Unsteady Cavitating Turbulent Flow Research Articles

Verification and validation of Delayed Detached Eddy Simulation for cavitating turbulent flow around a hydrofoil and a marine propeller behind the hull

The Delayed Detached Eddy Simulation (DDES) was used to study unsteady cavitating turbulent flow around a hydrofoil and a highly skewed marine propeller behind a ship hull. The numerical results reproduced the transient cavitating flow around the hydrofoil and marine propeller, which agreed fairly well with the available experimental data. To further evaluate the accuracy of these DDES results quantitatively, the verification and validation methodologies and procedures were firstly applied in DDES model. The modified three-equation method was derived and illustrated to do verification and validation in DDES model. The hydrofoil cavitation simulation results indicated that the unsteady cavitating flow significantly influenced the distribution of the DDES errors. The periodic cavity shedding and large-scale vortex increased the DDES errors with both being the main source of the large errors. Similar verification and validation methodologies and procedures were then used to analyze propeller cavitation. These results showed that the errors in the predicted thrust coefficient were much smaller than the errors in the total cavity volume. Further analyses with different mesh resolutions showed that the finer flow structures were reproduced with better mesh resolution. Two small peaks between the crest and trough on the fluctuating pressure curves were induced in the predictions using Mesh 6 and 7 due to the area change of the shedding cavity, which indicated that the pressure fluctuations was closely correlated to the cavitation.

Numerical and experimental study on unsteady behavior of cavitating flow around a two-dimensional wedge-shaped body

In this study, the numerical analysis of unsteady cavitating turbulent flow behind a two-dimensional wedge-shaped body is performed using the commercial program STAR-CCM+ as a part of a fundamental study on the control fin of supercavitating underwater vehicles. We explore the vortex structures in the near and far wake fields and investigate the effect of cavity growth on the periodic characteristics of wake flow (σ = 1.0 ~ 2.0). Pressure fluctuations above the wedge are converted to sound pressure levels in the frequency domain via the fast Fourier Transform. As a result, we confirm that the shedding frequency of the vortices behind the body is strongly affected by the development of cavitation. As the cavitation number decreases, the frequency of the vortex in the near wake region decreases, and the force accelerating the Karman vortex in the far wake region decreases. In addition, we clearly validate the wake flow characteristics of a two-dimensional wedge-shaped body by comparing our numerical results with the experimental results carried out at the Chungnam National University Cavitation Tunnel (CNU-CT) at three different cavitation numbers (σ = 1.3, 1.5, and 2.0). Observations using a high-speed camera and measurements of pressure fluctuation above the test model are carried out to demonstrate the wake flow characteristics.

An integral calculation approach for numerical simulation of cavitating flow around a marine propeller behind the ship hull

In this paper, the unsteady cavitating turbulent flow around a marine propeller is simulated based on the unsteady Reynolds averaged Navier-Stokes (URANS) with emphasis on the hull-propeller interaction by an integral calculation approach, which means the propeller and hull are treated as a whole when the cavitating flow is calculated. The whole calculational domain is split to an inner rotating domain containing a propeller and an outer domain containing a hull. And the two split sections are connected together in ANSYS CFX by using the GGI interfaces and the transient rotor stator frame change/mixing model. The alternate rotation model is employed for the advection term in the momentum equations in order to reduce the numerical error. Comparison of predictions with measurements shows that the propeller thrust coefficient can be predicted satisfactorily. The unsteady cavitating flow around the propeller behind the ship hull wake shows quasi-periodic features including cavity inception, growth and shrinking. These features are effectively reproduced in the simulations which compare well to available experimental data. In addition, significant pressure fluctuations on the ship hull surface induced by the unsteady propeller cavitation are compared with experimental data at monitoring points on the hull surface. The predicted amplitudes of the first components corresponding to the first blade passing frequencies match well with the experimental data. The maximum error between the predictions and the experimental data for the pressure pulsations is around 8%, which is acceptable in most engineering applications.

