Active Control of Unsteady Cavitating Flows Over Hydrofoil

A preliminary two-dimensional (2D) numerical investigation of the active control of unsteady cavitation by means of one single synthetic jet actuator (SJA) is presented. The investigation involves the cloud-cavitating flow of water around a NACA 0015 hydrofoil with an angle of attack of 8-deg and ambient conditions. The SJA locates on the suction side at a distance of 16% of the chord from the leading edge; it has been modeled by means of a user-defined velocity boundary conditions based on a sinusoidal waveform. A Eulerian homogeneous mixture model has been used, coupled with an extended Schnerr–Sauer cavitation model and a volume of fluid interface tracking method. As first, a sensitivity analysis allowed to evaluate the influence of the main control parameters, namely, the momentum coefficient Cμ, the dimensionless frequency F+, and the jet angle αjet. As a result, the best performing SJA configuration was retrieved at Cμ=0.0002, F+=0.309, and αjet=90 deg, which led to a reduction of both the average vapor content and the average torsional load in the measure of 34.6% and 17.8%. The analysis of the coupled dynamics between vapor cavity–vorticity and their proper orthogonal decomposition (POD)-based modal structures highlighted the benefit of the SJA lies in preventing the growth of a thick sheet cavity, which causes the development of the highly cavitating cloud dynamics after the cavity breakup. This is mainly due to an additional vorticity close to the hydrofoil surface just downstream the SJA, as well as a local pressure modification close the SJA during the blowing stroke.

Coupling Quasi-Two-Dimensional Friction Model and Discrete Vapor Cavity Model for Simulation of Transient Cavitating Flows in Pipeline Systems

Accurate simulation of cavitating flows in pipeline systems is important for cost-effective surge protection. However, this is still a challenge due to the complex nature of the problem. This paper presents a numerical model that combines the discrete vapor cavity model (DVCM) with the quasi-two-dimensional (quasi-2D) friction model to simulate transient cavitating flows in pipeline systems. The proposed model is solved by the method of characteristics (MOC), and the performance is investigated through a numerical case study formulated based on a laboratory pipeline reported in the literature. The results obtained by the proposed model are compared with those calculated by the classic one-dimensional (1D) friction model with the DVCM and the corresponding experimental results provided by the literature, respectively. The comparison shows that the pressure peak, waveform, and phase of pressure pulsations predicted by the proposed model are closer to the experimental results than those obtained by the classic 1D model. This demonstrates that the proposed model that combines the quasi-2D friction model with the DVCM has provided a solution to more accurately simulate transient cavitating flows in pipeline systems.

A Simulation Calculation Method of a Water Hammer with Multpoint Collapsing

The traditional discrete vapor cavity model (DVCM) is widely used in water hammer simulation. Water column separation in pipelines is usually predicted with this model. Nevertheless, the main weaknesses of this model consist of numerical instability and nonconvergence. Regarding the weaknesses of the traditional model, this paper discusses an improved method. The new method uses a new water hammer velocity formula, a new cavity model, and a floating grid method. Through simulations to test the effects of the new model, an experimental platform can be established to realize a water hammer with multipoint collapsing. The numerical simulation was programmed in C++ and the test was carried out with an actual pipeline model built in the laboratory. After certain modelling and calibration, the parameters in the simulation calculation were consistent with the measured parameters in the test. The numerical simulation results were compared with the experimental results. For the hydraulic transient system with multipoint collapsing, the superposition effect of the wave crest of the pseudo-water hammer in the traditional calculation model was obvious. The pressure of the water hammer in the simulation calculation was significantly higher than the actual value and the convergence effect of the water hammer wave was not good. Compared with the results of the traditional model, the simulation results of the new model were closer to the measured values. Therefore, the new model has better numerical solution accuracy, stability, and convergence, which is worth further study and promotion.

CFD Investigations of Transient Cavitation Flows in Pipeline Based on Weakly-Compressible Model

In hydraulic systems, transient flow often occurs and may results in cavitation in pipelines. In this paper, the Computational Fluid Dynamics (CFD) method based on the Fluent software was used to investigate the cavitation flow in pipeline; the density-pressure model was incorporated into the continuity equation by using further development of UDF (user defined function), which reflects the variable wave speed of the transient cavitation flow, and the related algorithms were established based on weakly compressible fluid Reynolds Average Navier-Stokes (RANS) techniques. Firstly, the numerical simulations of the transient non-cavitation and cavitation flows caused by the fast closing valve in the reservoir-pipe-valve system were carried out by using the grid slip technique. The simulation results can enrich the flow field information such as velocity, pressure and vapor volume fraction. Through the evolution process of the pressure field, the propagation characteristics of pressure waves can be analyzed qualitatively and quantitatively. Through the evolution process of the velocity field, it can be seen that the velocity distribution in the wall area changes rapidly and has a high gradient, which mainly depends on the viscosity. However, the change of the velocity distribution in the core region is related to the velocity distribution of the history of the past time, which mainly depends on the diffusion. The formation, development and collapse of the cavity can be successfully captured, and it can be clearly and visually observed that the uneven distribution of vapor cavity in the direction of pipe length and pipe diameter, and the vapor cavity move slowly along the top of the pipe wall. Rarefaction wave’s propagation into pressure decreasing region and pressure increasing region can lead to different results of cavitation flow. The accuracy and reliability of the weakly compressible fluid RANS method were verified by comparing the calculated results with the experimental data.

