Spanwise Position Research Articles

Experimental study of effects of tip geometry on the flow field in a turbine cascade passage

This study investigates the effects of blade tip geometry on the flow field of a turbine cascade at the incidence angle of 0 degree experimentally. The tests were performed in a low-speed turbine cascade wind tunnel. The Reynolds number based on the blade chord was about 172300 at the exit. Traverses of the exit flow field were made in order to measure the overall performance. The effects of using flat tip and grooved tip with a chord-wise channel were studied. The case with the flat tip is referenced as the baseline. The tip clearances are all 1 mm measuring 0.84 percent of the blade span. The depth of channel is 2mm. The flow field at 10% chord downstream from the cascade trailing edge was measured at 38 span-wise positions and 26 pitch-wise positions using a mini five-hole pressure probe. The static pressure distribution on the tip end wall is measured at 16 pitch-wise stations and 17 chord-wise stations. Results show that there exists great pressure gradient in the pressure side for the flat tip and the pressure side squealer tip, which means strong leakage flow. The pressure gradient from the pressure side to the suction side is greatly decreased for the grooved tip, and the resulting leakage flow is weaker. The core of the leakage vortex moves closer to the suction side for the pressure side squealer tip and farther away from the suction side for the suction side squealer tip. The pressure side squealer has little advantages over the flat tip in improving the flow capacity and reducing the overall losses. The suction side squealer tip and grooved tip can effectively decrease the intensity of the tip leakage vortex, improve the flow capacity and reduce loss of the turbine cascade passage and the grooved tip performs the best.

Time Resolved PIV Investigation on the Skin Friction Reduction Mechanism of Outer-Layer Vertical Blades Array

The drag reducing efficiency of the outer-layer vertical blades, which were first devised by Hutchins (2003), have been demonstrated by the recent towing tank measurements. From the drag measurement of flat plate with various vertical blades arrays by Park et al. (2011), a maximum 9.6% of reduction of total drag was achieved. The scale of blade geometry is found to be weakly correlated with outer variable of boundary layer. The drag reduction of 2.8% has been also confirmed by the model ship test by An et al. (2014). With a view to enabling the identification of drag reduction mechanism of the outer-layer vertical blades, detailed flow field measurements have been performed using 2D time resolved PIV in this study. It is found that the skin friction reduction effect is varied according to the spanwise position, with 2.73% and 7.95% drag reduction in the blade plane and the blade-in-between plane, respectively. The influence of vertical blades array upon the characteristics of the turbulent coherent structures was analyzed by POD method. It is observed that the vortical structures are cut and deformed by blades array and the skin frictional reduction is closely associated with the subsequent evolution of turbulent structures.

Bilevel Optimization Strategy for Aircraft Wing Design Using Parallel Computing

A new bilevel optimization strategy for wing design is developed, in which the optimizations of the wing-planform and wing-airfoil shapes are decoupled from each other. The design of the wing-planform shape and the shape of the airfoils in several spanwise positions are considered as the goal of the optimization. In the new approach, the design problem is decomposed into a series of subproblems based on the design variables. The design variables defining the wing-planform shape are optimized in a top-level optimization, and the design variables defining the shape of airfoils in several spanwise positions are optimized in several sublevel optimizations. To take into account the influence of the airfoil shape in a specific spanwise position on the shape of the airfoils in other spanwise positions, a series of design variables are added to the design vector of the top-level optimization. The top-level optimizer is responsible for the consistency of the optimization. Using this approach, the number of design variables in the top-level optimization is reduced; the airfoils in several spanwise positions are optimized in parallel; and, instead of complex three-dimensional aerodynamic and structural solvers, much simpler and faster two-dimensional airfoil analysis tools can be used.

