bluuebunny/crossref_metadata_2025 huggingface dataset:
Development, Testing, and Performance Prediction of Radial Compressor Stages for Multistage Industrial Compressors
Authors: Dalbert P. , Casey M. V. , Schurter E.
Published: November 2009

A review of some theoretical and experimental techniques currently in use for the aerodynamic development of standard stages for industrial centrifugal compressors is presented. Design methods for standardized families of radial compressor stages are summarized. The closed-loop test stands used for performance measurements are described and methods of test data analysis for identifying component performance and improving component matching are discussed. Test data obtained from standardized families of stages are analyzed using elementary dimensional analysis. This allows a simple structure for the interpolation of the large amounts of test data to be formulated. Correction formulae for secondary effects (such as changes in gas properties, Reynolds number, and small deviations from geometric similarity) are also derived from measurements. The data are incorporated into a stage-stacking calculation method for performance prediction of multistage compressors.
Unsteady Flow in a Centrifugal Compressor With Different Types of Vaned Diffusers
Authors: Haupt U. , Seidel U. , Abdel-Hamid A. N. , Rautenberg M.
Published: November 2009

Experiments were conducted to investigate the characteristics of self-excited flow oscillations in a high-performance centrifugal compressor system with a straight channel radial vaned diffuser. Fast response dynamic pressure transducers on the shroud wall and blade-mounted strain gages were used to identify the onset of the oscillations and their characteristics in space and time. In addition, flow characteristics near the shroud wall were visualized by an oil injection method, showing the extent of upstream directed reverse flow in the impeller range during significant unsteady flow compressor operation. Rotating nonuniform flow patterns were found in a wide range of operating speeds before the occurrence of surge. The number of lobes in the nonuniform flow patterns was dependent on the operating conditions and varied from two to four. Results of this experimental investigation were compared with those obtained from a previous investigation of the same compressor but with a cambered vane diffuser. Considerable similarity between the two configurations was found in the spatial distribution of the unsteady pressure field and in the frequencies of the fluctuations. The stability margin before the occurrence of surge and the operating regimes in which very intense pressure fluctuations were found were however different. In both cases, flow visualization techniques revealed the occurrence of reversed flow near the shroud wall of the impeller. Reverse flow extent up to the leading edge of the splitter blades systematically correlated with the occurrence of a nonuniform pressure pattern rotating with relatively high speed. Low rotational speed pressure patterns were observed when the extent of the reverse flow was up to the leading edge of the long blade. These different flow characteristics can be related to the occurrence of distinct rotating stall cell numbers. This result could be confirmed by unsteady pressure and blade vibration measurements.
Off-Design Performance of Supersonic Compressors With Fixed and Variable Geometry
Authors: Broichhausen K.-D. , Gallus H. E. , Mo¨nig R.
Published: November 2009

Regarding the extremely high pressure ratios of jet-engine compressors for the next decade, increasing interest belongs to the further development of supersonic compressors with supersonic relative flow at rotor inlet and supersonic absolute flow at stator inlet. In the past, different suitable design procedures for these components have been developed and tested successfully. However, there is a lack of information concerning the off-design performance of supersonic compressors. The present paper first systematically shows blading and flow path geometry of different experimentally investigated supersonic axial flow compressors. These investigations refer to combinations of characteristic rotors and stators with fixed and variable geometry. A comparison of these geometric data with the main characteristics of the flow pattern shows that, for the investigated stages, the three-dimensional passage geometry has an essential influence on the off-design performance. On the basis of this information semi-empirical models are established for a numerical description of the flow phenomena with predominant influence, as for example shock-, profile-, and endwall boundary layer losses and rotor-stator interactions. For the determination of the off-design performance, these models are incorporated into a streamline curvature calculation method. The computer model established is able to describe the off-design characteristics of the different investigated supersonic compressor stages in the most important operating range.
Inter and Intrablade Row Laser Velocimetry Studies of Gas Turbine Compressor Flows
Authors: Williams M. C.
Published: November 2009

Characterization of the aerodynamics of the flows within gas turbine intrablade passages requires the development of sensitive laser-Doppler velocimeters, a data acquisition system capable of efficiently and accurately handling substantial quantities of data, and appropriate methods of posttest data analysis and display to maximize the value of the recorded data. Pratt & Whitney developed a confocal LDV and data acquisition system that were first employed in the mid-1970s for gas turbine intrablade fan studies. Experience gained through use of the instruments in several aerodynamic studies has provided the basis for the development of a second generation system. The performance of the recently enhanced instrument configured as a single-component LDV has been demonstrated by a NASA-sponsored fan study in which approximately 200 million valid velocity signals were rapidly acquired. The high resolution and data rates achieved permitted detailed mappings of the flow at a variety of conditions and locations. Following this successful study, a second NASA-sponsored program was initiated to assess the feasibility of utilizing this LDV capability to perform detailed stator–rotor interaction studies in a multistage compressor. The equipment employed in these studies will be described and data typical of the findings will be presented.
Calculation of Turbulent Flow and Heat Transfer in Channels With Streamwise-Periodic Flow
Authors: Habib M. A. , Attya A. E. , McEligot D. M.
Published: November 2009

