| 1. |
- Ottersten, Martin, 1981, et al.
(författare)
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Inlet Gap Influence on Low-Frequency Flow Unsteadiness in a Centrifugal Fan
- 2022
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Ingår i: Aerospace. - : MDPI AG. - 2226-4310. ; 9:12
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Shao, Xinyuan, 1997, et al.
(författare)
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Near-wall approximations to speed up simulations for atmosphere boundary layers in the presence of forests using lattice Boltzmann method on GPU
- 2022
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Forests play an important role in influencing the wind resource in atmospheric boundary layers and the fatigue life of wind turbines. Due to turbulence, a difficulty in the simulation of the forest effects is that flow statistical and fluctuating content should be accurately resolved using a turbulence-resolved CFD method, which requires a large amount of computing time and resources. In this paper, we demonstrate a fast but accurate simulation platform that uses a lattice Boltzmann method with large eddy simulation on Graphic Processing Units (GPU). The simulation tool is the open-source program, GASCANS, developed at the University of Manchester. The simulation platform is validated based on canonical wall-bounded turbulent flows. A forest is modelled in the form of body forces injected near the wall. Since a uniform cell size is applied throughout the computational domain, the averaged first-layer cell height over the wall reaches to ⟨Δy+⟩=165. Simulation results agree well with previous experiments and numerical data obtained from finite volume methods. We demonstrate that good results are possible without the use of a wall-function, since the forest forces overwhelm wall friction. This is shown to hold as long as the forest region is resolved with several cells. In addition to the GPU speedup, the approximations also significantly benefit the computation efficiency.
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| 3. |
- Xue, Xiao, 1989, et al.
(författare)
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Wall-modeled large-eddy simulation integrated with synthetic turbulence generator for mutiple-relaxation-time lattice Boltzmann method
- 2023
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Ingår i: Physics of Fluids. - 1070-6631 .- 1089-7666. ; 35:6
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Tidskriftsartikel (refereegranskat)abstract
- The synthetic turbulence generator (STG) lies at the interface of the Reynolds averaged Navier–Stokes (RANS) simulation and large-eddy simulation (LES). This paper presents an STG for the multiple-relaxation-time lattice Boltzmann method (LBM) framework at high friction Reynolds numbers, with consideration of near-wall modeling. The Reichardt wall law, in combination with a force-based method, is used to model the near-wall field. The STG wall-modeled LES results are compared with turbulent channel flow simulations at Reτ=1000,2000,5200 at different resolutions. The results demonstrate good agreement with direct numerical simulation, with the adaptation length of 6–8 boundary layer thickness. This method has a wide range of potentials for hybrid RANS/LES-LBM related applications at high friction Reynolds numbers.
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| 4. |
- Vasudevan, Sudharsan, 1991, et al.
(författare)
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Numerical model to estimate subcooled flow boiling heat flux and to indicate vapor bubble interaction
- 2021
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 170
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Tidskriftsartikel (refereegranskat)abstract
- There are numerous technical applications where hot components, with uneven temperature distribution, require cooling. In such applications, it is desired to provide efficient local cooling of the hot spots, while avoiding unnecessary over-cooling of the other regions. Such an approach, known as precision cooling, has several advantages. In addition to the fact that it reduces the effort for cooling, it limits the unintended heat lost to the cooling medium. In liquid cooled systems, such as Internal Combustion Engines (ICE), subcooled flow boiling offers immense potential for precision cooling. The primary challenges in extracting this potential are understanding the complexities in the subcooled flow boiling phenomenon and estimating the risk of encountering film boiling. The present study introduces a numerical model to estimate the wall heat flux in subcooled flow boiling and the model includes a mechanistic formulation to account for vapor bubble interaction. The formulation for vapor bubble interaction serves two purposes: (a) blends two well-established models in the literature, one in the isolated bubbles regime and other in the fully developed boiling regime, to estimate the wall heat flux; and (b) provides information to limit boiling in order to not encounter film boiling. The results from the new model are validated with two different experiments in the literature and the wall heat flux estimated by the model is in agreement with experimental results and responsive to different input parameters, such as bulk velocity, operating pressure and inlet subcooling. The new model requires only input of local flow quantities and hence implementation in Computational Fluid Dynamics (CFD) is straightforward.
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| 5. |
- Ask, Jonas, 1970, et al.
(författare)
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Flow and dipole source evaluation of a generic SUV
- 2007
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Ingår i: 13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference). - Reston, Virigina : American Institute of Aeronautics and Astronautics.
