Vortex Shedding Frequency Research Articles

Characteristics of geometry-and pressure-induced laminar separation bubbles at an enhanced level of free-stream turbulence

Responses of a geometry-induced separation bubble (GISB) and a pressure-induced separation bubble (PISB) at enhanced levels of free-stream turbulence (FST) have experimentally been investigated for a comparative study using the particle image velocimetry (PIV) technique. The outlines of separation bubbles based on the dividing streamlines are self-similar for different levels of FST and Reynolds numbers. The spectral analyses of the time-resolved PIV data show that the vortex shedding frequency of a separated shear layer remains unchanged for the GISB cases even with an enhanced level of FST. In contrast, it is different for the PISB cases. We propose a criterion that determines whether the frequency will remain the same even for the cases with FST. Linear stability analyses reveal that the inviscid-inflectional instability dominates the transition process, and the linear stages of transition are not completely bypassed even at an enhanced level of FST. The most amplified frequencies, while scaling with the displacement thickness and the boundary layer edge velocity, collapse in a single curve for all the cases. Furthermore, measurements in the spanwise plane show that the streamwise velocity streak/Klebanof mode at an enhanced level of FST is not a general flow feature for all types of separation bubbles. However, at an enhanced level of FST for the PISB case, the boundary layer streaks are found to distort the two-dimensional vortex structure associated with the Kelvin–Helmholtz instability, eventually leading to a three-dimensional $\varLambda$ -like structure in the spanwise plane.

Vortex dynamics and associated turbulence production in the separated transitional flow and the wake at a moderate Reynolds number

The vortex dynamics in the separated transitional flow and the near-wake over a C4 compressor blade at Rec = 24,000 and incidence angles of 0°, 2.5°, 5°, 7.5°, and 10° are investigated experimentally. The laminar separation vortex shedding, the trailing edge vortex shedding, and the interaction of the two vortices are captured using the time-resolved particle image velocimetry. To separate the contributions to the overall turbulence production due to various flow dynamics, the orthonormality of proper orthogonal decomposition modes of the velocity components is utilized. When the laminar separation is close to the trailing edge, the laminar separation vortex locks in the wake instability with an approximately subharmonic of the von Karman frequency. Approximately 80 % of the turbulence is produced by the large-scale flow structure governed by the laminar separation vortex and trailing edge vortex. When the shear layer separates near the leading edge, the laminar separation vortex shedding frequency is lower with a smaller streamwise wavelength. The turbulence production rate contributed by the laminar separation vortex shedding increases with incidences. These novel experimental findings offer insight into transition mechanisms and turbulence modeling of the separated flow located near periodic wake flow.

Experimental near-field analysis for flow induced noise of a structured porous-coated cylinder

Porous coating on cylinders reduces the aerodynamic noise due to vortex shedding. A key understanding of how porous media suppress vortex shedding is related to near-field hydrodynamics. In this paper, an experimental study was undertaken over the Reynolds number range of 0.52×105≤Re≤1.74×105 to measure outer diameter surface pressure fluctuations of a structured porous-coated cylinder (SPCC). The near- field pressure and far-field noise were measured simultaneously to obtain a deep understanding of the noise-reduction mechanism of the SPCC. The results showed that the SPCC causes a delay in the boundary layer separation compared to the bare cylinder. While a substantial reduction in far-field noise was provided by the SPCC, a high-frequency noise attributed to the non-propagating hydrodynamic energy field within the SPCC was observed. The use of an SPCC led to a reduction in the surface pressure fluctuations over the entire frequency range, acting as a noise reduction mechanism. It was shown that the surface pressure fluctuations and acoustic signals are strongly correlated at the vortex shedding frequency for the bare cylinder, while it was insignificant for the SPCC, indicating the strong role of the SPCC in reducing the surface pressure energy content so that it cannot propagate to the far-field. The evaluation of vortex shedding in the time–frequency domain was carried out with the aid of wavelet transform. It was observed that amplitude modulation in time was verified only for the bare cylinder at the fundamental vortex shedding frequency in the pre- and post-separation regions.

