bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulation of Separation Control in Axisymmetric Diffuser Flow
Authors: Wassen Erik P. , Thiele Frank
Published: February 2009

Large–Eddy Simulations were performed for the separated turbulent flow in an axisymmetric diffuser. An active control technique was investigated in order to decrease the size of the separation region. The technique was based on periodic perturbation using sinusoidal blowing and suction through slits on the diffuser’s circumference. The slit position and slit geometry were varied as well as the frequency and the amplitude of the control flow. The length of the separation region could be reduced by up to 37%, along with a 20% reduction of pressure loss in the diffuser. A vortex visualization technique was applied to detect changes in the turbulent structures of the flow in the axisymmetric expansion.
Simulations of Complex Flows and Fluid-Structure Interaction Problems on Fixed Cartesian Grids
Authors: Najjar Fady M. , Mittal Rajat
Published: February 2009

A finite-difference based approach for computing flows with complex moving solid three-dimensional boundaries on fixed Cartesian grid has been developed. Internal solid boundaries are represented by “blocking off” the grid cells inside the boundary. This results in considerably increased computing efficiency over conventional body-conformal structured grid methods. A mixed explicit-implicit fractional step method is employed for time integration while the spatial discretization scheme is based on a second-order accurate central-difference scheme. The pressure Poisson equation is solved using algebraic multigrid as well as Krylov subspace based methods. The current simulation methodology is validated by simulating various canonical flows. Further, we compute the flow generated by a moving body as well as the flow generated by a synthetic jet in order to demonstrate the capabilities of this solver.
Flow Separation Within the Engine Inlet of an Uninhabited Combat Air Vehicle (UCAV)
Authors: Brear Michael J. , Warfield Zachary , Mangus John F. , Braddom Steve , Paduano James D. , Philhower Jeffry S.
Published: February 2009

This paper discusses the structure of the flow within the engine inlet of an uninhabited combat air vehicle (UCAV). The UCAV features a top-mounted, serpentine inlet leading to an engine buried within the fuselage. The performance of the inlet is found to depend strongly on a flow separation that occurs within the inlet. Both the time-averaged and the unsteady structure of this separation is studied, and an argument relating the inlet performance to the behaviour of this separation is suggested. The results presented in this paper also suggest that there are considerable aerodynamic limitations to further shortening or narrowing of the inlet. Since there are substantial, system level benefits from using a smaller inlet, the case for separated flow control therefore appears clear.
The Effect of Wake Passing on a Flow Separation in a Low Pressure Turbine Cascade
Authors: Brear Michael J. , Hodson Howard P.
Published: February 2009

This paper describes an investigation into the effect that passing wakes have on a separation bubble that exists on the pressure surface and near the leading edge of a low pressure turbine blade. Previous experimental studies have shown that the behaviour of this separation is strongly incidence dependent and that it responds to its disturbance environment. The results presented in this paper examine the effect of wake passing in greater detail. Two dimensional, Reynolds averaged, numerical predictions are first used to examine qualitatively the unsteady interaction between the wakes and the separation bubble. The separation is predicted to consist of spanwise vortices whose development is in phase with the wake passing. However, comparison with experiments shows that the numerical predictions exaggerate the coherence of these vortices and also overpredict the time-averaged length of the separation. Nonetheless, experiments strongly suggest that the predicted phase locking of the vortices in the separation onto the wake passing is physical.
Towards Sensing and Control of Separation in Subsonic Flows
Authors: Young Monica J. , Glauser Mark N. , Higuchi Hiroshi , Taylor Jeffrey
Published: February 2009

The purpose of this study is to validate the use of Proper Orthogonal Decomposition POD and Modified Linear Stochastic Estimation mLSE based low-dimensional methods to model an external flow over a NACA 4412 airfoil. By using a combination of Particle Image Velocimetry PIV and multiple airfoil surface pressure measurements, the full velocity field (mean plus fluctuating) is estimated through implementation of a modified complementary technique. We will identify a low-dimensional mean flow just from the wall pressure, specifically observing when the profiles are at the incipient condition. This gives a reasonable estimate of the low-dimensional velocity field. The importance of this work lies in that the flow is estimated from the wall pressure only, providing a practical means for estimating the flow state. This is particularly important for flow control applications.
Asymmetry and Unsteadiness of Incipient Separation of Nominally Axisymmetric Turbulent Boundary Layer
Authors: Masuda Shigeaki , Takagi Rei , Okada Kouhei , Obi Shinnosuke
Published: February 2009

Unsteady and asymmetric behavior of separation of nominally axisymmetric and steady turbulent boundary layer in an axisymmetric expansion is investigated. The test diffusers have the total divergence angle from 8 to 50deg and length-to-diameter ratio of 8. Bulk Reynolds number is ranging between 2.5 × 104 and 2.4 × 105. The wall static pressure fluctuation increases monotonically with increasing divergence angle up to 25° and then decreases. The wall static pressure exhibits large scale law frequency fluctuations, occurring almost concurrently in the entire passage. Distributions of both mean velocity and reverse flow fraction in a cross-stream plane show axial-asymmetry of the separated flow, suggesting the rapid growth of circumferential non-uniformity upstream. The velocity measurements employing a split film probe to distinguish the flow directions exhibits intermittent flow reversal with higher velocity as compared with forward flow. The conditional average of wall hot film signals reveals the steep decrease at the incipience of flow reversal.
Design and Uncertainty Analysis of Oil Flow Standard System
Authors: Lim Ki Won
Published: February 2009

