Periodic Flow Structures Research Articles

Mechanism of periodic oscillation in low-Reynolds-number buffet around an airfoil at angle of attack 0

In this study, the low-frequency oscillations found in the compressible low-Reynolds-number regime, defined as low-Reynolds-number buffet, were investigated by numerical calculations and modal analysis. Dynamic mode decomposition (DMD) and compressive sensing methods were employed to extract periodic flow structures. Numerical simulation results showed low-Reynolds-number buffet and Kármán vortex shedding. Low- and high-frequency oscillations [St=O(0.01), O(1.0)] were extracted by DMD and named the buffet mode and vortex shedding mode, respectively. Low-Reynolds-number buffet does not necessarily exhibit supersonic regions or shock wave. Simulation results show that the thickness of the separated shear layer changes significantly under low-Reynolds-number buffet. The change in the thickness of the separated shear layer was confirmed by the buffet mode of DMD results. Two types of compression pressure waves, advecting upstream, were identified. DMD indicated that they resulted from vortex shedding. According to simulation and DMD results, the origin of the shock waves appears to be the condensation of compression pressure waves due to the vortex shedding mode and expansion in the supersonic region due to the buffet mode. The formation of the shock waves seems to be subordinated to the vortex shedding mode and the buffet mode. A feedback model for low-Reynolds-number buffet inherent in separated shear layers, which does not require supersonic regions or shock waves, was proposed. Supersonic regions highly condensed the compression pressure waves, inducing a larger separation region and amplifying the oscillation. The role of supersonic regions in determining oscillation amplitudes was evidenced, although supersonic regions are not essential to the mechanism of low-Reynolds-number buffet.

Study of wandering motion effects on the tornado-borne debris using proposed simplified numerical models

Tornado-borne debris is one of the most important factors in tornado-induced building damage. However, the aerodynamic characteristics of tornado-borne debris and the effects of turbulence in tornadoes remain unclear due to the limitation of the observation and experiment. Wandering motion is an apparent periodic flow structure in the tornado-core region that enhances the non-Gaussian characteristic of tornado flow. To clarify its effects on debris distribution, three simplified models are examined in the present study, that is, (a) the direct offset model, (b) the mean velocity correction model, and (c) the fluctuation correction model. It was found that wandering motion reduced the concentration in the tornado-core region, and both mean velocity and fluctuation of the wandering model affect debris distribution in the tornado-core region. Moreover, the period of wandering motion has little effect on debris distribution, but the eccentrical radius of wandering motion affects significantly. Among the three models, the fluctuation correction model provides the best prediction in the tornado-core region, and prediction accuracy improves by over 36% for predicting small debris in touch-down tornadoes.

Effects of Background Periodic Flow on MHD Fast-wave Propagation to a Coronal Loop

This paper investigates the propagation of MHD fast waves into a cylindrical coronal loop through an inhomogeneous stationary flow region. The background flow is assumed to have a small, spatially periodic structure in addition to a constant speed. This study focuses on the absorption of the wave energy in Alfvén resonance, comparing it with the constant flow case. A new flow (absorption) regime is induced by the periodic flow structure, which enhances the absorption for the antiparallel flow and inverse absorption (overreflection) for the parallel flow with respect to the axial wavevector, depending on the transitional layer and flow profiles. A giant overreflection and anomalous absorption behavior arises for some flow configurations. In the other flow regimes, its effect on the absorption is shown to be weak.