A Comprehensive Analytical Model for Vortex Shedding From Low-Speed Axial Fan Blades

Abstract The paper presents a comprehensive analytical model for the characterization of von Karman vortex shedding in the wake of models of low-speed axial fan blades. The elaborated minimal model is based on the Reynolds-averaged Navier–Stokes and continuity equations. For validation purposes, hot-wire measurements have been carried out in a wind tunnel on representative blade profiles. The measurement data obtained for various streamwise positions downstream of the blade trailing edge, i.e., transversal profiles of mean velocity as well as root-mean-square of fluctuating velocity, are evaluated. As the experimental validation demonstrates, the minimal model fairly localizes the transversal position of the vortex centers and represents the motion of the vortices along the wake. The validated minimal model serves the following benefits: (a) an extensive understanding of the underlying physics related to the flow field featuring vortex shedding in the near-wake region (easy-to-use quantitative correlation among the characteristics of wake flow affected by the shed vortices); (b) extension of the literature-based methodology for determination of the transversal distance between the shed vortex rows, being used as a scaling parameter for the Strouhal number utilized in calculation of vortex shedding frequency; and (c) modeling the behavior of rows of shed vortices farther away from the trailing edge. Such behavior may influence the acoustic signature of VS, and, as such, it is to be considered in fan noise modeling.

Vortex-Induced Vibrations of an Elastic Micro-Beam with Gas Modeled by DSMC.

The fluid-structure interaction is one of the most important coupled problems in mechanics. The topic is crucial for many high-technology areas. This work considers the interaction between an elastic obstacle and rarefied gas flow, seeking specific problems that arise during this interaction. The Direct Simulation Monte Carlo method was used to model the rarefied gas flow and the linear Euler-Bernoulli beam theory was used to describe the motion of the elastic obstacle. It turned out that the vibrations caused by the gas flow could provoke a resonance-like phenomenon when the frequency of vortex shedding of the flow was close to the natural frequency of the beam. This phenomenon could be useful in certain high-technology applications.

Vortex-induced vibrations of a free spanning submarine power cable

Vortex-induced vibrations (VIV) of a flexible structure occur when the vortex shedding frequency is close to the natural frequency of the structure. Submarine power cables exhibit more complex vortex-induced dynamics than the top-tensioned risers and submarine pipelines, because their modal properties such as the frequencies and modal shapes are affected by the structural sags. In order to better understand this problem and improve the frequency-domain analyses of vortex-induced cable fatigue damage risks, a laboratory study on VIV of a free spanning submarine power cable in steady currents with four sags is conducted. The scaled laboratory model is designed based on the gravity similarity and elastic similarity. With the geometric similarity ratio of 1:7, it represents in prototype a 500 kV self-contained oil-filling submarine power cable. The measured data indicates that the lock-in of the first antisymmetric, first symmetric and second symmetric modes all take place within the flow ranges in the experiments. The analysis further shows that the VIV features of the submarine power cable are significantly affected by the sags. At sag d = 0.23m, the cable mainly exhibits a single-mode VIV response. As the sag decreases from 0.23m to 0.09m, the natural frequencies of the cable are more closely spaced and its VIV tends towards a multi-mode response. During the multi-mode response, the first antisymmetric, first symmetric and second symmetric modes are observed to coexist. Among the excited modes, because the natural frequency of the first symmetric mode is the intermediate one, its vibration is apparently suppressed. The maximum root-mean-square value of the modal weight of the first symmetric mode decreases from 0.26 to 0.086 as the sag decreases from 0.23m to 0.09m.

