Browsing by Author "Knowles, Kevin"

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    Aerodynamic characteristics of a wing-and-flap configuration in ground effect and yaw
    (IMechE, 2015-08-11) Roberts, L.; Correia, J.; Finnis, Mark V.; Knowles, Kevin
    The influence of the yaw angle on a model representative of a monoposto racing car’s front wing and nose section operating in close proximity to the ground is discussed. The yawed condition is representative of a car operating in a crosswind or with side-slip while cornering. Because of the need for downforce in corners rather than on a straight, it is standard practice to test a racing car at various orientations of yaw, pitch and roll quasi-statically. Wind tunnel testing with a 50%-scale model at a unit Reynolds number of 1.69 × 106 was used to investigate the forces and the surface flow structures. The results were then used to validate simulations with the three-equation k–kL–ω transitional turbulence model to observe the surface pressures and the wake structures. It was found that a change in the surface pressure caused asymmetric loading of the wing, the strengthening or inhibiting of vortices depending on their rotational sense and an overall reduction in both the downforce and the drag of the wing; all these were amplified as the yaw angle was increased or the ground clearance reduced. The fundamental aerodynamic flow features of a racing car’s front wing operating at yaw are established.
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    The aerodynamic interference effects of side walll proximity on a generic car model
    (2010-11-03) Strachan, R K; Knowles, Kevin
    The flow around a generic car model both in isolation and in proximity to a near side wall has been investigated utilising experimental and computational methods. Phase one of this investigation tested a range of Ahmed generic road vehicle models with varying backlight angles in isolation, employing laser-Doppler anemometry, static pressure and aerodynamic force and moment measurements in the experimental section. Additionally, numerical simulations were conducted using a commercial Reynolds-averaged Navier Stokes (RANS) code with the RNG k-ε turbulence model. This phase served both to extend the previous knowledge of the flow around the Ahmed model, and analyse the effects of both the supporting strut and rolling road. Phase two then used similar methods to investigate the Ahmed model in proximity to a non-moving side wall. Results from phase two are compared with previous near-wall studies in order that an understanding of the effects of wall proximity can be presented, an area lacking in the existing literature. It is found that the flow on the isolated model must be understood before the effects of side wall proximity can be assessed. There is though, in general, a breakdown of any longitudinal vortices on the near-wall side of the model as model-to-wall distance reduces, with an increase in longitudinal vortex strength on the model side away from the wall. There also exists a large pressure drop on the near-wall model side, which increases in magnitude as model-to-wall distance reduces, before dissipating at separations where the boundary layer restricts the flow. Additionally, there is found to be a pressure drop on the top and bottom of the model with decreasing wall distance, with the relative magnitudes of these dependent on model geometry.
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    Aerodynamic problems of urban UAV operations
    (2011-09-09) Kittiyoungkun, S; Saddington, Alistair J.; Knowles, Kevin
    Unmanned Air Vehicles, UAVs are designed to operate without any onboard controllers. Consequently, they are considered to operate in a wide range of applications. Missions in undesirable conditions such as bad weather and/or highly unsteady gustiness could cause an unsuccessful operation. In many ways, aerodynamics is a key feature in the performance of UAVs such as influencing deformation vehicle, guidance and control. Two aspects of this research are, therefore, to understand flying conditions of UAVs in an urban environment and how the flying performance is affected by such conditions. The first objective relies on understanding air flow behaviour in the lower part of the urban environment which has the most important role on the response of UAVs. The second objective will be to look at the characteristics of a three-dimensional airfoil when it encounters an unsteady sinusoidal gust at different oscillation frequencies and freestream velocities. As the first step of the studies on the aerodynamic problem of UAV operations in the lower part of an atmospheric boundary layer in an urban environment, the boundary layer thickness in a suitable wind tunnel facility were the first experimental results obtained. Experimental measurements of the mean velocity profile in a turbulent boundary layer were investigated for three different floor roughness conditions as well as a smooth wall condition. As a result, three different boundary layer thicknesses were then classified depending on the wall surface roughness and a combination with turbulence generators providing a maximum thickness of 280 mm at the centre of the tunnel test section. However,the experimental investigations into the