Diverterless Supersonic Inlet (DSI)
The Diverterless Supersonic Inlet (DSI) is a novel air inlet design principle, used most famously on the Lockheed Martin F-35 Lightning II. A Diverterless Supersonic Inlet (DSI) compared to a conventional intake can reduce the weight and weight is the primary driver to reduce cost and increase performance of a fighter aircraft.
Modern fighters require excellent inlet/engine adaptability under wide flight conditions, taking into consideration the low radar cross section (RCS). The DSI, also referred to as the Bump inlet, provides this solution. A three-dimensional surface, or a bump, functions as a compression surface on the surface of the fuselage which creates a pressure distribution that pushes boundary layer air away from the inlet. The bump inlet separate the boundary layer by the high pressure zone generated by a 3-demensional bump before the inlet. The inlet cowl lips are designed to allow most of boundary layer flow to spill out of the aft notch.
The DSI structure complexity is greatly reduced by the removal of moving parts, a boundary layer diverter and a bleed or bypass system thus decreasing the aircraft’s empty weight, production cost, and requirements of maintenance-supporting equipment. Furthermore, by eliminating the surface discontinuity of the diverter, the forward sweep cowl lips and the S-shaped duct, which house the mental blades within the engine’s compressor, the bump can efficiently decrease the RCS. The DSI has been the focus of renewed research after initial NASA work in the 1950s. Simon et al studied an external bump inlet in a direct comparison with a traditional two-dimensional compression ramp. It was determined that the bump inlet outperformed the ramp inlet over a range of Mach numbers from 1.5 to 2, with both surfaces employing boundary layer bleed.
More recently, DSI work was done by Lockheed Martin engineers in the early 1990s as part of an independent research and development project called the Advanced Propulsion Integration project. The concept was developed and refined with Lockheed Martin-proprietary computer modeling tools made possible by advances in Computational Fluid Dynamics, or CFD. The overall inlet design moved from concept to reality when it was installed and flown on a Block 30 F-16 in a highly successful demonstration program. A Block 30 F-16 was modified with a diverterless supersonic inlet (DSI) by Lockheed-Martin in 1996 to test the feature for inclusion in the F-35 Lightning II. The inlet was found to successfully replace conventional methods of controlling supersonic and boundary layer airflow at speeds of up to Mach 2.0 with lower cost and complexity.
By 2010 there were five aircraft designs under development that had successfully integrated DSI, one was F-35 Joint Strike Fighter, and the others were the J-10, JF-17, J-20 and J-31 multi-role fighter aircraft developed by China [and Pakistan]. This renewed focus stemmed from the lack of a boundary layer diverter, which makes it stealthier compared to other conventional inlets. Researchers also found certain favorable characteristics of DSI compared to other “fixed” inlets, such as higher total pressure recovery, lower flow distortion, better compatibility at supersonic speeds etc. that made it suitable for application on supersonic aircraft.
Pressure recovery is defined as the average total pressure at the engine face, Aerodynamic Interface Plane (AIP) divided by the freestream total pressure. Distortion is a measure of how uniform the total pressure is at the AIP. Factors which reduces the recovery is flow separation, boundary layer ingestion and shock interactions. At high speeds, the intake needs to slow down the flow before it reaches the engine face, favorable around Mach 0.5.
On aircraft with engines installed on wing pylons, which is the most common configuration on transport- and passenger aircraft, the inlet is short and leads directly to the engine and the pressure recovery is nearly 100%. For engines that are integrated with the body, for example on fighter aircraft, the airflow is travelling along the body of the aircraft before it reaches the air intake. A boundary layer builds up along the body, something which is not desirable. The inlet stands off from a particular surface, allowing the boundary-layer on that surface to escape down the intermediate channel.
The subsonic flow field around the forward fuselage and the intake consists of free stream flow being modified by the presence of these bodies. The flow adjacent to the fuselage/intake generally follows their contour while far away it merges with the free stream flow. The supersonic flow field around the forward fuselage and intake consists of free stream flow being modified by shock and expansion waves due to the presence of these bodies.
The external flow field of a fuselage / intake combination is important since it determines not only the quality of air available to the engine but also the intake drag of the aircraft. This becomes even more important for a supersonic aircraft. The internal flow field of an intake duct plays an equally important role in determining the quality of air (total pressure, Mach number and flow distortion) to the engine.
To prevent the boundary layer from entering the inlet, or at least to minimize the amount that does, it is common to use a boundary layer diverter. The diverter separates the inlet from the fuselage and the boundary layer, but this design feature causes the inlet weight and drag to increase and with higher maintenance requirements. It is also a negative factor when it comes to radar cross-section.
Historically, inlet complexity is a function of top speed for fighter aircraft. Higher Mach numbers require more sophisticated devices for compressing supersonic airflow to slow it down to subsonic levels before it reaches the face of the engine. The F-15 inlet, for example, contains a series of movable compression ramps and doors controlled by software and elaborate mechanical systems. The ramps move to adjust the external and internal shape of the inlet to provide the optimum airflow to the engine at various aircraft speeds and angles of attack. Doors and ducting allow excess airflow to bypass the inlet.
Another way to solve this problem is to use a compression surface, also known as a bump, that redirects the boundary layer around the intake. This design is called Diverterless Supersonic Inlet (DSI). The pressure field in the vicinity of the bump compression surface shows positive pressure gradients away from the bump, which effectively blow the upstream boundary layer away from the inlet.
This design has several advantages compared to the diverter. It decreases the inlet weight, since the structure becomes less complex and it has no moving parts therefore requiring less maintenance. This further reduces the cost of the aircraft and is better concerning radar cross-section. The bump can also be used to improve the negative effects caused by the bends of the duct, for example to decrease the flow separation thus creating a more uniform pressure.
As with all supersonic aircraft, airframe, intake and engine integration is a challenging task and DSI is no exception. The design of the contoured bump is primarily based on conical wave theory but has to incorporate the forward fuselage effects such as modified pressure and Mach number distribution. The inlet cowling lip design is also involved since the boundary layer has to be bled away at all Mach numbers.
The smooth transitions between a combat jet's fuselage and intake, such configurations can provide a drastic reduction in radar cross section, lowering an aircraft's detectability on radar, especially at higher frequencies. Another useful attribute of the interaction between the forward swept intake cowl and the smooth hump blended surface between the intake and the fuselage is that it provides far less exposure to radar waves, especially from oblique angles, of an aircraft's jet engine fan face. The engine face is traditionally one of a combat jet's most radar reflective components. When the DSI concept is integrated into a clean sheet design and/or an aircraft utilizes curved intakes, baffle systems, and radar blocking devices, the radar return caused by a combat jet's motor face and traditional intake can be almost totally eliminated.
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