Effect of the driving algorithm on the turbulence generated by a random jet array

Abstract

Different driving algorithms for a large random jet array (RJA) were tested and their performance characterized by comparing the statistics of the turbulence generated downstream of the RJA. Of particular interest was the spatial configuration of the jets operating at any given instant (an aspect that has not been documented in previous RJAs studies), as well as the statistics of their respective on/off times. All algorithms generated flows with non-zero skewnesses of the velocity fluctuation normal to the plane of the RJA (identified as an inherent limitation of the system resulting from the unidirectional forcing imposed from only one side of the RJA), and slightly super-Gaussian kurtoses of the velocity fluctuations in all directions. It was observed that algorithms imposing spatial configurations generated the most isotropic flows, however they suffered from high mean flows and low turbulent kinetic energies. The algorithm identified as RANDOM generated the flow that, on an overall basis, most closely approximated zero-mean-flow homogeneous isotropic turbulence, with variations in horizontal and vertical homogeneities of RMS velocities of no more than 6%, deviations from isotropy (wRMS/uRMS) in the range of 0.62-0.77, and mean flows on the order of 7% of the RMS velocities (determined by averaging their absolute values over the three velocity components and three downstream distances). A relatively high turbulent Reynolds number (ReT = uT ℓ/ν = 2360, where ℓ is the integral length scale of the flow and uT is a characteristic RMS velocity) was achieved using the RANDOM algorithm and the integral length scale (ℓ = 11.5 cm) is the largest reported to date. The quality of the turbulence in our large facility demonstrates the ability of RJAs to be scaled-up and to be the laboratory system most capable of generating the largest quasihomogeneous isotropic turbulent regions with zero mean flow.

Laboratory experiments on the temporal decay of homogeneous anisotropic turbulence

“…i = j. Therefore, based on this result and the axisymmetric nature of the jet forcing around the x-axis reported in previous studies (Variano & Cowen 2008;Bellani & Variano 2013;Carter et al 2016;Alvarado et al 2016), we estimate the TKE dissipation rate following the equation derived in George & Hussein (1991) for local axisymmetric turbulence, = ν − s 2 11 + 2 s 2 12 + 2 s 2 21 + 8 s 2…”

A turbulent quarter century of active grids: from Makita (1991) to the present

“…But the conception of the RJA, in which an array (or grid) of pumps operates randomly, was clearly "inspired by the extremely successful active wind tunnel grid" (Variano et al, 2004). Its development has inspired the construction of others (see Asher and Litchendorf, 2009, Bellani and Variano, 2013, Khorsandi et al, 2013, Pérez-Alvarado et al, 2016, Carter et al, 2016, and it can be readily argued that RJAs will replace oscillating-grid turbulence in the generation of zero-mean-flow homogeneous, isotropic turbulence.…”

Sediment suspension and bed morphology in a mean shear free turbulent boundary layer

“…In random jet array driven facilities, a ratio H/J > 6 is required to ensure full mixing of the jets (Variano & Cowen 2008;Perez-Alvarado et al 2016). Prior to achieving a distance from the jets equal to six times the jet spacing, or roughly H − 6J < z < H, the jets are still merging and the flow is dominated by instantaneous jet activity driving distinct upward and downward flows.…”

“…Indeed, there is a growing body of literature that explores boundary layers that result from highly turbulent flows in the absence of mean shear altogether. Common methods of considering flows with negligible mean shear include, but are not limited to, channel flows along moving beds (Uzkan & Reynolds 1967;Thomas & Hancock 1977;Hunt & Graham 1978), mixing boxes via active grids (Rouse & Dodu 1955;Thompson & Turner 1975;Hopfinger & Toly 1976;McDougall 1979;Brumley & Jirka 1987;McKenna & McGillis 2004;McCorquodale & Munro 2017), stirring via jet arrays (Variano, Bodenschatz & Cowen 2004;Perez-Alvarado, Mydlarski & Gaskin 2016;Johnson & Cowen 2018), or insertions of non-fluid interfaces in developed or developing turbulent flows (Perot & Moin 1995a;Teixeira & Belcher 2000).…”