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Konferensbidrag (refereegranskat)abstract
- Accurately predicting both average flow quantities and acoustic sources at the front side window of today's ground vehicles is still a considerable challenge to automotive companies world-wide. One of the most important aspects for obtaining trustworthy results, but also the most tedious one and therefore perhaps overlooked, is the control and outcome of the mesh generation process. Generating unstructured volume meshes suitable for Large Eddy Simulations with high level representation of geometrical details is both a time consuming and an extremely computer demanding activity. This work investigates two different mesh generation processes with the main aim to evaluate their outcome with respect to the prediction of the two dominating dipole sources in a temporal form of the Curie's equation. Only a handful of papers exists with high level representation of the vehicle geometry and the aim of predicting the fluctuating exterior noise sources. To the author's knowledge no studies have been conducted in which both these source terms are evaluated quantitatively against measurements. The current paper investigates the degree to which the amplitude of these two source terms can be predicted by using the traditional law-of-the-wall and hex-dominant meshes with isotropic resolution boxes for a detailed ground vehicle geometry. For this purpose the unstructured segregated commercial FLUENT Finite Volume Method code is used. The flow field is treated as incompressible, and the Smagorinsky-Lilly model is used to compute the sub-grid stresses. Mean flow quantities are measured with a 14-hole probe for 14 rakes downstream of the side mirror. Dynamic pressure sensors are distributed at 16 different positions over the side window to capture the fluctuating pressure signals. All measurements in this work were conducted at Ford's acoustic wind tunnel in Cologne. All simulations accurately predict the velocity magnitude closest to the side window and downstream of the mirror head recirculation zoner. Some variations in the size and shape of this recirculation zone are found between the different meshes, most probably caused by differences in the detachment of the mirror head boundary layer. The Strouhal number of the shortest simulation was computed from the fundamental frequency of the mirror lift force component. The computed Strouhal number agrees well with the corresponding results from similar objects and gives an indication of an acceptable simulation time. Dynamic pressure sensors at 16 different locations at the vehicle side window were also used to capture the levels of the two dipole source terms. These results are compared against the three simulations. With the exception of three positions, at least one of the three simulations accurately captures the levels of both source terms up to about 1000Hz. The three positions with less agreement as compared to measurements were found to be in regions sensitive to small changes in the local flow direction.
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| 6. |
- Huang, Zhongjie, 1985, et al.
(författare)
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Low-Noise Propeller Design for Quiet Electric Aircraft
- 2020
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Ingår i: AIAA AVIATION 2020 FORUM. - Reston, Virginia : American Institute of Aeronautics and Astronautics.
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Konferensbidrag (refereegranskat)abstract
- Inspired by the tremendous success of electric cars, the electrification technology has recently attracted a great deal of attention in aviation industry. This leads to the advent of massive efforts on the development of electric aircraft. Electrification enables zero CO2 and NOx emissions for aircraft in the near future. However, the annoying noise radiated from electrified propulsion systems is still a great challenge. The major noise sources are propellers. In particular, short-haul electric aircraft are usually flying from small airports near communities, which would exaggerate the noise impacts on residents. This paper presents a study on the propeller noise for a hypothetical electric aircraft. First of all, the acoustic study focuses on the effects of blade number, blade diameter, and rotation speed for an isolated propeller. Then, an unique configuration of dual-rotating propeller is briefly investigated to explore its potential of low-noise generation. Lastly, multiple propellers installed on an aircraft with various distributive propulsion strategies are investigated. The overall noise emission of these distributive propulsion strategies are promising to reduce the total noise from electric aircraft. Nonetheless, the noise level is very sensitive to the installation location of every propeller. An interesting finding is that the acoustic-wave interaction among the propellers are limited to the near field, and is insignificant in the far field. The present results are expected to enhance the understanding of propeller noise generation mechanisms, which are beneficial to propose guidelines on the development of low-noise distributive propulsion systems for electric aircraft.
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| 7. |
- Li, Shuai, 1990, et al.
