High-speed Streak Research Articles

Nanosecond-Laser-Induced Breakdown of Aqueous Colloidal Solutions of Dysprosium Nanoparticles: The Influence of Nanoparticle Concentration on the Breakdown Plasma and the Intensity of Physical and Chemical Processes

This paper studies the dynamics of the development of laser breakdown plasma in aqueous colloids of dysprosium nanoparticles by analyzing the time patterns of plasma images obtained using a high-speed streak camera. In addition, the distribution of plasma flashes in space and their luminosity were studied, and the amplitude of acoustic signals and the rate of generation of new chemical products were studied depending on the concentration of dysprosium nanoparticles in the colloid. Laser breakdown was initiated by pulsed radiation from a nanosecond Nd:YAG laser. It is shown that the size of the plasma flash, the speed of motion of the plasma–liquid interface, and the lifetime of the plasma flash decrease with an increasing concentration of nanoparticles in the colloid. In this case, the time delay between the beginning of the laser pulse and the moment the plasma flash reaches its maximum intensity increases with increasing concentrations of nanoparticles. Varying the laser fluence in the range from 67 J/cm2 to 134 J/cm2 does not lead to noticeable changes in these parameters, due to the transition of the breakdown plasma to the critical regime. For dysprosium nanoparticles during laser breakdown of colloids, a decrease in the yield of hydrogen peroxide and an increase in the rate of formation of hydroxyl radicals per water molecule, characteristic of nanoparticles of rare earth metals, are observed, which may be due to the participation of nanoparticles and hydrogen peroxide in reactions similar to the Fenton and Haber–Weiss reactions.

Flow over a single dimple recessed in a flat plate

Direct numerical simulations have been conducted to investigate a zero-pressure-gradient boundary layer flow over a single shallow dimple. Here, the dimple depth to dimple diameter ratio (d/D) as well as the Reynolds number (based on D and free-stream velocity) are fixed at 0.05 and 20 000, respectively. The effect of inlet boundary layer thickness δ on a given dimple is investigated by considering δ/D∈[0.023,0.1]. The flow within the dimple exhibits either a horseshoe vortex (a continuous core line through the two spirals within the dimple) or a tornado-like vortex pair (discontinuous core line). For the given parameter range, four different flow patterns have been identified within the single dimple: (i) a steady symmetric horseshoe vortex pattern for δ/D∈[0.053,0.1], (ii) a steady asymmetric horseshoe vortex pattern for δ/D=0.04, (iii) a quasi-periodic asymmetric horseshoe vortex pattern for δ/D=0.033, and (iv) a mixed horseshoe and tornado-like vortex pattern for δ/D=0.023. The growth of the streamwise vorticity, mainly caused by the tilting of the vertical vorticity, plays a key role in the transition between the different flow patterns. Dimple-induced velocity streaks above the single dimple have been investigated in detail for the first time, showing four different streaks: (i) a high-speed streak above the dimple, (ii) two side-low-speed streaks located outside the dimple span, (iii) two side-high-speed streaks, and (iv) a mid-low-speed streak in between them. These are mainly caused by a flow acceleration effect and a flow diffuser effect over the dimple, as well as a “lift-up” mechanism within the downstream part of the dimple, tilting the boundary layer upward.

