Non-cavitating Conditions Research Articles

Mesh Refinement Investigation to Simulate Tip Vortex Cavitation under Non-Cavitating Conditions

Mesh Refinement Investigation to Simulate Tip Vortex Cavitation under Non-Cavitating Conditions

Impacts of impeller blade number on centrifugal pump performance under critical cavitation conditions

The number of impeller blades is a significant geometric parameter that considerably impacts centrifugal pump performance. A transient numerical analysis of a centrifugal pump is conducted to examine the influence of the variable impeller blade number on pump performance under critical cavitation conditions. Six different impellers with 3, 5, 7, 8, 9, and 11 blades are examined numerically at a rotational speed of 2900 r/min when other impeller parameters remain unchanged. Fields of interior flow and properties of centrifugal pumps are studied concerning static pressure, velocity magnitude, and vapor volume fraction using ANSYS Fluent to perform the numerical simulation. The results show that numerical analysis can accurately predict centrifugal pump internal flow. The current results match experimental and numerical data for the NPSH, with a 4.65% discrepancy. Blade numbers affect the flow field and pressure amplitude, especially at the outlet region. As blade numbers increase, pressure increases, and the impeller with 11-blade has the maximum pressure amplitude. The impeller with a seven-blade achieved its highest efficiency level, exhibiting a 0.48% improvement under non-cavitation conditions and a 1.4% improvement under critical cavitation conditions compared to the original model. Furthermore, the cavity size on an individual blade of a model with three-blade is more extensive compared to other models. In addition, the impeller with a nine-blade exhibits the lowest value of the vapor volume percentage. This research analyses the influence of different blade numbers on the performance of centrifugal pumps while operating under critical cavitation conditions. It aims to provide novel insights into the flow characteristics associated with such circumstances.

Comparison Between CFD and Open Water Tests For PPTC Propeller In Non-Cavitating Conditions

Comparison Between CFD and Open Water Tests For PPTC Propeller In Non-Cavitating Conditions

Numerical study of the Amick–Schonbek system

AbstractThe aim of this paper is to present a survey and a detailed numerical study on a remarkable Boussinesq system describing weakly nonlinear, long surface water waves. In the one‐dimensional case, this system can be viewed as a dispersive perturbation of the hyperbolic Saint‐Venant (shallow water) system. The asymptotic stability of the solitary waves is numerically established. Blow‐up of solutions for initial data not satisfying the noncavitation condition as well as the appearance of dispersive shock waves are studied.

Study of a proportional spool valve noise by means of Functional Data Analysis: Cavitation and intensity detection

Cavitation inside hydraulic components is a well-known issue, that arises under specific operating conditions. When this phenomenon occurs in valves, it leads to several issues, such as materials damage, performance reduction, and noise. The latter of them is becoming increasingly important in the transport sector, which is moving towards new technologies (i.e., electrification) that are notoriously quiet. Therefore, cavitation detection is becoming a matter of interest for several applications. The purpose of the work presented is to develop a diagnostic technique to detect the onset of cavitation in proportional valves. A 2-way 2-position proportional spool valve has been placed inside an acoustic enclosure; noise signals have been recorded by means of a high frequency microphone, for different flow rates and valve openings. Sound pressure levels have been analysed by means of statistical techniques; a Functional Data Analysis (FDA) has been performed in the open-source R environment, in which the experimental data, collected under different non-cavitation conditions, have been firstly used to perform a Functional Principal Component Analysis (FPCA) and then to define a threshold. Then, a further experimental dataset has been evaluated through a Hotelling control chart, from which it is possible to distinguish cavitating conditions as they fall outside the limits.

Developing a numerical framework to study the cavitation and non-cavitation behaviour of a centrifugal pump inducer

The present work explores the impact of tip clearance on the mean blade height ratio, inlet tip blade angle, and surface roughness of the inducer. The objective is to find an optimized inducer to limit the secondary flow over the blades, which in turn improves the pump efficiency and reduces the life cycle costs. A numerical framework is developed to investigate efficient operational and geometrical parameters on an inducer's cavitation and non-cavitation presentations. The catalyser functioning is simulated by applying a 3D CFD model, and the results are assessed against empirical data. The results show a reliable agreement with empirical data and suggest that the increment of tip clearance in the mean blade height ratio causes the hydraulic performance and the analytical cavitation number to decline in cavitation and non-cavitation conditions. Moreover, the optimum value of 85o is found for the inlet tip blade angle, which improves the non-cavitation performance.

