Phase-locked Particle Image Velocimetry Measurements Research Articles

Tip Vortex Meandering Characteristics of a Model Helicopter Rotor with Different Tip Shapes

This paper presents the results of an experimental investigation regarding the tip vortex meandering characteristics for a model helicopter rotor with different blade tip shapes. Phase-locked particle image velocimetry measurements are performed at the rotor phase angles between 0 and 72° with 6° increments. The meandering characteristics are presented along the first and the second principal axes of the meandering motion, which are found using the principal component analysis method. For all blades, the results show that the vortex position scattering along the principal axes can be defined with a Gaussian distribution. The scatterings about the second principal axes have a better agreement with the Gaussian distribution, with a fit up to the 95th percentile of the data, than the first axes for all configurations. Streamwise variations of the standard deviation of the meandering motion along the principal axes and the meandering angle, which is the angle between the slipstream and the first principal axis, are analyzed, and a simple empirical meandering model as a function of wake age is proposed. The meandering angle decreases with the vortex age, so the first principal axis becomes more aligned with the slipstream direction () from close to being perpendicular () at the early wake ages.

Boundary-Layer Control by Means of Pulsed Jets at Different Inclination Angles

Finite amounts of compressed air are impulsively emitted through a rectangular outlet at an axial velocity of . Their interaction with a steady crossflow with a zero-pressure-gradient turbulent boundary layer is studied on the basis of phase-locked particle image velocimetry measurements for a parameter space that is spanned by the jet inclination angle defined with respect to the surface tangent downstream of the outlet and the ratio between the axial jet velocity and the crossflow velocity . Two types of vortex structures are observed. For , , jets penetrate into the crossflow, and distinct, asymmetrical vortex rings are produced. For , as well as for , , starting jets attach to the wall as leading vortex half-rings are formed. The latter type is better suited to energize the boundary layer as low-momentum fluid is shifted away from the surface and high-momentum fluid from the freestream is entrained into the jet wake. In terms of the overall gain in streamwise momentum, the impulse provided due to overpressure during the rapid jet initiation is of major importance in the present study, and its exploitation may enable a significant enhancement in future flow control applications.

Heat transfer and flow field measurements of a pulsating round jet impinging on a flat heated surface

Heat transfer and flow field characteristics of a pulsating round air jet, impinging on a flat heated target surface were measured and compared to those of a steady impinging jet. The jet was partially confined and the target surface stand-off distance equaled two nozzle diameters, D. Investigated Reynolds numbers ranged from Re = 4,606 to 13,513, while pulsating jet velocities varied nearly sinusoidal with Strouhal numbers ranging between 2×10−3< Sr<15.6×10−3. Results indicated that besides the well-known secondary Nusselt number peak at approximately 2D from the stagnation point, an additional distinct increase in Nusselt number was detected at approximately 4D from the stagnation point as a result of partial jet confinement. For the studied Re and Sr-number range, jet pulsation resulted in both enhanced and slightly reduced overall heat transfer. Our results indicated that the physical mechanism responsible for overall heat transfer enhancement or reduction due to jet pulsation is increased or reduced primary vortex generation Strouhal numbers (Srv), respectively. The latter were extracted from phase-locked particle image velocimetry measurements. Overall heat transfer modulation linearly depended on the ratio of the ensemble averaged pulsating jet and steady jet primary vortex generation Strouhal numbers, Sr¯v/Sr¯v,0. Overall heat transfer enhancement and reduction was obtained when Sr¯v/Sr¯v,0>1 and Sr¯v/Sr¯v,0<1, respectively.

