Phase-locked Particle Image Velocimetry Research Articles

Vortex shedding flow patterns and their transitions in oscillatory flows past parallel-plate thermoacoustic stacks

Abstract This paper investigates vortex shedding phenomena occurring at the end of the parallel-plate thermoacoustic stacks, in oscillatory flow conditions induced by an acoustic standing wave. Phase-locked Particle Image Velocimetry (PIV) measurement technique is used to identify a range of vortex shedding flow patterns within an acoustic cycle, in particular during the “ejection” stage, when the fluid flows out of the stack. As an extension of the earlier work [X. Mao, Z. Yu, A.J. Jaworski, D. Marx, PIV studies of coherent structures generated at the end of a stack of parallel plates in a standing wave acoustic field, Experiments in Fluids 45 (5) (2008) 833–846] this paper reports experimental results obtained for a much wider range of parameters in order to identify detailed relationships between vortex flow patterns and experimental conditions. In addition to the flow patterns observed previously, current results show several new flow phenomena related to the evolution of vortices shed in the ejection stage. This work also attempts to classify systematically the types of the vortex wake patterns using three dimensionless parameters: the Reynolds number (Re), Keulegan–Carpenter number (KC) and Womersley number (Wo). In the (Re, KC) plane (Re is up to 5000, KC is up to 250), four flow pattern regions are defined according to the characteristics of nine types of vortex wake “transitions”. It is proposed that Wo plays an important role in determining the detailed classification of such transitions within each flow pattern region. Furthermore, the physical mechanisms behind different types of wake patterns and their relationship with the dimensionless parameters investigated have been discussed.

Advanced Experimental and Numerical Validation and Analysis of Propeller Slipstream Flows

A comparison of velocity(-derived) quantities is made in the slipstream of a turboprop equipped transport aircraft between phase-locked experimental Particle Image Velocimetry (PIV) data and unsteady Reynolds Averaged Navier-Stokes (uRANS) calculations. Velocity results indicate a high level of agreement between the numerical and experimental results. In addition to velocity vector data, also derived quantities, such as vorticity, allow a verification and validation of the modeling in CFD. Although a good agreement is achieved, future needs for the comparison and validation of computational (CFD) and experimental (PIV) data are highlighted.

Vortex Phase-Jitter in Acoustically Excited Bluff Body Flames

This paper describes an experimental study of the effect of acoustic excitation on bluff body stabilized flames, specifically on the flow field characteristics. The Kelvin-Helmholtz (KH) instability of the shear layer is excited due to the incident acoustics. In turn, the KH instability imposes a convecting, harmonic excitation on the flame, which leads to spatially periodic flame wrinkling and heat-release oscillations. Understanding the factors influencing these heat release oscillations requires an understanding of the generation, convection, and dissipation of these vortical disturbances. Phase locked particle image velocimetry was carried out over a range of conditions to characterize the vortical dynamics. It was found that the vortex core location exhibits “phase jitter”, manifested as cycle-to-cycle variation in flame and vorticity field at the same excitation phase. Phase jitter is shown to be a function of separation point dynamics, downstream convection time, and amplitude of acoustic excitation. It leads to fairly significant differences between instantaneous and ensemble averaged flow fields and, in particular, the decay rate of the vorticity in the axial direction.

Synchronizing Separation Flow Control With Unsteady Wakes in a Low-Pressure Turbine Cascade

Particle image velocimetry (PIV) measurements were made on a highly loaded low-pressure turbine blade in a linear cascade. The Pack B blade has a design Zweifel coefficient of 1.15 and a peak Cp at 63% axial chord on the suction surface. Data were taken at Rec=20K with 3% inlet freestream turbulence and a wake-passing flow coefficient of 0.8. Without unsteady wakes, a nonreattaching separation bubble exists on the suction surface of the blade beginning at 68% axial chord. The time-averaged separation zone is reduced in size by approximately 35% in the presence of unsteady wakes. Phase-locked hot-wire and PIV measurements were used to document the dynamics of this separation zone when subjected to synchronized, unsteady forcing from a spanwise row of vortex generator jets (VGJs) in addition to the unsteady wakes. The phase difference between VGJ actuation and the wake passing was optimized. Both steady state Cp and phase-locked velocity measurements confirm that the optimal combination of wakes and jets yields the smallest separation.

