Numerical and experimental evaluation of shock dividers

Mitigation of pressure pulsations in the exhaust of a pulse detonation combustor is crucial for operation with a downstream turbine. For this purpose, a device termed the shock divider is designed and investigated. The intention of the divider is to split the leading shock wave into two weaker waves that propagate along separated ducts with different cross sections, allowing the shock waves to travel with different velocities along different paths. The separated shock waves redistribute the energy of the incident shock wave. The shock dynamics inside the divider are investigated using numerical simulations. A second-order dimensional split finite volume MUSCL-scheme is used to solve the compressible Euler equations. Furthermore, low-cost simulations are performed using geometrical shock dynamics to predict the shock wave propagation inside the divider. The numerical simulations are compared to high-speed schlieren images and time-resolved total pressure recording. For the latter, a high-frequency pressure probe is placed at the divider outlet, which is shown to resolve the transient total pressure during the shock passage. Moreover, the separation of the shock waves is investigated and found to grow as the divider duct width ratio increases. The numerical and experimental results allow for a better understanding of the dynamic evolution of the flow inside the divider and inform its capability to reduce the pressure pulsations at the exhaust of the pulse detonation combustor

The Balloon Dilatation and Large Profile Catheter Maintenance Method for the Management of the Bile Duct Stricture Following Liver Transplantation

We dated a continuous, ∼22-m long sediment sequence from Lake Challa (Mt. Kilimanjaro area, Kenya/Tanzania) to produce a solid chronological framework for multi-proxy reconstructions of climate and environmental change in equatorial East Africa over the past 25,000 years. The age model is based on a total of 168 AMS 14C dates on bulk-organic matter, combined with a 210Pb chronology for recent sediments and corrected for a variable old-carbon age offset. This offset was estimated by i) pairing bulk-organic 14C dates with either 210Pb-derived time markers or 14C dates on grass charcoal, and ii) wiggle-matching high-density series of bulk-organic 14C dates. Variation in the old-carbon age offset through time is relatively modest, ranging from ∼450 yr during glacial and late glacial time to ∼200 yr during the early and mid-Holocene, and increasing again to ∼250 yr today. The screened and corrected 14C dates were calibrated sequentially, statistically constrained by their stratigraphical order. As a result their constrained calendar-age distributions are much narrower, and the calibrated dates more precise, than if each 14C date had been calibrated on its own. The smooth-spline age-depth model has 95% age uncertainty ranges of ∼50-230 yr during the Holocene and ∼250-550 yr in the glacial section of the record. The δ13C values of paired bulk-organic and grass-charcoal samples, and additional 14C dating on selected turbidite horizons, indicates that the old-carbon age offset in Lake Challa is caused by a variable contribution of old terrestrial organic matter eroded from soils, and controlled mainly by changes in vegetation cover within the crater basin

Imaging post liver transplantation part II: biliary complications

Biliary complications post liver transplantation are a significant source of morbidity and mortality and early recognition is paramount to the long-term success of the liver graft. Part II of this series will focus on liver transplant biliary anatomy, including the blood supply to the biliary system and potential problems if it is interrupted. The imaging rationale for investigating suspected biliary complications, potential pitfalls, and treatment options will be discussed. The various biliary complications will be illustrated using a collection of cases

Biliary Strictures after Liver Transplantation

Biliary strictures are one of the most common complications following liver transplantation, representing an important cause of morbidity and mortality in transplant recipients. The reported incidence of biliary stricture is 5% to 15% following deceased donor liver transplantations and 28% to 32% following living donor liver transplantations. Bile duct strictures following liver transplantation are easily and conveniently classified as anastomotic strictures (AS) or non-anastomotic strictures (NAS). NAS are characterized by a far less favorable response to endoscopic management, higher recurrence rates, graft loss and the need for retransplantation. Current endoscopic strategies to correct biliary strictures following liver transplantation include repeated balloon dilatations and the placement of multiple side-by-side plastic stents. Endoscopic balloon dilatation with stent placement is successful in the majority of AS patients. In patients for whom gaining biliary access is technically difficult, a combined endoscopic and percutaneous/surgical approach proves quite useful. Future directions, including novel endoscopic retrograde cholangiopancreatography techniques, advanced endoscopy, and improved stents could allow for a decreased number of interventions, increased intervals before retreatment, and decreased reliance on percutaneous and surgical modalities. The aim of this review is to detail the present status of endoscopy in the diagnosis, treatment, outcome, and future directions of biliary strictures related to orthotopic liver transplantation from the viewpoint of a clinical gastroenterologists

Diffraction of shock waves through a non-quiescent medium

An investigation of shock diffraction through a non-quiescent background medium is presented using both experimental and numerical techniques. Unlike diffracting shocks in quiescent media, a spatial distortion of the shock front occurs, producing a region of constant shock angle. An example of this process arises in the exhaust from a pulse-detonation combustor. As the background velocity is increased, such as through the inclusion of a converging nozzle at the exhaust, the spatial distortion becomes more apparent. Numerical simulations using a compressible Euler solver demonstrate that the distortion is not due to the geometrical influence of the nozzle, but rather is a function of the magnitude of the background flow velocity. The distortion is studied using a modified geometrical shock dynamics formulation which includes the background flow and is validated against experiments. A simple model is presented to predict the shock distortion angle in the weak-shock limit. Finally, the axial decay behaviour of the shock is investigated and it is shown that the advection of the shock by the background flow delays the arrival of the head and tail of the expansion characteristic at the centreline. This leads to an increase in the rate of decay of the shock Mach number as the background flow velocity is increased