A gravimetric flow standard system is established and in operation for the calibration of flowmeters with light oil (Calibration Oil K, Castrol). The Light Oil Flow Standard System (LOFSS), as one of the national flow standard system in Korea, can provide a direct primary calibration for any conventional oil flowmeter. The system, which is operated by the standing start and finish mode with the static weighing method, measures mass electronically from a gyroscopic scale. The flow of up to 350 m3/h is generated by two pumps and the main test line is designed with a 200 mm pipe diameter. For the uncertainty analysis of flow measurement by LOFSS, the equilibrium equation based on the calibration principle of the standard system was derived. The standard flow quantity is determined from the independent variables. Therefore the uncertainty of the independent variables were scrutinized with consideration of the operation conditions and combined for the expression of the uncertainty of the flow quantity determination. The evaluation procedure method follows the ISO Guide to the Expression of Uncertainty in Measurement. The expanded uncertainties in volume and mass flow measurement in the range of 18 and 350 m3/h are 0.04 and 0.01%, respectively. The uncertainty budget shows the contribution of the uncertainty of the independent variables to that of the flow quantity measurement.
Considerations for ISO 9300: The Effects of Roughness and Form on the Discharge Coefficient of Toroidal-Throat Sonic Nozzles
Authors: Gibson Jeff , Stewart David , Barton Neil
Published: February 2009

The effect of roughness and form was investigated for toroidal-throat sonic nozzles, using both experimental and computational techniques. Eight, 10 mm diameter nozzles were specially made to investigate the effects of deviating from ISO 9300; these nozzles were measured for diameter, axial profile, roughness and roundness in order to assess the effects on discharge coefficient, Cd. Flow calibrations have revealed that roughness effects are negligible for Ra/d ≤ 4.5×10−5 (three times the limit of ISO 9300) in the turbulent regime (Re > 106), in keeping with previously published data taken in the laminar regime. Test data from three unpolished nozzles, of average Ra of ∼ 0.3 μm, also demonstrated that excellent reproducibility in Cd vs. Re (≤ 0.04%) might be achieved without the need for polishing. Data is also presented from recent primary gravimetric calibrations of NEL’s 16 reference nozzles. The data supports the proposed equation for normally machined nozzles over the Reynolds number range tested (0.17×106 < Re < 17×106), with the standard deviation from the ISO/CD 9300 curve being 0.27% (k = 2; 95% confidence) over the range 0.35×106 < Re < 17×106, provided d ≥ 3.8 mm.
A Design Method of an Orifice Flowmeter With Separable Diffuser for Air Flow
Authors: Nishi Michihiro , Okamoto Minoru , Watanabe Atsushi , Kanzaki Junya , Inoue Tsuyoshi , Yoshida Kouichi
Published: February 2009

An orifice is a good tool to measure flow through pipeline systems. It will be a better one, if its loss is sufficiently reduced. Authors proposed a method to install a diffuser component downstream of an orifice plate for loss reduction. This orifice flowmeter with a separable diffuser will be widely applicable to the flow measurement, if the strong noise is reduced in an air flow system. For this purpose, we have computed the steady flow from an orifice by a turbulent flow analysis and decided the inlet diameter of downstream diffuser for those orifice plates with diameter ratios of 0.4, 0.5 and 0.7 respectively. This paper shows typical experimental results, which show the present design method is better than the previous one.
Velocity Measurements of Shear Flows by a Novel Velocity Profile Sensor With Micrometer Spatial Resolution
Authors: Czarske Ju¨rgen , Bu¨ttner Lars , Razik Thorsten , Mu¨ller Harald , Dopheide Dietrich , Becker Stefan , Durst Franz
Published: February 2009

A measuring system based on a differential laser-Doppler velocimeter has been extended to determine one-dimensional velocity profiles with a spatial resolution inside the measurement volume. The principle of the velocity profile sensor is based on the generation of two fringe systems with different gradients of the fringe spacings. The determination of the corresponding two Doppler frequencies yields the position as well as the velocity component of individual tracer particles, which results in the velocity profile for detecting several particles. The sensor was used to determine velocity profiles of flat-plate laminar boundary layers for varying Reynolds numbers. A precise determination of the wall shear stress was accomplished. All results are in good agreement with the theory. The main application of the velocity profile sensor is the spatial high-resolved investigation of turbulent boundary layers.
Performance Prediction for Optical Wall Shear Stress Sensor Using Direct Numerical Simulation of Fluid Flow
Authors: Shirai Katsuaki , Tsuru Keisuke , Obi Shinnosuke
Published: February 2009

