The increasing use of Intelligent Transport System (ITS) can enable very close vehicle spacings which generally results in a net drag reduction for the resulting convoys. The majority of vehicle development has, to date, been for vehicles in isolation, thus the study of interaction effects is becoming increasingly important. The main objective of this research is to investigate the use of Computational Fluid Dynamics (CFD) for understanding convoy aerodynamics and to further the understanding of airflow interaction between vehicles via CFD. In this study, time-averaged characteristics of a simplified, generic passenger vehicle, called the Ahmed car model, after Ahmed et.al (1984) is investigated computationally using the available commercial CFD code, Fluent version 6.1.22. Three different platoon combinations were analysed for the current study which includes a two, three and six model platoons for various rear end configurations of the Ahmed model geometry. Experiments were conducted in RMIT University Industrial Wind Tunnel for analysing the effects of drafting on drag coefficients using two different scales of Ahmed car models. This is an extension to the previous study performed on two 100% scales of Ahmed models (Vino and Watkins, 2004) and the results for both the current and previous experiments were compared using CFD. The CFD proved to be a useful technique since its results compared reasonably well for both the current and the previous experiments on drafting, using Ahmed models of identical (30°) rear slant configurations. However, near critical rear slant angles (~30°) for isolated vehicles some discrepancies were noted. The reasonable validation of experimental results enabled the study to be extended further computationally using CFD, to analyse the effects of inter-vehicle spacing on a platoon of 3 and 6 models for various rear end configurations (between 0° and 40°), in an attempt to provide useful information on vehicle-wake interaction for the Future Generation Intelligent Transport System (FGITS). Critical gaps were identified via CFD for the case of a two, three and six model platoons and the simulations clearly exposed the reasons for these critical gaps. At extremely close proximity, the models experienced more pressure recovery at their rear vertical base, which reduced the drag coefficient. Surprisingly, at some of the close vehicle spacings, the drag coefficients reached values that were higher than that of a vehicle in isolation. This was found due to the high momentum flow impingement to the fore body of the model and was similar to results found in physical experiments. Thus the current CFD analysis revealed that rear slant angle of the model and the inter-vehicle spacing greatly influences the wake structures and ultimately the vehicles aerodynamic drag coefficients in platoons. Even though the current CFD model (Realizable k-B turbulence model) predicted the basic flow structures such as the C-pillar vortices from the rear slant and 2D horse shoe vortices in the model's vertical rear base, the separation bubble on the rear slant that supplies energy to the strong C-pillar vortices was not replicated accurately, which is evidenced from the flow structure analysis. Hence it is recommended for further work, that the study should be extended using the Reynold's stress models or the Large Eddy Simulation (LES) turbulence models for flow structure observation and analysing vortex interactions between the models.

