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Any realizable increase in gas turbine efficiency has significant potential to reduce fuel burn and environmental impact. Streamwise micro-groove surfaces (‘riblets’) are well-known as a passive surface treatment to reduce drag, which may be useful in the context of increasing overall gas turbine efficiency. This paper presents the first direct numerical simulation of potentially performance-enhancing riblets on an axial flow high pressure compressor blade, where the micro-geometry of the riblets is fully resolved. The midspan section of a NACA6510 profile is considered at an engine-relevant true chord Reynolds number of 700,000 and Mach number 0.5 based on inlet conditions. Fixed triangular (or sawtooth) riblets are considered in the present numerical campaign. The current high-fidelity computational method permits the extraction of data such as the wall shear stress directly from the riblet surface. At the design incidence, the riblets tend to promote earlier transition to a turbulent flow over the suction side, yet significantly reduce the skin friction over the entire downstream chord to the trailing edge. The riblets reduce the viscous force over the blade by up to 18% at this nominal inflow incidence. Thus the current dataset permits new insight into the action of the riblets, since most studies of riblets on turbomachinery blades have been conducted experimentally where direct measurements of skin friction are not possible. The riblets are also able to reduce the skin friction over the high pressure compressor blade at off-design incidences, a promising result given axial flow compressors must cope with variable operating conditions.

Airblast atomizers of the ‘prefilming’ type are commonly employed for liquid fuel injection in gas turbine engines propelling modern aircraft. Supplied from holes or slits upstream, the liquid fuel forms a thin film over the prefilming surface before being driven to the atomizing edge by a turbulent flow through the turbine. We consider a simplified numerical setup using a volume-of-fluid method to simulate the essential physics of this multiphase problem. A liquid film is ‘sandwiched’ between sheared turbulent gas flows which deform and then rupture the liquid film. The simplified setup allows us to systematically vary parameters such as film thickness and turbulent gas flow Reynolds number to gauge their role in deforming and ultimately rupturing the initially stationary liquid film. This work presents a detailed study of the developing pressure field over the deforming film and related aerodynamic effects, as previously suggested by other authors, in particular the role of the inviscid lift force. Understanding and controlling the route to breakup and atomization of liquid fuels in such systems is of primary practical concern in modern gas turbine engine design.

The turbulent boundary layer developing under a turbulence-laden free stream is numerically investigated using the temporal boundary layer framework. This study focuses on the interaction between the fully turbulent boundary layer and decaying free-stream turbulence. Previous experiments and simulations of this physical problem have considered a spatially evolving boundary layer beset by free-stream turbulence. The state of the boundary layer at any given downstream position in fact reflects the accumulated history of the co-evolution of boundary layer and free-stream turbulence. The central aim of the present work is to isolate the effect of local free-stream disturbances existing at the same time as the ‘downstream’ boundary layer. The temporal framework used here helps expose when and how disturbances directly above the boundary layer actively impart change upon it. The bulk of our simulations were completed by seeding the free stream above boundary layers that were ‘pre-grown’ to a desired thickness with homogeneous isotropic turbulence from a precursor simulation. Moreover, this strategy allowed us to test various combinations of the turbulence intensity and large-eddy length scale of the free-stream turbulence with respect to the corresponding scales of the boundary layer. The relative large-eddy turnover time scale between the free-stream turbulence and the boundary layer emerges as an important parameter in predicting if the free-stream turbulence and boundary layer interaction will be ‘strong’ or ‘weak’ before the free-stream turbulence eventually fades to a negligible level. If the large-eddy turnover time scale of the free-stream turbulence is much smaller than that of the boundary layer, the interaction will be ‘weak’, as the free-stream disturbances will markedly decay before the boundary layer is able be altered significantly as a result of the free-stream disturbances. For a ‘strong’ interaction, the injected free-stream turbulence causes increased sp

The rupture of a liquid film due to coflowing turbulent shear flows in the gas phase is studied using a volume-of-fluid method. To simulate this multiphase problem, we use a simplified numerical setup where the liquid film is "sandwiched"between two fully developed boundary layers from a turbulent channel simulation. The film deforms and eventually ruptures within the shear zone created by the coflows. This efficient setup allows systematic variation of physical parameters to gauge their role in the aerodynamically driven deformation and rupture of a liquid film under fully developed sheared turbulence. This work presents a detailed study of the developing pressure field over the deforming film and related aerodynamic effects, as previously suggested by other authors, in particular the role of the inviscid lift and drag forces. A cumulative lift force is introduced to capture the effect of the alternating pressure minima and maxima forming over the film, which amplify and eventually rupture the film. A velocity scale derived from the lift-induced drag force reflects the state of the turbulent boundary layer over the film and collapses the temporal development of this cumulative lift force as well as the amplitude of film deformation with some success for the different film thicknesses and Reynolds numbers.

We propose a novel robust three-dimensional particle tracking technique based on a scanning laser setup. The method yields Lagrangian statistics in densely-seeded turbulent flows with good spatial and temporal resolution, overcoming some of the inherent difficulty with line-ofsight based volumetric methods. To do this we have developed an effective triangulation method greatly reducing ghost particle reconstruction using images from two cameras. A laser sheet is rapidly traversed (‘scanned’) across a measurement volume illuminating only a thin slice of the flow at a time. Particle images are taken at closely-spaced, overlapping nominal laser sheet locations giving multiple intensity recordings for each individual particle. The lasersheet intensity varies as a Gaussian across its thickness, which is here exploited to deduce the particle’s probable location along the scan direction to sub-sheet number resolution by fitting a similarly-Gaussian profile to its multiple intensity recordings. Following successful reconstruction of a time series of three-dimensional particle fields, particle tracks are formed from which all components of Lagrangian velocity and acceleration are calculated. The method is presently verified via synthetic experiment using a database born of direct numerical simulation, and is intended for high-Reynolds number experimental flows.

The problem of the turbulent boundary layer under decaying free-stream turbulence is numerically investigated using the temporal boundary layer framework. This tool is particularly suited to the problem since the evolution of homogeneous isotropic turbulence is classically described by temporal decay. This study focuses on the interaction between the fully turbulent boundary layer and free-stream turbulence, which has so far received little attention compared to the behaviour of the transitional boundary layer subject to free-stream turbulence. The bulk of our simulations were completed by seeding the free-stream of boundary layers ‘pre-grown’ to a desired thickness with homogeneous isotropic turbulence from a precursor simulation. This strategy allowed us to test different combinations of the turbulence intensity and largeeddy lengthscale of the free-stream turbulence with respect to the velocity- and large-eddy length-scales of the boundary layer; such a parametric investigation would remain otherwise inaccessible given available computing resources. Additionally, the present strategy permits assessment of the direct effect of the locally present free-stream disturbances, as opposed to a ‘downstream’ effect from free-stream disturbances far ‘upstream’. The relative large-eddy turnover timescale between the free-stream turbulence and the boundary layer emerges as an important parameter in predicting if the free-stream turbulence and boundary layer interaction will be ‘strong’ or ‘weak’ before the free-stream turbulence eventually fades away. For a ‘strong’ interaction, the main action of the free-stream turbulence on the boundary layer is to cause increased spreading of the boundary layer away from the wall, which then permits incursions of free-stream fluid deep within it, changing the boundary layer velocity profiles in the outer part of the flow. This has the important effect of increasing the boundary layer thickness _ by flattening the intermittency profile.