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EDITORIAL

J. Fluids Eng. 1999;121(2):233. doi:10.1115/1.2822193.
FREE TO VIEW
Abstract
Commentary by Dr. Valentin Fuster

ANNOUNCEMENTS

J. Fluids Eng. 1999;121(2):234-235. doi:10.1115/1.2822194.
FREE TO VIEW
Abstract
Commentary by Dr. Valentin Fuster

INDUSTRY PERSPECTIVES

RESEARCH PAPERS

J. Fluids Eng. 1999;121(2):237-247. doi:10.1115/1.2822197.

Pump research and development efforts are primarily driven by the needs of the customer. Today, these needs are centered around cost and reliability issues with the understanding that certain threshold levels of performance are achieved. As centrifugal pumps have reached high levels of maturity in most industrial applications, we can anticipate, that in the future, customer expectations will change subtly but significantly. They will demand continuously reducing costs with the understanding that reliability and technology needs will be satisfied. This would lead to a strong emphasis on consistent predictability of performance in the field and to less of a focus on innovations in design. R&D efforts in the past were intended to stretch the envelope to produce better hydraulic performance, to improve mean-time-between-failures, and to operate at higher speeds. In contrast, R&D efforts in the future would be aimed towards cost reduction, accurate hydraulic, guarantees, and flawless performance in the field. In this paper, the R&D efforts of the past, present, and future are discussed in terms of three core competencies, which are essential for today’s pump manufacturer. These are hydraulics (with an emphasis on improving predictability of performance and improving impeller life), vibrations (with a view to providing cost effective problem solving/avoidance capability), and pump designs which capitalize on improved understanding of the underlying technologies.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):248-253. doi:10.1115/1.2822198.

As in all areas relevant to the development and production of pumps the tasks the hydraulic engineer has to deal with have undergone a remarkable change in recent decades: from a more or less unrestricted design of components to design roles dictated by manufacturing techniques. These general trends are accompanied by developments related to changes of the market requirements. This is demonstrated for three particular pump types. The consequences of the demand to save energy—with all the different aspects—are described for the example of heating circulation pumps. The still growing environmental awareness is a challenge for an enlargement of the presently valid operation limits of sealless pumps and for the development of intelligent monitoring systems. It is demonstrated that the developments in the field of boiler feed pumps are closely related to the growing unit sizes. Availability and reliability, and as far as very large pumps are concerned the efficiency, have always been and still are the dominant criteria.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):254-258. doi:10.1115/1.2822199.

The evolution of pump research and production in Japan after 1955 is surveyed. The post-war period has been divided into three stages of development with unique social and industrial characteristics: expansion, conversion, and globalization. The growth of pump production in sales amount and quantity is shown for various types of pumps. The post-war direct and indirect research on pumps is classified into eight groups of topics and their past trends are analyzed. These changes are correlated with the characteristics of the corresponding background stage. These analyses with the past suggest new trends of research for pumps of tomorrow.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):259-265. doi:10.1115/1.2822200.

In recent years, increasing interest has been given to the rotordynamic forces on impellers, from the view point of the shaft vibration analysis. Previous experimental and analytical results have shown that the fluid-induced forces on closed-type (with shroud) centrifugal impellers in whirling motion contribute substantially to the potential destabilization of subsynchronous shaft vibrations. However, to date nothing is known of the rotordynamic forces on open-type (without shroud) centrifugal impellers. This paper examines the rotordynamic fluid forces on an open-type centrifugal compressor impeller in whirling motion. For an open-type impeller, the variation of the tip clearance due to the whirling motion is the main contribute to the rotordynamic forces. Experiments were performed to investigate the rotordynamic forces by direct measurements using a force balance device, and indirectly from the unsteady pressure on the casing wall over a range of whirl speed ratio (Ω/ω) for several flow rates. In this paper, the following results were obtained: (1) Destabilizing forces occur at small positive whirl speed ratio (0 ≤ Ω/ω ≤ 0.3) throughout the flow range of normal operation; (2) At smaller flow rate with inlet backflow, the magnitude of the fluid force changes dramatically at a whirl speed ratio close to Ω/ω = 0.8, resulting in destabilizing rotordynamic forces. From the measurement of unsteady inlet pressure, it was shown that the drastic changes in the fluid force are related to the coupling of the whirling motion with a rotating flow instability, similar to “rotating stall”; (3) The forces estimated from the unsteady pressure distribution on the casing wall and those estimated from the pressure difference across the impeller blades were compared with the results from the direct fluid force measurements. The direct fluid forces correlate better with the forces due to the pressure distribution on the casing wall.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):266-275. doi:10.1115/1.2822201.

