Wake Generator Research Articles

Effect of Squealer Geometry on Tip Flow and Heat Transfer for a Turbine Blade in a Low Speed Cascade

Abstract A detailed investigation on the effect of squealer geometries on the blade tip leakage flow and associated heat transfer is presented for a scaled up high pressure turbine blade in a low-speed wind tunnel facility. The linear cascade is made of four blades with the two corner blades acting as guides. The tip profile of a first stage rotor blade is used to fabricate the two-dimensional blade. The wind tunnel accommodates an 116° turn for the blade cascade. The mainstream Reynolds number based on the axial chord length based on cascade exit velocity is 4.83×105. An upstream wake effect is simulated with a spoked wheel wake generator placed upstream of the cascade. A turbulence grid placed even farther upstream generates a free-stream turbulence of 4.8%. The center blade has a tip clearance gap of 1.56% with respect to the blade span. Static pressure measurements are obtained on the blade surface and the shroud. Results show that the presence of the squealer alters the tip gap flow field significantly and produces lower overall heat transfer coefficients. The effects of different squealer arrangements are basically to study the effect of squealer rim placement on tip leakage flow and associated heat transfer. Detailed heat transfer measurements are obtained using a steady state liquid crystal technique. The effect of periodic unsteady wake effect is also investigated by varying the wake Strouhal number from 0–0.4. Results show that suction side squealers may be favorable in terms of overall reduction in heat transfer coefficients over the tip surface. However, the presence of a full squealer is most beneficial in terms of reducing overall heat load on the tip surface. There is reasonable effect of wake induced periodicity on tip heat transfer.

Characterization of Aircraft Wake Vortices by 2-μm Pulsed Doppler Lidar

The 2-μm pulsed Doppler lidar, already successfully used for wind and turbulence measurements, has been modified for long-range wake-vortex characterization. In particular, a four-stage data processing algorithm has been developed to achieve precise profiles of tangential velocities from which the vortex parameters such as trajectories, core separation, tilt angle, and circulation can be derived. The main advantage of the pulsed lidar is its long-range capability of more than 1 km. This allows for observations over long periods from the moment of wake generation to a progressed state of vortex decay. With the field experiment at Tarbes airfield the potential of the 2-μm pulsed Doppler lidar for full-scale wake-vortex characterization has been demonstrated. Two examples showing the parameters of wake vortices generated by large transport aircraft (LTA)-type aircraft will be presented.

Criteria of negative wake generation behind a cylinder

It was demonstrated by simulation in our previous study that both the normal stress and its gradient are responsible for the negative wake generation (overshoot in the axial velocity) and streamline shifting. Extensional properties of the fluids dominate the generation of the negative wake, while other factors strengthen or weaken the formation of velocity overshoot. In this study, the criteria for the negative wake generation are discussed in detail for various fluid models, including the PTT, the FENE-CR, the FENE-P, and the Giesekus models. With the FENE-CR fluid, it is easier to generate negative wake than with the FENE-P fluid. This confirms that the constant shear viscosity FENE-CR fluid enhances the velocity overshoot, and that the shear-thinning viscosity FENE-P fluid delays the negative wake generation. The Giesekus fluid has a similar behaviour to the PTT fluid with regarding to the critical conditions of negative wake generation when appropriate fluid parameters are selected. The mechanism of wall proximity in enhancing the negative wake generation is also demonstrated with the analysis for the first time.

