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The flow characteristics behind the runner of an air model of a propeller-type micro-hydroturbine were studied in detail by varying the operation conditions from part-load to high overload. The Reynolds number was varied from 3×104 to 9×104, and the swirl number from 0.7 to -0.4. An automated laser-Doppler anemometer (LDA) system for non-contact optical diagnostics was used to perform detailed measurements of the flow field distribution, including the profiles of two components of averaged velocities and pulsations and LDA signal spectra. Based on the results, a correlation was found between the identified features of the development of the flow structure under changing operating conditions of the hydroturbine and the nature of the evolution of the integral swirl number, which determines the state of the swirling flow. This can be used to develop recommendations for expanding the range of regulation of hydroturbine operation while maintaining high efficiency.

This paper presents the results of a parametric experimental study of free swirling flow at the exit of a flat-vane axial swirler. A total of 16 data sets were acquired by combining four swirler vane angles (22°, 29°, 50.5°, and 58.3°) and four exit nozzles of different diameters (30, 40, 52, and 76 mm). Sophisticated pressure probes consisting of precise microphones and a two-component LDV system were used to investigate the effect of these geometrical parameters on swirling flow regimes characterized by the swirl number. Particular attention was paid to the precessing vortex core (PVC) phenomenon observed at the exit of the swirler nozzle. It has been shown that by varying the vane angle and the diameter of the exit nozzle, it is possible to independently control the swirl number value and the occurrence of a PVC. A distinct correlation has been found between the PVC-induced pressure pulsations detected by acoustic probes and the tangential velocity fluctuations measured by LDV. The use of microphones provides a quick way to measure the frequency response of swirl flow in a wide range of geometries and flow configurations. The PVC effect does not occur at low subcritical values of the integral swirl number (S

In this work we employ mean field stability theory (MFST) to predict the onset of the precessing vortex core (PVC) in the draft tube of Francis turbines. MFST is based on the linear stability analysis of the mean field of turbulent flows. Recent work shows that MFST very accurately predicts the formation of coherent structures in turbulent shear flows, such as the PVC. MFST may further reveal the flow regions that are most susceptible to flow actuation to suppress the PVC, which is of great practical relevance. In this work, MFST is accompanied by a data-driven approach to predict the linear growth rate of the PVC based on pointwise wall pressure measurements. The method is based on statistical evaluation of the probability density function of the PVC amplitude at limit cycle. It makes use of the intense noise induced by the background turbulence, which is expected to be a major driver of hydrodynamic instabilities. The empirical and analytic results are compared to phase-locked LDV measurements conducted inside the draft tube at various operating conditions, to assess the quantitative accuracy of the approach. The methodologies outlined in this work will be of relevance for future design of hydro turbines to run stable over a wide range of operating conditions.

Swirl flow with the formation of a precessing vortex core (PVC) in a hydro turbine draft tube model was studied using Stereo-PIV and four acoustic sensors. Experiments were performed on an aerodynamic setup in a wide range of operating conditions of the hydro turbine. Using a spatial Fourier decomposition of pressure pulsation data obtained from four acoustic sensors, a PVC was observed for part-load operating regimes (0.3-0.7Qc ). Mean flow features were shown for the range of operating regimes from a deep part-load regime (0.3Qc ) to an overload regime (1.5Qc ). Based on phase-averaged velocity distributions, the PVC spatial structure was identified for maximum pressure pulsations on the cone walls of the draft tube.

In this work we used digital particle image visualization (PIV) to experimentally establish the self-similarity of far wake behind a tandem of two disks of a diameter D (300 mm) with a common axis along the incident flow. The research was performed in a water flume (Re ≈ 2 · 105) with variation of L, the longitudinal dimension of the tandem. The self-similarity of the velocity profile in the wake behind the tandem has been established; the level of turbulent fluctuations of the profile has been measured. Due to the influence of the second disk, the velocity deficit in the wake behind the tandem exceeded the corresponding value for a single disk, being independent of the distance between the disks (L = 4–8D). The velocity fluctuations behind the tandem did not differ much from the level of fluctuations in the case of a single disk up to a distance of forty calibers downstream, where the wake ceased to differ from the background of natural turbulent fluctuations of the incident flow. It has been found that the position of the second disk in the tandem affects the energy loss in the wake due to its expansion but does not influence the decay. The revealed patterns in the wake development behind tandems of bodies will enable optimization of construction of systems of repetitive elements and their movement in different flows.

