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Research on water flow in a pump inlet sump is presented. The main effort has been devoted to the study of the vortical structures’ appearance and their behavior. The study was conducted in a dedicated model of the pump sump consisting of a rectangular tank 1272 × 542 × 550 mm3 with a vertical bellmouth inlet 240 mm in diameter and a close-circuit water loop. Both Computational Fluid Dynamics (CFD) and experimental research methods have been applied. The advanced unsteady approach has been used for mathematical modeling to capture the flow-field dynamics. For experiments, the time-resolved Particle Image Velocimetry (PIV) method has been utilized. The mathematical modeling has been validated against the obtained experimental data; the main vortex core circulation is captured within 3%, while the overall flow topology is validated qualitatively. Three types of vortical structures have been detected: surface vortices, wall-attached vortices and bottom vortex. The most intense and stable is the bottom vortex; the surface and wall-attached vortices are found to be of random nature, both in their appearance and topology; they appear intermittently in time with various topologies. The dominant bottom vortex is relatively steady with weak, low-frequency dynamics; typical frequencies are up to 1 Hz. The origin of the vorticity of all large vortical structures is identified in the pump propeller rotation.

The airfoil NACA 64-618 is realized by using two levels of quality: first the theoretical profile is printed on a 3D printer in a direct, “naive” way, this geometry has been scanned by using 3D scanner and based on the deviations, a better model has been processed. The flow within the turbulent wake is measured by using Particle Image Velocimetry (PIV) technique at two velocities separated by one order of magnitude.

The feasibility study of Particle Image Velocimetry (PIV) measurements inside a test turbine at the University of West Bohemia. The current VT-400 turbine is not prepared for optical measurement with seeding particles, thus several technical issues had to be addressed until low-quality data were obtained only at low speed of 2000 RPM (rounds per minute). Even the low quality data are able to show the fluctuation anisotropy or the size of fluctuation structures, which are quantities not measurable by classical pressure methods.

A feasibility study of velocity field measurements using the Particle Image Velocimetry (PIV) method in an axial air turbine model is presented. The wakes past the blades of the rotor wheel were observed using the PIV technique. Data acquisition was synchronized with the shaft rotation, thus, the wakes were phase averaged for statistical analysis. The interaction of the rotor blade wakes with the stator ones was investigated by changing the stator wheel’s angle. The measurement planes were located just behind the rotor blades, covering approximately 3 cm × 3 cm in axial × tangential directions. The spatial correlation function suggests that the resolution used is sufficient for the large-scale flow-patterns only, but not for the small ones. The scales of fluctuations correspond to the shear layer thickness at the mid-span plane but, close to the end-wall, they contain larger structures caused by the secondary flows. The length-scales of the fluctuations under off-design conditions display a dependence on the area of the stator and rotor wakes cross-sections, which, in turn, depend on their angle. The obtained experimental data are to be used for the validation of mathematical simulation results in the future.

An experimental study on the flow pattern dynamics in a standard mixing vessel with radial baffles filled with water and induced by a pitched blade impeller pumping downward is presented. Investigation is mainly focused on detection and analysis of quasi-periodical or periodical low-frequency phenomenon connected with time- and length-scales considerably exceeding the Blade Passage Frequency (hereinafter BPF) and common turbulent eddies. This phenomenon, which is expressed as large-scale mean-flow variations, is generally known as flow Macro-Instability (hereinafter MI). It could break-down just below the liquid surface, or it crashes to the liquid surface and causes its Macro-Swelling (hereinafter MS). Our investigation was based on classical two-dimensional (2D) Particle Image Velocimetry (hereinafter PIV) measurement within 3 selected vertical planes in the vessel and subsequent analysis of the velocity field. The dominant frequencies evaluated in the selected points and overall analysis of the quasi-periodical macro-flow pattern behavior is to be shown. Identification of the quasi-periodical substructures appeared within the flow pattern was performed using the Oscillation Pattern Decomposition (hereinafter OPD) method. Observation of the macro-flow patterns confirmed presence of the macro-flow structures detected within flow pattern at the identical mixing pilot plant setup by previous investigations of the MIs phenomenon, i.e., the primary circulation loop and strong impeller discharge jet located in the lower vessel segment and the strong ascending wall current at the baffle, which break-down below the surface very often. A further important contribution of the presented work is the investigation of both flow pattern within the baffles vicinity and in the middle of the sector far from the baffle, showing a significant difference. Low-frequency periodical (or quasi-periodical) behavior of the investigated macro-structures was qualitatively confirmed by the presented results and it was quantified using the velocity dominant frequencies evaluation, as noted below. This frequency analysis brings insight into the supposed and detected interconnections between dynamics of the adjacent flow structures. Detected different flow patterns within the main plane near the baffles and in the inclined plane reveal a strong influence of the baffle presence to the local vertical flow, especially within the upper part of the vessel. Quite a different flow pattern appears tangentially in front of and behind the baffle, where a wake is revealed, indicating significant influence of the baffle on the local tangential flow. The new findings represent a contribution to better understanding the physical phenomena behind the standard mixing process.

