Regions Of High Vorticity Research Articles

Synergistic mechanism of the energy dissipation and sediment erosion in pump turbine under sand-laden water based on entropy production theory

Sediment erosion of hydraulic machinery can cause a lot of energy loss, thereby affecting their operational stability. Herein, solid–liquid two-phase flow simulations are conducted on a pump turbine using the Euler–Lagrangian model to predict the sediment-laden flow and sediment erosion. The predicted results are verified through experiments. The correlation between the energy loss characteristics and sediment erosion in hydraulic turbines is analysed using the entropy production and vorticity theories. Results show that the total vorticity in the runner area increases with the guide vane opening. The entropy production corresponds to a high-vorticity region. Energy dissipation is positively correlated with the degree of turbulence in the flow regime. Certain phenomena, such as vortex flow, impact, and friction in the runner area, are important causes of energy loss. Vortices can also cause local sediment erosion on runner blade walls. Severe wall sediment erosion typically occurs at locations with high entropy production. The total entropy production along the particle flow path is consistent with the variation law of the wall sediment erosion rate.

Numerical Analysis of Bionic Inlet Nozzle Effects on Squirrel-Cage Fan Flow Characteristics

In order to improve the inlet distortion of the squirrel-cage fan, this study proposes a parametric design method for the bionic structure of the inlet nozzle generatrix, which is spliced by multiple sinusoidal curves, based on the bionic structure of the humpback whale flipper leading-edge nodule. The geometric shape of the bionic generatrix is controlled by three parameters: the number of segments n, the amplitude ratio Tm, and the amplitude of the last curve An. These parameters are optimized through orthogonal tests and numerical simulations, with the aim of improving the fan’s aerodynamic efficiency. Based on the selected solution, a comparative analysis is conducted to examine the impact of cylindrical, conical, and bionic inlet nozzles on inlet distortion and flow evolution within the centrifugal fan. Numerical calculations demonstrate that the fan’s maximum total efficiency, with a bionic inlet nozzle designed in a rational manner, is 5.46% higher than that of the original fan and is 2.01% higher than that of the fan with a conical inlet nozzle. The proposed bionic structure can create a buffer zone at the fan’s inlet, thereby reducing the region of high vorticity caused by the separated flow. Consequently, this improvement leads to enhanced uniformity at the impeller’s inlet. Furthermore, the design method proposed in this study for the inlet nozzle’s bionic structure effectively regulates the airflow angle near the impeller shroud, thereby enhancing the fan’s inlet distortion and improving its overall aerodynamic performance.

LED-based PIV Of A Particle-Laden Turbulent Free-Surface

An experimental method is proposed to study dispersed two-phase flows at an airwater interface, a family of flows of practical significance in environmental and industrial settings. The applicability of this technique is demonstrated through the study of a lightly-deformed turbulent free-surface laden with floating particles (`floaters'). A low-mean turbulent flow is generated in a turbulence box actuated by a 10×10 synthetic jet array. Using LEDs and a single camera, free-surface flow measurements are carried out by Particle Image Velocimetry (PIV) simultaneously with Lagrangian tracking of the floaters, allowing the potential to characterise the coupling between the floater dynamics and the (sub)surface flow. Discrimination of the dispersed and continuous phases is carried out based on size. Individual floaters and clusters of floaters are successfully tracked throughout the field of view while they navigate through elongated and circular regions of high and low vorticity, characteristic features typically observed when a subsurface turbulent flow interacts with a free surface. Preliminary results of the floater-fluid interactions are presented to highlight the potential of this technique to better our understanding of floaterladen turbulent free surfaces.

Efficient point-based simulation of four-way coupled particles in turbulence at high number density.

In many natural and industrial applications, turbulent flows encompass some form of dispersed particles. Although this type of multiphase turbulent flow is omnipresent, its numerical modeling has proven to be a remarkably challenging problem. Models that fully resolve the particle phase are computationally very expensive, strongly limiting the number of particles that can be considered in practice. This warrants the need for efficient reduced order modeling of the complex system of particles in turbulence that can handle high number densities of particles. Here we present an efficient method for point-based simulation of particles in turbulence that are four-way coupled. In contrast with traditional one-way coupled simulations, where only the effect of the fluid phase on the particle phase is modeled, this method additionally captures the back-reaction of the particle phase on the fluid phase, as well as the interactions between particles themselves. We focus on the most challenging case of very light particles or bubbles, which show strong clustering in the high-vorticity regions of the fluid. This strong clustering poses numerical difficulties which are systematically treated in our work. Our method is valid in the limit of small particles with respect to the Kolmogorov scales of the flow and is able to handle very large number densities of particles. This methods paves the way for comprehensive studies of the collective effect of small particles in fluid turbulence for a multitude of applications.