turbulent boundary layer over a rough wall have shown that the boundary layer thickness is dependent on the surface roughness and is different from that obtained under the smooth wall condition. An experimental study into a simulated urban flow regime was then carried out after the measurement of the boundary layer. Wind tunnel experiments on the airflow around a single and twin buildings including an investigation of the airflow between the gap of the buildings were obtained. Wind in the lower part of the atmospheric boundary layer is more a micro-scale problem which increases or decreases the wind speed induced by buildings nearby. The studies have found some strong concentrated vortices caused by the flow separation essentially independent of the nature of the upstream flow and usually as a direct result of the building geometry and orientation. As the measurement location increased further downstream from the back of the buildings, the concentrated vortices were found to be weak and disappeared into the wake region. Finally, an experiment was conducted using a sinusoidal gust generator to describe the effects of wind oscillation parameters such as oscillation amplitude, oscillation frequency and reduced frequency under static and dynamic conditions. An evaluation was made of the onset of dynamic stall due to rapid changes in angle of attack during an unsteady pitch motion. The NACA 23012 wing profile was tested at a fixed angle of attack condition with varying oscillation flow parameters. Results demonstrate that those parameters influence the dynamic stall and hysteresis loop based on lift coefficient and angle of attack
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    Aerodynamics and performance enhancement of a ground-effect diffuser
    (2018-04) Ehirim, O H; Knowles, Kevin; Saddington, Alistair J.
    This study involved experimental and equivalent computational investigations into the automobile-type 3―D flow physics of a diffuser bluff body in ground-effect and novel passive flow-control methods applied to the diffuser flow to enhance the diffuser’s aerodynamic performance. The bluff body used in this study is an Ahmed-like body employed in an inverted position with the slanted section together with the addition of side plates along both sides forming the ramped diffuser section. The first part of the study confirmed reported observations from previous studies that the downforce generated by the diffuser in proximity to a ground plane is influenced by the peak suction at the diffuser inlet and subsequent static pressure-recovery towards the diffuser exit. Also, when the bluff body ride height is gradually reduced from high to low, the diffuser flow as indicated by its force curve and surface flow features undergoes four distinct flow regimes (types A to D). The types A and B regimes are reasonably symmetrical, made up of two low-pressure core longitudinal vortices travelling along both sides of the diffuser length and they increase downforce and drag with reducing ride height. However, below the ride heights of the type B regime, types C and D regimes are asymmetrical because of the breakdown of one vortex; consequently a significant loss in downforce and drag occurs. The second part of the study involved the use ― near the diffuser exit ― of a convex bump on the diffuser ramp surface and an inverted wing between the diffuser side plates as passive flow control devices. The modification of the diffuser geometry with these devices employed individually or in combination, induced a second-stage pressure-drop and recovery near the diffuser exit. This behaviour was due to the radial pressure gradient induced on the diffuser flow by the suction surface ii curvature of the passive devices. As a result of this aerodynamic phenomenon, the diffuser generated across the flow regimes additional downforce, and a marginal increase in drag due to the profile drag induced by the devices.
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    Aerodynamics of a convex bump on a ground-effect diffuser
    (Journal of Fluids Engineering, 2018-04-19) Ehirim, Obinna; Knowles, Kevin; Saddington, Alistair J.; Finnis, Mark V.
    A ground-effect diffuser is an upward-sloping section of the underbody of a racing car that enhances aerodynamic performance by increasing the downforce, thus improving tire grip. The downforce generated by a diffuser can be increased by geometric modifications that facilitate passive flow control. Here we modified a bluff body equipped with a 17° diffuser ramp surface (the baseline/plane diffuser) to introduce a convex bump near the end of the ramp surface. The flow features, force and surface pressure measurements determined in wind-tunnel experiments agreed with previous studies but the bump favorably altered the overall diffuser pressure recovery curve by increasing the flow velocity near the diffuser exit. This resulted in a static pressure drop near the diffuser exit followed by an increase to freestream static pressure, thus increasing the downforce across most of the ride heights we tested. We observed a maximum 4.9% increase in downforce when the modified diffuser was compared to the plane diffuser. The downforce increment declined as the ride height was gradually reduced to the low-downforce diffuser flow regime.
