Multigrid methods for compressible Navier-Stokes equations in low-speed flows

AbstractThe multigrid performance of pointwise, linewise and blockwise Gauss-Seidel relaxations for compressible laminar and turbulent Navier-Stokes equations is illustrated on two low-speed test problems: a flat plate and a backward facing step. The line method is an Alternating Symmetric Line Gauss-Seidel relaxation. In the block methods, the grid is subdivided into geometric blocks of n × n points with one point overlap. With in the blocks, the solution is obtained by a direct method or with an alternating modified incomplete lower-upper decomposition. The analysis is focused on flows typical for boundary layers, stagnation and recirculation regions. These are characterized by very small Mach numbers, high Reynolds numbers and high mesh aspect ratios

Design optimization of multi-functional multi-lobe cryogenic fuel tank structures for hypersonic vehicles

[Abstract:] Hypersonic hydrogen-powered cruise vehicles offer promise for economical and reliable high-speed atmospheric transport. In recent years, several vehicle concepts have been developed in which the integration of fuel tanks is a major challenge, as they feature complex aerodynamic designs. In this work, we explore the viability of multi-lobe hydrogen tanks as a solution to obtain lightweight and volume-efficient structures. To do so, a parametric finite-element model was developed to fit multi-lobe geometries inside hypersonic vehicles. The parametric model was then incorporated into an optimization that minimizes the mass and maximizes the fuel capacity of the tank. The methodology is organized in two steps: the global search is driven by a two-level optimization consisting of a genetic algorithm with a nested gradient-based method; and a local search where each design is further improved to obtain a Pareto front. As presented in the results, this is a promising approach for designing multi-lobe tanks for complex geometries.The research leading to these results has been conducted under Grant PID2019-108307RB-I00 funded by MCIN/AEI/10.13039/501100011033. The authors also acknowledge funding received from the Galician Government through research grant ED431C 2021/33. Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature.Xunta de Galicia; ED431C 2021/3

Life-Cycle Cost Estimation for High-Speed Vehicles: from the engineers’ to the airline’s perspective

This paper aims at upgrading the holistic Cost Estimation methodology for High-Speed Vehicles already developed by Politecnico di Torino and the European Space Agency (ESA) to encompass different stakeholders’ perspectives. In details, the presented methodology combines International Air Transport Association (IATA) best practices with a detailed Life- Cycle Cost (LCC) assessment, which includes the evaluation of Research, Development, Test and Evaluation (RDTE) Costs, Production costs and of Direct and Indirect Operating Costs (DOC and IOC). The integrated approach allows to further extend the capabilities of the inhouse developed HyCost tool to support all the actors of the product value-chain (including engineers, manufacturers, airlines and customers) in assessing the economic sustainability of a newly under-development high-speed vehicle. However, considering the need of providing all these cost analyses perspectives since the early design stages, the derived Cost Estimation Relationships are mainly derived on statistical bases. To cope with the uncertainties that affect the initial statistical population and consequently, the CERs, this paper presents each cost item together with the estimation of related prediction intervals. Finally, results of the application of the upgraded cost estimation methodology and of the upgraded tool to the LAPCAT MR2.4 high-speed civil transport are reported and discussed

Liquid column separation due to fluid hammer occurrence in propellant lines

This version of the article has been accepted for publication, after peer review, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.2514/1.B38451[Abstract]: When a pressurized liquid enters a pipeline with a closed-end and under vacuum conditions, the resulting liquid front suddenly is brought to rest at the end of the pipe. This type of flow configuration is found in propulsion systems of satellites during priming operation and induces a fluid hammer followed by a column separation, generating a multiphase gas/vapor bubble. This paper aims at explaining the column separation mechanism by solving the momentum equation for the liquid column moving in the pipeline when column separation occurs, and by applying the integral form of the conservation principles to expansion and compression waves within the flow. The resulting model provides the velocity and position of the liquid front during column separation. Thus, the size and duration of the multiphase bubble can be determined, and the variables involved in the process are identified, which helps on the analysis of applications where this complex phenomenon is involved. It is shown that the initial velocity of the liquid front during column separation is the main parameter, which itself is a function of the fuel tank pressure and the fluid hammer pressure rise. The comparison of the predictions with experimental data shows an excellent agreement.This research was supported by the Xunta de Galicia and the European Regional Development Funds under grant EDC431C-2021/39 and the Spanish Science and Education Ministry through grant RTI2018-101114-B-I00. We wish to acknowledge the support received from the Centro de Investigación de Galicia, funded by Xunta de Galicia and the European Regional Development Fund Galicia 2014–2020 Program through grant ED431G 2019/01. The present research activity was promoted by the European Space Research and Technology Centre of the European Space Agency through the General Support Technology Programme (GSTP) activity AO/1-6210/09/NL/CP.Xunta de Galicia; EDC431C-2021/39Xunta de Galicia; ED431G 2019/0

