1,720,981 research outputs found
Stability and transition of the flow behind isolated roughness elements in hypersonic boundary layers
No full textIn this work the effect of isolated surface roughness on the behaviour of a hypersonic boundary layer is investigated, with a particular focus on the effect of the three-dimensional roughness shape on the instability of the roughness wake and the subsequent transition process. The analysis is performed computationally using direct numerical simulations, which solve the compressible Navier-Stokes equations, and a new code, developed in the scope of the current work, to analyse the linear stability of these equations. The full three-stage roughness-induced transition process has been investigated: firstly, the receptivity process and generation of boundary layer instabilities from freestream disturbances; secondly, the generation of a roughness wake and its initial linear instability; and finally the non-linear breakdown to turbulence of the roughness wake. In particular the effect of the three-dimensional roughness shape on these processes has been studied, looking at the roughness height, frontal profile, planform shape and upward/downward ramps. Also the effect of freestream disturbance amplitude andwall cooling has been investigated. It has been found that the roughness height and frontal profile have a large influence on the stability characteristics of the resulting wake and the subsequent transition. The roughness planform shape has a marginal effect, although cylindrical and diamond-shaped elements yield more unstable wakes than a square roughness element. Bi-local stability analysis can be used in most cases to predict the most unstable wake mode, but it under-predicts the instability growth rates due to non-parallel effects. The roughness shape has been observed to affect the transition onset location. The criteria commonly used to predict roughness-induced transition, do not take into account the three-dimensional shape, and an alternative transition prediction, based on the amplitude of the roughness-induced streamwise streak, is considered
DNS database of roughness-induced instability and transition at Mach 6
No full textRaw data generated by direct numerical simulations of the flow behind isolated roughness elements at Mach 6. More information and details about the numerical set-up and the resulting data can be found in:
Jeroen Van den Eynde, "Stability and Transition of the Flow behind Isolated Roughness Elements in Hypersonic Boundary Layers", PhD thesis (University of Southampton), 2015
To request access go to http://library.soton.ac.uk/datarequest</span
Numerical simulations of transition due to isolated roughness elements at Mach 6
No full textAn accurate prediction of transition onset behind an isolated roughness element has not yet been established. This is particularly important in hypersonic flow, where transition is accompanied by increased surface heating. In the present contribution, a number of direct numerical simulations have been performed of a Mach 6 boundary layer over a flat plate with isolated roughness elements. The effects of roughness shape, planform, ramps, and freestream disturbance levels on instability growth and transition onset are investigated. It is found that the frontal shape has a large effect on the transition onset, which is in agreement with previous studies, whereas the roughness element planform has a marginal influence. A new result is that the roughness shape in the streamwise direction (in particular, the aft section) is also an important characteristic, since an element with a ramped-down aft section allows the detached shear layer to spread out and weaken, leading to a lower instability growth rate. Above a critical value, the instability growth rate is found to be correlated with the amplitude of the low-speed streak formed by the roughness element, suggesting that a more physically based transition criterion should take account of the local liftup effect of the particular roughness shape.<br/
Outcome of high-speed boundary layer transition workshop at HiSST 2022
No full textThis white paper outlines the urgent needs for future development in high-speed boundary layer transition (BLT) on the basis of the open discussion during a mini-workshop on the topic, held at Bruges during the HiSST conference on Wednesday 14 September 2022
Exploration of non-conventional techniques for the generation of element-based analytical ephemerides
No full textA comprehensive description is presented of the many approaches to modern ephemeris generation that are found in the literature. Particularly, the generation of ephemerides build upon series of basis functions has been discussed, comparing Chebyshev, Fourier and Poisson series and supporting the arguments with factual data. For the case of Taylor series approximations, the relationship between the degree of the approximating polynomial and the total number of terms in the approximation have been qualitatively discussed as a function of the number of segments in which the time interval is split. A few examples are provided to show the accuracy and investigate the computational cost of various analytical and numerical ephemerides
B-plane visualisation tool for uncertainty evaluation
No full textLaunchers used for interplanetary missions may be inserted into orbits in resonance with the Earth or that may to cross other planets' orbits. To verify the compliance with planetary protection requirements, the impact of the uncertainty on the trajectory evolution is assessed and two visualisations are produced. The first one is the common representation of the trajectory distribution on the Earth b-plane, highlighting also areas of gravitational interactions with other bodies. The second one represents a colour map of different states (i.e. impact with the Earth, resonances, interaction with other planets) on the grid of velocity variation used to define the initial conditions
Characterization of SRM plumes with alumina particulate in subscale testing
Full text linkThe current paper provides an outline and first results of the ESA-EMAP project. This project pursues activities regarding the experimental modeling of alumina particulates in solid boosters (EMAP). The issue regards the particles residing in the atmosphere after the passage of a launch vehicle with solid rocket propulsion, which might contribute to local and overall ozone depletion. The question is to what extent since the particle size distribution left behind is essentially unclear. For this
reason, the ESA-EMAP investigations focus on the characterization of the solid exhaust plume properties for well-defined combustion chamber conditions. Thus, details of the rocket motor assembly, of the developed solid propellant grains, and of first measurement results are provided. The paper presents technical findings concerning the rocket motors and reveals aspects to the feasibility of the applied measurement techniques
Two-Equation Transition Model Based on Intermittency and Empirical Correlations
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