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Dive into the research topics where Nicola de Divitiis is active.

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Featured researches published by Nicola de Divitiis.


Journal of Aircraft | 2006

Performance and Stability Analysis of a Shrouded-Fan Unmanned Aerial Vehicle

Nicola de Divitiis

This paper deals with the estimation of the performance and stability for a shrouded-fan unmanned rotorcraft whose mission profile also prescribes the flight in ground effect. The not so simple estimation of the aerodynamic coefficients and of the thrust in the various situations makes the performance calculation and the stability analysis difficult tasks. This is because of the strong interaction between the fan flow and shroud that causes quite different flow structures about the airframe depending on flight conditions. A further difficulty is related to the ground effect which produces substantial modifications in the rotor thrust and aerodynamic coefficients. To evaluate performance and stability, two models have been developed. One determines the aerodynamic coefficients of the shroud, whereas the other one calculates thrust and moment of the rotors system. Both models take into account the mutual interference between fan flow and fuselage and ground effect. Performance and stability are then discussed with reference to significant flight conditions.


Journal of Aircraft | 2005

Performance and Stability of a Winged Vehicle in Ground Effect

Nicola de Divitiis

The present work deals with the dynamics of vehicles that intentionally operate in the ground proximity. The dynamics in ground effect is influenced by the vehicle orientation with respect to the ground because the aerodynamic force and moment coefficients, which in turn depend on height and angle of attack, also vary with the Euler angles. This feature, usually neglected in the applications, can be responsible for sizable variations of the aircraft performance and stability. A further effect, caused by the sink rate, determines the unsteadiness that modifies the aerodynamic coefficients. In this work, an analytical formulation is proposed for the force and moment calculation in the presence of the ground and taking the aircraft attitude and sink rate into account. The aerodynamic coefficients are first calculated for a representative vehicle, and its characteristics in ground effect are investigated. Performance and stability characteristics are then discussed with reference to significant equilibrium conditions, while the nonlinear dynamics is studied through the numerical integration of the equations of motion.


Theoretical and Computational Fluid Dynamics | 2011

Lyapunov analysis for fully developed homogeneous isotropic turbulence

Nicola de Divitiis

The present work studies the isotropic and homogeneous turbulence for incompressible fluids through a specific Lyapunov analysis. The analysis consists in the calculation of the velocity fluctuation through the Lyapunov theory applied to the local deformation using the Navier-Stokes equations, and in the study of the mechanism of energy cascade through the finite scale Lyapunov analysis of the relative motion between two particles. The analysis provides an explanation for the mechanism of energy cascade, leads to the closure of the von Kármán-Howarth equation, and describes the statistics of the velocity difference. Several tests and numerical results are presented.


Journal of Spacecraft and Rockets | 2010

Fully Structured Aerodynamic Model for Parameter Identification of a Reentry Experimental Vehicle

Nicola de Divitiis; Antonio Vitale

Vehicle preflight aerodynamic databases are frequently validated and improved using system identification techniques that require the availability of flight data and the definition of suitable aerodynamic models. In this framework, the paper presents a novel theoretical aerodynamic model to be used for the aerodynamic identification of a lifting body in subsonic, transonic, and supersonic regimes. The proposed model is based on the general properties of the continuity equation in the von Karman form. Each aerodynamic coefficient incorporates free parameterstobeestimatedfromthe flightdataanalysisandtakesintoaccountthesimultaneouseffectsoftheseveral vehicle state variables. The proposed model was applied to the identification from the flight data of an experimental vehicle of the Italian Aerospace Research Centre. Identification results show that the model is able to perform an excellent fitting of the aerodynamic coefficients experimented in flight by the vehicle.


Journal of Aircraft | 2003

Wind estimation on a lightweight vertical-takeoff- and-landing uninhabited vehicle

Nicola de Divitiis

Wind-velocity measurement on a rotary-wing aircraft is a difficult task because of the flow induced by the rotors. The purpose of this paper is to develop a method to estimate the wind velocity components from the measurement of the state variables of a rotorcraft in the moving atmosphere. The algorithm presented is in the framework of the output error method. The wind-velocity components were estimated using a novel variational formulation. The method uses airframe and rotor models that calculate the aerodynamic and thrust coefficients by means of an artificial neural-network technique. To validate the method, the results are compared to wind-velocity estimates from a Kalman-Bucy filter


