Design of a High-Throughput Chemical Trace Detection Portal That Samples the Aerodynamic Wake of a Walking Person

The aerodynamic wake behind a walking person is first investigated using flow visualization and spectrophotometry in order to characterize the plume-to-wake transition and the decay of a trace body contaminant downstream of a walking human. The buoyant human thermal plume is shown to transition to an aerodynamic wake at a walking speed of 0.2 m/s. At higher walking speeds, the human wake is highly turbulent, resulting in an exponential decay of a passive scalar downstream of the walking human subject. From these results, a high-throughput chemical trace detection portal that uses the natural momentum of the wake to assist its collection is designed. This wake collection portal is shown to be capable of substantial improvement in security-checkpoint throughput over previous trace-detection-screening technology. To collect and sample the trace-bearing wake for a chemical signal, air flow rates nearly an order of magnitude larger than those of the previous collection technology must be accommodated. A high-flow-rate particle impactor was designed and tested to effectively collect the entire human aerodynamic wake. This impactor accommodates a flow rate of 750 L/s with a particle cutoff diameter of 5 μm. An iterative CFD process is described which streamlined the impactor geometry to reduce the total pressure drop across it. Experimental results are presented on the impactor collection efficiency. Preliminary experimental results show that trace chemicals can be detected at a useful level from a range of locations across the body when a human subject passes through the system wearing a chemical particle patch source.