Effects Of Blade Sweep Research Articles

Effect of Sweep on Axial Fan Noise Sources Using the Lattice Boltzmann Method

The effect of blade sweep has been studied numerically with the Lattice Boltzmann Method on a family of low-speed free-vortex axial fans with sweeps of ±45°. Good overall aerodynamic agreement is first demonstrated on all fans at the design condition, particularly in the tip gap. The local larger wall-pressure fluctuations seen in the unswept and backward swept fans compared to the forward case are traced to the stronger tip vortices that remain in the rotational plane or even move upstream. These stronger and faster vortices interacting with the fan blades are then responsible for the larger noise levels observed in the acoustic spectra of these fans, and particularly for large subharmonic humps. Excellent agreement between experimental and numerical noise predictions is finally reported stressing the dominant tip noise.

Experimental and computational investigation of the effect of blade sweep on acoustic characteristics of axial fan

The swept blade is one of the most efficient and practical techniques to alleviate the noise radiated from rotating fans. In this study, the swept effects of an automotive cooling fan on its acoustic characteristics were investigated using numerical and experimental approaches. Straight, forward, and backward-swept blades with a serrated shroud configuration were fabricated for the automotive cooling systems, and their performance and noise levels were measured. Moreover, measurements on the acoustic spectra were used to validate the numerical predictions. The pressure fluctuation on the cooling fan system, including the fan and shroud, was calculated from unsteady computational fluid dynamics simulation, and the acoustic signal at observer positions was predicted using an acoustic analogy. The swept effects in terms of noise reduction were compared for all three swept blades. Results indicate that the sweep angle in the forward direction can reduce the noise level of the cooling fan with a serrated shroud. Hence, the characteristics of the noise source caused by swept blades surrounding the shroud were identified in automotive cooling fans. Our findings in the present study provided significant insight into the acoustic characteristics depending on the direction of the swept blade, they can be used as a guide for designing a low-noise fan blade.

The effect of blade geometry on the structure of vertical axis wind turbine wakes

Vertical axis wind turbines (VAWT) may offer operational and design advantages over their horizontal axis counterparts, but the occurrence of dynamic stall and the resulting cyclical stresses present engineering and operational challenges. We present the results of an experimental study of the effects of blade sweep on the wake characteristics of VAWTs performed using high resolution particle image velocimetry. Several sweep configurations were tested while holding the turbine solidity constant. As the sweep angle was increased dynamic stall was reduced, which could yield a potential reduction in the amplitude of cyclical torque variations. Increased sweep also resulted in a more uniform wake exhibiting a faster wake recovery, and an increase in the planform energy flux in the turbine wake when compared to the straight-bladed turbine. The results support previous suggestions that the power generation of VAWT farms is bounded by the planform energy flux from above.

Acoustic theory of the many-bladed contra-rotating propeller: analysis of the effects of blade sweep on wake interaction noise

An analytical model is presented for the wake interaction tones produced by a contra-rotating propeller. We re-cast the usual far-field radiation formulae as a double integral over a nominal propeller source annulus. Assuming that the number of blades on both propellers is large, we evaluate the integral asymptotically in terms of its leading-order contributions from interior stationary or boundary critical points which represent the specific locations on the propeller annulus that dominate the sound radiation. The asymptotic approach is powerful producing results in the form of one-line algebraic formulae that contain no integrals or special functions yet remain accurate. The asymptotics show that sweep is not necessarily beneficial and can cause the blade design to become critical for particular tones and directions in terms of a continuum of interior points distributed along a line on the propeller source annulus producing a higher-order result and thus an enhanced radiated sound field. The paper also shows how the interior points are completely consistent with the sub- or super-critical gust response of a swept blade. Tones with low and zero azimuthal mode order are treated as special cases and the asymptotics show that, as the mode order reduces, the radiated sound becomes concentrated around the flight axis where even higher-order solutions are possible, including rings and annuli of stationary points around the propeller annulus. Full numerical calculations confirm the accuracy of the asymptotic approach.