We conducted a performance prediction for an optical wall shear stress sensor with using the velocity data of a direct numerical simulation. The Doppler signals were generated with respect to the path of tracer particles passing through the measurement volume. A signal processing technique was proposed to estimate the magnitude and yaw angle of local wall shear stress simultaneously from each Doppler signal. The simulated Doppler signals were processed with the technique, however the accuracy of estimating the yaw angle is not sufficient. In contrast, the estimated magnitude of wall shear stress showed a good agreement with the direct estimate from the DNS data if the yaw angle was accurately estimated. The measurement accuracy of the sensor mainly depends on estimating the yaw angle of each tracer particle. Another technique for detecting the yaw angle is needed for the accurate measurement of both the yaw angle and magnitude of local wall shear stress.
A Versatile Fully Submersible Stereo-PIV Probe for Tow Tank Applications
Authors: Pereira Francisco , Costa Tiziano , Felli Mario , Calcagno Guido , Di Felice Fabio
Published: February 2009

A unique, highly modular and flexible underwater system for stereoscopic particle image velocimetry (PIV) measurements has been designed, manufactured and tested. The instrument is intended for planar three-dimensional velocity measurements in large facilities such as water tow tanks and tunnels. The performance of the system is assessed in four major stereoscopic configurations. Errors under 2% for the in-plane components and 4% for the out-of-plane components are found. The system is tested in the INSEAN large circulating water channel where the measurement of the flow around a model ship oriented at a moderate yaw angle is performed and puts into evidence the main features of the flow.
Application of a Split-Fiber Probe to Velocity Measurement in the NASA Research Compressor
Authors: Lepicovsky J.
Published: February 2009

A split-fiber probe was used to acquire unsteady data in a research compressor. The probe has two thin films deposited on a quartz cylinder 200 μm in diameter. A split-fiber probe allows simultaneous measurement of velocity magnitude and direction in a plane that is perpendicular to the sensing cylinder, because it has its circumference divided into two independent parts. Local heat transfer considerations indicated that the probe direction characteristic is linear in the range of flow incidence angles of ±35 dg. Calibration tests confirmed this assumption. Of course, the velocity characteristic is nonlinear as is typical in thermal anemometry. The probe was used extensively in the NASA GRC low-speed, multistage axial compressor, and worked reliably during a test program of several months duration. The velocity and direction characteristics of the probe showed only minute changes during the entire test program. An algorithm was developed to decompose the probe signals into velocity magnitude and velocity direction. The averaged unsteady data were compared with data acquired by pneumatic probes. An overall excellent agreement between the averaged data acquired by a split-fiber probe and a pneumatic probe boosts confidence in the reliability of the unsteady content of the split-fiber probe data. To investigate the features of unsteady data, two methods were used: ensemble averaging and frequency analysis. The velocity distribution in a rotor blade passage was retrieved using the ensemble averaging method. Frequencies of excitation forces that may contribute to high cycle fatigue problems were identified by applying a fast Fourier transform to the absolute velocity data.
Experimental Study of Mixing Between a Heated Turbulent Flow and a Cold Airstream Transpirated Through a Porous Wall
Authors: Michard M. , Ait Ameur M. , Brillant G. , Bataille F.
Published: February 2009

Hot air flows along a porous wall, which is submitted to a secondary flow of cold air. Measurements, carried out by hot and cold wires, enable to evaluate the aerothermic characteristics of the turbulent boundary layer. An increase of the blowing rate ensures good average thermal protection effectiveness. However, the average and turbulent aerodynamic properties of the turbulent boundary layer are damaged. The analysis of the results close to the wall shows the existence of thermal intermittencies, which are intensified when the blowing rate goes up. This observation shows a strong interaction between the protective film that is created by the blowing process above the porous wall and the turbulent boundary layer of the main flow.
Turbulent Flow Over a NACA 4412 Airfoil at Angle of Attack 15 Degree
Authors: Badran O. O. , Bruun H. H.
Published: February 2009

This paper presents the measured mean flow and Reynolds stresses results, obtained on the center-line plane of the airfoil, covering the boundary layers over the upper surface, the potential flow region and the wake downstream of the trailing edge, at αa = 15°. The flying X-hot-wire probe was used to measure the U and V components of the flow field over the airfoil. An improved understanding of the physical characteristics of separation on the airfoil sections and in the region of the trailing edge is of direct value for the improvement of high lift wings for aircraft. From the study of the separation flow at angle of attack αa = 15°, the following can be concluded: (1) An intermittent reverse flow region occurred near the trailing edge of the airfoil. A separation bubble occurred for a short period of time and was then swept away with the stream wise flow. (2) The angle of attack αa = 15° corresponds to the position of maximum lift for a NACA 4412 airfoil section. (3) It is found that values of the Reynolds normal and shear stresses move away from the surface with downstream distance, and (4) In the wake region, relatively large values of Reynolds stresses occurred, which were related to the vertical oscillation in the lower wake.
Simultaneous Measurements of Velocity and Fluctuating Static-Pressure in a Circular Jet
Authors: Hiramoto Riho , Toyoda Kuniaki , Mori Hayato
Published: February 2009

