Abstract:We simultaneously measure both spinning and tumbling components of rotation for long near-neutrally buoyant fibers in homogeneous and isotropic turbulence. The lengths and diameters of the measured fibers extend to several orders of the Kolmogorov length of the surrounding turbulent flow. Our measurements show that the variance of the spinning rate follows a −4/3 power-law scaling with the fiber diameter (d) and is always larger than the variance of the tumbling rate. This behavior surprisingly resembles that … Show more
“…The mean square angular velocity of spheroids with different shapes show a clear scaling behaviour with an exponent , which agrees with predictions based on the K41 theory and with the available experiments (Parsa & Voth 2014; Bordoloi & Variano 2017; Oehmke et al. 2021). However, it turns out that the ratio of the mean square spinning rate to the mean square tumbling rate shows a systematic decrease as the particle size increases, and this is for all considered aspect ratios.…”
Section: Discussionsupporting
confidence: 89%
“…The inset shows the autocorrelation function of a single Cartesian acceleration component.Figure 12.Ratio of spinning rate variance over tumbling rate variance: comparison of present DNS results at with the experimental measurements by Oehmke et al. (2021) at . ( a ) Particle rotation ratio as a function of aspect ratio, here the colour code maps the particle size in equivalent radius units.…”
Section: Tablementioning
confidence: 79%
“…The experiments by Oehmke et al. (2021), the only ones to date to have measured both the spinning and tumbling of finite-size particles, find that, for prolate fibres, the variance of the spinning rate is always larger than the variance of the tumbling rate. These authors comment that the observed behaviour ‘surprisingly resembles that observed previously for sub-Kolmogorov fibers’.…”
Section: Resultsmentioning
confidence: 99%
“…This choice of representation is adapted from figure 5( b ) in Oehmke et al. (2021). We observe here an excellent agreement for the behaviour of all prolate particles.…”
Section: Tablementioning
confidence: 99%
“…Such a picture has even been extended to the case of non-spherical axisymmetric particles with various aspect ratios to elucidate the observed different scaling trends in tumbling and spinning rotation rates (Parsa & Voth 2014; Bordoloi & Variano 2017; Bounoua, Bouchet & Verhille 2018; Kuperman, Sabban & van Hout 2019; Pujara, Voth & Variano 2019; Oehmke et al. 2021). This interpretation assumes that neutrally buoyant material particles are only weakly coupled to the flow, so that the feedback effects (also referred as two-way coupling) stemming from the formation of boundary layers and wakes around the particle do not play a significant role, at least in statistical terms.…”
We study the translational and rotational dynamics of neutrally buoyant finite-size spheroids in hydrodynamic turbulence by means of fully resolved numerical simulations. We examine axisymmetric shapes, from oblate to prolate, and the particle volume dependences. We show that the accelerations and rotations experienced by non-spherical inertial-scale particles result from volume filtered fluid forces and torques, similar to spherical particles. However, the particle orientations carry signatures of preferential alignments with the surrounding flow structures, which are reflected in distinct axial and lateral fluctuations for accelerations and rotation rates. The randomization of orientations does not occur even for particles with volume-equivalent diameter size in the inertial range, here up to
$60$
dissipative units (
$\eta$
) at Taylor-scale Reynolds number
${Re_{\lambda }=120}$
. Additionally, we demonstrate that the role of fluid boundary layers around the particles cannot be neglected in reaching a quantitative understanding of particle statistical dynamics, as they affect the intensities of the angular velocities and the relative importance of tumbling with respect to spinning rotations. This study brings to the fore the importance of inertial-scale flow structures in homogeneous and isotropic turbulence and their impacts on the transport of neutrally buoyant bodies with sizes in the inertial range.
“…The mean square angular velocity of spheroids with different shapes show a clear scaling behaviour with an exponent , which agrees with predictions based on the K41 theory and with the available experiments (Parsa & Voth 2014; Bordoloi & Variano 2017; Oehmke et al. 2021). However, it turns out that the ratio of the mean square spinning rate to the mean square tumbling rate shows a systematic decrease as the particle size increases, and this is for all considered aspect ratios.…”
Section: Discussionsupporting
confidence: 89%
“…The inset shows the autocorrelation function of a single Cartesian acceleration component.Figure 12.Ratio of spinning rate variance over tumbling rate variance: comparison of present DNS results at with the experimental measurements by Oehmke et al. (2021) at . ( a ) Particle rotation ratio as a function of aspect ratio, here the colour code maps the particle size in equivalent radius units.…”
Section: Tablementioning
confidence: 79%
“…The experiments by Oehmke et al. (2021), the only ones to date to have measured both the spinning and tumbling of finite-size particles, find that, for prolate fibres, the variance of the spinning rate is always larger than the variance of the tumbling rate. These authors comment that the observed behaviour ‘surprisingly resembles that observed previously for sub-Kolmogorov fibers’.…”
Section: Resultsmentioning
confidence: 99%
“…This choice of representation is adapted from figure 5( b ) in Oehmke et al. (2021). We observe here an excellent agreement for the behaviour of all prolate particles.…”
Section: Tablementioning
confidence: 99%
“…Such a picture has even been extended to the case of non-spherical axisymmetric particles with various aspect ratios to elucidate the observed different scaling trends in tumbling and spinning rotation rates (Parsa & Voth 2014; Bordoloi & Variano 2017; Bounoua, Bouchet & Verhille 2018; Kuperman, Sabban & van Hout 2019; Pujara, Voth & Variano 2019; Oehmke et al. 2021). This interpretation assumes that neutrally buoyant material particles are only weakly coupled to the flow, so that the feedback effects (also referred as two-way coupling) stemming from the formation of boundary layers and wakes around the particle do not play a significant role, at least in statistical terms.…”
We study the translational and rotational dynamics of neutrally buoyant finite-size spheroids in hydrodynamic turbulence by means of fully resolved numerical simulations. We examine axisymmetric shapes, from oblate to prolate, and the particle volume dependences. We show that the accelerations and rotations experienced by non-spherical inertial-scale particles result from volume filtered fluid forces and torques, similar to spherical particles. However, the particle orientations carry signatures of preferential alignments with the surrounding flow structures, which are reflected in distinct axial and lateral fluctuations for accelerations and rotation rates. The randomization of orientations does not occur even for particles with volume-equivalent diameter size in the inertial range, here up to
$60$
dissipative units (
$\eta$
) at Taylor-scale Reynolds number
${Re_{\lambda }=120}$
. Additionally, we demonstrate that the role of fluid boundary layers around the particles cannot be neglected in reaching a quantitative understanding of particle statistical dynamics, as they affect the intensities of the angular velocities and the relative importance of tumbling with respect to spinning rotations. This study brings to the fore the importance of inertial-scale flow structures in homogeneous and isotropic turbulence and their impacts on the transport of neutrally buoyant bodies with sizes in the inertial range.
The aim of this study is to investigate experimentally the transition from a rigid regime to a deformed regime for flexible discs freely advected in turbulent flows. For a given disc, the amplitude of the deformation is expected to increase when its bending modulus decreases or when the turbulent kinetic energy increases. To quantify this qualitative argument, experiments are performed where the deformation of flexible discs is measured using three cameras. The amplitude of the deformation has been characterised by the eigenvalues of the moment of inertia tensor. Experimental results exhibit a transition from a rigid regime to a deformed regime that depends on the size, the density and the flexibility of the disc and the turbulent kinetic energy. The modelling of this transition is a generalisation and an extension of the previous models used to characterise the deformation of flexible fibres in turbulent flows.
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