Studies on Effects of Periodic Wake Passing upon a Blade Leading Edge Separation Bubble (Suppresion of Separation Bubble Due to Wake Passing)

This paper describes an experimental investigation on aerodynamic interaction between incoming periodic wakes from moving bars and separation bubble on a scaled leading edge model. Numerical simulations are also attempted to grasp an idea how the incoming wakes interact with the separation bubble. The model, which consists of a semi-circular leading edge and two flat-plates, is used to simulate the flow field around a compressor or a turbine blade. Cylindrical bars of the wake generator produce the periodic wakes in front of the test model. The study aims at enriching the knowledge on how and to what extent the periodic wake passing suppresses the leading edge separation bubble. Special attention is paid to emergence of wake-induced turbulent spots and subsequent calmed regions. Hot-wire probe measurements are executed under five different flow conditions to examine effects of Reynolds number, Strouhal number, direction of the bar movement and incidence of the test model against the incoming flow. The measurements reveal that the wake moving over the separation bubble does not directly suppress the separation bubble. Instead, wake-induced turbulence spots and the subsequent calmed regions have dominant impacts on the separation bubble suppression for the all test cases.

Effect of Blade Tip Geometry on Tip Flow and Heat Transfer for a Blade in a Low-Speed Cascade

A comprehensive investigation of the effect of various tip sealing geometries is presented on the blade tip leakage flow and associated heat transfer of a scaled up HPT turbine blade in a low-speed wind tunnel facility. The linear cascade is made of four blades with the two corner blades acting as guides. The tip section of a HPT first stage rotor blade is used to fabricate the two-dimensional blade. The wind tunnel accommodates an 116 deg turn for the blade cascade. The mainstream Reynolds number based on the axial chord length at cascade exit is 4.83×105. The upstream wake effect is simulated with a spoked wheel wake generator placed upstream of the cascade. A turbulence grid placed even farther upstream generates the required freestream turbulence of 4.8%. The center blade has a tip clearance gap of 1.5625% with respect to the blade span. Static pressure measurements are obtained on the blade surface and the shroud. The effect of crosswise trip strips to reduce leakage flow and associated heat transfer is investigated with strips placed along the leakage flow direction, against the leakage flow and along the chord. Cylindrical pin fins and pitch variation of strips over the tip surface are also investigated. Detailed heat transfer measurements are obtained using a steady-state HSI-based liquid crystal technique. The effect of periodic unsteady wake effect is also investigated by varying the wake Strouhal number from 0. to 0.2, and to 0.4. Results show that the trip strips placed against the leakage flow produce the lowest heat transfer on the tips compared to all the other cases with a reduction between 10–15% compared to the plain tip. Results also show that the pitch of the strips has a small effect on the overall reduction. Cylindrical pins fins and strips along the leakage flow direction do not decrease the heat transfer coefficients and in some cases enhance the heat transfer coefficients by as much as 20%.

Aerodynamic Blade Row Interactions in an Axial Compressor—Part II: Unsteady Profile Pressure Distribution and Blade Forces

This two-part paper presents experimental investigations of unsteady aerodynamic blade row interactions in the first stage of the four-stage low-speed research compressor of Dresden. Both the unsteady boundary layer development and the unsteady pressure distribution of the stator blades are investigated for several operating points. The measurements were carried out on pressure side and suction side at midspan. In Part II of the paper the investigations of the unsteady pressure distribution on the stator blades are presented. The experiments were carried out using piezoresistive miniature pressure sensors, which are embedded into the pressure and suction side surface of a single blade. The unsteady pressure distribution on the blade is analyzed for the design point and an operating point near the stability limit. The investigations show that it is strongly influenced by both the incoming wakes and the potential flow field of the downstream rotor blade row. If a disturbance arrives the leading edge or the trailing edge of the blade the pressure changes nearly simultaneously along the blade chord. Thus the unsteady profile pressure distribution is independent of the wake propagation within the blade passage. A phase shift of the reaction on pressure and suction side is observed. The unsteady response of the boundary layer and the profile pressure distribution is compared. Based on the unsteady pressure distribution the unsteady pressure forces of the blades are calculated and discussed.

