Downstream Rotor Research Articles

Wake Effect on HAT Tidal Current Power Device Performance

The rotor that initially converts the flow energy into rotational energy is a very important component that affects the efficiency of the entire tidal current power system. Rotor performance is determined by various design variables. Power generation is strongly dependent on the incoming flow velocity and the size of the rotor. To extract a large quantity of power, a tidal current farm is necessary with a multi-arrangement of devices in the ocean. However, the interactions between devices also contribute significantly to the total power capacity. Therefore, rotor performance, considering the interaction problems, needs to be investigated to maximize the power generation in a limited available area. The downstream rotor efficiency is affected by the wake produced from the upstream rotor. This paper introduces the performance of a downstream rotor affected by wakes from an upstream rotor, demonstrating the interference affecting various gabs between devices.

Analysis of Unsteady Tip and Endwall Heat Transfer in a Highly Loaded Transonic Turbine Stage

In a previous study, vane-rotor shock interactions and heat transfer on the rotor blade of a highly loaded transonic turbine stage were simulated. The geometry consists of a high pressure turbine vane and a downstream rotor blade. This study focuses on the physics of flow and heat transfer in the rotor tip, casing, and hub regions. The simulation was performed using the unsteady Reynolds-averaged Navier–Stokes code MSU-TURBO. A low Reynolds number k-ε model was utilized to model turbulence. The rotor blade in question has a tip gap height of 2.1% of the blade height. The Reynolds number of the flow is approximately 3×106/m. Unsteadiness was observed at the tip surface that results in intermittent “hot spots.” It is demonstrated that unsteadiness in the tip gap is governed by inviscid effects due to high speed flow and is not strongly dependent on pressure ratio across the tip gap contrary to published observations that have primarily dealt with subsonic tip flows. The high relative Mach numbers in the tip gap lead to a choking of the leakage flow that translates to a relative attenuation of losses at higher loading. The efficacy of new tip geometry is discussed to minimize heat flux at the tip while maintaining choked conditions. In addition, an explanation is provided that shows the mechanism behind the rise in stagnation temperature on the casing to values above the absolute total temperature at the inlet. It is concluded that even in steady (in a computational sense) mode, work transfer to the near tip fluid occurs due to relative shearing by the casing. This is believed to be the first such explanation of the work transfer phenomenon in the open literature. The difference in pattern between steady and time-averaged heat fluxes at the hub is also explained.

Numerical Investigations of the Efficiency of Circulation Control in a Compressor Stator

Airfoil active flow control has been attempted in the past in order to increase the permissible loading of boundary layers in gas turbine components. The present paper presents a stator with active flow control for a high-speed compressor using a Coanda surface near the trailing edge in order to inhibit boundary layer separation. The design intent is to reduce the number of vanes while—in order to ensure a good matching with the downstream rotor—the flow turning angle is kept constant. In a first step, numerical simulations of a linear compressor cascade with circulation control are conducted. The Coanda surface is located behind an injection slot on the airfoil suction side. Small blowing rates lead to a gain in efficiency associated with a rise in static pressure. In a second step, this result is transferred to a four-stage high-speed research compressor, where the circulation control is applied in the first stator. The design method and the first results are based on steady numerical calculations. The analysis of these results shows performance benefits of the concept. For both the cascade and the research compressor, the pressure gain and efficiency are shown as a function of blowing rate and jet power ratio. The comparison is performed based on a dimensionless efficiency, which takes into account the change in power loss.

Investigation of Effect of Unsteady Interaction on Turbine Blade Film Cooling

Film cooling is extensively used to provide protection against the severe thermal environment in gas turbine engines. It is known that the cooling effectiveness of the downstream rotor is influenced by the unsteady wake of the stator. Presence of wake due to an upstream vane is studied by placing a periodic set of rods upstream of the blade in most of the investigations. In order to explore the influence of the unsteady stator wake on the cooling effectiveness of the rotor blade under real situation, the film cooling of moving turbine blades, with preceding stationary vanes, was investigated numerically, and the influence of the unsteady interaction between the rotor and the stator on the film cooling effectiveness of the flow field surrounding the cooling holes on moving blades was discussed. Results show that the unsteady interaction between the stator wake and the cooling jet flow becomes stronger for the case of larger blowing ratio under the same stator wake condition, and the fluctuation of the film cooling effectiveness is increased correspondingly. Because of the wider overlying range of the cooling jet flow at the time of the stator wake passing through the cooling holes, better film cooling effectiveness achieves downstream of the 30% arc length. Cooling jet flow can hardly reach the endwall zone due to the effect of the vortex at this zone. It is entrained downstream of the jet position, resulting in better effectiveness in this zone.

