Subsonic Axial Flow Research Articles

Study of Sweep and Induced Dihedral Effects in Subsonic Axial Flow Compressor Passages—Part II: Detailed Study of the Effects on Tip Leakage Phenomena

This article presents the detailed study of rotor tip leakage related phenomena in a low speed axial compressor rotor passages for three sweep configurations [Unswept (UNS), Tip Chordline Swept (TCS) and Axially Swept (AXS)]. Fifteen domains are numerically studied with 5 sweep configurations (0°, 20°TCS, 30°TCS, 20°AXS, and 30°AXS) and for 3 tip clearances (0.0%, 0.7% and 2.7% of the blade chord). Results were well validated with experimental data. Observations near the tip reveal that UNS rotor shows high sensitivity than the swept rotors in the blade pressure distribution with change in tip clearance. AXS rotor has high loading capability and less tip clearance effect on blade loading at the near stall mass flow. Downstream shift of the vortex rollup along the chord is observed with increased flow coefficient and increment in the tip gap height. In particular, the effect of flow coefficient is more predominant on this effect. Tip vortex-related flow blockage is less with the swept rotors. Among the rotors, the AXS rotor is found to incur low total pressure losses attributable to tip leakage. Effect of incidence is observed on the flow leakage direction.

The dynamics of variants of two-dimensional cantilevered flexible plates in axial flow

Cantilevered thin flexible plates in subsonic axial flow may lose dynamic stability at sufficiently high flow velocity. Once the critical point is exceeded, flutter takes place, and the flutter amplitude grows as the flow velocity is further increased. Richer dynamics are obtained for modified forms (“variants”) of this basic system. In particular, in the present paper four cases are considered: with (i) gravity, (ii) a spring support of either linear or cubic type, (iii) a concentrated mass mounted on the plate, and (iv) a small oscillating incidence angle in the undisturbed flow. For each specific variant of the basic system, the influence of these added features is investigated; e.g., for the spring support and concentrated mass the effects of magnitude and/or location are discussed. Some interesting phenomena found in the dynamics of these variants of the system are summarized in this paper.

Study of Sweep and Induced Dihedral Effects in Subsonic Axial Flow Compressor Passages—Part I: Design Considerations—Changes in Incidence, Deflection, and Streamline Curvature

This article presents the study of Tip Chordline Sweeping (TCS) and Axial Sweeping (AXS) of low‐speed axial compressor rotor blades against the performance of baseline unswept rotor (UNS) for different tip clearance levels. The first part of the paper discusses the changes in design parameters when the blades are swept, while the second part throws light on the effect of sweep on tip leakage flow‐related phenomena. 15 domains are studied with 5 sweep configurations (0∘, 20∘ TCS, 30∘ TCS, 20∘ AXS, and 30∘ AXS) and for 3 tip clearances (0.0%, 0.7%, and 2.7% of the blade chord). A commercial CFD package is employed for the flow simulations and analysis. Results are well validated with experimental data. Forward sweep reduced the flow incidences. This is true all over the span with axial sweeping while little higher incidences below the mid span are observed with tip chordline sweeping. Sweeping is observed to lessen the flow turning. AXS rotors demonstrated more efficient energy transfer among the rotors. Tip chordline sweep deflected the flow towards the hub while effective positive dihedral induced with axial sweeping resulted in outward deflection of flow streamlines. These deflections are more at lower mass flow rates.

Prediction of the tonal noise radiation of an axial flow fan located downstream of a Bluff Body

This paper investigates the tonal noise radiated by a subsonic axial flow fan when installed downstream of a bluff body. Typically industrial axial flow fans operate in cluttered environments, and are usually driven through direct coupling from an engine. For this investigation, the fan is located in a knife-edged shroud and placed downstream of a bluff body having representative dimensions of a typical engine. Axial flow fans in general radiate broadband and discrete frequency noise, the latter of which modelling efforts are maturing. The numerical simulation is based on the aero-acoustic analogy where the unsteady flow is first computed using CFD and then passed to a BEM solver to compute the acoustic radiation. URANS and DES methods are examined for the turbulence modelling. The time-varying force on a single blade in the CFD solver over one complete rotation is used to construct the equivalent fan source in the BEM model. Experimental measurements of Sound Pressure Level are performed in a hemi-anechoic chamber, and a comparison between numerical predictions made. This numerical procedure can be used to further aid designers in understanding the effects of fan tonal noise in cluttered environments.