A computational method for the calculation of the flow and heat transfer in a channel, with elements of various heights inducing a streamwise-periodic flow, is presented and evaluated. The time-averaged conservation equations of mass, momentum, and energy were solved together using a finite-control-volume method. Reynolds stresses were obtained using a two-equation model, which solves the time-averaged equations of the turbulence kinetic energy and its dissipation rate. The calculated flow field is shown to be in satisfactory agreement with the experimental data. The results indicate that the local and overall heat loss parameters increase with increasing Reynolds and Prandtl numbers and element height and with decreasing spacing.
Design Point Variation of Three-Dimensional Loss and Deviation for Axial Compressor Middle Stages
Authors: Roberts W. B. , Serovy G. K. , Sandercock D. M.
Published: November 2009

Three-dimensional spanwise pressure loss and flow angle deviation variations have been deduced from NASA, university, and industrial sources from middle-stage research compressors operating near design point. These variations are taken as the difference above or below that predicted by blade element theory at any spanwise location. It was observed that the magnitude of the three-dimensional loss and deviation in the endwall regions is affected by hub and casing boundary layer thickness, camber, solidity, and blade channel aspect ratio for stators and rotor hubs. Rotor tip variations were found to depend on casing boundary layer thickness and tip clearance. Simple design point loss models derived from these data can aid in the design of axial compressor middle stages.
Effect of the Inlet Velocity Profile in the Three-Dimensional Flow in a Rear Axial Compressor Stage
Authors: Cyrus V.
Published: November 2009

A detailed investigation of the three-dimensional flow was carried out in a low-speed rear axial compressor stage with an aspect ratio of 1. Experimental data were obtained for both an inlet velocity profile with thin endwall boundary layer thickness and a distorted inlet velocity profile with a high turbulence intensity level. The distortion was produced by a specially designed screen. The flow mechanism in the rotor and stator blade rows is analyzed for these two velocity profiles at the design flow coefficient.
Performance of a Compressor Cascade Configuration With Supersonic Entrance Flow—A Review and Comparison of Experiments in Three Installations
Authors: Serovy G. K. , Okiishi T. H.
Published: November 2009

A cascade geometry derived from a research program on high-throughflow, transonic, axial-flow compressors was tested with similar supersonic entrance flow conditions in three linear cascade test facilities. The airfoil section used was representative of advanced rotor-blade, tip-region profiles designed to operate at inlet relative Mach numbers of 1.4 to 1.8. Objectives in the experiments were to study the reproducibility of test conditions and measured performance in facilities that are considered to be “state-of-the-art,” and to generate data sets that could be used as test cases or “benchmark” results to validate computational methods for turbomachine application. It was recognized from the beginning of the project that the aerodynamic regime involved represents a very difficult combination of problems in both experimentation and computation. This difficulty was certainly encountered; the experimental problems are fully discussed in this paper and the companion papers originating in two of the test groups. An excellent series of data sets has been obtained, and our confidence in the results is supported by the exchange of information and personnel that occurred during all phases of the experiments. The results presented here and in the forthcoming AGARD Propulsion and Energetics Panel “test case” compendium should serve as a standard for evaluating current and future computational efforts.
Test Results on the ARL 19 Supersonic Blade Cascade
Authors: Fourmaux A. , Gaillard R. , Losfeld G. , Meauze´ G.
Published: November 2009

This paper presents the ONERA contribution in a joint experimental program on the aerodynamics of supersonic airfoil cascades. The first part deals with the specific ONERA way of running cascade tests: description of the test facility, the test model, the instrumentation, and data reduction. Then, after a brief theoretical analysis of the ARL 19 cascade, some experimental results are presented and discussed.
Experimental Investigation of the Performance of a Supersonic Compressor Cascade
Authors: Tweedt D. L. , Schreiber H. A. , Starken H.
Published: November 2009

Results are presented from an experimental investigation of a linear, supersonic compressor cascade tested in the supersonic cascade wind tunnel facility at the DFVLR in Cologne, Federal Republic of Germany. The cascade was derived from the near-tip section of a high-throughflow axial flow compressor rotor and has a design relative inlet Mach number of 1.61. Test data were obtained over the range of inlet Mach numbers from 1.30 to 1.17. Side-wall boundary layer suction was used to reduce secondary flow effects within the blade passages and to control the axial-velocity-density ratio (AVDR). Flow velocity measurements showing the wave pattern in the entrance region were obtained with a laser anemometer. The unique-incidence relationship for this cascade, relating the supersonic inlet Mach number to the inlet flow direction, is discussed. The influence of static pressure ratio and AVDR on the blade performance is described, and an empirical correlation is used to show the influence of these (independent) parameters for fixed inlet conditions on the exit flow direction and the total-pressure losses.
Experimental Investigation of the Three-Dimensional Flow in an Annular Compressor Cascade
Authors: Schulz H. D. , Gallus H. D.
Published: November 2009

