Mechanical Engineering - Theses
Permanent URI for this collection
Search Results
1 - 3 of 3
-
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.
-
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.
-
ItemDirect numerical simulation of turbulent natural convection bounded by differentially heated vertical walls( 2013)Using new, high-resolution direct numerical simulation (DNS) data, this study appraises the different scaling laws found in literature for turbulent natural convection of air in a differentially heated vertical channel. The present data is validated using past DNS studies, and covers the Rayleigh number (Ra) range between 5.4 × 10^5 to 1.0 × 10^8. This is followed by an appraisal of various scaling laws proposed by four studies: Versteegh and Nieuwstadt (77), Holling and Herwig (34), Shiri and George (63) and George and Capp (23). These scaling laws are appraised with the profiles of the mean temperature defect, mean streamwise velocity, normal velocity fluctuations, temperature fluctuations and Reynolds shear stress. Based on the arguments of an inner (near-wall) and outer (channel-centre) region, the DNS data is found to support a −1/3 power law for the mean temperature in an overlap region. Using the inner and outer temperature profiles, an implicit heat transfer equation is obtained and a correction term in the equation is shown to be not negligible for the present Ra range when compared with explicit equations found in literature. In addition, I determined that the mean streamwise velocity and normal velocity fluctuations collapse in the inner region when using the outer velocity scale. A similar collapse is noted in the profiles of temperature fluctuations with increasing Ra when normalised with inner temperature and length scale. Lastly, I show evidence of an incipient proportional relationship between friction velocity and the outer velocity scale with increasing Ra. The study is extended to the spectrum of turbulent kinetic energy and temperature fluctuations of the flow. The one-dimensional streamwise spectra collapse onto the −5/3 slope, coinciding with the standard Kolmogorov form of the power spectra reported in literature. This collapse is found to occur in the outer region of the flow in the bounds between the peaks of the mean streamwise velocity. In spectrogram form, I find evidence that the spectral peaks correspond to energetic velocity structures in the channel — the structures of streamwise velocity fluctuations appear to stretch half of the streamwise domain and occur at a quarter intervals in the spanwise direction. From 2-dimensional autocorrelations, the structures of spanwise velocity fluctuations are found to be organised in a hatched pattern in an inner location (z^× i ≈ 7) and at the channel-centre. The respective pattern angles are \theta_ i ≈ 54◦ and \theta_ o ≈ 48◦, both measured from the horizontal. For the temperature spectrum, the −5/3 collapse is also observed in the same bounds as the velocity spectrum. In pre-multiplied form, the spectral peak is found to occur at the wall-normal location which coincides with the peak temperature fluctuations in the channel. With increasing Ra, the wall-parallel isocontours of temperature are found to show standard features of turbulent pressure driven boundary layers — streaks with spanwise length of 100+ units.