(författare)
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Numerical modeling of a wire mesh for aerodynamic noise reduction
- 2023
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 35:1
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Tidskriftsartikel (refereegranskat)abstract
- A novel wire mesh consisting of very fine wires and pores is numerically investigated for the purpose of noise reduction. To develop a numerical model for this wire mesh, a set of experimental flow-field data has been deployed for the model validation. The experimental data were measured with only 22% of the wind-tunnel cross section covered by the wire mesh, taking into account the vortex shedding from both sides of the wire-mesh fairing. It is found that existing wire-mesh models using a damping-type source term proportional to the square of flow velocity do not perform well in modeling this novel wire mesh. To tackle this issue, an improvement is proposed by additionally introducing a linear term to account for the permeability of the wire mesh, based on another set of experiments with the wind-tunnel cross section fully covered by the wire mesh. The proposed model is then validated against the experimental data, demonstrating its capability in modeling the wire mesh. Subsequently, the model is applied to a tandem cylinder configuration. Results show that a wide but short-span wire mesh significantly reduces the dominant tone of tandem cylinders, noise at higher frequencies, as well as the overall sound pressure levels.
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| 8. |
- Ottersten, Martin, 1981, et al.
(författare)
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Unsteady Simulation of Tonal Noise from Isolated Centrifugal Fan
- 2020
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- In this study, an isolated centrifugal fan is investigated for the aerodynamic and acoustic performances using RANS and URANS simulations. The noise is predicted by coupling the URANS and the Ffowcs Williams and Hawkings acoustic analogy. The aerodynamic properties obtained from RANS and URANS are consistent with the experimental data. The magnitudes of the tonal noise at the blade passing frequencies are well predicted. Recirculating flows, which are responsible for reducing the fan efficiency and increasing the noise generation, are observed between the shroud and the blade trailing edges. It is found that the recirculating flows are associated with the gap between the shroud and the inlet duct.
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| 9. |
- Vasudevan, Sudharsan, 1991, et al.
(författare)
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Comparative analysis of single and multiphase numerical frameworks for subcooled boiling flow in an internal combustion engine coolant jacket
- 2023
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Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311. ; 219
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Tidskriftsartikel (refereegranskat)abstract
- Computational analysis of nucleate boiling occurring in liquid cooled applications, such as internal combustion engines is often implemented within a single phase Computational Fluid Dynamics (CFD) framework, owing to low vapor fractions involved. With increase in specific power and the resulting higher thermal loads, accounting for the presence of the vapor phase using a multiphase framework is required in certain conditions, despite the higher computational costs. While detailed resolution of the liquid and vapor phases in nucleate boiling using a two fluid model is excessively computationally expensive, the homogeneous mixture multiphase framework is a good compromise between resolution and computational cost. In this article a numerical wall boiling model is implemented within both, a single phase and the mixture multiphase frameworks. Results from the two approaches are compared with measurements in a channel flow. The results from both approaches are in good agreement with experiments. The single phase approximation is valid when the vapor generation is low. The sensitivity of the results to the computational grid is also discussed in detail. Further, the two frameworks are used to simulate the heat transfer in the coolant jacket of a four-cylinder petrol engine. The results from the numerical simulations are compared with measurements. Both computational frameworks compare reasonably well with the measurements in terms of local metal temperature. However, the advantage of accounting for the vapor phase using the mixture multiphase framework is evident when the parameter related to vapor bubble interactions is analyzed in detail.
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| 10. |
- Wasala, Sahan, 1983, et al.
(författare)
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Modelling of DBD plasma actuator for controlling noise from a tandem cylinders configuration
- 2019
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Ingår i: INTER-NOISE 2019 MADRID - 48th International Congress and Exhibition on Noise Control Engineering.
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Konferensbidrag (refereegranskat)abstract
- Aeroacoustic noise, generated by aircraft landing gears during the take-off or landing manoeuvres, is considered excessive, causing environmental concerns for the people living close to airports. Therefore, there is an increasing need to innovate new technologies to reduce landing gear noise. For noise reduction, it is primarily important to understand the mechanism of flow-induced landing-gear noise generation and, further, to adapt relevant new technologies to the system in order to reduce noise levels by means of effective manipulation of related aerodynamic flow features. Dielectric Barrier Discharge (DBD) plasma actuators have shown efficiency to control flow separation from bluff bodies, consequently, mitigating subsequent vortex motions and noise generation. In the present paper, a simplified landing-gear model represented by a tandem-cylinders configuration has been used. The airflow has been simulated using hybrid RANS/LES. Effects of the plasma actuation are modelled using two different models. These include the Suzen & Huang model, which solves for the electric field and charge density fields in order to obtain the body force, and the Greenblatt model, which simply assumes that the body force decays exponentially both downstream and normal to the actuator.
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