Development and characterization of a passive, bio-inspired flow-tracking sensor

The effective natural transport of seeds in turbulent atmospheric flows is found across a myriad of shapes and sizes. However, to develop a sensitive passive sensor required for large-scale (in situ) flow tracking measurements, systems suffer from inertial lag due to the increased size and mass needed for optical visibility, or by carrying a sensor payload, such as an inertial measurement unit (IMU). While IMU-based flow sensing is promising for beyond visual line-of-sight applications, the size and mass of the sensor platform results in reduced flow fidelity and, hence, measurement error. Thus, to extract otherwise inaccessible flow information, a flow-physics-based tracer correction is developed through the application of a low-order unsteady aerodynamic model, inspired by the added-mass concept. The technique is evaluated using a sensor equipped with an IMU and magnetometer. A spherical sensor platform, selected for its symmetric geometry, was subject to two canonical test cases including an axial gust as well as the vortex shedding generated behind a cylinder. Using the measured sensor velocity and acceleration as inputs, an energized-mass-based dynamic model is used to back-calculate the instantaneous flow velocity from the sensor measurements. The sensor is also tracked optically via a high-speed camera while collecting the inertial data onboard. For the 1D test case (axial gust), the true (local) wind speed was estimated from the energized-mass-based model and validated against particle image velocimetry measurements, exhibiting good agreement with a maximum error of 10%. For the cylinder wake (second test case), the model-based correction enabled the extraction of the velocity oscillation amplitude and vortex-shedding frequency, which would have otherwise been inaccessible. The results of this study suggest that inertial (i.e. large and heavy) IMU-based flow sensors are viable for the extraction of Lagrangian tracking at large atmospheric scales and within highly-transient (turbulent) environments when coupled with a robust dynamic model for inertial correction.

Numerical simulations of flow around dual tandem circular cylinders under a strong axial magnetic field

This work is dedicated to understand the magnetohydrodynamic (MHD) flow around two identical tandem circular cylinders confined in a duct under external axial magnetic field. The influences of magnetic field (represented by the Hartmann number Ha), inlet velocity of conducting fluid (represented by the Reynolds number Re), and the gap ratio (L/d, the ratio of the distance between the centers of two cylinders to the diameter of the circular cylinder) on the flow regimes, vortex shedding frequency, pressure coefficients, drag coefficients, and pressure drop are investigated. The simulations are conducted in the parameter ranges 2≤L/d≤8, 180≤Re≤1440, and 101≤Ha≤1818, respectively. Four flow modes are observed, namely, no vortex shedding, single body, reattachment regime, and vortex shedding (VS) modes, with various Ha/Re2 and L/d values. For L/d>5, the flow field presents the VS flow model, a vortex street with synchronous periodic shedding appears behind the two cylinders, and the vortex shedding frequency matches well with that of the single one. The pressure coefficient shows different tendencies because of the arrangement of two circular cylinders. It can be found that the mean drag coefficients for the upstream circular cylinder and the downstream circular cylinder vary with Re/Ha0.8. For Re=361, Ha=504 and Re=722, Ha=1212, the flow field remains almost unchanged, which implies the balance between inertial force and Lorentz force because the inertial force would promote the flow, while the electromagnetic force leads to the reverse effect. Furthermore, the effects of magnetic field and inlet velocity of metal fluid on pressure drop ΔP between inlet and outlet can be fitted as a linear relation ΔP∼ReHa.

Relationship between vortex shedding noise and remotely-sensed surface pressure fluctuations of a structured porous-coated cylinder

Tonal noise suppression of a cylinder placed in uniform flow has been achieved, to some extent, by coating it with a structured porous material as a form of passive flow and noise control. A previously studied structured porous-coated cylinder is investigated in an anechoic wind tunnel to determine the relationship between the far-field vortex shedding noise and the pressure recorded on the outer porous surface. To date, no experimental studies have been conducted on the surface pressure of any type of porous-coated cylinder. Acoustic measurements are obtained using an equispaced microphone arc array and simultaneously unsteady surface pressure fluctuations are obtained around the cylinder mid-span circumference using remote-sensing techniques. By obtaining simultaneous time-dependent signals, more light is shed on the underlying noise-reduction mechanism of the structured porous-coated cylinder. In this paper, strong relationships between surface pressures and acoustic signals are revealed at the vortex shedding frequency. A spatio-temporal relationship between surface pressure and vortex shedding phenomena is also presented that helps explain the role of the structured porous media in passive flow and noise control.

