Streak Structures Research Articles

Control of turbulent coherent structures on flat panel by porous medium

Due to the unique internal structure of porous medium, the disturbance to the boundary layer involves both changing the dynamic behavior of the physical quantity in the conservation equation and changing the boundary conditions. In order to investigate the effect and mechanism of porous medium on the coherent structure near the wall of a flat panel, particle image velocimetry experiments of a flat panel and a flat panel with porous medium were carried out in a low turbulence return flow wind tunnel. The coherent structures of flat panel and porous medium-laden flat panel were compared in the study. The flow-normal plane flow display results show that porous medium can significantly lower the near-wall flow velocity, lift up the vortex structure in the near-wall region, reduce the vorticity, and effectively reduce the shear stress. Meanwhile, the velocity statistical quantities were extracted and fitted using the velocity profile method to show that at a wind velocity of 5.8 m/s and a porosity of 10 pores per inch (ppi), the maximum resistance reduction rate reaches 23.1%. The flow-spanwise plane flow display results show that the porous medium can destroy large-scale streak structures.

Laser-zoned treatment of magnesium surfaces with predictable degradation applications

Magnesium metal is a promising material for medical applications due to its biocompatibility and similar modulus of elasticity to human bone. However, its complex corrosion process must be addressed before it can be used clinically to match post-implantation tissue repair. This study aims to regulate material degradation by utilizing laser surface treatment. The surface of pure magnesium was modified using nanosecond and femtosecond laser methods to create various micro-nanostructures, such as chain, streak, column, and groove structures. Surface roughness and wettability tests revealed that the groove structures had higher roughness values. All structures exhibited hydrophilicity, but the femtosecond laser-generated structures were more hydrophilic. Electrochemical tests and immersion experiments demonstrated that femtosecond laser modification significantly improved the corrosion resistance of magnesium metal compared to polished samples. Cytotoxicity experiments showed that the laser-treated magnesium was not cytotoxic. Based on the results, we constructed various structures on the magnesium rods in different regions. As a result, the rods exhibited multi-stage biodegradation behavior in simulated body fluids (SBF). This study presents a novel approach to controlling the degradation sequence of medical metals.

LES investigations of turbulent heat transfer structures with exponential power escalation

The BORAX-type accident is a key scenario in the safety design of pool-type experimental nuclear reactors. It considers a reactivity insertion large enough to initiate an exponential power escalation with a period as small as tens of milliseconds, necessitating effective heat removal by the coolant system. This study presents a computational analysis of single-phase transient heat transfer under thermal-hydraulic conditions relevant to such reactors, focusing on turbulent channel flow between planar fuel plates with highly subcooled water. Our investigation uses Large Eddy Simulation (LES) performed with TrioCFD, the open-source CFD code developed by the French Atomic Energy Commission (CEA). Several modeling options are tested: incompressible flow with temperature as a passive scalar, incompressible flow with varying viscosity, and quasicompressible flow with temperature-dependent viscosity and mass density. To evaluate the accuracy of our physical models, all simulations were performed under uniform conditions, using the same experimental parameters, mesh, and numerical strategies. The specific examined scenario involves a highly subcooled turbulent channel flow at moderate pressure, experiencing an exponential power increase. We ensure detailed turbulence resolution at the Batchelor scale in the wall normal direction close to the heating wall. This communication presents a comparative study of LES simulations of the single-phase flow under exponential power excursion at the wall. Subsequent analyses focused on the impact of different velocity-temperature couplings on the prediction of wall heat transfer, incorporating comparisons with experimental infrared thermometry measurements. Early in the transient phase, temperature peaks are confined within turbulent streaks, as reported in the existing literature. However, as the exponential power transient unfolds, the viscous layer and streak structures destabilize, leading to a more dispersed distribution of hot spots. Preliminary findings suggest a transition in heat transfer modes from turbulence-driven to conduction during the final stages of the single-phase transient.