The hydrodynamic performance of a three-element automotive torque converter is analyzed by measuring flow between the elements with five-hole Pitot tubes. The performance of each element, including head, head loss, and efficiency, is defined and evaluated. The results show that the pump is the major source of loss in the speed ratio range where vehicles are most frequently operated in everyday driving. The loss coefficients for the three elements are also evaluated using a one-dimensional flow model. The friction loss coefficient of the turbine shows small variation over the entire tested speed ratio range, whereas the coefficients of the pump and stator vary considerably according to the operating speed ratio. The cause of loss in the pump and stator is investigated by flow visualization and three-dimensional numerical flow analysis. A low kinetic energy region in the pump and leading edge separation in the stator are clearly visualized or computed.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):276-281. doi:10.1115/1.2822202.

In this paper, an aeroacoustic study on a forward-curved blades centrifugal fan has been carried out. As a first step, the fan performance curves, i.e., total pressure, power, efficiency and sound power level versus flow rate were obtained, showing its unstable behavior over a wide operating range. Second, the fan sound power level spectra for several working conditions were determined. For this purpose a normalized installation for testing in laboratory was designed and constructed. Afterwards, the velocity and pressure fields, both at the inlet and outlet planes of the impeller were measured using hot wire probes and pressure transducers, for different operating conditions. Finally, the aeroacoustic behavior of the fan was determined measuring the vorticity field at the impeller outlet, which is known to be related to tonal noise generation. This relation is worked out using the theory of vortex sound, developed by several authors during the second half of this century. The paper shows that the generation of tonal noise is produced at the blade passing frequency and it increases with the flow rate. Although the main contribution to fan noise generation is due to mechanical sources, the bands in which aerodynamic noise is generated by these fans correspond to frequencies especially unpleasant to the human ear. Therefore, the research presented in this paper may be of considerable interest, establishing a starting point for the design of quieter and more efficient fans.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):282-288. doi:10.1115/1.2822204.

A boundary element method is used to predict the time-dependent cavitation on a propeller subject to nonaxisymmetric inflow. The convergence of the method is studied. The predicted cavities agree well with those observed in CAPREX, an experiment performed at MIT’s variable pressure water tunnel. The method is modified so that prediction of cavities detaching at mid-chord regions is possible. An algorithm for predicting the cavity detachment location on the blade is described and applied on a blade geometry which exhibits mid-chord cavitation.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):289-296. doi:10.1115/1.2822206.

Sheet cavitation on a foil section and, in particular, its unsteady characteristics leading to cloud cavitation, were experimentally investigated using high-speed visualizations and fluctuating pressure measurements. Two sources of sheet cavitation instability were evidenced, the re-entrant jet and small interfacial waves. The dynamics of the re-entrant jet was studied using surface electrical probes. Its mean velocity at different distances from the leading edge was determined and its role in promoting the unsteadiness of the sheet cavitation and generating large cloud shedding was demonstrated. The effect of gravity on the dynamics of the re-entrant jet and the development of interfacial perturbations were examined and interpreted. Finally, control of cloud cavitation using various means, such as positioning a tiny obstacle (barrier) on the foil surface or performing air injection through a slit situated in the vicinity of the leading edge, was investigated. It was shown that these were very effective methods for decreasing the amplitude of the instabilities and even eliminating them.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):297-304. doi:10.1115/1.2822207.

The unsteady cavitating flow of a propeller subject to a nonaxisymmetric inflow inside of a tunnel is addressed. A numerical method is developed which solves for the fully unsteady propeller problem and the tunnel problem separately, with the unsteady effects of one on the other being accounted for in an iterative manner. The propeller influence on the propeller is considered via velocity. The iterative process is found to converge very fast, usually within three iterations, even for a heavily loaded propeller. The effect of the tunnel extent and the number of panels on the predicted mean propeller forces is investigated. In the case of uniform inflow the equivalent open water velocity is calculated and then compared to that predicted from Glauert’s formula. The two velocities are found to be very close to each other in the case of light propeller loading, and to deviate from each other as the propeller loading increases. In the case of nonuniform flow the predicted unsteady propeller forces are found not to be affected appreciably by the tunnel effects in the case of noncavitating flow. In the case of cavitating flows the tunnel effects have been found to be appreciable, especially in terms of the predicted cavity extent and volume. The predicted cavity patterns are shown to be very close to those observed in CAPREX, a CAvitating PRopeller Experiment performed at MIT’s cavitation tunnel.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):305-311. doi:10.1115/1.2822208.

