Stability Analysis of Nonlinear Rotating Systems Using Lyapunov Characteristic Exponents Estimated From Multibody Dynamics

The use of Lyapunov Characteristic Exponents to assess the stability of nonlinear, time-dependent mechanical systems is discussed. Specific attention is dedicated to methods capable of estimating the largest exponent without requiring the Jacobian matrix of the problem, which can be applied to time histories resulting from simulations performed with existing multibody solvers. Helicopter ground resonance is analyzed as the reference application. Improvements over the available literature are: the problem is formulated in physical coordinates, without eliminating periodicity through multiblade coordinates; the rotation of the blades is not linearized; the problem is modeled considering absolute positions and orientations of parts. The dynamic instability that arises at some angular velocities when the isotropy of the rotor is broken (e.g., caused by the failure of one lead-lag damper, a design test condition) is observed to evolve into a large amplitude limit cycle, where the usual Floquet-Lyapunov analysis of the linearized time-periodic simply predicts instability

Stability Analysis of Nonlinear Rotating Systems Using Lyapunov Characteristic Exponents Estimated From Multibody Dynamics

The use of Lyapunov Characteristic Exponents to assess the stability of nonlinear, time-dependent mechanical systems is discussed. Specific attention is dedicated to methods capable of estimating the largest exponent without requiring the Jacobian matrix of the problem, which can be applied to time histories resulting from simulations performed with existing multibody solvers. Helicopter ground resonance is analyzed as the reference application. Improvements over the available literature are: the problem is formulated in physical coordinates, without eliminating periodicity through multiblade coordinates; the rotation of the blades is not linearized; the problem is modeled considering absolute positions and orientations of parts. The dynamic instability that arises at some angular velocities when the isotropy of the rotor is broken (e.g., caused by the failure of one lead-lag damper, a design test condition) is observed to evolve into a large amplitude limit cycle, where the usual Floquet-Lyapunov analysis of the linearized time-periodic simply predicts instability

Dermal regeneration template: reconstruction in oral cancer defects

BackgroundPost ablative oral mucosal defect resulting from the removal of tumors can be treated with various techniques.PurposeIn this paper, we are showing what, in our experience, are the advantages and disadvantages given using biosynthetic skin substitutes when dealing with this kind of lesions.Materials and methodsPatients included in the sample came to our attention with both neoplastic lesions (11 subjects) and important scar retraction after previous oncologic surgery (1 subject). All patients underwent trans-oral resection surgery following the same surgical protocol and post ablative oral mucosal defect were treated using the dermal regeneration template. The surgical defect location, size, and time of removal of the silicone layer varied from one subject to the other.ResultsMost patients showed good healing with reduced scarring and adequate remucosalisation of the defect. The main complications were shown in a palatal lesion treated with concomitant osteal resection, which developed an oroantral fistula at follow up, and tongue lesions which showed some scarring.ConclusionsGiven our experience, we would advise using dermal substitutes when reconstructing oral defects only after a cautious evaluation of the area of the lesion, the gap size, the possible adherence of the membrane to the gap, and the presence of tissue supporting the overlying membrane

Gingival reactive lesions in orally rehabilitated patients by free revascularized flap

The aim is to discuss four cases of gingival reactive hyperplastic lesions in patients with a history of excision of oral neoplastic lesions and rehabilitation by a free revascularized flap of the iliac crest. One female and 3 male patients were referred due to the presence of exophytic lesions at the rehabilitated sites. The clinical examination revealed that the poor oral hygiene was the common trigger factor in all the cases, in addition to trauma from the upper left second molar in the first case, pericoronitis related to a partially erupted lower right third molar in the third case, and poor stability of an upper removable partial denture in the fourth case. All the cases were subjected to elimination of these suspected triggering factors, exclusion of dysplasia, excisional biopsy by CO2 laser, and five follow-up visits. The histological examination of all the cases confirmed the diagnosis of pyogenic granuloma. These presented cases suggest that the limitations in oral functions and maintaining the oral hygiene measures following the free revascularized flap reconstruction surgery probably played a role in the development of gingival reactive hyperplastic lesions with presence of trigger factors such as local trauma, chronic infection, or inadequate prosthesis

Rotorcraft stability analysis using Lyapunov characteristic exponents estimated from multibody dynamics

Stability analysis of complex, nonlinear dynamical systems is a challenge. The use of Lyapunov Characteristic Exponents through a Jacobian-less method is proposed as a means to identify the Maximum Lyapunov Characteristic Exponent, namely the fundamental stability indicator of a generic problem, solely from time series obtained through general-purpose multibody dynamics simulations of complex rotorcraft aeromechanics models. The method is first applied to a relatively simple scenario concerning the identification of ground resonance. Then, its application to more complex models is addressed by studying the aeroelastic stability and identifying the whirl flutter of the XV-15 tiltrotor using a comprehensive aeroelastic model

Surgical management of the acromegalic face: Could the aesthetic improvement of the face influence the patient's QoL? Combined surgical approach

Acromegaly is a chronic and progressive disease related to a disorder of growth hormone production. It may present with a variety of clinical signs and a dento-facial deformity and this results in a loss of self-esteem and a reduction of Quality of Life. Presentation of case: A 38-years–old male patient, affected by acromegaly with class III malocclusion, noticeable nose deformity and macroglossia was treated. Bi-maxillary orthognathic surgery and partial glossectomy have been performed in one-step surgey. Open rhinoplasty was done in a second step. Both dento-skeletal class III and restoration of the facial appearance have been solved. Dento-skeletal class III was completly solved together with the restoration of the facial appearance and the patient satisfaction has been achieved. Conclusions: This case report describes the successful and stable treatment of an adult patient affected by acromegaly

Naso-Ethmoidal Phosphaturic Mesenchymal Tumor: A Rare Tumor Site for an Uncommon Paraneoplastic Syndrome

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