Large eddy simulation for jet noise characteristics of liquid rocket engine with guide groove

The study of jet noise characteristics of liquid rocket engines is a prerequisite for evaluating the noise level of rockets and effectively controlling the noise. To obtain the effect of engine operating and structural parameters on jet noise, the Large Eddy Simulation combined with the FW-H integration method is applied for the scaled engine with guide groove under different chamber pressures and nozzle expansion ratios. The simulation results show that the noise field distribution of the jet with a guide groove is highly symmetrical and has obvious directivity. The noise caused by jet impingement on the guide groove is strongly correlated to the distribution of high vorticity regions. Remarkably, higher chamber pressure leads to longer potential jet length, higher noise sound pressure levels, and greater enhancement effect of impact on noise. However, the frequency spectral characteristics are basically invariant. When the chamber pressure is constant, the increment of the nozzle expansion ratio in a certain range results in lower noise peak frequency and weaker impact effect.

Impact of the North Atlantic Oscillation on the Decadal Variability of the Upper Subtropical‐Tropical Atlantic Ocean

AbstractIn the northeastern tropical Atlantic, a region of high potential vorticity (PV) determines the size of the exchange window for the interior thermocline flow of the subtropical cell via its variations in strength and extent. Variability of this PV barrier has the potential to impact the ventilation of the tropical Atlantic on decadal timescales. Here, the impact of the North Atlantic Oscillation (NAO) on the PV barrier related to isopycnals within the thermocline of the subtropical‐tropical Atlantic Ocean is assessed from Argo observations for the time period of 2006–2022. Relative to the negative NAO phase (2009–2010), during the positive NAO phase (2014–2019), the North Atlantic subtropical high and the northeast trades are intensified. Satellite‐derived wind stress curl shows increased upwelling/downwelling on the equatorward/poleward side of the trade wind zone, respectively. In the subtropical‐tropical Atlantic, a symmetric pattern of isopycnal heave is observed: rising isopycnals within 20°N and 20°S and sinking poleward of that. With rising isopycnals, the PV barrier in the northeastern tropical Atlantic becomes stronger. Analyses of geostrophic velocities and the Sverdrup streamfunction show that during the positive NAO phase there are increased equatorward velocities at thermocline level along the western boundary and reduced velocities through the interior as a result of intensified northeast trades and therefore a strengthened PV barrier. Intensified trades lead to enhanced subduction of thermocline waters and, independent of that, to a strengthened Equatorial Undercurrent transport as observed at the mooring site at 0°, 23°W, likely via the pulling effect of the subtropical cells.

Vortex dynamics characteristics in the tip region based on Wray–Agarwal model

In order to solve the blockage effect and energy dissipation phenomenon caused by cavitation in the low-pressure vortex core region, this paper analyzes the spatial evolution of vorticity intensity and turbulent kinetic energy intensity under different cavitation conditions based on the Wray–Agarwal (WA) model. First, the tip leakage flow characteristics are studied, the evolution of vorticity and vorticity intensity is analyzed, then the distribution of turbulent kinetic energy distribution in the blade tip region is studied, and finally, the vorticity transport characteristics of the tip region are analyzed. It is found that the tip leakage rate is less affected by the vortex cavitation of the tip leakage, and there is a strong interaction between the leakage flow at the tip leading edge and the trailing edge, and the separation vortices and low-speed regions formed in the end-wall region cause blockage of the flow passage. Low pressure causes cavitation to cover most regions of the suction surface, inhibiting the formation and development of the tip leakage vortices. The distribution range of high turbulent kinetic energy region is almost the same as that of high-vorticity region, and there is a positive correlation between the two intensities. Severe cavitation causes the high turbulent kinetic energy region at the outlet of the flow passage to develop in the radial and axial directions of the impeller, which increases the turbulent dissipation and energy loss. The change of vorticity transport intensity caused by cavitation is mainly reflected in the expansion contraction term, and the Coriolis force term plays a dominant role in the vorticity transport process. This paper provides a reference for further improving the performance of mixed-flow pumps.

