Dana Dabiri

On errors of digital particle image velocimetry

The goal of the present study is to quantify and reduce, when possible, errors in two-dimensional digital particle image velocimetry (DPIV). Two major errors, namely the mean bias and root-mean-square (RMS) errors, have been studied. One fundamental source of these errors arises from the implementation of cross correlation (CC). Other major sources of these errors arise from the peak-finding scheme, which locates the correlation peak with a sub-pixel accuracy, and noise within the particle images. Two processing techniques are used to extract the particle displacements. First, a CC method utilizing the FFT algorithm for fast processing is implemented. Second, a particle image pattern matching (PIPM) technique, usually requiring a direct computation and therefore more time consuming, is used. Using DPIV on simulated images, both the mean-bias and RMS errors have been found to be of the order of 0.1 pixels for CC. The errors of PIPM are about an order of magnitude less than those of CC. In the present paper the authors introduce a peak-normalization method which reduces the error level of CC to that of PIPM without adding much computational effort. A peak-compensation technique is also introduced to make the mean-bias error negligible in comparison with the RMS error. Noise in an image suppresses the mean-bias error but, on the other hand, significantly amplifies the RMS error. A digital video signal usually has a lower noise level than that of an analogue one and therefore provides a smaller error in DPIV.

Digital particle image thermometry : the method and implementation

A computerized flow visualization technique capable of automatically quantifying the temperature field in a two-dimensional cross section of a flow field is described. The temperature sensors used are fast-response temperature-sensitive micro-encapsulated liquid crystal particles. Illuminating the flow by a thin sheet of white light, the reflected colors from the liquid-crystal particles were captured through a 3-chip video color camera and stored onto a videotape for subsequent data processing. The temperature field was obtained through an automatic color-temperature calibration scheme in HSI rather than RGB space, thus allowing for data processing of approximately one-third the time of RGB processing. The technique is finally applied to the study of a heated vortex-ring and some preliminary results are discussed.

The Application of Advanced Methods in Analyzing the Performance of the Air Curtain in a Refrigerated Display Case

Computational Fluid Dynamics (CFD) modeling is effectively coupled with the experimental technique of Digital Particle Image Velocimetry (DPIV), to study the flowfield characteristics and performance of the air curtain of a medium-temperature open vertical refrigerated display case used in supermarkets. A global comparison of the flowfield and quantification of the entrained air into the case indicate that there is a considerable amount of cold air spillage from a typical display case that is replaced by the ambient warm entrained air across the air curtain, lowering the energy efficiency of the case. The computational model that is developed from the marriage of CFD and DPIV techniques provides a reliable simulation tool that can be used for the design optimization of air curtains. A correct estimate of the infiltration rate by changing different parameters in a validated computational simulation model will provide a feasible tool for minimizing the spillage of the cold air, and thereby designing more energy efficient open display cases.

A full three-dimensional characterization of defocusing digital particle image velocimetry

The paradigm set forth by Pereira et al (2000 Exp. Fluids 29 S78–84) was an important milestone in capturing the optical geometry of a three-dimensional defocusing digital particle image velocimetry (DDPIV) design within a set of systematic equations. However, the opportunity to improve upon their pseudo-three-dimensional conceptual implementation of the two-dimensional equations exists by revisiting the derivations of these equations and revising some of their assumptions in order to define a modified set of equations for a true full-three-dimensional derivation. This paper introduces this newly revised set of equations that will explicitly and more accurately represent the three-dimensional DDPIV measurement system. A three-dimensional geometric uncertainty model has also been established through uncertainty analysis. Finally, a discussion of the differences and benefits of the new system of equations is presented.

Experimental investigation of the vorticity generation within a spilling water wave

Sources of vorticity are examined for a laboratory-generated spilling breaking wave. Two cases are studied. For the first case, based on the breaker height, the Reynolds and Froude numbers are 7370 and 2.04, respectively. The breaker is preceded by 1 mm wavelength capillary waves, with the largest amplitude-to-wavelength ratio equal to 0.18. For this case, it is found that the dominant source of vorticity flux is a viscous process, due to the deceleration of a thin layer of the surface fluid. For the second case, the Reynolds and Froude numbers based on the wave height are 1050 and 1.62, respectively. No breaking is observed for this case; rather a capillary–gravity wave is observed with 4 mm wavelength capillaries preceding the gravity wave. The largest amplitude-to-wavelength ratio of these capillaries is 0.28. This case shows that capillary waves do not contribute to the vorticity flux; rather the only dominant source of the vorticity flux into the flow is the free-surface fluid deceleration. Lastly, a thin free-surface jet that is relatively vorticity-free is found to precede the spilling breaker. Analyses suggest that our wave-breaking phenomena can be modelled by a hydraulic jump phenomenon where the Froude number is based on the thickness of the free-surface jet, and on the velocity of the free-surface jet just prior to breaking. We believe this to be a more physically descriptive value of the Froude number. For the high-speed case, the Froude number based on the thickness of the free-surface jet is 4.78, while for the lower-speed case it is 2.14.

Dual luminophore polystyrene microspheres for pressure-sensitive luminescent imaging

Polystyrene microspheres containing both an oxygen-sensitive platinum porphyrin luminescence and a pressure-insensitive silicon porphyrin luminescence are prepared in high yield. The ratio of these two luminescences responds reversibly in aerodynamic flows over a wide dynamic range of oxygen concentrations, with a response time of <10 ms. These microspheres have been used in a non-intrusive imaging method to potentially obtain the pressure distributions in three-dimensional aerodynamic flows.

Universal outlier detection for particle image velocimetry (PIV) and particle tracking velocimetry (PTV) data

A generalization of the universal outlier detection method of Westerweel and Scarano (2005 Universal outlier detection for PIV data Exp. Fluids 39 1096–100) has been made, allowing the use of the above algorithm on both gridded (PIV) and non-gridded (PTV) data. The changes include a different definition of neighbors based on Delaunay tessellation, a weighting of neighbor velocities based on the distance from the point in question and an adaptive tolerance to account for the different distances to neighbors. The new algorithm is tested on flows varying from impinging jets to turbulent boundary layers and wakes to wingtip vortices, both PIV and PTV. The residuals for these flows also show universality in their probability density functions, similarly suggesting the use of a single threshold value to identify outliers. Also the new algorithm is found to work with data up to about a 15% spurious vector content.

On the interaction of a vertical shear layer with a free surface

New experiments have been conducted using a combined free-surface gradient detector (FSGD) and digital particle image velocimetry (DPIV) technique to study the interaction between a vertical shear layer, created by a surface-piercing splitter plate, and a free surface. The emphasis of this study is on understanding aspects of the interaction between the free-surface deformation (FSD) and the near-surface turbulence through the correlations between the elevation and the vorticity fields, and the spectral behaviour of the near-surface pressure. The Reynolds number of the present study, based on visual thickness and the velocity average of the two streams, is 12 100. Mean results for the velocity and vorticity fields show that self-similarity is achieved. Instantaneous data sets show that at the free surface, vortex tubes within the main rollers connect normally with the free surface as is evidenced by strong vorticity as well as the strong deformations at the free surface. The instantaneous data sets also show that the streamwise vortices near the braid regions, while weaker than those seen in the main rollers, also reconnect with the free surface. Statistical analyses show that the FSD is strongly correlated with the near-surface vorticity field, as the correlation coefficients are quite high (

Optical mapping of fluid density interfaces: Concepts and implementations

\sim 0.7--0.8

An improved three-dimensional characterization of defocusing digital particle image velocimetry (DDPIV) based on a new imaging volume definition

). The pressure spectrum slope within the shear layer near the surface is found to be