Planar Particle Image Velocimetry Research Articles

Particle image velocimetry study of the flow control effects of singular and multiple curved serration models

This paper reports on preliminary observations of an investigation of flows associated with models that mimic serrations on the leading edge of barn owl feathers. The objective was to use particle image velocimetry measurements to determine the capacity of singular and multiple curved (three-dimensional) serration models to modify the noise-reducing indicators of a narrow-channeled flow past a cylinder. Four models were tested: 3 singular serration models of respective angles of inclination, α = 24°, 27.5° and 31°, and a model consisting of an array of 3 serrations of α = 24°, 27.5° and 31°. Each case was subjected to flow of Reynolds number (based on the serration height and maximum velocity of the flow) of ∼2,000, simulating the flow regime of local flow around barbs of real barn owl flights. A planar particle image velocimetry technique was used to capture the midspan plane velocities to determine the effects of each model. The results show that using singular serration models of inclination angles than 30° may lead to disorganized spatial structures and enhanced turbulence levels. On the other hand, an array of only 3 curved serrations of different geometries can modify the spatial flow structure into a well-ordered one, resulting in a 50% reduction in turbulence intensities. These initial results suggest that under complex flow conditions, the insertion of single and multiple curved serrations can lead to significant flow changes that may result in potential noise modifications.

The role of flow field dynamics in enhancing volatile organic compound conversion in a surface dielectric barrier discharge system

Abstract This study investigates the correlation between flow fields induced by a surface dielectric barrier discharge (SDBD) system and its application for the volatile organic compound (VOC) gas conversion process. As a benchmark molecule, the conversion of n-butane is monitored using flame ionization detectors, while the flow field is analysed using planar particle image velocimetry. Two individual setups are developed to facilitate both conversion measurement and investigation of induced fluid dynamics. Varying the gap distance between two SDBD electrode plates for three different n-butane mole fractions reveals local peaks in relative conversion around gap distances of 16 mm to 22 mm, indicating additional spatially dependent effects. The lowest n-butane mole fractions exhibit the highest relative conversion, while the highest n-butane mole fraction conversion yields the greatest number of converted molecules per unit time. Despite maintaining constant energy density, the relative conversion exhibits a gradual decrease with increasing distances. The results of the induced flow fields reveal distinct vortex structures at the top and bottom electrodes, which evolve in size and shape as the gap distances increase. These vortices exhibit gas velocity magnitudes approximately seven times higher than the applied external gas flow velocity. Vorticity and turbulent kinetic energy analyses provide insights into these structures' characteristics and their impact on gas mixing. A comparison of line profiles through the centre of the vortices shows peaks in the middle gap region for the same gap distances, correlating with the observed peaks in conversion. These findings demonstrate a correlation between induced flow dynamics and the gas conversion process, bridging plasma actuator studies with the domain of chemical plasma gas conversion.

Plasma-based base flow modification on swept-wing boundary layers: dependence on flow parameters

This work examines the control of cross-flow instabilities (CFIs) and laminar–turbulent transition on a swept wing, through the plasma-based base flow modification (BFM) technique. The effect of experimentally derived plasma body forces on the steady boundary layer base flow is explored through numerical simulations. Linear stability theory is subsequently used to predict the net BFM effect on CFIs. Based on these preliminary predictions, experiments are conducted in a low-turbulence wind tunnel where a spanwise-invariant plasma actuator is installed near the wing leading edge and operated at constant input voltage and frequency. Various flow parameters governing the plasma-based BFM technique are investigated, namely the Reynolds number, angle of attack and wavelength of excited stationary CFI modes. Stationary and travelling CFIs are quantified by planar particle image velocimetry while the transition topology and location are recorded by infrared thermography. The results confirm the stabilising effect of BFM on the swept-wing boundary layer. However, the plasma-based BFM is found to render the boundary layer more susceptible to travelling CFIs. In the presence of both net BFM effect and intrinsic plasma unsteady perturbations, the plasma-based BFM technique achieves transition delay with specific combinations of Reynolds number, angle of attack and wavelength of excited stationary CFI modes. The present findings provide insights into the fundamental principles of operating plasma actuators within the context of BFM control.

