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Jun 2007

Volume 19, Issue 6, Articles (06xxxx)

Issue Cover Spotlight Figure

Phys. Fluids 19, 067101 (2007); http://dx.doi.org/10.1063/1.2736341 (20 pages)

H. Salman and P. H. Haynes
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Pair velocity correlations among swimming Escherichia coli bacteria are determined by force-quadrupole hydrodynamic interactions

Qian Liao, Ganesh Subramanian, Matthew P. DeLisa, Donald L. Koch, and Mingming Wu

Phys. Fluids 19, 061701 (2007); http://dx.doi.org/10.1063/1.2742423 (4 pages) | Cited 14 times

Online Publication Date: 14 June 2007

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This Letter examines the pair velocity correlations among Escherichia coli (E. coli) bacteria swimming freely in a microfluidic channel. A large number of bacterial tracks are obtained using a particle tracking algorithm, and the longitudinal and transverse pair velocity correlation functions are evaluated. A theoretical analysis traces the origin of correlated motion between bacterial pairs to the translation of a bacterium driven by the force-quadrupole velocity disturbance caused by the swimming of neighboring bacteria. Both theory and experiments indicate that the longitudinal and transverse correlation functions are positive and negative, respectively.
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87.17.Jj Cell locomotion, chemotaxis
47.63.Gd Swimming microorganisms
87.18.Ed Cell aggregation
47.60.-i Flow phenomena in quasi-one-dimensional systems

The wimple: A rippled deformation of a wetting film during its drainage

Vladimir S. Ajaev, Roumen Tsekov, and Olga I. Vinogradova

Phys. Fluids 19, 061702 (2007); http://dx.doi.org/10.1063/1.2741151 (4 pages) | Cited 3 times

Online Publication Date: 19 June 2007

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It has long been accepted that hydrodynamic pressure in a draining fluid film can cause inversion of curvature of a fluid-fluid interface, creating the so-called dimple. However, it was recently discovered that a different shape, dubbed a wimple, can be formed if a bubble/drop is initially in the field of repulsive surface forces, so that a wetting film is formed. The film profile then includes a central region in which the film remains thin, surrounded by a ring of greater film thickness and bounded at the outer edge by a barrier rim. This shape later evolves to a conventional dimple, which then drains in the usual way. Here we carry out numerical simulations of the draining film evolution that allow us to uncover the physical mechanism responsible for wimple formation. Simple analytical estimates are then obtained for characteristic times of different stages of drainage, and are shown to be in good agreement with experimental data. We demonstrate that wimpling is a general phenomenon that can be encountered in many different systems.
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47.55.nd Spreading films
47.55.dr Interactions with surfaces
68.08.Bc Wetting
68.03.Cd Surface tension and related phenomena
47.11.-j Computational methods in fluid dynamics
68.15.+e Liquid thin films

Continuous breakdown of Purcell’s scallop theorem with inertia

Eric Lauga

Phys. Fluids 19, 061703 (2007); http://dx.doi.org/10.1063/1.2738609 (4 pages) | Cited 14 times

Online Publication Date: 21 June 2007

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Purcell’s scallop theorem defines the type of motions of a solid body—reciprocal motions—which cannot propel the body in a viscous fluid with zero Reynolds number. For example, the flapping of a wing is reciprocal and, as was recently shown, can lead to directed motion only if its frequency Reynolds number, Ref, is above a critical value of order one. Using elementary examples, we show the existence of oscillatory reciprocal motions which are effective for all arbitrarily small values of the frequency Reynolds number and induce net velocities scaling as Refα (α>0). This demonstrates a continuous breakdown of the scallop theorem with inertia.
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47.10.-g General theory in fluid dynamics
87.19.rs Movement
87.19.ru Locomotion

Spatially periodic reversing core in a twisted-fin generated swirling pipe flow

Cyrus K. Aidun and Mehran Parsheh

Phys. Fluids 19, 061704 (2007); http://dx.doi.org/10.1063/1.2738611 (4 pages) | Cited 1 time

Online Publication Date: 21 June 2007

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Experimental results for swirling turbulent flow in a pipe, generated by a 180° twisted fin inside the pipe, are presented. The results show that the core region undergoes a spatially periodic change in direction of rotation from counter-rotating to co-rotating and back to counter-rotating flow relative to the main swirling flow. Up to four transitions in direction of rotation have been recorded with a two-component laser-Doppler velocimeter downstream of the swirl generating fin. The Reynolds number is varied from 25 000 to 85 000 and the average swirl number varies from 0.5 to 0.25 downstream of the fin. The underlying cause of the periodicity in the direction of rotation of flow in the core region is conjectured to be based on the secondary flows generated by a pair of co-rotating helical vortices forming upstream at the spiral-shaped swirl generating fin.
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47.32.-y Vortex dynamics; rotating fluids
47.27.nf Flows in pipes and nozzles
47.60.-i Flow phenomena in quasi-one-dimensional systems

Turbulence generator using a precessing sphere

Susumu Goto, Nobukazu Ishii, Shigeo Kida, and Michio Nishioka

Phys. Fluids 19, 061705 (2007); http://dx.doi.org/10.1063/1.2746040 (4 pages) | Cited 14 times

