A computational analysis is performed of self-sustained oscillatory flow over a cavity driven by a shear layer at Mach 1·5. The unsteady flow is studied through solutions of the Reynolds-averaged Navier-Stokes equations with turbulence... more
A computational analysis is performed of self-sustained oscillatory flow over a cavity driven by a shear layer at Mach 1·5. The unsteady flow is studied through solutions of the Reynolds-averaged Navier-Stokes equations with turbulence modelled by a two-equation k-ω model. The trailing edge (face) of a baseline rectangular cavity is modified using wedge and ramp shapes to investigate means for the suppression and attenuation of the self-sustained oscillation. Through modification of the shear layer impingement, both wedge and ramp are effective in reducing the level of oscillation. The time-averaged pressure (form) drag coefficient of the cavity is also reduced significantly. The main cause of the drag reduction is the elimination or reduction of the high pressure area near the downstream corner of the cavity due to the presence of a vortex. Two types of unsteady flow exist when a curved ramp is employed: ‘regular’ and ‘random’. The use of a h= 0·6D ramp generates a random type pres...
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This study deals with the wall resolved Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulation of boundary layer flow separation over a circular hump model and its active control. An array of Synthetic Jet Actuators (SJAs) is... more
This study deals with the wall resolved Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulation of boundary layer flow separation over a circular hump model and its active control. An array of Synthetic Jet Actuators (SJAs) is implemented in the hump model to introduce a train of vortex rings into the boundary layer to control flow separation. The OpenFOAM solver is used to numerically simulate and analyze the fluid flow using the k–ω shear stress transport model. Hot wire anemometry and particle image velocimetry measurements are carried out to evaluate the accuracy of the URANS technique as well as the effectiveness of SJAs by comparing numerical predictions to experimental data. The time-averaged results are in a good agreement with experimental results and demonstrate a successful application of SJAs to delay the flow separation by the interactions of vortical structures with the separated shear flow. The three-dimensional simulation also reveals that near wall coherent flow...
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Nose cones, turbine blades and bearings have rotating components and represent very practical geometries for which the modal behavior of vortex structures is not completely understood. These three rather different physical cases are being... more
Nose cones, turbine blades and bearings have rotating components and represent very practical geometries for which the modal behavior of vortex structures is not completely understood. These three rather different physical cases are being studied. A common theme of competition between modes and vortex types, whether counter-rotating or co-rotating, emerges. The objective of ongoing work is to obtain physical confirmation, enhanced understanding and predictive capability for the vortex structures encountered in rotating machines.
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April 10-15, 2016 Abstract Areas where substantial research on planar turbine cascades is ongoing, or is still needed, are identified. Compressibility effects are particularly important and are addressed in the three main sections. The... more
April 10-15, 2016 Abstract Areas where substantial research on planar turbine cascades is ongoing, or is still needed, are identified. Compressibility effects are particularly important and are addressed in the three main sections. The modeling of the shock-boundary layer interaction is not yet reliable. For supersonic speeds the agreement is excellent apart from the crucial region downstream of the shocks. At subsonic speeds the vortices were shed in a classical von Kármán vortex street. This resulted in strong base pressure deficits causing high wake losses and energy separation in the wake. The base pressure deficit and the measurements of wake energy separation coincide and it is concluded that the two phenomena are both manifestations of von Kármán vortex shedding. At Mach numbers above unity the vortex shedding was found to be one of a number of transient shedding patterns.
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Cold sprayed metallic coats can provide an appropriate surface finish for a range of applications, including marine propellers, boat keels, and ablating surfaces in aerospace. Cold spraying can coat surfaces at a comparatively lower... more
Cold sprayed metallic coats can provide an appropriate surface finish for a range of applications, including marine propellers, boat keels, and ablating surfaces in aerospace. Cold spraying can coat surfaces at a comparatively lower environmental impact with respect to plasma spray or chemically bonded sprays. In a metal cold spray, the kinetic energy imparted to the metal particles by a high speed gas provides the means for particles to plastically deform and deposit on the target substrate. Therefore, a uniform particle distribution of uniform velocity is typically desirable to achieve a good quality deposition rate and to reduce wastage from particles failing to splatter on the target substrate. This work revisits the method of characteristics technique for designing axisymmetric nozzles for generating lightly laden jets. By Computational Fluid Dynamics, the performance of current commercial cold spray nozzles is compared with new nozzle profiles, designed with a smooth throat an...