Pulverized coal combustion in boundary layer turbulence using direct numerical simulation

The present study investigates pulverized coal combustion under oxy-fuel conditions interacting with turbulent boundary layer flows over a flat plate using direct numerical simulation (DNS). The coal particles are tracked in the Lagrangian framework while the flow is solved in an Eulerian way. Three cases with reacting particles, inert particles and single-phase flows were performed. The interplay between pulverized coal combustion, turbulent boundary layer flows and the wall was examined. It was shown that the coal particles first ignite in the outer layer of the boundary layer turbulence, where individual coal particle ignition occurs. The wall-normal velocity in the reacting case is larger compared with that of the non-reacting cases due to flow acceleration induced by combustion. The volatile matter mass fraction is high in the inner layer in the downstream region. However, the heat release rate is low due to the cold wall effect. The strong correlation between the wall heat flux and streak structures in the near-wall region is observed in all cases. The values of radiative heat transfer are similar in the upstream region of both the cases with reacting and inert particles. However, as combustion occurs and particle temperature increases in the reacting case, the radiative heat transfer from particles to the wall increases significantly in the downstream region. Particle accumulation due to turbophoresis in the near-wall region is analyzed. It was shown that the magnitude of streamwise vorticity is attenuated by coal combustion. Therefore, particle wall-accumulation is less prominent in the reacting case compared with the inert case.

Observation of periodic structures and pseudogaps in pristine compound α-TiNCl by STM/STS and break junction methods

Scanning tunneling microscopy and spectroscopy (STM/STS) and break-junction tunnel spectroscopy (B.TTS) measurements are performed on the pristine layered α-TiNCl semiconductor (pri-TiNCl), being the precursor of superconducting nitride chlorides. The STM topography of pri-TiNCl shows basic crystal structures with the lattice periods of a0≈ 0.38 nm and b0≈ 0.31 nm, ensuring that a clean a–b surface of micro-single crystals is obtained. From the STS measurements, the averaged conductance versus voltage dependence dI/dV(V) reveals kink structures at relatively high bias voltages of |F| ∼ −50 mV and −100 mV. The spatial (r-) conductance distributions dI/dV (V, r) (dI/dV maps) proportional to those of quasiparticle local densities of states (LDOSes) exhibit the bias-independent streak structures with the period of (4.8 ± 0.2) b0, which are predominantly observed within the range of |V| < 95 mV. The temperature dependence of the dI/dV B.TTS spectra shows the gap structure with the energy scale of 4Σ = 180 meV vanishing at Ts ∼ 120 K. Thus, the ratio of the gap Σ to the transition temperature Ts is 2Σ/kBTs ∼ 10. Here, kB is the Boltzmann constant. Such a ratio is typical of the pseudogap features in cuprate superconductors and dielectric gap characteristics in layered chalcogenides with charge-density waves.

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.

POD analysis of spatiotemporal characteristics of wake turbulence over hilly terrain and their relationship to hill slope, hill shape and inflow turbulence

In this study, the turbulent wake flow fields over isolated 2-D ridges and 3-D hills with different slopes and oncoming flow conditions are predicted by large-eddy simulations (LES). The coupling effects of hill slopes, hill shapes and inflow turbulence on the spatiotemporal characteristics of wake turbulence over hilly terrain are clarified in terms of turbulent statistics and unsteady flow patterns. The snapshot-based proper orthogonal decomposition (POD) technique is then adopted to elucidate the physical meaning of wake turbulence over hilly terrain. The numerical results show that the vertical distributions of turbulent statistics and the unsteady flow patterns in the wake of hilly terrain are jointly determined by hill slopes, hill shapes and oncoming flow conditions. Additionally, the snapshot-based POD method is capable of reconstructing a large amount of fluctuating kinetic energy in the wake of hilly terrain with a few low-order POD modes. Moreover, the primary wake patterns and their relations to hill slopes, hill shapes and oncoming flow conditions are identified. On the horizontal plane, the wake turbulence structures of 3-D gentle/steep hill are dominated by the asymmetric elongated streak structures and a pair of counter-rotating vortices, respectively. On the vertical plane, the main patterns of hill wake are often featured by the recirculating flow, and the size of recirculation bubble is highly dependent on hill slopes and hill shapes.

Detecting turbulent structures on single Doppler lidar large datasets: an automated classification method for horizontal scans

Abstract. Medium-to-large fluctuations and coherent structures (mlf-cs's) can be observed using horizontal scans from single Doppler lidar or radar systems. Despite the ability to detect the structures visually on the images, this method would be time-consuming on large datasets, thus limiting the possibilities to perform studies of the structures properties over more than a few days. In order to overcome this problem, an automated classification method was developed, based on the observations recorded by a scanning Doppler lidar (Leosphere WLS100) installed atop a 75 m tower in Paris's city centre (France) during a 2-month campaign (September–October 2014). The mlf-cs's of the radial wind speed are estimated using the velocity–azimuth display method over 4577 quasi-horizontal scans. Three structure types were identified by visual examination of the wind fields: unaligned thermals, rolls and streaks. A learning ensemble of 150 mlf-cs patterns was classified manually relying on in situ and satellite data. The differences between the three types of structures were highlighted by enhancing the contrast of the images and computing four texture parameters (correlation, contrast, homogeneity and energy) that were provided to the supervised machine-learning algorithm, namely the quadratic discriminant analysis. The algorithm was able to classify successfully about 91 % of the cases based solely on the texture analysis parameters. The algorithm performed best for the streak structures with a classification error equivalent to 3.3 %. The trained algorithm applied to the whole scan ensemble detected structures on 54 % of the scans, among which 34 % were coherent structures (rolls and streaks).

