182 research outputs found
Upwind sail aerodynamics: A RANS numerical investigation validated with wind tunnel pressure measurements
Get PDFThe aerodynamics of a sailing yacht with different sail trims are presented, derived from simulations performed using computational fluid dynamics. A Reynolds-averaged Navier-Stokes approach was used to model sixteen sail trims first tested in a wind tunnel, where the pressure distributions on the sails were measured. An original approach was employed by using two successive simulations: the first one on a large domain to model the blockage due to the wind tunnel walls and the sails model, and a second one on a smaller domain to model the flow around the sails model. A verification and validation of the computed aerodynamic forces and pressure distributions was performed. The computed pressure distribution is shown to agree well with the measured pressures. The sail surface pressure was correlated with the increase of turbulent viscosity in the laminar separation bubble, the flow reattachment and the trailing edge separation. The drive force distribution on both sails showed that the fore part of the genoa (fore sail) provides the majority of the drive force and that the effect of the aft sail is mostly to produce an upwash effect on the genoa. An aerodynamic model based on potential flow theory and a viscous correction is proposed. This model, with one free parameter to be determined, is shown to fit the results better than the usual form drag and induced drag only, even if no friction drag is explicitly considered. (C) 2012 Elsevier Inc. All rights reserved.</p
Mathematical modelling of unsteady problems in thin aerofoil theory
No full textThe de-icing of aircraft wings by the injection of fluid through a slot in the leading edge of the wing is analysed. A review of current de-icing methods is presented and the semi-infinite slot-injection equation derived, which is a singular partial integro-differential equation. The Stefan condition is used to close the system. A discretisation of the equation is presented and the subsequent numerical results are analysed. The model is then revised to account for the retraction of the ice layer away from the slot. An asymptotic result for the thin ice layers is also presented.The problem of describing the motion of a thin, flexible membrane fixed at both ends (a 'sail') is then considered. The steady sail is analysed for the case of an inextensible sail and previous work on this topic is extended by using a discretisation of the singular integro-differential equation that is pertinent to the later analysis of the unsteady sail. An asymptotic expression for the eigenvalues of the system, defined as the values of the tension parameter for which the sail generates zero lift, is also presented. The problem is then extended to that of an extensible sail and numerical results are presented for both the sail with excess length and the membrane without slack.The case where the angle of incidence of the sail to the free stream is a prescribed function of time is then analysed. Previous work on this subject is extended to include the extensible sail and numerical results are presented. A linear stability analysis is then undertaken for both the extensible and elastic sails; the resulting quadratic eigenvalue problem is solved numerically and is in agreement with the numerical experiments.The trailing edge of the membrane is now permitted to move freely and thus the motion of a 'flag' is analysed. The inclusion of bending stiffness is found to be crucial to the stability properties of the flag. The steady equation of motion is numerically approximated for both a hinged flag and a flag that is clamped at the leading edge. The unsteady flag equation is then discretised and numerical results are presented. A linear stability analysis is performed, the conclusions of which are consistent with the numerical approximations of the unsteady flag equation
A New Model for the Planetary Radiation Pressure Acceleration for Optical Solar Sails
No full textSolar sailing is a propellantless propulsion method that exploits solar radiation pressure to generate thrust. In recent years, several solar sails have been launched into Earth-bound orbit to demonstrate this technology’s potential. Because planetary radiation pressure can reach magnitudes comparable to that of solar radiation pressure in proximity of the Earth, it cannot automatically be neglected in near-Earth solar-sail mission design studies. Nevertheless, its effect on the solar-sail dynamics has been investigated only to a very limited, first-order extent, and every study considered an “ideal” – i.e., perfectly reflecting – sail model. Although employing the ideal sail model proves useful for preliminary orbital analyses, its limited fidelity prevents more in-depth research into the near-Earth solar-sail dynamics and trajectory optimization. In light of this, this paper provides a new planetary radiation pressure acceleration model for optical solar sails. This model forms an extension of the “spherical” planetary radiation pressure acceleration model for ideal solar sails devised by Carzana et al. in Reference [1]. In the current paper, the underlying assumptions and full derivation of the newly devised optical model are presented. Subsequently, the accuracy of the optical model is analyzed through a comparison with the ideal model, using NASA’s upcoming ACS3 mission as reference scenario.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Astrodynamics & Space MissionsSpace Engineerin
A New Model for the Planetary Radiation Pressure Acceleration for Solar Sails
No full textSolar sailing is a propellantless propulsion method that takes advantage of solar radiation pressure to generate thrust. The last decades have seen the launch of several solar-sail missions to demonstrate the technology’s potential for space exploration and exploitation. Even more missions are scheduled for launch in the near future, including NASA’s ACS3 and NEA Scout missions and Gama’s Alpha sailcraft. Although most of these sailcraft have flown – or will fly – in LEO, where the planetary radiation pressure is strong (up to approximately 20% of the solar radiation pressure), studies on the perturbing accelerations produced by the Earth’s albedo and blackbody radiation have been conducted only to a very limited first-order extent. This paper therefore provides a novel, detailed analytical model for these perturbing accelerations, valid for double-sided perfectly reflecting solar sails. The underlying assumptions of the model are presented and its full derivation is described. A thorough analysis of the blackbody and albedo radiation pressure accelerations is conducted for a variety of orbital conditions and Sun-Earth-sail configurations. In order to quantify the accuracy of the model, a comparison with the state of the art (the finite-disk radiation source model) is provided. Ultimately, a variety of analyses to quantify the effect of Earth’s albedo and blackbody radiation on the maneuvering capabilities of solar sails are provided, using the orbit of the ACS3 mission as reference scenario. These analyses show that, for an orbit-raising steering law, losses in the altitude gain of 19.6% of the total gain are incurred over a 10-day orbit-raising period. Similarly, losses in the inclination gain of up to 25% of the total gain are observed when implementing an inclination-changing steering law. These results highlight the non-negligible effect of uncontrolled planetary radiation pressure acceleration on the maneuvering capabilities of solar sails in LEO.Astrodynamics & Space MissionsSpace Engineerin