This paper presents an experimental study of a strongly swirling turbulent flow with the formation of a precessing vortex core (PVC) that emerges at the outlet of a tangential swirler nozzle. The studies were carried out using a stereoscopic particle image velocimetry (SPIV) and two acoustic pressure sensors. An analysis of the velocity fields measured by the proper orthogonal decomposition (POD) showed that the precession motion of the vortex makes a significant contribution (more than 34%) to the turbulence kinetic energy, making it possible to consider the PVC effect as a prominent and convenient object for testing theoretical models describing precessing vortex motion. Estimates of the model parameters of the precessing vortex such as the vortex core radius, vortex precession radius, and vortex intensity based on statistical data obtained from uncorrelated PIV images are presented in the paper. The estimated parameters were compared with the parameters obtained by phase averaging the PIV images, and as a result, three-component velocity distributions were obtained relative to the vortex position. The analysis showed that the precession radius, vortex core size, and vortex circulation are normally distributed. The conditional averaging technique made it possible to determine the structural parameters of the PVC, which was confirmed to be a left-handed helical vortex. Due to the rapid disintegration of the PVC above the nozzle, only a portion of the spiral vortex was observed. Therefore, the helical vortex pitch was estimated in a local sense. Basically, all of the approaches gave similar results for the vortex parameters, providing access to the vortex dynamics. In particular, these cross-checked parameters were used to calculate the precession frequency on the basis of the available helical vortex model. The obtained frequency was found to be in good agreement with the experimentally measured precession frequency, confirming the adequacy of the theory.

This work is devoted to the study of unsteady flow with the precessing vortex core (PVC) formed at the exit of a compact vane swirler with varying vanes angle and nozzles diameters. Amplitude-frequency characteristics of the PVC were obtained using two microphones. The modified Strouhal number dependence have showed a good generalization of the data for all nozzle diameters. The averaged and phase-averaged distributions of three components of velocity have been measured via the LDA system. The increasing the recirculation zone at increasing nozzle diameter for the swirl parameter Sg=0.53 and Re=1.5·104 was detected. The degeneration of PVC was detected for all studied nozzle diameters D = 30, 40, 50 mm. In case of smallest diameter D = 30 mm the PVC ceases to be periodic due to the absence of a recirculation zone. The three-dimensional structure of the PVC is reconstructed by the phase averaging method and visualized using the Q-criterion. Formation of the shifted recirculation zone, outer secondary vortex (OSV) and inner secondary vortex (ISV) is observed.

The special probes utilization together with measuring microphones allows to hold local measurements of pressure pulsations and to deform a flow structure marginally. Knowledge about a transfer function of the probe is required for the original signal reconstruction. This article presents the experimental technique of the probe transfer function obtainment. The theoretical assessment of the transfer function was carried out, using the model presented in the literature. Also the original signal reconstruction of pressure pulsations was carried out at the exit of the vortex tangential chamber.

Results are presented of experiments carried out using an air model of a hydraulic turbine draft tube. These included velocity measurements in the conical part of the tube using a laser-Doppler anemometer and measurement of pressure pulsations using acoustic sensors (microphones). It is shown that a system of two runner vane cascades can be used to simulate a velocity distribution that resembles the distributions downstream of a runner under full-scale conditions. Experiments show that modes involving the formation of a vortex rope occur in the region of significant hydroelectric generating unit underload. Analysis of spectra, the cross-correlation function, and mean velocity distributions confirmed the vortex nature of the generation of powerful pressure pulsations in the draft tube cone.

