Strut Geometry Research Articles

Numerical Investigation into the Effects of Design Parameters on the Flow Characteristics in a Turbine Exhaust Diffuser

In this study, we numerically investigated the effects of design parameters, such as the strut geometry or diffusion angle, on the performance of an industrial turbine exhaust diffuser. Turbine exhaust diffusers are commonly used to change the kinetic energy of exhaust gases from the outlet of turbine stages into the static pressure. The turbine exhaust diffuser investigated in this work consisted of an annular diffuser with five identical struts equally spaced around the front circumference and a conical diffuser with a hub extension at the rear. Four design parameters were considered and several values for each parameter were tested in this study. The aerodynamic performances of the studied diffusers were evaluated according to their pressure recovery coefficients and rates of total pressure loss. Contours for the velocity, pressure, and entropy increase were plotted and compared for the various diffuser shapes. The numerical results showed that the strut thickness and the axially swept angle of the strut significantly influence the aerodynamic performance of the turbine exhaust diffuser, whereas the strut lean angle and the diffuser hade angle are less important.

Stent strut streamlining and thickness reduction promote endothelialization.

Stent thrombosis (ST) carries a high risk of myocardial infarction and death. Lack of endothelial coverage is an important prognostic indicator of ST after stenting. While stent strut thickness is a critical factor in ST, a mechanistic understanding of its effect is limited and the role of haemodynamics is unclear. Endothelialization was tested using a wound-healing assay and five different stent strut models ranging in height between 50 and 150 µm for circular arc (CA) and rectangular (RT) geometries and a control without struts. Under static conditions, all stent strut surfaces were completely endothelialized. Reversing pulsatile disturbed flow caused full endothelialization, except for the stent strut surfaces of the 100 and 150 µm RT geometries, while fully antegrade pulsatile undisturbed flow with a higher mean wall shear stress caused only the control and the 50 µm CA geometries to be fully endothelialized. Modest streamlining and decrease in height of the stent struts improved endothelial coverage of the peri-strut and stent strut surfaces in a haemodynamics dependent manner. This study highlights the impact of the stent strut height (thickness) and geometry (shape) on the local haemodynamics, modulating reendothelialization after stenting, an important factor in reducing the risk of stent thrombosis.

Effect of the Co3O4 load on the performance of PdO/Co3O4/ZrO2 open cell foam catalysts for the lean combustion of methane: Kinetic and mass transfer regimes

In this study, kinetic and mass transfer resistances (pore and interphase diffusion) were evaluated by varying the Co3O4 amount in the catalyst (PdO/Co3O4) supported on the zirconia open cell foam (Zir-OCF) of 30 ppi. The catalytic performance was examined toward the methane combustion in lean conditions (0.5 and 1.0 vol.% CH4 inlet concentration, O2/CH4 molar ratio 8, WHSV 30, 60 and 90 NL h−1 gcat−1). Pore and strut geometry of the bare foam were evaluated by SEM images and X-CT data analysis. Solution combustion synthesis was employed to line the Co3O4 spinel on the foam, while the wetness impregnation for the PdO on the spinel. The various resistances were evaluated and compared at different WHSV by varying the Co3O4 amount. Finally, we derived a correlation that describes the mass transfer in OCFs at low Reynolds numbers.

Microstructural damage behaviour of Al foams

Microheterogeneous materials such as metal foams exhibit a strong structure-property-relationship. The macroscopic material properties depend on the pore geometry and especially on the strut geometry and the microstructure of the struts such as the grain structure. Since the grain structure in the struts differs from that of the corresponding bulk material, it is of utmost importance to perform microtensile tests on individual struts. Previous works on aluminium foams outlined strong scattering in this micromechanical properties. A possible reason for the scattering might be micro-porosity and inclusions in the struts resulting from the manufacturing of metal foams by investment casting. The present contribution deals with ex situ and In situ microtensile tests to elucidate reasons for the scattering in the micromechanical properties determined from microtensile tests on individual struts. In situ tensile tests using high resolution X-ray computed tomography were used to study the microstructural failure mechanisms in aluminium foam struts. The results were further proven by additional ex situ tensile experiments on struts, which were analysed by low resolution X-ray computed tomography prior to tensile testing to allow for more specimens and hence for better statistics. Micro-porosity and primary inclusions were found to be the main reason for differences in the micromechanical properties of individual aluminium foam struts.

