Time-resolved Particle Image Velocimetry Measurements Research Articles

Boundary layer state detection using piezoelectric sensors

Two piezoelectric series bimorph sensors were embedded below the skin of a NACA 0012 symmetrical airfoil to detect the local state of the boundary layer during wind tunnel testing. Small vanes piercing the airfoil skin were glued onto the bimorphs providing a mechanical coupling to the local mechanical force fluctuations imparted by the local unsteady boundary layer flow. The state of the boundary layer at the sensor sites was varied by changing the angle of attack. The objective of this work was to establish the ability of this sensor concept to accurately distinguish among typical boundary layer states such as attached laminar flow, turbulent flow and separated flow. The output of the sensor was compared to concurrent time-resolved particle image velocimetry measurements, which served as a validation technique. Using the developed sensor response envelope, a single data point time series of the piezo electrical signal was proven to be sufficient to accurately detect the boundary layer state on classical airfoils in the low Reynolds number regime. In projected future applications, single or arrays of bimorph sensors can be used to map the boundary layer of more complex or morphing shape airfoils. The fast response of the sensor can in principle be utilised in closed-loop flow control systems, aimed at drag reduction or lift enhancement.

Mechanisms of instability growth, interaction and breakdown induced by a backward-facing step in a swept-wing flow

Time-resolved particle image velocimetry measurements were performed downstream of a swept backward-facing step. The measurements allow detailed analysis of the interactions between the unsteady instabilities and the stationary crossflow vortices. Different mechanisms are identified that lead to the modulation of the different families of unsteady instabilities that occur downstream of the step. For the low-frequency spanwise-travelling mode, the modulation occurs due to a redistribution of momentum when the instability encounters regions of large spanwise shear of the wall-normal and streamwise velocity. However, the higher-frequency streamwise-travelling instabilities undergo the familiar ‘lift-up’ mechanism when they encounter the regions of large vertical velocity due to the presence of the stationary crossflow vortices. The process leading to large velocity spikes, and ultimately to a laminar breakdown to turbulence, is identified as a constructive interaction between the different unsteady instabilities, coupled with an interaction with the stationary crossflow vortices when the phases align properly.

Impact of a Centerbody on the Unsteady Flow Dynamics of a Swirl Nozzle: Intermittency of Precessing Vortex Core Oscillations

Abstract The precessing vortex core (PVC) is a self-excited flow oscillation state occurring in swirl nozzles. This is caused by the presence of a marginally unstable hydrodynamic helical mode that induces precession of the vortex breakdown bubble (VBB) around the flow axis. The PVC can impact emissions and thermoacoustic stability characteristics of combustors in various ways, as several prior studies have shown. In this paper, we examine the impact of centerbody diameter (Dc) on the PVC in a nonreacting flow in a single nozzle swirl combustor. Time-resolved high-speed stereoscopic PIV measurements are performed for combinations of two swirl numbers, S = 0.67 and 1.17 and Dc = 9.5 mm, 4.73 mm, and 0 (i.e., no centerbody). The bulk flow velocity at the nozzle exit plane is kept constant as Ub = 8 m/s for all cases (Re∼20,000). The centerbody end face lies in the nozzle exit plane. A new modal decomposition technique based on wavelet filtering and proper orthogonal decomposition provides insight into flow dynamics in terms of global modes extracted from the data. The results show that without a centerbody, a coherent PVC is present in the flow as expected. The introduction of a centerbody makes the PVC oscillations intermittent. These results suggest two routes to intermittency as follows. For S = 0.67, the VBB and centerbody wake recirculation zone regions are nominally distinct. Intermittent separation and merger due to turbulence result in PVC oscillations due to the destabilization of the hydrodynamic VBB precession mode of the flow. In the S = 1.17 case, the time averaged VBB position causes it to engulf the centerbody. In this case, the emergence of intermittent PVC oscillations is a result of the response of the flow to broadband stochastic forcing imposed on the time averaged vorticity field due to turbulence.

