The frame grabbers


Tests done on models of a military cargo aircraft in the Duits-Nederlandse Windtunnels (DNW) in Marknesse, the Netherlands, have given a new meaning to the adage ‘a picture is worth a thousand words’. Huge volumes of position and flow visualization data, produced by high-speed machine-vision cameras set up in a wind tunnel, have explained the behavior of hoses that trail behind the aircraft while it is used as an airborne tanker in air-to-air refueling operations. Frame grabbers played a vital role in these tests by helping engineers capture, analyze and understand the complex picture of movements and aerodynamic interactions in the wind tunnel itself.

When the Brazilian Air Force contracted Embraer Defense and Security to design, develop and build the twin turbofan KC-390 military transporter, it set out specific requirements for the aircraft.

Commenting on the project, Paulo Gastão Silva, vice-president, KC-390 Program, Embraer, says, “The development of the KC-390 military transporter is one of the biggest engineering challenges our company has faced. In particular, the aircraft has to be versatile, as different missions require different configurations.”

According to Gastão Silva, wind tunnel testing is one of the most important parts of the development process and a range of special tests have already been carried out at DNW to address aspects of the design.

He continues, “For the challenges we face, we employ DNW because it offers the best testing technologies for our wind tunnel campaign. DNW’s large low-speed facility (LLF) enables development models to be tested and updated, giving us confidence in the design before the real aircraft is built and takes to the air.”

Wind tunnel testing

Wind tunnel testing has been, and will continue to be, an essential part of the development process for the KC-390 before it makes its maiden flight, which is scheduled for 2014.

Engineers at Embraer are aiming to create a multipurpose aircraft that is able to operate in a range of environments and climatic conditions. The designers also need to ensure that it is not only capable of being refueled in flight, but can also be quickly configured as a tanker itself using a trailing hose and drogue. The latter, which resembles a windsock or shuttlecock, stabilizes the refueling hose in flight.

An air-to-air refueling operation is fraught with difficulties and risks. A detailed analysis of the movement of the KC-390’s refueling hoses, caused by vortices or turbulence, is now helping engineers understand their complex behavior and ensure that the approaching aircraft safely establishes a connection with the moving drogue. In addition, though it is not possible to produce a theoretical model to accurately describe the movement of the hoses, wind tunnel tests showing the actual motion are used to investigate different angles of attack of the aircraft’s wings. This is necessary to study the effects of the cruising speeds required to enable slow- or fast-flying aircraft to be refueled by the tanker.

In the case of significant or erratic movements, empirical wind tunnel tests can be used to reveal design flaws. For instance they are able to show that the housing on the aircraft’s wing, from which the hose extends, may need to be modified or its position changed.

Twofold test regime

Relying on an imaging system developed and integrated by Forschungsinstitut für Bildverarbeitung, Umwelttechnik & Strömungsmechanik (FIBUS) (Institute for Image Analysis, Environmental Control & Flow Mechanics), which uses high-speed machine-vision cameras in conjunction with Teledyne DALSA’s Xcelera-CL+ PX8 full frame grabbers, the work carried out at DNW’s LLF comprised two components.

The first involved using stereo pattern recognition (SPR) to measure the behavior and track the motion of air-to-air refueling hoses while extended from the model aircraft. The second was used to determine the exact position of the model of the aircraft in the wind tunnel’s test-section, with respect to the particle image velocimetry (PIV) laser sheet. This is essential so that the complex flow field behind the wing can be analyzed (using PIV) to determine how tip vortices and turbulence influence the motion of the fuel hoses.

These tests presented various challenges, as Frenk Wubben, senior project manager, DNW, explains: “Previously used techniques applied to solving such challenging problems proved inadequate. The customer’s request was unique, so we had to find a better technique to measure the behavior of the hoses trailing behind the wind tunnel model.

“Furthermore, in the past we have calculated the position of models using support kinematics. The accuracy of this approach is limited because of the deflection of the support system, which can be only partly corrected. For this test, the position of the model needed to be determined with greater precision to accurately position it with respect to the PIV laser sheet. In other words, the position of the laser sheet with respect to the model had to be known accurately.”

Machine vision

Measurement data was obtained from images captured by two Bonito CMC-4000, four-megapixel cameras from Allied Vision Technologies, mounted on the ceiling of the test section of the wind tunnel. By illuminating the markers under UV light generated by high-power UV LEDs (395–405nm), high-contrast black-and-white images with no reflections were obtained.

Each camera was connected to two Xcelera-CL+ PX8 boards. These frame grabbers are based on the PCI Express x8 interface. Commenting on the machine vision setup, Dr Reinert Müller, FIBUS, says, “The complete system is complex because it consists of a fast SandyBridge-E computer system including fast graphics, four Xcelera-CL+ PX8 boards and a digital I/O card from National Instruments.