Effect of Strut Geometry in Supersonic Mixing

Effective mixing in a short mixing chamber is a major challenge in supersonic air breathing engines, especially the mixing between two high speed co-axial streams. The residence time is a major factor to get the two streams properly mixed. The selection of mixing method is crucial in the supersonic conditions. An experimental study has been performed on the supersonic mixing of air from strut injectors of various combinations in their trailing ramp angles with a free stream air of Mach number 2. Two different configurations of geometries, a plain geometry and a lobed geometry considered and further two different combinations, slot and hole injections, are taken for the study in each geometry. Among different combinations, it is found that a better mixing enhancement is achieved for the 9o lobed geometry struts.

Flexibility of Biodegradable Polymer Stents with Different Strut Geometries.

Objective: Biodegradable stents (BDSs) represent a new technological development in the field of cardiovascular angioplasty; good flexibility helps stents pass through tortuous vessels during delivery and reduces the amount of damage caused to blood vessels. This study investigates the relationship between flexibility and the geometry of BDS struts. Methods: Four stent struts with different geometry (circular, triangular, hexagonal, and spline curved) and the same links were modeled to evaluate their flexibility via a three-point bending experimental method and a numerical method. Results: The bending state of the four stents was well-balanced. The bending effect of the four stents was different. Under the same conditions, the circular and spline curved stents showed the best bending effects while the hexagonal stent was the worst. However, these differences were not significant. Conclusion: The flexibility of BDSs is related to the geometry of the struts and links; however, the geometry of the struts has less effect on flexibility than the links. The greater the area enclosed by the strut centerline, the better flexibility of the stent.

Effects of build orientation and sample geometry on the mechanical response of miniature CP-Ti Grade 2 strut samples manufactured by laser powder bed fusion

Additive manufacturing can produce a regular three-dimensional mesh containing tiny interconnected wires/struts that form a porous structure. It is reasonable to assume that the properties of these single wires/struts will affect the performance of each unit-cell contained within the porous structure and thereby affect the mechanical properties of the whole porous structure. The effects of strut geometry and build orientation on resulting mechanical properties are examined in this study. Laser powder bed fusion was used to manufacture single strut samples of CP-Ti Grade 2 in the vertical and horizontal orientations with cross-sectional areas ranging from 0.07 mm2 to 2.3 mm2. For samples with cross-sectional areas smaller than 1.5 mm2, the difference in yield strength between the vertical and the horizontal build orientations was 79%, consistent with differences in microstructure scale in these orientations. Build orientation effects were much less pronounced in larger samples where the microstructural scale and features were similar. The presence of surface roughness and process-induced porosity contributed to the significant differences in ductility exhibited in the different orientations. The results indicate that build orientation effects may be significant for as-deposited surfaces and should be considered when designing porous structures consisting of thin struts since these will likely be used with as-deposited surface roughness.

Damage Tolerance Evaluation of E-PBF-Manufactured Inconel 718 Strut Geometries by Advanced Characterization Techniques.

By means of electron beam powder bed fusion (E-PBF), highly complex lightweight structures can be manufactured within short process times. Due to the increasing complexity of producible components and the entangled interplay of damage mechanisms, common bulk material properties such as ultimate tensile or fatigue strength are not sufficient to guarantee safe and reliable use in demanding applications. Within this work, the damage tolerance of E-PBF-manufactured Ni-based alloy Inconel 718 (IN 718) strut geometries under uniaxial cyclic loading was investigated supported by several advanced measurement techniques. Based on thermal and electrical measurements, the failure of single struts could reliably be detected, revealing that continuous monitoring is applicable for such complex geometries. Process-induced surface roughness was found to be the main reason for early failure during cyclic loading. Thus, adequate post-processing steps have to be established for complex geometries to significantly improve damage tolerance and, eventually, in-service properties.

Modelling of cellular materials by a microsphere‐based material model

AbstractMetal foams are a very interesting class of cellular materials which, due to their structure, can be used both for lightweight construction and for the absorption of kinetic energy. They have a microheterogeneous structure, which makes it difficult to simulate these materials efficiently. Although microstructure models are very precise in terms of strut size and pore geometry, they are very computationally intensive due to their high resolution and therefore do not allow the simulation of entire components. While continuum models that do not resolve the specific microstructure are very efficient, they do not allow the influence of variations in strut size, strut geometry or pore size to be modelled directly by the simulation. Therefore, simulation approaches such as microsphere models are necessary, which combine the macroscopic component scale with the microscopic microstructure.

