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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.

This article shows the results of a study of the structure of turbulent flow behind a square profile ALUTEC 45 × 45 mm with T-slots. The angle of rotation of the profile relative to its axis varied α = 0 ° , 15 ° , 30 ° and 45 ° . During the experiment, the flow velocity was 5 m⋅s − 1 , Reynolds number was 7 . 7 ⋅ 1 0 4 . The Constant Temperature Anemometry technique was used for experimental studies. To avoid backflow, the measuring plane was positioned at the rear of the profile at a distance of x ⋅ d − 1 ≈ 2 . 2 . As a result of the studies, it was found that the highest Taylor microscale Reynolds number and standard deviation for turbulent flow was observed in the area behind the cylinder. The width of this area is 3.5 times the width of the cylinder. With the distance from the center of the cylinder in the spanwise direction to flow the Taylor microscale Reynolds number and standard deviation sharply decreases. The maximum values of the Taylor microscale Reynolds number are observed at α = 45 ° is 426 and 398. It has also been found that behind the cylinder there is some area in which the some parameters of the turbulent flow vary greatly with the change angle α . The lowest energy dissipation rate in this range is observed for α = 15 ° − 68 m 2 ⋅ s − 3 and the largest for α = 0 ° − 138 m 2 ⋅ s − 3 . We also found that the minimum value of the Kolmogorov scale and the Kolmogorov time is observed at α = 0 ° . The minimum values of the Kolmogorov scale η = 71 μ m and the Kolmogorov time τ η = 0 . 33 ms . The maximum values for previous parameters observed at α = 15 ° is η = 84 μ m and τ η = 0 . 47 ms . We also found that the flow rate and standard deviation distributions between the ALUTEC profile and the ordinary square cylinder different. This can be observed when the upper part of the profile is tightly closed and the lower part is open. In this case, along with the profile inside of the T-slot, there is a generation of internal flow. This reduces the total backflow area behind the ALUTEC profile by 20% compared to an ordinary square profile.

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.

Feature of turbulent flow anisotropy behavior behind an asymmetric NACA 64-618 airfoil investigated in this paper. Experimental studies were performed using a hot-wire anemometery with X-probe at the chord-based Reynolds number $$1.7 \times 10^5$$ 1.7 × 10 5 . The average ensemble velocity and Reynolds stress components are used to determine the wake topology and anisotropy of turbulence. The obtained data allowed to identify the outside wake region, which is characterized by low instability and a high degree of anisotropy of the turbulent flow. This tendency is observed at different angles incident. Further, to gain better insight into the physics of this phenomenon the structure of turbulence have been evaluated. Integral turbulence length and time scales were estimated by the area of the autocorrelation function of velocity fluctuations. Then, using the second-order structural function, we obtained the dissipation characteristics of the flow. In addition, the features of the energy spectrum in the region with high and low degrees of turbulence anisotropy were analyzed.

Purpose Some recent effort showed that usage of Krueger flaps helps to maintain laminar flow in cruise flight. Such flaps are positioned higher relative to the chord to shield the leading edge from the insect contamination during take-off. The flap passes several through critical intermediate position during the deployment to its design position. The purpose of this paper is to analyse the aerodynamics. Design/methodology/approach To better understand such flow phenomena, the combined approach of computational fluid dynamics and experimental methods were used. Flow simulation was performed with in-house finite volume Navier–Stokes solver in fully turbulent unsteady RANS regime. The experimental data were obtained by means of force and pressure measurements and some areas of the flow field were examined with 2 C particle image velocimetry. Findings The airfoil with flap in critical position has a very limited maximum lift coefficient. The maximum achievable lift coefficient during the deployment is significantly affected by the vertical position of the trailing edge of the flap. The most unfavourable position during the deployment is not the flap perpendicular to the chord, but the flap inclined closer to it is the retracted position. Research limitations/implications The flap movement was not simulated either in the simulation or in the experiment. Only intermediate static positions were examined. Practical implications A better understanding of aerodynamic phenomena connected with the deployment of a Krueger flap can contribute to the simpler and lighter of kinematics and also to decrease time-to-market. Originality/value Limited experimental and computational results of Krueger flap in critical positions during the deployment are published in the literature.

By using the Stereo Particle Image Velocimetry (PIV) technique we observe the secondary flow of second kind in a corner of a channel of square cross-section. The boundary layer thickness is much lower than the channel size. Therefore, the flow is still developing, not filling the entire channel cross-section, which is the case more widely reported in literature studied in a very long channel. The non-linear secondary effects of interacting boundary layers are observed as a single stream-wise vortex close to the channel corner in case of laminar boundary layers. In the case of turbulent boundary layers, the secondary flow takes shape of a symmetric pair of counter-rotating vortices. This pattern is observable only in the average velocity field, while the instantaneous ones display large amount of vortices, whose spatial distribution close to the corner in dependence on their orientation leads to statistically emergent net vorticity. At the same time, this pattern is reproduced by using a numerical simulation.

