Kyuro Sasaki

Structure of a turbulent separation bubble

Flow in the separation bubble formed along the sides of a blunt flat plate with right-angled corners has been studied in terms of extensive single- and two-point measurements of velocity and surface-pressure fluctuations. The cross-correlations between the surface-pressure and velocity fluctuations are found to be useful for the study of large-scale vortex structure in the bubble. Large-scale vortices are shed downstream from the separation bubble with a frequency of about 0.6 U ∞ / x R , where U ∞ is the approaching velocity and x R is the time-mean length of the bubble. On top of this regular vortex shedding, there exists a large-scale unsteadiness in the bubble. Vortices which are much larger than the regular vortices are shed with frequencies less than about 0.2 U ∞ / x R . The large-scale unsteadiness is accompanied by enlargement and shrinkage of the bubble and also by a flapping motion of the shear layer near the separation line. The intermittent nature of the flow in the bubble is clarified in some detail. The distributions of the cross-correlations between the pressure and velocity fluctuations demonstrate the vortex structure in the reattaching zone. The longitudinal distance between the vortices is estimated to be (0.7–0.8) x R and their convection velocity is about 0.5 U ∞ near the reattachment line. The cross-correlations also suggest the existence of a longitudinal counter-rotating system in the bubble. The distance between the axes of the rotation is of the order of 0.6 x R . Variations of timescales, lengthscales and phase velocities of the vortices are presented and discussed.

Structure of large-scale vortices and unsteady reverse flow in the reattaching zone of a turbulent separation bubble

This paper describes experiments concerning the structure of large-scale vortices and the unsteady reverse-flow properties in the reattaching zone of a nominally two-dimensional separation bubble formed at the leading edge of a blunt flat plate with right-angled corners. The experiment was performed in a wind tunnel with a constant Reynolds number 2.6 × 10 4 (based on the main-flow velocity and the thickness of the plate). Split-film probes, being sensitive to instantaneous reversals of flow direction, were extensively employed. An important feature of this study is a judicious use of surface-pressure fluctuations as a conditioning signal to educe the structure of the large-scale vortices. Distributions of fluctuating-velocity vectors and contour lines of high-frequency turbulent energy in a few space–time domains are presented and discussed. The most economical interpretation of these space-time distributions is that the large-scale vortices in the reattaching zone are hairpin vortices whose configuration is sketched in the text. The unsteady flow in the reattaching zone is mainly governed by two agents; the motion of the large-scale vortices and the low-frequency unsteadiness. The unsteady flow is clarified in terms of the motion (in a space–time domain) of zeros of the longitudinal velocity close to the surface of the plate; the effects of the two agents on this motion are presented separately. On the basis of these results, a mathematical model of the unsteady flow in the reattaching zone is suggested and found to yield good comparison with measured reverse-flow intermittency and frequency of local-flow reversals. It appears that the separation bubble experiences shrinkage and enlargement in connection with the low-frequency unsteadiness and that the speed of shrinkage is much greater than that of enlargement. The strength of the large-scale vortices in the reattaching zone seems to be dependent on the phase of the low-frequency unsteadiness.

Discrete-vortex simulation of a turbulent separation bubble

The discrete-vortex model is applied to simulate the separation bubble over a two- dimensional blunt flat plate with finite thickness and right-angled corners, which is aligned parallel to a uniform approaching stream. This flow situation is chosen because, unlike most previous applications of the model, the separation bubble is supposed to be strongly affected by a nearby solid surface. The major objective of this paper is to examine to what extent the discrete-vortex model is effective for such a flow. A simple procedure is employed to represent the effect of viscosity near the solid surface; in particular, the no-slip condition on the solid surface. A reduction in the circulation of elemental vortices is introduced as a function of their ages in order to represent the three-dimensional deformation of vortex filaments, An experiment was also performed for comparison purposes. The calculation yielded reasonable predictions of the time-mean and r.m.s. values of the velocity and the surface-pressure fluctuations, together with correlations between their fluctuating components, over most of the separation bubble. The interrelation between instantaneous spatial variations of the surface-pressure and velocity fluctuations were also obtained. A comparison between the calculated and measured results suggests that, in the real flow, the three-dimensional deformation of vortex filaments will become more and more dominant as the reattachment point is approached.

Three-Dimensional Vortex Structure in a Leading-Edge Separation Bubble at Moderate Reynolds Numbers

We describe the results of a flow visualizalion study which concerns three-dimensional vortex structures in a leading-edge separation bubble formed along the sides of a blunt flat plate. Dye and hydrogen bubbles were used as tracers. Reynolds number, based on the plate thickness, was varied from 80 to 800.

Gas Production System From Methane Hydrate Layers by Hot Water Injection Using Dual Horizontal Wells

In this study, we investigate a system of gas production from methane hydrate layers involving hot water injection using dual horizontal wells. Physical and numerical models simulating the gas production process from methane hydrate layers within a hot water chamber are proposed. Experiments with scaled two-dimensional physical models using an imitated hydrate layer (NaHCO 3 ice formation) were performed to investigate fluid flow characteristics and production performance. The thermal simulator was used to simulate experimental chamber growth and field production. Numerical simulations for the processes were successfully performed with a two-component (water and oil or methane hydrates), three-phase (water, methane hydrates and methane gas) and three-dimensional model, matching the physical model. Results of the history-matched numerical simulations were in good agreement with data on production and chamber shapes obtained using the Intermediate3-Stonel wettability model. Simulations of field production using dual horizontal wells 500 m in length were performed to evaluate cumulative gas production over 3 years of injection with 500 x 10 3 kg/day of hot water, which varied from 5 x 10 6 to 9 x 10 6 std m 3 . The production process appears economical, in view of the expected convective heat transfer from the chamber boundary and buoyancy force on dissociated methane gas.

