An official website of the United States government
We investigate the effects of fluid elasticity on the flow forces and the wake structure when a rigid cylinder is placed in a viscoelastic flow and is forced to oscillate sinusoidally in the transverse direction. We consider a two-dimensional, uniform, incompressible flow of viscoelastic fluid at$$Re=100$$, and use the FENE-P model to represent the viscoelastic fluid. We study how the flow forces and the wake patterns change as the amplitude of oscillations,$$A^*$$, the frequency of oscillations (inversely proportional to a reduced velocity,$$U^*$$), the Weissenberg number,$$Wi$$, the square of maximum polymer extensibility,$$L^2$$, and the viscosity ratio,$$\beta$$, change individually. We calculate the lift coefficient in phase with cylinder velocity to determine the range of different system parameters where self-excited oscillations might occur if the cylinder is allowed to oscillate freely. We also study the effect of fluid elasticity on the added mass coefficient as these parameters change. The maximum elastic stress of the fluid occurs in between the vortices that are observed in the wake. We observe a new mode of shedding in the wake of the cylinder: in addition to the primary vortices that are also observed in the Newtonian flows, secondary vortices that are caused entirely by the viscoelasticity of the fluid are observed in between the primary vortices. We also show that, for a constant$$Wi$$, the strength of the polymeric stresses increases with increasing reduced velocity or with decreasing amplitude of oscillations.
more »
« less
This content will become publicly available on March 25, 2026
Pulsatile flow in a thin-walled viscoelastic tube
Low-inertia pulsatile flows in highly distensible viscoelastic vessels exist in many biological and engineering systems. However, many existing works focus on inertial pulsatile flows in vessels with small deformations. As such, here we study the dynamics of a viscoelastic tube at large deformation conveying low-Reynolds-number oscillatory flow using a fully coupled fluid–structure interaction computational model. We focus on a detailed study of the effect of wall (solid) viscosity and oscillation frequency on tube deformation, flow rate, phase shift and hysteresis, as well as the underlying flow physics. We find that the general behaviour is dominated by an elastic flow surge during inflation and a squeezing effect during deflation. When increasing the oscillation frequency, the maximum inlet flow rate increases and tube distention decreases, whereas increasing solid viscosity causes both to decrease. As the oscillation frequency approaches either$$0$$(quasi-steady inflation cycle) or$$\infty$$(steady flow), the behaviours of tubes with different solid viscosities converge. Our results suggest that deformation and flow rate are most affected in the intermediate range of solid viscosity and oscillation frequency. Phase shifts of deformation and flow rate with respect to the imposed pressure are analysed. We predict that the phase shifts vary throughout the oscillation; while the deformation always lags the imposed pressure, the flow rate may either lead or lag depending on the parameter values. As such, the flow rate shows hysteresis behaviour that traces either a clockwise or counterclockwise curve, or a mix of both, in the pressure–flow rate space. This directional change in hysteresis is fully characterised here in the appropriate parameter space. Furthermore, the hysteresis direction is shown to be predicted by the signs of the flow rate phase shifts at the crest and trough of the oscillation. A distinct change in the tube dynamics is also observed at high solid viscosity which leads to global or ‘whole-tube’ motion that is absent in purely elastic tubes.
more »
« less
- Award ID(s):
- 1922839
- PAR ID:
- 10582480
- Publisher / Repository:
- Cambridge University Press
- Date Published:
- Journal Name:
- Journal of Fluid Mechanics
- Volume:
- 1007
- ISSN:
- 0022-1120
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
This study explores heat and turbulent modulation in three-dimensional multiphase Rayleigh–Bénard convection using direct numerical simulations. Two immiscible fluids with identical reference density undergo systematic variations in dispersed-phase volume fractions,$$0.0 \leq \varPhi \leq 0.5$$, and ratios of dynamic viscosity,$$\lambda _{\mu }$$, and thermal diffusivity,$$\lambda _{\alpha }$$, within the range$$[0.1\unicode{x2013}10]$$. The Rayleigh, Prandtl, Weber and Froude numbers are held constant at$$10^8$$,$$4$$,$$6000$$and$$1$$, respectively. Initially, when both fluids share the same properties, a 10 % Nusselt number increase is observed at the highest volume fractions. In this case, despite a reduction in turbulent kinetic energy, droplets enhance energy transfer to smaller scales, smaller than those of single-phase flow, promoting local mixing. By varying viscosity ratios, while maintaining a constant Rayleigh number based on the average mixture properties, the global heat transfer rises by approximately 25 % at$$\varPhi =0.2$$and$$\lambda _{\mu }=10$$. This is attributed to increased small-scale mixing and turbulence in the less viscous carrier phase. In addition, a dispersed phase with higher thermal diffusivity results in a 50 % reduction in the Nusselt number compared with the single-phase counterpart, owing to faster heat conduction and reduced droplet presence near walls. The study also addresses droplet-size distributions, confirming two distinct ranges dominated by coalescence and breakup with different scaling laws.more » « less