Active Flow Control at Low Angles of Attack: Stingray Unmanned Aerial Vehicle

Active flow control using fluidic actuators, via arrays of synthetic jet actuators, was used to provide control power for the Stingray unmanned aerial vehicle in the longitudinal (pitch) and lateral (roll) directions at low angles of attack. Using this technique, the pitch and roll moments were altered such that the effect is similar to that of a deflection of conventional control effectors. The control effectiveness of the synthetic jets on the aerodynamic performance of the Stingray unmanned aerial vehicle was investigated experimentally in a wind tunnel. Global flow measurements were conducted, where the moments and forces on the vehicle were measured using a six-component sting balance. The effect of the actuation was also examined on the surface static pressure at two spanwise locations. In addition, a particle image velocimetry technique was used to quantify the flowfield over the model, both the global flowfield as well as the localized interaction domain near the synthetic jet orifice. The synthetic jets were able to alter the local streamlines and displace the boundary layer through the formation of a small quasi-steady interaction region on the suction surface of the Stingray unmanned aerial vehicle's wing. Phase-locked particle image velocimetry data were acquired to provide insight into the growth, propagation, and decay of the synthetic jets impulse and their interaction with the crossflow. Furthermore, the changes induced on the moments and forces can be proportionally controlled by either changing the momentum coefficient or by driving the synthetic jets with a pulse modulation waveform. This can lead the way for future development of closed-loop control models.

Steady streaming near model cod otoliths

Typical fish distinguish sounds at about 10‐1000 Hz with particle motions as small as 0.1 nm and angular separation near 10° using their ears, which contain dense, bony otoliths weakly suspended in endolymph and tissue. The otoliths oscillate relative to incident sound, inducing flows in the surrounding fluid which are in turn sensed by the hair cells on the overlying macular membrane which project into the groovelike sulcus on the otolith. These fluid flows are then sampled by the hair cells and ''heard'' by the fish. The irregular geometry of the otolith shapes the flow patterns. The hair cells, which are organized into different ciliary orientation groups on the macula, preferentially sample the flow patterns along their axes. The steady component of the fluid motion near enlarged models of an actual cod otolith oscillated at 2‐‐20 Hz along different directions was studied in the vicinity of the hair cells using phase‐locked particle‐image velocimetry (PIV) and pathline visualizations. The possible relationship between the flow patterns and hearing capabilities is discussed. Although the oscillation amplitudes studied are much larger than those for underwater sound, evidence suggests that the flow patterns are amplitude independent. [Supported by ONR.]

Cross-talk in multiple dielectric barrier discharge actuators

Phase locked particle image velocimetry (PIV) is used to study flow alteration by dielectric barrier discharge actuators mounted on a bluff body. Images from these PIV experiments confirm the importance of negative ions in the actuation process and demonstrate cross-talk interactions between adjacent actuators.

Dynamic vortex structures for flow-control applications

It is well known that the application of active flow-control strategies has the potential to increase the efficiency of many devices when compared to static actuator concepts. However, many aspects of the flow-control process are not well understood. In the case of pneumatic vortex generators the importance of coherent structures and their interaction with turbulent boundary layers remains an open question. A flat plate experiment was performed to determine the dynamic evolution of the induced vortex structures using phase-locked stereoscopic Particle Image Velocimetry. The qualification of the actuator system was performed by means of the time-resolved Particle Image Velocimetry measurements of the jet flow. The results show that an initial overshooting of the jet velocity dominates the unsteady start-up process, which results in a vortex structure of larger size and impact. This effect differs essentially from the case of steady blowing. In addition, the ability to shift high-momentum fluid into the near-wall region is a result of strong mixing combined with a minimum distance between the vortex core and the model surface.

On the role of oxygen in dielectric barrier discharge actuation of aerodynamic flows

Phase-locked particle image velocimetry is used to study the mechanism of induced flow in the near field of a rf dielectric barrier discharge actuator mounted in the separated flow region of a bluff body. Flow actuation is found to be asymmetric, with suction toward the buried downstream electrode when it is biased positively relative to the upstream exposed electrode. Lesser flow is seen on the reverse voltage swing, where the buried electrode should attract positive ions. This phenomenon is enhanced when oxygen is added to the flow, suggesting that oxygen negative ions, possibly O2−, play a dominant role in plasma actuation.

