Flow field measurements

Flow field measurements map the air flow near a model. The measurements can be classified into two categories:

• Intrusive measurements ─ instruments are located in the air flow and affect the flow;
• Non-intrusive measurements ─ instruments are located outside the air flow and do not affect the flow.

Some projects make use of one of these two measurement types, whereas other projects require both techniques. For instance, measurements of wake vortex often consist of an initial scan with a probe (intrusive) to obtain a general impression, followed by Particle Image Velocimetry (non-intrusive) for more precise and detailed measurements.

Intrusive measurements

Intrusive measurements utilize instruments placed in the air flow field. DNW uses the following types of intrusive measurements:

• Total and static pressure probes, single or multiple, mounted on a rake
• Directional probes with 5, 9 or 15 holes, mounted on a rake
• Hot wire turbulence measurements
• Hot film measurements

Probes

The multiple probes on a rake are used to perform simultaneous measurements of pressure in a number of locations (vector measurements). This method is extremely suitable for measuring wake vortices of aircraft. Calculations are based on average values of individual measurements of air flow in terms of time and space. The measurements are usually performed behind the model, as this minimizes impact on the air flow.

Wake vortex measurements are often used as an initial scan for more specific and more detailed non-intrusive measurements. Advantages of intrusive probe measurements compared to non-intrusive measurements such as PIV are the relatively short throughput time and lower cost.

Hot wire turbulence measurements

This method is based on the correlation between changes in temperature (measured by the wire) and air flow velocity. Measurements using one wire only measure speed, whereas measurements that use three wires also measure direction. 

Hot film measurements

This method is an alternative for hot wire turbulence measurements and is also used for measuring the status of the boundary layer (e.g. laminar and turbulent). The hot film used for surface flow consists of stickers on the model’s surface.These measurements are also based on the correlation between changes in temperature and air flow velocity. 

Non-intrusive measurements

Non-intrusive measurement instruments do not affect the air flow, and therefore do not affect the phenomena to be analyzed. 

DNW uses the following types of non-intrusive measurement techniques:

Particle Image Velocimetry

Particle Image Velocimetry (PIV) is used to measure the flow field. Therefore, PIV records the displacement and hence the air flow of very small tracer particles that are injected into the flow. Separate photos taken at ultra-short intervals of as little as 20 microseconds enable measurements of the speed and direction of the particles. The advantage of PIV is that it measures the entire flow field instantaneously, including movements of vortices.

Infrared Thermography

DNW applies InfraRed Thermography (IRT) for detailed surface flow visualizations of aerodynamic phenomena such as laminar-turbulent boundary layer transition. The physical principle of infrared thermography is based on the measurement of the (electro-magnetic) heat radiation of a wind tunnel model surface. Observed local surface heat radiation differences can be used to discriminate between locations with laminar and turbulent boundary layers respectively, due to their specific skin-friction and heat-transfer properties.

With IRT as transition detection technique, DNW provides customers a non-intrusive method to establish the performance of their designs for natural laminar flow (NLF) and/or hybrid laminar flow control (HLFC) technologies. It is also a cost-effective method for validation of forced laminar-turbulent transition tripping effectiveness at a multitude of flow conditions. Finally, IRT provides opportunities to investigate shockwave positions and/or locations with flow separation.     

Test models constructed from carbon-fiber reinforced plastics or polymers are naturally well suited for application of IRT, due to their favorably high surface emissivity properties. To enable IRT application on conventional metallic models, DNW applies on-site spray-painting of a 80 µm thin insulating coating.

For the measurement of heat radiation, DNW makes use of infrared cameras of the type A655sc, by FLIR. The cameras have a spectral range from 7.5 to 14 μm and a temperature resolution of about 30 mK. The 640x480 pixel cameras can be equipped with different lenses to tailor the spatial resolution for the objectives of a specific test. Typical area coverage is 70*70 cm and the accuracy of the measured transition position is approximately 2-3% (x/c). Data-acquisition and processing takes place with DNW-developed IRT software, which allows for recording of sequences of radiation images, adjustment of color-maps and storage of the processed recordings in either still-image frames or continuous video format. Results can be presented online in the control rooms of all DNW wind tunnels.

Laser Doppler Velocimetry

Laser Doppler Velocimetry (LDV) also measures displacement of tracer particles, but uses a series of measurements in one particular location. This means that LDV only measures phenomena in one position. The advantage of LDV is the high frequency of its measurements. However, measurements of an entire field require a series of separate measurements for which the system needs to be moved.

Stereo Pattern Recognition

In the animation above the trajectory of a dropped store from a helicopter is displayed. The trajectory was determined as part of a tailored solution in response to a customer request in a project managed by the Netherlands Aerospace Centre (NLR). The object to be tested was a scaled model of a helicopter, which in the event of an emergency must drop its payload in a controlled manner. The target was to accurately determine the trajectory and attitude of the payload after this emergency store separation under many different situations varying in speed, angle of attack and angle of sideslip to name a few.

DNW applied a measurement technique using high-speed digital cameras in a stereoscopic arrangement. This technique is based on tracking three-dimensional coordinates of markers applied to the store. The tracking technique has also been successfully employed by DNW in various other projects ranging from high-speed tracking of objects to wing deformation measurements of passenger airplane models. Furthermore, also high-accuracy helicopter rotor tracking experiments belong to the testing envelope of DNW 

In order to obtain the targeted information, a scaled experiment in a wind tunnel was performed. The model stores were Froude-scaled, 3D printed and subsequently had markers applied to them. A two-camera measurement system was installed on the test section of the DNW-LST, observing a pre-determined volume around the helicopter in which to track the store. This volume was calibrated using a high-precision three-dimensional marker grid with accurately known coordinates. The calibration allows one to establish the relation between the two-dimensional image planes of the digital cameras and the measurement volume.

After the wind tunnel conditions were set, the rest of the measurement was executed in an automated sequence. Wind tunnel illumination was dimmed and a high-power UV-light source was activated to generate sufficient illumination and contrast for marker detection by the camera system. A trigger signal starting the acquisition of 500 frames at a frame rate of 500 Hz was generated together with the activation of the release mechanism of the helicopter store.  This sequence enabled a good resolution of the trajectory, since a single frame only spans 2 milliseconds resulting in sharp enough images. A high-intensity illumination source is required to accomplish this high resolution. 

The 500 frames were subsequently processed to identify the markers on the store, calculate the precise three-dimensional coordinates of the markers on the store and then calculate the position and 3D attitude of the store. In this way over one hundred trajectories have been measured under varying aerodynamic conditions resulting in a productive campaign and a satisfied customer, who by now has the data to predict the envelope of conditions in which the store may be safely released.