Browsing by Author "Bacci, David"

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    Angle of attack effects on the induced structural loads of a weapons bay
    (Springer, 2024) Bacci, David; Saddington, Alistair J.
    Aero-acoustic analysis was conducted on a weapons bay numerical model with doors, incorporating radar cross section reduction features. The effect of the angle of attack on the aero-acoustic response of the cavity was analysed at Mach numbers of 0.85 and 1.20. It was found that incidence influenced both mean-flow features and acoustic response. Further, linear and angular accelerations induced by the flow on the bay doors revealed potential adverse fluid–structure coupling when the results were compared with modal analysis. Again, the angle of attack did influence the aeroacoustic effects on the cavity door structure.
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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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    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, Alistair 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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    Exploring angle-of-attack effects in the aero-acoustic response of a weapons bay at transonic and supersonic Mach number
    (AIAA, 2022-06-13) Bacci, David; Saddington, Alistair J.
    Numerical aero-acoustic analysis was conducted on a weapon bay model with doors, incorporating radar cross section reduction features. The effect of angle of attack on the aero-acoustic response of the cavity was analysed at a transonic Mach number of 0.85, and at a supersonic Mach number of 1.20. It was found that incidence had influence on both mean-flow features and acoustic response. Further, linear and angular accelerations induced by the flow on doors revealed potential adverse fluid-structure coupling when results were compared with modal analysis. Again, angle of attack did influence the aeroacoustic effects on the cavity door structure.
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    Hilbert–Huang spectral analysis of cavity flows incorporating fluidic spoilers
    (AIAA, 2022-10-07) Bacci, David; Saddington, Alistair J.
    Numerical aeroacoustic analysis was conducted on an M219 cavity geometry, incorporating signature suppression features and leading-edge fluidic spoilers. The numerical model was validated against existing experimental data. The palliative properties of fluidic spoilers were investigated at Mach numbers of 0.85, 1.20, and 1.80 with blowing coefficients of 0.03 and 0.06. The results are presented for the acoustic spectrum, and further analysis was conducted using the Hilbert–Huang methodology. The fluidic spoilers were able to considerably reduce the overall level of acoustic noise and to reduce and/or suppress the resonant modes typical of cavity flows. The effectiveness of the spoilers was a direct consequence of their effect on the detached shear layer, of which the trajectory and coherence were altered. The Hilbert–Huang spectral analysis provided an enhanced understanding of the complex nature of the aeroacoustic behavior of the cavity. Acoustic modes were identified that, together with the Rossiter–Heller tones, governed the behavior of the spectrum. This demonstrated how the generated tones, appearing inside the cavity, were a result of complex nonlinear interactions between shear-layer acoustic instabilities and centrifugal instabilities originating in the flow recirculating in the internal part of the cavity. This also demonstrated that the fundamental frequencies had frequency and amplitude modulation characteristics that spread the energy in a wide bandwidth. This is not captured by classical Fourier analysis.
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    Identification of the formation of resonant tones in compressible cavity flows
    (Elsevier, 2018-03-14) Bacci, David; Saddington, Alistair 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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    Influence of door gap on aeroacoustics and structural response of a cavity
    (AIAA, 2023-11-20) Bacci, David; Saddington, Alistair J.
    Numerical aeroacoustic analysis using the Shear Stress Transport–Scale Adaptive Simulation turbulence model was conducted on a weapon bay model based on the M219 geometry with doors incorporating radar cross-section reduction features. The effect of the introduction of a gap between the doors and the cavity edge on the aeroacoustic and structural response of the cavity was analyzed at Mach 0.85. The effect of introducing 3 deg of sideslip was also investigated. Both mean and unsteady flow analyses were conducted. The results showed a strong palliative effect of the door gap with and without sideslips. The overall analysis of the spectral signature on the forces and moments acting on the doors indicated the possibility of fluid–acoustic coupling, as all acoustic spectra showed a predominant tone located at the same frequency of the first structural mode.
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    A trade-off analysis between lateral/directional stability and radar cross section requirements of an air-to-air combat airframe
    (Elsevier, 2023-05-09) Bacci, David; Vagias, Ioannis
    Numerical aerodynamic and radar analyses were conducted on 3 low-radar cross-section airframes, derived from the AVT-183 diamond wing. The geometries were developed by adding different configurations of vertical stabilators, to improve lateral and directional characteristics at high angles of attack whilst minimising the deterioration of radar signature characteristics. The analyses were conducted at a Mach number of 0.30, with angle of attack varying between 0 deg to 30 deg and angle of sideslip varying from 0 deg to 6 deg. Monostatic radar cross-section analysis was conducted in L and X bands. The study explored the trade-off between radar signature control (RCS) and the spin/departure requirements typical of a modern combat aircraft designed for air-superiority. Several of the proposed configuration were shown to improve both lateral and directional stability, whilst maintaining acceptable RCS.
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    Transonic aero-acoustics of weapon bays
    (2017-09) Bacci, David; Saddington, Alistair 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, Alistair 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.