Investigation of the influence of the bluff-body temperature on a lean premixed DME/air flame approaching blowoff

In flames stabilized by a two-dimensional (2D) bluff body, the lean blowoff (LBO) limit can be extended by increasing the bluff-body temperature, which is verified experimentally in premixed dimethyl ether (DME)/air flames in this work. Two bluff-body temperatures of 573 K and 773 K are tested, respectively. High-speed OH* chemiluminescence (CL) imaging is applied to record the spatial distribution of the heat release. Particle image velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) are performed simultaneously to explore the flow field and the combustion characteristics. Based on the results of OH*-CL, the heat release of the area near the bluff body is strengthened in the case of higher bluff-body temperature, and the enhanced chemical reactivity will further influence the downstream area. The turbulent flame speed obtained by OH-PLIF is increased in the entire area of interest. Due to the higher velocity gradient adjacent to the bluff body in the upstream region (y < 35 mm), the flame surface density (FSD), brush thickness, and flame wrinkling ratio are not sensitive to the bluff-body temperature. However, in the downstream region (y > 35 mm), a noticeable change can be found in these three parameters due to a higher turbulent flame speed and a much lower velocity gradient. Based on the PIV results, a limited effect on the velocity field by increasing the bluff-body temperature by 200 K. Furthermore, the probability density functions (PDF) of the vorticity and strain rate on the flame front are computed. The results indicate that the enhanced flame stability leads to a more pronounced overlapping between the flame front and the high-vorticity region, further increasing the values of these two parameters.

Dynamic separation on an accelerating prolate spheroid

Time-varying flow separation on an accelerating prolate spheroid has been studied at various angles of incidence. Instantaneous pressure and scanning stereoscopic particle image velocimetry were used to shed light on the evolution of cross-flow structures for the Reynolds number ( $Re$ ) range of $1.0\times 10^6\leq Re \leq 1.5\times 10^6$ . The movement of separation lines is examined for various model accelerations to investigate on the interplay between acceleration and flow separation. The results demonstrate that for axial accelerations, the streamwise pressure distribution in the rear part of the prolate spheroid switches from an adverse to a favourable pressure gradient. At the same time, the circumferential adverse pressure gradient present during steady motion vanishes during said accelerations. In contrast, both streamwise and circumferential adverse pressure gradients strengthen when the model is axially decelerated. These dynamic pressure distributions influence the location of the separation line, which in turn moves closer to the model meridian during accelerations while moving outwards during decelerations. The streamwise vorticity distribution and the streamwise circulation both show how the separation-line position impacts the vortex formation. A high-vorticity region near the model surface is established during acceleration. In contrast, a decelerating model leads to transport of high-vorticity fluid into the outer area of the cross-flow separation. We further assess the memory effects following the near-impulsive velocity changes. The cross-flow retains the memory of moving separation lines shortly after the acceleration. However, the separation recovers quickly to a steady state.

An investigation on flame structure and NOx formation in a gas turbine model combustor using large eddy simulation

In this work, large eddy simulations (LES) of a Gas Turbine Model Combustor (GTMC) are done using a five-step global mechanism that includes separate thermal and non-thermal NOx formation parts. To verify the accuracy of the solution, time-averaged profiles of the flow variables and fluctuations are compared to the available experimental and numerical data. The LES results show that the vortical structures inside the chamber are highly connected to the temperature field and chemical reactions, and despite having a major role in fast premixing and consequent NOx reductions, they contribute to NOx generation by forming high temperature spots inclusive of chemical radicals. Also, the importance of the baroclinic torque in vorticity creation is demonstrated by comparing the corresponding values to vortex stretching in upstream parts of the chamber. It is shown that the baroclinic torque mostly takes action between high vorticity and high strain regions and can possibly intensify the strong vortices, while the vortex stretching is mostly active near the strong vortices. Furthermore, observation of detailed statistics shows that most of the heat release occurs in samples with mixture fractions near the global value, while NO generation is highly biased toward the strong vortices and the stoichiometric mixture fraction. To investigate the role of the radicals in more details, a chemical reactor network (CRN) is created by clustering the LES solution. Also, the integration of Partially Stirred Reactors (PaSRs) with Perfectly Stirred Reactor (PSR) networks is used to improve the accuracy of predicting the reactant jet penetration and ignition radicals.

