Thermodynamic effects during growth and collapse of a single cavitation bubble

The thermodynamic effects associated with the growth and collapse of a single cavitation bubble are investigated in the present paper by an experimental approach. The study focuses on the temperature variations in the liquid surrounding the bubble. Experiments are conducted in a cylinder partially filled with water at an ambient temperature and atmospheric pressure. The bubble growth results from the expansion of an initial air bubble, due to the pressure wave generated by a so-called ‘tubearrest’ method. Several locations of the bubble, at different distances from the bottom wall of the cylinder, are considered. The bottom wall is made of sapphire, which is transparent to both the visible and infrared light spectra which enables temperature measurements by a high-speed thermovision camera at a wavelength of 3–5 m. Water is opaque to the infrared light spectrum, hence only temperatures in the boundary layer and on the liquid vapour interface could be determined. A temperature decrease of 3 K was recorded during the bubble growth while an increase up to 4 K was detected during the collapse. Experimental results are compared to the predictions of the ‘thermal delay’ model based on the assumption that the bubble growth and collapse are due to phase changes only. In this approach, the temperature variations are related to the latent heat exchanges during the vapourization and condensation processes. On the basis of these results, the respective effects of phase change and air dilatation/compression in the bubble dynamics are discussed

Cavitation regime detection through Proper Orthogonal Decomposition: dynamics analysis of the sheet cavity on a grooved convergent-divergent nozzle

The unsteady character of the sheet cavity dynamics on the suction side of hydrofoils, on convergent–divergent nozzles or on blades in turbines and propellers is responsible for many issues like erosion, noise and vibrations. This two-phase flow dynamics is investigated using a robust method based on Proper Orthogonal Decomposition (POD). This method is applied to sequences of sheet cavity images, in order to identify the cavitation regimes (sheet cavity or cloud cavitation regimes). Once this method is validated on a reference case, POD calculation is used to evaluate the efficiency of a passive control method. Different longitudinal grooved surfaces are machined on the diverging wall of a Venturi. The grooves geometry allows to change the cavitation regime for a fixed cavitation number, and even to avoid the cloud cavitation shedding, which may damage structures

Cavitation inception in fast startup

The start-up of rocket engine turbopumps is generally performed only in a few seconds. It implies that these pumps reach their nominal operating conditions after only a few rotations. During these first rotations of the blades, the flow evolution in the pump is governed by transient phenomena, based mainly on the flow rate and rotation speed evolution. These phenomena progressively become negligible when the steady behaviour is reached. The pump transient behaviour induces significant pressure fluctuations which may result in partial flow vaporization, i.e. cavitation. An existing experimental test rig has been updated in the LML laboratory (Lille, France) for the start-ups of a centrifugal pump. The study focuses on cavitation induced during the pump start-up. Instantaneous measurement of torque, mass flow rate, inlet and outlet unsteady pressures, and pump rotation velocity enable to characterize the pump behaviour during rapid starting periods

Experimental characterization and modelling of a cavitating centrifugal pump operating in fast start-up conditions

The start-up of rocket engine turbopumps is generally performed only in a few seconds. It implies that these pumps reach their nominal operating conditions after only a few rotations. During these first rotations of the blades, the flow evolution in the pump is governed by transient phenomena, based mainly on the flow rate and rotation speed evolution. These phenomena progressively become negligible when the steady behavior is reached. The pump transient behaviour induces significant pressure fluctuations which may result in partial flow vaporization, i.e. cavitation. An existing experimental test rig has been updated in the LML laboratory (Lille, France) for the start-ups of a centrifugal pump. The study focuses on cavitation induced during the pump start-up. Instantaneous measurement of torque, flow rate, inlet and outlet unsteady pressures, and pump rotation velocity enable to characterize the pump behaviour during rapid starting periods. Three different types of fast start-up behaviours have been identified and have been presented at ISROMAC 12 (Duplaa et al, 2008). According to the final operating point, the start-up is characterized either by a single drop of the delivery static pressure, by several low-frequency drops, or by a water hammer phenomenon that can be observed both a the inlet and outlet of the pump. A physical analysis to explain these three different types of transient flow behaviour has been recently proposed (Duplaa et al, 2010). In the present paper, a modelling of the fast start ups in cavitating conditions is proposed. It consists of a two steps adaptation of fast start-up model in non cavitating conditions proposed by Dazin et al (2007). For that, fast X-rays imaging has been performed in the impeller with the collaboration of the French Atomic Agency (CEA) in order to determinate the high frequency evolution of the volume fraction during fast the start-ups. Although the results of the modelling presented here are not definitive, they are very promising

Numerical and experimental investigations of the cavitating flow on a two-dimensional hydrofoil

