Fluctuating Velocity Field Research Articles

Experimental investigation of the distorted flow field in a new type of direct-connect facility for scramjet studies

A supersonic distorted flow field generated by a new type of direct-connect facility at Mach 2.1 is investigated experimentally via particle image velocimetry (PIV). This facility is designed to accurately reproduce the real flow field in scramjet especially the flow distortion within the isolator, while avoiding the unstart problem in traditional direct-connect facility. The time-averaged velocity fields reveal the principle of flow distortion generated by this facility and show the detailed flow field structures. The mean results suggest that the choose of target parameters distribution will directly affect the performance of this facility. Then, the distorted flow fields in the isolator with a specific non-uniform incoming flow condition are investigated in detail, with the method of proper orthogonal decomposition (POD). The POD analysis of the fluctuating velocity fields finds that there are complex flow field structures within the isolator, including the large-scale vortex structure and the small-scale structure near the wall surface. And these structures in the eigenmodes are associated with the shock wave and the low-velocity region close to the wall surface. Meanwhile, the distorted main flow is also considered to contribute to the flow field instability. Finally, according to the analysis results, the practical guidance of this new type of direct-connect facility is proposed for the subsequent experimental research about scramjet.

Co‐Current Fluid Flow Mechanisms Within a Complex Porous Structure at the Pore Level

AbstractThis study has been addressed the mechanisms of the co‐current fluid flow in a heterogeneous porous media using the lattice Boltzmann method. Since the smallest interaction between viscous and capillary forces at the interface is of great importance, the simulations are performed with the hybrid phase‐field model at the pore level. This is because the main advantage of this model is that due to its connection with the phase‐field theory, it has a solid physical basis to record interface dynamics. Model verification is followed by recording viscous coupling effects. The relative permeability curves and the velocity profiles are compared with the analytical solution, so that the observations confirm the accuracy claim of the model. In the following, the fluid flow through the heterogeneous porous structure is simulated, so that its sweep efficiency and corresponding velocity field at the breakthrough time are discussed. The results show that increasing the injection velocity reduces the displacement efficiency and increases the velocity field fluctuations due to the viscous fingering regime. Changing the wetting conditions to strong imbibition (SI) will increase the efficiency due to the role of capillary pressure and corner flow mechanism. Also, increasing the interfacial tension in the SI and the strong drainage will increase and decrease the sweep efficiency, respectively, so the rate of changes in fully immiscible flow is much higher than the near miscible flow. The velocity fluctuations in the fully immiscible flow are more significant than in the near miscible flow, which is reasonable given the flow properties.

Junction flow inside and around three-row cylindrical group on rigid flat surface

Groups of bluff bodies are widespread in nature and technology. These are the supports of bridge crossings, high-rise buildings in cities, offshore drilling and wind platforms, algae and vegetation in the seas and rivers, forests and other objects. The flow of air or water around such structures has a complex vortex and jet character and requires significant efforts in the process of scientific research to improve the environmental situation and reduce material and technical costs in the process of operating such structures. The purpose of the research is study the features of the generation and evolution of vortex and jet flows near and inside the three-row group of cylinders, which are installed on the rigid flat surface. The results of experimental studies showed that the flow around the group of cylinders had a complex unsteady nature, which is due to the interaction of vortex and jet flows typical flow elements with the three-row cylindrical group, which was located installed on the rigid flat surface. The three-row cylindrical group (31 piles with a diameter of 0.027 m) is a model of a bridge support, which was streamlined at a velocity of 0.06 m/s to 0.5 m/s (Reynolds number Red=(1600–6700) and Froude number Fr=(0.04–0.18)). Visual investigations and measurements of the velocity field were carried out inside and around the three-row structure. The features of the formation and evolution of vortex and jet flows inside and near the cylindrical group were established. Integral, spectral and correlation characteristics of the velocity fluctuation field were obtained. Mean, root-mean-square values of velocity and probability density functions of velocity fluctuations integrally displayed the changes in the velocity field in the spatial and temporal domain in the junction area of grillage and plate. The power spectral densities of velocity fluctuations and mutual correlation functions made it possible to study the features of the generation of the velocity fluctuation field in the frequency domain and its interrelationships in space. It was revealed that the velocity field inside the horseshoe vortex structures was multimodal. The spectral levels of velocity fluctuations at the periphery of the quasistable horseshoe vortex structures were higher than in the cores of these structures. The highest levels of the velocity fluctuation spectra were observed in front of the second lateral cylinder where the interaction of the vortex and jet flows took place. Discrete peaks in the spectral levels of velocity fluctuations are found at the frequencies of formation of large-scale wake vortices and the frequencies of formation of small-scale vortex structures of the shear layer, which are due to the Kelvin-Helmholtz instability. It has been established that the frequency of formation of shear layer vortices is (10–40) times higher than the frequency of formation of wake vortices.