Numerical investigation of energy loss distribution in the cavitating wake flow around a cylinder using entropy production method

The wake flow of a circular cylinder is numerically investigated by Large Eddy Simulation (LES) combined with the Schnerr–Sauer cavitation model. By comparing entropy production in the presence or absence of cavitation, the energy loss distribution in the wake flow field of a cylinder is explored, shedding light on the interactions between multiscale vortex systems and cavitation. The comparative results reveal that, under non-cavitating conditions, the energy loss region in the near-wake area is more concentrated and relatively larger. Energy dissipation in the wake flow field occurs in regions characterized by very high velocity gradients, primarily near the upper and lower surfaces of the cylinder near the leading edge. The influence of cavitation bubbles on entropy production is predominantly observed in the trailing-edge region (W1) and the near-wake region (W2). The distribution trends of wall entropy production on the cylinder’s surface are generally consistent in both conditions, with wall entropy production primarily concentrated in regions exhibiting high velocity gradients.

Pressure Pulsation Characteristics of Agricultural Irrigation Pumps under Cavitation Conditions

Agricultural irrigation pumps are the main agricultural irrigation machinery, and their performance is closely related to the flow characteristics inside them. This paper combines experimental research and numerical simulation analysis. Based on the good agreement between the experimental results and numerical simulation data, this paper focuses on studying the characteristics of pressure fluctuations of agricultural irrigation pumps under cavitation conditions. The study found that under non-cavitation conditions, the pressure fluctuations at different monitoring points in pumps with different numbers of blades showed periodic variations, which are related to the number of blades. Under cavitation conditions, the pressure fluctuation coefficients of agricultural irrigation pumps with different impeller blade numbers increased, with a significant impact on the distribution of radial forces on the impeller. The research results supplement the relevant theoretical analysis and have certain practical significance for the application of agricultural irrigation pumps to practical production.

Possibilities and limits of modeling cavitation in high-pressure homogenizers – a validation study

High-pressure homogenizers are widely used in industrial processes to produce emulsions with small droplet sizes. During the process, cavitation occurs under industrial process conditions. In order to investigate the flow conditions inside a homogenizer geometry, CFD simulations are commonly used, since it is not possible to evaluate local flow conditions experimentally. However, these studies have, so far, investigated flow under non-cavitating conditions, which do not adequately reflect industrial process conditions. This study investigates the extent to which the two cavitation models, the Schnerr-Sauer model and the Zwart-Gerber-Belamri model, can represent cavitation in the gap of the homogenizing geometry using RANS simulation. Simulations are validated with experimental cavitation visualization data. Results show that the Schnerr-Sauer model (with appropriately set modeling constants) is able to accurately predict the operating conditions responsible for cavitation inception in the valve, as well as the length and width of the cavitation zone.

Experimental investigation on tip vortex cavity deformation and flow dynamics using high-speed imaging and laser Doppler velocimetry measurements

Cavitation affects engineering applications relating to aquatic operations. Tip vortex cavitation (TVC) leads to several technical problems, such as efficiency loss and noise. Hence, there is a need for a comprehensive understanding of the formation mechanism, shape distortions, and underlying physical phenomena of TVC. The dispersion relation of inertial waves on inviscid cavitating vortices is a valuable tool for predicting various TVC shapes. This study explains the patterns of flow around the tip of an elliptical foil and the cavity shape deformations under different flow conditions through experimental analysis. Experiments are conducted using a National Advisory Committee for Aeronautics hydrofoil in the cavitation tunnel at Chungnam National University. The appearance, development, and shapes of vortex cavitation are closely examined using high-speed imaging technology. There is good agreement between the vortex cavity shapes captured by this high-speed imaging and those derived in previous studies. Using laser Doppler velocimetry (LDV) measurements and analysis through image processing of high-speed images, we compare the vortex core trajectories in cavitating and non-cavitating conditions. There is a good match between the two, demonstrating the feasibility of predicting the flow behaviors around vortex cavities using LDV data. As the noise from the TVC is considered a significant source of underwater radiated noise we also measured sound pressure level, which exhibits abrupt changes at specific cavitation numbers, supporting the occurrence of the singing vortex phenomenon and highlighting its sensitivity to small variations in flow conditions. Furthermore, we obtain precise measurements of the instantaneous TVC diameter under different cavitation numbers, allowing the physical functionality of distinct TVC shapes to be determined. Our results significantly extend the scientific understanding of the flow around TVC and the fundamental causes of TVC distortions.

