Tip Geometry Research Articles

Parametric study of small-scale rotors in axial descent

Despite extensive research in multirotor aerodynamics in the recent past, axial descent, specifically the vortex ring state, still poses great challenges for multirotor configurations as this flight stage is typically accompanied by severe losses in rotor thrust and strong thrust fluctuations. This paper presents a parametric study to investigate the influence of relevant geometric parameters of a small-scale rotor blade on the rotor performance in axial descent. Design variables subject to variation were the collective pitch, chord length, taper ratio, number of blades, as well as the tip geometry. Custom rotors for each parameter modification were manufactured and experimentally evaluated in wind tunnel tests with mean thrust recordings and measurements of the thrust fluctuations serving as performance metrics. Results indicated that rotor blades with larger aspect ratio and higher blade loading coefficient are less affected by the adverse aerodynamics in the vortex ring state, experiencing lower thrust losses and vibrational loads. Particle image velocimetry flow visualization confirmed that the aerodynamic losses in the vortex ring state can be attributed to blade vortex interactions. Comparison of the rotor flow structure in hover of all investigated rotor designs suggested that improvements in the descent performance of a rotor stem from a combination of reduced tip vortex strength and increased axial tip vortex convection rate. Using the experimental findings of this study, a predictive model for approximating the maximum extent of mean thrust losses in axial descent for a given blade geometry and hover thrust coefficient could be established.

A parametric study of squealer tip geometries applied in a hydraulic axial turbine used in a rocket engine turbopump

Axial turbines are machines widely used in different engineering applications. Due to their constructive characteristics, they must have a space between the rotor blades and the turbine casing, called tip clearance. Unfortunately, this gap allows a part of the fluid to leak from the pressure side to the suction side of the rotor blades. This leakage is undesirable and represents an energy loss. A way to avoid part of this loss is through the use of desensitization techniques. Although the use of these techniques is widely known, no studies in the open literature have evaluated these techniques in hydraulic turbines. This work presents a numerical analysis of squealer desensitization techniques applied in a hydraulic axial turbine. The turbomachine under study is the first stage of the hydraulic axial turbine used in the Low Pressure Oxidizer Turbopump (LPOTP) of the Space Shuttle Main Engine (SSME). Numerical simulations were performed using CFX v.19.2 software, and computational meshes were generated in ICEM v.19.2 software. Initially, the computational model was validated, using the experimental results published by the National Aeronautics and Space Administration (NASA). A parametric analysis was performed considering the variation in squealer cavity depth and rim thickness. The study found that the squealer cavity depth has a greater influence on the stage performance than its rim thickness. The tendency is that the greater the cavity depth, the greater the stage efficiency. One of the squealer geometries analyzed allowed an average increased efficiency of 1.43%, over the entire turbine operational range. The results obtained also show that the application of the proposed geometries would enable the reduction in cavitation close to the trailing edge of the rotor blades. This result is extremely valuable, as it can impact the life cycle of the turbine.

Using a Tip Characterizer to Investigate Microprobe Silicon Tip Geometry Variation in Roughness Measurements.

Given their superior dynamics, microprobes represent promising probe candidates for high-speed roughness measurement applications. Their disadvantage, however, lies in the fact that the volume of the microprobe’s silicon tip decreases dramatically during roughness measurement, and the unstable tip geometry leads to an increase in measurement uncertainty. To investigate the factors that influence tip geometry variation during roughness measurement, a rectangular-shaped tip characterizer was employed to characterize the tip geometry, and a method for reconstructing the tip geometry from the measured profile was introduced. Experiments were conducted to explore the ways in which the tip geometry is influenced by tip wear, probing force, and the relative movement of the tip with respect to the sample. The results indicate that tip fracture and not tip wear is the main reason for tip volume loss, and that the lateral dynamic load on the tip during scanning mode is responsible for more tip fracture than are other factors.