Mine ventilation measurements with tracer gas method and evaluations of turbulent diffusion coefficient

Tracer gas measurements have been carried out at the Pongkor underground gold mine, Indonesia, to evaluate mine ventilation flows and to investigate the effective turbulent diffusion coefficients in mine airways. The airflow routes and quantity, and the diffusion coefficient have been obtained by matching the measurements with numerical simulations using the advection-diffusion equation. Two leakages with flow quantity of 26.7 and 36.7 m3/s were detected. Reduction of leakages have been measured with the method after stopping the leakage routes. The turbulent diffusion coefficients for the simple airways have good agreement with the Taylor equation. However, for complex airways in operating mines, the coefficients show higher values (1.5 to 32 times) than that obtained by the Taylor equation and these have been compared with the data measured in the Kushiro coal mine, Japan. It is mainly affected by the ratio of airway length over equivalent diameter and airway frictions, but airflow mixing along the airway also has an effect on the diffusion coefficient.

CO2 Temperature Prediction in Injection Tubing Considering Supercritical Condition at Yubari ECBM Pilot-Test

The Japanese consortium for CO 2 -Enhanced Coal Bed Methane (ECBM) carried out a pilot project on CO 2 injection from 2002 to 2007 in the city ofYubari, Hokkaido, Japan. However, supercritical CO 2 could not be obtained because of low CO 2 injectivity and heat loss along the deep injection tubing. The absolute pressure and CO 2 temperature at the bottomhole was approximately 15.5 MPa and 28°C, respectively. Therefore, it can be assumed that CO 2 was injected into the coal seam in its liquid phase. Liquid CO 2 is less permeable in the coal seam because of its high viscosity and the resultant swelling of the coal matrix to decrease permeability. This study provides a numerical system to predict CO 2 flow characteristics of pressure, temperature, supercritical or liquid by considering heat transfer from the injector into surrounding casings and strata. This study focused on keeping supercritical CO 2 in the tubing because the viscosity of supercritical CO 2 is 40% less than that of liquid CO 2 . The CO 2 temperature required to keep CO 2 in its supercritical condition from the surface to the bottom was successfully predicted for various CO 2 injection rates and electric heating powers. Finally, injected CO 2 is expected to be supercritical at an injection rate of over 12 ton/d without any heating.

Modeling microbial-induced oil viscosity reduction: Effect of temperature, salinity and nutrient concentration

ABSTRACT This research simulated oil recovery with emphasis on oil viscosity reduction by direct microbe action and metabolites; predicted hydrogeochemical reactions involved with nutrient – brine interaction in reservoirs. PHREEQC was used to simulate reactions between the reservoir brine and nutrient minus microbe. Hitherto, UTCHEM was employed for the enhancement of oil viscosity by assuming production of gases and by the direct microbe action. The model depicted the precipitation of calcite plus dissolution of k-feldspar combined with the evolution of CO2 and CH4 influenced by temperature and pH. Oil recovery was directly proportional to salinity reduction and increasing nutrient concentration.

Experimental Modelling of the SAGD Process 3/4 Enhancing SAGD Performance with Periodic Stimulation of the Horizontal Producer

Experiments on initial stages of the steam-assisted gravity drainage (SAGD) process were carried out, using 2-D scaled reservoir models, to investigate production process and performance. Expansion of the initial steam chamber, its shape and area, and temperature distributions were visualized using video and thermal-video pictures. The relationship between isotherms and steam chamber interface was investigated to study the drainage mechanism. The temperature at the interface where the steam chamber was expanding was observed to remain nearly constant at 80°C. Effect of vertical spacing between the two horizontal wells on oil recovery was also investigated. For the case of conventional SAGD, oil production rate increased with increasing vertical spacing; however, the lead time for the gravity drainage to initiate oil production became longer. The results suggest that L can be used as a governing factor to evaluate production rate and lead time in the initial stage of the SAGD process. Based on these experimental results, the SAGD process was modified: the lower production well was intermittently stimulated by steam injection, in conjunction with continuous steam injection in the upper horizontal injector. Using the modified process (named SAGD-ISSLW), the time to generate near breakthrough condition between two wells was shortened, and oil production was enhanced at the rising chamber stage compared with that of the conventional SAGD process.

Physicochemical and microemulsion properties of dimeric quaternary ammonium salts with trimethylene spacer for enhanced oil recovery

Dimeric surfactants, also termed as Gemini surfactants, are regarded as organic materials which have two hydrophilic head groups and two hydrophobic groups in the molecules linked together with a spacer. In this study, dimeric surfactants of quaternary ammonium bromide connected with a trimethylene spacer group (m-3-m) have been investigated as potential micellar solutions for enhanced oil recovery. Static surface tension, interfacial tension as well as optimal salinity characterized their physicochemical and microemulsion properties. Using modeled petroleum fluids, the critical micelle concentration (CMC) was found dependent not only of the chemical architecture of the surfactant but also of the composition in the liquid phase. The nature and/or the length of spacer group participates significantly to the spatial rearrangement of the dimeric surfactants which subsequently altered the surface properties. For the same spacer group, an ultralow interfacial tension was achieved. Encouraging oil solubilization was found for surfactants used with an effect pronounced for longer alkyl chain. Furthermore, both the effects and the presence of metallic divalent ions on the phase behavior were discussed.