Centrifugal pumps as turbines (PATs) are widely used in chemical engineering for recycling the abundant energy from high-pressure fluid. The operation of PATs is significantly affected by their upstream conditions, which are not steady (i.e., with a constant flow rate) in reality, thus, research on the flow mechanism of PATs under transient conditions should be considered of higher importance. In this study, a numerical model of a PAT was developed by employing the sliding mesh method to describe turbine rotation, and a user-defined function (UDF) for characterizing transient flow conditions. Corresponding experiments were also conducted to provide validation results for the simulation, and the simulation results agreed well with the experimental outcomes. The instantaneous characteristic curves under the current working conditions were obtained for different transient flow rates. The results show that the turbine’s efficiency is significantly affected by transient flow conditions. In particular, a rapid increase (large time derivative) of flow rate results in a large energy dissipation at the turbine outlet, and therefore, the turbine efficiency decreases. In addition, as the flow rate increases, the hydrodynamic force on the impeller, and the pressure fluctuation amplitude in the volute first decrease and then increase, reaching the minimum near the design flow rate. The current study provides a reliable and precise approach for the estimation of hydrodynamic performance of fluid machinery under transient flow conditions.

Centrifugal pumps are widely used in the oil and mining industries. In contrast to water pumps, the centrifugal pumps in the oil and mining industries are used for the transportation of drilling fluid, which is typically non-Newtonian fluid. Drilling fluids are usually modeled as power-law fluids with varying shear viscosity and imposed shear rates. In this paper, a numerical simulation of power-law fluid flow in a centrifugal pump was simulated, varying only in the flow-rate magnitude, using water flow as a comparison. The simulation results show that the pump used for drilling fluid presents a lower head and efficiency but a higher shaft power than that used for water. The flow patterns of both the water pump and the drilling fluid pump were investigated in terms of pressure fluctuation, turbulent kinetic energy, and radial force on the impeller. In contrast to the literature, this paper also analyzes the pressure pulsations in the individual blades of the impeller, as well as those in the volute path. In the case of drilling fluid, it was found that the viscous effect made the flow at the end of the blades highly irregular, and this could be attributed to the pressure generated by them. At the same time, the fluid flow at the small cross-section of the volute was more sensitive to the rotation of the impeller. In addition, the effects of the shear collision exerted on the outlet fluid of the impeller and the fluid in the volute, as well as the dynamic and static interferences, made the non-Newtonian power-law fluid consume more mechanical energy than the water. The results of this paper can be used as a reference for improving the design of centrifugal pumps using non-Newtonian fluids as media.

The propulsive performance of a bio-inspired autonomous underwater glider (AUG) with active twin undulatory wings undergoing undulatory motion was investigated by numerically solving the viscous incompressible Navier–Stokes equations, coupled with the immersed boundary method. The aspect ratio (AR) effects of the undulatory wings were studied. The simulation results showed that with the increase of AR, the thrust force generated by the active twin undulatory wings showed a linear growth, while the propulsion efficiency of the AUG increased to the peak and then decreased. The optimum magnitude of AR around 2 was obtained in the current study. The vortex structures in the wake of the active twin wings are also presented and discussed. The conclusions acquired here could provide guidance for the new conceptual design of bio-inspired AUGs.

The working condition of a centrifugal pump as a turbine (PAT) is often unsteady. The rotating speed of a PAT constantly varies as the flow and load change, resulting in transient hydrodynamic behaviors between different working conditions. During the transition, the PAT undergoes a severe change in performance and complicated internal flow structures. In previous work, the fixed rotating speed of a PAT was mostly considered using computational fluid dynamics. To investigate the transient behavior of a PAT, relevant simulation tools are developed to depict transient flow conditions, and the corresponding transient speed of the impeller is calculated. Both large and small fluctuation transitions are simulated for the practical application of the PAT. The simulated results are first verified by experiments. The results show that the rotating speed significantly affects the performance and stability of the PAT. The rapid increment in flow rate and rotating speed lead to large energy dissipation in the internal flow field of the PAT. The range of high efficiency of the PAT expands and migrates to the high flow rate range. The efficiency in the transition condition started a cyclic growth after the flow reached 60 m³/h, and it reached a peak at around 80 m³/h, which was about 5% lower than the calculated value in a quasi-steady state. In the range of high rotating speeds, the rotating speed of the impeller and the operational stability are sensitive to flow fluctuation. The internal flow fields during transition conditions are analyzed as well. The obtained results can be utilized as a reference for studying the hydrodynamic characteristics and stability of fluid machinery in the transition under transient flow conditions.

A multi-body system model is proposed for the mimicking of swimming fish with coupled active and passive movements. The relevant algorithms of the kinematics and dynamics of the multi-body system and coupled fluid solver are developed and fully validated. A simplified three-body model is applied for the investigation of the hydrodynamic performance of both an active pitch motion and passive movement. In general, there is an optimal stiffness, under which the model swims with the fastest velocity. The effect of the damper can be drawn only when the stiffness is small. Comparing with the rigid tail, the flexible tail leads to a faster speed when the stiffness and damping coefficients are in a suitable range.

A mono tiltrotor (MTR) design which combines concepts of a tiltrotor and coaxial rotor is presented. The aerodynamic modeling of the MTR based on blade element momentum theory (BEMT) is conducted, and the method is fully validated with previous experimental data. An automated optimization approach integrating BEMT modeling and optimization algorithms is developed. Parameters such as inter-rotor spacing, blade twist, taper ratio and aspect ratio are chosen as design variables. Single-objective (in hovering or in cruising state) optimizations and multi-objective (both in hovering and cruising states) optimizations are studied at preset design points; i.e., hovering trim and cruising trim. Two single-objective optimizations result in different sets of parameter selections according to the different design objectives. The multi-objective optimization is applied to obtain an identical and compromised selection of design parameters. An optimal point is chosen from the Pareto front of the multi-objective optimization. The optimized design has a better performance in terms of the figure of merit (FM) and propulsive efficiency, which are improved by 7.3% for FM and 13.4% for propulsive efficiency from the prototype, respectively. Further aerodynamic analysis confirmed that the optimized rotor has a much more uniform load distribution along the blade span, and therefore a better aerodynamic performance in both hovering and cruising states is achieved.