Boundary Layer Measurements Research Articles

1202 航空用低圧タービン翼の高負荷化に関する研究 : 円柱後流特性の影響(OS12-1 流体機械の諸問題,オーガナイズドセッション)

This paper details experimental studies on the flow field around a low-pressure linear turbine cascade. Highly loaded low-pressure turbine (LPT) blades are one of the key paths to successful future aero-engines, however those blades are usually accompanied with separation bubble, especially at cruise condition, eventually leading to the increase in aerodynamic loss. The purpose of this study is to clarify the effect of incoming wakes upon the aerodynamic loss of the rotor blade cascade, through the measurements of wake-affected boundary layers upon including separation bubble for low Reynolds number conditions. Two types of cylindrical bars on the timing belts work as wake generator to emulate upstream stator wakes that impact the rotor blade. Hot-wire probe measurement is conducted over the blade suction surface to understand to what extent and how the incoming wakes affect the boundary layers containing separation bubble. This paper focuses on difference of bar diameter in the characteristics of the wake affected separated boundary layer.

Intermittency in the atmospheric surface layer: Unresolved or slowly varying?

We present streamwise velocity structure functions 〈 δ v L ( τ ) 〉 = 〈 | v ( t + τ ) − v ( t ) | p 〉 (with p = 1 : 5 ) obtained in the near neutral atmospheric surface layer at the Utah SLTEST site at the highest terrestrial Reynolds number R e τ = O ( 10 6 ) . We show that the occurrence of very large scale coherent oscillations in the streamwise velocity throughout the wall region, interpreted as genuine structural features of the canonical turbulent boundary layer, affects the scaling exponents of the p > 3 order structure functions. This results in a slight alteration of the intermittent behavior of the velocity field. It was found that for positive (fast) large scale oscillation of the low-pass filtered velocity signal, deviations from the Kolmogorov K41 prediction (absence of multiscaling) are more marked, as compared to negative (slow) excursion. The results are discussed in terms of convergence of statistics from atmospheric boundary layer measurements.

Development of Laser Techniques for Acoustic Boundary Layer Measurements. Part II: Comparison of LDV and PIV Measurements to Analytical Calculation

The adaptation of Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV) for acoustic boundary layer measurements is considered. The specificities of acoustic boundary layer are presented and the theoretical expression of acoustic particle velocity is reminded. Appropriate parameters of the PIV system for sound measurements are determined. Results of LDV and PIV measurements of particle velocity profiles in acoustic boundary layers are compared with theoretical predictions based on the literature for different phases along the acoustic period. These results are very satisfactory and show that these two techniques are suitable for acoustic boundary layer measurements.

The Effect of Leading-Edge Geometry on Wake Interactions in Compressors

The effect of leading-edge geometry on the wake/boundary-layer interaction was studied in a low-speed single-stage HP compressor. Both a 3:1 elliptic and a circular leading edge were tested on a controlled diffusion aerofoil stator blade. Experiments were undertaken on the stator suction surface; these included hotwire boundary-layer traverses, surface hotfilm measurements, and high resolution leading-edge pressure measurements. Steady computational fluid dynamics (CFD) predictions were also performed to aid the interpretation of the results. The two leading-edge shapes gave rise to significantly different flows. For a blade with an elliptic leading edge (Blade A), the leading-edge boundary layer remained attached and laminar in the absence of wakes. The wake presence led to the formation of a thickened laminar boundary layer in which turbulent disturbances were observed to form. Measurements of the trailing-edge boundary layer indicated that the wake/leading-edge interaction for Blade A raised the suction-surface loss by 20%. For a blade with a circular leading edge (Blade B), the leading-edge boundary-layer exhibited a separation bubble, which was observed to reattach laminar in the absence of wakes. The presence of the wake moved the separation position forward while inducing a turbulent reattachment upstream of the leading-edge time-average reattachment position. This produced a region of very high momentum thickness at the leading edge. The suction-surface loss was found to be 38% higher for Blade B than for Blade A. Wake traverses downstream of the blades were used to determine the total profile loss of each blade. The profile loss of Blade B was measured to be 32% higher than that of Blade A.

Development of Laser Techniques for Acoustic Boundary Layer Measurements.<BR>Part I: LDV Signal Processing for High Acoustic Displacements

A statistical model of the Laser Doppler signal in the case of pure acoustics is proposed. It appears that two different cases should be considered depending on the ratio of acoustic displacement amplitude to probe volume diameter. The processing of Laser Doppler Velocimetry signal in the case of high particle displacements (with oscillations across the measuring volume) is then considered. A specific signal post-processing strategy is proposed to determine the acoustic frequency, the acoustic velocity amplitude and its phase. First, the acoustic frequency is estimated by means of synchronous analysis weighted by arrival times. Then, the signal is uniformly re-sampled and the phase of the acoustic velocity is calculated. Lastly, a least-square method weighted by local probability density function is used to determine the acoustic velocity amplitude. This method permits an accurate estimation of the three acoustic parameters (frequency, velocity amplitude and phase) even in the adverse conditions induced by the proximity of a wall and is applied to oscillating viscous boundary layer measurements.

Two-point correlations in high Reynolds number flat plate turbulent boundary layers

Two-point correlations of turbulent boundary layer are presented for Reθ of 9800 and 19,100. The results are based on wind tunnel measurements performed in the 30 cm thick boundary layer of the 21.6 m long LML (Laboratoire de Mécanique de Lille) boundary layer research facility. Simultaneous hot-wire probe measurements of the entire boundary layer at 143 different points on an array are used for computation of the two-point correlations. The two-point correlations in the streamwise–spanwise plane at 11 different wall-normal positions covering one boundary layer thickness in the spanwise direction show that the maximal extension of the correlations in the streamwise direction is bounded within ±3.5δ for both of the Reynolds number tested. The shapes of the positive correlations in the streamwise–spanwise plane at different wall-normal positions are similar throughout the boundary layer from nearly the freestream to the wall. The correlations in the streamwise–wall-normal plane for 11 different wall-normal reference positions also show that the correlations in some cases cover the entire boundary layer. The streamwise extent of the correlations in the streamwise–wall-normal plane is about 7–8 boundary layer thicknesses. Two-point correlation maps for the streamwise–wall-normal plane reveal the existence of non-zero correlations between even the intermittent region and near-wall region.

PIV measurements of turbulent boundary layer over a rod-roughened wall

The effects of surface roughness on a turbulent boundary layer (TBL) were investigated by performing particle image velocimetry (PIV) measurements. The Reynolds number based on the momentum thickness was about Re θ = 1000–1500. The roughness elements used were periodically arranged two-dimensional spanwise rods, and the ratio of the boundary layer thickness to the roughness height was k/ δ = 0.025. Along the streamwise direction, self-preserving forms of the streamwise mean velocity and the turbulent Reynolds stress were obtained. Spatially developing characteristics of the rough wall TBL were examined. Introduction of the roughness elements augmented turbulent stresses in the region of y < 4–5 k s, where k s is an effective sand roughness height. We found that the effective sand roughness height ( k s) is a more appropriate length scale for representing the extent of roughness effect rather than the roughness height ( k). By using PIV measurements, the effects of surface roughness on the turbulence structure in the roughness sublayer were examined and compared with previous direct numerical simulation (DNS) results. Iso-contours of the mean velocity and Reynolds stress in the roughness sublayer obtained by PIV were in very good agreement with DNS findings. Vortex structures induced by the rod-roughness and strong sweep events were also examined.

The Heat/Mass Transfer Analogy for a Simulated Turbine Endwall With Fillets

Mass transfer measurements are employed as alternative methods for heat transfer measurement because of the difficulty of heat transfer measurements in thin boundary layers, complicated secondary flows, and large thermal gradients. Even though mass transfer experiments are fast and show detailed local measurement data, the conversion of mass transfer results to heat transfer data requires the heat/mass transfer analogy factors in detail. Therefore, the usefulness of mass transfer data depends on finding a simple analogy factor. The heat/mass transfer analogy on a simulated turbine endwall with fillets is evaluated in the present paper. Since the heat/mass transfer analogy factor may not be always the same, the heat/mass transfer analogy should be verified for other different geometries and experimental conditions. To utilize the heat/mass transfer analogy fully, it is necessary to check that the presence of different aerodynamic conditions caused by the fillets affects the heat/mass transfer analogy on a simulated turbine endwall with fillets. To compare heat transfer data and mass transfer data, heat transfer measurements on the endwall with fillets are conducted with a thermal boundary layer measurement technique and mass transfer measurements employing naphthalene sublimation technique on the endwall with the fillets are extracted from literature with equivalent experimental conditions and a similar geometry. As expected by heat transfer and mass transfer equations, the heat/mass transfer analogy factor is applied and shows a good agreement between heat transfer and mass transfer results on the endwall with the fillets from the leading edge to the trailing edge.

Simultaneous phosphor and CARS thermometry at the wall–gas interface within a combustor

Phosphor thermometry and vibrational coherent anti-stokes Raman spectroscopy (CARS) were applied simultaneously to examine gas–solid interfaces in a generic combustor. For this purpose, an internally air-cooled obstacle was installed within an optically accessible, pressurized combustion chamber. During the operation of a turbulent, swirled n-heptane flame, the obstacle’s surface temperature and the surface-normal gas temperature distribution were measured. The surface temperature was determined by Thermographic Phosphors, materials whose phosphorescence decay times depend on their temperature. Following a pulsed UV laser excitation (355nm), the 659nm emission band of Mg4FGeO6:Mn was monitored by a photomultiplier tube.Non-invasive temperature measurements in the flue gas region of the n-heptane spray flame near the surface were performed pointwise by vibrational CARS of diatomic nitrogen. Beams from a frequency doubled Nd:YAG laser (532nm) and a modeless broadband dye laser (607nm) were phase-matched within a surface-parallel, planar BOXCARS configuration. This allowed gas phase thermometry as close as 30μm to the surface.The thermal boundary layer and wall temperature measurements were consistent with each other. This demonstrates the potential of spectrocopic techniques to study gas–solid interfaces with high temporal and spatial resolution. Using the interior surface temperature within the cooling channel measured by a thermocouple, the heat flux through the wall and the local heat transfer coefficient at the front side of the obstacle were estimated.

The Influence of Viscous Effects and Physical Scale on Cavitation Tunnel Contraction Performance

The general performance of an asymmetric cavitation tunnel contraction is investigated using computational fluid dynamics (CFD) including the effects of fluid viscosity and physical scale. The horizontal and vertical profiles of the contraction geometry were chosen from a family of four-term sixth-order polynomials based on results from a CFD analysis and a consideration of the wall curvature distribution and its anticipated influence on boundary layer behavior. Inviscid and viscous CFD analyses were performed. The viscous predictions were validated against boundary layer measurements on existing full-scale cavitation tunnel test section ceiling and floor and for the chosen contraction geometry against model-scale wind tunnel tests. The viscous analysis showed the displacement effect of boundary layers to have a fairing effect on the contraction profile that reduced the magnitude of local pressure extrema at the entrance and exit. The maximum pressure gradients and minimum achievable test section cavitation numbers predicted by the viscous analysis are correspondingly less than those predicted by the inviscid analysis. The prediction of cavitation onset is discussed in detail. The minimum cavitation number is shown to be a function of the Froude number based on the test section velocity and height that incorporate the effects of physical scale on cavitation tunnel performance.

Transition on the T106 LP Turbine Blade in the Presence of Moving Upstream Wakes and Downstream Potential Fields

Boundary layer measurements were performed on a cascade of the T106 high lift low-pressure (LP) turbine blades that was subjected to upstream wakes and a moving downstream potential field. Tests were carried out at a low level of inlet freestream turbulence (0.5%) and at a higher (4.0%). It is found that perturbations in the freestream due to both disturbances are superposed on each other. This affects the magnitude of the velocity perturbations at the edge of the boundary layer under the wakes as well as the fluctuations in the edge velocity between the wakes. Furthermore, the fluctuations in the adverse pressure gradient on the suction surface depend on the relative phase of the upstream and downstream disturbances, providing an additional stimulus for clocking studies. Time-mean momentum thickness values calculated from laser Doppler anemometry (LDA) traverses performed near the suction surface trailing edge are used to identify the optimum relative phase angle of the combined interaction. Unsteady suction surface pressures, quasiwall shear stress and LDA data illustrate the resulting multimode process of transition, which is responsible for the observed clocking effects. The optimum relative phase angle of the upstream wake and the downstream potential field can produce 0.25% of efficiency improvement through the reduction of the suction surface boundary layer loss. This reduction is mainly related to the calmed region and the laminar flow benefits that can be more effectively utilized than when only the upstream wakes are present. During the remaining parts of the cycle, the features that are usually associated with the wake and the potential field effects are still present.

Stereoscopic PIV measurements of a turbulent boundary layer with a large spatial dynamic range

The flow in a streamwise/wall-normal plane of a turbulent boundary layer at moderate Reynolds number (Reθ = 2,200) is characterized using two stereo PIV systems just overlapping in the streamwise direction. The aim is to generate SPIV data for near-wall turbulence with enough spatial dynamic range to resolve most of the coherent structures present in the flow and to facilitate future comparisons with direct numerical simulations. This is made possibly through the use of four cameras with large CCD arrays (4,008 px × 2,672 px) and through a rigorous experimental procedure designed to minimize the impact of measurement noise on the resolution of the small scales. For the first time, both a large field of view [Sx; Sy] = [2.6δ; 0.75δ] and a high spatial resolution (with an interrogation window size of 13.6+) have been achieved. The quality of the data is assessed through an analysis of some of the statistical results such as the mean velocity profile, the rms and the PDF of the fluctuations, and the power spectra.