A detailed experimental investigation was carried out to examine the influence of blade loading on the three-dimensional flow in an annular compressor cascade. Data were acquired over a range of incidence angles. Included are airfoil and endwall flow visualization, measurement of the static pressure distribution on the flow passage surfaces, and radial-circumferential traverse measurements. The data indicate the formation of a strong vortex near the rear of the blade passage. This vortex transports low-momentum fluid close to the hub toward the blade suction side and seems to be partly responsible for the occurrence of a hub corner stall. The effect of increased loading on the growth of the hub corner stall and its impact on the passage blockage are discussed. Detailed mapping of the blade boundary layer was done to determine the loci of boundary layer transition and flow separation. The data have been compared with results from an integral boundary layer method.
Effects of Incidence on Three-Dimensional Flows in a Linear Turbine Cascade
Authors: Yamamoto A. , Nouse H.
Published: November 2009

The paper describes effects of the incidence on cascade three-dimensional flows and on the associated loss mechanisms occurring in a low-speed linear turbine rotor cascade. For each of five nominal incidences ranging from 7.2 to −53.3 deg, the cascade flow was surveyed at 15 or 16 planes located axially throughout the cascade. Blade-to-blade flows at the cascade midspan and near the endwalls, meridional flows along the blade surfaces, and static and total pressures within the cascade were analyzed in detail. The results were represented by two- or three-dimensional tomograms, on the surfaces of which the cascade flows were drawn by vectors, scalar contours, and streaklines, and from which one can easily understand the extraordinarily complicated cascade flows and the loss generation mechanisms. The present study will not only give new insight into the incidence effects, but will also contribute many solid experimental facts of a quantitative nature to our current knowledge of turbine cascade flows.
Detection of Separation Bubbles by Infrared Images in Transonic Turbine Cascades
Authors: Bra¨unling W. , Quast A. , Dietrichs H.-J.
Published: November 2009

In a test facility for straight cascades, equipped with profiles designed for a highly loaded gas turbine rotor of a high-pressure stage, experiments were conducted to clarify some effects of shock wave–boundary layer interactions. The specific aim was to determine both the position and strength of compression shocks originating from profile wake flows and the position and extent of separation bubbles. The latter are most often detected by visualization methods like surface oil flow patterns or Schlieren photographs, as well as by typical properties in wall pressure distribution curves. In addition, the infrared image technique, which has found many applications in a wide range of technical activities in the recent years, may also be used. Compared with other methods, this technique has distinct advantages in fluid mechanics applications. The whole model can be observed without disturbing the boundary layer by tappings, measuring materials, or probes. Some typical infrared images are presented and interpreted using results of pressure distribution measurements, hot-film measurements, and surface oil flow visualizations.
Automated Design of Controlled-Diffusion Blades
Authors: Sanz J. M.
Published: November 2009

A numerical automation procedure has been developed to be used in conjunction with an inverse hodograph method for the design of controlled-diffusion blades. With this procedure a cascade of airfoils with a prescribed solidity, inlet Mach number, inlet air flow angle, and air flow turning can be produced automatically. The trailing edge thickness of the airfoil, an important quantity in inverse methods, is also prescribed. The automation procedure consists of a multidimensional Newton iteration in which the objective design conditions are achieved by acting on the hodograph input parameters of the underlying inverse code. The method, although more general in scope, is applied in this paper to the design of axial flow compressor blade sections, and a wide range of examples is presented.
Computation of Three-Dimensional Turbulent Turbomachinery Flows Using a Coupled Parabolic-Marching Method
Authors: Kirtley K. R. , Lakshminarayana B.
Published: November 2009

A new coupled parabolic-marching method was developed to compute the three-dimensional turbulent flow in a turbine endwall cascade, a compressor cascade wake, and an axial flow compressor rotor passage. The method solves the partially parabolized incompressible Navier–Stokes equation and continuity in a coupled fashion. The continuity equation was manipulated using pseudocompressibility theory to give a convergent algorithm for complex geometries. The computed end-wall boundary layers and secondary flow compared well with the experimental data for the turbine cascade as did the wake profiles for the compressor cascade using a k–ε turbulence model. Suction side boundary layers, pressure distributions, and exit stagnation pressure losses compared reasonably well with the data for the compressor rotor.
The Use of Foreign Gas to Simulate the Effects of Density Ratios in Film Cooling
Authors: Teekaram A. J. H. , Forth C. J. P. , Jones T. V.
Published: November 2009

This paper provides a unique experimental assessment of the use of a foreign gas to vary the injection-to-mainstream density ratio in film-cooling experiments. It is widely recognized that it is important to scale both the velocity and density field in such tests in order that they will be representative of conditions in the gas turbine. Where it is difficult to achieve the temperature required for the correct injection-to-mainstream density ratio, some experimental techniques resort to the use of a heavier foreign gas to simulate the colder injection flow. In the experiment reported, the Isentropic Light Piston Tunnel has provided an environment in which a desired injection-to-mainstream density ratio can be achieved both by varying the temperature of the two airstreams and by using a heavier injection gas, carbon dioxide. Direct heat transfer measurements, which do not require the use of a mass transfer analogy, are taken. Use of the superposition model of film cooling allows the results to be interpreted easily to yield heat transfer parameters all obtained under isothermal wall boundary conditions. Comparison of results for air and carbon dioxide injection show that with this approach, it is possible to use a foreign injection gas to simulate the required injection-to-freestream density ratio.
Phase-Resolved Heat-Flux Measurements on the Blade of a Full-Scale Rotating Turbine
Authors: Dunn M. G. , Seymour P. J. , Woodward S. H. , George W. K. , Chupp R. E.
Published: November 2009

This paper presents detailed phase-resolved heat-flux data obtained on the blade of a Teledyne 702 HP full-stage rotating turbine. A shock tube is used as a short-duration source of heated air and platinum thin-film gages are used to obtain the heat-flux measurements. Results are presented along the midspan at several locations on the blade suction and pressure surfaces from the stagnation point to near the trailing edge. For these measurements, the turbine was operating at the design flow function and at 100 percent corrected speed. Results are presented for the design vane/blade spacing (0.19 Cs) and at a wide spacing (0.50 Cs). Data are also presented illustrating the phase-resolved blade heat-flux distribution with upstream cold gas injection from discrete holes on the vane surface. The results illustrate that several successive passages can be superimposed upon each other and that a heat-flux pattern can be determined within the passage. A Fourier analysis of the heat-flux record reveals contributions from the fundamental and first harmonic of the passage cutting frequency. Time-resolved surface pressure data obtained on the blade pressure surface are compared with heat-flux data.
Surface Heat Transfer Fluctuations on a Turbine Rotor Blade Due to Upstream Shock Wave Passing
Authors: Johnson A. B. , Rigby M. J. , Oldfield M. L. G. , Ainsworth R. W. , Oliver M. J.
Published: November 2009

A theoretical model to explain observed rapid large-scale surface heat transfer rate fluctuations associated with the impingement of nozzle guide vane trailing edge shock waves on a transonic turbine rotor blade is described. Experiments were carried out in the Oxford Isentropic Light Piston Cascade Tunnel using an upstream rotating bar system to simulate the shock wave passing. High-frequency surface heat transfer and pressure measurements gave rapidly varying, large, transient signals, which schlieren photography showed to be associated with the impingement of passing shock waves on the surface. Heat transfer rates varying from three times the mean value to negative quantities were measured. A simple first-order perturbation analysis of the boundary layer equations shows that the transient adiabatic heating and cooling of the boundary layer by passing shock waves and rarefactions can give rise to high-temperature gradients near the surface. This in turn leads to large conductive heat transfer rate fluctuations. The application of this theory to measured fluctuating pressure signals gave predictions of fluctuating heat transfer rates that are in good agreement with those measured. It is felt that the underlying physical mechanisms for shock-induced heat transfer fluctuations have been identified. Further work will be necessary to confirm them in rotating experiments.
Heat Transfer, Pressure Drop, and Mass Flow Rate in Pin Fin Channels With Long and Short Trailing Edge Ejection Holes
Authors: Lau S. C. , Han J. C. , Batten T.
Published: November 2009

Experiments have been conducted to study the turbulent heat transfer and friction characteristics in pin fin channels with small trailing edge ejection holes that are commonly found in modern internally cooled turbine airfoils. The main objective of the investigation is to examine the effects of varying the length and the configuration of the trailing edge ejection holes on the overall heat transfer, the overall pressure drop, the local pressure distribution, and the local mass flow rate distribution in the pin fin channel. The staggered pin fin array (L/D = 1.0, X/D = S/D = 2.5) in the test channel has 15 rows of three pins. The diameter of the ejection holes is one-half the diameter of the pins. There are 30 or 23 ejection holes on one of the side walls of the test channel and six similar ejection holes at the radial flow exit. Experimental results are obtained for two trailing edge ejection hole lengths, four ejection hole configurations, and Reynolds numbers between 10,000 and 60,000. The results show that the overall heat transfer increases when the length of the trailing edge ejection holes is increased and when the trailing edge ejection holes are configured so that much of the cooling air is forced to flow farther downstream in the radial flow direction before exiting the pin fin channel through ejection holes. The overall Nusselt number can be correlated with an equation of the form NuD = a (ReD)b, where the values of the exponent b are about the same for all the test cases with trailing edge flow ejection. Results also show that the increase in the overall heat transfer is generally accompanied by an increase in the overall pressure drop (that is, an increase in the required pumping power), except that the overall heat transfer is lower and the overall pressure drop is higher when there is no radial flow ejection. In the cases with both radial and trailing edge flow ejection, about 15 to 20 percent of the flow exits through the tip bleed holes.
The Two-Dimensional Behavior of Film Cooling Jets on Concave Surfaces
Authors: Schwarz S. G. , Goldstein R. J.
Published: November 2009