Development of a Large-Scale High-Speed Linear Cascade Rig for Turbine Blade Tip Heat Transfer Study

The high-speed Over-Tip-Leakage (OTL) flow has a significant impact on the aerodynamic performance of the High-Pressure Turbine (HPT) passage and generates high thermal load for the blade tip. Different tip sealing and cooling design strategies are applied to reduce the OTL loss and help turbine survive in a high temperature environment. High-speed linear cascade experimental rigs play an important role in understanding the flow physics and evaluating their performance. Multiple blades and passages are often required to maintain a reasonable flow periodicity. To match the engine representative Reynolds number and Mach number, a high-speed multi-passage cascade design inevitably demands more compressed air supply. A very large amount of heating power is also required if the engine condition wall-to-gas temperature ratio needs to be matched. In this study, a simplified 2-Passage linear cascade rig for high-speed tip heat transfer research was developed. Both the design method and the rig performance are presented. Different from existing design method to match two-dimensional blade loading, this study shows there are other design flexibilities, such as assist blade tip gap, tailboard adjustment, and profiling adjustment, to match the periodic three-dimensional OTL flow structures. The design method was validated by experimental effort. High resolution tip heat transfer coefficient distribution at stationary and rotating conditions (Rotating Mach number = 0.35) are reported. The enlarged test model can offer much more improved resolution of optical measurement near the tip region.

Influence of flow structures and combustion on cross-scale turbulent kinetic energy transfer in premixed swirl flames.

This paper experimentally analyzes the simultaneous influence of combustion and large-scale vortex structures on the transfer of kinetic energy across scales around the laminar flame thickness δL0 in a turbulent premixed swirl flame and a non-reacting swirl flow. High-resolution tomographic particle image velocimetry and formaldehyde planar laser induced fluorescence measurements are used to obtain 3D velocity fields and estimates of the progress variable and density fields. The kinetic energy transfer across a filter scale of Δ=1.5δL0 was then quantified using physical space analysis. Coherent flow structures were identified using the swirling strength and proper orthogonal decomposition was used to identify the dominant periodic flow structure. While non-reacting regions of the flow show mean down-scale energy transfer (forward-scatter), mean back-scatter is observed internal to the flame. Importantly, the back-scatter magnitude in the flame increases in regions undergoing flame/vortex interaction. That is, the mean back-scatter magnitude at locations simultaneously inside the flame and a large-scale coherent vortex is higher than regions in the flame and not in a vortex, with the mean back-scatter magnitude increasing with the swirling strength. This increased back-scatter may be due to locally higher heat release rates in locations of flame/vortex interaction. Overall, the results demonstrate the importance of kinetic energy back-scatter at scales around the flame thickness and the complicated relationship between back-scatter and local flame/flow structure; these results should be considered when modeling turbulence in flames.

Investigation of flow unsteadiness in a highly-loaded compressor cascade using a dynamic mode decomposition method

Unsteady flow in the hub endwall region has long been a hot topic in the turbomachinery community. However important it is to the performance of the whole engine, the coherent unsteady flow phenomena are still not well understood. In this paper, the complex flow field in the hub endwall of a cantilevered compressor cascade has been investigated through numerical approach. The predicted results were validated by experimental data. To highlight the dominant flow structures among irregular and chaotic motions of various vortices, a Dynamic Mode Decomposition (DMD) method was utilized. The results show that there exist three dominant periodic flow structures: the oscillation of the leakage vortex, a circumferential migration of a Breakdown Induced Vortex (BIV) and the fluctuation of the passage vortex. These three coherent structures all together form a self-sustained closed loop which accounts for the flow unsteadiness of the studied cascade. During this process, the BIV plays a key role in inducing the flow unsteadiness. Only if the BIV is strong enough to affect the passage vortex, the flow unsteadiness occurs. This study expands current knowledge base of flow unsteadiness in a compressor environment, and shows the efficacy of the DMD method for revealing the origin of flow unsteadiness.

