57 research outputs found
Lagrangian Refined Kolmogorov Similarity Hypothesis for Gradient Time-evolution in Turbulent Flows
We study the time evolution of velocity and pressure gradients in isotropic
turbulence, by quantifying their decorrelation time scales as one follows fluid
particles in the flow. The Lagrangian analysis uses data in a public database
generated using direct numerical simulation of the Naiver-Stokes equations, at
a Reynolds number 430. It is confirmed that when averaging over the entire
domain, correlation functions decay on timescales on the order of the mean
Kolmogorov turnover time scale, computed from the globally averaged rate of
dissipation and viscosity. However, when performing the analysis in different
subregions of the flow, turbulence intermittency leads to large spatial
variability in the decay time scales. Remarkably, excellent collapse of the
auto-correlation functions is recovered when using the `local Kolmogorov
time-scale' defined using the locally averaged, rather than the global,
dissipation-rate. This provides new evidence for the validity of Kolmogorov's
Refined Similarity Hypothesis, but from a Lagrangian viewpoint that provides a
natural frame to describe the dynamical time evolution of turbulence.Comment: 4 Pages, 4 figure
Lattice Boltzmann equation simulations of turbulence, mixing, and combustion
We explore the capability of lattice Boltzmann equation (LBE) method for complex
fluid flows involving turbulence, mixing, and reaction.
In the first study, LBE schemes for binary scalar mixing and multi-component
reacting flow with reactions are developed. Simulations of initially non-premixed
mixtures yield scalar probability distribution functions that are in good agreement
with numerical data obtained from Navier-Stokes (NS) equation based computation.
One-dimensional chemically-reacting flow simulation of a premixed mixture yields a
flame speed that is consistent with experimentally determined value.
The second study involves direct numerical simulation (DNS) and large-eddy
simulation (LES) of decaying homogenous isotropic turbulence (HIT) with and without
frame rotation. Three categories of simulations are performed: (i) LBE-DNS in
both inertial and rotating frames; (ii) LBE-LES in inertial frame; (iii) Comparison
of the LBE-LES vs. NS-LES. The LBE-DNS results of the decay exponents for kinetic
energy k and dissipation rate ε, and the low wave-number scaling of the energy
spectrum agree well with established classical results. The LBE-DNS also captures
rotating turbulence physics. The LBE-LES accurately captures low-wave number
scaling, energy decay and large scale structures. The comparisons indicate that the
LBE-LES simulations preserve flow structures somewhat more accurately than the
NS-LES counterpart.
In the third study, we numerically investigate the near-field mixing features in low
aspect-ratio (AR) rectangular turbulent jets (RTJ) using the LBE method. We use
D3Q19 multiple-relaxation-time (MRT) LBE incorporating a subgrid Smagorinsky
model for LES. Simulations of four jets which characterized by AR, exit velocity,
and Reynolds number are performed. The investigated near-field behaviors include:
(1) Decay of mean streamwise velocity (MSV) and inverse MSV; (2) Spanwise and
lateral profiles of MSV; (3) Half-velocity width development and MSV contours; and
(4) Streamwise turbulence intensity distribution and spanwise profiles of streamwise
turbulence intensity. The computations are compared against experimental data and
the agreement is good. We capture both unique features of RTJ: the saddle-back
spanwise profile of MSV and axis-switching of long axis from spanwise to lateral
direction.
Overall, this work serves to establish the feasibility of the LBE method as a
viable tool for computing mixing, combustion, and turbulence
Study on coalescence dynamics of unequal-sized microbubbles captive on solid substrate
The dynamics of bubble coalescence are of importance for a number of industrial processes, in which the size inequality of the parent bubbles plays a significant role in mass transport, topological change and overall motion. In this study, coalescence of unequal-sized microbubbles captive on a solid substrate was observed from cross-section view using synchrotron high-speed imaging technique and a microfluidic gas generation device. The bridging neck growth and surface wave propagation at the early stage of coalescence were investigated by experimental and numerical methods. The results show that theoretical half-power-law of neck growth rate is still valid when viscous effect is neglected. However, the inertial-capillary time scale is associated with the initial radius of the smaller parent microbubble. The surface wave propagation rate on the larger parent microbubble is proportional to the inertial-capillary time scale
VOLUMETRIC LATTICE BOLTZMANN SIMULATION FOR BLOOD FLOW IN AORTA ARTERY PUMPED THROUGH AORTIC HEART VALVE
poster abstractComplicated moving boundaries pose a major challenge in compu-tational fluid dynamics for complex flows, especially in the biomechan-ics of both blood flow in the cardiovascular system and air flow in the respiratory system where the compliant nature of the vessels can have significant effects on the flow rate and wall shear stress. We develop an innovative approach to treat arbitrarily moving boundaries in Lat-tice Boltzmann Method (LBM) using a volumetric lattice Boltzmann representation, which distributes particles in fluid lattice cells. A volu-metric bounce-back procedure is applied in the streaming step while momentum exchange between the fluid and moving solid boundary are accounted for in the collision step. Additional boundary-induced migra-tion is introduced to conserve fluid mass as the boundary moves across fluid cells. We use the volumetric LBM to simulate blood flow in aorta pumped from heart focusing on the flow rate, flow structure, pressure distribution within the aorta for different heart pumping con-ditions. For validation, the volumetric LBM is compared with Navier-Stokes computation and good agreements are achieved. We study the flow dynamics within the aorta in the cardiac cycle (systole and diasto-le) through alternatively opening and closing the inlet boundary to mimic the heart pumping mechanism
