This page describes my previous research projects in the Experimental Aero Group (currently the Aerodynamics group) of the NUS Temasek Laboratories, in the Gas Dynamics and Turbulence Laboratory (GDTL) of The Ohio State University, in the Supersonic Laboratory of the University of California Irvine, and in the Fatigue Team of the Department of Mechanical Engineering of the University of Padova. Click on the figures or the links below to access the corresponding pages.

Grid fins are aerodynamic control surfaces consisting of an outer frame with an internal lattice of intersecting thin walls. Grid fins have several advantages compared to conventional planar fins but they present higher drag at transonic speeds. A swept-back grid-fin has been developed by the NUS Temasek Laboratories for reducing this problem. This configuration has been explored by using CFD simulations followed by validation with wind-tunnel measurements. The results indicate that, compared to a conventional grid fin design, the swept-back type offers about 30% drag reduction in the transonic velocity range.

A dielectric-barrier-discharge (DBD) plasma is used to create an electrically-induced wind close to a surface for flow control. The main advantage of this type of actuation is that it directly converts electric energy into kinetic energy without involving moving mechanical parts. Secondly, its response time is very short and enables real-time control at high frequency. DBD plasma actuators are also light, small, and have little power consumption. However current DBD plasma actuators have low efficiency of energy conversion. Increasing their performance for aerodynamic control is one of the goals of the NUS Temasek Laboratories.

Feedback-based flow control has the potential of improving the aerodynamics of vehicles and the flows of other engineering systems. However, since a flow can not be described by simple models, the development of flow-control techniques is difficult. By choosing as a benchmark problem the control of cavity-flow resonances, this multi-disciplinary research has contributed to advance the methodologies and the tools for feedback-based flow control.

Flow actuation is crucial to flow control and benefits from it. In this case the characteristics of a a two-dimensional synthetic-jet forced by the titanium diaphragm of a compression driver and issuing from a high aspect-ratio converging nozzle have been studied in detail. The information obtained allowed devising a feedback-based compensator that flattens the highly frequency-dependent response of this actuator.

I managed the project of a feedback-controlled air-supply system that greatly enhanced the experimental capabilities of the GDTL. The system enables the use of cold and hot air in a dual stream jet facility and the operation of other different subsonic and supersonic flow facilities. The system is currently in use and allows the GDTL to run a wide spectrum of experiments with an unprecedented level of convenience and comfort.

Similar to the flow over a cavity, impinging jets are characterized by self-sustained oscillations with strong pressure fluctuations that can cause significant damage to nearby objects as well as high levels of noise. The exhaust of STOVL aircraft hovering close to the ground is a typical example of impinging jet. Clearly the attenuation or elimination of the impinging resonance would be beneficial. For this purpose Hartmann tubes have been used in this research by exploring their effect in a Mach 1.3 ideally-expanded axisymmetric impinging jet with both strong and weak resonance.

A novel approach for enhancing jet mixing has been recently developed at the University of California Irvine. This exploits the strong flow instability developed in a convergent-divergent nozzle operated at off-design conditions. The method does not use mechanical devices to directly disturb the flow and introduces minimal thrust penalty (1-2%) with potential for even smaller or negligible losses with appropriate design or control of the nozzle shape. The technique can be used in single- or dual-stream jets. In dual-stream jets, where the outer (secondary) stream is the unstable flow, the method is called Mixing Enhancement using Secondary Parallel Injection (MESPI).
During my last year in UCI I worked on MESPI for enhancing liquid mixing with a secondary air stream. In the NUS Temasek Laboratories I investigated the benefit of MESPI in enhancing the mixing of the plume of high-bypass turbofan jet engines.

Scaling the experimental results to a full-size engine shows that Targeted Mach Wave Elimination (TMWE) with eccentric secondary flows reduces the noise perceived in the direction of peak emission by up to 11 dB. In this study I performed a preliminary analysis of the application of this silencing technique to engine design. The results indicate that TMWE is feasible and that engines for supersonic business aircraft can apply this method and can be designed using the core of existing military turbofans.

Efficient suppression of high-speed jet noise remains an unsolved problem that has prevented wide-scale development of supersonic transport. Mach wave emission, which radiates in the downstream direction and is caused by the supersonic convection of eddies relative to their surrounding medium, represents a dominant contribution to supersonic jet noise. With Prof. Dimitri Papamoschou of the University of California, Irvine, I studied and developed a technique where a secondary flow added to a supersonic jet reduces Mach wave emission when the convective velocity of the jet eddies with respect to the secondary flow drops to subsonic values, provided that the secondary flow eddies are also subsonic with respect to the ambient. Equal-thrust comparison of different experimental results shows that Mach Wave Elimination (MWE) is very effective in reducing noise in the direction of strongest emission.

This work aims to separate the overall stress of structural components into a structural contribution (due to the overall shape) and a local contribution (due to the geometric details of the welding). Two models are proposed by using as a study case the symmetrical cross joint. In the first model the local stress field corresponds to that of a butt-weld joint, whereas in the second model it is that of a sharply notched joint. The geometries of these components are defined from that of the actual cross joint. The results support the hypotheses since the combined structural and local stress fields of both the models closely match that of the actual cross joint.

Life prediction of mechanical components requires the knowledge of their loading history. The ability to create realistic loading sequences for accelerated testing of the components is also crucial in industrial applications. To this aim I developed a digital modulator (DIMO) that creates load timetraces by amplitude and frequency modulation of a sinusoidal signal to which a mean-value distortion is then superimposed. The principle can be also used in reverse by using a digital demodulator (DIDE) that extracts and counts the loading cycles in a timetrace based on their amplitude, mean value and frequency.

External fixators are flexible and capable prostheses used to heal bone fractures or deformations. Several types of fixator are available each having peculiar properties and capabilities. In this study, done in cooperation with colleagues from the Fatigue Team of the University of Padua, I evaluated the force and deformation characteristics of the different types based on structural considerations. Particular attention was given to the assessment of the stress put by the fixators' pins in their points of anchorage to the bones. The results provide some basic guidelines for the optimal selection and the use of external fixators.