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The dynamics and multiple-cycle evolution of the incompressible flow induced by a moving piston through the open valve of a motored piston-cylinder assembly was investigated using direct numerical simulation. A spectral element so...
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The dynamics and multiple-cycle evolution of the incompressible flow induced by a moving piston through the open valve of a motored piston-cylinder assembly was investigated using direct numerical simulation. A spectral element solver, adapted for moving geometries using an Arbitrary Lagrange/Eulerian formulation, was employed. Eight cycles were simulated and the ensemble- and azimuthally-averaged data were found to be in good agreement with experimentally determined means and fluctuations at all measured points and times. During the first half of the intake stroke the flow field is dominated by the dynamics of the incoming jet and the vortex rings it creates. With decreasing piston speed a large central ring becomes the dominant flow feature until the top dead center. The flow field at the end of the previous cycle is found to have a dominant effect on the jet breakup and the vortex ring dynamics below the valve and on the observed significant cyclic variations. Based on statistical averaging, the evolution of the turbulent flow field during the first half of the intake stroke is dominated by the jet breakup process leading to a strongly anisotropic behavior. In the second part of the intake stroke, the decrease of the incoming jet velocity results in a more isotropic behavior.
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In this study, a numerical method is developed to perform the direct numerical simulation (DNS) of gas-solidliquid flows involving capillary effects. The volume-of-fluid method employed to track the free surface and the immersed b...
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In this study, a numerical method is developed to perform the direct numerical simulation (DNS) of gas-solidliquid flows involving capillary effects. The volume-of-fluid method employed to track the free surface and the immersed boundary method is adopted for the fluid-particle coupling in three-phase flows. This numerical method is able to fully resolve the hydrodynamic force and capillary force as well as the particle motions arising from complicated gas-solid-liquid interactions. We present its application to liquid bridges among spherical particles in this paper. By using the DNS method, we obtain the static bridge force as a function of the liquid volume, contact angle, and separation distance. The results from the DNS are compared with theoretical equations and other solutions to examine its validity and suitability for modeling capillary bridges. Particularly, the nontrivial liquid bridges formed in triangular and tetrahedral particle clusters are calculated and some preliminary results are reported. We also perform dynamic simulations of liquid bridge ruptures subject to axial stretching and particle motions driven by liquid bridge action, for which accurate predictions are obtained with respect to the critical rupture distance and the equilibrium particle position, respectively. As shown through the simulations, the strength of the present method is the ability to predict the liquid bridge problem under general conditions, from which models of liquid bridge actions may be constructed without limitations. Therefore, it is believed that this DNS method can be a useful tool to improve the understanding and modeling of liquid bridges formed in complex gas-solid-liquid flows.
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We explore one-point and two-point statistics of the Navier–Stokes-αβ regularizationmodel at moderateReynolds number (Re≈200) in homogeneous isotropic turbulence. The results are compared to the limit cases of the Navier–Stok...
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We explore one-point and two-point statistics of the Navier–Stokes-αβ regularizationmodel at moderateReynolds number (Re≈200) in homogeneous isotropic turbulence. The results are compared to the limit cases of the Navier–Stokes-α model and the Navier–Stokes-αβ model without subgridscale stress, as well as with high-resolution direct numerical simulation. After reviewing spectra of different energy norms of theNavier–Stokes-αβ model, the Navier–Stokes-α model, and Navier–Stokes-αβ model without subgrid-scale stress, we present probability density functions and normalized probability density functions of the filtered and unfiltered velocity increments along with longitudinal velocity structure functions of the regularization models and direct numerical simulation results. We highlight differences in the statistical properties of the unfiltered and filtered velocity fields entering the governing equations of the Navier–Stokes-α and Navier–Stokes-αβ models and discuss the usability of both velocity fields for realistic flow predictions. The influence of the modified viscous term in the Navier–Stokes-αβ model is studied through comparison to the case where the underlying subgrid-scale stress tensor is neglected. Whereas, the filtered velocity field is found to have physically more viable probability density functions and structure functions for the approximation of direct numerical simulation results, the unfiltered velocity field is found to have flatness factors close to direct numerical simulation results.