Experimental and numerical investigation of a subsonic compressor with bend-skewed slot-casing treatment

On the basis of the test results of discrete axial and blade angle slot casing treatment, a new type of casing treatment was designed for a subsonic axial flow compressor rotor by optimizing various geometry parameters. To obtain a wide operating range and to minimize penalties in terms of isentropic efficiency, seven compressor configurations incorporating casing treatments of 0, 16.6, 33.3, 50, 66.6, 83.3, and 100 per cent rotor exposures were experimentally investigated. The results showed that significant improvements in stall margin are possible in all exposures and insignificant isentropic efficiency sacrifices are recorded in some exposures. Nearly 21.43 per cent stall margin improvement in terms of the corrected mass flow-rate was achieved with 33.3 per cent rotor blade tip axial chord exposure. The compressor built with 16.6 per cent rotor exposure was the best configuration in terms of maximum isentropic efficiency gain. The second issue of the paper was to offer a contribution to the understanding of the physical mechanism by which bend-skewed slot-casing treatment improves stall margin under subsonic conditions. By applying a concept similar to ‘Domain scaling’ approach (as often used in multistage turbomachinery flow-fields) to the interface between the rotor blade passage and end-wall treatments, a time-dependent three-dimensional numerical simulation was performed for the subsonic axial-flow compressor rotor with bend-skewed slot-casing treatment. The numerical results agreed well with the available experimental results. Detailed analyses of the coupled flow through bend-skewed slot-casing treatment and rotor blade passage under subsonic conditions led to some preliminary conclusions as to the flow physics involved in the stall margin improvements afforded by the use of bend-skewed slot-casing treatment.

Classification of aeroacoustically relevant surface modes in cylindrical lined ducts

Wave modes in a straight cylindrical duct with a locally reacting boundary and a steady subsonic axial flow are investigated. The duct modes are separated into ordinary duct modes and surface modes confined to a neighbourhood of the boundary. Previous asymptotic results of Rienstra for the surface modes assume that the dimensionless frequency ω is large, and that the azimuthal order m ≪ ω. In this paper, these results are generalized to arbitrary values of m, as applicable to rotor-alone noise in aeroengines where m is a multiple of the number of blades and is typically O ( ω). A dimensionless number λ is found to govern the surface modes’ behaviour, and qualitatively different behaviour is seen for λ in four distinct regions, separated by critical values depending only on the steady flow Mach number. For aeroacoustically relevant parameters, our generalized asymptotics are shown to provide a distinctly better approximation to the full equations than previous approximations.

Numerical and Experimental Investigations of Steady Micro-Tip Injection on a Subsonic Axial-Flow Compressor Rotor

Steady tip injection has been demonstrated to be an effective means of extending the stable operating range of a tip-critical compressor. This study presents a state-of-the-art design for the tip injection through the casing with flush-mounted inclined holes and the effectiveness of steady micro-air injection to enhance stability in a subsonic axial-flow compressor rotor using an external-air supply. For the tested rotor, experimental results demonstrate that at 53% design speed, the stalling mass flow can be reduced by 7.69% using an injected mass flow equivalent to 0.064% of the annulus flow. Time-dependent CFD simulations were conducted to identify the physical mechanic that accounts for the beneficial effects of the steady micro-air injection on the performance and stability of the compressor. Detailed analyses of the flow visualization at the tip have exposed the different tip flow topologies between the cases without tip injection and with tip injection. It was found that the primary stall margin enhancement afforded by the steady micro-air injection is a result of the tip-clearance flow manipulation. The repositioning of the tip-clearance vortex further towards the trailing edge of the blade passage and delaying the movement of incoming/tip-clearance flow interface to the leading edge plane are the physical mechanisms responsible for extending the compressor stall margin.