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Axisymmetric screech from an under-expanded contoured sonic nozzle is modelled by Implicit Large Eddy Simulations (ILES). A self-sustained shear-layer instability develops naturally, without artificial excitation, in the time-marching... more
Axisymmetric screech from an under-expanded contoured sonic nozzle is modelled by Implicit Large Eddy Simulations (ILES). A self-sustained shear-layer instability develops naturally, without artificial excitation, in the time-marching ILES simulation. This reproduces some of the main flow characteristics of the A2 axisymmetric screech mode documented in experiment. This includes a tonal pressure field, which is resolved by the model. The axisymmetric modelling approach prevents the development of azimuthal structures in the jet shear-layer downstream of the potential core, which are known to be important features for controlling jet mixing and broad-band jet noise.
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Three rather different physical cases have been studied. All represent very practical cases for which the modal behaviour of vortical structures is not completely understood. The work on these problems is on-going and it is hoped that... more
Three rather different physical cases have been studied. All represent very practical cases for which the modal behaviour of vortical structures is not completely understood. The work on these problems is on-going and it is hoped that physical confirmation, enhanced understanding and predictive capability for the vortical structures encountered will eventuate. EXAMPLE 1: SURFACE FLOWS OVER ROTATING CONES Experimental studies have shown that the boundary-layer flow over a rotating cone is susceptible cross-flow and centrifugal instability modes of spiral nature, depending on the cone sharpness. For half-angles (ψ) ranging from propeller nose cones to rotating disks (ψ ≥40 ° ), the instability triggers co-rotating vortices, whereas for sharp spinning missiles (ψ 40 ° . Below this half-angle it appears that an alternative instability mechanism is at work, which is not amenable to investigation using the broad-cone formulation. An alternative formulation has been developed and propertie...
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Cold spray metal coating requires accelerating metal particles to high speeds using a carrier gas to provide sufficient kinetic energy for the particles to embed in a substrate and thereby coat it. The dynamics of the carrier gas and of... more
Cold spray metal coating requires accelerating metal particles to high speeds using a carrier gas to provide sufficient kinetic energy for the particles to embed in a substrate and thereby coat it. The dynamics of the carrier gas and of the dispersed phase have a significant effect on the effectiveness and efficiency of this metal processing technique. This paper investigates the flow and particle dynamics of a sonic jet lightly laden with copper particles, by computational fluid dynamic (CFD). The model uses an Eulerian-Lagrangian coupled formulation, in which copper particles are accelerated by the compressible flow up to sonic speeds. The predictions for the jet spreading rate, the velocity centerline distribution and decay rate, and the shear layer half-velocity are consistent with published experimental work. Three numerical experiments using different copper particle distributions of Rosin-Rammler type highlighted the significant role of the particle distribution on the partic...
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Axial compressors are used in power generation and aircraft engines. In compressor design, three dimensional blading is already common. Since the flow structure in axial compressors is inherently unsteady, it is of interest to study... more
Axial compressors are used in power generation and aircraft engines. In compressor design, three dimensional blading is already common. Since the flow structure in axial compressors is inherently unsteady, it is of interest to study salient features of axial compressor flows by time-resolved approaches. This paper shows how flow separation can be affected by the periodic interaction between the wake of a preceding blade row and the vane boundary layer, and how stall inception in a highly loaded axial compressor develops. Laminar flow separation on a flat plate under periodic wake impingement is studied in the closed-loop low-speed wind tunnel of the University of Leicester. Static pressure measurements by flush-mounted taps and microphones visualize the flow separation on the flat plate. These measurements also show that the naturally occurring transitional flow separations can be calmed by periodic forcing the inflow using small gusts produced by leading-edge mounted synthetic jets...