Reconciling Parker Solar Probe Observations and Magnetohydrodynamic Theory

The Parker Solar Probe mission provides a unique opportunity to characterize several features of the solar wind at different heliocentric distances. Recent findings have shown a transition in the inertial range spectral and scaling properties around 0.4–0.5 au when moving away from the Sun. Here we provide, for the first time, how to reconcile these observational results on the radial evolution of the magnetic and velocity field fluctuations with two scenarios drawn from the magnetohydrodynamic theory. The observed breakdown is the result of the radial evolution of magnetic field fluctuations and plasma thermal expansion affecting the distribution between magnetic and velocity fluctuations. The two scenarios point toward an evolving nature of the coupling between fields that can be also reconciled with Kraichnan and Kolmogorov pictures of turbulence. Our findings have important implications for turbulence studies and modeling approaches.

Experimental and numerical study of a compact inline swirler for gas–liquid separation

The liquid mean and fluctuating velocity field, the pressure drop, the gas-phase distribution and the bubble size distribution in a compact inline swirler for gas–liquid separation were experimentally measured. The results served as validation for the proposed computational approach and the most suitable turbulence model was identified. The gas phase distributions obtained considering both a constant bubble diameter and the local bubble size distribution (BSD) through the solution of a Population Balance Equation (PBE) confirmed the lighter gas phase accumulation towards the pipe centre. The study highlights that to realistically predict the BSD a coupled solution of the PBE with the flow equations is needed and that the resulting distribution is relatively narrow. The gas accumulation zone obtained coupling the solution of the PBE is in good agreement with the experimental one. It is shown that CFD may help the design of inline swirlers for gas–liquid separation.

Large-eddy simulation of turbulent heat transfer in negative buoyant fountain

This paper presents the results of large-eddy simulation (LES) of a downward wall jet that encounters an upward flow in a vertical rectangular channel, named negative buoyant fountain (NBF). The downward jet fluid has higher temperature than the opposing upward stream so that the influence of the buoyancy force on the physics of the mixing process can be investigated. Two subgrid-scale models available in the OpenFOAM platform, i.e. the dynamic k-equation model (DKM) and the wall-adapting local eddy-viscosity (WALE) model, are used for cross-comparison. The numerical results of the time-averaged and fluctuating velocity fields agree satisfactorily with the experimental data. The LES results form an accurate, high-resolution, three-dimensional database of buoyant turbulent mixing, which has expanded the sparse flow field measurements provided by the experiment, hence the numerical database could be used to validate the Reynolds-averaged Navier–Stokes (RANS) simulation method and to contribute to the turbulence modeling. In particular, the statistics of turbulent heat flux have been analyzed and various turbulent heat flux models have been a priori tested based on the LES numerical database. Such information could be useful for RANS simulation guidelines or for deriving anisotropic turbulent heat flux models applicable for turbulent buoyant flows sharing the similar principal flow structures in complex geometries.

CFD‐based analysis of entropy generation due to mean and fluctuating fields during turbulent natural convection in a square enclosure

AbstractThe current study aims to numerically investigate the entropy generation during the natural convection flow of air in a square cavity. The governing equations for the conservation of mass, momentum, energy, and turbulence are solved using a control volume‐based technique employing the commercial code Fluent. Runs have been performed for both laminar and turbulent flow regimes by varying the Rayleigh number (Ra) from 103 to 1010. On the other hand, various viscous distribution coefficients (ϕ = 10−4, 10−3, and 10−2) and constant Prandtl number (Pr = 0.71) were considered. Given the conflicting perspectives in the literature regarding the entropy generation under turbulent regimes, more research is needed to better understand the impact that the fluctuating flow has on entropy production. The four terms of entropy generation inherent to turbulent natural convection (entropy generation due to dissipation in the mean and the fluctuating velocity fields in addition to the heat flux due to the mean and the fluctuating temperature) are computed in the present work and compared to calculations based on only mean values of temperature and velocity gradients. It was found that taking into account the fluctuating terms of temperatures and velocities augment the total entropy generation by 10.10%, 14.43%, and, 17.70%, up to 32.60%, respectively, for Ra = 5 × 108, Ra = 109, Ra = 1.58 × 109, and Ra = 1010. The gain shows the tendency to increase with the Rayleigh number. Thus, the fluctuating terms cannot be neglected particularly for high Rayleigh numbers. Furthermore, unlike entropy production due to the mean flow field, numerical outcomes reveal that the generated irreversibilities due to fluctuating flow are located around the upper hot and the lower cold corners of the heated walls. In addition, a numerical relationship between the first and the second laws of thermodynamics has been derived. A promising result that emerged from this study has shown that the Nusselt number and therefore the first law of thermodynamics is sufficient to estimate the heat part of entropy generation without the necessity of using the second law.

