Turbulent fluctuations above the buffer layer of wall-bounded flows

The behaviour of the velocity and pressure fluctuations in the logarithmic and outer layers of turbulent flows is analysed using spectral information and probability density functions from channel simulations at Reτ _2000. Comparisons are made with experimental data at higher Reynolds numbers. It is found, in agreement with previous investigations, that the intensity profiles of the streamwise and spanwise velocity components have logarithmic ranges that are traced to the widening spectral range of scales as the wall is approached. The same is true for the pressure, both theoretically and observationally, but not for the normal velocity or for the tangential stress cospectrum, although even those two quantities have structures with lengths of the order of several hundred times the wall distance. Because the logarithmic range grows longer as the Reynolds number increases, variables which are ‘attached’ in this sense scale in the buffer layer in mixed units. These results give strong support to the attached-eddy scenario proposed by Townsend (1976), but they are not linked to any particular eddy model. The scaling of the outer modes is also examined. The intensity of the streamwise velocity at fixed y/h increases with the Reynolds number. This is traced to the large-scale modes, and to an increased intensity of the ejections but not of the sweeps. Several differences are found between the outer structures of different flows. The outer modes of the spanwise and wall-normal velocities in boundary layers are stronger than in internal flows, and their streamwise velocities penetrate closer to the wall. As a consequence, their logarithmic layers are thinner, and some of their logarithmic slopes are different. The channel statistics are available electronically at http://torroja.dmt.upm.es/ftp/channels/

Reynolds number effects on the Reynolds-stress budgets in turbulent channels

Budgets for the nonzero components of the Reynolds-stress tensor are presented for numerical channels with Reynolds numbers in the range Reτ ≤180–2000. The scaling of the different terms is discussed, both above and within the buffer and viscous layers. Above (x_2^+)≈150, most budget components scale reasonably well with u_t^3/h, but the scaling with (u_t^4)/v is generally poor below that level. That is especially true for the dissipations and for the pressure-related terms. The former is traced to the effect of the wall-parallel large-scale motions, and the latter to the scaling of the pressure itself. It is also found that the pressure terms scale better near the wall when they are not separated into their diffusion and deviatoric components, but mostly only because the two terms tend to cancel each other in the viscous sublayer. The budgets, together with their statistical uncertainties, are available electronically from http://torroja.dmt.upm.es/channels

Evidences of persisting thermal structures in Couette flows

[EN] DNS of passive thermal turbulent Couette flow at several friction Reynolds numbers (180, 250, and 500), and the Prandtl number of air are presented. The time averaged thermal flow shows the existence of long and wide thermal structures never described before in Couette flows. These thermal structures, named CTFS (Couette Thermal Flow Superstructures), are defined as coherent regions of hot and cold temperature fluctuations. They are intrinsically linked to the velocity structures present in Couette flows. Two different 2D symmetries can be recognized, which get stronger with the Reynolds number. These structures do not affect the mean flow or mean quantities as the Nusselt number. However, turbulent intensities and thermal fluxes depend on the width of the structures, mainly far from the walls. Since the width of the structures is related to the channel width, the statistics of thermal Couette flow are to some point box-dependent.This work was supported by the MINECO/FEDER, under project ENE2015-71333-R. The computations of the new simulations were made possible by a generous grant of computing time from the Barcelona Supercomputing Centre, reference FI-2018-1-0037. FAA is partially funded by GVA/FEDER project ACIF2018. We are very grateful for the advices and revision provided by one of the referees of the article, as it has helped to enrich its content.Alcántara-Ávila, F.; Gandía-Barberá, S.; Hoyas, S. (2019). Evidences of persisting thermal structures in Couette flows. International Journal of Heat and Fluid Flow. 76:287-295. https://doi.org/10.1016/j.ijheatfluidflow.2019.03.001S2872957

Letter: The link between the Reynolds shear stress and the large structures of turbulent Couette-Poiseuille flow