Numerical investigation of thermo-acoustic instability in a model afterburner with a simplified model for observed lock-in Phenomena

Thermoacoustic oscillations in a gas turbine afterburner are numerically investigated using CFD. A simplified 2-dimensional axisymmetric afterburner with bluff-body stabilized flame is considered in the investigation. Occurrences of both low and high-frequency thermo-acoustic oscillations in the afterburner chamber are observed at specific fuel flow rates. The flow field from the CFD shows the bluff-body vortex shedding frequency to lock-in with the acoustics of the chamber during the thermo-acoustic oscillations. The synchronization and lock-in of bluff-body wake with chamber acoustics happen with increase in fuel injection rates resulting in thermoacoustic coupling. The Proper Orthogonal Decomposition of the flow field revealed the presence of chamber acoustics in the pressure field confirming the coupling. Then a simplified mathematical model based on the van-der Pol oscillator is attempted to reproduce the observed lock-in behavior of the bluff-body wake. The chamber acoustic field is considered as the forcing term for the simplified oscillator. The oscillator model qualitatively captures the synchronization of the flame-holder wake oscillations with the chamber acoustics. This model could be extended to combustors with bluff-body wake in predicting the thermo-acoustic oscillations.

Effects of side and corner modification on the aerodynamic behavior of high-rise buildings considering serviceability and survivability

This study explores the complementary effects of side and corner modification on the aerodynamic behavior for high-rise buildings across representative design wind speeds. Twelve doubly-symmetric prismatic models were examined using high-frequency force balance (HFFB) wind tunnel testing at the University of Florida. The effectiveness of the aerodynamic strategies was quantified using roof drift and roof acceleration under different design wind speeds covering serviceability and survivability. The results show that both corner and side modifications can achieve promising aerodynamic performance under high design wind speeds. However, the effectiveness of the aerodynamic strategies is significantly reduced under low design wind speeds. With a corner modification strategy, the vortex shedding frequency is increased, leading to worse across-wind response at lower design wind speeds when compared to the square benchmark model. To address this issue, side modifications (i.e., side protrusions) can be used to preserve the vortex shedding frequency and achieve competitive aerodynamic performance while simultaneously maintaining the floor area and geometry. This research explores new aerodynamic modification options for owners, architects, and structural engineers with the aim of better aerodynamic performance for high-rise buildings without compromising other design objectives.

Study on Nonlinear Correlation in Modal Coefficients of the Bionic Airfoil

Applying bionic airfoils is essential in enlightening the design of rotating machinery and flow control. Dynamic mode decomposition was used to reveal the low dimensional flow structure of Riblets, Seagull, and Teal bionic airfoils at low Reynolds numbers 1 × 105 and is compared with NACA4412 airfoils. The attack angle of the two-dimensional airfoil is 19°, and the SST k-ω turbulence model and ANSYS fluent were used to obtain the transient flow field data. The sparse identification of nonlinear dynamics reveals the nonlinear correlation between modal coefficients and establishes manifold dynamics. The results show that the bionic airfoil and NACA4412 airfoil have the same type of nonlinear correlation, and the dimension and form of the minimum reduced-order model are consistent. The modal coefficients always appear in the manifold equation in pairs with a phase difference of 90°. The dimension of the manifold equation is two-dimensional, and the absolute value of the coefficient corresponds to the fundamental frequency of airfoil vortex shedding. The reconstructed flow field based on the manifold equation is highly consistent with the numerical simulation flow field, which reveals the accuracy of the manifold equation. The relevant conclusions of this study emphasize the unity of the nonlinear correlation of bionic airfoils.