Wave energy conversion: Design and shape optimization

Modified Wells turbines allow an efficient use of the power contained in the ocean and sea waves. The present study introduces the performance of an axial turbine which called Wells turbine. This turbine is used in oscillating water column (OWC) wave energy conversion devices. This type of axial turbine is investigated through numerical analysis and optimization. Unsteady 3D Reynolds-averaged Navier-Stokes equations were solved with k-ω SST turbulence closer model. A comparative study of optimized and conventional blades with steady and unsteady flows has been presented. For shape optimization, blade profile-thickness and sweep modifications along with and without grooved-casing, (GC) designs are considered. The results concluded that the reference blade with GC performs better in terms of torque coefficient and efficiency if the flow is attached. The unsteady flow gives a stream-wise circulation near the blade suction surface. The groove changes the tip vortex and helps to suppress the flow separation. In addition, the effect of blade sweep, profile variation and groove depth on the hysteresis behavior of the Wells turbine has been investigated in this work.

다단 천음속 축류홴에서 블레이드 스윕이 공력 성능에 미치는 영향

다단 천음속 축류홴에서 블레이드 스윕이 공력 성능에 미치는 영향

Marine Energy Turbine Performance: Effect of Blade Sweep

The present paper investigates blade sweep effect of a Wells turbine used in an oscillating water column (OWC) to extract wave energy. The numerical analysis of flow was carried out by using a commercial software ANSYS CFX® v14. 0. Two different blades were used: unswept blade and 10o backward-swept blade, respectively. It was found that the backward swept blade strongly affects the blade stall margin. It was also observed that the flow separations of the swept and unswept blades occur near the trailing edge and the leading edge, respectively.

Investigating the effect of blade sweep and lean in one stage of an industrial gas turbine׳s transonic compressor

In this paper, the simultaneous effects of the sweep and lean of the blades in one stage of a transonic compressor on its performance have been investigated. Then, with the help of numerical solution, fluid flows over these two modified geometries generated from the original sample were analyzed. Considering the applied constraints, the two generated rotor geometries have different geometrical characteristics; so that in rotor No. 1, the blade has a backward sweep and it is less affected by lean, while in the modified rotor No. 2, the blade has a forward sweep and it is more affected by lean. In the first sample, it is observed that the stage efficiency increases by 0.5% for operating design, while the stall margin reduces, and the chocking mass flow rate diminishes by 1.5%. Also regarding the second modified blade, the results indicate that the stall margin increases, the choking flow rate at the nominal rotational speed of the stage increases by 0.18% and the stage efficiency increases by 1%. The comparison of numerical results also shows that, in the first modified rotor, the pressure ratio of the stage diminishes by 0.01%; while in the second sample, the pressure ratio of the stage increases by the same amount. These results were then compared with the experimental results, showing a good agreement.

Effect of blade sweep on inlet flow in axial compressor cascades

This paper presents comparative numerical studies to investigate the effects of blade sweep on inlet flow in axial compressor cascades. A series of swept and straight cascades was modeled in order to obtain a general understanding of the inlet flow field that is induced by sweep. A computational fluid dynamics (CFD) package was used to simulate the cascades and obtain the required three-dimensional (3D) flow parameters. A circumferentially averaged method was introduced which provided the circumferential fluctuation (CF) terms in the momentum equation. A program for data reduction was conducted to obtain a circumferentially averaged flow field. The influences of the inlet flow fields of the cascades were studied and spanwise distributions of each term in the momentum equation were analyzed. The results indicate that blade sweep does affect inlet radial equilibrium. The characteristic of radial fluid transfer is changed and thus influencing the axial velocity distributions. The inlet flow field varies mainly due to the combined effect of the radial pressure gradient and the CF component. The axial velocity varies consistently with the incidence variation induced by the sweep, as observed in the previous literature. In addition, factors that might influence the radial equilibrium such as blade camber angles, solidity and the effect of the distance from the leading edge are also taken into consideration and comparatively analyzed.

