Spatially separated continuum sources revealed by microlensing in the gravitationally lensed broad absorption line quasar SDSS J081830.46+060138.0

Gravitational microlensing is a powerful tool for probing the inner structure of distant quasars. In this context, we have obtained spectropolarimetric observations of the two images of the broad absorption line (BAL) quasar SDSS J081830.46+060138.0 (J0818+0601) at redshift z ≃ 2.35. We first show that J0818+0601 is actually gravitationally lensed, and not a binary quasar. A strong absorption system detected at z = 1.0065 ± 0.0002 is possibly due to the lensing galaxy. Microlensing is observed in one image and it magnifies the emission lines, the continuum, and the BALs differently. By disentangling the part of the spectrum that is microlensed from the part that is not microlensed, we unveil two sources of continuum that must be spatially separated: a compact one, which is microlensed, and an extended one, which is not microlensed and contributes to two thirds of the total continuum emission. J0818+0601 is the second BAL quasar in which an extended source of rest-frame ultraviolet continuum is found. We also find that the images are differently polarized, suggesting that the two continua might be differently polarized. Our analysis provides constraints on the BAL flow. In particular, we find that the outflow is seen with a nonzero onset velocity, and stratified according to ionization.

Recent Advances in the Prediction and Mitigation of Flow Induced Pulsations in Flexible Risers and Flowlines

Flow-induced pulsations in flexible risers and flowlines (FLIP) has been the subject of ongoing research roughly over the last two decades. This phenomenon, also called singing, can cause high vibration and cyclic stress levels in the attached up- and downstream pipe systems, thereby imposing serious limitations on the flowrates through the pipes. FLIP occurs when the flow velocity along the flexible is above a certain onset flow velocity. Large pressure pulsations can appear as a result of a feedback loop mechanism established between vortex shedding at the inner cavities and longitudinal acoustic standing waves. Since FLIP is recommended to be avoided in all cases, the main focus of the research has been to understand the phenomenon and build models to predict the onset velocity. An overview of the experiences in the behavior and prediction of onset data on a large number of field cases is given. Recent advances on the theory and prediction of the onset and behavior of flow induced pulsations are discussed. This ranges from new advanced test methods for acoustic source allocation to full large scale experiments. In addition, new and upcoming anti-FLIP technologies as well as possible mitigation actions available to operators are discussed, by introducing recent field experiences as well as large scale tests.

Investigation of Dynamic Buckling of a Wing Model in Air Flow

The paper deals with studying a dynamic stability of the wing model in the ram airflow. As known, at a certain flow rate that is called critical, there is a phenomenon of self-excited non-damping flexural-torsional self-vibrations, named flutter. A two-mode elastic system wing model is under consideration, as is common in the literature in the field concerned. The paper continues and develops investigations of well-known scientists in this field, such as V.L. Biderman, S.P. Strelkov, Ya.G. Panovko, I. I. Gubanova, E.P. Grossman, J.Ts. Funen, etc. A great deal of papers dedicated to this problem and published by abovementioned and other scientists, give only the problem formulation and the derivation of equations, often in a fairly simplified form, do not offer solutions of these equations for specific numerical parameters of the wing model, and do not study how these parameters affect the flutter onset velocity.The paper details the derivation of linear differential equations of small vibrations of the wing model in the flow, determines the natural frequencies and shapes of flexural-torsional vibrations, checks their orthogonality, studies vibrations under the influence of aerodynamic force and moment, determines the critical flow velocity for a number of system parameters, and draws a conclusion about the influence of these parameters on the critical velocity. In particular, it studies how such a parameter as the distance between the center of gravity and the center of stiffness affects the critical velocity, as well as how the stiffness of the model's spring suspension, which simulates the stiffness characteristics of the wing impacts on bending and torsion. The calculation results allow us to draw conclusion concerning the methods of dealing with this phenomenon. One of the promising options may be, in addition to varying the geometric and rigid parameters of the system, the introduction of additional mass to be an analogue of the vibration damper. The paper may be of interest both for engineering students who learn the theory of mechanical vibrations, and for engineering-specialists in aero-elasticity and dynamic stability of elements in mechanical systems.

