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Rezaeiravesh, S., Vinuesa, R., Liefvendahl, M. & Schlatter, P. (2018). Assessment of uncertainties in hot-wire anemometry and oil-film interferometry measurements for wall-bounded turbulent flows. European journal of mechanics. B, Fluids, 72, 57-73
Open this publication in new window or tab >>Assessment of uncertainties in hot-wire anemometry and oil-film interferometry measurements for wall-bounded turbulent flows
2018 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 72, p. 57-73Article in journal (Refereed) Published
Abstract [en]

In this study, the sources of uncertainty of hot-wire anemometry (HWA) and oil-film interferometry (OFI) measurements are assessed. Both statistical and classical methods are used for the forward and inverse problems, so that the contributions to the overall uncertainty of the measured quantities can be evaluated. The correlations between the parameters are taken into account through the Bayesian inference with error-in-variable (EiV) model. In the forward problem, very small differences were found when using Monte Carlo (MC), Polynomial Chaos Expansion (PCE) and linear perturbation methods. In flow velocity measurements with HWA, the results indicate that the estimated uncertainty is lower when the correlations among parameters are considered, than when they are not taken into account. Moreover, global sensitivity analyses with Sobol indices showed that the HWA measurements are most sensitive to the wire voltage, and in the case of OFI the most sensitive factor is the calculation of fringe velocity. The relative errors in wall-shear stress, friction velocity and viscous length are 0.44%, 0.23% and0.22%, respectively. Note that these values are lower than the ones reported in other wall-bounded turbulence studies. Note that in most studies of wall-bounded turbulence the correlations among parameters are not considered, and the uncertainties from the various parameters are directly added when determining the overall uncertainty of the measured quantity. In the present analysis we account for these correlations, which may lead to a lower overall uncertainty estimate due to error cancellation Furthermore, our results also indicate that the crucial aspect when obtaining accurate inner-scaled velocity measurements is the wind-tunnel flow quality, which is more critical than the accuracy in wall-shear stress measurements.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Hot-wire anemometry, Oil-film interferometry, Uncertainty quantification, Wall-bounded turbulence
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-228692 (URN)10.1016/j.euromechflu.2018.04.012 (DOI)2-s2.0-85047057608 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20180530

Available from: 2018-05-30 Created: 2018-05-30 Last updated: 2018-05-30Bibliographically approved
Rinaldi, E., Schlatter, P. & Bagheri, S. (2018). Edge state modulation by mean viscosity gradients. Journal of Fluid Mechanics, 838, 379-403
Open this publication in new window or tab >>Edge state modulation by mean viscosity gradients
2018 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 838, p. 379-403Article in journal (Refereed) Published
Abstract [en]

Motivated by the relevance of edge state solutions as mediators of transition, we use direct numerical simulations to study the effect of spatially non-uniform viscosity on their energy and stability in minimal channel flows. What we seek is a theoretical support rooted in a fully nonlinear framework that explains the modified threshold for transition to turbulence in flows with temperature-dependent viscosity. Consistently over a range of subcritical Reynolds numbers, we find that decreasing viscosity away from the walls weakens the streamwise streaks and the vortical structures responsible for their regeneration. The entire self-sustained cycle of the edge state is maintained on a lower kinetic energy level with a smaller driving force, compared to a flow with constant viscosity. Increasing viscosity away from the walls has the opposite effect. In both cases, the effect is proportional to the strength of the viscosity gradient. The results presented highlight a local shift in the state space of the position of the edge state relative to the laminar attractor with the consequent modulation of its basin of attraction in the proximity of the edge state and of the surrounding manifold. The implication is that the threshold for transition is reduced for perturbations evolving in the neighbourhood of the edge state in the case that viscosity decreases away from the walls, and vice versa.

