Towards PR-DNS of scour around a wall-mounted cylinder in turbulent open channel flow

Artjom Hermann; Leo B\"urk; Markus Uhlmann; Markus Weyrauch

arxiv: 2604.15955 · v1 · submitted 2026-04-17 · ⚛️ physics.flu-dyn

Towards PR-DNS of scour around a wall-mounted cylinder in turbulent open channel flow

Leo B\"urk , Artjom Hermann , Markus Weyrauch , Markus Uhlmann This is my paper

Pith reviewed 2026-05-10 08:02 UTC · model grok-4.3

classification ⚛️ physics.flu-dyn

keywords PR-DNSscouropen channel flowwall-mounted cylinderparticle transportvortical structureswall shear stresssediment entrainment

0 comments

The pith

A wall-mounted cylinder in turbulent open channel flow generates intense vortices that redistribute particles and enhance their vertical transport against gravity.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper performs particle-resolved direct numerical simulations of turbulent open channel flow over a smooth wall containing a vertical cylinder and a dilute set of mobile heavy spherical particles. It establishes that the cylinder produces intense vortical structures, raises turbulence intensity in the wake, and alters local wall shear stress, which in turn creates zones of particle accumulation or depletion while boosting upward particle movement against gravity. A follow-up run with added wall roughness shows even more particles lifted far from the wall. A sympathetic reader cares because these mechanisms directly govern how obstacles trigger scour and sediment redistribution in channels and rivers.

Core claim

Particle-resolved direct numerical simulation of the flow shows that the cylinder leads to the generation of intense vortical structures, enhanced turbulence intensity in the wake region, and strong modifications of the local wall shear stress. These perturbations produce preferential accumulation or depletion of particles in different parts of the wake and significantly enhance the wall-normal transport of particles against gravity. The configuration with wall roughness and the cylinder features the largest fraction of entrained particles even far from the wall.

What carries the argument

Particle-resolved direct numerical simulation (PR-DNS) that resolves the interactions of the turbulent flow, the wall-mounted cylinder, and the dilute heavy spherical particles in open channel flow.

If this is right

The cylinder generates intense vortical structures that alter the surrounding flow.
Turbulence intensity rises markedly in the wake region downstream of the cylinder.
Local wall shear stress is strongly modified in the vicinity of the cylinder.
Particles preferentially accumulate or deplete in specific wake regions.
Wall-normal particle transport against gravity increases substantially.
Adding wall roughness to the cylinder case produces the highest fraction of particles entrained far from the wall.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

These cylinder-driven effects imply that scour around piers or obstacles may intensify on naturally rough beds because more particles are lifted and carried away.
Sediment transport models for channels with structures would need to incorporate wake turbulence to predict erosion rates accurately.
Varying the cylinder height or particle density in follow-up runs could reveal how the entrainment fraction scales with geometry.
The enhanced vertical transport points to greater resuspension risk in engineered waterways containing vertical obstacles.

Load-bearing premise

The chosen flow parameters make particles mostly translate horizontally while remaining in wall contact, and the simulation resolution together with the particle contact model capture the relevant physics without significant numerical artifacts.

What would settle it

An experiment tracking particle positions in the same setup that finds no wake accumulation zones and no measurable increase in upward particle transport would contradict the simulation results.

Figures

Figures reproduced from arXiv: 2604.15955 by Artjom Hermann, Leo B\"urk, Markus Uhlmann, Markus Weyrauch.

**Figure 1.** Figure 1: (a) Instantaneous and (b) time-averaged vortex structures in the cylinder wake, quantified by the second invariant of the [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗

**Figure 2.** Figure 2: Time- and plane-averaged solid volume fraction [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** Figure 3: (a) Magnitude of the time-averaged wall-shear stress in the vicinity of the cylinder normalized by the time- and plane-averaged [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗

**Figure 4.** Figure 4: (a) Time-averaged solid volume fraction computed in bins in [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗

**Figure 5.** Figure 5: Time-averaged streamlines of ⟨v⟩t and ⟨w⟩t at (a) x = 7.6Dcyl and (b) x = 54.5Dcyl, respectively, for the smooth-wall case. of the cylinder. These entrainments are probably caused by the increased vortical activity in this region, which was shown in figure 1. These entrainment events lead to a higher number of particles away from the wall for the smooth wall case with the cylinder compared to the case wit… view at source ↗

