Flow physics of discrete boundary layer suction – measurements and predictions

Abstract

The primary objective of this work is to determine the detailed characteristics of the flow features induced in a boundary layer by suction through laminar flow control (LFC) perforations. An additional goal is to validate a predictive method for generic LFC suction surfaces and to apply this technique to typical flight condition configurations. Fundamental insights into the flow physics of LFC suction surfaces are obtained from a unique series of high-resolution three-component laser Doppler velocimetry (LDV) flow field measurements. The flow fields induced by isolated super-scale perforations under low-speed conditions are mapped and found to be strongly three-dimensional and profoundly different from the idealized concept of continuously distributed suction. Over a range of sub- and super-critical suction flow rates a variety of suction-dependent complex flow features are identified, including a pair of contra-rotating streamwise vortices, multiple co-rotating streamwise vortices, spanwise variations of the mean flow and inherently unstable boundary layer profiles. The measurements reveal that suction-induced transition commences with an instability of these attached vortices, resulting in the development of a pair of turbulent wedges downstream of the perforation. A finite-volume Navier–Stokes method is validated by simulating a variety of low-speed experiments and comparisons are made between the LDV measurements and the predicted flow field. The computational technique reproduces all of the observed flow features, although it slightly under-predicts their magnitude and extent. By analysing the predicted flow fields the mechanism for the formation of the trailing vortex pair is established. Earlier flow visualization experiments, which exhibited vortex shedding, are also simulated by solving the time-dependent governing equations and it is found that the principal unsteady flow features are captured. Despite the challenge posed to the computational method by the diverse range of flow phenomena induced by discrete suction, the predictions provide good agreement with the measurements and observations. The computational tool is subsequently applied to predict the flow fields of single and multiple rows of actual-scale micro-perforations under low-speed and typical transonic flight conditions. A range of suction-induced flow features are predicted and a variety of distinct flow modes are identified. The low-speed critical suction limits are also measured and a design criterion, based on the sucked streamtube characteristics, is established. The basis of this critical suction criterion is also validated for transonic flight configurations.