The principle behind particle tracking is simple: the fluid flow is seeded with particles, the location of which is tracked through time. Depending on the nature of the particles, the fluid and the flow, the particles may follow the individual fluid elements, or they may have a relative velocity due to their buoyancy, their inertia or both. Particle tracking may thus be used both as a technique for measuring the fluid velocity if the particles are sufficiently small (and approximately neutrally buoyant) to follow the fluid elements. In the other extreme, multiphase and sedimenting flows may be investigated.
Particle tracking is a major feature within DigImage. A large subset of the general image processing techniques are utilised during the tracking process in addition to highly specialised routines carefully optimised to perform the tracking in an accurate, reliable and efficient manner.
The tracking process may be broken into three phases: the experiments, the tracking process and the subsequent analysis. Experimental considerations and a general overview of the tracking process are covered by the external particle tracking documentation. Subsequent analysis of the tracking data may be performed by the DigImage utility Trk2DVel, or a user supplied module.
The role of DigImage itself is confined largely to the actual tracking proccess. From the user's point of view the tracking proccess may be divided into two steps: setting up the controlling parameters and proccessing the video tape to determine the particle locations and paths.
This may be used to move the tape under computer control, and/or search for specific transitions or marks on the tape, either interactively or from a command file.
This option starts up the particle tracking system for up to 511 particles. The algorithm used by this option is somewhat more robust than that implemented by [H Start particle tracking - 4095 particles] and so should generally be preferred. Moreover the optional recording of local background intensity and the use of background intensity pricing are only implemented for 511 or fewer particles. The tracking itself is in batch mode, using the parameters set by [S Set tracking parameters] and requires no user intervention. If you wish to stop the tracking early, then <Q> or <escape> winds up the tracking process gracefully, optionally closing down any command files, and returns control to this menu. Note that the tracking is performed by a separate program, but this is linked in transparently to DigImage. The only time this structure is apparent (other than a small time delay in starting and terminating) is if <ctrl><break> is used to exit the tracking process program: control will return to this menu rather than MS-DOS.
If the .IND tracking file for the currently specified base name is found to exist, then the user will be warned. Under this situation pressing <escape> aborts the tracking process; any other key will allow the tracking to start, overwriting the existing files. Note that this warning will not be issued if the tracking is started from a command file: the files will always be overwritten in this case. If the tracking files for the specified base name do not already exist, then no warning will be generated.
This option starts up the particle tracking system for up to 4095 particles. The algorithm used by this option is somewhat less robust than that implemented by [G Start particle tracking - 511 particles] and so should only be used if tracking of more than 511 particles is required. In particular, if the maximum new particle error velocity is relatively high and there are a lot of new particles, then the algorithm may not be able to match all the particles successfully (a message is generated to this effect). This limitation will generally arise only for the first few samples during the tracking process. Further, with high particle densities the probability of mismatching is increased and the maximum velocities which may be tracked are somewhat lower. The optional recording of local background intensity and the use of background intensity pricing are not available under this option. For details about operation, termination and the files produced, refer to [G Start particle tracking - 511 particles] above.
The first task DigImage performs upon choosing this option is to initialise the VTR for computer control, unless this has already been done. The VTR is then placed in shuttle/pause mode, prior to allowing the user to undertake manual repositioning. During the initialisation phase the OPERATION switch on the VTR should be set to REMOTE and then changed to LOCAL when so prompted by DigImage. Finally, once the manual positioning thas been completed, the OPERATION switch should be returned to REMOTE.
This option generates real time streaks in a similar manner to the Streak.CMD and Streak2.CMD macros. The main difference is that this option offers a more convenient method of changing the streak parameters. By default this option assumes dark particles. [P Particle streaks - bright particles] is identical except it defaults to bright particles.
Identical to [O Particle streaks - dark particles] except that bright (white) particles are assumed by default.
This option provides access to a hierachy of menus to enable the setting of the many different parameters controlling all facets and options available. Extensive help is available within this menu tree to enable the tracking to be configured in a manner appropriate for each individual experiment. Despite its complexity, the tracking is very easy to configure for basic experiments, and is suprisingly insensitive to the precise details of many of the parameter values.
This option should be used to start the particle tracking if it has been customised by the user using the Trk2Hook.FOR file. Prior to running this option it is essential that the customised tracking program is created using Trk2Make.BAT. See the document Trk2Hook.DOC in the Document directory for further details.
DigImage User Documentation