We report on a study of the impact of coherent helical vortex structures on the shape of the reaction zone and heat release in swirling methane/air flames in regimes with a vortex breakdown. Three kinds of atmospheric flames are considered, viz., fuel-lean and fuel-rich premixed flames and a partially premixed fuel-rich lifted flame. Based on the measurements of the velocity fields by a stereo PIV in combination with the OH PLIF and HCHO PLIF, the impact of the coherent flow structures on large-scale corrugations of the reaction zone is evaluated. Helical vortex structures, detected in both the non-reacting and reacting high-swirl flows by using proper orthogonal decomposition, are found to promote combustion both in the lean premixed and fuel-rich partially premixed flames. In the first case, based on the phase-averaged intensity of the HCHO×OH signal and the location of the helical vortex structure in the inner mixing layer, it is concluded that the vortex locally increases the heat release rate by enlarging the flame front and enhancing the mass exchange between the combustion products inside the recirculation zone and the fresh gases. The events of the local flame extinctions are detected in the instantaneous PLIF snapshots for the lean mixture, but they do not cause extinction of the entire flame or a blow-off. In case of the lifted flame, the outer helical vortex structure promotes combustion by locally intensifying the mass exchange between the fuel-rich jet with the surrounding air.

We report on the experimental study of turbulent transport in the near field of swirling jets at Re = 5000 by using simultaneously the stereoscopic particle image velocimetry and planar laser-induced fluorescence methods. The present focus is on the analysis of terms of the mean (axial) momentum and scalar (mass) transport equations, estimated from the mean velocity and concentration, the Reynolds stress and turbulent flux components, measured in the central axial plane of the jets. A proper orthogonal decomposition has also been applied to the velocity data sets to analyse contribution of the coherent velocity and scalar fluctuations to the turbulent momentum and mass fluxes for different swirl rates.

We studied cavitating flow over the suction side of a symmetric 2D foil - a scaled-down model of high-pressure hydroturbine guide vanes (GV) - in different cavitation regimes at several attack angles. High-speed imaging was used to analyze spatial patterns and time dynamics of the gas-vapor cavities, as well as for evaluating the characteristic integral parameters. A hydroacoustic pressure transducer was employed to register time-spectra of pressure fluctuations behind the hydrofoil and, thereby, determine dedicated frequencies of unsteady regimes. A PIV technique was applied to measure the velocity fields and its fluctuations, which were compared for the free and forced flow conditions. The active flow control was implemented by means of a continuous liquid supply with different flow rates through a slot channel located in the GV surface at the distaence of 60% of the chord length from the foil leading edge. It was found that the active mass injection does not influence the primary flow upstream of the slot channel position absolutely. At small angles of incidence, the injection flow at velocities in the range between zero to 0.76 of the mean bulk velocity was observed not to practically influence the distributions of turbulent characteristics so that the global difference is only between the free and forced flow conditions. For cavitation-free and cavitation inception cases, the active mass injection was shown to make the flow turbulence structure more developed and the wake past the GV section more intense. However, the active flow control system considered also allows a favorable and efficient flow manipulation, especially at the regimes with developed gas-vapor cavities. Moreover, the active flow management makes it possible to reduce substantially the amplitude or totally suppress the periodic cavity length oscillations and pressure pulsations associated with them.

We report on the experimental investigation of cavitating flow control over a 2D model of guide vanes of a Francis turbine by means of a continuous tangential injection of liquid along the foil surface. The generated wall jet, providing supplementary mass and momentum, issues from a nozzle chamber inside the hydrofoil through a spanwise slot channel on its upper surface. High-speed imaging was used to distinguish cavity flow regimes, study the spatial patterns and time dynamics of partial cavities, as well as to evaluate the characteristic integral parameters of cavitation. Time-resolved LIF visualization of the jet discharging from the nozzle was employed to check if the generated wall jet is stable and spanwise uniform. Hydroacoustic measurements were performed by a hydrophone to estimate how the amplitudes and frequencies of pressure pulsations associated with cavity oscillations change with the injection rate. A PIV technique was utilized to measure the mean velocity, its fluctuations and the dominant turbulent shear stress component, which were all compared for different flow conditions and with the results for the unmodified (standard) foil. The effect of injection rate on cavitation and flow dynamics was examined for three attack angles, 0, 3 and 9°, and a range of cavitation numbers corresponding to different regimes. The low-speed injection was shown to lead to an intensification of turbulent fluctuations in the boundary layer and shrinking of the attached cavity length by up to 25% compared to the case without injection. The injection with a high velocity, in turn, causes a rise of the local flow velocity and a reduction of turbulent fluctuations near the wall, which, consequently, increases the foil hydrodynamic quality at a relatively low energy consumption for generation of the wall jet. However, in this case the vapor cavity becomes longer. Thus, the low-speed injection turns out to be effective to mitigate cavitation but the injection at a high velocity is more preferable from the standpoint of the flow hydrodynamics. In the whole, the implemented control method showed to be quite an efficient tool to manipulate cavitation and hydrodynamic structure of the flow and, thereby, under certain conditions, to suppress the cavitation-caused instabilities.

We study the flow over a cylinder placed between two parallel rigid walls using Large-eddy simulations and Particle Image Velocimetry. The Reynolds number based on the inflow velocity and diameter of the cylinder is 3750 corresponding to the subcritical regime with laminar separation. Three-dimensional visualization shows the presence of the horseshoe vortex system prior to the cylinder. The comparison of time-averaged velocity fields and fluctuations shows good agreement between simulations and experiments. Spectral analysis suggests the presence of low-frequency modulations of the recirculating bubble.

