Two-dimensional simulations of Rayleigh-Taylor instability in elastic-plastic media.

Two-dimensional numerical simulations for the Rayleigh-Taylor instability in an elastic-plastic medium are presented. Recent predictions of the theory regarding the asymmetric growth of peaks and valleys during the linear phase of the instability evolution are confirmed. Extension to the nonlinear regime reveals singular features, such as the long delay in achieving the nonlinear saturation and an intermediate phase with growth rate larger than the classical one.

Formation of spikes and bubbles in the linear phase of Rayleigh-Taylor instability in elastic-plastic media.

The generation of spikes and bubbles, a typical characteristic of the nonlinear regime in the Rayleigh-Taylor instability, is found to occur as well during the linear regime in an elastic-plastic solid medium caused, however, by a very different mechanism. This singular feature originates in the differential loads at different locations of the interface, which makes that the transition from the elastic to the plastic regime takes place at different times, thus producing an asymmetric growth of peaks and valleys that rapidly evolves in exponentially growing spikes, while bubbles can also grow exponentially at a lower rate.

Cylindrical convergence effects on the Rayleigh-Taylor instability in elastic and viscous media.

Convergence effects on the perturbation growth of an imploding surface separating two nonideal material media (elastic and viscous media) are analyzed in the case of a cylindrical implosion in both the Rayleigh-Taylor stable and unstable configurations. In the stable configuration, the perturbation damping effect due to angular momentum conservation becomes destroyed for sufficiently high values of the elastic modulus or of the viscosity of the media. For the unstable configuration, Rayleigh-Taylor instability can be suppressed by the elasticity or mitigated by the viscosity, but without practically affecting the perturbation growth due to the geometrical convergence. However, the convergence effects manifest themselves in a manner somewhat different from the classical Bell-Plesset effect by making the process more sensitive to the media compressibility than in the case involving ideal media.

Elastic-plastic Rayleigh-Taylor instability at a cylindrical interface.

The boundaries of stability are determined for the Rayleigh-Taylor instability at a cylindrical interface between an ideal fluid in the interior and a heavier elastic-plastic solid in the outer region. The stability maps are given in terms of the maximum dimensionless initial amplitude ξ_{th}^{*} that can be tolerated for the interface to remain stable, for any particular value of the dimensionless radius B of the surface, and for the different spatial modes m of the perturbations. In general, for the smallest dimensionless radius and larger modes m, the interface remains stable for larger values of ξ_{th}^{*}. In particular, for m>1 and B→0, it turns out ξ_{th}^{*}→1, and a cylindrical geometry equivalent to Drucker's criterion is found, which indeed ends up being independent of the interface geometry.

Rayleigh-Taylor instability in elastic-plastic solid slabs bounded by a rigid wall.

The linear evolution of the incompressible Rayleigh-Taylor instability for the interface between an elastic-plastic slab medium and a lighter semi-infinite ideal fluid beneath the slab is developed for the case in which slab is attached to a rigid wall at the top surface. The theory yields the maps for the stability in the space determined by the initial perturbation amplitude and wavelength, as well as for the transition boundary from the elastic to the plastic regimes for arbitrary thicknesses of the slab and density contrasts between the media. In particular, an approximate but very accurate scaling law is found for the minimum initial perturbation amplitude required for instability and for the corresponding perturbation wavelength at which it occurs. These results allows for an interpretation of the recent experiments by Maimouni et al. [Phys. Rev. Lett. 116, 154502 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.154502].

Studies of equation of state properties of high-energy-density matter generated by intense ion beams at the facility for antiprotons and ion research.