Local impermeable wall effectiveness is measured along a concave surface downstream of a row of film cooling jets. Three different injection hole diameters, two density ratios (0.95 and 2.0), and a wide range of blowing rates (0.3 to 2.7) are considered. Except close to injection, where normal momentum is strong, an increase in blowing rate results in improved film cooling performance. This is attributed to an increase in thermal mass (capacity) of coolant, tangential momentum of the jet, and blockage. Far downstream of injection, the normal and tangential momentum weaken, but blockage and large thermal mass at high injection rates still keep effectiveness high. Lateral mixing of the jets caused by the unstable concave mainflow results in film-cooling performance, which correlates well with the same parameter, which is influential in slot injection on a flat surface. Deviation from this two-dimensional behavior is found only at very low blowing rates and weak wall curvature.
Hydrodynamic Measurements of Jets in Crossflow for Gas Turbine Film Cooling Applications
Authors: Pietrzyk J. R. , Bogard D. G. , Crawford M. E.
Published: November 2009

This paper presents the results of a detailed hydrodynamic study of a row of inclined jets issuing into a crossflow. Laser-Doppler anemometry was used to measure the vertical and streamwise components of velocity for three jet-to-mainstream velocity ratios: 0.25, 0.5, and 1.0. Mean velocity components and turbulent Reynolds normal and shear stress components were measured at locations in a vertical plane along the centerline of the jet from 1 diameter upstream to 30 diameters downstream of the jet. The results, which have application to film cooling, give a quantitative picture of the entire flow field, from the approaching flow upstream of the jet, through the interaction region of the jet and mainstream, to the relaxation region downstream where the flow field approaches that of a standard turbulent boundary layer. The data indicate the existence of a separation region in the hole from which the jet issues, causing high levels of turbulence and a relatively uniform mean velocity profile at the jet exit.
Turbulence Measurements in a Multistage Low-Pressure Turbine
Authors: Binder A. , Schroeder Th. , Hourmouziadis J.
Published: November 2009

The flow in the rotor blades of a five-stage low-pressure turbine was investigated experimentally using hot-film probes. Time averaging, Fourier transforms, and ensemble averaging are applied for data reduction. The techniques prove to be a very helpful instrument for the assessment of the flow characteristics in the relative frame. A strong interaction is identified between two successive rows of rotor blades. A physical model, developed from velocity and turbulence results, gives a comprehensive understanding of the phenomenon. The main parameter is the nonuniformity of the flow entering the downstream blade row. Separation occurs when the wake of the upstream rotor blades enters the blade passage near the leading edge, preferably on the pressure side. The interaction is quasi-steady in the relative frame and rotates with the rotor speed. It was observed only in one of three investigated blade rows. Further studies are necessary to identify the mechanism correlating the nonuniformity to the separation.
Potential Pressure Field by Stator/Downstream Strut Interaction
Authors: Kodama H. , Nagano S.
Published: November 2009

The potential pressure field caused by stator/downstream strut interaction is investigated analytically and experimentally. It is shown that the small disturbance theory using an actuator disk model gives a good prediction of the forward propagation of downstream pressure perturbations. The calculation method of stator tailoring to shield the upstream field from the effect of downstream struts is also shown and verified by experiments.
A Theoretical Analysis of Flow Through the Nucleating Stage in a Low Pressure Steam Turbine
Authors: Skillings S. A. , Walters P. T. , Jackson R.
Published: November 2009

In order to improve steam efficiency and reliability, the phenomena associated with the formation and growth of water droplets must be understood. This paper describes a theoretical investigation into flow behavior in the nucleating stage, where the predictions of a one-dimensional theory are compared with measured turbine data. Results indicate that droplet sizes predicted by homogeneous condensation theory cannot be reconciled with measurements unless fluctuating shock waves arise. Heterogeneous effects and flow turbulence are also discussed along with their implications for the condensation process.
Structure of Tip Clearance Flow in an Isolated Axial Compressor Rotor
Authors: Inoue M. , Kuroumaru M.
Published: November 2009

Ensemble-averaged and phase-locked flow patterns in various tip clearances of two axial compressor rotors were obtained by aperiodic multisampling technique with a hot wire in the clearance and with a high-response pressure sensor on the casing wall. A leakage flow region distinct from a throughflow region exists at every clearance. In the case of a small tip clearance, the leakage jet flow interacts violently with the throughflow near the leading edge, and a rolling-up leakage vortex decays downstream. As the clearance increases, a stronger leakage vortex comes into existence at a more downstream location, and a reverse flow due to the vortex grows noticeably. A scraping vortex is recognized at the pressure side near the trailing edge only for the small clearance. A horseshoe vortex appears in the upstream half of the through flow region for every tip clearance. The solidity does not affect the flow pattern substantially except for the interaction of the leakage vortex with the adjacent blade and wake.
The Influence of Turbine Clearance Gap Leakage on Passage Velocity and Heat Transfer Near Blade Tips: Part I—Sink Flow Effects on Blade Pressure Side
Authors: Metzger D. E. , Rued K.
Published: November 2009

A study has been conducted to investigate influences of tip leakage flow on heat transfer and flow development along the pressure side of a gas turbine blade. An analysis of the sink character of the flow situation indicates that high velocities and accelerations are generated very near the gap, and an apparatus was specifically designed to model the phenomena and to permit resolution of the expected localized near-gap heat transfer enhancement. In the experiments, leakage flow was drawn from an adjustable stream wise corner slot in a straight square test channel. A thin stainless steel ohmic-heated test surface adjacent to the slot simulated the airfoil surface. Supporting nonintrusive mean and fluctuating flowfield measurements were conducted with a laser-Doppler anemometer to aid interpretation of the heat transfer results and to provide a basis for comparison with future numerical predictions. The flowfield measurements confirm that near the gap the flow is highly accelerated, and indicate apparent relaminarization of the initially turbulent boundary layer. The heat transfer measurements show that leakage generates large increases in local heating near the gap. The presence of this undesirable enhancement helps to explain observed in-service material distress and failures of blades that appear to initiate at the pressure side tip.
The Influence of Turbine Clearance Gap Leakage on Passage Velocity and Heat Transfer Near Blade Tips: Part II—Source Flow Effects on Blade Suction Sides
Authors: Rued K. , Metzger D. E.
Published: November 2009

An experimental study has been designed and conducted to investigate turbine blade suction side heat transfer and flow near the tip clearance gap. Modeling of the phenomena was carried out in a water tunnel with injection through an adjustable streamwise corner slot in a square test channel. A thin stainless steel ohmic-heated test surface adjacent to the slot simulated the airfoil surface and permitted fine resolution of local heat transfer rates. Mean and fluctuating flow field measurements were conducted with a laser-Doppler anemometer to aid interpretation of the heat transfer results and to provide a basis for comparison with future numerical predictions. The results indicate that flow leakage from the turbine tip clearance gap into the suction side hot gas path results in more extensive and complex heat transfer effects than those measured for the blade pressure side in the companion Part I study. The character of the heat transfer andflow field deviations from closed gap conditions is strongly dependent on the particular combination of flow and geometry parameters present. The observed characteristics have been partitioned into categories of similar behavior, and the parameter combinations that define the boundaries between categories have been tentatively identified for the benefit of designers. The overall conclusions of this study and of the parallel study reported in Part I are that the effects of tip leakage flow on airfoil surface heat transfer near the blade tip can be very significant on both pressure and suction sides, and should be taken into account in blade cooling specification and design.
An Approximate Solution for the Flow Between a Rotating and a Stationary Disk
Authors: Owen J. M.
Published: November 2009

The linear Ekman-layer equations are solved for the case of a rotor-stator system with a superimposed radial outflow of fluid. For laminar flow, the predicted rotational speed of the core between the boundary layers on the rotor and stator agrees well with existing experimental measurements when the superimposed flow rate is zero, but the theoretical solutions underestimate the core rotation when the flow rate is nonzero. For turbulent flow, the linear theory underestimates the core rotation under all conditions. Solutions of the turbulent momentum-integral equations for the rotor are used to provide an approximation for the core rotation that agrees reasonably well with the measured values over a range of flow rates and rotational speeds. Despite the fact that the equations take no account of the presence of the peripheral shroud, the approximate solutions for the moment coefficients are in reasonable agreement with the available experimental data. It is shown that the core rotation is suppressed and the moment coefficient equals that of a free disk when the superimposed flow rate equals the free-disk entrainment rate.
Turbulent Flow Velocity Between Rotating Co-Axial Disks of Finite Radius
Authors: Louis J. F. , Salhi A.
Published: November 2009

The turbulent flow between two rotating co-axial disks is driven by frictional forces. The prediction of the velocity field can be expected to be very sensitive to the turbulence model used to describe the viscosity close to the walls. Numerical solutions of the Navier–Stokes equations, using a k–ε turbulence model derived from Lam and Bremhorst, are presented and compared with experimental results obtained in two different configurations: a rotating cavity and the outflow between a rotating and stationary disk. The comparison shows good overall agreement with the experimental data and substantial improvements over the results of other analyses using the k–ε models. Based on this validation, the model is applied to the flow between counterrotating disks and it gives the dependence of the radial variation of the tangential wall shear stress on Rossby number.
The Use of Fins to Reduce the Pressure Drop in a Rotating Cavity With a Radial Inflow
Authors: Chew J. W. , Farthing P. R. , Owen J. M. , Stratford B.
Published: November 2009

A combined theoretical and experimental study of radial inflow through a rotating cavity is reported. It is shown that radial fins attached to one of the disks are effective in reducing the pressure drop across the cavity. The mathematical model, is an extension of earlier plane-disk momentum-integral methods; the fins are treated as rectangular rib elements and a rough-disk model is derived. Numerical solutions of the integral equations are given. An approximate linear solution is also derived. Experiments were conducted when both disks were plane and when one of the disks was fitted with 60 radial fins. Flow visualization revealed the flow structure in the cavity and confirmed some of the assumptions used in the theoretical model. Measurements and predictions of the pressure drop across the cavity were in reasonable agreement.
Low Aspect Ratio Axial Flow Compressors: Why and What It Means
Authors: Wennerstrom A. J.
Published: November 2009

