Showing papers by "Julio Soria published in 1998"

Turbulence structures of wall-bounded shear flows found using DNS data

Abstract: This work extends the study of the structure of wall-bounded flows using the topological properties of eddying motions as developed by Chong et al. (1990), Soria et al. (1992, 1994), and as recently extended by Blackburn et al. (1996) and Chacin et al. (1996). In these works, regions of flow which are focal in nature are identified by being enclosed by an isosurface of a positive small value of the discriminant of the velocity gradient tensor. These regions resemble the attached vortex loops suggested first by Theodorsen (1955). Such loops are incorporated in the attached-eddy model versions of Perry & Chong (1982), Perry et al. (1986), and Perry & Marusic (1995), which are extensions of a model first formulated by Townsend (1976). The direct numerical simulation (DNS) data of wall-bounded flows studied here are from the zero-pressure-gradient flow of Spalart (1988) and the boundary layer with separation and reattachment of Na & Moin (1996). The flow structures are examined from the viewpoint of the attached eddy hypothesis.

Accuracy of out-of-plane vorticity measurements derived from in-plane velocity field data

Abstract: A study of the errors in out-of-plane vorticity (ω z ) calculated using a local χ2 fitting of the measured velocity field and analytic differentiation has been carried out. The primary factors of spatial velocity sampling separation and random velocity measurement error have been investigated. In principle the ω z error can be decomposed into a bias error contribution and a random error contribution. Theoretical expressions for the transmission of the random velocity error into the random vorticity error have been derived. The velocity and vorticity field of the Oseen vortex has been used as a typical vortex structure in this study. Data of different quality, ranging from exact velocity vectors of analytically defined flow fields (Oseen vortex flow) sampled at discrete locations to computer generated digital image frames analysed using cross-correlation DPIV, have been investigated in this study. This data has been used to provide support for the theoretical random error results, to isolate the different sources of error and to determine their effect on ω z measurements. A method for estimating in-situ the velocity random error is presented. This estimate coupled with the theoretically derived random error transmission results for the χ2 vorticity calculation method can be used a priori to estimate the magnitude of the random error in ω z . This random error is independent of a particular flow field. The velocity sampling separation is found to have a profound effect on the precise determination of ω z by introducing a bias error. This bias error results in an underestimation of the peak vorticity. Simple equations, which are based on a local model of the Oseen vortex around the peak vorticity region, allowing the prediction of the ω z bias error for the χ2 vorticity calculation method, are presented. An important conclusion of this study is that the random error transmission factor and the bias error cannot be minimised simultaneously. Both depend on the velocity sampling separation, but with opposing effects. The application of the random and bias vorticity error predictions are illustrated by application to experimental velocity data determined using cross-correlation DPIV (CCDPIV) analysis of digital images of a laminar vortex ring.

Dynamics of the velocity gradient tensor invariants in isotropic turbulence

Abstract: The evolution of the invariants (R and Q) of the velocity gradient tensor in homogeneous isotropic turbulence is investigated using data from direct numerical simulation (DNS). The concepts of conditional average time rate of change of the invariants and conditional mean trajectories (CMT) in invariant phase space are introduced to study the dynamics of this flow. The resulting dynamical system in the (R,Q) phase space is a clockwise spiral with a stable focus at the origin, illustrating that in the mean, the cyclic sequence of topological evolution following a fluid particle is unstable-node/saddle/saddle (UN/S/S)→stable-node/saddle/saddle (SN/S/S)→stable-focus/stretching (SF/S)→unstable-focus/contracting (UF/C). The mean rates of change of R and Q, i.e., Ṙ, Q, are found to be negligible near the right branch of the null discriminant (D=0) curve, indicating that this curve is an attractor in the (R,Q) space. The effects of both the diffusion term and the anisotropic part of the pressure Hessian term on ...

