Time-resolved Particle Image Velocimetry Research Articles

Insights into the fluid dynamics of the Bach impeller

The Bach impeller is a novel impeller that was purposefully designed to reduce power consumption as well as shear stress levels in stirred bioreactors. Contrary to conventional impellers where stirring is achieved by the pushing action of the blades, the Bach impeller relies on a central spiral element which induces an up-pumping central vortex and discharges the flow in the tangential direction through vanes. This feature is new and relevant to the field of biochemical engineering where cell culture and microbial fermentation often make use of shear-sensitive, living organisms to produce biopharmaceuticals and medicines. This article offers a thorough characterisation of the fluid mechanics of this impeller by means of 2D phased- and time-resolved particle image velocimetry (PIV) experiments. Key flow dynamics parameters including velocity magnitudes, kinetic energy, and shear stresses were estimated for different combinations of impeller size, clearance, and speed. The standard impeller size, D/T=0.52, performed most promisingly in combination with an off-bottom clearance of C=0.6T, as the elevated impeller height allowed to pull high momentum fluid to the upper part of the tank and therefore improved overall transport phenomena in the entire reactor volume. For this impeller diameter-clearance combination peak ensemble-averaged shear stresses at the vessel base were lower due to the impeller greater distance from the bottom. A distinctive feature of the Bach impeller was its "smooth operation" mode, where velocity fluctuations due to the vane passages are nearly negligible (~5% of ensemble-averaged kinetic energy). These aspects make of the Bach impeller an appealing option in the context of stem cell bioprocess applications.

Debris-induced consequences on turbulence and vorticity in solar photovoltaic module-generated array wakes

Particle-laden flows in solar photovoltaic (PV) systems are inevitable, where wind-swept debris in open environments are carried by high winds and turbulence, coating panel surfaces or damaging structures. Particle deposition, or soiling, is a well-known issue for large-scale plants which rely on uninhibited solar rays for optimal production. But understanding the mechanisms leading to soiling requires a physical and fluid dynamics-centered focus, since turbulence dominates PV panel wakes and is also known to alter particle concentration and trajectories. This study presents an experimental campaign toward consequences of particle-laden flow between two model PV panels using time-resolved particle image velocimetry. The model array was subjected to varied particle volume fractions, including a tracer particle case and a water droplet case. Characterization of mean velocity, turbulence statistics, and mean kinetic energy within the single phase and, separately, particle phase flows showed modified features due to particle inertia. Images captured at a frequency of 1 kHz in the near wake of the upstream panel allow for a first experimental look at vorticity and convective velocity of vortex structures for single-phase and particle-phase flows which are crucial to debris transport and soiling in PV environments.

Regulating turbulent separation by surface microstructures on a blunt plate

Microstructured surfaces can induce secondary flow and regulate flow structures of the turbulent separation flow. However, the mechanism governing the relationship between the microstructure size and the characteristic flow size remains unclear. In this study, the separated flow over a blunt flat plate with surface microstructures is studied using time-resolved particle image velocimetry experiments and implicit large-eddy simulations for the plate-thickness-based Reynolds number from 5.08×103 to 1.31×104. The ratio of the height of microstructures to the plate thickness (h/d) ranges from 0.01 to 0.1. Combining experimental and numerical results, the relationships between the separation bubble size and the microstructure size under different Reynolds numbers exhibit similarity when normalized by the separation bubble size of the smooth plate. The dimensionless separation bubble size decreases when the microstructure height increases and large microstructures (h/d = 0.1) exhibit good performance on reducing the flow separation. Near the leading edge, the distortion of two-dimensional vortices and the generation of three-dimensional hairpin vortices are promoted by the first several rows of large microstructures. Additionally, in the main separation region, secondary positive spanwise vortices emerge from large microstructures. Subsequently, the secondary vortices lift up and evolve into streamwise vortices. The characteristic scale of secondary vortices is represented by a significant peak in the spectra of spanwise wavenumbers, which is of the same magnitude as the height of large microstructures. Furthermore, increasing the microstructure height weakens the streamwise correlation of the flow, and the characteristic scale of the correlation is comparable to the height of large microstructures.