Water surface response to turbulent flow over a backward-facing step

The water surface response to subcritical turbulent flow over a backward-facing step (BFS) is studied via high-resolution large-eddy simulation (LES). The LES method is validated first using data of previously reported experiments. The LES-predicted water surface is decomposed into different types of gravity waves as well as turbulence-driven forced waves. Analysis of the LES data reveals the interplay between low-frequency large-scale turbulence structures, which are the result of flow separation from the step and reattachment behind the step, and the dynamics of the water surface. The water surface deformation is mainly the result of freely propagating gravity waves and forced waves, owing to turbulence in the form of rollers and/or hairpin vortices. Gravity waves with zero group velocity define the characteristic spatial and temporal scales of the surface deformations at higher frequencies, while large eddies determine their low-frequency modulation. These deformations are mainly confined in lateral bands that propagate downstream following the advection of the near-surface streamwise vortices (rollers) that are shed from the step. Steeper surface waves are observed in regions of negative perturbation velocity gradient and down-welling, downstream of the larger rollers, and are associated with thin isolated regions of high vorticity near the surface. The investigation of such a complex flow has shown that the decomposition of the water surface fluctuations into its different physical components may be used to identify the dynamics of the underlying flow structure.

Uncertainty quantification of separation control with synthetic jet actuator over a NACA0025 airfoil

A compressed sensing method based on polynomial chaos expansion (CS-PCE) is used to study separation control over an airfoil with a synthetic jet actuator (SJA). Uncertainty quantification (UQ) of a two-dimensional cavity driven flow model based on a Monte Carlo (MC) method with a sample size of 3000 is used as the indicator to evaluate the applicability and accuracy of four different probability collocation (PRC) methods. The sample points corresponding to the chosen model are used in a three-dimensional SJA control model. The effects of input factors on the aerodynamic performance and flow field parameters of airfoil are studied and show that a 5% coefficients of variation of stochastic input variables result in a roughly 30% fluctuation of lift-to-drag ratio. High uncertainty mainly exists in the middle and downstream regions of the upper surface corresponding to high vorticity regions. As sample size is increased, the accuracy of the CS-PCE model is improved. Compared with traditional methods, the CS-PCE method reduces computing costs up to 70% for similar results.

Investigation of airflow onto uncooled and cooled perpendicular substrates for bioaerosol sampling

A better understanding of the mechanics of condensation is needed to devise active bioaerosol samplers. Here, dropwise water condensation produced by directing humid air flow perpendicularly on ambient (25 °C) and cooled (4 °C) copper plate substrates was studied. Numerical data obtained from solving the two-dimensional incompressible Navier-Stokes equations with low Reynolds numbers (= 40) showed significant vorticity strength developing at the edges of the copper plate. This resulted in a higher degree of aerosol deposition and an increased likelihood of drop nucleation at the impaction zone. When a drop is already present at the edge, simulations predicted that the high vorticity region shifted to the apexes of drops with concomitant increase in the magnitude of vorticity strength. These results explained the experimentally-observed dropwise condensation of larger drops at the edges of the ambient substrate. Analyses of drop size distributions at the center and edges of ambient and cooled substrates showed that drop growth was enhanced by improved condensation on the cooled substrate surface in addition to the flow vorticity effect. Preliminary findings indicate that the recovery of viable aerosolized Escherichia coli from ambient and cooled substrate was found to be invariant, portending its utility for sampling when the electrical power available for cooling is limited.

Caustics in turbulent aerosols: an excitable system approach

Aerosol particles are inertial particles. They cannot follow the surrounding fluid in a region of high vorticity. These encounters render the particle velocity field ${{\boldsymbol {v}}}$ locally compressible. Caustics occur if the trace of the gradient matrix $\textsf{$\boldsymbol{\sigma}$}$ of the field ${{\boldsymbol {v}}}$ locally diverges. For three-dimensional isotropic and homogeneous turbulence, the dynamics of the gradient matrix can be expressed in terms of three geometric invariants. In the present paper we establish a parametrisation of this problem where the dynamics takes the form of an excitable stochastic dynamical system with a three-dimensional phase space. The deterministic part of the dynamics is solved analytically. We show that the deterministic system has a globally attractive stable fixed point. Small noise induces excursions from the fixed point that typically relax straight back towards the fixed point. Caustics emerge as non-trivial return to a global fixed point when noise excites a trajectory across a stability threshold. The relaxation to the global fixed point will then involve at least one, and it may involve two or even three divergences of $\boldsymbol {Tr}\,\textsf{$\boldsymbol{\sigma}$}$ . Based on a combination of analytical insights and numerical analysis, we determine the rate of occurrence, duration and relative observation probability of caustic events in turbulent aerosols. Our analysis reveals that each approach towards a divergence proceeds along a straight line in the phase space of the dynamical system, which can help identify caustics. Moreover, there are infinite ways in which caustics can arise, namely whenever $\boldsymbol {Tr}\,\textsf{$\boldsymbol{\sigma}$}$ tends to $-\infty$ , so that no two caustics look the same.