Accelerating turbulence in heated micron tubes at supercritical pressure

The purpose of this research was to provide further understanding of turbulent dynamics and heat transfer mechanisms in accelerating flows with thermophysical variations and pressure drops in micron tubes. Direct numerical simulations were conducted to investigate the turbulence to supercritical pressure ${\rm CO}_2$ in heated micron tubes with inner diameter $99.2~\mathrm {\mu }$ m. In general, the turbulent heat transfer enhancement/deterioration at supercritical pressure is dominated by variations in thermophysical properties, buoyancy and thermal acceleration; however, the mechanism differs in micron tubes ( $d^* < 100\ \mathrm {\mu }$ m). The results showed that the pressure drop and scale effect made significant contributions to the development of turbulence flows heated at supercritical pressure in micron tubes, leading to the prominent property change and flow acceleration in the inlet fully developed turbulent flow. The deviation on temperature distribution because of pressure changes was non-negligible. The primary contribution of the acceleration was the decay of a boundary layer, which significantly suppressed the production of turbulence and decreased heat transfer. The acceleration had stabilizing effects on the ejection and sweep motions of the turbulent flow. The high-speed fluid contributed to a new disturbance scenario of the flow with a larger spanwise wavenumber superimposed on existing perturbations. The high-speed streak width in the quasilaminar region was approximately $150$ – $160\nu /u_\tau$ in accelerating flow. In the micron tubes, the Reynolds stress events of quadrant Q4 contributed 60 % of the Reynolds stress, greater than those of quadrant Q2.

Influence of channel bend angle on the turbulent statistics in sharply bent channel flows

Turbulence in wall-bounded flows shows a wide range of regimes, where the interaction between scales significantly occur. Reynolds number is used to characterize the dynamics of fluid corresponding to single phase channel flows. Meanwhile, different flow behavior exists in curved channels even at fixed Reynolds number, where the curvature varies as shown by Brethouwer [“Turbulent flow in curved channels,” J. Fluid Mech. 931, A21 (2022)]. In the present study, we show how wall-bounded turbulent flow behaves on sharply bending the channel by investigating the time averaged velocity profiles at the straight section, at the bend, and in the inclined section. The well-known logarithmic behavior of the time averaged normalized velocity profile is retained, where the von Kármán and the additive constants assume altered values depending on the sharp bend inclination. The near-wall fluctuations at the bend are enhanced, which is due to diffusion of counter-rotating vortices leading to increased turbulent activity. In terms of spatial structure of the random fluctuating field, the two-point correlation statistics suggest that multiple high speed and low speed streak pairs are generated, and there are multiple streamwise vortices of different sizes when bend inclination is increased. Budgets of turbulent kinetic energy are presented for various inclinations of the bend at different sections of the bend channel, which depicts that turbulent kinetic energy is modulated at the bend.

Experimental investigation on evolution characteristics of high- and low-speed streaks in supersonic turbulent boundary layer

High-speed and low-speed streak structures in the near wall region of a turbulent boundary layer with a Mach number of 3 are experimentally examined by employing the spatiotemporally resolved nanoparticle plane laser scattering technique. The time evolution characteristics of the high-speed and low-speed streaks in the supersonic turbulent boundary layer are systematically investigated through the speed field sequence results at various time intervals. The obtained results reveal that the dynamic behavior of the bands is chiefly represented by the translation along the flow direction. The process of dissipation of the existing streaks and the formation of a new streak is also observed and analyzed. The duration values of the high-speed and low-speed streak structures are assessed by utilizing the time-resolved characteristics of the speed field, and the predicted duration of the streak structure and the maximum flow length exceed 306 µs and 23.6 times the thickness of the boundary layers, respectively. Finally, the merging phenomenon of medium and low-speed streaks in the turbulent boundary layer is carefully scrutinized. The merging of low-speed streaks observed under supersonic conditions is consistent with the vortex packet merging model proposed by Tomkins and Adrian.

