Large Eddy Simulation of Highly Compressible Jets with Tripped Boundary Layers

In high-speed aircraft, supersonic jets used for propulsion can lead to very intense aerodynamically generated acoustic noise. Thus, there is a need to study the aerodynamic and aeroacoustic properties of highly compressible jets. In previous studies, several simulations of supersonic jets have been conducted. Unfortunately, the turbulence intensity at the nozzle exit was dependent on the internal geometry of the nozzle and could not be tuned. This is a pity given that, as shown experimentally and numerically for subsonic and supersonic jets, the boundary layer state of the jet affects the jet flow and noise. In this study, a boundary-layer tripping method permitting to obtain an initially turbulent supersonic jet is studied. The influence of the tripped jet boundary layers on the flow and acoustic fields of the jet is analyzed. The impact of nozzle-exit turbulence levels on the noise radiation and notably on the acoustic components specific to supersonic jets (screech noise, broadband shock-associated noise, mixing noise and Mach wave radiation) is discussed

An investigation of the mechanisms of sound generation in initially laminar subsonic jets using the Goldstein acoustic analogy

This version has been accepted for publication.Published in Journal of Fluid Mechanics / Volume 714 / January 2013, pp 24 - 57 Copyright © Cambridge University Press 2013 DOI: http://dx.doi.org/10.1017/jfm.2012.448, Published online: 02 January 201

Two-dimensional features of correlations in the flow and near pressure fields of Mach number 0.9 jets

International audienceIn the present study, two-dimensional spatial correlations are calculated in the flow and the near pressure fields of two isothermal round jets at a Mach number of 0.9, computed by highly-resolved simulations using cylindrical coordinates (r , θ, z). The two jets have diameter-based Reynolds numbers of 3, 125 and 100, 000, and they are initially weakly and strongly disturbed, respectively. For both jets, correlations are evaluated between signals at a given point, namely flow fluctuations on the jet axis at the end of the potential core and pressure fluctuations in the jet near field, and 2-D fields acquired in sections (z, r). The full signals but also the axisymmetric and first azimuthal modes are considered. Overall, despite the significant differences in Reynolds number and nozzle-exit conditions, the results for the two jets are very similar. Strong levels of correlations are obtained over large spatial regions and long time periods, providing information on noise generation mechanisms. In particular, the 2-D correlation fields reveal the presence of a wavepacket-like structure growing in the jet mixing layers, centered on a correlation spot in the potential core, and peaking in amplitude around the end of the jet core, which leads to the emission of sound waves in the downstream direction

Intensity and bunch length measurement for lepton beam in the injection lines of the SPS and LEP

We describe a system which is used operationally to measure the absolute intensity of single lepton bunches in a beam transfer line. It is based on the detailed knowledge of every single item of a complex measuring chain that comprises a beam coupler on one end and an acquisition system on the other end. This knowledge can be acquired by a well tested theoretical model and careful measurement of the transfer function of each processing module. A precision better than 3 % can be obtained and no external calibration is required. A value for the bunch length can be deduced from a spectral intensity measurement at two well chosen frequencies

Orbit and trajectory measurement with low intensity lead ion beams in the SPS

The orbit measurement system of the CERN SPS was designed to measure the position of dense proton beams with an intensity of up to 0.28 A. The lower design limit for the lead ion beam intensity has been fixed at 35mA. This requires a substantial extension of the dynamic range. We describe the properties of the system and its modifications together with the results obtained for sulphur ion beams in the past and lead ions more recently

Numerical studies towards practical large-eddy simulation

Large-eddy simulation developments and validations are presented for an improved simulation of turbulent internal flows. Numerical methods are proposed according to two competing criteria: numerical qualities (precision and spectral characteristics), and adaptability to complex configurations. First, methods are tested on academic test-cases, in order to abridge with fundamental studies. Consistent results are obtained using adaptable finite volume method, with higher order advection fluxes, implicit grid filtering and "low-cost" shear-improved Smagorinsky model. This analysis particularly focuses on mean flow, fluctuations, two-point correlations and spectra. Moreover, it is shown that exponential averaging is a promising tool for LES implementation in complex geometry with deterministic unsteadiness. Finally, adaptability of the method is demonstrated by application to a configuration representative of blade-tip clearance flow in a turbomachine

Detection of interstellar H_2D^+ emission

We report the detection of the 1_{10}-1_{11} ground state transition of ortho-H_2D^+ at 372.421 GHz in emission from the young stellar object NGC 1333 IRAS 4A. Detailed excitation models with a power-law temperature and density structure yield a beam-averaged H_2D^+ abundance of 3 x 10^{-12} with an uncertainty of a factor of two. The line was not detected toward W 33A, GL 2591, and NGC 2264 IRS, in the latter source at a level which is 3-8 times lower than previous observations. The H_2D^+ data provide direct evidence in support of low-temperature chemical models in which H_2D^+ is enhanced by the reaction of H_3^+ and HD. The H_2D^+ enhancement toward NGC 1333 IRAS 4A is also reflected in the high DCO^+/HCO^+ abundance ratio. Simultaneous observations of the N_2H^+ 4-3 line show that its abundance is about 50-100 times lower in NGC 1333 IRAS 4A than in the other sources, suggesting significant depletion of N_2. The N_2H^+ data provide independent lower limits on the H_3^+ abundance which are consistent with the abundances derived from H_2D^+. The corresponding limits on the H_3^+$ column density agree with recent near-infrared absorption measurements of H_3^+ toward W 33A and GL 2591.Comment: Standard AAS LaTeX format (15 pages + 2 figures

