--- _id: '9728' abstract: - lang: eng text: "Most real-world flows are multiphase, yet we know little about them compared to their single-phase counterparts. Multiphase flows are more difficult to investigate as their dynamics occur in large parameter space and involve complex phenomena such as preferential concentration, turbulence modulation, non-Newtonian rheology, etc. Over the last few decades, experiments in particle-laden flows have taken a back seat in favour of ever-improving computational resources. However, computers are still not powerful enough to simulate a real-world fluid with millions of finite-size particles. Experiments are essential not only because they offer a reliable way to investigate real-world multiphase flows but also because they serve to validate numerical studies and steer the research in a relevant direction. In this work, we have experimentally investigated particle-laden flows in pipes, and in particular, examined the effect of particles on the laminar-turbulent transition and the drag scaling in turbulent flows.\r\n\r\nFor particle-laden pipe flows, an earlier study [Matas et al., 2003] reported how the sub-critical (i.e., hysteretic) transition that occurs via localised turbulent structures called puffs is affected by the addition of particles. In this study, in addition to this known transition, we found a super-critical transition to a globally fluctuating state with increasing particle concentration. At the same time, the Newtonian-type transition via puffs is delayed to larger Reynolds numbers. At an even higher concentration, only the globally fluctuating state is found. The dynamics of particle-laden flows are hence determined by two competing instabilities that give rise to three flow regimes: Newtonian-type turbulence at low, a particle-induced globally fluctuating state at high, and a coexistence state at intermediate concentrations.\r\n\r\nThe effect of particles on turbulent drag is ambiguous, with studies reporting drag reduction, no net change, and even drag increase. The ambiguity arises because, in addition to particle concentration, particle shape, size, and density also affect the net drag. Even similar particles might affect the flow dissimilarly in different Reynolds number and concentration ranges. In the present study, we explored a wide range of both Reynolds number and concentration, using spherical as well as cylindrical particles. We found that the spherical particles do not reduce drag while the cylindrical particles are drag-reducing within a specific Reynolds number interval. The interval strongly depends on the particle concentration and the relative size of the pipe and particles. Within this interval, the magnitude of drag reduction reaches a maximum. These drag reduction maxima appear to fall onto a distinct power-law curve irrespective of the pipe diameter and particle concentration, and this curve can be considered as the maximum drag reduction asymptote for a given fibre shape. Such an asymptote is well known for polymeric flows but had not been identified for particle-laden flows prior to this work." acknowledged_ssus: - _id: M-Shop alternative_title: - ISTA Thesis article_processing_charge: No author: - first_name: Nishchal full_name: Agrawal, Nishchal id: 469E6004-F248-11E8-B48F-1D18A9856A87 last_name: Agrawal citation: ama: Agrawal N. Transition to turbulence and drag reduction in particle-laden pipe flows. 2021. doi:10.15479/at:ista:9728 apa: Agrawal, N. (2021). Transition to turbulence and drag reduction in particle-laden pipe flows. Institute of Science and Technology Austria. https://doi.org/10.15479/at:ista:9728 chicago: Agrawal, Nishchal. “Transition to Turbulence and Drag Reduction in Particle-Laden Pipe Flows.” Institute of Science and Technology Austria, 2021. https://doi.org/10.15479/at:ista:9728. ieee: N. Agrawal, “Transition to turbulence and drag reduction in particle-laden pipe flows,” Institute of Science and Technology Austria, 2021. ista: Agrawal N. 2021. Transition to turbulence and drag reduction in particle-laden pipe flows. Institute of Science and Technology Austria. mla: Agrawal, Nishchal. Transition to Turbulence and Drag Reduction in Particle-Laden Pipe Flows. Institute of Science and Technology Austria, 2021, doi:10.15479/at:ista:9728. short: N. Agrawal, Transition to Turbulence and Drag Reduction in Particle-Laden Pipe Flows, Institute of Science and Technology Austria, 2021. date_created: 2021-07-27T13:40:30Z date_published: 2021-07-29T00:00:00Z date_updated: 2024-02-28T13:14:39Z day: '29' ddc: - '532' degree_awarded: PhD department: - _id: GradSch - _id: BjHo doi: 10.15479/at:ista:9728 file: - access_level: closed checksum: 77436be3563a90435024307b1b5ee7e8 content_type: application/x-zip-compressed creator: nagrawal date_created: 2021-07-28T13:32:02Z date_updated: 2022-07-29T22:30:05Z embargo_to: open_access file_id: '9744' file_name: Transition to Turbulence