Velocity and fluctuating static-pressure were measured simultaneously in a circular jet in order to discuss the pressure transport of turbulent energy and the generation mechanism of aerodynamic sound in relation to vortical structure. A combined sensor, which consists of X-type hot-wire and pressure probe, was used for the simultaneous measurements. The air jet was issued from a sharp-edged circular orifice under the excitation at the interaction mode so that velocity and fluctuating static-pressure can be determined in space and time domains by using phase-average technique. The results suggest that the simultaneous measurements with the phase-average technique give us useful information to reveal the turbulent shear flow phenomena. In particular, the pressure transport is closely related to vortical structure, and intense sound sources are caused by the vortex merging and by the acceleration of vortices during vortex pairing.
An Improved Technique to Measure Axial Variation in Wall Flow Along the Channels of a Diesel Particulate Filter
Authors: Subbu Ganapathi , Collings Nick
Published: February 2009

A technique to measure wall flow variation in Diesel Particle Filters (DPFs) is described. In a recent paper, it was shown how the flow distribution in DPFs could be measured in a non-destructive manner. This involved measuring the progressive dilution of a tracer gas introduced at the “outlet” channel upstream end. In the present paper, a significant further improvement to this technique is described, in which only a single probe is required, rather than the two of the previous technique. The single, traversable, probe consists of a controllable flow sink, and slightly downstream, a tracer gas supply. By controlling the sink flow rate such that a very small concentration of tracer gas is aspirated into it, the total flow up to that location in the channel is determined. Typical results showing the axial variation in the wall flow for known wall blockage cases are presented. It is suggested that this technique could be used to interpret the soot loading in the filter channels in a non-intrusive way.
Anaytical Study of the Oscillating Channel Flow Solution With Application to the Turbulent Case
Authors: Poroseva Svetlana V. , Girimaji Sharath S.
Published: February 2009

In turbulent channel flows with imposed pressure-gradient oscillations the extension of the Stokes second problem solution has been used as a laminar benchmark. The present study shows that the Stokes solution is an appropriate approximation to the laminar channel flow solution only in the limit of an infinitely wide channel or very high frequencies of imposed oscillations. In a finite-width channel with finite-frequency oscillations, the exact channel flow solution should be used. The paper provides the thorough examination of the spatio-frequency variations of such solution. Different frequency regimes are described. It is shown that the borders between these regimes vary depending on the distance from a channel wall and a flow characteristic considered. Difference in high-frequency regimes in profiles of the phase and the amplitude is pointed out. Changes in the behavior of the laminar oscillating flow characteristics in the presence of the turbulent mean flow are considered. Oscillating channel and pipe flows are qualitatively compared.
Effect of Blockage Factor on Thermal-Fluid Flow Transport Phenomenon Over Slot-Perforated Flat Surface in Channel
Authors: Torii Shuichi , Umeda Shinzaburo , Yang Wen-Jei
Published: February 2009

A numerical and experimental study is performed to investigate unsteady, two-dimensional, incompressible laminar flow over both sides of a slot-perforated flat surface, which is placed in a channel. Enhances is placed on the effect of the blockage factor, i.e., the ratio of plate thickness, δ, to channel width, W, on the heat transfer performance and the velocity and thermal fields. It is found from the study that: (i) when the slot width is increased, the alternating change in the fluid flow disturbs the thermal boundary layer formed along the plate and induces mixing of the upper and lower streams of the plate downstream from the slot, resulting in an amplification of heat transfer performance; (ii) heat transfer performance at the rear plate is induced with an increase in d/δ and Re; and (iii) by contrast, heat transfer performance is attenuated with an increase in the blockage factor, whose effect becomes larger in the lower region of the Reynolds number. These results are confirmed by the flow visualization using ion-exchange resins.
Unsteady Natural Convection in a Horizontal Cylinder With Internal Heat Generation
Authors: Obabko Aleksandr V. , Almasri Eyad A. , Cassel Kevin W.
Published: February 2009

The unsteady natural convection of a fluid within a horizontal cylinder with internal heat generation is considered. The unsteady Navier-Stokes equations in the Boussinesq approximation are computed numerically for Rayleigh numbers in the range from 106 to 1010 to determine the fluid flow and heat transfer characteristics. For Rayleigh numbers below a critical value, RaR ≈ 108, the unsteady solutions approach a steady-state solution that is symmetric about the vertical center line. Above the critical Rayleigh number, however, the flow becomes highly unsteady and evolves toward a quasi-periodic asymmetric oscillation.
Drag and Lift Acting on a Circular Cylinder in Laminar Boundary Layer: Part 1 — Verification of Force Transducer for Measuring Small Force
Authors: Hatakenaka Kiyoshi , Tanaka Kazuhiro , Fuchiwaki Masaki , Shimizu Fumio
Published: February 2009