Flapping dynamics of a flexible membrane attached to the leading edge of a forward-facing step

An experimental investigation of velocity fields over and wall pressure fluctuations on a forward-facing step (FFS) disturbed by a flexible membrane has been conducted. A two-dimensional flexible membrane of 40mm length and 0.15mm thickness attached to a FFS of 40mm height was tested with oncoming velocity in the range of 7.3 < U < 12.9 m/s, corresponding to Reynolds numbers of 20391 < ReH < 35312 based on the step height. A high-speed camera was used to reconstruct the configuration of the flapping membrane. The flow field velocity dataset in the horizontal-vertical plane and the pressure fluctuation on the step surface were measured by planar particle image velocimetry and pressure sensors, respectively. The flexible membrane displayed two distinct modes, viz. a bending mode or flapping mode. In the bending mode, the membrane elevates the shear layer, thereby delaying the reattachment of separation flow generated by a FFS. In the flapping mode, the membrane periodically generates flapping-induced vortices (FIVs) of different scales with varying Reynolds numbers. The pressure on the FFS also undergoes periodic fluctuations corresponding to these FIVs.

DMD-based spatiotemporal superresolution measurement of a supersonic jet using dual planar PIV and acoustic data

The present study applies a framework of the spatiotemporal superresolution measurement based on the total-least-squares dynamic mode decomposition, the Kalman filter and the Rauch-Tung-Striebel smoother to an axisymmetric underexpanded supersonic jet of a jet Mach number of 1.35. Dual planar particle image velocimetry was utilized, and paired velocity fields of the flow with a short time interval were obtained at a temporal resolution of 5000 Hz. High-frequency acoustic data of 200,000 Hz were simultaneously obtained. Then, the time-resolved velocity fields of the supersonic jet were reconstructed at a temporal resolution of 200,000 Hz. Also, time coefficients of dynamic modes in high temporal resolution were calculated. The correlation between time coefficients implies that the mixing promotion by screech tone causes the lift-up of the high-velocity fluid from the jet center and accelerates at the downstream side.

Experimental evaluation of the flow field induced by an active vortex generator

This investigation examined the flow field generated by a ramp-shaped vortex generator (VG) that underwent active oscillation within a laminar boundary layer. The oscillations were applied through a servomotor, which pivoted the VG around its leading edge. The study evaluated the influence of varying the maximum VG height during the oscillations (h), actuation frequency (f), and the waveform governing the periodic oscillation of the VG. Planar particle image velocimetry (PIV) measurements were conducted to estimate flow mixing and the drag induced by the VG. The height-based Reynolds number (Reh) ranged from 300 to 600, and the chord-based Strouhal number (Stc) for the oscillations varied from 0.67 to 3.33. The findings of the study indicate that active VGs lead to a greater wall-normal transport of streamwise momentum and result in lower drag compared to static VGs. Furthermore, increasing h results in larger momentum transport and drag of the active VGs. The investigation also revealed that the highest momentum transport and drag occurred when f was close to the instability frequency of the shear layer. The results show the potential of active VGs for separation control under various flow conditions.

Three-Dimensional Nature of Low-Frequency Unsteadiness in a Turbulent Separation Bubble

Three-dimensional behavior of low-frequency unsteadiness in the incompressible turbulent separation bubble (TSB) produced by a wall-mounted hump is investigated using time-resolved planar and stereoscopic particle image velocimetry measurements at several planes across the separated region. The aspect ratio (wind tunnel width/separation length, Lz,WT/Lb=4.4) provides nominally two-dimensional flow for more than half of the spanwise extent of the test section. Analysis in the streamwise/wall-normal plane along the center of the test section shows low-frequency (St<0.1) large-scale motion of the separated region. The flowfield contains features of both geometry-induced and pressure-gradient-induced separation, but unsteady dynamics produce dominant frequencies closer to geometry-induced TSBs (St≈0.1) compared to purely pressure-gradient-induced TSBs (St≈0.01). Measurements along the spanwise direction and parallel to the initial shear layer development show strong evidence that the low-frequency motion is inherently three-dimensional, providing an additional dimension to the understanding of the flapping/breathing typically observed in planar streamwise/wall-normal measurements. Spectral proper orthogonal decomposition and low-order modeling identify spanwise undulations with wavelengths of the order of Lb and frequencies of St<0.1. The three-dimensional behavior causes a peak/valley formation along the span and a more localized expansion/contraction, leading to only small variations in the integral volume of the TSB.

