Flow Measurement

NRC Aerospace has developed and implemented several specialized techniques to measure the relevant properties of aerodynamic flows, typically in a wind-tunnel setting. Properties of interest could include the flow pressure and temperature, three components of velocity, or even sound waves. We continue to develop and improve implementation of these methods to increase our capacity for innovative research and client testing.

Following are some considerations when selecting a flow measurement technique:

• type of information required (ie. pressure, temperature, velocity) and location (ie. model surface, surrounding flow or downstream of model)
• quantity and resolution of information (ie. spatial or temporal resolution, global or point flow measurements, over a large or small area/volume)
• to what extent the studied flow will be perturbed by the measurement technique
• available access of the measuring device to the flow, either physical or optical
• whether mainly quantitative or qualitative results are required
• whether time-averaged or instantaneous flow information is required
• the acceptable amount of time to acquire and process the information, whether results are required in real time or after processing
• the particular flow conditions, since some methods can only be used for a certain range of flow velocity, pressure and temperature
• the desired accuracy of measurements, as each technique has an inherent accuracy and repeatability.

For example, particle image velocimetry (PIV) provides a great deal of spatial information on instantaneous flow velocity but at relatively low frequency. It is relatively unobtrusive but requires optical access to the flow. 

Rakes of pressure probes traversed through the flow of interest provide useful information on time-averaged quantities over a range of flow area but cause more disturbance in the flow. 

Traversing probes (eg. pitot-static, 5-hole pressure probes, hot-wire, microphones) through the flow are also more time consuming than global methods such as PIV or pressure sensitive paint (PSP), but the latter provides excellent global surface flow information with high spatial resolution. PSP results are both qualitative and quantitative, but require optical access. 

If optical access is not a problem, but physical access is limited, PIV and PSP can be good choices, allowing measurements to be made in locations inaccessible to conventional instruments.