Bottom-up approach for traceable calibration of tip geometry of stylus profilometer

A novel approach for accurate and traceable calibration of stylus tip geometry is introduced in the paper. The approach consists of several steps. Firstly, the geometry of an AFM tip is calibrated to a kind of line width standard whose geometry is traceably calibrated to the lattice constant of crystal silicon. Then, the stylus tip to be calibrated is measured by the calibrated AFM tip in an AFM, thus its tip geometry can be accurately determined after the contribution of the AFM tip geometry being corrected from the measured AFM image. After being calibrated, the stylus tip can be applied in measurements of vast microstructures and surfaces, where the measurement results can be in turn corrected using the characterized stylus tip geometry. In such a way, the stylus tip geometry and its measurement results can be finally traceable to the lattice constant of silicon, using this bottom-up approach. Detailed experimental examples are illustrated. For a stylus type RFTHB-50 studied in this paper, its tip radius is measured as 1.727 μm with a standard deviation of 0.007 μm. It is significantly smaller than its nominal value of 2 μm, indicating the need of the calibration. The application of calibrated stylus tip in measurements of microspheres is demonstrated. Compared to conventional tip characterization methods based on tip characterizers the proposed method has advantages of (i) no risk of damaging sharp edges of tip characterizers, (ii) capable of directly characterizing the 3D geometry of stylus tip, (iii) high accuracy.

Quantitative evaluation of nanoindents: Do we need more reliable mechanical parameters for the characterization of materials?

Abstract Various sources of errors in the standard procedure according to the ISO 14577 draft for the iteration of elastic modulus and hardness of solid materials, with reference to fused quartz with its particular and unique indentation response, are pointed out on the basis of practical examples. Similar objections exist towards the use of the S 2 F N – 1 parameter, where S is the stiffness and F N is the normal force. It is suggested to use unambiguous mechanic characterization (at least additionally) by quantitative analysis of the loading curves in nanoindentations. These exhibit a general dependence between normal force and (displacement)3/2, the proportionality constant k being a non-iterated nanoindentation coefficient with dimension [force/length3/2] and unit [μN/nm3/2] that depends on the indenter tip geometry (pyramidal or conospherical) and is a characteristic material’s property. The validity has been demonstrated for virtually all types of materials (metals, semimetals, oxides, salts, organic molecular crystals, polymers) independent of their different bonding states. Pressure-induced phase transitions are detected by kinks in the linear plots for the loading curves well in the nanoindentation region. Crystalline materials, such as strontium titanate, exhibit far-reaching anisotropic effects along lattice axes that would strongly forbid the reference to an amorphous standard. The nanoindentation coefficient characterizes the different polymorphs and the face anisotropies. Another unambiguous mechanical characterization is given by the full work of indentation (as differentiated from the less secure plastic work of indentation). It also characterizes the face anisotropies with high precision for a given indenter as it is found proportional to (normal force)3/2, which allows for useful extrapolations.

Comparison of EUV Photomask Metrology Between CD-AFM and TEM

Accurate metrology of extreme ultraviolet (EUV) photomask is a crucial task. In this paper, two different methods for reference EUV photomask metrology are compared. One is the critical dimension atomic force microscopy (CD-AFM). In the measurements, the contribution of its AFM tip geometry is usually the dominant error source, as measured AFM images are the dilated results of measured structures by the AFM tip geometry. To solve this problem, a bottom-up approach has been applied in calibrating the (effective) AFM tip geometry where the result is traceably calibrated to the lattice constant of silicon crystals. The other is transmission electron microscopy (TEM). For achieving measurement traceability, structure features are measured in pairs in TEM images; thus the distance between the structure pair calibrated by a metrological AFM in prior can be applied to determine the magnification of the TEM image. In this study, selected photomask structures are calibrated by the CD-AFM, and then sample prepared and measured by high-resolution TEM nearly at the same location. The results are then compared. Of six feature groups compared, the results agree well within the measurement uncertainty, indicating excellent performance of the developed methodology. This research supports the development of a photomask standard, which is applied as a “reference ruler” with improved low measurement uncertainty in photomask fabs.

Control system optimization for the nanomechanical devices based on alloys with shape memory effect

Recent progress in design of nanomechanical tools with shape memory effect (SME) resulted in successful realization of the new systems for 3D nanomanipulation and bottom-up nanointegration of the nanoobjects like CNT, nanowires etc. SME demands controlled heating of the active layer of the shape memory nanotool. The heating can lead to thermal drift and errors in positioning of the nanotool. The paper presents the results of numerical modeling and experimental data on the thermal expansion of a heating element, including the tungsten microneedle depending on the configuration of its tip geometry. It is proved that the control system for nanomechanical devices including the microneedle with optimized profile demonstrates both improved precision of positioning and smaller electric energy consumption.

