•Flow inside a ducted axial fan is computed by highly resolved LES.•The effect of tip-gap size and flow rate on the flow field are analyzed in detail.•The diameter and strength of the tip vortex ...increase with the tip gap.•The largest tip gap at the design condition shows breakdown of the main tip vortex.•The increase of tip-gap size resulted in more intense turbulent mixing and higher losses.
Detailed and comprehensive analyses of the tip vortex system of a ducted axial fan are performed based on highly resolved large-eddy simulations. Various tip gap sizes, which are typically found in technical fans, and two operating conditions are considered. The Reynolds number based on the tip speed of the blade and the diameter of the fan D0 is Re=9.36×105, the Mach number is M=0.136, and the rotational speed Ω=3000 rpm. The chosen tip clearances s are s/Do=0.001,0.005,0.01 and the design and off-design operating conditions are defined by the flow rate coefficients Φ=0.195 and Φ=0.165, respectively. The conservation equations of a compressible, viscous fluid are integrated on a multi-block structured mesh with 140 × 106 grid points in a rotating frame of reference for a single out of five blades using periodic boundary conditions in the circumferential direction and prescribed undisturbed inflow conditions based on experimental data. The results show that increasing the tip-gap size results in several vortices in the tip-gap region, i.e., tip leakage, separation and induced vortices, which enlarge the diameter and the strength of the main tip vortex and decrease the efficiency of the fan. For the off-design operating condition, the tip-gap vortex for the smallest tip-gap size decays faster than for the design operating condition. Increasing the tip-gap width, changes the direction of the tip-leakage vortex trajectory. The angle between the blade chord and the tip-leakage vortex is decreased at design condition, while the opposite behavior is observed for the off-design condition, in which the tip-leakage vortex moves further away from the suction side of the blade. Furthermore, the interaction between the axial flow and the main tip leakage vortex convecting downstream of the blade leads to an enhanced turbulent mixing at larger tip-gap size. For the largest tip clearance, i.e., s/Do=0.01, spiral vortex breakdown occurs at the design operating condition caused by the interaction of the main tip vortex with the secondary vortex generated in the tip-gap region. At off-design operating conditions, a larger tip-leakage-loss coefficient is obtained due to a more intensive turbulent mixing, where the maximum loss coefficient occurs at around midchord.
•We present a study in characterizing tip wear of a high speed AFM on four different samples and under different measurement conditions. Our results indicate the dominance of tip breakage in ...high-speed AFM measurements.•To understand the mechanism of tip breakage, we modelled and simulated the tip-sample interaction. The results indicate that the tip-sample interaction force increases dramatically in measurement scenarios of steep surfaces.•Detailed methods for tip characterization and wear test procedures are presented in the paper.
Tip abrasion is a critical issue particularly for high-speed atomic force microscopy (AFM). In this paper, a quantitative investigation on the tip abrasion of diamond-like-carbon (DLC) coated tips in a high-speed metrological large range AFM device has been detailed. Wear tests are conducted on four different surfaces made of silicon, niobium, aluminum and steel. During the tests, different scanning speeds up to 1 mm/s and different vertical load forces up to approximately 33.2 nN are applied. Various tip characterization techniques such as scanning electron microscopy (SEM) and AFM tip characterizers have been jointly applied to measure the tip form change precisely. The experimental results show that tip form changes abruptly rather than progressively, particularly when structures with steep sidewalls were measured. This result indicates the increased tip breakage risk in high-speed AFM measurements. To understand the mechanism of tip breakage, tip-sample interaction is modelled, simulated and experimentally verified. The results indicate that the tip-sample interaction force increases dramatically in measurement scenarios of steep surfaces.
Angiogenesis is a critical, fine-tuned, multi-staged biological process. Tip-stalk cell selection and shuffling are the building blocks of sprouting angiogenesis. Accumulated evidences show that ...tip-stalk cell selection and shuffling are regulated by a variety of physical, chemical and biological factors, especially the interaction among multiple genes, their products and environments. The classic Notch-VEGFR, Slit-Robo, ECM-binding integrin, semaphorin and CCN family play important roles in tip-stalk cell selection and shuffling. In this review, we outline the progress and prospect in the mechanism and the roles of the various molecules and related signaling pathways in endothelial tip-stalk cell selection and shuffling. In the future, the regulators of tip-stalk cell selection and shuffling would be the potential markers and targets for angiogenesis.
•A method for decomposing the pressure force on the immersed body is derived for RANS/LES.•The new method keeps good accuracy in single-phase and multiphase flows.•The effect of different cavitation ...types on lift and drag is quantified.•Tip clearance region is nonnegligible, where the lift is generated by the interaction of shear and rotating flows.
