# Passive Control of Base Pressure: A Review
## 2 Various Types of Passive Controls
### Other Types of Passive Control Technique
However, it is shown that that the semi-circular lobes improve the turbulent mixing and creation of the shear layer. Consequently, the attachment length is reduced significantly by around 75% [36]. The semi-circular lobes reduce unwanted differences of low frequency on the top of the nozzle. Nevertheless, this change is at the expense of the high-frequency pressure's enhanced fluctuations due to intensified small, turbulent sizes. The DMD velocity field analysis shows that the lobes can suppress the broad, coherent structure with two-step wavelength measurements in the referral configuration without flow control. The wavelength of the stable structures on the span seems to depend on the lobe geometry, as all spatial DMD modes observed display a periodicity that is equal to the distance between two lobes [36]. Even in the intermediate lobes, where the wall's contour is shaped like a standard backward-facing step, the base pressure is higher than that of the reference geometry. This trend is because of the lobes' upstream effect due to the transonic flow conditions and the wake's three-dimensional existence. The recompression due to the shear layer's intrusion occurs in a smaller stream region and moves upstream because of the minimal reattachment length. The pressure is reduced due to an acceleration in the flow until the lobes' edge and the boat tail. Persistent recompression to the main body's rear is evident further downstream [36]. For form-drag reduction in the flow over a two-dimensional bluff body with a blunt trailing edge, a new passive control device is introduced by [PERSON] et al. [37]. The system consists of small tabs on the top and bottom edges of a bluff trail to effectively interrupt a two-dimensional wake. A wind tunnel experiment and a large-scale simulation were conducted to test its drag reduction efficiency. Extensive parametrical research is conducted experimentally by changing the passive control's height and width in the form of a tab and spacing along the span between the adjacent tabs. The Reynolds numbers considered for the study are 20,000, 40,000, and 80,000. The base pressure increases for a wide range of parameters (i.e., drag reduces) at all three Reynolds numbers. In contrast, the base pressure is significantly increased by more than 30% for the optimal tab configuration. Numerical simulations are done at lower Reynolds of 320 and 4200 to investigate the tab's base pressure rise. Tab presence resulted in a significant decline in vortical strength in the wake and results in a considerable increase in the length and width of the vortex formation. The base pressure variance in a vertical direction for all tabs configurations is negligible (less than 0.5%). The base pressure of the uncontrolled flow is distributed nearly uniformly in the direction of the span, indicating that the wake behind the bluff body has remained uniform. The maximum base pressure over the spanwise direction is 5% greater than that of [PERSON] [38]. The trend is reasonable considering the present research of [PERSON] et al. [37], where the ratio of span to base height is W/h = 5, whereas that of [PERSON] [38] is W/h = 28. As a result, with tab, the base pressure is substantially increased by 23%. An experimental study was performed by [PERSON] et al. [39] to test the effectiveness of the changes in base geometry in the control of mean and unsteady base pressure on a circular arc 12\({}^{\circ}\) to zero angle incidence boat-tailed afterbody under jet-off and jet-on conditions. Tests were conducted for freestream Mach numbers range from 0.6 to 1.06, while jet-on pressure ratios from 1 to 12 were completed. In the jet-off case relative to the sharp base, the base pressure increase was 53% and 42% for the rounded base and base cavity configuration. The round base showed better results up to a nozzle pressure ratio of 8 for M \(\leq\) 0.9 in jet-on condition. However, for M = 1.06, the base cavity showed a better effect than the rounded base configuration.