Effect of tip geometry on the performance of low-speed axial flow fan

Low-speed axial flow fans are widely used especially in air conditioning and ventilation systems. The performance of axial flow fans is negatively affected by the blockage and loss resulted from the tip leakage vortex (TLV) and the reverse flow. This paper investigates the development of TLV in a partially shrouded fan used in the outdoor unit of an air-conditioning system. It was found that the TLV follows the blade tip chord, due to the radial inflow in the initial unshrouded part of the blade, unlike the fully covered fan. Thus, the TLV trajectory can be manipulated by modifying the blade tip chord shape. It was also found that the reverse flow from the outlet near the shroud is driven by the negative component of TLV. The blade tip chord was modified to be in the axial direction before reaching the shrouded part to reduce the blockage and negative component of TLV. Comparing the results for the modified blade shows that the reverse flow and TLV blockage are effectively reduced by adjusting the TLV trajectory to be along the fan's rotational axis. The maximum flow rate increased up to 12% accompanied by a noticeable reduction in the loss by the tip modification.

On the effects of texture and microstructure on hydrogen transport towards notch tips: A CPFE study

Hydrogen embrittlement is an important degradation mechanism affecting the lifetime of engineering components. The diffusion of hydrogen atoms into the metal lattice can be affected by the localized stresses that develop around service-induced flaws. In addition, the magnitudes of flaw tip stresses are affected by the flaw geometry, as well as the texture and microstructure of the metal. Microstructural effects can be significant, particularly in metals such as zirconium with a high degree of elastic and plastic anisotropy. This study uses a coupled diffusion-crystal plasticity finite element approach to conduct a comprehensive study on the redistribution of hydrogen atoms in the vicinity of service-induced flaws in zirconium polycrystals. The effects of texture, grain size, and notch tip geometry on the concentration of hydrogen atoms in the lattice sites are investigated. The results indicate that material texture can significantly affect the distribution of hydrogen atoms as well as the location of maximum hydrostatic stress and maximum hydrogen concentration. The modeled highly textured microstructures have connected regions of peak hydrogen concentration. It is shown that as the notch tip becomes sharper, the effects of texture on the hydrogen localization near the notch become less significant. It is also shown that with changing grain orientations, it is possible to move the location of peak hydrogen concentration away from the notch tip.

FEA Modeling of Soft Tissue Interaction for Active Needles With a Rotational Tip Joint

A finite element analysis (FEA) model was developed for investigating the design and the tissue interaction of actively steerable needles with a rotational tip joint in soft tissue, based on the coupled Eulerian-Lagrangian (CEL) mesh. The new model is algorithmically simple in that the proposed CEL strategy allows needle insertion simulations along non-predetermined paths in soft tissue without the re-mesh at every direction change of the needle tip. For the features, various steering motions can be simulated and studied for different needle designs in a simpler approach by changing the needle geometry and boundary conditions. The developed FEA model, using the thoroughly measured material properties, was validated for predicting insertion path and estimating the tissue interaction forces inside two different gelatin tissue phantoms. Further, using the validated model, the effect of tip geometry on tissue was briefly investigated. Given bevel angles, it was found that the ratio of tip length to the diameter dominates the tissue damage gradient. The results demonstrate that the proposed model effectively examines the steering and tissue insertion of actively steerable needles and investigates the tip design to minimize the tissue damage by the needle steering.

Development of multifunctional nanopipettes for controlled intracellular delivery and single-entity detection.