Experimental investigations on a circular cylinder in a flow have been carried out extensively over the years because of the simplicity of its shape. The wake past the cylinder has been investigated in detail, it has been reported that the vortices shedding from the surface of the cylinder in a uniform flow form a symmetric vortex or a Karman vortex sheet, depending on Reynolds number, and there are several modes of the vortex sheet observed in the wake of the cylinder in a laminar or a turbulent boundary layer. On the other hand, flow forces acting on the cylinder have been measured in detail and drag and lift coefficients acting on it in a uniform flow can be found easily in any data-books. The coefficients in the case of the turbulent boundary layer have also been measured. The forces acting in the case of the laminar boundary layer, however, have yet to be reported. This is mainly because it is very difficult to measure a small flow force of some mN with commercial force transducers. The principle of Hatamura-type force transducers that are made use of frequently in measuring forces is that force acting on the block supported by two parallel spring plates is calculated from the strains detected by four strain gauges that are attached on each of the surfaces of the plates and constitute a Wheatstone’s bridge. It can be applied to measure the small force, that is, it can be calculated from the displacement of the block that is measured by a non-contact type of commercial gap sensor when the stiffness of the supporting springs is made moderately small. In the present report, which is a part of the study that aims at measuring the small flow force acting on a circular cylinder in a laminar boundary layer, a force transducer is developed that the force of some mN can be precisely measured with, based on the concept of the Hatamura-type force transducer. The accuracy of the transducer is verified by measuring the forces acting on the cylinder in a uniform flow and comparing it with the given values in the data book.
Time Dependent Calculations for Incompressible Flows on Adaptive Unstructured Meshes
Authors: Zhao Xiang , Wang Jun , Zhang Sijun
Published: February 2009

An adaptive unstructured method has been developed for simulating two-dimensional unsteady, viscous, incompressible flows. The pressure-based approach coupled with a mesh adaptation method is employed to better capture the details of flow physics for time dependent problems by optimizing computational cost with respect to accuracy. The mesh adaptation for locally refining and coarsening hybrid unstructured grid is based on hanging node approach. The time dependent calculations is further enhanced by virtue of parallel computing which is the most powerful for large scale intensive computation at the present time. The proposed method is validated by comparison with experimental results of low Reynolds number flow over a shedding circular cylinder.
Investigation of Microbubble Boundary Layer Using Particle Image Velocimetry
Authors: Ortiz-Villafuerte Javier , Hassan Yassin A.
Published: February 2009

Particle tracking velocimetry has been used to measure the velocity fields of both continuous and dispersed phases, in a microbubble turbulent boundary layer, in a channel flow. Hydrogen and oxygen microbubbles were generated in the flow by electrolysis. The average size of the microbubble radius was 15 μm. Significant drag reductions (above 40%) were observed, when the accumulation of microbubbles took place in a critical zone within the buffer layer. The z-component of the mean vorticity field was derived from the measured velocity fields. There is a decrease in the magnitude of the vorticity, leading to a smother transition from the viscous to the buffer layer. This indicates the increase of the viscous sublayer thickness, as also observed in investigations of drag reduction by polymer and surfactant injection in liquid flows.
Bubble Effects on Wall Shear in Vortical Flows
Authors: Ferna´ndez Arturo , Lu Jiacai , Tryggvason Gre´tar
Published: February 2009

Direct numerical simulations of the motion of bubbles in turbulent flows are being carried out, using a finite volume/front tracking technique that accounts fully for the effect of fluid inertia, viscosity, bubble deformability, and surface tension. The objective of the simulations is both to address the fundamental interaction mechanisms between the bubbles and the flow and how the bubbles modify the wall turbulent structures, as well as to provide data for validation of simplified models. Results for bubbles placed in the so-called “minimum turbulent channel” show significant drag reduction as the bubbles disrupt the near-wall turbulent flow.
On the Skin Friction Reduction Mechanisms of Microbubbles (Keynote)
Authors: Kodama Yoshiaki
Published: February 2009

The skin friction reduction effect of microbubbles in various flow conditions are reviewed. At location immediately downstream from an air injection point, the reduction does not depend on the flow speed, but at further downstream, it becomes smaller at higher flow speed, and decays with downstream distance. Experiments in a towing tank using a very long flat plate show that the reduction persists up to nearly 50m. In a full scale experiment using a 105m-long ship, 5.5% drag reduction was obtained. In case the bubbles are away from the wall, the skin friction increases. An optical technique has shown that bubbles in a boundary layer increase velocity fluctuations but decrease the Reynolds shear stress. Three possible skin friction reduction mechanisms are listed.
Measurement of Turbulence Modification by Microbubbles Causing Frictional Drag Reduction
Authors: Kitagawa Atsuhide , Sugiyama Kazuyasu , Ashihara Masa-aki , Hishida Koichi , Kodama Yoshiaki
Published: February 2009

In this research, in order to clarify the mechanism for drag reduction caused by microbubbles, the turbulence structure of flow field including microbubbles in a horizontal channel is experimentally investigated using particle tracking velocimetry and laser induced fluorescence (PTV/LIF) technique. First, we discuss the particle image velocimetry (PIV) and PTV results for the liquid phase velocity detection. Second, using instantaneous measurement data, we obtain the profiles of the mean velocity, turbulence intensity and Reynolds stress of the liquid phase. In order to obtain the information of both the liquid and gas phases simultaneously, furthermore, we propose a new system based on the combination of PTV, LIF and infrared shadow technique (IST).
Microbubble Drag Reduction at High Reynolds Number
Authors: Sanders Wendy C. , Ivy Elizabeth M. , Ceccio Steven L. , Dowling David R. , Perlin Marc
Published: February 2009