Passive stability enhancement with sails of a hovering flapping twin-wing robot
No full textHovering flapping wing flight is intrinsically unstable in most cases and requires active flight stabilization mechanisms. This paper explores the passive stability enhancement with the addition of top and bottom sails, and the capability to predict the stability from a very simple model decoupling the roll and pitch axes. The various parameters involved in the dynamical model are evaluated from experiments. One of the findings is that the damping coefficient of a bottom sail (located in the flow induced by the flapping wings) is significantly larger than that of a top sail. Flight experiments have been conducted on a flapping wing robot of the size of a hummingbird with sails of various sizes and the observations regarding the flight stability correlate quite well with the predictions of the dynamical model. Twelve out of 13 flight experiments are in agreement with stability predictions.Control & Simulatio
A study of the effects of social skills training on self-esteem in an adolescent foster child, 1998
Get PDFOf social skills training en self-esteem in adolescent foster youth utilizing the 10-item Rosenberg Self-Esteem Scale. The subject for this research was a 17 years old African American male in foster care. This study hypothesized that social skills training, framed in cognitive restructuring, would increase the subject's level of competence, (efficacy) and would result in a heightened sense of self-esteem. The findings of this study concluded that social skills training was effective in increasing overall self-efficacy and self-esteem. The length of time, for cc baseline and intervention might have yielded different results
The Constant Pair
Get PDFA young woman sails to and brings back so she can marry her lover that her father had banished to the Indieshttps://egrove.olemiss.edu/kgbsides_uk/1613/thumbnail.jp
Development of a system for the investigation of spinnakers using fluid structure interaction methods
No full textWhile historically sailmaking and saildesign were considered as arts, in the 20th century, mainly from the 1980s onwards, engineering sciences have started to play an important role. Two fields are of particular interest: structural and fluid mechanics. Initially, the sails were tested in the wind tunnel, aggregate flow forces measured and the interaction of flow and structural behaviour implicitly captured by visual observation. No quantitative structural assessment was available in these experiments. With the advent of affordable powerful personal computers, programs were developed to compute the flow around sails and the structural reaction to the resulting forces. These programs were based on significantly simplified assumptions about the fluid mechanics - potential flow - as well as the complete neglect of any unsteady behaviour of flow or coupled result. These simplifications limit the applicability of these programs to upwind sails, essentially this airfoils working at small angles of attack. As downwind sails do not comply with these limitations they are still tested in the wind tunnel with the associated scale effects and limited outcome of quantitative results.Within this thesis a method is being developed to capture the interaction between the complex viscous flow around downwind sails and compute the structural answer to the resulting forces. First a structural model suitable for downwind sails is developed. This is coupled to a commercial solver for simulations of viscous flow. The individual parts (structural and flow simulation as well as coupling) and the entire method are verified and validated. Finally an application example is given. First, the structural model and coupling to the flow solver are developed. The particular challenge regarding the structural model is the requirement to compute the complex behaviour of downwind sails. By design these sails have negligible bending stiffness with the material being stiff in tension but without any meaningful compressive stiffness. To this end the classic CST-element is extended by a wrinkling model, a robust solver able to capture the resulting non-linearities is implemented. This model is coupled to a commercial RANS solver by a bespoke coupling algorithm. This algorithm ensures the conservative transfer of forces and deformations while keeping the coupled simulation stable.Next, to ensure applicability of the structural and flow simulation models as well as the coupling, they are verified for grid and time step dependency and validated against analytical or experimental data. As no experimental data was freely available on the particular case of downwind sails, wind tunnel tests were conducted to provide at least aggregate flow forces and flying shapes. Particularly the structural simulation and coupling were successfully verified and validated, the simulation of partially separated flow around highly curved surfaces like downwind sails exhibited a strong sensitivity to e.g. small changes of the angle of attack. Validation of the flow simulation was hampered by uncertainties in the experimental data.Finally, the method is used to compare three sail designs on a hypothetical yacht based on the AC90-rule. The impact of the sail design changes is clearly shown with small variations in sail (profile) depth resulting in very much different optimal angles of attack.Improvements to the method could in particular be achieved by implicit or strong coupling of flow and structural simulation, this would yield time-accurate information on the sails unsteady behaviour. Further, even more involved flow simulation methods, e.g. large or detached eddy simulation instead of turbulence modelling might improve the accuracy of the flow simulation.Ship Hydromechanics and Structure
- …