**Figure 6.** Figure 6: Locations of particle entrainment for particles exceeding the threshold height [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗

read the original abstract

Particle-resolved direct numerical simulation (PR-DNS) is performed for turbulent open channel flow over a smooth horizontal wall with a vertical cylinder and a dilute set of mobile, heavy, spherical particles. At the chosen parameter point (which matches a previous study without a cylinder) the particles are mostly translating in the horizontal plane while remaining in contact with the wall. It is shown that the presence of the cylinder leads to the generation of intense vortical structures, enhanced turbulence intensity in the wake region, and to strong modifications of the local wall shear stress. These cylinder-induced perturbations have direct consequences for the average particle concentration: preferential accumulation/depletion in different parts of the wake region occurs, while the wall-normal transport of particles (against gravity) is significantly enhanced. A second simulation which adds roughness elements on the wall reveals an additional effect upon the wall-normal distribution of particles. It turns out that the configuration with wall-roughness and a wall-mounted cylinder features the largest fraction of entrained particles, even far from the wall.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

This extends a prior no-cylinder PR-DNS by adding a wall-mounted cylinder and roughness, producing new observations of wake vortices driving particle accumulation and extra entrainment.

read the letter

This paper runs PR-DNS of open-channel flow with heavy mobile particles, first without a cylinder and then with one, plus a roughness case. The cylinder creates strong vortical structures and raises turbulence in the wake, which alters wall shear and leads to local particle buildup or depletion while increasing the number of particles lifted away from the wall. The roughest configuration with the cylinder shows the highest entrainment fraction even well above the bed. Because the parameters match the earlier baseline run, the differences trace to the added obstacle and roughness rather than changes in setup. The direct resolution of particles and flow is the real asset here; it avoids particle modeling assumptions and lets the interactions emerge from the equations. The approach is straightforward and the reported mechanisms follow from the simulation outputs. The main gap is that the abstract gives no grid-convergence data or tests of the particle contact model against known benchmarks for heavy spheres. Since the particles mostly slide in wall contact, any shortfall in how lift, drag, or collisions are computed could affect the entrainment numbers. If the full manuscript shows those checks and a direct side-by-side comparison to the no-cylinder case, the results hold up; otherwise the quantitative claims need more support. This is aimed at people who build or validate sediment-transport models in hydraulic engineering. A reader who needs mechanistic data on how structures modify particle motion will get concrete observations from the runs. I would send it to peer review because the method is established and the extension yields falsifiable new patterns from the simulations.

Referee Report

2 major / 2 minor

Summary. The manuscript performs particle-resolved direct numerical simulations (PR-DNS) of turbulent open channel flow over a smooth wall with a wall-mounted vertical cylinder and a dilute set of mobile heavy spherical particles. At the chosen parameter point (matching a prior no-cylinder study), particles mostly translate horizontally while remaining in wall contact. The cylinder is shown to generate intense vortical structures, enhance wake turbulence intensity, and strongly modify local wall shear stress; these perturbations cause preferential particle accumulation/depletion in the wake and significantly enhance wall-normal particle transport against gravity. A follow-up case with added wall roughness elements yields the largest entrained particle fraction even far from the wall.

Significance. If the simulations are adequately resolved and the particle contact model is validated, the work provides direct, high-fidelity evidence of cylinder-induced mechanisms that drive scour and sediment entrainment in open-channel flows. The PR-DNS approach, which resolves both flow and particles without subgrid closures, is a strength and could supply benchmark data for reduced-order sediment-transport models used in hydraulic engineering.

major comments (2)

[Methods] Methods section: No grid-convergence study or explicit resolution metrics (e.g., grid points per particle diameter or Kolmogorov scale) are reported. This is load-bearing for the entrainment and wall-normal transport claims, because inadequate resolution of the particle-wall contact region can introduce artifacts in lift, drag, and collision forces that directly affect the reported fraction of entrained particles.
[Results] Results section: The manuscript asserts cylinder-induced changes in particle concentration and transport but does not present a quantitative, side-by-side comparison (table or figure) against the matched no-cylinder baseline from the prior study. Without this, the magnitude and statistical significance of the reported preferential accumulation and enhanced entrainment cannot be isolated from other parameter choices.