Internal and external hydrodynamics of Taylor cone under constant and alternating voltage actuation

Electrohydrodynamic atomization (EHDA) is carried out in the Taylor cone mode for generating unimodal particle distribution, which can be achieved by either constant voltage actuation (CVA) or alternating voltage actuation (AVA). The present study reports an experimental investigation of the flow field both inside and outside the Taylor cone using light sheet fluorescence imaging and time-resolved particle image velocimetry measurements. Liquid ethanol is used as the working fluid and the amplitude of both constant and alternating electric potential difference is set at the same value, i.e., VC=VA=5.2 kV with an actuation frequency of 200 Hz in the case of alternating EHDA. The hydrodynamic behavior both inside and outside the Taylor cone is presented for the first time. The flow field measurements demonstrate meridional circulation from the nozzle exit toward the apex of the Taylor cone along the generatrix followed by flow from the apex of the Taylor cone along the central axis. A symmetric toroidal vortex is observed inside the Taylor cone in the case of CVA and an asymmetric toroidal vortex is observed for AVA. The flow field shows streamline-like flow in the ambient medium from the nozzle toward the ground electrode along the interface of the Taylor cone jet for CVA. In contrast, two vortical structures are observed around the apex of the Taylor cone for AVA. The velocity profile near the liquid–air interface of the Taylor cone indicates no direct correlation between the flow field inside the Taylor cone with the flow of the ambient medium. This difference may be attributed to the corona wind generated due to asymmetric electrode configuration. The unsteady flow field generated by alternating EHDA has great potential for enhanced heat transfer using spray cooling.

Characterization of milkweed-seed gust response

Inspired by the reproductive success of plant species that employ bristled seeds for wind-borne dispersal, this study investigates the gust response of milkweed seeds, selected for their near-spherical shape. Gust-response experiments are performed to determine whether these porous bodies offer unique aerodynamic properties. Optical motion-tracking and particle image velocimetry (PIV) are used to characterize the dynamics of milkweed seed samples as they freely respond to a flow perturbation produced in an unsteady, gust wind tunnel. The observed seed acceleration ratio was found to agree with that of similar-sized soap bubbles as well as theoretical predictions, suggesting that aerodynamic performance does not degrade with porosity. Observations of high-velocity and high-vorticity fluid deflected around the body, obtained via time-resolved PIV measurements, suggest that there is minimal flow through the porous sphere. Therefore, despite the seed’s porosity, the formation of a region of fluid shear, accompanied by vorticity roll-up around the body and in its wake, is not suppressed, as would normally be expected for porous bodies. Thus, the seeds achieve instantaneous drag exceeding that of a solid sphere (e.g. bubble) over the first eight convective times of the perturbation. Therefore, while the steady-state drag produced by porous bodies is typically lower than that of a solid counterpart, an enhanced drag response is generated during the initial flow acceleration period.

Study of the influence of an external flow rate perturbation on the vortex structure and heat transfer in impinging jets

The study of physical processes dominating in submerged turbulent jets impinging on a wall is an important task because this configuration is utilized in various applications. The efficiency of heat transfer in this configuration has been a subject of a long-term study. Active flow control technique and the optimization of the control signal can be applied to exploit inherent flow properties to further improve the heat transfer from the wall in impingent jets. In this paper, IR-thermography and time-resolved PIV measurements are used for the diagnostics of wall temperature fields and large-scale vortex dynamics under external flow rate forcing control. It is found that the low-frequency forcing (for the Strouhal number St = 0.6) increases integral temperature on the wall as compared to the unforced case and the high-frequency forcing (St = 0.9).

Flow features of a new fluidic oscillator using time-resolved PIV measurement and 3D numerical simulation

Flow features of a new fluidic oscillator using time-resolved PIV measurement and 3D numerical simulation

Outer-layer structure arrangements based on the large-scale zero-crossings at moderate Reynolds number

The structural arrangements in the outer layer of turbulent boundary layer flows were explored with large-field time-resolved particle image velocimetry measurements at moderate Reynolds number. The large- and small-scale structures were reconstructed by the modes of multiscale proper orthogonal decomposition. The association between hairpin packets and uniform momentum zones (UMZs) was examined by the conditional averaging results based on the large-scale positive-to-negative/negative-to-positive (PN/NP) zero-crossings. The scale arrangements provided the spatial evidence that the intense small-scale swirling motions are aligned in the confined internal shear layers along the backside of the large-scale, low-speed region, which was characterized by hairpin vortex packets. The uniform momentum zones (UMZs) conditioned on the large-scale PN/NP zero-crossings were detected from the histograms of the instantaneous streamwise velocity. The attached eddy behavior was consolidated based on the conditional events, by presenting the joint probability of UMZs thickness and wall-normal location. A close agreement of the conditional averaging raw velocity and modal velocity was examined. Moreover, the conditional averaging results of the UMZs interface probability exhibited a similar spatial distribution as the small-scale turbulent kinetic energy and swirling strength, which manifests the coincidence between the hairpin heads and the UMZs interfaces. This result was confirmed by the distribution of the wall-normal locations corresponding to the maximum value of interface probability and small-scale representations, which performs a streamwise inclination angle of 15°. The statistical spatial feature demonstrated the association between hairpin packets and uniform momentum as proposed by Adrian et al. [“Vortex organization in the outer region of the turbulent boundary layer,” J. Fluid Mech. 422, 1–54 (2000)].