A study of the turbulence structures of wall-bounded shear flows using DNS data

Abstract: This project extends the study of the structure of wall-bounded flows using the topological properties of eddying motions as developed by Chong et al. (1990), Soria et al. (1992, 1994), and as recently extended by Blackburn et al. (1996) and Chacin et al. (1996). In these works, regions of flow which are focal in nature are identified by being enclosed by an isosurface of a positive small value of the discriminant of the velocity gradient tensor. These regions resemble the attached vortex loops suggested first by Theodorsen (1955). Such loops are incorporated in the attached eddy model versions of Perry & Chong (1982), Perry et al. (1986), and Perry & Marusic (1995), which are extensions of a model first formulated by Townsend (1976). The DNS data of wall bounded flows studied here are from the zero pressure gradient flow of Spalart (1988) and the boundary layer with separation and reattachment of Na & Moin (1996). The flow structures are examined from the viewpoint of the attached eddy hypothesis.

High-resolution cross-correlation PIV on photographic film

Abstract: An adaptation of current image shifting techniques has been developed that permits cross-correlation PIV analysis of non- overlapping images recorded on photographic media. The image acquisition technique utilizes pulsed lasers to produce two singly exposed sub-images of particles in one photographic frame. The technique can also provide images suitable for cross-correlation analysis on digitally recorded images without the need for a specialized camera. An adaptive cross- correlation PIV analysis method is employed to extract the in- plane velocity field information with high velocity sampling resolution and large velocity dynamic range. Further capabilities and limitations of the technique are presented as well as an analysis of the associated measurement errors. Some of the more interesting problems that were encountered during the development of the experimental technique are also discussed. The application of this method to the unsteady flow of a vortex ring will be illustrated as an example. High spatial resolution measurements using film recording have yielded the detailed instantaneous 2-D velocity and vorticity fields. Measurements made from digital recordings using this technique provide the evolution of instantaneous in-plane velocity and vorticity fields.

Vortex ring formation at an orifice

Abstract: High spatial resolution PIV measurements using film recording have yielded detailed instantaneous 2-D velocity fields of the formation of orifice generated vortex rings. The resultant high spatial resolution vorticity distributions enable the location and magnitude of regions of both primary and secondary vorticity to be determined The experimental measurements demonstrate how secondary vorticity produced at the piston hastens the transition to turbulence.

Effect of anomalous particle light scattering on PIV image quality

Abstract: Optical mastermind techniques, such as PIV, are finding more frequent use in experimental fluid mechanics laboratories. The continuing development of both imaging technology and image processing algorithms over the past twenty years have extended its measurement capabilities. Developments in high power pulsed lasers, such as Nd:YAG have also contributed to extending the applicability of the technique to a variety of flows. It has been found that for measurements in water, problems arise due to a combination of the very high light intensity supplied by a high powered pulsed Nd:YAG laser and Mie scattering effects of sub-micron sized particles inherently present in un-distilled water (e.g. mains water supply). This creates problems for obtaining consistent PIV images. These problems arise when using large water tunnel facilities for which it is not feasible to use distilled water. In addition to this, particle selection is of prime importance when one wishes to use orthogonally polarized dual laser systems. Experimental results will be presented to illustrate the degradation in image quality due to these ubiquitous effects. Recommendation regarding particle selection and optimum laser polarization states will be presented for conditions in distilled and typical mains water supply.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Flow field measurements of separated and reattached flows

Abstract: A mean, 2-D separating and reattaching flow over a blunt flat plate at a \(\operatorname{Re} = \frac{{Uh}}{v} = 1000\) (plate thickness, h) has been investigated using Particle Image Velocimetry (PIV). Medium spatial resolution instantaneous velocity field measurements, as well as temporal velocity field measurements in and around the separation bubble have been obtained. The experimental results show a highly unsteady flapping of the shear layer causing considerable variation in the size of the separation bubble. This flapping occurs principally in the downstream region of the separated shear layer and results in a large uncertainty of the instantaneous reattachment point.

Topology of Dynamically Passive Scalar Fields in Isotropic Homogeneous Turbulence

Abstract: Transport of scalars by turbulent fluid motion is of fundamental importance and is encountered in applications such as pollutant formation, mass and heat transfer and chemical reactions. The physics of many of these applications is extremely complex and not well understood due to the intriguing complex topology of the fluid motions and scalar fields. In this paper, a new formulation is proposed to study the detailed topology of diffusing scalars and their relationship to the turbulent flow topology. Direct numerical simulations (DNS) are used to study those topologies in homogeneous isotropic turbulence, using an object-oriented code (Crespo 1994).