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This paper examines the response of a rotor blade boundary layer and a rotor near-wake to an impinging wake of an inlet guide vane (IGV) located upstream of the rotor blade. Two-dimensional particle image velocimetry (PIV) measurements are performed in a refractive index matched turbomachinery facility that provides unobstructed view of the entire flow field. Data obtained at several rotor phases enable us to examine the IGV-wake-induced changes to the structure of the boundary layer and how these changes affect the flow and turbulence within the rotor near-wake. We focus on the suction surface boundary layer, near the blade trailing edge, but analyze the evolution of both the pressure and suction sides of the near-wake. During the IGV-wake impingement, the boundary layer becomes significantly thinner, with lower momentum thickness and more stable profile compared with other phases at the same location. Analysis of available terms in the integral momentum equation indicates that the phase-averaged unsteady term is the main contributor to the decrease in momentum thickness within the impinging wake. Thinning of the boundary/shear layer extends into the rotor near-wake, making it narrower and increasing the phase-averaged shear velocity gradients and associated turbulent kinetic energy (TKE) production rate. Consequently, the TKE increases during wake thinning, with as much as 75% phase-dependent variations in its peak magnitude. This paper introduces a new way of looking at the PIV data by defining a wake-oriented coordinate system, which enables to study the structure of turbulence around the trailing edge in great detail. [DOI: 10.1115/1.3149282]

This paper presents an application of the potential field panel method commonly used in aerodynamics analysis to obtain streamlinelike trajectories and use them for path planning and collaborative target tracking for swarming micro air vehicles in an urban environment filled with complex shaped buildings and other architectural structures. In addition, we introduce a performance matching technique that relates the fluid velocities, which are obtained as a part of the panel method solution, to vehicle velocities along each trajectory. The approach is further extended to track moving targets yet avoid obstacles and collision between the vehicles. Because of the inherent nature of streamlines, obstacle avoidance is automatically guaranteed. To make the micro air vehicles follow and track a moving target, dynamically changing streamline patterns are calculated for each and every one of the micro air vehicles within a swarm. To prevent vehicle-to-vehicle collisions, each micro air vehicle is represented using a point source singularity element within the potential field. The simulation results are quite encouraging, in the sense that micro air vehicle swarms quickly locate and track the assigned target in an environment filled with complex-shaped structures while a voiding obstacles and collisions among themselves. One benefit of the method is that the trajectory computations can be relatively fast and even have the potential to be applied in real time, depending on the number and complexity of the urban structures. DOI: 10.2514/1.45469

The flow and turbulence in a rotor near wake located within a nonuniform field, generated by a row of inlet guide vanes, are investigated experimentally in a refractive-index-matched facility that provides unobstructed view of the entire flowfield. Stereoscopic particle image velocimetry measurements, performed in closely spaced radial planes, enable measurements of all the components of the phase-averaged strain rate, Reynolds stress, and triple correlation tensors. The rotor wake is bent and compressed as a result of exposure to regions with high axial momentum ('jets'), which fill the gaps between inlet guide vanes wakes. As the rotor wake propagates away from the blade, this process of bending and compression by the jets leads to formation of distinct wake kinks containing regions of high turbulence (turbulent 'hot spots'). We focus on an early stage of hot-spot formation. On the suction side of the wake, compression by a jet increases turbulence production, causing nonuniform and asymmetric distribution of Reynolds stresses. In a curvilinear coordinate system aligned with the wake centerline, all the Reynolds stresses are higher on the suction side of the wake where the decay rate of turbulence is much slower than that expected in wakes. The production rate of streamwise stress is dominated by interactions of the shear stress with the cross-stream gradients of the phase-averaged streamwise velocity. The latter also interact with the cross-stream stress to cause high production of shear stress, especially on the suction side. The locations of peak streamwise and shear stresses are consistent with those of the corresponding production rates. The similar locations of peak streamwise and cross-stream stresses, together with low production rate of the latter, suggest a significant contribution by intercomponent transfer from the streamwise to the cross-stream stress. The effects of streamwise curvature and rotation on the evolution of turbulent stresses are marginal. The paper also provides distributions of advection by phase-averaged flow and turbulent diffusion components. The diffusion terms oppose the peaks of production rates, but also have high values along the perimeter of the wake. We compare the distribution of diffusion of turbulent kinetic energy to a model based on the gradient-diffusion hypothesis. The model predicts the diffusion peak that opposes the production rate maximum quite well, but underpredicts the diffusion along the perimeter of the wake and overpredicts it near the wake center. DOI: 10.2514/1.31079

The flow structure and turbulence around the leading and trailing edges of a rotor blade operating downstream of a row of inlet guide vanes (IGV) are investigated experimentally. Particle image velocimetry (PIV) measurements are performed in a refractive index matched facility that provides unobstructed view of the entire flow field. Data obtained at several rotor blade phases focus on modification to the flow structure and turbulence in the IGV wake as it propagates along the blade. The phase-averaged velocity distributions demonstrate that wake impingement significantly modifies the wall-parallel velocity component and its gradients along the blade. Due to spatially non-uniform velocity distribution, especially on the suction side, the wake deforms while propagating along the blade, expanding near the leading edge and shrinking near the trailing edge. While being exposed to the nonuniform strain field within the rotor passage, the turbulence within the IGV wake becomes spatially nonuniform and highly anisotropic. Several mechanisms, which are consistent with rapid distortion theory (RDT) and distribution of turbulence production rate, contribute to the observed trends. For example, streamwise (in rotor frame reference) diffusion in the aft part of the rotor passage enhances the streamwise fluctuations. Compression also enhances the turbulence production very near the leading edge. However, along the suction side, rapid changes to the direction of compression and extension cause negative production. The so-called wall blockage effect reduces the wall-normal component.