Role of various scale-similarity models in stabilized mixed subgrid-scale model

We investigate the physical role of various scale-similarity models in the stabilized mixed model [K. Abe, “An improved anisotropy-resolving subgrid-scale model with the aid of a scale-similarity modeling concept,” Int. J. Heat Fluid Flow 39, 42 (2013); M. Inagaki and K. Abe, “An improved anisotropy-resolving subgrid-scale model for flows in laminar–turbulent transition region,” Int. J. Heat Fluid Flow 64, 137 (2017)] and evaluate their performance in turbulent channel flows. Among various models in the present study, the original model combined with the scale-similarity model for the subgrid-scale (SGS)-Reynolds term yields the best prediction for the anisotropy of the grid-scale (GS) velocity fluctuations and the SGS stress, even in coarse grid resolutions. Moreover, it successfully predicts large intensities of the spectra close to the cut-off scale in accordance with the filtered direct numerical simulation, whereas other models predict a rapid decay of the spectra in the low-wavelength region. To investigate the behavior of the models close to the cut-off scale, we analyze the budget equation for the GS Reynolds stress spectrum. The result shows that the scale-similarity model for the SGS-Reynolds term plays a role in the enhancement of the wall-normal velocity fluctuation close to the cut-off scale. Thereby, it activates turbulence close to the cut-off scale, leading to a reproduction of the proper streak structures observed in wall-bounded turbulent flows. The reproduction of velocity fluctuations close to the cut-off scale and turbulent structures is a key element for further development of SGS models.

Mechanism for Increased Viscous Drag over Porous Sheet Acoustic Liners

Traditional acoustic liners used in damping sound in aircraft applications consist of a porous face sheet, honeycomb core, and solid backing. Experiments have observed that, under conditions of high tonal sound levels, there occurs a dramatic increase in the viscous drag in the turbulent boundary layers over these surfaces. The mechanism for this increase in drag is the topic of interest. This involves experiments on a series of acoustic liners with fabricated porous sheets having a range of hole diameters, spacing, and patterns. The hole spacing spanned a range of viscous spanwise length scales from . The boundary-layer measurements include mean velocity and turbulence intensity profiles, velocity spectra, and turbulence burst, ejection, and sweep statistics based on the variable interval time-averaging and -quadrant methods. Coupled with this are direct drag measurements of the acoustic liners. The results show a dramatic increase in drag at frequencies near acoustic resonance and at high sound levels. This is presumed to be driven by net zero mass jets emanating from the porous sheets. For the experimental data available, the largest increase in drag is observed for the smaller spanwise distance between perforations, which is comparable to the typical spanwise wavelength of wall streak structures in turbulent boundary layers. The increase in drag is correlated with an increase in the turbulence wall layer burst and ejection events.

Supersonic boundary layer transition induced by self-sustaining dual jets**Project supported by the National Natural Science Foundation of China (Grant Nos. 11602299, 11872374, and 51809271).

To promote high-speed boundary layer transition, this paper proposes an active self-sustaining dual jets (SDJ) actuator utilizing the energy of supersonic mainflow. Employing the nanoparticle-based planar laser scattering (NPLS), supersonic flat-plate boundary layer transition induced by SDJ is experimentally investigated in an Ma-2.95 low-turbulence wind tunnel. Streamwise and spanwise NPLS images are obtained to analyze fine flow structures of the whole transition process. The results reveal the transition control mechanisms that on the one hand, the jet-induced shear layer produces unstable Kelvin–Helmholtz instabilities in the wake flow, on the other hand, the jets also generates an adverse pressure gradient in the boundary layer and induce unstable streak structures, which gradually break down into turbulence downstream. The paper provides a new method for transition control of high-speed boundary layer, and have prospect both in theory and engineering application.

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.

