3D Flow Dynamics in a Turbulent Slot Jet: Time-resolved Tomographic PIV Measurements (original) (raw)
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Tomographic PIV and Planar Time-resolved PIV Measurements in a Turbulent Slot Jet
Shestakov Maxim
The structure of a quasi-two-dimensional turbulent jet in a narrow channel was investigated experimentally in this work. Study of spatio-temporal flow structure was produced by the Time-resolved PIV technique. The 3D flow structure in the near field of the jet was studied by Tomographic PIV. Secondary flows in the bounded jet were obtained by Tomographic PIV for the first time.
2014
An experimental study is presented for turbulent flow control in backward-facing step (BFS) flow using timeresolved tomographic particle image velocimetry (TR tomo-PIV) and high-resolution 2D-2C-PIV in the 1-m low speed wind tunnel at DLR in Göttingen, Germany. Vortex generators were designed and implemented on a BFS with an oncoming zero pressure gradient turbulent boundary layer flow. The investigated passive flow control devices were wedge-shaped, backward-oriented vortex generators which were transversely mounted upstream of the step. The Reynolds number was Re h = 2.0×10 4 , based on the free stream velocity U ∞ = 10 m/s and the step height h = 30 mm. TR tomo-PIV was employed to measure the separated shear layer flow within a measurement volume of 50×60×10 mm 3 in x-, y-and z-directions respectively, which was located right behind the step and above the plate. Four high-speed CMOS cameras were used to record image pairs at a sampling rate of 1 kHz. In order to evaluate the effect of the given flow control devices on the reattachment length, additional high-resolution 2D-2C-PIV was applied to a streamwisevertical measurement plane downstream of the step. The time-averaged reattachment length was reduced by 29.1%, which proved that the vortex generators maintain the ability to attenuate separated flow regions. For analysis of spatial and temporal evolutions of the vortices, two-point cross-correlation functions revealed scales of turbulence and identified a relative constant convective velocity U c ≈ 0.5•U ∞. Reynolds shear stress and turbulent kinetic energy were increased correspondingly where streamwise vortices were induced, entrained and dissipated. Furthermore, Proper Orthogonal Decomposition (POD) was applied to the experimental data to extract coherent structures and the corresponding frequency bandwidth. Periodic travelling waves were visualized by reconstruction of the first two POD modes. Hence, the POD analysis provided clear insights into the evolution of the coherent structures as well as into the interaction between the mean and quasi-periodic flow fields. These coherent structures eventually increased the momentum transfer and consequently conveyed kinetic energy from the mean flow to the random fluctuating motions.
PIV study of large-scale flow organisation in slot jets
2015
The paper reports on particle image velocimetry (PIV) measurements in turbulent slot jets bounded by two solid walls with the separation distance smaller than the jet width (5-40%). In the far-field such jets are known to manifest features of quasi-two dimensional, two component turbulence. Stereoscopic and tomographic PIV systems were used to analyse local flows. Proper orthogonal decomposition (POD) was applied to extract coherent modes of the velocity fluctuations. The measurements were performed both in the initial region close to the nozzle exit and in the far fields of the developed turbulent slot jets for Re P 10,000. A POD analysis in the initial region indicates a correlation between quasi-2D vortices rolled-up in the shear layer and local flows in cross-stream planes. While the near-field turbulence shows full 3D features, the wall-normal velocity fluctuations day out gradually due to strong wall-damping resulting in an almost two-component turbulence. On the other hand, the longitudinal vortex rolls take over to act as the main agents in wall-normal and spanwise mixing and momentum transfer. The quantitative analysis indicates that the jet meandering amplitude was aperiodically modulated when arrangement of the large-scale quasi-2D vortices changed between asymmetric and symmetric pattern relatively to the jet axis. The paper shows that the dynamics of turbulent slot jets are more complex than those of 2D, plane and rectangular 3D jets. In particular, the detected secondary longitudinal vortex filaments and meandering modulation is expected to be important for turbulent transport and mixing in slot jets. This issue requires further investigations.
3 D Dynamics of Vortex Structures in a Quasi Two-dimensional Jet
2015
The work focuses on investigation of spatial-temporal 3D vortex structure of a quasi two-dimensional turbulent jet. Time-resolved tomographic PIV technique with repetition rate up to 10 kHz was used to measure 3D velocity distributions. It was shown that in quasi two-dimensional turbulent jet two types of coherent vortex structures exist. On the basis of instantaneous distributions of Q criterion 3D dynamics of vortex structure is studied. Longitudinal secondary vortex structures in far field of the quasi two-dimensional turbulent jet were detected for the first time.