Vorticity transport in the leading-edge vortex on a rotating blade

AbstractVorticity transport is analysed within the leading-edge vortex generated on a rectangular flat plate of aspect ratio 4 undergoing a starting rotation motion in a quiescent fluid. Two analyses are conducted on the inboard half of the blade to better understand the vorticity transport mechanisms responsible for maintaining the quasi-equilibrium state of the leading-edge vortex. An initialglobalanalysis between the$25$and$50\, \%$spanwise positions suggests that, although spanwise velocity is significant, spanwise convection of vorticity is insufficient to balance the flux of vorticity from the leading-edge shear layer. Subsequent detailed analyses of vorticity transport in planar control volumes at the$25$and$50\, \%$spanwise positions verify this conclusion and demonstrate that vorticity annihilation due to interaction between the leading-edge vortex and the opposite-sign layer on the plate surface is an important, often dominant, mechanism for regulation of leading-edge-vortex circulation. Thus, it provides an important condition for maintenance of an attached leading-edge vortex on the inboard portion of the blade.

Parameter Variation and the Leading-Edge Vortex of a Rotating Flat Plate

Particle image velocimetry was used to characterize the flowfield on flat plates of aspect ratios 2 and 4 undergoing a starting rotation motion at Reynolds numbers, based on tip velocity, of 4,000, 8,000, and 16,000. The starting motion was performed in a tank of quiescent water. For both aspect ratios, a leading-edge vortex was observed on the suction surface of the plate, and its evolution and circulation were characterized with variations in angle of attack, Reynolds number, and azimuthal and spanwise positions. A region of strong counter-rotating vorticity was also observed between the leading-edge vortex and the plate surface, which becomes entrained by the leading-edge vortex. The circulation of the leading-edge vortex, when rendered dimensionless by the plate chord length and tip speed, was found to be relatively insensitive to Reynolds number and the azimuthal position of the plate within the ranges of the measurements; however, a strong (approximately linear) dependence on angle of attack was observed such that the leading-edge vortex circulation generally exceeded the bound circulation predicted by thin airfoil theory at the same local velocity. A significant collapse of the leading-edge vortex circulation was observed when normalized by the theoretical bound circulation, suggesting that this may be a useful tool for predicting leading-edge vortex strength.

21pm2-E2 梯子型電極による誘電泳動現象を利用したマイクロ流路における粒子の整列と分取

The present study describes a new micro-device which can accurately align and sort particles in a very high rate using the dielectrophoretic (DEP) force. The present rail- and ladder-type electrodes can precisely locate the particles in certain spanwise and height positions, and moreover, can control the distance between particles and align them. The particle motion in the microchannel flow and the performance of the device were investigated numerically and experimentally. The results confirmed that the distance between the particles could be controlled to be equally spaced by adjusting the on/off frequency of the voltage with the pitch of the ladder-type electrode. In combination with the flip-type electrode, particles were able to be sorted with high rate and accuracy.

Flow performance of highly loaded axial fan with bowed rotor blades

In this paper, a partial bowed rotor blade was proposed for a newly designed high loaded axial fan. The blade was positively bowed 30 degrees from hub to 30 percent spanwise position. Flows of radial blade and bowed blade fans were numerically compared for various operation conditions. Results show that the fan's performance is improved. At the designed condition with flow coefficient of 0.52, the efficiency of the bowed blade fan is increased 1.44% and the static pressure rise is increased 11%. Comparing the flow structures, it can be found that the separated flow in the bowed fan is reduced and confined within 20 percent span, which is less than the 35 percent in the radial fan. It means that the bowed blade generates negative blade force and counteracts partial centrifugal force. It is alleviates the radial movements of boundary layers in fan's hub region. Flow losses due to 3D mixing are reduced in the rotor. Inlet flow to downstream stator is also improved.

Experimental Investigation of a Tandem Cylinder System With a Yawed Upstream Cylinder

The unsteady flow field produced by a tandem cylinder system with the upstream cylinder yawed to the mean flow direction is investigated for upstream cylinder yaw angles from α=60 deg to α=90 deg. Multipoint fluctuating surface pressure and hot-wire measurements were conducted at various spanwise positions on both the upstream and downstream cylinders. The results indicate that yawing the front cylinder to the mean flow direction causes the pressure and velocity spectra on the upstream and downstream cylinders to become more broadband than for a regular tandem cylinder system, and reduces the magnitude of the peak associated with the vortex-shedding. However, spanwise correlation and coherence measurements indicate that the vortex-shedding is still present and was being obscured by the enhanced three-dimensionality that the upstream yawed cylinder caused. When the cylinder was yawed to α=60 deg, the pressure fluctuations became extremely broadband and exhibited shorter spanwise correlation.

Vortex development induced by part gap geometry and endwall configurations for variable stator vanes in a compressor cascade

The article describes investigations on the three-dimensional (3D) flowfield development near the endwall of a linear compressor cascade which is caused by specific part gap and endwall concepts of adjustable stator vanes. Their beneficial or harmful characteristics, previously measured with outlet loss and flow turning distributions, are investigated inside the passage at different stagger angles to analyse the origin and the interaction of the geometry-induced vortex system. Qualitative blade-to-blade measurements were conducted with particle image velocimetry in several spanwise positions as well as oil flow visualisation on the blade surfaces and the endwall. Improved three-dimensional (3D) numerical Reynolds-averaged Navier–Stokes (RANS) calculations with Reynolds-stress turbulence models were carried out and enhance the experimental findings. Results indicate extensive interactions between secondary flow and leakage flow through a penny gap depending on the aerodynamic loading. The part clearance vortex development and its impact to the blade boundary layers downstream the passage is visualised and enhances the understanding of the geometry effect. Also, the impact of the endwall concepts without radial clearances to the endwall boundary layer is shown and explains their beneficial characteristics compared to a reference geometry.

Turbulent flow over a rough backward-facing step

This work characterizes the impacts of the realistic roughness due to deposition of foreign materials on the turbulent flows at surface transition from elevated rough-wall to smooth-wall. High resolution PIV measurements were performed in the streamwise-wall-normal (x–y) planes at two different spanwise positions in both smooth and rough backward-facing step flows. The experiment conditions were set at a Reynolds number of 3450 based on the free stream velocity U∞ and the mean step height h, expansion ratio of 1.01, and the ratio of incoming boundary layer thickness to the step height, δ/h, of 8. The mean flow structures are observed to be modified by the roughness and they illustrate three-dimensional features in rough backward-facing step flows. The mean reattachment length Xr is significantly reduced by the roughness at one PIV measurement position while is slightly increased by the different roughness topography at the other measurement position. The mean velocity profiles at the reattachment point indicate that the studied roughness weakens the perturbation of the step to the incoming turbulent flow. Comparisons of Reynolds normal and shear stresses, productions of normal stresses, quadrant analysis of the instantaneous shear-stress contributing events, and mean spanwise vorticity reveal that the turbulence in the separated shear layer is reduced by the studied roughness. The results also indicate an earlier separation of the turbulent boundary layer over the current rough step, probably due to the adverse pressure gradient produced by the roughness topography even before the step.

Inviscid Analysis of Extended-Formation Flight

Flying airplanes in extended formations, with separation distances of tens of wingspans, significantly improves safety while maintaining most of the fuel savings achieved in close formations. The present study investigates the impact of roll trim and compressibility at a fixed lift coefficient on the benefits of extended-formation flight. An Euler solver with adjoint-based mesh refinement combined with a wake propagation model is used to analyze a two-body echelon formation at a separation distance of 30 spans. Two geometries are examined: a simple wing and a wing-body geometry. Energy savings, quantified by both formation drag fraction and span efficiency factor, are investigated at subsonic and transonic speeds for a matrix of vortex locations. The results show that, at fixed lift and trimmed for roll, the optimal location of vortex impingement is about 10% inboard of the trailing airplane’s wing tip. Interestingly, the improvement in drag fraction is relatively robust in the vicinity of the optimal position. Over 90% of energy benefits can be obtained with a 5% variation in vertical and 10% variation in spanwise positions. Control surface deflections required to achieve roll trim reduce the benefits of formation flight by 3–5% at subsonic speeds and 9–11% at transonic speeds. Overall, simulations show peak induced drag saving for the trail aircraft are 54% in subsonic flow and 35% in transonic flow while accounting for trim.

Experimental study of monodisperse granular flow through an inclined rotating chute

In blast furnaces, particles like coke, sinter and pellets enter from a hopper and are distributed on the burden surface by a rotating chute. Such particulate flows suffer occasionally from particle segregation during transportation caused by differences in density or size. To get a more fundamental insight into these effects, we started an experimental study to investigate the effect of rotation on such particulate flows. Here, as a first step, we present an experimental study of granular flow of monodisperse 3 mm spherical glass particles flowing with a constant mass rate through a rotating smooth rectangular chute, which is inclined at a fixed angle. Experiments are performed for a sufficiently long time to study steady (but streamwise accelerating) flow. Particle image velocimetry (PIV), electronic ultrasonic height sensors, and a dynamic weighing scale are used to measure the surface velocity of the particle stream, the particle bed height and mass flow rate in the chute, respectively. The influence of rotation speed and angle of inclination of the chute is studied. We find an interesting interplay between the Coriolis force, which pushes the granular flow to the sidewall of the chute and tends to diminish the acceleration of the flow, and centrifugal forces that accelerate the flow. The velocity components display a complex dependence on the spanwise and streamwise position in the chute. The bed height in the chute is also influenced by these effects of system rotation. These measurements provide a well-defined set of observations for refining and validating computer simulations of granular flows, and point out some important limitations of physical experiments. We present preliminary three-dimensional discrete particle simulations, which show that the experimental measurements of bed height and surface particle velocity in a chute inclined at 30° can be predicted nearly quantitatively both without and with rotation of the chute.

Experimental investigation of the root flow in a horizontal axis wind turbine

ABSTRACTThis research investigates the flow behavior and its features in the blade's root region of a horizontal axis wind turbine by using stereoscopic particle image velocimetry (PIV) technique. Wind tunnel tests are conducted to measure the velocity field, phase‐locked with the blade motion, at different azimuth angles and at different spanwise positions. The pressure distribution is obtained from PIV velocity field by solving the Navier–Stokes momentum equations. In this paper, we aim to answer two questions: (i) How is the flow behavior in the root region? (ii) How is the evolution of the root vortex? The analysis of the velocity fields shows an outboard radial flow motion in the root region and a vorticity driven inboard motion at the bladeŠs maximum chord position. As a result of this vorticity driven flow, an increase in the axial velocity close to nacelle is measured. Wake sheets are observed and further discussed in the measured velocity and vorticity distributions. The formation and evolution of the root vortices conveyed downstream by the axial velocity are analyzed through vorticity and pressure distributions. Although the azimuthal vorticity in 3D representation is showing the trailing vorticity, the tilting of the root vortex tube is observed in the axial vorticity distribution. Moreover, the radial vorticity and azimuthal velocity from chordwise measurements show separation on the suction surface of the blade. This research concluded that the flow in the blade wake is driven by the root vortex; hence, the local effects of the root vortex cannot be ignored. Copyright © 2013 John Wiley & Sons, Ltd.

On the impacts of coarse-scale models of realistic roughness on a forward-facing step turbulent flow

The present work explores the impacts of the coarse-scale models of realistic roughness on the turbulent boundary layers over forward-facing steps. The surface topographies of different scale resolutions were obtained from a novel multi-resolution analysis using discrete wavelet transform. PIV measurements are performed in the streamwise–wall-normal (x–y) planes at two different spanwise positions in turbulent boundary layers at Reh=3450 and δ/h=8, where h is the mean step height and δ is the incoming boundary layer thickness. It was observed that large-scale but low-amplitude roughness scales had small effects on the forward-facing step turbulent flow. For the higher-resolution model of the roughness, the turbulence characteristics within 2h downstream of the steps are observed to be distinct from those over the original realistic rough step at a measurement position where the roughness profile possesses a positive slope immediately after the step’s front. On the other hand, much smaller differences exist in the flow characteristics at the other measurement position whose roughness profile possesses a negative slope following the step’s front.

Integral transform solutions of dynamic response of a clamped–clamped pipe conveying fluid

Analysis of dynamic response of pipe conveying fluid is an important aspect in nuclear power plant design. In the present paper, dynamic response of a clamped–clamped pipe conveying fluid was solved by the generalized integral transform technique (GITT). The governing partial differential equation was transformed into a set of second-order ordinary differential equations which is then numerically solved by making use of the subroutine DIVPAG from IMSL Library. A thorough convergence analysis was performed to yield sets of reference results of the transverse deflection at different time and spanwise position. We found good agreement between the computed natural frequencies at mode 1–3 and those obtained by previous theoretical study. Besides, modal separation analysis was carried out and the influence of mass ratio on deflection and natural frequencies was qualitatively and quantitatively assessed.

Experimental investigation on the wing-wake interaction at the mid stroke in hovering flight of dragonfly

This paper focuses on flow structures of the wing-wake interaction between the hind wing and the wake of the forewing in hovering flight of a dragonfly since there are arguments whether the wing-wake interaction is useful or not. A mechanical flapping model with two tandem wings is used to study the interaction. In the device, two identical simplified model wings are mounted to the flapping model and they are both scaled up to keep the Reynolds number similar to those of dragonfly in hovering flight since our experiment is conducted in a water tank. The kinetic pattern of dragonfly (Aeschna juncea) is chosen because of its special interesting asymmetry. A multi-slice phase-locked stereo particle image velocimetry (PIV) system is used to record flow structures around the hind wing at the mid downstroke (t/T=0.25) and the mid upstroke (t/T=0.75). To make comparison of the flow field between with and without the influence of the wake, flow structures around a single flapping wing (hind wing without the existence of the forewing) at these two stroke phases are also recorded. A local vortex identification scheme called swirling strength is applied to determine the vortices around the wing and they are visualized with the iso-surface of swirling strength. This paper also presents contour lines of ωz at each spanwise position of the hind wing, the vortex core position of the leading edge vortex (LEV) of hind wing with respect to the upper surface of hind wing, the circulation of the hind wing LEV at each spanwise position and so on. Experimental results show that dimension and strength of the hind wing LEV are impaired at the mid stroke in comparison with the single wing LEV because of the downwash from the forewing. Our results also reveal that a wake vortex from the forewing traverses the upper surface of the hind wing at the mid downstroke and its distance to the upper surface is about 40% of the wing chord length. At the instant, the distance of the hind wing LEV to the upper surface is about 20% of the wing chord length. Thus, there must be a wing-wake interaction mechanism that makes the wake vortex become an additional LEV of the hind wing and it can partly compensate the hind wing for its lift loss caused by the downwash from the forewing.

Unsteady Aerodynamics of a Low Aspect Ratio Turbine Stage: Modeling Issues and Flow Physics

In this paper the three-dimensional unsteady aerodynamics of a low aspect ratio, high pressure turbine stage are studied. In particular, the results of fully unsteady three-dimensional numerical simulations, performed with ANSYS-CFX, are critically evaluated against experimental data. Measurements were carried out with a novel three-dimensional fast-response pressure probe in the closed-loop test rig of the Laboratorio di Fluidodinamica delle Macchine of the Politecnico di Milano. An analysis is first reported about the strategy to limit the CPU and memory requirements while performing three-dimensional simulations of blade row interaction when the rotor and stator blade numbers are prime to each other. What emerges as the best choice is to simulate the unsteady behavior of the rotor alone by applying the stator outlet flow field as a rotating inlet boundary condition (scaled on the rotor blade pitch). Thanks to the reliability of the numerical model, a detailed analysis of the physical mechanisms acting inside the rotor channel is performed. Two operating conditions at different vane incidence are considered, in a configuration where the effects of the vortex-blade interaction are highlighted. Different vane incidence angles lead to different size, position, and strength of secondary vortices coming out from the stator, thus promoting different interaction processes in the subsequent rotor channel. However some general trends can be recognized in the vortex-blade interaction: the sense of rotation and the spanwise position of the incoming vortices play a crucial role on the dynamics of the rotor vortices, determining both the time-mean and the time-resolved characteristics of the secondary field at the exit of the stage.