Numerical simulation of cavitation surge and vortical flows in a diffuser with swirling flow

The strong swirling flow at the exit of the runner of a Francis turbine at part load causes flow instabilities and cavitation surges in the draft tube, deteriorating the performance of the hydraulic power system. The unsteady cavitating turbulent flow in the draft tube is simplified and modeled by a diffuser with swirling flow using the Scale-adaptive simulation method. Unsteady characteristics of the vortex rope structure and the underlying mechanisms for the interactions between the cavitation and the vortices are both revealed. The generation and evolution of the vortex rope structures are demonstrated with the help of the iso-surfaces of the vapor volume fraction and the Qcriterion. Analysis based on the vorticity transport equation suggests that the vortex dilatation term is much larger along the cavity interface in the diffuser inlet and modifies the vorticity field in regions with high density and pressure gradients. The present work is validated by comparing two types of cavitation surges observed experimentally in the literature with further interpretations based on simulations.

On the flow structure of cloud cavitating flow around an axisymmetric body near the free surface

The influence of the free surface on the cavitating flow is an important issue involved in the design of high speed surface vehicles. In the present paper, unsteady cavitating turbulent flow around an axisymmetric body near the free surface was investigated by both launching experiment and LES simulation. The vortex motion induced by cavity shedding under the effect of the free surface is emphatically analyzed by comparing with the submerged condition. The vortex shedding process around the projectile is not synchronized, while the asymmetric characteristic in collapse process is more remarkable, with the generation of multiple vortex ring structures.

Numerical study of unsteady cavitation on 2D NACA0015 hydrofoil using free/open source software

The free/open source software as OpenFOAM, Salome meshing and python language under Debian Linux system are evaluated to model unsteady cavitating turbulent flow around a NACA0015 hydrofoil. Based on the cavitation model proposed by Kunz and large eddy simulation (LES) method, we denote the benefits of free software and open source tools as an alternative to proprietary software of computational fluid mechanics, and provided a modified cavitation model to improve numerical accuracy. The simulation results of typical break-off cycle for cavitation shedding are compared to available experimental data, and validated using image processing to find percentage of similarities. The sheet cavity frequency of 7.752 Hz was obtained based on cavitation evolution and pressure fluctuations. The study gives relevant information for CFD software development in the future.

LES investigation on cavity shedding of a Clark-Y hydrofoil under different attack angle with an integration method

Numerical simulations of unsteady cavitating turbulent flow around a Clark-Y hydrofoil were performed using the Large Eddy Simulation method under different attack angles. The shedding of the cavity cloud was captured and the numerical results of the total vapour volume accords well with the experimental results. In this paper the concept of entrainment ability (generally adopted in water jet) was firstly introduced to describe the process of the cavity shedding. Based on the integral time averaged Navier-Stokes equations, the integration of p+ρ1v2 was adopted to describe the entrainment ability. A horizontal line from upstream to the leading edge was defined to monitor the flow rate and entrainment ability being transferred to the suction side. It is found that the entrainment ability directly determines the level of the re-entrant jet, which sees a sudden rise at the breakdown of the cavity cloud with the re-entrant jet reaching to the leading edge. Moreover, compared with no-cavitating flow, the entrainment ability was greatly hindered by the intensive cavity cloud over the foil. The pressure fluctuation at the trailing edge can be transferred upstream through the pressure side and influences the entrainment ability and flow rate across Line B(QB, Line B is defined as a horizontal line from upstream to the leading edge). In addition, as the attack angle increases, the time averaged QB greatly increases, while the time averaged entrainment ability experiences a marginal rise. Thus, the variation of attack angle only impacts the entrainment ability from Line A (mA), since mA varies linearly with QB.

Numerical analysis of unsteady cavitating turbulent flow and shedding horse-shoe vortex structure around a twisted hydrofoil

