The bidirectional velocity probe has been used in various flames to measure local velocity. The device is based on the pressure difference between a closed forward facing cavity and a closed rearward facing cavity. The probes have been noted to indicate a pressure difference greater than that which would be predicted based on Bernoulli's equation. Each device must be experimentally calibrated in a wind tunnel at similar Reynolds number to determine its “amplification factor.” This study uses PIV, flow visualization and CFD to examine the flow field around the probe, as well as an experimental study which compares various probe configurations for measurement of velocity by pressure differential. The conclusion is that the amplification factor is indeed greater than unity but use of the wind tunnel for calibration is questionable.

The bidirectional probe has been used in combustion environments to measure localized flame velocities. The device measures the pressure difference between a closed forward-facing cavity, and a closed rearward-facing cavity. Each device must be experimentally calibrated to determine the “amplification factor,” which is unity if the measured differential pressures match Bernoulli’s equation. It is hypothesized that the probe design disrupts the flow, creating turbulence and irreversibilities, resulting in measured pressure differences. This study uses computational fluid dynamics, particle image velocimetry, and flow visualization techniques to examine the flow field around the probe, as well as an experimental study exploring the impact of minor changes in probe design on differential pressure measurements for velocity calculations.