Piv experimental research of instantaneous flow characteristics of circular orifice synthetic jet

The instantaneous flow characteristics of circular orifice synthetic jet was experimentally studied by a phase-locked Particle Image Velocimetry (PIV) system. The instantaneous flowfields, including the forming, developing and breaking down of the vortex for the jet were clearly shown by the PIV experimental results. As the basis of the study of the instantaneous flow, 36 images were taken and phase-averaged for each condition. The PIV experiment was mainly focused on the time evolution of the vortex pairs formed in the push cycle, the saddle point existing in the suck cycle, the variation of the centerline velocity in the whole cycle and the cross-stream velocity profiles and their self-similarity. Finally, the orifice depth was changed from 1.5 mm to 2 mm and 3.5 mm in order to study the effect of different orifice depths on the flow structure, which shows that at all stream wise sections, the peak of the mass flux and momentum flux increases as the orifice depth increases. Furthermore, the nondimensional distance of the mass flux from the exit is the maximum, while the nondimensional distance of the centerline velocity peak from the exit is the minimum, and nondimensional distance of the momentum flux from the exit section is between them.

Numerical and Experimental Flow Analysis of Moving Airfoils with Laminar Separation Bubbles

Experimental measurements and unsteady Reynolds-averaged Navier-Stokes simulations of the low-Reynolds-number flow past an SD7003 airfoil with and without plunge motion at Re = 60 k are presented, where transition takes place across laminar separation bubbles. The experimental data consist of high-resolution, phase-locked particle image velocimetry measurements in a wind tunnel and a water tunnel. The numerical simulation approach includes transition prediction which is based on linear stability analysis applied to unsteady mean-flow data. The numerical results obtained for steady onflow are validated against particle image velocimetry data and published force measurements. Good agreement is obtained for specific turbulence models. Flows with plunge motion reveal strong effects of flow unsteadiness on transition and the resulting laminar separation bubbles which are well captured in the simulations.

Phase-Locked Particle Image Velocimetry Measurements of a Flapping Wing

The unsteady aerodynamics of a biomimetic inspired flapping-wing mechanism has been analyzed by performing detailed phase-locked diagnostics of its flow field. Flow visualization and particle image velocimetry results have shown the presence of a shed dynamic stall vortex that spans across most of the wing span. The shedding of this type of leading-edge vortex was accompanied by the formation of another leading-edge vortex before the first vortex reached the midchord, resulting in multiple shedding leading-edge vortices on the top surface of the wing during each wing stroke. A strong starting vortex was also formed at the trailing edge of the wing during the early part of its translational stroke. This vortex continuously gained strength from shed vorticity as the wing accelerated into its stroke. The starting vortex remained close to the trailing edge until the wing reached midstroke. A pair of vortices that continuously trailed from the root and tip of the wing were identified, both of which induced a significant downwash velocity over the wing surface. These trailed vortices were found to exhibit a contracting wake structure as they convected into the wake below the wing, consistent with an increase in slipstream velocity. The evolution of the tip and root vortex pair showed rapid diffusive characteristics with an increase in time (wake age).

Active control of the aeroacoustics of cavity flows from the downstream edge

Active control of the aeroacoustics of flow-induced oscillations in a rectangular cavity using a vibrating plate inserted at the downstream edge is demonstrated. Pressure and phase-locked Particle Image Velocimetry measurements show that the oscillations of the mixing layer can be controlled in order to attenuate or amplify the radiated sound. Results obtained from a previously developed analytical model are used to characterize the effects of the control. To cite this article: L. Chatellier et al., C. R. Mecanique 334 (2006).