Intensified flow dynamics by second-order acoustic standing-wave mode: Vortex-excited acoustic resonances in channel branches

The intensified flow dynamics by the second acoustic standing-wave mode, superimposed with vortex-excited acoustic resonances inside a channel with coaxial side-branches, were experimentally investigated. In the experiments, the frequency lock-on range of vortex-excited acoustic resonances was determined first by comparing wall pressure fluctuations and simulated acoustic standing-wave modes. The intensified flow dynamics by the second acoustic standing-wave mode were measured by particle image velocimetry (PIV); the flow dynamics were also measured when coupled with the first standing-wave mode for comparison. The results demonstrate that shear layer developments over the branch entrance can be classified into three regions, i.e., the developing region, the transition region, and the collapsing region. Both the momentum thickness and the growth rate of the shear layer were significantly intensified in the developing and transition regions by the second standing-wave mode. The subsequent spatiotemporal evolutions of the shedding vortex, recirculation zone, and synchronous flow streaks were identified by the phase-locked PIV measurements. With second-order acoustic modulations, the shedding vortex breaks away from beneath the recirculation zone to impinge the downstream branch corner while directly converging with the recirculation zone during first-order modulation. Finally, the aeroacoustic energy transfer between vortex dynamics with standing waves was revealed using Howe’s aeroacoustic analogy. The energy transferred from the standing-waves contributed to the formation and development of the shedding vortex, while the energy produced by convection and the collapsing of the shedding vortex maintained the standing waves’ propagations. The accumulated aeroacoustic energy produced during one second-order acoustic resonance cycle was found to be up to 150 times that produced during one first-order cycle.

A freely yawing axisymmetric bluff body controlled by near-wake flow coupling

Flow-induced oscillations of a wire-mounted, freely yawing axisymmetric round bluff body and the induced loads are regulated in wind tunnel experiments (Reynolds number $60\,000&lt;Re_{D}&lt;200\,000$) by altering the reciprocal coupling between the body and its near wake. This coupling is controlled by exploiting the receptivity of the azimuthal separating shear layer at the body’s aft end to controlled pulsed perturbations effected by two diametrically opposed and independently controlled aft-facing rectangular synthetic jets. The model is supported by a thin vertical wire upstream of its centre of pressure, and prescribed modification of the time-dependent flow-induced loads enables active control of its yaw attitude. The dynamics of the interactions and coupling between the actuation and the cross-flow are investigated using simultaneous, time-resolved measurements of the body’s position and phase-locked particle image velocimetry measurements in the yawing plane. It is shown that the interactions between trains of small-scale actuation vortices and the local segment of the aft-separating azimuthal shear layer lead to partial attachment, and the ensuing asymmetric modifications of the near-wake vorticity field occur within 15 actuation cycles (approximately three convective time scales), which is in agreement with measurements of the flow loads in an earlier study. Open- and closed-loop actuation can be coupled to the natural, unstable motion of the body and thereby affect desired attitude control within 100 convective time scales, as is demonstrated by suppression or enhancement of the lateral motion.

Fluid Dynamic Characterization of Transcatheter Aortic Valves Using Particle Image Velocimetry.

Transcatheter aortic valves provide superior systolic hemodynamic performance in terms of valvular pressure gradient and effective orifice area compared with equivalent size surgical bioprostheses. However, in depth investigation of the flow field structures is of interest to examine the flow field characteristics and provide experimental evidence necessary for validation of computational models. The goal of this study was to compare flow field characteristics of the three most commonly used transcatheter and surgical valves using phase-locked particle image velocimetry (PIV). 26-mm Edwards SAPIEN 3, 26-mm Medtronic CoreValve, and 25-mm Carpentier-Edwards PERIMOUNT Magna were examined in a pulse duplicator with input parameters matching ISO-5840, that is, heart rate of 70 beats/min, cardiac output of 5 L/min, and mean aortic pressure of 100 mm Hg. A 2D PIV system was used to obtain flow velocity and viscous shear stress fields during the entire cardiac cycle. In vitro testing showed that the mean transvalvular pressure gradient was lowest for SAPIEN 3, followed by CoreValve, and PERIMOUNT Magna surgical bioprosthesis. In addition, the viscous shear stress magnitude within the jet boundary layer was higher in PERIMOUNT Magna than CoreValve and SAPIEN 3 at the peak of the flow. However, the measured shear stress values were below the known threshold for platelet activation and red blood damage. Therefore, shear-induced platelet activation is unlikely to take place during systole in the three bioprosthetic heart valves. The PIV measurements can be used for verification and validation of computational simulations.

