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Rezaeiravesh, S., Gscheidle, C., Peplinski, A., Garcke, J. & Schlatter, P. (2025). In-situ estimation of time-averaging uncertainties in turbulent flow simulations. Computer Methods in Applied Mechanics and Engineering, 433, Article ID 117511.
Open this publication in new window or tab >>In-situ estimation of time-averaging uncertainties in turbulent flow simulations
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2025 (English)In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 433, article id 117511Article in journal (Refereed) Published
Abstract [en]

The statistics obtained from turbulent flow simulations are generally uncertain due to finite time averaging. Most techniques available in the literature to accurately estimate these uncertainties typically only work in an offline mode, that is, they require access to all available samples of a time series at once. In addition to the impossibility of online monitoring of uncertainties during the course of simulations, such an offline approach can lead to input/output (I/O) deficiencies and large storage/memory requirements, which can be problematic for large-scale simulations of turbulent flows. Here, we designed, implemented and tested a framework for estimating time-averaging uncertainties in turbulence statistics in an in-situ (online/streaming/updating) manner. The proposed algorithm relies on a novel low-memory update formula for computing the sample-estimated autocorrelation functions (ACFs). Based on this, smooth modeled ACFs of turbulence quantities can be generated to accurately estimate the time-averaging uncertainties in the corresponding sample mean estimators. The resulting uncertainty estimates are highly robust, accurate, and quantitatively the same as those obtained by standard offline estimators. Moreover, the computational overhead added by the in-situ algorithm is found to be negligible allowing for online estimation of uncertainties for multiple points and quantities. The framework is general and can be used with any flow solver and also integrated into the simulations over conformal and complex meshes created by adopting adaptive mesh refinement techniques. The results of the study are encouraging for the further development of the in-situ framework for other uncertainty quantification and data-driven analyses relevant not only to large-scale turbulent flow simulations, but also to the simulation of other dynamical systems leading to time-varying quantities with autocorrelated samples.

Place, publisher, year, edition, pages
Elsevier BV, 2025
Keywords
Autocorrelation, In-situ estimation, Time-averaging uncertainty, Turbulent flows, Uncertainty quantification
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-356691 (URN)10.1016/j.cma.2024.117511 (DOI)001356362600001 ()2-s2.0-85208533004 (Scopus ID)
Note

QC 20241122

Available from: 2024-11-20 Created: 2024-11-20 Last updated: 2025-02-09Bibliographically approved
Massaro, D., Peplinski, A., Stanly, R., Mirzareza, S., Lupi, V., Xiang, Y. & Schlatter, P. (2024). A comprehensive framework to enhance numerical simulations in the spectral-element code Nek5000. Computer Physics Communications, 302, Article ID 109249.
Open this publication in new window or tab >>A comprehensive framework to enhance numerical simulations in the spectral-element code Nek5000
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2024 (English)In: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944, Vol. 302, article id 109249Article in journal (Refereed) Published
Abstract [en]

A framework is presented for the spectral-element code Nek5000, which has been, and still is, widely used in the computational fluid dynamics (CFD) community to perform high-fidelity numerical simulations of transitional and high Reynolds number flows. Despite the widespread usage, there is a deficiency in having a comprehensive set of tools specifically designed for conducting simulations using Nek5000. To address this issue, we have created a unique framework that allows, inter alia, to perform stability analysis and compute statistics of a turbulent flow. The framework encapsulates modules that provide tools, run-time parameters and memory structures, defining interfaces and performing different tasks. First, the framework architecture is described, showing its non-intrusive approach. Then, the modules are presented, explaining the main tools that have been implemented and describing some of the test cases. The code is open-source and available online, with proper documentation, to-run instructions and related examples.