Effect of incoming flow conditions on air lubrication regimes

Different air phase regimes are formed by controlled air injection in a spatially developing flat plate turbulent boundary layer (TBL). The air is introduced via a slot type injector without the use of a backward-facing step or cavitator upstream of the air injection position. The effect of different incoming liquid flow characteristics on the different regimes is investigated by varying both the liquid freestream velocity and the incoming TBL thickness. The latter is realized through changing the position of the air injection along the length of the water tunnel facility. That resulted in a downstream distance based Reynolds number from 1 to 5 million. Three different air phase regimes are identified under different air flow rates and freestream velocities: the bubbly regime, the transitional, and the air layer regime. The morphological differences of each one are described and quantitative analysis is performed based on the non-wetted area in each condition. The incoming TBL as well as the flow around the air layer are measured with planar particle image velocimetry. The latter enabled the determination of the air layer thickness. In addition, the ratio of the air layer to the incoming boundary layer thickness tair/δ is also calculated (≈ 0.04 – 0.5). This is a significant dimensionless parameter for scaling, which indicates the extent to which the air layer is embedded within the incoming TBL. Depending on the incoming flow conditions, a two or three branch air layer is formed. The length of the air layer is found to increase with increasing liquid freestream velocities. A good agreement between the air layer length and a half gravity wave predicted by the dispersion relation is found. An increase of the air layer length is observed with a decreasing incoming TBL thickness. This is attributed to a decrease in the local mean velocity at the air–water interface due to the TBL growth. Finally, increasing the incoming TBL thickness delays the onset of the air layer regime.

Leading-edge curvature effect on aerodynamic performance of flapping wings in hover and forward flight

This study investigates the role of leading-edge (LE) curvature in flapping wing aerodynamics considering hovering and forward flight conditions. A scaled-up robotic model is towed along its longitudinal axis by a rack gear carriage system. The forward velocity of the robotic model is changed by varying the advance ratio J from 0 (hovering) to 1.0. The study reveals that the LE curvature has insignificant influence on the cycle-average aerodynamic lift and drag. However, the time-history lift coefficient shows that the curvature can enhance the lift around the middle of downstroke. This enhanced lift is reduced from 5% to 1.2% as J changed from 0 to 1.0. Further flow examinations reveal that the LE curvature is beneficial by enhancing circulation only at the outboard wing sections. The enhanced outboard circulation is found to emanate from the less stretched leading-edge vortices (LEVs), weakened trailing-edge vortices (TEVs), and the coherent merging of the tip vortices (TVs) with the minor LEVs as observed from the phase-lock planar digital particle image velocimetry measurements. The far-wake observation shows that the LE curvature enhances the vorticity within the TV, helping to reduce the overall flow fluctuations in the far field. These findings can be extended to explain the predominantly straight LE wing shape with a small amount of curvature only observed near the wing tip for flapping fliers with Re from 103 to 104.

Pressure And Noise Estimation Of Tandem Cylinder Flow Based On PIV

The present work proposes an optimized method for pressure reconstruction and far-field noise prediction for tandem cylinder flow based on time-resolved planar particle image velocimetry (PIV). Proper orthogonal decomposition (POD) low-order reconstruction is applied to mitigate the incoherent noise introduced during PIV measurement and processing and enhances the identification of dynamic characteristics of relative structures within the flow field. As a result, PIV-based pressure reconstruction through solving the Poisson equation encompasses fewer errors arising from contaminated boundary conditions and Poisson source terms, thereby mitigating the propagation of errors into pressure. The time-marching algorithm accelerates the convergence and stabilizes the iterative progress while solving the Poisson equation, leading a considerable computation savings for cases with multiple snapshots like aeroacoustics relative investigation. The reconstructed wall pressure is compared with the reference pressure fluctuation signal simultaneously measured, yielding good agreement. Finally, the far-field noise was predicted through Curle’s analogy based on reconstructed wall pressure, considering the spanwise correction derived through an additional spanwise planar PIV. The predicted far-field noise agrees well with the reference microphone measurement. The overall assessment demonstrates that the employment of POD low-order reconstruction and time-marching algorithm significantly improve the speed and accuracy for estimating pressure field and far-field noise.

