Frequency Of Shear Layer Research Articles

Flow Reattachment on a NACA 0025 Airfoil Using an Array of Microblowers

An array of commercially available synthetic jet actuators was developed for controlling flow separation on a NACA 0025 airfoil. The array is more compact in comparison to standard synthetic jet actuators previously studied for separation control. Wind tunnel experiments were conducted at chord Reynolds number and . Control of separation was applied by the array through forcing at the wake and shear layer frequencies ( and , respectively). Both effectively suppress flow separation, increase lift, and decrease drag. The dynamics of the reattached flow show different feature for the two forcing schemes. The results demonstrate that this array has comparable control effectiveness on separation to other synthetic jet actuators.

Turbulent flow over a cylinder confined in a channel at Re = 3,900

This paper presents data from numerical simulation of the flow over a cylinder confined in a channel at Reynolds number of 3,900. Direct numerical simulation (DNS) of the flow field from blockage ratio of 0 (i.e. the canonical flow over an unconfined cylinder) up to blockage ratio of 0.7 have been performed. At large blockage ratios, an adverse pressure gradient develops and the boundary layer on the channel walls separates and interacts with the cylinder wake. It is observed that the interaction gives rise to complex flow dynamics, affecting the size of the wake, making the wake asymmetric and also resulting in a decrease in the intensity of the turbulent fluctuations in the shear layer. Analysis of the spectra of the turbulent near wake shows an increase of the shedding frequency of the Kármán vortices as blockage ratio is increased. This occurs up to around blockage ratio of 0.7 where it is observed that the formation and shedding of coherent Kármán vortices no longer occur. For all except the blockage ratio of 0.5 case, the frequency of the shear layer is well approximated by the natural frequency of the shear layer instabilities obtained from linear stability theory. In all of these cases, there is a peak of energy associated with subharmonics of the shear layer frequency indicating pairing of the Kelvin–Helmholtz vortices in the shear layer.

Effect of varying frequency of a synthetic jet on flow separation over an airfoil

An experimental investigation on the effects of the synthetic jet actuator (SJA) was conducted on a National Advisory Committee for Aeronautics 0025 airfoil in a low-speed recirculating wind tunnel at a chord Reynolds number of 100 000 and at an angle of attack 12°. Particle image velocimetry was used to visualize the flow separation for the uncontrolled baseline flow, and the flow attachment for the SJA controlled flows. The location of the SJA was at −1.3% from the separation point, and a blowing ratio of 0.8 was chosen for this study. The blowing ratio proved to be effective in suppressing the separation of the flow. The reduced frequency (Ste) was varied between 1, 2, 14, and 58. The momentum bursts from the SJA based on the reduced frequency determined the effectiveness of the control method. The Reynolds stresses and turbulence production decreased dramatically with increasing frequency up to the shear layer frequency (Ste= 14), but further excitation (Ste= 58) resulted in a regain of turbulence levels. Proper orthogonal decomposition was performed which showed that the low frequency operations globally affect the modes in the shear layer while the high frequency operations are confined to the airfoil surface.

Laminar Separation Bubble Development on a Finite Wing

This experimental study considers the development of a laminar separation bubble formed over a finite wing and contrasts it with the flow evolution over a two-dimensional airfoil. The experiments were performed on a reference NACA 0018 airfoil model and a finite wing with an aspect ratio of 2.5 at a Reynolds number of 125,000 involving surface pressure and particle image velocimetry measurements. For equivalent effective angles of attack, the results reveal significant differences between the two- and three-dimensional configurations. Specifically, increasing the effective angle of attack causes the separation bubble to shift upstream on the two-dimensional airfoil, whereas its mean position and streamwise extent remain invariant to spanwise changes in the effective angle on the finite wing. Similarly, the dominant shear layer frequency and characteristics of shear layer rollers do not vary appreciably over the wingspan. This suggests that the sectional analogy between the local effective angle on the wing and that on the airfoil is not universally applicable to the wing sections subjected to the formation of laminar separation bubbles. Instead, the spanwise characteristics of the bubble on the wing are well approximated by those obtained on the airfoil at the angle of attack matching the effective angle of the wing root. It should be noted, however, that these findings are exclusive of the regions in the immediate vicinity of the wing tip and/or root, which were not considered in the present investigations.