Place, publisher, year, edition, pages
Elsevier BV, 2024
Keywords
Computational fluid dynamics, Numerical toolbox, Stability analysis, Statistical analysis
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-347059 (URN)10.1016/j.cpc.2024.109249 (DOI)001244454300001 ()2-s2.0-85193603654 (Scopus ID)
Note

QC 20240702

Available from: 2024-05-30 Created: 2024-05-30 Last updated: 2025-02-09Bibliographically approved
Mallor, F., Liu, J., Peplinski, A., Vinuesa, R., Örlü, R., Weinkauf, T. & Schlatter, P. (2024). In-Situ Analysis of Backflow Events and Their Relation to Separation in Wings Through Well-Resolved LES. In: ERCOFTAC Series: (pp. 17-22). Springer Science and Business Media B.V., 31
Open this publication in new window or tab >>In-Situ Analysis of Backflow Events and Their Relation to Separation in Wings Through Well-Resolved LES
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2024 (English)In: ERCOFTAC Series, Springer Science and Business Media B.V. , 2024, Vol. 31, p. 17-22Chapter in book (Other academic)
Abstract [en]

Wall-bounded turbulent flows as those occurring in transportation (e.g. aviation) or industrial applications (e.g turbomachinery), are usually subjected to pressure gradients (PGs). The presence of such PGs affects greatly the development and physics of the turbulent boundary layer (TBL), making it an open research area. An important phenomena associated with the presence of strong adverse PGs (APGs) as appearing in wings, is the separation of the boundary layer, which can lead to stall.

Place, publisher, year, edition, pages
Springer Science and Business Media B.V., 2024
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-340780 (URN)10.1007/978-3-031-47028-8_3 (DOI)2-s2.0-85178156992 (Scopus ID)
Note

QC 20231214

Available from: 2023-12-14 Created: 2023-12-14 Last updated: 2025-02-09Bibliographically approved
Toosi, S., Peplinski, A., Schlatter, P. & Vinuesa, R. (2024). The Effect of Wing-Tip Vortices on the Flow Around a NACA0012 Wing. In: ERCOFTAC Series: (pp. 183-188). Springer Science and Business Media B.V., 31
Open this publication in new window or tab >>The Effect of Wing-Tip Vortices on the Flow Around a NACA0012 Wing
2024 (English)In: ERCOFTAC Series, Springer Science and Business Media B.V. , 2024, Vol. 31, p. 183-188Chapter in book (Other academic)
Abstract [en]

The wing-tip vortices are formed near the tip of finite-span, lift-generating surfaces as a result of the pressure difference between the pressure and suction sides of the wing.

Place, publisher, year, edition, pages
Springer Science and Business Media B.V., 2024
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-340785 (URN)10.1007/978-3-031-47028-8_28 (DOI)2-s2.0-85178092210 (Scopus ID)
Note

QC 20231214

Available from: 2023-12-14 Created: 2023-12-14 Last updated: 2025-02-09Bibliographically approved
Toosi, S., Peplinski, A., Schlatter, P. & Vinuesa, R. (2024). The impact of finite span and wing-tip vortices on a turbulent NACA0012 wing. Journal of Fluid Mechanics, 997, Article ID A68.
Open this publication in new window or tab >>The impact of finite span and wing-tip vortices on a turbulent NACA0012 wing
2024 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 997, article id A68Article in journal (Refereed) Published
Abstract [en]

High-fidelity simulations are conducted to investigate the turbulent boundary layers around a finite-span NACA0012 wing with a rounded wing-tip geometry at a chord-based Reynolds number of Re-c = 200 000 and at various angles of attack up to 10 degrees. The study aims to discern the differences between the boundary layers on the finite-span wing and those on infinite-span wings at equivalent angles of attack. The finite-span boundary layers exhibit: (i) an altered streamwise and a non-zero spanwise pressure gradient as a result of the variable downwash induced by the wing-tip vortices (an inviscid effect typical of finite-span wings); (ii) differences in the flow history at different wall-normal distances, caused by the variable flow angle in the wall-normal direction (due to constant pressure gradients and variable momentum normal to the wall); (iii) laminar flow entrainment into the turbulent boundary layers near the wing tip (due to a laminar-turbulent interface); and (iv) variations in boundary layer thickness across the span, attributed to the variable wall-normal velocity in that direction (a primarily inviscid effect). These physical effects are then used to explain the differences in the Reynolds stress profiles and other boundary layer quantities, including the reduced near-wall peak of the streamwise Reynolds stress and the elevated Reynolds stress levels near the boundary layer edge, both observed in the finite-span wings. Other aspects of the flow, such as the downstream development of wing-tip vortices and their interactions with the surrounding flow, are reserved for future investigations.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2024
Keywords
turbulence simulation, turbulent boundary layers
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-355786 (URN)10.1017/jfm.2024.667 (DOI)001337398500001 ()2-s2.0-85207805211 (Scopus ID)
Note

