Luigi de Luca

Viscous interaction phenomena in hypersonic wedge flow

We deal with an experimental investigation carried out to study some aspects of shock/boundary-layer interaction in nominally two-dimensional hypersonic wedge flow, i.e., over flat plate/rump configurations. These flow conditions are two dimensional only geometrically because some spanwise periodic variations of the heat flux over the ramp in the reattaching flow region are observed. The measurements, basically made by means of a computerized infrared (IR) imaging system, have been performed in a blowdown wind tunnel at Mach number equal to 7.14 and unit Reynolds number ranging from 7.6 x 10 6 to 24 X 10 6 /m. The influence of the leading-edge shape (bluntness and geometry), flat plate length, and ramp angle on the separation region, average heat transfer at reattachment, and wavelength of the heat transfer variations has been analyzed. IR results are compared to other experimental data as well as to semiempirical correlations for the heat flux peak.

Influence of Shear Layer Dynamics on Impingement Heat Transfer

Measurements of convective heat transfer coefficients on a flat plate with an air jet impinging on it perpendicularly are made to investigate the influence of some governing parameters. Particular attention is focused on the effects of the shear layer dynamics. For certain flow conditions and/or test arrangements, coherent structures and/or recirculation currents are observed affecting the distribution of the heat transfer coefficients. Measurements of wall temperature as well as of adiabatic wall temperature of the stream are made by means of an infrared scanning radiometer, and the heat transfer coefficients are calculated by means of the so-called heated thin foil technique. The data are reduced in dimensionless form as Nusselt numbers and compared with data from the available literature. Both spatial distributions and averaged values of the Nusselt number are discussed. A new explanation for the second peak in the local Nusselt number is proposed.

Modeling and Experimental Validation of the Frequency Response of Synthetic Jet Actuators

A lumped-element mathematical model of the operation of a synthetic jet actuator driven by a thin piezoelectric disk is both analytically and numerically investigated to obtain information about the frequency response of the device. It is shown that the actuator behaves as a two-coupled oscillator system, and simple relationships are given to predict the two peak frequencies, corresponding to the modified Helmholtz and first-mode structural resonance frequencies. The model is validated through experimental tests carried out on three devices having different mechanical and geometrical characteristics, designed primarily to achieve an increasing coupling strength. A strict agreement between overall theoretical scaling laws and numerical computations is also found.

Modulation transfer function cascade model for a sampled IR imaging system

The performance of the infrared scanning radiometer (IRSR) is strongly stressed in convective heat transfer applications where high spatial frequencies in the signal that describes the thermal image are present. The need to characterize more deeply the system spatial resolution has led to the formulation of a cascade model for the evaluation of the actual modulation transfer function of a sampled IR imaging system. The model can yield both the aliasing band and the averaged modulation response for a general sampling subsystem. For a line scan imaging system, which is the case of a typical IRSR, a rule of thumb that states whether the combined sampling-imaging system is either imaging-dependent or sampling-dependent is proposed. The model is tested by comparing it with other noncascade models as well as by ad hoc measurements performed on a commercial digitized IRSR.

Insights on the impact of a plane drop on a thin liquid film

Numerical simulations of early and intermediate instants of a plane two-dimensional drop impact on a preexisting thin film of the same liquid are performed. The evolution of the phenomenon is analyzed by solving the free-surface Navier–Stokes equations by means of a volume of fluid (VOF) method. Viscous, inertial and surface tension forces are taken into account; gravity is neglected. The so-called splashing regime is emphasized, where the emergence of an initial horizontal ejecta sheet is followed by the formation of an almost vertical lamella sheet, which is the planar counterpart of the well known splashing-crown of spherical geometry. Overall velocity and pressure fields as well as detailed interface shapes are presented, and several insights on the relevant scaling laws are furnished. In the ejecta sheet (jet) regime a major result is the finding of a deviation from the standard square root behavior for the dependence on time of the contact length of sheet first emergence, which is proved to be cruci...

