and phase-averaged single- (including quadrant profiles, probability density functions, weighted joint probability density functions and skewness profiles) and two-point (including cross-correlations, one-dimensional and two-dimensional wave-number spectra) Reynolds shear stress and other statistics in both physical and Fourier domains.
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ItemCoherent structures from the linearized Navier-Stokes equations for wall-bounded turbulent flows( 2020)In this dissertation we present a study of the large-scale coherent structures that are modelled by the linearized Navier-Stokes equations in a turbulent channel flow. The results presented here are divided into three parts: i) a study of the wall-normal coherence of these large-scale structures, ii) estimation of the instantaneous and the statistical features of these structures and iii) an analysis of the effect of stratification on the structures. In all three cases the trends obtained from the linear model are compared to Direct Numerical Simulation (DNS) datasets. In the first part, the wall-normal extent of the large-scale structures modelled by the linearized Navier-Stokes equations subject to stochastic forcing is directly compared to DNS data of a turbulent channel flow. A friction Reynolds number of 2000 is considered. We use the two-dimensional (2-D) linear coherence spectrum (LCS) to perform the comparison over a wide range of energy-carrying streamwise and spanwise length scales within the logarithmic region of the flow. The study of the 2-D LCS from DNS indicates the presence of large-scale structures that are coherent over large wall-normal distances and that are self-similar. We find that, with the addition of an eddy viscosity profile, these features of the large-scale structures are captured by the linearized equations, except in the region close to the wall. In the second part of this dissertation, the understanding of wall-normal coherence is exploited to build and analyze linear estimators from the linearized Navier-Stokes equations. For this purpose, we use the coherence-based estimation technique of spectral linear stochastic estimation (SLSE) (Tinney et al., 2006, Baars et al., 2016). The estimator uses the instantaneous streamwise velocity field or the 2-D energy spectrum of the streamwise velocity component at one wall-normal location (obtained from DNS) as input, to predict the same quantity at other wall-normal locations. We find that the addition of an eddy viscosity profile significantly improves the estimation. The final part of this dissertation focuses on the effect of stratification on the large-scale structures that are modelled by the linearized equations. For this purpose, we build an eddy-viscosity enhanced linearized Navier-Stokes based model for stratified turbulent channel flows, and consider the model at different bulk Richardson numbers. Thereafter, we analyze the extent to which the model captures the large-scale features that emerge from competing shear and buoyancy driven mechanisms in these flows. In particular, we compare the large-scale structures modelled by linearized equations to the quasi-streamwise rolls observed in unstably stratified turbulent channel flows (Pirozzoli et al., 2017, Blass et al., 2019). As we increase the influence of buoyancy, the linear model captures plume-like channel-wide features that resemble these quasi-streamwise rolls.