[EN] The length and width of the long and wide structures appearing in turbulent Couette flows are studied by means of a new dataset of direct numerical simulation covering a stepped transition from pure Couette flow to pure Poiseuille one, at Re-tau approximate to 130, based on the stationary wall. The existence of these structures is linked to the averaged Reynolds stress, (uv) over bar : as soon as in any part of the channel (uv) over bar changes its sign, the structures disappear. The length and width of the rolls are found to be, approximately, 50h and 2.5h, respectively. For this Reynolds number, simulations with a domain shorter than 100h cannot properly describe the behaviour of the longest structures of the flow.This work was supported by MINECO, under Project No. ENE2015-71333-R. The work of M. Oberlack was supported by the German Research Foundation (DFG) under the Grant No. OB96/39-1. The computations of the new simulations were made possible by a generous grant of computing time from the Supercomputing centre of the Universitat Politecnica de Valencia. We are grateful to Mr. Simon Hoyas for fruitful conversations about the paper.Gandía-Barberá, S.; Hoyas, S.; Oberlack, M.; Kraheberger, S. (2018). Letter: The link between the Reynolds shear stress and the large structures of turbulent Couette-Poiseuille flow. Physics of Fluids. 30(4):1-4. https://doi.org/10.1063/1.5028324S1430

Stratification effect on extreme-scale rolls in plane Couette flows

[EN] The existence of the large-scale structures appearing in turbulent Couette flows is studied by means of a direct numerical simulation data set of active thermal Couette flows for different friction Richardson numbers, at the Prandtl number of air. The existence of these structures is linked to the nonexistence of an active thermal flow. As soon as the Richardson number is greater than 1.5, the structures are less energetic, and for a value of only 3, the structures have vanished. This is due to the reorganization of the intense Reynolds stress events. Thus, large-scale structures will hardly appear in real-life Couette flows of air with a stable wall-normal gradient of temperature.This work was supported by Grant No. RTI2018-102256-B-I00 of MINECO/FEDER. The computations of the new simulations were made possible by a generous grant of computing time from the Barcelona Supercomputing Centre, Grant No. AECT-2020-2-0005. F.A.A. is partially funded by Generalitat Valenciana, GVA/FEDER project ACIF2018.Gandía-Barberá, S.; Alcántara-Ávila, F.; Hoyas, S.; Avsarkisov, V. (2021). Stratification effect on extreme-scale rolls in plane Couette flows. Physical Review Fluids. 6(3):1-18. https://doi.org/10.1103/PhysRevFluids.6.0346051186

CFD Study of Needle Motion Influence on the Spray Conditions of Single-Hole Injectors

[EN] The spray characteristics and, consequently, the success of the Diesel combustion process is strongly affected by the manner in which fuel is introduced in the combustion chamber. This work consists in studying the effect of needle motion of typical single-hole sac-type injectors on nozzle exit conditions. Three-dimensional moving mesh simulations have been carried out to calculate the injection process using cylindrical and conical nozzle geometries. The CFD analysis includes a study of the effect of cavitation on kinetic turbulent energy and velocity profiles. Results show that the flow within the nozzle and at the exit varies depending on the nozzle geometry and needle position. The model predicts clouds of cavitation that grow and exit the nozzle at low needle lifts. A kind of hysteresis in the development of the flow has also been observed between needle opening and closing. The existing correlation between turbulence and cavitation at the nozzle hole exit during the needle motion has been quantifiedThis research has been funded by the Spanish Government in the frame of the Project "Caracterizacion experimental de la cavitacion en el flujo interno e influencia sobre modelos de chorro Diesel," Reference TRA2007-68006-C02-01. S.H. and P.F. were partially supported by the Universidad Politecnica de Valencia under the program "Primeros Proyectos de investigacion," in the frame of the project "Simulacion CFD de chorros Diesel en inyeccion directa: la atomizacion primaria," Reference PAID-2759 and by the Generalitat Valenciana under Contract No. GV/2010/039.Margot, XM.; Hoyas Calvo, S.; Fajardo, P.; Patouna, S. (2011). CFD Study of Needle Motion Influence on the Spray Conditions of Single-Hole Injectors. Atomization and Sprays. 21(1):31-40. doi:10.1615/AtomizSpr.v21.i1.30S314021

