Structures In Turbulent Channel Flow Research Articles

Large-scale coherent structures in turbulent channel flow: a detuned instability of wall streaks

Large-scale coherent structures in turbulent channel flow: a detuned instability of wall streaks

Coherent pressure structures in turbulent channel flow

Coherent pressure structures in turbulent channel flow

Structure of turbulent channel flow subjected to simultaneous inlet turbulence and localized injection

Localized wall injection exists both in naturally occurring flows and in industrial applications, such as seepage of water from permeable soil layers in a river and film cooling of turbine blades. In these cases, the incoming flow is practically always subjected to disturbances, which in turn increases the turbulence level. In this experimental study, the combined effects of localized wall injection and background turbulence on the structure of turbulent channel flow are investigated for the first time.

The effect of inlet turbulence on the quiescent core of turbulent channel flow

The impact of inlet turbulence on the structure of turbulent channel flow is investigated using particle image velocimetry. Streamwise–wall-normal plane measurements are performed in a channel, where different turbulence intensities were generated at the inlet with an active grid. Four cases are tested with matched centreline mean velocities, while the centreline turbulence intensities ranged from 3.7 % for the reference case, up to 6.4 %. The friction velocity is found to be approximately constant with varying centreline turbulence intensities, resulting in a matched friction Reynolds number of $Re_\tau \approx 770$ for all cases, which contrasts with similar experiments performed in a zero-pressure-gradient boundary layer. The log region remains intact for all cases. The so-called quiescent core of the turbulent channel flow is also investigated. In addition to increased core discontinuity, the increased fluctuations of the streamwise velocity give rise to new core states, which differ from the conventional ones in their characteristic velocity. They are associated with a bulk of low- or high-momentum fluid passing through the measurement domain, and their occurrence increases with turbulence intensity. Tracking the core boundaries indicates an overall tendency of the core to move closer to the wall for increased inlet turbulence intensities, resulting in an increased core thickness. Moreover, it is found that the low-momentum cores generally reside closer to the wall compared with the ordinary cores and appear to be thicker than them, whereas the opposite, i.e. residing farther from the wall and being thinner, is true for the high-momentum cores.

Heat transfer enhancement by feedback blowing and suction based on vortical structure in turbulent channel flow at low Reynolds number

Heat transfer enhancement by feedback blowing and suction based on vortical structure in turbulent channel flow at low Reynolds number

Resolvent modelling of near-wall coherent structures in turbulent channel flow

Turbulent channel flow was analysed using direct numerical simulations at friction Reynolds numbers Reτ=180 and 550. The databases were studied using spectral proper orthogonal decomposition (SPOD) to identify dominant near-wall coherent structures, most of which turn out to be streaks and streamwise vortices. Resolvent analysis was used as a theoretical approach to model such structures, as it allows the identification of the optimal forcing and most amplified flow response; the latter may be related to the observed relevant structures obtained by SPOD, especially if the gain between forcing and response is much larger than what is found for suboptimal forcings or if the non-linear forcing is white noise. Results from SPOD and resolvent analysis were compared for several combinations of frequencies and wavenumbers. For both Reynolds numbers, the best agreement between SPOD and resolvent modes was observed for the cases where the lift-up mechanism from resolvent analysis is present, which are also the cases where the optimal resolvent gain is dominant. These results confirm the outcomes in our previous studies (Abreu et al., 2019; Abreu et al., 2020), where we used a DNS database of a pipe flow for the same Reynolds numbers.

Hierarchy of coherent structures in turbulent channel flow

By analyzing a database of fully developed turbulent channel flow at the friction Reynolds number Reτ = 4179, we investigate the sustaining mechanism of a hierarchy of coherent structures in the wall-bounded turbulence. For this purpose, we decompose the turbulent fields into different scales by a band-pass filter. Using the filtered velocity and velocity gradients, we identify the hierarchy of coherent structures to observe that the largest-scale structures at each distance from the wall are composed of quasi-streamwise vortices and low-speed streaks. Since these are similar to well-known coherent structures in the buffer layer, they are likely to be maintained by the self-sustaining process. In contrast, structures smaller than the distance from the wall distribute isotropically. These observations are also confirmed by using a conditional sampling method. Moreover, quantifying the scale-dependent contributions to the vortex stretching and energy transfer, we show that the largest-scale coherent structures are strongly affected by the mean shear, whereas smaller-scale vortices are generated by the energy cascading events. Incidentally, in large-scale ejection regions (i.e. large-scale low-speed streaks), the cascading events are stronger than in large-scale sweep regions.