This paper presents the preliminary results from a friction drag reduction experiment conducted at high Reynolds numbers. The experiments were conducted in two phases at the U. S. Navy’s William B. Morgan Large Cavitation Channel (LCC) in Memphis, TN on a flat, hydraulically smooth plate (12.9 meters long by 3.0 meters wide by 0.18 meters thick) at flow speeds ranging from 0.5 to 20 m/s. The employed drag reduction technique involved the injection of microbubbles into the boundary from a line source over a range of injection flow rates from 100 to 800 scfm. These results include mean velocity data and spatially averaged shear stress data acquired in the single-phase flow, as well as spatially averaged shear stress data, bubble images, and void fraction measurements made in the bubble-injected flow.
Numerical Simulation of High Drag Reduction Regime in Polymer Solutions (Keynote)
Authors: Dubief Yves , White Christopher M. , Terrapon Vincent E. , Shaqfeh Eric S. G. , Lele Sanjiva K. , Moin Parviz
Published: February 2009

The simulation of drag reduced channel flows has to rely on consitituve models such as FENE-P. Their implementation is not straightforward to achieve convergence and stability of the solution. This paper discusses the problem of advection embedded in the FENE-P equation and the issue of the domain size. Finally we present results of simulations for High Drag Reduction regime, and show the subsequent modification of the vortical structures.
Recent Experimental Results on Surfactant Drag Reduction
Authors: Zakin Jacques L. , Qi Yunying , Zhang Ying
Published: February 2009

Certain surfactant solutions are drag reducing in turbulent shear flows. Because they “self-repair” after mechanical degradation, they are very attractive for reducing pumping energy losses in recirculating water applications such as district heating and cooling systems. Some surfactant systems reduce drag below the Virk limiting drag reduction asymptote for high polymers. The surfactant asymptote is 40% below that for polymers and the limiting velocity profile slope for highly drag reducing surfactant systems is about twice that for polymers. Like polymers, surfactants show low turbulence intensities normal to the wall and may exhibit zero Reynolds stresses requiring the postulation of an “elastic” stress to satisfy the total stress balance. The influences of counterion chemical structure and shear on microstructures, rheology and therefore drag reduction of cationic surfactant solutions are also addressed. Viscoelasticity, high extensional/shear viscosity ratios and threadlike microstructures have been proposed as necessary physical criteria for surfactant drag reduction. Recently, however, several non-viscoelastic drag reducing surfactant solution systems (zero first normal stress differences, no recoil and no stress overshoot) have been reported. Most drag reducing surfactant solutions have extensional to shear viscosity ratios of 100 or more. However, two solutions with a low ratio in the shear/extensional rate range of 20∼1000 s−1 have been observed. The ratio tends to increase at higher extensional rates, however, so the second criterion may be valid. Finally, cryo-TEM images of some drag reducing surfactant micelle microstructures which lacked threadlike structures in the quiescent state have been observed. However, Zheng et al. [1] showed that vesicle microstructures in the quiescent state can change to threadlike micelles under shear, supporting the third criterion.
Experimental Study of Polymer Drag Reduction in a Turbulent Channel Flow
Authors: Shen Xiaochun , Kim Kihyun , Miller Jamison L. , Sun-Chee-Fore Ryan , Sirviente Ana I.
Published: February 2009

The effect of polymer injection on the turbulence characteristics of a fully developed turbulent water channel flow was studied. The main focus of this study was to assess the influence of the polymer solution injection concentration and consequent mixing. This study was conducted by measuring the turbulence characteristics of two fully developed turbulent channel flows for a Reynolds number of 5×104, with different injection concentrations of the same polymer solution but with the same homogeneous concentration at the test section. These measurements were complemented by visualizations of the flow, which revealed the presence of supra-molecular polymer structures. The development of such structures seems to enhance the drag reducing abilities of the polymer solution.
Local Mesh Refinement and Penalty Methods Dedicated to the Direct Numerical Simulation of Incompressible Multiphase Flows
Authors: Vincent Ste´phane , Caltagirone Jean-Paul , Lubin Pierre , Randrianarivelo Nirina
Published: February 2009

Recent advances in numerical methods for the direct numerical simulation (DNS) of multiphase flows are presented. The mathematical formulation is based on an eulerian discretisation of the navier-stokes equations on fixed staggered curvilinear grids where the media are located thanks to volume of fluid functions. An adaptative and local mesh refinement method (AMR) is proposed to allow multi-space scale solutions in three dimensions and a tensorial penalty (or fictious domain) approach is detailed for the numerical modelling of incompressibility, velocity/pressure coupling and liquid/solid interactions in multi fluid flows. The article is focussed on three-dimensional applications of the direct numerical simulation. The DNS tool is applied to the mould filling under unstable viscous jet conditions, the stoke’s flow induced by a solid particle in a tube and the turbulent wave breaking in deep water. These flows are chosen to highlight the abilities of the numerical model for characterising unsteady incompressible multiphase flows.
Current Issues in Analytical Description of Particle/Droplet-Laden Turbulent Flows
Authors: Gao Z. , Pandya R. V. R. , Shotorban B. , Mashayek F.
Published: February 2009

