Abstract
Abstract. We present a new system for the generation of rapid, strong flow perturbations in the aerodynamic wind tunnel at École Centrale de Nantes. The system is called the chopper, and it consists of a rotating bar cutting through the inlet of a wind tunnel test section, thus generating an inverse gust that travels downstream. The flow generated by the chopper is investigated with respect to the rotational frequency using an array equipped with hot-wires that is traversed downstream in the flow field. It is found that the gust can be described as a superposition of the mean gust velocity, an underlying gust shape, and additional turbulence. Following this approach, the evolution of the mean gust velocity and turbulence intensity are presented, and the evolution of the underlying inverse gust shape is explained. The turbulence is shown to be characterized by an integral length scale of approximately half the chopper blade width and a turbulence decay according to E(f)∝f-5/3.
Highlights
The atmospheric boundary layer is naturally turbulent, and these turbulence conditions are highly complex and nonstationary
To gain a better understanding of the inverse gust structure itself, in the step, the gust events were brought into focus. It was shown how the inverse gust has an underlying shape with superimposed fluctuations, so that each inverse gust can be decomposed into a mean gust velocity u, the global underlying inverse gust shape u, and the high-frequency fluctuations u
The decomposition can be carried out in the spectral domain. This triple decomposition was used to discuss the evolution of the underlying inverse gust shape that disperses downstream while the average gust velocity simultaneously increases and the global turbulence intensity decreases
Summary
The atmospheric boundary layer is naturally turbulent, and these turbulence conditions are highly complex and nonstationary. While the term “gust” is most commonly used for a strong change of the wind velocity in the stream-wise direction (see, e.g., Bardal and Sætran, 2016; Letson et al, 2019), gusts can occur in the transverse direction, for example in the form of thunderstorm downbursts (see, e.g., Nguyen and Manuel, 2014), or in the form of sudden wind direction changes. This shows that the working conditions in the lower part of the atmospheric boundary layer are challenging.
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