Online Publication Date: 28 June 2007

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We propose a precessing sphere as a tabletop turbulence generator, which has less uncertainty in the setting of control parameters and the resulting high flow-reproducibility. The precession is realized by rotating the spin axis of a sphere around another axis (the precession axis). In our experiments, the two axes are fixed at right angles. The flow inside the sphere is governed only by two nondimensional parameters, one being Re (the Reynolds number defined by the maximum peripheral velocity around the spin axis) and the other being Γ (the rate of precession). The range of parameters for sustaining turbulence is revealed by the time-series analysis of velocity fields measured by particle image velocimetry. Well-developed turbulence can be sustained even for Γ of the order of a few percent when Re is beyond a few thousands.
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47.27.-i Turbulent flows
47.32.Ef Rotating and swirling flows
47.60.-i Flow phenomena in quasi-one-dimensional systems

Determination of the critical Shields number for particle erosion in laminar flow

Malika Ouriemi, Pascale Aussillous, Marc Medale, Yannick Peysson, and Élisabeth Guazzelli

Phys. Fluids 19, 061706 (2007); http://dx.doi.org/10.1063/1.2747677 (4 pages) | Cited 12 times

Online Publication Date: 28 June 2007

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We present reproducible experimental measurements for the onset of grain motion in laminar flow and find a constant critical Shields number for particle erosion, i.e., θ c = 0.12±0.03, over a large range of small particle Reynolds number: 1.5×10−5 ⩽ Rep ⩽ 0.76. Comparison with previous studies found in the literature is provided.
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47.57.Gc Granular flow
47.15.-x Laminar flows
45.70.Mg Granular flow: mixing, segregation and stratification
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back to top Viscous and Non-Newtonian Flows

A magnetically actuated ball valve applicable for small-scale fluid flows

Kristian Smistrup and Howard A. Stone

Phys. Fluids 19, 063101 (2007); http://dx.doi.org/10.1063/1.2717690 (9 pages) | Cited 3 times

Online Publication Date: 27 June 2007

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We present three possible designs for magnetically actuated ball valves that can be scaled down to nanometer length scales. Analytical expressions are presented for the hydraulic resistance of the ball valve as a function of geometric parameters and the state of the valve, and we also present analytical expressions for the hydrodynamic force on the magnetic bead that functions as the ball in the valve. We verify these expressions numerically and calculate the magnetic forces that can be exerted on the magnetic bead using the proposed structures. Finally, for typical parameters we show that these structures will be able to withstand a back pressure between 3 and 30 kPa regardless of the size of the bead/ball.
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85.70.Ay Magnetic device characterization, design, and modeling
47.85.Np Fluidics
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
47.61.Fg Flows in micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS)
back to top Particulate, Multiphase, and Granular Flows

Characteristics of transitional multicomponent gaseous and drop-laden mixing layers from direct numerical simulation: Composition effects

L. C. Selle and J. Bellan

Phys. Fluids 19, 063301 (2007); http://dx.doi.org/10.1063/1.2734997 (33 pages) | Cited 3 times

Online Publication Date: 22 June 2007

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Transitional states are obtained by exercising a model of multicomponent-liquid (MC-liquid) drop evaporation in a three-dimensional mixing layer at larger Reynolds numbers, Re, than in a previous study. The gas phase is followed in an Eulerian frame and the multitude of drops is described in a Lagrangian frame. Complete dynamic and thermodynamic coupling between phases is included. The liquid composition, initially specified as a single-Gamma (SG) probability distribution function (PDF) depending on the molar mass, is allowed to evolve into a linear combination of two SGPDFs, called the double-Gamma PDF (DGPDF). The compositions of liquid and vapor emanating from the drops are calculated through four moments of their PDFs, which are drop-specific and location-specific, respectively. The mixing layer is initially excited to promote the double pairing of its four initial spanwise vortices, resulting into an ultimate vortex in which small scales proliferate. Simulations are performed for four liquids of different compositions, and the effects of the initial mass loading and initial free-stream gas temperature are explored. For reference, simulations are also performed for gaseous multicomponent mixing layers for which the effect of Re is investigated in the direct-numerical-simulation–accessible regime. The results encompass examination of the global layer characteristics, flow visualizations, and homogeneous-plane statistics at transition. Comparisons are performed with previous pretransitional MC-liquid simulations and with transitional single-component (SC) liquid-drop-laden mixing layer studies. Contrasting to pretransitional MC flows, the vorticity and drop organization depend on the initial gas temperature, this being due to drop/turbulence coupling. The vapor-composition mean molar mass and standard deviation distributions strongly correlate with the initial liquid-composition PDF. Unlike in pretransitional situations, regions of large composition standard deviation no longer necessarily coincide with those of large mean molar mass. The rotational and composition characteristics are all liquid-specific and the variation among liquids is amplified with increasing free-stream gas temperature. The classical energy cascade is found to be of similar strength, but the smallest scales contain orders of magnitude less energy than SC flows, which is confirmed by the larger viscous dissipation for MC flows. The kinetic energy and dissipation are liquid-specific and the variation among liquids is amplified with increasing free-stream gas temperature. The gas composition, of which the first four moments are calculated, is shown to be close to, but distinct from, a SGPDF. Eulerian and Lagrangian statistics of gas-phase quantities show that the different observation framework may affect the perception of the flow.
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47.55.Ca Gas/liquid flows
47.27.ek Direct numerical simulations
47.55.D- Drops and bubbles
47.32.Ef Rotating and swirling flows
64.70.F- Liquid-vapor transitions
back to top Laminar Flows

