A scanning PIV method for fine-scale turbulence measurements

Abstract

mechanics. It is a desirable aim, as the fine scales are responsible for (amongst other things) the dissipation of kinetic energy and chemical mixing (Tsinober 2009) and have an important role in phenomena such as cloud formation (Bodenschatz et al. 2010) and flame extinction (Sreenivasan 2004). The challenge stems from the fact that as the Reynolds number increases, the smallest length-and timescales become smaller and faster, requiring significant measurement or computational capability to capture even low Reynolds number flows.For laboratory investigations of turbulence, it would be valuable to have an accurate, three-dimensional, three-component (3D-3C) velocimetry technique with good space and time resolution. Of particular importance is access to the full velocity gradient tensor (VGT), as it embodies the fine scales of turbulence and provides direct access to the turbulent kinetic energy dissipation rate (Wallace 2009). Because of the wide range of lengthscales involved, a large spatial dynamic range (SDR, the ratio of the smallest measureable lengthscale to the largest) is also valuable.Hot wire probes with nine, twelve or even twenty wires (Wallace 2009) offer access to the full VGT with good temporal resolution, but are invasive, complicated to calibrate and only offer pointwise measurements. Dual plane PIV can offer a 3D-3C measurement with good accuracy, time and spatial resolution (Mullin and Dahm 2006), but the setup is complex: it requires four cameras and some means of differentiating between planes, usually chromatically by using two different wavelength laser sources. Furthermore, the depth of the measurement volume is very limited (i.e. between two planes), and the full VGT is only available in one plane. Cinematographic stereo PIV can be used to yield snapshots of 3D-3C velocity fields (Ganapathisubramani et al. 2007), but is only applicable to convective flows as it relies upon Taylor's frozen turbulence hypothesis.Abstract A hybrid technique is presented that combines scanning PIV with tomographic reconstruction to make spatially and temporally resolved measurements of the fine-scale motions in turbulent flows. The technique uses one or two high-speed cameras to record particle images as a laser sheet is rapidly traversed across a measurement volume. This is combined with a fast method for tomographic reconstruction of the particle field for use in conjunction with PIV cross-correlation. The method was tested numerically using DNS data and with experiments in a large mixing tank that produces axisymmetric homogeneous turbulence at R ≃ 219. A parametric investigation identifies the important parameters for a scanning PIV setup and provides guidance to the interested experimentalist in achieving the best accuracy. Optimal sheet spacings and thicknesses are reported, and it was found that accurate results could be obtained at quite low scanning speeds. The two-camera method is the most robust to noise, permitting accurate measurements of the velocity gradients and direct determination of the di...

On velocity gradient dynamics and turbulent structure

“…This large-scale anisotropy persists to the fine scales, where ratios of second-order moments of velocity gradients are consistent with local axisymmetry rather than isotropy (Lawson & Dawson 2014 Ishihara et al (2007) and the snapshot of data taken from the JHU database.…”

“…This paper draws upon two datasets: an experimental dataset of homogeneous, axisymmetric turbulence at Re λ 180 reported in Lawson & Dawson (2014), and a DNS dataset of homogeneous, isotropic turbulence in a periodic cube at Re λ 433 from the Johns Hopkins University (JHU) turbulence database (Li et al 2008). Relevant parameters of both datasets are described in table 1.…”

“…3.1. The French washing machine experiment A brief description of the experimental method is necessary: a full exposition is available in Lawson & Dawson (2014). A hybrid scanning tomographic particle image velocimetry (PIV) system was developed and used to acquire 1003 independent, volumetric, time-series measurements at the centre of a large dodecagonal water tank that was stirred by two counter-rotating impellers in an arrangement commonly known as a 'French washing machine'.…”

“…These were post-processed to obtain the pressure field and to reduce measurement noise. Each velocity field was corrected to remove the bias error introduced by scanning (Lawson & Dawson 2014). Simulated passive tracers, which advect without slip asẊ(t) = u(X(t), t), are tracked numerically through the volume, starting from grid collocation points X(0) = x 0 .…”

New insights into the fine-scale structure of turbulence

“…This generates an axisymmetric turbulent flow at Re λ ≈ 180 with a Kolmogorov length scale of η ≈ 0.93 mm and a Kolmogorov time scale of τ η ≈ 0.88 s. The advantage of such a large experimental set-up is that even the finest scales of turbulence are still accessible to particle image velocimetry (PIV) techniques. For the current study, a hybrid PIV method recently introduced by Lawson & Dawson (2014) is used, which combines scanning PIV with tomographic reconstruction. The experimental results…”