La dynamique d’une poche de cavitation sur un hydrofoil 2D a été étudiée expérimentalement dans le tunnel de cavitation de l’Ecole Navale et numériquement, l’étude numérique est basée sur une approche homogène. Plusieurs modèles de cavitation sont confrontés aux expériences dans une configuration périodique complexe faisant à 2 fréquences caractéristiques. Une méthode basée sur la caractérisation en nombre d’onde-fréquence de la dynamique de la poche de cavitation est présentée

Cavitation in a hydraulic system: The influence of the distributor geometry on cavitation inception and study of the interactions between bubbles

Hydraulic systems are often subjected to pressure drops, which may lead to cavitation. In systems such as power steering, hoist loads, or ventricular assist devices, distributors are generally used. Significant pressure losses can happen in a distributor due to gap and overlap, which may lead to cavitation development. However, this issue is almost never included in the conception of the distributors. In this study, the multibubble model of the modified Rayleigh–Plesset equation is applied to the rotary distributor of an oil hydraulic system. The influence of the overlap length, the gap, the rotation speed, and distributor inlet pressure on the cavitation and particularly the interactions between bubbles at cavitation inception are studied. The study highlights a critical length of the overlap; over this value, the overlap length influences significantly the cavitation duration and the void fraction. More generally, some geometrical details have a strong influence on cavitation. Optimization of these details in engine parts, taking account the occurrence of cavitation, would be an appropriate solution to reduce its effects. The study also demonstrates that the growth of small bubbles may be delayed by the interactions with the nearby bigger ones, even if the ambient pressure is lower than their theoretical critical pressure. They eventually collapse at the first moments of the cavitation development. However, if the ambient pressure drops further, that is, beyond a critical pressure, a small bubble gains enough inertial energy to overcome these interaction phenomena and thus to grow. The growth of small bubbles increases the interactions between bubbles and slows down the growth of nearby bigger ones. The results show that the interactions between bubbles are of primary importance in the first moments of the cavitation development, which suggests that they should be taken into account in the definition of the critical pressure

Study of the cavitating instability on a grooved Venturi profile

Cavitation is a limiting phenomenon in many domains of fluid mechanics. Instabilities of a partial cavity developed on an hydrofoil, a converging-diverging step or in an inter-blade channel in turbomachinery, have already been investigated and described in many previous works. The aim of this study is to evaluate a passive control method of the sheet cavity. According to operating conditions, cavitation can be described by two different regimes: an unstable regime with a cloud cavitation shedding and a stable regime with only a pulsating sheet cavity. Avoiding cloud cavitation can limit structure damages since a pulsating sheet cavity is less agressive. The surface condition of a converging-diverging step, like a Venturi-type obstacle, is here studied as a solution for a passive control of the cavitation. This study discusses the effect of an organized roughness, in the shape of longitudinal grooves, on the developed sheet cavity. Analyzes conducted with Laser Doppler Velocimetry, visualisations and pressure measurements show that the grooves geometry, and especially the groove depth, acts on the sheet cavity dynamics. Results show that modifying the surface condition, by varying the grooves geometry, can reduce cavity sheet length and even suppress the cloud cavitation shedding.Comment: Submitted to Journal of Fluids Engineerin

Measurements of the temperature variations during the growth and collapse of cavitation bubbles

The present work focuses on the analysis of the extreme conditions encountered during the process of collapse of cavitation bubbles. The objective is to characterize the temperature variations inside the vapor/gas bubble, and also in the surrounding liquid. This information is especially relevant to address the issues of cavitation erosion and develop appropriate models that take into account the thermal effects in cavitation. The work is based on an experimental approach where temperature measurements are performed with cold wires technique. This technique is characterized by a very small time response, and can be applied to both phases during the growth and collapse of the bubbles

Bayesian optimisation of RANS simulation with ensemble-based variational method in convergent-divergent channel

This paper investigates the applicability of a hybrid data assimilation approach, namely ensemble-based variational method (EnVar), to optimise Reynolds Averaged Navier-Stokes (RANS) simulations in convergent-divergent channel from the perspective of Bayesian inference. Concretely, the ensemble-based variational method is applied to infer the inlet velocity and turbulence model corrections by assimilating Direct Numerical Simulation (DNS) results or limited experimental data. The approach is first adopted to infer the inlet velocity profile for the WallTurb Bump and Venturi geometry. The improvement can be achieved near the inlet region for the bump, but for Venturi in light of the view field limited in adverse pressure gradient region, the observation space is not sensitive to the perturbation of inlet condition. In a second step, the model corrections in k − ω SST model are investigated by assimilating the limited sparse experimental data. With the inferred model corrections, the predictions on both velocity and turbulent kinetic energy (TKE) get improved. The results indicate that the ensemble-based variational method is efficient in inferring unknown quantities of both low dimension (D=20) and high dimension (D=2400) with small ensemble size robustly and non-intrusively. This approach could prove very useful for Bayesian inference or optimisation in CFD problems