Rotor Wake Variability in a Multistage Compressor

Rotorwakevariability isnot typically accountedforin computational fluid dynamics models,yet small deviations from the modeled average wake could have impacts on performance and forced vibratory response of airfoils. This paper quantifies rotor wake variability, in terms of wake depth and width, for each stage in a three-stage research compressor. Time-resolved flow angle data are acquired with a cross-film sensor at midspan for all three rotor exits for 50 circumferential positions spanning a vane passage. This research focuses on rotor wake variability driven by blade row interactions. The scope of the study includes wake variations caused by blade-to-blade differences, interactions with upstream rotor wakes, and interactions with the potential field of the downstream vane row. The lowest levels of variability are associated with rotor 1. In the downstream rotors, ensemble-averaged revolutions of data show an amplitude modulation caused by interactions with the rotor wakes from the upstream blade rows and, thus, increased levels of blade-to-blade wake variability. The main effect of the downstream vane row is to shift the mean levels of the wake without significantly influencing the wake shape. Nomenclature mc = corrected mass flow rate Nc = corrected rotational speed R = rotor Rec = Reynolds number based on chord length S = stator t = time U = wheel speed V = flow velocity in the absolute reference frame W = flow velocity in the rotating reference frame � = absolute flow angle � = relative flow angle � = adiabatic efficiency � = standard deviation

A Method to Generate Propulsor Side Forces

A computational study was performed to investigate a method to generate vehicle maneuvering forces from a propulsor alone. A ducted, preswirl propulsor was configured with an upstream stator row and downstream rotor. During normal operation, the upstream stator blades are all situated at the same pitch angle and preswirl the flow into the propulsor while generating a roll moment to counter the moment produced by the rotor. By varying the pitch angles of the stator blade about the circumference, it is possible to both generate a mean stator side force and subsequently vary the axial velocity and swirl that is ingested into the propulsor. The rotor then generates a side force in response to the inflow. Both potential flow and fully viscous 3D Reynolds averaged Navier–Stokes (RANS) computations were used to predict the stator forces, velocity field, and rotor response. Potential flow methods were used for initial examination of a wide variety of stator configurations. The most promising were then modeled using RANS. The RANS inflow was then computed and used as velocity boundary conditions during rotor blade design using potential flow methods. Blade parameters including blade number, rake, skew, and a combination of the two were varied to characterize their effects. RANS was used to then validate the final propulsor design. Computations demonstrated that total side force coefficients on the order of 0.1 and moment coefficients about the stator leading edge of 0.066 could be generated by the propulsor alone. This translates to an additional 50% control authority at 3 kn for current Navy 21″ unmanned undersea vehicles.

Experimental Reduction of Transonic Fan Forced Response by Inlet Guide Vane Flow Control

The main contributor to the high cycle fatigue of compressor blades is the response to aerodynamic forcing functions generated by an upstream row of stators or inlet guide vanes. Resonant response to engine order excitation at certain rotor speeds can be especially damaging. Studies have shown that flow control by trailing edge blowing (TEB) can reduce stator wake strength and the amplitude of the downstream rotor blade vibrations generated by the unsteady stator-rotor interaction. In the present study, the effectiveness of TEB to reduce forced fan blade vibrations was evaluated in a modern single-stage transonic fan rig. Data were collected for multiple uniform full-span TEB conditions over a range of rotor speeds including multiple modal resonance crossings. Resonant response sensitivity was generally characterized by a robust region of strong attenuation. The baseline resonant amplitude of the first torsion mode, which exceeded the endurance limit on the critical blade, was reduced by more than 80% with TEB at 1.0% of the total rig flow. The technique was also found to be modally robust; similar reductions were achieved for all tested modal crossings, including more than 90% reduction in the second leading-edge bending response using 0.7% of the rig flow.

