Flow structure of the ridge integrated submerged inlet
Introduction
Submerged inlet concepts have become an important issue in subsonic drone design studies. Flights of the Boeing MQ-25 Stingray aerial refueling drone in 2019 demonstrated the implementation of such an induction system on a long range heavy UAV. Fig. 1 shows the inlet entrance on top of the drone [1]. Lower radar cross-section and drag, compact geometry for a tube launch systems and lighter weight are the important attributes of this design philosophy. The best known flush inlet was developed by the National Advisory Committee for Aeronautics (NACA) in 1945 for application on auxiliary air inlets [2], [3] and a similar design is used as the inlet for the Teledyne CAE J402 turbojet on the McDonnell Douglas Harpoon anti-ship missile (1977-pesent). A series of articles by Taskinoglu [4], [5], [6] between 2002 and 2004 reports on studies for design and optimization of a submerged inlet on a generic cylindrical fuselage. Another example with a rectangular entrance was presented by Sun in 2007 [7] and a similar concept can be found in Wang's paper published in 2016 [8]. The intensity of entrance-induced vortices and the thickness of the ingested upstream boundary layer are the main factors that affect the internal flow quality of the submerged inlets [9]. The flow separation at the bends and secondary flow formation is common problematic phenomena in curved diffuser ducts, related to the nature of the boundary layer, and these are well studied for a variety of induction systems [10], [11], [12], [13], [14], [15]. These problems are more serious for inlets that ingest the upstream boundary layer inlets because without effective flow diversion before the entrance, the resulting thickened internal boundary layer becomes highly sensitive to the adverse pressure gradient inside the duct, with a tendency to separate, as well as induce strong secondary flows [16], [17].
Up to now, a few practical solutions based on an internal array of vortex generators, active jets, bleeding or combination of these techniques have been used to improve the boundary layer stability and reduce the flow distortion at the compressor face [18], [19], [20], [21]. These solutions are mainly focused on controlling the internal flow and boundary layer patterns, and solutions for diverting the external boundary layer are also under research. For example, installation of a bump-shaped vortex generator upstream of the inlet as a boundary layer diverter were investigated by Shu Sun [22] in 2016. Results of this research show improvements in pressure recovery, but the paths of the vortices around the entrance were not determined.
In this paper, a ridge surface [23], [24], [25] is used as a diverter upstream of the inlet. The basic structure of the ridge and its flow pattern without integrated inlet are shown in Fig. 2-a and b. Our previous studies show that this surface diverts the upstream boundary layer by transferring it into a pair of counter rotating streamwise vortices along a diverging path. The combination of vortices and steep cross-sectional pressure gradients creates a powerful trap for capturing the low speed boundary layer (BL) from a uniform straight path into a vortical flow pattern along a preset direction. Controlling the vortex based on the aerodynamic characteristics of the ridge is the unique property of this shape [23], [24], [25]. To investigate the effects of a ridge surface on the propulsive efficiency, a submerged inlet with a triangular entrance is designed and used for flow simulations. The resulting flow structure of a ridge/inlet configuration is investigated and compared with available data.
Section snippets
Ridge/inlet model for CFD simulation
The area between two ridges (swept width) is the candidate location for the inlet entrance. The ridge geometry is similar to that in Ref. [17] but instead of a blunt edge (along the tip) a sharp tip with inclined angle of 21° is used. The ridge/inlet combination is shown in Fig. 3-a and b. In order to maximize the flow capture area, a triangular geometry is used in the design. Flow distortion downstream of the aft lip inside the duct is an important consideration from previous research [26].
Results and discussions
The boundary layer developed from a planar upstream surface results in considerable reduction of pressure recovery and significant flow distortion inside the baseline duct. In contrast, the flow conditions for the ridge/inlet combination are completely different, and details are described in the following subsections from the results of the density-based solver.
Conclusion
Flow structure and efficiency of a submerged inlet, designed for integration with a ridge surface is investigated using computational fluid dynamic simulations. The ridge configuration diverts the upstream boundary layer before the entrance by transferring it to a pair of streamwise vortices along a predefined path. In the current design, the safe passage of vortices at the sides of the inlet entrance is a requirement for design integration. The aerodynamic efficiency of the inlet is measured
Declaration of Competing Interest
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript.
Acknowledgements
Inlet team members in the college of Power and Energy of N.U.A.A are gratefully acknowledged for their cooperation. This research is supported by National Natural Science Foundation of China (No. 11872207) and Aeronautical Science Foundation of China (No. 20180952007).
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- 1
Lecturer in College of Aerospace Engineering, Nanjing University Aeronautics and Astronautics, 29 Yudao St., Nanjing 210016, China.
- 2
Ph.D. supervisor in State Key Laboratory of Mechanical and Control of Mechanical Structures. 29 Yudao St., Nanjing 210016, China.
- 3
Ph.D. supervisor in College of Energy and Power, Nanjing University Aeronautics and Astronautics. 29 Yudao St., Nanjing 210016, China.