Wall-attached and wall-detached eddies in proper orthogonal decomposition modes of a turbulent channel flow

To comprehensively understand the geometric and kinematic characteristics of inertial coherent motions that conform to the attached-eddy model, proper orthogonal decomposition (POD) is applied to volumetric streamwise fluctuating velocity fields in a turbulent channel flow with Reτ=2003 being resolved by direct numerical simulation. Wall-attached POD eddies (WAPEs) or wall-detached POD eddies (WDPEs) are identified from all of the POD eigenmodes by the wall-attached or wall-detached conditions, respectively. These POD eddies can be regarded as statistical structures that make independent energy contributions. WAPEs with a wide range of scale hierarchies are found to be self-similar in both geometries and kinematics. The generalized logarithmic law of high-even-order moments contributed by self-similar WAPEs further indicates their Gaussian-like behavior. These results suggest that WAPEs are the prime statistical representatives of attached eddies. In contrast, the scale distribution of WDPEs across a wide range of flow layers is invariant and their geometric shapes are self-similar over a wide range of length scales, but the kinematic self-similarity of WDPEs is not evident.

Swirl rotation direction effects on lean direct injection pilot mixer performance: Experiments and LES modeling

In order to improve the understanding of the effects of various practical design parameters on the performance of Lean Direct Injection (LDI)-based combustors, the impact of varying the relative rotation direction of the outer air swirler (OAS) and inner air swirler (IAS) on the performance of a methane-fueled LDI-type mixer/injector is investigated under non-reacting and reacting conditions by both experiments and computational fluid dynamics (CFD) simulations. Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) models are used and compared in CFD modeling of counter- and co-swirling LDI mixers, while the adoption of a nonpremixed Flamelet Generated Manifold (FGM) method to predict reacting flows is investigated for comparison with experimental measurements of the mean and fluctuating velocity fields and OH* chemiluminescence. In addition, a self-consistent and robust CFD mesh generation strategy is developed to create notionally grid-independent CFD meshes for non-reacting and reacting flow simulations. The experimental non-reacting flow fields and flame lift-off limits demonstrate that while the basic structure of the flow field does not change by altering the relative rotation direction of IAS and OAS, the details of the center recirculation zone including its width and length do, resulting in slightly reduced flame lift-off equivalence ratio for the counter-rotating configuration. Although LES-FGM is able to capture the major flow and flame patterns in both counter- and co-swirling reacting flows, some substantial disparities are apparent, particularly in the region within/near the venturi that adversely impact the predictive abilities of this LES-FGM approach.

Influence of Synthetic Jets on Multiscale Features in Wall-Bounded Turbulence

This experimental research focuses on the impacts of submerged synthetic jets on a fully-developed turbulent boundary layer (TBL) under a drag reduction working case. Two-dimensional velocity vectors in the flow field are captured with the aid of a particle image velocimetry (PIV) system. Proper orthogonal decomposition (POD) analyses provide evidence that synthetic jets notably attenuate the induction effect of prograde vortex on the low-speed fluid in large-scale fluctuation velocity field, thereby weakening the bursting process of near-wall turbulent events. Furthermore, the introduced perturbance redistributes the turbulent kinetic energy (TKE) and concentrates the TKE onto small-scale coherent structures. Modal time coefficients in various orders of POD are divided into components of multiple frequency bands by virtue of complementary ensemble empirical mode decomposition (CEEMD). It is found that the turbulence signals are shifted from low-frequency to high-frequency bands thanks to synthetic jets, thus revealing the relationship between scales and frequency bands. One further method of scale decomposition is proposed, that is, the large-scale fluctuating flow field will be obtained after removing the high-frequency noise data with the help of continuous mean square error (CMSE) criterion.