Investigation of Upstream Boundary Layer Influence on Mountain Wave Breaking and Lee Wave Rotors Using a Large-Eddy Simulation

Abstract Interactions between a turbulent boundary layer and nonlinear mountain waves are explored using a large-eddy simulation model. Simulations of a self-induced critical layer, which develop a stagnation layer and a strong leeside surface jet, are considered. Over time, wave breaking in the stagnation region forces strong turbulence that influences the formation and structure of downstream leeside rotors. Shear production is an important source of turbulence in the stagnation zone and along the interface between the stagnation zone and surface jet, as well as along the rotor edges. Buoyancy perturbations act as a source of turbulence in the stagnation zone but are shown to inhibit turbulence generation on the edges of the stagnation zone. Surface heating is shown to have a strong influence on the strength of downslope winds and the formation of leeside rotors. In cases with no heating, a series of rotor circulations develops, capped by a region of increased winds. Weak heating disrupts this system and limits rotor formation at the base of the downslope jet. Strong heating has a much larger impact through a deepening of the upstream boundary layer and an overall decrease in the downslope winds. Rotors in this case are nonexistent. In contrast to the cases with surface warming, negative surface fluxes generate stronger downslope winds and intensified rotors due to turbulent interactions with an elevated stratified jet capping the rotors. Overall, the results suggest that for nonlinear wave systems over mountains higher than the boundary layer, strong downslope winds and rotors are favored in late afternoon and evening when surface cooling enhances the stability of the low-level air.

Investigation of Loss Generation in an Embedded Transonic Fan Stage at Several Gaps Using High-Fidelity, Time-Accurate Computational Fluid Dynamics

The blade-row interaction (BRI) rig at the Air Force Research Laboratory, Compressor Aero Research Laboratory, has been simulated at three axial gaps between the highly loaded upstream stator row and the downstream transonic rotor using TURBO. Previous work with the stage matching investigation (SMI) demonstrated a strong dependence of mass flow rate, efficiency, and pressure ratio on the axial spacing between an upstream wake generator and the downstream rotor through the variation of the axial gap. Several loss producing mechanisms were discovered and related to the spacings, referred to as close, mid, and far. In the SMI work, far spacing had the best performance. The BRI experiments were a continuation of the SMI work with the wake generator replaced with a swirler row to turn the flow and a deswirler row to create a wake by diffusion. Results of the BRI experiments showed a performance degradation between mid- and far spacings, which was not observed in SMI. This trend is seen in the numerical work as well, and the time-averaged data show that the majority of this performance change occurred in the rotor. An unsteady separation bubble periodically forms and collapses as shocks reflect through the stator passage, creating additional aerodynamic blockage. The shed vortices induced by the unsteady loading and unloading of the stator trailing edge are chopped, with a frequency related to the spacing, by the rotor leading edge and ingested by the rotor. Once ingested the vortices interact in varying degrees with the rotor boundary layer. A treatment of the loss production in the BRI rig is given based on the time-accurate and time-averaged, high-fidelity TURBO results.

Experimental investigation about the effect of non-axisymmetric wake impact on a low speed axial compressor

Non-axisymmetric wake impact experiments were carried out after the best exciting frequency for a low speed axial compressor had been found by axisymmetric wake impact experiments. When the number and circumferential distribution of inlet guide vanes (IGV) are logical the wakes of non-axisymmetric IGVs can exert beneficial unsteady exciting effect on their downstream rotor flow fields and improve the compressor’s performance. In the present paper, four non-axisymmetric wake impact plans were found working better than the axisymmetric wake impact plan. Compared with the base plan, the best non-axisymmetric plan increased the compressor’s peak efficiency, and the total pressure rise by 1.1 and 2%, and enhanced the stall margin by 4.4%. The main reason why non-axisymmetric plans worked better than the axisymmetric plan was explained as the change of the unsteady exciting signal arising from IGV wakes. Besides the high-frequency components, the non-axisymmetric plan generated a beneficial low-frequency square-wave exciting signal and other secondary frequency components. Compared with the axisymmetric plan, multi-frequency exciting wakes arising from the non-axisymmetric plans are easier to get coupling relation with complex vortices such as clearance vortices, passage vortices and shedding vortices.

Water flow model turbine flow visualization study of the unsteady interaction of secondary flow vortices with the downstream rotor

This article presents detailed flow visualization photographs, root mean square processed photography, and computational fluid dynamics (CFD) results of the interaction of the vane passage vortex and horseshoe vortex with the rotor of an axial flow turbine model. Different modes of vortex breakdown behaviour have been experimentally observed inside the rotating passage of the turbine blade. These are spiral vortex mode and bubble mode breakdown. The breakdown mode changes as the vortices are influenced by the periodic pressure field of the rotor. The measurements were taken in a vertical water channel with ink injection for flow visualization. Unsteady CFD analyses have been made with some success in prediction of the unsteady flow structures. In particular, the pre-instability behaviour of the passage vortex in the experiments matches the results of the numerical investigations.