The paper reports on a parallel study of the dynamics of large-scale vortical structures in non-swirling, low-swirl and high-swirl turbulent jets by means of the time-resolved tomographic PIV technique. 3D realizations of the instantaneous velocity and velocity gradients, local strain and rotation rates, helicity and pressure fluctuations are analyzed. Deformation of the large-scale vortices is studied by evaluating the local stretching/compression rates and local values of the Lamb vector divergence. Linear stability analysis is also applied to the mean velocity profiles in order to identify the most unstable azimuthal modes.

We report on a parallel study on properties of large-scale vortical structures in low- and high-swirl turbulent jets by means of the time-resolved tomographic particle image velocimetry technique. The high-swirl jet flow is featured by a well-established bubble-type vortex breakdown with a central recirculation zone. In the low-swirl flow, the mean axial velocity, while intermittently acquiring negative values, remains positive in the mean but with a local velocity defect immediately downstream from the nozzle exit, followed by a spiralling vortex core system and its eventual breakdown. Measurements of the 3D velocity fields allowed direct analysis of the azimuthal/helical modes via Fourier transform over the azimuthal angle and proper orthogonal decomposition (POD) analysis in the Fourier space. A precessing vortex core is detected for both swirl cases, whereas the POD analysis showed that the one originating in the bubble-type vortex breakdown is much more energetic and easier to detect.

Graphical Abstract: [Figure not available: see fulltext.]

We studied cavitating flow over the suction side of two symmetric 2D foils - a NACA0015 hydrofoil and a scaled-down model of high-pressure hydroturbine guide vanes (GV) - in different cavitation regimes at several attack angles. High-speed imaging was used to analyze spatial patterns and time dynamics of the gas-vapor cavities, as well as for evaluating the characteristic integral parameters. A PIV technique was applied to measure the velocity fields and its fluctuations, which were compared for both foils and with the data measured in the non-cavitating flows at the same flow conditions. We compare the dynamics of growth and convection of traveling bubbles at a smaller attack angle and the transition of a sheet cavity pattern to a streaky one at the higher incidence on both foils. For the GV, asymmetric spanwise variations of the sheet cavity length were for the first time visualized for an unsteady regime that is characterized by alternating periodic detachments of clouds between the sidewalls of the test channel and Strouhal number of 0.27. According to numerical calculations by Decaix and Goncalvès (2013), this asymmetric behavior is very likely to be governed by the cross instability. Moreover, it was concluded that the existence of the cross instability is independent on the test body shape and its aspect ratio. For single-phase flow and cavitation inception conditions, the PIV measurements revealed the appearance of the second maximum in the fluctuating velocity distributions over the GV profile at the higher incidence angle due to the flow separation from its surface at roughly 71% of the chord length from the foil leading edge. This results in a more intensive turbulent wake past the GV compared to that behind the NACA foil. After the transition to unsteady regimes, both maxima of the fluctuating velocity over the GV vanish, the distributions become more flat and almost coincide for both foils. While at developed cavitation regimes the main flow features and cavitation dynamics do not differ much for the both foils, the detected second maximum of the fluctuating velocity for the GV at the higher attack angle makes this profile less suitable (from the hydrodynamics standpoint) for flow control in practical full-scale conditions, especially at subcavitating and cavitation inception regimes, than the NACA hydrofoil.

The onset of instabilities of various types, including those caused by cavitation, in ducts of hydraulic systems negatively affects the efficiency, reliability and safety of hydrotechnical and hydropower equipment. This fact makes it necessary to develop different means to control such flows. The main aim of the study is to determine the possibility of applying and assessing the effectiveness of the method of gas-vapor cavity dynamics management based on continuous tangential injection of liquid. The methods used in the study. In order to study the stages of evolution and spatial structure of partial cavities as well as to estimate their integral characteristics, the high-speed visualization was applied. Spatial distributions of the mean velocity and turbulent characteristics in one-And two-phase flows around the model hydrofoil were measured by Particle Image Velocimetry (PIV). The results. The investigation was carried out for a modified model of guide vanes of a high-pressure turbine equipped with a spanwise slot channel in its surface to produce a wall jet to feed slowed down layers of liquid with a supplementary momentum over the suction side. In the experiments, the angle of attack of the model profile was changed from zero to nine degrees and various flow conditions were achieved by varying the cavitation number in a wide range. Basing on visual analysis of occurring flow regimes, starting from cavitation inception and finishing with developed unsteady cavities, the influence of injection on cavitation was determined. The effect of liquid injection with different velocities on the flow hydrodynamics was evaluated by measuring ensembles of instantaneous velocity which were used to calculate distributions of mean and turbulent characteristics. It was shown that the low-speed injection of liquid along the hydrofoil surface leads to intensification of turbulent fluctuations in the boundary layer and, thereby, hinders the development of an attached cavity due to production of additional perturbations in the flow. Injection with a high velocity, in its turn, causes a rise of the local flow velocity and reduction of turbulent fluctuations near the wall, which allows increasing the lift coefficient of the foil and its hydrodynamic quality owing to a pressure drop over the suction side at relatively low energy consumptions to generate the wall jet. However, in such a case the gas-vapor cavity becomes longer. Thus, the low-speed injection turns out to be effective to mitigate cavitation but the injection at a high velocity is more preferable from the standpoint of the flow hydrodynamics. Consequently, the implemented method of flow control is quite an efficient tool to manipulate hydrodynamic characteristics of the foil and decrease the intensity of vaporization and, under certain conditions, even to suppress instabilities linked with cavitation.