Bypass transition in a boundary layer flow induced by plasma actuators

Bypass transition in flow over a flat plate triggered by a pair of dielectric-barrier-discharge plasma actuators mounted on the plate surface and aligned in the streamwise direction is investigated. A four-species plasma–fluid model is used to model the electrohydrodynamic force generated by the plasma actuation. A pair of counter-rotating streamwise vortices is created downstream of the actuators, leading to the formation of a high-speed streak in the centre and two low-speed streaks on each side. As the length of actuators increases, more momentum is added to the boundary layer and eventually a turbulent wedge is generated at an almost fixed location. With large spanwise distance between the actuators (wide layout), direct numerical simulations indicate that the low-speed streaks on both sides lose secondary stability via an inclined varicose-like mode simultaneously, leaving a symmetric perturbation pattern with respect to the centre of the actuators. Further downstream, the perturbations are tilted by the mean shear of the high- and low-speed streaks and consequently a ‘W’-shaped pattern is observed. When the pair of plasma actuators is placed closer (narrow layout) in the spanwise direction, the mean shear in the centre becomes stronger and secondary instability first occurs on the high-speed streak with an asymmetric pattern. Inclined varicose-like and sinuous-like instabilities coexist in the following breakdown of the negative streaks on the side and the perturbations remain asymmetric with respect to the centre. Here the tilting of disturbances is dominated by the mean shear in the centre and the perturbations display a ‘V’ shape. Linear analysis techniques, including biglobal stability and transient growth, are performed to further examine the fluid physics; the aforementioned phenomena at narrow and wide layouts, such as the secondary instabilities, the ‘V’ and ‘W’ shapes, and the symmetric and asymmetric breakdown, are all observed.

The uniform-momentum zones and internal shear layers in turbulent pipe flows at Reynolds numbers up to [formula omitted

The statistical characteristics of the internal shear layers (ISL) and the coherent structures around the ISLs in turbulent pipe flows are examined using direct numerical simulation data at four Reynolds numbers Reτ=180, 360, 500 and 1000. The ISLs are defined by the peaks on the velocity gradients with no ad hoc filter applied on the ISL selection. This is different from the previous studies where uniform momentum zones (UMZs) are identified first using a velocity histogram and then the ISL is defined as an UMZ interface. In this study, ISLs are ranked by both their strength and location. The signature behaviours of the ISL including an abrupt streamwise velocity jump and a sharp decrease in velocity fluctuation across the ISL are confirmed. Both the positive and the negative ISLs are stronger towards the wall. The local imbalance between ejections and sweeps are observed around ISLs. Near-wall ISLs have stronger ejections while ISLs in the pipe centre have stronger sweeps. The balance between ejection and sweep is achieved approximately at 0.55-0.6 of the pipe radius for the range of Reynolds numbers studied. The flow structures around an ISL have been identified via 3D conditional sampling. The average positive ISL is located between the high-speed streak above and the low-speed streak below. The low-speed streak is associated with a pair of counter-rotating streamwise vortices which result in a strong ejection around the ISL.

Large eddy simulation of the fluid–structure interaction in an abstracted aquatic canopy consisting of flexible blades

Abstract

Extreme wall shear stress events in turbulent pipe flows: spatial characteristics of coherent motions

Abstract

An Apparatus for Measuring the Characteristics of the High-Speed Image Converter Cameras Operating in the Range of Soft X-Rays and Vacuum UV

A pumped vacuum apparatus is described that contains an electron gun with an adjustable high-voltage voltage source, interchangeable metal targets, and flanges to which the object under study is connected. The apparatus is especially designed to measure the spatial and temporal characteristics of high-speed image converter streak cameras that operate in the soft X-ray range. It can be used for research and measurements in the design of soft X-ray photodetectors and their main parts, for example, microprobe detectors, imaging devices, solid-state linear and matrix photodetectors, photoemitters operating in vacuum, etc.

Can preferential concentration of finite-size particles in plane Couette turbulence be reproduced with the aid of equilibrium solutions?