후향 계단 주위 난류 박리 유동에 대한 비정상 후류의 영향

An experimental study was made of turbulent separated and reattaching flow over a backward-facing step, where unsteady wake was generated by a spoked-wheel type wake generator with cylindrical rods in front of the separated flow. The influence of unsteady wake was scrutinized in terms of the rotating speed of the wake generator (0 ≤ St_H ≤ 0.4). A conditional averaging technique in corporation with SBF was employed to elucidate the influence of the unsteady wake on the large-scale vortical structures of the separated flow. Special attention was made during two-dimensional measurements of wall-pressure with or without unsteady wake. The wall-pressure fluctuations were used to predict dipole sound source by Curle's integral formula. It was found that the reduction of the dipole sound source was due to the reduction of turbulent kinetic energy by unsteady wake in the recirculation region.

A Phased Approach to Increase Airport Capacity through Safe Reduction of Existing Wake Turbulence Constraints

Previous wake turbulence research has suggested that using knowledge of wake turbulence behavior to develop candidate arrival and departure procedures would increase capacity at a variety of United States airports. This paper outlines the operational issues involved as well as the process through which the Federal Aviation Administration (FAA) and The MITRE Corporation’s Center for Advanced Aviation System Development (MITRE/CAASD) analyzed an evolving set of candidate procedures. This work is part of the broader effort to implement an FAA/National Aeronautics and Space Administration (NASA) Wake Turbulence Research Management Plan (RMP).We describe in detail two candidate procedures, their operational variations and the expected capacity benefits at selected airports. The first procedure is a near-term proposed change to the 2500 foot runway centerline separation minimum for dependent approaches to two Closely Spaced Parallel Runways (CSPR). When visual separation can not be provided, the current FAA rule requires parallel runways spaced less than 2500 ft to be treated as a single runway. While in-trail wake separation standards are dependent on wake categories of the generating and encountering aircraft, this rule is not. This paper shows that, by refining this standard to reflect the wake generation differences of different weight classes, substantial capacity benefits may be possible. Variations on this procedure are analyzed to explore the sensitivity of the capacity benefits to the underlying assumptions.The second procedure is a mid-term proposed change to reduce wake constraints for departures from parallel runways spaced closer than 2500 ft using a short-term prognosis of crosswinds at and near the area of aircraft rotation. A brief outline is provided of activities planned during 2004 to advance towards the specific design and implementation of these procedures. Once such activity is additional wake data collection at St. Louis (STL).

Large-scale vortical structure of turbulent separation bubble affected by unsteady wake

The large-scale vortical structure of a turbulent separation bubble under the influence of an unsteady wake was investigated. The unsteady wake was generated by a spoked-wheel type wake generator installed in front of the separation bubble. This wake generator was rotated either clockwise or counter-clockwise at Re H=5.600. The mechanism of vortex shedding from the separation bubble was analyzed in detail by taking a conditional average as well as a phase average. Spatial box filtering (SBF) was used to extract the large-scale vortical structure from the turbulent separation bubble affected by the unsteady wake. To elucidate the influence of the unsteady wake on the large-scale vortical structure, conditional averages of the velocity, vorticity and turbulent kinetic energy were calculated. The nature of the convection of the vortical structure under the influence of an unsteady wake was analyzed. The dipole acoustic pressure level was predicted using Curle's integral of wall-pressure fluctuations.

Fan interaction noise reduction using a wake generator: experiments and computational aeroacoustics

A control grid (wake generator) aimed at reducing rotor–stator interaction modes in fan engines when mounted upstream of the rotor has been studied here. This device complements other active noise control systems currently proposed. The compressor model of the instrumented ONERA CERF-rig is used to simulate suitable conditions. The design of the grid is drafted out using semi-empirical models for wake and potential flow, and experimentally achieved. Cylindrical rods are able to generate a spinning mode of the same order and similar level as the interaction mode. Mounting the rods on a rotating ring allows for adjusting the phase of the control mode so that an 8 dB sound pressure level (SPL) reduction at the blade passing frequency is achieved when the two modes are out of phase. Experimental results are assessed by a numerical approach using computational fluid dynamics (CFD). A Reynolds averaged Navier–Stokes 2-D solver, developed at ONERA, is used to provide the unsteady force components on blades and vanes required for acoustics. The loading noise source term of the Ffowcs Williams and Hawkings equation is used to model the interaction noise between the sources, and an original coupling to a boundary element method (BEM) code is realized to take account of the inlet geometry effects on acoustic in-duct propagation. Calculations using the classical analytical the Green function of an infinite annular duct are also addressed. Simple formulations written in the frequency domain and expanded into modes are addressed and used to compute an in-duct interaction mode and to compare with the noise reduction obtained during the tests. A fairly good agreement between predicted and measured SPL is found when the inlet geometry effects are part of the solution (by coupling with the BEM). Furthermore, computed aerodynamic penalties due to the rods are found to be negligible. These results partly validate the computation chain and highlight the potential of the wake generator system proposed.