The results of heat transfer, total pressure loss, and wake flow field measurements downstream of two-row staggered elliptical and circular pin fin arrays are presented. Two different types of elliptical fins are tested, Standard Elliptical Fin (SEF) and a fin that is based on NACA four digit symmetrical airfoil shapes (N fin).

This paper continues our effort to study the dynamics of deterministic stresses in a multistage turbomachine using experimental data. Here we focus on the tip and hub regions and compare them to midspan data obtained in previous studies. The analysis is based on data obtained in particle image velocimetry (PIV) measurements performed in the second stage of a two-stage turbomachine. A complete data set is obtained using blades and fluid with matched optical index of refraction. Previous measurements at midspan have shown that at midspan and close to design conditions, the deterministic kinetic energy is smaller than the turbulent kinetic energy. The primary contributor to the deterministic stresses at midspan is the interaction of a blade with the upstream wakes. Conversely, we find that the tip vortex is the dominant source of phase-dependent unsteadiness and deterministic stresses in the tip region. Along the trajectory of the tip vortex, the deterministic kinetic energy levels are more than one order of magnitude higher than the levels measured in the hub and midspan, and are of the same order of magnitude as the turbulent kinetic energy levels. Reasons for this trend are explained using a sample distribution of phase-averaged flow variables. Outside of the region affected by tip-vortex transport, within the rotorstator gap and within the stator passages, the turbulent kinetic energy is still 3-4 times higher than the deterministic kinetic energy. The deterministic and turbulent shear stress levels are comparable in all spanwise locations, except for the wakes of the stator blades, where the turbulent stresses are higher. However, along the direction of tip-vortex transport, the deterministic shear stresses ate about an order of magnitude higher than the turbulent shear stresses. The decay rates of deterministic kinetic energy in the hub and midspan regions are comparable to each other, whereas at the tip the decay rate is higher. The decay rates of turbulent kinetic energy are much smaller than those of the deterministic kinetic energy. The paper also examines terms in the deterministic kinetic energy transport equation. The data indicate that 'deterministic production' and a new term, here called 'dissipation due to turbulence,' are the dominant source/sink terms. Regions with alternating signs of deterministic production indicate that the energy transfer between the phase-averaged and average-passage flow fields can occur in both directions. The divergence of the pressure-velocity correlation, obtained from a balance of all the other terms, is dominant and appears to be much larger than the deterministic production (source/sink) term. This trend indicates that there are substantial deterministic pressure fluctuations in the flow field, especially within the rotor-stator gap and within the stator passage. [DOI: 10. 1115/1.1625692]

This experimental study provides striking examples of the complex flow and turbulence structure resulting from blade-wake and wake-wake interactions in a multi-stage turbomachine. Particle image velocimetry (PIV) measurements are performed within the entire 2nd stage of a two-stage turbomachine. The experiments are performed in a facility that allows unobstructed view of the entire flow field, facilitated using transparent rotor and stator and a fluid that has the same optical index of refraction as the blades. This paper contains data on the phase-averaged flow structure including velocity, vorticity and strain-rate, as well as the turbulent kinetic energy and shear stress, at mid span, for several orientation of the rotor relative to the stator. Two different test setups with different blade geometries are used in order to highlight and elucidate complex phenomena involved, as well as to demonstrate that some of the interactions are characteristic to turbomachines and can be found in a variety of geometries. The first part of the paper deals with the interaction of a 2nd-stage rotor with the wakes of both the rotor and the stator of the 1st stage. Even before interacting with the blade, localized regions with concentrated mean vorticity and elevated turbulence levels form at the intersection of the rotor and stator wakes of the 1st stage. These phenomena persist even after being ingested by the rotor blade of the 2nd stage. As the wake segment of the 1st-stage rotor blade arrives to the 2nd stage, the rotor blades become submerged in its elevated turbulence levels, and separate the region with negative vorticity that travels along the pressure side of the blade, from the region with positive vorticity that remains on the suction side. The 1st-stage stator wake is chopped-off by the blades. Due to difference in mean lateral velocity, the stator wake segment on the pressure side is advected faster than the segment on the suction side (in the absolute frame of reference), creating discontinuities in the stator wake trajectory. The nonuniformities in phase-averaged velocity distributions generated by the wakes of the 1st stage persist while passing through the 2nd-stage rotor. The combined effects of the 1st-stage blade rows cause 10-12 deg variations of flow angle along the pressure side of the blade. Thus, in spite of the large gap between the 1st and 2nd rotors (compared to typical rotor-stator spacings in axial compressors), 6.5 rotor axial chords, the wake-blade interactions are substantial. The second part focuses on the flow structure at the intersection of the wakes generated by a rotor and a stator located upstream of it. In both test setups the rotor wake is sheared by the nonuniformities in the axial velocity distributions, which are a direct result of the 'discontinuities' in the trajectories of the stator wake. This shearing creates a kink in the trajectory of the rotor wake, a quadruple structure in the distribution of strain, regions with concentrated vorticity, high turbulence levels and high shear stresses, the latter with a complex structure that resembles the mean strain. Although the 'hot spots' diffuse as they are advected downstream, they still have elevated turbulence levels compared to the local levels around them. In fact, every region of wake intersection has an elevated turbulence level. [DOI: 10.1115/1.1509078]