Investigation of an optimal pulsed jet mixing and combustion in supersonic crossflow

The enhanced mixing and combustion mechanism of pulsed sonic jet with optimal frequency in supersonic crossflow with a 10° ramp has been investigated using Large Eddy Simulation (LES). The results show that the energetic structures from the barrel shock and shear vortex are further enlarged at the phase of 1/4T0 periodically due to the swing forward and backward effect of the bow shock. There coexists clockwise and counter-clockwise rotating shear layer vortex structures in the pulsed jet, while only counter-clockwise rotating shear layer vortex structure is found in the steady case. The mixing process and the flame distribution are significantly affected by these structures of different scales. The reflected shock waves in the transverse jet in supersonic crossflow have strong coupling effects with the heat release rate. The pulsed jet is also found to improve the non-premixed dominant heat release rate, but not the premixed one. The mechanism of enhanced mixing and combustion efficiency relevant to the optimal frequency of 50 kHz is studied with Power Spectral Density (PSD) and wavelet analysis, and it is interesting to notice that the optimal pulsed jet frequency has strong coupling effects with the bow shock swing back and forward frequency, the jet shear layer and barrel shock frequencies (i.e., 50 kHz). However, for the non-pulsed steady case, the bow shock characteristic frequency is found to be 40 kHz, which suggests the optimal pulsed jet frequency can be 40 kHz. In order to further validate it, Unsteady Reynolds Averaged Navier Stokes (URANS) simulations have been performed with the pulsed jet frequency of 40 kHz. It is optimising to see that the mixing and combustion efficiency are further improved.

Effects of Sinusoidal Excitation on Transverse Jet Dynamics, Structure, and Mixing

This experimental study explored the effects of axisymmetric sinusoidal excitation on structural and mixing characteristics in the equidensity jet in crossflow (JICF). Planar laser-induced fluorescence imaging of acetone seeded in the jet fluid was used in this quantification, where the jet Reynolds number was fixed at and crossflow velocity was varied. Jet-to-crossflow momentum-flux ratios ranged from , with a convectively unstable upstream shear layer (USL) in the absence of forcing, to , with a globally unstable USL. Sinusoidal excitation had a relatively significant influence on the convectively unstable JICF at larger values, especially when the USL was locked-in to the forcing frequency, causing the jet cross section to become more symmetric and causing molecular mixing, quantified in both centerplane and cross-sectional planes, to improve. For a globally unstable JICF, only sinusoidal forcing corresponding to clear lock-in conditions, close to the fundamental shear-layer frequency and/or at very high amplitudes, affected jet structure, with somewhat moderate improvements in molecular mixing. Typically, excitation conditions creating enhanced jet penetration and spread did not correlate directly with improved molecular mixing; there was a stronger correlation of improved mixing with creation of a more symmetric jet cross section, in some cases with a counter-rotating vortex pair structure.

Numerical investigation of frequency-amplitude effects of dynamic morphing for a high-lift configuration at high Reynolds number

PurposeThe purpose of this study illustrates the morphing effects around a large-scale high-lift configuration of the Airbus A320 with two elements airfoil-flap in the take-off position. The flow around the airfoil-flap and the near wake are analysed in the static case and under time-dependent vibration of the flap trailing-edge known as the dynamic morphing.Design/methodology/approachExperimental results obtained in the subsonic wind tunnel S1 of Institut de Mécanique des Fluides de Toulouse of a single wing are discussed with high-fidelity numerical results obtained by using the Navier–Stokes multi-block (NSMB) code with advanced turbulent modelling able to capture the predominant instabilities and coherent structure dynamics. An explanation of the dynamic time-dependent grid deformation is provided, which is used in the NSMB code to simulate the flap’s trailing-edge deformation in the morphing configuration. Finally, power spectral density is performed to reveal the coherent wake structures and their modification because of the morphing.FindingsFrequency of vibration and amplitude of deformation effects are investigated for different morphing cases. Optimal morphing regions at a specific frequency and a slight deformation were able to attenuate the predominant natural shear-layer frequency and to considerably decrease the width of the von Kármán vortices with a simultaneous increase of aerodynamic performances.Originality/valueThe new concept of future morphed wings is proposed for a large scale A320 prototype at the take-off position. The dynamic morphing of the flap’s trailing-edge is simulated for the first time for high-lift two-element configuration. In addition, the wake analysis performed helped to show the turbulent structures according to the organised eddy simulation model.

