308 research outputs found
Shock-fitting and shock-capturing simulations of transonic flows in a channel with a half lenticular profile
No full textThe present study focuses on particular properties of transonic flows through a planar
channel featuring a circular bump on the lower wall. The selected geometry is reminiscent of the
region surrounding the trailing edge of an airfoil at zero angle-of-attack and the resulting flow
pattern is indeed similar to the fishtail shock-pattern that characterizes airfoils flying at nearly
sonic speed. Numerical simulations have been conducted by solving the inviscid Euler equations
using both a commercial and an in-house CFD code; discontinuities are modeled using shock-
capturing in the former and shock-fitting in the latter. Numerical experiments reveal different
shock-patterns obtained by independently varying the pressure ratio defined as the ratio between
the static outlet pressure and the inlet total pressure. When shock-interactions occur, shock-polar
analysis reveals that the branching point can be modeled using either von Neumann’s three-shock-
theory or Guderley’s four-wave-theory, depending on the pressure ratio. Furthermore, for certain
values of the pressure ratio, double solutions have been observed
Features of “Fishtail” Shock Interaction in Transonic Flows on a NACA0012 Profile
No full textSlender profiles flying at nearly sonic speed exhibit a peculiar shock pattern, which is commonly referred to in literature as the "fishtail" shock structure. It consists of two oblique shock waves that originate at the trailing edge of the airfoil and interact with a detached, nearly normal shock wave in two triple points located along the oblique shocks. Making use of both shock-fitting and shock-capturing CFD simulations and classical shock polar analysis, we prove that, in the case of a NACA0012 airfoil, the two interaction points cannot be modeled using the three shocks theory that von Neumann developed nearly a century ago to describe triple points arising in Mach reflections. Our analysis suggests that the four-wave interaction model proposed by Guderley should be used instead
Shock-fitting techniques on 2D/3D unstructured and structured grids: algorithmic developments and advanced applications
No full textOver the last decades the simulations of compressible flows featuring shocks have been one of the major drivers for developing new computational algorithms and tools able to compute also complex flow configurations. Nowadays, Computational fluid dynamics (CFD) solvers are mainly based on shock capturing methods, which rely on the integral form of the governing equations and can compute all type of flows, including those with shocks, using the same discretization at all grid points. Consequently, these methods can be implemented with ease and provide physically meaningful solutions also for complex flow configurations, features particularly attractive for CFD community. Although shock capturing methods have been the subject of development and innovations for more than 40 years, they are plagued by several numerical problems due to the shocks capture process, such as discontinuities
finite-width, numerical instabilities and reduction of accuracy order in the shock downstream region, which are still unsolved and probably will never find a solution. For this reason, there is a renewed interest in shock-fitting techniques: in particular, these methods explicitly identify the discontinuities within the flow field and compute them by enforcing the Rankine-Hugoniot jump relations. Because of this modelling, shocks are represented by zero thickness discontinuities, so that significant advantages can be gained in terms of solution quality and accuracy improvements. Furthermore, this class of methods is immune to the numerical problems linked to shock capture process. Following this research line, the presented Thesis proposes new developments
and advanced applications of shock-fitting techniques, which prove that these methods are an effective option regarding shock capturing ones in simulating flows with shocks, able to provide also a better understanding of all the phenomena linked to shock waves
Simulation of Shock Boundary Layer Interaction Using Shock Fitting Technique
No full textAn unstructured, shock-fitting algorithm, originally developed to simulate Eulerian flows, has being further developed to make it capable of dealing with shock/boundary-layer interactions. This paper illustrates the algorithmic features of this technique and the results achieved in the computation of hypersonic flows past compression ramps
The transonic flow past a NACA0012 and the von Neumann paradox
No full textThe confluence of three shock waves at a common branching point is observed in different shock-interaction patterns, the most prominent example being the irregular reflection of an oblique shock from a solid surface, known as a Mach reflection. During WWII, John von Neumann developed an analytical model of the Mach reflection which fits well the experiments, except when the incident shock is weak. This inconsistency is referred to in the literature as the von Neumann paradox. In this paper we combine numerical simulations performed using a shock-fitting CFD code and shock polar analysis to show that the triple-point that arises in the so-called fishtail shock-structure, which is observed in the transonic flow past airfoils, is not amenable to be described using von Neumann’s model. Our analysis points to the fact that Guderley’s four waves model, which has already been experimentally and numerically validated as a plausible solution to the von Neumann paradox, should be used instead