Kármán Street-Boundary Layer Interactions In Turbulent Flow

Understanding the intricate interplay between turbulent boundary layers and wake structures caused by obstacles in the flow is key to assessing the local shear stress fields as well as the drag. These types of flows can be relevant to natural phenomena including sediment and nutrient transport. While existing literature predominantly explores the flow downstream of cylinders mounted perpendicular to bounding surfaces or downstream of cylinders positioned outside the boundary layer region, this work experimentally studies the turbulent boundary layer passing over a cylindrical obstacle positioned near and parallel to the surface of an underlying solid wall. Planar particle image velocimetry (PIV) was performed in an open recirculating water channel for three test cases at a friction Reynolds number Reτ = 1320. Key parameters investigated include the cylinder diameter relative to the boundary layer thickness (D/δ = 0.09 and 0.01) and the gap between the cylinder and the wall (G/δ = 0.09), where the range of D/δ used is smaller than most examples found in the literature. Instantaneous and averaged velocity and vorticity fields are examined based on the PIV measurements. The results in Fig. 1 show how von Kármán vortex streets and turbulent boundary layer structures interact to form unique flow features. The larger cylinder generates a clear separation downstream of its axis as well as a wake that remains coherent over a distance δ downstream. The smaller cylinder with D/δ = 0.01 generates a much narrower and weaker wake, but still shows a clear influence on time-averaged velocity profile. Future experiments will evaluate flow conditions further downstream as well as additional values of G/δ for the same D/δ values.

Turbulence Modulation By Large Heavy Particles In Wall-Bounded Turbulence

"Most studies on particle laden flows have focused either on small heavy particles or large neutrally buoyant particles. We present, for the first time (to the best of our knowledge), results in the regime of large and heavy particles, investigating the particle-turbulence dynamics over a parameter space covering a wide range of particle-volume fractions, Stokes and Froude numbers. We consider a flow in a pipe aligned with the gravitational vector, thus allowing for the investigation of the two-way coupling between particles and carrier phase turbulence, while excluding the effect of gravity on the wall-normal migration of particles. Time-resolved images of both tracers (10 micrometers particles) and the dispersed particles (700 micrometers in diameter) were taken in the streamwise-wall-normal pipe plane using a planar particle image velocimetry (PIV) system consisting of a high speed camera (Phantom VEO 440), and an Nd:YLF Laser as light source. Using Voronoi tessellations to analyze the inertial particle dynamics, we show a strong deviation from Poisson behaviour for cases with and without turbophoresis. From the time-resolved PIV measurements, we also show that the carrier phase turbulence is modulated even at particle concentrations as low as 0.3% indicating that particle-induced stresses due to distortion of fluid streamlines cannot be neglected. An increase or decrease in the peak streamwise velocity fluctuation in the particle laden flow was observed, depending on the Stokes number, while the radial velocity fluctuation was found either to be the same as in the single phase flow case or higher. Furthermore, pressure drop measurements indicate a linear increase in the skin friction coefficient with volume fraction of particles. This was true for all Stokes and Froude numbers considered. The growth rate (β) of the skin friction coefficient was however observed to be only a function of the Froude number. A power-law fit to the data shows that β scales inversely with the Froude number indicating that for Froude numbers less than 1, the influence of gravity cannot be neglected. "

Resolving Flow Structures Inside Three Dimensional Packed Beds With Gas Flow By Ray Tracing Particle Image Velocimetry

Two different configurations are investigated in this study by ray tracing Particle Image Velocimetry to access the interior of different spherical packing structures. Configuration 1 consists of a regular bidisperse packed bed of 10mm and 40mm spheres based on a body-centered cubic arrangement. Ray tracing Particle Image Velocimetry was performed behind two spheres in the 40mm layer for particle Reynolds numbers from 200 to 700. The averaged velocity flow fields show the jet structures according to the pores between the underlying smaller spheres and the influence of the large spheres. Since ray-tracing Particle Image Velocimetry suffers from experimental and numerical limitations, planar 2D Particle Image Velocimetry measurements are performed to obtain spatially highly resolved validation data in the optically accessible void pores. These measurements were performed at different height positions in the bed showcasing that similar flow fields appear in the different pores. Configuration 2 deals with the jet flow (Re=5900) around an individual sphere, part of a body-centred cubic arrangement of d=40mm spheres. The emphasis lays on the temporally resolved snapshot data of the turbulent jet flow obtained by ray tracing Particle Image velocimetry. Data obtained from both configurations is used for a comprehensive discussion of the chances and challenges of ray tracing Particle Image Velocimetry. Limitations appear on the experimental side, like generation of depth of field, illumination and reflections, sufficient measurement signal, and accuracy of the geometry, but also on the numerical side, like the reproduction of the geometry, material properties, and real-world conditions. Since all these phenomena are interlinked, performing the ray tracing method in more complex systems is questionable.