Periodic flow structures in a turbofan fan stage in windmilling

In a fan stage under windmilling conditions, the stator operates under negative incidence, leading to flow separation, which may present an unsteady behaviour due to rotor/stator interactions. An experimental study of the unsteady flow through the fan stage of a bypass turbofan in windmilling is proposed, using hot-wire anemometry. Windmilling conditions are reproduced in a ground engine test bed by blowing a variable mass flow through a bypass turbofan in ambient conditions. Time-averaged profiles of flow coefficient are independent of the mass flow, demonstrating the similarity of velocity triangle. Turbulence intensity profiles reveal that the high levels of turbulence production due to local shear are also independent of the inlet flow. A spectral analysis confirms that the flow is dominated by the blade passing frequency, and that the separated regions downstream of the stator amplify the fluctuations locked to the BPF without adding any new frequency. Phase-locked averaging is used to capture the periodic wakes of the rotor blades at the rotor/stator interface. A spanwise behaviour typical of flows through windmilling fans is evidenced. Through the inner sections of the fan, rotor wakes are thin and weakly turbulent, and the turbulence level remains constant through the stage. The rotor wakes thicken and become more turbulent towards the fan tip, where flow separation occurs. Downstream of the stator, maximum levels of turbulence intensity are measured in the separated flow. Large periodical zones of low velocity and high turbulence intensity are observed in the outer parts of the separated stator wake, confirming the pulsating motion of the stator flow separation, locked at the blade passing frequency. Space-time diagrams show that the flow is chorochronic, and a 2 D non-linear harmonic simulation is able to capture the main interaction modes, however, the stator incidence distribution could be affected by 3 D effects.

Passive control on wake structure of a wall-mounted cube by using a horizontal hole

To control the wake flow around a wall-mounted cube, a passive control method is proposed by drilling a horizontal hole from the front surface to rear surfaces of the cube. The controlled and uncontrolled wakes are measured by using particle image velocimetry (PIV). The cube model has a side length of D = 70 mm, and the diameter of horizontal control hole (HCH) is d = 10 mm. Reynolds number based on side length of cube is ReD=11,550. Furthermore, to study the positional effect of the HCH, three kinds of holes with different heights h are used. The time-averaged statistical characteristics show that the jet generated from the HCH reduces the scale of the arch-type vortex, turbulence intensity, Reynolds shear stress and turbulent kinetic energy in the wake, and enhances the recovery of streamwise velocity. Among three HCHs, the HCH of h = 35 mm has the optimal control effectiveness. The analysis of vortex dynamics indicates that the HCH jet effectively suppresses the periodic flow structures and accelerates the wake vortex shedding. Meanwhile, the HCH jet exhibits different effect on the spanwise vortex at its different heights. The POD analysis implies that under the influence of the HCH, the alternant spanwise vortices are suppressed, but symmetric vortices are relatively enhanced.

Flow field induced by a dielectric barrier discharge plasma actuator analyzed with bi–orthogonal decomposition

A typical dielectric barrier discharge (DBD) plasma actuator generates complex periodic flow structures in burst mode. Efficient use of these actuators depends on clear understanding of the relationship between the operational parameters of the actuator and flow structure. The present study reports the temporal and spatial evolution of these flow structures by utilizing the bi-orthogonal decomposition (BOD) technique. The flow induced by the actuator is captured using a time resolved particle image velocimetry (2D–2C–TR–PIV) system. The BOD of flow field is carried out using instantaneous velocity field data. The DBD plasma actuator is operated at different combinations of duty cycle, α (50% ≤ α ≤ 90%), and burst frequency, fb (10 Hz ≤ fb ≤ 90 Hz). The modal energy content is used to characterize the flow field as a function of operating variables, i.e., α and fb of the actuation signal. The mean mode of the decomposition successfully approximates the time averaged behavior of the induced flow field. The mean mode energy level increases with the increase in both α and fb with a more pronounced effect observed as a function of fb. The coherent structures are located close to the near wall at high burst frequency. The non-dimensional entropy decreases with the increase in both α and fb with a more pronounced effect of fb than that of α. The decrease in entropy value indicates that space–time complexities are reduced at higher burst frequency. The topos of higher order modes reveal the presence of coherent structures that grow in time and convect along the wall like a train of vortices. The chronos of mode 2 and mode 3 is locked in with respect to the burst frequency. However, the chronos of mode 4 and mode 5 shows frequency doubling at lower burst frequency actuation and frequency halving at higher burst frequency actuation. The entropy value or space–time complexity of flow structures generated by DBD plasma actuator is related to the nonlinear vortex interaction mechanism, i.e., period doubling and period halving of chronos.