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High-speed fighter aircraft feature low-bypass engines where the dominant noise source is the aerodynamic jet noise. As the jet is operated incorrectly expanded, it can feature an intense tonal noise, commonly referred to as a screech... more
High-speed fighter aircraft feature low-bypass engines where the dominant noise source is the aerodynamic jet noise. As the jet is operated incorrectly expanded, it can feature an intense tonal noise, commonly referred to as a screech tone. The origin of screech stems from operating the jet incorrectly expanded, whereupon a system of shock cells interacts with convected instabilities in the jet outer shear-layer, generating noise [1].
This is the author's final draft of the paper published as Proceedings of the 9th International Congress on Sound and Vibration, University of Central Florida, Orlando, FL, USA, 8-11 July, 2002, pp. 1-7.
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The laboratory e-tutor is a bespoke collection of self-running presentations in Adobe Communicator on Blackboard, written for the second year thermodynamic and fluid dynamic laboratories. The e-tutor has significantly changed the way... more
The laboratory e-tutor is a bespoke collection of self-running presentations in Adobe Communicator on Blackboard, written for the second year thermodynamic and fluid dynamic laboratories. The e-tutor has significantly changed the way laboratories are taught in Engineering by providing a bespoke step by step video guidance to each experiment in the form of an interactive presentation. Building upon the existing LabView computer based data collection and processing, the e-tutor uses the existing PCs attached to each experiment. It enhances the paper instruction sheet that is handed out at the beginning of each laboratory session by providing a multi-media content that enables students to start and run safely through the experiment and data analysis, developing their independent learning and their confidence in performing hands on engineering tasks. Students liked very much the blended learning environment of an e-tutor and a member of staff at hand to ask clarifications if in doubt an...
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Equivalent measurements to the Real Ear Measurement (REM) procedure are sought using faster and contactless Laser Doppler Vibrometry. A feasibility study has been performed on tympanicmembrane-like rubber and latex models, in which the... more
Equivalent measurements to the Real Ear Measurement (REM) procedure are sought using faster and contactless Laser Doppler Vibrometry. A feasibility study has been performed on tympanicmembrane-like rubber and latex models, in which the vibro-acoustic response was measured using both standard REM equipment and a PSV-500-HM Laser Doppler Vibrometer (LDV). The membrane surface velocity distribution was measured by the LDV and the transfer function between the forcing acoustic field and the vibration response of the membrane was calculated. The variability in the transfer function due to membrane shape, membrane type, and measurement position was determined. This study indicates that the transfer functions from measurements in the neighbourhood of the modelled umbo spot repeated several days apart were found to be consistent within typical clinical measurement tolerances. These results seem to indicate that Laser Doppler Vibrometry has the potential for replacing the standard REM proced...
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Reducing stage loss by secondary flow structures is important for improving the efficiency and the stall margin in highly loaded axial compressor stages. A numerical investigation of the NASA 37 transonic axial compressor shows the... more
Reducing stage loss by secondary flow structures is important for improving the efficiency and the stall margin in highly loaded axial compressor stages. A numerical investigation of the NASA 37 transonic axial compressor shows the significant growth of separated flow on the stator suction surface. Simulations show that about two thirds of the stator blade span is covered by separated flow at the trailing edge. The flow structure on the hub surface is affected by a cross-flow from the blade pressure side. In this work, a flow structure driven approach to the design of an end-wall treatment is used to address this flow separation. To decrease the interaction of the cross-flow with the suction surface flow, a guide groove is defined such as to guide the cross-flow driven by the passage pitchwise pressure gradient towards the trailing edge of the adjacent stator blade. The groove shape is defined by five geometry functions. These functions use six independent parameters in total to define the groove shape. A Beta distribution function is used to create a smooth connection between the blade root and the groove. The effect of these parameters on the total pressure loss and on the flow structures is evaluated through RANS simulations. The comparison between the numerical predictions with the groove against corresponding results with an axisymmetric hub, which are validated against experiment, shows a reduction in the stator trailing edge flow separation area. This gives a reduction in the total pressure loss of about 5%. Limit streamlines over the hub indicate that the groove reduces the mass flow feed to the suction side trailing edge region of flow separation. A parametric study on the groove slope and the depth is carried out to maximise this beneficial effect
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December 16-21 2017 Abstract The effects of compressibility are intrinsic to many axial flow turbomachines, is. Both subsonic and supersonic speed ranges are considered in this investigation. Subsonic surface base pressures, and wake... more
December 16-21 2017 Abstract The effects of compressibility are intrinsic to many axial flow turbomachines, is. Both subsonic and supersonic speed ranges are considered in this investigation. Subsonic surface base pressures, and wake energy separation, are a direct result of periodic von Kármán vortex shedding. This is the principal cause of both wake energy separation and the related subsonic base static pressure deficit. At high subsonic speeds a 17oC temperature difference across the wake was observed. This time-averaged temperature separation was a manifestation of the energy separation (Eckert-Weise) effect. At supersonic speeds the trailing edge base pressure, and the wake energy separation, exhibit different characteristics from the subsonic behavior. Shock waves from the trailing edge may impinge on the adjacent suction surface adversely affecting the downstream boundary layer. Supersonic flows usually cause shock and expansion waves and this may occur in steady flows. Other...
