The decay of turbulence generated by a class of multiscale grids

Abstract

A new experimental investigation of decaying turbulence generated by a low-blockage space-filling fractal square grid is presented. We find agreement with previous works ) but also extend the length of the assessed decay region and consolidate the results by repeating the experiments with different probes of increased spatial resolution. It is confirmed that this moderately high Reynolds number Re λ turbulence (up to Re λ 350 here) does not follow the classical high Reynolds number scaling of the dissipation rate ε ∼ u 3 /L and does not obey the equivalent proportionality between the Taylor-based Reynolds number Re λ and the ratio of integral scale L to the Taylor microscale λ. Instead we observe an approximate proportionality between L and λ during decay. This non-classical behaviour is investigated by studying how the energy spectra evolve during decay and examining how well they can be described by self-preserving single-length-scale forms. A detailed study of homogeneity and isotropy is also presented which reveals the presence of transverse energy transport and pressure transport in the part of the turbulence decay region where we take data (even though previous studies found mean flow and turbulence intensity profiles to be approximately homogeneous in much of the decay region). The exceptionally fast turbulence decay observed in the part of the decay region where we take data is consistent with the non-classical behaviour of the dissipation rate. Measurements with a regular square mesh grid as well as comparisons with active-grid experiments by Mydlarski & Warhaft (J. are also presented to highlight the similarities and differences between these turbulent flows and the turbulence generated by our fractal square grid.

The structure of turbulent flames in fractal- and regular-grid-generated turbulence

“…The energy spectra of the FG2, on the other hand, showed a good collapse for all frequencies. This behavior is in agreement with previous measurements of fractal grid generated turbulence [3][4][5][6] and indicative of a single length scale decay [43,44]. Recently however, Valente [46] pointed out that a purely visual validation of the concept is not sufficient to prove a single length scale decay of turbulence.…”

“…In an attempt to demonstrate whether this behaviour can also be observed in our data, we followed the approach of Valente et al [6] and plot in Fig. 7 the one-dimensional compensated energy spectra.…”

“…This particularity was pointed out in ref. [3,5] and recently verified by Valente et al [6] using extensive wind-tunnel data.…”

“…This was first noticed by Hurst et al [3] and is extensively discussed in [4,6] which is why we do not further comment on it here. Secondly, and maybe more important for combustion applications, it means that in principal the level of turbulence intensity can be chosen independently of the length scale of the flow by moving to different locations downstream of the fractal grid.…”

“…Vassilicos et al [3][4][5][6] proposed fractal grids as a new type of low blockage turbulence generators with a number of potential applications [7][8][9][10][11][12][13][14]. Fractal grids consist of structures with multiple length scales rather than one length scale.…”