Cavitating turbulent flow around hydrofoils was simulated using the Partially-Averaged Navier–Stokes (PANS) method and a mass transfer cavitation model with the maximum density ratio (ρl/ρv,clip) effect between the liquid and the vapor. The predicted cavity length and thickness of stable cavities as well as the pressure distribution along the suction surface of a NACA66(MOD) hydrofoil compare well with experimental data when using the actual maximum density ratio (ρl/ρv,clip=43391) at room temperature. The unsteady cavitation patterns and their evolution around a Delft twisted hydrofoil were then simulated. The numerical results indicate that the cavity volume fluctuates dramatically as the cavitating flow develops with cavity growth, destabilization, and collapse. The predicted three dimensional cavity structures due to the variation of attack angle in the span-wise direction and the shedding cycle as well as its frequency agree fairly well with experimental observations. The distinct side-lobes of the attached cavity and the shedding U-shaped horse-shoe vortex are well captured. Furthermore, it is shown that the shedding horse-shoe vortex includes a primary U-shaped vapor cloud and two secondary U-shaped vapor clouds originating from the primary shedding at the cavity center and the secondary shedding at both cavity sides. The primary shedding is related to the collision of a radially-diverging re-entrant jet and the attached cavity surface, while the secondary shedding is due to the collision of side-entrant jets and the radially-diverging re-entrant jet. The local flow fields show that the interaction between the circulating flow and the shedding vapor cloud may be the main mechanism producing the cavitating horse-shoe vortex. Two side views described by iso-surfaces of the vapor volume fraction for a 10% vapor volume, and a non-dimensional Q-criterion equal to 200 are used to illustrate the formation, roll-up and transport of the shedding horse-shoe vortex. The predicted height of the shedding horse-shoe vortex increases as the vortex moves downstream. It is shown that the shape of the horse-shoe vortex for the non-dimensional Q-criterion is more complicated than that of the 10% vapor fraction iso-surface and is more consistent with the experiments. Further, though the time-averaged lift coefficient predicted by the PANS calculation is about 12% lower than the experimental value, it is better than other predictions based on RANS solvers.

Numerical simulation of cavitating turbulent flow through a Francis turbine

The unsteady cavitating turbulent flow in a Francis turbine is simulated based on governing equations of the mixture model for cavity-liquid two-phase flows with the RNG k-ϵ turbulence model in the present paper. An improved mass transfer expression in the mixture model is obtained based on evaporation and condensation mechanics with considering the effects of the non-dissolved gas, the turbulence, the tension of interface at cavity and the effect of phase change rate and so on. The governing equations of the mixture model for the unsteady cavitating-liquid flow is solved by a direct coupling method numerically with the finite volume method (FVM) using the unstructured tetrahedron grid and the structured hexahedral grid system. This direct coupling simulation was successfully applied to simulate the cavitating two-phase turbulent flow through a Francis turbine. The simulated external results agreed well with the experimental results.

Numerical investigation of unsteady cavitating turbulent flow around a full scale marine propeller

This paper treats the unsteady cavitating turbulent flow around a full scale marine propeller operated in non-uniform ship wake. The RANS method combined with k-ω SST turbulence model and the mass transfer cavitation model was applied for the flow simulation. It is noted that both the propeller performance and the unsteady features of cavitating turbulent flow around the propeller predicted by the numerical calculation agreed well with the experimental data. Due to the non-uniform wake inflow and gravity effect, there occurred periodical procedure for cavity development such as cavitation inception, growth, shrinking, etc near the blade tip for the propeller. The study also indicated that there was considerably large pressure fluctuation near the propeller during the operation. The 1st order frequency of pressure fluctuation predicted by numerical simulation equaled the rotating frequency of propeller blades. Both amplitude and frequency agreed with the experimental results fairly well.

Simulations of unsteady cavitating turbulent flow in a Francis turbine using the RANS method and the improved mixture model of two-phase flows

This paper reports the simulation results for the unsteady cavitating turbulent flow in a Francis turbine using the mixture model for cavity–liquid two-phase flows. The RNG k–e turbulence model is employed in the Reynolds averaged Navier–Stokes equations in this study. In the mixture model, an improved expression for the mass transfer is employed which is based on evaporation and condensation mechanisms with considering the effects of the non-dissolved gas, the turbulence, the tension of interface at cavity and the effect of phase change rate and so on. The computing domain includes the guide vanes, the runner, and the draft tube, which is discretized with a full three-dimensional mesh system of unstructured tetrahedral shapes. The finite volume method is used to solve the governing equations of the mixture model and a full coupled method is combined into the algorithm to accelerate the solution. The computing results with the mixture model have been compared with those by the single-phase flow model as well as the experimental data. The simulation results show that the cavitating flow computation based on the improved mixture model agrees much better with experimental data than that by the single-phase flow calculation, in terms of the amplitude and dominated frequency of the pressure fluctuation. It is also observed from the present simulations that the amplitude of the pressure fluctuation at small flow rate is larger than that at large flow rate, which accords with the experimental data.