Lagrangian Deposition and TRajectory ANalysis
Rotorcraft Flow Analyses for Tactical Hazard Assessment

    In the mid-1990s, the U.S. Army Research Laboratory initiated an effort to directly include the effects of rotorwash on Chemical-Biological (CB) cloud deposition on helicopter fuselages, as part of its ongoing work on assessment of CB challenges to be faced by current and projected U.S. Army rotorcraft. These rotorwash effects may have a significant influence on CB agent deposition on the fuselage, affecting both CB survivability design features as well as decontamination procedures.

    One element of this work was the development at CDI of the computer program LDTRAN (Lagrangian Deposition and TRajectory ANalysis) and its companion helicopter wake flow field prediction model VTCALC (Vortex Trajectory CALCulation). These models predict the structure of the helicopter-induced flow field and then process the movement of CB particles through the predicted flow field. Recent work has also led to the addition of ground pickup of CB material, and its movement through the helicopter flow field and possible deposition on the fuselage. All of these features were incorporated in an overall mission-oriented model denoted CADARS (Chemical Agent Deposition Analysis for Rotorcraft Surfaces) operated by the U.S. Army Research Laboratory Survivability and Lethality Analysis Directorate (ARL-SLAD) at Edgewood Arsenal. The CADARS model has been applied to a wide range of developmental and operational Army rotorcraft since its inception in the late 1990s. The model is currently being incorporated by CDI into a brownout module to provide a high fidelity rendering of the brownout cloud in rotorcraft flight simulation and analysis software.

Typical flow field/particle transport prediction using the CDI LDTRAN model embedded in CADARS.

    The starting point for this problem is predicted clouds of CB material aloft, taking advantage of pre-existing U.S. Army cloud transport and diffusion models such as VLSTRACK. For small-scale problems close to the surface, however, it was felt appropriate to focus on local transport effects due to particle mass and aerodynamic drag. Modeling of such phenomena was done by drawing on development work for the FSCBG computer code, a general PC-based analysis of the dispersion of aerially released material maintained and marketed by CDI. This particular code originated in the 1970s, and has undergone extensive revision and development since 1989, including incorporation of an advanced near-field droplet transport model embodied in the AGDISP code and the drift code AgDRIFT, both of which were developed at CDI. These models supplied the key particle/droplet transport algorithms used in LDTRAN.

    Regarding the particular problems of rotorcraft operation in a CB environment, the first class of issues addressed involved basic features of the modeling of the rotor wake. In this work, it was judged likely that a simplified version of the full-span free wake model (originally developed in RotorCRAFT, parent of the current CDI CHARM comprehensive helicopter model) would ultimately prove the most cost-effective choice for projected modeling needs. Substantial reductions in the spatial refinement of the wake model were made without adversely impacting the level of accuracy required for this application. In addition, a number of other simplifications were made to the baseline wake and fuselage models, consistent with the goal of maximizing computational efficiency while focusing on the needs of the debris pickup and transport problem. These changes, coupled with the simplification of the wake model, left a tightly focused set of core routines whose primary task is computing the geometry of the vortex wake and its resultant velocity field. This software was packaged within a re-designated VTCALC subroutine set and used as a preprocessor for the trajectory analysis.

    An investigation of the effects of the presence/absence of the helicopter fuselage showed that for ground pickup the presence of the fuselage contributed little if any to the flow field seen by the debris. The following figure illustrates the trajectories of passive tracers lifted by the helicopter wake; as is evident. The structure of the flow field is dominated by the downwash and vortical effects generated by the rotor blades.

Ground transfer for a UH-60L Black Hawk in hover; particle size is 100 microns.

    These flow modeling tools provided the enabling technology for the overall CADARS model, which has been applied to full-spectrum studies of a wide range of U.S. Army rotorcraft, including the RAH-66 Comanche, the CH-47G Chinook, the UH-60L Blackhawk, and the AH-64D Apache.

Schematic depiction of vehicle deposition analysis for the RAH-66 Comanche using CADARS.