Inlet Gap Effect on Aerodynamics and Tonal Noise Generation of a Voluteless Centrifugal Fan

In this paper, the gap effects on the aerodynamics and tonal noise generation of voluteless centrifugal fans are studied based on different gap geometries. The study is motivated by the state of the art of this type of fan, for which the tonal noise generation due to the gap turbulence has not been addressed concerning the gap geometry, while a recent study reported that there is tonal noise at the blade passing frequency (BPF) from the gap turbulence. We simulate the configurations using a hybrid method coupling the improved delayed detached eddy simulation (IDDES) with Formulation 1A of Farassat. Our simulation shows regions with high vorticity magnitudes in the channel between two blades near the trailing edges close to the shroud. The turbulence renders a uniform pressure rise. By changing the gap design, the turbulent regions can be reduced. The configurations show a similar trend of the root mean square (RMS) pressure on the blade leading edge (BLE), largest at the shroud, and decays when the distance to the gap increases. The gap designs affect the amplitude of the RMS pressure, which is connected to the BPF. Spectral analysis is performed for the surface pressure fluctuations and the sound pressure upstream of the fan. The surface pressure fluctuations show that, for all cases, the regions with high energy are identical to the locations where the gap turbulence evolves and accounts for the impingement on the BLE. The amplitude of the tonal noise at the BPF differs between the cases

Inlet Gap Influence on Low-Frequency Flow Unsteadiness in a Centrifugal Fan

In this study, unsteady low-frequency characteristics in a voluteless low-speed centrifugal fan operating at a high mass flow rate are studied with improved delayed detached eddy simulation (IDDES). This study is motivated by a recent finding that the non-uniformly distributed pressure inside this type of fan could be alleviated by improving the gap geometry. The present simulation results show that the velocity magnitudes of the gap have distinct low and high regions. Intensive turbulent structures are developed in the low-velocity regions and are swept downstream along the intersection between the blade and shroud, on the pressure side of the blade. Eventually, the turbulence gives rise to a high-pressure region near the blade’s trailing edge. This unsteady flow behavior revolves around the fan rotation axis. Additionally, its period is 5% of the fan rotation speed, based on the analysis of the time history of the gap velocity magnitudes and the evolution of the high-pressure region. The same frequency of high pressure was also found in previous experimental measurements. To the authors’ knowledge, this is the first time that the trigger of the gap turbulence, i.e., the unsteady local low velocity, has been determined

Tonal Noise of Voluteless Centrifugal Fan Generated by Turbulence Stemming from Upstream Inlet Gap

Volutes for radial-flow turbomachines (e.g., centrifugal fans and pumps) are spiral funnel-shaped casings that house rotors. Their function is to guide the flow from rotors to outlets and maintain constant flow speeds. Under specific conditions, however, volutes are removed (termed voluteless) to reduce flow losses and noise. In this paper, a generic voluteless centrifugal fan is investigated for the tonal noise generation at an off-design operation point. In contrast to typical tonal noise sources induced by the fan blades, we find out that another predominant source is the turbulence stemming from the clearance gap between the fan front shroud and the inlet duct. The turbulence evolves along with the front shroud and is swept downstream to interact with the top side of the blade leading edge. An obvious additional tone is observed at\ua0273 Hz other than the blade passing frequency (BPF0) and relevant harmonic frequencies. By coarsening the mesh resolution near the inlet gap and front shroud in the simulations, we artificially deactivate the gap turbulence. Consequently, the tone at\ua0273 Hz disappears completely. The finding indicates that the interaction between the gap turbulence and blades accounts for the tone. As the gap turbulence exists near the front shroud, this rotating wall introduces rotational momentum into the turbulence due to skin friction. Hence, this tonal interaction frequency is smaller than\ua0BPF0 with a decrement of the fan rotation frequency. To the authors\u27 knowledge, this is the first time that voluteless centrifugal fans are studied for the gap-turbulence noise generation

Unsteady RANS and IDDES studies on a telescopic crescent-shaped wingsail

Over the years, several research projects have evaluated different concepts for wind-assisted propulsion, generally concluding that it can lead to significant fuel savings. The time-averaged propulsive performance of a single rigid wingsail has been analysed in previous studies. However, the unsteady characteristics of the external loads which may induce structural vibration are also important to be considered. In this study, full-scale simulations, with both unsteady RANS and IDDES methods, are performed to analyze the flow field. The paper\u27s analysis includes flow separation and vortex shedding, the development and dissipation of wake vortices, and the lift reduction due to tip vortices. It also studies the telescopic function of the wingsail by analyzing sails with different heights and wind conditions. The paper concludes that the unsteady RANS and IDDES simulations make similar predictions for time-averaged loads but disagree on the unsteady characteristics. The IDDES simulations indicate more complex vortex-shedding phenomena