Experimental study of the self-excited resonance effect on the dynamic lift and flow structure around inline cylinders

The excitation of acoustic resonance by air flow around a row of inline cylinders is investigated experimentally using phase-locked particle image velocimetry and direct measurements of the dynamic lift forces. The experiments are performed during self-excited acoustic resonance for arrangements of two, three, four, and five inline cylinders with a spacing ratio L∕D=2.0, where L is the center-to-center distance between the cylinders and D is the cylinder diameter, to clarify the effect of increasing the number of cylinders on the resonance mechanism and the characteristics of the periodic flow cycle. The PIV measurements show that increasing the number of cylinders results in weaker and distorted periodic flow structures around the most downstream cylinders. However, the phase and strength of the flow structures within the most upstream gap are minimally affected. Intrinsic flow features, such as gap flow that is observed in some cases, lead to the disruption of the shear layer impingement on downstream cylinders. Moreover, direct measurements of the dynamic lift forces on all cylinders simultaneously identified the upstream gap as the location where most of the energy exchange between the flow and the acoustic fields occurs. Decomposition of the dynamic lift force with respect to the acoustic pressure revealed that the distorted flow structures were accompanied by weaker, sometimes negative, contribution to the acoustic resonance excitation. The findings presented in this article explain the observation that for inline configurations of cylinders, as well as inline tube bundles, increasing the number of cylinders in the flow direction does not result in stronger resonance, and demonstrate that countermeasures for resonance excitation should target the most upstream rows of cylinders.

Three-Dimensional CFD Simulation and Experimental Assessment of the Performance of a H-Shape Vertical-Axis Wind Turbine at Design and Off-Design Conditions

The paper presents the results of a computational study on the aerodynamics and the performance of a small-scale Vertical-Axis Wind Turbine (VAWT) for distributed micro-generation. The complexity of VAWT aerodynamics, which are inherently unsteady and three-dimensional, makes high-fidelity flow models extremely demanding in terms of computational cost, limiting the analysis to mainly 2D or 2.5D Computational Fluid-Dynamics (CFD) approaches. This paper discusses how a proper setting of the computational model opens the way for carrying out fully 3D unsteady CFD simulations of a VAWT. Key aspects of the flow model and of the numerical solution are discussed, in view of limiting the computational cost while maintaining the reliability of the predictions. A set of operating conditions is considered, in terms of tip-speed-ratio (TSR), covering both peak efficiency condition as well as off-design operation. The fidelity of the numerical predictions is assessed via a systematic comparison with the experimental benchmark data available for this turbine, consisting of both performance and wake measurements carried out in the large-scale wind tunnel of the Politecnico di Milano. The analysis of the flow field on the equatorial plane allows highlighting its time-dependent evolution, with the aim of identifying both the periodic flow structures and the onset of dynamic stall. The full three-dimensional character of the computations allows investigating the aerodynamics of the struts and the evolution of the trailing vorticity at the tip of the blades, eventually resulting in periodic large-scale vortices.

Large-scale unsteady flow structures in the leakage flow of a low-speed axial fan with rotating shroud

