Grid turbulence studied by Particle Image Velocimetry (original) (raw)
Related papers
Analysis of Turbulence Energy Spectrum by Using Particle Image Velocimetry
Procedia Engineering, 2014
Transport phenomena occur frequently in industrial problems. Most of the turbulent transport properties can be directly associated with the turbulent energy dissipation rate; hence it is a very significant parameter in the design of chemical processing equipment. To develop a better chemical processing equipment design, a thorough knowledge of the effect flow structure on local turbulence parameters like turbulent kinetic energy, eddy diffusivity and the energy dissipation rate are required. Turbulence is heterogeneous in most of the process equipment. Hence, the use of spatial average energy dissipation rate causes error in modelling of turbulent transport processes. In this present work, particle image velocimetry (PIV) is used to obtain the energy spectrum from grid generated homogeneous turbulence velocity data. The model of energy spectrum given by has been fitted to this energy spectrum using energy dissipation rate. A different approach, based on a third order structure function and velocity gradient technique has been used to compute the energy dissipation rate. The model predictions have been verified by experimental PIV velocity data from oscillating grid apparatus.
2021
Grid turbulence is investigated using cross-correlation digital Particle Image Velocimetry (PIV) over a range of Taylor Reynolds Number (Reλ) from 5 to 44. Instantaneous velocity is measured directly and vorticity and velocity gradients are obtained indirectly. Measurements are taken at various downstream locations from the generating grid. Probability distribution functions (PDFs) are calculated for the fluctuating component of the velocity, the spatial velocity gradients and vorticity. The PDF of the velocity fluctuations has a Gaussian distribution while velocity gradients and vorticity are found to have non-Gaussian PDF distributions. The structure of the flow is investigated by calculating spatial autocorrelations for all measured and derived data. The spatial velocity autocorrelations differ from previous experimental measurements of grid turbulence, most of which have been determined from single-point measurements. This difference is believed to be due to differences in the w...
Tomographic and Stereoscopic PIV measurements of Gridgenerated Homogeneous Turbulence
This paper presents tomographic PIV measurements in grid-generated homogeneous turbulence. The measurements are performed in a small-scale water tunnel and turbulence is generated by a rectangular grid of mesh size M = 8.25mm and solidity, σ = 0.426. The mean velocity in the channel is U = 0.12ms -1 , which corresponds to a bulk flow Reynolds number of approximately 6000 and a Taylor microscale Reynolds number of about Reλ=11. The results are compared with theoretical data and stereo-PIV reference measurements in the same facility and flow conditions. The effects of Gaussian velocity field filtering on the velocity spectra and auto-correlation is investigated and shown to reduce the measurement noise. The focus of the current HIT measurements is on the intermediate to far range (20 ≤ x/M ≤ 60). Results are presented in form of power spectra and auto-correlation functions as well a s fluctuating velocity fields and fluctuation PFDs.
Experiments in Fluids, 2019
Simultaneous measurements of velocity and concentration using stereoscopic particle image velocimetry (stereo-PIV) and planar laser-induced fluorescence (PLIF) were used to investigate the mixing performance of a scaled-up multi-inlet vortex reactor (MIVR). Data were collected in three measurement planes located at different heights from the reactor bottom (¼, ½, and ¾ of the reactor height) for Reynolds numbers of 3250 and 8125 based on the reactor inlet velocity and hydraulic diameter. The collected data were analyzed to determine turbulent flow statistics such as turbulent viscosity, turbulent diffusivity, and turbulent Schmidt number. When analyzed across 16 different azimuth angles and radial positions (r) normalized by the reactor radius (Ro), the turbulent viscosity was found to be nearly axisymmetric. In the free-vortex region (r/ Ro > 0.2), the turbulent viscosity results were nearly constant. Near the center of the reactor in the forced-vortex region (r/ Ro < 0.1), the turbulent viscosity significantly increased, with peak values occurring near the center. The turbulent viscosity and Reynolds shear stress were highest near the reactor exit at the ¾ plane. The dominance of high turbulent fluxes and low concentration gradients near the reactor center led to high turbulent diffusivity. Away from the center, the turbulent diffusivity was reduced because of large concentration gradients and low turbulence intensity in the spiral arm region. The turbulent Schmidt numbers were also found to correlate with concentration gradients. The turbulent Schmidt number values were found to vary from 0.1 to 1.2. The highest spatial variation in Sc t was observed in the spiral arms region, where the concentration gradients are also the highest. This spatial variation in Schmidt number contrasts with the common assumption of constant Sc t in Reynolds-averaged CFD models.
Particle image velocimetry (PIV) has become a standard technique in turbulence experiments for its non-intrusive ability to simultaneously measure multiple points in a flow field. However, inherent volume averaging of the interrogation windows as well as fixed limitations of camera spatial resolution and related hardware set the minimum and maximum scales of structures that can be resolved with this measurement technique. Resolution limitations present themselves as an attenuation of the measured turbulence statistics, seen particularly in the Reynolds wall-normal stress components. This experimental study investigates the impact of hardware-level resolution limitations on the resolved turbulence statistics of PIV measurements made of both rough and smooth wall turbulent boundary layers influenced by external pressure gradients. Velocity fields are measured with PIV using two lens magnifications; one with a baseline lens and one with the addition of a teleconverter lens to view the ...