Energy cascade and vortex structure in turbulent channel flow

Energy cascade and vortex structure in turbulent channel flow

Dual-plane stereoscopic PIV measurement of vortical structure in turbulent channel flow on sinusoidal riblet surface

Abstract Vortical structure in wall turbulence over a sinusoidal riblet surface is investigated by means of a dual-plane stereoscopic particle image velocimetry (DPS-PIV) measurement. The experiment is made in a channel flow at a friction Reynolds number of 150. The lateral spacing of the adjacent walls of the sinusoidal riblet varies in the streamwise direction and 12% of the drag reduction rate has been confirmed. The DPS-PIV measurement system consists of four high-speed CCD cameras. The laser sheets are provided on streamwise and wall-normal planes and separated 0.5 mm in the spanwise direction to each other. The profiles of the velocity statistics in the flat case show a good agreement with previous data. Since all velocity components can be measured on adjacent laser sheets simultaneously, an instantaneous velocity deformation tensor can be obtained and vortical structures can be identified by a second invariant of the tensor i.e., the Q value. The probability of Q value in the riblet side is almost unchanged from that of the flat side. The analysis of the tracking of vortical structures by using the Q value is performed. As similar to a pathline analysis in a previous study, we confirmed that the riblet surface prevents the vortical structure hitting the wall and vortical structures follow the upward and downward flows induced by the sinusoidal riblet surface.

Nonlinear optimal large-scale structures in turbulent channel flow

Coherent structures in turbulent shear flows take the form of packets of hairpin vortices reaching the outer region of the boundary layer along with streaks of different size, going from the near-wall to the outer region. The latter can be explained by the linear transient growth of the perturbations of the mean turbulent profile. Whereas, the former are recently found to be optimally-growing only in the presence of nonlinear effects, as ascertained for a turbulent channel flow at a low friction Reynolds number. The present work aims at investigating whether large-scale streaks can be optimally-growing in a nonlinear framework for a turbulent channel flow. Changing the friction Reynolds number from 180 to 590, the nonlinear optimal perturbation tends towards more robust large-scale streaks and vortical structures of smaller size. These streaks are generated by a coherent large-scale lift-up mechanism, acting as a source term in the energy balance, inducing a positive turbulent kinetic energy production at the outer scale. This indicates that the outer energy production peak arising between the two considered Reynolds numbers can be associated with the growth of optimal large-scale streaks, which represent a robust feature of turbulent channel flows.

Hierarchy of the Vortical Structures in Turbulent Channel Flow

To investigate the physical mechanism of the energy cascade in wall-bounded turbulence, we conduct direct numerical simulations. We apply the Gaussian filter to the simulated physical quantities so that we can extract hierarchical structures for various scales. The comparison between the coarse-grained value of the second invariant of the velocity gradient tensor and that of the “internal energy”, which is defined in this study, shows that the energy for a certain scale is indeed possessed by vortices of the same scale.

Exploration of secondary instability of artificially excited coherent structure in turbulent channel flow

Exploration of secondary instability of artificially excited coherent structure in turbulent channel flow

Wavelet analysis on the drag-reducing characteristics of turbulent channel flow with surfactant additive based on experimental data

Two-dimensional velocity fields for turbulent channel flows of water and 50 ppm CTAC/NaSal (cetyltrimethyl ammonium chloride/sodium salicylate) solution were experimentally obtained by particle image velocimetry. Multi-scale decompositions of the fluctuating velocity signals were performed by two-dimensional binary orthogonal discrete wavelet to analyze the influence of surfactant additives on the multi-scale characteristics of turbulent channel flow. From the results of wavelet multi-scale decompositions, it can be observed that the quantity of coherent structures near the wall in CTAC solution flow is decreased obviously. The results of the flatness factor show that the addition of drag-reducing additives inhibits the intermittency in turbulence. By analyzing the distribution of local intermittence measure and local Reynolds shear measure, it is found that the intermittency mainly concentrates near the wall, and the intermittent region distinctly reduces in CTAC solution flow compared with that in water flow. By combining the analysis of the motion of coherent structures in turbulent channel flow, it is shown that surfactant additives mainly and distinctly impact the coherent structures near the wall, especially in the viscous layer.