The intracellular delivery of biomolecules and nanoscale materials to individual cells has gained remarkable attention in recent years owing to its wide applications in drug delivery, clinical diagnostics, bio-imaging and single-cell analysis. It remains a challenge to control and measure the delivered amount in one cell. In this work, we developed a multifunctional nanopipette - containing both a nanopore and nanoelectrode (pyrolytic carbon) at the apex - as a facile, minimally invasive and effective platform for both controllable single-cell intracellular delivery and single-entity counting. While controlled by a micromanipulator, the baseline changes of the nanopore ionic current (I) and nanoelectrode open circuit potential (V) help to guide the nanopipette tip insertion and positioning processes. The delivery from the nanopore barrel can be facilely controlled by the applied nanopore bias. To optimize the intracellular single-entity detection during delivery, we studied the effects of the nanopipette tip geometry and solution salt concentration in controlled experiments. We have successfully delivered gold nanoparticles and biomolecules into the cell, as confirmed by the increased scattering and fluorescence signals, respectively. The delivered entities have also been detected at the single-entity level using either one or both transient I and V signals. We found that the sensitivity of the single-entity electrochemical measurement was greatly affected by the local environment of the cell and varied between cell lines.

Electrode Design for Thermal and Nonthermal Plasma Discharge Inside a Constant Volume Combustion Chamber

Abstract Previous methods of achieving ignition in the Plasma, Combustion and Fluid imaging (PCFi) Laboratory’s Constant Volume Combustion Chamber (CVCC) utilizes either a standard or modified spark plug. The standard spark plug achieves a representation of side wall ignition (similar to a combustion engine), while the modified spark plug has an extended electrode to allow for a center camber ignition used for laminar burning speed (LBS) measurements. The creation of the modified spark plug required welding a stainless-steel (SS) wire to the electrode of the plug. This process is time consuming and requires a large quantity to effectively test a wide range of parameters such as gap size or electrode geometry. Two custom-design electrodes are presented in this paper which extend the experimental range of the PCFi’s CVCC system. Electrode Design A, gives the ability to test thin wire electrode with adjustability of gap size and different diameters through use of a compression fitting. This electrode design (i.e., tip-to-tip) is utilized with a traditional style of automotive ignition system (i.e., capacitive discharge) to study ignition process (i.e., thermal plasma) and spherical flame propagation. Electrode Design B, adds the ability to change tip geometry (i.e., plate-to-plate, tip-to-plate, tip-to-sphere, plate-to-sphere, etc.). In this paper, the plate-to-plate configuration is demonstrated to study uniform low-temperature nanosecond plasma discharge. Both electrode designs reduce structural weakness by removing the welded joint and allow for linear gap size adjustment. The electrode utilizes high-temperature epoxy, ceramic, and grafoil seals to make parameter adjustments easy and precise. The design was analyzed, prior to building and testing, based on the stress induced from the sealant, the total rated voltage, the rated temperature, and the fracture stress of the ceramic material. The stress induced in the electrodes was analyzed with finite element analysis (FEA) and the results were found to be within the limits of the material in terms of the compressive and fracture strengths. The maximum voltage was found to be around 30 kV. Design A is presented with three different electrode diameters of 1.3, 1, and 0.5 mm and Design B which utilizes a threaded connection for adjustable tip geometry. A sample of data, visual and electrical, is presented for the newly created electrode with a 0.5 mm diameter as well as combustion images for up to 10 atm of initial pressure for methane-air mixture. The new electrode design was able to survive several months of experimental use with few issues compared with the previous welded design.

Investigation of wear mechanisms in AlSi-polyester abradable - Ti(6Al4V) blade contacts using stroboscopic imaging

Abradable linings in aero engines have been an area of research interest over the past few decades as small reductions in clearances between stationary and rotating parts can lead to large increases in engine efficiency. The work performed in this article focuses on characterising the blade wear behaviour in contacts between Ti (6Al 4V) blades and AlSi-polyester abradables. This was done by performing three abrasion tests on the new test rig developed at the University of Sheffield. Tests have been performed on the AlSi-polyester abradables of the same nominal hardness over two incursion rates – 0.02μm/pass and 0.2 μm/pass and two blade tip speeds – 85 m/s and 170 m/s. The front-on stroboscopic imaging technique was used for these tests, which allowed capturing images of the entire blade front for a number of blade strikes during a test. It was found that at the incursion rate of 0.02μm/pass, both adhesions to the blade surface and blade wear were observed across the blade width. It was observed that adhesions were more likely to gradually wear off rather than fracture at 0.02μm/pass, and, fracture at 0.2 μm/pass. Tested surface profiles were obtained using an Alicona non-contact measurement system. This allowed the comparison of the blade profile results from the blade images to the surface of the respective tested abradable sample. It was concluded that adhesions that fractured could contribute to the localized gaps between the final blade and the final abradable surface where such adhesions have fractured close to the end of a test. Further testing areas have been identified such as the investigation into the effects of parameters such as incursion rate of a blade into an abradable, blade tip speed and abradable hardness on the results. The developed front-on imaging system also opened a possibility to investigate in-situ the rub performance of blades of varying tip geometry.

Fiber response to pin penetration in dry woven fabric using numerical analysis

A new type of washer with integrated pin structures was developed and presented in a previous study for the use in mechanical joints of fiber-reinforced plastics (FRP) and metal sheets. Thereby, the pin-structured washer significantly increases the load-bearing capacity of mechanically joined FRP-metal combinations. To do so, the FRP hole area is to be relieved by transferring the occurring loads into the laminate via the pin structures. In thermoset FRP, the insertion of the pin-structured washer usually leads to fiber breaks. For fiber-reinforced thermoplastics (FRTP) the chance of fiber failures during the penetration process can be reduced by inserting the pin-structured washer via an ultrasonic-based locally invasive heating system. By doing this, the polymer is locally molten, allowing the fibers to move around the pin structures. However, the ultrasonic-based insertion does not allow visual observation of the fiber movement within the process. With the aim of getting information about the fibers, that are moved, stressed or broken depending on the pin tip geometries, a numerical investigation was carried out. Therefore, a 3 × 3 twill model of a dry woven fabric segment based on a Bézier curve for yarn simplifications was developed. With respect to the molten polymer during the penetration process, the effect of the matrix was neglected. With consideration of necessary simplifications regarding the calculation time, the stress distribution on the dry woven fabric and a regular pattern of the damage situation of the fibers could be investigated. To understand the mechanisms of the fiber behavior within the penetration process, three different pin diameters and pin tip angles were examined and compared in numerical simulations.

Failure analysis of simple overlap bonding joints and numerical investigation of the adhered tip geometry effect on the joint strength

Abstract This study was carried out in two stages. In the first step, a numerical study was performed to verify the previous experimental study. In accordance with the previous experimental study data, single lap joints models were created using the ANSYS finite element analysis program. Then, nonlinear stress and failure analyses were performed by applying the failure loads obtained in the experimental study. The maximum stress theory was used to find finite element failure loads of the single lap joints models. As a result of the finite element analysis, an approximate 80 % agreement was found between experimental and numerical results. In the second step of the study, in order to increase the bond strength, different overlap end geometry models were produced and peel and shear stresses in the adhesive layer were compared according to the reference model. As a result of the analyses, significant strength increases were calculated according to the reference model. The strength increase in model 3 and model 5 was found to be 80 % and 67 %, respectively, relative to the reference model.

Open-source Toolkit: Benchtop Carbon Fiber Microelectrode Array for Nerve Recording.

Conventional peripheral nerve probes are primarily fabricated in a cleanroom, requiring the use of multiple expensive and highly specialized tools. This paper presents a cleanroom "light" fabrication process of carbon fiber neural electrode arrays that can be learned quickly by an inexperienced cleanroom user. This carbon fiber electrode array fabrication process requires just one cleanroom tool, a Parylene C deposition machine, that can be learned quickly or outsourced to a commercial processing facility at marginal cost. This fabrication process also includes hand-populating printed circuit boards, insulation, and tip optimization. The three different tip optimizations explored here (Nd:YAG laser, blowtorch, and UV laser) result in a range of tip geometries and 1 kHz impedances, with blowtorched fibers resulting in the lowest impedance. While previous experiments have proven laser and blowtorch electrode efficacy, this paper also shows that UV laser-cut fibers can record neural signals in vivo. Existing carbon fiber arrays either do not have individuated electrodes in favor of bundles or require cleanroom fabricated guides for population and insulation. The proposed arrays use only tools that can be used at a benchtop for fiber population. This carbon fiber electrode array fabrication process allows for quick customization of bulk array fabrication at a reduced price compared to commercially available probes.