Extended-POD based acoustic analysis of separated aerofoils via simultaneous time-resolved PIV and force measurements

We present a cross-correlation based analysis of the acoustic field generated in the wake of NACA 0012 and NACA 65410 aerofoils at a chord-based Reynolds number Rec = 75000 as obtained from pressure fields reconstructed from a series of planar time-resolved Particle Image Velocimetry (PIV) experiments. The experiments are performed in a water channel facility located at the University of Southampton with an overhead carriage system to allow precise control of the angle of attack α of the aerofoils as well as to mount them to a six-axis force/torque transducer to allow for simultaneous load measurements. Angles of attack in the range 4◦ < α < 17◦ are explored corresponding to a range of conditions from zero flow separation to fully stalled.

Active control of separated flow on a symmetric airfoil by pitching oscillation

Experiments are conducted on the control of massively separated flow over a symmetric airfoil by imposing pitching oscillation at smaller amplitudes. Two symmetric airfoils of different thickness to chord ratios are considered to account for the effect of thickness and the stall-type on flow control. Instantaneous aerodynamic forces and moments are calculated by integrating surface and wake pressure distributions. Time-resolved particle image velocimetry and unsteady pressure measurements are carried out simultaneously to characterize the flow field over the pitching airfoil. The results presented here provide an insight into flow dynamics and the dynamic response of a stalled airfoil to pitching oscillation at different reduced frequencies. The present research adopts a dynamical system approach and attempts to estimate an optimum pitching oscillation frequency for better flow control on a stalled airfoil. The adopted methodology can be used to determine the optimal value of input parameters pertaining to flow control on a flat plate/wing by any unsteady excitation. Besides, a novel data-driven aerodynamic model has been developed on a Fourier basis for a symmetrical airfoil undergoing pitching oscillation at the post-stall regime.

3D pressure field reconstruction from time-resolved stereoscopic PIV measurements by relaxation of Taylor's hypothesis

This work presents reconstructions of 3D pressure fields starting from 2D3C stereoscopic-PIV (SPIV) measurements. In Fratantonio et al. (2021), we presented a new reconstruction algorithm, the “Instantaneous convection” method, capable of producing 3D velocity fields from time-resolved SPIV measurements. For reconstructions in flows with strong shear layers and high turbulence intensity, this method is able to provide time-resolved 3D velocity volumes that are more accurate than those that can be obtained from the more frequently employed reconstruction method based on the Taylor’s hypothesis and on the use of a mean convective field. Here we investigate the possibility of reconstructing the 3D pressure field from the timeresolved series of reconstructed 3D velocity data. A pseudo-tracking method is employed for computing the velocity material derivative, and the pressure field is then reconstructed by solving the 3D Poisson equation. The velocity and pressure reconstructions are validated on the Direct Numerical Simulation data of the turbulent channel flow taken from the John Hopkins Turbulence Database (JHTDB), and an application to experimental SPIV measurements of an air jet flow in coflow carried out at the Turbulent Mixing Tunnel (TMT) facility at Los Alamos National Laboratory is presented.

Investigation of the shear-layer instabilities in supersonic impinging jets using dual-time velocity measurements

Motivated by applications in the propulsion industry, the fundamental study of phase-locked shear-layer instabilities in supersonic impinging jets has been of research interest for long time. While such flows have been experimentally investigated in various research studies using time-unresolved particle image velocimetry (PIV) techniques, the understanding of the shear-layer dynamics is limited, due to the absence of temporal information. Time-resolved PIV measurements for high-speed flows require a large bandwidth, which is challenging to achieve with the current state of technology. An alternate approach using time-unresolved double-PIV measurements is presented in the current study, which provides multiple samples of dual-time data, depicted in figure 1. Such data can be obtained using two co-visual PIV systems, triggered at a user-selectable time-offset, ∆t. As shown by Sikroria et al. (2020), the application of techniques like dynamic mode decomposition (DMD) on time-unresolved dual-time data provides valuable information about the flow structures governing the shear-layer instabilities. The experimental setup for such measurements in supersonic impinging jets, followed by the determination of the relevant dynamical flow structures from the data, will be presented in the conference.