Ten years of operational boundary-layer measurements at the Richard - Aßmann Observatory Lindenberg: The role of remote sensing

Remote sensing forms an essential part of the MOL-RAO boundary-layer measurements. Special emphasis will be put on the place of scintillometry as a technique to derive the sensible turbulent heat flux at the regional scale thus bridging the gap between local micrometeorological measurements and the typical spatial resolution of regional-scale meteorological models and of satellite images.

Ten years of operational boundary-layer measurements at the Richard - Aßmann Observatory Lindenberg: The role of remote sensing

Ten years of operational boundary-layer measurements at the Richard - Aßmann Observatory Lindenberg: The role of remote sensing

On the Physics of Heat Transfer and Aerodynamic Behavior of Separated Flow Along a Highly Loaded Low Pressure Turbine Blade Under Periodic Unsteady Wake Flow and Varying of Turbulence Intensity

AbstractThis paper attempts to provide a detailed insight into the heat transfer and aerodynamic behavior of a separation zone that is generated as a result of boundary layer development along the suction surface of a highly loaded low pressure turbine blade. This paper experimentally investigates the individual and combined effects of periodic unsteady wake flows and freestream turbulence intensity (Tu) on heat transfer and aerodynamic behavior of the separation zone. Heat transfer experiments were carried out at Reynolds numbers of 110,000, 150,000, and 250,000 based on the suction surface length and the cascade exit velocity. Aerodynamic experiments were performed at Re=110,000. For the above Re numbers, the experimental matrix includes Tu’s of 1.9%, 3.0%, 8.0%, and 13.0% and three different unsteady wake frequencies with the steady inlet flow as the reference configuration. Detailed heat transfer and boundary layer measurements are performed with particular attention paid to the heat transfer and aerodynamic behavior of the separation zone at different Tu’s at steady and periodic unsteady flow conditions. The objectives of the research are (a) to quantify the effect of Tu on the aerothermal behavior of the separation bubble at steady inlet flow conditions, (b) to investigate the combined effects of Tu and the unsteady wake flow on the aerothermal behavior of the separation bubble, and (c) to provide a complete set of heat transfer and aerodynamic data for numerical simulation that incorporates Navier–Stokes and energy equations. The experimental investigations were performed in a large-scale, subsonic, unsteady turbine cascade research facility at the Turbomachinery Performance and Flow Research Laboratory of Texas A&amp;M University.

Aerodynamics of Scaled Runback Ice Accretions

Runback ice accretions present a unique situation in iced-airfoil aerodynamics in that the airfoil typically has a clean leading edge before the ice accretion. To investigate the aerodynamic effects of runback ice accretions, simulations were scaled from accretions obtained in the NASA Glenn Icing Research Tunnel for testing in the Illinois subsonic wind tunnel. Simple geometric scaling, based on airfoil chord, as well as boundary-layer scaling, based on estimated boundary-layer thickness, was used. The NACA 3415 and the NACA 23012 airfoils were tested at a Reynolds number of 1.8 x 10 6 and Mach number of0.18. Simple two-dimensional simulations were tested as well as three-dimensional simulations that more accurately simulated the features of the full-scale ice accretion. Significant aerodynamic penalties due to runback accretions were identified. In the worst case these penalties included a loss of over 0.75 in C imaX and 7 deg in stalling angle of attack. In certain cases scaled runback accretions were found to increase the stalling angle of attack and maximum-lift coefficient. This phenomenon was investigated using boundary-layer measurements and fluorescent-oil flow visualization. It was concluded that the interaction between the boundary layer and the simulation was responsible for the phenomenon.