Explaining wall-bounded turbulence through deep learning

Despite its great scientific and technological importance, wall-bounded turbulence is an unresolved problem that requires new perspectives to be tackled. One of the key strategies has been to study interactions among the coherent structures in the flow. Such interactions are explored in this study for the first time using an explainable deep-learning method. The instantaneous velocity field in a turbulent channel is used to predict the velocity field in time through a convolutional neural network. Based on the predicted flow, we assess the importance of each structure for this prediction using the game-theoretic algorithm of SHapley Additive exPlanations (SHAP). This work provides results in agreement with previous observations in the literature and extends them by quantifying the importance of the Reynolds-stress structures, finding a connection between these structures and the dynamics of the flow. The process, based on deep-learning explainability, has the potential to shed light on numerous fundamental phenomena of wall-bounded turbulence, including the objective definition of new types of flow structures

Turbulent boundary layers and channels at moderate Reynolds numbers

The behaviour of the velocity and pressure fluctuations in the outer layers of wall-bounded turbulent flows is analysed by comparing a new simulation of the zero-pressure-gradient boundary layer with older simulations of channels. The 99 % boundary-layer thickness is used as a reasonable analogue of the channel half-width, but the two flows are found to be too different for the analogy to be complete. In agreement with previous results, it is found that the fluctuations of the transverse velocities and of the pressure are stronger in the boundary layer, and this is traced to the pressure fluctuations induced in the outer intermittent layer by the differences between the potential and rotational flow regions. The same effect is also shown to be responsible for the stronger wake component of the mean velocity profile in external flows, whose increased energy production is the ultimate reason for the stronger fluctuations. Contrary to some previous results by our group, and by others, the streamwise velocity fluctuations are also found to be higher in boundary layers, although the effect is weaker. Within the limitations of the non-parallel nature of the boundary layer, the wall-parallel scales of all the fluctuations are similar in both the flows, suggesting that the scale-selection mechanism resides just below the intermittent region, y/¿=0.3¿0.5. This is also the location of the largest differences in the intensities, although the limited Reynolds number of the boundary-layer simulation (Re¿ ¿ 2000) prevents firm conclusions on the scaling of this location. The statistics of the new boundary layer are available from http://torroja.dmt.upm.es/ftp/blayers/

High-resolution simulations of a turbulent boundary layer impacting two obstacles in tandem

High-fidelity large-eddy simulations of the flow around two rectangular obstacles are carried out at a Reynolds number of 10 000 based on the freestream velocity and the obstacle height. The incoming flow is a developed turbulent boundary layer. Mean-velocity components, turbulence fluctuations, and the terms of the turbulent-kinetic-energy budget are analyzed for three flow regimes: skimming flow, wake interference, and isolated roughness. Three regions are identified where the flow undergoes the most significant changes: the first obstacle's wake, the region in front of the second obstacle, and the region around the second obstacle. In the skimming-flow case, turbulence activity in the cavity between the obstacles is limited and mainly occurs in a small region in front of the second obstacle. In the wake-interference case, there is a strong interaction between the freestream flow that penetrates the cavity and the wake of the first obstacle. This interaction results in more intense turbulent fluctuations between the obstacles. In the isolated-roughness case, the wake of the first obstacle is in good agreement with that of an isolated obstacle. Separation bubbles with strong turbulent fluctuations appear around the second obstacle

High-resolution large-eddy simulations of simplified urban flows

High-fidelity large-eddy simulations of the flow around two rectangular obstacles are carried out at a Reynolds number of 10,000 based on the free-stream velocity and the obstacle height. The incoming flow is a developed turbulent boundary layer. Mean-velocity components, turbulence fluctuations, and the terms of the turbulent-kinetic-energy budget are analyzed for three flow regimes: skimming flow, wake interference, and isolated roughness. Three regions are identified where the flow undergoes the most significant changes: the first obstacle's wake, the region in front of the second obstacle, and that around the second obstacle. In the skimming-flow case, turbulence activity in the cavity between the obstacles is limited and mainly occurs in a small region in front of the second obstacle. In the wake-interference case, there is a strong interaction between the free-stream flow that penetrates the cavity and the wake of the first obstacle. This interaction results in more intense turbulent fluctuations between the obstacles. In the isolated-roughness case, the wake of the first obstacle is in good agreement with that of an isolated obstacle. Separation bubbles with strong turbulent fluctuations appear around the second obstacle