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    Boundary-layer transition on wings in ground effect
    (2017-12) Roberts, L. S.; Finnis, Mark V.; Knowles, Kevin
    The competitiveness of a high-performance racing car is extremely reliant on aerodynamics. Due to the current economic climate, track testing is often forsaken and the majority of aerodynamic development carried out using sub-scale wind tunnel testing and computational simulations. It is important, therefore, that experimental and computational approaches represent real-world conditions as closely as possible. Although racing cars travel at much higher speeds than typical passenger cars, in comparison to aircrafts they still operate at relatively low Reynolds numbers and, consequently, laminar and transitional phenomena are evident. Despite this, the bulk of relevant literature available for racing-car aerodynamics is undertaken with little regard to the influence of Reynolds number, and in the case of computational studies, the omission of laminar and transitional phenomena all together. The present work has demonstrated, using a super-scale two- dimensional wind-tunnel model, that laminar and transition flow phenomenon are important at Reynolds numbers equivalent to a full-scale racing car. Moreover, the influence of these aspects increased as the wing’s ground clearance reduced; meaning that in ground effect they are even more important. Further experiments with three-dimensional models of varying complexity, from a simple single-element wing to a highly complex F1-specification wing, showed that laminar phenomena are important for F1 applications as well as for lower-downforce capability racing cars. A transition-sensitive eddy-viscosity turbulence model, k-kL-w, was used to simulate inverted wings operating in ground effect. It was shown that that laminar and transitional flow states could be simulated easily inside a commercial solver, and that the model offered a substantial improvement over the classical fully-turbulent k-w SST in terms of both force coefficient prediction and surface-flow structures. This experiments and computational simulations described in this thesis show the Reynolds number sensitivity of, and importance of laminar phenomenon on, wings operating in ground effect. It has been shown that laminar boundary layers are an important aspect of the flow characteristics of wings in ground effect, at both full-scale and model-scale Reynolds numbers. As such, it is recommended that future studies incorporate laminar and transitional phenomena.
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    Characteristics of boundary-layer transition and Reynolds-number sensitivity of three-dimensional wings of varying complexity operating in ground effect
    (American Society of Mechanical Engineers, 2016-06-03) Roberts, Luke S.; Finnis, Mark V.; Knowles, Kevin
    The influence of Reynolds number on the aerodynamic characteristics of various wing geometries was investigated through wind-tunnel experimentation. The test models represented racing car front wings of varying complexity: from a simple single-element wing to a highly complex 2009-specification formula-one wing. The aim was to investigate the influence of boundary-layer transition and Reynolds-number dependency of each wing configuration. The single-element wing showed significant Reynolds-number dependency, with up to 320% and 35% difference in downforce and drag, respectively, for a chordwise Reynolds number difference of 0.81 × 105. Across the same test range, the multi-element configuration of the same wing and the F1 wing displayed less than 6% difference in downforce and drag. Surface-flow visualization conducted at various Reynolds numbers and ground clearances showed that the separation bubble that forms on the suction surface of the wing changes in both size and location. As Reynolds number decreased, the bubble moved upstream and increased in size, while reducing ground clearance caused the bubble to move upstream and decrease in size. The fundamental characteristics of boundary layer transition on the front wing of a monoposto racing car have been established.