The Cavermod (CAVitation ERosion MODel) is an erosion test device first described by Dominguez-Cortazar et al. (1992, 1997). Recently, it was modified in two steps: first by increasing its maximum rotation rate (from 4500 to 8000 rpm) and second by shortening its vapor core (from 156 to 66 mm). This paper plans to present the main results which are obtained in both configurations (long and short vortex) and for “slow” or “rapid” regime of rotation. They mainly concern 1. the hydrodynamic aspects of the vapor core collapse, as deduced from observation of rapid films (evolution of the vortex length, collapse, velocity), 2. the erosion patterns produced on metallic targets such as pure aluminium and copper. A second companion paper will present the results of force measurements in both configuration and an attempt to estimate the local erosive pressures.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):312-317. doi:10.1115/1.2822209.

The Cavermod device, as described in the companion paper (Filali et al., 1999), allows us to produce the axial collapse of a cavitating vortex at high velocities. From a global point of view, we can consider that it produces a high momentum in the axial direction. Large forces, concentrated on a small area and able to produce erosion pits on hard materials, result from the sudden momentum stopping against a solid wall. In this paper, the results of the forces measurements are given. Four different measurements devices are used to analyze the Cavermod performance in both cases of long and short vortex: dislocations in MgO (Magnesium Oxide) single crystal, two special piezoelectric ceramic transducers and a PVDF film transducer. Special attention is given to the PVDF film response which is found twice the response of other devices. In addition, an attempt is made to interpret the temporal force signal given by a ceramic transducer in terms of local erosive pressure.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):318-329. doi:10.1115/1.2822210.

The present paper summarizes steady and unsteady computations of turbulent flow induced by a pitched-blade turbine (four blades, 45° inclined) in a baffled stirred tank. Mean flow and turbulence characteristics were determined by solving the Reynolds averaged Navier-Stokes equations together with a standard k-ε turbulence model. The round vessel had a diameter of T = 152 mm. The turbine of diameter T/3 was located at a clearance of T/3. The Reynolds number (Re) of the experimental investigation was 7280, and computations were performed at Re = 7280 and Re = 29,000. Techniques of high-performance computing were applied to permit grid sensitivity studies in order to isolate errors resulting from deficiencies of the turbulence model and those resulting from insufficient grid resolution. Both steady and unsteady computations were performed and compared with respect to quality and computational effort. Unsteady computations considered the time-dependent geometry which is caused by the rotation of the impeller within the baffled stirred tank reactor. Steady-state computations also considered neglect the relative motion of impeller and baffles. By solving the governing equations of motion in a rotating frame of reference for the region attached to the impeller, the steady-state approach is able to capture trailing vortices. It is shown that this steady-state computational approach yields numerical results which are in excellent agreement with fully unsteady computations at a fraction of the time and expense for the stirred vessel configuration under consideration.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):330-342. doi:10.1115/1.2822211.

This paper treats the numerical analysis of the rising process of a solid-gas-liquid three-phase mixture along a vertical pipeline with an abrupt enlargement in diameter. The system of governing equations used is based upon the one-dimensional multifluid model and the transitions of gas flow pattern are taken into account in the system of governing equations. For the case of a sudden enlargement in diameter in a coaxial pipeline, the procedure of the numerical calculation to obtain the flow characteristics in the pipeline section after a sudden change in diameter has been established here. Furthermore, in order to confirm the validity of the present theoretical model by the comparison between the calculated and experimental values, the experiments have been made using four kinds of lifting pipes, including the straight one. Thereby, it has been found that the numerical model proposed here gives good fit to the prediction of the flow rates of lifted water and solid particles against that of air supplied for the case of a sudden change in diameter. In addition, the flowing process for each phase has been investigated from a photographic point of view. As a result, we found that the moving process of the solid particles depends strongly upon the volumetric flux of gas-phase as well as the submergence ratio.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):343-350. doi:10.1115/1.2822212.