Cancellation of Tollmien–Schlichting Waves in Direct Vicinity of a Plasma Actuator

The active cancellation of Tollmien–Schlichting waves is a promising and energy-efficient laminar flow control technology with the goal of drag reduction by delaying laminar–turbulent transition. This paper analyzes, for the first time, the control mechanism in the direct vicinity of the actuator using particle image velocimetry. Artificial Tollmien–Schlichting waves are generated at a frequency of in a two-dimensional zero-pressure-gradient flat-plate boundary layer (). The Tollmien–Schlichting waves travel downstream and are attenuated with an unsteady dielectric-barrier-discharge plasma actuator force. Phase-locked particle image velocimetry measurements at an acquisition rate of in double-frame mode are conducted in the direct vicinity of the plasma actuator, resolving 16 phase angles of the wave cycle. The use of two cameras allows measurement of two full wavelengths of the Tollmien–Schlichting waves and a detailed analysis of the cancellation process. The phase-averaged velocity fields of the artificially generated Tollmien–Schlichting waves are validated with a linear stability theory. The wavelength, amplification, and shape of the Tollmien–Schlichting wave disturbances and the counterwaves generated by the plasma actuator are analyzed.

A Comparative study on the aeromechanic performances of upwind and downwind horizontal-axis wind turbines

Traditional horizontal-axis wind turbines are mainly designed as upwind configuration. In order to avoid blade strikes, the rotor blades have to be positioned far enough away from the turbine tower and have to be designed as inflexible as possible. In addition, a complicated yaw control system is required to keep the turbine rotor facing the incoming wind. Due to these drawbacks, the turbine in downwind configuration is proposed to overcome these disadvantages because, first of all, rotor blades can be designed more flexible since there is no danger of blade strikes, and secondly, yaw control system could be eliminated if nacelle is designed appropriately to follow the incoming wind direction passively. In the present study, a comparative experimental investigation was conducted to quantify the aeromechanic performance of a downwind turbine (DWT), in comparison to that of a traditional upwind turbine (UWT). The thrust coefficient of the DWT model was found to be increased slightly in the time-averaged quantity, but have a significant augment in the fluctuations. Due to the shadow effect, the power outputs of the DWT model was found to be decreased by 3.2% when they were operated in a same atmospheric boundary layer (ABL) wind. In addition, a high-resolution particle image velocimetry (PIV) system was employed to characterize the ensemble-averaged and phase-locked wake flow structures to quantify the turbulent flow characteristics in the turbine wakes. The velocity deficit in the lower half turbine wake for the UWT case was found to be greater than that of the DWT case at the location of X/D < 1.0. The higher wind load fluctuations for the DWT system were found to be correlated well with the higher TKE distributions in the turbine wakes. The phase-locked PIV measurements illustrated that the wake regions can be divided into four zones, which are dominated by the vortices shedding from different turbine components. The detailed flow field measurements were correlated with the dynamic force and power measurement data to elucidate the underlying physics.

Dynamics and control of the vortex flow behind a slender conical forebody by a pair of plasma actuators

Detailed particle-image-velocimetry (PIV) and surface pressure measurements are presented to study the vortex flow behind a slender conical forebody at high angles of attack. The results confirm the existence of two randomly appearing mirror imaged asymmetric bi-stable states of the separation vortices, giving rise to large side force and moment. A pair of carefully designed dielectric barrier discharge plasma actuators mounted near the apex and on both sides of the conical body are used to manipulate the vortex flow and thus provide control of the side forces on the body without using flaps. By making use of a duty-cycle actuation scheme that alternately actuates the port and starboard plasma actuators and optimizing the duty-cycle frequency, the present work demonstrates the feasibility of achieving a nearly perfect linear proportional control of the side force and moment in response to the duty-cycle ratio. Phase-locked PIV and surface pressure measurements are used to study the unsteady dynamic evolution of the flow within one duty-cycle actuation to reveal the flow control mechanism. It is found that under the duty-cycle actuation with the optimized frequency, the vortex flow essentially follows the plasma actuation by alternating between the two bi-stable states controlled directly by the duty-cycle ratio.

