Showing papers by "Manoochehr Koochesfahani published in 1999"

Molecular Tagging Velocimetry (MTV ) measurements in gas phase flows

Abstract: Recent developments in Molecular Tagging Velocimetry (MTV) using the phosphorescence of biacetyl are described for gas-phase flows. With improvements in tagging, detection, and processing schemes, whole-field measurements of two components of the velocity vector are obtained simultaneously, typically at more than 300 points over a plane. Application of this measurement approach is demonstrated in mapping the velocity and vorticity fields of the intake flow into a “steady flow rig” model of an internal combustion engine.

The Accuracy of Remapping Irregularly Spaced Velocity Data onto a Regular Grid and the Computation of Vorticity

Abstract: The velocity data obtained from molecular tagging velocimetry (MTV) are typically located on an irregularly spaced measurement grid. To take advantage of many standard data processing techniques, the MTV data need to be remapped onto a grid with a uniform spacing. In this work, accuracy and noise issues related to the use of a least-squares-fit to various low order polynomials for the remapping of these data onto a uniformly spaced grid and the subsequent computation of vorticity from these data are examined. This information has relevance to PIV data processing as well. It has been previously noted that the best estimate of the velocity vector acquired through the use of tracer techniques such as PIV, is at the midpoint of the displacement vector. Thus, unless special care is taken, PIV data are also initially obtained on an irregular grid. The error in the remapped velocity and the calculated vorticity field is divided into a mean bias error and a random error. In the majority of cases, the mean bias error is a more significant source of error than the more often quoted random error. Results of the simulation show that the best choice for remapping is the use of a least-squares fit to a 2nd order polynomial and the best choice for vorticity calculation is to use a 4th order accurate, central, finite difference applied to uniformly sampled data. The actual value of the error depends upon the data density and the radius used for the selection of velocity measurements to be included in the remapping process. Increasing the data density and reducing the fit radius improve the accuracy.

Vorticity Field Evolution in a Forced Wake

Abstract: : The purpose of this work is to quantify the vorticity evolution in the flow field of the forced wake of a splitter plate inside a confining geometry. The interest in this flow stems from the fact that forcing a low Reynolds number 2-D wake can lead to a highly three-dimensional flow and a large increase in mixing. The authors' recent estimates, based on chemically reacting laser induced fluorescence (LIF) measurements, report the amount of molecularly mixed fluid in terms of mixed-fluid fraction to be 2.5 to 3 times larger than that in high Reynolds number natural two-stream mixing layers. Both reacting and non-reacting LIF data connect this increase in mixing to the downstream evolution of the streamwise vorticity, which is generated by the reorientation and stretching of spanwise vorticity near the side walls of the flow facility. It is believed that understanding the vorticity interaction with walls, its dynamics, and downstream evolution will be helpful to an overall strategy for mixing enhancement and control. The measurements were carried out by Molecular Tagging Velocimetry (MTV), a technique that takes advantage of molecules with long-lived excited states for nonintrusive, multi-point measurements of various fluid dynamical quantities. Small regions of the flow were tagged by a laser and their subsequent evolution was monitored over the luminescence lifetime of the molecule. A two-detector imaging system was used to acquire an image of the initially tagged regions and a subsequent image of these regions convected by the flow over a prescribed time delay. The Lagrangian displacement vectors from such image pairs were computed using a spatial correlation technique. The particular flow investigated here was highly three dimensional. This application highlights the capability of MTV to make measurements when strong out-of-plane motions are present. (5 refs.)