Aimee S. Morgans

Short circular apertures subject to turbulent bias flow underpin acoustic damping and fluid delivery across a range of applications, including next-generation hydrogen combustors. However, flow separation at sharp edges leads to a drop in the aerodynamic discharge coefficient (Cd) and can trigger ``whistling'' -- acoustic amplification driven by a feedback loop between shed vorticity and acoustic waves. To address these issues, the present study investigates shape optimisation of chamfers at the upstream and downstream circular aperture edges, initially for acoustic and aerodynamic performance under isothermal conditions. The approach combines a high-fidelity hybrid numerical framework coupled with Bayesian optimisation. Under adiabatic conditions, a Pareto analysis reveals acoustic and aerodynamic optimisation trade-offs governed by asymmetry in the flow-acoustic interactions. While large downstream chamfers act as acoustic dampers by increasing the distance between the shed vorticity and downstream edge, they also promote aerodynamic efficiency losses through large-scale flow separation. Conversely, large upstream chamfers function as aerodynamic nozzles, maximising Cd through separation suppression while achieving acoustic damping via trailing-edge shedding of vorticity. Exploration of the operational space further uncovers a topological bifurcation: geometric conditioning at the upstream edge leads to an abrupt transition to attached flow, eliminating the vena contracta and creating a distinct, physically inaccessible gap in the aerodynamic design space. Finally, the framework is extended to heat-transferring conditions by incorporation of the linearised energy equation in the numerical simulations. A new optimisation of the orifice geometry confirms that while positive mean temperature gradients provide passive thermodynamic damping, the core whistling suppression mechanisms remain the same as for the adiabatic case. Ultimately, these findings establish robust design guidelines for practical bias-flow apertures, demonstrating how tailored edge chamfering can balance aeroacoustic stability, aerodynamic efficiency, and convective heat transfer in real-world combustor environments.

Turbulent hydrogen flames with varying operating conditions in the swirl-axial air injection AHEAD combustor were studied computationally with a multi-regime flame closure method, combustion LES / Stochastic fields. The method was validated by comparisons with measurements in isothermal and reacting flows. The velocity fields, flames, mixing fields and thermo-chemical states were analysed in detail. Further comparisons were carried out for different operating conditions to study the effect of global equivalence ratio and axial air injection ratio. On the one hand, it introduces higher axial momentum, which restricts flashback. On the other hand, increasing the global equivalence ratio or axial air injection ratio negatively affects the spatial mixing quality where the axial momentum flux plays an important role. The results also suggest that thermo-chemical states are dominantly controlled by the global equivalence ratio rather than the inlet reactant temperature or flow conditions. The effect of differential diffusion was also studied. Differential diffusion slightly increases the possibility of the upstream occurrence of the flame inner branch, which results in the inner flame branch brush becoming broader. This was found to be related to the changes of the upstream mixing field due to differential diffusion. Nevertheless, the global system is negligibly influenced by differential diffusion due to the high Reynolds number.

The turbulent wake past a square-back Ahmed body in close proximity to the ground experiences random side-to-side switching between two asymmetric positions, a phenomenon known as bi-modality. It has been observed to be sensitive to the dynamics of the upstream boundary layers formed along the body surfaces. Close to the body fore end, these separate and reattach, with hairpin vortices emanating from the reattachment points and growing along the surfaces before breaking down upstream of the base. This study uses wall-resolved large eddy simulations to investigate the effect of using suction to suppress these upstream boundary layer separations on the wake bi-modality. It is seen that, in the unforced flow (in the absence of suction), the smaller top and side surface vortices resulting from breakdown interact as they convect downstream. Steady suction is confirmed to suppress the boundary layer separations on the different body surfaces. When the boundary layer separations on the two side (vertical) surfaces are suppressed, it is found that horizontal bi-modality is completely inhibited with weak vertical asymmetry preserved. The interaction of the small top/side surface vortices is interrupted, damping boundary layer fluctuations just upstream of the base. Applying suction on different combinations of side/top/bottom boundary layer separations is found to have different effects on the underbody flow and the wake vertical balance, with bi-modality suppression dependent on side surface suction. This confirms that bi-modality is triggered, at least in part, by boundary layer disturbances on the surfaces perpendicular to the switching direction.

With the severity and frequency of significant weather events increasing, methods for alleviating unsteady wind loading for high-rise buildings are gaining interest. This study numerically investigates the three-dimensional flow structures around a canonical high-rise building immersed in an atmospheric boundary layer at different oncoming wind angles, using wall-resolved large eddy simulations. A synthetic jet located on the top surface is used as open-loop active actuation with the aim of suppressing the building's side-force fluctuations when exposed to oncoming wind variations. Three different frequencies of jet forcing are considered, all half an order of magnitude larger than the vortex shedding frequency. The behaviour of the synthetic jet and its effect on the building's unsteady side force, time-averaged flow fields and unsteady flow structures are investigated numerically. The synthetic jet actuation is found to reduce the side-force fluctuation of the building, enhance the downwash flow and successfully attenuate the antisymmetric vortex shedding. This was achieved to different extents across the range of oncoming wind angles considered and may motivate future attempts to explore experimental active control strategies for attenuation of unsteady wind loading.

Ammonia is a carbon-free fuel which could be used in gas turbines. Ammonia’s potential as a fuel can be improved when mixed with hydrogen. However, this could cause an increased propensity to thermoacoustic oscillations in the combustor. The acoustic response of hydrogen/ammonia flames was evaluated by calculating the flame transfer functions (FTFs). A local level set approach, combined with an incompressible velocity perturbation model, was used to calculate the flame front response to different frequencies of acoustic oscillations. The unsteady heat release rate was calculated from the flame front surface area, obtained by solving the G-equation, thereby calculating the gain and phase of the FTF. The unstretched flame speed, was obtained from experimental values and two chemical kinetic models: CRECK-NH3 and GRI-Mech3.0. The models’ accuracy was assessed by comparing the modelled to experimental values from literature. CRECK-NH3 was fitter for ammonia/hydrogen modelling, as it was always within 12% of experimental values, compared to GRI-Mech3.0 which always differed by over 35%. The FTFs suggested an increase in hydrogen enrichment led to an increase in the flame acoustic response, as the gain drop off occurred at a higher frequency with higher hydrogen content. The flame acts as a low-pass filter to acoustic waves and the bandwidth of this filter (the frequency at which the gain drops off) increases with hydrogen content. This was due to higher flame speeds with higher hydrogen content. These FTFs were also compared to those of hydrogen/methane flames and the non-linear response was briefly analysed.

Entropy noise is the sound produced by the acceleration of convective temperature fluctuations (entropy waves) by non-uniform flows and is a major contributor to indirect combustion noise. Entropy waves are accelerated by nozzles in rocket engines or by the turbine blade rows in the combustors of gas turbines. While several analytical models have been developed for entropy noise generated by nozzles, not many efficient models exist for the case of blade rows. This is due to the more complex physical mechanisms involved: when entropy waves pass through cascades of blades, sound is produced due to the acceleration and turning of the entropy waves in the interblade region, but also unsteady forces are created in the blades and vorticity is shed from the trailing edge of the aerofoils. All these mechanisms are strongly coupled and affect the generated entropy noise. In this work, we explore numerically the importance of these mechanisms. To this end, we present time- and frequency-domain simulations of entropy waves interacting with several canonical aerofoils. Finally, a simple model is presented to estimate the entropy noise generated by each configuration.