The work presented in this paper shows with the help of two-dimensional hydrodynamic simulations that intense heavy-ion beams are a very efficient tool to induce high energy density (HED) states in solid matter. These simulations have been carried out using a computer code BIG2 that is based on a Godunov-type numerical algorithm. This code includes ion beam energy deposition using the cold stopping model, which is a valid approximation for the temperature range accessed in these simulations. Different phases of matter achieved due to the beam heating are treated using a semiempirical equation-of-state (EOS) model. To take care of the solid material properties, the Prandl-Reuss model is used. The high specific power deposited by the projectile particles in the target leads to phase transitions on a timescale of the order of tens of nanosecond, which means that the sample material achieves thermodynamic equilibrium during the heating process. In these calculations we use Pb as the sample material that is irradiated by an intense uranium beam. The beam parameters including particle energy, focal spot size, bunch length, and bunch intensity are considered to be the same as the design parameters of the ion beam to be generated by the SIS100 heavy-ion synchrotron at the Facility for Antiprotons and Ion Research (FAIR), at Darmstadt. The purpose of this work is to propose experiments to measure the EOS properties of HED matter including studies of the processes of phase transitions at the FAIR facility. Our simulations have shown that depending on the specific energy deposition, solid lead will undergo phase transitions leading to an expanded hot liquid state, two-phase liquid-gas state, or the critical parameter regime. In a similar manner, other materials can be studied in such experiments, which will be a very useful addition to the knowledge in this important field of research.

Stability boundaries for the Rayleigh-Taylor instability in accelerated elastic-plastic solid slabs.

The linear theory of the incompressible Rayleigh-Taylor instability in elastic-plastic solid slabs is developed on the basis of the simplest constitutive model consisting in a linear elastic (Hookean) initial stage followed by a rigid-plastic phase. The slab is under the action of a constant acceleration, and it overlays a very thick ideal fluid. The boundaries of stability and plastic flow are obtained by assuming that the instability is dominated by the average growth of the perturbation amplitude and neglecting the effects of the higher oscillation frequencies during the stable elastic phase. The theory yields complete analytical expressions for such boundaries for arbitrary Atwood numbers and thickness of the solid slabs.

Linear Rayleigh-Taylor instability in an accelerated Newtonian fluid with finite width.

The linear theory of Rayleigh-Taylor instability is developed for the case of a viscous fluid layer accelerated by a semi-infinite viscous fluid, considering that the top interface is a free surface. Effects of the surface tensions at both interfaces are taken into account. When viscous effects dominate on surface tensions, an interplay of two mechanisms determines opposite behaviors of the instability growth rate with the thickness of the heavy layer for an Atwood number A_{T}=1 and for sufficiently small values of A_{T}. In the former case, viscosity is a less effective stabilizing mechanism for the thinnest layers. However, the finite thickness of the heavy layer enhances its viscous effects that, in general, prevail on the viscous effects of the semi-infinite medium.

Finite-thickness effects on the Rayleigh-Taylor instability in accelerated elastic solids.

A physical model has been developed for the linear Rayleigh-Taylor instability of a finite-thickness elastic slab laying on top of a semi-infinite ideal fluid. The model includes the nonideal effects of elasticity as boundary conditions at the top and bottom interfaces of the slab and also takes into account the finite transit time of the elastic waves across the slab thickness. For Atwood number A_{T}=1, the asymptotic growth rate is found to be in excellent agreement with the exact solution [Plohr and Sharp, Z. Angew. Math. Mech. 49, 786 (1998)10.1007/s000330050121], and a physical explanation is given for the reduction of the stabilizing effectiveness of the elasticity for the thinner slabs. The feedthrough factor is also calculated.

Rayleigh-Taylor instability in accelerated elastic-solid slabs.

We develop the linear theory for the asymptotic growth of the incompressible Rayleigh-Taylor instability of an accelerated solid slab of density ρ_{2}, shear modulus G, and thickness h, placed over a semi-infinite ideal fluid of density ρ_{1}<ρ_{2}. It extends previous results for Atwood number A_{T}=1 [B. J. Plohr and D. H. Sharp, Z. Angew. Math. Phys. 49, 786 (1998)ZAMPA80044-227510.1007/s000330050121] to arbitrary values of A_{T} and unveil the singular feature of an instability threshold below which the slab is stable for any perturbation wavelength. As a consequence, an accelerated elastic-solid slab is stable if ρ_{2}gh/G≤2(1-A_{T})/A_{T}.

