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The DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Wed, 21 Apr 2021 08:29:11 GMT2021-04-21T08:29:11ZDirect numerical simulations of laminar and transitional flows in diverging pipes
http://hdl.handle.net/10985/18599
Direct numerical simulations of laminar and transitional flows in diverging pipes
VITTAL SHENOY, Dhanush; SHADLOO, M. S.; PEIXINHO, Jorge; HADJADJ, Abdellah
Purpose: Fluid flows in pipes whose cross-sectional area are increasing in the stream-wise direction are prone to separation of the recirculation region. This paper aims to investigate such fluid flow in expansion pipe systems using direct numerical simulations. The flow in circular diverging pipes with different diverging half angles, namely, 45, 26, 14, 7.2 and 4.7 degrees, are considered. The flow is fed by a fully developed laminar parabolic velocity profile at its inlet and is connected to a long straight circular pipe at its downstream to characterise recirculation zone and skin friction coefficient in the laminar regime. The flow is considered linearly stable for Reynolds numbers sufficiently below natural transition. A perturbation is added to the inlet fully developed laminar velocity profile to test the flow response to finite amplitude disturbances and to characterise sub-critical transition. Design/methodology/approach: Direct numerical simulations of the Navier–Stokes equations have been solved using a spectral element method. Findings: It is found that the onset of disordered motion and the dynamics of the localised turbulence patch are controlled by the Reynolds number, the perturbation amplitude and the half angle of the pipe. Originality/value: The authors clarify different stages of flow behaviour under the finite amplitude perturbations and shed more light to flow physics such as existence of Kelvin–Helmholtz instabilities as well as mechanism of turbulent puff shedding in diverging pipe flows.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10985/185992019-01-01T00:00:00ZVITTAL SHENOY, DhanushSHADLOO, M. S.PEIXINHO, JorgeHADJADJ, AbdellahPurpose: Fluid flows in pipes whose cross-sectional area are increasing in the stream-wise direction are prone to separation of the recirculation region. This paper aims to investigate such fluid flow in expansion pipe systems using direct numerical simulations. The flow in circular diverging pipes with different diverging half angles, namely, 45, 26, 14, 7.2 and 4.7 degrees, are considered. The flow is fed by a fully developed laminar parabolic velocity profile at its inlet and is connected to a long straight circular pipe at its downstream to characterise recirculation zone and skin friction coefficient in the laminar regime. The flow is considered linearly stable for Reynolds numbers sufficiently below natural transition. A perturbation is added to the inlet fully developed laminar velocity profile to test the flow response to finite amplitude disturbances and to characterise sub-critical transition. Design/methodology/approach: Direct numerical simulations of the Navier–Stokes equations have been solved using a spectral element method. Findings: It is found that the onset of disordered motion and the dynamics of the localised turbulence patch are controlled by the Reynolds number, the perturbation amplitude and the half angle of the pipe. Originality/value: The authors clarify different stages of flow behaviour under the finite amplitude perturbations and shed more light to flow physics such as existence of Kelvin–Helmholtz instabilities as well as mechanism of turbulent puff shedding in diverging pipe flows.Flow and air-entrainment around partially submerged vertical cylinders
http://hdl.handle.net/10985/17944
Flow and air-entrainment around partially submerged vertical cylinders
AGEORGES, Valentin; PEIXINHO, Jorge; PÉRRET, Gaële
In this study, a partially submerged vertical cylinder is moved at constant velocity through water, which is initially at rest. During the motion, the wake behind the cylinder induces free-surface deformation. Eleven cylinders, with diameters from D=1.4 to 16 cm, were tested under two different conditions: (i) constant immersed height h and (ii) constant h/D. The range of translation velocities and diameters are in the regime of turbulent wake with experiments carried out for 4500<Re<240000 and 0.2<Fr<2.4, where Re and Fr are the Reynolds and Froude numbers based on D. The focus here is on drag-force measurements and relatively strong free-surface deformation up to air-entrainment. Specifically, two modes of air-entraiment have been uncovered: (i) in the cavity along the cylinder wall and (ii) in the wake of the cylinder. A scaling for the critical velocity for air-entrainment in the cavity has been observed in agreement with a simple model. Furthermore, for Fr>1.2, the drag force varies linearly with Fr.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10985/179442019-01-01T00:00:00ZAGEORGES, ValentinPEIXINHO, JorgePÉRRET, GaëleIn this study, a partially submerged vertical cylinder is moved at constant velocity through water, which is initially at rest. During the motion, the wake behind the cylinder induces free-surface deformation. Eleven cylinders, with diameters from D=1.4 to 16 cm, were tested under two different conditions: (i) constant immersed height h and (ii) constant h/D. The range of translation velocities and diameters are in the regime of turbulent wake with experiments carried out for 4500<Re<240000 and 0.2<Fr<2.4, where Re and Fr are the Reynolds and Froude numbers based on D. The focus here is on drag-force measurements and relatively strong free-surface deformation up to air-entrainment. Specifically, two modes of air-entraiment have been uncovered: (i) in the cavity along the cylinder wall and (ii) in the wake of the cylinder. A scaling for the critical velocity for air-entrainment in the cavity has been observed in agreement with a simple model. Furthermore, for Fr>1.2, the drag force varies linearly with Fr.Drop dynamics of viscoelastic filaments
http://hdl.handle.net/10985/18383
Drop dynamics of viscoelastic filaments
PINGULKAR, Hrishikesh; PEIXINHO, Jorge; CRUMEYROLLE, Olivier
The stretching of viscoelastic polymer solutions close to breakup can create attached drops on a filament, whose properties and dynamics are little understood. The stretching of capillary bridges and the consecutive filament, until its breakup, can be quantified using diameter-space-time diagrams, which demonstrate hierarchy, as well as asymmetry of satellite drops around a big central drop. All drops experience migration, oscillation, and merging. In addition, the position of the minimum diameter on the filament is determined, along with the number of drops, their positions, the diameters of drops and the filament breakup time. The maximum number of drops on the filament can be predicted using the Deborah number. The diagrams also quantify the large Hencky strains in the filaments before pinch-off. The obtained minimum diameter is used to measure the extensional viscosity, which indicates the effect of polymer concentration and direction of filament thinning.
Wed, 01 Jan 2020 00:00:00 GMThttp://hdl.handle.net/10985/183832020-01-01T00:00:00ZPINGULKAR, HrishikeshPEIXINHO, JorgeCRUMEYROLLE, OlivierThe stretching of viscoelastic polymer solutions close to breakup can create attached drops on a filament, whose properties and dynamics are little understood. The stretching of capillary bridges and the consecutive filament, until its breakup, can be quantified using diameter-space-time diagrams, which demonstrate hierarchy, as well as asymmetry of satellite drops around a big central drop. All drops experience migration, oscillation, and merging. In addition, the position of the minimum diameter on the filament is determined, along with the number of drops, their positions, the diameters of drops and the filament breakup time. The maximum number of drops on the filament can be predicted using the Deborah number. The diagrams also quantify the large Hencky strains in the filaments before pinch-off. The obtained minimum diameter is used to measure the extensional viscosity, which indicates the effect of polymer concentration and direction of filament thinning.