A planar jet issuing from a fully developed two-dimensional turbulent channel flow is studied, with a focus on the transverse flapping of the jet core. The streamwise and transverse velocities were measured with hot-wire anemometry using an X-type probe. The mean velocity field and the velocity covariances were first characterised to assess the undisturbed flow field. Periodic excitations were introduced from a slot mounted at the channel exit and the coherent fluctuating part of the signal was obtained by using a phase-locked averaging technique, where the periodic initial forcing was used as trigger. This enabled the eduction of the coherent structure associated with the introduced perturbation. Its amplitude was found to be directly proportional to the intensity of the initial forcing and, within a certain range of the initial forcing amplitude, the growth curves were identical as well as the spatial distribution of the extracted fluctuations. Parallel and non-parallel linear stability theory captures qualitatively and quantitatively the features of the educed coherent structure. The existence of the linear mode in the turbulent jet implies that the large-scale perturbations observed in natural (unforced) jets can be regarded as an incoherent set of linear modes.

中文翻译：

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平面湍流射流中的线性模式

研究了从完全发展的二维湍流通道流中发出的平面射流，重点是射流核心的横向拍动。流向和横向速度是使用 X 型探头通过热线风速测量法测量的。平均速度场和速度协方差首先被表征以评估未受干扰的流场。从安装在通道出口处的槽引入周期性激励，并通过使用锁相平均技术获得信号的相干波动部分，其中使用周期性初始强迫作为触发。这使得推导出与引入的扰动相关的相干结构成为可能。发现其振幅与初始强迫的强度成正比，并且在初始强迫振幅的一定范围内，生长曲线和提取波动的空间分布相同。平行和非平行线性稳定性理论定性和定量地捕捉导出的相干结构的特征。湍流射流中线性模式的存在意味着在自然（非强制）射流中观察到的大规模扰动可以被​​视为一组不连贯的线性模式。