I. V. Naumov

Experimental and numerical results on three-dimensional instabilities in a rotating disk–tall cylinder flow

The three-dimensional axisymmetry-breaking instability of axisymmetric flow between a rotating lid and a stationary cylinder is analyzed both numerically and experimentally for the case of tall cylinders with the height/radius aspect ratio between 3.3 and 5.5. A complete stability diagram for the primary three-dimensional instability is obtained experimentally and computed numerically. The instability sets in due to different three-dimensional disturbance modes that are characterized by different azimuthal wavenumbers. The critical Reynolds numbers and associated frequencies are identified for each mode. The onset of three-dimensional flow behavior is measured by combining the high spatial resolution of particle image velocimetry and the temporal accuracy of laser Doppler anemometry. The results are compared to the numerical stability analysis. The measured onset of three dimensionality is in a good agreement with the numerical results. Disagreements observed in supercritical regimes can be explained by s...

Vortex breakdown in closed containers with polygonal cross sections

The vortex breakdown bubble in the confined flow generated by a rotating lid in closed containers with polygonal cross sections was analysed both experimentally and numerically for the height/radius aspect ratio equal to 2. The stagnation point locations of the breakdown bubble emergence and the corresponding Reynolds number were determined experimentally and in addition computed numerically by STAR-CCM+ CFD software for square, pentagonal, hexagonal, and octagonal cross section configurations. The flow pattern and the velocity were observed and measured by combining the seeding particle visualization and the temporal accuracy of laser Doppler anemometry. The vortex breakdown size and position on the container axis were determined for Reynolds numbers, ranging from 1450 to 2400. The obtained results were compared with the flow structure in the closed container of cubical and cylindrical configurations. It is shown that the measured evolution of steady vortex breakdown is in close agreement with the numerical results.

Investigation of a wake decay behind a circular disk in a hydro channel at high Reynolds numbers

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 %).

Stagnation zone formation on the axis of a closed vortex flow

The features of developing a counterflow zone (bubble-mode vortex breakdown or vortex explosion) at the center of an intensively swirled flow produced in a liquid-filled cylindrical container with a rotating endwall have been studied. The observation showed that the scenario of developing a bubble-mode breakdown zone with generation of counterflow is the same for cylinders with low or high aspect ratio, and it remains independent of stationary-nonstationary transition boundary for the main vortex flow.

Loss of efficiency in a coaxial arrangement of a pair of wind rotors

The efficiency of a pair of wind turbines is experimentally investigated for the case when the model of the second rotor is coaxially located in the wake of the first one. This configuration implies the maximum level of losses in wind farms, as in the rotor wakes, the deceleration of the freestream is maximum. As a result of strain gauge measurements, the dependences of dimensionless power characteristics of both rotors on the distances between them were determined for different modes at different tip speed ratios. The obtained results are of interest for further development of aerodynamics of wind turbines, for optimizing the work of existing wind farms and reducing their power losses due to interactions with wakes of other wind turbines during design and calculation.

Experimental investigation of wake evolution behind a couple of flat discs in a hydrochannel

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 (Lх = 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 Lx, and at Lх > 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 Lx, 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.

Development and interaction of rotor wakes

The present work shows the results of a series of experimental investigations of the wake development behind a model rotor subject to upstream disturbances created either by another rotor or by a disk. The experiments are carried out in a water flume in order to control the flow and to carry out visualizations and to perform optical diagnostics. The aim of the work is to clarify similarities and differences in the wake of a wind turbine subject to different inflow disturbances, and in particular to see if there is any difference in the rotor wake resulting from a upstream disturbance created by a rotor and one created by an immobile disk. The background for the study is an on-going discussion if disks can replace rotors in laboratory experiments. In the paper, we will also show new experimental data that support our main conclusion, which is that strong differences exist between the near wakes characteristics of a rotor and a disk.

Wake developments behind different configurations of passive disks and active rotors: Paper

The present paper takes a broad view on our previous experimental studies of flows behind different single and dual configurations from passive disks or active rotors to establish new aspects of the wake development [1-4]. The aim of the present examination is to obtain a better understanding of the wake formations and interactions between wind turbines in wind farms. A correlation between independent investigations of the near [1] and far wakes behind single [2] and dual [3-4] systems will be established to the same operating regimes and flow conditions. New examinations of the old data need because two main differences in the wake behaviour for the disk-disk and the rotor-rotor systems were found: the wake intensity grows for the dual disks in comparison with the single one, but in contrast to this, wake intensity behind the dual rotor system is smaller than the one behind a single rotor. These differences may be explained by an influence of the rotor tip vortices which are absent in the disk-disk model. The present retesting of the near and far wake data should provide an evidence of this conclusion.

An influence of the different incoming wake-like flows on the rotor vibrations: Paper

The aim of the current investigation is the rotor vibrations generated by the disturbances caused the different types of incoming wake-like flows. Those wakes arriving at the tested rotor were created by two ways: a passive wake generator (immobile disk) and an upstream rotating rotor as an active wake generator. The influence of both wakes on the tested rotor was studied in a water flume. A model of the tested three-bladed rotor designed using Glauerts optimum theory at an optimal tip speed ratio λ = 5 was placed in both “passive” and “active” wakes to recognize dissimilarities on the vibrations of the tested rotor. The distance from the wake generators to the tested rotor was varied from 4 to 8 rotor diameters. Also, the shift between the rotor axis and axis of the incoming wakes was changed to 0, 0.5 and 1 rotor diameters. The flow condition before rotor was measured with high temporal accuracy using LDA. The turbulent intensity of the incoming wake flows changed from 3 to 16% due to the types of the wake generators. Power and thrust characteristics and their pulsations of the tested rotor were measured by strain gauges. The dependences of power coefficients from tip speed ratios and positions of the wake generators were documented. The present study showed a strong influence of the initial flow from the two different wake generators on the rotor vibrations.

Comparison of the far wake behind dual rotor and dual disk configurations

There is an increasing interest in studying the development of far wakes behind two or more interacting wind turbines in order to determine the influence of wake interaction in relation to the design of wind farms. The focus of this experimental study is to understand and describe the resulting wake features for two rotors subjected to different operating and spatial conditions. As a part of this, a comparison with the wake development behind two disks replacing the rotor models was performed to determine the difference between the two wake systems. LDA and Stereo PIV experiments were carried out to study the development of far wakes behind configurations of dual HAWT wind turbine rotors and dual circular disks. The setups were placed in the middle of a water flume. The initial flow in the flume is subjected to a very low turbulence level, limiting the influence of all external disturbances in order to focus the study to the inherent wake instability. As a result of the investigation, we obtained decays of profiles for the velocity deficit and turbulent pulsations in the far wakes behind both dual rotor and dual disk configurations. By using regression techniques to fit the obtained velocity profiles the experimental data were approximated by identical analytical models and compared to each other. An identical rational dependence with the same powers, but with different coefficients, was found for the two configurations.