Numerical investigation of noise suppression and amplification in forced oscillations of single and tandem cylinders in high Reynolds number turbulent flows

The noise generated by a three-dimensional turbulent wake behind an oscillating single or tandem cylinder(s) is studied using detached eddy simulation and the Ffowcs Williams-Hawkings (FW-H) method. This hybrid methodology is validated using some experimental data of a turbulent flow past a single or tandem stationary cylinder(s) and some numerical data (obtained from a direct numerical simulation) of a laminar flow past tandem oscillating cylinders. For an oscillating cylinder at a fixed Reynolds number Re=120,000, the sound energy is suppressed compared to that of a stationary cylinder provided the value of oscillation frequency approaches 57% of the original vortex-shedding frequency of the stationary cylinder. This noise reduction arises from the structural oscillations suppressing the noise generated by the vortex shedding from the stationary cylinder—the additional source of noise stemming from the structural oscillations is too small to compensate for the mechanism responsible for the noise suppression. The noise generated by the cylinder motion includes contributions from the loading and thickness noise, whereas the noise generated by vortex shedding from the stationary cylinder only includes the loading noise. The flow past tandem stationary cylinders exhibits a larger number of spectral peaks in the sound pressure power spectrum and the peaks occur at a lower frequency compared to the flow past a single cylinder. The drag force fluctuations result in the propagation of the sound pressure in the in-line direction, whereas the lift force fluctuations result in the propagation of the sound pressure in the transverse direction. Five different scenarios of stationary and/or oscillating tandem cylinders are studied. The second and third invariants of the velocity gradient tensor are used to visualize the distinct vortex structures in the wake for the five different scenarios of tandem cylinders. A single cylinder oscillating in the transverse direction exhibits a larger sound pressure level (SPL) than the streamwise oscillation of the same cylinder. In contrast, the streamwise (in-line) oscillation of one or the other cylinder in a tandem array results in a larger SPL than the transverse oscillation of one or the other cylinder. Finally, the SPL is more sensitive to the oscillations of the rear (downstream) cylinder than the front (upstream) cylinder.

Flow visualization observation of the interference vortex flow from a pair of square cylinders

In order to investigate the aspect of the mutual interference flow from two square cylinders, the visual water-flow observation experiment was performed. In the experiment, the pitch ratio L/d and the arrangement angle α were made into the parameter, and investigation of vortex shedding frequency or the flow pattern was performed. The pitch ratio L/d is 3 kinds, 1.13, 1.89 and 4.15, and an arrangement angle α is 7 kinds per 15 degree from 0 degrees to 90 degrees. The water flow velocity U was 0.04 m/s which correspond to the Reynolds number about 850. As the result of experiment, the complicated flow becomes clear by classification by color of tracer ink, and the aspect of the mutual interference vortex flow can understand better. Even if the pitch ratio was the same, the vortex shedding characteristics changed with arrangement angles. If an arrangement angle is smaller than a geometric duplication angle, the vortex shedding frequency from each square cylinder is the same. On the other hand, when an arrangement angle is larger than the geometric projection angle, the vortex shedding frequency from each square cylinder differs. In the smallest pitch ratio as L/d= 1.13, it is not dependent on the arrangement angle, and produce mutual interference.

Effect of synthetic jet on circular cylinder radiated noise in laminar flow state