The influence of combustion effect on unsteady vortex structure in the form of precessing vortex core was studied using the non-intrusive method of laser Doppler anemometry and special procedure of extracting the non-axisymmetric mode of flow fluctuations. The studies show that combustion has a significant effect on the parameters of such a core, reducing the amplitude (vortex deviation from the burner center) and increasing precession frequency. At the same time, the acoustic sensors detect almost an order reduction in the level of pressure pulsations generated by the precessing vortex core. Moreover, distributions of tangential velocity fluctuations and cross-correlation analysis show that vortex precession is quite pronounced even under the combustion conditions, bringing a significant coherent component to distributions of velocity fluctuations.

One of the main sources of powerful pressure pulsations in the fluid flow parts of the hydroturbine at partial load or overload operation regimes is the precessing vortex rope forming due to high residual swirl of the flow downstream the hydroturbine wheel. Numerical simulation and experiments have been performed in order to study the influence of inlet flow conditions in the draft tube on level of the flow pulsations. Numerical calculations have shown that it is possible to reproduce velocity distributions at the draft tube inlet corresponding to different operation regimes of the real hydroturbines using a combination of two blade grids – rotor and stator. Experimental verification of numerical calculations was performed using an aerodynamic test rig and method of 3-d printing for fabrication of the blade grids and draft tube model. Velocity field measurements at entrance of the draft tube conical part making use of an laser-Doppler anemometer and level of the pressure pulsations using acoustic probes have allowed to define correlation between the velocity distribution type and amplitude-frequency characteristics of the flow pulsations which generated by the precessing rope.

LDA and PIV techniques were used to study the decay of an axisymmetrical turbulent wake originated downstream an immobile disk in a water flume for Reynolds numbers Re = 1.5−2.4⋅105. Data were compared with experiments performed with a set of thermo-anemometers behind a disk in a wind tunnel at lower Reynolds numbers (Re = 1.3−2.6⋅104). Observations for a new range of Reynolds number confirmed that the velocity distribution in the disk wake keeps self-similarity. The decay of a wake by the power law −2/3 in the presented experiments remains until the maximum deficit of velocity becomes comparable with the turbulent pulsation level in the free-stream (less than 2 %).

The models of two types of rotors with blades constructed using different optimization methods are compared experimentally. In the first case, the Glauert optimization by the pulsed method is used, which is applied independently for each individual blade cross section. This method remains the main approach in designing rotors of various duties. The construction of the other rotor is based on the Betz idea about optimization of rotors by determining a special distribution of circulation over the blade, which ensures the helical structure of the wake behind the rotor. It is established for the first time as a result of direct experimental comparison that the rotor constructed using the Betz method makes it possible to extract more kinetic energy from the homogeneous incoming flow.

Current work deals with an LDA characterization of the velocity distributions and pressure pulsation maps in an axisymmetric draft tube for the flow conditions corresponding to wide range of the operating regimes of the micro-hydro turbine unit. For the detailed analysis of the velocity profiles, three characteristic regimes corresponding to part-load regime with precessing vortex core formation, best efficiency regime and overload regime of hydro turbine operation were selected. To capture spatial characteristics of the unsteady flow for part-load regime a phase averaging procedure for the LDA data has been employed.

This article is devoted to the study of pressure pulsations behind the runner of a hydro turbine model caused by the precessing vortex core (PVC). Pressure pulsations are investigated under conditions of stationary load and transient modes of the hydro-turbine operation. Studies were performed in model conditions on the aerodynamic setup. The map of pressure pulsations was built for stationary modes using acoustic sensors and served to find rotational speeds of the swirler and flow rates at which the PVC occurs. On the basis of the data presented by parametric dependences, the initial and final parameters of the transition process were chosen. In the article, a sudden (fast) transition from part-load regime to the best efficiency point and back was considered. The characteristic times for the formation of PVC and the establishment of the flow regime when changing the controlling parameters of the installation have been determined using a continuous wavelet transform.