Detecting deep axisymmetric toroidal magnetic fields in stars

Context. Asteroseismology has revealed small core-to-surface rotation contrasts in stars in the whole Hertzsprung–Russell diagram. This is the signature of strong transport of angular momentum (AM) in stellar interiors. One of the plausible candidates to efficiently carry AM is magnetic fields with various topologies that could be present in stellar radiative zones. Among them, strong axisymmetric azimuthal (toroidal) magnetic fields have received a lot of interest. Indeed, if they are subject to the so-called Tayler instability, the accompanying triggered Maxwell stresses can transport AM efficiently. In addition, the electromotive force induced by the fluctuations of magnetic and velocity fields could potentially sustain a dynamo action that leads to the regeneration of the initial strong axisymmetric azimuthal magnetic field. Aims. The key question we aim to answer is whether we can detect signatures of these deep strong azimuthal magnetic fields. The only way to answer this question is asteroseismology, and the best laboratories of study are intermediate-mass and massive stars with external radiative envelopes. Most of these are rapid rotators during their main sequence. Therefore, we have to study stellar pulsations propagating in stably stratified, rotating, and potentially strongly magnetised radiative zones, namely magneto-gravito-inertial (MGI) waves. Methods. We generalise the traditional approximation of rotation (TAR) by simultaneously taking general axisymmetric differential rotation and azimuthal magnetic fields into account. Both the Coriolis acceleration and the Lorentz force are therefore treated in a non-perturbative way. Using this new formalism, we derive the asymptotic properties of MGI waves and their period spacings. Results. We find that toroidal magnetic fields induce a shift in the period spacings of gravity (g) and Rossby (r) modes. An equatorial azimuthal magnetic field with an amplitude of the order of 105 G leads to signatures that are detectable in period spacings for high-radial-order g and r modes in γ Doradus (γ Dor) and slowly pulsating B (SPB) stars. More complex hemispheric configurations are more difficult to observe, particularly when they are localised out of the propagation region of MGI modes, which can be localised in an equatorial belt. Conclusions. The magnetic TAR, which takes into account toroidal magnetic fields in a non-perturbative way, is derived. This new formalism allows us to assess the effects of the magnetic field in γ Dor and SPB stars on g and r modes. We find that these effects should be detectable for equatorial fields thanks to modern space photometry using observations from Kepler, TESS CVZ, and PLATO.

Coherence of unsteady wake of periodically plunging airfoil

We present an experimental investigation of the flow structure in the near wake of a NACA0012 airfoil plunging sinusoidally at a chord Reynolds number of Re = 20 000 and for a wide range of reduced frequency k and Strouhal number based on peak-to-peak amplitude St. Estimated mean thrust coefficients using the mean and fluctuating velocity fields confirm the St2 dependence as well as a significant effect of the reduced frequency for k ≤ 1. Generally, time-averaged flow quantities are better correlated with St than k in the range tested (k ≤ 3.14 and St ≤ 0.24). Analysis of the streamwise flow and cross-flow in the near wake using two-point cross-correlations and proper orthogonal decomposition reveals that the unsteady characteristics are even better correlated with St than the mean flow quantities. The percentage energy of the fundamental wake modes of the streamwise flow and the flapping mode of the cross-flow increases with increasing St, but at different rates in the drag-producing and thrust-producing wakes. There are similarities to the wake synchronisation behind oscillating bodies. The spanwise-averaged cross-correlation coefficient in the measurement domain grows linearly for small St (in drag-producing wakes), and is nearly constant at a high value for larger St (in thrust-producing wakes). Results show that the Strouhal number is the most important parameter that determines the degree of two-dimensionality of the wake, and suggest that spanwise vortices are quasi-two-dimensional for St ≥ 0.05 and x/c ≤ 4. The implications for experimental gust generators using oscillating airfoils are discussed.

