Extending the velocity range of Robotic Volumetric PIV

Abstract

Car industry and motorsport development nowadays strongly rely on wind tunnel testing. As the literature survey shows, in order to fully understand the characteristics of the flow around a car, large-scale and volumetric flow field measurements are required. For its features, PIV can be pointed as a useful technique for automotive wind tunnels. However, its use in an automotive wind tunnel is strongly conditioned by the industrial environment itself, which requires particular efforts for what concerns safety and economic aspects.

The advent of Robotic Volumetric PIV has permitted the measurement of time-averaged properties of large-scale complex aerodynamic flows by the combination of coaxial volumetric velocimetry and robotics. Firstly, the use of Helium Filled Soap Bubbles as seeding permits to enlarge the measurement volume due to their higher scattering capability. Furthermore, the compactness of the coaxial velocimetry probe together with the robotic actuation allows to measure large-scale volumes, partitioning the entire volume and reconstructing the time-averaged flow filed from multiple PIV acquisitions.
Since its presentation, Robotic Volumetric PIV has been used to investigate low velocities flows, with free-stream values ranging between 2 m/s and 14 m/s. More specifically, 15 m/s can be considered the largest velocity that can be analysed with the state-of-the-art of the system due to its hardware limitations in the maximum acquisition frequency.
For this reason, to meet the requirements in terms of flow velocities proper of industrial environments, the range of analysable velocities has to be extended.

To address the identified requirement of high-speed volumetric measurements, a new acquisition technique has been designed and proposed. Even if a standard double-pulse strategy can measure high-speed flows due to the short time that can elapse between the two subsequent pulses, in order to increase the accuracy of the final results, a two-$\Delta t$ method is proposed. Firstly, a predictor is built using a Double-Pulse, Double-Frame acquisition with a short pulse separation time $\Delta t_1$. Afterwards, the predictor is used to allow the stretching of the second pulse separation time $\Delta t_2$ without encountering errors given by the false pairing that would be present due to the longer displacement of the particles.

To test the performance of the new method, the near wake of a $50\%$ replica of the Ahmed body with a 25$^{\circ}$ slant angle has been studied through an experimental campaign in the Open Jet Facility (OJF) of TU Delft. Firstly, a measurement at 12 m/s has been performed in order to be able to assess the performances of the proposed method w.r.t. the time-resolved acquisition strategy and the DP standard -DF approach.
Afterwards, an airspeed of 20 m/s has been considered, in order to demonstrate the capability of the new technique of extending the velocity range of Robotic Volumetric PIV.

The conducted measurements demonstrate the possibility of extending the velocity range of the Robotic Voluemtric PIV system with the proposed method. However, this is achieved at the expense of a lower accuracy, due to the lack of temporal information, and a longer measurement time, given by the necessity of multiple acquisitions.