Instrumented FLIP Test and Static Pressure Influence on the Onset Velocity and Frequency on an Industrial Scale

Since the early 2000, Flow Induced Pulsations (FLIP) has been more and more encountered on platforms. This phenomenon generates high acoustic pressure pulsations that may cause noises up to one hundred and ten dB and significant fatigue stress levels in small piping either at topside or subsea equipment. The source of the phenomenon is inside of the flexible pipe but FLIP has no effect on it. Nevertheless, in case of FLIP experience companies may have to reduce their flow rate. Therefore, FLIP must be understood in order for the companies to avoid this constraint. In this frame, a FLIP test was performed with protagonists who are involved in the understanding of this phenomenon. The test was done in 2016 at CESAME Poitiers (France) in an eighteen meter-long and six-inch flexible pipe on an air open loop. The prototype was fully instrumented and pressures up to forty bars were tested and mass flow rates up to 6 kg.s−1 to reproduce the FLIP phenomenon. The test setup and signals analysis are presented in this paper. Moreover, FLIP onset velocities and frequencies are compared with TechnipFMC models. As a conclusion of this paper pressure influence for the six-inch tested on the FLIP initiation will be presented.

Air Bubble Entrainment, Breakup, and Interplay in Vertical Plunging Jets

The entrainment, breakup, and interplay of air bubbles were observed in a vertical, two-dimensional supported jet at low impact velocities. Ultra-high-speed movies were analyzed both qualitatively and quantitatively. The onset velocity of bubble entrainment was between 0.9 and 1.1 m/s. Most bubbles were entrained as detached bubbles from elongated air cavities at the impingement point. Explosion, stretching, and dejection mechanisms were observed for individual bubble breakup, and the bubble interaction behaviors encompassed bubble rebound, “kiss-and-go,” coalescence and breakup induced by approaching bubble(s). The effects of jet impact velocity on the bubble behaviors were investigated for impact velocities from 1.0 to 1.36 m/s, in the presence of a shear flow environment.

Flow Induced Pulsations (FLIP) in Rough Bore Gas Flexible Pipes, Tests and Model

Since the early 2000’s singing risers phenomenon has been encountered. The so called Flow Induced Pulsations (FLIP) phenomenon occurs in dry gas risers (such as Gas Export lines) and may generate high tonal noises up to 110 dB but may also lead to high vibration of adjacent equipment leading to significant fatigue failure. This publication presents the recently developed model that aims at performing FLIP assessment for rough bore gas flexible pipes. The developed model provides the FLIP onset velocity and frequency for a given rough bore. It will also describe the main contributing factors such as the inner layer corrugation profile, the operating conditions (pressure, temperature and flow rate) and adjacent equipment’s acoustic reflection contributions. In addition, a Flow Induced Pulsation test carried out in 2003 to 2006 will be presented. Test outcomes will be compared to model presented in the first part. Finally, reliability of the model will be presented detailing benchmark with past tests and FLIP experienced on fields. To conclude, the model enables predictions and recommendations to avoid FLIP at an early stage prior to project execution.

Uncertainty Quantification of Aeroacoustic Power Sources in Corrugated Pipes

Gas transport in corrugated pipes often exhibit whistling behavior, due to periodic flow-induced pulsations generated in the pipe cavities. These aero-acoustic sources are strongly dependent on the geometrical dimensions and features of the cavities. As a result, uncertainties in the exact shape and geometry play a significant role in determining the singing behavior of corrugated pipes. While predictive modelling for idealized periodic structures is well established, this paper focusses on the sensitivity analysis and uncertainty quantification (UQ) of uncertain geometrical parameters using probabilistic models. The two most influential geometrical parameters varied within this study are the cavity width and downstream edge radius. Computational Fluid Dynamics (CFD) analysis was used to characterize the acoustic source. Stochastic collocation method was used for propagation of input parameter uncertainties. The analysis was performed with both full tensor product grid and sparse grid based on level-2 Clenshaw-Curtis points. The results show that uncertainties in the width and downstream edge radius of the cavity have an effect on the acoustic source power, peak Strouhal number and consequently the whistling onset velocity. Based on the assumed input parameters distribution functions, the confidence levels for the prediction of onset velocity were calculated. Finally, the results show the importance of performing uncertainty analysis to get more insights in the source of errors and consequently leading to a more robust design or risk-management oriented decision.