In this paper, the force decomposition method (FDM) is proposed for decomposing the pressure forces acting on the immersed body. The major improvement in FDM is the applicability in RANS and LES simulations with non-constant density flows, e.g. cavitating flow. In the single-phase flow over a circular cylinder (Re = 3900), the FDM results show excellent agreement with the pressure force given by conventional method. In the cavitating flow over a hydrofoil with tip clearance, FDM can also reproduce the same tendency of the pressure force with an average deviation below 17%. In this case, the vorticity and kinematic effect induced force dominates the pressure force. In order to isolate the effect of attached cavitation near the suction side and tip clearance cavitation, a method combing domain cutting (manually) and cell extracting (by a threshold of vorticity magnitude) is proposed. The lift caused by vorticity force is mainly affected by the vortices shedding from the suction side of the hydrofoil. Also, the tip clearance region should not be ignored, where the lift generation by TLV is the interaction of strong shear and rotating flows.
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Owing to its ability to handle large flows, an axial flow pump as turbine (PAT) can generate considerable amounts of electricity in small-scale hydropower plants. However, a PAT's efficiency can be ...hindered by tip leakage flow (TLF), namely, flow through the clearance between the impeller blade tip and shroud. Accordingly, this study investigates the influences of TLF on the PAT's energy performance through numerical simulations in which the entropy production method has been adopted. TLF and the associated tip leakage vortex (TLV) are found to both decrease the hydraulic efficiency and increase the flow rate; the shaft power output is also affected, especially near the machine's best efficiency point. The effect of TLF on the pressure distribution along the blade depends on the flow conditions, and the form of the TLV directly generated by TLF is affected by the flow incidence angle. The vorticity transport equation reveals that the vortex stretching term plays a dominant role in the spatial evolution of the TLV and has the greatest impact on the pressure distribution. Finally, different operating conditions lead to different energy loss mechanisms: turbulent dissipation is the main cause of energy loss, and high flow conditions are marked by an increase in TLF-dependent wall shear stress dissipation.
•The clearance flow is considered to evaluate the PAT energy performance.•The mechanism of how clearance flow affects the pressure distribution is revealed.•The entropy production method is utilized for the visualization of high energy loss.•Different operating conditions have led to different energy loss mechanisms.
Rhinoplasty is arguably the most complex and intricate surgery performed by facial plastic surgeons. Nasal tip refinement of a broad nasal tip has remained the most challenging part of rhinoplasty as ...sophisticated techniques are critical to achieve aesthetically pleasing and structurally sound nasal tips that can withstand the contractile forces of healing. Successful tip refinement relies on an in-depth preoperative and intraoperative understanding of the patient's nasal anatomy, well developed arsenal of techniques, the experience of the surgeon, and the aesthetic desires of the patient. Although the approach to gain access to the nasal tip so as to successfully reshape the tip has been a topic of debate over many years, the aim of this article is to outline and demonstrate how the broad nasal tip can be successfully recontoured through an endonasal approach using nondestructive techniques that have been effectively used in open rhinoplasty. We believe that there continues to be a place for endonasal tip rhinoplasty especially in this era in which patients desire less invasive procedures with shorter healing time.
In the present work, the tip leakage vortex (TLV) in a mixed flow pump as turbine at pump mode is decomposed and reconstructed by Dynamic Mode Decomposition (DMD) for the first time. The flow field ...is solved by the k-ω SST turbulence model with validation of experimental data. The unstable primary tip leakage vortex (PTLV) can be mainly classified as two parts, oscillating PTLV-A and shedding PTLV-B. The evolution frequency of PTLV is 8.4fi (fi is the impeller rotating frequency). Results of DMD show that DMD can exactly decompose the dominant frequency of TLV evolution and its harmonic frequencies. The coherent structures captured by DMD mainly distribute near the trajectory of PTLV. Meanwhile, DMD can successfully reconstruct the flow field on basis of mean flow mode and the first mode. Proper Orthogonal Decomposition (POD) can also capture the main coherent structures of PTLV, due to the dominant frequencies of the most energetic modes contain the characteristic frequency of TLV.
•The evolution frequency of tip leakage vortex is 8.4 times of impeller rotating frequency.•Dynamic mode decomposition is used to decompose and reconstruct the coherent structure of a mixed flow pump.•The coherent structures of tip leakage flow can be divided into two types.•Major vortex structures by dynamic mode decomposition and proper orthogonal decomposition are compared.
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