and Drag Reduction in Particle-Laden Pipe Flows.zip file_size: 22859658 relation: source_file - access_level: open_access checksum: 72a891d7daba85445c29b868c22575ed content_type: application/pdf creator: nagrawal date_created: 2021-07-28T13:32:05Z date_updated: 2022-07-29T22:30:05Z embargo: 2022-07-28 file_id: '9745' file_name: Transition to Turbulence and Drag Reduction in Particle-Laden Pipe Flows.pdf file_size: 18658048 relation: main_file file_date_updated: 2022-07-29T22:30:05Z has_accepted_license: '1' keyword: - Drag Reduction - Transition to Turbulence - Multiphase Flows - particle Laden Flows - Complex Flows - Experiments - Fluid Dynamics language: - iso: eng license: https://creativecommons.org/licenses/by/4.0/ month: '07' oa: 1 oa_version: Published Version page: '118' publication_identifier: issn: - 2663-337X publication_status: published publisher: Institute of Science and Technology Austria related_material: record: - id: '6189' relation: part_of_dissertation status: public status: public supervisor: - first_name: Björn full_name: Hof, Björn id: 3A374330-F248-11E8-B48F-1D18A9856A87 last_name: Hof orcid: 0000-0003-2057-2754 title: Transition to turbulence and drag reduction in particle-laden pipe flows tmp: image: /images/cc_by.png legal_code_url: https://creativecommons.org/licenses/by/4.0/legalcode name: Creative Commons Attribution 4.0 International Public License (CC-BY 4.0) short: CC BY (4.0) type: dissertation user_id: c635000d-4b10-11ee-a964-aac5a93f6ac1 year: '2021' ... --- _id: '6189' abstract: - lang: eng text: 'Suspended particles can alter the properties of fluids and in particular also affect the transition fromlaminar to turbulent flow. An earlier study [Mataset al.,Phys. Rev. Lett.90, 014501 (2003)] reported howthe subcritical (i.e., hysteretic) transition to turbulent puffs is affected by the addition of particles. Here weshow that in addition to this known transition, with increasing concentration a supercritical (i.e.,continuous) transition to a globally fluctuating state is found. At the same time the Newtonian-typetransition to puffs is delayed to larger Reynolds numbers. At even higher concentration only the globallyfluctuating state is found. The dynamics of particle laden flows are hence determined by two competinginstabilities that give rise to three flow regimes: Newtonian-type turbulence at low, a particle inducedglobally fluctuating state at high, and a coexistence state at intermediate concentrations.' article_number: '114502' article_processing_charge: No author: - first_name: Nishchal full_name: Agrawal, Nishchal id: 469E6004-F248-11E8-B48F-1D18A9856A87 last_name: Agrawal - first_name: George H full_name: Choueiri, George H id: 448BD5BC-F248-11E8-B48F-1D18A9856A87 last_name: Choueiri - first_name: Björn full_name: Hof, Björn id: 3A374330-F248-11E8-B48F-1D18A9856A87 last_name: Hof orcid: 0000-0003-2057-2754 citation: ama: Agrawal N, Choueiri GH, Hof B. Transition to turbulence in particle laden flows. Physical Review Letters. 2019;122(11). doi:10.1103/PhysRevLett.122.114502 apa: Agrawal, N., Choueiri, G. H., & Hof, B. (2019). Transition to turbulence in particle laden flows. Physical Review Letters. American Physical Society. https://doi.org/10.1103/PhysRevLett.122.114502 chicago: Agrawal, Nishchal, George H Choueiri, and Björn Hof. “Transition to Turbulence in Particle Laden Flows.” Physical Review Letters. American Physical Society, 2019. https://doi.org/10.1103/PhysRevLett.122.114502. ieee: N. Agrawal, G. H. Choueiri, and B. Hof, “Transition to turbulence in particle laden flows,” Physical Review Letters, vol. 122, no. 11. American Physical Society, 2019. ista: Agrawal N, Choueiri GH, Hof B. 2019. Transition to turbulence in particle laden flows. Physical Review Letters. 122(11), 114502. mla: Agrawal, Nishchal, et al. “Transition to Turbulence in Particle Laden Flows.” Physical Review Letters, vol. 122, no. 11, 114502, American Physical Society, 2019, doi:10.1103/PhysRevLett.122.114502. short: N. Agrawal, G.H. Choueiri, B. Hof, Physical Review Letters 122 (2019). date_created: 2019-03-31T21:59:12Z date_published: 2019-03-22T00:00:00Z date_updated: 2024-03-28T23:30:48Z day: '22' department: - _id: BjHo doi: 10.1103/PhysRevLett.122.114502 external_id: arxiv: - '1809.06358' isi: - '000461922000006' intvolume: ' 122' isi: 1 issue: '11' language: - iso: eng main_file_link: - open_access: '1' url: https://arxiv.org/abs/1809.06358 month: '03' oa: 1 oa_version: Preprint publication: Physical Review Letters publication_identifier: eissn: - '10797114' issn: - '00319007' publication_status: published publisher: American Physical Society quality_controlled: '1' related_material: record: - id: '9728' relation: dissertation_contains status: public scopus_import: '1' status: public title: Transition to turbulence in particle laden flows type: journal_article user_id: 2DF688A6-F248-11E8-B48F-1D18A9856A87 volume: 122 year: '2019' ...