Forced Low-Frequency Spray Characteristics of a Generic Airblast Swirl Diffusion Burner

The low-frequency response of the spray from a generic airblast diffusion burner with a design typical of an engine system has been investigated as part of an experimental study to describe the combustion oscillations of aeroengine combustors called rumble. The atomization process was separated from the complex instability mechanism of rumble by using sinusoidal forcing of the air mass flow rate without combustion. Pressure drop across the burner and the velocity on the burner exit were found to follow the steady Bernoulli equation. Phase-locked particle image velocimetry measurements of the forced velocity field of the burner show quasisteady behavior of the air flow field. The phase-locked spray characteristics were measured for different fuel flow rates. Here again quasi-steady behavior of the atomization process was observed. With combustion, the phase-locked Mie-scattering intensity of the spray cone was found to follow the spray behavior measured in the noncombusting tests. These findings lead to the conclusion that the unsteady droplet Sauter mean diameter mean and amplitude of the airblast atomizer can be calculated using the steady-state atomization correlations with the unsteady burner air velocity.

The response of turbulent premixed flames to normal acoustic excitation

To model the thermo-acoustic excitation of flames in practical combustion systems, it is necessary to know how a turbulent flame front responds to an incident acoustic wave. This will depend partly on the way in which the burning velocity responds to the wave. In this investigation, the response of CH 4/air and CH 4/H 2/air mixtures has been observed in a novel flame stabilisation configuration, in which the premixture of fuel and air is made to decelerate under controlled conditions in a wide-angle diffuser. Control is provided by an annular wall-jet of air and by turbulence generators at the inlet. Ignition from the outlet of the diffuser allows an approximately flat flame to propagate downwards and stabilise at a height that depends on the turbulent burning velocity. When the flow is excited acoustically, the ensemble-averaged height oscillates. The fluctuations in flow velocity and flame height are monitored by phase-locked particle image velocimetry and OH-planar laser induced fluorescence, respectively. The flame stabilised against a lower incident velocity as the acoustic amplitude increased. In addition, at the lowest frequency of 52 Hz, the fluctuations in turbulent burning velocity (as represented by the displacement speed) were out-of-phase with the acoustic velocity. Thus, the rate of displacement of the flame front relative to the flow slowed as the flow accelerated, and so the flame movement was bigger than it would have been if the burning velocity had not responded to the acoustic fluctuation. With an increase in frequency to 119 Hz, the relative flame movement became even larger, although the phase-difference was reduced, so the effect on burning velocity was less dramatic. The addition of hydrogen to the methane, so as to maintain the laminar burning velocity at a lower equivalence ratio, suppressed the response at low amplitude, but at a higher amplitude, the effect was reversed.

Theoretical and experimental investigations of low Mach number turbulent cavity flows

Theoretical and experimental investigations are conducted for rectangular cavities of varying sizes in low Mach number turbulent flows. Emphasis is put on the characterization of the generation of self-sustained oscillations in order to develop methods of active control applied to the aeroacoustics of cavity flows. A linearized stability analysis for low Mach number flows is proposed in which the interface of the cavity is modeled by a vorticity layer. Subsequently, the cavity flow is investigated experimentally in a subsonic wind tunnel, using pressure measurements and a phase-locked particle image velocimetry system. Experimental results indicate that the oscillation process is governed by convective waves, with no definite influence of convected vortical structures. The good agreement between the experimental data and the predictions given by the model allows the identification of the oscillations of the cavity interface via the parameters issued from the theoretical analysis.

Structure of a screeching rectangular jet: a stereoscopic particle image velocimetry study

The unsteady velocity field generated by an underexpanded jet has been investigated using stereoscopic particle image velocimetry (PIV). A 4:1 aspect ratio converging–diverging rectangular nozzle designed to operate at a fully expanded condition of was used. The nozzle was operated at off-design conditions to generate imperfectly expanded jets with intense screech tones. Phase-locked PIV measurements show the spatial and temporal evolution of the three-dimensional jet with high fidelity. In addition to the globally averaged mean and turbulence velocity field data, the phase-averaged data for the velocity and vorticity fields were also obtained. The turbulence quantities were resolved into contributions from the periodic and random motions. The deformation of the periodic spanwise structures results in the formation of strong streamwise vortices that appear to govern the mixing of the jet. It is shown that the presence of coherent vorticity of significant strength, in addition to the shock cell strength, is largely responsible for determining the screech intensity.