Studies on High-Lift LP Turbine Airfoils of Aero Engines (Understanding of Boundary Layer Transitional Behaviour on Time-Space Diagram under the High Lift and Ultra High Lift Conditions)

This paper details experimental studies on the flow field around a linear cascade of low-pressure turbine airfoils whose solidity is changeable. Highly loaded low-pressure turbine (LPT) blades are one of the key paths to successful future aero-engines, however these blades are usually accompanied with separation bubble, eventually leading to the increase in aerodynamic performance. The purpose of this study is therefore to clarify any favorable effects of incoming wakes upon the aerodynamic loss of high-lift or ultra high-lift rotor blade cascade through the measurements of wake-affected boundary layers including separation bubble under low Reynolds number conditions. Cylindrical bars on the timing belts work as wake generator to emulate upstream stator wakes that impact the rotor blade. Hot-wire probe measurement is conducted over the blade suction surface to understand to what extent and how the incoming wakes affect the boundary layers containing separation bubble. Time-dependent transitional behaviors due to the wake passing are examined through the detailed inspection of several composite maps of flow properties displayed on time-space diagrams.

Studies on High-Lift LP Turbine Airfoils of Aero Engines (Understanding of Time-Averaged Behaviour of Separated Boundary Layer under the Influence of Incoming Wakes)

This paper details experimental studies on the flow field around a low-pressure linear turbine cascade whose solidity is changeable. Highly loaded low-pressure turbine (LPT) blades or ultrahigh lift airfoils are one of the key paths to successful future aero-engines, however those blades are usually accompanied with separation bubble, eventually leading to the increase in aero dynamic loss. The purpose of this study is to clarify the effect of incoming wakes upon the aerodynamic loss of the rotor blade cascade, through the measurements of wake-affected boundary layers upon including separation bubble for low Reynolds number conditions and/or low solidity conditions. Cylindrical bars on the timing belts work as wake generator to emulate upstream stator wakes that impact the rotor blade. Hot-wire probe measurement is conducted over the blade suction surface to understand to what extent and how the incoming wakes affect the boundary layers containing separation bubble. In particular, from the viewpoint of providing database for blade designing, this paper focuses on time-averaged characteristics of the separated boundary layer influenced by the wake passing.

Combined phosphor and CARS thermometry at the wall–gas interface of impinging flame and jet systems

For the determination of surface normal temperature gradients, a generic system was built up consisting of two opposed, vertical nozzles impinging onto a flat, horizontal copper plate. From below, the plate was heated by non-reacting, turbulent air jets (Re = 5,000) and by a laminar flame (? = 0.7, Re = 350), respectively. For well-defined boundary conditions, the plate was cooled by a turbulent cold jet from above in both cases. Wall temperature as well as gas temperature distributions within and outside of the thermal boundary layer of the hot side of the system were determined. The radial surface temperature profile of the plate was measured by coating it with thermographic phosphors (TP), materials whose phosphorescence decay time is dependent on their temperature. The TP was excited electronically by a frequency-tripled Nd:YAG laser (355 nm). The temporal decay of the phosphorescence intensity was measured pointwise by a photomultiplier tube. In this case, the 659 nm emission line of Mg4FGeO6:Mn was monitored. Non-intrusive point measurements of the gas temperature close to the surface were performed by rovibrational coherent anti-Stokes Raman spectroscopy (CARS) of diatomic nitrogen. Beams from a seeded, frequency-doubled Nd:YAG laser (532 nm) and from a modeless broadband dye laser (607 nm) were phase-matched into a surface-parallel, planar-boxcars configuration. The temperature data could be collected as close as 300 ?m to the surface. Thermographic phosphors as well as CARS proved to be consistent for wall temperature and boundary layer measurements in all test cases. The results and challenges of this approach are discussed.

Boundary layer observations in West Africa using a novel microwave radiometer

Boundary layer measurements in Nangatchori, Benin were performed over the period of one full year (2006) using a novel ground-based microwave profiler and additional remote-sensing instruments. In this paper, the diurnal cycle of the ITD (Inter-Tropical Discontinuity) in the transition period between dry and wet season during the month of April is described in detail. Dry air masses from the north (Sahel) and moist air from the south (tropical Atlantic Ocean) cause very sharp temperature and humidity gradients in the low troposphere over West Africa. Continuous observations of these phenomena in terms of temperature and humidity profiles have been achieved for the first time with a high temporal resolution of less than 15 minutes. Especially the ability of the microwave radiometer to provide temperature profiles with high vertical resolution through multi-angle measurements gives detailed boundary layer information in conjunction with meteorological tower observations. Together with additional lidar ceilometer observations of aerosol backscatter the change of air masses can be seen very well. The high data availability of > 85 % allows a statistical analysis of the full month of April in which Nangatchori comes increasingly under the influence of tropical air. Thus the data set is well suited for an improved process understanding, model evaluation in a data sparse area, and possibly together with additional observations the development of improved boundary layer parameterizations for atmospheric models.