We measure the two dimensional instantaneous velocity fields inside an axial turbine by using the technique PIV (Particle Image Velocimetry). The measured plane is located just behind the rotor wheel and it is oriented in axial × tangential direction. We measured a single overloaded regime as well as the nominal one at tip radius. We discuss several technical difficulties appearing during this measurement, mainly with the seeding particles and with the optical access into a massive steel body. The glass dividing the channel past rotor and the empty space of diffusor has been dirtied by large oil droplets condensing at the blades and splashed to this window. We “solved” this trouble by removing the glass, because it anyway divides volumes with same static pressure. Best quality result was obtained without this glass, but its removal changes the geometry. In our contribution we focus to the effect of such rude change to the trustworthiness of the results.

In this article, we show the results of a study of the structure of a turbulent flow behind a square cylinder. Namely, for the series of points that have the highest standard deviation value. During the experiment, we were changing the position of the square cylinder relative to the total flow. Experimental data were obtained using Constant Temperature Anemometry. As a result of the analysis, we obtained graphical distributions of the main parameters of the turbulent flow. They revealed that the maximum value of the Reynolds number based on the Taylor micro-scale is observed exactly at the cylinder angle of 30° and 45°. It was also found that the distribution of the energy dissipation rate, depending on the angle of rotation of the cylinder, has a certain sinusoidal character with local peak values. The results of the analysis also showed that the distribution of values of dissipation scale and dissipation time at some distance from the cylinder has the largest range of their values.

Flow in a branched channel is studied experimentally using the PIV technique. The presented study is concentrated on clarifying the dynamical behaviour in individual branches. The 11 branches issuing from the main channel perpendicularly, all channels are of rectangular cross-section. First, the time-mean flow-field is shown, then the flow dynamics is investigated using the OPD method. Flow patterns and frequencies are evaluated in three selected branches. The separated flow in branches exhibits highly dynamical behaviour, which differs substantially in the branches close to the inflow, in the main channel middle and close to its end. The typical topologies and frequencies of the detected quasi-periodical structures in the channel braches are shown in the study. Mostly, the flow-fields are populated by trains of vortices with alternating orientation and saddle-like structures. The flow-field close to the channel walls affects heat transfer process between the wall and fluid.

This paper shows the results of a study of the turbulent structure behind the NACA 0012 airfoil. During the experiment, the flow velocity was 20 m·s−1. That corresponds the Reynolds number 1.3·105. The point behind the trailing edge was chosen for experimental studies. Measurements were performed at six angles of attack α = 0°, 15°, 30°, 45°, 60°, 75° and various cases of positioning the measuring section. Namely at a constant crosssection and a constant distance behind the airfoil. The NetScanner pressure system and hot-wire technique were used for experimental studies. The obtained data allowed us to investigate the wake topology. The average velocity and standard deviation distributions are clearly grouped depending on the angle of attack. Thus, flowing around the airfoil is better up to α ≤15°. Distributions by power density and dissipation spectra also have a similar grouping tendency. Finally, we investigated the turbulent structure according to the research program. We found that at α ≥45°, depending on the measurement case, there is a clear difference in the distribution of standard deviation, Eulerian length scale, Taylor micro-scale, and Reynolds number based on the Taylor micro-scale. The obtained values at the constant distance, in contrast to the constant cross-section, are reduced.

