Edge Of Shear Layer Research Articles

Overview of the recent experimental research on the J-TEXT tokamak

The J-TEXT capability is enhanced compared to two years ago with several upgrades of its diagnostics and the increase of electron cyclotron resonance heating (ECRH) power to 1 MW. With the application of electron cyclotron wave (ECW), the ECW assisted plasma startup is achieved; the tearing mode is suppressed; the toroidal injection of 300 kW ECW drives around 24 kA current; fast electrons are generated with toroidal injected ECW and the runaway current conversion efficiency increases with ECRH power. The mode coupling between 2/1 and 3/1 modes are extensively studied. The coupled 2/1 and 3/1 modes usually lead to major disruption. Their coupling can be either suppressed or avoided by external resonant magnetic perturbation fields and hence avoids the major disruption. It is also found that the 2/1 threshold of external field is significantly reduced by a pre-excited 3/1 mode, which can be either a locked island or an external kink mode. The disruption control is studied by developing prediction methods capable of cross tokamak application and by new mitigation methods, such as the biased electrode or electromagnetic pellet injector. The high-density operation and related disruptions are studied from various aspects. Approaching the density limit, the collapse of the edge shear layer is observed and such collapse can be prevented by applying edge biasing, leading to an increased density limit. The density limit is also observed to increase, if the plasma is operated in the poloidal divertor configuration or the plasma purity is increased by increasing the pre-filled gas pressure or ECRH power during the start-up phase.

A study of turbulent filaments in the edge plasma of the Wendelstein 7-X stellarator

Filaments are studied by examining fast camera images on the Wendelstein 7-X stellarator. Fast cameras offer a unique perspective, revealing the complex 3D structure of filaments in the entire poloidal cross-section of the plasma. By correlating individual pixels, their location, shape, and movement are analyzed in standard and high-ι configurations. The presence of filaments is not uniform poloidally around. The number of active areas matches the number of magnetic islands in both configurations. Filaments are found to extend to multiple toroidal turns in standard configuration. No time delay is observed between the different toroidal sections. Such behavior is not seen in high-ι configuration. Filaments are observed within and without the edge shear layer, indicated by the direction of their poloidal rotation. Inside the shear layer, their velocity scatters around 1.25 km s−1, accompanied by a lifetime between 80 and 120 µs. Outside, their velocity shows greater absolute values and variance, but still in a few km s−1 range. The similarities and differences between the two configurations are discussed and compared to previous results.

Vortex dynamics and boundary layer transition in flow around a rectangular cylinder with different aspect ratios at medium Reynolds number

The numerical investigation focuses on the flow patterns around a rectangular cylinder with three aspect ratios ( $L/D=5$ , $10$ , $15$ ) at a Reynolds number of $1000$ . The study delves into the dynamics of vortices, their associated frequencies, the evolution of the boundary layer and the decay of the wake. Kelvin–Helmholtz (KH) vortices originate from the leading edge (LE) shear layer and transform into hairpin vortices. Specifically, at $L/D=5$ , three KH vortices merge into a single LE vortex. However, at $L/D=10$ and $15$ , two KH vortices combine to form a LE vortex, with the rapid formation of hairpin vortex packets. A fractional harmonic arises due to feedback from the split LE shear layer moving upstream, triggering interaction with the reverse flow. Trailing edge (TE) vortices shed, creating a Kármán-like street in the wake. The intensity of wake oscillation at $L/D=5$ surpasses that in the other two cases. Boundary layer transition occurs after the saturation of disturbance energy for $L/D=10$ and $15$ , but not for $L/D=5$ . The low-frequency disturbances are selected to generate streaks inside the boundary layer. The TE vortex shedding induces the formation of a favourable pressure gradient, accelerating the flow and fostering boundary layer relaminarization. The self-similarity of the velocity defect is observed in all three wakes, accompanied by the decay of disturbance energy. Importantly, the decrease in the shedding frequency of LE (TE) vortices significantly contributes to the overall decay of disturbance energy. This comprehensive exploration provides insights into complex flow phenomena and their underlying dynamics.