A particle-based model for endothelial cell migration under flow conditions

Endothelial cells (ECs) play a major role in the healing process following angioplasty to inhibit excessive neointima. This makes the process of EC healing after injury, in particular EC migration in a stented vessel, important for recovery of normal vessel function. In that context, we present a novel particle-based model of EC migration and validate it against in vitro experimental data. We have developed a particle-based model of EC migration under flow conditions in an in vitro vessel with obstacles. Cell movement in the model is a combination of random walks and directed movement along the local flow velocity vector. For model calibration, a set of experimental data for cell migration in a similarly shaped channel has been used. We have calibrated the model for a baseline case of a channel with no obstacles and then applied it to the case of a channel with ridges on the bottom surface, representative of stent strut geometry. We were able to closely reproduce the cell migration speed and angular distribution of their movement relative to the flow direction reported in vitro. The model also reproduces qualitative aspects of EC migration, such as entrapment of cells downstream from the flow-disturbing ridge. The model has the potential, after more extensive in vitro validation, to study the effect of variation in strut spacing and shape, through modification of the local flow, on EC migration. The results of this study support the hypothesis that EC migration is strongly affected by the direction and magnitude of local wall shear stress.

Numerical investigation of scramjet strut injector cooling for different fuel mass fluxes and strut material properties at mach 8 flight conditions

The thermal load on a lobed scramjet (supersonic combustion ramjet) strut injector is investigated numerically for Mach 8 flight conditions. For this purpose, coupled 3D RANS (Reynolds Averaged Navier-Stokes) simulations of the supersonic external flow field around the strut injector and the subsonic strut internal hydrogen flow are performed. The temperature distribution in the solid strut material is simulated by a separate code, which exchanges data with the fluid solvers. Due to the predominantly low Mach numbers inside the strut, an all-Mach number preconditioning is required to enable convergence of the compressible flow solver. The air flow Mach number in the combustion chamber is three. Different strut materials (copper, silicone carbide, molybdenum, and tungsten) with different heat capacities and thermal conductivities are compared. Moreover, the impact of the height of the blunt tip of the strut on the heat load is investigated. Due to a small detached normal shock wave upstream of the strut, the temperatures at the tip become very high; this part is notably difficult to cool. In order to keep the strut temperatures acceptable, it is cooled actively by the hydrogen that is injected for combustion. A sophisticated internal strut structure is needed to direct the hydrogen towards regions of highest heat load. Different strut geometries, fuel mass fluxes (equivalence ratios), and strut materials are investigated.

Semianalytical Geometry-Property Relationships for Some Generalized Classes of Pentamodelike Additively Manufactured Mechanical Metamaterials

We study a special class of mechanical metamaterials, namely lattices, based on beams (struts) with nonuniform cross sections, of which pentamode mechanical metamaterials are a special case. Five symmetric beam types including simple cylinder, concave double cone, convex double cone, concave hyperbolic, and convex hyperbolic are considered. Three types of loads including lateral force, axial force, and moment are applied to the free end of cantilever beams with the five aforementioned geometries and their responses are compared in terms of displacement, normal stress variation along the beam length, and shear stress variation along the beam length. Using the displacement diagrams obtained for different strut geometries and loading conditions, semi-analytical relationships are derived for the displacement at the end of cantilever beams. The semi-analytical relationships are then used to calculate the elastic modulus and yield strength of lattice structures based on two types of unit cells, namely diamond and cube. We also build numerical models and perform experiments to benchmark our semi-analytical results. Comparison of the analytical, numerical, and experimental results demonstrate the accuracy of the semi-analytical relationships presented for the mechanical properties of this class of mechanical metamaterials. Moreover, in both the cube- and diamond-based structures, increasing the α value (i.e., the ratio of the largest to the smallest cross-section radius in each strut) at a constant relative density decreases the elastic modulus, yield strength, the initial maximum stress, and the plateau stress.

Characterization of laminar flow in periodic open-cell porous structures

The present work deals with the numerical simulations of 3D incompressible flow through periodic porous structures, consisting of cubic cells with different strut geometry and structure tilting with respect to the flow direction. Simulation results are obtained for different porosities, while covering a range of Reynolds numbers from laminar-steady flow (Darcy regime) up to the transition to laminar-unsteady flow (moderate Forchheimer regime). These results are post-processed in order to characterize the influence of the periodic porous structures on the flow statistics related to the Darcy-Forchheimer law. Based on this analysis, along with 3D simulation results previously obtained for different random foams, correlations for the Darcy and Forchheimmer permeability coefficients are developed from a theoretical background, being expressed only as functions of the cell geometrical parameters and tortuosity estimation. However, the influences of flow tortuosity and cell orientation on the Darcy coefficient are found to cancel each other. The proposed correlation for the Darcy coefficient performs very well for both periodic and random porous structures, when compared with recent correlations available in the literature. The performance of the correlation proposed for the Forchheimmer coefficient is found to be satisfactory, however, different correlation constants were found for periodic and random structures. The proposed correlation for the Forchheimer permeability coefficient depends on the flow tortuosity inside the porous structure, which must be estimated either through 3D fluid flow simulations or using tortuosity correlations. In this respect, a correlation is proposed for predicting the flow tortuosity inside porous structures, as an alternative for avoiding 3D simulations.

Supersonic combustion of hydrogen using an improved strut injection scheme

Numerical investigation of mixing is performed at Mach 2.0 model Scramjet combustor employing parallel strut injection schemes for fuel. In the present investigation, basic strut injector is modified in such a way to produce additional vortices in streamwise direction and improve fuel-air mixing. Air is injected at Mach 2.0 at the combustor inlet and fuel is injected at sonic speed from the blunt end of the strut. The flow field involving high-speed turbulent mixing and heat addition was modeled by three-dimensional Reynolds averaged Navier-Stokes equations. A realizable k-ε model was chosen to close the turbulence problem with the default model constants. Non-premixed combustion of hydrogen and air is modeled using the mixture fraction β-pdf framework. Turbulence-chemistry interactions are handled by a strained flamelet model. Comparisons of numerical results with experimental results have demonstrated the accuracy and applicability of computational grid and a numerical scheme for hot and cold flow solutions. The shock-shear layer interaction present within the combustor increases the local turbulent intensity and has a positive effect on mixing. The mixing efficiency obtained with improved strut injector is compared with the basic strut. Improved strut injection scheme showed a mixing efficiency of >95% with a 45% reduction in length. Further combustion efficiency is calculated in the streamwise direction and plot follows the similar trend as the mixing efficiency. The proposed modification of strut geometry showed improved mixing and combustion performance.

Effect of Pretreatment on Interface Stability and Morphology of Ni/Al Hybrid Foams by in situ Microcantilever Fracture Experiment

Ni/Al hybrid foams are a new class of innovative cellular composite materials consisting of open-cell aluminium (Al) foams electrochemically coated with nanocrystalline nickel (Ni). They may be used for lightweight construction elements or as crash energy absorbers. The Ni coating strengthens the Al foam achieving an up to ten times higher energy absorption capacities compared to the Al basis foam. Cellular materials such as foams provide a strong structure-property relationship as the macroscopic material properties strongly depend on the strut geometry and the material properties of the individual struts. The interface stability between the coating and the substrate foam is the dominant contribution in the strengthening mechanism of Ni/Al hybrid foams. Hence, micromechanical characterization is an important task for the design of components made of Ni/Al hybrid foams. A strong interface corresponds to a shear-stiff connection between substrate foam and coating, whereas a soft interface allows sliding between the two phases and hence reduces the buckling stiffness of individual foam struts resulting in lower strength and energy absorption capacity of the macroscopic foam. An increased critical energy release rate for interface cracking was revealed by in situ microcantilever bending tests prepared by focused ion beam (FIB) during bending tests in the scanning electron microscope (SEM) after the chemical pretreatment of the base foam.

Basic research of the consideration of additive manufactured lattice structures under thermoand fluid dynamic loads

This scientific research deals with lattice structures manufactured with laser powder bed fusion. Laser powder bed fusion is part of additive manufacturing. The so called layered construction is an increasingly used innovative manufacturing process that can be used to realize new design possibilities. Lightweight structures or bionic structures play a key role here. The focus of this work is on periodic lattice structures. In addition to saving resources and reducing the weight of components, lattice structures have particularly pronounced mechanical properties. However, little is known about their thermo- and fluid-dynamic properties. This work describes the first influencing factors of lattice structures ina thermo- and fluid-dynamic context using a case study. The aim of this paper is to evaluate important design and simulation criteria of lattice structures. Different lattice structures are considered, whose strut geometry is varied. The case study is carried out using a heat exchanger. While classical heat exchangers have lamellar structures, the substitution of these by lattice structures is evaluated. Thiswork represents a first consideration of the most important parameters and gives an overview of the most important core points.

In a Preliminary Simulation Data, Stent with Thinnest Strut Beta Crystalline Titanium Gold Alloy (β-Ti-Au) Outperforms Typical Implants