-
Flow dynamics around a stationary flat plate near a free surface is investigated using time-resolved two-dimensional particle image velocimetry. The study examines variations in angle of attack ($$\theta =0^\circ {-}35^\circ {}$$), Reynolds number ($$Re$$$$\approx$$$$10^3$$$$-$$3$$\times$$$$10^4$$) and plate proximity to the free surface ($$H^*$$). Under symmetric boundary conditions ($$H^*\geqslant {15}$$), increasing$$\theta$$intensifies fluid–plate interaction, resulting in the shedding of leading-edge and trailing-edge vortices (LEV and TEV), each characterised by distinct strengths and sizes. In both symmetric ($$H^*\geqslant {15}$$) and asymmetric ($$H^*=5$$) boundary conditions at$$\theta \lt 5^\circ {}$$, fluid flow follows the contour of the plate, unaffected by Reynolds number. However, at$$H^*=5$$, three flow regimes emerge: the first Coanda effect (CI), regular shedding (RS) and the second Coanda effect (CII), each influenced by$$\theta$$and$$Re$$. The CI regime dominates at lower angles ($$5^\circ {}\leqslant \theta \leqslant 25^\circ {}$$) and$$Re \leqslant 12\,500$$, featuring a Coanda-induced jet-like flow pattern. As the Reynolds number increases, the flow transitions into the RS regime, leading to detachment from the upper surface of the plate. This detachment results in the formation of LEV and TEV in the wake, along with surface deformation, secondary vortices and wavy shear layers beneath the free surface. At$$22\,360\lt Re \leqslant 32\,200$$and$$5^\circ {} \leqslant \theta \leqslant 25^\circ {}$$, in the CII regime, significant surface deformation causes the Coanda effect to reattach the flow to the plate, forming a unique jet-like flow.more » « less
-
The motion and deformation of a neutrally buoyant drop in a rectangular channel experiencing a pressure-driven flow at a low Reynolds number has been investigated both experimentally and numerically. A moving-frame boundary-integral algorithm was used to simulate the drop dynamics, with a focus on steady-state drop velocity and deformation. Results are presented for drops of varying undeformed diameters relative to channel height ($$D/H$$), drop-to-bulk viscosity ratio ($$\lambda$$), capillary number ($$Ca$$, ratio of deforming viscous forces to shape-preserving interfacial tension) and initial position in the channel in a parameter space larger than considered previously. The general trend shows that the drop steady-state velocity decreases with increasing drop diameter and viscosity ratio but increases with increasing$$Ca$$. An opposite trend is seen for drops with small viscosity ratio, however, where the steady-state velocity increases with increasing$$D/H$$and can exceed the maximum background flow velocity. Experimental results verify theoretical predictions. A deformable drop with a size comparable to the channel height when placed off centre migrates towards the centreline and attains a steady state there. In general, a drop with a low viscosity ratio and high capillary number experiences faster cross-stream migration. With increasing aspect ratio, there is a competition between the effect of reduced wall interactions and lower maximum channel centreline velocity at fixed average velocity, with the former helping drops attain higher steady-state velocities at low aspect ratios, but the latter takes over at aspect ratios above approximately 1.5.more » « less
-
The rheological behaviour of dense suspensions of ideally conductive particles in the presence of both electric field and shear flow is studied using large-scale numerical simulations. Under the action of an electric field, these particles are known to undergo dipolophoresis (DIP), which is the combination of two nonlinear electrokinetic phenomena: induced-charge electrophoresis (ICEP) and dielectrophoresis (DEP). For ideally conductive particles, ICEP is predominant over DEP, resulting in transient pairing dynamics. The shear viscosity and first and second normal stress differences$$N_1$$and$$N_2$$of such suspensions are examined over a range of volume fractions$$15\,\% \leq \phi \leq 50\,\%$$as a function of Mason number$$Mn$$, which measures the relative importance of viscous shear stress over electrokinetic-driven stress. For$$Mn < 1$$or low shear rates, the DIP is shown to dominate the dynamics, resulting in a relatively low-viscosity state. The positive$$N_1$$and negative$$N_2$$are observed at$$\phi < 30\,\%$$, which is similar to Brownian suspensions, while their signs are reversed at$$\phi \ge 30\,\%$$. For$$Mn \ge 1$$, the shear thickening starts to arise at$$\phi \ge 30\,\%$$, and an almost five-fold increase in viscosity occurs at$$\phi = 50\,\%$$. Both$$N_1$$and$$N_2$$are negative for$$Mn \gg 1$$at all volume fractions considered. We illuminate the transition in rheological behaviours from DIP to shear dominance around$$Mn = 1$$in connection to suspension microstructure and dynamics. Lastly, our findings reveal the potential use of nonlinear electrokinetics as a means of active rheology control for such suspensions.more » « less
