Giovanni Lombardi

On the analysis of fluctuating velocity signals through methods based on the wavelet and Hilbert transforms

Abstract In the present paper we describe signal processing procedures based on the wavelet and Hilbert transforms, which may be used to characterize the time variation of the amplitude and frequency modulations that are present in the fluctuating components of experimental velocity signals. The classical Hilbert demodulation technique is described together with the wavelet-ridge extraction procedure, and a new joint wavelet-Hilbert technique is presented. Methods to obtain the instantaneous contribution to the correlation between the components present in two signals through the wavelet and Hilbert transforms are also described. The procedures are then applied to the analysis of the velocity fluctuations in the wake of a bluff body, and it is shown that they allow a physical interpretation to be given of the connection between different fluctuating components present in the velocity signals and the dynamics of the vorticity structures in the wake.

Low aspect-ratio triangular prisms in cross-flow: measurements of the wake fluctuating velocity field

Abstract An experimental investigation is carried out to study the characteristics of the fluctuating wake flow field of finite-length prismatic bodies in cross-flow, placed vertically on a plane and having isosceles triangular cross-section with 60° or 90° apex angle orientated in the downstream direction. The wake flow field is studied with hot-wire anemometry for aspect ratios h/w =1.0, 1.5, 2.0 and 3.0 at Re= wU/ν ≅1.2×10 5 , and the velocity signals are analysed by means of procedures based on the wavelet transform. Velocity fluctuations with a clear dominating frequency are found immediately outside the wake along all the models, even if their magnitude decreases at a height corresponding to the free end of the bodies. The wavelet analysis of the phase difference between signals acquired at opposite sides of the wake shows that alternate vortex shedding occurs for all the models. An increase in the phase shift between the fluctuations on the same side of the wake with increasing vertical distance between the probes suggests that the shed vortices are curved. No evidence of symmetrical shedding or of the presence of arch-type vortices is found, even for the models with the smaller aspect ratios.

Wind-Tunnel Tests on a Model Antenna Rotating in a Cross-Flow

Abstract Aerodynamic loads acting on a rotating antenna depend essentially on its speed of rotation. Due to the complexity of wind-tunnel tests on a rotating model, the evaluation of these loads is usually obtained through tests carried out on a non-rotating model, suitably modifying the results with theoretical or semi-empirical methods to account for rotation. In order to investigate the accuracy of this methodology, wind-tunnel tests on a rotating antenna placed in a cross flow were carried out and the experimental results were compared with those obtained from tests on a static model with subsequent application of the quasi-steady theory to determine the loads whilst in rotation. Differences emerged from the comparison between the two procedures, some of which were quite significant; an attempt has been made to determine the reasons leading to these differences. Furthermore, the tests on the rotating model were used to verify the efficiency of load reduction of a new fin configuration previously analysed with tests on a static model. Tests showed that significant reductions in torque can be obtained with the new fin configuration, confirming the possibility of its use in practical applications.

Characterization Of Boundary Layer Conditions In Wind Tunnel Tests Through Ir Thermography Imaging

A computerized infrared (IR) imaging system is evaluated as a diagnostic tool for aerodynamic research by performing experimental tests in a subsonic wind tunnel. In particular, the IR technique is used to characterize the behaviour of the boundary layer on two wing models, having different cross-sections, by measuring the temperature distribution over their heated surfaces. The results show that IR thermography is capable of immediately yielding accurate information on the location and extent of the transition between laminar and turbulent flow and on the region of separation, for the whole boundary layer over the surface of a model wing.

High-performance road-vehicle optimised aerodynamic design: Application of parallel computing to car design

Abstract The HIPEROAD project has developed a software system capable of performing a semi-automatic optimisation of the shape of sport cars with respect to their aerodynamical properties. The system utilises an aerodynamic solver implemented on parallel MIMD systems, and features advanced tools for the evolution and meshing of car surfaces. The system allows one to include aerodynamic optimisation in the early stages of car design. It has been tested on a Ferrari auto model and full agreement between computed and measured aerodynamical properties was found. The code has been used for designing an optimised model with improved car safety.

A Methodology for the Preliminary Analysis and Comparison of Wing-Tail and Canard Configurations

A methodology for the analysis of the performance of different aircraft configurations in the conceptual design phase is addressed. A scalar objective function, takeoff weight, is minimised by means of a numerical optimisation technique which takes into account the high number of geometrical parameters and the flight mechanics requirements involved in the problem. The study is confined to the cruise segment of the mission, and results are shown both for tail-aft and canard configurations. The above technique produces equally optimised configurations, thus giving a meaningful means to compare tail-aft and canard configurations.

Wind-tunnel tests on a model antenna with different fin configurations

Abstract Wind-tunnel tests on a model of a rotating radar antenna are described. The purpose of the tests was to establish the effectiveness of various devices in reducing the variation of torque, during a revolution, caused by aerodynamic loads when the antenna is immersed in a stream. A widely-used solution—fins added to the back of the antenna, the drag of which is utilized to reduce the variation of torque—was first tested, but visualization of the flow suggested the idea that lifting fins could be more efficient. Tests confirmed that significant results in the reduction of maximum torque and its gradient can be obtained with this new configuration, together with a flow of remarkable stability and the possibility of choosing a wide range of design parameters.