Place, publisher, year, edition, pages
Cambridge University Press, 2018
Keywords
nonlinear dynamical systems, nonlinear instability, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-222027 (URN)10.1017/jfm.2017.921 (DOI)2-s2.0-85040834445 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20180131

Available from: 2018-01-31 Created: 2018-01-31 Last updated: 2018-01-31Bibliographically approved
Otero, E., Gong, J., Min, M., Fischer, P., Schlatter, P. & Laure, E. (2018). OpenACC accelerator for the Pn-Pn-2 algorithm in Nek5000. In: Proceedings of the 5th International Conference on Exascale Applications and Software: . Paper presented at The 5th International Conference on Exascale Applications and Software, 17th to 19th April 2018 in Edinburgh, Scotland.
Open this publication in new window or tab >>OpenACC accelerator for the Pn-Pn-2 algorithm in Nek5000
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2018 (English)In: Proceedings of the 5th International Conference on Exascale Applications and Software, 2018Conference paper, Oral presentation with published abstract (Refereed)
National Category
Computer and Information Sciences
Identifiers
urn:nbn:se:kth:diva-232362 (URN)978-0-9926615-3-3 (ISBN)
Conference
The 5th International Conference on Exascale Applications and Software, 17th to 19th April 2018 in Edinburgh, Scotland
Note

QC 20180725

Available from: 2018-07-20 Created: 2018-07-20 Last updated: 2018-07-25Bibliographically approved
Vidal, A., Nagib, H. M., Schlatter, P. & Vinuesa, R. (2018). Secondary flow in spanwise-periodic in-phase sinusoidal channels. Journal of Fluid Mechanics, 851, 288-316
Open this publication in new window or tab >>Secondary flow in spanwise-periodic in-phase sinusoidal channels
2018 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 851, p. 288-316Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulations (DNSs) are performed to analyse the secondary flow of Prandtl's second kind in fully developed spanwise-periodic channels with in-plane sinusoidal walls. The secondary flow is characterized for different combinations of wave parameters defining the wall geometry at Re-h = 2500 and 5000, where h is the half-height of the channel. The total cross-flow rate in the channel Q(yz) is defined along with a theoretical model to predict its behaviour. Interaction between the secondary flows from opposite walls is observed if lambda similar or equal to h similar or equal to A, where A and lambda are the amplitude and wavelength of the sinusoidal function defining the wall geometry. As the outer-scaled wavelength (lambda/h) is reduced, the secondary vortices become smaller and faster, increasing the total cross-flow rate per wall. However, if the inner-scaled wavelength (lambda(+)) is below 130 viscous units, the cross-flow decays for smaller wavelengths. By analysing cases in which the wavelength of the wall is much smaller than the half-height of the channel lambda << h, we show that the cross-flow distribution depends almost entirely on the separation between the scales of the instantaneous vortices, where the upper and lower bounds are determined by lambda/h and lambda(+), respectively. Therefore, the distribution of the secondary flow relative to the size of the wave at a given Re-h can be replicated at higher Re-h by decreasing lambda/h and keeping lambda(+) constant. The mechanisms that contribute to the mean cross-flow are analysed in terms of the Reynolds stresses and using quadrant analysis to evaluate the probability density function of the bursting events. These events are further classified with respect to the sign of their instantaneous spanwise velocities. Sweeping events and ejections are preferentially located in the valleys and peaks of the wall, respectively. The sweeps direct the instantaneous cross-flow from the core of the channel towards the wall, turning in the wall-tangent direction towards the peaks. The ejections drive the instantaneous cross-flow from the near-wall region towards the core. This preferential behaviour is identified as one of the main contributors to the secondary flow.

Place, publisher, year, edition, pages
Cambridge University Press, 2018
Keywords
turbulence simulation, turbulent flows
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-232762 (URN)10.1017/jfm.2018.498 (DOI)000439307100010 ()2-s2.0-85050633237 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationSwedish e‐Science Research Center
Note