Birth places of extreme ultraviolet waves driven by impingement of solar jets upon coronal loops

Solar extreme ultraviolet (EUV) waves are large-scale propagating disturbances in the corona. It is generally believed that the vital key for the formation of EUV waves is the rapid expansion of the loops that overlie erupting cores in solar eruptions, such as coronal mass ejections (CMEs) and solar jets. However, the details of the interaction between the erupting cores and overlying loops are not clear, because that the overlying loops are always instantly opened after the energetic eruptions. Here, we present three typical jet-driven EUV waves without CME to study the interaction between the jets and the overlying loops that remained closed during the events. All three jets emanated from magnetic flux cancelation sites in source regions. Interestingly, after the interactions between jets and overlying loops, three EUV waves respectively formed ahead of the top, the near end (close to the jet source), and the far (another) end of the overlying loops. According to the magnetic field distribution of the loops extrapolated from Potential Field Source Surface method, it is confirmed that the birth places of three jet-driven EUV waves were around the weakest magnetic field strength part of the overlying loops. We suggest that the jet-driven EUV waves preferentially occur at the weakest part of the overlying loops, and the location can be subject to the magnetic field intensity around the ends of the loops

Generation of interior cavity noise due to window vibration excited by turbulent flows past a generic side-view mirror

We investigate the interior noise caused by turbulent flows past a generic side-view mirror. A rectangular glass window is placed downstream of the mirror. The window vibration is excited by the surface pressure fluctuations and emits the interior noise in a cuboid cavity. The turbulent flows are simulated using a compressible large eddy simulation method. The window vibration and interior noise are predicted with a finite element method. The wavenumber-frequency spectra of the surface pressure fluctuations are analyzed. The spectra are identified with some new features that cannot be explained by the Chase model for turbulent boundary layers. The spectra contain a minor hydrodynamic domain in addition to the hydrodynamic domain caused by the main convection of the turbulent boundary layer. The minor domain results from the local convection of the recirculating flow. These domains are formed in bent elliptic shapes. The spanwise expansion of the wake is found causing the bending. Based on the wavenumber-frequency relationships in the spectra, the surface pressure fluctuations are decomposed into hydrodynamic and acoustic components. The acoustic component is more efficient in the generation of the interior noise than the hydrodynamic component. However, the hydrodynamic component is still dominant at low frequencies below approximately 250 Hz since it has low transmission losses near the hydrodynamic critical frequency of the window. The structural modes of the window determine the low-frequency interior tonal noise. The combination of the mode shapes of the window and cavity greatly affects the magnitude distribution of the interior noise

Parameter Sensitivity Study on Inflow Distortion of Boundary Layer Ingested Turbofans

The inflow distortion to the fan introduced by the ingestion of the fuselage boundary layer is the most critical challenge in realizing the benefits of boundary later ingesting (BLI) concepts. Minimizing the level of distortion while maintaining the desired amount of ingested boundary layer and free stream flow is crucial in minimizing the penalties to fan efficiency and noise emissions. In this paper, a parametric sensitivity study is performed to examine the integration of two semi-buried BLI turbofans at the rear end of a typical tube-and-wing body (TWB) fuselage. The key parameters influencing BLI, such as the nacelle installation positions, wing position, fuselage length, rear fuselage shape, intake shape and operating conditions were evaluated by computational fluid dynamics (CFD). Among the investigated parameters, increasing the nacelle spanwise installation spacing improved inflow distortion by reducing the diffusion separation, but this needs to be offset against the added weight and nacelle drag. A high wing position variant showed strong interference between the wing and the nacelle, which must be avoided as this significantly increases the complexity of the inflow distortion. A moderate angle of attack (AOA) variation did not affect the fan inflow distortion but there was a tendency for interference from the wing to increase when the AOA was increased. The general conclusions from this study will be useful in the conceptual design of a similar type of BLI configuration, as well as a more comprehensive optimization of this type of aircraft–engine integration