This article focuses on the effect of active control via synthetic jets (SJs) on sound generated by a two-dimensional circular cylinder by using the acoustic analogy method. The cylinder is immersed in a uniform upstream flow, where the corresponding Reynolds number is 100 and the Mach number is 0.1. A pair of SJs is placed near the cylinder’s separation point issuing periodically varying forcing, with different combinations of forcing frequency and phase difference. The lattice Boltzmann method (LBM) is coupled with the multi-direct forcing immersed boundary method to solve the near-field flow dynamics. The mechanism of the sound generation lies in the fact that pressure pluses are induced by the periodic vortex shedding from the cylinder’s surface, i.e. dipoles. In the case with active flow control, extra monopoles are generated by the unsteady flow rate resulting from the SJs' periodic blow/suction. The interaction between monopoles and dipoles is confirmed to have a big influence on the acoustic field. The acoustic analogy method is used in various cases with a wide range of control parameters, because it has a considerably lower computational cost than the direct simulation method. Taking into account the effect of the monopole, the acoustic analogy method is developed for solving two-dimensional sound field by substituting the Green’s function. Results indicate that the primary lock-on and the secondary lock-on occur in the case of specified control parameters. The frequency of vortex shedding is related to the SJs’ frequency, deviating from the unforced frequency. Owing to the noise induced by flow, the frequency and phase difference of the SJs also have significant influence on sound field. The far-field noise is enlarged although the SJs reduce drag, due to the induced extra monopole, as well as the strengthened hydrodynamic fluctuation. Further increasing SJs’ frequency or reducing the phase difference will enlarge the far-field noise and make the directivity transformed from dipole to monopole, since the SJs’ self-noise is stronger. Moreover, it is found that the acoustic power increases approximately 4–18 dB compared with the unforced circular cylinder and the drag dipole is strengthened in all combinations of control parameters. This study deepens the understanding of the effect of SJs on sound field, and provides a reference for future studying the control strategies of suppressing noise generated from bluff bodies.

Unsteady flow past slotted circular cylinders in laminar regime: Effect of slit shapes and Reynolds number

A numerical study of incompressible and unsteady flow past a slotted circular cylinder was carried out in a laminar regime. Three different-shaped slits (converging, diverging, and parallel) were installed symmetrically about the cylinder's horizontal axis. A finite volume-based open-source computational toolbox, OpenFOAM, was used to compute the flow field in the Reynolds number (Re) range of 60–180. The results showed the presence of the periodic vortex shedding for all Re ranges and the slotted cylinders were effectively able to suppress it. The surface vorticity over the slotted cylinder surface increased by 16%–23% as compared to the normal cylinder. The momentum injection to the cylinder base reduced the strength of vortex shedding and shedding frequency for the slotted cylinders. Pressure recovery was improved due to an increase in the pressure coefficient which further led to a decrease in the pressure drag. The total drag coefficient for the slotted cylinders was lower than the normal cylinders for Re > 70. Also, a comparatively lower Strouhal number was observed for the slotted cylinders.

On wave–current interaction with a horizontal cylinder: Load characteristics and vortex pattern

This study performed a two-dimensional experiment to reveal the load characteristics of a submerged horizontal cylinder with vortex patterns in the wake region under wave–current flows. Wave–current conditions were tested with both following and opposing currents. Instantaneous representations of the velocity field, streamline topology, and vorticity patterns, obtained using particle image velocimetry, provided insight into the origin of the unsteady loading of the cylinder. The spatiotemporal characteristics of the pressure distribution on the cylinder were analyzed using Fourier series expansion across the circumference of the cylinder. The results demonstrate that, unlike the action of current only, the presence of waves modifies the frequency and position of vortex shedding, resulting in three distinct modes in the frequency domain of the vertical force on the cylinder, where each mode exhibits a distinct mode of vortex shedding. The variation in frequency domain of Fourier coefficients of pressure around the cylinder is consistent with the force corresponding to different modes. Moreover, velocity rate β and the frequency relationship between the wave and current-induced vortex shedding play important roles in the frequency characteristics of the vertical force.