Time-frequency analysis techniques are proposed as a necessary tool for the analysis of acoustics generated by helicopter transient maneuvering flight. Such techniques are necessary as the acoustic signals related to transient maneuvers are inherently unsteady. The wavelet transform is proposed as an appropriate tool, and it is compared to the more standard short-time Fourier transform technique through an investigation using several appropriately sized interrogation windows. It is shown that the wavelet transform provides a consistent spectral representation, regardless of employed window size. The short-time Fourier transform, however, provides spectral amplitudes that are highly dependent on the size of the interrogation window, and so is not an appropriate tool for this situation. An extraction method is also proposed to investigate blade-vortex interaction noise emitted during helicopter transient maneuvering flight. The extraction method allows for the investigation of blade-vortex interactions independent of other sound sources. The method is based on filtering the spectral data calculated through the wavelet transform technique. The filter identifies blade-vortex interactions through their high amplitude, high frequency impulsive content. The filtered wavelet coefficients are then inverse transformed to create a pressure signature solely related to blade-vortex interactions. This extraction technique, along with a prescribed wake model, is applied to experimental data extracted from three separate flight maneuvers performed by a Bell 430 helicopter. The maneuvers investigated include a steady level flight, fast- and medium-speed advancing side roll maneuvers. A sensitivity analysis is performed in order to determine the optimal tuning parameters employed by the filtering technique. For the cases studied, the optimized tuning parameters were shown to be frequencies above 7 main rotor harmonics, and amplitudes stronger than 25% (−6 dB) of the energy in the main rotor harmonic. Further, it is shown that blade-vortex interactions can be accurately extracted so long as the blade-vortex interaction peak energy signal is greater or equal to the energy in the main rotor harmonic. An in-depth investigation of the changes in the blade-vortex interaction signal during transient advancing side roll maneuvers is then conducted. It is shown that the sound pressure level related to blade-vortex interactions, shifts from the advancing side, to the retreating side of the vehicle during roll entry. This shift is predicted adequately by the prescribed wake model. However, the prescribed wake model is shown to be inadequate for the prediction of blade-vortex interaction miss distance, as it does not respond to the roll rate of the vehicle. It is further shown that the sound pressure levels are positively linked to the roll rate of the vehicle. Similar sound pressure level directivities and amplitudes can be seen when vehicle roll rates are comparable. The extraction method is shown to perform admirably throughout each maneuver. One limitation with the technique is identified, and a proposal to mitigate its effects is made. The limitation occurs when the main rotor harmonic energy drops below an arbitrary threshold. When this happens, a decreased spectral amplitude is required for filtering; which leads to the extraction of high frequency noise unrelated to blade-vortex interactions. It is shown, however, that this occurs only when there are no blade-vortex interactions present. Further, the resulting sound pressure level is identifiable as it is significantly less than the peak blade-vortex interaction sound pressure level. Thus the effects of this limitation are shown to be negligible. / text

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In this study we investigate the interactions between two co-rotating vortices. These vortices are subject to rapid rotation and stable stratification such as are found in planetary atmospheres and oceans. By conducting a large number of simulations of vortex interactions, we intend to provide an overview of the interactions that could occur in geophysical turbulence. We consider a wide parameter space covering the vortices height-to-width aspect-ratios, their volume ratios and the vertical offset between them. The vortices are initially separated in the horizontal so that they reside at an estimated margin of stability. The vortices are then allowed to evolve for a period of approximately 20 vortex revolutions. We find that the most commonly observed interaction under the quasi-geostrophic (QG) regime is partial-merger, where only part of the smaller vortex is incorporated into the larger, stronger vortex. On the other hand, a large number of filamentary and small scale structures are generated during the interaction. We find that, despite the proliferation of small-scale structures, the self-induced vortex energy exhibits a mean `inverse-cascade' to larger scale structures. Interestingly we observe a range of intermediate-scale structures that are preferentially sheared out during the interactions, leaving two vortex populations, one of large-scale vortices and one of small-scale vortices. We take a subset of the parameter space used for the QG study and perform simulations using a non-hydrostatic model. This system, free of the layer-wise two-dimensional constraints and geostrophic balance of the QG model, allows for the generation of inertia-gravity waves and ageostrophic advection. The study of the interactions between two co-rotating, non-hydrostatic vortices is performed over four different Rossby numbers, two positive and two negative, allowing for the comparison of cyclonic and anti-cyclonic interactions. It is found that a greater amount of wave-like activity is generated during the interactions in anticyclonic situations. We also see distinct qualitative differences between the interactions for cyclonic and anti-cyclonic regimes.

The aim of this study is to extend a parallel Fortran90 code to compute three&ndash / dimensional laminar and turbulent flowfields over rotary wing configurations. The code employs finite volume discretization and the compact, four step Runge-Kutta type time integration technique to solve unsteady, thin&ndash / layer Navier&ndash / Stokes equations. Zero&ndash / order Baldwin&ndash / Lomax turbulence model is utilized to model the turbulence for the computation of turbulent flowfields. A fine, viscous, H type structured grid is employed in the computations. To reduce the computational time and memory requirements parallel processing with distributed memory is used. The data communication among the processors is executed by using the MPI ( Message Passing Interface ) communication libraries. Laminar and turbulent solutions around a two bladed UH &ndash / 1 helicopter rotor and turbulent solution around a flat plate is obtained. For the rotary wing configurations, nonlifting and lifting rotor cases are handled seperately for subsonic and transonic blade tip speeds. The results are, generally, in good agreement with the experimental data.