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    Comparison of passive flow control methods for a cavity in transonic flow
    (AIAA, 2016-03-21) Saddington, Alistair J.; Thangamani, Varun; Knowles, Kevin
    Acomparative study of different passive control techniques was conducted on a cavity with a length of 320mmwith length-to-depth and length-to-width ratios of five and two, respectively. The tests were conducted at a freestream Mach number of 0.71. Both leading-edge and trailing-edge modifications were included in the studies. Results from surface pressure measurements showed that leading-edge control techniques were more effective at suppressing cavity tone amplitudes than trailing-edge modifications.Asquare-tooth spoiler showed the greatest reduction in tonal amplitude (8.8 dB); however, a sawtooth spoiler showed the greatest reduction in overall sound pressure level (8.13 dB). Velocity measurements inside the cavity were made using particle image velocimetry for the clean cavity and the cavity with sawtooth spoilers. The results showed a reduction in momentum exchange between the freestream flow and the cavity when spoilers were used. This is proposed to be the main reason for the reduced tonal amplitudes.
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    Computational aeroacoustic study of a landing gear
    (2009-11-10T00:00:00Z) Khanal, Bidur; Knowles, Kevin; Saddington, Alistair J.; Obayashi, S.
    Computational study of a single wheel landing gear con guration was completed to understand the noise source and it's nature. The ow eld visualisation showed the present of large structural shedding in the wake side of the the landing gear wheel. These large structures were responsible for the low frequency noise. Spectral peaks at frequencies lower than 200 Hz were found to exist from the analysis of the frequency content of the pressure signals at far eld. These low frequency peaks were due to the large structural shedding.
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    Computational investigation of cavity flow control using a passive device
    (Royal Aeronautical Society, 2012-12-31T00:00:00Z) Khanal, Bidur; Knowles, Kevin; Saddington, Alistair J.
    In this paper, the results of computational studies on the unsteady flow features in threedimensional empty cavities and cavities with a representative store are presented. Flow simulations with a turbulence model based on a hybrid method, which behaves as a standard Reynolds-averaged Navier-Stokes (RANS) model within the attached boundary layer and as a Large-Eddy Simulation LES Sub-Grid Scale model in the rest of the flow (commonly known as Detached-Eddy Simulation [DES]) are used in this study. The time-mean flow study showed the presence of three-dimensional effects inside the cavities. The mean flowfield visualisation also clearly showed the presence of a pair of 'tornado-like' vortices in the upstream half of the cavity which merge to a single, large recirculation further downstream. Visualisation for the cavity-with-store case revealed that the mean flowfield was effectively divided into two halves with signifiant reduction of the spanwise flow across the cavity width. In the unsteady flow study, near-field acoustic spectra were computed for the empty cavity and cavity-with-store cases. Study of unsteady pressure spectra for the cavitywith- store case found the presence of many peaks and the corresponding mode frequencies were found to agree well with the Rossiter modes. The blockage effect of store and strut on the spanwise flow is thought to have reduced the interaction, and subsequent non-linear coupling, between the Rossiter modes. This may be the reason for the co-existence of multiple modes without the coupling among them.
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    Computational study of cavity flowfield at transonic speeds
    (2009-12-31T00:00:00Z) Khanal, Bidur; Knowles, Kevin; Saddington, Alistair J.
    In this paper, the results of a computational study on the unsteady flow features in three-dimensional empty cavities and a cavity with a store are presented. Flow simulations with a turbulence model based on a hybrid method, which behaves as a standard RANS model within the attached boundary layer and as a LES Sub-Grid Scale model in the rest of the flow, including the separated regions, are used in this study. The time-mean flow study showed the existence of spanwise flow in the 3D cavity. In the unsteady flow study, computed near- field acoustic spectra were for empty cavity as well as cavity-withstore cases. Unsteady results from an empty cavity case are compared with experimental data and the frequency of the dominant mode is in good agreement with the experiment. Study of unsteady pressure spectra for the cavity-with-store case found the presence of many peaks and the corresponding mode frequencies were found to agree well with the Rossiter modes. The mean flowfield visualisation for the cavity-with-store case clearly showed that the mean flowfield was effectively divided into two halves with signifiant reduction of the spanwise flow across the cavity width. This blockage effect of store and strut on the spanwise flow is thought to have reduced the interaction, and subsequent non-linear coupling between, the Rossiter modes. This may be the reason for the coexistence of multiple modes without the coupling among them.
