High fidelity multidisciplinary analyses of flow in weapon bays

Babu, Savio High fidelity multidisciplinary analyses of flow in weapon bays. PhD thesis, University of Liverpool.

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Abstract

Modern military aircraft employ weapon bays for carriage and release of stores. The clearance of these stores for release from aircraft may require several flight tests at a range of conditions where the trajectories of released stores are obtained through accelerometers located on the store. Although effective, this is expensive and time consuming and only limited flight tests can be accomplished at critical conditions. Predictions made using store release analysis through wind tunnels and Computational Fluid Dynamics (CFD) have the potential to reduce the number of flight tests required for store clearance. The motivation for the current work, stems from the need to investigate carriage and release of a store from a weapon bay, idealised as a rectangular cavity, through a unique blend of disciplines comprising Computational Fluid Dynamics, Computational Structural Dynamics (CSD), Computational Aero- Aeroacoustics (CAA) and High Performance Computing (HPC). Detached-Eddy Simulations (DES) of flow in clean cavities were carried out to compare two cavities of different aspect ratios for configurations with doors-off and doors-on. Both cavities had similar acoustic signatures and the addition of doors channelled the flow causing acoustic waves to propagate further away from the cavity. DES computations were carried out for a store at different positions relative to a cavity that showed that a store at carriage position pacified the cavity acoustics the most. Fin tip displacements were small for a store at carriage position and exhibited buzzing characteristics. This was similar to the case where a store was positioned at the shear layer of a cavity but with slightly larger displacements. While fin displacements were not large, the results highlighted concerns for fin fatigue life. Comparisons between rigid and elastic fins showed small differences in loads, however, aeroelastic simulations showed that where resonance of structural and cavity modes occurred, large amplitude fin oscillations were predicted. Scale-Adaptive Simulations (SAS) were validated against experimental data for clean cavities and were found to be similar to DES results and could be run at a larger time-step. The cost savings and similarities of SAS to DES encouraged its use for store release computations. Store release computations from a cavity were conducted and the variability of a stores trajectory due to the unsteady cavity flow-field was investigated. Visualisations using Q-criteria highlighted instantaneous structures that were in contact with the store fins causing the trajectory to vary for different release times. Overall, the thesis suggests the use of SAS as an affordable method for analysing store release computations from a cavity and highlights the need for a stochastic evaluation of trajectories from transonic cavities. A study comparing different signal lengths for post-processing unsteady pressure data revealed that, the minimum CFD signal length required to capture all dominant tones was around 0.05s. Different post-processing methods for spectral content were compared and the use of Maximum Entropy Methods (MEMs), based on Burgs Estimator, was suggested as it not only captured dominant tones but also predicted the highest Sound Pressure Levels (SPLs), that could be used to produce the maximum boundary of a given signal.

Item Type:	Thesis (PhD)
Additional Information:	Date: 2014-08 (completed)
Subjects:	?? TL ?? ?? U1 ??
Depositing User:	Symplectic Admin
Date Deposited:	22 Jan 2016 16:33
Last Modified:	17 Dec 2022 00:47
DOI:	10.17638/02008160
URI:	https://livrepository.liverpool.ac.uk/id/eprint/2008160