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Research Interests: Engineering, Mechanical Engineering, Aerospace Engineering, Mechanics, Particle Image Velocimetry, and 9 moreNuclear Energy and Chemical Engineering, Flow Separation, Boundary Layer, Flow Separation Control, Hot Wire Anemometry, Uncategorized, Operating parameters, Institutional Collaboration, and Synthetic Jet
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Research Interests: Mechanical Engineering, Materials Science, Heat Transfer, Nanofluid, Microchannel, and 12 moreThermal Resistance, Heat transfer enhancement, Interdisciplinary Engineering, Applied Thermal Engineering, Nusselt Number, Nano Coolant, Uncategorized, Twisted Tape, Microchannel heat sink, Heat Sink, Single Phase, and Swirl flow
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A proof of concept is provided by computational fluid dynamic simulations of a new recirculating type casing treatment. This treatment aims at extending the stable operating range of highly loaded axial compressors, so to improve the... more
A proof of concept is provided by computational fluid dynamic simulations of a new recirculating type casing treatment. This treatment aims at extending the stable operating range of highly loaded axial compressors, so to improve the safety of sorties of high-speed, high-performance aircraft powered by high specific thrust engines. This casing treatment, featuring an axisymmetric recirculation channel, is evaluated on the NASA rotor 37 test case by steady and unsteady Reynolds Averaged Navier Stokes (RANS) simulations, using the realizable k-ε model. Flow blockage at the recirculation channel outlet was mitigated by chamfering the exit of the recirculation channel inner wall. The channel axial location from the rotor blade tip leading edge was optimized parametrically over the range −4.6% to 47.6% of the rotor tip axial chord c z . Locating the channel at 18.2% c z provided the best stall margin gain of approximately 5.5% compared to the untreated rotor. No rotor adiabatic efficienc...
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The tip leakage flow over the blades of an axial compressor rotor adversely affects the axial rotor efficiency and can determine the onset of tip leakage stall. The performance of a new casing treatment concept in the shape of an... more
The tip leakage flow over the blades of an axial compressor rotor adversely affects the axial rotor efficiency and can determine the onset of tip leakage stall. The performance of a new casing treatment concept in the shape of an axisymmetric recirculation channel is explored by steady Reynolds-Averaged Navier–Stokes (RANS) realizable k-ε modelling on the NASA Rotor 37 test case. The modelling exposed a number of attractive features. The casing treatment increased the stall margin at no penalty to the rotor isentropic efficiency over the rotor operating line. A recirculation in the casing channel self-activated and self-adjusted with the rotor loading to provide more passive flow control at higher rotor loading conditions. The nozzle-shaped recirculation channel outflow opposed the tip leakage jet, re-located the casing surface flow interface further downstream, and reduced the rotor blade tip incidence angle. This combination of features makes the new casing treatment particularly ...