Experimental investigation on frequency characteristics of plasma synthetic jets

The performance of a two-electrode plasma synthetic jet actuator (PSJA) is investigated for a wide range of dimensionless actuation frequencies ( f*) using high-speed phase-locked particle imaging velocimetry measurements. The jet-induced velocity fields in the axisymmetric plane are measured during both transient and steady working stages of the PSJA. When f* increases, the jet duration time (Tjet) is reduced, while the peak suction velocity (Us) increases consistently. Three integral parameters including the total expelled gas mass, impulse, and issued mechanical energy also decline considerably with increasing frequency, which is shown to relate to both the reduced cavity density and the decreasing jet duration. Theoretical analysis reveals that the mean cavity density decreases monotonically with the square root of the discharge frequency. The decreasing rate is inversely proportional to a thermal cut-off frequency ( fc, 210 Hz for the current study), which scales with the convective heat transfer coefficient between the actuator cavity walls and the cavity gas, as well as the area of the cavity internal surface. In the time-averaged velocity fields, the jet centreline velocity (U¯c) exhibits a local maximum in the axial coordinate. The nondimensional maximum centreline velocity reduces with increasing frequency of operation. The jet spreading rate of the plasma synthetic jets (PSJs) decreases from 0.14 to 0.09 with increasing frequency. During the transient working stage of a PSJ, the exit velocity trace elapses 20 successive actuation cycles to stabilize. In contrast to the exit velocity, approximately 130 cycles are needed for the mean cavity density/temperature to reach steady values.

Control of Dynamic Stall over a NACA 0015 Airfoil Using Plasma Actuators

Dynamic stall is observed in numerous applications, including sharply maneuvering fixed-wing aircraft, biomimetics, wind turbines, and most notably, rotorcraft. The associated unsteady loading can lead to aerodynamic flutter and mechanical failure in the system. The present work explores the ability of nanosecond pulse-driven dielectric barrier discharge plasma actuators to control dynamic stall over a NACA 0015 airfoil. The Reynolds number, reduced frequency, and excitation Strouhal number were varied over large ranges: , , and , respectively. Surface pressure measurements were taken for each combination of Reynolds number, reduced frequency, and excitation Strouhal number. Phase-locked particle image velocimetry measurements were acquired for select cases. It was observed that the trends of effect of were similar for all combinations of Reynolds number and reduced frequency, and three major conclusions were drawn. First, it was observed that low Strouhal number excitation () results in oscillatory aerodynamic loading in the stalled stage of dynamic stall. This oscillatory behavior was gradually reduced as increased and was not observed beyond . Second, all excitation resulted in earlier flow reattachment. Last, it was shown that excitation, especially at high , resulted in reduced aerodynamic hysteresis and dynamic stall vortex strength. The decrease in the strength of the dynamic stall vortex is achieved by the formation of large-scale structures induced by the excitation that bleed the leading-edge vorticity before the ejection of the dynamic stall vortex. At sufficiently high excitation Strouhal numbers (), the dynamic stall vortex was completely suppressed.