QC 20241104

Available from: 2024-11-04 Created: 2024-11-04 Last updated: 2025-05-27Bibliographically approved
Massaro, D., Peplinski, A. & Schlatter, P. (2023). Coherent structures in the turbulent stepped cylinder flow at ReD=5000. International Journal of Heat and Fluid Flow, 102, 109144, Article ID 109144.
Open this publication in new window or tab >>Coherent structures in the turbulent stepped cylinder flow at ReD=5000
2023 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 102, p. 109144-, article id 109144Article in journal (Refereed) Published
Abstract [en]

The coherent structures arising in the turbulent flow around a three-dimensional stepped (or step) cylinder are studied through direct numerical simulation. This geometry is widespread in many applications and the junction substantially modifies the wake behaviour, generating three main cells. The mechanisms of vortex connections on the junction are difficult to be captured and interpreted. We thus use a high-order spectral -element methodology (SEM), and the adaptive mesh refinement technique (AMR) to adequately resolve each region of the domain, capturing the smallest turbulent scales. In this way, we can analyse the vortical interactions on the junction via the A2-criterion and understand the evolution of the train of hairpins, which appears only when the cylinder shear layer gets unstable. Together with the hairpins, four horseshoe and edge vortices coexist on the flat junction surface. A complete picture of the vortices' evolution in time is provided. To extract the large-scale, and most energetic, structures in the wake we perform a three-dimensional proper orthogonal decomposition (POD) of the flow. The first six POD modes correspond to three travelling modes which identify the large (L), the small (S) and the modulation (N) cells. The ReD trend shows that these cells persist at higher Reynolds numbers with a larger separation between the vortex shedding frequencies fN and fL. At the same time, the downwash POD mode gets less strong with a more intense and localised modulation region which affects a more extended portion of the large cylinder wake.

Place, publisher, year, edition, pages
Elsevier BV, 2023
Keywords
Stepped cylinder, Modal analysis, Vortex dynamics, Coherent structures
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-329379 (URN)10.1016/j.ijheatfluidflow.2023.109144 (DOI)000998905300001 ()2-s2.0-85154617597 (Scopus ID)
Note

QC 20230620

Available from: 2023-06-20 Created: 2023-06-20 Last updated: 2025-02-09Bibliographically approved
Offermans, N., Massaro, D., Peplinski, A. & Schlatter, P. (2023). Error-driven adaptive mesh refinement for unsteady turbulent flows in spectral-element simulations. Computers & Fluids, 251, 105736, Article ID 105736.
Open this publication in new window or tab >>Error-driven adaptive mesh refinement for unsteady turbulent flows in spectral-element simulations
2023 (English)In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 251, p. 105736-, article id 105736Article in journal (Refereed) Published
Abstract [en]