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    Controlling unsteady cavity flows using internal structures
    (La Societe Savante de L'Aeronautique et de L'Espace, 2017-03-29) Knowles, Kevin; Bacci, David; Saddington, Alistair J.; Newby, B.; Taylor, N. J.
    We report experimental measurements and preliminary analysis on a series of geometric modifications to a rectangular cavity, aimed at alleviating the severity of the aeroacoustic environment. The cavity had a length-to-depth ratio of 5 and a width-to-depth ratio of 1, and featured a simplified representation of a generic missile on the centre line. The modifications consisted of full width and depth ribs or “collars” with a cutout for the missile. Collars could be fitted at various combinations of locations in the cavity and were either straight (i.e. perpendicular to the cavity centre line), leaned or yawed. The cavity flowfield was characterised by surface pressure measurements along the ceiling. Judging from the available measurements the presence of collars modified the mean pressure distribution, and appeared to reduce the acoustic tones and generally lower the broadband noise.
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    Density Measurements for Rectangular Free Jets Using Background Oriented Schlieren
    (Royal Aeronautical Society, 2013-08-30T00:00:00Z) Tipnis, T. J.; Finnis, Mark V.; Knowles, Kevin; Bray, Derek
    An experimental study incorporating the use of the Background-Oriented Schlieren (BOS) technique was performed to measure the density field of a rectangular supersonic jet. This technique is easier to set up than conventional schlieren since the optical alignment involving the various mirrors, lenses and knife-edge is replaced by a background pattern and a single digital camera. The acquired images which contain information of density gradients in the flow are solved as a Poisson equation and further processed using deconvolution and tomographic algorithms to generate a 3-D domain which contains information about the actual density. 2-D slices can then be extracted to quantitatively visualise the density along any required planes. The results from supersonic axisymmetric jets are used for validation of the code; these show excellent agreement with pre-validated CFD data. The results for a rectangular supersonic jet are then obtained. These show good agreement with the CFD data, in terms of shock-cell spacing and overall structure of the jet. The technique has proved useful for investigating axis-switching, a phenomenon generally associated with non-axisymmetric jets.
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    Design of a Tube Launched Man Portable Unmanned Aerial Vehicle
    (2007-03-29T12:49:08Z) Proctor, S; Knowles, Kevin; Bray, Derek
    The aim of the MSc was to design a man portable tube launched Unmanned Aerial Vehicle (UAV). The resulting UAV has a vertically staggered wing layout and a V-tail. The UAV has folding wings and tail to allow the deployment from a tube and is small in size to allow it to be man portable. Several design variants were experimented with to verify the staggered wing configuration. The outcome is that a small UAV suitable for tube launching is possible, although the UAV has yet to be launched from a tube.
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    Determining the extent to which simulation can be used to train RAF pilots to fly and fight the Eurofighter Typhoon
    (2017-06-19) Allsop, A J; Knowles, Kevin; Saddington, Alistair J.
    This research examines the extent to which simulation can be used to train pilots of the Royal Air Force to fly and fight the Eurofighter Typhoon, and is the culmination of a series of trials over a period of 4 years. The approach was threefold, firstly examining the performance of students trained entirely on the Operational Conversion Unit’s full syllabus in the simulator and then tested against their peers on each of the four phases in live flight, secondly investigating the cultural acceptance levels of the present Typhoon pilots and lastly using lessons learnt to generate and test a syllabus to train Typhoon pilots to Mult Role Combat Ready in 40% of the present time. It was found that increasing the proportion of synthetics from the lowest Live Synthetic Balance (LSB) of 75:25 used on the front-line meets a cultural and resource barrier at 50:50. This did not represent the maximum LSB achievable however with the heavily synthetic Multi-Role Syllabus reaching an LSB of 21:79 with successful completion of the end of course test. Cultural acceptance of the simulator had correlations with the squadron a pilot was assigned to, the manner in which the simulators were programmed for use and the experience level of the pilot. No evidence was found within the sample to suggest age had an effect. Recommendations on minimum proportions of live and synthetic training was mapped for each of the required tasks and comparisons of these were made across complexity levels. Resource savings found by the generating and testing a Multi Role Combat Ready syllabus that recognised and incorporated all the strengths, weaknesses and lessons identified in the previous trials generated a saving of approximately 9 months and 100 Typhoon live flying hours per student, equivalent to approximately 1300 man maintenance hours that could be reinvested into personnel in the form of leave, adventurous training or development.