A New Method for Impeller Inlet Design of Supercritical CO2 Centrifugal Compressors in Brayton Cycles

Supercritical Carbon Dioxide (SCO2) is considered as a potential working fluid in next generation power and energy systems. The SCO2\ua0Brayton cycle is advantaged with higher cycle efficiency, smaller compression work, and more compact layout, as compared with traditional cycles. When the inlet total condition of the compressor approaches the critical point of the working fluid, the cycle efficiency is further enhanced. However, the flow acceleration near the impeller inducer causes the fluid to enter two-phase region, which may lead to additional aerodynamic losses and flow instability. In this study, a new impeller inlet design method is proposed to achieve a better balance among the cycle efficiency, compressor compactness, and inducer condensation. This approach couples a concept of the maximum swallowing capacity of real gas and a new principle for condensation design. Firstly, the mass flow function of real gas centrifugal compressors is analytically expressed by non-dimensional parameters. An optimal inlet flow angle is derived to achieve the maximum swallowing capacity under a certain inlet relative Mach number, which leads to the minimum energy loss and a more compact geometry for the compressor. Secondly, a new condensation design principle is developed by proposing a novel concept of the two-zone inlet total condition for SCO2\ua0compressors. In this new principle, the acceptable acceleration margin (AAM) is derived as a criterion to limit the impeller inlet condensation. The present inlet design method is validated in the design and simulation of a low-flow-coefficient compressor stage based on the real gas model. The mechanisms of flow accelerations in the impeller inducer, which form low-pressure regions and further produce condensation, are analyzed and clarified under different operating conditions. It is found that the proposed method is efficient to limit the condensation in the impeller inducer, keep the compactness of the compressor, and maintain a high cycle efficiency

Propulsive performance of a rigid wingsail with crescent-shaped profiles

Wind-assisted ship propulsion is considered an effective method for reducing greenhouse gas emissions. This paper presents numerical analyses of the aerodynamics of a single rigid wingsail conducted using the unsteady Reynolds-averaged Navier–Stokes (uRANS) equations. The wingsail is designed with a new sectional profile: a crescent-shaped foil. This new profile and the classical NACA 0015 profile were compared. Simulations were performed in two and three dimensions, with a focus on key physical quantities such as the external loads on the wingsail, the flow field, and the propulsive performance. It is concluded that the wingsail with the crescent-shaped section has higher propulsion efficiency than the NACA 0015. However, stronger flow separation was detected for the crescent-shaped section. As the separation deteriorates, the flow unsteadiness, challenges the strength and stability of the wingsail structure. The three-dimensional simulations of both profiles, particularly NACA 0015, show that the tip vortices induced from the side edge of the wingsail account for substantial negative effects on the propulsion performance. A case study revealed that installing a wingsail with a crescent-shaped profile reduced fuel consumption by 9% compared with no wingsail

Blade-Tip Vortex Noise Mitigation Traded-Off against Aerodynamic Design for Propellers of Future Electric Aircraft

We study noise generation at the blade tips of propellers designed for future electric aircraft propulsion and, furthermore, analyze the interrelationship between noise mitigation and aerodynamics improvement in terms of propeller geometric designs. Classical propellers with three or six blades and a conceptual propeller with three joined dual-blades are compared to understand the effects of blade tip vortices on the noise generation and aerodynamics. The dual blade of the conceptual propeller is constructed by joining the tips of two sub-blades. These propellers are designed to operate under the same freestream flow conditions and similar electric power consumption. The Improved Delayed Detached Eddy Simulation (IDDES) is adopted for the flow simulation to identify high-resolution time-dependent noise sources around the blade tips. The acoustic computations use a time-domain method based on the convective Ffowcs Williams–Hawkings (FW-H) equation. The thrust of the 3-blade conceptual propeller is\ua04%\ua0larger than the 3-blade classical propeller and\ua08%\ua0more than the 6-blade one, given that they have similar efficiencies. Blade tip vortices are found emitting broadband noise. Since the classical and conceptual 3-blade propellers have different geometries, especially at the blade tips, they introduce deviations in the vortex development. However, the differences are small regarding the broadband noise generation. As compared to the 6-blade classical propeller, both 3-blade propellers produce much larger noise. The reason is that the increased number of blades leads to the reduced strength of tip vortices. The findings indicate that the noise mitigation through the modification of the blade design and number can be traded-off by the changed aerodynamic performance