Special properties of transonic flows in a channel with a lenticular bump
No full textThe present study focuses on particular properties of transonic flows through a planar channel featuring a circular bump on the lower wall. The selected geometry is reminiscent of the region surrounding the trailing edge of an airfoil at zero angle of attack and the resulting flow pattern is indeed similar to the fishtail shock-pattern that characterizes airfoils flying at nearly sonic speed. Numerical simulations have been conducted by solving the inviscid Euler equations using both a
commercial and an in-house CFD code; discontinuities are modeled using shock-capturing in the former and shock-fitting in the latter. Numerical experiments reveal different shock-patterns obtained by independently varying the inlet Mach number and the outlet-to-inlet static pressure ratio. When shock-interactions occur, shock-polar analysis reveals that the branching point can be modeled using either von Neumann’s three-shock-theory or Guderley’s four-wave-theory, depending on the inlet
Mach number. Furthermore, for certain pairs of boundary conditions, multiple solutions have been observed
Front-Tracking Technique for Computing Inflatable Structures in Hypersonic Flows
No full textIn recent years, both NASA and ESA have run Technology Demonstration Missions employing Hypersonic Inflatable Aerodynamic Decelerators (HIADs), an inflatable structure covered by a flexible heat shield, which is used to decelerate and protect space vehicles entering the atmosphere at hypersonic speed. This paper describes a
recently developed numerical technique to simulate the fluid/structure interaction between the hypersonic stream and the flexible structure of the HIAD. The authors do so by using a front-tracking/shock-fitting technique that models both the bow shock and the thin membrane of the HIAD as double-sided surfaces of negligible thickness. Shock motion is governed by the Rankine–Hugoniot jump relations, whereas a nonlinear membrane model is used to simulate the deformation of the HIAD under the combined effect of the inflation and aerodynamic pressure. Local remeshing is used to ensure that the surface triangulations used to geometrically describe both the material
interfaces and the bow shock are conforming with the background tetrahedral grid that fills the fluid domain.
The computational examples cover three different HIADs of increasing geometrical complexity. We also show that
fitting the bow shock allows to preserve the design order of the spatial discretization scheme in the entire shockdownstream
region
Economic factors affecting obesity: an application in Italy
No full textThe World Health Organization has stated that obesity is spreading around the world like a “global epidemic”. In 2004 the percentage of obese people in the Italian population was 9%, but the trend s increasing in recent years. Focusing on this country, the purpose of the paper is to analyze the socio-economic variables affecting obesity by means of a survey conducted in a consumer sample. Our analysis is based on a survey conducted in Italy, and the sample was composed of 999 consumers. We used a binary logit model and the dependent variable is body mass index (BMI), expressed in a dichotomic way (seriously overweight and obese, value 1, and normal weight, value 0). The results show that the condition of the seriously overweight and obese increases with age, especially in people over 65 of age. Also gender is correlated with the pathology: being seriously overweight and obese is far more likely for men than for women. An inverse relation was shown between obesity and education, and between obesity and the level of food knowledge. The results highlight that disadvantaged social categories are more susceptible to the problem of overweight and obesity. A policy implication of the analysis, to limit the spread of obesity, could lie in programs aimed at improving health and food awareness and focused on these minority groups.economics of obesity, BMI and consumer, logit model, Food Consumption/Nutrition/Food Safety, Health Economics and Policy,
Quantitative assessment of the mass-saving derived from Mars aerocapture maneuvers
No full textAerocapture maneuver represents one of the most disrupting idea in space missions as it allows for significant mass-savings by avoiding the use of propellant in orbit insertion maneuvers. Generally, in the literature, there are not many works that quantify the mass-benefit of an aerocapture maneuver. For this reason, the aim of this study is to provide a quantification of the propellant mass-saving and the increase of the payload with respect to a mission that took place between 2013 and 2014 (MAVEN mission). This mission involved an insertion to a Mars orbit through a common propulsive manuever. Two different aerocapture trajectories were considered to obtain arrival orbit with a period of about 35 hours and an alternative one with a period of 4.5 hours. These two aerocapture maneuvers provide a mass saving due to unused propellant of 738.07 kg in the first case and 1281.15 kg in the second. The increase in payload capacity was calculated while also considering the additional weight of an inflatable shield used in the aerocapture maneuver. Both the two aerocapture maneuvers have offered a significant payload increase for the same initial mission mass (up to 1546.38%)
A fluid dynamics technique for modelling inflatable shield for re-entry or aerocapture missions
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