横流失稳是导致大型客机后掠翼流动转捩的重要原因，以横流为主导的边界层转捩过程会出现横流涡，使边界层内的流动呈现复杂的三维性，其速度的三维空间分布是这种流动的基本特征。本文在低湍流度回流式风洞中，利用双丝边界层热线针对45°后掠、-4°攻角的后掠翼模型的主流和横流速度的空间分布进行了精细测量，模型平行于来流方向的截面形状为NLF0415层流翼型。实验雷诺数Re = 2.15×106，测量距离壁面的最小距离为0.05mm，垂直于壁面方向和平行于翼展方向的空间分辨率分别为0.05mm和0.707mm。研究发现，边界层主流速度剖面沿垂直于壁面方向上呈现不同程度的扭曲，主流平均速度等值线沿展向呈现波状分布。在转捩过程中，壁面附近横流速度方向一致，随着壁面距离的增加，横流速度剖面沿展向呈现正负交替变化，横流速度是导致主流速度剖面出现扭曲的重要原因。主流、横流雷诺正应力主要分布在各自对应的平均速度沿翼展方向梯度较大的空间位置上，在边界层内沿展向呈现周期性分布规律。速度频谱中存在400Hz、5500Hz、11000HZ、16500Hz的特征频率，其中11000HZ、16500Hz是5500Hz的倍频模态，其分布位置和增长率非常一致。主、横流脉动速度的频谱特征基本一致，说明诱导转捩的边界层结构存在明显的三维性。400Hz和5500Hz及其倍频模态之间存在一定程度的相互干扰；Breakdown发生之前，5500Hz及其倍频模态的脉动能量和空间分布范围出现了一定程度的减小，与400Hz模态对应流动结构的破碎有关。根据这些频谱对应的脉动能量增长率和空间分布位置的分析，推测400Hz和5500Hz对应的流动结构的破碎是导致边界层转捩的主要原因。

Crossflow instability is an important factor that causes the transition of boundary layer on the swept Wing of large passenger aircraft. Crossflow-dominated transboundary vortices occur in the transition process of the boundary layer, and the flow in the boundary layer presents a complex three-dimensionality. The three-dimensional spatial distribution of velocity is the basic feature of this kind of flow.In this paper, in the low turbulence return-flow wind tunnel, the V-type boundary layer hotwire is used to precisely measure the spatial distribution of the main flow and crossflow velocity of the boundary layer on the swept-wing model with 45° swept and -4° angle of attack. The cross-sectional shape of the model parallel to the incoming flow direction is the NLF0415 laminar airfoil. The experimental Reynolds number Re=2.15×10^6, the minimum distance from the model surface is 0.05mm, and the spatial resolution perpendicular to the wall direction and parallel to the spanwise direction is 0.05mm and 0.707mm, respectively.In this study, we find that the mainstream velocity profile of the boundary layer exhibits different degrees of distortion along the direction perpendicular to the wall surface, and the average velocity isoline of the mainstream presents a wavy distribution along the direction. In the transition process, the cross-flow velocity near the wall is in the same direction. With the increase of the wall distance, the cross-flow velocity profile alternates positively and negatively along the span, and the cross-flow velocity is an important reason for the distortion of the mainstream velocity profile. Mainstream and crossflow Reynolds normal stress are mainly distributed in the spatial locations where the corresponding average speeds have large gradients along the spanwise direction. The distribution of Reynolds shear stress is basically consistent with the distribution of cross-flow velocity. The periodical distribution patterns appear along the span in the boundary layer. There are 450Hz, 5500Hz, 11000HZ, and 16500Hz characteristic frequencies in the frequency spectrum, among which 11000HZ and 16500Hz are the frequency-multiplying modals of 5500Hz. The distribution location and growth rate are very consistent. The frequency characteristics of the main and cross-flow velocities are basically the same, indicating that there is a clear three-dimensionality in the boundary layer structure that induces transition. There is a certain degree of mutual interference between 450Hz and 5500Hz and their frequency doubling modes; before the occurrence of Breakdown, the pulsation energy and spatial distribution range of 5500Hz and its frequency doubling modes have been reduced to a certain extent, corresponding to the 450Hz modal flow. The structure is broken. Based on the analysis of the pulsating energy growth rate and spatial distribution positions of these spectra, it is speculated that the breakage of the flow structure corresponding to 400 Hz and 5500 Hz is the main reason for the boundary layer transition.