Browsing by Author "Bray, Derek"

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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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    The effect of angle of attack on the aeroacoustic environment within the weapons bay of a generic UCAV
    (Elsevier, 2019-07-24) Bacci, David; Saddington, A. J.; Bray, Derek
    Cavity flow studies are generally concerned with observing the effect of geometry changes whilst maintaining a fixed zero angle of attack. Cavities employed as weapons bays will, however, experience a range of angles of attack. This paper presents the first known results showing the effect of flight angle of attack on the aeroacoustic characteristics of an internal weapons bay installed in an uninhabited combat air vehicle (UCAV). The UCAV geometry consisted of a Boeing M219-type cavity in a Boeing UCAV1303 airframe. Numerical simulation was conducted using a full-scale detached eddy simulation model and representative transonic flight conditions. As well as the reference case of zero degrees, data for angles of attack of 3.0, 4.5 and 6.0 degrees were analysed. Experimental data was used to validate the reference computational model, which agreed with the overall fluctuating sound pressure level (OAFPL) to within the experimental uncertainty of 4 dB. Data from the computational model was post-processed with frequency-domain and time-frequency-domain techniques showing that the flow structure within the weapons bay was altered significantly by the angle of attack changes, affecting the mean pressure distribution, frequency spectra and resonant modes. Overall, increasing the angle of attack from 0.0 to 3.0 degrees produced an increment in the acoustic load whilst a further increase tended to affect the resonance mechanism and thereby reduce the coherence and the temporal footprints of the resonant modes.
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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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    Identification of the formation of resonant tones in compressible cavity flows
    (Elsevier, 2018-03-14) Bacci, David; Saddington, A. J.; Bray, Derek
    Identification of the fluid dynamic mechanisms responsible for the formation of resonant tones in a cavity flow is challenging. Time-frequency non-linear analysis techniques were applied to the post-processing of pressure signals recorded on the floor of a rectangular cavity at a transonic Mach number. The results obtained, confirmed that the resonant peaks in the spectrum were produced by the interaction of a carrier frequency (and its harmonics) and a modulating frequency. High-order spectral analysis, based on the instantaneous wavelet bi-coherence method, was able to identify, at individual samples in the pressure–time signal, that the interaction between the fundamental frequency and the amplitude modulation frequency was responsible for the creation of the Rossier–Heller tones. The same technique was also able to correlate the mode switching phenomenon, as well as the deactivation of the resonant tones during the temporal evolution of the signal.
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    Statistical modelling for prediction of axis-switching in rectangular jets
    (Professional Engineering Publishing, 2013-08-01T00:00:00Z) Tipnis, T. J.; Knowles, Kevin; Bray, Derek
    Rectangular nozzles are increasingly used for modern military aircraft propulsion installations, including the roll nozzles on the F-35B vertical/short take-off and landing strike fighter. A peculiar phenomenon known as axis-switching is generally observed in such non-axisymmetric nozzle flows during which the jet spreads faster along the minor axis compared to the major axis. This might affect the under-wing stores and aircraft structure. A computational fluid dynamics study was performed to understand the effects of changing the upstream nozzle geometry on a rectangular free jet. A method is proposed, involving the formulation of an equation based upon a statistical model for a rectangular nozzle with an exit aspect ratio (ARe) of 4; the variables under consideration (for a constant nozzle pressure ratio (NPR)) being inlet aspect ratio (ARi) and length of the contraction section. The jet development was characterised using two parameters: location of the cross-over point (Xc) and the difference in the jet half-velocity widths along the major and minor axes (ΔB30). Based on the observed results, two statistical models were formulated for the prediction of axis-switching; the first model gives the location of the cross-over point, while the second model indicates the occurrence of axis-switching for the given configuration.
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    Transonic aero-acoustics of weapon bays
    (2017-09) Bacci, David; Saddington, A. J.; Bray, Derek
    The requirement for modern combat aircraft to have low radar cross sections and improved aerodynamic performance has introduced the necessity to incorporate weapons bays in almost every new military aircraft design project. This, on its own, has led to a renewed interest in the field of cavity flows, especially during transonic and supersonic speed regimes. Although considerable data already exist on the fundamental physical aspects of cavity flows, whenever a cavity is integrated in an aircraft design, various other related issues must also be considered. Airframe aerodynamics requirements may impose changes on the shape of the cavity, while flight dynamics parameters, like incidence and sideslip, may prompt a different response of such a non-linear phenomenon. A study was therefore conducted in order to assemble knowledge and understanding of some of the main aspects related to weapon bay design. A representative cavity, exposed to a typically representative transonic Mach number, was tested to determine the effects of the introduction of typical stealth design features. These included the saw-toothing of the leading and trailing edges of the bay and the indentations of the doors accompanying the cavity. Such features were tested with and without the presence of a model of a representative store inside the bay. Subsequently, these aspects were tested, in numerical models, by installing the cavity on a representative stealthy airframe, which was used to explore incidence angle effects of the flow characteristics. Finally, an innovative solution, designed to mitigate the adverse aspects of the flow was tested. Due to the extreme complexity of the aero-acoustic environment typical of cavity flows, a technique based on the complementary use of frequency-domain and time-frequency domain linear and non-linear analyses was used to process the pressure histories recorded. Such a procedure was able to highlight the complexity of the flow, which, in accordance with previous studies, was rich in non-statistical stationary phenomena, like amplitude modulation, frequency modulation, and mode switching.
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    Wavelet analysis of complex geometry transonic cavity flows
    (2016-09-09) Bacci, David; Saddington, A. J.; Bray, Derek
    The aero-acoustic analysis of a weapon bay of an Unmanned Combat Air Vehicle (UCAV) was predicted using Computational Fluid Dynamics (CFD) methods. Along the reference geometry, consisting in the installation of the Boeing M219 modified type cavity in the Boeing UCAV1303 airframe, two additional configurations, developed modifying the leading and trailing edge geometries of the bay, were tested. Pressure signals inside the cavity were post-processed using Joint Time Frequency Analysis (JTFA) techniques, consisting in a combination of frequency domain and time-frequency domain techniques based respectively on the Fourier and wavelet transform. Results showed an intermittency nature of the modes present in the spectra as well as a continuous change, during the temporal evolution of the signal, of the dominant mode. Also were recorded, using second order wavelet spectral moments, non-linear phenomena between the main modes like phase coupling.