Active control of fan noise from high-bypass ratio aeroengines: a numerical study

AbstractUnder the national research project, dubbed Turbotech II, in which MTU Aero Engines, DLR Institute of Propulsion Technology and EADS Corporate Research Centre participate, active noise control (ANC) has been tested with a scale model fan of one metre diameter for a high bypass ratio aeroengine. MTU’s task in this project was to develop a computer code to predict the sound field in the intake duct of the fan-rig by the use of active control. The primary objective of the numerical study was to specify numbers of actuators (loudspeakers) and error sensors (microphones) and their positioning to control the harmonic sound power, radiated upstream to the duct intake. The computer model is based on the geometry of an annular or circular duct of rigid walls and infinite length, containing a subsonic axial uniform flow. The modal amplitudes of the primary sound field are input data. The actuators are modelled by acoustic monopoles. Two control algorithms have been used for achieving the control objective. The first consists simply in the reduction of the in-duct mean squared pressures. The second, so called modal control, is designed to cancel dominant modes selectively. Numerical results are presented using a typical configuration of wall mounted actuators and error sensors in the form of a number of rings uniformly distributed along the length of the intake duct. Guidelines have also been derived to design a favourable configuration of actuators and sensors. The findings of the numerical study are compared with the results of the ANC tests.

Active control of fan tones radiated from turbofan engines. I. External error sensors

This paper is the first of two papers dealing with the active control of fan tones radiated from the intake of turbofan engines. The present paper describes an idealized mathematical model for the prediction of the sound transmission and radiation of harmonic sound from a finite-length, unflanged hollow hard-walled circular duct containing a subsonic axial mean flow. The paper proceeds to use this model in computer simulations aimed at assessing the potential for reducing the far-field radiated sound by active means using two rings of discrete secondary sources on the duct wall. Perfect knowledge of the sound field to be controled is assumed throughout. The paper investigates two control objectives. The first is concerned with minimizing the total sound power transmitted along the duct, and hence the total sound power radiated. The other is concerned with minimizing the radiated sound power radiated into a specified band of sideline solid angles. Implicit in this investigation is the assumption of perfect observability of the radiated far field equivalent to a large number of error sensors located in the far field. The corresponding performance for the two control objectives when the observability of the sound field is limited to measurements made by a number of the error sensors situated on the duct wall is the subject of the companion paper.

Aeroacoustics of axial propeller: Applying to an acoustic wind tunnel and optimization of an axial propeller for cooling system

A simplified model is more convenient to predict design parameter effects on operating characteristics and radiated noise of a propeller. Thus, the aeroacoustic design software, decivent, is used for subsonic axial flow fan, mounted as well in reduced systems like cooling systems as in greater structures like test wind tunnel. For a given flow rate and with the blades specified in cylindrical sections, the total pressure and torque are provided by the following computations: (i) outlet velocity triangle by solving radial balance, (ii) lift coefficient by a conformal transformation, and (iii) lift and drag forces, evaluated with considerations of laminar or turbulent boundary layer stall. The outlet relative velocity and force fields allow one to compute the radiated noise whose components are (i) loading noise, computed by Lighthill’s model, and (ii) broadband noise computed by Fukano’s model. Correlations with tests show the validity of this methodology. This software, weak customer in computational duration, may be connected with an optimization software which, after some hundred iterations, allows one to exhibit the best compromise. Two propellers design by these means will be presented: one for a preliminary plan of an acoustic wind tunnel, and one for a cooling system.