Wake transitions and laminar separation bubble in the flow past an Eppler 61 airfoil

The flow past an Eppler 61 airfoil at 10° angle to the free-stream is investigated numerically for 100 ≤ Re ≤ 87 000. Vortex shedding is observed beyond Re ∼ 600 and three-dimensionality sets in at Re ∼ 1280.9 via the mode C instability and hairpin vortex structures that grow with an increase in Re. At larger Re, the shear layer vortices, arising from the instability of the separated shear layer, interact with the flow close to the airfoil and cause it to reattach. A Laminar Separation Bubble (LSB) forms at Re ∼ 20 000 and beyond. The airfoil experiences a very significant increase in the lift and a decrease in the drag at the formation of LSB. Although the flow is three-dimensional, the primary mechanism of the formation of LSB appears to be two-dimensional. The length of the LSB decreases with an increase in Re. The variation of the shear layer and primary wake frequency, with Re, is studied. Both follow the power law. However, the variation before and after the formation of LSB is quite different. Unlike the primary wake frequency, the shear layer frequency suffers a jump at the formation of LSB. Wake formation length (Lf), estimated via the spatial distribution of Reynolds stress, exhibits spanwise periodicity at the onset of three-dimensionality.

Flow Characteristics of a Supersonic Open Cavity

An open cavity flow exhibits intense self-sustained oscillations. This transient behavior stimulates violent pressure fluctuations because of multiple-order cavity tones. Detached eddy simulation was conducted to simulate the cavity flow at the freestream Mach number of 1.19. In order to improve the understanding of the shear layer convection processes and frequency characteristics the velocity of the flow field at cavity mid-span was studied using the dynamic mode decomposition (DMD) algorithm. The first three modes of the supersonic cavity flow were extracted to describe the flow configuration at the dominant frequencies. The two-vortices, three-vortices, and four-vortices are the corresponding first three DMD modes. The simplified mode structures are proposed to explain the flow dynamics in a supersonic cavity. When a feedback compression wave encounters the extrusion wave at a special location, an “analogous sonic boom” phenomenon appears causing violent noise in the cavity.

Turbulent wake behind side-by-side flat plates: computational study of interference effects

The complex wake behind two side-by-side flat plates placed normal to the inflow direction has been explored in a direct numerical simulation study. Two gaps, $g=0.5d$ and $1.0d$ , were considered, both at a Reynolds number of 1000 based on the plate width $d$ and the inflow velocity. For gap ratio $g/d=0.5$ , the biased gap flow resulted in an asymmetric flow configuration consisting of a narrow wake with strong vortex shedding and a wide wake with no periodic near-wake shedding. Shear-layer transition vortices were observed in the wide wake, with characteristic frequency 0.6. For $g/d=1.0$ , two simulations were performed, started from a symmetric and an asymmetric initial flow field. A symmetric configuration of Kármán vortices resulted from the first simulation. Surprisingly, however, two different three-dimensional instability features were observed simultaneously along the span of the upper and lower plates. The spanwise wavelengths of these secondary streamwise vortices, formed in the braid regions of the primary Kármán vortices, were approximately $1d$ and $2d$ , respectively. The wake bursts into turbulence some $5d$ – $10d$ downstream. The second simulation resulted in an asymmetric wake configuration similar to the asymmetric wake found for the narrow gap $0.5d$ , with the appearance of shear-layer instabilities in the wide wake. The analogy between a plane mixing layer and the separated shear layer in the wide wake was examined. The shear-layer frequencies obtained were in close agreement with the frequency of the most amplified wave based on linear stability analysis of a plane mixing layer.

Resolvent analysis of separated and attached flows around an airfoil at transitional Reynolds number

Cross-stream component of the optimal response at the natural shear layer frequency (solid black line: displacement thickness of the boundary layer, solid brown line: zero-streamwise mean velocity contour).

