15:00
Multiphase and non-Newtonian flows 3
Chair: Markus Uhlmann
15:00
15 mins
|
Direct Numerical Simulations of two-phase Taylor-Couette turbulence
Vamsi Spandan, Rodolfo Ostilla-Monico, Roberto Verzicco, Detlef Lohse
Abstract: Two-phase Taylor-Couette flow is simulated using the Euler-Lagrange approach, where the dispersed phase is treated as point particles with effective forces such as drag, lift, added mass and buoyancy acting on them. Two-way coupling is implemented between the carrier and the dispersed phase allowing us to study the interaction between the point like particles and the large scale flow structure in the carrier phase. Light buoyant particles are observed to be very effective in disrupting the coherent Taylor rolls, thus reducing the overall dissipation in the system and the overall driving torque.
|
15:15
15 mins
|
Dynamical properties of preferential concentration and clustering of inertial particles in turbulent flows
Romain Monchaux
Abstract: We analyze one-way coupling DNS of heavy particles in homogeneous turbulent flow over a large range of Stokes numbers at $R_{\lambda}=180$. We focus on preferential concentration and clustering aspects, and more particularly on their dynamical properties.
|
15:30
15 mins
|
Multifractal Droplet Dynamics in Two-Dimensional, binary-fluid turbulence
Nairita Pal, Prasad Perlekar, Rahul Pandit
Abstract: We present the most extensive direct numerical simulations,
attempted so far, of statistically steady, homogeneous, isotropic turbulence in
two-dimensional, binary-fluid mixtures with air-drag-induced friction. We model
this mixture by using the Cahn-Hilliard-Navier-Stokes equations and choose
parameters, e.g., the surface tension, such that we have a droplet of the
minority phase moving inside a turbulent background of the majority phase. Our
study reveals that a single droplet, whose mean radius lies in the inertial
range of scales, (a) enhances the the forward-cascade part of the energy
spectrum of two-dimensional turbulence and (b) stretches the tails of the PDF
of the Okubo-Weiss parameter $\Lambda$. We show that the dynamics of the
droplet is affected significantly by the turbulence in the fluid. In
particular, the PDFs of the components of the acceleration shows wide,
non-Guassian tails. We characterize the time dependence of the deformation of
the droplet and show that it exhibits multifractality.
|
15:45
15 mins
|
UNSTEADY PARTICLE ACCUMULATION IN WALL TURBULENCE
Dmitrii Sikovsky
Abstract: We propose the asymptotic theory of unsteady accumulation of inertial particles in the viscous sublayer of wall-bounded turbulent flow. We derive the diffusion equation for the particle concentration in the viscous sublayer and find the self-similar exact solution of this equation at large times. It is shown that near the wall the maximal concentration grows as the square root of time, while the distance from the wall to the concentration pike as well as its width decay as inverse square root of time. The obtained solution is corroborated by the results of stochastic Lagrangian simulations.
|
16:00
15 mins
|
Particle collision statistics in turbulent flows with kinematic simulation
Maximilian Eggersdorfer, Daniel Meyer
Abstract: Colliding droplets or particles in turbulent flows are important in applications ranging from rain formation in clouds to aerosol production in process engineering. To reduce the computational costs when simulating such flows, kinematic simulation (KS) is frequently applied as a cheap surrogate for direct numerical simulation (DNS) of the turbulent flow field. In this work, we provide for the first time a systematic validation of the particle collision statistics that result from KS. We show that while the particle collision frequencies for particles with different Stokes numbers are in good agreement with DNS reference data, a more detailed inspection of the flow field and particle concentration characteristics reveals significant differences between KS and DNS.
|
16:15
15 mins
|
Sedimentation of large particles in turbulent environments
Walter Fornari, Francesco Picano, Luca Brandt
Abstract: The aim of the present study is to investigate the sedimentation of large non-colloidal spherical particles
in both quiescent and turbulent environments. To this aim, Direct Numerical Simulations are performed using an
Immersed Boundary Method to account for the dispersed phase. The solid volume fractions considered
are in the range 0.5%-1.0%, while the solid to fluid density ratio is set equal to 1.02.
The particle diameter is chosen to be approximately 12 Komlogorov lengthscales in nominal conditions. The results show that
the mean settling velocities decrease in the turbulent cases. The overall drag is increased both by the non-linear
finite Reynolds number behavior and by unsteady effects, which are negligible in quiescent cases.
|
16:30
15 mins
|
The reorganisation of turbulent pipe flow by a drag-reducing polymer additive
David Dennis, Francesca Sogaro
Abstract: The effect of a drag-reducing polymer additive on the organisational states of turbulent pipe flow is investigated by performing stereoscopic particle image velocimetry measurements in a large-scale pipe flow experiment at $Re_D=10000$ using both water and a visco-elastic, shear-thinning, semi-dilute aqueous polymer solution. The effect of the polymer is to decrease the drag (by 62\%) whilst significantly increasing the probability that the flow exists in a flow state with a low azimuthal wavenumber ($k_\theta=2$). This result indicates that the $k_\theta=2$ state is potentially a favourable (i.e.~low drag) flow state.
|
|