Effet des imperfections initiales sur la stabilité dynamique et la réponse des plaques rectangulaires

Revue historique -- But et sommaire de la recherche -- Équations temporelles du mouvement -- Définition du problème -- Les équations de base -- Les conditions aux limites -- Solution des équations de base -- Solution asymptotique des équations de mouvement -- Construction de la solution asymptotique en première approximation -- Réponse stationnaire -- Passage de vibrations forcées à vibrations paramétriques

Investigation of the Effect of the Force-Frequency on the Behaviour of a New Viscous Damper for Railway Applications

The primary purpose of this work is to experimentally investigate the damping coefficient of a viscous damper, intended to be used in railway applications to reduce noise emission. The viscous shock absorber used in this study is a commercial vehicle damper to which minor modifications were added. This investigation was focused on detecting the variation in the damping coefficient value over a wide range of frequencies. The experimental setup tends to simulate the railway vibration represented by a strong steel metal sheet structure attached to a shaker from its lower side. The shaker itself is connected to the damper rod through a dual acceleration-force sensor. A sinusoidal load with wide range of frequencies was applied by the shaker to the top of the damper's rod. Both acceleration and force time-responses were collected, stored and analyzed to extract the Force-Displacement and the Force-Velocity graphs. Based on the damping coefficients obtained for the different values of excitation frequencies, the results show that the damping coefficient is not constant and depends on the excitation frequency

Vibration wave characteristics in Conventional and Periodic Drill string models

Drill string models are of two major types- Conventional and Periodic. Conventional drill string models are widely used due to their simplicity in manufacturing and dynamics analysis. Drill strings are affected mainly by torsional, lateral and axial vibrations. The dispersive or propagative nature of these vibrations can only be analyzed by using periodic drill string models. Drill pipe makes up the majority of the drill string. In reality, at each end of the drill pipe, tubular, larger-diameter portions called the tool joints are located. Since the dimensions of the tool joint are negligible when compared to the drill pipe, conventional drill string models assume drill strings to be uniform structures. The tool joints in drill strings introduce a geometrical periodicity in the structure. A periodic structural component is comprised of a repeating array of cells, which are themselves an assembly of elements. The elements may have differing material properties as well as geometric variations. The periodic structures act as mechanical filters for the wave propagation. A material or geometrical change in the structure causes a disruption to the continuous propagation of vibration waves. There are three types of waves propagating in rotating structures: torsional, flexural (bending) and longitudinal. Of these waves, torsional and longitudinal waves are dispersive while flexural waves are non- dispersive. The proportions of the waves which are dispersed vary according to the Wavenumber. The wavenumber is a property of the wave and depends on the structural, material and geometrical properties of the wave guide. Waveguide is the medium through which a wave travels. The wave number, and hence the dispersion and propagation characteristics of waves can be tuned by varying the material and geometrical periodicity in the drill strings. This paper presents the vibration propagation and attenuation characteristics in conventional and periodic drill string models. The periodic drill string model is developed such that it can be easily scaled to a real drilling rig model. The research also features a laboratory setting which comprises a mini drilling rig and actual drilling samples. These features ensure robustness of the dynamics analyzed for the periodic drill string model. The research findings are analyzed under zero and non- zero borehole drill string interactions. Vibration information is collected from upper and lower parts of the drill string models to compare the propagation and attenuation characteristics. Proximity sensors are used for the data acquisition which is then analyzed using frequency and power spectrum graphs. The time and frequency domain vibration propagation characteristics are investigated for both the conventional and periodic drill string models. Laboratory testing analyzes (1) introduction of stop and pass band regions in frequency spectra by periodic drill string models, and (2) vibration attenuation in torsional and lateral vibration modes. The tests confirm the introduction of stop bands in frequency spectra for the periodic drill strings, while they were absent and had an all pass-band for conventional drill string models. The research opens up a new method of passive control of drilling vibrations- by introducing tool joint designs which could be tuned to efficiently dampen out the harmful vibrations affecting the drilling rigs.qscienc

A rare cause of proximal intestinal obstruction in adults - annular pancreas: a case report

Annular pancreas is a rare congenital anomaly characterized by the presence of ectopic pancreatic tissue surrounding the descending part of the duodenum. It is one of the few congenital anomalies of the gastrointestinal tract which can produce symptoms late in life. In adults, the factors initiating symptoms are recurrent pancreatitis, duodenal stenosis at the site of the annulus, or duodenal or gastric ulceration. We report a new case involving a 24-year-old woman hospitalised for epigastric pain, nausea and vomiting. Radiological examination was consisted with an annular pancreas. At operation a complete obstruction of the second part of the duodenum was found, caused by an annular pancreas, no other congenital anomaly of the intra-abdominal organs was noted. A gastroenterostomy was performed

Wave propagation in double helical rods

The propagation of waves in helical rods has been studied extensively. However, studying the wave propagation in double helical rods have received less attention although this can be useful in multiple fields of science and engineering. Obtaining an analytical model for a double helical rod is challenging since the curvature and tortuosity are not constant. Thus, resolving the wave behaviour analytically is nearly impossible. In this paper, wave propagation in a double helical rod will be studied using the wave and finite element method which is a technique that can be used to model homogeneous and periodic one and two dimensional structures based on the periodic structure theory. For modelling a double helical rod, the finite element model of a single turn is processed using Bloch waves. The dispersion curves and wavemodes are obtained and the similarities and differences of waves in helical and double helical rods are highlighted.The financial support provided by the Qatar National Research Fund through the National Priorities Research Program under grant number NPRP 11S-1220-170112 and Qatar University Internal Grant QUCG-CENG-19/20-6