This work employs for the first time invariant solutions of the Navier-Stokes equations to study the interaction between finite-size particles and near-wall coherent structures. We consider horizontal plane Couette flow and focus on Nagata's upper-branch equilibrium solution (Nagata, 1990) at low Reynolds numbers where this solution is linearly stable. When adding a single heavy particle with a diameter equivalent to 2.5 wall units, we observe that the solution remains stable and is essentially unchanged away from the particle. This result demonstrates that it is technically feasible to utilize exact coherent structures in conjunction with particle-resolved DNS. While translating in the streamwise direction, the particle migrates laterally under the action of the quasi-streamwise vortices until it reaches the region occupied by the low-speed streak, where it attains a periodic state of motion. As a result of the ensuing preferential particle location, the time-average streamwise particle velocity differs from the plane-average fluid-phase velocity at the same wall-distance as the particle center, as previously observed in experiments and in numerical data for fully turbulent wall-bounded flows. Additional constrained simulations where the particle is maintained at a fixed spanwise position while freely translating in the other two directions reveal the existence of two equilibria located in the low-speed and in the high-speed streak, respectively, the former being an unstable point. A parametric study with different particle to fluid density ratios is conducted which shows how inertia affects the spanwise fluctuations of the periodic particle motion. Finally, we discuss a number of potential future investigations of solid particle dynamics which can be conducted with the aid of invariant solutions (exact coherent structures) of the Navier-Stokes equations.

Monitoring a Laser-Pulse Front Profile with a Picosecond Resolution and a Wide Dynamic Range at Multichannel Facilities

The results of the development of a technique for measuring the power contrast of a laser pulse by registering the pulse-front profile with a large dynamic range are presented. The technique was developed on the basis of a high-speed streak camera and fiber-optic transmission lines; it provides the following registration parameters: the dynamic range of measuring the laser-pulse power contrast at the wavelength λ = 0.53 µm is 107, while the temporal resolution of the registration channel is no worse than 60 ps. A method of increasing the power contrast to 1011 using an optical scheme based on elements with nonlinear transmission is shown.

Heated transcritical and unheated non-transcritical turbulent boundary layers at supercritical pressures

Nominally zero-pressure-gradient fully developed flat-plate turbulent boundary layers with heated and unheated isothermal walls at supercritical pressures are studied by solving the full compressible Navier–Stokes equations using direct numerical simulation. With a heated isothermal wall, the wall temperature sets such that the flow temperature varies through the pseudo-critical temperature, and thus pseudo-boiling phenomena occur within the boundary layers. The pseudo-boiling process induces strongly nonlinear real-fluid effects in the flow and interacts with near-wall turbulence. The peculiar abrupt density variations through the pseudo-boiling process induce significant near-wall density fluctuations $\sqrt{\overline{\unicode[STIX]{x1D70C}^{\prime }\unicode[STIX]{x1D70C}^{\prime }}}/\overline{\unicode[STIX]{x1D70C}}\approx 0.4{-}1.0$ within the heated transcritical turbulent boundary layers. The large near-wall density fluctuations induce a turbulent mass flux $\unicode[STIX]{x1D70C}^{\prime }u_{i}^{\prime }$, and the turbulent mass flux amplifies the Favre-averaged velocity fluctuations $u_{i}^{\prime \prime }$ in the near-wall predominant structures of streamwise low-speed streaks that are associated with the ejection (where $u^{\prime \prime }<0$ and $v^{\prime \prime }>0$), while reducing the velocity fluctuations in the high-speed streaks associated with the sweep ($u^{\prime \prime }>0$ and $v^{\prime \prime }<0$). Although the near-wall low-speed and high-speed streak structures dominate the Reynolds-shear-stress generation, the energized Favre-averaged velocity fluctuations in the low-speed streaks enhance both the mean-density- and density-fluctuation-related Reynolds shear stresses ($-\overline{\unicode[STIX]{x1D70C}}\overline{u^{\prime \prime }v^{\prime \prime }}$ and $-\overline{\unicode[STIX]{x1D70C}^{\prime }u^{\prime \prime }v^{\prime \prime }}$) in the ejection event and, as a result, alter the Reynolds-shear-stress profile. The large density fluctuations also alter the near-wall viscous-stress profile and induce a near-wall convective flux $-\overline{\unicode[STIX]{x1D70C}}\widetilde{u}\widetilde{v}$ (due to non-zero $\widetilde{v}$). The changes in the contributions in the stress-balance equation result in a failure of existing velocity transformations to collapse to the universal law of the wall. The large density fluctuations also greatly contribute to the turbulent kinetic energy budget, and especially the mass flux contribution term becomes noticeable as one of the main positive terms. The unheated non-transcritical turbulent boundary layers show a negligible contribution of the real-fluid effects, and the turbulence statistics agree well with the statistics of an incompressible constant-property turbulent boundary layer with a perfect-gas law.