A domain decomposition approach has been developed to solve for flow around multiple objects. The method combines features of mask and multigrid algorithm implemented within the general framework of a primitive variable, pseudospectral elements formulation of fluid flow problems. The computational domain consists of a global rectangular domain, which covers the entire flow domain, and local subdomains associated with each object, which are fully overlapped with the rectangular domain. There are two key steps involved in calculating flow past multiple objects. The first step approximately solves the flow field by the mask method on the Cartesian grid alone, including on those grid points falling inside an object (a fuzzy boundary between the fluid-object interface), but with the restriction that the velocity on grid points within and on the surface of an object should be small or zero. The second step corrects the approximate flow field predicted from the first step by taking account of the object surface, i.e., solving the flow field on the local body-fitted (curvilinear) grid surrounding each object. A smooth data communication between the global and local grids can be implemented by the multigrid method when the Schwarz Alternating Procedure (SAP) is used for the iterative solution between the two overlapping grids. Numerical results for two-dimensional test problems for flow past elliptic cylinders are presented in the paper. An interesting phenomenon is found that when the second elliptic cylinder is placed in the wake of the first elliptic cylinder a traction force (a negative drag coefficient) acting on the second one may occur during the vortex formation in the wake area of the first one.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):351-358. doi:10.1115/1.2822213.

Monotonic convergence of numerical solutions with the computational grid refinement is an essential requirement in estimating the grid-dependent uncertainty of computational fluid dynamics. If the convergence is not monotonic, the solution could be erroneously regarded as convergent at the local extremum with respect to some measure of the error. On the other hand, if the convergence is exactly monotonic, estimation methods such as Richardson extrapolation properly evaluate the uncertainty of numerical solutions. This paper deals with the characterization of numerical schemes based on the property of the monotonic convergence of numerical solutions. Two typical discretization schemes of convective terms were considered; the second-order central difference scheme and the third-order Leonard’s QUICK scheme. A fully developed turbulent flow through a square duct was calculated via a SIMPLER based finite volume method without a turbulence model. The convergence of the numerical solution with the grid refinement was investigated for the mean flow property as well as fluctuations. The comparison of convergence process between the discretization schemes has revealed that the QUICK scheme results in preferable monotonic convergence, while the second-order central difference scheme undergoes non-monotonic convergence. The latter possibly misleads the determination of convergence with the grid refinement, or causes trouble in applying the Richardson extrapolation procedure to estimate the numerical uncertainty.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):359-364. doi:10.1115/1.2822214.

The three-dimensional effects of secondary flow, passive injection, and particle size on the motion of solid particles entrained in a laminar, incompressible flow through a curved, converging, rectangular passage were numerically investigated. Emphasis was placed on observing the physical mechanisms that cause particles 5 μm and smaller in diameter to deposit on passage surfaces and to concentrate near the endwalls and mid-span at the passage exit. Particle trajectories were calculated for 5, 30, and 300 μm diameter solid particles. It was observed that the paths of 5 μm particles were similar to the streamlines of the three-dimensional flow in the channel until the particles encountered the boundary layers on the blade surfaces and endwalls, where they would graze the surfaces (contributing to particle deposition) and concentrate at the exit of the channel. Particles of 30 μm diameter, however, were only slightly affected by secondary flows, but were affected enough to be made to concentrate at the exit near the endwall and mid-span surfaces. Particles of 300 μm diameter were not affected by secondary flows at all. The particle trajectories showed that the passage secondary flow convected particles across endwalls toward the pressure and suction surface boundary layers of the blades. It was observed that small particles were made to decelerate and/or concentrate in the boundary layers near the passage exit. It was found that this concentration of particles along the suction surface and endwalls could be significantly reduced by means of passive injection. (Passive injection is a method of inducing the flow of jets in the curved portion of an airfoil shaped surface due to the pressure difference on opposing sides. This is accomplished by means of holes or slots that have been drilled through the surface at strategic locations.)

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):365-372. doi:10.1115/1.2822215.

A mathematical model to predict the air-solids performance of central air-jet pumps has been developed based on the fundamentals of fluid and particle mechanics. The influence of throat entry configuration on performance has been incorporated into the analytical model by introducing a throat entry function and suction area ratio. Nondimensional parameters to represent air-solids jet pump performance has been defined and used in the analytical procedure. The performance predictions obtained by this model show good agreement with experimental results.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):373-378. doi:10.1115/1.2822216.