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A three-dimensional (3D) direct numerical simulation (DNS) study of the propagation of a reaction wave in forced, constant-density, statistically stationary, homogeneous, isotropic turbulence is performed by solving Navier-Stokes ...
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A three-dimensional (3D) direct numerical simulation (DNS) study of the propagation of a reaction wave in forced, constant-density, statistically stationary, homogeneous, isotropic turbulence is performed by solving Navier-Stokes and reaction-diffusion equations at various (from 0.5 to 10) ratios of the rms turbulent velocity U' to the laminar wave speed, various (from 2.1 to 12.5) ratios of an integral length scale of the turbulence to the laminar wave thickness, and two Zeldovich numbers Ze = 6.0 and 17.1. Accordingly, the Damk?hler and Karlovitz numbers are varied from 0.2 to 25.1 and from 0.4 to 36.2, respectively. Contrary to an earlier DNS study of self-propagation of an infinitely thin front in statistically the same turbulence, the bending of dependencies of the mean wave speed on U' is simulated in the case of a nonzero thickness of the local reaction wave. The bending effect is argued to be controlled by inefficiency of the smallest scale turbulent eddies in wrinkling the reaction-zone surface, because such small-scale wrinkles are rapidly smoothed out by molecular transport within the local reaction wave.
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We perform direct numerical simulation of flow past the NACA (National Advisory Committee for Aeronautics) 0012 airfoil with partially covered wavy roughness elements near the leading edge. The Reynolds number Rec based on the fre...
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We perform direct numerical simulation of flow past the NACA (National Advisory Committee for Aeronautics) 0012 airfoil with partially covered wavy roughness elements near the leading edge. The Reynolds number Rec based on the freestream velocity (U0) and airfoil chord length (C), and the angle of attack (AoA) is fixed, i.e., Re_c = 5 × 10~4; AoA = 10°. The leading edge roughness elements are characterized by streamwise sinusoidal-wave geometry that is uniform in the spanwise direction with fixed height, whereas varying wave numbers (k) from 0 to 12. Based on the validation of the smooth baseline case (k=0), the roughness effects on the aerodynamic performance are evaluated in terms of the lift and drag coefficients. The drag coefficient decreases monotonically with k, while the variation of the lift coefficient with k is similar to the "drag crisis" phenomenon observed in cylinder flows. The sharp variations of lift and drag coefficients from k=6 to 8 indicate that k=8 is a critical case, beyond which massive separation occurs and almost covers the airfoil's suction side and dominates the airfoil aerodynamic performance. To further reveal the underlying mechanism, the flow structures, pressure, skin friction coefficients, shear layer transition onset, and boundary layer separation are quantified to investigate the roughness effects. The roughness elements strongly modify the separation behavior, whereas they have little effect on the transition onset. The unsteady interactions and convections of separation bubbles downward the trailing edge also change the wake evolution. Based on the scaling of the asymmetric wake profiles, we find that the wake defect is gently decreasing with k, but the increase in the wake width is much stronger. This scaling analysis gains a better insight into the roughness effects on the momentum deficit flow rate, which confirms the drag mechanism with different roughness wave numbers.
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A consistent formulation is presented for the direct numerical simulation of an arbitrarily shaped colloidal particle at a deformable fluidic interface. The rigid colloidal particle is decomposed into a collection of solid spheric...
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A consistent formulation is presented for the direct numerical simulation of an arbitrarily shaped colloidal particle at a deformable fluidic interface. The rigid colloidal particle is decomposed into a collection of solid spherical beads and the three-phase boundaries are replaced with smoothly spreading interfaces. The major merit of the present formulation lies in the ease with which the geometrical decomposition of the colloidal particle is implemented, yet allows the dynamic simulation of intricate three-dimensional colloidal shapes in a binary fluid. The dynamics of a rodlike, a platelike, and a ringlike particle are presently tested. It is found that platelike particles attach more rapidly to a fluidic interface and are subsequently harder to dislodge when subject to an external force. Using the Bond number, i.e., the ratio of the gravitational force to the reference capillary force, a spherical particle with equal affinity for the two fluids breaks away from a fluidic interface at the critical value Bo = 0.75. This value is in line with our numerical experiments. It is here shown that a plate and a ring of equivalent masses detach at greater critical Bond numbers approximately equal to Bo = 1.3. Results of this study will find applications in the stabilization of emulsions by colloids and in the recovery of colloidal particles by rising bubbles.