Numerical Investigation of Nonisothermal Cavitating Flows on Hydrofoils by Means of an Extended Schnerr–Sauer Model Coupled With a Nucleation Model

This work aimed to investigate cavitating flows of water, liquid hydrogen, and nitrogen on hydrofoils numerically, using the open source code openfoam. The Eulerian homogeneous mixture approach has been used, consisting in a mass transfer model, which is based on the combination of a two-phase incompressible unsteady solver with a volume of fluid interface tracking method. Thermal effects have been introduced by means of the activation of energy equation and latent heat source terms plus convective heat source term. The dependency of the saturation conditions to the temperature has been defined using Antoine-like equations. An extended Schnerr–Sauer model based on the classical nucleation theory (CNT) has been implemented for the computation of the interfacial mass transfer rates. In order to investigate the nucleation effects, an extension of the CNT has been considered by coupling the population balance equation (PBE)/extended quadrature-based method of moments with the computational fluid dynamics (CFD) model, which has been defined in combination with a transport equation for the nuclei density. Results showed that nucleation determined a nonuniform field of nuclei density so as to produce a reduction of the temperature drop inside the vapor bubbles, as well as a warmed wake downstream the vapor cavity. Unsteady computations also revealed an influence of the nucleation on the dynamics of the vapor cavity and the bubble detachment.

Active Control of Unsteady Cavitating Flows in Turbomachinery

A preliminary 2D numerical investigation of the active control of unsteady cavitation by means of one single synthetic jet actuator (SJA) is presented. The SJA has been applied to hinder the intrinsic instabilities of a cloud cavitating flow of water around a NACA 0015 hydrofoil with an angle of attack of 8° and ambient conditions. It has been placed inside the inception region at a distance of 16% of the chord from the leading edge. Concerning the numerical approach, a Eulerian homogeneous mixture/mass transfer model has been used, in combination with an extended Schnerr-Sauer cavitation model and a Volume of Fluid (VOF) interface tracking method. The synthetic jet has been modeled by means of a user-defined velocity boundary conditions based on a sinusoidal waveform. A sensitivity analysis has been first performed in order to evaluate the influence of the main control parameters, namely the momentum coefficient Cμ, the dimensionless frequency F+ and the jet angle αjet. By combining the cavitating vapor content and the impact on the hydrodynamic performance, the best performing SJA configuration has been retrieved. Then, a deeper analysis of the vapor cavity dynamics and the vorticity field has been conducted in order to understand the modification of the main flow produced by the synthetic jet. The best SJA configuration was observed at Cμ = 0.0002, F+ = 0.309 and αjet = 90°, which led to a reduction of both the average vapor content and the average torsional load in the measure of 34.6% and 17.8% respectively. A reduction of the average pulsation frequency of the pressure upstream confirmed the beneficial effect of the SJA. The analysis of the coupled dynamics between vapor cavity-vorticity and their POD-based modal structures highlighted that the benefit of the SJA lies on preventing the growth of a thick sheet cavity which tends to cause the development of the highly cavitating cloud dynamics after the cavity breakup. This is mainly due to an additional vorticity close to the hydrofoil surface just downstream the SJA, as well as a local pressure modification close the SJA during the blowing stroke.

Cavitating Flow through a Micro-Orifice

Microfluidic systems have witnessed rapid development in recent years. As one of the most common structures, the micro-orifice is always included inside microfluidic systems. Hydrodynamic cavitation in the micro-orifice has been experimentally discovered and is harmful to microfluidic systems. This paper investigates cavitating flow through a micro-orifice. A rectangular micro-orifice with a l/d ratio varying from 0.25 to 4 was selected and the pressure difference between the inlet and outlet varied from 50 to 300 kPa. Results show that cavitation intensity increased with an increase in pressure difference. Decreasing exit pressure led to a decrease in cavitation number and cavitation could be prevented by increasing the exit pressure. In addition, the vapor cavity also increased with an increase in pressure difference and l/d ratio. Results also show the pressure ratio at cavitation inception was 1.8 when l/d was above 0.5 and the cavitation number almost remained constant when l/d was larger than 2. Moreover, there was an apparent difference in cavitation number depending on whether l/d was larger than 1.