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A numerical investigation is conducted for the flow between two concentric cylinders with a wide gap, relevant to bearing chamber applications. This wide gap configuration has received comparatively less attention than narrow gap journal... more
A numerical investigation is conducted for the flow between two concentric cylinders with a wide gap, relevant to bearing chamber applications. This wide gap configuration has received comparatively less attention than narrow gap journal bearing type geometries. The flow in the gap between an inner rotating cylinder and an outer stationary cylinder has been modelled as an incompressible flow using an implicit finite volume RANS scheme with the realisablek-εmodel. The model flow is above the critical Taylor number at which axisymmetric counterrotating Taylor vortices are formed. The tangential velocity profiles at all axial locations are different from typical journal bearing applications, where the velocity profiles are quasilinear. The predicted results led to two significant findings of impact in rotating machinery operations. Firstly, the axial variation of the tangential velocity gradient induces an axially varying shear stress, resulting in local bands of enhanced work input to...
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A new class of cost-optimized prefactored high-order compact schemes, developed for shock-free error-bounded aeroacoustic applications, is presented. The cost-optimization theory of Pirozzoli (2007), based on the minimization of the... more
A new class of cost-optimized prefactored high-order compact schemes, developed for shock-free error-bounded aeroacoustic applications, is presented. The cost-optimization theory of Pirozzoli (2007), based on the minimization of the computational cost for a given level of numerical error, is applied to obtain a new class of time-explicit prefactored compact schemes. Suitable high-order prefactored boundary closures, which are accurate and stable within a selected Fourier space envelope, are coupled with the new interior schemes. More conventional non-reflecting boundary conditions are shown to display an impedance mismatch, thus reducing the order of accuracy of the overall scheme. An eigenvalue analysis is performed, to verify the stability of the prefactored cost-optimized schemes coupled with the boundary closures. A parallelization strategy, based on a finite-sized overlapping interface, is presented, and weak scalability tests results are shown. Good agreement is shown between the predicted percentage cost-saving of the one-dimensional cost function and the measured savings in computational time for a one-dimensional monochromatic wave propagation test. Sample applications to broadband and two-dimensional space benchmark problems clearly highlight the favourable properties of the baseline scheme for large-scale aeroacoustic applications.
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The performance of axial flow turbines is affected by losses from secondary flows that result in entropy generation. Reducing these secondary flow losses improves the turbine performance. This paper investigates the effect of applying a... more
The performance of axial flow turbines is affected by losses from secondary flows that result in entropy generation. Reducing these secondary flow losses improves the turbine performance. This paper investigates the effect of applying a non-axisymmetric contour to the hub of a representative one-and-half stage axial turbine on the turbine performance. An analytical end-wall hub surface definition with a guide groove is used to direct the pressure side branch of the horseshoe vortex away from the blade suction side, so to retard its interaction with the suction side secondary flow and thus decrease the losses. This groove design is a development of the concept outlined in Obaida et al. (2016). A baseline three-dimensional steady RANS k-ω SST model, with axisymmetric walls, is validated against reference experimental measurements from a one-and-half stage turbine at the Institute of Jet Propulsion and Turbomachinery at RWTH Aachen, Germany. The CFD predictions of the non-axisymmetric ...
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As much as two-phase mixture models resolve more physics than single-phase homogeneous models, their inconsistent heat transfer predictions have limited their use in modelling nanofluid cooled minichannel heat sinks. This work... more
As much as two-phase mixture models resolve more physics than single-phase homogeneous models, their inconsistent heat transfer predictions have limited their use in modelling nanofluid cooled minichannel heat sinks. This work investigates, addresses, and solves this key shortcoming, enabling reliable physically sound predictions of minichannel nanoflows, using the two-phase mixture model. It does so by applying the single-phase and the two-phase mixture model to a nine-passages rectangular minichannel, 3 mm deep and 1 mm wide, cooled by a 1% by volume suspension of Al2O3 nanoparticles in water, over the Reynolds number range 92 to 455. By varying the volume fraction αnf of the second phase between 2% and 50%, under a constant heat flux of 16.67 W/cm2 and 30 Celsius coolant inflow, it is shown that the two-phase mixture model predicts heat transfer coefficient, pressure loss, friction factor, exergy destruction rate, exergy expenditure rate, and second law efficiency values convergi...