Tratamiento quirúrgico de fracturas transversas más pared posterior de acetábulo / Surgical treatment of transverse and posterior acetabular wall fractures

Objective: Describe our experience in the surgical treatment of transverse with posterior wall fractures of the acetabulum, the elements that help in the choice of the approach and functional and radiological results of this type of injury. Methods: We retrospectively evaluated 9 patients (9 fractures) with transverse and posterior wall fractures operated between August 2008 and September 2010. Six patients were men and 3 women with an average age of 40.4 years. We reviewed the surgical protocols, the perioperative data and the pre and post operative radiological images. We registered the complications and the functional results with the Merle D’Aubigne and Postel score. Results: All cases were considered secondary to high energy mechanisms. Five patients had posterior dislocation of the hip and 4 had central dislocation. Eight patients had associated injuries. In the early postoperative period there were no complications. One patient had posttraumatic osteoarthritis and required total hip arthroplasty. The average of Merle d’Aubigne score was 16.1 points. Conclusion: Surgical treatment for this type of injury had good results in this group of patients.

Objetivo: Describir nuestra experiencia en el tratamiento quirúrgico de las fracturas transversas más pared posterior de acetábulo, los elementos que ayudan en la elección del abordaje y los resultados funcionales y radiológicos de este tipo de lesiones. Material y Métodos: Estudio retrospectivo y descriptivo de una serie consecutiva de 9 pacientes (9 fracturas) operados de una fractura transversa más pared posterior de acetábulo entre agosto de 2008 y septiembre de 2010. Seis hombres y 3 mujeres con una mediana de edad de 40,4 años se operaron en este período. Se revisaron los protocolos operatorios, las fi chas clínicas y las imágenes pre y postoperatorias. Se evaluaron las complicaciones tempranas y tardías y los resultados funcionales de acuerdo a la escala de Merle D’Aubigne y Postel a un seguimiento promedio de 17 meses. Resultados: Todos los casos se consideraron secundarios a mecanismos de alta energía. Cinco pacientes presentaron luxación posterior de cadera y 4 presentaron luxación central. Ocho pacientes presentaron lesiones asociadas. En el postoperatorio temprano no se presentaron complicaciones. Un paciente presentó artrosis post traumática y requirió artroplastía total de cadera. El puntaje promedio de Merle d’Aubigne fue de 16,1 puntos. Conclusión: El tratamiento quirúrgico para este tipo de lesiones tiene buenos resultados en el corto plazo en este grupo de pacientes.

Técnica quirúrgica para facilitar lacolocación de tornillos sacroilíacospercutáneos / Percutaneous iliosacral screw fixation

Percutaneous iliosacral screw fixation is a useful technique for fixation of posterior pelvis ring injuries but it has potential complications. We describe a surgical technique modification of the original technique described by Routt etal. Using the surface of the C-arm as a guide in the inlet and outlet views, exact positioning of the guide wire is easier and surgical time as radiation exposure are decreased. We hope that this modification, in addition to a better understanding of sacral anatomical variations and risk factors for misplacement of iliosacral screws would reduce complications related to this procedure.

La fijación percutánea con tornillos sacroilíacos es una técnica de utilidad demostrada para la fijación de las fracturas de la pelvis posterior, sin embargo, no está exenta de riesgos. Describimos una modificación de la técnica quirúrgica original descrita por Routt et al. Utilizando las superficies del fluoroscopio como referencia en las vistas inlet y outlet, se facilita la orientación de la aguja guía disminuyendo el tiempo operatorio y el tiempo de radiación necesaria para la colocación de este tipo de osteosíntesis. Esperamos que esta modificación, asociado a un mejor conocimiento de la anatomía del sacro y los factores asociados a mal posición de los tornillos sacroilíacos permita disminuir los riesgos asociados a este procedimiento.

Fijación externa supra-acetabular de pelvis: nota técnica / Supra acetabular pelvic external fixation: technical note