The air flow inside single-stage test turbine is studied experimentally by Particle Image Velocimetry (PIV). Here we report the measurements results at axial-tangential plane just behind the rotor wheel at middle blade radius. We studied one nominal state and two off-design states. We discuss the applicability of resolution of our PIV system to this flow – we can distinguish the largest vortical structures in the shear layers between blade wake and jet, but the bottom part of the cascade is invisible for us at this moment. Spatial distributions of statistical moments of vorticity show a non-classical behavior at the cross-points of stator and rotor jets. We use our previously published algorithm for calculating spatial energy spectrum and display the turbulent kinetic energy colored by different length-scales of the fluctuations.

Grid turbulence is considered to be a canonical case of turbulent flow. In the presented paper, the flow structure is analyzed from the point of view of mixing properties, where vortical structures and their properties play a significant role. That is why the effect of various length-scales in turbulence is studied separately. The experimental study uses the Particle Image Velocimetry (PIV) method. The original method for spatial spectrum evaluation is applied. Results on vortex spatial spectrum and isotropy are presented. The scaling of turbulent kinetic energy (TKE) is measured, furthermore, the TKE is decomposed according to the length-scales of the fluctuations. By this method, we found that the decay of TKE associated with the smallest length-scales is more sensitive to the Reynolds number than that at larger length-scales. The TKE at the largest investigated length-scales decays more slowly. The turbulence decay-law is studied for various Reynolds numbers. The second and fourth statistical moments of vorticity are evaluated at various Reynolds numbers and distances from the grid. The isotropy is investigated in the sense of ratio of fluctuations in stream-wise to span-wise directions as the used data are captured using the planar PIV method. The full 3D fluctuation invariants were investigated in a representative position by means of the Stereo-PIV method.

The Particle Image Velocimetry is utilized to investigate flow behind a cylinder of finite length with subcritical value of Aspect Ratio (AR). The cylinder is submitted to uniform freestream with very low intensity of turbulence and thin laminar boundary layer. The flow field cannot be expected as 2D even for time-averaged quantities. The Strouhal number is evaluated in various planes of measurement. The wake is fully turbulent and is composed of many streamwise and spanwise-oriented structures. Such complex flow is very convenient to be subjected to dynamical decomposition analysis.

Particle Image Velocimetry (PIV) is an experimental method of fluid research resulting into a spatially resolved two-dimensional velocity field. We measured the velocity inside a single-stage axial test turbine at the Czech Aerospace Research Centre. We studied the axial × tangential plane just behind the rotor wheel at the tip radius under three regimes: underloaded, designed and overloaded ones. We found individual vortices in the instantaneous snapshots and we analyze their properties statistically.

The wake dynamics behind a long circular cylinder in cross-flow was studied using the POD method. The temporal parts of POD modes, Chronoses, are subjected to frequency analysis. Five groups of modes are distinguished according to the frequency contents. The low order high energy modes contain the vortex shedding frequency or its harmonics up to 3rd order plus Kolmogorov spectrum. The higher order modes are characterized by combination of Kolmogorov spectrum with the white noise spectrum, its importance grows with the mode order. The very high order modes are characterised by the white noise spectrum only.

This article presents results of the experimental research and numerical simulations of the flow in a pumping system&rsquo, s discharge object with the welded siphon. The laboratory simplified model was used in the study. Two stationary flow regimes characterized by different volume flow rates and water level heights have been chosen. The study concentrates mainly on the regions below and behind the siphon outlet. The mathematical modelling using advanced turbulence models has been performed. The free-surface flow has been carried out by means of the volume-of-fluid method. The experimental results obtained by the particle image velocimetry method have been used for the mathematical model validation. The evolution and interactions of main flow structures are analyzed using visualizations and the spectral analysis. The presented results show a good agreement of the measured and calculated complex flow topology and give a deep insight into the flow structures below and behind the siphon outlet. The presented methodology and results can increase the applicability and reliability of the numerical tools used for the design of the pump and turbine stations and their optimization with respect to the efficiency, lifetime and environmental demands.