The Effects of Blade 3D Designs in Different Orthogonal Coordinates on the Performance of Compressor Cascades

AbstractThe chordwise based Sweep-Dihedral Coordinates (SDC) and axial-tangential based Axial Sweep-Lean Coordinates (ASLC) are the two widely used coordinates systems for blade 3D design. In order to clarify the relationships between them, some numerical simulations were conducted in a simplified planar cascade model. The cascade model has a large aspect ratio of 3.0 and free-slip endwalls, which are used to minimize the endwall flow effects. The simulation results of totally 9 cases with different blade 3D design schemes, including sweep, dihedral, axial sweep and lean, were analyzed. Firstly, the effects of each type of blade 3D designs were summarized. And then, based on the rule of vector decomposition in an orthogonal coordinates, the effects of blade axial sweep and lean in ASLC were deduced from the effects of blade sweep and dihedral in SDC. It was found that forward/backward axial sweep is combined by positive/negative dihedral and forward/backward sweep, which have opposite trends of effects, resulting in a counterbalance phenomenon. Moreover, positive/negative lean is combined by positive/negative dihedral and forward/backward sweep, which have the same trends of effects, resulting in a superposition effect.

Effect of Centerbody Scattering on Advanced Open-Rotor Noise

Formulas for calculating the effect of centerbody scattering on the sound radiated from an advanced open rotor are presented. The effects of blade sweep and distributed blade loading are considered.Mach number effects are also implicitly included in the model. The work extends a previously published method and applies it to a practical situation in which scattering by the centerbody has a significant effect on the radiated sound field.

Aerodynamic effects of blade sweep and skew in low-speed axial flow rotors at the design flow rate: An overview

A detailed literature survey is presented herein in order to overview the aerodynamic impact of non-radial blade stacking techniques applied to axial flow fan and compressor rotors. The literature suggests a consensus that forward blade sweep and skew provides a means for the following advantages in the part load operational range of low-speed axial flow turbofan and compressor rotors: improvement of efficiency and performance, and extension of stall-free operating range. However, the published research results are rather diversified regarding the judgment of performance and loss modifying effects of sweep and skew at the design point. The current paper summarizes the major aerodynamic phenomena related to such blade stacking techniques, in order to contribute to a general reasoning of performance and efficiency modification at the design flow rate. Furthermore, it provides guidelines how to consider these phenomena in tailoring the blade geometry for potential efficiency gain and for achievement of the prescribed total pressure rise at the design point. The role of adequate computational fluid dynamics tools was considered in the paper to be essential in evaluation of aerodynamic effects of non-radial blade stacking, as well as in incorporation of sweep and skew in systematic blade design techniques.

Effects of blade sweep on the performance characteristics of axial flow turbomachinery rotors

Experimental studies were carried out in order to survey the performance and efficiency aspects of spanwise constant forward and backward sweep in axial flow rotors of low aspect ratio (AR) blading for incompressible flow, for part-load, near-design, and overload operational ranges. The experiments involved overall performance tests, laser Doppler velocimetry, and stationary total pressure probe measurements. The overall performance data and pitchwise averaged and resolved flow characteristics were evaluated in detail. For moderate and high flow rates, it was pointed out that positive or negative sweep tends to reduce or increase the blade load in the vicinity of the endwalls, respectively. It has been concluded that the loss-modifying effect of sweep can be judged by considering the three-dimensional viscous phenomena, and the influence of sweep on local blade efficiency depends on the balance of changes in blade load and losses. For low flow rates, forward sweep was found beneficial over the entire span from the aspect of improved stall margin and efficiency. The influence of AR on the performance reducing effect of sweep was studied on the basis of literature data.

