摘要
首先对比了天上飞行状态与地面风洞状态下钝锥边界层的转捩特性,然后利用基于线性稳定性理论的
高马赫数飞行器绕流存在着激波、边界层、流动分离、稀薄气体效应和高温真实气体效应等多种复杂流动现象的空气动力学问题,其中边界层的转捩问题是高马赫数流动研究的热点和难点。当飞行器以高马赫数飞行,边界层从层流状态转捩为湍流状态时,其表面摩擦阻力和表面热流急剧增加,直接影响着飞行器的飞行性能。准确预测边界层的转捩位置将为飞行器设计提供理论依据,可以有效地改进飞行器性能,提高其升阻比,降低燃料消耗,并有利于进行热防护设计,是飞行器设计的关键问题之一。
圆锥作为飞行器的重要组成部分,前人对其边界层在高马赫数条件下的流动稳定性及转捩特性开展了大量的研
近些年来,关于壁温比对边界层转捩影响的研究,涂国华
由于地面实验条件无法完全复现天上飞行的实际状态,天地差异客观存在。边界层转捩问题机理复杂,其天地差异呈现新的特点。揭示边界层转捩天地差异的主要机理是开展转捩问题天地相关性研究的基础,是建立相关模型和方法的前提。本文首先对天上飞行状态与地面风洞状态下钝锥边界层的转捩特性进行了对比,然后利用基于线性稳定性理论的
钝锥转捩实验在国防科技大学Ф300 mm高超声速静风洞完成。该风洞采用上吹下吸的运行方式,喷管采用轴对称短化设计,喷管出口直径为 300 mm。为了尽可能保持喷管壁面为层流边界层,在喷管喉道上游采用抽吸方式抽除稳定段收缩段发展起来的边界层。风洞运行的名义马赫数为6,正常运行时间长达 30 s,可以通过调节来流总温总压改变来流雷诺数。
温敏漆技术是测量边界层转捩经常采用的一种方法。温敏漆在一定波长的入射光照射下,能够发射出波长与入射光显著不同的激发光,并且激发光的强度随温度升高而单调降低。在高超声速条件下,气流的摩擦加热使得层流区和湍流区之间的温度差异非常显著,利用温敏漆这一特性,将温敏漆材料涂覆在模型表面并在一定波长的入射光进行照射,利用相机拍摄激发光强的变化情况。当边界层由层流转变为湍流时,表现为图像上的局部温度陡升,通过壁面温度变化这一现象可以确定边界层转捩位置。
为了研究圆锥边界层转捩,将直角坐标系下的可压缩守恒型Navier‑Stokes(N‑S)方程变化到贴体坐标系下,得到计算所用的控制方程。若贴体坐标系与直角坐标系的对应关系为,经过推导可以得出贴体坐标系下的N‑S方程为
(1) |
式中:,,,,
,,, |
。 |
数值模拟的流动环境为来流马赫数Ma=6,来流静温T∞=52 K,来流单位雷诺数Reunit=1×1
根据线性稳定性理论,小扰动可以写成行进波形式
(2) |
式中:为扰动量,为扰动波的形状函数,α和β分别为流向和展向的波数,ω为频率,c.c表示共轭复数。将
(3) |
式中:s0为扰动开始增长的位置或参考位置,s为当前位置。得到N(ω, x, z)值后取所有频率下的N值包络作为预测转捩位置用的N值,即
(4) |
当N值达到由实验标定的某个阈值NT时,即认为流动发生了转捩。不同类型的边界层流动NT值一般需要重新标定,因此

图1 来流单位雷诺数为1.0×1
Fig.1 Temperature-sensitive paint image of windward side of blunt cone at 10° angle of attack under freestream unit Reynolds number of 1.0×1

图2 HIFiRE-1钝锥外形迎风面的红外热图测
Fig.2 Infrared map of windward side of HIFiRE-1 blunt con

图3 HIFiRE-1飞行试验与地面实验状态下钝锥转捩阵面对
Fig.3 Comparison of HIFiRE-1 transition morphology in the cases of flight test and ground experimen
采用基于线性稳定性理论的

图4 地面实验与飞行实验状态下钝锥边界层的
Fig.4 Transition morphology of blunt cone boundary layer under flight and ground conditions predicted by the
飞行试验状态和地面实验状态下的圆锥转捩特性具有显著差异。由风洞实验和飞行试验条件可以得出,风洞状态的壁温比Tw/T0一般在0.6~0.7,而飞行状态下的壁温比Tw/T0一般在0.1~0.2,下面将对不同Tw/T0对钝锥边界层转捩的影响开展分析。

图5 不同壁温比条件下的马赫云分布
Fig.5 Distributions of Mach number contours under different ratios of wall temperature to total temperature

图6 不同壁温比条件下的壁面极限流线分布
Fig.6 Distributions of surface limiting streamlines under different ratios of wall temperature to total temperature

图7 不同壁温比条件下x=300 mm,φ=60°与120°处的特征物理量剖面
Fig.7 Profiles of characteristic physical quantities at x=300 mm along the φ=60° and 120° meridians under different ratios of wall temperature to total temperature

图8 壁温比Tw/T0=0.2时流向x=300 mm,周向φ=60°与120°处不稳定扰动增长率分布
Fig.8 Growth rate distributions of unstable waves at x=300 mm along φ=60° and 120° meridians under Tw/T0=0.2

图9 不同壁温比条件下流向x=300 mm,周向φ=60°与120°处第一模态与第二模态中性曲线分布
Fig.9 Neutral curves of the first and the second modes at x=300 mm along φ=60° and 120° meridians under different ratios of wall temperature to total temperature

图10 不同壁温比条件下的N值包络分布
Fig.10 N-value envelopes under different ratios of wall temperature to total temperature

图11 不同壁温比条件下N=3时不同频率扰动引起的转捩线位置
Fig.11 Transition positions induced by various frequency disturbances when N=3 under different ratios of wall temperature to total temperature
本文首先对比了天上飞行状态与地面风洞状态下钝锥边界层的转捩特性,然后利用基于线性稳定性理论的
(1) 在飞行试验状态下,圆锥0°子午线和90°子午线上的边界层均先于上述两子午线之间区域转捩,圆锥迎风面的转捩形貌呈“W”形。在风洞实验状态下,圆锥迎风面迟于背风面转捩,迎风面上的边界层转捩形貌呈“V”形。
(2) 在低壁温比条件下,圆锥迎风中心与侧面的边界层先于两者之间区域转捩,转捩形貌与飞行试验结果相似。在高壁温比条件下,圆锥迎风面区域迟于侧面及背风面区域转捩,转捩形貌与风洞实验结果相似。
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