The flow field characteristics of three different geometries of square jets in a crossflow at various blowing ratios are examined. The geometries considered are: perpendicular, streamwise-inclined, and spanwise-inclined jets. The inclined jets are at a 30 deg angle to the wind tunnel floor. Mean velocity and turbulence measurements along with film cooling effectiveness and scalar transport data were obtained. Jet-to-crossflow blowing ratios of 1.5, 1.0 and 0.5 are used with a density ratio of 1. It is shown that the flow field at the jet exit is strongly influenced by the crossflow as well as by the inlet conditions at the entrance to the jet orifice. The strong streamline curvature which is present in the perpendicular and spanwise injection cases appears to result in the greatest turbulence anisotropy. The film cooling effectiveness is best at the lowest blowing ratios as the jet is deflected strongly towards the floor of the wind tunnel, although the improvement is more significant for the streamwise injection case.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):379-383. doi:10.1115/1.2822217.

This paper reports the results of a linear instability analysis for a viscous liquid jet injecting into a quiescent inviscid gas medium with three-dimensional disturbances. A dispersion equation that accounts for the growth of asymmetric waves is derived, and the maximum rates of growth of various modes are calculated. The asymmetric breakup phenomenon of the jet and its structures at different modes is also studied by using a high-speed multi-frame holographic system. The theoretical predictions agree well with the experimental observations. The results of this study thus confirm the existence and even domination of unstable asymmetric modes under certain physical conditions in the breakup process.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):384-390. doi:10.1115/1.2822218.

In this study naturally occurring large-scale structures and some turbulence characteristics within an impinging jet array are investigated. The dynamics of a three-by-three elliptic jet array are analyzed relative to the flow structures within the array. With applications to electronic component cooling, low Reynolds number conditions, Re = 300 to 1500, are presented. Two jet aspect ratios are used, 2 and 3, with identical jet hydraulic diameters and jet-to-jet space. The effects of impinging distance are studied in the range of one to six jet hydraulic diameters. Flow visualization and PIV are used for the identification of structures and quantitative analysis. These results are used to evaluate the integrated surface layer vorticity, Γ*, which is shown to depend on the jet aspect ratio and impingement distance. Also, a transport coefficient is presented, based on a turbulence velocity and length scales. This coefficient is shown to experience a maximum value versus impingement distance that coincides with the location of axis switching.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):391-395. doi:10.1115/1.2822219.

Theoretical, experimental, and field test studies of the self-resonating pulsating jet have been carried out. The interaction mechanism of vortices and outlet wall is described. On the basis of theoretical calculations and experimental data, a new mini-extended and energy concentrated self-resonating pulsating jet nozzle can be recommended. Hydrodynamic experiments, rock erosion tests, and field tests in a drilling engineering environment were carried out using the new nozzle. The results show that the new nozzle has stronger erosion capability than the cone-type or conventional organ-pipe self-resonating nozzle jet. The field test results in drilling engineering show that the outlets of the newly designed nozzles have strong anti-scouring ability. The new exit design of the nozzle can increase its life span.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):396-404. doi:10.1115/1.2822221.

In this paper the flow field in the intake duct of a model water-jet unit is studied by using a commercial 3D CFD code. In order to model the intake duct/hull interaction, the computational domain includes a large section of the hull in the vicinity of the intake duct opening. Appropriate boundary conditions are used on the far upstream and downstream of the duct inlet in order to minimize the effect of the domain boundaries on the flow field in the vicinity of the intake duct. Computations are performed for different boat speeds and flowrates. In addition, the effects of the impeller shaft, shaft rotation, boat trim as well as the traverse flow across the hull are investigated. The results of the computations are compared with some preliminary experimental results obtained from model self-propulsion tests in a towing tank. Good correlation is obtained between the predictions and the experimental results.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):405-409. doi:10.1115/1.2822222.

The problem of an axisymmetrical laminar flow past a square cavity was treated experimentally. The cavity flow patterns obtained in flow visualization were of a special interest. Some geometrical features of dividing streamline like coordinates of separate and reattachment points as well as the coordinates of the vortex center were measured for a range of Re numbers and compared with the numerical results obtained by Stevenson. The detected flow patterns were also related to the global pressure-flow characteristic. Two zones of laminar flow characterized by the different nonlinear corrections to Darcy’s law were distinguished.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):410-417. doi:10.1115/1.2822223.