Turbulentní proudění se skládá z vírů a jiných koherentních struktur, jejichž pohyb je tak složitě uspořádaný, že ho vnímáme jako náhodný. Naše práce se snaží o rozšíření našeho porozumnění tomuto problému pomocí principů objevených při výzkumu supratekuté turbulence, která se skládá z kvantovaných vírů - lineárních topologických singularit makroskopického parametru uspořádání. V tomto článku předkládáme algoritmus hlednání jednotlivých vírů ve změřeném okamžitém vektorovém poli rychlostí klasické tekutiny (např. vzduchu). Metoda je založena na přímém fitování proudového pole Gaussovským modelem víru a minimalizování energie zbytku. Statistické vlastnosti nalezeného souboru vírů nám snad pomohou v pochopení turbulence z trochu jiné perspektivy. The turbulent flows consist of vortices and other coherent structures, whose motion is such highly ordered, that it is usually considered to be random. We try to bring light into this problem by using the ideas coming from research of superfluid flows, where the turbulence consists of quantized vortices - linear topological singularities in the macroscopic order parameter. We present an algorithm developed for searching of individual vortices in an experimentally obtained 2D flow-field of a classical fluid (e.g. air). This algorithm directly fits the flow-field by the Gauss's model of vortex minimizing energy of the rest. The statistics of the reconstructed vortices can help us to understand the turbulence from another point of view.

The dynamics of the tip vortices in the wake behind a wall-mounted finite-length circular cylinder of the aspect ratio 2 was studied experimentally using time resolved stereo PIV technique. The cylinder was mounted normal to a ground plane and it was subjected to a cross-flow with thin boundary layer developed on the wall, the Reynolds number based on inflow velocity and cylinder diameter was 9.7 thousands. The dynamics of tip vortices were analysed using the POD method applied to the plane perpendicular to the flow close to the cylinder. Besides the decaying power spectrum, slower that the Kolmogorov-type one, the two distinct frequencies were detected on Strouhal numbers 0.09 and 0.15. These frequencies could be linked to the vortical structures dynamics in the wake. The frequency Sh = 0.15 corresponds to predominantly spanwise vortices dynamics with anti-symmetrical patterns with respect to the cylinder axis, while the frequency Sh = 0.09 corresponds to mainly streamwise vortical structures dynamics with symmetrical patterns respectively. Thus, the von Kármán vortex street was detected on Strouhal frequency 0.15.

The structure of boundary layer on an airfoil suction side was studied using stereo time resolved PIV technique experimentally in low Reynolds number, about 33 thousands. The mean velocity field is close to the 2D case, however the instantaneous structure is highly dynamical and 3D. Dynamics of the vortices in the boundary layer has been analysed using POD method. The results show lag of coherence in the streamwise direction, oblique patterns are detected instead.

Reynolds number is the basic parameter determining the flow-field topology and its evolution in time unambiguously if only inertial, pressure and viscous effects are involved. However the Reynolds number definition is not unique and its physical meaning depends strongly on its actual definition. In the paper Reynolds number definition is given. Then, its role in typical situations in fluid dynamics is to be explained. The laminar flow, transition from laminar state to turbulence and fully turbulent flow are to be taken into account respectively. The physical meaning of Reynolds number vary accordingly. In conclusion the final statement is adopted, that the Reynolds number should be understood as a measure of convective and viscous effects in terms of length-, time- and/or velocity-scales presented in a given flow-field as a result of competing convection and diffusion effects.Reynolds number is the basic parameter determining the flow-field topology and its evolution in time unambiguously if only inertial, pressure and viscous effects are involved. However the Reynolds number definition is not unique and its physical meaning depends strongly on its actual definition. In the paper Reynolds number definition is given. Then, its role in typical situations in fluid dynamics is to be explained. The laminar flow, transition from laminar state to turbulence and fully turbulent flow are to be taken into account respectively. The physical meaning of Reynolds number vary accordingly. In conclusion the final statement is adopted, that the Reynolds number should be understood as a measure of convective and viscous effects in terms of length-, time- and/or velocity-scales presented in a given flow-field as a result of competing convection and diffusion effects.

