Abstract
Active grids are commonly used in wind tunnels to generate turbulence with different characteristic features. In contrast to the common objective to generate turbulence with a very high Reynolds number, this work focuses on a method of blockage induced flow design for the generation of special flow structures. Particularly, we aim to investigate the underlying constraints of this excitation method. For this purpose, the scale dependency of the excitation is studied by clearly defined structures such as periodic sinusoidal velocity variations, velocity steps, and single gusts. It is shown that the generation process is limited by the reduced frequency of the active grid motion. For low values of reduced frequencies the imprinted flow structures remain undamped, whereas for higher reduced frequencies they are damped. This insight leads to the constraint that the active grid motion needs to be modified to compensate for the underlying dynamic damping effects. Thus, the inserted energy has to be increased for the corresponding reduced frequencies. This finding can be transferred to the generation of turbulent flows, for which an exemplary adaption is shown .Graphic abstract
Highlights
For many engineering problems the impact of turbulence is of high relevance
The first approach to generate turbulent flows in a wind tunnel was made by applying a simple regular grid to the nozzle (Simmons and Salter 1934)
To allow for the generation of defined flow conditions, the dependence between wind velocity behind the active grid and shaft angle needs to be determined. This relation between active grid and resulting flow is expressed by a transfer function, which we determine with the following procedure (see Reinke et al (2017)): For a constant wind tunnel fan speed the
Summary
For many engineering problems the impact of turbulence is of high relevance. Quite often, like for wind turbines, the environmental conditions are highly unsteady. The change from passive to actively controlled grid elements allowed to generate a variety of new flow conditions and to increase the Reynolds numbers significantly This was a big step, as high Reynolds numbers could be generated in smaller wind tunnels. Hearst and Lavoie (2015) performed a comprehensive study of the initial conditions for active grid excitation and found that the highest homogeneity of the flow was generated by this double-random asynchronous mode. Weitemeyer et al (2013) introduced an operation of the active grid where they kept the global blockage constant, while changing the local blockage This approach was further used for the direct reproduction of measured atmospheric wind by using a transfer function of the local blockage of the grid and the velocity behind it (see Reinke et al 2017). The outcome of the investigation is further used to show a possible application to more complex turbulence time series (Sect. 7)
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