The use of intravascular ultrasound imaging to improve use of inferior vena cava filters in a high-risk bariatric population

ObjectivePulmonary embolism is the leading cause of death after gastric bypass procedures for obesity, approximating 0.5% to 4%. All bariatric patients, but especially the super-obese, which have a body mass index (BMI) >50 kg/m2, are at significant risk for postoperative venous thromboembolism (VTE). Visualization and weight limitations of fluoroscopy tables exclude most bariatric and all super-obese patients from inferior vena cava (IVC) filter placement using fluoroscopy. Intravascular ultrasound (IVUS)-guided IVC filter placement is the only modality that allows these high-risk patients to have an IVC filter placed.MethodsHospital and outpatient records of the 494 patients who underwent gastric bypass procedures from January 1, 2004, to May 31, 2006, were reviewed. All patients who had concurrent IVC filter placement with the use of IVUS guidance were selected. Comorbidities, outcomes, and complications were recorded.ResultsWe identified 27 patients with mean BMI of 70 ± 3 kg/m2; of these, 25 were super-obese (BMI >50 kg/m2). Procedures included five laparoscopic and 22 open gastric bypass operations. All patients underwent concurrent IVC filter placement using IVUS guidance. In addition to super-obesity, indications for IVC filter placement included history of VTE (n = 4), known hypercoagulable state (n = 2), and profound immobility (n = 21). Mean follow up was 293 ± 40 days. Technical success rate was 96.3%. There were no catheter site complications. In one surviving patient, a nonfatal pulmonary embolism was detected by computed tomography 2 months postoperatively. Two patients died, and autopsy excluded VTE as the cause of death in both.ConclusionThis study suggests efficacy of IVUS-guided IVC filter placement in preventing mortality from pulmonary embolism in high-risk bariatric patients, including the super-obese. IVUS-guided IVC filter placement can be safely performed with an excellent success rate in all bariatric patients, including the super-obese, who otherwise would not be candidates for IVC filter placement due to the limitations imposed by their large body habitus

Mid-term results with laser atherectomy in the treatment of infrainguinal occlusive disease

BackgroundLaser atherectomy offers a potential intervention for multivessel infrainguinal disease in patients with poor revascularization options. Despite promising early results reported in the literature, the proper patient population who might benefit from laser atherectomy has yet to be determined.MethodsFrom July 2004 to June 2006, patients undergoing laser atherectomy were retrospectively reviewed and assessed for comorbidities, operative and follow-up variables potentially associated with the end points of nondefinitive therapy, and limb salvage.ResultsDuring the study period, 40 patients (21 women, 19 men) underwent laser atherectomy, and the average follow-up was 461 ± 49 days (range, 17 to 1050 days). Their average age was 68 ± 2 years (range, 43 to 93 years). The indication for laser atherectomy was critical limb ischemia in 26 (65%) and lower limb claudication in 11 (35%). A total of 47 lesions were treated in the following arterial segments: 34 femoropopliteal and 13 infrapopliteal. Femoropopliteal distribution by the Trans-Atlantic Society Classification (TASC) was A in 3, B in 17, C in 10, D in 4, and infrapopliteal lesions distribution was A in 1, B in 3, C in 4, and D in 5. Adjunctive angioplasty was used in 75% of cases. The overall technical success rate (<50% residual stenosis) was 88%. Laser atherectomy–based treatment was the definitive therapy for 23 patients (58%), and the overall 12-month primary patency was 44%. The limb salvage rate at 12 months in 26 patients with critical limb ischemia was 55%. Renal failure was a risk factor for amputation (P < .001) and failed primary patency (P < .05), type 2 diabetes mellitus was a risk factor for amputation (P < .05), and poor tibial runoff was associated with failed primary patency and amputation (P < .05). Outcome was associated with the number of patent infrapopliteal runoff vessels.ConclusionThese data demonstrate that laser atherectomy can be used with high initial technical success rate. Chronic renal failure and diabetes are risk factors for a negative outcome. Poor results in patients with diabetes and renal failure necessitate careful case selection in this subgroup, in which laser atherectomy is less likely to provide a definitive revascularization result or limb salvage

An open and parallel multiresolution framework using block-based adaptive grids

A numerical approach for solving evolutionary partial differential equations in two and three space dimensions on block-based adaptive grids is presented. The numerical discretization is based on high-order, central finite-differences and explicit time integration. Grid refinement and coarsening are triggered by multiresolution analysis, i.e. thresholding of wavelet coefficients, which allow controlling the precision of the adaptive approximation of the solution with respect to uniform grid computations. The implementation of the scheme is fully parallel using MPI with a hybrid data structure. Load balancing relies on space filling curves techniques. Validation tests for 2D advection equations allow to assess the precision and performance of the developed code. Computations of the compressible Navier-Stokes equations for a temporally developing 2D mixing layer illustrate the properties of the code for nonlinear multi-scale problems. The code is open source