Insights into turbulent flow structure and energy dissipation in centrifugal pumps: A study utilizing time-resolved particle image velocimetry and proper orthogonal decomposition

The purpose of the study is to investigate the flow structure and energy dissipation mechanism within centrifugal pump impellers. The velocity fields at the mid-sections of the flow passages of 4- and 5-blade impellers were measured under various flow rates using the Time-resolved Particle Image Velocimetry (TR-PIV) technique; the proper orthogonal decomposition (POD) analysis was performed to extract flow structures with different scales; the dissipation rate was estimated using the large-eddy PIV method; the correlation between turbulent kinetic energy (TKE) and dissipation rate was measured using the cross-correlation analysis. The findings are as follows: impeller with more uniform and stable flow, the scale and energy contribution of large-scale structures in lower order modes are smaller, leading to higher impeller efficiency; large-scale structures identified in the low-order modes correspond to locations of high TKE and dissipation areas; the high velocity gradients within the impeller passage and the interactions between large-scale flow structures encourage the breakup of large-scale turbulent structures into dissipative-scale structures; the correlation between TKE and dissipation rate reflects the extent to which large-scale flow structures within the impeller passage break up into dissipative scales.

Experimental study of turbulent flow in lower plenum with flow skirt of reactor pressure vessel of pressurized water reactor

The turbulent flow in lower plenum of reactor pressure vessel (RPV) of pressurized water reactor (PWR) is an important phenomenon for design of flow mixing device. A high-precision matched index of refraction (MIR) technique is requisite for time-resolved particle image velocimetry (TR-PIV) measurements in RPV with complicated internals. In this study, turbulent flow in a scaled-down RPV model was measured by TR-PIV under different Reynolds numbers (Re) and asymmetrical flow rate with aid of the high-precision MIR technique of sodium iodide (NaI) solution and acrylic. The time-averaged flow structures include flow separation at the corner of lower support plate, fluid bifurcation around the lower edge of flow skirt, flow turning upward below the vortex suppression plate, horizontal jets at holes of flow skirt, vertical jets at holes of vortex suppression plate, improving turbulent mixing and Reynolds stresses in the lower plenum. The coherent structures were studied by proper orthogonal decomposition (POD) analysis. The flow skirt and vortex suppression plate break down large vortices through them. With Re increasing, Reynolds stresses decrease fast, and the energy fractions of low-order modes transfer into high-order modes, because turbulent vortices break up. In the case of asymmetrical flow rate, the time-averaged velocity and Reynolds stress decrease slightly, and the cumulative energy fraction decreases at flow skirt and vortex suppression plate.

Global dynamics and topology of two-phase mixing layer flow through simultaneous gas and liquid velocity measurements

This study reports the first time-resolved particle image velocimetry characterization of a planar two-phase mixing layer flow, whose velocity field is measured simultaneously in gas and liquid streams. Two parallel air and water flows meet downstream of a splitter plate, giving rise to an initially spanwise invariant configuration. The aim is to elucidate further the mechanisms leading to the flow breakup in gas-assisted atomization. The complete experimental characterization of the velocity field represents a database that could be used in data-driven reduced-order models to investigate the global behaviour of the flow system. After the analysis of a selected reference case, a parametric study of the flow behaviour is performed by varying the liquid ( $Re_l$ ) and gas ( $Re_g$ ) Reynolds numbers, and as a consequence also the gas-to-liquid dynamic pressure ratio ( $M$ ), shedding light on both time-averaged (mean) and unsteady velocity fields. In the reference case, it is shown that the mean flow exhibits a wake region just downstream of the splitter plate, followed by the development of a mixing layer. By increasing both $Re_l$ and $Re_g$ , the streamwise extent of the wake decreases and eventually vanishes, the flow resulting in a pure mixing layer regime. The spectral analysis of the normal-to-flow velocity fluctuations outlines different flow regimes by variation of the governing parameters, giving more insights into the global characteristics of the flow field. As a major result, it is found that at high $Re_g$ and $M$ values, the velocity fluctuations are characterized by low-frequency temporal oscillations synchronized in several locations within the flow field, which suggest the presence of a global mode of instability. The proper orthogonal decomposition of velocity fluctuations, performed in both gas and liquid phases, reveals finally that the synchronized oscillations are associated with a low-frequency dominant flapping mode of the gas–liquid interface. Higher-order modes correspond to interfacial wave structures travelling with the so-called Dimotakis velocity. For lower gas Reynolds numbers, the leading modes describe higher frequency fingers shedding at the interface.