Effect of skew on the tonal noise characteristics of a full-scale submarine propeller

Recently, interest in underwater propeller noise has been increasing, especially submarine propeller noise which is directly related to the survivability. The submarine propeller is operating in a strong non-uniform inflow, thereby generating unsteady loading noise, which is dominant under non-cavitating conditions. It is widely known that unsteady loading noise can be reduced using a skewed propeller, therefore, accurate modeling of non-uniform inflows is important to understand the effects of skew. In this study, the entire submarine and propeller were modeled at full scale to consider the non-uniform inflow and analyze the effect of skew on tonal noise characteristics. Numerical studies of the submarine with Case-B (Base) and Case-S (Skewed) propellers were carried out in STAR-CCM+, and each case was analyzed according to the advance speed. The results show that the thrust of a blade fluctuates significantly and causes unsteady loading noise when the blade passes through non-uniform inflow regions. The noise level of the Case-S propeller decreases by 9–10 dB from that of the Case-B propeller near the axial direction but increases at specific radiation directions. Furthermore, it was observed that the effect of the skew on the tonal noise decreased as the advance speed increased.

Numerical analysis of propulsion performance of a waterjet-propelled vehicle in steady drift

This paper analyzes the propulsion performance and the correlating flow details of a waterjet–propelled vehicle in straight ahead and steady turning conditions. The viscous flow field of the fully discretized waterjet – propelled vehicle is numerically solved utilizing RANS method. Drift angles ranging from 0° to 30° with an interval of 10° are considered. The theoretical control volume of waterjet system under oblique flow and the corrected thrust equation are proposed. The results show that the enhanced bilge vortex in drift alters the ingested flow pattern of the waterjet-propelled vehicle. A steady hydraulic performance of the pump is observed under non-cavitating conditions, and the inlet is one of the major source of the energy loss of the internal flow. It is problematic to evaluate the thrust and the inlet efficiency due to the irregular shape of capture area. The tangential velocity component is in proportion to the drift angle at pump inflow plane, and leads to the port/starboard asymmetry of the axial velocity and vorticity. The swirling flow rolls up near the windward inlet duct and local vortices align to the spiral axial flow. The streamwise vortex occurs and distributes near the lower-windward side of the inlet in steady turning conditions.

Long time well-posedness of Whitham–Boussinesq systems

Consideration is given to three different full dispersion Boussinesq systems arising as asymptotic models in the bi-directional propagation of weakly nonlinear surface waves in shallow water. We prove that, under a non-cavitation condition on the initial data, these three systems are well-posed on a time scale of order , where ɛ is a small parameter measuring the weak nonlinearity of the waves. For one of the systems, this result is new even for short time. The two other systems involve surface tension, and for one of them, the non-cavitation condition has to be sharpened when the surface tension is small. The proof relies on suitable symmetrizers and the classical theory of hyperbolic systems. However, we have to track the small parameters carefully in the commutator estimates to get the long time well-posedness. Finally, combining our results with the recent ones of Emerald provide a full justification of these systems as water wave models in a larger range of regimes than the classical (a, b, c, d)-Boussinesq systems.