Scaling of square-prism shear layers

Scaling characteristics, essential to the mechanisms of transition in square-prism shear layers, were explored experimentally. In particular, the evolution of the dominant instability modes as a function of Reynolds number were reported in the range $1.5\times 10^{4}\lesssim Re_{D}\lesssim 7.5\times 10^{4}$. It was found that the ratio between the shear layer frequency and the shedding frequency obeys a power-law scaling relation. Adherence to the power-law relationship, which was derived from hot-wire measurements, has been supported by two additional and independent scaling considerations, namely, by particle image velocimetry measurements to observe the evolution of length and velocity scales in the shear layer during transition, and by comparison to direct numerical simulations to illuminate the properties of the front-face boundary layer. The nonlinear dependence of the shear layer instability frequency is sustained by the influence of $Re_{D}$ on the thickness of the laminar front-face boundary layer. In corroboration with the original scaling argument for the circular cylinder, the length scale of the shear layer was the only source of nonlinearity in the frequency ratio scaling, within the range of Reynolds numbers reported. The frequency ratio scaling may therefore be understood by the influence of $Re_{D}$ on the appropriate length scale of the shear layer. This length scale was observed to be the momentum thickness evaluated at a transition point, defined where the Kelvin–Helmholtz instability saturates.

Nonlinear dynamics and hydrodynamic feedback in two-dimensional double cavity flow

This paper reports results obtained with two-dimensional numerical simulations of viscous incompressible flow in a symmetric channel with a sudden expansion and contraction, creating two facing cavities; a so-called double cavity. Based on time series recorded at discrete probe points inside the double cavity, different flow regimes are identified when the Reynolds number and the intercavity distance are varied. The transition from steady to chaotic flow behaviour can in general be summarized as follows: steady (fixed) point, period-1 limit cycle, intermediate regime (including quasi-periodicity) and torus breakdown leading to toroidal chaos. The analysis of the intracavity vorticity reveals a ‘carousel’ pattern, creating a feedback mechanism, that influences the shear-layer oscillations and makes it possible to identify in which regime the flow resides. A relation was found between the ratio of the shear-layer frequency peaks and the number of small intracavity structures observed in the flow field of a given regime. The properties of each regime are determined by the interplay of three characteristic time scales: the turnover time of the large intracavity vortex, the lifetime of the small intracavity vortex structures and the period of the dominant shear-layer oscillations.

Effect of trailing edge shape on the separated flow characteristics around an airfoil at low Reynolds number: A numerical study

Direct numerical simulations of the flow field around a NACA 0012 airfoil at Reynolds number 50 000 and angle of attack 5° with 3 different trailing edge shapes (straight, blunt, and serrated) have been performed. Both time-averaged flow characteristics and the most dominant flow structures and their frequencies are investigated using the dynamic mode decomposition method. It is shown that for the straight trailing edge airfoil, this method can capture the fundamental as well as the subharmonic of the Kelvin-Helmholtz instability that develops naturally in the separating shear layer. The fundamental frequency matches well with relevant data in the literature. The blunt trailing edge results in periodic vortex shedding, with frequency close to the subharmonic of the natural shear layer frequency. The shedding, resulting from a global instability, has an upstream effect and forces the separating shear layer. Due to forcing, the shear layer frequency locks onto the shedding frequency while the natural frequency (and its subharmonic) is suppressed. The presence of serrations in the trailing edge creates a spanwise pressure gradient, which is responsible for the development of a secondary flow pattern in the spanwise direction. This pattern affects the mean flow in the near wake. It can explain an unexpected observation, namely, that the velocity deficit downstream of a trough is smaller than the deficit after a protrusion. Furthermore, the insertion of serrations attenuates the energy of vortex shedding by de-correlating the spanwise coherence of the vortices. This results in weaker forcing of the separating shear layer, and both the subharmonics of the natural frequency and the shedding frequency appear in the spectra.

Numerical study on the effect of shape modification to the flow around circular cylinders

