A fluid dynamic valve passively allows fluid flow out of a moving stream in one flow direction and not in the reverse. This allows the collection of fluid from a single direction of an AC fluid flow. The siphoned portion of the flow has a flow rate proportional to the mainstream flow. This device can collect exhaled breath or selective entrenchment during inhale. In one orientation, it can meter aerosolized particles into an inhale breath stream for pulmonary delivery, without complicated breath timing or drug loss due to drug adsorption to the back of the throat. Alternatively, a user can breathe through the device and a proportional amount, relative to the volumetric flow rate, of each exhale can flow into an auxiliary chamber for analysis. In addition, the device has a low respiratory burden and is comfortable to use.