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DGV Measurement Examples: Engine Inlet

This application example describes the measurement of the flow field in the cross section of an engine inlet duct within an 1:10 scale aircraft model. The model itself was mounted in the 3 × 3 m² wind tunnel test section of DNW-NWB at DLR Braunschweig (Image of the model within the tunnel). The investigation was aimed at uncovering velocity inhomogeneities upstream of the engine as a function of the aircraft's angle of attack, yaw angle and air speed.

The application of DGV within a wind tunnel mounted aircraft model posed a number of engineering challenges: 1) the light sheets had to remain stationary with respect to the inlet duct as the angle of attack is varied, and 2) due to size restrictions the camera system had to be located externally. A flexible endoscope was utilized to transmit the image data to the camera system. Three miniature scanning light sheet devices, roughly 50 × 50 × 15 mm³ in size, supplied with laser light via 10 µm fibers, along with 90° folding mirrors were mounted on the exterior of the duct such that the cross section was illuminated by three light sheets intersecting at 120°.

The cross section spanned by the three sheets was imaged from a downstream position via a flexible endoscope which was mounted into a stem as indicated in the figure. The rather poor spatial resolution of the endoscope did not pose a problem here because DGV does not need to resolve discrete particles. Techniques requiring the imaging of discrete particles, such as PIV, would not have been able of tolerating this loss in spatial resolution. This aspect makes the use of DGV method especially attractive in environments with poor optical access.

Initial validation tests were performed on the inlet duct during a ground testing phase. A vaccuum system attached downstream of the inlet and optical module simulated the engine air supply at flow speeds up to 210 m/s. The DGV measurements were compared to simultaneously acquired orifice plate mass flow measurements as well as laser-transit anenometry (LTA = L2F) data. The correlation between the differently acquired data is quite good except at higher flow rates. Interestingly enough the deviations occurred past the point where the flow in the smallest cross section became sonic (choked).

Mass flow

One of the reconstructed time-averaged flow fields obtained during the wind tunnel phase of the project is shown in the figure below. A strong vortical structure along with an unbalanced axial flow profile can be observed. Important design parameters can be derived from this type of data such as the total pressure loss and total pressure distortion.

Flow field in the cross section of an engine inlet of an aircraft model placed in the 3 × 3 m2 DNW-NWB wind tunnel of DLR-Braunschweig.