Numerical investigation of self-sustained oscillations of stall cells around a leading edge-separating airfoil

The flow around a stalled airfoil is investigated using zonal detached-eddy simulation (mode 2), including transition effects through a coupling with the γ−Reθ,t framework. The airfoil exhibits mixed trailing edge-leading edge stall type properties. The chord length-based Reynolds number and Mach number, respectively, amount to 1·106 and 0.16. Two computations with different initial conditions are performed for 40 and 120 chord-passing durations, respectively (or equivalently 0.23 and 0.67 s), allowing the capture of several periods of the low frequency dynamics of the flow—compared to typical von Kármán vortex shedding. A stall hysteresis is observed: the computation initiated from an attached flow remains thus, but the computation which starts from a separated flow yields a quasi-permanent low-frequency oscillatory behavior, which bifurcates to the previously attached topology after 90 chord-passing durations (0.45 s). The oscillatory phase displays events of emergence and disappearance of stall cells. The partly- and fully attached flows are validated against experimental data. The oscillatory bistable flow is then analyzed with regard to the characteristics and frequency contents of both massive separation and partial transient reattachments. It is shown that the low-frequency separated shear layer flapping at the leading edge is forced by high-frequency fluctuations, which travel from the trailing edge upstream, close to the wall in the separated flow. The flapping phenomenon displays a Strouhal number based on the front-section height of the airfoil around St=fc sin (α)/u∞≃0.02. Conversely, the high-frequency fluctuations have Strouhal numbers closer to 3, which is in close agreement with leading-edge shear-layer instability frequencies. The spectral content of the flow is then explored in search of the source of these high-frequency fluctuations. It is proposed that they stem from the instability of the trailing edge shear layer between the pressure side boundary layer and the separated flow from the suction side. Finally, a scenario describing a cycle of the low-frequency oscillation of a stall cell is proposed.

An almost subharmonic instability in the flow past rectangular cylinders

The three-dimensional instability of the flow past a 5 : 1 rectangular cylinder is investigated via Floquet analysis and direct numerical simulations. A quasi-subharmonic (QS) unstable mode is detected, marking an important difference with the flow past bodies with lower aspect ratio and/or with smooth leading edge. The QS mode becomes unstable at Reynolds number (based on the cylinder thickness and free-stream velocity) $Re \approx 480$ ; its spanwise wavelength is approximately three times the cylinder thickness. The structural sensitivity locates the wavemaker region over the longitudinal sides of the cylinder, indicating that the instability is triggered by the mutual inviscid interaction of vortices generated by the leading edge shear layer.

Structure of turbulence in the flow around a rectangular cylinder

The separating and reattaching turbulent flow past a rectangular cylinder is studied to describe how small and large scales contribute to the sustaining mechanism of the velocity fluctuations. The work is based on the anisotropic generalised Kolmogorov equations, exact budget equations for the second-order structure function tensor in the space of scales and in the physical space. Scale-space energy fluxes show that forward and reverse energy transfers occur simultaneously in the flow, with interesting modelling implications. Over the longitudinal cylinder side, the Kelvin–Helmholtz instability of the leading edge shear layer generates large spanwise rolls, which get stretched into hairpin-like vortices and eventually break down into smaller streamwise vortices. Independent sources of velocity fluctuations act at different scales. The flow dynamics is dominated by pressure–strain: the flow impingement on the cylinder surface in the reattachment zone produces spanwise velocity fluctuations very close to the wall, and at larger wall distances reorients them to feed streamwise-aligned vortices. In the near wake, large von Kármán-like vortices are shed from the trailing edge and coexist with smaller turbulent structures, each with its own independent production mechanism. At the trailing edge, the sudden disappearance of the wall changes the structure of turbulence: streamwise vortices vanish progressively, while spanwise structures close to the wall are suddenly turned into vertical fluctuations by the pressure–strain.