Invariant solutions of minimal large-scale structures in turbulent channel flow for up to 1000

Understanding the origin of large-scale structures in high-Reynolds-number wall turbulence has been a central issue over a number of years. Recently, Rawat et al. (J. Fluid Mech., vol. 782, 2015, pp. 515–540) have computed invariant solutions for the large-scale structures in turbulent Couette flow at $Re_{\unicode[STIX]{x1D70F}}\simeq 128$ using an overdamped large-eddy simulation with the Smagorinsky model to account for the effect of the surrounding small-scale motions. Here, we extend this approach to Reynolds numbers an order of magnitude higher in turbulent channel flow, towards the regime where the large-scale structures in the form of very-large-scale motions (long streaky motions) and large-scale motions (short vortical structures) emerge energetically. We demonstrate that a set of invariant solutions can be computed from simulations of the self-sustaining large-scale structures in the minimal unit (domain of size $L_{x}=3.0h$ streamwise and $L_{z}=1.5h$ spanwise) with midplane reflection symmetry at least up to $Re_{\unicode[STIX]{x1D70F}}\simeq 1000$. By approximating the surrounding small scales with an artificially elevated Smagorinsky constant, a set of equilibrium states are found, labelled upper- and lower-branch according to their associated drag. It is shown that the upper-branch equilibrium state is a reasonable proxy for the spatial structure and the turbulent statistics of the self-sustaining large-scale structures.

Local topology via the invariants of the velocity gradient tensor within vortex clusters and intense Reynolds stress structures in turbulent channel flow

Previous works have shown that momentum transfer in the wall–normal direction within turbulent wall–bounded flows occurs primarily within coherent structures defined by regions of intense Reynolds stress [1]. Such structures may be classified into wall–attached and wall–detached structures with the latter being typically weak, small–scale, and isotropically oriented, while the former are larger and carry most of the Reynolds stresses. The mean velocity fluctuation within each structure may also be used to separate structures by their dynamic properties. This study aims to extract information regarding the scales, kinematics and dynamics of these structures within the topological framework of the invariants of the velocity gradient tensor (VGT). The local topological characteristics of these intense Reynolds stress structures are compared to the topological characteristics of vortex clusters defined by the discriminant of the velocity gradient tensor. The alignment of vorticity with the principal strain directions within these structures is also determined, and the implications of these findings are discussed.

Inner–outer interactions of large-scale structures in turbulent channel flow

Direct numerical simulation data of turbulent channel flow ($Re_{{\it\tau}}=930$) are used to investigate the statistics of long motions of streamwise velocity fluctuations ($u$), and the interaction of these structures with the near-wall disturbances, which is facilitated by their associated large-scale circulations. In the log layer, the negative-$u$ structures are organized into longer streamwise extent (${>}3{\it\delta}$) in comparison to the positive-$u$ counterparts. Near the wall, the footprint of negative-$u$ structures is relatively narrow in comparison to the footprint of positive-$u$ structures. This difference is due to the opposite spanwise motions in the vicinity of the footprints, which are either congregative or dispersive depending on the circulation of the outer roll cells. Conditional sampling of the footprints shows that the spanwise velocity fluctuations ($w$) are significantly enhanced by the dispersive motions of high-speed structures. On the other hand, the near-wall congregative motions of negative-$u$ structures generate relatively weak $w$ but intense negative-$u$ regions due, in part, to the spanwise collective migration of near-wall streaks. The concentrated near-wall regions of negative-$u$ upwell during the merging of the outer long scales – an effect that is demonstrated using statistical analysis of the merging process. This leads to a reduction of the convection speed of downstream negative-$u$ structures and thus promotes the merging with upstream ones. These top-down and bottom-up interactions enhance the spatial coherence of long negative-$u$ structures in the log region.

Effect of hierarchical structured superhydrophobic surfaces on coherent structures in turbulent channel flow

In this study, the coherent structures, which were effected by superhydrophobic (SH) surfaces in turbulent channel flow, were investigated by time-resolved particle image velocimetry (TRPIV). Hierarchical structured SH surfaces which have micro-nano secondary structures were applied in the experiment. Drag reduction rate is approximately 11% at Reθ≅1300, which is based on momentum thickness. Coherent structures (low-speed streak, hairpin vortex and hairpin vortex packets) with scale information were obtained by conditional sampling and averaging method. Compared to the hydrophilic (PH) surfaces, the conditional averaged hairpin vortex structures of SH surfaces have a lager spacing of vortex legs, a smaller angle of inclination along streamwise and spanwise directions. Accordingly, a smaller-scale hairpin vortex packet with three swirling motions was emerged. Through the comparative research on angle of inclination, the lifting movement of vortex in wall-normal direction was also weaker than PH case. These shows that the existence of SH surfaces restricts the development of coherent structures and induces drag reduction effects.