Multi-resolution, time-resolved PIV measurements of a Decelerating Turbulent Boundary Layer near Separation

We report on measurements of the time-evolving velocity profile of a turbulent boundary layer subjected to a strong adverse pressure gradient (APG) at Reynolds numbers up to Reθ ≈ 55 000 with an upstream friction Reynolds number exceeding Reτ ≈ 10 000. Near the point of flow separation high-resolution imaging at high camera frame rates captured the time evolving velocity profile using the so-called “profile-PIV” technique in a nested imaging configuration of two cameras operating at different image magnifications. One camera used an image magnification better than unity to resolve the viscous scales directly at the wall while the remainder of the roughly 200 mm thick boundary layer is simultaneous captured by the second camera. In the APG the variance of the stream-wise velocity exhibits no “inner peak” commonly found in turbulent boundary layers without pressure gradient influence. Spectral analysis further shows that the peak energy within the boundary layer shifts away from the wall toward lower frequencies. The overlap between the simultaneously imaged areas allows to assess and, to first order, correct for the effect of spatial smoothing on statistical quantities, spectra and related quantities. A multi-frame cross-correlation algorithm was used to process the extensive data base. In addition, a newly developed 2D-2C “Shake-The-Box” algorithm (STB) provided highly resolved particle tracking data beyond the reach of conventional PIV processing.

Unsteady flow structures behind a shark denticle replica on the wall: Time-resolved particle image velocimetry measurements

Experiments were performed to determine the unsteady flow structures of a shark denticle replica with three longitudinal ridges of different elevations and sharp trailing edges. Time-resolved particle image velocimetry measurements were conducted to capture the time-varying flow fields behind a shark denticle replica and contoured denticle, using the latter as a benchmark configuration for comparison. The flow fields in the streamwise and spanwise planes were obtained experimentally to determine the highly three-dimensional flow behavior. The flow dynamics of the unsteady superimposed flow structures behind the denticle replica are illustrated in terms of the time-averaged flow fields, statistical flow quantities, dominant vortex structures, and their phase-dependent evolution processes. The findings demonstrate that the denticle replica reduces the size and stability of the recirculation zone, suppressing flow separation. The velocity fluctuations and Reynolds shear stresses of the denticle replica are higher than those of the contoured denticle. Proper orthogonal decomposition analysis on the fluctuating velocity fields reveals alternating clockwise and counterclockwise vortices that convect along the streamwise direction, which correspond to decomposed flow structures, such as counter-rotating vortex pairs in the spanwise plane. These aspects are considered to be signatures of a hairpin-shaped vortex based on the phase-averaging analysis of the unsteady three-dimensional behavior. A hairpin-shaped vortex makes a concerted contribution to fluid mixing between the central wake region and surrounding area. As the fluid flows downstream, fast detachment of the hairpin-shaped vortex is recognized away from the wall to the main stream, which in turn attenuates the flow structure signatures in the spanwise plane. A low-frequency swinging motion of the fluid is also identified in the central wake region, which enhances fluid mixing on the two sides of the wake.

Evolution of flow structures in twin-rotors wakes in drones by time-resolved PIV

An experimental investigation of the noise emissions of a twin-rotor together with the evolution and spatial organization of the flow structures wakes has been carried out by means of aeroacoustics and time-resolved PIV (TR-PIV) measurements. Each rotor is characterized by three bladed propellers with diameter D = 393.7 mm running at four different rotational speeds (2620, 3500, 4360, 5200 RPM). Intricate flow patterns characterized by periodic vortical structures are formed in the wake of the rotors in twin configuration at the rotor-to-rotor distance of 1.02D. Their interaction determines a strong impact on the aerodynamic performances as well as on the noise generation. Hence, the need for TR-PIV measurements relies on what is the role of these instantaneous flow patterns to unveil the spatial organization and the dynamic behaviour inspected by imaging the region between the rotors. It is found that the flow organization and the interaction between evolution of vorticity intensity of tip vortices characterize the twin-rotors wakes.

Time-Resolved PIV Measurements and Turbulence Characteristics of Flow Inside an Open-Cell Metal Foam.