A Linearized Unsteady Aerodynamic Analysis for Real Blade Supersonic Cascades

The prediction capabilities of a linearized unsteady potential analysis have been extended to include supersonic cascades with subsonic axial flow. The numerical analysis of this type of flow presents several difficulties. First, complex oblique shock patterns exist within the cascade passage. Second, the acoustic response is discontinuous and propagates upstream and downstream of the blade row. Finally, a numerical scheme based on the domain of dependence is required for numerical stability. These difficulties are addressed by developing a discontinuity capturing scheme and matching the numerical near-field solution to an analytical far-field solution. Comparisons with semi-analytic results for flat plate cascades show that reasonable predictions of the unsteady aerodynamic response at the airfoil surfaces are possible, but aeroacoustic response calculations are difficult. Comparisons between flat plate and real blade cascade results show that one effect of real blades is the impulsive loads due to motion of finite strength shocks.

Discrete‐frequency radiated noise and unsteady rotor force from a subsonic axial flow fan

First Page

Asymptotic solutions for unsteady flow in cascades

Asymptotic methods are presented for obtaining approximate solutions to unsteady flows in oscillating cascades. The formulation is in the framework of linearized potential flow, and the problems considered include low-frequency and low-solidity expansions for subsonic and supersonic cascades. For the supersonic cascade, simple formulas are obtained for the unsteady lift and moment, valid to first order in a frequency parameter. It is shown that terms of successive orders can be obtained by solving a sequence of quasi-static problems, with the effective upwash at each step modified by the lower-order solutions. The approach is in the spirit of matched asymptotic expansions, and different expansions based on different limit processes are sought for the subresonant and superresonant regions. For cascades in subsonic axial flow, the acoustic resonance phenomenon leads to a nonuniformity with respect to the interblade phase angle. The location of the singularity can be moved by suitably redefining the limit process, permitting uniformly valid expansions to be obtained separately for the subresonant and superresonant regions. Numerical comparisons with the full unsteady solution indicate that the present approximations are remarkably accurate in the range of reduced frequencies of interest in aeroelastic analyses.

Dynamics of a Flexible Cylinder in Subsonic Axial Flow

This paper examines the dynamics of a flexible cylinder with pinned ends immersed in axial subsonic flow, either bounded or unconfined. The problem proves to be surprisingly resistant to exact solution, as compared to the incompressible flow case, because of difficulties in determining precisely the inviscid aerodynamic forces. This paper presents a number of distinct formulations of these forces, involving different approximations: (1) a slender-body approximation; (2) an approximate three-dimensional formulation where, in the determination of the aerodynamic forces, the axial shape is prescribed in advance; and (3) an exact integral formulation of the generalized aerodynamic forces. In each case, Galerkin-type solutions yield the system eigenfrequencies which describe the dynamical behavior of the system. It is found that for sufficiently high flow velocities, divergence and flutter are possible. The different methods yield similar, but not quantitatively identical results. Interestingly, dependence of the dynamical characteristics on Mach number is shown to be weak for slender cylinders; for nonslender ones, it is stronger. Finally, a brief discussion of wave propagation in an unconstrained cylinder indicates the existence of a cutoff flow velocity for backward propagating waves, followed by wave amplification at higher flow, which is closely related to loss of stability inmore » the constrained system.« less

Unsteady Flow in a Supersonic Cascade with Subsonic Leading-Edge Locus

Linearized theory is used to predict the unsteady flow in a supersonic cascade with subsonic axial flow velocity. A closed-form analytical solution is obtained by using a double application of the Wiener-Hopf technique. Although numerical and semianalytical solutions of this problem have already appeared in the literature, this paper contains the first completely analytical solution. It has been stated in the literature that the blade source should vanish at the infinite duct resonance condition. The present analysis shows that this does not occur. This apparent discrepancy is explained in the paper.