[Departement_IRSTEA]Ecotechnologies [TR1_IRSTEA]SPEE [ADD1_IRSTEA]Sûreté alimentaire; International audience; This article describes the planning, set-up, turbulence characterization and analysis of measurements of a passive grid turbulence experiment that was carried out in the S1MA wind-tunnel from ONERA in Modane, in the context of the ESWIRP European project. This experiment aims at a detailed investigation of the statistical properties of turbulent flows at large Reynolds numbers. The primary goal is to take advantage of the unequaled large-scale dimensions of the ONERA S1MA wind-tunnel facility, to make available to the broad turbulence community high-quality experimental turbulence data with unprecendented resolution (both spatial and temporal) and accuracy (in terms of statistical convergence). With this goal, we designed the largest grid-generated turbulence experiment planned and performed to date. Grid turbulence is a canonical flow known to produce almost perfectly homogeneous and isotropic turbulence (HIT) which remains a unique framework to investigate fundamental physics of turbulent flows. Here, we present a brief description of the measurements, in particular those based on hot-wire diagnosis. By comparing results from classical hot-wires and from a nano-fabricated wire (developed at Princeton University), we show that our goal of resolving down to the smallest dissipative scales of the flow has been achieved. We also present the full characterization of the turbulence here, in terms of turbulent energy dissipation rate, injection and dissipation scales (both spatial and temporal) and Reynolds number. © 2017, Deutsches Zentrum f

Experimental study of the structure of the boundary layer on the suction side of a plate is to be presented. The flat plate with rounded edges and angle of attack of 7° is used. The boundary layer flow will be explored using the time-resolved PIV technique, measuring plane was located very close to the wall. Analysis of the flow dynamics is to be presented using the POD technique applied on both velocity and vorticity fields.

2D3C TR-PIV technique was utilized to investigate streamwise-oriented vortical structures behind an inclined flat plate. The angle of attack was set to 7 deg, several fields of view in the wake were investigated. The instantaneous velocity vector fields were captured, dynamics of the flow was studied using POD method. The streamwise structures are determined by vorticity and low- and high-velocity streaks are defined. The acquired results are in a good agreement with the new hypothesis of a principle of flight.

The stereo PIV measurement were performed behind a wing in the plane perpendicular to the flow to study the vortical structures oriented in the streamwise direction, which take place both in suction and pressure sides of the wing. The Reynolds numbers during the experiments in the 3 m wind tunnel range from 0.5 million up to 1.5 million.

The presented study is focused on experimental investigation of a boundary layer on a flat plate in adverse pressure gradient. The flat plate is placed in regular flow, the pressure gradient is generated by the plate inclination. The study [9] deals with structure of the wake behind the plate, the presented study concentrates on structure of the flow close to the suction surface of the plate. Dynamical behaviour of the flow structures is studied in details with respect to the streamwise topology changes. In spite of the fact that the time-mean flow field is 2D, constant along the span, the instantaneous structures topology is fully 3D.

The experimental method of PIV (Particle image velocimetry) is used to study the air flow over a forward facing step creating shear layer between the outer flow and the recirculation bubble. This layer decays into vortices, which are highlighted by using Reynolds (temporal) decomposition or Agrawal (spatial) decomposition. The later can be tuned to different-sized structures, and, additionally, their energies roughly follow the Kolmogorov -5/3 law.

The motivation of the presented study is supporting new ideas about principle of flight by Hoffman and Johnson, see [1]. The new hypothesis of physical mechanism of flight relies on existence of streamwise vortical structures on the suction side of the airfoil and within its wake. The vortices origin is supposed to be the instability of the boundary layer subjected to adverse pressure gradient. The vortices are of highly dynamical nature, changing theirs position, size and other parameters in time very rapidly. For this experiment the simplest airfoil possible was chosen represented by a flat plate in uniform flow and moderate angle of attack. In the suggested paper detailed measurement of the zone of interest will be carried out using stereo PIV method. System of measuring planes perpendicular to the flow will be explored. POD analysis is to be utilized.

Stereo PIV measurements has been performed behind the inclined plate, angle of attack 5 and 10 deg. Occurrence and dynamics of streamwise structures behind the plate trailing edge have been studied in details using POD method. The streamwise structures are represented by vortices and low- and highvelocity regions, probably streaks. The obtained results support the hypothesis of an airfoil-flow force interaction by Hoffman and Johnson [1,2].

Coanda effect takes place in flow within valves diffuser for certain conditions. The valve plug in half-closed position forms wall-jet, which could be stable or instable, depending on geometry and other conditions. This phenomenon was subject of experimental study using time-resolved PIV technique. For the acquired data analysis the special spatio-temporal methods have been used.