Three dimensional analysis of the exhalation flow in the proximity of the mouth

The human exhalation flow is characterized in this work from the three-dimensional velocimetry results obtained by using the stereo particle image velocimetry (SPIV) measurement technique on the flow emitted from a realistic airway model. For this purpose, the transient exhalation flow through the mouth of a person performing two different breaths corresponding to two metabolic rates, standing relaxed (SR) and walking active (WA), is emulated and studied. To reproduce the flow realistically, a detailed three-dimensional model obtained from computed tomography measurements on real subjects is used. To cope with the variability of the experimental data, a subsequent analysis of the results is performed using the TR-PIV (time resolved particle image velocimetry) technique. Exhalation produces a transient jet that becomes a puff when flow emission ends. Three-dimensional vector fields of the jet velocity are obtained in five equally spaced transverse planes up to a distance of ▪ from the mouth at equally spaced time instants ▪ which will be referred to as phases (φ), from the beginning to the end of exhalation. The time evolution during exhalation of the jet area of influence, the velocity field and the jet air entrainment have been characterized for each of the jet cross sections. The importance of the use of realistic airway models for the study of this type of flow and the influence of the metabolic rate on its development are also analyzed. The results obtained contribute to the characterization of the human exhalation as a pathway of the transmission of pathogens such as SARS-CoV-2 virus.

Amplitude modulation in turbulent boundary layer over anisotropic porous wall

In this study, the amplitude modulation effect in a turbulent boundary layer over anisotropic porous walls is investigated experimentally at the Reynolds number based on friction velocity of Reτ = 236–319. The streamwise and wall-normal velocity fields were measured using time-resolved particle image velocimetry. To clarify the coherent structures related to the amplitude modulation over the porous wall with skin friction reduction effect, the large-scale structures are extracted from the low-pass filtered streamwise velocity fluctuations. The small-scale events related to high fluctuation energy are detected by the variable-interval space-averaging technique. Over the porous wall, the induced upwash and downwash motion leads to a notable suppression of large-scale structures. The small-scale motions in the near-wall region are mainly caused by the ejection events, while the sweep events are significantly suppressed. The amplitude modulation effects indicate that the positive and negative large-scale velocity streaks produce suppression and enhancement effects to the near-wall small-scale turbulence, respectively, which is contrary to the conventional phenomenon over the smooth wall case. The interaction between outer large-scale and inner small-scale structures is significantly weakened by the porous wall, contributing to the overall skin friction reduction.

Vortex ring breakdown dominating the entrainment of a synthetic jet

The understanding of the entrainment mechanism of synthetic jets can help optimise the synthetic jet actuators in engineering applications. It is generally believed that vortex rings or strong velocity fluctuations in the near field of the synthetic jet are responsible for its enhanced entrainment. However, in recent years, it has been found that the enhanced entrainment of the synthetic jet may be caused by the instability or the vortex ring breakdown in the transition region. To shed new light on this issue, synthetic jets with different Reynolds numbers and dimensionless stroke lengths are investigated with time-resolved two-dimensional particle image velocimetry. Based on the analyses of velocity triple-decomposition, Fourier mode decomposition and phase-averaged $\lambda _{ci}D/U_0$ field, the streamwise positions of the vortex ring breakdown are determined for the synthetic jets, and the entrainment coefficient can be divided into three components, i.e. the coherent turbulent kinetic energy production, the random turbulent kinetic energy production and the shape of the velocity profile. It is found that the entrainment coefficient is dominated by the component related to the random turbulent kinetic energy production, and reaches its peak value at the position of vortex ring breakdown. The results obtained in different cases show a strong correlation between vortex ring breakdown and entrainment enhancement. From the perspective of instantaneous snapshot, the mechanism of vortex ring breakdown enhanced entrainment is revealed, that is, vortex ring breakdown enhanced the small-scale vortex near the turbulent/non-turbulent interface, resulting in an increase of enstrophy production, and thus enhanced local entrainment.

