TY - JOUR
AB - Phase-field methods have long been used to model the flow of immiscible fluids. Their ability to naturally capture interface topological changes is widely recognized, but their accuracy in simulating flows of real fluids in practical geometries is not established. We here quantitatively investigate the convergence of the phase-field method to the sharp-interface limit with simulations of two-phase pipe flow. We focus on core-annular flows, in which a highly viscous fluid is lubricated by a less viscous fluid, and validate our simulations with an analytic laminar solution, a formal linear stability analysis and also in the fully nonlinear regime. We demonstrate the ability of the phase-field method to accurately deal with non-rectangular geometry, strong advection, unsteady fluctuations and large viscosity contrast. We argue that phase-field methods are very promising for quantitatively studying moderately turbulent flows, especially at high concentrations of the disperse phase.
AU - Song, Baofang
AU - Plana, Carlos
AU - Lopez Alonso, Jose M
AU - Avila, Marc
ID - 6413
JF - International Journal of Multiphase Flow
SN - 03019322
TI - Phase-field simulation of core-annular pipe flow
VL - 117
ER -
TY - JOUR
AB - Segregation of maternal determinants within the oocyte constitutes the first step in embryo patterning. In zebrafish oocytes, extensive ooplasmic streaming leads to the segregation of ooplasm from yolk granules along the animal-vegetal axis of the oocyte. Here, we show that this process does not rely on cortical actin reorganization, as previously thought, but instead on a cell-cycle-dependent bulk actin polymerization wave traveling from the animal to the vegetal pole of the oocyte. This wave functions in segregation by both pulling ooplasm animally and pushing yolk granules vegetally. Using biophysical experimentation and theory, we show that ooplasm pulling is mediated by bulk actin network flows exerting friction forces on the ooplasm, while yolk granule pushing is achieved by a mechanism closely resembling actin comet formation on yolk granules. Our study defines a novel role of cell-cycle-controlled bulk actin polymerization waves in oocyte polarization via ooplasmic segregation.
AU - Shamipour, Shayan
AU - Kardos, Roland
AU - Xue, Shi-lei
AU - Hof, Björn
AU - Hannezo, Edouard B
AU - Heisenberg, Carl-Philipp J
ID - 6508
IS - 6
JF - Cell
SN - 00928674
TI - Bulk actin dynamics drive phase segregation in zebrafish oocytes
VL - 177
ER -
TY - JOUR
AB - Recent studies suggest that unstable recurrent solutions of the Navier-Stokes equation provide new insights
into dynamics of turbulent flows. In this study, we compute an extensive network of dynamical connections
between such solutions in a weakly turbulent quasi-two-dimensional Kolmogorov flow that lies in the inversion symmetric subspace. In particular, we find numerous isolated heteroclinic connections between different
types of solutions—equilibria, periodic, and quasiperiodic orbits—as well as continua of connections forming
higher-dimensional connecting manifolds. We also compute a homoclinic connection of a periodic orbit and
provide strong evidence that the associated homoclinic tangle forms the chaotic repeller that underpins transient
turbulence in the symmetric subspace.
AU - Suri, Balachandra
AU - Pallantla, Ravi Kumar
AU - Schatz, Michael F.
AU - Grigoriev, Roman O.
ID - 6779
IS - 1
JF - Physical Review E
SN - 2470-0045
TI - Heteroclinic and homoclinic connections in a Kolmogorov-like flow
VL - 100
ER -
TY - THES
AB - In many shear flows like pipe flow, plane Couette flow, plane Poiseuille flow, etc. turbulence emerges subcritically. Here, when subjected to strong enough perturbations, the flow becomes turbulent in spite of the laminar base flow being linearly stable. The nature of this instability has puzzled the scientific community for decades. At onset, turbulence appears in localized patches and flows are spatio-temporally intermittent. In pipe flow the localized turbulent structures are referred to as puffs and in planar flows like plane Couette and channel flow, patches arise in the form of localized oblique bands. In this thesis, we study the onset of turbulence in channel flow in direct numerical simulations from a dynamical system theory perspective, as well as by performing experiments in a large aspect ratio channel.