The paper deals with aerodynamics of a reacting coal-dust-gas flame in a combustion chamber with a rated output of 50 kW. Spatial distributions of the axial and tangential components of the flame velocity were measured with the help of the laser Doppler anemometer system “LAD” -06i. Fine brown coal with an average particle size of 60 µm was used as solid fuel and tracers of the Doppler signal. Aerodynamics of the flame was studied varying orientation of the scroll swirler (co- and counter-swirl relative to the first stage of the chamber). It is shown that under the conditions of counter-swirl, the flow is evenly distributed over the width of the combustion chamber, which has a positive effect on stability of combustion of a pulverized-coal flame.

Experimental study of flow in the draft-tube cone of a laboratory air model hydro-turbine was performed. The evolution of the integral swirl number varied from BEP regime to deep part load operation conditions (discharge Q was a constant and speed of runner rotation n was varied) was found. Averaged velocity profiles were measured by two-component LDA system and were used for calculations of the swirl number. Tangential velocity pulsation spectra in the center of draft tube cone were analyzed to describe PVC formation. Swirl number as an analytical expression of the operating parameters (flow rate, speed of rotation) was estimated by model from work [1] and showed good agreement in the region of zero-swirl condition.Experimental study of flow in the draft-tube cone of a laboratory air model hydro-turbine was performed. The evolution of the integral swirl number varied from BEP regime to deep part load operation conditions (discharge Q was a constant and speed of runner rotation n was varied) was found. Averaged velocity profiles were measured by two-component LDA system and were used for calculations of the swirl number. Tangential velocity pulsation spectra in the center of draft tube cone were analyzed to describe PVC formation. Swirl number as an analytical expression of the operating parameters (flow rate, speed of rotation) was estimated by model from work [1] and showed good agreement in the region of zero-swirl condition.

The phase-averaged velocity distributions were applied for calculation of parameters of a precessing vortex core; these were the input data for calculation of precession frequency by a formula derived from the helical vortex theory. It was demonstrated that the calculated frequency is within 10% error from the frequency measured in experiments. These results confirm the theory validity, and this opens opportunities for analysis of mechanisms controlling the precessing vortex core and external factors.

The paper reports on some results of the experimental study of flow and pressure pulsations in a laboratory model of the draft tube (DT) of Francis-99 hydro-turbine over a broad range of operating regimes. Velocity distributions at the model inlet varied within 866 modes of the turbine load conditions, including those with maximum coherent pressure pulsations on the model walls. The contact and non-contact methods were used to measure pressure pulsations on the model walls using acoustic sensors and to measure the averaged velocity distribution with a laser Doppler anemometer “LAD-06i”. Analysis of the results have showed that in the model cone there are flow modes with forming of precessing vortex cores, accompanied by a sharp increase in the amplitude of coherent pressure pulsations on the wall and the velocity field rearrangement. It is shown that the vortex core starts forming with an increase in the integral parameter of swirl up to S>0.5. A novelty of the work is the combination of the rapid prototyping (3-d printing) of the inflow swirl generators and computerized measurement techniques that makes it possible to acquire rapidly a large amount of experimental data for a variety of designs and operating conditions. The results can provide insight into the effect of various design and operating parameters on the flow physics, as well as serve for verification of the numerical simulations.

The article presents the results from experimental and numerical investigations of the parameters characterizing large-scale vortex structures formed in the models of various burners with flow swirling. The experiments included flow visualization and velocity field measurements carried out using a modern contact-less diagnostic system constructed on the basis of a laser Doppler anemometer. In addition, the frequency responses of unsteady vortex flow modes were investigated using dedicated acoustic sensors. The distribution of static pressure induced by an unsteady vortex was obtained using the phase averaging method. Along with experiments, the swirl flow parameters were calculated using an analytic theory and the Star CCM+ commercial software package. The adequacy of the mathematical modeling results was checked by comparing them with the physical experiment data.