Experimental analysis of trailing edge hydroelastic coupling on a hydrofoil

This paper explores the conditions for hydroelastic trailing edge vibrations generating tonal noise on a NACA0015 aluminium hydrofoil clamped in a hydrodynamic tunnel. Tests were performed for Reynolds numbers Re, ranging from 2×105 up to 12×105 and various angles of attack α, from 0 up to 10°. A laser vibrometer was used to characterize the hydrofoil vibratory response. Time Resolved Particle Image Velocimetry (TR-PIV) was used to scrutinize the origin of the hydrodynamic excitation mechanism. Hydroelastic trailing edge vibrations of significant amplitude were observed at moderate angles of attack 4≤α≤8.5°, for Reynolds number such that the pressure side boundary layer transition was located close to the trailing edge, with a frequency signature allowing a lock-in with the hydrofoil trailing edge structural mode. Two passive solutions were tested to mitigate this hydroelastic flow-induced vibration: a truncated hydrofoil and a triggered one. The truncated configuration slightly impacts the vibration while triggering the pressure side boundary layer transition ahead of the trailing edge eliminates the trailing edge vibrations with negligible impact on the hydrofoil hydrodynamics performances.

Effects of Blockage Ratio on the Spatiotemporal Dynamics of Turbulent Flow Separation Around a Square Cylinder at Moderate Reynolds Numbers

Abstract The effects of blockage ratio (BR) on turbulent flows around square cylinders at moderate Reynolds numbers are investigated using a time-resolved particle image velocimetry (TR-PIV). The blockage ratios range from 2.5% to 15%, and the Reynolds numbers based on the freestream velocity and cylinder thickness are 3000, 7500, and 15,000. The flow dynamics are examined in terms of the mean flow, Reynolds stresses, frequency spectra, reverse flow area, and proper orthogonal decomposition (POD). The results show that the wake characteristics are nearly independent of the Reynolds number and blockage ratio. Spectral analyses of the velocity fluctuations demonstrate that the von Kármán (VK) shedding frequency is independent of the Reynolds number and blockage ratio, however, the Kelvin–Helmholtz (KH) frequencies increase with increasing Reynolds number and blockage ratio. The probability density function of the reverse flow area shows unimodal and bimodal distributions for the lower (BR ≤ 5%) and higher (BR ≥ 10%) blockage ratios, respectively, and the mean reverse flow area and its standard deviation decrease with increasing blockage ratio. The results also show that the contributions from the first POD mode pair to the total energy increase with blockage ratio but independent of the Reynolds number. The POD mode coefficients show significant cycle-to-cycle variation at lower blockage ratios, suggesting that the energetic structures are comparatively less organized at lower blockage ratios. The spectra of the velocity fluctuations, reverse flow area, and POD mode coefficients all show dominant peaks at the fundamental shedding frequency.

Unsteady wake interference of unequal-height tandem cylinders mounted in a turbulent boundary layer

The unsteady wake interference of unequal-height tandem finite wall-mounted cylinders (FWMCs) fully submerged in a turbulent boundary layer (TBL) was investigated using time-resolved particle image velocimetry. The aspect ratios of the cylinders were fixed at $h/d = 5.3$ for the upstream cylinder (UC) and $H/d = 7.0$ for the downstream cylinder (DC) to achieve a height ratio of $h/H = 0.75$ , where d is the diameter of the cylinders. The Reynolds number based on the cylinder diameter was $Re = 5540$ and the submergence ratio was $\delta /H = 1.2$ , where $\delta $ is the TBL thickness. Three main flow regimes of tandem FWMCs were examined by varying the centre-to-centre spacing ( $s$ ) between the cylinders: extended-body ( $s/d = 2$ ), reattachment ( $s/d = 4$ ) and co-shedding ( $s/d = 6$ ) regimes. These test cases denoted as SR2, SR4 and SR6, respectively, were compared with a reference isolated cylinder (SC) with an aspect ratio similar to that of the DC. Spatio-temporal analysis of the flow field showed that the gap region of SR2 is characterized by a strong downwash of alternating low- and high-momentum fluid induced by the approach flow that is deflected from the unsheltered portion of the DC. In contrast, the gap region of SR4 and SR6 exhibited both downwash and upwash flow with a saddle point that moves closer to the mid-height of the UC as the spacing ratio increases. The upwash and downwash shear layers were associated with small-scale vortices with Strouhal numbers larger than that of the Kármán vortex shedding in the spanwise shear layers. The wake structure behind the DC was significantly altered compared with the SC due to sheltering effects, and the spacing ratio had a significant impact on the spatio-temporal evolution of the vortices.