Introduction: External fixation is a useful technique for the treatment of instable pelvic fractures. There are several configuration alternatives regarding to Schanz pin placement. In order to assure safe placement of the Shanz pins, it is useful to obtain an intraoperative fluoroscopic image of the supra-acetabular teardrop, wich indicates the linear osseous path between the anterior inferior iliac spine (AIIS) and the posterior superior iliac spine (PSIS). Material and Method: The surgical technique for safe supra-acetabular pin placement is described using an anatomic piece corresponding to the pelvis of a middle-aged male. A combination of outlet and obturador projections is sued to show an adequate fluoroscopic vision of the supra-acetabular teardrop. The result of this technique is demonstrated with a case report. Results: Having achieved a proper intraoperative vision of the supra-acetabular teardrop, the Schanz pins can be safely placed from the anterior inferior iliac spine (AIIS) through the osseous corridor in order to reach the posterior superior iliac spine (PSIS). Conclusions: Placing both Schanz pins in the supra-acetabular area and guided under fluoroscopic vision as described in this technique, allows reaching the posterior pelvis, achieving better control of bone fragments and compression of the posterior pelvic ring through an anterior approach. Therefore, this technique is an excellent alternative for external fixation of an unstable pelvic fracture.

Introducción: La fijación externa es de gran utilidad para el tratamiento de fracturas inestables de pelvis. Existen diversas opciones de configuración y alternativas para la colocación de los fijadores externos. Para la colocación segura de estas agujas de Schanz, la imagen fluoroscópica intra operatoria de la “lágrima” supra-acetabular, que indica el trayecto lineal entre la espina ilíaca antero inferior (EIAI) y la espina ilíaca postero superior (EIPS), resulta de gran utilidad. Material y Método: Se describe la técnica para la colocación de estas agujas en una pieza anatómica bajo visión fluoroscópica y cómo obtener una adecuada visión de la “lágrima” supra-acetabular al colocar el aparato de rayos en outlet-obsturatriz. Además se muestra el resultado de esta técnica en un caso clínico. Resultados: Obteniendo una adecuada visión intra-operatoria de la “lágrima” supra-acetabular se pueden colocar agujas de Schanz o cualquier material de osteosíntesis desde la espina ilíaca antero inferior hasta la espina ilíaca postero superior a través de un corredor óseo supra-acetabular. Conclusiones: Las agujas de Schanz colocadas en la zona supra-acetabular bajo control de rayos con la técnica aquí descrita permiten alcanzar la zona posterior de la pelvis (EIPS), logrando un mejor control de los fragmentos óseos y permitiendo la compresión desde anterior de la pelvis posterior. Son por lo tanto una excelente alternativa al momento de decidir una fijación externa de fracturas inestables de pelvis.

Hemodinamia inestable en fracturas de pelvis estables / Results in the surgical treatment of the unstable shoulder

We report two cases of hemodynamic instability secondary to stable pelvic fractures. Both patients had isolated arterial lesions with large pelvic hematomas. In the two cases embolization of the injured vessel was performed. Knowing the existence of this kind of traumatic lesion is crucial for early diagnosis improving the results in the treatment of this life-threatening condition. Patients with pelvic fractures should be under observation for long periods of time, because delayed hemorrhage can occur.

Presentamos dos casos de inestabilidad hemodinámica en pacientes con fracturas de pelvis que fueron consideradas estables. Los pacientes presentaban lesión arterial traumática aislada y hematoma para-vesical de gran tamaño. Ambos fueron tratados de manera exitosa mediante embolización de su lesión arterial. Conocer este tipo de lesiones y diagnosticarlas de manera oportuna son claves en el buen resultado clínico de una lesión potencialmente mortal pero frecuentemente subvalorada. Es aconsejable observar por períodos prolongados de tiempo a pacientes con fracturas de pelvis ya que pueden producirse sangrados tardíos.

In vitro antimicrobial susceptibility of anaerobic bacteria isolated from caprine footrot.

The in vitro antimicrobial susceptibility of Gram-negative anaerobic bacilli commonly isolated from footrot in goats was studied. A total of 97 isolates belonging to the genera Dichelobacter, Fusobacterium, Prevotella, Porphyromonas and Bacteroides, obtained from clinical cases of footrot in south-western Spain between March 2000 and May 2001, were tested against 25 antimicrobial agents comprising beta-lactams, aminoglycosides, macrolides, chloramphenicol, quinolones, lincosamides, sulphonamides and tetracyclines in order to optimise antibiotic treatment of this disease in goats. beta-lactams, tetracyclines and metronidazole displayed the highest in vitro efficacy against the species involved in the pathogenesis of footrot.

Aetiology of caprine footrot in Extremadura region, Spain.