NUMERICAL SIMULATION AND ANALYSIS OF THE PRESSURE FLUCTUATION OF FOUR WATERJETS IN V-TYPE SHIP

The noise and vibration induced by pressure fluctuation is one of the major issues that threatens the waterjet-propelled ship comfortability. The self-propulsion simulations of four waterjets which are installed at the V-type Vessel’s Stern are carried out by solving the unsteady RANS equations. The analysis on pressure fluctuation characteristic for non-cavitation conditions are presented on the inner surface of the waterjets 5 locations along the axial direction, which are duct outlet, impeller inlet and outlet sections, impeller tip, stator outlet. This paper also focuses on comparing the pressure fluctuation characteristic between the four waterjets which are installed at the V-type Vessel’s Stern and the flush type of intake waterjet. This research could be useful for structural design of the waterjet propulsion ship.

Experimental Study on Unsteady Cavitating Flow and Its Instability in Liquid Rocket Engine Inducer

To study instability in the unsteady cavitating flow in a liquid rocket engine inducer, visualization experiments of non-cavitating and cavitating flows inside a model inducer were carried out at different flow conditions. Visual experiments were carried out to capture the evolution of non-cavitating and cavitating flows in a three-bladed inducer by using a high-speed camera. The external characteristic performance, cavitation performance, and pressure pulsation were analyzed based on the observation of non-cavitation and cavitation development and their instabilities. Under non-cavitation conditions, the change of flow rate has a significant impact on the pressure pulsation characteristics in the inducer. The occurrence of cavitation aggravated the instability of the flow and caused the intensity of pressure pulsation at each measuring point to increase. This cavitation structure has strong instability, and the tail region is often accompanied by shedding cavitation clouds perpendicular to the blade surface.

Numerical study on the effect of guide vane opening and draft tube outlet pressure on cavitation in Kaplan turbine

variations in electricity demand and site conditions like mass flow rate and net head, force the turbines to operate under off-design conditions. This increases cyclic loading in stationary and rotating components and severe cavitation in low head turbines like Kaplan turbine. The present analysis aims to examine the influence of various operating conditions on hydraulic efficiency and cavitation characteristics using ANSYS CFX. The stages of cavitation development in the runner are extensively studied for different operating conditions. The key point of present work is the critical analysis of various flow parameters for cavitating and non-cavitating conditions. The value of critical cavitation factor is found to increase from 0.34 to 0.61 with an increase in guide vane opening from 35º to 60º. The blade loading and flow parameters are found to be affected due to changes in guide vane opening and cavitation. The computed results are validated with the experimental model testing results and found to bear close comparison.

Performance prediction of cavitating marine current turbine by BEMT based on CFD

In the present study, the hydrodynamic performance of cavitating and non-cavitating marine current turbines (MCTs) has been investigated by blade element momentum theory (BEMT) coupled with Reynolds-averaged Navier–Stokes (RANS) technique. Before employing the BEMT, two-dimensional (2-D) lift and drag forces of cavitating and non-cavitating sections of MCT have been computed by RANS. The lift and drag forces, including cavity shapes, depend on Reynolds number (Re), cavitation number, and angle of attack. The hydrodynamic characteristics of cavitating 2-D hydrofoils (sections) were calculated via a multiphase solver “interPhaseChangeFoam” from the open-source code OpenFOAM. Cavitating flow simulations around a hydrofoil were carried out with Shear Stress Transport (SST) k−ω turbulence and Schnerr–Sauer cavitation model. The grid convergence index (GCI) was applied to confirm the numerical accuracy of the simulations. Validation of the proposed BEMT model has been done with two different model MCTs under non-cavitating conditions. Then, cavitating flow over a hemispherical head-form axisymmetric body, the NACA 66, and the Clark-Y were selected to evaluate the accuracy of the numerical methods and turbulence models available in the solver. Then, cavitating National Renewable Energy Laboratory (NREL) S814 2-D hydrofoil was later simulated for various Reynolds numbers, cavitation numbers, and angles of attacks. By using cavitating NREL S814 2-D hydrofoil data, the cavitation phenomenon was included in the method of BEMT. The distribution of cavitation on the blade has been modeled with BEMT coupled with RANS. Power coefficients at a wide range of tip speed ratios, including cavitation distribution on turbine blades obtained from BEMT, have been compared with each other and experiments. A reliable agreement has been found for all cases studied here. It is shown that cavitation has a negative effect on the power output of MCT for all tip speed ratios and cases discussed in this study. In addition, under cavitating conditions, the tip speed ratio of the operation point that corresponds to the highest power coefficient shifts to the right side of the power curve.