Validation of Rotating Detonation Combustor Computational Fluid Dynamics Simulations for Predicting Unsteady Supersonic–Subsonic Flow Field at the Exit

Abstract Rotating detonation combustors (RDCs) have gained increased interest for integration with power-generating gas turbines due to the potential to increase thermal efficiency. The unsteady flow field exiting the RDC is fundamentally different compared to traditional swirl-stabilized combustors. Successful integration of RDC with gas turbines will depend on the ability to properly condition the unsteady flow to achieve performance levels comparable to swirl-stabilized combustors. RDC simulations require significant computational resources due to the small spatial and temporal time scales required to resolve the detonation phenomenon. Furthermore, traditional steady-state computational fluid dynamics (CFD) analyses are not possible for RDC simulations. The present study develops and validates a computationally efficient approach for predicting unsteady flow fields exiting the combustor using 2D, transient reacting CFD with periodic boundary conditions in the combustor and a downstream plenum. Validation is performed by comparing the CFD results to various experimental measurements: (i) wave speed obtained from high-speed ion probe and dynamic pressure data, (ii) average wall static pressure measurements, and (iii) time-resolved particle image velocimetry (PIV) at 100 kHz at the RDC exit. Results indicate good agreement between CFD and experiments with respect to velocity field exiting the RDC, detonation wave speed, and static pressure distribution.

Columnar-jet to wall-jet state transition in transversely excited swirling flow

The current work investigates the transition of a swirling jet between two flow states, namely, columnar-jet (CJ) and wall-jet (WJ) states, under the influence of external transverse acoustic forcing. Here, we have performed simultaneous time-resolved stereoscopic particle image velocimetry and dynamic pressure measurements to understand the structure and dynamics of these swirling jets under external forcing. It is observed that at low Reynolds number (Re), the swirl flow transitions from CJ to WJ state due to transverse forcing. And the flow returns to the CJ state when the acoustic is turned OFF. However, above a critical flow Re, the swirl flow does transition from CJ to WJ state when subjected to transverse forcing, but upon turning OFF the acoustics, the flow stays in the WJ state. Thus, the swirling flow demonstrates bistability in the flow states only above a critical flow Re. It is observed that upon forcing, there is an increase in the streamwise velocity fluctuations (u′x) near the centerline and in the radial velocity fluctuations (u′r) near the injector lip. This finding is also confirmed through spectral proper orthogonal decomposition analysis. In addition, it is observed that as the flow transitions from CJ to WJ state, the relative contribution of the convective term Ur¯∂Ur¯∂r toward the radial pressure gradient (∂P¯∂r) increases in comparison to the centrifugal force term (U¯θ2r). The study highlights the effect of acoustic-induced velocity fluctuations on the bistability of swirl flows over a range of flow Re.

Particle image velocimetry in the impeller of a centrifugal pump: A POD-based analysis