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.

The wake behind a wall-mounted finite-length circular cylinder of the aspect ratio 2 stuck out of a wall was studied experimentally using 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 time-averaged velocity and streamwise vorticity fields showed the development of the mean flow structure consisting of the tip vortex pair, the extent of the near-wake recirculation zone with back-flow, the downwash phenomenon and base vortex structures close to the wall.

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 scope of this paper is to perform a detailed experimental investigation of the shape error effect on the turbulence evolution behind NACA 64-618 airfoil. This airfoil is 3D-printed with predefined typical shape inaccuracies. A high-precision optical 3D scanner was used to assess the shape and surface quality of the manufactured models. The turbulent flow was studied using hot-wire anemometry. The developed force balance device was provided to measure the aerodynamic characteristics of the airfoil. Experimental studies were carried out for three angles of attack, +10∘, 0∘, −10∘, and different chord-based Reynolds numbers from 5.3×104 to 2.1×105. The obtained results show that the blunt trailing edge and rough surface decline the aerodynamic performance of the blades. In addition, the experimental results revealed a strong sensitivity of the Taylor microscale Reynolds number to the type of shape inaccuracy, especially at Re≈1.7×105. We also discuss the evolution of the Reynolds stress components, the degree of flow anisotropy, and the power spectrum distributions depending on the airfoil inaccuracies.

We studied the grid-generated turbulence by using Particle Image Velocimetry (PIV) technique. We test on this already well studied flow the new ways of analyzing spatially resolved PIV data, such as the spatial spectra and structure functions. We compare some of the turbulence characteristics with results of Laser Doppler Anemometry (LDA).

Two buildings differing in height in tandem configuration are subjected to a turbulent boundary layer flow. The physical modelling in a wind tunnel has been performed, Particle Image Velocimetry in horizontal planes has been applied. The time-mean flow-field is dominated by the contrarotating vortex pair in the near-wake of the upstream building forming strong back-flow in this wake. Dynamical activity is located close to the bigger downstream building below the level of the smaller building roof. The dynamical structures in the space between them are studied using advanced analytical methods, Oscillation Pattern Decomposition (OPD). The 3 important OPD modes were detected differing by topology and frequency. The 2 OPD modes are characterized by vortical structures formed within the wake of the smaller upstream building and moving downstream towards the bigger building. The last OPD mode shows strong pulsation transversal flow in front of the bigger building. Typical frequency of the cyclo-stationary behaviour could be characterized by the Strouhal number in the range from 0.09 up to 0.15.

The swirling flow in a diffuser has been studied using the stereo PIV technique. The DANTEC hardware and software were used for the experiment. The water pipe flow in the diffuser inlet has been set spinning by introducing tangential jet flow. Both piping and diffuser were of circular cross-section, however diffuser casing had rectangular (square) cross-section. The plane of measurement was located perpendicular to the flow within the diffuser. The two cameras with the Scheinpflug lens mounting were used, however the angle between the lenses axes and the diffuser outer surface was far from to be perpendicular, as recommended in similar situations. This configuration has been dictated by technical conditions in the laboratory. The two-level calibration target with dots was used for the cameras calibration. The calibration covers the diffusers central region in a good quality, however regions close to the walls were not well resolved. Many combinations of axial and tangential inflow conditions have been studied. The acquisition covered about 3 seconds in real time with frequency 500 Hz. Both statistics and dynamics of the flow have been studied in details. Results are shown for both statistical characteristics and dynamical analysis of various conditions. During experiments the air-glass-water interface was not normal to the camera viewing axis. The discussion how to solve this problem and of the bias of the calibration will be introduced.

The grid turbulence past a grid made of row of prismatic circular rods (rod diameter = 10 mm, rod spacing = 20 mm) perpendicular to the flow is observed by using a pair of PIV cameras. The first one has field of view larger (81 mm), the second one smaller (31 mm), which increases the dynamical range, where the spectral turbulence properties can be explored. Energy spectra displays usual behavior approximately following k −5/3 law, the anisotropy originates at larger scales and the flatness describing strong rare events is connected with smaller scales. The spectral properties of vorticity do not collapse due to the different lengthscale of differentiation, which make questionable the previous research based on the vorticity statistics. Research background: Grid turbulence is the best experimentally accessible prototype of ideal homogenous and isotropic turbulence, although it is known, it is not exactly the ideal one. Purpose of the article: This contribution explores the possibility of expanding the limited dynamical range of PIV method. Methods: Particle Image Velocimetry is based on observing the motion of small particles carried by the flow. Findings & Value added: Anisotropy connected with large scales, flatness with small scales; it does not work for vorticity.

