Large eddy simulation of the tip-leakage cavitating flow with an insight on how cavitation influences vorticity and turbulence

Cavitation within a tip leakage flow remains a challenging issue in a variety of axial hydraulic machines. It is still not possible nowadays to predict cavitation occurrence in such a flow with acceptable accuracy. In the present study, we have carried out numerical simulations of a tip leakage cavitating flow, generated by a straight NACA0009 hydrofoil. We have used the LES method combined with the Schnerr-Sauer cavitation model. The numerical results agree well with experimental data. The evolution of the tip leakage cavitating flow, involving tip-leakage vortex (TLV), tip-separation vortex (TSV) and induced vortex (IV), is analyzed from Eulerian and Lagrangian viewpoints. The results show that the spatial evolution of the tip leakage cavitating flow can be divided into three stages: Stage I, Independent development of the TLV and TSV: Stage II, Fusion of the TLV and TSV; and Stage III, Development of the IV and dissipation of the TLV. The Lagrangian coherent structures (LCSs) obtained from the numerical results indicate that the TLV cavitation significantly influence the local flow patterns. The vorticity transport equation was then used to further analyze the influence of the cavitation on the vortices. The results demonstrate that the stretching term dominates the TLV evolution and the dilatation term is responsible for the vorticity reduction inside the TLV cavity. The results also show how the cavitation influences the local turbulence and that the transport term in the turbulent kinetic energy equation influences the turbulence distribution near the TLV cavity. (C) 2019 Elsevier Inc. All rights reserved.

Physical modelling of leading edge cavitation

Youcef Ait Bouziad

Cavitation is usually the main physical phenomenon behind performance alterations in hydraulic machinery. For this reason, it is crucial to accurately predict its inception and development and to highlight a comprehensive relation between the cavitation development and the performances drop associated. The common cavitation models, based on numerical flow simulations, are intended to reproduce the general cavitation behavior, and their major focus is the cavitation onset and developed cavity shape prediction. In the present study, various methods in cavitation modelling are investigated. Specific computational methods are outlined for the two sensitive zones of cavity detachment and closure. Finally, an industrial case is investigated in order to highlight the mechanisms of head drop phenomenon in hydraulic machines. Current modelling techniques are reviewed together with physical arguments concerning the cavitation phenomenon, and a 2D hydrofoil test case is used to evaluate the models. A mono-fluid interface tracking model, a multiphase state-equation based model, and a multiphase transport-equation based model are discussed in terms of reproducing the cavitation flow characteristics as the cavitation inception, development, pressure distribution and velocity profiles in cavitation regimes. An innovative approach based on the local stress formulation is proposed. The non-viscous anisotropic stress is taken into account through the maximum tensile stress criterion for cavitation inception instead of the classical pressure threshold. The maximum tensile stress criterion, formulated using the shear strain rate formulation is used for CFD computations. The method is evaluated with the case of a parabolic nose leading edge flow with comparison to the boundary layer computations. The developed model is tested in the case of a 2D hydrofoil in both smooth and rough walls under different flow conditions. The ability of the model to take into account Reynolds and surface roughness effects, as observed in experimental investigations, is demonstrated. A comparative study of turbulence modelling for unsteady cavitation is presented which indicates a strong correlation between the cavitation unsteadiness predictions and the turbulence modelling. The adapted techniques in reproducing the unsteady cavitation flow are found to be either using an accurate filtering turbulence model to correctly capture the large eddies, or to modify the turbulent viscosity function, and thereby introducing an artificial compressibility effect. The simulated leading edge cavitation instability, in our case, occurs at a certain cavity length where the cavity closure corresponds to the high pressure gradient region and is governed mainly by the occurrence of the reentrant jet at the cavity closure. This phenomenon is found to be periodic and the shedding frequencies matches to the Strouhal law as observed in experiments. Finally, the multiphase mixture model is used in the case of an industrial inducer. The model provides satisfactory results for the prediction of the cavitation flow behavior and performance drop estimation for the operating points studied. An analysis based on global energy balance and local flow analysis demonstrates that the head drop is mainly caused by the lower torque generation and the hydraulic losses induced by the secondary flows. These phenomena occur when the cavity extends towards the throat region, leading to important changes in the flow structure.