The flow field within the channels of a centrifugal pump impeller is usually complex, containing turbulent structures in a wide range of time and length scales. Identifying the different structures and their dynamics in this rotating frame is, therefore, a difficult task. However, modal decomposition can be a useful tool for detecting coherent structures. In this paper, we make use of proper orthogonal decomposition (POD) of time-resolved flow fields in order to investigate the flow in a centrifugal pump. For that, we carried out experiments using time-resolved particle image velocimetry (TR-PIV) in a pump of transparent material operating at different conditions and obtained the statistical characteristics of the turbulent flow from phase-ensemble averages of velocities and turbulent kinetic energy. The results reveal that at the pump’s best efficiency point (BEP) the flow is well-organized, with no significant flow separation. For flow rates below the BEP, flow separation and vortex structures appear in the impeller channels, making the flow unstable. At flow rates above the BEP, intense jets appear close to the suction blades, while small instabilities occur on the pressure side. The POD analysis shows that at low flow rates, the flow is dominated by large-scale structures with intense energy levels, while at the BEP and higher flow rates, the flow is dominated by small-scale structures. Our results shed light on the turbulence characteristics inside the impeller, providing relevant information for reduced-order models capable of computing the flow in turbomachinery at much lower costs when compared to traditional methods.

On the unsteady wake flow behind a sphere with large transverse-rotating speeds

The unsteady wake downstream of a spinning sphere with a Reynolds number of 7930 (Re=UoD/ν, based on the sphere diameter and the free-stream velocity) and a range of spinning ratios (the ratio between the surface velocity and free-stream velocity, α=Vs/Uo) from 0 to 6.0 were studied using time-resolved particle image velocimetry. The Reynolds stresses and proper orthogonal decomposition results indicated that the wake exhibited distinct features in different spinning ratio regimes, and the state of the boundary layer played an important role in forming these features. In regime I (α≤0.25) where the boundary layer around the sphere was laminar, a pair of counter-rotating vortices emerged in the wake with unsteady size and position. The bottom boundary layer transitioned from laminar to turbulent as α increased from 0.25 to 0.75 (regime II). The transition caused a significant increase in the stream-wise stress as the unsteadiness becomes more coherent. The sphere was gradually covered by the turbulent boundary layer as α increased in regimes III and IV (0.75<α≤3.0). A large separation bubble was created in cases with α>3.0 (regime V); flow structures with a large stream-wise momentum separated from the bubble and was shed downstream with a strong flapping motion in the vertical direction, leading to large vertical stress behind the sphere.

Influence of hemisphere disturbance on laminar boundary layer at low Reynolds numbers

We experimentally investigate the two-dimensional flow characteristics caused by hemisphere disturbance in the laminar boundary layer, with the aim of analyzing the periodic vortex structures generated by the hemisphere at different freestream velocities. For flow fields with Reynolds numbers of ReD= 1919, 2386, and 2819, instantaneous snapshots of the streamwise–wall-normal plane and streamwise–spanwise plane are acquired by time-resolved particle image velocimetry. The velocity distribution near the hemisphere model in the laminar flow state and the conditions for the generation of periodic structures are discussed. Strong shear occurs in the dense area of velocity contours, including a stable horizontal shear layer and inclined shear layer of shedding vortex structures, and the Reynolds shear stress attains a local maximum. The feasibility of three frequency extraction methods for hemisphere disturbance is also compared, and the periodic structures corresponding to each frequency are analyzed in detail. At higher values of ReD, the disordered flow field is formed by a multi-frequency superposition. Spatial two-point cross correlation analysis, which can be regarded as a flow visualization of frequency spectrum analysis, illustrates that the correlation and periodicity of the coherent structures are strongest in the inclined shear layer. Spectral proper orthogonal decomposition appears to be more effective in capturing periodic information about the streamwise–spanwise plane of the hemisphere disturbance. The three frequency extraction methods show that with an increase in ReD gradually transforms the periodic vortex structures from a single frequency state to a multi-frequency superposition state.

Flow structure and impinging interactions of two confined turbulent converging jets in crossflow