The aim of the experimental work is to determine the critical Reynolds number where turbulence first becomes sustained. Recently, the onset of turbulence has been described in analogy to absorbing state phase transition (i.e. directed percolation). In particular, it has been shown that the critical point can be estimated from the competition between spreading and decay processes. Here, by performing experiments, we identify the mechanisms underlying turbulence proliferation in channel flow and find the critical Reynolds number, above which turbulence becomes sustained. Above the critical point, the continuous growth at the tip of the stripes outweighs the stochastic shedding of turbulent patches at the tail and the stripes expand. For growing stripes, the probability to decay decreases while the probability of stripe splitting increases. Consequently, and unlike for the puffs in pipe flow, neither of these two processes is time-independent i.e. memoryless. Coupling between stripe expansion and creation of new stripes via splitting leads to a significantly lower critical point ($Re_c=670+/-10$) than most earlier studies suggest.
While the above approach sheds light on how turbulence first becomes sustained, it provides no insight into the origin of the stripes themselves. In the numerical part of the thesis we investigate how turbulent stripes form from invariant solutions of the Navier-Stokes equations. The origin of these turbulent stripes can be identified by applying concepts from the dynamical system theory. In doing so, we identify the exact coherent structures underlying stripes and their bifurcations and how they give rise to the turbulent attractor in phase space. We first report a family of localized nonlinear traveling wave solutions of the Navier-Stokes equations in channel flow. These solutions show structural similarities with turbulent stripes in experiments like obliqueness, quasi-streamwise streaks and vortices, etc. A parametric study of these traveling wave solution is performed, with parameters like Reynolds number, stripe tilt angle and domain size, including the stability of the solutions. These solutions emerge through saddle-node bifurcations and form a phase space skeleton for the turbulent stripes observed in the experiments. The lower branches of these TW solutions at different tilt angles undergo Hopf bifurcation and new solutions branches of relative periodic orbits emerge. These RPO solutions do not belong to the same family and therefore the routes to chaos for different angles are different.
In shear flows, turbulence at onset is transient in nature. Consequently,turbulence can not be tracked to lower Reynolds numbers, where the dynamics may simplify. Before this happens, turbulence becomes short-lived and laminarizes. In the last part of the thesis, we show that using numerical simulations we can continue turbulent stripes in channel flow past the 'relaminarization barrier' all the way to their origin. Here, turbulent stripe dynamics simplifies and the fluctuations are no longer stochastic and the stripe settles down to a relative periodic orbit. This relative periodic orbit originates from the aforementioned traveling wave solutions. Starting from the relative periodic orbit, a small increase in speed i.e. Reynolds number gives rise to chaos and the attractor dimension sharply increases in contrast to the classical transition scenario where the instabilities affect the flow globally and give rise to much more gradual route to turbulence.
AU - Paranjape, Chaitanya S
ID - 6957
KW - Instabilities
KW - Turbulence
KW - Nonlinear dynamics
TI - Onset of turbulence in plane Poiseuille flow
ER -
TY - JOUR
AB - In pipes and channels, the onset of turbulence is initially dominated by localizedtransients, which lead to sustained turbulence through their collective dynamics. In thepresent work, we study numerically the localized turbulence in pipe flow and elucidate astate space structure that gives rise to transient chaos. Starting from the basin boundaryseparating laminar and turbulent flow, we identify transverse homoclinic orbits, thepresence of which necessitates a homoclinic tangle and chaos. A direct consequence ofthe homoclinic tangle is the fractal nature of the laminar-turbulent boundary, which wasconjectured in various earlier studies. By mapping the transverse intersections between thestable and unstable manifold of a periodic orbit, we identify the gateways that promote anescape from turbulence.
AU - Budanur, Nazmi B
AU - Dogra, Akshunna
AU - Hof, Björn
ID - 6978
IS - 10
JF - Physical Review Fluids
TI - Geometry of transient chaos in streamwise-localized pipe flow turbulence
VL - 4
ER -