Coherent structures in the linearized impulse response of turbulent channel flow

We study the evolution of velocity fluctuations due to an isolated spatio-temporal impulse using the linearized Navier–Stokes equations. The impulse is introduced as an external body force in incompressible channel flow at $Re_{\unicode[STIX]{x1D70F}}=10\,000$. Velocity fluctuations are defined about the turbulent mean velocity profile. A turbulent eddy viscosity is added to the equations to fix the mean velocity as an exact solution, which also serves to model the dissipative effects of the background turbulence on large-scale fluctuations. An impulsive body force produces flow fields that evolve into coherent structures containing long streamwise velocity streaks that are flanked by quasi-streamwise vortices; some of these impulses produce hairpin vortices. As these vortex–streak structures evolve, they grow in size to be nominally self-similar geometrically with an aspect ratio (streamwise to wall-normal) of approximately 10, while their kinetic energy density decays monotonically. The topology of the vortex–streak structures is not sensitive to the location of the impulse, but is dependent on the direction of the impulsive body force. All of these vortex–streak structures are attached to the wall, and their Reynolds stresses collapse when scaled by distance from the wall, consistent with Townsend’s attached-eddy hypothesis.

Streak and hierarchical structures of the Tohoku\u2013Hokkaido subduction zone plate boundary

Here, we investigated four families of moderate-size (M ~ 4.8) repeating earthquakes and nearby smaller events that occurred in the Naka-Oki, Kushiro-Oki, Urakawa-Oki, and Kamaishi-Oki regions of the Tohoku–Hokkaido subduction zone, Japan, between 2002 and 2017. We employed a hypocenter relocation method based on waveform cross-correlations of the above four families, as well as a waveform inversion method using the empirical Green’s function for the Naka-Oki and Kushiro-Oki earthquakes, where every moderate-size repeating earthquake showed complex and variable rupture processes. However, their rupture areas overlapped significantly and were elongate parallel to the slip direction. Some parts of these areas were also repeatedly ruptured by smaller earthquakes, with these smaller earthquakes and moderate events sometimes following almost identical rupture processes. The results indicate the existence of a characteristic structure that is represented by several elliptical patches that are elongate in the slip direction of the plate interface. It is likely that many streak structures have developed over a 1000 km along-strike section of the plate interface that extends from the Kushiro-Oki region to the Naka-Oki region of the subduction zone. These patches may be distributed hierarchically, with the smaller patches rupturing independently. The rupture of a smaller patch may either stop, producing a small earthquake, or it may grow into a large earthquake, with this rupture process depending on subtle differences in the physical conditions that we are unable to observe in a practical sense. Earthquakes are random processes, but they may not be completely random, as the preexisting characteristic structure of the seismic region may control the rupture process to some degree.

Cell-to-Dendrite Transition Induced by a Static Transverse Magnetic Field During Lasering Remelting of the Nickel-Based Superalloy

The effect of static transverse magnetic field on the microstructure of IN 713C nickel-based superalloy treated by laser remelting (LR) has been investigated. Dendrite microstructure’s transition from cellular to dendritic was observed with the application of a 0.45 T static transverse magnetic field during LR. The white streak structures resulting from thermal stress in the molten pool was found to disappear for the sample treated under magnetic field. The Hartman number (Ha) was calculated and found to be larger than 10, indicating that the damping effect of the static magnetic field on the melt flow was found to be dominant in the present situation. The slowed melt flow is beneficial to the dendritic growth, which may be attributed to the cell-to-dendrite transition during LR. The thermoelectric magnetic force (TEMF) acting on the dendrites was found to destabilize the solid/liquid interface and thus enhance dendritic growth during LR under the magnetic field as well. The EBSD analysis shows that new grains formed in the remelting region when the static magnetic field was applied. The TEMF was calculated to be as high as 1.12 × 106 N/m3, which is capable of fragmentting the dendrite arms and leading to the formation of new grains during LR.

Quasi-direct numerical simulation of forced convection over a backward-facing step: Effect of Prandtl number

Quasi direct numerical simulations (quasi-DNS) of forced heat convection over a backward-facing step are performed to study the turbulent heat transfer of low Prandtl number fluids in complex flow regimes. Reynolds number based on step height and inlet bulk velocity equals 4805. The expansion wall is heating by uniform dimensionless heat flux density. Effects of Prandtl number (Pr = 0.01, 0.025, 0.1, 1.0) on mean temperatures, heat transfer coefficients, heat fluxes, turbulent Prandtl numbers and instantaneous velocity and temperature fluctuations, are compared with each other. For low Prandtl numbers, diffusion effect is prominent, thermal boundary layer is much thicker, Nusselt number is smaller and Stanton number is larger. The maximum heat transfer location moves upstream of the reattachment point for larger Prandtl numbers while it is at the reattachment point for low Prandtl numbers. Wall-normal heat flux decomposition shows the turbulent part is comparable to the molecular one in the recirculation zone even for low Prandtl numbers. Turbulent Prandtl number profiles are irregular in the recirculation zone but in the recovering zone they increase first and then decrease. Their peak values decrease as Prandtl number increases. Near-wall instantaneous fields for streamwise velocity and temperature fluctuations show that the streak structures disappear in the recirculation zone but exist in the recovering zone. These highly-resolved data with heat transfer could serve for the validation and improvement of turbulence heat transfer models for low Prandtl media in future.