QC 20180803

Available from: 2018-08-03 Created: 2018-08-03 Last updated: 2018-08-06Bibliographically approved
Vinuesa, R., Schlatter, P. & Nagib, H. M. (2018). Secondary flow in turbulent ducts with increasing aspect ratio. PHYSICAL REVIEW FLUIDS, 3(5), Article ID 054606.
Open this publication in new window or tab >>Secondary flow in turbulent ducts with increasing aspect ratio
2018 (English)In: PHYSICAL REVIEW FLUIDS, ISSN 2469-990X, Vol. 3, no 5, article id 054606Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulations of turbulent duct flows with aspect ratios 1, 3, 5, 7, 10, and 14.4 at a center-plane friction Reynolds number Re-tau,Re- c similar or equal to 180, and aspect ratios 1 and 3 at Re-tau,Re- c similar or equal to 360, were carried out with the spectral-element code NEK5000. The aim of these simulations is to gain insight into the kinematics and dynamics of Prandtl's secondary flow of the second kind and its impact on the flow physics of wall-bounded turbulence. The secondary flow is characterized in terms of the cross-plane component of the mean kinetic energy, and its variation in the spanwise direction of the flow. Our results show that averaging times of around 3000 convective time units (based on duct half-height h) are required to reach a converged state of the secondary flow, which extends up to a spanwise distance of around similar or equal to 5h measured from the side walls. We also show that if the duct is not wide enough to accommodate the whole extent of the secondary flow, then its structure is modified as reflected through a different spanwise distribution of energy. Another confirmation of the extent of the secondary flow is the decay rate of kinetic energy of any remnant secondary motions for z(c)/h > 5 (where z(c) is the spanwise distance from the corner) in aspect ratios 7, 10, and 14.4, which exhibits a decreasing level of energy with increasing averaging time t(a), and in its rapid rate of decay given by similar to t(a)(-1). This is the same rate of decay observed in a spanwise-periodic channel simulation, which suggests that at the core, the kinetic energy of the secondary flow integrated over the cross-sectional area, < K >(yz), behaves as a random variable with zero mean, with rate of decay consistent with central limit theorem. Long-time averages of statistics in a region of rectangular ducts extending about the width of a well-designed channel simulation (i.e., extending about similar or equal to 3h on each side of the center plane) indicate that ducts or experimental facilities with aspect ratios larger than 10 may, if properly designed, exhibit good agreement with results obtained from spanwise-periodic channel computations.

Place, publisher, year, edition, pages
American Physical Society, 2018
Keywords
Direct Numerical-Simulation, Straight Square Duct, Boundary-Layers, Rectangular Ducts, Reynolds-Numbers, Shear, Wall
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-230444 (URN)10.1103/PhysRevFluids.3.054606 (DOI)000433036100004 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilSwedish e‐Science Research Center
Note

QC 20180614

Available from: 2018-06-14 Created: 2018-06-14 Last updated: 2018-06-15Bibliographically approved
Appelquist, E., Schlatter, P., Alfredsson, P. H. & Lingwood, R. (2018). Turbulence in the rotating-disk boundary layer investigated through direct numerical simulations. European journal of mechanics. B, Fluids, 70, 6-18
Open this publication in new window or tab >>Turbulence in the rotating-disk boundary layer investigated through direct numerical simulations
2018 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 70, p. 6-18Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulations (DNS) are reported for the turbulent rotating-disk boundary layer for the first time. Two turbulent simulations are presented with overlapping small and large Reynolds numbers, where the largest corresponds to a momentum-loss Reynolds number of almost 2000. Simulation data are compared with experimental data from the same flow case reported by Imayama et al. (2014), and also a comparison is made with a numerical simulation of a two-dimensional turbulent boundary layer (2DTBL) over a flat plate reported by Schlatter and Örlü (2010). The agreement of the turbulent statistics between experiments and simulations is in general very good, as well as the findings of a missing wake region and a lower shape factor compared to the 2DTBL. The simulations also show rms-levels in the inner region similar to the 2DTBL. The simulations validate Imayama et al.’s results showing that the rotating-disk turbulent boundary layer in the near-wall region contains shorter streamwise (azimuthal) wavelengths than the 2DTBL, probably due to the outward inclination of the low-speed streaks. Moreover, all velocity components are available from the simulations, and hence the local flow angle, Reynolds stresses and all terms in the turbulent kinetic energy equation are also discussed. However there are in general no large differences compared to the 2DTBL, hence the three-dimensional effects seem to have only a small influence on the turbulence.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Near-wall turbulence, Rotation, Turbulence statistics
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-227538 (URN)10.1016/j.euromechflu.2018.01.008 (DOI)000432105000002 ()2-s2.0-85042080446 (Scopus ID)
Funder
Swedish e‐Science Research CenterSwedish Research Council
Note

