Xinping Long

Large eddy simulation of turbulent attached cavitating flow with special emphasis on large scale structures of the hydrofoil wake and turbulence-cavitation interactions

In this paper, the turbulent attached cavitating flow around a Clark-Y hydrofoil is investigated by the large eddy simulation (LES) method coupled with a homogeneous cavitation model. The predicted lift coefficient and the cavity volume show a distinctly quasi-periodic process with cavitation shedding and the results agree fairly well with the available experimental data. The present simulation accurately captures the main features of the unsteady cavitation transient behavior including the attached cavity growth, the sheet/cloud cavitation transition and the cloud cavitation collapse. The vortex shedding structure from a hydrofoil cavitating wake is identified by the Q-criterion, which implies that the large scale structures might slide and roll down along the suction side of the hydrofoil while being further developed at the downstream. Further analysis demonstrates that the turbulence level of the flow is clearly related to the cavitation and the turbulence velocity fluctuation is much influenced by the cavity shedding.

Verification and validation of Urans simulations of the turbulent cavitating flow around the hydrofoil

In this paper, we investigate the verification and validation (V&V) procedures for the Urans simulations of the turbulent cavitating flow around a Clark-Y hydrofoil. The main focus is on the feasibility of various Richardson extrapolation-based uncertainty estimators in the cavitating flow simulation. The unsteady cavitating flow is simulated by a density corrected model (DCM) coupled with the Zwart cavitation model. The estimated uncertainty is used to evaluate the applicability of various uncertainty estimation methods for the cavitating flow simulation. It is shown that the preferred uncertainty estimators include the modified Factor of Safety (FS1), the Factor of Safety (FS) and the Grid Convergence Index (GCI). The distribution of the area without achieving the validation at the Uv level shows a strong relationship with the cavitation. Further analysis indicates that the predicted velocity distributions, the transient cavitation patterns and the effects of the vortex stretching are highly influenced by the mesh resolution.

Numerical investigation of unsteady cavitating turbulent flows around twisted hydrofoil from the Lagrangian viewpoint

Unsteady cavitating turbulent flow around twisted hydrofoil is simulated with Zwart cavitation model combined with the filter-based density correction model (FBDCM). Numerical results simulated the entire process of the 3-D cavitation shedding including the re-entrant jet and side-entrant jet dynamics and were compared with the available experimental data. The distribution of finite-time Lyapunov exponent (FTLE) was used to analyze the 3-D behavior of the re-entrant jet from the Lagrangian viewpoint, which shows that it can significantly influence the particle trackers in the attached cavity. Further analysis indicates that the different flow behavior on the suction side with different attack angle can be identified with Lagrangian coherent structures (LCS). For the area with a large attack angle, the primary shedding modifies the flow pattern on the suction side. With the decrease in attack angle, the attached cavity tends to be steady, and LCS A is close to the upper wall. A further decrease in attack angle eliminates LCS A in the boundary layer. The FTLE distribution also indicates that the decreasing attack angle induces a thinner boundary layer along the foil surface on the suction side.

Effects of coring directions on the mechanical properties of Chinese shale

This paper presents an experimental study of the effects of bedding planes on the mechanical properties of shale samples. Two specimens, sourced from Sichuan Basin in China, cored perpendicular and parallel to the beddings were tested using a uniaxial compressive strength (UCS) machine, and the corresponding crack propagation patterns were analysed using acoustic emission (AE) data. In addition, a digital camera system called ARAMIS was used to acquire the lateral and axial strains during the loading. It was observed that there was significant influence on the peak strength of shale when tested for various bedding planes. As expected, the sample with perpendicular load had nearly twice as high maximum axial strength than the sample with parallel load, and the peak cumulative energy release of the parallel load on the specimen was 1.5 times higher than that of the perpendicular load. The axial strain was higher than the lateral strain in perpendicular load, while under parallel load, the lateral strain was higher than the axial strain. When the axial stress was small, for the shale sample with perpendicular load, the axial strain increased gradually with the increase of stress. In contrast, the lateral strain remained near to zero during the loading period for the sample with parallel load, and both the axial strain and lateral strain were not affected by the increasing load and remained zero. The volumetric strain of the two samples indicated that shale with a perpendicular load shows a compaction behaviour, while parallel load leads to a dilatancy characteristic. The AE data showed that the compression period consists of three stages on both of the two tests and energy release occurs mainly in the third stage. From the ARAMIS results, no perpendicular cracking was observed in the normal stress test, while under parallel loading, the cracks were mostly parallel to the load direction.