Open-cell metal foams are porous medium for thermo-fluidic systems. However, their complex geometry makes it difficult to perform time-resolved (TR) measurements inside them. In this study, a TR particle image velocimetry (PIV) method is introduced for use inside open-cell metal foam structures. Stereolithography 3D printing methods and conventional post-processing methods cannot be applied to metal foam structures; therefore, PolyJet 3D printing and post-processing methods were employed to fabricate a transparent metal foam replica. The key to obtaining acceptable transparency in this method is the complete removal of the support material from the printing surfaces. The flow characteristics inside a 10-pore-per-inch (PPI) metal foam were analyzed in which porosity is 0.92 while laminar flow condition is applied to inlet. The flow inside the foam replica is randomly divided and combined by the interconnected pore network. Robust crosswise motion occurs inside foam with approximately 23% bulk speed. Strong influence on transverse motion by metal foam is evident. In addition, span-wise vorticity evolution is similar to the integral time length scale of the stream-wise center plane. The span-wise vorticity fluctuation through the foam arrangement is presented. It is believed that this turbulent characteristic is caused by the interaction of jets that have different flow directions inside the metal foam structure. The finite-time Lyapunov exponent method is employed to visualize the vortex ridges. Fluctuating attracting and repelling material lines are expected to enhance the heat and mass transfer. The results presented in this study could be useful for understanding the flow characteristics inside metal foams.

Phase-averaged and cycle-to-cycle analysis of jet dynamics in a scaled up vocal-fold model.

Phase-averaged and cycle-to-cycle analysis of key contributors to sound production in phonation is examined in a scaled-up vocal-fold model. Simultaneous temporally and spatially resolved pressure and velocity measurements permitted examination of each term in the streamwise integral momentum equation. The relative sizes of these terms were used to address the issue of whether transglottal pressure is a surrogate for vocal-fold drag, a quantity directly related to sound production. Further, time traces of transglottal pressure and volume flow rate provided insight into the role of cycle-to-cycle variations in voiced sound production which affect voice quality. Experiments were conducted using a 10× scaled-up model in a free-surface water tunnel. Two-dimensional vocal-fold models with semi-circular ends inside a square duct were driven with constant opening and closing speeds. The time from opening to closed, To , was half the oscillation period. Time-resolved digital particle image velocimetry (DPIV) and pressure measurements along the duct centreline were made for 3650 ≤ Re ≤ 8100 and equivalent life frequencies from 52.5 to 97.5 Hz. Results showed that transglottal pressure does serve as a surrogate for the vocal-fold drag. However, smaller but non-negligible momentum flux and inertia terms, caused by the jet and vocal-fold motions, may also contribute to vocal-fold drag. Further, cycle-to-cycle variations including jet switching and modulation are inherent in flows of this type despite their high degrees of symmetry and repeatability. The origins of these variations and their potential role in sound production and voice quality are discussed.

Time-resolved particle image velocimetry measurements of the turbulent Richtmyer–Meshkov instability

Abstract

Quantitative evaluation of predictability of linear reduced-order model based on particle-image-velocimetry data of separated flow field around airfoil

A quantitative evaluation method for a reduced-order model of the flow field around an NACA0015 airfoil based on particle image velocimetry (PIV) data is proposed in the present paper. The velocity field data obtained by the time-resolved PIV measurement were decomposed into significant modes by a proper orthogonal decomposition (POD) technique, and a linear reduced-order model was then constructed by the linear regression of the time advancement of the POD modes or the sparsity promoting dynamic mode decomposition (DMD). The present evaluation method can be used for the evaluation of the estimation error and the model predictability. The model was constructed using different numbers of POD or DMD modes for order reduction of the fluid data and different methods of estimating the linear coefficients, and the effects of these conditions on the model performance were quantitatively evaluated. The results illustrates that forward (standard) model works the best with two to ten significant DMD modes selected by sparsity promoting DMD.

Investigation of Film Cooling Using Time-Resolved Stereo Particle Image Velocimetry

Abstract In the present study, an existing test rig at the Institute of Thermal Turbomachinery (ITS), Karlsruhe Institute of Technology (KIT), designed for generic film cooling studies is adopted to accommodate time-resolved stereoscopic particle image velocimetry (SPIV) measurements. Through a similarity analysis, the test rig geometry is scaled by a factor of about 20. Operating conditions of hot gas and cooling air inlet and exit can be imposed that are compliant with realistic engine conditions including density ratio (DR). The cooling air is supplied by a parallel-to-hot gas coolant flow-configuration with a coolant Reynolds number of 30, 000. Time-resolved and time-averaged stereo article image velocimetry data for a film cooling flow at high DR and a range of blowing ratios are presented in this study. The investigated film cooling hole constitutes a 10 deg–10 deg–10 deg laidback fan-shaped hole with a wide spacing of P/D = 8 to insure the absence of jet interaction. The inclination angle amounts to 35 deg. The time-resolved data indicate transient behavior of the film cooling jet.