Background: Large volume of data support the overall safety of coronary stents for cardiovascular disease. Yet, one cannot lose sight of their shortcomings such as restenosis; hence stents continue to evolve in lattices, materials, and drugs. Studies outlining the successful use of titanium gold alloy stents to counter these issues are lacking. Methods: In this analysis we obtained available historical manufacturing records on routinely used stents to compare to two revolutionary titanium-gold alloy stents. By using a 3D CAD finite element analysis space, each stent type was tested for flexibility, rigidity, and radial forces. Except for lengths and diameter, each type was held to their own strut geometry and thickness. Our analysis focused on using Von Mises Stress and resulting deformation or expansion. Our assessments were performed by using discrete changes and Pearson’s chi-squared statistics to obtain significance of our findings. Three lengths: 15mm,27.5mm and 40mm were tested for each type. Comparisons were obtained from the mean percentage length or diameter (3.5mm) changes. Results: β-Ti-Au alloy in our hexagonal mesh was significantly more expansive (78.29 percent gain in diameter under 7 atm. than Orsiro Hybrid (the baseline). p 9.09 percent >7.80 percent). Maximum changes in displacements, 0.19 and 0.25. Both values were significant. (95% CI 0.11-0.27, 0.17-2.33). Conclusion: Routine use of Titanium in coronary stents has been hindered by its low density, elastic modulus and strength; contrary these results suggest that by mixing titanium with gold and on the right structure the alloy can be constructed with a thin strut for percutaneous coronary intervention.

A biodegradable elastic stent model

In this paper we derive and analyse a one-dimensional model of biodegradable elastic stents. The model is given as a nonlinear system of ordinary differential equations on a graph defined by the geometry of stent struts. The unknowns in the problem are the displacement of the middle curve of the struts, the infinitesimal rotation of the cross-sections of the stent struts, the contact couples and contact forces at struts and a function describing the degradation of the stent. The model is based on the one-dimensional model of a biodegradable elastic curved rod model by Tambača and Žugec (‘One-dimensional quasistatic model of biodegradable elastic curved rods’, Zeitschrift für Angewandte Mathematik und Physik 2015; 66(5): 2759–2785) and the ideas from the one-dimensional elastic stent modelling by Tambača et al. (‘Mathematical modeling of vascular stents’, SIAM Journal on Applied Mathematics 2010; 70(6): 1922–1952) used to formulate contact conditions at vertices. We prove the existence and uniqueness results for the model.

Role of jet spacing and strut geometry on the formation of large scale structures and mixing characteristics

The present study primarily focuses on the effect of the jet spacing and strut geometry on the evolution and structure of the large-scale vortices which play a key role in mixing characteristics in turbulent supersonic flows. Numerically simulated results corresponding to varying parameters such as strut geometry and jet spacing (Xn = nDj such that n = 2, 3, and 5) for a square jet of height Dj = 0.6 mm are presented in the current study, while the work also investigates the presence of the local quasi-two-dimensionality for the X2(2Dj) jet spacing; however, the same is not true for higher jet spacing. Further, the tapered strut (TS) section is modified into the straight strut (SS) for investigation, where the remarkable difference in flow physics is unfolded between the two configurations for similar jet spacing (X2: 2Dj). The instantaneous density and vorticity contours reveal the structures of varying scales undergoing different evolution for the different configurations. The effect of local spanwise rollers is clearly manifested in the mixing efficiency and the jet spreading rate. The SS configuration exhibits excellent near field mixing behavior amongst all the arrangements. However, in the case of TS cases, only the X2(2Dj) configuration performs better due to the presence of local spanwise rollers. The qualitative and quantitative analysis reveals that near-field mixing is strongly affected by the two-dimensional rollers, while the early onset of the wake mode is another crucial parameter to have improved mixing. Modal decomposition performed for the SS arrangement sheds light onto the spatial and temporal coherence of the structures, where the most dominant structures are found to be the von Kármán street vortices in the wake region.

Strain hardening reduces energy absorption efficiency of austenitic stainless steel foams while porosity does not

Strain hardening significantly affects the mechanical function of steel foams. We compare hardening and failure of two commercially available austenitic stainless steel foams (316L and 310) spanning strut porosities of 9.4 to 14.4%. Damage is correlated with strut microstructure and geometry, combining in-situ quasi-static compression testing in the SEM, 3D-evaluation by synchrotron μCT and bending simulations. We provide an analytical model for the experimentally observed strain hardening. Upon compression, 316L steel foams exhibit a plateau regime of continuously increasing stress due to the hardening effect, whereas 310 steel foams show almost constant plateau stress. This is explained by the much less ductile behaviour of the 310 steel foam struts as compared to 316L steel foam struts. Finite element modelling suggests that significant stress concentrations develop around microporosities in the 310 struts. Due to its finer and less porous microstructure, the 316L foam exhibits a larger energy absorption capacity than the 310 foams. This results in distinctly different efficiency-strain curves. However, up to about 25% strain, the efficiency values are surprisingly similar. Thus, modification of microstructure and/or pore micro-geometry can be used to optimise the stress-strain response to achieve the desired energy absorption property of steel foams.