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Computer-based optimization techniques can be employed to improve the efficiency of energy conversions processes, including reducing the aerodynamic loss in a thermal power plant turbomachine. In this paper, towards mitigating secondary... more
Computer-based optimization techniques can be employed to improve the efficiency of energy conversions processes, including reducing the aerodynamic loss in a thermal power plant turbomachine. In this paper, towards mitigating secondary flow losses, a design optimization workflow is implemented for the casing geometry of a 1.5 stage axial flow turbine that improves the turbine isentropic efficiency. The improved turbine is used in an open thermodynamic gas cycle with regeneration and cogeneration. Performance estimates are obtained by the commercial software Cycle – Tempo. Design and off design conditions are considered as well as variations in inlet air temperature. Reductions in both the natural gas specific fuel consumption and in CO<sub>2</sub> emissions are predicted by using the gas turbine cycle fitted with the new casing design. These gains are attractive towards enhancing the competitiveness and reducing the environmental impact of thermal power plant.
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The interaction of secondary flow with the main passage flow results in entropy generation; this accounts for considerable losses in turbomachines. Low aspect ratio blades in an axial turbine lead to a high degree of secondary flow... more
The interaction of secondary flow with the main passage flow results in entropy generation; this accounts for considerable losses in turbomachines. Low aspect ratio blades in an axial turbine lead to a high degree of secondary flow losses. A particular interest is the reduction in secondary flow strength at the turbine casing, which adversely affects the turbine performance. This paper presents a selective review of effective techniques for improving the performance of axial turbines by turbine end wall modifications. This encompasses the use of axisymmetric and non-axisymmetric end wall contouring and the use of fences. Specific attention is given to non-axisymmetric end walls and to their effect on secondary flow losses. A baseline three-dimensional steady RANS k-ω SST model, with axisymmetric walls, is validated against experimental measurements from the Institute of Jet Propulsion and Turbomachinery at the Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen, Germany, with...
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Three rather different physical cases have been studied. All represent very practical geometries for which the modal behavior of vortex structures is not completely understood. The work on these problems is ongoing with the objective of... more
Three rather different physical cases have been studied. All represent very practical geometries for which the modal behavior of vortex structures is not completely understood. The work on these problems is ongoing with the objective of obtaining physical confirmation, enhanced understanding and predictive capability for the vortex structures encountered in rotating machines. 1. Surface Flows over Rotating Cones. Experimental studies have shown that the boundarylayer flow over a rotating cone is susceptible to crossflow and centrifugal instability modes of a spiral nature, depending on the cone sharpness. For apex half-angles (ψ) ranging from propeller nose cones to rotating disks (ψ ≥40 ◦ ), the instability triggers co-rotating vortices, whereas for sharp spinning missiles (ψ <40 ◦ ), counterrotating vortices are observed. A mathematical description is provided of the onset of co-rotating vortices for a family of cones rotating in quiescent fluid, with a view towards explaining ...
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The aerodynamic noise radiating from single and dual-stream jets is by acoustic analogy from time-resolved Computational Fluid Dynamic (CFD) simulations. For this purpose, two Ffowcs Williams and Hawkings (FW-H) solvers were developed,... more
The aerodynamic noise radiating from single and dual-stream jets is by acoustic analogy from time-resolved Computational Fluid Dynamic (CFD) simulations. For this purpose, two Ffowcs Williams and Hawkings (FW-H) solvers were developed, based on an advanced time formulation (AFW-H) and on a convective formulation (CFW-H). The methods are coded in Python and embedded in Antares, a CFD post-processor developed by Cerfacs, France. The new FW-H solvers were tested on a hierarchy of noise sources of increasing complexity. The radiating field from elementary acoustic sources was considered first, progressing then to single-stream and dual-stream jets. CFD results obtained at Cerfacs by Large Eddy Simulation and at the University of Leicester by Detached Eddy Simulation provided the input to the acoustic analogy to estimate the noise radiation from jets. The jet noise predictions are compared against acoustic results obtained numerically by the elsA software (ONERA, France) and against soun...