Negative wake in the uniform flow past a cylinder

The upstream/downstream streamline shift and the associated negative wake generation (streamwise velocity overshoot in the wake) in a viscoelastic flow past a cylinder are studied in this paper, for the Oldroyd-B, UCM, PTT, and FENE-CR fluids, using the Discrete Elastic Viscous Split Stress Vorticity (DEVSS-ω) scheme (Dou HS, Phan-Thien N (1999). The flow of an Oldroyd-B fluid past a cylinder in a channel: adaptive viscosity vorticity (DAVSS-ω) formulation. J Non-Newtonian Fluid Mech 87:47–73). The numerical algorithm is a parallelized unstructured Finite Volume Method (FVM), running under a distributed computing environment through the Parallel Virtual Machine (PVM) library. It is demonstrated that both the normal stress and its gradient are responsible for the negative wake generation and streamline shifting. Fluid extensional rheology plays an important role in the generation of the negative wake. The negative wake can occur in flows where the fluid extensional viscosity does not increase rapidly with strain rate. The formation of the negative wake does not depend on whether the streamlines undergo an upstream or a downstream shift. Shear-thinning viscosity weakens the velocity overshoot and while shear-thinning first normal stress coefficient enhances the velocity overshoot. Wall proximity is not necessary for the velocity overshoot; however, it enhances the strength of the negative wake. For the Oldroyd-B fluid, the ratio of the solvent viscosity to the zero-shear viscosity plays an important role in the streamline shift. In addition, mesh dependent behaviour of normal stresses along the centreline at high De in most cylinder/sphere simulations is due to the convection of normal stress from the cylinder to the wake, which results in the maximum of the normal stress being located off the centreline by a short distance at high De.

Comparison between triangular cylinder and screen far wakes

Triangular cylinder and screen (50% solidity) far wakes have been examined and compared with a view to understand the effect of initial conditions on the self-preserving turbulence structures of various scales. Velocity data were obtained in the ( x, y)-plane, i.e. the plane of mean shear, using an array of 8 X-wires. Measurements were made at x/ h ( x is the streamwise distance downstream of the cylinder and h is the height of the wake generator) =330 for the triangular cylinder and 220 for the screen. The wake has been previously verified to be approximately self-preserving at these downstream stations. A two-dimensional orthogonal wavelet transform is used to analyse the measured velocity data. This technique enables the turbulence structures of different scales to be separated and characterized. Sectional streamlines and vorticity contours are examined and compared between the two wake generators. Discernible differences are observed in the turbulence structures of large-down to intermediate-scales. The differences are further quantified in terms of the contributions from the turbulence structures of different scales to the Reynolds stresses and vorticity variance.

