Derek Bray

Experimental investigation into transonic flows over tandem cavities

An experimental study was conducted to analyse the pressure distribution along the floor of a cavity, with and without the presence of an upstream tandem cavity, at a constant freestream Mach number of about 0-911. Measurements were made for single cavities and the results compared with those obtained in the presence of an upstream tandem cavity. This comparison was made over a wide range of geometries, covering open to closed classes of cavities with both identical and different dimensions for the two cavities. The effect of the spacing between the two cavities was also studied. The downstream cavity is shown to be significantly affected by the presence of an upstream cavity, with both the overall net static pressure and its gradient being affected, dependent upon the class of cavity geometry and spacing under consideration.

Modelling and simulation of a fuel cell powered electric drivetrain for wide body passenger aircraft

With each passing day, new and more stringent emissions regulations are being introduced in all transportation sectors. The present day commercial airline sector is no stranger to that as exemplified by ICAO agreements signed in 2010; therefore reducing ground pollutant gas and noise emissions has become more important to a point where it has grown into the subject area of many new technological developments. One such development is the recent demonstration of electrical taxi drivetrains for wide body passenger airliners powered by a modified APU turbine electrical power generation system. To further understand the full potential of electric taxi drivetrains, work has been carried out to simulate replacing a regular APU by using PEM fuel cells to generate all the electrical power required by the aircraft and drivetrain. Simulation results will bring more light into the possible requirements, implications and benefits of the new architectures.

Statistical modelling for prediction of axis-switching in rectangular jets

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 (AR e ) of 4; the variables under consideration (for a constant nozzle pressure ratio (NPR)) being inlet aspect ratio (AR i ) and length of the contraction section. The jet development was characterised using two parameters: location of the cross-over point (X c ) 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.

Density measurements for rectangular free jets using background-oriented schlieren

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 3D domain which contains information about the actual density. 2D 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.

Transient and time-averaged characteristics of a compressible ground vortex flow

Particle image velocimetry data are presented from a scaled jet-lift aircraft ground vortex compressible flow. The scaled ground vortex is generated by a vertical compressible jet in cross-flow impinging on a moving ground plane. Particle image velocimetry is used to generate both transient and time-averaged flow statistics from the ground vortex region over a range of nozzle pressure ratios from 2.3 to 3.7, nozzle height-to-diameter ratios (h/dn) from 3 to 10 (where dn = 12.7 mm) and cross-flow velocities (V∞) from 10 to 20 m/s. These conditions correspond to effective (jet-to-cross flow) velocity ratios of 15 <  V e - 1  < 60. For each condition, mean and root mean square ground vortex core position was analysed from sets of 72 instantaneous particle image velocimetry vector maps. Over the range of effective velocity ratios, V e - 1 , the particle image velocimetry results showed that the ground vortex mean streamwise position varied from 5dn to 16dn and the root mean square fluctuation in this position varied from 0.7dn to 1.5dn. Further analysis of the ground vortex temporal characteristics did not reveal any dominant non-dimensional frequencies.

Wavelet analysis of complex geometry transonic cavity flows

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.