Journal of Visualization, 2018
Three-dimensional flow fields downstream of micro VGs in backward-facing step (BFS) flow are experimentally investigated by time-resolved tomographic particle image velocimetry (PIV). Streaks within the separated shear layer are visualized by time-averaged velocity field. Complex streamwise vorticity fields are analyzed by proper orthogonal decomposition (POD) and coherent vortex structures are reconstructed by predominant POD mode pairs.Graphical Abstract
Identification of vortex structures in flow fields using tomographic PIV method
Journal of Fluid Science and Technology
We carried out the implementation of visualization techniques that aim at the interpretation of the complex turbulent structures in fluid flow. Turbulent structures help in understanding various phenomena of the flow field such as energy dissipation and drag reduction. However, few studies were found regarding vortex sheet structure identification for experimental data. In the current study, these sheet structures were visualized. Experimental data for the three-dimensional instantaneous velocity vector field was measured using the Tomographic PIV method. Two different kinds of flow settings, pipe flow and whirlpool flow, were measured and analyzed. For comparison of results, pipe measurements were recorded at varying Reynolds number and whirlpool flow were recorded with constant stirring and decaying turbulence. criterion analysis, which evaluates regions of higher vorticity than strain, was employed for visualization of vortex tubes in a time series flow field measurement. Vortex sheets were visualized using the matrix analysis that utilizes second-order velocity gradient tensor and its eigenvalues for identifying regions with a high correlation and magnitudes of vorticity and strain. The relative position of the tube and sheet structures visualized using the two methods were reviewed for the experimental data of pipe flow and whirlpool flow. Comparison of the experimentally resolved structures with structures of DNS data was studied for pipe flow and a close resemblance was observed.
Investigation of turbulent puffs in pipe flow with time-resolved stereoscopic PIV
Time-resolved stereoscopic particle image velocimetry (SPIV) was used to study the 3D flow field and the flow structure of a turbulent spot, or puff, at low Re in a pipe. The high sampling frequency of the SPIV system (500 Hz) makes it possible to obtain timeresolved velocity measurements over the entire circular cross-section of the pipe. When time is converted into a spatial coordinate with help of the bulk velocity, i.e. assuming frozen turbulence, the result is the first quasi-instantaneous 3-D velocity measurement of a turbulent puff. The 3-D plots of the iso-contours of the streamwise vorticity, and various cuts of the 3-D vector fields show the complex structure of the flow in the turbulent puff. At the trailing edge of the puff, where the laminar flow undergoes transition to turbulence, pairs of counter rotating streamwise vortices result in large mushroom-like structures as seen in flow visualizations. Integration of the velocity fields over the cross-section of the pipe further shows that very large spikes occur in the energy of the radial and azimuthal velocity components. These spikes appear to be related to the presence of hairpin vortices.
Time-resolved 3C-2D PIV Measurements in the Far-Field of a Turbulent Zero-Net-Mass-Flux Jet
This paper presents time-averaged and temporally evolving mean flow and turbulence statistics for a turbulent zero-net-mass-flux (ZNMF) jet at high Reynolds and Strouhal numbers in the far-field in a plane perpendicular to the jet axis. The measurements have been obtained using time-resolved stereo particle image velocimetry (TR-SPIV). The jet is generated by the oscillation of a piston, which discharges filtered water to the quiescent fluid in a tank through a round orifice. A multigrid cross correlation digital particle image velocimetry algorithm (MCCDPIV) has been used to compute the images from each camera that subsequently have been combined to obtain the three components of the velocity. Velocity and turbulence statistics are in good agreement with results obtained in previous work. A comparative study has been performed to determine the contribution to the spreading rate due to the displacement of the center of the jet as rigid body. Different criteria to define the instanta...
3D organization of high-speed compressible jets by tomographic PIV
This work investigates the three dimensional organization of compressible jets at high-speed regime by tomographic particle image velocimetry (TOMO PIV). Experiments are conducted at Mach numbers 0.3, 0.9 and 1.1 (underexpanded regime) across the end of the potential core within a large cylindrical domain (1.6Dx8D, with D=22mm the jet diameter). At M=1.1, shock and expansion waves are observed in the potential core between Z/D=2.6 and Z/D=5. The velocity fields are analysed by proper orthogonal decomposition (POD) to investigate the three-dimensional organization of large scale structures. Modal patterns are described based on visualizations of the axial velocity, which, among the velocity components, is the main contributor to the total fluctuating kinetic energy. Azimuthal Fourier decomposition is performed prior to POD to reduce the measurement noise of the extracted modes. At any Mach number, the most energetic modes show energy peaks by the end of the potential core and beyond (Z/D>5). At M=1.1, shock and expansion waves are not detected in the most energetic modes.