Topological Feature Identification

  V2K provides tools for the detection and identification of topological features which exist within a structured volume mesh. These features consist of:

singularities (i.e. collapsed block faces),

point continuous block-to-block interfaces and

patched block interfaces.

Initialization

 The topological features listed above are associated with generic interface objects within the V2K environment. Objects of this type not only support the specification of user and software assigned attributes describing the topological features, but they also become pickable thereby enabling the user to interact with them via the graphics display. 
 Initially, an imported volume mesh will have no interface objects defined, thus the first step is to generate an initial set of objects on which the details of the topology can be placed. This initialization is accomplished by pressing the Initialize pushbutton. This instructs the software to generate an interface object at each of the six faces comprising each block wihtin the mesh. These initial interface objects are labeled as unprocessed and are essentially place-holders in that they are devoid of any topological information.

 The initialization should be performed only once for a given volume mesh. For this reason, the Initialize button becomes desensitized immediately after its activation and also after the import of a previosuly generated neutral map file. This data file contains a list of all interface objects and their associated topological data perviously defined. In any event, the user must assign the proper topological data to each interface object, and this task remains incomplete as long as unprocessed interface objects exist.

 V2K provides facilities to make the task of identifying and assigning topological features to interface objects nearly automatic. Flow field boundary conditions, though not considered as topological features, may also be assigned to interface objects, and the mechanics of their assignment is described elsewhere within this document.

Singularity Detection

 Singularites within the model are automatically detected by simply pressing the Find singularities button. The underlying algorithm examines each interface object whose current Type is labeled as "unprocessed" against the following defintion for a singularity:

The associated mesh face is collapsed along one coordinate direction only though it need not remain collapsed along the full length in the second coordinate direction.

The collapsed edge exists at the beginning and/or at the end of the edges defining the second coordinate direction.

Point-continuous Interfaces

 Point continuous interfaces which may exist between adjacent blocks can be identified by pressing the Find one-to-one interfaces pushbutton. In this context "unprocessed" interfaces are examined in pairs to determine whether the required point continuity exists. The algoithm is sufficiently general so as to recognize both full and partial block face point continuity. Interface objects or portions thereof which are found to be point continuous are combined into a single interface object whose type is then assigned as "ONE_TO_ONE". Additional attributes which define the index range for each block where point continuity exists are also defined. Within this range of indices, the Swap attribute is also set to describe the relative orientations of the transformed surface coordinates for each block.

User Controls

 Various user controls are provided to constrain when and in what manner singularity and point continuous interfaces are to be defined. These controls are described individually below.

Same Point Tolerance

 In an ideal world, point comparisions performed by V2K in detecting collapsed edges and point-to-point continuity would be satisfied to machine zero. Owing to different approaches employed in meshing software as well as those of the user, this frequently is not the case, and the user is thus required to prescribe a Same Point Tolerance prior to invoking either the singularity or point continuous interface detection algorithms. This tolerance is the offset between points below which points are to be considered to be coincident, and its magnitude will effect where each of the topological features may be found to exist. This quantity ought to be prescribed no larger than the smallest spacing which exists within the mesh, and it must be within acceptable limits of the flow solver.

Sub-face Increment

 There is an additional user prescribed quantity labeled as the Sub-face Increment which controls the manner in which singularities and point-to-point interfaces may be defined. This parameter places a size constraint on the definition of either interface type such that the ultimate dimensions (in terms of index range) are integer multiples of this quantity. The goal here is to ensure that the topological description remains multi-gridable to a user-controllable level, and this requirement dictates the integer which should be entered in the text field. An example demonstrating the effects of this parameter will be provided at the end of the next section.

Full/Partial Interface Definition

 The TOG sub-panel provides a checkbox labeled as define full interfaces only which should be set to the desired state before either of the feature detection algorithms is invoked. This setting controls whether an "unprocessed" interface is to be treated as a whole in the defintion of either a singular or point continuous interface, or whether it may be defined in "chunks" of size prescribed by the prescribed Sub-face Increment). This control plays a role if one or more point comparisons produce an offset which exceeds the Same Point Tolerance. The role of these control options are best illustrated by means of an example.

Log File Output

 A checkbox labeled as dump log file is included to facilitate the generation of a data file containing the results written to the scrolling text field. The data file is hardwired as "togresults.txt". As will be described in the section below, this represents a convenient means to gather data if the mesh generator is to be revisited in order to correct issues related to point tolerance; this data describes local tolerance variations as well as where they occur thereby providing something of a roadmap indicating where the analyst ought to focus his or her attention.

Usage Notes

 Practical suggestions for the efficient use of the topological feature detection algorithms described above is in order.

(1) If a model possesses singularities, these features should be identified first as such will reduce the number of interface objects with which the point continuous facility must contend. This will result in faster execution. In the same light, all flow field boundary conditions should be assigned as well as this, too, will reduce the number of remaining unprocessed objects.

(2) It is typically an interative process to determine a Same Point Tolerance which will result in the complete defintion of both singular and point continuous topological features. This process involves repeated applications of the feature detection algorithm(s) with the define full interfaces only checkbox in the off position. Each cycle is followed by an upward adjustment of the Same Point Tolerance as indicated by the offset values displayed in the scrolling text field. Once all unprocessed interfaces have been removed. V2K should be restarted and the feature detection algorithm(s) invoked once again using this new Same Point Tolerance. The define full interfaces only checkbox setting is then immaterial.