Effect of inertia on the dynamic contact angle in oscillating menisci

The contact angle between a gas-liquid interface and a solid surface is a function of the dynamic conditions of the contact line. Classic steady correlations link the contact angle to the contact line velocity. However, it is not clear whether they hold in presence of inertia and in the case of perfect wetting fluids. We analyze the shape of a liquid interface and the corresponding contact angle in accelerating conditions for two different fluids, i.e. HFE7200 (perfect wetting) and demineralized water. The set-up consists of a U-shaped quasi-capillary tube in which the liquid column oscillates in response to a pressure step on one of the two sides. We obtained the evolution of the interface shape from high-speed back-light visualization, and we fit interface models to the experimental data to estimate the contributions of all the governing forces and the contact angle. Traditional interface models fail to predict the interface shape and its contact angle at large interface and contact line accelerations. We propose a new model to account for the acceleration, and we discuss its impact on the measurement of the transient contact angle

A methodology for preliminary sizing of a Thermal and Energy Management System for a hypersonic vehicle

This paper addresses a methodology to parametrically size thermal control subsystems for high-speed transportation systems during the conceptual design phase. This methodology should be sufficiently general to be exploited for the derivation of Estimation Relationships (ERs) for geometrically sizing characteristics as well as mass, volume and power budgets both for active (turbopumps, turbines and compressors) and passive components (heat exchangers, tanks and pipes). Following this approach, ad-hoc semi-empirical models relating the geometrical sizing, mass, volume and power features of each component to the operating conditions have been derived. As a specific case, a semi-empirical parametric model for turbopumps sizing is derived. In addition, the Thermal and Energy Management Subsystem (TEMS) for the LAPCAT MR2 vehicle is used as an example of a highly integrated multifunctional subsystem. The TEMS is based on the exploitation of liquid hydrogen boil-off in the cryogenic tanks generated by the heat load penetrating the aeroshell throughout the point-to-point hypersonic mission. Eventually, specific comments about the results will be provided together with suggestions for future improvements

Analysis of fluid hammer occurrence with phase change and column separation due to fast valve opening by means of flow visualization

This is a PDF file of an unedited manuscript that has been accepted for publication[Abstrac]: This paper presents an experimental investigation on the fluid hammer phenomenon generated when filling a pipe line under vacuum conditions with a closed end. This physical configuration, although it can be found in many piping configurations, it is of special interest in propulsion systems of satellites during priming operation. The fluid hammer taking place here not only leads to high pressure peaks in the fluid but also to low pressures, which can cause cavitation, gas desorption and liquid column separation. The study is carried out on a facility allowing flow visualization, which is achieved by replacing the pipe closed end by a quartz cylinder drilled with the same tube inner diameter. In this way, the flow can be recorded with high speed imaging at this location. The visualizations confirm that the pressure evolution is accompanied by a complex multiphase flow pattern. First of all, a foamy mixture of non-condensable gas, vapor and liquid droplets precedes the liquid front arrival at the bottom end. During the fluid hammer compression wave, the vapor condensates and the non-condensable gas gets compressed. Afterwards, the arrival of an expansion wave induces the movement of the liquid column backwards, with the corresponding pressure drop that generates a gaseous bubble referred to as column separation. Finally, the collapse of this bubble is at the origin of the next pressure rise.The present research activity was initiated and promoted by the European 280 Space Research and Technology Centre of the European Space Agency (ESTEC/ESA) through the GSTP activity AO/1-6210/09/NL/C

Heat flux augmentation caused by surface imperfections in turbulent boundary layers

Aerodynamic heating of hypersonic vehicles is one of the key challenges needed to be overcome in the pursuit of hypersonic ascent, re-entry, or sustained flight. Small, unavoidable imperfections are always present on the surface of aircraft in the form of steps, gaps, and protuberances. These can lead to high levels of localised heat flux augmentation, up to many times the undisturbed level. Flat plate experiments have been carried out in the Oxford High Density Tunnel with the aim of characterising the heating effects caused by small scale protuberances and steps in turbulent boundary layers. The current work presents experimental heat flux augmentation data, an assessment of existing heat flux correlations, and introduces new engineering level correlations to describe heat flux augmentation for a range of surface geometries