The behavior of unconstrained parachute fabric under aerodynamic loading

As parachute systems are required to make precise drops of increasingly heavy equipment, it is important that the simulation tools that are used to develop these systems have relevant data that they can use for validation. Before a detailed validation experiment can be run, it is necessary to understand how these fabrics behave over a wide range of conditions to determine the proper parameter space for the validation tests. In an effort to determine fabric behavior so that specific test points can be selected for validation, minimally constrained squares of F111 nylon fabric were tested in the United States Air Force Academy’s (USAFA’s) low speed wind tunnel and the Army Research Laboratory’s (ARL’s) Microsystem Aeromechanics Wind Tunnel. The tests at USAFA covered different angles of attack, freestream velocities, and amounts of excess length. Testing at ARL was conducted to obtain particle image velocimetry (PIV) measurements around the fabric while simultaneously tracking the motion of the fabric. The results of the experiments with varying excess length indicated that spanwise excess length had a more significant impact on the fabric mean shape and vibrations than excess length in the chordwise direction. Variations in angle of attack and freestream velocity indicated that the minimally constrained fabric experiences large amplitude vibrations at low angles of attack and vibrations beyond the trailing edge are insignificant at angles above 6°. Data from both the USAFA and ARL experiments indicated that this was true even when the leading edge shear layer was separated and undergoing unsteady oscillations. The PIV results from the ARL experiments indicated that at low angles of attack the fabric often experienced unsteady oscillations causing a leading edge region to flip into an up-or-down quasi-stable position. When this flipping occurred it induced vortex shedding which resulted in highly unsteady forces generated by the model. The lack of vibration of the fabric at angles of attack where the leading edge shear layer was undergoing unsteady separation indicates that the material properties of the F111 nylon make it less sensitive to small disturbances when under load. The presence of spanwise pre-strain eliminated large amplitude vibrations of the fabric at low angles of attack and the vortex formation was linked to leading edge oscillations, indicating that the leading edge state drives much of the fluid–structure interaction for minimally constrained fabric.

Zonal shear layer collapse and the power scaling of the density limit: old L-H wine in new bottles

Edge shear layer collapse causes edge cooling and aggravates radiative effects. This paper details on the microscopic dynamics of the emergence of power (Q) scaling of density limit (DL) from the shear layer collapse transport bifurcation scenario. The analysis is based on a novel 4-field model, which evolves turbulence energy, zonal flow energy, temperature gradient and density, including the neoclassical screening of zonal flow response. Bifurcation analysis yields power scaling of critical density for shear layer collapse as . The favorable Q scaling of the DL emerges from the fact that the shear layer strength increases with Q, thus preventing shear layer collapse. This in turn reduces particle transport and improves particle confinement. RMP induced ambient stochastic fields degrade the shear layer by inducing decoherence in the Reynolds stress. As a result the particle transport increases and particle confinement degrades. This leads to the emergence of unfavorable stochastic field intensity () scaling of the critical density as . All fields, including zonal flow shear, exhibit hysteresis when the power (Q) is ramped cyclically across the bifurcation point. The hysteresis is due to dynamical delay in bifurcation on account of critical slowing down. Thus, the dynamical hysteresis here is fundamentally different from the hysteresis associated with the existence of bi-stable states.

Electrode biasing maintains the edge shear layer at high density in the J-TEXT tokamak

Collapse of the edge flow shear as the line-averaged density approaches the Greenwald density limit has been observed as a precursor to the enhanced edge particle flux characteristic of proximity to the density limit regime. Here, we report the use of a biased electrode to sustain the edge shear layer in high density discharges, in which the shear layer would otherwise collapse. A stable increase in line-averaged density is observed along with a strong increase in edge density. These experiments were carried out on the J-TEXT tokamak. The Reynolds stress at the edge is enhanced, and the zonal flow sustained, while density perturbation levels, the flux of turbulence internal energy (i.e., turbulence spreading), and particle and heat flux all decrease significantly. Electron adiabaticity increases, and bias voltage modulation experiments show that an increase in the edge shear leads the increase in adiabaticity. These results suggest that external edge E × B flow shear drive may be of interest for sustaining edge plasma states at high density, and support the hypothesis that collapse of the edge shear layer triggers the onset of the strong transport and turbulence characteristic of the density limit regime.