The paper presents detailed measurements of the incompressible flow development in a large-scale 90 deg curved diffuser with strong curvature and significant streamwise variation in cross-sectional aspect ratio. The flow path approximates the so-called fishtail diffuser utilized on small gas turbine engines for the transition between the centrifugal impeller and the combustion chamber. Two variations of the inlet flow, differing in boundary layer thickness and turbulence intensity, are considered. Measurements consist of three components of velocity, static pressure and total pressure distributions at several cross-sectional planes throughout the diffusing bend. The development and mutual interaction of multiple pairs of streamwise vortices, redistribution of the streamwise flow under the influence of these vortices, the resultant streamwise variations in mass-averaged total-pressure and static pressure, and the effect of inlet conditions on these aspects of the flow are examined. The strengths of the vortical structures are found to be sensitive to the inlet flow conditions, with the inlet flow comprising a thinner boundary layer and lower turbulence intensity yielding stronger secondary flows. For both inlet conditions a pair of streamwise vortices develop rapidly within the bend, reaching their peak strength at about 30 deg into the bend. The development of a second pair of vortices commences downstream of this location and continues for the remainder of the bend. Little evidence of the first vortex pair remains at the exit of the diffusing bend. The mass-averaged total pressure loss is found to be insensitive to the range of inlet-flow variations considered herein. However, the rate of generation of this loss along the length of the diffusing bend differs between the two test cases. For the case with the thinner inlet boundary layer, stronger secondary flows result in larger distortion of the streamwise velocity field. Consequently, the static pressure recovery is somewhat lower for this test case. The difference between the streamwise distributions of measured and ideal static pressure is found to be primarily due to total pressure loss in the case of the thick inlet boundary layer. For the thin inlet boundary layer case, however, total pressure loss and flow distortion are observed to influence the pressure recovery by comparable amounts.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):418-421. doi:10.1115/1.2822224.
Abstract
Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):422-426. doi:10.1115/1.2822225.

The ultrasonic flow meter is a newcomer among flow meters for measuring large quantities of natural gas. It has notable advantages compared to traditional meters. The ultrasonic flow meter is much more compact and has a wider dynamic range for flow measurements than the orifice plate meter. When manufactured, the ultrasonic sensors are often set back from the pipe wall in a cavity. When the fluid flows past the cavities, a secondary flow of vortices with characteristic size equal to the cavity width is established inside the cavities. The aim of this study has been to investigate the influence of this secondary flow on the accuracy of the ultrasonic flow meter. Both measurements and numerical simulations of the cavity flow have been conducted. It has been found from the present work, that the influence of the flow in the cavities on the measurements increases nonlinearly with the pipe flow rate.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):427-433. doi:10.1115/1.2822227.

A complete set of macroscopic two-equation turbulence model equations has been established for analyzing turbulent flow and heat transfer within porous media. The volume-averaged transport equations for the mass, momentum, energy, turbulence kinetic energy and its dissipation rate were derived by spatially averaging the Reynolds-averaged set of the governing equations. The additional terms representing production and dissipation of turbulence kinetic energy are modeled introducing two unknown model constants, which are determined from a numerical experiment using a spatially periodic array. In order to investigate the validity of the present macroscopic turbulence model, a macroscopically unidirectional turbulent flow through an infinite array of square rods is considered from both micro- and macroscopic-views. It has been found that the stream-wise variations of the turbulence kinetic energy and its dissipation rate predicted by the present macroscopic turbulence model agree well with those obtained from a large scale microscopic computation over an entire field of saturated porous medium.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):434-440. doi:10.1115/1.2822228.

The high-lift performance of a multi-element airfoil was optimized by using neural-net predictions that were trained using a computational data set. The numerical data was generated using a two-dimensional, incompressible, Navier-Stokes algorithm with the Spalart-Allmaras turbulence model. Because it is difficult to predict maximum lift for high-lift systems, an empirically-based maximum lift criteria was used in this study to determine both the maximum lift and the angle of attack at which it occurs. Multiple input, single output networks were trained using the NASA Ames variation of the Levenberg-Marquardt algorithm for each of the aerodynamic coefficients (lift, drag, and moment). The artificial neural networks were integrated with a gradient-based optimizer. Using independent numerical simulations and experimental data for this high-lift configuration, it was shown that this design process successfully optimized flap deflection, gap, overlap, and angle of attack to maximize lift. Once the neural networks were trained and integrated with the optimizer, minimal additional computer resources were required to perform optimization runs with different initial conditions and parameters. Applying the neural networks within the high-lift rigging optimization process reduced the amount of computational time and resources by 83% compared with traditional gradient-based optimization procedures for multiple optimization runs.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):441-445. doi:10.1115/1.2822229.