The simulation of turbulent flows requires high spatial resolution in potentially a priori unknown, solution -dependent locations. To achieve adaptive refinement of the mesh, we rely on error indicators. We assess the differences between an error measure relying on the local convergence properties of the numerical solution and a goal-oriented error measure based on the computation of an adjoint problem. The latter method aims at optimizing the mesh for the calculation of a predefined integral quantity, or functional of interest. This work follows on from a previous study conducted on steady flows in Offermans et al. (2020) and we extend the use of the so-called adjoint error estimator to three-dimensional, turbulent flows. They both represent a way to achieve error control and automatic mesh refinement (AMR) for the numerical approximation of the Navier-Stokes equations, with a spectral element method discretization and non-conforming h-refinement.The current study consists of running the same physical flow case on gradually finer meshes, starting from a coarse initial grid, and to compare the results and mesh refinement patterns when using both error measures. As a flow case, we consider the turbulent flow in a constricted, periodic channel, also known as the periodic hill flow, at four different Reynolds numbers: Re = 700, Re = 1400, Re = 2800 and Re = 5600. Our results show that both error measures allow for effective control of the error, but they adjust the mesh differently. Well-resolved simulations are achieved by automatically focusing refinement on the most critical regions of the domain, while significant saving in the overall number of elements is attained, compared to statically generated meshes. At all Reynolds numbers, we show that relevant physical quantities, such as mean velocity profiles and reattachment/separation points, converge well to reference literature data. At the highest Reynolds number achieved (Re = 5600), relevant quantities, i.e. reattachment and separation locations, are estimated with the same level of accuracy as the reference data while only using one-third of the degrees of freedom of the reference. Moreover, we observe distinct mesh refinement patterns for both error measures. With the spectral error indicator, the mesh resolution is more uniform and turbulent structures are more resolved within the whole domain. On the other hand, the adjoint error estimator tends to focus the refinement within a localized zone in the domain, dependent on the functional of interest, leaving large parts of the domain marginally resolved.

Place, publisher, year, edition, pages
Elsevier BV, 2023
Keywords
Adaptive mesh refinement, Spectral error indicator, Adjoint error estimator, Error control, Spectral element method, Direct numerical simulations
National Category
Fluid Mechanics Computational Mathematics
Identifiers
urn:nbn:se:kth:diva-323021 (URN)10.1016/j.compfluid.2022.105736 (DOI)000897039400009 ()2-s2.0-85143485679 (Scopus ID)
Note

QC 20230112

Available from: 2023-01-12 Created: 2023-01-12 Last updated: 2025-02-09Bibliographically approved
Massaro, D., Lupi, V., Peplinski, A. & Schlatter, P. (2023). Global stability of 180-bend pipe flow with mesh adaptivity. Physical Review Fluids, 8(11), Article ID 113903.
Open this publication in new window or tab >>Global stability of 180-bend pipe flow with mesh adaptivity
2023 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 8, no 11, article id 113903Article in journal (Refereed) Published
Abstract [en]

The global stability of the flow in a spatially developing 180∘-bend pipe with curvature δ=R/Rc=1/3 is investigated by performing direct numerical simulations to understand the underlying transitional mechanism. A unique application of the adaptive mesh refinement technique is used during the stability analysis for minimizing the interpolation and quadrature errors. Independent meshes are created for the direct and adjoint solutions, as well as for the base flow extracted via selective frequency damping. The spectrum of the linearized Navier-Stokes operator reveals a pair of complex conjugate eigenvalues, with frequency f≈0.233. Therefore, the transition is attributed to a Hopf bifurcation that takes place at Reb,cr=2528. A structural sensitivity analysis is performed by extracting the wavemaker. We identify the primary source of instability located on the outer wall, θ≈15 downstream of the bend inlet. This region corresponds to the separation bubble on the outer wall. We thus conclude that the instability is caused by the strong shear resulting from the backflow, similar to the 90-bend pipe flow. We believe that understanding the stability mechanism and characterizing the base flow in bent pipes is crucial for studying various biological flows, like blood vessels. Hence, this paper aims to close the knowledge gap between a 90-bend and toroidal pipes by investigating the transition nature in a 18-bend pipe flow.

Place, publisher, year, edition, pages
American Physical Society (APS), 2023
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-340973 (URN)10.1103/PhysRevFluids.8.113903 (DOI)001110146700002 ()2-s2.0-85178080751 (Scopus ID)
Note

Not duplicate with DiVA 1757985

QC 20231218

Available from: 2023-12-18 Created: 2023-12-18 Last updated: 2025-02-09Bibliographically approved
Massaro, D., Peplinski, A. & Schlatter, P. (2023). Interface Discontinuities in Spectral-Element Simulations with Adaptive Mesh Refinement. In: Spectral and High Order Methods for Partial Differential Equations ICOSAHOM 2020+1 - Selected Papers from the ICOSAHOM Conference 2021: . Paper presented at 13th International Conference on Spectral and High Order Methods, ICOSAHOM 2021, Jul 12 2021 - Jul 16, 2021, Vienna, Austria (pp. 375-386). Springer Nature
Open this publication in new window or tab >>Interface Discontinuities in Spectral-Element Simulations with Adaptive Mesh Refinement
2023 (English)In: Spectral and High Order Methods for Partial Differential Equations ICOSAHOM 2020+1 - Selected Papers from the ICOSAHOM Conference 2021, Springer Nature , 2023, p. 375-386Conference paper, Published paper (Refereed)
Abstract [en]