Influence of asymmetric valve strategy on large-scale and turbulent in-cylinder flows

Phase-locked particle imaging velocimetry measurements are carried out in a direct-injected spark-ignition single-cylinder optical research engine equipped with fully variable valve timing to assess the impact of asymmetric intake valve lift strategies on the in-cylinder flow. The engine was operated under a range of asymmetric strategies, with one valve following a full lift profile, while the second intake valve is scaled as a factor of the first, expressed as % maximum valve lift. Proper orthogonal decomposition combined with a proposed methodology allows instantaneous velocity fields to be decomposed into what are nominally demonstrated as coherent and turbulent constituent velocity fields. Analysis of the coherent fields reveals the behaviour of large-scale structures within the flow, subject to cyclic variation. In the case of 40% maximum valve lift, an increase in the flow cyclic variability is observed. This is found to be as a result of a switch between a flow dominated by a counter-rotating vortex pair and a single vortex. The impact of maximum valve lift on the bulk motion is further evident by an increase in the magnitude of swirl ratio from 0.5 to −6.0 (at 75°CA). Analysis of the turbulent constituent shows how the increased valve life asymmetry leads to increased turbulence during the intake stroke by over 250%. Finally, it is shown how the ensemble turbulence statistics may be misleading as stochastic fluctuations were found to be typically 66% of the total turbulent kinetic energy calculated from the ensemble statistic in the tested conditions.

Experimental Investigation of Flow Instability in a Turbocharger Ported Shroud Compressor

Turbocharger centrifugal compressors are equipped with a “ported shroud” to reduce flow instabilities at low mass flow rates. This passive stability control device using flow recirculation has been demonstrated to extend the surge margin of a compressor without substantially sacrificing performance. However, the actual working mechanisms of the system are not well understood. In this paper, the relationship between inlet flow recirculation and instability control is studied using stereoscopic particle image velocimetry (PIV) in conjunction with dynamic pressure transducers at the inlet of the turbocharger compressor with and without ported shroud. Both stable and unstable operational points are analyzed using phase-locked PIV measurements during surge. Detailed description of unstable flow in the centrifugal compressor is presented by reconstructing the complex flow structure evolution in the compressor inlet during surge. Rather than one-dimensional, the surge flow is characterized by a three-dimensional structure of both entering and exiting swirling flows, alternating in magnitude during a self-excited pressure cycle. The correlation between pressure and velocity measurements shows that the development of compressor unsteadiness is concurrent with swirling reversed flow at the impeller tip. The impact of the ported shroud on the inlet velocity flowfield is evidenced by the presence of localized flow recirculation. Stability improvement due to the ported shroud is thus a result of removing swirling backflow from the impeller inducer tip and recirculating it into the impeller inlet to increase the near shroud inlet blade loading and the incidence angle.

High fidelity phase locked PIV measurements analysing the flow fields surrounding an oscillating piezoelectric fan

Piezoelectric fans have been studied extensively and are seen as a promising technology for thermal management due to their ability to provide quiet, reliable cooling with low power consumption. The fluid mechanics of an unconfined piezoelectric fan are complex which is why the majority of the literature to date confines the fan in an attempt to simplify the flow field. This paper investigates the fluid mechanics of an unconfined fan operating in its first vibration frequency mode. The piezoelectric fan used in this study measures 12.7mm × 70mm and resonates at 92.5Hz in air. A custom built experimental facility was developed to capture the fan's flow field using phase locked Particle Image Velocimetry (PIV). The phase locked PIV results are presented in terms of vorticity and show the formation of a horse shoe vortex. A three dimensional A2 criterion constructed from interpolated PIV measurements was used to identify the vortex core in the vicinity of the fan. This analysis was used to clearly identify the formation of a horse shoe vortex that turns into a hairpin vortex before it breaks up due to a combination of vortex shedding and flow along the fan blade. The results presented in this paper contribute to both the fluid dynamics and heat transfer literature concerning first mode fan oscillation.

Development of a feedback model for the high-speed impinging planar jet

This article experimentally investigates the self-excited impinging planar jet flow, specifically the development and propagation of large-scale coherent flow structures convecting between the nozzle lip and the downstream impingement surface. The investigation uses phase-locked particle image velocimetry measurements and a new structure-tracking scheme to measure convection velocity and characterize the impingement mechanism near the plate, in order to develop a new feedback model that can be used to predict the oscillation frequency as a function of flow velocity ( $$U_o$$ ), impingement distance ( $$x_o$$ ) and nozzle thickness ( $$h$$ ). The resulting model prediction shows a good agreement with experimental tone frequency data.