Trailing Edge Flap Effects on Dynamic Stall Vortex and Unsteady Aerodynamic Forces on a Pitching Airfoil

This study focuses on the effect of the upward deflection of trailing edge flap (TEF) on the strength and trajectory of dynamic stall vortex (DSV) around a pitching airfoil by means of numerical simulations based on unsteady Reynolds-averaged Navier-Stokes (URANS). The effect of the upward deflection of the TEF on the unsteady aerodynamic forces due to DSV is analyzed. The numerical simulation method for large mesh deformation is constructed. Radial basis function- (RBF-) based mesh deformation algorithm, as well as Laplacian and optimization-based mesh smoothing algorithm, is adopted to ensure the mesh quality in flow field simulations. The results reveal that the upward deflection of the TEF can reduce the peaks of drag and pitching moment coefficients. Although the maximum lift coefficient of the airfoil is slightly reduced, its maximum drag and pitching moment coefficients are significantly reduced by up to 34.8% and 31.8%, respectively. The vorticity transport behavior in a planar control region during the DSV formation and detachment is analyzed. It is found that the TEF can change the development process of the DSV. The upward deflection of the TEF reduces the vorticity flux from the leading edge shear layer, which causes the circulation of the DSV and the translational velocity of the vortex center to decline. The peaks of the unsteady aerodynamic forces on the airfoil induced by the DSV are reduced. The upward deflection of the TEF plays the role of alleviating the pitching moment load. The longer TEF can result in a better control effect. The bigger the upward deflection angle of the TEF, the better its control effect.

Enhanced particle transport events approaching the density limit of the J-TEXT tokamak

Enhanced particle transport events are discovered and analyzed as the density limit of the J-TEXT tokamak is approached. Edge shear layer collapse is observed and the ratio of Reynolds power to turbulence production decreases. Simultaneously, the divergence of turbulence internal energy flux (i.e. turbulence spreading) increases, indicating that shear layer collapse triggers an outward spreading event. Studies of correlations show that the enhanced particle transport events are quasi-coherent, and manifested primarily in density fluctuations which exhibit positive skewness. Electron adiabaticity emerges as the critical parameter which signals transport event onset. For α < 0.35 as density approaches the Greenwald density, both turbulence spreading and density fluctuations rise rapidly. Taken together, these results elucidate the connections between edge shear layer, density fluctuations, particle transport events, turbulence spreading and plasma edge cooling as the density limit is approached.

Bounds on edge shear layer persistence while approaching the density limit

This paper details the theory of edge shear layer collapse as the density approaches the Greenwald density limit. It significantly extends earlier work, which was restricted in applicability. The zonal shear flow screening length is calculated for banana, plateau and Pfirsch–Schluter regimes. Poloidal field scaling persists in the plateau regime. Neoclassical screening and drift wave–zonal flow dynamics are combined in a theory, which is then reduced to a predator–prey model. Zonal noise, due to incoherent mode coupling, is retained. The threshold condition for edge shear layer collapse is computed, and linked to a critical value of the dimensionless parameter . Here is the ion sound radius, is the zonal flow screening length and is the equilibrium density scale length. The limiting initial edge density for shear layer collapse is derived and shown to scale favorably with the plasma current. The results are discussed in light of the density limit and Ohmic phenomenology.

Volumetric entropy generation rate associated with tip clearance flow in linear cascade

The tip clearance flow structure and its corresponding entropy generation rate of a linear cascade with 1.85mm radial tip clearance operating under three different incidence angle are investigated. The incidence angle influences the tip leakage vortex position as well as the size of the tip leakage vortex. Larger incidence angle results in larger tip leakage vortex and pushes the tip leakage vortex further away from the suction side. The entropy generation rate related to the tip clearance flow structure is calculated and it is confirmed that tip clearance flow is the main major source of aerodynamic loss in linear cascade. Larger incidence angle leading to higher entropy generation rate. Results also show that it’s not the core region of tip leakage vortex but the edge vortex and shear layer of the tip clearance flow which has the highest entropy generation rate. When operating under an incidence angle of 4°, there exist an extra high entropy generation rate region in main flow. It is found that the compression and expansion of fluid due to the existence of tip clearance vortex was the main reason for this extra high entropy generation.