Experimental Investigation of Mechanical Properties of Black Shales after CO2-Water-Rock Interaction

The effects of CO2-water-rock interactions on the mechanical properties of shale are essential for estimating the possibility of sequestrating CO2 in shale reservoirs. In this study, uniaxial compressive strength (UCS) tests together with an acoustic emission (AE) system and SEM and EDS analysis were performed to investigate the mechanical properties and microstructural changes of black shales with different saturation times (10 days, 20 days and 30 days) in water dissoluted with gaseous/super-critical CO2. According to the experimental results, the values of UCS, Young’s modulus and brittleness index decrease gradually with increasing saturation time in water with gaseous/super-critical CO2. Compared to samples without saturation, 30-day saturation causes reductions of 56.43% in UCS and 54.21% in Young’s modulus for gaseous saturated samples, and 66.05% in UCS and 56.32% in Young’s modulus for super-critical saturated samples, respectively. The brittleness index also decreases drastically from 84.3% for samples without saturation to 50.9% for samples saturated in water with gaseous CO2, to 47.9% for samples saturated in water with super-critical carbon dioxide (SC-CO2). SC-CO2 causes a greater reduction of shale’s mechanical properties. The crack propagation results obtained from the AE system show that longer saturation time produces higher peak cumulative AE energy. SEM images show that many pores occur when shale samples are saturated in water with gaseous/super-critical CO2. The EDS results show that CO2-water-rock interactions increase the percentages of C and Fe and decrease the percentages of Al and K on the surface of saturated samples when compared to samples without saturation.

Shear Cavitation in an Annular Jet Pump Under Recirculation Conditions

Recirculation accompanied by shear cavitation is a key flow feature in annular jet pumps (AJPs). In this study, a high-speed camera was used to capture the recirculation region and various types of cavity clouds. By monitoring the trajectories of the small bubbles, the main recirculation regions under each flow rate ratio were obtained. As the flow rate ratio decreases, the recirculation region continued expanding with the separation point moving upstream, while the reattachment point remained nearly stationary regardless of the decreasing flow rate ratio. Hills spherical vortex theory was adopted to evaluate the variations of the recirculation regions. Moreover, the minimum local static wall pressure in the recirculation region decreases as well, which can promote the inception and development of shear cavitation. There are numerous vortices simultaneously induced by the large velocity gradient in the shear layer, the core of which becomes a potential site for cavitation. Consequently, with the growth of the recirculation region, three types of cavity clouds, viz., the ribbonlike, annular, and merged cavity clouds, appear in turn. The movement characteristics of these cavity clouds, including their inception, distortion, and collapse, are illustrated based on the high-speed imaging results. The ribbonlike and annular cavity clouds are both induced by the small vortices in the shear layer because of the low local pressure in the vortex core. However, the merged cavity clouds are caused by a combination of several ribbonlike and annular cavity clouds, which provides a larger scale and a longer life span. Hence, the collapse of the merged cavity clouds can cause a large pressure pulsation near the reattachment point of the recirculation region. The corresponding frequency spectra were also demonstrated based on the fast Fourier transform (FFT) method.

Numerical investigation on the expansion of supercritical carbon dioxide jet

Supercritical carbon dioxide (SC-CO2) fluid is characterized by low rock breaking threshold pressure and high rock breaking rate. Meanwhile, SC-CO2 fluid has relatively low viscosity near to gas and high density near to liquid. So, it has great advantages in drilling and rock breaking over water. In this paper, numerical study of SC-CO2 flowing through a nozzle is presented. The purpose of this simulation is to ascertain why the SC-CO2 jet flow has better ability in drilling and rock breaking than the water jet flow. The simulation model was controlled by the RANS equations together with the continuity equation as well as the energy equation. The realizable k-epsilon turbulence model was adopted to govern the turbulent characteristics. Pressure boundary conditions were applied to the inlet and outlet boundary. The properties of carbon dioxide and water were described by UDF. It is found that: (1) under the same boundary conditions, the decay of dimensionless central axial velocity and dynamic pressure of water is quicker than that of the SC-CO2, and the core length of SC-CO2 jet is about 4.5 times of the nozzle diameter, which is 1 times longer than that of the water; (2) With the increase of inlet pressure or the decrease of outlet pressure, the dimensionless central axial velocity and dynamic pressure attenuation of water keeps the same, while the decay of central axial velocity of SC-CO2 turns gentle; (3) the change of central axial temperature of SC-CO2 is more complex than that of the water.