The particle/droplet-laden turbulent flows occur in many important natural and technological situations, e.g., cloud [1], aerosol transport and deposition [2], spray combustion [3, 4], fluidized bed combustion [5], plasma spray coating and synthesis of nanoparticles [6]. Undoubtedly, turbulence itself remains as a difficult and unsolved problem of classical mechanics despite many persistent efforts by physicists and engineers. The presence of particles/droplets (hereafter simply referred to as particles) further adds to the complexity of the turbulence. The particle behavior in turbulent flows gives rise to many interesting phenomena, such as, turbophoresis [7], turbulent thermal diffusion and barodiffusion [8, 9], preferential distribution, and anomaly and intermittency [9, 10]. It is challenging to come up with a unique predictive theory for the particle phase which is useful for engineering purposes and also capable in quantifying various related phenomena.
Turbulent Coagulation of Aerosol Particles: New Insights From Direct Numerical Simulations and Holographic Imaging Experiments
Authors: Collins Lance R. , Meng Hui , Ahluwalia Aruj , Cao Lujie , Pan Gang
Published: February 2009

Particle collisions driven by turbulent fluctuations play a key role in such diverse problems as cloud formation, aerosol powder manufacturing and inhalation drug therapy to name a few. In all of these examples (and many others) turbulent fluctuations increase the rate of collisions relative to the background collision rate driven by Brownian motion. Furthermore, turbulence can spontaneously generate very large fluctuations in the particle concentration field. This “clustering” is caused by the inertial mismatch between the heavy particles and the lighter surrounding gas; vortices in the flow “centrifuge” the heavier particles out of vortex cores and into the straining regions that lie in between the vortices. Because collision is a binary process, concentration fluctuations further enhance the turbulent coagulation rate by as much as two orders of magnitude. An effect of this size must be accounted for in a rational model of turbulent coagulation. Sundaram & Collins (J. Fluid Mech. 1997) showed that the radial distribution function (RDF) of the particle population, evaluated at contact, precisely corrects the collision kernel for clustering. Subsequent work has explored the dependence of the RDF on the system parameters (e.g., particle size, concentration, response time and Reynolds number) using direct numerical simulations. These results have improved our understanding and ability to predict the effect of the first three parameters; however, owing to the limited range of Reynolds number that can be reached in a numerical simulation, questions remain over the scaling of the RDF with Reynolds number. This is a critical issue for high-Reynolds-number applications such as cloud physics, where values of the Reynolds number can be 1–2 orders of magnitude greater than can be simulated. We will present our highest Reynolds number simulations to date and show our attempts to resolve this issue. Recently, the ability to measure three-dimensional particle positions using holography has been realized (e.g., Meng & Pu, J. Opt. Soc. Am. 2003). With holography, the optical image that is produced contains fringes that, upon inverting the laser, reproduce the original image in three dimensions. The hologram can then be scanned using a digital camera to obtain the particle positions. An important consideration with this study is the need to differentiate individual particles. We developed a search algorithm that locates particle centers, even in the presence of optical aberations and speckle noise. The algorithm has been used to obtain the first experimental RDF measurements to date. Thus far we see good agreement between the experimentally obtained RDF and the simulations. Besides validating the simulations, experiments can span a much broader range of Reynolds numbers, providing critical data that may help resolve the open questions associated with this parameter.
Gravity Driven Bubbly Flows: The Role of Vortical Structures
Authors: Mudde Robert F.
Published: February 2009

Gravity driven bubbles are found in many industrial applications. Two typical reactors are the bubble column, in which the liquid is stagnant and the air lift reactor in which the liquid circulates, under the action of gravity, through the reactor. These reactors are attractive for a number of reasons: they have no moving parts and are thus low in maintenance; the size can be enormous (diameters of several meters, heights of tens of meters) allowing large volume flows to be processed; good mixing and heat transfer characteristics, etc. Our knowledge about the structure of the flow induced is rather limited. This makes design, fine tuning of operation and scale up still difficult. The two-phase flow in a bubble reactor is complicated. In the bubble columns, the liquid exhibits a large-scale circulation in a time averaged sense, with upward flow in the center and downward flow in the wall region. The first reliable data on this large-scale circulation date back to the work of Hills (174). In 1984, Franz et al. reported on the motion of what was later called vortical structures, eddy like structures (with sizes on the order of the column diameter) that move through the bubbly mixture. These vortical structures have been research more extensive during the last ten year by e.g. Fan and coworkers, Dudukovic and coworkers and Mudde & Van den Akker. The structures are found for a wide range of gas fractions, ranging from a few percent to well above 20%. The vortical structures seem to be a universal feature of the gravity driven bubble flows as they were also found in air lift reactors. For this reactor it has been reported that the liquid flow behaves more or less like the superposition of a net liquid flow and the complicated flow features found in the bubble column. The similarities will be high lighted. The vortical structures have important consequences for e.g. the (pseudo-)turbulence and the mixing in the bubbly flow. In 2-dimensional equipment they appear very regular and a separation between the low frequency fluctuations and the high frequency ‘turbulence’ is easily made. However, in 3-dimensional columns the situation is more complicated. LDA data show that the vortical structures are still responsible for a the occurrence of low frequency oscillations (on the order of 0.1Hz), but they are no longer appearing regularly and a separation of frequencies is no longer possible. Finally, the newest experiment seem to show that the vortical structures can be suppressed up to gas fraction of (at least) 10%. These new experiments suggest that the gravity driven bubbly flow is not inherently unstable, but rather sensitive to the conditions at the gas inlet.
Molecular Dynamics Simulation of Nanobubbles
Authors: Takagi Shu , Kikugawa Gota , Matsumoto Yoichiro
Published: February 2009