Phenomenology and scaling of optimal flapping wing kinematics

Biological flapping wing fliers operate efficiently and robustly in a wide range of flight conditions and are a great source of inspiration to engineers. The unsteady aerodynamics of flapping wing flight are dominated by large-scale vortical structures that augment the aerodynamic performance but are sensitive to minor changes in the wing actuation. We experimentally optimise the pitch angle kinematics of a flapping wing system in hover to maximise the stroke average lift and hovering efficiency with the help of an evolutionary algorithm and in situ force and torque measurements at the wing root. Additional flow field measurements are conducted to link the vortical flow structures to the aerodynamic performance for the Pareto-optimal kinematics. The optimised pitch angle profiles yielding maximum stroke-average lift coefficients have trapezoidal shapes and high average angles of attack. These kinematics create strong leading-edge vortices early in the cycle which enhance the force production on the wing. The most efficient pitch angle kinematics resemble sinusoidal evolutions and have lower average angles of attack. The leading-edge vortex grows slower and stays close-bound to the wing throughout the majority of the stroke-cycle. This requires less aerodynamic power and increases the hovering efficiency by 93% but sacrifices 43% of the maximum lift in the process. In all cases, a leading-edge vortex is fed by vorticity through the leading edge shear layer which makes the shear layer velocity a good indicator for the growth of the vortex and its impact on the aerodynamic forces. We estimate the shear layer velocity at the leading edge solely from the input kinematics and use it to scale the average and the time-resolved evolution of the circulation and the aerodynamic forces. The experimental data agree well with the shear layer velocity prediction, making it a promising metric to quantify and predict the aerodynamic performance of the flapping wing hovering motion.

Individual influence of pitching and plunging motions on flow structures over an airfoil during dynamic stall

The individual influence of pitching and plunging motions on flow structures is studied experimentally by changing the phase lag between the geometrical angle of attack and the plunging angle of attack. Five phase lags are chosen as the experimental parameters, while the Strouhal number, the reduced frequency and the Reynolds number are fixed. During the motion of the airfoil, the leading edge vortex, the reattached vortex and the secondary vortex are observed in the flow field. The leading edge vortex is found to be the main flow structure through the proper orthogonal decomposition. The increase of phase lag results in the increase of the leading edge velocity, which strongly influences the leading edge shear layer and the leading edge vortex. The plunging motion contributes to the development of the leading edge shear layer, while the pitching motion is the key reason for instability of the leading edge shear layer. It is also found that a certain increase of phase lag, around 34.15° in this research, can increase the airfoil lift.

Separation mechanics of non-slender delta wings during streamwise gusts

The dynamic-stall behavior on a generalized non-slender delta wing subjected to streamwise gusts has been investigated experimentally. A 45∘ sweep, NACA0012 delta wing serves as the canonical shape. In lieu of varying inflow conditions, the model is accelerated in the streamwise direction from a chord-based Reynolds number of 300,000 to 450,000 through a ramp motion at incidence angles near maximum steady-state lift. Force augmentation both during and after the gust is shown to be dependent on the initial structure of separated leading edge shear layers, with strong sensitivity at an angle of attack of 30∘. It is shown that lift enhancement stems from initially separated flow near the wing apex reattaching during the gust. Surface pressure measurements show that the reattachment is driven by a strong favorable pressure gradient that is maintained after the gust ends. The equations governing near-wall vortex dynamics state that vorticity flux from the wall increases with favorable pressure gradients, indirectly predicting an increase in circulation near the wing surface during a streamwise gust. However, circulation obtained through Particle Image Velocimetry (PIV) on the z∕c=0.1, 0.3, and 0.5 spanwise sections does not support this prediction, rather showing that the circulation in the wake (near the wing surface) remains nearly constant throughout the gust. Overall, strong three-dimensional gradients dictate flow structure before and after the gust. Spanwise variation of flow structure is especially pronounced for the 30∘ angle of attack case, where each planar section exhibits unique separation behavior in response to the gust.

Effect of butterfly-scale-inspired surface patterning on the leading edge vortex growth

The effects of longitudinal square grooves on the growth of the leading edge vortex (LEV) produced by an impulsively-started flat plate are investigated experimentally. The Reynolds numbers are 1416, 2833 and 5667, and the maximum vortex formation time is 2.8. The square grooves are inspired by the surface patterning (scales) on butterfly wings. It is hypothesized that the grooves play a role in the LEV formation and development by creating a stronger secondary vortex near the LE that impedes the growth of the LEV. To evaluate this hypothesis, circulation curves of the total positive vorticity field and LEV of a smooth plate and grooved plate are compared. Also, the secondary vorticity generated by the LEV interaction with the patterned surface is studied, as well as the subsequent effect on the LEV’s growth rate and peak circulation. The vortex development process varies for each Reynolds number. At the lowest number, the effects of the grooves are minimal; however, they are most significant at the highest Reynolds number where the unsteady interactions between the secondary vortex and the leading edge shear layer become stronger for the grooved plate causing an earlier separation of the LEV. Therefore, contrary to the hypothesis, the data shows that stronger secondary vortex causes a stronger LEV with an earlier separation. The stronger secondary vortex interacts more rigorously with the LE shear layer causing the latter to feed more concentrated vorticity to the LEV.