QC 20180517

Available from: 2018-05-17 Created: 2018-05-17 Last updated: 2018-05-31Bibliographically approved
Vinuesa, R., Negi, P. S., Atzori, M., Hanifi, A., Henningson, D. S. & Schlatter, P. (2018). Turbulent boundary layers around wing sections up to Re-c=1, 000, 000. International Journal of Heat and Fluid Flow, 72, 86-99
Open this publication in new window or tab >>Turbulent boundary layers around wing sections up to Re-c=1, 000, 000
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2018 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 72, p. 86-99Article in journal (Refereed) Published
Abstract [en]

Reynolds-number effects in the adverse-pressure-gradient (APG) turbulent boundary layer (TBL) developing on the suction side of a NACA4412 wing section are assessed in the present work. To this end, we analyze four cases at Reynolds numbers based on freestream velocity and chord length ranging from Re-c = 100, 000 to 1,000,000, all of them with 5 degrees angle of attack. The results of four well-resolved large-eddy simulations (LESs) are used to characterize the effect of Reynolds number on APG TBLs subjected to approximately the same pressure-gradient distribution (defined by the Clauser pressure-gradient parameter beta). Comparisons of the wing profiles with zero pressure-gradient (ZPG) data at matched friction Reynolds numbers reveal that, for approximately the same beta distribution, the lower-Reynolds-number boundary layers are more sensitive to pressure-gradient effects. This is reflected in the values of the inner-scaled edge velocity U-e(+), the shape factor H, the components of the Reynolds-stress tensor in the outer region and the outer-region production of turbulent kinetic energy. This conclusion is supported by the larger wall-normal velocities and outer-scaled fluctuations observed in the lower-Re-c cases. Thus, our results suggest that two complementing mechanisms contribute to the development of the outer region in TBLs and the formation of large-scale energetic structures: one mechanism associated with the increase in Reynolds number, and another one connected to the APG. Future extensions of the present work will be aimed at studying the differences in the outer-region energizing mechanisms due to APGs and increasing Reynolds number.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE INC, 2018
Keywords
Large-eddy simulation, Turbulent boundary layer, Pressure gradient, Wing section
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-234198 (URN)10.1016/j.ijheatfluidflow.2018.04.017 (DOI)000441488400008 ()2-s2.0-85048126226 (Scopus ID)
Note

QC 20180911

Available from: 2018-09-11 Created: 2018-09-11 Last updated: 2018-09-11Bibliographically approved
Vidal, A., Vinuesa, R., Schlatter, P. & Nagib, H. M. (2018). Turbulent rectangular ducts with minimum secondary flow. International Journal of Heat and Fluid Flow, 72, 317-328
Open this publication in new window or tab >>Turbulent rectangular ducts with minimum secondary flow
2018 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 72, p. 317-328Article in journal (Refereed) Published
Abstract [en]

In the present study we perform direct numerical simulations (DNSs) of fully-developed turbulent rectangular ducts with semi-cylindrical side-walls at Re-t,Re- c similar or equal to 180 with width-to-height ratios of 3 and 5. The friction Reynolds number Re-tau,Re- (c) is based on the centerplane friction velocity and the half-height of the duct. The results are compared with the corresponding duct cases with straight side-walls (Vinuesa et al., 2014), and also with spanwise-periodic channel and pipe flows. We focus on the influence of the semi-cylindrical side-walls on the mean cross-stream secondary flow and on further characterizing the mechanisms that produce it. The role of the secondary and primary Reynolds-shear stresses in the production of the secondary flow is analyzed by means of quadrant analysis and conditional averaging. Unexpectedly, the ducts with semi-cylindrical side-walls exhibit higher cross-flow rates and their secondary vortices relocate near the transition point between the straight and curved walls. This behavior is associated to the statistically preferential arrangement of sweeping events entering through the curved wall and ejections arising from the adjacent straight wall. Therefore, the configuration with minimum secondary flow corresponds to the duct with straight side-walls and sharp corners. Consequences on experimental facilities and comparisons between experiments and various numerical and theoretical models are discussed revealing the uniqueness of pipe flow.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE INC, 2018
Keywords
Wall-bounded turbulence, Turbulent duct flow, Side-wall geometry, Secondary flow, Direct numerical simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-234199 (URN)10.1016/j.ijheatfluidflow.2018.06.014 (DOI)000441488400025 ()2-s2.0-85049313018 (Scopus ID)
Note