Numerical investigation of cavitation flow on hydroelastic response of a flexible 3D hydrofoil

ABSTRACT The dynamical response of a 3D hydrofoil in the cavitation flow is investigated. Fluid and Structure Interaction (FSI) simulation was performed using ANSYS Fluent Software. First, the validation of cavitation flow around NACA 0015 hydrofoil was carried out. To predict the essential features of cavitation flow, like the shedding of vortex and cloud cavities, the turbulent viscosity of SST (Shear-Stress Transport) k-ω model was modified by writing a UDF (User-Defined Function) code. To ensure the simulation of two-way Fluid-Structure Interaction (FSI), the results of NACA 66 hydrofoil in the uniform flow at non-cavitation conditions (wetted flow) are compared with experimental results, which indicates a good agreement. The results show that metallic alloys (aluminium and magnesium) have a slight effect on hydrofoil performance in comparison to each other. At the flexible foils, the hydrodynamic loads would cause bending and twisting deformations of the foil and the foil vibration.

Comparison of the hydrodynamic performance of front and rear-stator pump-jet propulsors in an oblique wake under the cavitation condition

Pump-jet propulsors, which exhibit high efficiency and low noise, are widely used in underwater vehicles. In a marine environment, a nonuniform wake and an oblique flow affect a propulsor's hydrodynamic force. Therefore, the hydrodynamic performance of front and rear-stator pump-jet propulsors under complex operational conditions was compared in this study. The unsteady hydrodynamic performance was predicted using a shear stress transport k–ω model. When the calculation converged, the Schnerr–Sauer cavitation model was used to simulate the cavitation performance. After an analysis of grid independence and a verification of the calculation model's reliability, the hydrodynamic performance of two pump-jet propulsors in an oblique wake was simulated under the non-cavitation and cavitation conditions. The results indicate that under the non-cavitation condition, the performance of the front-stator pump-jet propulsor worsens more slowly with an increase in the inflow angle. Moreover, the rear-stator pump-jet propulsor stops exhibiting high efficiency when the flow angle (β) is 30°. When cavitation occurs, the performance of the two pump-jet propulsors worsens and their hydrodynamic coefficients decrease rapidly. When β = 30°, the front and rear-stator pump-jet propulsors lose 60% and 40% of their thrust, respectively. The results of this study indicate that cavitation generation has a greater impact on the performance of a front-stator pump-jet propulsor.

Experimental study on the boundary layer of the NACA16-012 hydrofoil during the cavitation disappearance phenomenon

The generated cavitation on the NACA16-012 hydrofoil suddenly disappears at a specific angle of attack, even at a low cavitation number, where cavitation normally develops. This was named the cavitation disappearance phenomenon by the authors. It has not been clear why, once cavitation occurs, it disappears. In this study, pressure distributions on a suction surface under noncavitation and disappearance conditions were measured in the cavitation tunnel at an angle of attack at which the cavitation disappearance phenomenon occurred. Moreover, the boundary layer and pressure distribution on a suction surface under noncavitation conditions were comprehensively measured in the wind tunnel at a wide range of the angle of attack. As a result of the wind tunnel experiment, the angle of attack where the cavitation disappearance phenomenon occurred corresponded to the angle of attack where the onset of the short bubble occurred under noncavitation conditions. As the result of the cavitation tunnel experiment, the pressure distribution was found to differ between noncavitation and disappearance conditions. Under disappearance conditions, the suction peak disappeared and the low-pressure region extended to the downstream region. Additionally, the pressure distribution under the disappearance condition seemed to correspond with the distribution in a burst state, which was measured in a wind tunnel. At the same time, the boundary layer in the sheet/cloud cavitation periodically repeated the short bubble (during cloud release) and burst (during sheet cavitation) states. The disappearance of cavitation may be caused by the situation in which the burst boundary layer cannot return to a short bubble after the cloud cavity release and sheet cavitation cannot occur because the pressure is high in the burst boundary layer.