Patched Interfaces

 Patched interfaces are, in general, block-to-block interfaces for which point continuity does not exist. That flux transport occurs across such interfaces requires that they be identified; this is the function of the patched interface facilities. Unlike singular and point-continuous interface objects, however, the specification of patched interfaces is a purely interactive, user-driven operation. The process involves the definition of an association between a single primary interface object and one or more secondary interface objects. The facilities developed to support patched interface defintion are located in the right-most portion of the TOG tab as shown below.

 Pressing the Define patched interfaces button (1) highlights a primary interface object in red and (2) activates a specialized picking mode which permits the selection of multiple secondary interface objects. These secondary interfaces become highlighted in yellow as they are selected. Note that as secondary interfaces are assigned to the primary interface, V2K also transparently assigns the primary interface to each secondary interface, thus, as the process advances to subsequent primary interfaces, "pre-selected" secondary interfaces may alrfeady exist. V2K automatically reconciles the primary/secondary assignments as they are made regardless of whether secondary interfaces are selected or unselected.

Capture Box controls

 Selection of secondary interface objects is permitted from the remaining collection of unprocessed interfaces only. However, to reduce display clutter, only those unprocessed interfaces in "close" proximity to the primary interface are rendered. The controls provided in the Capture Box permit the user to adjust the proximity threshold. The right and left arrow buttons expand and contract, respectively, the bounding box surrounding the primary interface, and for each adjustment, the display is dynamically updated to include those remaining unprocessed interfaces whose bounding boxes intersect that of the primary interface. The Reset button returns the bounding box expansion/contraction factor to its nominal value. Note that once a secondary interface has been selected, its display is never supressed regardless of how the primary interface bounding box is manipulated.

Patched interface defintion

 Patched interface defintion involves the initial selection of secondary interface objects for every primary (i.e. unprocessed) interface object. Until all such assignments have been made, the software operates in the definition mode in which the primary interface is selected by the software. Selected secondary interfaces must be formally saved by pressing the Save button after which the software advances to the next unprocessed interface object, makes it the primary interface, and then stands ready to accept the secondary selections. This process continues until each unprocessed interface has been visited and assigned one or more secondary interface assignments. When V2K detects that there are no more unprocessed interfaces requiring assignments, the specialized seleciton mode will terminate, and numerous buttons within the panel will become desensitized. A message will appear in the scrolling text field indicating that the patched definition process has been completed.

 The user need not, however, remain in the definition mode until all patched interfaces have been defined. The Done button allows the user to suspend the selection process, perform other operations, or even exit the program, but the process remains incomplete. When the patched interface panel is revisited, and the Define patched interfaces button pressed again, the software will resume the specification process starting with the next unprocessed interface for which no secondary interface assignments have bene made.

 It is a subtle though important point that though a primary interface and its secondary interface assignments are saved as a patched interface, they remain as unprocessed interfaces within the database until the Update button is pressed. Since the patched interface facility operates only upon unprocessed interfaces, those previously used in a primary or secondary context must remain available for assignment to other primary interfaces; hence the rationale for delaying their update to the Patched status. Note that the Update button becomes sensitive only when V2K has terminated the definition process or the user has done so by pressing the Done button.

Patched interface editing

 Once V2K has terminated the patch defintion mode, pressing the Define patched interfaces button again invokes an editing mode which allows the user to alter any of the patched interfaces previously defined. At this point the facilities within the Interface frame become sensitized. The left and right arrow buttons allow the user to scroll down and up, respectively, within the set of patched interfaces to review and perhaps modify the secondary selections as are displayed for the indicated primary interface. The central pick button causes all unprocessed interfaces to be temporarily rendered from which the user may select an interface which is become the primary interface. For a successful selection, the display reverts to that of the defintion mode; all unprocessed interfaces which are sufficiently "close" to the primary interface are rednered and all current secondary interface assignments are rendered in yellow. The rendering of all remaining unprocessed interfaces is again supressed.

 Ijn contrast to the defintion mode, V2K will not self-terminate in the patched interface editing mode. The user must do this manually by pressing the Done button. Once again, the Update button will become sensitized, and the user should press this button is all defintions are satisfactory. The neutral map file may then be written at any time.

Usage Notes

(1) When the patched interface facilities are invoked, it should be the users expectation, in general, that each of the remaining unprocessed interfaces is to become a patched interface. If this is not the case, V2K will never self-terminate the defintion mode, and there will be no opportunity to review any patche dinterface defintions via the editing features as the edit mode nor the facilities within the Interface frame will become available. Even so, the Update button will correctly introduce each patched interface definition as a Patched type within the neutral map file. The corresponding unprocessed interfaces will, however, remain in the file and must be removed manually.

(2)The state of an unprocessed interface (insofar as whether it has been successfully patched) is retained only within the current session of V2K. That is to say that if the user terminates a session before all patched interfaces have been defined (and V2K has self-terminated the defintion mode), at restart the state of each patched interface definition will be unknown. Though all secondary interface assignments are recoverable, it is unknown whether the defintion is complete. Thus, when the facilities are again activated, V2K will begin processing at the very first unprocessed interface. The user will have to re-save each valid defintion before continuing with the remainder of the new patched interface defintions.

(3)Patched interface defintions, whether completed or temporarily suspended by the user, are made available for inclusion into the neutral map file by pressing the Update button. If this is not done and the neutral map file is subsequently written it will be devoid of any patched interface defintions as well as all primary/secondary interface associations - all data will be lost upon the import of this file.