Detailed measurements of the flow field within the entire 2nd stage of a two-stage axial turbomachine are performed using particle image velocimetry. The experiments are performed in a facility that allows unobstructed view on the entire flow field, facilitated using transparent rotor and stator and a fluid that has the same optical index of refraction as the blades. The entire flow field is composed of a 'lattice of wakes,' and the resulting wake-wake and wake-blade interactions cause major flow and turbulence nonuniformities. The paper presents data on the phase averaged velocity and turbulent kinetic energy distributions, as well as the average-passage velocity and deterministic stresses. The phase-dependent turbulence parameters are determined from the difference between instantaneous and the phase-averaged data. The distributions of average passage flow field over the entire stage in both the stator and rotor frames of reference are calculated by averaging the phase-averaged data. The deterministic stresses are calculated from the difference between the phase-averaged and average-passage velocity distributions. Clearly, wake-wake and wake-blade interactions are the dominant contributors to generation of high deterministic stresses and tangential nonuniformities, in the rotor-stator gap, near the blades and in the wakes behind them. The turbulent kinetic energy levels are generally higher than the deterministic kinetic energy levels, whereas the shear stress levels are comparable, both in the rotor and stator frames of references. At certain locations the deterministic shear stresses are substantially higher than the turbulent shear stresses, such as close to the stator blade in the rotor frame of reference. The nonuniformities in the lateral velocity component due to the interaction of the rotor blade with the 1st-stage rotor-stator wakes, result in 13 percent variations in the specific work input of the rotor. Thus, in spite of the relatively large blade row spacings in the present turbomachine, the nonuniformities in flow structure have significant effects on the overall performance of the system. [DOI: 10.1115/1.1509077]

Investigation of the internal fluid mechanic losses for a turbine blade with trailing edge coolant ejection was present in Uzol et al. (2001). The current study is a detailed experimental investigation of the external subsonic flowfield near the trailing edge and the investigation of the external aerodynamic loss characteristics of the turbine blade with trailing edge, coolant ejection system. Particle Image Velocimetry experiments and total pressure surveys in the near wake of the blade are conducted for two different Reynolds numbers and four different ejection rates. Two different trailing edge configurations with different cut-back lengths are also investigated. Numerical simulations of the flowfield are also performed for qualitative flow visualization purposes. Two-dimensional, incompressible, and steady solutions of Reynolds-averaged Navier-Stokes equations are obtained. A two-equation standard k-[Epsilon] turbulence model coupled with an Algebraic Reynolds Stress Model is used for the simulation of the turbulent flowfield. The results show that the aerodynamic penalty levels in the wake region near the trailing edge are increased due to the mixing of the coolant and mainstream flows for 0-3 percent ejection rates. However, after a threshold level (5 percent ejection rate), the ejected coolant flow has enough momentum to fill the wake of the blade, which in turn results in a decrease in the aerodynamic penalty levels. [DOI: 10.1115/1.1351817]

The internal fluid mechanics losses generated between the blade plenum chamber and a reference point located just downstream of the trailing edge are investigated for a turbine blade trailing edge cooling system. The discharge coefficient [C.sub.d] is presented as a function of the free-stream Reynolds number, cut-back length, spanwise rib spacing, and chordwise rib length. The results are presented in a wide range of coolant to free-stream mass flow rate ratios. The losses from the cooling system show strong free-stream Reynolds number dependency, especially at low ejection rates, when they are correlated against the coolant to free-stream pressure ratio. However, when [C.sub.d] is correlated against a coolant to free-stream mass flow rate ratio, the Reynolds number dependency is eliminated. The current data clearly show that internal viscous losses due to varying rib lengths do not differ significantly. The interaction of the external wall jet in the cutback region with the free-stream fluid is also a strong contributor to the losses. Since the discharge coefficients do not have Reynolds number dependency at high ejection rates, [C.sub.d] experiments can be performed at a low free-stream Reynolds number. Running a discharge coefficient experiment at low Reynolds number (or even in still air) will sufficiently define the high blowing rate portion of the curve. This approach is extremely time efficient and economical in finding the worst possible [C.sub.d] value for a given trailing edge coolant system. [DOI: 10.1115/1.1348017]