飞机空气动力学中的分离流和旋涡流:CFD视角

A. Rizzi
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引用次数: 0

摘要

在早期的航空时代,弗雷德里克·兰彻斯特既是一位发明家,也是一位理论家。为了减少设计新飞机时的猜测,他需要一种飞行理论。他在1907年出版的《空气动力学》一书奠定了这种理论的早期基础。这一理论得到了其他人的贡献,尤其是路德维希·普朗特尔(Ludwig Prandtl)的改进,成为二战飞机光滑设计的基础,几乎没有猜测。随着喷气推进速度的提高,非线性空气动力学进入跨音速范围,新技术从根本上改变了飞机的设计。在20世纪40年代末和50年代初,大量的猜测又回到了飞机设计中。然而,随着计算流体动力学(CFD)的发展,Lanchester等人的遗产得以延续,该技术可以指导设计人员处理非线性跨音速效应。本文介绍了CFD发展的历史概况,并举例说明了第一代后掠翼跨音速战斗机设计中克服的一些困难。历史研究是法医CFD,旨在寻找设计师选择很久以前投入生产的机翼形状的可能解释。考察了当前CFD技术在细翼飞机空气动力学中的应用能力。讨论了混合机翼和中等掠翼上具有相干涡的流型。涡流空气动力学涉及到对集中涡与飞机部件相互作用的理解。现代reynolds - average Navier-Stokes (RANS)技术可用于预测附著流。但是涡旋与其他涡旋的相互作用和击穿会导致非定常的大分离流动,这已经超出了RANS的范围。在可预见的未来,对Navier-Stokes方程的直接模拟是无法通过计算机实现的,因此对更好的物理建模的需求是显而易见的。巡航性能和失速特性都受到强相互作用的影响。讨论了机翼流动物理的两个重要方面:从光滑表面分离产生涡流,以及涡流破裂,即涡流突然破裂导致升力损失。非细长机翼的涡旋空气动力学遇到了特别具有挑战性的问题,并展示了早期作战飞机的设计如何克服这些困难。本文通过使用模拟CFD,总结了两个气动设计案例:Saab J29A机翼如何在失速附近保持控制权威,以及Saab J32如何在高发生率下减轻俯仰不稳定性。
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Separated and vortical flow in aircraft aerodynamics: a CFD perspective
Abstract In the early era of aviation, Frederick Lanchester was both an inventor and a theoretician driven by the need for a theory of flight that would reduce the guesswork in designing new aircraft. His book Aerodynamics in 1907 laid down the early foundations of such a theory. The theory with contributions from others, notably Ludwig Prandtl, was refined to become the basis for the sleek designs of WWII aircraft brought about with little guesswork. New technology changed aircraft design radically with the increased speed of jet propulsion reaching into the transonic range with nonlinear aerodynamics. In the late 1940s and early 1950s substantial guesswork returned to aircraft design. The legacy of Lanchester et al., however, lived on with the development of computational fluid dynamics (CFD) that could guide designers through nonlinear transonic effects. This article presents a historical sketch of how CFD developed, illustrated with examples explaining some of the difficulties overcome in the design of the first-generation swept-wing transonic fighters. The historical study is forensic CFD in search for the likely explanation of the designer’s choice for the wing shape that went into production a long time ago. The capability of current CFD applied to the aerodynamics of aircraft with slender wings is surveyed. The cases discussed involve flow patterns with coherent vortices over hybrid wings and wings of moderate sweep. Vortex-flow aerodynamics pertains to understanding the interaction of concentrated vortices with aircraft components. Modern Reynolds-Averaged Navier-Stokes (RANS) technology is useful to predict attached flow. But vortex interaction with other vortices and breakdown lead to unsteady, largely separated flow which has been found out of scope for RANS. Direct simulation of the Navier-Stokes equations is out of computational reach in the foreseeable future, and the need for better physical modeling is evident. Both cruise performance and stalling characteristics are influenced by strong interactions. Two important aspects of wing-flow physics are discussed: separation from a smooth surface that creates a vortex, and vortex bursting, the abrupt breakdown of a vortex with a subsequent loss of lift. Vortex aerodynamics of not-so-slender wings encounter particularly challenging problems, and it is shown how the design of early-generation operational aircraft surmounted these difficulties. Through use of forensic CFD, the article concludes with two case studies of aerodynamic design: how the Saab J29A wing maintains control authority near stall, and how the Saab J32 mitigates pitch-up instability at high incidence.
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