Characteristics of two-dimensional flow around a rotating circular cylinder near a plane wall

Ming Cheng and Li-Shi Luo

Phys. Fluids 19, 063601 (2007); http://dx.doi.org/10.1063/1.2738608 (17 pages) | Cited 8 times

Online Publication Date: 12 June 2007

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We simulate a two-dimensional incompressible flow around a rotating circular cylinder near a plane wall at the Reynolds number Re = 200 by using the lattice Boltzmann equation with multiple relaxation times. We investigate the flow pattern in the parameter space of the rotational rate γ ≔ ωa/U and the normalized gap h ≔ H/D, where ω is the angular velocity of the cylinder, a and D are the cylinder radius and diameter, respectively, U is the inflow velocity, and H is the gap between the cylinder and the wall. We quantify the effects of γ and h on the hydrodynamic forces and the frequency of vortex shedding from the cylinder. Our results indicate that two critical values of h, hdown and hup, exist, which depend on γ. The flow is steady when h<hdown, while it has a wake of a regular vortex street when h>hup. When hdown<h<hup, the flow is aperiodic. We observe that the mean drag coefficient mathD is a monotonically increasing function of h when γ ⩽ 0. When γ>0, mathD is no longer a monotonic function of h. The mean drag coefficient mathD varies significantly in the range hdown<h<hup, and so do the root-mean-square values of the lift and drag coefficients, mathD and mathL. When h>hup, the wall effect diminishes.
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47.32.Ef Rotating and swirling flows
47.15.Cb Laminar boundary layers
47.11.Qr Lattice gas
47.32.ck Vortex streets
47.15.Fe Stability of laminar flows
47.15.Tr Laminar wakes
47.54.Bd Theoretical aspects

Slip and accommodation coefficients from rarefaction and roughness in rotating microscale disk flows

Danny Blanchard and Phil Ligrani

Phys. Fluids 19, 063602 (2007); http://dx.doi.org/10.1063/1.2739416 (12 pages) | Cited 7 times

Online Publication Date: 13 June 2007

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Accommodation coefficients are determined from experimental results and analysis based on the Navier-Stokes equations for rotation-induced flows in C-shaped fluid chamber passages formed between a rotating disk and a stationary surface. A first-order boundary condition is used to model the slip flow. The fluid chamber passage height ranges from 6.85 to 29.2 μm to give Knudsen numbers from 0.0025 to 0.031 for air and helium. In all cases, roughness size is large compared to molecular mean free path. The unique method presented for deducing tangential momentum accommodation coefficients gives values with less uncertainty compared to procedures that rely on flows in stationary tubes and channels. When channel height is defined at the tops of the roughness elements, slip velocity magnitudes and associated accommodation coefficients are a result of rarefaction at solid-gas interfaces and shear at the gas-gas interfaces. With this arrangement, tangential accommodation coefficients obtained with this approach decrease, and slip velocity magnitudes increase, at a particular value of Knudsen number, as the level of surface roughness increases. At values of the mean roughness height greater than 500 nm, accommodation coefficients then appear to be lower in air flows than in helium flows, when compared for a particular roughness configuration. When channel height is defined midway between the crests and troughs of the roughness elements, nondimensional pressure rise data show little or no dependence on the level of disk surface roughness and working fluid. With this arrangement, slip is largely independent of surface roughness magnitude and mostly due to rarefaction, provided the appropriate channel height is chosen to define the roughness height.
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47.45.Gx Slip flows and accommodation
47.32.Ef Rotating and swirling flows
47.10.ad Navier-Stokes equations
47.40.-x Compressible flows; shock waves
47.60.-i Flow phenomena in quasi-one-dimensional systems
07.10.Cm Micromechanical devices and systems
back to top Instability and Transition

Spatial resolution enhancement/smoothing of stereo–particle-image-velocimetry data using proper-orthogonal-decomposition–based and Kriging interpolation methods

Hasan Gunes and Ulrich Rist

Phys. Fluids 19, 064101 (2007); http://dx.doi.org/10.1063/1.2740710 (19 pages) | Cited 5 times

