## 4 Conclusions  
This study examined the effect of the inlet shape of an isokinetic sampler on the aspiration ratio. The aspiration ratio was analyzed at free-stream velocities of 5, 10, and 15 m/s by modeling a circular inlet with identical lengths in the radial direction and an ellipsoidal inlet with a rectangular shape in the middle along the major-axis and a semicircle shape on both ends. We performed a simulation by setting the suction flow rate of the sampling probe to 16.7 L/min, the pressure to atmospheric pressure, and temperature to 300 K in order to isolate the effect of the inlet shape on the aspiration ratio. The uniformity was analyzed to determine if the flow was fully developed, and both the circular and ellipsoidal inlets exhibited a uniformity of at least 0.92 in all flow-velocity ranges of the free stream. Furthermore, an analysis of the velocity magnitude near the inlet tip of the sampling probe indicated that the velocity boundary layer inside the sampling probe formed earlier in the ellipsoidal inlet than in the circular inlet at a flow velocity of 5 m/s; moreover, the velocity of the latter was 0.92% closer to that of the free-stream velocity than that of the former when the flow velocity was 15 m/s. The aspiration ratio according to inlet shape increased as the particle size and flow velocity increased; the ellipsoidal inlet was 1.8% to 3.7% more effective at isokinetic sampling than the circular inlet. This result indicates that the ellipsoidal nozzle is capable of isokinetic sampling, and its economic feasibility lies in the ability to adjust the length of the major axis. This is an advantage over the circular nozzle, which requires inlets of different diameters depending on the measurement environment. Lastly, the effect of turbulence intensity on the aspiration ratio was examined by adjusting the turbulence intensity of the free stream from 5 to 15%; an insignificant effect was observed for both inlets. In future studies, further analyses are required for the cases of an ellipsoidal nozzle with an area gradually expanding past the sampling inlet plane, and when the inlet tip forms an acute rather than orthogonal angle with the flow. Moreover, the aspiration ratio must be analyzed at various flow velocities to account for aerosol sampling by devices installed on fast-moving vehicles such as automobiles or unmanned air vehicles.  
**Author Contributions:** Conceptualization, J.-H.N.; Methodology, M.-C.C. and J.-H.Y.; Software, J.-H.Y.; Validation, M.-C.C.; Formal analysis, M.-C.C.; Investigation, M.-C.C.; