Surface Pressure Measurements using Coaxial Volumetric Velocimetry

Abstract

In experimental aerodynamics, pressure is an important parameter which provides insight into the various features of the flow around the test object. By obtaining the surface pressure information, various inferences can be drawn such as flow separation, local flow velocity etc. Traditionally, the most common method of obtaining pressure has been through direct measurements by incorporating the test model with pressure taps, which makes the model expensive and complicated. However, often there arises a requirement to obtain qualitative information and visualise the aerodynamic features during the test. Particle Image Velocimetry (PIV) solves provides this qualitative information as well as quantitative information in the form of a flow field. Pressure can be further computed using this velocity field. This technique holds a number of advantages over traditional wind tunnel measurements, especially with reduction in resources for manufacturing, ease of handling and non-intrusiveness. However, the conventional PIV technique came with the disadvantage of only having 2D2C information. Other techniques like stereo PIV and tomographic PIV were developed to tackle these limitations and the most recent development was that of Coaxial Volumetric Velocimetry (CVV). Along with the advent of Helium Filled Soap Bubbles (HFSB), this novel technique allows for large scale 3D3C measurements. This research is conducted to assess the feasibility of this promising technique to obtain surface pressure.
The flow field around an inverted wing in ground effect is investigated using CVV by robotic manipulation. Using HFSB as tracer particles, the experiment was carried out in the W-Tunnel of TU Delft. The wing was tested at two angles of attack, α = 5° and α = 8° and three ground clearance configurations
(h/c= 1, 0.6 and 0.3 at α = 5°) at airspeed of 10 m/s. The wing is equipped with a total of 30 surface pressure taps along the chord-wise direction, both on the pressure and suction side. The velocity field is obtained using Lagrangian particle tracking algorithm, Shake-the-Box and time-averaged velocity field is obtained by ensemble averaging of the obtained particle tracks. Static pressure is calculated using Poisson’s equation using the information obtained from velocity field. The pressure thus obtained is then compared with the readings obtained from pressure taps.
Such an assessment not only shows the feasibility of the technique but also highlights the shortcomings and potential improvements. To the best of the author’s knowledge through a literature study, this particular technique has not been used to obtain surface pressure information. Most of the work published in the field of CVV is to study flow features, mostly in the wake. Hence, it is expected that this research and the recommendations help to further development of such a technique in the future.