One of the more visible changes that has occurred in fans and compressors for aircraft turbine engines that have entered development since about 1970 has been a significant reduction in the aspect ratio of the blading. This has brought with it a greatly reduced engine parts count and improved ruggedness and aeroelastic stability. This paper traces the evolution of thinking concerning appropriate aspect ratios for axial flow compressors since the early years of the aircraft turbine engine. In the 1950’s, moderate aspect ratios were favored for reasons of mechanical design. As mechanical design capability became more sophisticated, several attempts were made, primarily in the 1960s, to employ very high aspect ratios to reduce engine size and weight. Four of these programs are described that were largely unsuccessful for both mechanical and aerodynamic reasons. After 1970, the pendulum swung strongly in the other direction and designs of very low aspect ratio began to emerge. This has had a significant impact on compressor design systems, and a number of the ways in which design systems have been affected are discussed. Some concluding remarks are made concerning the author’s opinion of trends in the near future in aerodynamic design technology.
Experimental Investigation of Rotating Stall in a Mismatched Three-Stage Axial Flow Compressor
Authors: Giannissis G. L. , McKenzie A. B. , Elder R. L.
Published: November 2009

This paper reports on an examination of rotating stall in a low-speed three-stage axial flow compressor operating with various degrees of stage mismatch. The objective of this study was to simulate the mismatching that occurs in high-speed multistage compressors when operating near surge. The study of the stall zones involved the use of fast response measurement techniques. The study clearly shows how stages can operate in an axisymmetric fashion even when heavily stalled, since rotating stall inception requires the stall of more than one stage. The study also compares conditions required for full-span and part-span stall and suggests that the part-span stall structure is more relevant to high-speed multistage compressors.
Self-Sensing Active Magnetic Dampers for Vibration Control
Authors: Bonfitto Angelo , De Lépine Xavier , Silvagni Mario , Tonoli Andrea
Published: November 2009

The aim of this paper is to investigate the potential of a self-sensing strategy in the case of an electromagnetic damper for the vibration control of flexible structures and rotors. The study has been performed in the case of a single degree of freedom mechanical oscillator actuated by a couple of electromagnets. The self-sensing system is based on a Luenberger observer. Two sets of parameters have been used: nominal ones (based on simplifications on the actuator model) and identified ones. In the latter case, the parameters of the electromechanical model used in the observer are identified starting from the open-loop system response. The observed states are used to close a state-feedback loop with the objective of increasing the damping of the system. The results show that the damping performance are good in both cases, although much better in the second one. Furthermore, the good correlation between the closed-loop model response and the experimental results validates the modeling, the identification procedure, the control design, and its implementation. The paper concludes on a sensitivity analysis, in which the influence of the model parameters on the closed-loop response is shown.
An Iterative-Based Feedforward-Feedback Control Approach to High-Speed Atomic Force Microscope Imaging
Authors: Wu Ying , Zou Qingze
Published: November 2009

This article presents an iterative-based feedforward-feedback control approach to achieve high-speed atomic force microscope (AFM) imaging. AFM-imaging requires precision positioning of the probe relative to the sample in all x-y-z axes directions. Particularly, this article is focused on the vertical z-axis positioning. Recently, a current-cycle-feedback iterative-learning-control (CCF-ILC) approach has been developed for precision tracking of a given desired trajectory (even when the desired trajectory is unknown), which can be applied to achieve precision tracking of sample profile on one scanline. In this article, we extend this CCF-ILC approach to imaging of entire sample area. The main contribution of this article is the convergence analysis and the use of the CCF-ILC approach for output tracking in the presence of desired trajectory varation between iterations—the sample topography variations between adjacent scanlines. For general case where the desired trajectory variation occurs between any two successive iterations, the convergence (stability) of the CCF-ILC system is addressed and the allowable size of desired trajectory variation is quantified. The performance improvement achieved by using the CCF-ILC approach is discussed by comparing the tracking error of using the CCF-ILC technique to that of using feedback control alone. The efficacy of the proposed CCF-ILC control approach is illustrated by implementing it to the z-axis control during AFM-imaging. Experimental results are presented to show that the AFM-imaging speed can be substantially increased.
Physical and electrical characterizations of metal-oxide-semiconductor capacitors fabricated on GaAs substrates with different surface chemical treatments and Al2O3 gate dielectric
Authors: Garcia-Gutierrez Domingo I. , Shahrjerdi Davood , Kaushik Vidya , Banerjee Sanjay K.
Published: November 2009

