# A Critical Review of Supersonic Flow Control for High-Speed Applications
## 3 Passive Control Methods
### Flow Control  
The examination of abrupt axi-symmetric expansion is a challenging subject that is becoming popular in a range of flow systems in the current scenario. The duct is employed with a smooth inner surface and a low base pressure in most cases. Sudden relief to the shear layer at the nozzle exit is due to the availability of a duct with a larger area. Due to a sudden increase in the duct area, the base pressure and flow-field may be articulated at the base area through the vortex dynamics produced, due to a sudden increase in the duct area. In contrast, the value of the base pressure is almost similar in the base area; however, the mean values are taken for the analysis. As a result of its wide application, abruptly increased flows have been thoroughly investigated. The investigations were carried out in an abrupt expansion duct in several cases with passive devices such as a rib, cavity, step body, and boattail. Several studies were carried out experimentally to control the flow, and these results serve as the benchmark results. [PERSON] (2018) studied the impacts of riblets in pressure gradients and three-dimensionality on airfoils, wings, and wing-body combinations in different speed regimes to control the viscous drag in 3M riblets. [PERSON] and [PERSON] [PERSON] and [PERSON] (2018) tweaked the leading edge flaps to enhance the delta wing design at a low Reynolds number of 1.9105 applied to a chord (geometric mean) of 286 mm as the typical length for micro and crewless air vehicles. The flow past desired blunt-nosed body with and without spikes was tested in a primary 2D water channel, at a given Reynolds number [PERSON] and [PERSON] (2018), with spikes of various forms, such as conical, hemispherical, flat, and square nose. [PERSON] and [PERSON] [PERSON] and [PERSON] (2018) used a forward-facing aerospike on the nose to examine the heat transfer and its control through the aerospikes for lifting the body configuration in a hypersonic flow. They used experimental techniques and a simplified model of the resulting basic symmetric-delta configuration. A similar study by the same authors [PERSON] and [PERSON] (2018) was carried out to assess the aerodynamic efficacy for aerospike on a hypothetical lifting-body in a hypersonic flow at various Mach numbers. In Ref. [28], the experimental research was carried out to determine the importance of the pressure hill height and the zone expansion impact for a flow-through in different forms of spikes with Reynolds numbers ranging from 2278 to 4405 to detect the vortex shedding process. In a prior work, the previous researchers [PERSON] and [PERSON] (2018) studied a theoretical estimate of the shock standoff distance in a supersonic range with the modified Newtonian impact theory. Based on this passive control strategy, the researchers [PERSON] and [PERSON] (2018) conducted the tests using a transonic wind tunnel of 38 \(\times\) 30 cm in the Mach number ranges of 0.7 to 1.0 and controlled the entire afterbody drag of the outlet flow. In the absence of the jet flow at the base, the same author [PERSON] and [PERSON] (2018) examined the development of flow management with passive control devices for the axi-symmetric base and afterbody drag reduction in different speed regimes. Experimentally, [PERSON] (2018) studied the control of flow in a sonic jet in a circular convergent nozzle by locating two tabs at the nozzle exit. The effects of annular ribs as a passive control device on the base pressure control was examined, using an experimental technique in a subsonic and sonic nozzle with an abrupt expansion duct [PERSON] (2018). In addition, the researchers investigated the pressure fluctuations in a typical missile base area design at a freestream Mach number of 0.7, with and without a base cavity [PERSON] (2018). The study was discovered in resonant high-speed cavity flows controlled by high frequency pulsed supersonic microjets [PERSON] and [PERSON] (2018) in addition to CD nozzles.