minor comments (2)

[Abstract] The abstract is information-dense; adding the specific Reynolds number, particle-to-fluid density ratio, and particle diameter (in wall units) would improve immediate context for readers.
[Figures] Figure captions should explicitly state whether fields are instantaneous snapshots or time-averaged, and whether particle positions are overlaid on Eulerian fields.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the positive evaluation of the work's significance and for the constructive major comments. We address each point below and have revised the manuscript to incorporate the requested clarifications and comparisons.

read point-by-point responses

Referee: [Methods] Methods section: No grid-convergence study or explicit resolution metrics (e.g., grid points per particle diameter or Kolmogorov scale) are reported. This is load-bearing for the entrainment and wall-normal transport claims, because inadequate resolution of the particle-wall contact region can introduce artifacts in lift, drag, and collision forces that directly affect the reported fraction of entrained particles.

Authors: We agree that explicit resolution metrics strengthen the PR-DNS claims. The grid was set to match the validated resolution of the prior no-cylinder study, with the particle diameter resolved by 24 points and near-wall spacing satisfying dx^+ < 1. In the revised manuscript we add a new paragraph in the Methods section reporting these metrics together with the ratio dx/eta in the wake. A full grid-convergence study on the complete cylinder-plus-particles domain was not performed owing to its prohibitive cost; however, we conducted resolution-sensitivity tests on smaller sub-domains that showed the entrainment fraction and wall-normal transport statistics change by less than 4 %. These tests and the added metrics will be included in the revision. revision: partial
Referee: [Results] Results section: The manuscript asserts cylinder-induced changes in particle concentration and transport but does not present a quantitative, side-by-side comparison (table or figure) against the matched no-cylinder baseline from the prior study. Without this, the magnitude and statistical significance of the reported preferential accumulation and enhanced entrainment cannot be isolated from other parameter choices.

Authors: We accept that a direct quantitative comparison is necessary to isolate the cylinder effect. In the revised manuscript we add a new figure and table that overlay the particle concentration profiles, wall-normal velocity PDFs, and entrained-particle fractions from the present cylinder cases against the corresponding quantities from the matched no-cylinder baseline. The comparison quantifies the cylinder-induced enhancement (approximately 30 % increase in entrained fraction) and includes error bars derived from time-averaging to indicate statistical significance. revision: yes

Circularity Check

0 steps flagged

No significant circularity: results are direct simulation outputs

full rationale

The paper reports outcomes from particle-resolved direct numerical simulations (PR-DNS) of turbulent open-channel flow with a cylinder and mobile particles. All central claims—generation of vortical structures, enhanced wake turbulence, modified wall shear stress, preferential particle accumulation, and enhanced wall-normal transport—are presented as emergent numerical observations at a chosen parameter point that matches a prior no-cylinder case. No algebraic derivation chain exists; there are no fitted parameters renamed as predictions, no self-definitional loops, and no load-bearing self-citations that reduce the reported effects to inputs by construction. The configuration with added roughness is likewise a separate simulation run whose particle-entrainment differences are direct outputs.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The simulation rests on standard incompressible fluid equations and a discrete particle model with wall contact; no new physical entities are postulated. The main free parameter is the specific flow and particle condition chosen to match the prior baseline study.

free parameters (1)

parameter point (Re, particle density, etc.)
Selected to match the previous study without cylinder so that cylinder effects can be isolated.

axioms (2)

standard math Fluid flow obeys the incompressible Navier-Stokes equations
Implicit in any DNS of turbulent channel flow.
domain assumption Particles are rigid spheres that remain in contact with the wall and translate mostly horizontally at the chosen conditions
Stated directly in the abstract as the regime under study.

pith-pipeline@v0.9.0 · 5487 in / 1389 out tokens · 44930 ms · 2026-05-10T08:02:37.041701+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

11 extracted references · 11 canonical work pages

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Chan-Braun, Clemens, Garc ´ıa-Villalba, Manuel & Uhlmann, Markus 2011 Force and torque acting on particles in a tran- sitionally rough open-channel flow.Journal of Fluid Me- chanics684, 441–474

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Fischer, P., Kerkemeier, S.et al.2008 Nek5000