Shape modification has been one of the effective ways of manipulating the flow past bluff bodies. Aiming at exploration of the flow control mechanisms of shape modified circular cylinders, a series of numerical simulations are conducted at the Reynolds number of 5000. The modifications adopted in the current paper could be divided into two categories, the 2-dimensional cross-sectional modifications (polygonal and ridged) and 3-dimensional span-wise modifications (linear wavy, sinusoidal wavy and O-ringed). By exploiting the numerical data obtained from the Large Eddy Simulations, several aspects, including the wake flow properties, the aerodynamic forces, the flow instabilities and span-wise correlations are compared in detail. The two wavy cylinders are found to modify the flow wake significantly, leading to considerable reduction in aerodynamic forces. The polygonal and ridged cylinders, however, are of no such effect in aerodynamic force mitigation. The O-rings only have slight effect on the flow wake and the forces. As for the flow instabilities, the modifications adopted in this paper barely affects the Kármán vortex shedding frequency. The shear layer frequency, however, differs from case to case. Particularly, in the cases of the two wavy cylinders, disparity exists in the shear layer frequency at the node and saddle. This is explained by the boundary layer properties at separation. While recognizing the insufficiency in the span-wise length of the cylinders, the span-wise correlations of the aerodynamic forces are inspected. It is found that the lift forces of the 3-dimensional cylinders are generally less correlated than that of the 2-dimensional ones.

High-frequency forcing of a turbulent axisymmetric wake

A high-frequency periodic jet, issuing immediately below the point of separation, is used to force the turbulent wake of a bluff axisymmetric body, its axis aligned with the free stream. It is shown that the base pressure may be varied more or less at will: at forcing frequencies several times that of the shear layer frequency, the time-averaged area-weighted base pressure increases by as much as 35 %. An investigation of the effects of forcing is made using random and phase-locked two-component particle image velocimetry (PIV), and modal decomposition of pressure fluctuations on the base of the model. The forcing does not target specific local or global wake instabilities: rather, the high-frequency jet creates a row of closely spaced vortex rings, immediately adjacent to which are regions of large shear on each side. These shear layers are associated with large dissipation and inhibit the entrainment of fluid. The resulting pressure recovery is proportional to the strength of the vortices and is accompanied by a broadband suppression of base pressure fluctuations associated with all modes. The optimum forcing frequency, at which amplification of the shear layer mode approaches unity gain, is roughly five times the shear layer frequency.

Trailing-edge dynamics of a morphing NACA0012 aileron at high Reynolds number by high-speed PIV

Particle image velocimetry (piv) measurements are made at the trailing edge of a piezoelectric actuated aileron in order to investigate the physical effect on the flow via high-frequency low-amplitude actuation at high Reynolds numbers. The measurements at different actuation frequencies show the modification of the primary frequency components of the flow with the actuation frequency. A statistical analysis reveals the reduction of the Reynolds stress components which increases with the actuation frequency. Proper orthogonal decomposition (pod) analysis shows the modification of the spatial modes illustrating the vortex breakdown in the shear-layer and the reduction of the temporal mode spectral energy depending on the actuation. It has been shown that a specific low amplitude actuation frequency produces a significant reduction of the predominant shear-layer frequency.

Large-eddy simulation of the turbulent near wake behind a circular cylinder: Reynolds number effect

The purpose of the present work is <!–<query>The highlights are in an incorrect format. Hence they have been deleted. Please refer the online instructions: http://www.elsevier.com/highlights and provide 3-5 bullet points.</query>–>to study the effect of the Reynolds number on the near-wake structure and separating shear layers behind a circular cylinder. Three-dimensional unsteady large-eddy simulation is carried out and two different subgrid scale models are applied in order to evaluate the turbulent wake reasonably. The Reynolds number based on the free-stream velocity and the cylinder diameter is ranging from Re=5500–41,300, corresponding to the full development of the shear-layer instability in the intermediate subcritical flow regime. For a complete validation of this numerical study, hydrodynamic bulk coefficients are computed and compared to experimental measurements and numerical studies in the literature. Special focus is made on the variations of both the large-scale near-wake structure and the small-scale shear-layer instability with increasing Reynolds numbers. The present numerical study clearly shows the broadband nature of the shear-layer instability as well as the dependence of the shear-layer frequency especially on the high Reynolds numbers.