Online Publication Date: 12 June 2007

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Methods for data reconstruction and spatial enhancement of experimental data for a transitional boundary layer with laminar separation bubble are investigated. Particularly, proper orthogonal decomposition (POD) is applied to direct numerical simulation (DNS) data to extract the DNS-based POD modes, which are projected onto the experimental data (via a least-squares procedure) in order to obtain model coefficients. These model coefficients are then used to reconstruct, “interpolate,” and smooth the experimental data based on the DNS modes. In addition, in order to compare and assess the effectiveness of the present DNS-based procedure, Kriging interpolation is performed on the experimental (as well as numerical) data. These procedures are applied to time periodic (experimental) instantaneous spanwise vorticity (ωz) at a constant spanwise location. We have demonstrated that particle-image-velocimetry (PIV)-based POD modes can be smoothed by Kriging interpolation, thus a noise-free reconstruction of PIV data can be achieved. It is also found that for very low resolution experimental data, DNS-based interpolation is superior over Kriging interpolation. On the other hand, Kriging interpolation based on the Gaussian correlation model works very well for sufficiently high resolution experimental data. The correlation parameter can be used to control the degree of smoothness in the data reconstruction. Both procedures effectively eliminate the unwanted noise in the experimental data. One important difference between the two procedures is that, with quite some confidence, the DNS-based procedure can also be used for “extrapolation” since the model coefficients do not depend on spatial variation. In fact, we show that near-wall spanwise vorticity, which is not available from experimental data, can be recovered faithfully. Moreover, the enhancement (interpolation and smoothing) of full three-dimensional PIV data has been performed by Kriging interpolation employing a Gaussian correlation model.
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47.15.Cb Laminar boundary layers
47.11.-j Computational methods in fluid dynamics
47.55.D- Drops and bubbles
47.32.C- Vortex dynamics
02.60.Ed Interpolation; curve fitting

Holmboe modes revisited

O. M. Umurhan and E. Heifetz

Phys. Fluids 19, 064102 (2007); http://dx.doi.org/10.1063/1.2730544 (15 pages) | Cited 6 times

Online Publication Date: 27 June 2007

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A scaling analysis is presented better identifying the conditions in which the Boussinesq approximation may be used to study shear disturbances like that of Holmboe modes. The classic Holmboe normal mode instability is then reanalyzed by including baroclinic effects whose introduction alters the onset of Holmboe’s traveling-wave instability depending on the direction of the propagating modes. Since the introduction of baroclinicity is tantamount to relaxing the Boussinesq assumption, it means that in the presence of shear there is now a vertical variation of the horizontal momentum flux that alters the phase speed and structure of the classic Holmboe modes; the physical source of their broken right-left propagatory symmetry is associated with this physical effect. Furthermore, the regions of parameter space in which Holmboe’s classic analysis predicts there to be nonpropagating double instabilities now supports propagating Holmboe modes when baroclinic effects are included. We also find that a globally constant shear profile behaves as a stabilizing agent, in contradiction to the destabilizing role that shear normally plays in the classic Kelvin-Helmholtz problem of a shear-density interface. The general relationship between the normal modes of this type of system to that of the continuous spectrum is also noted. We also find that the baroclinic effects explored here probably do not manifest in terrestrial oceanographic and laboratory conditions, although they may do so in atmospheres.
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47.20.Ft Instability of shear flows (e.g., Kelvin-Helmholtz)
47.35.De Shear waves
47.20.Ma Interfacial instabilities (e.g., Rayleigh-Taylor)

Evolution of mushroom-type structures behind a heated cylinder

Maosheng Ren, Camilo Rindt, and Anton van Steenhoven

Phys. Fluids 19, 064103 (2007); http://dx.doi.org/10.1063/1.2741397 (11 pages)

Online Publication Date: 27 June 2007

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The three-dimensional transition in the wake flow behind a heated cylinder occurs at a much lower Reynolds number than for the unheated case. The three-dimensional transition is initialized in the near-wake by the formation of Λ-shaped structures and manifests itself in the far-wake as escaping mushroom-type structures from the upper vortices. In this study, both experimental and numerical techniques are used to investigate the origin and development of these mushroom-type structures. The formation of the mushroom-type structures is associated with the occurrence of Λ-shaped vortices in the near-wake. Hot fluid between the legs and the head of the Λ-shaped structure is lifted up. This lift-up process together with the action of buoyancy pulls out hot fluid from the upper vortex cores, resulting in a mushroom-type structure, which is comprised of a so-called stem and cap. Hot fluid is continuously transported through the stem to the advancing front of the mushroom-type structure. Finally, a pinch-off phenomenon is observed of the cap, ending up as a buoyant vortex ring. An analytical model is presented for the pinch-off process.
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47.15.Tr Laminar wakes
47.32.C- Vortex dynamics
47.15.Fe Stability of laminar flows
back to top Turbulent Flows

Inner and outer scalings in rough surface zero pressure gradient turbulent boundary layers

Brian Brzek, Raúl Bayoán Cal, Gunnar Johansson, and Luciano Castillo

Phys. Fluids 19, 065101 (2007); http://dx.doi.org/10.1063/1.2732439 (17 pages) | Cited 9 times