Simultaneous Two-Colour Two-Dye Thermometry PLIF And PIV To Determine The Nusselt Number In Steady And Oscillatory Poiseuille-Rayleigh-Bénard Convection

In a flow field driven by natural or mixed convection, the measurement of heat transfer and hence the Nusselt number, requires simultaneous measurement of the vertical component of velocity and the vertical temperature gradient. The spatiotemporal nature of the formation of large-scale circulating structures, their interaction with the cross flow and its impact on heat transport, temperature distribution and wall shear stress also can be identified by simultaneous velocimetry and thermometry. An optical measurement system has been developed to apply a high sensetive ((~7 %)⁄℃) two-colour two-dye planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) to identify the flow and temperature organization and measure the heat transfer and wall shear stress. The system is to be used to investigate a Poiseuille-Rayleigh-Bénard convection system designed and fabricated to operate in both continuous and oscillatory conditions. The aim is to allow investigation of the influence and effect of large-scale flow structures and thermal plumes on heat and flow transport properties.

Application Of Particle Image Velocimetry For The Anatomical Comparison Of Haemodynamics In Healthy And Treated Aortas.

The leading cause of mortality and morbidity globally is cardiovascular disease (CVD). CVD can present in the thoracic aorta as atherosclerosis, an aneurysm or a dissection, which can be related to certain fluid mechanics metrics. This study investigates the fluid flow profiles within a healthy and a branched endovascular graft-treated rigid phantom, manufactured from Sylgard 184, to determine the effect geometry has on the resulting fluid mechanics and the subsequent biological processes. Planar particle image velocimetry data was recorded to visualise steady flow conditions at Reynolds numbers 4500 (peak), 3400 (mean), and −225 (reverse). Evaluation of anatomical features affecting the flow fields, concluded that the tapering of the healthy thoracic aorta naturally aids in the prevention of low flow on the inner arch wall more effectively than the untapered graft-treated model. Focus on the aortic branches demonstrated the angle of the aortic branches in relation to the aortic arch, highly affects the regions of low and high blood flow present within each branch. However, due to the biological response (or lack of) associated with a cellular boundary and synthetic material, the haemodynamic changes in the aortic arch and branches have different challenges. Therefore, direct replication of a healthy geometry is not necessarily optimal for the design of an endovascular graft.

Light Reflections Detection And Correction For Robotic Volumetric PIV

Laser light reflection mitigation in Particle Image Velocimetry (PIV) is crucial for accurate flow field measurements. While numerous methods exist for planar PIV, fewer have been developed for volumetric PIV systems, and in particular for coaxial setups like robotic volumetric PIV. Light reflections in volumetric PIV experiments result in high-intensity regions that corrupt particle detection and analysis. This study presents a novel approach for treating light reflections in robotic volumetric PIV experiments. The proposed method uses image filtering and masking techniques in the wavenumber space to separate particle images from reflection regions. The process involves decomposing the image signal into low- and high-wavenumber components using the 2D discrete Fourier transform (DFT) to then use a high-pass filter to attenuate the intensity of the reflection regions. Finally, a step of automated adaptive masking is applied to remove residual reflection areas that the filtering is not able to eliminate. The proposed approach is tested on experimental data obtained from experiments performed using robotic volumetric PIV on two different geometries: a side-view mirror and a rotating two-blade propeller. Comparison between raw and pre-processed images, as well as particle tracking results, is presented. The results from this data comparison show successful removal of reflection-induced artifacts in instantaneous images by using the spatial Fourier filter automated masking approach. The developed image pre-processing strategy effectively removes unsteady light reflection regions in robotic volumetric PIV images, preventing the appearance of spurious particle tracks and improving the accuracy of flow field measurements. The spatial gaps introduced by the masking procedure can be easily filled in via measurements from multiple directions, which are promptly achieved via the robotic volumetric PIV approach.

Analysis of coherent structures over interacting barchan dunes based on tomographic particle image velocimetry

The influence of barchan dune interaction upon unsteady flow separation and wake dynamics around the fixed-bed downstream barchan dune (DBD) model are experimentally investigated at a Reynolds number of 2640 based on the tomographic particle image velocimetry (PIV) system. The time-averaged statistics including the mean velocities, recirculation area, vortex spatial topology, Reynolds stress, and turbulent kinetic energy were used to characterize the flow field and large-scale anisotropy. It was found that arch-shaped vortex “chains” with strong spanwise coherence shedding from isolated barchan crestline populate the whole wake region, while elongated rod-shaped vortex structures with strong streamwise coherence induced by the up-downwash flow around the DBD were found to fill the whole measurement range, which is closely related to “sheltering” effect on the incoming flow acting at DBD due to the presence of upstream barchan dune (UBD). Additionally, in order to study the complex dynamic features of these predominated vortex structure transformations, time-resolved planar particle image velocimetry was applied. This technique allows for providing complementary insights into the temporal behavior of the unsteady coherent flow structures populating the wake field in different experimental configurations. It was found that the basic unsteady flapping motion, vortex roll-up, and complex vortex interactions including vortex pairing, merging, and breaking up can all be analyzed by dividing into certain scales with ease in a combination wavelet and Lagrangian framework.

