A vortex sheet method applied to unsteady flow separation from sharp edges

A computational method is presented which describes the unsteady two-dimensional vortex generation and convection in stationary geometries with sharp edges. A second-order panel method is used to describe the motion of the two-dimensional vortex sheet, while the generation of vorticity at the sharp edges is enforced through a Kutta condition. In order to easily satisfy the normal-velocity boundary condition on the stationary walls, the flow domain is transformed to a half-plane (x > 0) by a Schwarz-Christoffel conformal mapping. In the computational plane the solid walls are situated on the vertical coordinate axis so that image vorticity can be utilized to satisfy the boundary condition in a simple way. The method is applied to describe the separating impulsively started flow past a sharp-edged wedge and the flow in a channel with a deep cavity. These applications show that the method is able to describe vortex shedding in complex geometries in an accurate way

Elimination of flow-induced pulsations and vibrations in a process installation: a combination of on site measurements, calculations and scale modeling

The aim of the work described in this paper was to trace and eliminate vibration sources in a low pressure system with high flow velocities. Considerable vibration on the pipe system between a flashing vessel (6.5 m diameter) and heat-exchangers resulted in fatigue failure, leakage and subsequent shut down of a chemical production plant. As compressors or pumps were not directly involved in the process, we considered aero-acoustic pulsations as the main cause of the high vibration levels. Analysis of the flow in the pipe system showed that mean flow velocities could rise up to 50m/s, which is rather high considering the speed of sound in the gas of 195 ms/s. Furthermore separation of the flow from the pipe wall occurs at the outlets of a flashing vessel, which resulted in estimated flow velocities of over 100 m/s in the vena contracta. This paper provides an overview of the analysis done by a onedimensional acoustical model and scale modeling of part of the piping. The recommended modification consists of a perforated pipe section between the outlets of the vessel and has been tested in a scale model. Tests have been performed with airflow at identical Mach numbers for different configurations with perforation ratios form 70 to 36%. The modification with a perforation ratio of 70% shows a reduction of a factor 5 of the pressure pulsation and vibration levels over the entire frequency range. The net pressure loss for this configuration is similar to that in the original lay-out without the perforated tube. This modification was implemented during a plant shutdown and proved to be very effective. The results are well in line with the predicted levels and on the basis of this success a similar design has been planned for two new installations. © TNO Science & Industry 2008

An experimental method for validating compressor valve vibration theory

This paper presents an experimental method for validating traditional compressor valve theory for unsteady flow conditions. Traditional valve theory considers the flow force acting on the plate and the flow rate as quasi-steady variables. These variables are related via semi-empirical coefficients which are determined by steady flow experiments. The new experimental methodology permitted the simultaneous measurement of instantaneous valve opening, instantaneous volume-flow rate and instantaneous pressure difference across the valve. Results for an oscillating valve (at 1.9 times the valve resonance frequency) show that the gas force is predicted reasonably accurately. However, the flow rate model should be improved in order to predict the observed hysteresis (30 %) and fluctuations in the vena contracta factor

Flow induced pulsations caused by corrugated tubes

Corrugated tubes can produce a tonal noise when used for gas transport, for instance in the case of flexible risers. The whistling sound is generated by shear layer instability due to the boundary layer separation at each corrugation. This whistling is examined by investigating the frequency, amplitude and onset of the pulsations generated by 2" artificially corrugated tubes and cable feeds. Special attention is given to the influence of the geometry of the corrugations and to the influence of the boundary conditions of the tubes. Two distinct modes are measured. One high mode with a typical Strouhal number Sr=0.35 and one with a Strouhal number of Sr=0.1. The relative length scale for the corrugations to be used in the Strouhal number is a modified gap width, which is the gap width excluding the downstream edge radius. The exact Strouhal number for a corrugation is furthermore dependent on details of the corrugation, as the convective velocity of the flow disturbances is influenced by details in the geometry such as edge rounding. The amplitude of the generated pulsations scales with the acoustic pressure (ρcU) and will saturate for higher flow rates (p'/ρcU=constant). The saturation level is independent of pressure and tube length and is solely dependent on the corrugation geometry. Larger cavities will generate higher amplitude pulsations. The onset of the whistling is dependent on the tube itself and the system boundaries. Only for very long tubes is the onset insensitive to the system boundaries and will the onset be determined by corrugations. In that case the onset is determined by a critical boundary layer thickness. For smaller tubes, this critical layer thickness is still relevant, but the boundary conditions will have a large effect. A system with a high reflection coefficient will start at lower gas velocities than a system with a low reflection coefficient. Based on the current results the frequency and amplitude of the pulsations can be predicted. Copyright © 2007 by ASME

Dynamic Reservoir Well Interaction

In order to develop smart well control systems for unstable oil wells, realistic modeling of the dynamics of the well is essential. Most dynamic well models use a semi-steady state inflow model to describe the inflow of oil and gas from the reservoir. On the other hand, reservoir models use steady state lift curves for modeling of the wells. When producing oil from thin oil rims, this description does not sufficiently describe the well behavior observed in practice. For this reason, a model was built that describes both the dynamic flow of oil and gas towards the well bore and the dynamic flow inside the well. The integrated model provides a realistic description of the well dynamics on a time scale of minutes, which is the time scale that is required for development of a control system. As a result, the integrated model allows the development of model based gas coning control or water coning control schemes, as well as model based interpretation of well data

The Geopolitics of Shale Gas : The Implications of the US' Shale Gas Revolution on Intrastate Stability within Traditional Oil- and Natural Gas-Exporting Countries in the EU Neighborhood

The US’ shale gas revolution could in the long term destabilize traditional oil- and gas exporters in the European Union (EU) neighborhood: A combination of substitution effects and greater energy efficiency, could put pressure on the price of oil, leading to fiscal difficulties in traditional hydrocarbon exporting countries

Effect of water-droplets on flow-induced pulsations in pipe with two closed-side branches: an experimental study

In this study experiments are conducted on the flow within a horizontal pipe with two closed side branches of equal length. This configuration is referred to as "quasi-cross configuration" as the distance between the two closed side branches is much smaller than the acoustic wavelength of the first acoustic mode. Focus is placed upon investigating of the effect of high amounts of injected water on Flow-Induced Pulsations (FIPs). A first test with only dry air is performed and pressure fluctuations are measured at the closed end of each side branch to investigate FIPs. Several tests are conducted with increasing amounts of water, in droplet form, which is injected with a nozzle positioned upstream the branches. To understand the effect of the multiphase flow pattern, tests are performed with the injector located far from and close to the side branches. With far injector and at low injection rates, no decrease of pulsations is observed while the amount of injected water increases. At high injection rates, a shift of the pulsation peak to lower velocities and an increase of the Strouhal number calculated by using the diameter of the side branch and the superficial gas velocity are reported. With close injector, pulsations are eliminated as the injection rate increases. At low injection rates, an increase of the Strouhal number is also observed