Equilibrium similarity, effects of initial conditions and local Reynolds number on the axisymmetric wake

Equilibrium similarity considerations are applied to the axisymmetric turbulent wake, without the arbitrary assumptions of earlier theoretical studies. Two solutions for the turbulent flow are found: one for infinite local Reynolds number which grows spatially as x1/3; and another for small local Reynolds number which grows as x1/2. Both solutions can be dependent on the upstream conditions. Also, the local Reynolds number diminishes with increasing downstream distance, so that even when the initial Reynolds number is large, the flow evolves downstream from one state to the other. Most of the available experimental data are at too low an initial Reynolds number and/or are measured too near the wake generator to provide evidence for the x1/3 solution. New results, however, from a laboratory experiment on a disk wake and direct numerical simulations (DNS) are in excellent agreement with this solution, once the flow has had large enough downstream distance to evolve. Beyond this the ratio of turbulence intensity to centerline velocity deficit is constant, until the flow unlocks itself from this behavior when the local Reynolds number goes below about 500 and the viscous terms become important. When this happens the turbulence intensity ratio falls slowly until the x1/2 region is reached. No experimental data are available far enough downstream to provide unambiguous evidence for the x1/2 solution. The prediction that the flow should evolve into such a state, however, is confirmed by recent DNS results which reach the x1/2 solution at about 200 000 momentum thicknesses downstream. After this the turbulence intensity ratio is again constant, until box-size affects the calculation and the energy decays exponentially.

Hypocretinergic Neurons are Primarily involved in Activation of the Somatomotor System

The hypocretinergic system has been implicated in the generation and/or maintenance of wakefulness. Our results challenge this hypothesis. Utilizing cats as an animal model and immunocytochemical procedures for the simultaneous detection of hypocretin and Fos, we determined that hypocretinergic neurons are activated during wakefulness but only when somatomotor activity is present. These neurons are not activated during alert or quiet wakefulness in the absence of motor activity or during quiet sleep. We conclude that the hypocretinergic system is not responsible for the generation and/or maintenance of wakefulness, per se; on the contrary, we suggest that hypocretinergic neurons are primarily involved in motor functions irrespective of the animal's behavioral state.

Comparison Between Triangular Cylinder and Screen Near-Wakes in the Orthogonal Wavelet Representation

The near wake generated by a triangular cylinder and a screen, respectively, has been investigated with a view to provide information on the effect of initial conditions on turbulence structures of various scales based on the wavelet multi-resolution analysis. The velocity data were simultaneously obtained at x/h (x is the streamwise distance downstream of the wake generator and h is the height of the wake generator)=20 using a rake of eight X-wires aligned in the plane of mean shear. A vector wavelet multi-resolution technique was applied to decompose the velocity data into a number of wavelet components based on the central frequencies. The instantaneous sectional streamlines and vorticity field were used to reveal the turbulent structures at each wavelet component/central frequency. It was found that the behavior of large- and intermediate-scale structures depend on the initial conditions and the small-scale structures are independent of the initial conditions. The contributions from the wavelet components to the time-averaged Reynolds stresses and vorticity variance were estimated. Both the large-scale and intermediate longitudinal structures make the most significant contributions to Reynolds stresses in the triangular cylinder wake, but the contribution from the large-scale structures appears dominating in the screen wake. The relatively small-scale structures of the triangular cylinder wake contribute most to the total spanwise vorticity variance.

Unsteady dissipation measurements on a flat plate subject to wake passing

Boundary layer measurements were performed on a flat plate with an imposed pressure gradient typical of a high-lift low-pressure (LP) turbine blade and subject to incoming turbulent wakes shed from a moving bar wake generator. A multiple-orientation one-dimensional laser doppler anemometry (LDA) technique was used to measure the ensemble-average mean flow and Reynolds stresses. These ensembleaverage measurements were used to calculate the boundary layer dissipation, thereby providing unprecedented experimental evidence of the loss-reducing mechanisms associated with wake-induced transition. The benign character of the calmed zone was confirmed and the early stages of boundary layer separation were found to have laminar levels of dissipation. A deterministic natural transition phenomenon was identified between wake passing events, highlighting the existence of natural transition phenomena in LP turbine style pressure distributions.