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The effects on particle dispersion by turbulence transition in a three-dimensional plane jet are investigated by means of direct numerical simulation. The governing equations of fluid motion are solved by a finite volume method an...
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The effects on particle dispersion by turbulence transition in a three-dimensional plane jet are investigated by means of direct numerical simulation. The governing equations of fluid motion are solved by a finite volume method and a fractional-step projection scheme. The particles are traced in the Lagrangian framework. It is found that the transition phenomenon of particle dispersion occurs during turbulence transition for particles at certain Stokes numbers. For particles at the intermediate Stokes numbers of 1 and 10, the dispersion changes from non-uniform to uniform patterns. These transition behaviors of particle dispersion reflect the self-selective mechanism between multi-scale structures in turbulent flows. (c) 2006 Elsevier B.V. All rights reserved.
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We focus in this paper on the effect of the resolution of direct numerical simulations (DNS) on the spatio-temporal development of the turbulence downstream of a single square grid. The aims of this study are to validate our numer...
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We focus in this paper on the effect of the resolution of direct numerical simulations (DNS) on the spatio-temporal development of the turbulence downstream of a single square grid. The aims of this study are to validate our numerical approach by comparing experimental and numerical one-point statistics downstream of a single square grid and then investigate how the resolution is impacting the dynamics of the flow. In particular, using the Q-R diagram, we focus on the interaction between the strain-rate and rotation tensors, the symmetric and skew-symmetric parts of the velocity gradient tensor,respectively. We first show good agreement between our simulations and hot-wire experiment for one-point statistics on the centreline of the single square grid. Then, by analysing the shape of the Q-R diagram for various streamwise locations, we evaluate the ability of under-resolved DNS to capture the main features of the turbulence downstream of the single square grid.
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Three-dimensional turbulent boundary layers under strong pressure gradients and curvature are the rule in real-world flow applications but are typically not well predicted by turbulence models due to isotropic eddy viscosity or eq...
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Three-dimensional turbulent boundary layers under strong pressure gradients and curvature are the rule in real-world flow applications but are typically not well predicted by turbulence models due to isotropic eddy viscosity or equilibrium assumptions. Validation-quality data in complex 3D flows is necessary for continued efforts to improve simulation accuracy. The Benchmark Validation Experiments for RANS/LES Investigations (BeVERLI) Hill bump model, designed specifically for validation experiments, was tested in the Virginia Tech Stability Wind Tunnel to collect validation experiment data on the three-dimensional (3D) boundary layer flow over a 3D hill. Laser Doppler velocimetry measurements on the bump model were used to study the mean flow and turbulence structure and evaluate the impact of pressure gradient and curvature upon the total shear stress in the boundary layer and evaluate the impact of pressure gradient and curvature upon the total shear stress behavior in the near-wall region. From analysis of the BeVERLI Hill flow, including the boundary layer just upstream of the hill, and comparison with the 3D flow around a wing-body junction of olcmen et al. (Exp Fluids 31:219-228, 2001), it is shown that none of the stations studied exhibit a constant shear stress region over any significant region of the boundary layer.
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Blowing a recorder at a low to moderate blowing speed with the toneholes all closed yields the lowest note in the range of the instrument. As the blowing speed is increased, the tone abruptly changes to the tone an octave higher. ...
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Blowing a recorder at a low to moderate blowing speed with the toneholes all closed yields the lowest note in the range of the instrument. As the blowing speed is increased, the tone abruptly changes to the tone an octave higher. This "jump" in the frequency of the dominant spectral component of the tone is referred to as "regime change." Interestingly, in conversations with recorder players, several have mentioned that regime change seems to occur at a significantly lower blowing speed for bass recorders than for instruments that sound an octave or more higher. In this paper we study regime change in the recorder and use Navier-Stokes modeling to confirm and study differences in the behavior of different instruments in the recorder family. We show, using modeling, how the regime change threshold in a model of the bass recorder can be increased by changing the geometry in the vicinity of the labium. These results are then confirmed through experimental studies of real recorders with designs inspired by the modeling results. The insights gained from these results suggest new recorder designs that may produce instruments that in some respects are more playable than current instruments.
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