The leakage flow in low-speed fans provided with rotating shroud has been surveyed by means of two-dimensional LDV and PIV. Measurements have been taken upstream of the rotor and in the gap area at different operating conditions. The flow contains both periodic and non-periodic large-scale flow structures, which are responsible for the noise generation. The former structures have been extracted by means of the ensemble average technique, and, in order to study the latter ones, the non-periodic part of the instantaneous velocity field has been analysed by means of the proper orthogonal decomposition technique.In the present fan, the leakage flow consists in a jet which leaves the gap in the diagonal direction. Depending on rotational speed and blade loading, the jet may form a recirculation bubble anchored to the rotor ring or it may turn radially, forming a large recirculating flow zone.Large-scale, periodic vortical structures leaving the gap are convected by the leakage flow, either wrapping around the recirculation bubble or flowing radially outwards with a spiral pattern. In the former case, such structures, though weakened, likely reach the rotor, while, in the latter case, they have not been found at the rotor entrance. Hence, a different degree of mixing takes place which is consistent with the extension of the blade part where the leakage flow is reingested. This is consistent with the differences in acoustic measurements.The POD technique has shown that, at high loading, large-scale non-periodic vortical structures convect towards the rotor inlet. They are weaker though comparable to the periodic ones released from the gap; at low rotational speed they are stronger than at high rotational speed. Non-periodic flow structures may be present in the flow leaving the gap also, but it has not been possible to identify them clearly. Likely, non-periodic flow structures are originated by the ripping of the periodic ones.A possible explanation for the noise generation in fans with rotating shroud is that the periodic pressure field anchored to the rotor results in the release of periodic flow structures from the gap. Then, such structures may reach the rotor or rip during convection, likely resulting in non-periodic ones which are ingested by the rotor. In both cases, the interaction with the rotor blades likely results in noise generation. The degree of prerotation of the leakage flow is consistent with the frequency shift of the narrowband humps observed in the sound pressure spectra.

MODELLING OF SHEAR AND SHEARLESS FLOW WITH PERIODIC VELOCITY NONSTATIONARITY

The results of experimental modeling of shear and shearless flow with periodic velocity nonstationarity, organized using a generator of periodic wakes such as the "squirrel" cage, are presented. The purpose of this paper is to compare the structure of the flow behind the "squirrel" cages, as well as the analysis of the characteristics of the transition boundary layer for two different ways of locating the working surfaces: in the zone of the shearless core and shear periphery zone.
 The physical modelling of turbulized flow with velocity periodic nonstationarity is carried out in two experimental installations. It is shown that behind rotating “squirrel” cages there are two regions in the distributions of mean time velocities: the shearless flow core located in the center of “squirrel” cage and peripheral shear part. The aim of this paper is to compare the flow structure behind “squirrel” cages as well as to analyze the features of transient boundary layer for two different installations of working surfaces. The latter were flat plates installed on the different distances from the center of the “squirrel” cages: in the shearless flow core and in shear zone. Total longitudinal fluctuations are characterized by peaks reason of which is intersections of wakes. Behind the “squirrel” cages the levels of fluctuations decrease along the plates at x~100-600 mm from ~12 to 4,5% (II) and from ~6 to 3,5% (I). Despite the development of boundary layer happens under different external conditions (in uniform (I) and shear (II) flows), wake-induced transition takes place in both installations. Transformation of velocity profiles from pseudolaminar to turbulent is similar to one taking place in bypass transition. Distributions of total longitudinal fluctuations across the boundary layer differ by quantity of peaks and their intensity. Today the physical modeling is one of the most perspective methods for studying transport processes under complex conditions. That is why the experimental investigations of periodic external flow structure are necessary for the further optimization of different equipment and their reliability enhancement.

Large eddy simulation of a buoyancy-aided flow in a non-uniform channel – Buoyancy effects on large flow structures