Wavelet multi-resolution analysis of initial condition effects on near-wake turbulent structures

The effects of initial conditions on turbulence structures of various scales in a near wake have been investigated for two wake generators with the same characteristic dimension, i.e., a circular cylinder and a screen of 50% solidity, based on the wavelet multi-resolution analysis. The experimental investigation used two orthogonal arrays of sixteen X-wires, eight in the (x, y)-plane, and eight in the (x, z)-plane. Measurements were made at x/h (x is the streamwise distance downstream of the cylinder and h is the height of the wake generator) = 20. The wavelet multi-resolution technique was applied to decomposing the velocity data, obtained in the wakes generated by the two generators, into a number of wavelet components based on the central frequencies. The instantaneous sectional streamlines and vorticity field were thus 'visualized' for each wavelet component or central frequency. It was found that the behavior of large- and intermediate-scale structures depend on the initial conditions and the small-scale structures are independent of the initial conditions. The contributions from the wavelet components to the time-averaged Reynolds stresses and vorticity were estimated. Both the large-scale and intermediate longitudinal structures make the most significant contributions to Reynolds stresses in the circular cylinder wake, but the contribution from the large-scale structures appears dominating in the screen wake. The relatively small scale structures of the circular cylinder wake contribute most to the total rms spanwise vorticity.

DPIV study of near-stall wake-rotor interactions in a transonic compressor

Wake-rotor interactions at near-stall conditions are studied using digital particle image velocimetry (DPIV) in a transonic compressor at the Compressor Aero Research Lab (CARL) at Wright-Patterson Air Force Base. The wake generators (WGs) are designed to simulate wakes of highly loaded stators without producing flow swirl. Flow visualization, instantaneous, and average velocity field data are presented for near-stall compressor operating conditions. These results describe the interaction of the wake shed from the upstream WG with the downstream potential field of the rotor leading edge for two axial spacings at 75 and 90% span. The advantage of ensemble averaging is demonstrated when operating at challenging conditions such as near-stall, which poses special difficulties in obtaining DPIV data. Ensemble averaging, which is possible because of a natural phase locking provided by the blade potential field, allows robust statistics.

Hybrid simulations of solar wind interaction with magnetized asteroids: General characteristics

Using electromagnetic hybrid (kinetic ions, fluid electrons) simulations, the interaction between solar wind and a magnetized asteroid is investigated. At no or very low levels of asteroid magnetization the solar wind remains undisturbed. As the level of magnetization is increased, a phase standing whistler wake is generated which owing to its propagation characteristics remains confined to planes close to that containing the solar wind flow velocity and the interplanetary magnetic field (IMF). No density perturbations are associated with this wake. Further increase in asteroid magnetization results in the additional generation of fast and slow magnetosonic wakes, which fall behind the whistler wake and are associated with the formation of a plasma tail. Because of their propagation characteristics, these wakes are more symmetric than the whistler wake. At still higher levels of magnetization where plasma pileup occurs upstream of the asteroid, the fast magnetosonic wake becomes dominant and eventually exhibits shock‐like behavior. In general, the size of the interaction region increases with the level of asteroid's magnetic field strength. This interaction region is highly structured and asymmetric owing to wave propagation characteristics and the IMF direction. In addition, once the fast magnetosonic wake is strong enough to result in ion reflection the subsequent E × B motion of these ions results in considerable asymmetry in the interaction region. This is a purely ion kinetic effect which comes about as a result of the overall interaction region being comparable to ion scale lengths.

비정상 후류가 난류박리기포의 응집구조에 미치는 영향

Large-scale vortical structure of a turbulent separation bubble affected by unsteady wake is essential to understand flow mechanisms in various tluid devices. A spoked-wheel type of wake generator provides unsteady wake, which modifies the turbulent separation bubble significantly by changing rotation directions and passing frequencies. A detailed mechanism of vortex shedding from the separation bubble with unsteady wake is analyzed by taking a conditional average with spatial box filtering, which spatially integrates measured signals at pre-determined wavelength. A convecting nature of the large-scale vortical structure is analyzed carefully. Spatial evolution of the large-scale vortical structure with frequency variance is also exemplified.<br/>