Revisit of axis-switching in low aspect-ratio rectangular jets
poster abstractAxis switching refers to the change in the orientation of the major axis of the jet from initial spanwise to lateral direction. This phenomenon is of interest both from fundamental physics and practical application points of view. This behavior is most noticeable in square and low aspect-ratio (AR) rectangular jet flows. It has been reported computationally that square jet (AR=1) and rectangular jet (AR=1.5) switch the major axis 450 and 900 respectively [1-2]. In this work we revisit the axis-switching phenomenon through direct numerical simulation using kinetic-based lattice Boltzmann method for a square jet and 4 rectangular jets with AR =1.5, 2, 2.5, and 3 respectively at Re=200. It is observed that larger AR jet develops longer characteristic decay (CD) region where jet decay and mixing depend on the jet AR. As all rectangular jets exhibit 900 axis-switching close to the end of the CD region as expected although the location varies with ARs, 450 axis-switching is discovered in each jet before the 900 axis-switching appears downstream. Quantitative examinations of the transition from 450 to 900 in rectangular jets are conducted to reveal the physical underlying mechanism of jet development and provide physical insights for practical application
Scalings of Inverse Energy Transfer and Energy Decay in 3-D Decaying Isotropic Turbulence with Non-rotating or Rotating Frame of Reference
Energy development of decaying isotropic turbulence in a 3-D periodic cube with non-rotating or rotating frames of reference is studied through direct numerical simulation using GPU accelerated lattice Boltzmann method. The initial turbulence is isotropic, generated in spectral space with prescribed energy spectrum E(Îș)~Îșm in a range between Îșmin and Îșmax. The Taylor microscale Reynolds number Reλ and Rossby number Ro are introduced to characterize the inertial, viscous, and rotational attributes of the system. The focus of this study is on the scalings of early inverse energy transfer and late energy decay in the development of turbulent energy under various conditions through combinations of m, Îșmin, Îșmax, Reλ and Ro. First, we demonstrate the validity of the simulation by confirming the quantitative dependence of the decay exponent n on the initial energy spectrum exponent m, at Reλ =255 and Ro=â, varying the values of m, Îșmin and Îșmax. Second, at relatively low Reλ, the decay exponent for different initial spectra statistically fall in respective ranges, all of which agree well with the corresponding analytical predictions. Third, we quantitatively investigate the 3-D inverse energy transfer. Our findings include (i) the exponent of inverse energy transfer spectrum E(Îș)~ÎșÏ depends on the initial spectrum exponent E(Îș) ~ Îșm: if m<4, Ï=m while if mâ„4, Ï=4; (ii) rotation alters the inverse energy transfer rate when Reλâ€255 and Roâ„0.8; (iii) the energy increase in large scale during inverse energy transfer exhibits a bell shape, the peak of which varies with Reλ and Ro
Local and nonlocal pressure Hessian effects in real and synthetic fluid turbulence
The Lagrangian dynamics of the velocity gradient tensor A in isotropic and
homogeneous turbulence depend on the joint action of the self-streching term
and the pressure Hessian. Existing closures for pressure effects in terms of A
are unable to reproduce one important statistical role played by the
anisotropic part of the pressure Hessian, namely the redistribution of the
probabilities towards enstrophy production dominated regions. As a step towards
elucidating the required properties of closures, we study several synthetic
velocity fields and how well they reproduce anisotropic pressure effects. It is
found that synthetic (i) Gaussian, (ii) Multifractal and (iii) Minimal Turnover
Lagrangian Map (MTLM) incompressible velocity fields reproduce many features of
real pressure fields that are obtained from numerical simulations of the Navier
Stokes equations, including the redistribution towards enstrophy-production
regions. The synthetic fields include both spatially local, and nonlocal,
anisotropic pressure effects. However, we show that the local effects appear to
be the most important ones: by assuming that the pressure Hessian is local in
space, an expression in terms of the Hessian of the second invariant Q of the
velocity gradient tensor can be obtained. This term is found to be well
correlated with the true pressure Hessian both in terms of eigenvalue
magnitudes and eigenvector alignments.Comment: 10 pages, 4 figures, minor changes, final version, published in Phys.
Fluid
Spatial and Temporal Scaling of Unequal Microbubble Coalescence
We numerically study coalescence of air microbubbles in water, with density ratio 833 and viscosity ratio 50.5, using lattice Boltzmann method. The focus is on the effects of size inequality of parent bubbles on the interfacial dynamics and coalescence time. Twelve cases, varying the size ratio of large to small parent bubble from 5.33 to 1, are systematically investigated. The âcoalescence preference,â coalesced bubble closer to the larger parent bubble, is well observed and the captured power-law relation between the preferential relative distance Ï and size inequality Îł, math formula, is consistent to the recent experimental observations. Meanwhile, the coalescence time also exhibits power-law scaling as math formula, indicating that unequal bubbles coalesce faster than equal bubbles. Such a temporal scaling of coalescence on size inequality is believed to be the first-time observation as the fast coalescence of microbubbles is generally hard to be recorded through laboratory experimentation
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