Online Publication Date: 1 June 2007

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A new set of experiments have been performed in order to study the effects of surface roughness and Reynolds number on a zero pressure gradient turbulent boundary layer. In order to properly capture the x dependence of the single point statistics, consecutive measurements of 11 streamwise locations were performed which enabled the use of the full boundary layer equations to calculate the skin friction. This quantity was obtained within 3% and 5% accuracy for smooth and rough surfaces, respectively. For the sand grain type roughnesses used, only the Zagarola and Smits scaling, U∞δ*/δ, was able to remove the effects of roughness and Reynolds number from the velocity profiles in outer variables. However, each scaling used for the velocity deficit profiles resulted in self-similar solutions for fixed experimental conditions. When examining the Reynolds stresses in the inner region [i.e., 0<(y+ϵ)+<0.1δ+], the 〈u2〉 component showed the largest influence of roughness, where the high peak near the wall was decreased and became nearly flat for the fully rough regime profiles. In addition, the Reynolds stresses in outer variables showed self-similarity for fixed experimental conditions. However, as the roughness parameter, k+ increases, all Reynolds stress profiles became similar in shape indicating increased isotropy near the wall. Furthermore, the boundary layer parameters also showed a considerable increase due to roughness.
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47.27.nb Boundary layer turbulence
47.53.+n Fractals in fluid dynamics
47.10.ad Navier-Stokes equations
47.27.Gs Isotropic turbulence; homogeneous turbulence

Direct numerical simulation of a plane turbulent wall-jet including scalar mixing

Daniel Ahlman, Geert Brethouwer, and Arne V. Johansson

Phys. Fluids 19, 065102 (2007); http://dx.doi.org/10.1063/1.2732460 (13 pages) | Cited 6 times

Online Publication Date: 7 June 2007

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Direct numerical simulation is used to study a turbulent plane wall-jet including the mixing of a passive scalar. The Reynolds and Mach numbers at the inlet are Re = 2000 and M = 0.5, respectively, and a constant coflow of 10% of the inlet jet velocity is used. The passive scalar is added at the inlet enabling an investigation of the wall-jet mixing. The self-similarity of the inner and outer shear layers is studied by applying inner and outer scaling. The characteristics of the wall-jet are compared to what is reported for other canonical shear flows. In the inner part, the wall-jet is found to closely resemble a zero pressure gradient boundary layer, and the outer layer is found to resemble a free plane jet. The downstream growth rate of the scalar is approximately equal to that of the streamwise velocity in terms of the growth rate of the half-widths. The scalar fluxes in the streamwise and wall-normal direction are found to be of comparable magnitude. The scalar mixing situation is further studied by evaluating the scalar dissipation rate and the mechanical to mixing time scale ratio.
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47.27.nb Boundary layer turbulence
47.27.wg Turbulent jets
47.27.wj Turbulent mixing layers
47.40.Dc General subsonic flows
47.53.+n Fractals in fluid dynamics
47.27.ek Direct numerical simulations

Unsteadiness of an axisymmetric separating-reattaching flow: Numerical investigation

Sébastien Deck and Pascal Thorigny

Phys. Fluids 19, 065103 (2007); http://dx.doi.org/10.1063/1.2734996 (20 pages) | Cited 24 times

Online Publication Date: 12 June 2007

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The separated flow over a cylinder elongated by another cylinder of a smaller diameter is investigated numerically at the high subsonic regime using zonal detached eddy simulation (ZDES) and compared with the experimental data of Deprés, Reijasse, and Dussauge [AIAA J. 42, 2541 (2004) ]. First, it is shown that this axisymmetric step flow has much in common with the two-dimensional facing step flows as regards the shear layer instability process. Second, the statistical and spectral properties of the pressure fluctuations are scrutinized. Close to the step, the surface pressure signature is characterized by low frequencies f.Lr/U∞ = O(0.08) (where Lr and U∞ denote, respectively, the mean reattachment length and free-stream velocity) and an upstream velocity of 0.26U∞ while in the second half-part of the recirculation higher frequencies fluctuations at f.Lr/U∞ ≈ 0.6 and a downstream convection velocity 0.6U∞ are the dominant features. The current calculation shows that the separated bubble dynamics depends on very complex interactions of large eddies formed in the upstream free shear layer with the wall in the reattachment region. These structures are shed with a nondimensional frequency of about 0.2. Besides, it has been observed that the secondary corner vortex experiences a cycle of growth and decay. The correspondence between the frequencies of this secondary corner vortex dynamics and the flapping motion (f.Lr/U∞ ≈ 0.08) suggests that there should be different aspects of the same motion. These results show that there is an ordered structure in this axisymmetric separating/reattaching flow which is dominated by large scale coherent motion. This is confirmed by a two-point correlation analysis of the pressure signals showing that the flow is dominated by highly coherent antisymmetric modes at the flapping and vortex shedding frequencies whose signatures are evidenced in the spectrum of the computed buffet loads. Possible onsets of a large-scale self-sustained motion of the separated area are finally discussed and the existence of an absolute instability of the axisymmetric recirculation bubble originating from a region located near the middle of the recirculating zone is conjectured.
Show PACS
47.20.Ib Instability of boundary layers; separation
47.32.Ff Separated flows
47.11.Kb Spectral methods
47.40.Dc General subsonic flows
47.55.dd Bubble dynamics

Effects of open-loop and closed-loop control on subsonic cavity flows

J. Little, M. Debiasi, E. Caraballo, and M. Samimy

Phys. Fluids 19, 065104 (2007); http://dx.doi.org/10.1063/1.2740302 (15 pages) | Cited 6 times

