Abstract
The influence of upstream turbulence on the flow produced by a plane jet is investigated experimentally with hot-wire anemometry and smoke flow visualisation. An innovative active grid, where each wing can be independently controlled, is used to change the upstream turbulence conditions. Three cases are investigated: a canonical reference case, a case with the same integral scale as the reference case but an order of magnitude increase in turbulence intensity, and a case with both an order of magnitude increase in turbulence intensity and an order of magnitude increase in integral scale compared to the reference case. It is demonstrated that the wake width increases with turbulence intensity, but decreases with integral scale for constant turbulence intensity. In addition, the positional variability of the wake width is highest with high turbulence intensity and a short integral scale. Along the jet centreline, the potential core region is shorter with elevated upstream turbulence intensity; this is reflected in both the mean velocity and the variance. The decay of the centreline mean velocity is also retarded by incoming turbulence. In all, increased incoming turbulence results in increased jet spreading, and a shorter integral scale further increases the spreading.Graphic abstract
Highlights
The plane jet is a canonical flow of both historical and contemporary significance
Changing the rotational speed of the wings resulted primarily in changes to the integral scale ( Luu ) at the jet exit, while the turbulence intensity ( u 0∕Ub ) and Taylor microscale Reynolds number ( Re = ⟨u 2⟩1∕2 ∕ ) remained approximately constant. This contrasts with typical observations for active grids in wind tunnel flows where both the turbulence intensity and integral scale are strongly affected by ± (Larssen and Devenport 2011; Hearst and (a)
For the active grid cases (Fig. 11a and b), there is a peak in the spectra that corresponds to the mean rotational rate of the wings, demonstrating that the periodicity imposed on the flow upstream remains present in the shear layers
Summary
Experiments in Fluids (2021) 62:18 for the development of analytical expressions for its evolution that can be validated against experiments, c.f., George (1989); Pope (2000); Cafiero and Vassilicos (2019). Cafiero et al (2015a) investigated the effect of placing a square fractal grid at the nozzle exit and measured up to a position approximately 10D downstream They showed that this grid enhanced the turbulence intensity downstream, roughly behaving similar to the partial mesh of Rajagopalan et al (2013) instead of the full meshes or the control rings. Koochesfahani and Dimokaris (1989) investigated the effects of disturbance on the plane mixing layer by pitching an airfoil within it They found that the growth rate of shear layers was dependent on the forcing frequency and generally low frequencies resulted in smaller wake widths. The active grid is used because it can produce high levels of turbulence and orders of magnitude changes in the length scales (Larssen and Devenport 2011; Hearst and Lavoie 2015)
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