QC 20180907

Available from: 2018-09-07 Created: 2018-09-07 Last updated: 2018-09-07Bibliographically approved
Negi, P. S., Vinuesa, R., Hanifi, A., Schlatter, P. & Henningson, D. S. (2018). Unsteady aerodynamic effects in small-amplitude pitch oscillations of an airfoil. International Journal of Heat and Fluid Flow, 71, 378-391
Open this publication in new window or tab >>Unsteady aerodynamic effects in small-amplitude pitch oscillations of an airfoil
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2018 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 71, p. 378-391Article in journal (Refereed) Published
Abstract [en]

High-fidelity wall-resolved large-eddy simulations (LES) are utilized to investigate the flow-physics of small-amplitude pitch oscillations of an airfoil at Rec=100,000. The investigation of the unsteady phenomenon is done in the context of natural laminar flow airfoils, which can display sensitive dependence of the aerodynamic forces on the angle of attack in certain “off-design” conditions. The dynamic range of the pitch oscillations is chosen to be in this sensitive region. Large variations of the transition point on the suction-side of the airfoil are observed throughout the pitch cycle resulting in a dynamically rich flow response. Changes in the stability characteristics of a leading-edge laminar separation bubble has a dominating influence on the boundary layer dynamics and causes an abrupt change in the transition location over the airfoil. The LES procedure is based on a relaxation-term which models the dissipation of the smallest unresolved scales. The validation of the procedure is provided for channel flows and for a stationary wing at Rec=400,000.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Dynamic-response, Laminar separation bubble, Local stability, Transition, Unsteady aerodynamics, Wall-resolved les
National Category
Fluid Mechanics and Acoustics Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-228734 (URN)10.1016/j.ijheatfluidflow.2018.04.009 (DOI)2-s2.0-85046802460 (Scopus ID)
Funder
VINNOVA, 2014-00933EU, European Research Council, 694452-TRANSEP-ERC-2015-AdGSwedish e‐Science Research Center
Note

QC 20180529

Available from: 2018-05-29 Created: 2018-05-29 Last updated: 2018-07-02Bibliographically approved
Offermans, N., Peplinski, A., Marin, O. & Schlatter, P. (2017). Adjoint error estimators and adaptive mesh refinement in Nek5000.
Open this publication in new window or tab >>Adjoint error estimators and adaptive mesh refinement in Nek5000
2017 (English)Report (Other academic)
Abstract [en]

The development of adaptive mesh refinement capabilities in the field of computational fluid dynamics is an essential tool for enabling the simulation of larger and more complex physical problems. In this report, we describe recent developments that have been made to enable adaptive mesh refinement in Nek5000 and we validate the method on simple, two-dimensional, steady test cases.We start by describing the modifications brought to Nek5000 that enable the presence of hanging nodes in the mesh. Thanks to this new feature, we can use the h-refinement technique for mesh adaptation, where selected elements are split via quadtree (2D) or octree (3D) structures. Then, two methods are considered to estimate and control the error. The first method is local and based on the spectral properties of the solution on each element. The second method is goal-oriented and takes into account both the local properties of the solution and the global dependence of the error in the solution via the resolution of an adjoint problem. Finally, the use of automatic mesh refinement is demonstrated in Nek5000 on two test cases: the lid-driven cavity at Re = 7, 500 and the flow past a cylinder at Re = 40. Both error estimation methods are compared andare shown to efficiently reduce the number of degrees of freedom required to reach a given tolerance on the solution compared to conforming refinement. Moreover, the gains in terms of mesh generation, accuracy and computational cost are discussed by analysing the convergence of some functional of interest and the evolution of the mesh as refinement proceeds.

Keywords
Error estimators; mesh refinement; adaptivity; spectral element method
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-217498 (URN)
Funder
Knut and Alice Wallenberg Foundation
Note

QC 20171123

Available from: 2017-11-13 Created: 2017-11-13 Last updated: 2017-11-23Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-9627-5903

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