Mechanical Engineering - Theses
Permanent URI for this collection
Search Results
1 - 6 of 6
-
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.
-
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.
-
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.
-
ItemOptimisation of closed-loop aerodynamic systems( 2018)The design of a missile system is a multi-disciplinary engineering activity that involves structural, aerodynamics, rocket propulsion, guidance, electronic, and closed-loop control engineering to name a few. In modern engineering practice, a systems engineering approach is utilised to manage the design of a missile, but this does not necessarily guarantee that the final design is optimal. The process may also be inefficient, requiring many iterations of design, prototyping and testing in order to achieve the required specifications. In this thesis, multi-disciplinary optimisation frameworks are developed that target the aerodynamics and closed-loop control system of a supersonic tail-fin controlled missile. The aerodynamics and control system are highly coupled systems, but it is rare to see these subsystems optimised together in the literature. This is due in part to the computational requirements of the aerodynamic simulations and in part due to many control system design techniques that tend to treat the missile dynamics as immutable. A model representing a supersonic tail-fin controlled missile is developed. The model utilises computational fluid dynamics (CFD) simulations in order to capture the aerodynamic behaviour and a state-space model for the dynamics of the missile. Control algorithms are utilised to perform the autopilot function of the missile. This model serves as a basis on which the aerodynamic shape and controller gains can be optimised. Aerodynamic shape optimisation problems typically have large computational demands thus making them impractical to be used with global optimisation algorithms. The first optimisation framework developed is based on sample based global extremum seeking. It is shown that under certain conditions, the convergence behaviour of CFD simulations can be viewed as plant dynamics and thus extremum seeking techniques can be applied to find the optimal aerodynamic shape. The results are a step toward obtaining globally optimal solutions within comparative computation times of gradient-based optimisers. While useful for shape optimisation, the previous result would still struggle with combined aerodynamic shape and control optimisation problems. The next framework proposed is an adjoint-based gradient optimisation framework. The adjoint method has previously been utilised for static shape optimisation problems, but the result presented here is an extension for dynamic and controlled missile problems. The result shows that with appropriate time-scale separation between the actuator and slow states of the missile, the gradient of the cost function can be found with just two times the computational requirements of mapping the aerodynamic characteristics of the missile. This computational requirement is independent of the number of shape design variables and thus shows its practicability. An example of a missile tail-fin profile and autopilot gain optimisation problem is presented. There exists limitations of the adjoint based framework which prevent its use for certain missile geometries. Consequently, an implicit filtering framework is utilised in combination with the adjoint framework to cater for general missile geometries while still maintaining competitive computational speeds. This framework shows that general missile problems can be optimised without restriction. A number of optimisation examples involving a missile tail-fin profile and platform, missile nose cone and autopilot gains are presented. Lastly, goal-oriented mesh adaptation which has often been utilised in the CFD community to refine their computational meshes is utilised in non-linear model predictive control (NMPC). Goal-oriented mesh adaptation is a result derived from the adjoint method. The control algorithm that is developed is computationally faster than the standard NMPC and therefore can be utilised in so-called "fast" systems.
-
ItemEvaluation of coronary stents and atherosclerosis employing optical coherence tomography and computational fluid modelling( 2017)Atherosclerotic coronary artery disease (CAD) is a major health burden worldwide and percutaneous coronary intervention (PCI) is an established treatment for this condition. Both PCI and invasive imaging techniques have evolved tremendously over the past few decades. Limitations of angiography were largely overcome, first by intra-vascular ultrasound, and then, optical coherence tomography (OCT), which is now recognized as the most sensitive and validated tool to examine the vessel lumen, plaque composition and stent-vessel wall interface. This thesis centers on randomized, OCT trials of coronary atherosclerosis and stents. In-vivo, comparative studies of drug eluting stents were conducted to directly observe their mechanical and healing characteristics. Results of these trials subsequently laid foundation for computational fluid dynamics (CFD) experiments and some illuminating observations were made around the effects of stent malapposition on intra coronary flow dynamics. It is the information of this kind that guides scientists to refine stent designs and clinicians, to improve procedural outcomes.
-
ItemNumerical analysis of turbulent heat transfer in pipe and channel flows( 2014)Turbulent heat transfer (THT) for internal geometry (pipe or channel) has a wide range of applications such as heat exchangers, combustion chambers, transport of hot fluid from underground geothermal wells, nuclear reactors, etc. Hence, the understanding of the role of turbulence through direct numerical simulation (DNS) of flow and heat transfer in a canonical pipe geometry will have a significant influence on the designs of many engineering systems. A review of literature reveals that very few DNS studies were conducted for THT in pipe flow. In fact, most researchers prefer to conduct DNS and THT studies for channel flow due to simplified geometry and coordinate system. As a result, there is now plenty of DNS data on channel for a wide range of Reynods and Prandtl numbers. The present work aims to fill a gap in the literature by carrying out DNS of THT in pipe flow for a wide range of Prandtl number (0.025 <= Pr <= 7). It has been found that previous studies only considered either the effect of mesh resolutions or the applied thermal boundary condition on thermal statistics in turbulent flow apart from the influence of governing parameters, Reynolds and Prandtl numbers. Often, it is very difficult to compare those results for making a decision solely based on the dependence of the particular parameter since the computational domain length varies in each simulation. A parametric investigation conducted in this thesis clearly illustrates the influence of domain length on the convergence of thermal statistics eventually explains the reasons for variation of different results in published literature. A review of the scaling properties of the governing equation for passive scalar transport in wall-bounded turbulent flows is also conducted in this thesis. Traditional scaling approaches fail to provide the invariant profiles as the relevant non-dimensional parameters are varied. Based on magnitude ordering and scaling analysis of the mean energy equation, I explore the properties of four distinct thermal balance layers and determine the limiting value of Prandtl number for the onset of the four layer regime. Moreover, the generalized form of the intermediate length scaling has been deduced and verified with existing heat transfer data. Further comparison between inner and intermediate normalizations reveals that the existing and proposed scaling approaches fail to simultaneously and self-consistently reconcile all the profiles, which has become an open challenge for many researchers. A systematic comparison of thermal statistics between pipe and channel flow for different Reynolds and Prandtl number has been presented to provide a better understanding of the similarities and differences of the heat transfer mechanism of these two types of geometry. The intermediate scaling theory matches uniformly for the mean temperature and turbulent heat flux profiles irrespective of pipe and channel flows. However, the inner scaling of turbulence thermal statistics such as mean temperature, thermal intensities, and heat flux profiles shows that there are significant discrepancies between pipe and channel flows unlike the turbulent flow statistics. Finally a set of DNS have been performed for a common engineering application like turbulent flow in a heated wavy-wall pipe. The main objective is to present the flow and thermal turbulence statistics for establishment of a new approach by comparing with available and present scaling framework. Like surface roughness, the corrugation height shows similar tendency to influence the flow and thermal behavior and as a result, the mean momentum and energy balance theory provide a basis to quantify its characteristics to derive the scaling behaviors as a function of roughness scale.