Some results have been reported recently related to the bubble formation with Molecular Dynamics (MD) simulation method. Some of them conduct the MD simulations of the bubble nucleation including impurity molecules with L-J potential [1,2]. In the present study, we investigate the stability of the nanometer size bubble in water, using molecular dynamics (MD) simulation method. MD simulation of an aqueous surfactant system: water liquid and alcohols below the liquid saturation density is carried out to investigate the stability of “nanobubbles” and the structure of the gas-liquid interface. To analyze the effect of surfactant structure, volume, and polarization on the stability of bubble nuclei, we use water by SPC/E model as the solvent molecules and 1-propanol, 1-pentanol, 1-heptanol as the surfactant molecules. Fig.1 shows the numerical result of instantaneous behavior of nanobubbles under the presence of surfactant in water. The calculation system is the cubic cell which has a side length of 25.057[Å], and a three-dimensional periodic boundary condition is applied. To include the intramolecular motion, AMBER force field [3] is adopted as a potential function. The momentum equations are integrated by velocity-Verlet argorithm [4]. Further, the time integration is extended to the Multi Time Scale algorithm by r-RESPA method [5]. As the surfactant molecules, to evaluate the influence of the hydrophobic effect of surfactants on the stability of bubble nuclei, we adopt 1-propanol (C3H7OH), 1-pentanol (C5H11OH), and 1-heptanol (C7H15OH), and to investigate the influence of the polarization of hydrophilic groups (-OH), “pseudo” 1-pentanol of which charge is cancelled away is also calculated. As a result, it was found that from the MD simulation at the condition that the bubble nuclei could not exist stably in pure water, a stable bubble is formed in aqueous surfactant system and hydroxyl groups of surfactants tend to point to the liquid phase at the gas-liquid interface. It is also shown that the longer hydrophobic chains the surfactants have, the more stably the bubble nuclei can exist.
Dynamics of Bubble Motion and Gas-Liquid Interfacial Phenomena
Authors: Tsuchiya Katsumi
Published: February 2009

Two aspects of the dynamics associated with oscillating bubbles are discussed in this paper: oscillatory motion of bubble itself and bubble-surface wave. The primary issue here is whether it is the case that the surface wave occurs in sychronization with the bubble’s oscillatory motion. The dynamic process of wave formation and propagation along the surface of an oscillating bubble is studied based on high-speed imaging, through which the wave characteristics such as wavelength and phase/propagation speed are evaluated as mostly the vertical projection of rather regularly generated bubble-surface ripples. The bubble oscillating motion is characterized quantitatively by the bubble-gyration (or edge-rotation) frequency, diameter and velocity. In addition, dynamics of mass transfer across gas–liquid interface in a gas-dispersed (continuous liquid) system are examined via high-sensitivity, high-speed imaging. The dispersive dynamics of the dissolved component from the gas into the liquid phase are visualized using laser-induced fluorescence (LIF) with pH-sensitive pyrene (HPTS) for both a single and multi-bubble systems. The coupling between these dynamics of surface/interfacial flow and mass transfer is attempted towards better understanding of such complex phenomena prevailing in the vicinity of the fluctuating gas–liquid interface. Enhancement of the mass transfer is found to be associated with the (nonlinear) wave formation, influence of which could be included in modeling the mass-transfer coefficient, apart from an physical account of the near-surface concentration gradient. Due to significant bubble–bubble interactions in a multi-bubble system, the dispersive pattern of low-pH region arising from gas dissolution becomes extremely complex; the visual estimate of time variation in fluorescence level is then mainly made over a fixed space in the gas–liquid flow system.
Condensation-Induced Water Hammer in Horizontal Refrigerant Pipe With Warm Gas Entry
Authors: Martin C. Samuel
Published: February 2009

Careful experiments have been conducted for the purpose of investigating the phenomenon of condensation-induced waterhammer in an ammonia refrigeration system. To initiate a waterhammer event warm ammonia gas was introduced over static subcooled ammonia liquid placed in a horizontal 146.3 mm diameter carbon steel pipe 6.0 m in length. By means of fast response piezoelectric pressure transducers and a high speed data acquisition system rapid dynamic pressures were recorded whenever a shock event occurred. The occurrence of condensation-induced waterhammer depended upon three major variables; namely, (1) initial liquid depth, (2) liquid temperature, and (3) mass flow rate of warm gas. For given liquid depth and temperature, once the warm gas threshold conditions were exceeded shocks occurred with greater magnitude as the mass flow rate of gas input was increased. With adequate subcooling condensation-induced waterhammer occurred for initial liquid depths ranging from 25% to 95% of internal pipe diameter. The threshold mass flow rate of warm gas necessary to initiate waterhammer was greater as the initial liquid was lowered.