A review of flow control techniques and optimisation in s-shaped ducts
Introduction
With operational costs, fuel efficiency, and noise levels of modern airplane engines a prime concern, the s-shaped duct, an example of which is shown in Fig.Ā 1, has a number of distinct advantages. Due to the curvature in the duct, the incoming air is slowed down much faster than in conventional straight ducts which leads to shorter designs and considerable weight savings (Hui, Feng, Yaoying, Baigang, 2012, Vaccaro, Elimelech, Chen, Sahni, Jansen, Amitay, 2015). It is estimated that the net weight of an aircraft would decrease by 15% if the length of the fuselage is reduced by one inlet diameter (ChenĀ and Wang,Ā 2012). Moreover, the curved inlet is a line-of-sight blockage to the engine fan/compressor and thus effectively lowers the noise level as well as radar and infrared signatures (Hui, Feng, Yaoying, Baigang, 2012, Chen, Wang, 2012). Further weight saving potential exists for unmanned aerial vehicles (UAVs) as their total size is often determined by the propulsion system (VaccaroĀ etĀ al., 2015): a shorter propulsion system would directly relate to a smaller UAV.
S-shaped ducts have been used as intakes on a number of commercial and military airplanes (engines) such as the Boeing 727 (PW JT8D), the Lockheed Tristar L-1011 (RR-RB211), the General Dynamics F-16 (PW F100), and the McDonell-Douglas F-18 (GE F404) with the engine buried in the fuselage. The convoluted s-duct has also been employed to join compressor and turbine stages in turbomachinery. However, the duct design introduces a number of undesirable flow features, such as a non-uniform pressure distribution at the aerodynamic interface plane (AIP), i.e. the plane between the exit of the intake and the compressor/fan of the engine, and flow separation for particularly aggressive convoluted ducts. These undesirable flow features reduce the efficiency of the s-shaped inlets, increase the pressure loss, cause higher levels of fatigue, and hence decrease the operational lives of the engine compressor/fan (ChenĀ and Wang,Ā 2012). Non-uniform flow conditions at the AIP also lower engine surge and stall limits (ReichertĀ and Wendt,Ā 1994).
Section snippets
Performance and geometrical parameters
S-shaped ducts are commonly characterised by their diffusion ratio, their radius ratio as well as their length-to-diameter, LDR, and length-to-offset, LOR, ratios. The diffusion ratio depends on the inlet and outlet diameters denoted by D1 and D2, respectively in Fig.Ā 1. The ductās centreline is specified by two arcs with respective radii R1 and R2. The radius ratio is defined by the ratio of the arc radius to the inlet radius.
The two main criteria for evaluating the performance of engine
Flow physics in s-ducts
First, a schematic drawing (Fig.Ā 2) of widely accepted flow features in s-ducts is presented. Fig.Ā 2 was created based on findings in the literature.
Depending on the diffusion rate, the radius of curvature and the strength of secondary flows, flow separation is expected near the inflection point (Wellborn, Reichert, Okiishi, 1992, Vaccaro, Elimelech, Chen, Sahni, Jansen, Amitay, 2013). The radius of curvature generates centrifugal pressure gradients that, helped by the diffusion rate, develop
Working principle of common passive flow control devices
Spoilers are commonly used as a swirl control device. The spoiler trips the flow which causes separation and a decrease of the total pressure. A reduction in pressure gradients has a direct effect on swirl formation. Fences and rails are also used to the same effect.
Vortex generators (VG) are mainly used to control separation. They re-energise the low momentum fluid in the boundary layer by generating vortical structures that draw higher momentum fluid from the core flow in to the boundary
Active flow control
One of the earliest studies to employ active flow control in s-ducts was conducted by BallĀ (1985), using wall suction and blowing at a Mach number of 0.7. Pressure measurements suggest that separation was eliminated and effective boundary layer control achieved, using a slot blowing a mass flow equal to 2% of the inlet flow. Automatic adjustable blades (AABs) were used as a swirl control mechanism by WengĀ and GuoĀ (1991). This method reduced the bulk swirl to the point of elimination at 7.5
Hybrid flow control
A few researchers have combined passive and active flow control techniques in an attempt to control both the steady and time-dependant flow properties in s-shaped intake ducts. This combination of active and passive flow control methods is called a hybrid system.
Gissen, Vukasinovic, McMillan, Glezer, 2014b, Gissen et al., 2014a proved that hybrid systems can have an additional benefit on performance parameters, compared to passive or active flow control alone. In GissenĀ etĀ al.Ā (2014b), the
Conclusions
Passive flow control devices have been heavily studied by a number of researchers and show good results. From the passive flow control devices reviewed, tapered fin vortex generators and submerged vortex generators improved pressure loss and distortion by double digit percentages. It is widely agreed that the optimum location for vortex generators is upstream of the separation point just ahead of the high adverse pressure gradient. At this location the boundary layer grows rapidly before
Acknowledgement
The authors are grateful to the Engineering and Physical Sciences Research Council (EPSRC) for their sponsorship of the Ph.D. project. Authors also acknowledge Rolls-Royceās support.
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