PIV investigation of the flow induced by a passive surge control method in a radial compressor

Due to recent emission regulations, the use of turbochargers for force induction of internal combustion engines has increased. Actually, the trend in diesel engines is to downsize the engine by use of turbochargers that operate at higher pressure ratios. Unfortunately, increasing the impeller rotational speed of turbocharger radial compressors tends to reduce their range of operation, which is limited at low mass flow rate by the occurrence of surge. In order to extend the operability of turbochargers, compressor housings can be equipped with a passive surge control device such as a “ported shroud.” This specific casing treatment has been demonstrated to enhance the surge margin with minor negative impact on the compressor efficiency. However, the actual working mechanisms of the system remain not well understood. Hence, in order to optimize the design of the ported shroud, it is crucial to identify the dynamic flow changes induced by the implementation of the device to control instabilities. From the full dynamic survey of the compressor performance characteristics obtained with and without ported shroud, specific points of operation were selected to carry out planar flow visualization. At normal working, both standard and stereoscopic particle imaging velocimetry (PIV) measurements were performed to evaluate instantaneous and mean velocity flow fields at the inlet of the compressor. At incipient and full surge, phase-locked PIV measurements were added. As a result, satisfying characterization of the compressor instabilities was provided at different operational speeds. Combining transient pressure data and PIV measurements, the time evolution of the complex flow patterns occurring at surge was reconstructed and a better insight into the bypass mechanism was achieved.

Flow field investigations in rotating facilities by means of stationary PIV systems

The flow field inside rotating test sections can be investigated by means of particle image velocimetry (PIV) operated in the phase-locked mode. With this experimental approach, the measurement system is kept fixed and it is synchronized with the periodical passage of the test section. Therefore, the direct output of the PIV measurements is the absolute velocity field, while the relative one is indirectly obtained from proper data processing that relies on accurate knowledge of the peripheral velocity field. This work provides an uncertainty analysis about the evaluation of the peripheral displacement field in phase-locked PIV measurements. The analysis leads to the detection of the levels of accuracy required in the estimation of both the angular velocity and the position of the center of rotation to ensure correct evaluation of the peripheral displacement field. In this regard, a simple methodology is proposed to evaluate the center of rotation position with an accuracy below 1 px. Finally, a procedure to pre-process the PIV images by subtracting the peripheral displacement is described. The advantages of its implementation are highlighted by the comparison with the performance of a more standard methodology where the peripheral field is subtracted from the absolute velocity field and not directly from the PIV raw data.

Particle-Image Velocimetry Study of a Pediatric Ventricular Assist Device

Particle-image velocimetry (PIV) was used to visualize the flow within an optically transparent pediatric ventricular assist device (PVAD) under development in our laboratory. The device studied is a diaphragm type pulsatile pump with an ejection volume of 30 ml per beating cycle intended for temporary cardiac assistance as a bridge to transplantation or recovery in children. Of particular interest was the identification of flow patterns, including regions of stagnation and/or strong turbulence that often promote thrombus formation and hemolysis, which can degrade the usefulness of such devices. For this purpose, phase-locked PIV measurements were performed in planes parallel to the diaphragm that drives the flow in the device. The test fluid was seeded with 10 microm polystyrene spheres, and the motion of these particles was used to determine the instantaneous flow velocity distribution in the illumination plane. These measurements revealed that flow velocities up to 1.0 m/s can occur within the PVAD. Phase-averaged velocity fields revealed the fixed vortices that drive the bulk flow within the device, though significant cycle-to-cycle variability was also quite apparent in the instantaneous velocity distributions, most notably during the filling phase. This cycle-to-cycle variability can generate strong turbulence that may contribute to greater hemolysis. Stagnation regions have also been observed between the input and output branches of the prototype, which can increase the likelihood of thrombus formation.

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.

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.