LEM Characterization of Synthetic Jet Actuators Driven by Piezoelectric Element: A Review

In the last decades, Synthetic jet actuators have gained much interest among the flow control techniques due to their short response time, high jet velocity and absence of traditional piping, which matches the requirements of reduced size and low weight. A synthetic jet is generated by the diaphragm oscillation (generally driven by a piezoelectric element) in a relatively small cavity, producing periodic cavity pressure variations associated with cavity volume changes. The pressured air exhausts through an orifice, converting diaphragm electrodynamic energy into jet kinetic energy. This review paper considers the development of various Lumped-Element Models (LEMs) as practical tools to design and manufacture the actuators. LEMs can quickly predict device performances such as the frequency response in terms of diaphragm displacement, cavity pressure and jet velocity, as well as the efficiency of energy conversion of input Joule power into useful kinetic power of air jet. The actuator performance is also analyzed by varying typical geometric parameters such as cavity height and orifice diameter and length, through a suited dimensionless form of the governing equations. A comprehensive and detailed physical modeling aimed to evaluate the device efficiency is introduced, shedding light on the different stages involved in the process. Overall, the influence of the coupling degree of the two oscillators, the diaphragm and the Helmholtz frequency, on the device performance is discussed throughout the paper.

On transient growth oscillations in linear models

The study of oscillating norm behavior in linear systems is addressed. The analysis is carried out by considering a model equation that naturally arises in the context of the linearized formulation of some convection-dominated systems over finite length domains. The occurrence of oscillating patterns in the energy evolution of the solutions is linked to the structure of the spectrum and to the non-normal character of the linear operator involved. A distinction between transient and asymptotic oscillations is made and some relations between the frequency signatures shown in both cases and global characteristics of the spectrum are highlighted. The physical importance of such oscillating behaviors is stressed by reconsidering a model for the linear stability of a falling liquid curtain studied by other authors in a previous work.

Intermediate and late follow-up of the use of apico-aortic conduits in the surgical treatment of hypertrophic cardiomyopathy

Between December 1977 and July 1983, four apico-aortic prosthetic valved conduits were implanted in four patients affected by severe hypertrophic cardiomyopathy. The disease was isolated in two cases, associated with critical stenosis of the right coronary artery in one case and with a subaortic tunnel plus aortic valve regurgitation in the final case. Three patients were male and one female; the ages ranged from 6 to 49 years. All patients survived surgery and were followed-up. At present, all are living and have no postoperative symptoms. The left ventricular-aortic gradient was relieved in all cases. Echocardiographic studies during the follow-up showed an enlargement of the left ventricular cavity with a satisfactory cardiac contraction. Three years following surgery the female patient gave birth to a normal male baby. Her pregnancy was normal and without complications. Seven years after surgery the first patient was successfully reoperated because of conduit thrombosis caused by degeneration of the biological porcine valve. In the follow-up, which ranges from 18 months to 7 years (mean 4.5 years) all patients are in Functional Class I. All preoperative symptoms have been relieved for all patients and there has been no incidence of late or sudden death.

Image restoration in thermo-fluid-dynamic applications of IR digital imagery

The importance of image restoration is increasing with the widespread use of imaging systems in various sectors of modern technology. However, as far as the quantitative application of computerized Infrared Scanning Radiometers to the study of thermo-fluid--dynamic problems is concerned, it seems that the image restoration is not addressed in a satisfactory way by the currently available commercial systems. The aim of the present paper is to yield a contribution in this direction. While defining a degradation model, in particular, the effects (if any) due to digital sampling are also taken into account by means of a cascade model including both imaging and sampling effects. Two heat transfer applications (in presence of natural and forced convection) are discussed.

Global dynamics analysis of nappe oscillation

The unsteady global dynamics of a gravitational liquid sheet interacting with a one-sided adjacent air enclosure, typically referred to as nappe oscillation, is addressed, under the assumptions of potential flow and absence of surface tension effects. To the purpose of shedding physical insights, the investigation examines both the dynamics and the energy aspects. An interesting re-formulation of the problem consists of recasting the nappe global behavior as a driven damped spring-mass oscillator, where the inertial effects are linked to the liquid sheet mass and the spring is represented by the equivalent stiffness of the air enclosure acting on the average displacement of the compliant nappe centerline. The investigation is carried out through a modal (i.e., time asymptotic) and a non-modal (i.e., short-time transient) linear approach, which are corroborated by direct numerical simulations of the governing equation. The modal analysis shows that the flow system is characterized by low-frequency and high...