The aim of the present paper is to obtain a better understanding of wakes generated by 2, 3 and 4 elements (disks and/or rotors) modeling an array of wind turbines in a wind farm. We here extend the results from our previous experimental model studies on the correlation between the behavior of the wake behind dual configurations of passive disks and active rotors to establish new knowledge regarding the wake development behind the 3rd and 4th rotor operating in a wake. To accomplish this, 2D PIV velocity measurements were carried out to investigate the flow properties upstream and downstream the tested rotors and discs in the array. The study also provides measured values of power and thrust coefficients of the model turbines using strain gauges installed directly on the rotors. The aim of the laboratory experiment is to provide data to calibrate and validate wake models for design and analysis arrays of wind turbines located in wind farms.

Aerodynamics of flame combustion of a pulverized coal was studied experimentally using a laboratory setup with a thermal power of 25 kW. The axial and tangential velocity profiles were measured with the co-and counter-swirl of the first and second degrees of the burner. Averaged velocity profiles were measured by a two-component LDA system. The temperature was also measured on the walls and axis of sections of the pulverized coal burner. The temperature distributions were obtained at different coal dust flow rates as well as at co-and counter-swirl. The results obtained allow us to conclude about the efficiency of flow mixing at pulverized coal combustion.

We report on experimental and analytical studies of strongly swirling air flow in the near-field of a generic axisymmetric tangential swirler used often as swirl generator in combustors, phase separators or mixing devices. The focus was on the characterization of the precessing vortex core (PVC) for a range of swirl numbers S = 1.4–2.4 and Reynolds numbers of 14,460–38,300. The velocity field and the spectra measured with a Laser–Doppler anemometer confirmed in all cases the occurrence of PVC and the central recirculation zone (CRZ) that for higher swirl numbers penetrates back into the swirler chamber. The instantaneous pressure, its frequency characteristics and the phase-average values have been measured with a sensitive microphone equipped with a special probe tip. We show then that the PVC frequency, helix diameter and the core displacement computed from the analytical model of a confined helical vortex ( Alekseenko et al., 1999 , Alekseenko et al., 2007 ) using only the time-averaged velocity field agree well with the measurements, whereas the analytically reconstructed phase-averaged velocity and pressure distributions show good qualitative agreement with the measured pressure fields. The exception is the discrepancy in the pressure amplitude, which for the highest S = 2.4 reaches almost 50%. Despite the latter imperfection, it has been confirmed that the analytical model can be used to identify the existence of a PVC and to compute its structure parameters solely from the measured (or otherwise available) time-mean axial and tangential velocities.

The decay of a far wake and its turbulent fluctuations behind two thin discs of the same diameter D, oriented normal to the incident flow, have been studied using the Particle Image Velocimetry (PIV). The experimental study was carried out in a water flume (Re ≈ 2·105) with varying distances between the discs (L х = 4–8D) and their axes shift relative to each other (0, 0.5D and 1D). It is found that the velocity deficit behind two discs depends weakly on L x , and at L х > 40D, it becomes indistinguishable from the level of turbulent fluctuations of the incident flow. It is found that the decay of the average velocity deficit and its turbulent fluctuations in a wake of a tandem of discs can be described by the same analytical dependence with exponent–2/3 as for the wake decay of a single disc. However, at the same distance downstream, the value of deficit behind two discs is substantially higher than the corresponding value behind a single disc. Velocity fluctuations in a far wake behind a pair of discs depend weakly on longitudinal dimension L x , but at the same time, in contrast to the velocity deficit, their level does not differ significantly from the level of fluctuations behind a single disc.

The paper reports on results of the experimental study of flow and pressure pulsations in a laboratory model of the draft tube of Francis-99 hydro turbine over a broad range of operating regimes. A novelty is the rapid prototyping and 3-d printing of the inflow swirl generators that makes it possible to acquire rapidly a large amount of experimental data for a variety of designs and operating conditions. The frequency characteristics of flow and the phase-average pressure distribution in draft tube cone have been measured with the aid of special acoustic sensors.