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April 10-15, 2016 Abstract A wide-ranging taxonomy of turbomachinery types is given. The requirements of steam turbines and aircraft engines established a traditional approach to aerodynamic design analyzing orthogonal planes. Computers... more
April 10-15, 2016 Abstract A wide-ranging taxonomy of turbomachinery types is given. The requirements of steam turbines and aircraft engines established a traditional approach to aerodynamic design analyzing orthogonal planes. Computers facilitated this and led to a greater reliance on Computational Fluid Dynamics, offering exciting developments in three dimensional modeling. A balance should be sought between analytical, computational and experimental work. In work on an axial flow pump rig photographic investigations of cavitation over the rotor tip have given insights including the very abrupt collapse of cavitation bubbles. Although essential for supersonic regions blade sweep can also be used effectively at lower speeds and can provide significant performance improvements. Further integration of the design and research communities should lead to an improved understanding and predictability. One such area is the unexpected appearance of streamwise vortices on blade suction surfa...
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Presented at Fluid mechanics : an interdisciplinary approach. Experimentatin, analytics, numerical analysis and high performance computing, Madingley Hall, Cambridge 23rd to 25th July 2014.
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This paper concerns unsteady near-wake flows on, and close to, the thick trailing edges of turbine blades, circular cylinders and similar bodies. Subsonic surface base pressures, and EckertWeise energy separation in the wake, are... more
This paper concerns unsteady near-wake flows on, and close to, the thick trailing edges of turbine blades, circular cylinders and similar bodies. Subsonic surface base pressures, and EckertWeise energy separation in the wake, are principal manifestations of the same phenomenon. Both are a direct result of von Kármán vortex shedding. The subsonic flow past a turbine blade having a thick trailing edge is still not well-predicted and this results from a lack of understanding of the flow past the trailing edge and into the wake. It is here argued that von Kármán vortex shedding is the principal cause of the subsonic base pressure deficit and the related energy separation in the wake. Parallels can be found in the behaviour of elastically-mounted circular cylinders and the caudal fin oscillation propelling fish. These should also affect supersonic flows although the physical causes are different. At supersonic speeds the trailing edge base pressure, and the energy separation in the downs...
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Tip stall inception in a high speed jet axial rotor, if induced at a critical phase in a sortie, can compromise the stability of the engine and undermine the aircraft safety. Steady and time-resolved Computational Fluid Dynamics (CFD) is... more
Tip stall inception in a high speed jet axial rotor, if induced at a critical phase in a sortie, can compromise the stability of the engine and undermine the aircraft safety. Steady and time-resolved Computational Fluid Dynamics (CFD) is used to investigate the flow through a highly loaded axial rotor, at a stage pressure ratio representative of future highly manoeuvrable combat aircraft systems. The base flow from which tip stall onsets and the early stages of tip stall are modelled, reproducing the salient stages of the flow processes leading to an axial compressor rotating stall, by flow separation at the rotor tip. A casing treatment concept aimed at mitigating this stall type is explored by CFD. The treatment features a channel that provides passive flow control by flow recirculation. Some novel design aspects of the channel geometry are considered. A numerical investigation of the channel parameter space, by varying the axial length, achieved a 9.11% improvement in the compres...
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Progress is made in the validation and use of a shareable, royalty-free, non-commercial Computational AeroAcoustics (CAA) post-processor for predicting the far-field noise radiated by acoustically active transonic flows. Building upon... more
Progress is made in the validation and use of a shareable, royalty-free, non-commercial Computational AeroAcoustics (CAA) post-processor for predicting the far-field noise radiated by acoustically active transonic flows. Building upon past validation tests of this Ffowcs Williams and Hawkings CAA post-processor, which is part of the Antares software suite, predictions are obtained of a single isothermal Mach 0.9 jet at a Reynolds number of approximately half million. A more challenging application to a dual-flux transonic isothermal jet issuing from two axisymmetric coaxial nozzles, with axially staggered nozzle exit planes, shows salient spectral roll-off rates and directivity patterns typical of this geometry, which represents an idealised turbofan engine exhaust configuration. This builds confidence in the application of this new tool to aeroacoustic research as well as to aircraft design.