Drag Reduction for Flow Past Circular Cylinder Using Static Extended Trailing Edge

AbstractA numerical study has been carried out on the two-dimensional flow past a circular cylinder. In this case, a splitter plate is provided at the rear stagnation point in the downstream direction. ansys fluent has been used to carry out the numerical simulations based on finite volume method approach. Grid independence was achieved and the numerical model was validated with results available in open literature at Reynolds numbers of 100, 5000, and 100,000 respectively. In the present investigation, the characteristics of vortex shedding due to the presence of splitter plate in the circular cylinder were investigated. The main focus of this research was to find the optimal splitter plate length for low, moderate, and high Reynolds numbers. It was observed that at low, moderate, and high Reynolds numbers, the drag coefficient (cd) for optimal plate length decreased drastically as compared to the baseline circular cylinder case. Moreover, the fluctuating nature of lift coefficient (cl) had also ceased. This research work has a good potential to decrease time-varying structural loads on bluff bodies by decreasing the vortex shedding frequency and consequently decreasing drag. The scope of our research extends to structures of bridges and large vehicles, radiator pipes of heat exchangers, landing gears of aircraft, and many more.

Piezoelectric energy harvesting from vortex-induced vibrations on a GFRP beam with embedded piezoelectric patch

In the present era, the demand for energy continues to increase and nevertheless, energy resources are gradually decreasing. Therefore, extracting energy from the operating ambient is of great importance especially for industrial applications. Among the numerous available ambient energy sources, wind energy is one of the most promising and prevalent energy sources existing in the environment. In this study, a piezoelectric energy harvester (PEH) consisting of an electromechanical coupling of GFRP cantilever beam with an embedded piezoelectric patch is developed for wind energy harvesting. The cantilever beam under the wind flow vibrates due to the pressure field that occurs on the leeward side of the beam. The generation of the pressure field is based on the vortex shedding phenomenon. Theoretical model of the regarding electromechanical coupling subjected to vortex induced vibration is presented and the effect of the pressure field having various vortex shedding frequencies on harvested power is investigated by means of numerical simulations validated with an experimental study. In order to determine the effect of the direction in which the wind excites the PEH, two wind flow conditions are considered; cross wind and head wind. According to the results, it was found that the PEH generates considerably more voltage outputs under cross wind loading than that obtained from the head wind excitation. In cross wind case, maximum open circuit voltage of 82.4 V is obtained at the wind speed of 6 m/s with the vortex shedding frequency of 18 Hz, which is very close to the second resonance frequency of the PEH. With a calculated load resistance of 100 kΩ, the resulting maximum direct voltage and electric power is 58.7 V and 11.5 mW, respectively. As far as the energy efficiency of PEH is concerned, it is determined that the efficiency is about 0.75 for the frequency of 18 Hz, which is quite acceptable for energy harvesting. It is concluded that a composite PEH with an embedded piezoelectric patch can be used as an effective energy harvester for the vortex induced vibration when the vortex shedding frequency is close to its resonance frequency.

Mechanisms of Morphing Wall Flow Control by Traveling Waves over an Airfoil

The main two mechanisms of morphing wall flow control are direct injection of momentum in the streamwise direction and indirect momentum transfer via triggering instabilities. Traveling waves have been shown to perform better than standing waves, probably because they can use both mechanisms. However, the relative importance of the two mechanisms is not known. To differentiate between the mechanisms, a range of parameters (frequency, amplitude, and starting location) at stall (15 deg angle of attack) and poststall (20 deg angle of attack) is tested using wall-resolved large-eddy simulations with a sharp-interface curvilinear immersed boundary method at a low Reynolds number of over a NACA0018 airfoil. The results of the simulations demonstrate that the flow is reattached within a range of nondimensional frequencies, actuation amplitudes, and starting locations of oscillation at the stall and poststall angles of attack. Significant lift enhancement and drag reduction are also observed within these ranges. The nondimensional frequency range at which the flow is reattached is found to be similar to the dominant nondimensional frequencies of leading-edge vortex shedding of the unactuated airfoil. These indicate that the indirect transfer of momentum is the dominant mechanism because direct injection of momentum increases with the increase of amplitude and frequency; that is, separation should reduce as they increase. Nevertheless, direct injection of momentum improves the performance relative to pure excitations of standing waves when instabilities are triggered.