Engineering reports, 2020
Particle image velocimetry (PIV) measurement technique provides an excellent opportunity for investigating instantaneous spatial structures which are not always possible with point measurements techniques like laser Doppler velocimetry. In this review, it was shown that PIV technique provides an effective means of visualizing important structures of Newtonian and drag-reducing fluid flows. Such structures include large-scale-events that constitute an important portion of the Reynolds stress tensor; shear layers of drag-reducing flows, which have been suggested to constitute the mechanism of drag reduction (DR); and near wall vortices/low speed streaks which constitute the mechanism of turbulence production. PIV investigations of turbulence statistics in Newtonian and drag-reducing fluid flows were reviewed with the view of providing explanation to DR by additives. Results of turbulence statistics, for Newtonian fluid flow, showed that streamwise velocity fluctuations and turbulence intensity had peak values close to the wall, in a region of high mean velocity gradient, while radial fluctuating velocity and Reynolds stress tensor had peaks further from the wall and at approximately the same detachment from the wall. In single-and two-phase flows in horizontal channels, the velocity profile of polymer solution, in the turbulent regime, show asymmetric behavior. This review highlighted important interfacial characteristics in gas-liquid flows such as S-shaped velocity profile as well as the turbulences statistics in each phase and across the interface region. Drag-reducing agents (DRAs)-imposed changes on turbulence statistics and flow structures were also examined. PIV studies of drag-reducing flows showed that DRAs act to dampen wall-normal-, streamwise fluctuating velocity, and Reynolds stress tensor. The reduction in Reynolds stress tensor is higher than the reduction of both wall-normal and streamwise velocity fluctuations and this discrepancy has been associated with the decorrelation of the component of fluctuating velocity. Furthermore, the addition of DRA produces a shift of the peak of wall-normal velocity fluctuations further from the wall due to increased buffer layer thickness. DRAs do not only act to reduce drag but also to modify the flow structure. The major influence of DRAs on flow structures is seen in the This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Engineering Reports, 2020
Particle image velocimetry (PIV) measurement technique provides an excellent opportunity for investigating instantaneous spatial structures which are not always possible with point measurements techniques like laser Doppler velocimetry. In this review, it was shown that PIV technique provides an effective means of visualizing important structures of Newtonian and drag-reducing fluid flows. Such structures include large-scale-events that constitute an important portion of the Reynolds stress tensor; shear layers of drag-reducing flows, which have been suggested to constitute the mechanism of drag reduction (DR); and near wall vortices/low speed streaks which constitute the mechanism of turbulence production. PIV investigations of turbulence statistics in Newtonian and drag-reducing fluid flows were reviewed with the view of providing explanation to DR by additives. Results of turbulence statistics, for Newtonian fluid flow, showed that streamwise velocity fluctuations and turbulence intensity had peak values close to the wall, in a region of high mean velocity gradient, while radial fluctuating velocity and Reynolds stress tensor had peaks further from the wall and at approximately the same detachment from the wall. In single-and two-phase flows in horizontal channels, the velocity profile of polymer solution, in the turbulent regime, show asymmetric behavior. This review highlighted important interfacial characteristics in gas-liquid flows such as S-shaped velocity profile as well as the turbulences statistics in each phase and across the interface region. Drag-reducing agents (DRAs)-imposed changes on turbulence statistics and flow structures were also examined. PIV studies of drag-reducing flows showed that DRAs act to dampen wall-normal-, streamwise fluctuating velocity, and Reynolds stress tensor. The reduction in Reynolds stress tensor is higher than the reduction of both wall-normal and streamwise velocity fluctuations and this discrepancy has been associated with the decorrelation of the component of fluctuating velocity. Furthermore, the addition of DRA produces a shift of the peak of wall-normal velocity fluctuations further from the wall due to increased buffer layer thickness. DRAs do not only act to reduce drag but also to modify the flow structure. The major influence of DRAs on flow structures is seen in the This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Particle Image Velocimetry: Fundamentals and Its Applications
2011
Particle image velocimetry (PIV) is the newest entrant to the field of fluid flow measurement and provides instantaneous velocity fields over global domains. As the name suggests, PIV records the position over time of small tracer particles introduced into the flow to extract the local fluid velocity. Thus, PIV represents a quantitative extension of the qualitative flow visualization techniques that have been practised for several decades. This paper gives a brief background on evolution of PIV and then its principle of operation, main features and basic elements are explaind. Errors in PIV occurring while measurements are discussed and, the state-of-the-art of the technique today is overviewed and illustrated by reference to recent, seminal publications describing both the development and application of PIV.
Measurement of the zero-pressure gradient turbulent boundary layer using particle image velocimetry
33rd Aerospace Sciences Meeting and Exhibit
The structure of a tripped zero-pressure gradient boundary layer has been measured using particle image velocimetry PIV). This paper describes the PIV experiment and the coherent structures which are identified from instantaneous streamwise wall-normal velocity fields. More than 340 large-format PIV photographs were taken at three Reynolds numbers ranging between 930 < Re, < 6845. The instantaneous d Research Associate. Member AlAA W t proiessor. Associate Fellow CopyrlghIO Amcrlcan lnrUluc of AamauUcS and A s m U a , Jm., 1994. AI1 rlghvi rcscrvcd
Particle image velocimetry measurements of flow over a wavy wall
Physics of Fluids, 2001
The velocity field in a plane that is oriented in the flow direction and perpendicular to the wall was measured for flow over a sinusoidal wavy boundary. The conditions were such that the surface may be considered as fully rough. Visual observations, as well as two-point correlations of the fluctuating velocity field, reveal that the turbulent structure is similar to what is found for flow over a smooth surface even though the mechanisms by which the wall maintains the turbulence are quite different. This provides support for the notion that, far enough away from the wall, the turbulence is universal.