Riccardo Panciroli

A particle image velocimetry study of the flow physics generated by a thin lamina oscillating in a viscous fluid

In this paper, we study the flow physics produced by a thin rigid lamina oscillating in an otherwise quiescent viscous fluid. Particle image velocimetry (PIV) is used to extract the flow kinematics, which is, in turn, utilized to reconstruct the pressure distribution around the lamina through the integration of Navier-Stokes equations. The hydrodynamic loading experienced by the lamina is ultimately estimated from PIV data to investigate added mass and fluid damping phenomena. Experiments are conducted for varying Reynolds and Keulegan-Carpenter numbers to elucidate the relative weight of inertial, convective, and viscous phenomena on the resulting flow physics. In agreement with prior numerical studies, experimental results demonstrate that increasing the Reynolds and the Keulegan-Carpenter numbers results into the formation of coherent structures that are shed at the edges of the lamina and advected by the flow. This phenomenon is associated with nonlinearities in the hydrodynamic loading, whereby fluid...

Hydroelastic Impacts of Deformable Wedges

This work investigates the slamming phenomenon experienced during the water entry of deformable bodies. Wedges are chosen as reference geometry due to their similarity to a generic hull section. Hull slamming occurs when a ship re-enters the water after having been partially or completely lifted out the water. There are three more cases commonly defined as slamming phenomena: bow-flare, wet-deck and green water slamming. These are all special cases of the general topic of water entry of a body. While the analysis of rigid structures entering the water has been extensively studied in the past and there are analytical solutions capable of correctly predicting the hydrodynamic pressure distribution and the overall impact dynamics, the effect of the structural deformation on the overall impact force is still a challenging problem to be solved. In fact, in case of water impact of deformable bodies, the dynamic deflection could interact with the fluid flow, affecting the hydrodynamic load. This work investigates the hull-slamming problem by experiments and numerical simulations of the water entry of elastic wedges impacting on an initially calm surface with pure vertical velocity. The objective is to determine an accurate model to predict the overall dynamics of the wedge and its deformations. More than 1,200 experiments were conducted by varying wedge structural stiffness, deadrise angle, impact velocity and mass. On interest are the overall impact dynamics and the local structural deformation of the panels composing the wedge. Alongside with the experimental analysis, numerical simulations based on a coupled Smoothed Particle Hydrodynamics (SPH) and FEM method are developed. Ranges of applicability of a simplified model neglecting the air are found. The results provide evidence of the mutual interaction between hydrodynamic load and structural deformation. It is found a simple criterion for the onset of fluid structure interaction (FSI), giving reliable information on the cases where FSI should been taken into account. The occurrence of ventilation and cavitation varying the impact parameters are also outlined.

Dynamic response of sandwich shells to underwater blasts

This paper presents a general approach for analysing the response of beam, plates and shells to short duration pressure pulses typically encountered as a result of underwater explosions. The problem is formulated in terms of curvilinear coordinates and the resulting equations can then be specialized for each particular geometry. Results are presented for several examples.

Fluid-structure interaction during the water entry of flexible cylinders

In this work, we experimentally study the water entry of flexible cylinders. Experiments are performed in free fall and we explore variations of the entry velocity by varying the drop height. High speed imaging is utilized to study the fluid kinematics, the pile-up evolution, the cavity formation, and the overall structural deflection. The impact dynamics is analyzed through accelerometers, whereby fibre bragg gratings (FBG) measure the punctual deformation at characteristic locations on the cylinder surface. A modal decomposition approach is utilized to reconstruct the overall structural deflection from the punctual strain measurements. The proposed reconstruction methodology is compared against high-speed images. Results show that during the water entry the cylinder mainly deforms in the direction of the hydrodynamic loading, whereby marked vibrations whose amplitude increase with the entry velocity dominate the dynamic response.

Dynamic monitoring of compliant bodies impacting the water surface through local strain measurements

The understanding and the experimental characterization of the evolution of impulsive loading is crucial in several fields in structural, mechanical and ocean engineering, naval architecture and aerospace. In this regards, we developed an experimental methodology to reconstruct the deformed shape of compliant bodies subjected to impulsive loadings, as those encountered in water entry events, starting from a finite number of local strain measurements performed through Fiber Bragg Gratings. The paper discusses the potential applications of the proposed methodology for: i) real-time damage detection and structural health monitoring, ii) fatigue assessment and iii) impulsive load estimation.

Dynamic Response of Sandwich Structures to Impulsive Loads

This article considers the dynamic response of sandwich structures to various pulses. During the early phase, waves propagating through the thickness of the sandwich can cause damage to the core. Once the overall bending deformation is established, a different type of response is observed. The challenge in the analysis is to capture both the early phase and the long term response. It is shown that, because of the kinematic assumptions made in their development, plate and shell theories cannot adequately capture the early phase of the response. Here the dynamic behavior of sandwich structures is investigated in details. Two- and three-dimensional finite element models are used to assess the applicability of beam, plate, and shell theories and determine the response to various pulses. For beams and plates, only a few modes contribute to the overall response but for shells, curvature effects can cause many modes to participate. It is shown that shell theories predict the overall response adequately and that for the early phase of the deformation the method of characteristics and a simple model suitable for design calculations is developed. Previous investigators have considered examples in which the duration of the loading was large compared to the period of the first mode. In practical applications, the reverse is likely to occur and then a different type of response is observed. In that the response depends on the pressure impulse.Copyright

Structural health monitoring through fiber Bragg grating strain sensing

In this work, we leverage a modal decomposition analysis to develop a methodology for the live structural health monitoring. The proposed methodology relies on punctual strain measurements performed utilizing a fiber bragg grating (FBG) system. We propose an experimental measurement chain for the live monitoring of the structural kinetics and the distributed stress field. Its effectiveness is verified by performing experiments on the water entry of compliant bodies. Structural punctual strains are synchronously acquired at several locations by FBG sensors through a homemade high-speed optical interrogator. The structural deformation is reconstructed through its decomposition over a finite number of mode shapes. The combination of the high-speed FBG interrogator with the computationally inexpensive reconstruction technique based on modal decomposition allows for real time monitoring of structures subjected to impulsive loadings.

An Introduction to Self-Excited Oscillations

This paper describes an approach used to introduce a type of nonlinear problems in an undergraduate class on mechanical vibrations. Self-excited oscillations are encountered in a number of practical applications including brakes, clutches, belts, tires, and violins. To go beyond the derivation of the equations of motion for simplified models and examine the effect of various parameters requires the ability to find numerical solutions. It was found that developing numerical solutions using a simple integration technique such as Euler’s method with a spreadsheet program like Excel was most effective because: (1) Euler’s method is easy to implement; (2) Excel is widely available; (3) students are able to develop the solution themselves; (4) it can be done quickly. In this case students were able to explore problems with one or more degrees of freedom and compare their results with those found in recent publications which presents several advantages: students develop confidence in their ability to explore different models and examine the effects of different complicating factors, they develop their own solutions and are able to focus on understanding the physics of the problem, and they develop a sense that they are working on problems of current interest instead of some overly simplified textbook problem. Examples dealing with brake squeal problem were used and the effects of mass, stiffness, damping and friction were studied. Many different friction models are available and several of them were used to determine the effect of friction on the appearance of self-excited vibrations. The appearance of a limit cycle in the phase portrait is discussed along with the dynamics of the system. It is also shown that a short high frequency excitation can be used to squelch those self-excited oscillations.Copyright