This paper is concerned with measurements of the flow field in the separated flow region behind a backward-facing step. The main instrument used in this research was Flying X Hot-Wire Anemometry (FHWA). Stationary (single normal) Hot-Wire Anemometry (SHWA) was also used. Comparative measurements between the SHW probe and the FHW system were conducted downstream of the step (step height H = 120 mm) and results are presented for axial locations of 1H and 2H. Two step configurations were considered; (i) a blunt leading edge with flow underneath (Case I) and (ii) a blunt leading edge with no flow underneath (Case II). It is observed from the results presented that the two Hot-Wire methods produce significantly different mean velocity and turbulence results inside the separation bubble. In particular, the SHWA method cannot detect the reverse flow velocity direction, while the Flying Hot-Wire clearly identifies the existing reverse flow. Also, in the shear flow region, the results presented indicate that measurements with a SHW probe must be treated with great caution.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):446-449. doi:10.1115/1.2822230.

A novel velocimeter consisting of multiple orthogonal disks fitted with pressure transducers has been developed. Dynamic pressure differences are measured between the center of one disk face and the center of the other face, on each of the disks. While previously-developed anemometers based on dynamic pressure differences (such as yaw or three-hole probes) can only measure velocities with a small range of directions, the new disk probe can measure three components of velocity, even in highly three-dimensional flows where the approximate direction of the flow is not known. Wind tunnel tests have shown the velocimeter to be quite accurate; it can measure velocities to ±1.4% and wind directions to ±4 deg. The velocimeter is very robust and therefore can make measurements in environments too harsh for most other velocity transducers.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):450-459. doi:10.1115/1.2822231.

This paper presents the application of a three-dimensional Navier-Stokes finite element code (NS3D) in the context of turbomachinery rotor-stator multistage interaction. A mixing-plane approach is used, in which boundary conditions at a common interface plane between adjacent blade rows are iteratively adjusted to yield a flow satisfying the continuity, momentum, and energy conservation equations, in an average sense. To further improve the solutions, a mesh adaptation technique then redistributes the mesh points of the structured grid within each component, according to an a posteriori edge-based error estimate based on the Hessian of the local flow solution. This matrix of second derivatives controls both the magnitude and direction of the required mesh movement at each node, is then implemented using an edge-based spring analogy. The methodology is demonstrated for two test cases with two types of data: a well-instrumented experimental large-scale rotating rig for a second stage compressor at UTRC and an actual engine. The latter, a two-stage compressor of a turboprop, has been only tested as a single-stage configuration, because of the quality of the experimental data available. All results compare well to the data and demonstrate the utility of the approach. In Particular, the mesh adaptation shows large improvements in agreement between the calculations and the experimental data.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):460-468. doi:10.1115/1.2822232.

A three-dimensional numerical simulation of linearly sheared flow past a circular cylinder has been performed for a shear parameter β of 0.02 and a mean Reynolds number of 131.5. A cylinder of 24 diameters span is considered. A second-order accurate finite volume scheme is used to integrate the unsteady Navier-Stokes equations. Present computations confirm both qualitatively and quantitatively, the aspects of cellular shedding as reported by several investigators through experimental studies. Up to five constant frequency cells of obliquely shedding vortices are observed. The nondimensional frequencies of these cells are observed to be lower than those given by parallel shedding correlations at the equivalent Reynolds numbers. It is also observed that the cell boundaries continuously move in time. Detailed distributions of vorticity and velocity components are presented to describe the flow. The influence of end-wall boundary conditions is studied by computing two cases, one with free-slip condition, and the other with no-slip condition on disks of radius of five cylinder diameters.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):469-478. doi:10.1115/1.2822233.