Further Developments in the Aerodynamic Analysis of Unsteady Supersonic Cascades—Part 2: Aerodynamic Response Predictions

This paper is the second of a two-part report on a theoretical analysis of the aerodynamic response to an oscillating supersonic cascade in subsonic axial flow. Supersonic resonance criteria are discussed and lead to the distinction between subresonant and superresonant cascade motions. Numerical predictions, based on the unsteady solution reported in Part 1, are presented for two typical cascade configurations. These reveal the possibility of both subresonant and superresonant single-degree-of-freedom torsional instabilities. Subresonant instabilities occur over a broad range of frequencies and interblade phase angles, whereas superresonant instabilities occur only over a narrow range of such cascade parameter values. For a given blade motion frequency and free-stream Mach number, it appears that the least stable condition will usually lie in the subresonant region.

Unsteady Supersonic Cascade in Subsonic Axial Flow

This paper presents an analysis for determining the unsteady flowfield produced by an oscillating cascade placed in a supersonic stream which has a subsonic velocity component normal to the cascade. The analysis is based on the assumptions of an inviscid, two dimensional, linearized flowfield. Solutions for the velocity potential and the blade pressure distributions which satisfy the blade-to-blade periodicity condition are developed explicitly in terms of disturbance functions distributed on blade and wake surfaces. The boundary conditions of flow tangency at blade surfaces and continuity of pressure across wake surfaces provide integral relations which can be solved numerically to evaluate the disturbance functions. Predicted blade pressure distributions are in good agreement with results determined from a previous finite cascade solution. Further, in the two limiting cases of sonic axial velocity and zero frequency, the present solution approaches the lower limit of Lane's solution for supersonic axial flow, and it reduces to an Ackeret type of steady-state solution, respectively. The numerical examples indicate that a single~degree- of-freedom torsional instability will exist over a broad range of cascade parameter values.

Closure to “Discussion of ‘The Unsteady Aerodynamics of a Finite Supersonic Cascade With Subsonic Axial Flow’” (1974, ASME J. Appl. Mech., 41, pp. 539–540)

Closure to “Discussion of ‘The Unsteady Aerodynamics of a Finite Supersonic Cascade With Subsonic Axial Flow’” (1974, ASME J. Appl. Mech., 41, pp. 539–540)

Second-Generation Theoretical Study of Sound Emission from Turbomacbinery Ducts

Development of advanced gas-turbine power-plant systems requires the capability to predict accurately turbomachinery noise levels in the farfield. The purpose of the research described herein was to extend the understanding of the propagation of sound through realistically contoured inlet and exhaust ducts and to produce a methodology which relaxes the simplifications inherent in previous analyses of the propagation of sound in hard- and soft-walled ducts. Real inlet and exhaust ducts were simulated by matching an appropriate series of cylindrical and conical elements, terminating in a plane baffle, with specified acoustic wall impedance and mean airflow for each element. At the juncture between each finite element, appropriate solutions to the linear wave equation were coupled to describe the propagation of particular acoustic modes. Some initial results of the theoretical studies were that (a) acoustic power can propagate along a finite duct at frequencies below cutoff; (b) cutoff frequency is increased by constricting duct cross section; (c) specific acoustic modes may be ideally cancelled due to the effect of subsonic axial flow; (d) duct length may be selected to minimize the modal power radiated at a given frequency; and (e) wave reflections at the duct end are strongly enhanced for certain wall impedances.

The Unsteady Aerodynamics of a Finite Supersonic Cascade With Subsonic Axial Flow

A method is presented for determining the unsteady flow field and the aerodynamic response which occurs when a finite oscillating cascade is placed in a supersonic stream, which has a subsonic velocity component normal to the cascade. Solutions are obtained through the combined use of closed-form and numerical procedures. Computed results indicate that the finite cascade analysis should provide a reasonable indication of the influence of the cascade parameters on the response of the infinite array. A brief parametric study for a typical configuration reveals possible aerodynamic instabilities when the blades perform single-degree-of-freedom pitching oscillations over a broad range of frequencies and interblade phase angles.