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    The effect of aspect ratio on the leading-edge vortex over an insect-like flapping wing
    (IOP, 2015-10-12) Phillips, Nathan; Knowles, Kevin; Bomphrey, Richard J.
    Insect wing shapes are diverse and a renowned source of inspiration for the new generation of autonomous flapping vehicles, yet the aerodynamic consequences of varying geometry is not well understood. One of the most defining and aerodynamically significant measures of wing shape is the aspect ratio, defined as the ratio of wing length (R) to mean wing chord (). We investigated the impact of aspect ratio, AR, on the induced flow field around a flapping wing using a robotic device. Rigid rectangular wings ranging from AR = 1.5 to 7.5 were flapped with insect-like kinematics in air with a constant Reynolds number (Re) of 1400, and a dimensionless stroke amplitude of (number of chords traversed by the wingtip). Pseudo-volumetric, ensemble-averaged, flow fields around the wings were captured using particle image velocimetry at 11 instances throughout simulated downstrokes. Results confirmed the presence of a high-lift, separated flow field with a leading-edge vortex (LEV), and revealed that the conical, primary LEV grows in size and strength with increasing AR. In each case, the LEV had an arch-shaped axis with its outboard end originating from a focus-sink singularity on the wing surface near the tip. LEV detachment was observed for around mid-stroke at span, and initiated sooner over higher aspect ratio wings. At the larger, stronger vortex persisted under the wing surface well into the next half-stroke leading to a reduction in lift. Circulatory lift attributable to the LEV increased with AR up to AR = 6. Higher aspect ratios generated proportionally less lift distally because of LEV breakdown, and also less lift closer to the wing root due to the previous LEV's continuing presence under the wing. In nature, insect wings go no higher than likely in part due to architectural and physiological constraints but also because of the reducing aerodynamic benefits of high AR wings.
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    The effects of scaling and high subsonic cavity flow and control
    (2014-08-15) Thangamani, Varun; Saddington, Alistair J.; Knowles, Kevin
    The effects of scaling a cavity with respect to a fixed incoming boundary layer thickness on its flow dynamics and control was studied experimentally. Three cavity models with constant length-to-depth ratio of 5 and length-to-width ratio of 2 and with corresponding linear dimensions in the ratio 0.5 : 1 : 2 were tested at freestream Mach number 0.71. Additionally, the 0.5 and 1 scale models were tested at freestream Mach number of 0.85. The experiments involved timeaveraged pressure measurements, unsteady pressure measurements, and PIV measurements. Time-averaged pressure measurements made at the floor were used to study the ’flow-type’ of the cavities. Unsteady pressure measurements were used to study the acoustic characteristics of the cavity. The cavity length-to-boundary layer thickness ratios tested were 10, 20 and 40. The Cp distribution on the clean cavities indicated a change in the cavity flowtype with change in the cavity scale. Varying the L/δ from 10 to 40 changed the cavity flow-type from open to transitional. Analysis of the frequency spectra of the cavity revealed an increase in tonal amplitudes and OASPL with increasing L/δ . The PIV measurements indicated that this could be caused by an increase in energy exchange between the freestream and the cavity. The velocity magnitudes inside the cavities were found to increase with increase in L/δ . A comparative study of different passive control methods on the largest cavity showed that leading-edge spoilers were superior in cavity tone suppression. Of these, the effectiveness of a sawtooth spoiler on the three cavities of different scales was tested. The results showed that while the spoiler was effective in eliminating tones and suppression of noise for the smaller cavities, it was unable to eliminate the tones completely for the largest cavity. To find the correct method for scaling the spoilers with the cavity dimensions, different spoiler heights were tested on the three cavities. The results showed that the cavity noise suppression for a given cavity attains saturation level at a particular spoiler height, called the critical spoiler height. An increase in spoiler height beyond the critical spoiler height was found to have no effect on the noise suppression. It is also found that this critical spoiler height can be scaled with the length of the cavity (for given L/D, M and spoiler profile) irrespective of the boundary layer thickness.