On the role of curvature in the response of air-backed composites to hydrodynamic loading: An experimental study

Composite materials are increasingly utilized in high-performance naval structures, due to their superior properties over traditional materials like steel and aluminum. However, their widespread use is hindered by our limited understanding of their behavior when exposed to marine environments. One of the main loading conditions encountered during operations at sea is the hydrodynamic loading on the side in contact with water. Despite the practical significance of curved composite structures, the state-of-the-art on air-backed composites relies on the study of flat plates. Here, we use three-dimensional digital image correlation and planar particle image velocimetry to study the influence of curvature on the dynamic response of composite plates to hydrodynamic loading. Our findings reveal that curvature significantly influences both structural deformations and flow physics. The curved plate experiences localized vibrations with lower amplitude and higher frequency, caused by its increased stiffness. Additionally, the hydrodynamic pressure at the center of the plate decays faster in time for the curved configuration, highlighting the importance of curvature in shaping fluid–structure interactions. Our results advance the understanding of fluid–structure interactions in composite materials and highlight the importance of curvature in the design of resilient marine structures.

Swirling flow field reconstruction and cooling performance analysis based on experimental observations using physics-informed neural networks

The design of thermal protection modules (such as film cooling) for combustion chambers requires a high-fidelity swirling flow field. Although numerical methods provide insights into three-dimensional mechanisms of swirling flow, their predictions of key features such as recirculation zones and swirling jets are often unsatisfactory due to inherent anisotropy and the isotropic nature of the Boussinesq hypothesis. Experimental methods, such as hot wire, laser Doppler velocimetry, and planar particle image velocimetry (PIV), offer more accurate reference data but are limited by sparse or planar observations. In this study, considering the outperformed capability of solving inverse problems, physics-informed neural network (PINN) was adopted to reconstruct the mean swirling flow field based on limited experimental observations from two-dimensional and two-component (2D2C) results. It was found that adding partial information characterizing the swirling flow, such as the swirling jet, could significantly improve the reconstruction of flow field. In addition, film cooling effectiveness was the key variable to evaluate the film cooling performance, which was relatively measurable in the scalar field. To further improve the accuracy of the reconstruction, the multi-source strategy was adopted into the neural network, where the film cooling effectiveness (FCE) of the effusion plate was imported as the scalar source. It was found that the prediction of the flow field near the target plate was improved, where the highest error reduction could reach 76.5%. Finally, through the reconstructed three-dimensional vortex distribution, it was found that swirling flow vortex structures near the swirler exit had a significant impact on cooling effectiveness, causing a non-uniform cooling distribution. This study aims to diagnose the three-dimensional swirling flow field with deep learning by leveraging limited experimental data and deepen the understanding of effusion cooling under swirling flow condition so that obtains a more accurate reference in the design of thermal protection modules.

Flapping dynamics of a flexible membrane attached to the leading edge of a forward-facing step

The flapping dynamics of a flexible membrane (FM) and its effect on the flow fields over and pressure fluctuations on a forward-facing step (FFS) have been investigated experimentally. The two-dimensional FM with 40 mm in length and 0.15 mm in thickness was vertically attached to the leading edge of a FFS with 40 mm in height. The deformation of the flapping FM was recorded by a high-speed camera. Velocity data in the vertical central plane and the pressure fluctuations on the step surface were measured by planar particle image velocimetry and pressure sensors, respectively. The results demonstrate that as the dimensionless bending rigidity (γ) of the FM decreases, the FM displayed two distinct modes, i.e., the bending mode and the flapping mode. In the bending mode, the bent FM is similar to a curved barrier, which elevates the shear layer and delays the reattachment of separation flow. In the flapping mode, the amplitude of the FM increases with the decrease in γ, which in turn effects the scale of flapping-induced vortices (FIVs). In proper orthogonal decomposition analysis, the results reveal a transition in the dominant flow structure from large-scale separation to FIVs with reducing γ. The FIVs significantly affect the pressure distribution on the step surface of the FFS, and the range where the coherent contribution dominates expands with the decreasing γ.