Non-intrusive surface pressure measurement using Pressure Sensitive Paint (PSP) delivers distributed information of the aerodynamic force, which is one of the balancing forces (inertia-, elastic- and aerodynamic forces) acting on the object. The same force can be obtained experimentally from flow-field information, using the PIV- and LPT-based pressure measurement approach. This method essentially relies on invoking the flow governing equations for the exchange of momentum, which allows the pressure gradient to be expressed in terms of the measurement of velocity and acceleration (material derivative).
High spatial and temporal resolution with a direct measurement of the material derivative (particle acceleration) via STB (see Fig. 2 for a large scale volumetric experiment) or Tomo PTV has been achieved in past experiments that demonstrate the suitability of this measurement principle to tackle both steady and unsteady aerodynamic loads. Moreover, a new multi-pulse STB approach is available which is suited for investigation of high-speed flows and delivers the particle velocity and acceleration along short 4-pulse particle tracks. This new method has been assessed for pressure from LPT methods within the frame of the predecessor project NIOPLEX (see Fig. 3). In the scope of this project a numerical simulation of a transonic base flow was used to provide the basis for synthetic experimental data sets with the purpose of testing and validating pressure-reconstuction techniques. Several different techniques were assessed and compared, ranging from relatively standard two-frame PIV methods, to more advanced procedures based on multi-frame and Lagrangian Particle Tracking approaches. Either time-resolved time sequences were used, or multi-pulse bursts of limited duration, the latter being deemed more realistic to be reached under true high-speed flow applications.
This assessment revealed that under these conditions most methods could satisfactorily reconstruct instantaneous pressure fields, with the LPT-based techniques producing the more accurate results (up to r.m.s. error in pressure coefficient below 0.01) than the PIV-based approaches, which is attributed to a combination of higher spatial resolution of the input data and better use of time information in the data sets. Also, for PIV-based methods, the use of longer series of time-resolved input data allows more accurate reconstructed pressure fields. Therefore, the multi-pulse STB technique is opening a new field for aerodynamic and -elastic flow investigations at high Reynolds numbers and up to supersonic speeds e.g. for Shock-Wave-Boundary-Layer-Interaction (SWBLI) and buffeting with unprecedented accuracies. At the same time the (multi-pulse) STB method enables a very high resolution of mean statistical flow properties as the individual tracks can be averaged in bins of subpixel size which allows for a precise and highly resolved pressure integration e.g. along the whole wing with nose and trailing edge (or any model with strong curvatures) based on the mean or phase-locked velocity and acceleration fields.