Scale-Resolving Simulations of a Statistically Targeted Forcing Method for Synthetic Turbulence Generation in a Freestream Turbulence

Abstract Computational fluid dynamics results for synthetic turbulence generation of freestream turbulence by a proposed statistically targeted forcing (STF) method are presented. The STF method has been previously documented for homogeneous isotropic and anisotropic turbulence and formulated to introduce a fluctuating velocity field with a distribution of first and second moments that match a user-specified target mean velocity and Reynolds stress tensor, by incorporating deterministic time-dependent forcing terms into the momentum equation for the resolved flow. This study extends its applicability to generation of freestream turbulence in scale-resolving simulations in which flow is spatially developing. The method provides flexibility in regions where synthetic turbulence needs to be generated or damped, for use in engineering level scale-resolving simulations such as large eddy simulation (LES), and hybrid Reynolds-averaged Navier–Stokes (RANS)-LES. The objective of this study is to evaluate the performance of the proposed STF method in simulations that incorporate monotonically integrated LES (MILES), Smagorinsky (SMAG) LES subgrid stress model, and dynamic hybrid RANS-LES (DHRL) model in reproducing inflow freestream turbulent flow, and the capability of each model to reproduce proper energy decay characteristics downstream of forcing. Results are inter-rogated and compared to target statistical velocity and turbulent stress distributions for inflow turbulence and evaluated in terms of energy spectra. Analysis of the influence of STF model parameters, mesh resolution, and LES subgrid stress model on the results is investigated. Results show that the STF method can successfully reproduce desired statistical distributions in a nearly isotropic freestream turbulent flow.

Anisotropy in turbulent Rayleigh–Bénard convection with and without rotation

We present results of direct numerical simulations on anisotropy in the velocity and the convective temperature fields of turbulent Rayleigh–Bénard convection in low-Prandtl-number fluids with and without uniform rotation about the vertical direction. Our results are in the intermediate range of Rayleigh number (Ra∼106−108) and high Rossby number (Ro>1). The probability distribution for the fluctuating velocity field v shows exponential tails. The distribution function for the vertical velocity is significantly different from those for the horizontal velocity components, which we take as a mark of anisotropy. The probability distribution function for the fluctuating temperature field θ is also quite different from that of any component of the velocity field and is proportional to exp [−(θ/θ0)4], where θ0 is a constant. To study the anisotropy in Fourier space, we look at the Fourier modes of the velocity fields and compare our numerical results with a calculation based on an effective linear model.

Experimental study of vortex formation in pulsating jet flow by time-resolved particle image velocimetry

We experimentally investigate the pulsating circular jet flow at moderate Reynolds numbers. By applying time-resolved particle image velocimetry in the axial-radial plane, we measure the near-field velocity fields with the jet source temporally modulated by sinusoidal pulsations. As a baseline, the steady jet flow with the same mean Reynolds number is tested. The direct comparisons of the mean and fluctuating velocity fields show that the whole potential core as well as the axisymmetric shear layer is modulated by the pulsation effect. Meanwhile, larger-scale vortices are formed in the shear layer with phase correlation of the pulsation cycle. As a result, the pulsation increases the turbulent mixing in the latter half of the potential core, and it extends the fluid entrainment further in the radial direction. The increased fluid entrainment of the ambient quiescent fluid is clearly identified by the attracting Lagrangian coherent structures as the bounds of the growing vortices within the shear layer. By analyzing the dynamic modes, we find that the low-frequency off-the-axis helical structures, which are dominant in the steady jet flow, are inhibited. The axisymmetric jet column mode and its harmonics along the axis are strengthened by the pulsation effect. Furthermore, the vortex formation mainly takes place particularly in the deceleration phase, whereas a shock-like wave front is formed during the acceleration, indicating the distinct roles of the pulsation phases in the jet instability.

Evaluation of a Statistically Targeted Forcing Method for Synthetic Turbulence Generation in Scale-Resolving Simulations

Abstract Computational fluid dynamics (CFD) results are presented for synthetic turbulence generation by a proposed statistically targeted forcing (STF) method. The new method seeks to introduce a fluctuating velocity field with a distribution of first and second moments that approximate a user-specified target mean velocity and Reynolds stress tensor, by incorporating deterministic time-dependent forcing terms into the momentum equation for the resolved flow. The STF method is formulated to extend the applicability of previously documented methods and provide flexibility in regions where synthetic turbulence needs to be generated or sustained, for use in engineering level large eddy and hybrid large eddy/Reynolds-averaged Navier–Stokes CFD simulations. The objective of this study is to evaluate the performance of the proposed STF method in large-eddy simulation (LES) simulations of isotropic and anisotropic homogeneous turbulent flow test cases. Results are interrogated and compared to target statistical velocity and turbulent stress distributions and evaluated in terms of energy spectra. Analysis of the influence of STF model parameters, mesh resolution, and LES subgrid stress model on the results is investigated. Results show that the new method can successfully reproduce desired statistical distributions in a homogeneous turbulent flow.