The authors present experimental evidence on the impact of three different chemical surface treatments on the interface between the GaAs substrate and the aluminum oxide dielectric layer used in the fabrication of metal-oxide-semiconductor capacitors. The three different chemical surface treatments studied prior to atomic layer deposition (ALD) of the dielectric layer include (a) GaAs native oxide removal in a dilute HF solution only, (b) HF etch followed by a NH4OH treatment, and (c) HF etch followed by a (NH4)2S treatment. Moreover, interfacial self-cleaning of nontreated GaAs wafers upon ALD of aluminum oxide using trimethyl aluminum precursor was examined. Transmission electron microscopy, electron energy loss spectroscopy (EELS) and capacitance-voltage (C-V) data showed slight differences among the nontreated, HF-only, and NH4OH treated samples. However the (NH4)2S treated sample showed improved capacitance-voltage characteristics as well as an improved aluminum oxide/GaAs interface compared to the other three samples. Additionally, the characteristic oxygen K EELS peak suggests the presence of a thin additional layer close to the center of the high-κ layer containing oxygen, tantalum, and aluminum, as a consequence of probable plasma damage to the high-κ layer during the TaN metal gate deposition.
Optimum Onager: The Classical Mechanics of a Classical Siege Engine
Authors: Denny Mark
Published: November 2009

The onager is a throwing weapon of classical antiquity, familiar to both the ancient Greeks and Romans. Here we analyze the dynamics of onager operation and derive the optimum angle for launching a projectile to its maximum range. There is plenty of scope for further considerations about increasing onager range, and so by thinking about how this machine might be improved, a student can gain insight beyond the equations of motion and can test hypotheses on readily available working models. Some of these performance improvements are considered in this paper.
Quiz Corrections: Improving Learning by Encouraging Students to Reflect on Their Mistakes
Authors: Henderson Charles , Harper Kathleen A.
Published: November 2009

Most introductory physics instructors administer several quizzes and/or exams each term. Instructors are willing to invest the significant time it takes to develop, administer, and grade these assessments because they believe that regular assessments help students learn. However, instructors also believe that students do not make full use of the learning potential of these assessments. For example, in an interview study, six college physics instructors were asked how they expect their students to utilize an instructor solution after a test. All said they expect students to compare the instructor solution with their own and learn from any mistakes made. However, the instructors all thought that most students do not do this sort of comparison and only look at solutions superficially, if at all.1
Tension of a Soft Spring in Contact with a Cylinder
Authors: Lee Wen-Tang , Hsiao Huang-Wen
Published: November 2009

It is possible for a relatively small force, applied to one end of a rope, to support a much larger force if the rope is merely wrapped a few times around a post. This setup, called a capstan, has been discussed in a number of papers in this journal and elsewhere.1–3 If we wrap a cord around a rough curved surface, T1 and T2 are the tensions in the cord on the two sides of the surface (see Fig. 1). At the point the cord is about to slip, the theory predicts1 T2 = T1eμθ, (1) where μ is the coefficient of static friction between the cord and surface. The amount of wrapping is indicated by the angle θ measured in radians. A common experiment is to measure T1 and T2 for various values of θ and to obtain the coefficient of friction μ from the slope of a plot of ln (T2/T1) versus θ. [Note that Eq. (1) implies ln (T2/T1) = μθ.] The tension in the string varies exponentially with position along the cylinder. It would be difficult to measure that variation using the experiment represented by Fig. 1. However, if the string is replaced by a tightly wound spring, the tension in the spring at any point can be found from the separation between adjacent coils (greater separation means greater tension). Among the capstan-related experiments described by Levin3 is one in which a spring with unequal weights connected to its ends is draped over a rough-surfaced horizontal cylinder (Fig. 2). With this arrangement, Levin is able to demonstrate qualitatively the variation in the tension in the spring. We describe here a similar experiment in which quantitative measurements of the tension variation in the spring can be obtained. From those measurements, the coefficient of static friction between the spring and the cylinder can be calculated.
Fats Deliver a Painful Message
Authors: Gough Nancy R.
Published: November 2009

Oxidized products of linoleic acid are endogenous ligands of TRPV1, a channel that activates nerves that signal pain.
Single File or En Masse
Authors: Wong Wei
Published: November 2009

TGF-β signaling causes metastasizing breast cancer cells to change from collective to single cell movement.
Aiming Straight for the Heart: Prolyl Hydroxylases Set the BAR
Authors: Garcia Joseph A.
Published: November 2009

β
2
-adrenergic receptor density is regulated by oxygen availability.
A Gluconeogenic Tryst in the Nucleus, with ER Stress as the Third Wheel
Authors: Rutkowski D. Thomas
Published: November 2009

A transcriptional co-regulator links gluconeogenesis and ER stress responses in the liver.
FBXO31: A New Player in the Ever-Expanding DNA Damage Response Orchestra
Authors: Shiloh Yosef
Published: November 2009

FBXO31 is a damage-induced checkpoint protein that enhances cyclin D1 degradation in response to genotoxic stress.
Nutrition-Minded Cell Cycle
Authors: Shiozaki Kazuhiro
Published: November 2009

Crosstalk between TOR and MAPK signaling pathways determines mitotic onset in fission yeast during nutrition stress.