In this work, time-resolved particle image velocimetry (TR-PIV) measurements are carried out to study the flow structure and impinging interactions of two confined turbulent converging jets in crossflow, with the upstream and downstream jets inclined at 20° and 170° with respect to the oncoming crossflow direction. Experiments are conducted for fixed dimensionless nozzle separation distance of s=L/D=3, fixed dimensionless nozzle-to-target surface distance of H/D=6.73, fixed duct Reynolds number of Re=3500 and two jets' Reynolds numbers of Rej=3000 and 6000 that correspond to jets'-to-crossflow velocity ratios of rj=3.45 and 6.91, respectively. Flow visualization images showing ensemble-averaged and instantaneous flow distributions and turbulent characteristics including Reynolds stresses and turbulence intensities for equal and non-equal discharge velocities of the jets are presented on several orthogonal planes of the test section. The swirling strength criterion has been employed to identify coherent structures, and cross spectral analysis of the fluctuating velocity signals detecting subharmonic and harmonic sideband frequencies identify the merging and breakup interactions of the evolving coherent structures. The snapshot proper orthogonal decomposition technique (POD) has been applied to extract the dominant vortical structures and to quantify their relative and cumulative energy contributions to the total kinetic energy fluctuation. The spatial structure of the main four POD modes and the temporal projections of these POD coefficients have been explored to understand the spatio-temporal characteristics of the dominant turbulent fluctuations and their spectral characteristics. Our results show the effect of the velocity ratios on the jets' trajectories, penetration, spreading characteristics, crossflow entrainment through the inner and outer shear layers, transverse velocity decay, size of the converging region and location of the combining point. In addition, the dimensionless shedding frequencies of the oscillating combined jet, the developing wall jets and wake vortices have been obtained. Furthermore, it has been shown that for all values of the jets' velocity ratios employed in this study, a recirculation region does not develop in the converging region.

Fluid-Dynamic and Aeroacoustic Characterization of Side-by-Side Rotor Interaction

An investigation of twin corotating rotors’ interaction effects was performed by load (thrust and torque) measurements, flow field dynamics through Time-Resolved Particle Image Velocimetry, and acoustic emissions using a microphone array. Two rotors, each with a diameter of D = 393.7 mm and equipped with three blades, were investigated in a side-by-side configuration, to simulate a multirotor propulsion system. The mutual distance between the propellers is 1.02 D, and four different rotating speeds, i.e., 2620, 3500, 4360, and 5200 RPM, were explored. In such a configuration, thrust and torque undergo a reduction compared to that found for a single propeller configuration. The level of aerodynamic load fluctuations increases as well. The interaction of the wakes produces a recirculation region at the external periphery of the shear layers. An innovative approach involving the coupling of Proper Orthogonal Decomposition (POD) and Wavelet Transform has been employed to investigate the dominant structures within the flow and their mutual influence. The results reveal that the interacting wakes are dominated by a wave-like motion pulsating at Harmonics of the Blade Passing Frequency (HBPF) of 1/3. Higher orders of POD modes capture coherent vortical structures, including tip vortices pulsating at HBPF = 1. The aeroacoustic investigation shows that the noise level, in terms of the Over All Sound Pressure Level, presents a remarkable increment concerning that generated by the single propeller.

Tomographic PIV investigation on near-wake structures of a hemisphere immersed in a laminar boundary layer

The near wake of a hemisphere immersed in a laminar boundary layer is studied utilizing time-resolved tomographic particle image velocimetry (TPIV). Focus is placed on the three-dimensional vortical structures and the formation details of hairpin vortices before the onset of transition. The three-dimensional instantaneous pressure field of the hemisphere wake is reconstructed for better understanding the flow mechanism. Experiments are carried out with Reynolds number $Re_{r}=1370$ , based on the hemisphere radius $R$ . Features of periodicity of the near wake are analysed using proper orthogonal decomposition and Fourier transformation. The velocity fluctuation in the wall-normal direction is shown to be crucial to the formation of hairpin vortices in the near wake. By investigating the transport of mass and vorticity, and the correlation between pressure and hairpin vortex strength, the formation mechanism is revealed clearly. Specifically, the main hairpin vortices (MHVs) are formed within the reaction of outer high-speed flow and near-wall flow. The formation of the head portion is followed by the leg portion formation. The shedding of the MHVs is highly correlated with the pressure, as well as the pressure gradient in the wall-normal direction. For the side hairpin vortices (SHVs), the leg portion is formed first, followed by the generation of the head portion thanks to induction of the re-oriented standing vortices. The generation of the SHVs can be regarded as the downstream bridging of the standing vortices, similar to the generation of hairpin vortices due to the connection of streamwise vortices in turbulent boundary layers.