CFD study on flow approach angle of a bi-directional velocity probe for fire testing

The present study examines the flow characteristics near the bi-directional velocity probe and estimates the probe constant associated with the Reynolds number and flow approach angle. A series of numerical experiments using CFX model are conducted to simulate the 3D flow field near the probe. The CFD model was validated with the best fit polynominal of the experimental data of McCaffrey and Heskestad. The calculated probe constants for a Reynolds number higher than 3000 are almost constant at 1.07 and match well with the conventionally-accepted asymptotic value of 1.08. Using the validated numerical code, a series of parameter studies were conducted. The angular sensitivity of the bi-directional probe increases when increasing the flow approach angle from 0 to 30 degrees, and the maximum angular sensitivity was observed near the flow approach angle of 30 degree. Based on the detailed analysis of flow structure, the present study shows that the pressure at the rear hole of the probe is more affected by the flow approach angle and influenced by the asymmetric wake flow behind the probe body. It is expected that this study can contribute to enhancing the reliability of velocity measurement in fire testing and be utilized as a preliminary study for design optimization of the bi-directional probe based on the understanding of detailed flow characteristics.