Vapor Mapping in a Microscopic Space with a Scanning Nanoprobe Interferometer

Evaporation of a liquid inside a microscopic space plays a key role to drive various natural phenomena or chemical/thermal processes in microfluidic systems. So far, fundamental studies in such fields have been indirect due to the lack of techniques geometrically accessible in microscopic spaces. Here, we report on a nanoprobe interferometer that can directly visualize vapor in 3D (dimensions) in a microscopic space. We first optimize the tip geometry of the nanoprobe to enhance its sensitivity just by analyzing the interference patterns during the nanoprobe growth in real time. Based on scanning with the nanoprobe interferometer, vapor concentration from a water meniscus in a glass microcapillary is experimentally mapped, which leads to the measurement of the local evaporation flux from the meniscus. The evaporation flux increases from the capillary center to the wall and decreases with the capillary diameter, mostly due to the Kelvin effect and strong evaporation at the three-phase contact line. Specifically, the flux at the center follows Jv(center) ∝ e–7.7×10–4d, in sharp contrast to the power-law decrease reported. Our nanotechnological methodology would pave the way to explore various questions associated with microscopic, geometrically confined evaporation dynamics which has been experimentally inaccessible so far.

Exploiting rotational asymmetry for sub-50\u2009nm mechanical nanocalligraphy

Nanofabrication has experienced extraordinary progress in the area of lithography-led processes over the last decades, although versatile and adaptable techniques addressing a wide spectrum of materials are still nascent. Scanning probe lithography (SPL) offers the capability to readily pattern sub-100 nm structures on many surfaces; however, the technique does not scale to dense and multi-lengthscale structures. Here, we demonstrate a technique, which we term nanocalligraphy scanning probe lithography (nc-SPL), that overcomes these limitations. Nc-SPL employs an asymmetric tip and exploits its rotational asymmetry to generate structures spanning the micron to nanometer lengthscales through real-time linewidth tuning. Using specialized tip geometries and by precisely controlling the patterning direction, we demonstrate sub-50 nm patterns while simultaneously improving on throughput, tip longevity, and reliability compared to conventional SPL. We further show that nc-SPL can be employed in both positive and negative tone patterning modes, in contrast to conventional SPL. This underlines the potential of this technique for processing sensitive surfaces such as 2D materials, which are prone to tip-induced shear or beam-induced damage.

Experimental investigation of a rotor blade tip vortex pair

This paper presents the results of an experimental study of two closely spaced vortices generated by a rotating blade with a modified tip geometry. The experiments are carried out in two water channel facilities and involve a generic one-bladed rotor operating in a regime near hover. It is equipped with a parametric fin placed perpendicular to the pressure surface near the tip, which generates a co-rotating vortex pair having a helical geometry. Based on previous results obtained with a fixed wing, a series of small-scale experiments is first carried out, to validate the method of vortex pair generation also for a rotating blade, and to obtain a qualitative overview of its evolution going downstream. A more detailed quantitative study is then performed in a larger facility at three times the initial scale. By varying the fin parameters, it was possible to obtain a configuration in which the two vortices have almost the same circulation. In both experiments, the vortex pair is found to merge into a single helical wake vortex within one blade rotation. Particle image velocimetry measurements show that the resulting vortex has a significantly larger core radius than the single tip vortex from a blade without fin. This finding may have relevance in the context of blade–vortex interactions, where noise generation and fatigue from fluid–structure interactions depend strongly on the vortex core size.

Segment tip geometry of sheet intrusions, II: Field observations of tip geometries and a model for evolving emplacement mechanisms

Igneous sheet intrusions are segmented across several orders of magnitude, with segment tip geometry commonly considered indicative of the propagation mechanism (brittle or non-brittle). Proposed propagation mechanisms are inferred to represent host rock mechanical properties during initial magma emplacement; typically, these models do not account for segment sets that show a range of tip geometries within the same lithology. We present a detailed structural characterization of basaltic sill segments and their associated host rock deformation from the Little Minch Sill Complex, Isle of Skye, UK, and a broader comparison with segment geometries in three additional intrusive suites (Utah, USA; and Mull and Orkney, UK). Each separate host lithology shows multiple tip geometries and styles of host rock deformation, from elastic-brittle fracture, to viscous indentation and fluidisation. We attribute this range of host rock deformations to evolving conditions that occur at the tips both during sheet growth and arrest.