Computational simulation and analysis of double-swept blade in BVI noise reduction

The TURNS computational fluid dynamics (CFD) code with the Beddoes prescribed wake and the WOPWOP computational acoustics code is used to study blade-sweep blade–vortex interaction (BVI) noise reduction design. The CFD three-dimensional unsteady solutions of blade surface pressure distributions are used as the input to WOPWOP acoustics computational code to produce the overall sound pressure level (OASPL) on a 3-rotor radiation observer hemisphere around the helicopter rotor. To study the effects of blade sweep on BVI noise reduction, computations are performed on a baseline rectangular blade and a corresponding double-swept blade to better understand the impact of blade sweep on BVI noise reduction in relation to the interaction angle between blade leading edge and the shed tip-vortex. The present study indicates that tip-region blade forward sweep produces favorable BVI angles for dominate BVIs to reduce the maximum BVI noise level on the advancing side, while increasing noise level on the retreating side. Increasing in the noise level on the retreating side as a trade-off for decreasing in the maximum noise level on the advancing side results favorably in the reduction of the overall maximum noise level and in changing the ‘hot’ noise spots into a more desirable ‘less hot’ noise region.

Numerical investigation on the effect of blade sweep on the performance of Wells turbine

A Wells turbine is one of the simplest and promising self-rectifying air turbines which is basic to the needs of the near future and likely to be economically viable. With the recent development in computer hardware and software, it has now become practicable to conduct a reasonable computation of three-dimensional turbulent flows through complex geometry. To investigate the effect of blade sweep on the performance of the Wells turbine, the numerical investigation was carried out under steady flow condition with a fully 3-D Navier–Stokes code for two kinds of blades, NACA0020 and CA9. As a result, it was found that the performance of the Wells turbine is considerably influenced by the blade sweep. The optimum blade sweep ratio ( f=0.35) for the NACA0020 was found. This value is just the same as one obtained experimentally by the authors in the past. It was also found that the overall turbine performance for the NACA0020 is better than that for the CA9. It was shown that the numerical method is able quite well to predict the effect of blade sweep of the Wells turbine. The detailed flow patterns for several blade sweeps were also shown and discussed in this paper.

THE RESPONSE OF A SWEPT BLADE ROW TO A THREE-DIMENSIONAL GUST

An analytical expression is obtained for the unsteady loading, acoustic mode amplitude, and sound power output of a three-dimensional rectilinear cascade of blades with finite chord excited by a three dimensional gust. The effects of spanwise wave number and cross-flow are included so that the effect of blade sweep and a gust with a spanwise variation can be considered. The results show that the effect of the spanwise gust and blade sweep are very similar and reduce the effective frequency of the incoming gust. The dominant effect is caused by the cut off of acoustic modes when the trace velocity of the gust along the blade span is subsonic.

An analytical model for upstream and downstream sound power from swept blade rows

This paper will describe an analytical approach which can be used to calculate the upstream or downstream sound power generated by a swept blade row. The blade row is modeled using a linear cascade of flat plates with finite chord. The correct allowance for spanwise loading and gust convection is included. The value of this approach is that it can be used at very high reduced frequencies which are typically found in broadband fan noise applications. Results will be given which show the problems associated with only considering the noise sources at the leading edges of the blades and ignoring the trailing edges. The effects of blade sweep will also be illustrated by showing the radiated modal sound power as a function of blade sweep. [Work supported by NASA.]

The effect of blade sweep on the reduction and enhancement of supersonic propeller noise

An asymptotic frequency-domain approach is used to describe the radiation from a supersonic swept propeller within the framework of linear acoustics. With this approach the radiation of singularities, their points of origin on the blades, and their relation to blade geometry and loading are easily obtained. In particular, it is shown that a swept propeller with a completely subsonic leading edge can still radiate singularities, if the leading edge is blunt, due to a supersonic edge effect at the blade tips. In addition, the radiation from a family of ‘critical’ swept-blade designs is shown to be more singular than that from a straight-bladed design. Numerical and asymptotic results for such designs show that the peak radiation is, typically, increased by 5–10 dB.