Bunch Length Measurement Employing Cherenkov Radiation from a Thin Silica Aerogel

The temporal profile measurement for ultra-short electron bunches is one of the key issues for accelerator-based coherent light sources. A bunch length measurement system using Cherenkov radiation (CR) is under development at the Research Center for Electron Photon Science, Tohoku University. This system allows for the real-time diagnostics of electron bunches. The system comprises a thin silica aerogel as the Cherenkov radiator, a specially designed optical transport line, and a high-speed streak camera. The electron bunch length can be obtained by measuring the time spread of the CR from the electrons passing through the radiator medium using the streak camera. In this paper, we describe the novel bunch length measurement system using CR, discuss the expected time resolution of the system, and finally present the measurement results.

The stability of laser welding with an off-axis wire feed

The concept using an off-axis filler wire during laser welding is introduced here in order to provide added process robustness considering gap width variations. Its stability is investigated with respect to gap width, welding speeds and powers. Geometry of the welds is analysed by tracing of weld cap edges and joint cross sections, connecting trends to weld parameters. High speed imaging and streak images are used to further study and describe sequences of events, including undercut formation. Formation of imperfections are found to be mainly correlated to wire feed position variations at the surface due to irregular melting of the wire tip.

Rotating Mirror Digital Streak Camera for Investigation of Fast Processes

A TKPF269 rotating mirror digital streak camera with a photosensitive element based on a CMOS array was developed for the purpose of replacing film in high-speed streak cameras (SFR-2M, USF-2, etc.). A brief description of the TKPF269 streak camera design is given. The space-time picture of the emergence of light from the ends of optical fibers upon firing of an explosive in a model assembly was simultaneously recorded by a TKPF269 streak camera and an USF-2 streak camera to compare the metrological characteristics of these devices. The results of experimental data processing are presented.

Large-eddy simulation of reactive pollutant exchange at the top of a street canyon

The exchange of reactive pollutants (NO, NO2, and O3) at the top (roof level) of a street canyon are investigated using the parallelized large-eddy simulation model (PALM). The transport equations of NO, NO2, and O3 with simple photochemical reactions are combined within the LES model for this study. NO and NO2 are emitted from an area source located near the canyon floor, and O3 is included within the ambient air and inflow. A clockwise-rotating vortex appears in the street canyon and transports NO, NO2, and O3. NO and NO2 are transported along the ground and leeward wall and escape from the canyon at the roof level. O3 enters the canyon at the roof level and is transported along the windward wall. The mean O3 production rate is generally negative with large magnitudes at and near the roof level and near the windward wall. The chemical reactions reduce the mean NO and O3 concentrations in the canyon by 31% and 84%, respectively, and increase the mean NO2 concentration in the canyon by 318%. The exchange of reactive pollutants at the roof level is significantly affected by small-scale eddies at the roof level and low- or high-speed streaks above the canyon. Air in the canyon with high NO and NO2 concentrations escapes from the canyon when low-speed air parcel appears due to small-scale eddies at the roof level or low-speed streak above the canyon. In contrast, air outside the canyon with a high O3 concentration enters the canyon when high-speed air parcel appears because of small-scale eddies at the roof level or high-speed streak above the canyon. The time-lagged correlation analysis reveals that NO, NO2, and O3 concentrations near the ground are affected by low- or high-speed streaks above the canyon but not significantly affected by small-scale eddies at the roof level.