Online Publication Date: 12 June 2007

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This work presents an experimental investigation of the effects of open- and closed-loop control techniques on the flow structure and surface pressure signature in subsonic cavity flows. The cases include the uncontrolled (baseline) Mach 0.30 flow over a shallow cavity of aspect ratio 4 with Reynolds number based on the cavity depth of 105, and four actively controlled flows. The controlled cases include open-loop at two discrete frequencies and two closed-loop cases: parallel proportional with time delay and reduced-order model-based linear quadratic. Measurements and analyses include particle image velocimetry, spectra and spectrograms of surface pressure and velocity fluctuations, flow visualization, and proper orthogonal decomposition. Data are presented and analyzed in an effort to better understand the behavior of the cavity flow in response to a variety of actuation cases. Results show that both open- and closed-loop control have significant effects on the flow dynamics and surface pressure behavior. In addition, the results reveal substantial differences between the effects of each type of open-loop and closed-loop control.
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47.85.L- Flow control
47.40.Dc General subsonic flows
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.80.Jk Flow visualization and imaging

Near-wall passive scalar transport at high Prandtl numbers

Robert Bergant and Iztok Tiselj

Phys. Fluids 19, 065105 (2007); http://dx.doi.org/10.1063/1.2739402 (18 pages) | Cited 8 times

Online Publication Date: 13 June 2007

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Very accurate numerical simulations of a passive scalar field in the turbulent channel and flume flow were performed at friction Reynolds numbers Reτ = 150 and Reτ = 395 and Prandtl numbers Pr = 100, Pr = 200. Direct numerical simulation is used for description of the velocity field. The temperature field is described with the LES-like approach with the smallest resolved temperature scales equal to the smallest scales of the velocity field. The consistency of the applied physical modelling and pseudospectral scheme is first tested with comparison of the results with the existing DNS simulations of F. Schwertfirm and M. Manhart [Proceedings of Turbulence, Heat, and Mass Transfer (2006) ] at Reτ = 180 and Pr = 25. The sensitivity of the method to the grid refinement and time step variations is performed with simulations at Reτ = 150 and Pr = 200. Both tests show that the proposed approach produces very accurate mean temperature profiles, heat transfer coefficients, and other low-order moments of the turbulent thermal field. It is shown that the mean temperature profiles near the wall can be accurately predicted even when the temperature scales between the Batchelor and Kolmogorov scale are not resolved. The key to the success of the proposed approach lies in the fact that the large-scale structures govern the turbulent heat transfer at high Prandtl numbers. Resolved spectra of the temperature fluctuations and the rms temperature fluctuations in the diffusive sublayer and the thermal buffer layer (y+<5) are practically unaffected by the unresolved temperature scales. The contribution of the sub-Kolmogorov thermal scales becomes relevant above the thermal buffer layer (y+>5), where the unresolved temperature scales affect spectra and rms temperature fluctuations, but not the log-law shape of the mean temperature profile and the mean heat transfer coefficient. Further results are obtained at Reτ = 150, Pr = 100, Pr = 500, and Reτ = 395, Pr = 100, Pr = 200. These results are compared with Kader empirical temperature profiles and other available experimental and numerical results. Significant difference in the mean temperature profiles is demonstrated between the profiles calculated at friction Reynolds numbers 150 and 395. Kader correlation is shown to be very accurate at higher Reynolds number but underpredicts temperatures at low Reynolds number.
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47.27.nd Channel flow
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.27.E- Turbulence simulation and modeling
47.27.te Turbulent convective heat transfer

Particle image velocimetry study of turbulent flow over transverse square ribs in an asymmetric diffuser

Mark F. Tachie

Phys. Fluids 19, 065106 (2007); http://dx.doi.org/10.1063/1.2738610 (15 pages) | Cited 7 times

Online Publication Date: 14 June 2007

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The objective of this paper is to study the combined effects of rib roughness and adverse pressure gradient produced in an asymmetric diffuser on turbulent flows. The two-dimensional asymmetric diffuser was comprised of a straight flat floor and a curved roof. The diffuser section was preceded and followed by straight parallel walls. The complete test conditions were comprised of a reference smooth floor and repeated arrays of transverse square ribs glued onto the floor to produce three pitch-to-height ratios, p/k = 3, 6, and 8. The curved roof was kept smooth in all the experiments. For each of the four test conditions, a particle image velocimetry was used to conduct detailed velocity measurements within the diverging section and also at locations upstream and downstream of the diverging section. From these measurements, the mean streamlines, mean velocities, turbulent intensities, Reynolds shear stress, and production terms in the transport equations for the turbulent kinetic energy and Reynolds stresses were obtained. The results obtained in the diverging section showed that the boundary layers that developed on the ribs thickened considerably at the expense of those adjacent to the roof opposite to the ribs. The streamlines and mean velocity profiles over the ribs showed that adverse pressure gradient increased the roughness sublayer substantially. Adverse pressure gradient and rib roughness also increased the drag and levels of the turbulent intensities, Reynolds shear stress and production terms compared to smooth-wall zero pressure gradient turbulent boundary layer. It appears, however, that adverse pressure gradient enhanced turbulence more effectively than it increased drag.
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47.27.nb Boundary layer turbulence
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.80.Jk Flow visualization and imaging