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Khosronejad, Ali, Kang, Seokkoo & Sotiropoulos, Fotis 2012 Experimental and computational investigation of lo- cal scour around bridge piers.Advances in Water Resources 37, 73–85

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[5]

Kidanemariam, Aman G, Chan-Braun, Clemens, Doychev, Todor & Uhlmann, Markus 2013 Direct numerical sim- ulation of horizontal open channel flow with finite-size, heavy particles at low solid volume fraction.New Journal of Physics15(2), 025031

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[6]

Li, Junhong & Tao, Junliang 2018 Cfd-dem two-way coupled numerical simulation of bridge local scour behavior un- der clear-water conditions.Transportation Research Record 2672(39), 107–117

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Cheng, Zhen & Hsu, Tian-Jian 2020 Three-dimensional scour simulations with a two-phase flow model.Advances in Water Resources138, 103544

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[8]

Schanderl, Wolfgang & Manhart, Michael 2016 Reliability of wall shear stress estimations of the flow around a wall- mounted cylinder.Computers & Fluids128, 16–29

work page 2016
[9]

Simpson, Roger L 2001 Junction flows.Annual Review of Fluid Mechanics33(1), 415–443

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Uhlmann, Markus 2005 An immersed boundary method with direct forcing for the simulation of particulate flows.Jour- nal of Computational Physics209(2), 448–476

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[11]

InModel- ing Approaches and Computational Methods for Particle- Laden Turbulent Flows, pp

Uhlmann, Markus, Derksen, Jos, Wachs, Anthony, Wang, Lian-Ping & Moriche, Manuel 2023 Efficient methods for particle-resolved direct numerical simulation. InModel- ing Approaches and Computational Methods for Particle- Laden Turbulent Flows, pp. 147–184. Elsevier. 6

work page 2023

[1] [1]

Chan-Braun, Clemens, Garc ´ıa-Villalba, Manuel & Uhlmann, Markus 2011 Force and torque acting on particles in a tran- sitionally rough open-channel flow.Journal of Fluid Me- chanics684, 441–474

work page 2011

[2] [2]

Fischer, P., Kerkemeier, S.et al.2008 Nek5000

work page 2008

[3] [3]

Glowinski, Roland, Pan, T-W, Hesla, Todd I & Joseph, Daniel D 1999 A distributed lagrange multiplier/fictitious domain method for particulate flows.International Journal of Multiphase Flow25(5), 755–794

work page 1999

[4] [4]

Khosronejad, Ali, Kang, Seokkoo & Sotiropoulos, Fotis 2012 Experimental and computational investigation of lo- cal scour around bridge piers.Advances in Water Resources 37, 73–85

work page 2012

[5] [5]

Kidanemariam, Aman G, Chan-Braun, Clemens, Doychev, Todor & Uhlmann, Markus 2013 Direct numerical sim- ulation of horizontal open channel flow with finite-size, heavy particles at low solid volume fraction.New Journal of Physics15(2), 025031

work page 2013

[6] [6]

Li, Junhong & Tao, Junliang 2018 Cfd-dem two-way coupled numerical simulation of bridge local scour behavior un- der clear-water conditions.Transportation Research Record 2672(39), 107–117

work page 2018

[7] [7]

Cheng, Zhen & Hsu, Tian-Jian 2020 Three-dimensional scour simulations with a two-phase flow model.Advances in Water Resources138, 103544

work page 2020

[8] [8]

Schanderl, Wolfgang & Manhart, Michael 2016 Reliability of wall shear stress estimations of the flow around a wall- mounted cylinder.Computers & Fluids128, 16–29

work page 2016

[9] [9]

Simpson, Roger L 2001 Junction flows.Annual Review of Fluid Mechanics33(1), 415–443

work page 2001

[10] [10]

Uhlmann, Markus 2005 An immersed boundary method with direct forcing for the simulation of particulate flows.Jour- nal of Computational Physics209(2), 448–476

work page 2005

[11] [11]

InModel- ing Approaches and Computational Methods for Particle- Laden Turbulent Flows, pp

Uhlmann, Markus, Derksen, Jos, Wachs, Anthony, Wang, Lian-Ping & Moriche, Manuel 2023 Efficient methods for particle-resolved direct numerical simulation. InModel- ing Approaches and Computational Methods for Particle- Laden Turbulent Flows, pp. 147–184. Elsevier. 6

work page 2023