A blade cascade allowing free pitch movement of five blades was developed in Institute of Thermomechanics. This model is introduced to study the phenomenon of stall flutter existing in rotary bladed wheels of steam turbines and other devices. This article describes how to induce the stall flutter for prescribed boundary conditions (as inlet velocity, various angles of attack, etc.) and gives survey about flow field differences around the cascade set to both the turbine and the compressor geometry. This experimental research utilized time-resolved Particle Image Velocimetry (PIV) to measure and to evaluate statistical quantities of the wake behind the NACA0010 profiles of the cascade. Also dynamical analysis was performed in the form of Fast-Fourier transform and also phase-locked measurement was applied. Gained knowledge will be utilized to design advanced model of the cascade allowing stall flutter examination without the use of ball bearings.

This paper presents the results of the drag coefficient estimations for different types of NREL airfoils based on the experimental data. Namely, it was S803, S807, S813, and S817 profiles with the same chord length. The investigations were conducted at three angles of attack α=0°, α=±5°, different chord-based Reynolds numbers 0.6×105, 1.3×105 and 2.6×105. While, measuring cross-sections were placed behind the trailing edge at x∙c−1≈0.2, 0.4 and 1.0. Experimental data were collected using a hot-wire split fiber probe 55R55, which allowed us to estimate the characteristics of turbulent flow in stream-wise and crosswise directions. According to the obtained results, the highest and lowest Cd values correspond to profiles S817 and S803, respectively. Moreover, the results show that more asymmetric profiles S803 and S807 have the lowest resistance at zero angles of attack. Research background: Application of Antonia and Rajagopalan methodology to drag assessment of various stream bodies. Purpose of the article: Comparative evaluation of the drag coefficients of the NREL airfoils based on instantaneous velocity distribution behind. Methods: Hot-wire anemometry with split fiber probe 55R55. Findings & Value added: The highest drag coefficient corresponds to the S817 profile. The asymmetry of the airfoil shape has a significant impact on its drag characteristics.

Aluminum construction profiles become a standard construction material for short-term stands in science and in industry as well. It has the shape of square prism with an groove in each side to easy connecting the nut or bolt in any location. We studied the wake past the profile of 45 mm side. We studied the topology of the wake in the dependence on the angle of attack by using the Particle Image Velocimetry technique.

The 3D structure of the wake behind a circular cylinder in a cross-flow is studied experimentally using stereo PIV technique, Reynolds number was around 5 thousands. The statistical characteristics along the cylinder axis are effectively constant and thus 2D, however instantaneous flow-field in the wake is fully 3D. Within the wake dynamical flow, the streamwise vortices and spots of streamwise velocity component pulsations are detected.

A circular cylinder in crossflow is subjected to the study of 3D dynamical structure of its wake. The typical dynamics is characterized by quasi-periodic behaviour called Kármán – Bénard vortex street with the typical frequency in dimensionless form known as Strouhal number. The experimental study relies on stereo Particle Image Velocimetry method, the plane of measurement is perpendicular to the flow in the distance 3.8 cylinder diameters in streamwise direction. Reynolds number was around 5 thousand. The structure and dynamical behaviour of the wake along the cylinder axis is studied in details. On the top of the known streamwise velocity deficit definition of the wake, the streamwise oriented dynamically evolving vortices are detected. For the detailed dynamics examination, the Oscillation Pattern Decomposition method was used. Waves of various structures travelling along the cylinder axis as well as some pulsations were identified. The wake dynamics is characterised by the streamwise velocity component deficit and the streamwise vorticity concentration. Both structures are moving periodically in transversal direction with the same frequency, the velocity deficit is shifted by a quarter of period behind the vorticity concentration in the shedding process.

The circular cylinder turbulent wake is studied experimentally using Particle Image Velocimetry (PIV) method. The vorticity component parallel to the cylinder axis is evaluated from measured velocity field. Dynamics of the vorticity field is analysed using the Oscillation Pattern Decomposition (OPD) method. The 3 dominant modes corresponding to the 1st, 2nd and 3rd harmonics of Strouhal frequency are detected and presented. The corresponding topologies are characterized by waves of vortical systems. The results are similar to the dynamics of velocity field, however the procedure is much easier to implement, as only half number of degrees of freedom are active.