The present study reports on the location of major foci of footrot in goats in the Extremadura region of Spain by the determination of locally occurring strictly anaerobic microorganisms involved in the pathogenesis and development of this disease. The most commonly isolated microorganisms belonged to the genera Dichelobacter, Fusobacterium, Porphyromonas and Prevotella; these were found in conjunction with other species of minor importance. The species most frequently isolated were Fusobacterium necrophorum (40%), Dichelobacter nodosus (31.7%), Porphyromonas asaccharolytica (21.1%) and Prevotella melaninogenica (12.9%). Virulence factors identified in the isolated microorganisms included haemolysins, elastases and lecithinases, which enabled the organisms involved to initiate and/or aggravate the disease. Serotyping was performed for Dichelobacter nodosus isolates, since this species is responsible for triggering the process of infection. A and C were the most frequently isolated serovarieties (representing 40.7% and 25.9% of the cases, respectively).

Comparative study of biochemical methods for the identification of Dichelobacter nodosus.

The aim of this study was to compare four identification procedures to detect Dichelobacter nodosus and develop a rapid, simple and effective method to identify D. nodosus strains isolated from cases of ovine footrot. The four methods used were: (a) the classic guidelines set down by Holdeman et al. (1977) and Summanen et al. (1993) which are based on gas liquid chromatography (GLC) and different biochemical tests, this method was considered as landmark; (b) Baron and Citron's flowchart for the rapid identification of Gram-negative rod-shaped anaerobes (1997); (c) the API rapid 32 A system (bio Mérieux), and (d) Mast ID Anaerobe ID Ring (MID8) (Mast Diagnostics). None of the four methods used allowed us to correctly identify the D. nodosus strains (neither the strains isolated from cases of ovine footrot nor those originating from type collection). Because of the difficulties encountered in obtaining a correct identification of D. nodosus, we propose a simple, rapid and effective way to achieve this task. Our flowchart will provide the means to identify this microorganism in any laboratory of general microbiology without having to use any specialised equipment.

Effectiveness of different adjuvants in stimulating Dichelobacter nodosus antibody in sheep vaccinated against ovine footrot.

This research consists of an evaluation of the effectiveness of different substances administered as adjuvants in the stimulation of humoral immune response induced by the vaccine composed of strains A1, A2 and C of Dichelobacter nodosus. To do this, a total of 120 Merino sheep were vaccinated and revaccinated. These sheep were selected from a farm located in the region of Extremadura (Spain), and they were divided into 12 groups of 10 animals each. An additional group with 10 sheep was used as control. The immune response (titre of antibodies) was determined by agglutination tests and ELISA. The most pronounced immune response was obtained by the use of Freund's incomplete adjuvant and aluminium hydroxide as adjuvants.

Minimum inhibitory concentrations for 25 selected antimicrobial agents against Dichelobacter nodosus and Fusobacterium strains isolated from footrot in sheep of Portugal and Spain.

The agar dilution method was used to determine the inhibitory activity of 25 antimicrobial agents against 69 strains of Dichelobacter nodosus and 108 strains of the genus Fusobacterium, all of which were isolated from 90 clinical cases of ovine footrot between October 1998 and November 2000. In the case of the micro-organisms belonging to the genus Fusobacterium, the six beta-lactams studied (benzyl penicillin, ampicillin, cloxacillin, cefadroxil, cefuroxime and cephalexine) proved to be, in general, the most effective antimicrobial agents. Chloramphenicol, clindamycin and doxycycline were also quite active against Fusobacterium spp. With regard to the 69 strains of D. nodosus tested, the levels of resistance remain low.

Aetiology of ovine footrot in the Portuguese region of Alto Alentejo.

In this work, we found it appropriate to carry out a study directed towards isolating and identifying the entailed microorganisms which trigger off footrot in sheep, placing special emphasis on the serotipification of the different Dichelobacter nodosus species. With this goal in mind four flocks from the Portuguese region of 'Alto Alentejo' were selected, all of them had one common feature: their main health problem was ovine footrot. We also set out to determine the elastolitic capacity of isolated strict-anaerobic bacteria, in order to be able to clarify the direct involvement of these microorganisms in the outbreak of this infectious process.