Development and Application of an Arbitrary Lagrangian Eulerian Solver for Turbomachinery Aeromechanics

This paper describes the development and application of turbomachinery aeromechanics arbitrary Lagrangian Eulerian 3-D, an arbitrary Lagrangian Eulerian finite-element-coupled fluid―structure interaction solver, to axial and radial flow turbomachinery aeromechanics issues. The incorporation of turbomachinery-specific algorithms enables the analysis of crack generation in the inlet guide vanes of a modern transonic compressor due to resonant vibrations generated by the potential/shock field of the downstream rotor. The capability to model turbulent wake forcing functions makes possible predictions of the resonant vibratory stress resulting from the subsonic rotor wakes interacting with the downstream stators. The modification of outflow boundary conditions to model incoming acoustic waves allows the unsteady aerodynamic response of a high-speed centrifugal compressor impeller to the potential field/shock excitation generated by the downstream radial-vaned diffuser to be simulated. Finally, the impeller blade aerodynamic damping is calculated to quantify the effect of blade-motion-induced aerodynamics at the exducer. The results make a case that, with increasing computing capacity, coupled fluid―structure interaction methods can become the approach of choice for high cycle fatigue design predictions.

A turbulent inflow model based on velocity modulation

This article presents a novel turbulent inflow model based on modulation of the velocity field for use with time-domain propulsor calculations. Given an experimental mean and rms turbulent inflow, a model can be constructed by modulating the velocity field over a range of frequencies. Assuming the turbulence is homogeneous, the inflow can be constructed as a Fourier series where the frequencies can also be modulated to smooth the broadband output. To demonstrate the effectiveness of the model, experimental inflow velocity data were acquired for an upstream stator, downstream rotor configuration mounted on an undersea vehicle afterbody. Two main sources of turbulence originated from the vorticity shed from the stator wakes and the boundary layer vorticity produced on the hull body. Three-dimensional, unsteady velocity data were acquired using hot-wire anemometry and reduced to provide mean and rms velocity values. Time-series data were processed to provide velocity power spectra used to calibrate the model. Simulations were performed using a modified version of the propulsor unsteady flow code capable of computing fully turbulent inflows. This solver models the propulsor blade as a vortex lattice and sheds the vorticity into the wake to solve the unsteady potential flow. The no-flux boundary conditions are satisfied at the lattice control points and the resulting unsteady circulation is a function of the instantaneous inflow velocity field over the blade. Vorticity is shed into the wake to account for the full time history of the inflow velocity field. To demonstrate the full effectiveness of the model, computed surface pressure data were exported to a code to compute the far-field radiated noise (both tonal and broadband). Simulated data were compared with experimentally obtained noise data with favorable results. Applications of this methodology in the incompressible flow domain include broadband analysis of propulsor-radiated noise on undersea vehicles and radiator fan noise in cars.

PIV Study of Wake-Rotor Interactions in a Transonic Compressor at Various Operating Conditions

Details of the unsteady flowfield between an upstream wake generator and a downstream rotor that are closely spaced in a transonic compressor are studied at various operating conditions using particle-image velocimetry. Flow-visualization images and particle-image velocimetry data facilitate analysis of the details of shed vortices, wake motion, and wake-shock interaction phenomena. Such analysis not only aids the understanding of the effect of blade-row interactions on compressor performance but also allows verification of time-accurate CFD codes that are used to characterize transonic compressors. As the operating point changes from choke to stall and the rotor-bow shock moves upstream, distinct vortex-shedding patterns are observed that affect the wake deviation and rotor incidence. With close spacing between the wake generator and the rotor, vortex shedding from the wake generator and the passage of the rotor-bow shock are strongly synchronized and blade-passage phase-locked measurements are possible. The resulting multiple images of the flow corresponding to any blade position are averaged to yield vortex and rotor-bow-shock locations at various back pressures. Using various postprocessing methods, specific shed vortices and wake topological features are isolated and details of the wake-shock interaction are captured.

Experimental Investigation of the Effects of a Moving Shock Wave on Compressor Stator Flow

Linear cascade testing was performed to simulate the flow conditions experienced by stator blades in an axial compressor with supersonic relative Mach numbers at the inlet to the downstream embedded rotors. Experiments were conducted in a transonic blow-down wind tunnel with a nominal inlet Mach number of 0.65. A single moving normal shock introduced at the exit of the stator cascade simulated the bow shock from a downstream rotor. The shock was generated using a shock tube external to the wind tunnel. Pressure measurements indicated that the stator matched its design intent loading, turning, and loss under steady flow conditions. Effects of the passing shock on the stator flowfield were investigated using shadowgraph photography and digital particle image velocimetry (DPIV). Measurements were taken with three different shock strengths. In each case, the passing shock induced a vortex around the trailing edge of the stator. The size and strength of these vortices were directly related to the shock strength. A suction side separation on the trailing edge of the stator was observed and found to correlate with the vortex blockage.

