Unsteady Incidence and the Correction of Pneumatic Probe Measurements

Abstract

Abstract Pneumatic probes such as five-hole probes (5HP) have many advantages and can measure flow angles, total and static pressure, velocity and Mach number. In most applications, pressure transducers are connected to the probe head by lengths of tubes, allowing a simple, miniaturised, low-cost probe construction. Such “steady” probes therefore measure a “pneumatically-averaged” flow field, which is of sufficient accuracy for many purposes. However, this paper demonstrates that when the probe incidence angle fluctuates, pneumatic averaging causes errors in the indicated total and static pressure. The aim of this paper is to understand these pneumatic averaging errors and to demonstrate practical means to post-correct data. A quasi-steady model provides an analytical framework to explain the effect of unsteady flow on 5HPs. Total and static pressure coefficients have a largely symmetric response to positive and negative incidence, causing a bias error in the pneumatic average. The steady calibration cannot account for these effects, leading to erroneous measurements. These errors are demonstrated by comparing 5HP and Kiel-shrouded pitot traverses in the shedding wake of a D-shaped body. The 5HP overestimates total pressure loss and drag coefficients by up to 44% and 60% respectively. Similar pneumatic-averaging errors will have affected a substantial body of data in the literature. A post-correction method is demonstrated that can be applied to historical data. Estimates of unsteady flow angles are obtained from a low-cost unsteady computation. The quasi-steady model accurately corrects the 5HP data, reducing errors by an order of magnitude. Similar corrections are obtained using simple correlations based on the root-mean-squared angle fluctuation.