Investigation on Jet Characteristics of Turbofan Exhaust System under Take-off Condition with High Angle of Attack

To understand the jet flow characteristics of turbofan separate exhaust system, a parametric design method based on the initial Class Shape Transformation function was developed. HBPR and UHBPR turbofan separate exhaust systems were designed. Furthermore, the jet flow characteristics of the HBPR turbofan exhaust system under take-off condition with zero angle of attack were studied based on numerical simulation. The jet flow characteristics of the HBPR and UHBPR turbofan exhaust system under take-off condition with high angle of attack were also simulated. The effects of angle of attack and bypass ratio on the jet flow characteristics were investigated and the related flow mechanisms were analyzed. Results show that the axisymmetric plumes of the HBPR turbofan exhaust system are distributed around the engine axis under take-off condition with zero angle of attack. With the plug wake as the center, the core flow, the fan/core shear layer, the fan flow, the fan/free stream shear layer and the free stream are wrapped around the plug wake from inside out. Vortexes appear in the lee area at the back of the cowl and jet flow under take-off condition with high angle of attack. These vortexes cause cross sectional secondary flow and expose the high-velocity core flow to the low-velocity free stream. The contact area and velocity gradient in the mixing region among the free stream, fan flow and core flow increase. Therefore, the mixture among jet flow and free stream strengthens. So the high-velocity region, the high-vorticity region, and the high turbulence kinetic energy region shorten by 55.1%, 47.7% and 50.9% respectively. The vorticity values and turbulence kinetic energy level peak on the upper side of the exhaust plumes increase by about 30% and 87% respectively. Relative to these parameters from the HBPR turbofan exhaust system, the jet velocity peak value of UHBPR turbofan decreases by 5.5% under take-off condition with high angle of attack. The vorticity values and turbulence kinetic energy level reduce due to decreased velocity gradient in shear layers downstream of the nozzle exit plane. The turbulence kinetic energy level peak on the upper side of the exhaust plumes decreases by 29.3%. The reasons are that the contact area between high-velocity core flow and the free stream decreases due to thicker fan flow and the velocity gradient in the core flow and free stream mixing region decreases because of the lower core flow velocity.

Clustering of fast gyrotactic particles in low-Reynolds-number flow.

Systems of particles in turbulent flows exhibit clustering where particles form patches in certain regions of space. Previous studies have shown that motile particles accumulate inside the vortices and in downwelling regions, while light and heavy non-motile particles accumulate inside and outside the vortices, respectively. While strong clustering is generated in regions of high vorticity, clustering of motile particles is still observed in fluid flows where vortices are short-lived. In this study, we investigate the clustering of fast swimming particles in a low-Reynolds-number turbulent flow and characterize the probability distributions of particle speed and acceleration and their influence on particle clustering. We simulate gyrotactic swimming particles in a cubic system with homogeneous and isotropic turbulent flow. Here, the swimming velocity explored is relatively faster than what has been explored in other reports. The fluid flow is produced by conducting a direct numerical simulation of the Navier-Stokes equation. In contrast with the previous results, our results show that swimming particles can accumulate outside the vortices, and clustering is dictated by the swimming number and is invariant with the stability number. We have also found that highly clustered particles are sufficiently characterized by their acceleration, where the increase in the acceleration frequency distribution of the most clustered particles suggests a direct influence of acceleration on clustering. Furthermore, the acceleration of the most clustered particles resides in acceleration values where a cross-over in the acceleration PDFs are observed, an indicator that particle acceleration generates clustering. Our findings on motile particles clustering can be applied to understanding the behavior of faster natural or artificial swimmers.

Transport of condensing droplets in Taylor-Green vortex flow in the presence of thermal noise.