Huge desmoid tumor of the anterior abdominal wall mimicking an intraabdominal mass in a postpartum woman: a case report

Desmoid tumors are benign neoplasms that most often arise from muscle aponeurosis and have been associated with both trauma and pregnancy. The etiology of desmoids has not been determined. We report the case of anterior abdominal wall desmoid tumor in a female patient with previous history of cesarean section. Preoperative ultrasound and computed tomography demonstrated a large mass mimicking a large hematoma or an intraabdominal mass. The tumor was removed by wide excision with safe margins. The abdominal wall defect was reconstructed with polypropylene mesh. Subsequent histology revealed a desmoid tumor. Desmoid tumors in females are often associated with pregnancy or occur post-partum. The reasons behind this association are unclear. The most common sites are in the abdominal muscles.Pan African Medical Journal 2013; 14:5

Modal analysis of spur gears for varied teeth root cracks characteristics: finite element analysis (FEA) simulations

Gear failure is considered the main alarming and undesirable event in gearboxes. Usually, cracks happen by fatigue caused due to cyclic loading. Fatigue stress is focused on the teeth root because of the small tooth fillet radius. This causes progressive damage to gear teeth which causes teeth failure, and hence, a damaged gear. This work shows a numerical framework to identify and quantify cracks' existence at the teeth root of spur gears. The problem is numerically analysed through finite element-based simulation with ANSYS by conducting a modal analysis. There are nine cases of gears with different cracks on their tooth; these cracks are varied by their number (single or multiple), Crack Length Percentage (CLP %), and location. The FEA is used to simulate all nine cases to predict the bending natural frequency of teeth and to investigate the effects of the variation of the crack by looking at the natural frequency of the teeth bending as well as the deformation level. Results revealed that the teeth bending natural frequency decreases as the CLP% increases. In addition, the gear stiffness is calculated based on the natural frequency and gear’s mass and it is indicated that the stiffness decreases as the CLP% increases, while the deformation level increase with CLP%

A wandering spleen presenting as a hypogastric mass: case report

Wandering spleen is a rare condition characterized by the absence or underdevelopment of one or all of the ligaments that hold the spleen in its normal position in the left upper quadrant of the abdomen. It is an uncommon clinical entity that mainly affects children. Among adults it most frequently affects women of reproductive age, in whom acquired laxity of the splenic ligaments is usually the cause. Patients with a wandering spleen may be asymptomatic, present with a movable mass in the abdomen, or have chronic or intermittent abdominal pain because of partial torsion and spontaneous detorsion of the spleen. A 26-year-old woman was admitted to our hospital with vomiting and abdominal pain. Abdominal examination revealed a large ovoid hypogastric mass. A CT scan showed a wandering spleen in the hypogastric region. Exploratory laparotomy revealed an ischemic spleen. A total splenectomy was performed

Vibration of Periodic Drill-Strings with Local Sources of Resonance

A new class of drill-strings is proposed for attenuating undesirable vibrations to ensure effective operation. The drill-string is provided with passive periodic inserts, which are integrated with sources of local resonance (LR). The inserts make the drill-string act as a low frequency pass mechanical filter for the transmission of vibration along the drill-string. Proper design of the periodic inserts with sources of LR tend to shift these stop bands towards zones of lower frequencies to enable confining the dominant modes of vibration of the drill-string within these bands. In this manner, propagation of the vibration along the drill-string can be completely blocked. A finite element model (FEM) is developed using ANSYS to investigate the bandgap characteristics of the proposed drill-string with sources of LR. The developed FEM accounts for bending, torsional, and axial vibrations of the drill-string in order to demonstrate the effectiveness of the periodic inserts with LR in simultaneous control of these combined modes as compared to conventional solid periodic inserts, which are only limited to controlling bending vibrations. The effect of the design parameters of the periodic inserts with LR on the bandgap characteristics of the drill-string is investigated to establish guidelines of this class of drill-strings

Intussusception caused by an inverted Meckel's diverticulum: a rare cause of small bowel obstruction in adults

Adult intussusception due to Meckel's diverticulum is an uncommon cause of intestinal obstruction. However, the surgeon should still be suspicious of this condition since the non specific symptoms and the rarity of it make a preoperative diagnosis uncertain. Considering the secondary nature of adult intussusception and the necessity of early surgical intervention to avoid morbidity and mortality, we report one case of intussusception due to Meckel's diverticulum in an adult. A 22-year-old patient was admitted to our hospital with vomiting and abdominal pain. The abdomen was hard with tenderness. We diagnosed an acute small bowel obstruction and performed emergency surgery. The intra operative findings were distention of the small bowel and intussusception of ileus due to an inverted Meckel's diverticulum located 70 cm from the ileocecal valve. 30 cm ischemic loop was identified. A segmental small bowel resection and hand-sewn anastomosis was performed. Histopathology distinguished Meckel's diverticulum measuring 5 cm x 3.5 cm x 1 cm and no signs of malignancy