An Investigation of Wake-Shock Interactions in a Transonic Compressor With Digital Particle Image Velocimetry and Time-Accurate Computational Fluid Dynamics

The effects of varying axial gap on the unsteady flow field between the stator and rotor of a transonic compressor stage are important because they can result in significant changes in stage mass flow rate, pressure rise, and efficiency. Some of these effects are analyzed with measurements using digital particle image velocimetry (DPIV) and with time-accurate simulations using the 3D unsteady Navier-Stokes computational fluid dynamics solver TURBO. Generally there is excellent agreement between the measurements and simulations, instilling confidence in both. Strong vortices of the wake can break up the rotor bow shock and contribute to loss. At close spacing vortices are shed from the trailing edge of the upstream stationary blade row in response to the unsteady, discontinuous pressure field generated by the downstream rotor bow shock. Shed vortices increase in size and strength and generate more loss as spacing decreases, a consequence of the effective increase in rotor bow shock strength at the stationary blade row trailing edge. A relationship for the change in shed vorticity as a function of rotor bow shock strength is presented that predicts the difference between close and far spacing TURBO simulations.

Flex Circuit Sensor Array for Surface Unsteady Pressure Measurements

Introduction T HE acquisition of high-fidelity, high-frequency response surface pressure measurements are of interest to experimentalists in gas-turbine aeromechanical research because these data provide a means to measure indirectly the unsteady aerodynamic loading acting on an airfoil. Airfoils are typically (traditionally) instrumented from commercially available miniature pressure transducers that are flush mounted to the airfoil to prevent the transducer from aerodynamically disturbing the flow. Cavities are machined into the airfoil to accommodate the transducers, and trenches are machined to route the lead wires away from the transducer.1−5 The main difficulty in the use of traditional transducers for high spatial resolution comes from the congestion and interference caused by the cavities and trenches required for each sensor location and the number of locations desired. A high spatial resolution array of high-frequency pressure transducers was designed and fabricated that eliminates most of the difficulty associated with having a large concentration of transducers in a given area. Two pressure sensor arrays were applied to the suction and pressure surfaces of two inlet guide vanes (IGVs) of a transonic research compressor to measure the unsteady loading distribution induced by the passage of the bow shocks and the potential field emanating from the downstream rotor.

Impact of wake on downstream adjacent rotor in low-speed axial compressor

This paper investigates the effect of upstream wake on unsteady separated flow field of downstream adjacent vanes by solving 2D unsteady Reynolds-averaged Navier-Stokes equations discretized by a high-order scheme. The results indicated that the maximum relative reduction of loss coefficient was 27.2% when the relative passing frequency (the ratio of wake passing frequency to the characteristic frequency of trailing-vortex shedding of downstream adjacent rotor) is less than 2.5. The amplitude of wake defect exists a threshold value and the aerodynamic performance is enhanced monotonically with the amplitude of wake defect basically. The effective range of incidence must be greater than 8°, which is near the stall boundary.

Three-Dimensional Time-Resolved Inlet Guide Vane-Rotor Potential Field Interaction

To investigate and quantify the three-dimensional time-varying characteristics of inlet guide vane (IGV) wakes, including the effect of the potential field of a downstream rotor, a series of experiments are performed in a highspeed fan stage. The unsteady three-dimensional velocity and total pressure fields downstream of the IGVs and upstream of the rotor have been measured across one vane passage and time resolved over several rotor blade passage periods. These unique data will be compared with several wake correlations and analyzed to determine the time-varying characteristics of the IGV wakes. Earlier two-component velocity measurements identified a unique interaction occurring between the rotor potential field and the IGV wakes. The rotor potential field increases as the IGV wakes approach the rotor, resulting in a constructive/destructive interaction. Measurements of the IGV wake at times of maximum and minimum interaction show that the constructive interaction increases the velocity deficit and semiwake width as the distance from the rotor decreases. A two-dimensional vortical‐potential gust splitting analysis will be implemented to determine the vortical and potential harmonic wake gust forcing functions upstream of the rotor over one rotor blade passing period.

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.