Pressure measurement is one of the fundamental measurements in fluid dynamics experiments. Different tools and devices are available to access at the local pressure level on/around a model.
Surface pressure measurement:
- PSP (Pressure Sensitive Paint) techniques,
- Static pressure taps.
PSP - Pressure Sensitive Paint
Pressure sensitive paint (PSP) is a relatively new tool that has the unique capability of providing a field measurement over the entire surface of a model. Our PSP system is an intensity based system.
Data Acquisition & Post Processing
The pressure field is computed from the luminescence of the test article captured using digital cameras. Thus, PSP is essentially an image based technique. Usually the processed data is mapped to a 3D grid of the test article. Efficient image processing software already existed at ONERA to avoid any manual involvement during data processing.
The complete data processing time of a PSP point is about 1mn for 1 camera. As the results are provided on the 3D grid of the model, it is very simple to match together results coming from different cameras and to perform additional processing like force and moment integrations from the pressure field.
The PSP technique is available on a routine basis in all high speed facilities. At ONERA Wind Tunnel Division, PSP technique is really considered like a quantitative technique, this is the result of specific optimization of all the system parts (paint, illumination, cameras and moreover dedicated software). Accuracy of about 0.02 in CP is commonly achieved in high speed.
In low speed, the PSP technique has been successfully demonstrated at ambient pressure. At high pressure (for testing in F1 at high Re conditions), satisfactory results have been obtained, however, the equipment must still be optimised in order to obtain acquisition time compatible with industrial requirements.
Technique available at: S1MA, S2MA, S3MA, F1 (in atmospheric conditions and pressurized conditions, up to 3,8 bars) and F2.
Some developments in progress at ONERA
- PSP unsteady measurements: in development in the ONERA scientific department. Preliminary wind tunnel tests on-going in research facilities (qualitative results).
- PSP on rotating models: objective is to measure a steady state pressure distribution.
Static pressure taps
These pressure taps could be used for steady or unsteady measurement (with Kulite sensor).
Pressure measurement on the model surface can be measured if the model includes pressure taps. This can be useful for pressure dominated phenomena but this only accounts for normal forces of the body.
The hole diameter on the model surface is around 0.4mm. This hole is connected to the pressure sensor by tubes:
Usually, the length between the sensor and the pressure hole location have to be minimize to reduce time response time.
- Inox tube: internal diameter = 0.4mm / external diameter = 0.6 mm
- Silicon tube internal diameter = 0.6mm / external diameter = 1 mm
A new technology for pressure tap installation: The classical manufacturing process for the installation of pressure tubes in a model involves removable parts (cavity covers, separate wing tips, etc.).
The new process proposed by the engineering department consisted of the laying of pressure tubes along the machined-out grooved core of the part (wing, control surfaces, etc.), that is to be equipped with pressure taps. This equipped core was then filled with electro-deposited copper, and milled to the desired shape with the rest of the model part. The end result was the model part had internally integrated pressure tubes. Finally, the pressure taps were drilled from the outside into the precisely located internal pressure tubes.
Individual transducers or scanner are used in ONERA wind tunnels.
Available hardware is:
- Druck, Entran, Bell and Howell, Digiquartz with the range from 1PSI to 3,500 PSI.
- PSI units (48 and 64 ports) with a range 5, 15 and 30 PSI.
- Kulite, thermally compensated for sensitivity and zero drift.