Contents lists available at ScienceDirect International Journal of Heat and Fluid Flow journal homepage: www.elsevier.com/locate/ijhff Accuracy of tomographic particle image velocimetry data on a turbulent round jet M. Khashehchi a, ⁎ , Z. Harun b a Department of Agro-Technology, College of Aburaihan, University of Tehran, Tehran, Iran b Faculty of Engineering and Built Environment, National University of Malaysia, UKM, Bangi 43600, Malaysia ARTICLE INFO Keywords: Round turbulent jet Tomographic Particle Image Velocimetry Planar PIV VGT ABSTRACT Tomographic particle image velocimetry (Tomo-PIV) was applied on a turbulent round air jet to quantitatively assess the accuracy of velocity gradients obtained in the self-similar turbulent region. The jet Reynolds number based on the nozzle diameter (d) was Re d = 3000. Mean velocity, turbulent intensities, and Reynolds shear stress at the center plane of the jet were measured. In addition, statistical results of Tomo-PIV along the axial direction were assessed by performing a separate set of two-dimensional two-component PIV experiments on a “side view” plane along the jet axis. Moreover, the probability distribution functions of four components of the measured velocity gradients in the axial and radial directions were validated by these “side view” planar PIV data. The root mean square of the velocity divergence values relative to the norm of the velocity gradient tensor was 0.36. Furthermore, the on- and off-diagonal components of the velocity gradients satisfied the axisymmetric isotropy conditions. The divergence error in the data affected only areas with low gradient magnitude. Therefore, tur- bulent structures in the regions with intense vorticity and dissipation can be closely monitored. On this basis, the joint pdfs of the invariants of the velocity gradient and strain and rotation tensor rates were produced and compared well with those in isotropic turbulence studies. 1. Introduction Access to three-dimensional (3D) velocity fields enables us to re- cognize the dynamics and physical structures of the turbulent shear layers of jets. Moreover, understanding the turbulence mechanisms that significantly alter the global behavior of such flows requires full access to the 3D structures and dynamics of the intermediate and small scales of the flows. In such turbulent flows, dynamics of 3D vortex structures are essential for heat and mass transfer (Yule, 1978; Liepmann et al., 1992). The generation and interconnections of vortex structures in the developing region of jets are the main sources of acoustic noise (Violato et al., 2011). Although direct numerical simulations (DNS) and large eddy si- mulations are the main sources of high-resolution instantaneous 3D flow information, these methods require a verification procedure and substantial computing cost for flows in complex geometries. The data are instead obtained from experimental techniques, such as particle tracking velocimetry (Maas et al., 1993), dual-plane stereoscopic PIV (DSPIV) (Kahler, 2004; Ganapathisubramani et al., 2005), holographic particle image velocimetry (Meng and Hussain, 1995; Zhang et al., 1997), scalar image velocimetry (Su and Dahm, 1996), and tomographic particle image velocimetry (Tomo-PIV) (Elsinga, 2006), and quality of data has improved significantly during the last two decades. In cases with sufficient optical access, Tomo-PIV provides the information of the instantaneous spatial distribution of all three velo- city vectors in highly complex flow fields. In this technique, simulta- neous projections of illuminated particles are used to reconstruct the light intensity distribution by means of optical tomography. Tomo-PIV also provides a denser instantaneous velocity field than other 3D three- component (3D3C) measurement techniques. Worth and Nickels (2008), Atkinson and Soria (2009), Hang-Yu et al. (2017), and Shengxian et al. (2018) further outlined this reconstructive method. This technique obtains a considerable number of velocity vectors within a volume of interest and provides a denser instantaneous velocity field than other 3D3C measurement techniques. Moreover, turbulent quan- tities that require all three velocity components (e.g., Reynolds stresses and turbulent intensities) or velocity gradients (e.g., velocity diver- gence and vorticity) can be characterized by Tomo-PIV data. Vector fields obtained from Tomo-PIV experiments contribute to gaining access to the velocity gradient tensor (VGT), thereby resulting in the identification of evidence regarding the dynamics and char- acteristics of flows. The perspective of 3D experimental measurements https://doi.org/10.1016/j.ijheatfluidflow.2019.03.005 Received 22 December 2018; Received in revised form 16 February 2019; Accepted 15 March 2019 ⁎ Corresponding author. E-mail address: m.khashehchi@ut.ac.ir (M. Khashehchi). International Journal of Heat and Fluid Flow 77 (2019) 61–72 0142-727X/ © 2019 Elsevier Inc. All rights reserved. T