This work aimed to investigate the turbulence characteristics behind different parts of the wind turbine blade. Airfoils S807, S813, S817, and S803, which characterize the shape of the blade at different lengths, were selected for estimation. During the experiment, the chord-based Reynolds number was 2.6×105, while the angle of attack was zero. Measuring crosssections were placed behind the trailing edge at x·c-1≈0.2, 0.4 and 1.0. For the determination flow topology, we used a Hotwire anemometry with a split fiber probe 55R55 and a miniature X-wire probe 55P64. The obtained data allowed us to determine and compare the evolution of the wake behind different types of airfoils in streamwise and spanwise directions. Thus, the largest and smallest velocity deficit located behind S817 and S803 airfoils, respectively. This trend is also evident in the Normalized Reynolds shear stress distributions. Finally, we determined the spectrum and calculated the Integral length scale, the Taylor and Kolmogorov microscale of turbulent flow. According to the results, profile S817 contributes to the formation of a flow with a large scale of turbulence, while the S803 is contrary.

This paper is devoted to the development of a new probe to measure a drift from cooling towers. The probe was named DPIK (Drop Hunter Isokinetic) and its development consists of a design of the aerodynamic and microelectronic part of the probe, verification methods, design and testing, verification tests in laboratory conditions and measurements on a model of the cooling tower. The paper deals mainly with the verification tools, as these methods are very closely related to the aerodynamic part of the probe design. Two options were chosen for verifying the probe principle. The first of these was the Interferometric Particle Imaging (IPI) method, the second was another optical method using similar experimental equipment Particle Image Velocimetry (PIV).

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.

Flow in the wake of a circular cylinder is studied experimentally using time-resolved stereo PIV method. Special attention is paid to 3D topology of dynamical structures. While the distribution of statistic quantities along the cylinder is uniform, and i.e. 2D, the instantaneous flow structure is fully 3D. Within the velocity fluctuating flow field the structures containing streamwise vorticity and velocity components are dominant. The POD modes connected with von Kármán vortex street are identified.

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.

We present a simple method of obtaining the turbulence kinetic energy spectrum from spatially resolved particle image velocimetry (PIV) data without need of use of Taylor hypothesis of frozen turbulence. Additionally, this method allows us to extract the velocity field components related to individual length scales resolved in the spectrum. We convolute the measured PIV velocity field with a band-pass filter, i.e., a difference of two Gauss functions with different widths, and the energy content of a such product we relate to the relevant length-scale of the used band. This is a “kind of wavelet transformation”, which, in respect to Fourier transformation, gives a good physical meaning to individual components.

We observe that decreasing the inner nozzle surface by adding longitudinal ribs increases the jet stability in terms of the amount of turbulent kinetic energy in the near shear layer. We try to explain our observation as a stabilization effect of secondary flow vortices emerging in the corners of the ribs. These stream-wise vortices damage the development of larger-scale structures in the near shear layer. This explanation is supported by autocorrelation function of the stream-wise velocity component, which displays slightly smaller integral length-scale in the case with ribs than in the case of smooth nozzle. The experiment is performed at Reynolds number 2.2 × 105 (based on the nozzle diameter 50 mm); the Stereo-PIV (Particle Image Velocimetry) measurement takes place at the plane perpendicular to the jet axis one diameter past the nozzle exit. Optical 3D scanner controls the real nozzle geometry. This article presents preliminary measurement at single position and single velocity only; further exploration of this problem is needed.

Wind tunnel is constructed. It is characterized by a high degree of airflow laminarization. The main constructive element of this aerodynamic tube is the laminarization module. It allows the destruction of turbulent fluxes by providing a homogeneous laminar flow. In order to evaluate the performance of the developed aerodynamic tube, a series of experimental studies was conducted in the range of air velocity of 10-50?m·s-1. To establish the laws of the distribution of flow parameters relative to the section the cross-sectional analysis method was used. It consists of forming a series of measuring planes that characterize the distribution of dynamic pressure and air flow velocity relative to the testing section. After that, the received experimental data are interpreted as three-dimensional graphic dependencies of the studied quantities. There is clearly observed a certain local area of stable values of dynamic pressure and flow velocity, after which they are sharply reduced in the direction of the walls of the section for testing. As a result of the analysis, a series of digital profiles of the distribution of dynamic pressure in the measuring planes of the testing section were obtained. Analysis of the obtained graphic dependencies allowed establishing of the uneven distribution of the dynamic pressure difference in the boundary layer zone along the inner perimeter of the test section is worth noting. Because of the side of the upper wall of the test section, the dynamic pressure difference is greater than the sidewall. This phenomenon is due to the difference in area of their blown air. Consequently, at a wall with a larger contact surface, a constant air stream experiences more loss of its kinetic energy than a wall with a smaller area. Especially this tendency is observed in the turbulent boundary layer.

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.