Modulating the Near-Wall Velocity Fields in Wall-Bounded Turbulence via Discrete Surface Roughness

The present work investigates the effect of discrete surface roughness, which falls in the transitionally rough regime, on the near-wall velocity fields of a smooth-wall turbulent boundary layer with zero pressure gradient. A spanwise array of cylindrical elements, whose diameters were and heights were , were attached onto the wall with spanwise spacing of . Three roughness Reynolds numbers , 1000, and 1500 were studied, and the ratio between the roughness height and the boundary-layer thickness was . Two-dimensional velocity fields in multiple wall-parallel planes at were measured by two-dimensional particle image velocimetry. For each case, it is found that roughness elements always introduce -shaped low-speed regions in the near-wall mean velocity fields by direct wake interaction, in the downstream of which a phase shift occurs to form one high-speed mean streak with reduced streamwise velocity fluctuation intensity behind each roughness element, marking the onset of the modulation effect on the mean velocity fields. Further analysis on the swirling strength fields suggests that such a modulation effect can be attributed to the roughness-introduced streamwise vortex pairs, which compete with hairpin vortices shedding from the top of the roughness elements and promote the phase shift in a bottom-up way.

Turbulent channel flow over an anisotropic porous wall – drag increase and reduction

The effect of the variations of the permeability tensor on the close-to-the-wall behaviour of a turbulent channel flow bounded by porous walls is explored using a set of direct numerical simulations. It is found that the total drag can be either reduced or increased by more than 20 % by adjusting the permeability directional properties. Drag reduction is achieved for the case of materials with permeability in the vertical direction lower than the one in the wall-parallel planes. This configuration limits the wall-normal velocity at the interface while promoting an increase of the tangential slip velocity leading to an almost ‘one-component’ turbulence where the low- and high-speed streak coherence is strongly enhanced. On the other hand, strong drag increase is found when high wall-normal and low wall-parallel permeabilities are prescribed. In this condition, the enhancement of the wall-normal fluctuations due to the reduced wall-blocking effect triggers the onset of structures which are strongly correlated in the spanwise direction, a phenomenon observed by other authors in flows over isotropic porous layers or over ribletted walls with large protrusion heights. The use of anisotropic porous walls for drag reduction is particularly attractive since equal gains can be achieved at different Reynolds numbers by rescaling the magnitude of the permeability only.

Experiments on localized secondary instability in bypass boundary layer transition

An experimental study on localized secondary instability of unsteady streamwise streaks in bypass boundary layer transition under an elevated level of free-stream turbulence has been carried out mainly using the particle image velocimetry (PIV) technique. Simultaneous orthogonal dual-plane PIV measurements were performed for a concurrent examination of the transitional flow features in both wall-normal and spanwise planes. These quantitative and simultaneous visualizations clearly show the wall-normal view of a low-speed streak undergoing sinuous/varicose motion in the spanwise plane. An oscillating shear layer in the wall-normal plane is found to be associated with the sinuous/varicose streak oscillation in the spanwise plane. Further, these measurements indicate that a localized secondary instability wavepacket can originate near the boundary layer edge. The time-resolved PIV measurements in the wall-normal plane clearly show how an instability develops on a lifted-up inclined shear layer and leads to flow breakdown. The estimated wavelength and convection velocity of such instabilities are found to compare well with those calculated from the one-dimensional linear stability analysis of the spatially averaged velocity profiles associated with the lifted-up shear layers. The time-resolved PIV measurements in the spanwise plane also facilitate quantitative visualizations of sinuous and varicose instabilities. These measurements experimentally confirm that a varicose instability at the juncture of an incoming high-speed streak and a downstream low-speed streak can eventually lead to the formation of lambda structures. The estimated convection velocity, wavelength and growth rate of these instabilities are found to be consistent with the numerical results reported in the literature. Moreover, the streak secondary instability is found to be apparent in the velocity contours, while the estimated streak amplitude is approximately 30 % of the free-stream velocity.