Evolution of a forced stratified mixing layer

J. Rotter, H. J. S. Fernando, and E. Kit

Phys. Fluids 19, 065107 (2007); http://dx.doi.org/10.1063/1.2740305 (10 pages)

Online Publication Date: 14 June 2007

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Laboratory measurements were carried out in a spatially developing stably stratified shear layer generated downstream of a splitter plate. The instabilities were controlled using a flapper spanning the entire shear layer, with the flapper forced at the fastest growing frequency of the primary [Kelvin-Helmholtz (KH)] instability. The measurements were taken as the KH instabilities roll up, break down, and degenerate into stratified turbulence. Both stratified and homogeneous shear layers were considered, the latter acting as the “baseline” case. The measurements included the streamwise and vertical velocities (made using X-wire hot film probes), which allowed calculation of the mean and rms velocities, turbulent kinetic energy (TKE) dissipation, and TKE production. The density and its gradients were measured using miniature conductivity probes. The measurements and flow visualization elicited interesting features of KH evolution, namely that KH billows may be turbulent from the onset, the TKE dissipation is largest at early stages of evolution, the production of TKE is a maximum at the breakdown of billows, the decay of turbulence to fossilized motions and concomitant formation of fine (layered) structure occur rapidly after the breakdown of billows, and episodic rebirth of (zombie) turbulence develops before a final permanently fossilized state is achieved.
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47.55.Hd Stratified flows
47.20.Ft Instability of shear flows (e.g., Kelvin-Helmholtz)
47.27.wj Turbulent mixing layers
47.80.Jk Flow visualization and imaging
47.35.De Shear waves
47.85.L- Flow control

Statistical analysis of small bubble dynamics in isotropic turbulence

Murray R. Snyder, Omar M. Knio, Joseph Katz, and Olivier P. Le Maître

Phys. Fluids 19, 065108 (2007); http://dx.doi.org/10.1063/1.2729733 (25 pages) | Cited 6 times

Online Publication Date: 14 June 2007

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The dynamics and dispersion of small air bubbles in isotropic turbulence are analyzed computationally. The flow field is simulated using a pseudospectral code, while the bubble dynamics are analyzed by integration of a Lagrangian equation of motion that accounts for buoyancy, added mass, pressure, drag, and lift forces. Probability density functions (pdfs) of bubble velocities, lift and drag forces, and of field velocities and vorticities along bubble trajectories are used to analyze bubble dynamics. Lagrangian bubble trajectories are also employed to determine dispersion characteristics, following the theoretical development of Cushman and Moroni [Phys. Fluids 13, 75 (2001) ]. Consistent with available experimental data, bubble rise velocities are increasingly suppressed with increasing turbulence intensity. The analysis also reveals that the vertical bubble velocities are characterized by asymmetric pdfs that are positive or negative-skewed dependent upon the nondimensional turbulence intensity and the Taylor length scale. The role of the lift force in moving the bubbles to the down-flow side of turbulent eddies, and consequently retarding their rise, is consistently observed in all analyses. The dispersion of 40 μm bubbles and transition to Fickian behavior is shown to be weakly affected by the turbulence level. Larger, 400 μm bubbles are shown to be more sensitive to turbulence level with transition to Fickian behavior delayed in low turbulence fields.
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47.55.dd Bubble dynamics
47.27.Gs Isotropic turbulence; homogeneous turbulence
47.32.cb Vortex interactions
47.27.tb Turbulent diffusion
47.11.Kb Spectral methods
47.85.Np Fluidics

Two-way coupling of finitely extensible nonlinear elastic dumbbells with a turbulent shear flow

Thomas Peters and Jörg Schumacher

Phys. Fluids 19, 065109 (2007); http://dx.doi.org/10.1063/1.2735562 (12 pages) | Cited 7 times

Online Publication Date: 21 June 2007

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We present numerical studies for finitely extensible nonlinear elastic dumbbells which are dispersed in a turbulent plane shear flow at moderate Reynolds number. The polymer ensemble is described on the mesoscopic level by a set of stochastic ordinary differential equations with Brownian noise. The dynamics of the Newtonian solvent is determined by the Navier-Stokes equations. Momentum transfer of the dumbbells with the solvent is implemented by an additional volume forcing term in the Navier-Stokes equations, such that both components of the resulting viscoelastic fluid are connected by a two-way coupling. The dynamics of the dumbbells is given then by Newton’s second law of motion including small inertia effects. We investigate the dynamics of the flow for different degrees of dumbbell elasticity and inertia, as given by Weissenberg and Stokes numbers, respectively. For the parameters accessible in our study, the magnitude of the feedback of the polymers on the macroscopic properties of turbulence remains small as quantified by the global energy budget and the Reynolds stresses. A reduction of the turbulent drag by up to 20% is observed for the larger particle inertia. The angular statistics of the dumbbells shows an increasing alignment with the mean flow direction for both, increasing elasticity and inertia. This goes in line with a growing asymmetry of the probability density function of the transverse derivative of the streamwise turbulent velocity component. We find that dumbbells get stretched preferentially in regions where vortex stretching or biaxial strain dominate the local dynamics and topology of the velocity gradient tensor.
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47.27.W- Boundary-free shear flow turbulence
47.50.Cd Modeling
47.10.ad Navier-Stokes equations
83.50.Ax Steady shear flows, viscometric flow
83.10.Mj Molecular dynamics, Brownian dynamics
47.32.C- Vortex dynamics
02.50.Ey Stochastic processes