Measurements have been carried out in a tow tank on cylindrical bodies submerged in proximity of traveling surface waves. Two bodies are considered: a reference plain cylinder and another cylinder containing a pair of wings (or hydrofoils) below the cylinder, not above. The latter body owes its origin to certain species of fish which has small wings for maneuverability. The wavelength of the surface waves (λ) is of the order of the cylinder length (L) or higher (1 < λ/L < 10). Temporal measurements of axial and vertical forces and pitching moments, phase matched to the surface elevation of traveling waves, have been carried out. The time periods of the waves and depth of water pertain to deep water and intermediate depth waves. The forces and moments exhibit characteristic phase relationship with water elevation. Towing affects only vertical forces in the speed range of 0 to 1 m/s. The effect of towing and surface waves on vertical forces is roughly additive. Within the low speed range of towing evaluated, the effects of surface waves dominate those of towing. The presence of the hydrofoil and intermediate depth waves bring in some additional effects which are not well understood. In intermediate depth waves, a small plain cylinder may encounter a resonance with traveling waves which can be averted by attaching a pair of small wings to dampen pitching moment and make it speed invariant, although at a cost of increased vertical forces.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):479-487. doi:10.1115/1.2822234.

The theoretical control of low-speed maneuvering of small underwater vehicles in the dive plane using dorsal and caudal fin-based control surfaces is considered. The two dorsal fins are long and are actually mounted in the horizontal plane. The caudal fin is also horizontal and is akin to the fluke of a whale. Dorsal-like fins mounted on a flow aligned vehicle produce a normal force when they are cambered. Using such a device, depth control can be accomplished. A flapping foil device mounted at the end of the tailcone of the vehicle produces vehicle motion that is somewhat similar to the motion produced by the caudal fins of fish. The moment produced by the flapping foils is used here for pitch angle control. A continuous adaptive sliding mode control law is derived for depth control via the dorsal fins in the presence of surface waves. The flapping foils have periodic motion and they can produce only periodic forces. A discrete adaptive predictive control law is designed for varying the maximum tip excursion of the foils in each cycle for the pitch angle control and for the attenuation of disturbance caused by waves. Strouhal number of the foils is the key control variable. The derivation of control laws requires only imprecise knowledge of the hydrodynamic parameters and large uncertainty in system parameters is allowed. In the closed-loop system, depth trajectory tracking and pitch angle control are accomplished using caudal and dorsal fin-based control surfaces in the presence of system parameter uncertainty and surface waves. A control law for the trajectory control of depth and regulation of the pitch angle is also presented, which uses only the dorsal fins and simulation results are presented to show the controller performance.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):488-495. doi:10.1115/1.2822235.

A combined experimental and computational study was carried out to investigate the laminar flow of a nonlinear viscoplastic fluid through an axisymmetric sudden expansion. The yield-stress, power-law index, and the consistency index of the yield shear-thinning test fluid were 0.733 Pa, 0.68, and 0.33 Pa · s0.68 , respectively, resulting in a Hedstrom number of 1.65. The Reynolds number ranged between 1.8 and 58.7. In addition, the flow of a Newtonian fluid through the same expansion was also studied to form a baseline for comparison. Velocity vectors were obtained on the vertical center plane using a digital particle image velocimeter (PIV). From these measurements. two-dimensional distributions of axial and radial velocity as well as the stream function were calculated covering the separated, reattached and redeveloping flow regions. These results were compared to finite difference numerical solutions of the governing continuity and fully-elliptic momentum equations. The calculations were found to be in good agreement with the experimental results. Both computational and experimental results indicate the existence of two distinct flow regimes. For low Reynolds numbers, a region of nonmoving fluid is observed immediately downstream of the step and no separated flow zone exists. For the higher Reynolds numbers, a recirculating flow zone forms downstream of the expansion step, which is followed by a zone of stagnant fluid adjacent to pipe wall characterizing reattachment.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1999;121(2):496-502. doi:10.1115/1.2822236.

The initial volume of a condensing bubble released from an orifice and the shape oscillations of the bubble during the collapse process were studied. Results of a large number of original experiments of bubbles condensing in miscible and immiscible liquids were analyzed to evaluate the initial volume, the instantaneous shape, and the average aspect-ratio of the bubbles. The results were compared to common empirical correlations that were originally developed to evaluate these parameters for noncondensing bubbles. It is demonstrated that such correlations can satisfactorily predict the initial volume of condensing bubbles and their mean aspect ratio throughout the collapse process.

Commentary by Dr. Valentin Fuster

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