We study the role of phase change and thermal noise in particle transport in turbulent flows. We employ a toy model to extract the main physics: Condensing droplets are modelled as heavy particles which grow in size, the ambient flow is modelled as a two-dimensional Taylor-Green flow consisting of an array of vortices delineated by separatrices, and thermal noise are modelled as uncorrelated Gaussian white noise. In general, heavy inertial particles are centrifuged out of regions of high vorticity and into regions of high strain. In cellular flows, we find, in agreement with earlier results, that droplets with Stokes numbers smaller than a critical value, St<St_{cr}, remain trapped in the vortices in which they are initialized, while larger droplets move ballistically away from their initial positions by crossing separatrices. We independently vary the Péclet number Pe characterizing the amplitude of thermal noise and the condensation rate Π to study their effects on the critical Stokes number for droplet trapping, as well as on the final states of motion of the droplets. We find that the imposition of thermal noise, or of a finite condensation rate, allows droplets of St<St_{cr} to leave their initial vortices. We find that the effects of thermal noise become negligible for growing droplets and that growing droplets achieve ballistic motion when their Stokes numbers become O(1). We also find an intermediate regime prior to attaining the ballistic state, in which droplets move diffusively away from their initial vortices in the presence of thermal noise.

Response and flow characteristics of a dual-rotor turbine flowmeter

The technical problems of flow measurement in hypersonic flight could be mitigated through dual-rotor turbine flowmeters (DRT-FMs). In this study, a visual experiment platform was designed to reproduce the flow of a 1.3 cm diameter DRT-FM. A mathematical model considering two rotors was developed to perform a parameterised study and evaluate the rotor responses. Further, the entire flow passage was numerically simulated through an added automatic iterative rotor-dynamic calculation based on the angular momentum balance theory. According to the experimental results, the response range of the rotational speeds was divided into three stages: unresponsive, unstable, and stable. The downstream rotor responded at a lower flow rate, increasing the measurement range of the rotor turbine flowmeter. Considering the region of linear increase in the rotor speed in stable state under different flow media, tip clearances, and number of blades, the mathematical results indicated that the downstream rotor exhibited a compensation effect of the rotational speed on the calculation of rotational speed at the current flow rate conditions, which mitigates the measurement instability of the upstream rotor. Through the simulations, the rotation speed difference of the two rotors resulted in a slight periodic disturbance to the downstream rotor, which was alleviated after flowing through the spacer and could be ignored. Moreover, the high vorticity regions appeared around the rotors and areas of abrupt structural changes in the flow passage; the distribution gradually extended as the vorticity decreased. The present study provides an understanding of the DRT-FM and some recommendations to improve its characteristics.

Numerical evaluation of suppression mechanism of shark-inspired riblet on tip leakage vortex of a NACA0009 hydrofoil with tip clearance

The tip leakage vortex (TLV) formed by tip clearance has adverse effects on the operation stability of bulb turbines in tidal power stations. In this paper, the NACA0009 hydrofoil is selected as the research object, a method of suppressing TLV by shark-inspired riblet is proposed, and the suppression mechanism is numerically studied. The results show that the shark-inspired riblet effectively suppresses the TLV, decreases the high-vorticity region generated by TLV, and increases the pressure in the centre of the vortex core. The area of the low-pressure region on hydrofoil tip decreases, with a decreasing trend moving downstream and to the suction surface. The tip leakage flow is controlled by reverse rotating vortex inside the riblet. The leakage velocity decreases, which leads to a reduction in the driving force of the tip leakage flow. In addition, the leakage rate declines 5.9%. Moreover, the TLV trajectory is close to the suction surface, the vertical distance of TLV movement is reduced, and the maximum TLV intensity decreases by 7%. With increasing riblet height, the suppression effect of shark-inspired riblet on TLV is more obvious, but the lift-to-drag ratio of hydrofoil decreases rapidly when the riblet height is greater than a critical value.

Experimental investigation of supersonic boundary-layer tripping with a spanwise pulsed spark discharge array

In this paper, a pulsed spark discharge plasma actuator array is deployed to control laminar–turbulent transition in a Mach 3.0 flat-plate boundary layer, and the subtle flow structures are visualized by nanoparticle planar laser scattering (NPLS) technique. Results show that the onset location of turbulence can be brought upstream by plasma actuation, corresponding to forced boundary-layer transition. Hairpin vortex packets evolved from the thermal bulbs play a vital role in the breakdown of laminar flow. With the help of a machine learning tool, all the relevant structures induced by plasma actuation are extracted from NPLS images, and a conceptual model of the hairpin vortex generation is proposed, including three stages: production and lift-up of the high-vorticity region, formation of the $\varLambda$ vortex and evolution of the hairpin vortex.