“In order to reach the desired high frame rate of 400fps, the cameras use a special left-half/right-half imaging mode. Each camera is connected to two frame grabbers, which are responsible for transferring their two ‘half images’ to a contiguous image buffer without any CPU overhead, using the unique child-buffer mechanism of the Sapera API. This configuration enables tracking to be done in real time.”

Müller continues, “The 1500MB/s bandwidth of the cameras required the use of high-quality Camera Link cables to enable data to be conveyed over the 8m distance to the computer. Illumination using the UV light was difficult because of a shortage of the high-power LEDs that were specially designed for this measurement. As a result the acquisition and processing speed was limited to a maximum of 200fps. In future the use of a greater light output should enable us to reach the maximum speed of the camera of 400fps, making tracking the moving fuel hose much easier.”

In order to take the fuel-hose and PIV measurements, SPR is used in dynamic and static modes respectively. Wubben adds, “In dynamic mode an acquisition time of 1.5 seconds was used to measure the position of the markers on the hose, with a sample frequency between 100Hz and 200Hz. In static mode, an acquisition time of 1.5 seconds was again used to measure the position of the markers on the model, at a sample frequency of 10Hz. “With a maximum sample frequency of 400Hz for the cameras and a resolution of four megapixels, the accuracy of the system is in the order of 1mm during dynamic and static measurements.”

PIV measurements

In parallel with PIV measurements, the positions of the markers on the model were measured using SPR, as Müller explains: “The twofold tests are necessary to understand what is going on. If you do the deflection measurement only with the SPR system you will learn about the motion of the fuel hose and wing, but you will never understand what causes the measured motion. The use of PIV was necessary to learn about the flow-field behind the wing. This complex flow-field, with tip vortices and turbulence, influences the motion of the fuel hose.”

PIV measurements are performed at positions. The desired PIV plane center coordinate is provided by the customer. Using the measured coordinates from the markers, the position of the model reference point (MRP) in the tunnel axis system is calculated using dedicated DNW software.

Wubben says the desired coordinates of the PIV plane are calculated using three data sets – the actual position of the MRP in the tunnel axis system, the relevant PIV plane center coordinate in the model axis system, and the model’s angle of attack and side slip angle.

“The PIV plane is measured within the laser sheet. The position of this sheet in the tunnel axis system is traversed in a stream-wise x-direction. The (y, z) position with respect to the MRP is reached by traversing the model in the y and z directions,” explains Wubben. “Adjustments to the position of the laser sheet and model are necessary until the actual PIV plane position matches the desired PIV plane position. The procedure is checked with a known coordinate on the model.”

Problem solved

The machine-vision system was delivered in May 2012 and was used for SPR measurements over the following months. “It worked flawlessly,” says Müller. “It is the only system capable of solving the problem we faced, which was to track the extended refueling hoses at the required high resolution and tracking speed of 200fps.”

FIBUS says it is now starting to set up a complete real-time PIV system using DALSA grabbers and the Bonito cameras. According to Müller the first tests are scheduled for this summer.

The wind tunnel

DNW German-Dutch Wind Tunnels recently completed

the aerodynamic testing of an advanced wind tunnel model of

the Embraer KC-39. The wind tunnel model was designed and manufactured by the National Aerospace Laboratory of the Netherlands (NLR). The Embraer KC-390 wind tunnel model is an update of a previously tested older model. The latest wind tunnel tests included simulated inflight refueling of the aircraft, as well as payload parachute testing. In addition, a number of specially developed complex measurement techniques were applied to examine specific phenomena such as the velocity flow field. This wind tunnel test is one in a series of tests being conducted for Embraer. The project is based on close collaboration between DNW and NLR, with each party contributing specific expertise to offer the client a ‘one-stop-shop’ approach. DNW is equipped with a system that is capable of monitoring turbulence and vortex effects using an optical flow measurement technique called particle image velocimetry (PIV). DNW developed a high-power LED UV illumination source capable of illuminating approximately a 10 x 10m area of the aircraft under test.

Measurement techniques

The hose and drogue system, designed and manufactured by the Nationaal Lucht- en Ruimtevaartlaboratorium (NLR, the National Aerospace Laboratory of the Netherlands) according to strict specifications set out by Embraer, was remotely controlled during the tests to simulate air-to-air refueling during flight.

Markers coated with UV paint were applied to both fuel hoses to enable their positions to be measured while they were extended at a constant speed from the wind tunnel model. Because of their dynamic movement it was not always possible to track the markers, so as a fall-back position, measurements could also be taken with the hoses fully extended. Similar markers were applied to the upper side of the model’s fuselage at well-defined positions along its axis to enable the MRP to be calculated. SPR was then used to optically determine the spatial position of the markers, using cameras set up in stereo mode.

Inder Kohli is product manager at Teledyne DALSA. Dr Reinert Müller is director at FIBUS, the Research Institute for Image Analysis, Environmental Control & Flow Mechanics based in Germany

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