The study of flow separation control by a nanosecond pulse discharge actuator

The basic dynamic characteristic of a nanosecond (NS) pulse discharge actuator in still air is investigated, which introduces viscid and inviscid effects to the flow, namely, heated air and pressure waves, respectively. The efficacy of a dielectric barrier discharge plasma actuator driven by a nanosecond pulse for active flow separation control is investigated experimentally on a NASA SC(2)-0712 aerofoil at Re=0.5×106 (25m/s). The pressure distribution on the aerofoil surface is measured, and the result indicates that the separation is well controlled at a high angle of attack. The PIV test result shows that the forcing frequency selectively affects the flow over the aerofoil, and a higher forcing frequency has a better influence on the leading edge shear layer. Schlieren imaging experiments at Re=0.1×106 (5m/s) show that the main disturbance factor for the nanosecond pulse plasma actuator is the heating effect, which changes the local temperature and density and induces several vortex structures. The vortex structures entrain the outer high-momentum fluid into the shear layer, thereby strengthening the mixing of the shear layer with the main flow and delaying separation or even reattaching a separated flow. The research reveals the major disturbance factor introduced by NS discharge applied to flow separation control. Furthermore, the spatiotemporal disturbance process induced by the pulse discharge is investigated.

Electric Probe Measurements of the Poloidal Velocity in the Scrape‐Off Layer of ASDEX Upgrade

AbstractA reciprocating probe head with six pins was used for localized measurements of electric fields and densities in the scrape‐off layer (SOL) of ASDEX Upgrade (AUG) up to the edge shear layer (SL) near the Last Closed Flux Surface (LCFS). The edge SL is characterized by a strong sudden change in the poloidal velocity vθ close to the separatrix. The probes were used to determine this velocity by different methods which are critically compared to each other concerning their reliability. By the first method the poloidal velocity was deduced from the radial electric field Er measured by two radially staggered probe pins, with vθ being due to the Er × Bϕ‐drift (Bϕ is the toroidal field). The two other methods utilized the cross correlation of two poloidally staggered ion‐biased probes and two poloidally staggered floating probes, respectively. In this case the time lags with maximum cross correlation were used to determine the poloidal velocity and its jump, yielding comparable results to the first method. Also the method of conditional averaging was applied to the latter signals. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Study of a stall cell using stereo particle image velocimetry

The structure of Stall Cells (SCs) on wings is analyzed on the basis of stereo particle image velocimetry measurements. All experiments regard a Reynolds number 0.87 × 106 flow around a rectangular wing with endplates and an aspect ratio of 2.0. The inherently unstable stall cell is stabilized by means of a localized spanwise disturbance. Velocity, vorticity, and Reynolds stress data above the wing and in the wake are presented and discussed, also in combination with Computational Fluid Dynamics data. The present study completes and clarifies the previously suggested models regarding the SC structure. The SC emerges in between the separation and trailing edge shear layer where three different types of vortices are identified: (a) the stall cell vortices that start normal to the wing surface and continue downstream aligned with the free stream, (b) the separation line vortex, and (c) the trailing edge line vortex that both run parallel to the wing trailing edge and grow significantly at the center of the stall cell. Analysis of the Reynolds stress data reveals high anisotropy. Concentration of high streamwise shear stress values is connected to the two shear layers and high cross shear Reynolds stresses are connected to vortex stretching. High normal Reynolds stress values are observed (a) in the separation but not in the trailing edge shear layer indicating the flapping of the former and (b) along the stall cell vortices indicating their wandering motion. The eddy viscosity based Reynolds averaged Navier-Stokes simulations are found in good qualitative agreement with the experiments in terms of the type and position of the identified vortex structures, an agreement which is linked to the correct trend in the predicted shear Reynolds stresses distributions. Quantitative deviations of the numerical results from the measurements are attributed to the isotropic definition of the turbulence model. Therefore, use of large eddy simulation is suggested for better prediction of the flow.