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    Effects of scaling on high subsonic cavity flow oscillations and control
    (AIAA, 2014-02-28) Thangamani, Varun; Knowles, Kevin; Saddington, Alistair J.
    The effects of scaling on cavity oscillations and control have been studied by measuring the unsteady pressure on the floor of three cavities of different scales. The cavities have a length-to-depth ratio of 5 and a length-to-width ratio of 2, and the corresponding linear dimensions are in the ratio0.5∶1∶2. The experiments were conducted with clean cavities and cavities fitted with leading-edge sawtooth spoilers so as to study the influence of scaling on clean cavities as well as the effectiveness of the passive control method on different sized cavities. The results showed significant variation of certain spectral characteristics of the clean cavities. The control effectiveness of the spoilers also showed variations with a change in scale of the model. It is recommended that, before implementing a passive control device for practical applications, the device should be tested in the possible range of cavity length-to-boundary-layer-thickness ratio (L/δ) that can be experienced in actual flight.
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    Effects of upstream nozzle geometry on rectangular free jets
    (2010-09-17) Tipnis, T. J.; Bray, Derek; Knowles, Kevin
    This study is aimed at understanding the effects of changing the upstream nozzle geometry on the development of rectangular free jets. An existing converging rectangular nozzle with an exit aspect ratio of 4 and a circular inlet (AR4 nozzle) has been used as the basic configuration for this work. The study is primarily based on the results of numerical simulations wherein the internal geometry variation is accomplished by changing the inlet aspect ratio (AR,) and the length of the converging section, expressed as a ratio with respect to the length of the nozzle (called 'converging section ratio*, CSR); all the other parameters are kept constant. The results from LDA experiments done on the AR4 nozzle are presented and used as validation data for the CPD simulations. Analyses of the numerical results help in understanding the variation of the jet spreading for different combinations of AR, and CSR. Two parameters are identified for describing the jet development: the cross-over point (XC), defined as the location downstream of the exit where the jet half-velocity-widths (B) along the major and minor axes are equal, and the difference in the half-velocity-widths at 30 nozzle equivalent diameters (Dm) from the exit (AB30), to ascertain the occurrence of axis-switching. For a given AR,, XC varies linearly with CSR; the variation of XC is non-linear with AR, for a constant CSR. The A1330 variation is non-linear with both AR, and CSR; the other variable being kept constant. The data obtained from the simulations are further used to propose two parametric models which can be used to predict the occurrence of axis-switching, within the scope of this work. The parametric models are validated and future work is proposed.
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    An efficiently parallelized high-order aeroacoustics solver using a characteristic-based multi-block interface treatment and optimized compact finite differencing
    (MDPI, 2017-05-28) Khanal, Bidur; Saddington, Alistair J.; Knowles, Kevin
    This paper presents the development of a fourth-order finite difference computational aeroacoustics solver. The solver works with a structured multi-block grid domain strategy, and it has been parallelized efficiently by using an interface treatment based on the method of characteristics. More importantly, it extends the characteristic boundary condition developments of previous researchers by introducing a characteristic-based treatment at the multi-block interfaces. In addition, most characteristic methods do not satisfy Pfaff’s condition, which is a requirement for any mathematical relation to be valid. A mathematically-consistent and valid method is used in this work to derive the characteristic interface conditions. Furthermore, a robust and efficient approach for the matching of turbulence quantities at the multi-block interfaces is developed. Finally, the implementation of grid metric relations to minimise grid-induced errors has been adopted. The code was validated against a number of benchmark cases, which demonstrated its accuracy and robustness across a range of problem types