A dynamic global-coefficient subgrid-scale eddy-viscosity model for large-eddy simulation in complex geometries

Donghyun You and Parviz Moin

Phys. Fluids 19, 065110 (2007); http://dx.doi.org/10.1063/1.2739419 (8 pages) | Cited 22 times

Online Publication Date: 29 June 2007

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An improvement of the dynamic procedure of Park et al. [Phys. Fluids 18, 125109 (2006)] for closure of the subgrid-scale eddy-viscosity model developed by Vreman [Phys. Fluids 16, 3670 (2004)] is proposed. The model coefficient which is globally constant in space but varies in time is dynamically determined assuming the “global equilibrium” between the subgrid-scale dissipation and the viscous dissipation of which utilization was proposed by Park et al. Like the Vreman model with a fixed coefficient and the dynamic-coefficient model of Park et al., the present model predicts zero eddy-viscosity in regions where the vanishing eddy viscosity is theoretically expected. The present dynamic model is especially suitable for large-eddy simulation in complex geometries since it does not require any ad hoc spatial and temporal averaging or clipping of the model coefficient for numerical stabilization and more importantly, requires only a single-level test filter in contrast to the dynamic model of Park et al., which employs two-level test filters.
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47.11.-j Computational methods in fluid dynamics
back to top Compressible Flows

Nonequilibrium reaction rates in the macroscopic chemistry method for direct simulation Monte Carlo calculations

M. J. Goldsworthy, M. N. Macrossan, and M. M. Abdel-jawad

Phys. Fluids 19, 066101 (2007); http://dx.doi.org/10.1063/1.2742747 (9 pages) | Cited 3 times

Online Publication Date: 29 June 2007

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The direct simulation Monte Carlo (DSMC) method is used to simulate the flow of rarefied gases. In the macroscopic chemistry method (MCM) for DSMC, chemical reaction rates calculated from local macroscopic flow properties are enforced in each cell. Unlike the standard total collision energy (TCE) chemistry model for DSMC, the new method is not restricted to an Arrhenius form of the reaction rate coefficient, nor is it restricted to a collision cross section which yields a simple power-law viscosity. For reaction rates of interest in aerospace applications, chemically reacting collisions are generally infrequent events and, as such, local equilibrium conditions are established before a significant number of chemical reactions occur. Hence, the reaction rates which have been used in MCM have been calculated from the reaction rate data which are expected to be correct only for conditions of thermal equilibrium. Here we consider artificially high reaction rates so that the fraction of reacting collisions is not small and propose a simple method of estimating the rates of chemical reactions which can be used in the MCM in both equilibrium and nonequilibrium conditions. Two tests are presented: (1) The dissociation rates under conditions of thermal nonequilibrium are determined from a zero-dimensional Monte Carlo sampling procedure which simulates “intramodal” nonequilibrium; that is, equilibrium distributions in each of the translational, rotational, and vibrational modes but with different temperatures for each mode; (2) the 2D hypersonic flow of molecular oxygen over a vertical plate at Mach 30 is calculated. In both cases the new method produces results in close agreement with those given by the standard TCE model in the same highly nonequilibrium conditions. We conclude that the general method of estimating the nonequilibrium reaction rate is a simple means by which information contained within nonequilibrium distribution functions predicted by the DSMC method can be included in the macroscopic chemistry method.
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47.70.Fw Chemically reactive flows
47.45.-n Rarefied gas dynamics
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A numerical study of passive scalar evolution in peripheral regions

H. Salman and P. H. Haynes

Phys. Fluids 19, 067101 (2007); http://dx.doi.org/10.1063/1.2736341 (20 pages) | Cited 8 times

Online Publication Date: 12 June 2007

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We study the effect of slip and no-slip wall boundaries on the decay rate of a passive scalar in a spatially smooth and random in time velocity field. Numerical simulations are carried out to verify the effect of the peripheral (near-wall) regions on the decay of the scalar variance. Using two kinematic flow models with simple velocity fields, we show that, in the case of slip boundaries, the passive scalar is characterized by an initial rapid stirring followed by an exponential decay of the scalar variance. In stark contrast, results for the case with no-slip boundaries show that, following an initial rapid stirring of the scalar within the bulk, there is an intermediate-time regime where the variance follows a power-law decay. This intermediate regime is established as a result of the trapping of the scalar in the peripheral regions near the no-slip walls. Finally, the behavior of the scalar variance switches to a final regime that is characterized by an exponential decay rate. The results presented here indicate that the recent ensemble-based theories regarding the evolution of a passive scalar in the peripheral regions correctly predict the main stages of the scalar evolution that arise in a single flow realization.
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47.11.-j Computational methods in fluid dynamics
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