EPFL2005

Physical Mechanism and Flow Control of Tip Vortex Cavitation

Ali Amini

Occurrence of cavitation in hydraulic machines is a challenging issue because it is often accompanied with loss of efficiency, noise emissions, vibrations, and erosion damages. Tip vortices, in particular, are an ideal site for the development of cavitation as the static pressure at their cores usually drops much below the freestream pressure. The first part of the present thesis is focused on the effect of dissolved gas content and other physical parameters on Tip Vortex Cavitation (TVC) trailing from an elliptical hydrofoil. The inception and desinence thresholds measured at different flow conditions reveal that TVC often disappears at cavitation indices higher than the inception thresholds. The measurements show that TVC desinence pressure increases with the gas content and may even reach to atmospheric pressure. This is explained by the convective diffusion of dissolved gases from water into the cavity due to local supersaturation. The extent of the delay in desinence is, however, dictated by the bulk flow parameters. Owing to flow visualizations, it is asserted that the formation of a laminar separation bubble at the hydrofoil tip is a necessary condition for the delayed desinence. The separation bubble acts like a shelter and creates a relatively calm area at the vortex core. The second part of the thesis is dedicated to TVC mitigation. First, the effectiveness of non-planar winglets in suppressing TVC is investigated. For this purpose, an elliptical hydrofoil is selected as the baseline and various winglets are realized by simply bending the last 5 or 10% of the span at ±45° and ±90° dihedral angles. TVC inception-desinence tests reveal undeniable advantages for the winglets while the hydrodynamic performances of the hydrofoils remain intact. It is observed that a longer winglet bent at a higher angle and toward the pressure side is more effective in TVC suppression. For instance, the 90°-bent-downward winglet reduces the TVC inception index from 2.5 for the baseline down to 0.8 at 15 m/s freestream velocity and 14° incidence angle. Stereo-PIV measurements show that for this winglet, the maximum tangential velocity of the tip vortex falls to almost half of the baseline and the axial velocity reduces tangibly at the vortex core. Second, the capacity of a flexible trailing thread in alleviating TVC is examined. To this end, one nylon thread with various diameters and lengths is attached to the tip of the elliptical hydrofoil. Under almost all the tested flow conditions, the thread experiences flutter due to hydro-elastic instabilities. Thereafter, the vortical flow forces the oscillating thread to coil around the vortex axis. This rotational motion decelerates the axial velocity at the vortex core due to increased drag and augmented turbulent mixing. A sufficiently thick thread may also be sucked into the vortex core under the effect of the pressure field. This results in the whipping motion, which consists of the quasi-periodic coincidence of a part of the thread and the tip vortex axis and is considerably superior to the rotational motion in TVC mitigation. It is shown analytically and confirmed experimentally that the whipping motion leads to viscous core thickening. Altogether, the results presented in this thesis provide a better understanding of the physics of TVC and pave the way for future designs with less vulnerability to cavitation.

EPFL2020

Analysis of large scale hydrodynamic phenomena in turbine draft tubes

Monica Iliescu

Hydraulic power is a renewable energy with a very advantageous price, and its flexibility of use coupled with its storage possibilities easily enables to meet the peak demand. Refurbishment of old hydraulic power plants is an important topical issue and the upgrading of the power stations of the years 1940 to 1960 can considerably increase both efficiency and power with minimal costs. The current challenge for the turbine manufacturers is the accurate flow prediction in the whole turbine and across a broad operating range, taking into account the unsteady features of the flow. For further developing flow computation tools, the major hydraulic turbine manufacturers : ALSTOM Power Hydro, GE Hydro, VATECH Escher Wyss, VOITH-SIEMENS Hydro have joined Electricité de France and EPFL, in the European initiative framework EUREKA n° 1625. This project has been financially supported by CTI – Swiss Commission for Technology and Innovation. The aim of the project was to analyze the flow within the draft tube of a turbine theoretically, experimentally and numerically, and the present work is a part of the project. In this general framework, the objective of the thesis was to set up PIV measurements for analyzing the flow behavior in the draft tube of a Francis turbine. Several enhancements of the PIV technique have been carried out for acceding to the 3D velocity field, both in single and two-phase flows, in the cone and at the outlet of the draft tube. Due to the complex geometry of the draft tube, particular attention has been given to calibration, allowing obtaining 3% uncertainty on the measured velocity field. In two-phase flow, corresponding to partial load operation in cavitation conditions, two measurement methods have been set up, allowing simultaneous measurement of the velocity field and volume of the vapors-core vortex. The first one relies on the 2D PIV method: two cameras focused on the same measurement zone provide separately the shape of the vapors core and the velocity of fluorescent seeding particles in the flow. The second method, 3D PIV, is based on the separation on the same image of the fluorescent seeding particles from the shape of the vapors core, taking advantage of background lighting in a specific wavelength. Using two cameras in stereoscopic arrangement allows, in this case, obtaining the third component of the velocity. Adaptive image processing algorithms have been developed in both cases for detecting the vapors core boundary and calculating its geometrical characteristics in the measurement section. These experimental developments have allowed studying several phenomena and characterizing several flow types in the draft tube. For this draft tube, the efficiency characteristic shows a severe drop close to the best efficiency point, and this part has been particularly addressed. The 3D velocity field has been measured at the runner outlet and diffuser outlet for a flow rate range of ± 20 % of the best efficiency point flow rate, and these results allowed explaining the source of the hydraulic losses that lead to the drop of the efficiency characteristic. A second issue concerns the spatial distribution of the non-uniformity of the velocity field at the runner outlet, synchronous with the runner blades passage. The velocity field development in the cone of the draft tube from the runner outlet to the cone outlet has been determined and the mixing length of the sheared flow has been studied. In low charge operating conditions (70% of the best efficiency point flow rate), relevant results of unsteady velocity field and related vapors volume are obtained for different pressure levels in the draft tube, which correspond to different cavitation conditions from single-phase flow to vortex-circuit interaction in two-phase flow. The shape of the vortex core has been determined and a parametrical model is proposed. Its influence on the flow, as well as its stability, has been evaluated depending on the pressure level. Particular attention has been paid to the characterization of the interaction of the synchronous part of the vortex-induced pulsations and the circuit. These results have been used to calibrate and validate flow computation codes and they form a database available to the project partners. An example is given of the validation of the vortex flow simulation at partial load in single-phase flow conditions. Unlike the usual data comparison based on wall pressure signals, the velocity field and vortex filament are also assessed.

EPFL2007