Numerical investigation on the impact of the converging angle of the suction chamber on annular jet pumps

The internal flow of an annular jet pump is an annular wall jet developed in a limited space, which is affected by its structure, shape, area ratio and flow rate ratio. In this paper, numerical simulation of an annular jet pump with area ratio A=1.75 was conducted based on realizable k-e turbulence model. And the impact of the converging angle of the suction chamber to the pump performance and the expansion of annular jet is investigated. According to the simulated results, the following conclusion is obtained. With the converging angle of the suction chamber (α) being 20°, the pump performance turns out to be the best. And with the increasing α, the maximal axial velocity and the squeezing effect to the annular jet are larger, while the mixing level of the two flows drops. In addition, the half-width of the annular jet, in the suction chamber, is proportional to tan(α/2) which is the tangent of the half angle of the suction chamber.

Application of Constant Rate of Velocity or Pressure Change Method to Improve Annular Jet Pump Performance

To improve annular jet pump (AJP) performance, new ways named constant rate of velocity/pressure change method (CRVC/CRPC) were adopted to design its diffuser. The design formulas were derived according to the assumption of linear velocity/pressure variation in the diffuser. Based on the two-dimensional numerical simulations, the effect of the diffuser profile and the included angle on the pump performance and the internal flow details has been analyzed. The predicted results of the RNG k-epsilon turbulence model show a better agreement with the experiment data than that of the standard and the realizable k-epsilon turbulence models. The AJP with the CRPC diffuser produces a linear pressure increase in the CRPC diffuser as expected. The AJP with CRPC/CRVC diffuser has better performance when the diffuser included angle is greater or the diffuser length is shorter. Therefore, the AJP with CRPC/CRVC diffuser is suitable for applications requiring space limitation and weight restriction.

A Damage Constitutive Model for the Effects of CO2-Brine-Rock Interactions on the Brittleness of a Low-Clay Shale

CO2 is a very promising fluid for drilling and nonaqueous fracturing, especially for CO2-enhanced shale gas recovery. Brittleness is a very important characteristic to evaluate the drillability and fracability. However, there is not much relevant research works on the influence of CO2 and CO2-based fluids on shale’s brittleness been carried out. Therefore, a series of strength tests were conducted to obtain the stress-strain characteristics of shale soaked in different phases of CO2 including subcritical or supercritical CO2 with formation of water for different time intervals (10u2009days, 20u2009days, and 30u2009days). Two damage constitutive equations based on the power function distribution and Weibull distribution were established to predict the threshold stress for both intact and soaked shale samples. Based on the results, physical and chemical reactions during the imbibition cause reductions of shales’ peak axial strength (20.79%~61.52%) and Young’s modulus (13.14%~62.44%). Weibull distribution-based constitutive model with a damage threshold value of 0.8 has better agreement with the experiments than that of the power function distribution-based constitutive model. The energy balance method together with the Weibull distribution-based constitutive model is applied to calculate the brittleness values of samples with or without soaking. The intact shale sample has the highest value of 0.9961, which is in accordance with the high percentage of brittleness minerals of the shale samples. The CO2-NaCl-shale interactions during the imbibition decrease the brittleness values. Among the three soaking durations, the minimum brittleness values occur on samples with 20u2009days’ imbibition in subcritical and supercritical CO2u2009+u2009NaCl solutions and the reductions of which are 2.08% and 2.49%, respectively. Subcritical/supercritical CO2u2009+u2009NaCl imbibition has higher effect on shale’s strength and Young’s modulus than on the brittleness. The low-clay shale still keeps good fracture performance after imbibition.