This paper investigates how the external geometry of a five-hole probe affects its accuracy and how internal geometry affects its settling time. An analytical model, which predicts settling time, is used to design an accurate, fast-settling, millimetre-scale probe. The paper has three components:
First, results are presented from a series of area traverses performed with five-hole probes which range in head diameter from 0.99 to 2.67 mm. It is found that the smallest probe gives the greatest accuracy when traversing the shear layers in blade wakes. However, it takes 3.4 times longer to complete this traverse than compared to the largest probe. This is because traverses with small probes require more time to allow the pressure readings to settle between each traverse position.
Second, an analytical model is developed which predicts settling time based upon the internal geometry of the probe. The approach adopted is capable of modeling any number of connected tubes with different lengths and diameters. It is validated against experimental measurements and is shown to give good agreement. This model can be used to ensure that probes are designed with acceptable settling times.
Finally, the analytical model is used to design an optimised five hole probe. Use of the model highlights two important results which are required to reduce settling time: First, the length of the smallest diameter tubes, i.e. the ones in the probe head, should be minimised. Second, the volume of tubing downstream of the head should be minimised. Applying these principles to a new probe design cuts the total traverse time by 71%, whilemaintaining the highest value of accuracy.