Non-locality of the turbulent electromotive force

ABSTRACTThe generation of large-scale magnetic fields ($\overline{\boldsymbol {B}}$) in astrophysical systems is driven by the mean turbulent electromotive force ($\overline{\rm{\boldsymbol {\cal E}} {}}$), the cross-correlation between local fluctuations of velocity and magnetic fields. This can depend non-locally on $\overline{\boldsymbol {B}}$ through a convolution kernel Kij. In a new approach to find Kij, we directly fit the time-series data of $\overline{\rm{\boldsymbol {\cal E}} {}}$ versus $\overline{\boldsymbol {B}}$ from a galactic dynamo simulation using singular value decomposition. We calculate the usual turbulent transport coefficients as moments of Kij, and show the importance of including non-locality over eddy length-scales to fully capture their amplitudes and that higher order corrections to the standard transport coefficients are small in this case.

Investigation of unsteady secondary flows and large-scale turbulence in heterogeneous turbulent boundary layers

Following the findings by Wangsawijaya et al. (J. Fluid Mech., vol. 894, 2020, A7), we re-examine the turbulent boundary layers developing over surfaces with spanwise heterogeneous roughness of various roughness half-wavelengths $0.32 \leq S/\bar {\delta } \leq 3.63$, where $S$ is the width of the roughness strips and $\bar {\delta }$ is the spanwise-averaged boundary-layer thickness. The heterogeneous cases induce counter-rotating secondary flows, and these are compared with the large-scale turbulent structures that occur naturally over the smooth wall. Both appear as meandering elongated high- and low-momentum streaks in the instantaneous flow field. Results based on the triple decomposed velocity fluctuations suggest that the secondary flows are spanwise-locked turbulent structures, with $S/\bar {\delta }$ governing the strength of the turbulent structures and the efficacy of the surface in locking the structures in place (most effective when $S/\bar {\delta } \approx 1$). In terms of unsteadiness, we find additional evidence from conditional averages of the fluctuating velocity fields showing that the secondary flows exhibit maximum unsteadiness (or meandering) when $S/\bar {\delta } \approx 1$. The conditional averages of both spanwise heterogeneous and smooth-wall cases result in structures that are reminiscent of those proposed for the streak-vortex instability model for the inner cycle of wall-bounded turbulence. However, in this case these structures are larger and do not necessarily share the same formation mechanism with the inner cycle. Secondary flows and large-scale structures coexist in the limits where either $S/\bar {\delta } \gg 1$ or $S/\bar {\delta } \ll 1$, where the secondary flows scale on $\delta$ or $S$, respectively. When $S/\bar {\delta } \gg 1$, the secondary flows are locked about the roughness transition, while relatively unaltered large-scale structures occur further from the transition. In the case where $S/\bar {\delta } \ll 1$, $S$-scaled secondary flows are confined close to the surface, coexisting with unaltered larger-scale turbulent structures that penetrate much deeper into the layer.

Pressure-informed velocity estimation in a subsonic jet

This work aims to estimate the time-resolved velocity field that is directly associated with pressure fluctuations in a subsonic round jet. Synchronous measurements of the velocity field and in-flow pressure fluctuations were performed on streamwise and cross-stream measurement planes. The spectral linear stochastic estimation and neural network architectures were implemented to estimate time-resolved velocity fields. In all cases, pressure fluctuations inside the flow are found to be connected to the streamwise convection of large-scale coherent structures in the flow, and the unique advantage of the bidirectional long-short-time-memory (LSTM) method is also reported in this work.

抵抗低減乱流境界層流れにおける大規模乱流構造と壁面せん断応力との関係

We have performed PIV measurements for the drag-reducing turbulent boundary layer flows by injecting surfactant aqueous solution. For the case of the high drag reduction ratio, large-scale turbulence structures were often observed in the fluctuating velocity field. However, the interaction between large-scale turbulence structures and wall-shear stress has not been investigated. In this study, we performed wide-range and high-magnification PIV measurements in order to obtain large-scale turbulence structures and wall-shear stress simultaneously. Decreasing and increasing of wall-shear stress correspond to passing turbulence structures which consist of low and high fluctuation velocities, respectively. This indicates that there would be the correlation between large-scale turbulence structures and instantaneous wall shear stress.