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Sustained demand for more fuel efficient, environmentally friendly, and quieter aircraft power plants is promoting the development of higher bypass engines for large transport aircraft. These turbofan engines feature a fan of larger... more
Sustained demand for more fuel efficient, environmentally friendly, and quieter aircraft power plants is promoting the development of higher bypass engines for large transport aircraft. These turbofan engines feature a fan of larger diameter, providing a larger fraction of the thrust by expansion through the secondary (annular) nozzle. This fixed-geometry nozzle is typically run choked at cruise conditions, generating an under-expanded co-axial jet with a significant broadband shock-associated noise signature. The three-dimensional shock-shear layer flow dynamics of a dual stream jet was modelled by Detached Eddy Simulations, using a simplified staggered coaxial round dual-nozzle geometry without a central plug. Predictions were obtained using an in-house up to third order space accurate finite-volume scheme based on the Roe approximate Riemann solver, using a tuneable Sweby flux limiter for capturing the unsteady shocks. Flow conditions were taken as representative of secondary noz...
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Training the next generation of aeroacoustic researchers is vital for addressing some difficult yet very important societal challenges, such as the provision of environmentally sustainable air transport operations, both for what concerns... more
Training the next generation of aeroacoustic researchers is vital for addressing some difficult yet very important societal challenges, such as the provision of environmentally sustainable air transport operations, both for what concerns airport noise pollution and cabin noise. Sustained demand for more fuel efficient, environmentally friendly, and quieter aircraft power plants is promoting the development of ultra-high bypass engines for large transport aircraft. These turbofan engines require trained aero-acousticians and improved industrial design tools, to tackle some of the difficult design challenges posed by the larger diameter and by the larger fraction of the thrust delivered through the secondary (annular) nozzle. Whilst aeroacoustics is a well-established discipline in its own right, it tends not to be offered as an undergraduate course. This creates a recruitment challenge in doctoral training, with new post-graduate students being recruited across a range of qualificati...
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The generation of a high velocity carrier gas flow for cold metal particle applications is addressed, with specific focus on titanium cold spraying. The high hardness of this material makes cold spraying titanium difficult to achieve by... more
The generation of a high velocity carrier gas flow for cold metal particle applications is addressed, with specific focus on titanium cold spraying. The high hardness of this material makes cold spraying titanium difficult to achieve by industry standard nozzles. The redesign of a commercial conical convergent-divergent cold spray nozzle is achieved by the application of aerospace design codes, based on the Method of Characteristics, towards producing a more isentropic expansion by contouring the nozzle walls. Steady three-dimensional RANS SST k-ω simulations of nitrogen are coupled two-way to particle parcel tracking in the Lagrangian frame of reference. The new contoured nozzle is found to produce higher particle velocities with greater radial spread, when operated at the same conditions/cost of operation as the commercial nozzle. These numerical results have shown the potential for extending cold spray to high density and low ductility particles by relatively minor rig modificati...
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Abstract The hydraulic and thermal performance of microchannel heat sink configurations for high performance electronic cooling applications is investigated by numerical modelling. Conjugate heat transfer simulations are obtained through... more
Abstract The hydraulic and thermal performance of microchannel heat sink configurations for high performance electronic cooling applications is investigated by numerical modelling. Conjugate heat transfer simulations are obtained through the silicon walls and the fluid domain of a square base prism heat sink traversed by 50 parallel rectangular cooling ducts, under a 150 W/cm2 constant heat flux input through the base. Al2O3 nanofluid coolant with a nanoparticle volume fraction ranging from 0 to 3% is supplied at 298 K, over the Reynolds number range 100 to 350, modelled as a single-phase homogeneous medium. Rectangular, twisted, and zig-zag fins are inserted into the plain rectangular duct to enhance the heat transfer rate. The zig-zag fin and 3% Al2O3 nanofluid provide the best thermal performance, with a 6.44 K lower average heated wall contact temperature, 60% higher Nusselt number, and 15% higher second law efficiency than without fins and plain water cooling. Twist in the microchannel fin unexpectedly reduced the microchannel pressure drop by 2% to 15% compared to a straight fin, possibly due to the more evenly distributed axial mass flux across the microchannel.