Effects of free-stream turbulence and Reynolds number on the separated shear layer from a circular cylinder

In this experimental investigation, hotwire measurements were used to study the development of a separating shear layer in the near wake of a circular cylinder. The main purpose was to investigate the effects of the free stream turbulence intensity, Ti, on the separating shear layer. The measurements were performed over a range of Re, based on the cylinder diameter and mean free-stream velocity, between 4.5×103 and 4.7×104, and free stream turbulence intensities of 0.25%, 2.2%, 3.4% and 6.2%.Mean and turbulent velocities in the separating shear layer plotted in self-similar coordinates showed that the shear layer evolved downstream approximately as a plane mixing layer when Ti≤2.2%, while for high Ti (Ti≥3.4%) a departure from similarity was observed for downstream locations x/D≥1.1. The shear layer transition point moved closer to the cylinder and the longitudinal distance at which the separating shear layer can no longer be approximated by a mixing layer decreased as Ti increased. Power spectra measurements showed peaks related to the shear layer frequency, fsl, and its harmonics. In agreement with the self-similarity study, these peaks became broader and indiscernible as Ti was increased, and at the highest Ti of 6.2%, peaks related to fsl and its harmonics could no longer be seen in the power spectra at all Re investigated in this work.The present measurements suggested that the appearance of peaks at fsl and its harmonics in the power spectra is well tied to the extent by which the separating shear layer behaves as a mixing layer. The free stream turbulence seemed to amplify the turbulence content of the separating shear layer thus promoting and accelerating the breakdown of the shear layer vortices. It also decreased the large vortex formation length which limited the downstream development region of the separating shear layer hence preventing the latter from developing in a manner similar to that of a mixing layer. The overall quantitative effect of the free stream turbulence on the development of the separating shear layer was measured by the change in the exponent of the functional relationship relating fsl and Re (i.e. fsh/fv~Ren where fv is the von Kármán vortex shedding frequency) with Ti. The present measurements have shown that the exponent n increased with increasing Ti namely, n was found to increase from 0.56 to 0.65 as Ti increased from 0.25% to 3.4%. Finally, a model has been developed showing that an increase in the free-stream turbulence intensity has, qualitatively, similar effects on the separated shear layer as an increase in Re.

Effects of Actuation Frequency on Flow Control Applied to a Wall-Mounted Hump

Active flow control, and particularly unsteady mass injection (synthetic jets), promotes reattachment of naturally separated flows and can increase lift and decrease drag on airfoils at high angles of attack. Synthetic-jet actuators are typically characterized by the unsteady momentum coefficient =(ρ_s h_s)/(0.5ρ_(∞)U^2_(∞)c) and the actuation frequency F^(+)= fX_sep/U_∞, where ρ_s and u_s are the density and velocity of the injected fluid, respectively; h_s is the slot velocity; f is the actuation frequency; and X_sep is the separation distance or chord length, depending on the author or application. Actuation frequencies are often chosen to be on the order of the natural large-scale shedding frequency, or F^(+) ~ O(1). At this frequency, large-scale vortex shedding is induced, which increases the entrainment rate and promotes deflection of the separated shear layer toward the surface. Recent investigations have looked at a second regime of actuation frequencies, which are typically an order of magnitude higher than the most dominant natural frequency in the separated flow and are designed to excite Kelvin–Helmholtz instabilities in the boundary layer. In the airfoil experiments of Amitay and Glezer and Glezer et al., high-frequency excitation was found to be more effective in improving the aerodynamic performance than actuation at lower frequencies by increasing the suction force immediately after the leading edge actuation location. Simulations performed by Visbal and Visbal et al. have shown that plasma-based actuation pulsed at frequencies in the range of F^(+) = 4–8 are effective in promoting laminar-turbulent transition and suppressing separation. However, when applied to a fully turbulent boundary layer, the plasma actuation required significantly more power to achieve a reduction in separation bubble size. Other airfoil simulations at low or transitional Reynolds number have found that actuation close to the fundamental shear-layer frequency is ineffective, and forcing closest to the natural shedding frequency is more optimal; however, such two-dimensional simulations may not be accurately capturing the three-dimensional flow physics. Similar results are seen in the numerical experiments by Dandois et al. on a backward-facing step, in which the separation bubble length increased with high-frequency forcing. Here, we briefly report results for a compressible, large-eddy simulation (LES) of flow past a wall-mounted hump at high Reynolds number where oscillatory control is applied at F^(+) ~ O(10). The LES with F^(+) ~ O(1) was previously validated and found to compare favorably with extensive experimental results of this geometry. Many other computational results in the low-frequency regime were reported in the NASA Langley Computational Fluid Dynamics Validation workshop. We simulate the flow at Re_c = 500,000 and Mach numbers of 0.25 and 0.6, the latter of which is significantly higher than previous simulations of high-frequency actuation.