# Passive Control of Base Pressure: A Review
## 2 Various Types of Passive Controls
### Passive Control in the Form of Spikes  
An experimental study was done on the bluff body at subsonic Mach numbers. Passive control in the form of aerospikes was placed on a plate of 1 mm thickness. Two spikes are placed on the plate at 11.5 mm between the nozzle and the duct. The tests were conducted at Mach numbers 0.6, 0.7, 0.8, and 0.9 for an area ratio of 6.25. L/W ratios of the duct taken into consideration are 4, 6, 8, and 10. These experiments were undertaken to control the base drag, an essential component in bluff bodies, to reduce the total drag [21]. Figure 12 below shows the placements of aerospikes. Findings stated that without control case, the base pressure first increases till L/W = 6 and then decreases till L/W = 8, and then is an increasing trend in base pressure until L/W = 10. With passive control of aerospike, the base pressure starts to increase right from the beginning and becomes more than atmospheric pressure at L/W = 6, and then again, as an increasing trend, it continues till L/W = 10. It acquires the pressure value, which is 5% more than atmospheric pressure. Figure 11: Experimental setup [20]. Figure 12: Control plates with aerospike [21]. The main reason for this phenomenon is that until L/W = 6, the aerospikes can break the base region's recirculation zone into two parts. Hence, the suction at the base is reduced, and the base pressure increases. Also, from L/W = 4 to L/W = 6, the duct's length is small. So, the two recirculating bubbles formed at the base get less space to travel. Whereas from L/W = 7, till L/W = 10, the duct's length is more and hence gets more space and time to travel, and therefore the base pressure gradually increases with the L/W ratio. However, the aerospike in passive control is very useful at the subsonic and transonic regimes. [PERSON] [22] studied a forward-facing aerospike to a satellite launch vehicle. The payload fairing significantly modifies its flow field and reduces the aerodynamic drag at transonic and supersonic speeds. The numerical simulation was carried out on 0.8 \(\leq\) M \(\leq\) 3.0 with and without aerospikes. A forward-facing aerospike attached to a payload fairing of a satellite launch vehicle significantly altered its flow field and decreased the aerodynamic drag in transonic and low supersonic speeds. The present payload fairing is an axisymmetric configuration and consists of a blunt-nosed body along with a conical section, payload shroud, and boat tail, followed by a booster. All pressure transducer loss is below the average order of \(\pm\)0.51%. In general, there is good agreement between the current computations and the available experimental data. When an aerospike is attached to the payload fairing, the payload fairing pressure is changed considerably. The reduction of surface pressure is not minimal for M \(<\) 1. There is a significant drop in surface pressure for M \(>\) 1. Another study was done by [PERSON] et al. [23] on threaded spikes as a passive control. In their research, as shown in Figure 13, two threaded spikes were attached in the opposite direction of length 40 mm on the circular plate of thickness 1 mm and diameter 80 mm. They conducted tests for Mach numbers (0.6 and 0.7) and two transonic Mach numbers (0.8 and 0.9). Ducts of lengths 100 mm, 150 mm, and 200 mm are examined. [PERSON] et al. [23] findings for L/W = 4 show that base pressure continuously decreases with an increase in Mach number without control. With the control case, there is an increase in base pressure, i.e., it is more than atmospheric pressure right from the beginning and decreases till Mach number 0.7 and then again increases till Mach number 0.9. For L/W = 6, the increase in base pressure with the control case was from Mach 0.6 to 0.9. For L/W = 8, there is an increase in base pressure until Mach 0.7 and then a slight decrease to 0.8 and a steep increase in base pressure until Mach 0.9.