Three-dimensional features of a Mach 2.1 shock/boundary layer interaction

2D particle image velocimetry was used to study the three-dimensionality of the shock-boundary layer interaction generated by a small 20° compression ramp in a low aspect ratio continuously operated wind tunnel. High-resolution data were taken in four streamwise-wallnormal planes: three planes located in the sidewall boundary layer and one near the tunnel centerline. The incoming boundary layer was found to show three-dimensionality, with significant overshoot in the velocity profiles observed near the sidewall. The size of the wedge influenced the interaction, which was weaker than that observed in the case of a large compression wedge. The flow turning angle was ≈8° near the tunnel centerline and changed significantly across the span. Measurements behind the compression wedge in the centerline plane showed that both velocity and turbulence properties were nearly fully recovered ≈14δ behind the compression corner. The shock angle varied with spanwise position, and a multi-shock structure was observed in the sidewall planes. The size of the interaction decreased in the sidewall boundary layer. Non-monotonic variations in both velocity and turbulence profiles across the sidewall planes suggest the presence of significant spanwise flows, possibly corner vortices.

Numerical investigation on synthetic jet flow control inside an S-inlet duct

The high degree of centerline curvature and cross-stream pressure gradient in S-inlet ducts gives rise to boundary layer separation and secondary flows, which result in poor pressure recovery and non-uniform flow in the outlet interface with the engine. The flowfield in ducts is three-dimensional due to the existence of secondary flow, so ordinary two-dimensional actuations have poor effect on reforming the flow. Synthetic jet actuations extended in different spanwise positions were employed to manipulate the flow, and compared with the two-dimensional actuation. The interaction mechanics between flow separation and secondary flow was studied at first. It was found that the secondary flow enhanced or weakened flow separation depending on the spanwise position of synthetic jet actuators. Moreover, the flow separation enhanced the secondary flow, thus causing lower pressure recovery and flow distortion in the duct outlet. The actuators located at different spanwise positions will weaken the secondary flows by improving the flow separation to get energetic and uniform main flow.

Nonlinear system identification of the dynamics of a vibro-impact beam: numerical results

We study the dynamics of a cantilever beam with two rigid stops of certain clearances by performing nonlinear system identification (NSI) based on the correspondence between analytical and empirical slow-flow dynamics. The NSI method in this work can proceed in two directions: One for the numerical data obtained from a reduced-order model by means of the assumed-mode method, and the other for the experimental data measured at the same positions as in numerical simulations. This paper focuses on the analysis of the numerical data, providing qualitative comparison with some experimental results; the latter task will be discussed in detail in a companion paper. First, we perform empirical mode decomposition (EMD) on the acceleration responses measured at ten, almost evenly-spaced, spanwise positions along the beam leading to sets of intrinsic modal oscillators governing the vibro-impact dynamics at different time scales. In particular, the EMD analysis can separate any nonsmooth effects caused by vibro-impacts of the beam and the rigid stops from the smooth (elastodynamic) response, so that nonlinear modal interactions caused by vibro-impacts can be explored only with the remaining smooth components. Then, we establish nonlinear interaction models (NIMs) for the respective intrinsic modal oscillators, where the NIMs invoke slowly-varying forcing amplitudes that can be computed from empirical slow-flows. By comparing the spatio-temporal variations of the nonlinear modal interactions for the vibro-impact beam and those of the underlying linear model (i.e., the beam with no rigid constraints), we demonstrate that vibro-impacts significantly influence the lower-frequency modes, introducing spatial modal distortions, whereas the higher frequency modes tend to retain their linear dynamics between impacts. We introduce a linear correlation coefficient as a measure for studying the linear dependency between the slowly-varying complex forcing amplitudes for the linear and vibro-impact beams and demonstrate that only a set of lower-frequency modes are strongly influenced by vibro-impacts, capturing most of the essential nonlinear dynamics. These results demonstrate the efficacy of the proposed approach to analyze strongly nonlinear measured time series and provide physical insight for strong nonlinear dynamical interactions.