Transient Growth and Receptivity of Steady Disturbances to Irregular Rough Walls

Direct numerical simulations (DNS) are performed to investigate the transient growth of a steady disturbance induced by a numerically generated Gaussian rough wall in a laminar boundary layer. In the calculation of the interaction between the rough wall and the fluid, the multiple direct force and immersed boundary method (MDF/IBM) are adopted. The evolution of the streak structures and the energy of the disturbances generated by the rough wall are presented. A similar evolution into an almost sinusoidal modulation for the cylindrical roughness element is found for the current irregular rough wall, and the disturbance energy also undergoes the classical transient growth mode. Moreover, the influences of the skewness, kurtosis, and correlation length on the evolution of spanwise harmonics are also analyzed. The results show that the effects of skewness and kurtosis are on the distribution of energy among the wavelengths and the subsequent growth processes, while the wavelengths of the harmonics are linked to both the streamwise and spanwise correlation lengths of the rough wall.

広範囲の強さとスケールを持つ主流乱れによる境界層遷移の実験

It is known that in a boundary layer subjected to a few percent of free stream turbulence intensity, streaks appear and eventually break down to turbulence after wavy motions. However, the boundary layer transition subjected to lower free stream turbulence intensity is still not been fully elucidated. The present paper focused on revealing effects on intensity and scales of free stream turbulence on boundary layer transition, especially on how it effects the boundary layer transition as well as the growth rate of disturbance energy in the streamwise direction. In this study we performed flow visualization and hot-wire measurements in order to investigate transition points and process with wide-range of free stream turbulence intensity and scales. From the flow visualizations at intensity 1.42%<Tu<3.99% indicate that, the streak structures elongated in streamwise direction and collapse to turbulent downstream. This is similar to that at a few percent of turbulent intensity. Even though the turbulence intensity is the same, the Transition Reynolds number increases with free stream velocity. So, it is strongly suggests that, the Transition Reynolds number is depends on the ration between the boundary layer thickness and turbulence scales.

Turbulence modulation in a particle-laden flow over a hemisphere-roughened wall

Turbulence modulation by the inertia particles in a spatially developing turbulent boundary layer flow over a hemisphere-roughened wall was investigated using the direct numerical simulation method. The Eulerian and Lagrangian approaches were used for the gas- and particle-phases, respectively. An immersed boundary method was employed to resolve the hemispherical roughness element. The hemispheres were staggered in the downstream direction and arranged periodically in the streamwise and spanwise directions with spacing of px/d=4 and pz/d=2 (where px and pz are the streamwise and spanwise spacing of the hemispheres, and d is the diameter). The effects of particles on the turbulent coherent structures, turbulent statistics and quadrant events were analyzed. The results show that the addition of particles significantly damps the vortices structures and increases the length scales of streak structures. Compared with the particle-laden flow over the smooth wall, the existence of the wall roughness decreases the mean streamwise velocity in the near wall region, and makes the peaks of Reynolds stresses profiles shift up. In addition, the existence of particles also increases the percentage contributions to Reynolds shear stress from the Q4 events, however, decreases the percentage contributions from other quadrant events.

Study on local topology model of low/high streak structures in wall-bounded turbulence by tomographic time-resolved particle image velocimetry

The relationship between the bursting event and the low/high-speed streak in the logarithmic law (log-law) region of a turbulent boundary layer is investigated. A tomographic time-resolved particle image velocimetry (TRPIV) system is used to measure the instantaneous three-dimensional-three-component (3D-3C) velocity field. The momentum thickness based Reynolds number is about 2 460. The topological information in the log-law region is obtained experimentally. It is found that the existence of the quadrupole topological structure implies a three-pair hairpin-like vortex packet, which is in connection with the low/high-speed streak. An idealized 3D topological model is then proposed to characterize the observed hairpin vortex packet and low/high-speed streak.