It has been a long time since the ‘abnormal’ turbulent intensity distribution and high inter-sub-channel mixing rates were observed in the vicinity of the narrow gaps formed by the fuel rods in nuclear reactors. The extraordinary flow behaviour was first described as periodic flow structures by Hooper and Rehme (1984). Since then, the existences of large flow structures were demonstrated by many researchers in various non-uniform flow channels. It has been proved by many authors that the Strouhal number of the flow structure in the isothermal flow is dependent on the size of the narrow gap, not the Reynolds number once it is sufficiently large. This paper reports a numerical investigation on the effect of buoyancy on the large flow structures. A buoyancy-aided flow in a tightly-packed rod-bundle-like channel is modelled using large eddy simulation (LES) together with the Boussinesq approximation. The behaviour of the large flow structures in the gaps of the flow passage are studied using instantaneous flow fields, spectrum analysis and correlation analysis. It is found that the non-uniform buoyancy force in the cross section of the flow channel may greatly redistribute the velocity field once the overall buoyancy force is sufficiently strong, and consequently modify the large flow structures. The temporal and axial spatial scales of the large flow structures are influenced by buoyancy in a way similar to that turbulence is influenced. These scales reduce when the flow is laminarised, but start increasing in the turbulence regeneration region. The spanwise scale of the flow structures in the narrow gap remains more or less the same when the buoyancy parameter is smaller than a critical value, but otherwise it reduces visibly. Furthermore, the mixing factors between the channels due to the large flow structures in the narrow gap are, generally speaking, reduced by buoyancy.

Analysis of topology transitions of swirl flames interacting with the combustor side wall

Topology transitions of swirl-stabilized flames in a cylindrical combustion chamber are analyzed for lean CH4/H2/air combustible mixtures. It is demonstrated that when the tip of a swirl flame is quenched at the combustor side wall, V to M flame topology transitions are triggered by flashback of the flame front along this wall. An analysis of the power spectral densities of the chemiluminescence luminosity reveals that these topological transitions are not consecutive to the appearance or disappearance of any axisymmetric or helical periodic flow structures interacting with the flame. It is shown that the appropriate space to analyze the stabilization regimes of these swirling flames corresponds to a diagram based on Karlovitz and Lewis numbers. Transitions from V to M flame shapes are found for a critical Karlovitz number below which the flame takes an M shape. The value of this critical Karlovitz number decreases when the Lewis number of the combustible mixture increases. The same type of law is found for M to V shape flame transitions, but this boundary does not coincide with the V to M shape transition boundary. In between these boundaries, the flame takes intermittently V or M shapes randomly. It is then shown that the critical Karlovitz number corresponding to the transition of V to M shape may be expressed by the product of a velocity gradient g calculated with the bulk injection velocity U and a characteristic scale of the outer recirculation zone, the laminar flame thickness α/SL, and the inverse of a vortex induced flame displacement speed STθ. This displacement speed is related to the maximal azimuthal velocity of the swirling jet at the injector outlet and to the laminar burning velocity. Using this definition of the Karlovitz number, V to M flame topology transitions collapse roughly on the same curve for all flow conditions explored when the swirl number (0.3 ≤ S ≤ 0.68), injection bulk velocity (7 ≤ U ≤ 25 m · s−1), equivalence ratio (0.5 ≤ ϕ ≤ 0.9), and hydrogen enrichment in the fuel blend (0≤XH2fuel≤0.37) are modified for rod stabilized and aerodynamically stabilized swirl flames. Results presented in this study might be used to ease predictions of the shape taken by swirl flames interacting with the chamber side wall when the flow operating conditions are modified.

Computational fluid dynamics simulation of hydrodynamics in USP apparatus 3-the influence of dip rate.

This study investigated the influence of dip rate on USP Apparatus 3 hydrodynamics in the presence of a solid dosage form (e.g. tablet) using Computational Fluid Dynamics (CFD) simulations. The primary variables of interest were the liquid phase velocity in the computational domain and wall shear stresses on the tablet surfaces. Geometry building and model setup were based on a number of simplifying assumptions. Computational grid-independent solutions were achieved for dip rates ranging from 5 to 10 dips per minute (dpm). For all cases studied, the hydrodynamics exhibited a periodicity dictated by the dip rate. Cycle-to-cycle variations were found to be negligible. Higher velocities were predicted in the wake of the tablet and they peaked at midway positions both during the up- and downstrokes of the cylinder. Three sub-regions of velocity were identified inside the reciprocating cylinder. Results also showed localized vortices/recirculations specific to the up- and downstroke, in addition to local stagnation zones. The wall shear stresses and velocity magnitudes scaled proportionately with increasing dip rates while exhibiting qualitatively similar behavior in their spatial and temporal distributions. Based on the predictions of the 2D axisymmetric CFD model, the hydrodynamics in USP Apparatus 3 is characterized by complex and periodic flow structures.