The problem of flow field in the urban boundary-layer (UBL) in aspects of wind comfort around buildings and pollutant dispersion has grown in importance since human activity has become so intense that it started to have considerable impact on environment. The issue of wind comfort in urban areas is the result of complex interactions of many flow phenomena and for a long time it arouses a great interest of the research centres. The aim of article is to study urban atmospheric flow at the local scale, which allows for both a detailed reproduction of the flow phenomena and the development of wind comfort criteria. The proposed methodology involves the use of PIV wind tunnel experiments as well as numerical simulations (Computational Fluid Dynamics, CFD) in order to enhance understanding of the flow phenomena at this particular scale in urban environments. The analysis has been performed for the 3D case of two surface-mounted buildings arranged in tandem, which were placed with one face normal to the oncoming flow. The local characteristics of flow were obtained by the use of commercial CFD code (ANSYS Fluent). The validation was carried out with reference to the PIV results.

The effect of ionic wind generated by plasma actuator on developed boundary layer inside a narrow channel was investigated recently. Since the main investigated plane was parallel to the channel axis, the description of flow field was not evaluated credibly. This paper is dealing with cross-section planes downstream the actuator measured via 3D time-resolved PIV. The actuator position is in spanwise or in streamwise orientation so that ionic wind is blown in the same direction as the main flow or in opposite direction or perpendicularly. The interaction between boundary layer and ionic wind is evaluated for three different velocities of main flow and several parameters of plasma actuation (steady and unsteady regime, frequency etc.). Statistical properties of the flow are shown as well as dynamical behaviour of arising longitudinal vortices are discussed via phase-locked measurement and decomposition method.

Wavelet analysis is applied to data from PIV measurement in order to recognize a specific structure in the flow. The PIV snaphots achieved on the model of the street canyon was used as a test case. Four flow characteristics that are at disposal from one-point simultaneous two-components measurement (e.g. from 2D LDA) were analyzed by Wavelet method: longitudinal and vertical velocity, momentum flux u’w’ and δS – the difference between momentum fluxes associated with a sweep and an ejection. Each of characteristic is useful for detection of certain type of event. We have focused on the sweep and the ejection that seem to be the most convenient for investigation of a significant inflow or an outflow from the street canyon.

Dynamics of the flow-field behind a backward-facing step in a narrow channel is studied experimentally using time resolved PIV technique. Secondary flow represented by vortical structures is studied using Oscillation Pattern decomposition technique. The low-frequency quasi-periodical structures appearing in the region just behind the step close to the channel bottom are studied. Typical dynamics of vortex structures involving contra-rotating vortex pair train, vortices coalescence and splitting have been observed.

Proper orthogonal decomposition (POD) of a very turbulent flow inside a street canyon is performed. The energy contribution of each mode is obtained. Also, physical meaning of the POD result is clarified. Particular modes of POD are assigned to the particular flow events like a sweep event, a vortex behind a roof or a vortex at the bottom of a street. Test of POD sensitivity to the acquisition time of data records is done. Test with decreasing sample frequency is also executed. Fur- ther, interpolation of POD expansion coefficient is performed in order to test possible increase in sample frequency and get new information about the flow from the POD analysis. We tested a linear and a spline type of the interpolation and the linear one carried out a slightly better result.

Infrared camera, Particle Image Velocimetry, smoke-wire, tuft filaments and oil-flow visualization techniques were used for wind-tunnel and in-flight investigation of boundary layer separation, both stall and separation bubbles, related to the low-Reynolds numbers transition mechanism. Airfoils of Wortmann FX66 series and FX66 series wing-fuselage interaction, as well as modern airfoils and their wing-fuselage geometry were subject to study. The presence of previously identified structures in the CFD modelling, such as horse-shoe vortices, was confirmed in the flow. Wind-tunnels and in-flight measurements on sailplanes were carried out and effect of passive flow control devices - vortex generators - was surveyed; namely counter-rotating vortex generators and Zig-zag type turbulators were applied. Separation suppression and consequent drag coefficient reduction of test aircrafts was reached. PIV investigation was further extended by Time-Resolved techniques. An important study on structure of the turbulent flow in the lower atmosphere, creating an environment of the soaring flight, was presented.