Journal of Aircraft | 2002

Effect of Microlift Force on the Performance of Ultralight Aircraft

Nicola de Divitiis

Microlift force, which is the aerodynamic force developed in the presence of a wind gradient, can be responsible for a sizeable improvement of gliding performance of ultralight aircraft. A mathematical formulation based on Lagranges equation method for the analysis of microlift is presented, where the effects of vertical velocity gradients on aircraft motion are taken into account. After giving some details on the modeled vehicle, the behavior of an ultralight sailplane during flight in a wind gradient is investigated. Optimal maneuvers are computed to interpret and discuss the performance of ultralight sailplanes soaring in a wind gradient. The proposed approach provides an effective means for understanding and improving the piloting techniques for sailplane and ultralight aircraft in nonuniform wind


Annals of Physics | 2015

Bifurcations analysis of turbulent energy cascade

Nicola de Divitiis

Abstract This note studies the mechanism of turbulent energy cascade through an opportune bifurcations analysis of the Navier–Stokes equations, and furnishes explanations on the more significant characteristics of the turbulence. A statistical bifurcations property of the Navier–Stokes equations in fully developed turbulence is proposed, and a spatial representation of the bifurcations is presented, which is based on a proper definition of the fixed points of the velocity field. The analysis first shows that the local deformation can be much more rapid than the fluid state variables, then explains the mechanism of energy cascade through the aforementioned property of the bifurcations, and gives reasonable argumentation of the fact that the bifurcations cascade can be expressed in terms of length scales. Furthermore, the study analyzes the characteristic length scales at the transition through global properties of the bifurcations, and estimates the order of magnitude of the critical Taylor-scale Reynolds number and the number of bifurcations at the onset of turbulence.


Applied Mathematical Modelling | 2014

Finite scale Lyapunov analysis of temperature fluctuations in homogeneous isotropic turbulence

Nicola de Divitiis

Abstract This study analyzes the temperature fluctuations in incompressible homogeneous isotropic turbulence through the finite scale Lyapunov analysis of the relative motion between two fluid particles. The analysis provides an explanation of the mechanism of the thermal energy cascade, leads to the closure of the Corrsin equation, and describes the statistics of the longitudinal temperature derivative through the Lyapunov theory of the local deformation and the thermal energy equation. The results here obtained show that, in the case of self-similarity, the temperature spectrum exhibits the scaling laws κ n , with n ≈ - 5 / 3 , - 1 and - 17 / 3 ÷ - 11 / 3 depending upon the flow regime. These results are in agreement with the theoretical arguments of Obukhov–Corrsin and Batchelor and with the numerical simulations and experiments known from the literature. The PDF of the longitudinal temperature derivative is found to be a non-gaussian distribution function with null skewness, whose intermittency rises with the Taylor scale Peclet number. This study applies also to any passive scalar which exhibits diffusivity.


Annals of Physics | 2016

Von Kármán-Howarth and Corrsin equations closure based on Lagrangian description of the fluid motion

Nicola de Divitiis

Abstract A new approach to obtain the closure formulas for the von Karman–Howarth and Corrsin equations is presented, which is based on the Lagrangian representation of the fluid motion, and on the Liouville theorem associated to the kinematics of a pair of fluid particles. This kinematics is characterized by the finite scale separation vector which is assumed to be statistically independent from the velocity field. Such assumption is justified by the hypothesis of fully developed turbulence and by the property that this vector varies much more rapidly than the velocity field. This formulation leads to the closure formulas of von Karman–Howarth and Corrsin equations in terms of longitudinal velocity and temperature correlations following a demonstration completely different with respect to the previous works. Some of the properties and the limitations of the closed equations are discussed. In particular, we show that the times of evolution of the developed kinetic energy and temperature spectra are finite quantities which depend on the initial conditions.


The Journal of Engineering | 2013

Refinement of a Previous Hypothesis of the Lyapunov Analysis of Isotropic Turbulence

Nicola de Divitiis

The purpose of this paper is to improve a hypothesis of the previous work of N. de Divitiis (2011) dealing with the finite-scale Lyapunov analysis of isotropic turbulence. There, the analytical expression of the structure function of the longitudinal velocity difference is derived through a statistical analysis of the Fourier transformed Navier-Stokes equations and by means of considerations regarding the scales of the velocity fluctuations, which arise from the Kolmogorov theory. Due to these latter considerations, this Lyapunov analysis seems to need some of the results of the Kolmogorov theory. This work proposes a more rigorous demonstration which leads to the same structure function, without using the Kolmogorov scale. This proof assumes that pair and triple longitudinal correlations are sufficient to determine the statistics of and adopts a reasonable canonical decomposition of the velocity difference in terms of proper stochastic variables which are adequate to describe the mechanism of kinetic energy cascade.

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Federico Corraro

Italian Aerospace Research Centre

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Guido De Matteis

Sapienza University of Rome

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Antonio Vitale

University of Naples Federico II

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