Homogeneity and isotropy in a laboratory turbulent flow

Bellani, Gabriele; Variano, Evan

doi:10.1007/s00348-013-1646-8

Cited by 45 publications

(78 citation statements)

References 44 publications

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“…Recently, in Bellani & Variano (2013) two RJA were separated by a distance of 162 cm, and faced each other. They used the same firing algorithm as proposed for the case of a single RJA (Variano & Cowen 2008), resulting in a nearly HIT with a negligible mean flow at the middle region of the tank.…”

Section: Generation Of Homogeneous Turbulencementioning

confidence: 99%

“…Following a similar approach as in Bellani & Variano (2013), Carter et al (2016) presented a facility in air consisting of two planar arrays of quasi-synthetic jets (256 in total) creating the largest region of homogeneous turbulence to date. Contrary to what was found in Bellani & Variano (2013), they observed anisotropy at large scales for all configurations tested.…”

Section: Generation Of Homogeneous Turbulencementioning

confidence: 99%

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Laboratory experiments on the temporal decay of homogeneous anisotropic turbulence

2019

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We experimentally investigate the temporal decay of homogeneous anisotropic turbulence, monitoring the evolution of velocity fluctuations, dissipation and turbulent length scales over time. We employ an apparatus in which two facing random jet arrays of water pumps generate turbulence with negligible mean flow and shear over a volume that is much larger than the initial characteristic turbulent large scale of the flow. The Reynolds number based on the Taylor microscale for forced turbulence is $Re_{\unicode[STIX]{x1D706}}\approx 580$ and the axial-to-radial ratio of the root mean square velocity fluctuations is $1.22$. Two velocity components are measured by particle image velocimetry at the symmetry plane of the water tank. Measurements are taken for both ‘stationary’ forced turbulence and natural decaying turbulence. For decaying turbulence, power-law fits to the decay of turbulent kinetic energy reveal two regions over time; in the near-field region ($t/t_{L}<10$, $t_{L}$ is the integral time scale of the forced turbulence) a decay exponent $m\approx -2.3$ is found whereas for the far-field region ($t/t_{L}>10$) the value of the decay exponent was found to be affected by turbulence saturation. The near-field exhibits features of non-equilibrium turbulence with constant $L/\unicode[STIX]{x1D706}$ and varying $C_{\unicode[STIX]{x1D716}}$ (dissipation constant). We found a decay exponent $m\approx -1.4$ for the unsaturated regime and $m\approx -1.8$ for the saturated regime, in good agreement with previous numerical and experimental studies. We also observe a fast evolution towards isotropy at small scales, whereas anisotropy at large scales remains in the flow over more than $100t_{L}$. Direct estimates of dissipation are obtained and the decay exponent agrees well with the prediction $m_{\unicode[STIX]{x1D716}}=m-1$ throughout the decay process.

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Section: Generation Of Homogeneous Turbulencementioning

confidence: 99%

Section: Generation Of Homogeneous Turbulencementioning

confidence: 99%

Laboratory experiments on the temporal decay of homogeneous anisotropic turbulence

2019

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“…Experiments are performed in a turbulent water tank, described in detail in [17][18][19]. The flow is driven by two symmetric arrays of randomly firing synthetic jets, which create homogeneous isotropic turbulence with almost zero mean flow.…”

Section: Facilitymentioning

confidence: 99%

“…For the experiments reported herein, we have modified the tank relative to that which is described in [17][18][19]. We have added two screens with 5 mm mesh in the x-y plane, to help confine the particle motion to the central region of the tank.…”

Section: Facilitymentioning

confidence: 99%

Slip velocity of large neutrally buoyant particles in turbulent flows

Bellani

Variano

2012

New J. Phys.

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We discuss possible definitions for a stochastic slip velocity that describes the relative motion between large particles and a turbulent flow. This definition is necessary because the slip velocity used in the standard drag model fails when particle size falls within the inertial subrange of ambient turbulence. We propose two definitions, selected in part due to their simplicity: they do not require filtration of the fluid phase velocity field, nor do they require the construction of conditional averages on particle locations. A key benefit of this simplicity is that the stochastic slip velocity proposed here can be calculated equally well for laboratory, field and numerical experiments. The stochastic slip velocity allows the definition of a Reynolds number that should indicate whether large particles in turbulent flow behave (a) as passive tracers; (b) as a linear filter of the velocity field; or (c) as a nonlinear filter to the velocity field. We calculate the value of stochastic slip for ellipsoidal and spherical particles (the size of the Taylor microscale) measured in laboratory homogeneous isotropic turbulence. The resulting Reynolds number is significantly higher than 1 for both particle shapes, and velocity statistics show that particle motion is a complex nonlinear function of the fluid velocity. We further investigate the nonlinear relationship by comparing the probability distribution of fluctuating velocities for particle and fluid phases.

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“…However, as summarized by Wiederseiner et al, 27 RIM techniques require strict experimental controls (e.g., mixing of different liquids, temperature control, and humidity control). These experimental controls are difficult to establish in turbulent flows, where relatively large scale facilities are commonly used, 20,28,29 thus limiting the feasibility of RIM techniques for the study of high Reynolds number turbulence.…”

Section: Introductionmentioning

confidence: 99%

Experimental study of turbulence decay in dense suspensions using index-matched hydrogel particles

Zhang

Rival

2018

Physics of Fluids

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In the present study, a refractive-index matching (RIM) technique using hydrogel particles was developed to quantitatively measure turbulence characteristics in dense suspensions. Compared to classic RIM methods, the use of superabsorbent polymer (SAP) material significantly simplifies experimental procedures and avoids strict experimental controls, which makes the method particularly suitable for turbulence measurements in dense suspensions. Because of the high absorbency of the approximately 1 mm SAP particles, optical visibility is achieved even in dense suspensions on the order of 20% by volume. Furthermore, the small hydrogel particle diameter allows for a particle diameter-to-integral scale ratio value of 1/20. The new method is then used to reveal the flow characteristics in decaying turbulence with suspension volume fractions up to 18.4% (the measurements pass through approximately 85 hydrogel particle-water interfaces). Evidence of turbulence attenuation in suspensions is demonstrated and attributed to the inhibition of turbulence production in said suspensions. The modulations in turbulence decay are apparent even in low suspension volume fractions (2.3%), whereas the turbulence characteristics of suspensions at higher volume fractions of 9.2% and 18.4% are observed to converge on each other.

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Homogeneity and isotropy in a laboratory turbulent flow

Cited by 45 publications

References 44 publications

Laboratory experiments on the temporal decay of homogeneous anisotropic turbulence

Laboratory experiments on the temporal decay of homogeneous anisotropic turbulence

Slip velocity of large neutrally buoyant particles in turbulent flows

Experimental study of turbulence decay in dense suspensions using index-matched hydrogel particles

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