Structure of Turbulent Channel Flow Perturbed by a Wall-Mounted Cylindrical Element

The current study reports structural modifications imposed in fully developed, turbulent channel flow by an isolated wall-mounted circular cylinder. The cylinder extended into the logarithmic layer of the flow, perturbing the larger flow scales that embody a significant fraction of the turbulent kinetic energy and Reynolds shear stress. Hot-wire measurements were made in the wake of the wall-mounted circular cylinder at multiple wall-normal and streamwise positions. Besides observing the expected mean velocity deficit in the wake of the cylinder, a secondary peak in streamwise Reynolds normal stress away from the wall was observed, coupled with suppression of the near-wall peak native to the incoming unperturbed, turbulent flow. These observations are similar to those made by Ryan et al. (“Effects of Simple Wall-Mounted Cylinder Arrangements on a Turbulent Boundary Layer,” AIAA Journal, Vol. 49, No. 10, 2011, pp. 2210–2220) for a wall-mounted cylinder protruding into the logarithmic layer of a turbulent boundary layer. Premultiplied velocity spectra elaborated on these energy modifications, specifically the occurrence of two distinct energy peaks at two-thirds of the cylinder height and an attenuation of energy associated with larger flow scales close to the wall. All of these perturbations were found to decay with streamwise distance downstream as the flow relaxed toward the unperturbed state. A clear persistence of the structures at the aforementioned peak at two-thirds cylinder height, similar in scale to the very large-scale motions in canonical wall turbulence, suggests an environment preferring structures of such scale. The influence of the cylinder aspect ratio on the characteristics of the perturbed flow is evaluated, and a distinction in wake structure is identified.

Velocity–vorticity correlation structure in turbulent channel flow

AbstractA new statistical coherent structure (CS), the velocity–vorticity correlation structure (VVCS), using the two-point cross-correlation coefficient $R_{ij}$ of velocity and vorticity components, $u_i$ and $\omega _j~ (i, j = 1, 2, 3)$, is proposed as a useful descriptor of CS. For turbulent channel flow with the wall-normal direction $y$, a VVCS study consists of using $u_i$ at a fixed reference location $y_r$, and using $|R_{ij} (y_r; x, y, z)|\geqslant R_0$ to define a topologically invariant high-correlation region, called $\mathit{VVCS}_{ij}$. The method is applied to direct numerical simulation (DNS) data, and it is shown that the $\mathit{VVCS}_{ij}$ qualitatively and quantitatively captures all known geometrical features of near-wall CS, including spanwise spacing, streamwise length and inclination angle of the quasi-streamwise vortices and streaks. A distinct feature of the VVCS is that its geometry continuously varies with $y_r$. A topological change of $\mathit{VVCS}_{11}$ from quadrupole (for smaller $y_r$) to dipole (for larger $y_r$) occurs at $y^{+}_r=110$, giving a geometrical interpretation of the multilayer nature of wall-bounded turbulent shear flows. In conclusion, the VVCS provides a new robust method to quantify CS in wall-bounded flows, and is particularly suitable for extracting statistical geometrical measures using two-point simultaneous data from hotwire, particle image velocimetry/laser Doppler anemometry measurements or DNS/large eddy simulation data.

Vortex structures in turbulent channel flow modulated by a steadily distributed spanwise Lorentz force

Turbulence control and drag reduction in a channel flow by using a steadily distributed spanwise Lorentz force are investigated numerically via a direct numerical simulation (DNS). The characteristics of controlled flow fields and vortex structures are described. Meanwhile, the mechanisms of turbulence suppression and drag reduction by the Lorentz force are also discussed. Calculated results indicate that: (1) The shear layers with a arge gradient of spanwise velocity are created in the laminar boundary layer induced by the spanwise Lorentz force, where the streamwise vortices are easily generated by perturbations. (2) Under the action of the distributed Lorentz force with proper control parameters, only periodically well-organized streamwise vortices are observed in the near-wall region of the turbulent channel flow. (3) After controlling, the averaged lift height of inclined streamwise vortices is reduced significantly as compared with the uncontrolled turbulence flow, resulting in the reduction of the burst strength and subsequent drag reduction on the wall.