## 5 Conclusions  
In this study, large-eddy simulations have been conducted to understand the plasma flow control effect on local wind pressures and to reveal the control mechanism of multi-SDBD plasma actuators. Three configurations of multi-SDBD plasma actuators are proposed and tested by the simulations, which can induce three directions of near-wall jets on the whole roof. Numerical results indicate that depending on wall jets, these configurations successfully change the flow structure around the roof and effectively reduce mean and fluctuating wind pressures on the local roof. Moreover, the skin friction coefficient on the roof surface can be increased by the configurations. As the wind speed increases, the control effect of the plasma actuators on the wind pressure continues to decrease. The following conclusions can be summarized. 1. The mean drag coefficient is not significantly affected by these configurations, because the actuators are only placed on the roof. In all configurations, the configuration with the near-wall jet direction from the trailing edge to the leading edge (i.e., case A) has a noticeable reduction of mean  
Figure 18: Contours of mean skin friction coefficients on the building surfaces with different cases. (**a**) no actuation; (**b**) case A; (**c**) case B; (**d**) case C.  
lift coefficient. The fluctuating lift and fluctuating drag coefficients can be significantly reduced by these configurations owing to the decline of the strength and size of the shedding vortices. 2. The roof area location with the reduced mean pressure coefficient is determined by the near-wall jet direction. Each single-SDBD plasma actuator can absorb the near-wall flow upstream of the actuator, and meanwhile can blow the flow downstream, resulting in the fact that the wall jet exerts a pressure on the downstream near-wall flow. Under the action of multi-SDBD plasma actuators, the wall jet is continuously accelerated to obtain the greatest momentum at the end and to exert the maximum pressure on the local roof area, leading to the apparent reduction of the mean pressure coefficient absolute value. This is also the reason why the limited roof regions with the reduced absolute value are located precisely at the end of the wall jet. Results show that under the effect of the wall jet blowing from the trailing edge to the leading edge (i.e., case A), the maximum reduction ratio of the mean pressure coefficient absolute value can reach 20% near the leading edge. Also, the pressure effect of multi-SDBD plasma actuators is far greater than their suction effect. 3. The influence of the wall jet on the shedding vortices is the key to understanding the reduction of the fluctuating pressure coefficient. In the recirculation region, the effect of the active actuators on the near-wall flow is much stronger than that of the incoming flow. Under the action of the wall jet, the small-scale spanwise vortices appear on the roof. These vortices suppress the intensity and size of streamwise shedding vortices produced by the flow separation at the leading edge, resulting in a reduction of the fluctuating pressure coefficient on most roof areas. Results show that after the actuators are ignited, the fluctuating pressure coefficient can be reduced by 20% at most from the no actuation situation. The results of the investigations above show that multi-SDBD plasma actuators can provide an improvement in the wind-resistance performance for the roof of a low-rise building. Although simulations and related experiments in the present research are about a small-scale building model with a dimension of 100 to 200 mm, these results can supply a theoretical basis for more engineering applications. In future applications, plasma actuators may be placed vertically on the lateral sides of a high building to control the vortex shedding in the crosswind direction and reduce the aerodynamic drag. The annular plasma synthetic jet actuators with the outward jet direction may be used to address the edge effect on the building roof.