Unsteady flow motions of an oscillating jet in crossflow

The temporal and spatial variations of the flow behavior and velocity field of an oscillating jet in a crossflow were experimentally investigated. A pulsed jet issuing into a crossflow caused the deflected jet to oscillate. The instantaneous oscillating flow patterns in the symmetry plane at several specific phase angles were identified using smoke flow visualization method. The instantaneous flow velocities were measured using the high-speed particle image velocimetry (PIV) and presented as phase-averaged velocity vectors and streamlines. Owing to the variation of jet exit velocity, the near tube-tip jet column flapped up and down within one cycle of jet pulsation and induced a periodic wavy flow structure in the downstream area. By identifying the variations of near-wake velocity vectors and streamline patterns within one excitation cycle, four characteristic flow modes (downwash, crossflow-dominated, jet-dominated, and transitional) were found at different excitation phase angles. Around the mid-oscillation cycle, the wavy flow structure was characterized by two adjacent vorticity concentrated areas of opposite signs in the phase-averaged vorticity contour. The turbulence properties of the excited elevated transverse jet at large jet-to-crossflow momentum flux ratio were larger than those at low jet-to-crossflow momentum flux ratio.

Computational Investigation on Fully Developed Periodic Laminar Flow Structure in Baffled Circular Tube with Various BR

This paper presents a 3D numerical analysis of fully developed periodic laminar flow in a circular tube fitted with 45° inclined baffles with inline arrangement. The computations are based on a finite volume method, and the SIMPLE algorithm has been implemented. The characteristics of fluid flow are presented for Reynolds number, Re = 100–1000, based on the hydraulic diameter (D) of the tube. The angled baffles were repeatedly inserted at the middle of the test tube with inline arrangement to generate vortex flows over the tested tube. Effects of different Reynolds numbers and blockage ratios (b/D, BR) with a single pitch ratio of 1 on flow structure in the tested tube were emphasized. The flows in baffled tube show periodic flow atx/D≈2-3, and become a fully developed periodic flow profiles atx/D≈6-7, depending on Re, BR and transverse plane positions. The computational results reveal that the higher of BR and closer position of turbulators, the faster of fully developed periodic flow profiles.

Measurement and decomposition of periodic flow structures downstream of a test turbine

This paper shows the experimental methodology which has been employed for the simultaneous aerodynamic and aeroacoustic analysis of a test turbine by means of a fast-response aerodynamic pressure probe. Two different phase references are used to resolve the fluctuations in time of the velocity and the pressure of phenomena occurring at different and unrelated periodicity. The phase-averaged fields are further post-processed by decomposing them into spatial-temporal Fourier coefficients. The deterministic unsteadiness is then simply represented by a limited number of Fourier coefficients, the physical meaning of which is discussed in details. In particular, this approach has been used to distinguish between propagating and non-propagating noise sources.

Characterization of periodic flow structure in a small-scale feedback fluidic oscillator under low-Reynolds-number water flow

Numerical simulation and experimentation using high time-resolved particle imaging velocimetry were conducted in order to clarify the flow characteristics in a small-scale feedback fluidic oscillator operated under low-Reynolds-number water flow. Flow oscillations were triggered by the Coanda effect and a periodically oscillating jet. Based on spectral analysis of the velocity fluctuations inside the oscillator body, the Strouhal number was not constant and the frequency was not linear with respect to the flow rate for the Reynolds number range from 238 to 714. The flow rate fluctuations of the feedback flows through the two feedback channels were determined quantitatively. By comparing the simulation and experimental results, the mechanism of the small-scale feedback fluidic oscillator was clarified by considering the time for a jet to oscillate from the center of the control region to either of the attachment walls and the time for a jet to oscillate from either of the attachment walls to the center of the control region separately.