We investigate the discontinuities arising at non-conforming (or non-conformal) interfaces in spectral element method (SEM) simulations. The derivate terms are by definition discontinuous and interface instabilities are usually not visible with a conformal mesh and sufficient resolution. Using the adaptive mesh refinement (AMR) technique the initial coarse mesh is progressively refined according to an error indicator or estimator. In our case, the spectral error indicator (SEI) is adopted. This leads to non-conformal interfaces, where hanging nodes are introduced through h-refinement implemented in the SEM code Nek5000. We consider the flow in a three-dimensional periodic straight pipe and use the turbulent kinetic energy budget as an indicator for assessing discontinuities (wiggles). They involve first and second-order derivatives and represent a fixed point in the statistical analysis of this canonical flow. Looking at the results, we observe that our AMR implementation does not affect the interface discontinuities. The jumps in derivatives are uniquely related to an inadequately resolved mesh. Relying on an error-driven approach, the SEI produces a mesh that allows computing the TKE budgets in excellent agreement with the literature and ensures saving in grid points by a factor of 2.

Place, publisher, year, edition, pages
Springer Nature, 2023
National Category
Fluid Mechanics Computational Mathematics
Identifiers
urn:nbn:se:kth:diva-336747 (URN)10.1007/978-3-031-20432-6_24 (DOI)2-s2.0-85169011552 (Scopus ID)
Conference
13th International Conference on Spectral and High Order Methods, ICOSAHOM 2021, Jul 12 2021 - Jul 16, 2021, Vienna, Austria
Note

Part of ISBN 9783031204319

QC 20230919

Available from: 2023-09-19 Created: 2023-09-19 Last updated: 2025-02-09Bibliographically approved
Massaro, D., Peplinski, A. & Schlatter, P. (2023). The flow around a stepped cylinder with turbulent wake and stable shear layer. Journal of Fluid Mechanics, 977, Article ID A3.
Open this publication in new window or tab >>The flow around a stepped cylinder with turbulent wake and stable shear layer
2023 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 977, article id A3Article in journal (Refereed) Published
Abstract [en]

The turbulent external flow around a three-dimensional stepped cylinder is studied by means of direct numerical simulations with the adaptive mesh refinement technique. We give a broad perspective of the flow regimes from laminar to turbulent wake at, which is the highest ever considered for this flow case. In particular, we focus on the intermediate Reynolds number that reveals a turbulent wake coupled with a stable cylinder shear layer (subcritical regime). This flow shows a junction dynamics similar to the laminar, where no hairpin vortex appears around the edges, and just two horseshoe vortices are visible. A new stable vortex in the form of a ring, which coils around the rear area, is also identified. In the turbulent wake, the presence of three wake cells is pointed out: the large and small cylinder cells together with the modulation region. However, the modulation dynamics varies between the subcritical and turbulent regimes. A time-averaged, three-dimensional set of statistics is computed, and spatially coherent structures are extracted via proper orthogonal decomposition (POD). The POD identifies the (long-debated) connection between the N-cell and the downwash behind the junction. Furthermore, as the Reynolds number increases, the downwash phenomenon becomes less prominent. Eventually, a reduced-order reconstruction with the most energetically relevant modes is defined to explain the wake vortex interactions. This also serves as a valuable starting point for simulating the stepped cylinder wake behaviour within complex frameworks, e.g. fluid-structure interaction.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2023
Keywords
turbulence simulation, vortex dynamics, wakes
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-341698 (URN)10.1017/jfm.2023.934 (DOI)001118961700001 ()2-s2.0-85179675334 (Scopus ID)
Note

QC 20231229

Available from: 2023-12-29 Created: 2023-12-29 Last updated: 2025-02-09Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-7448-3290

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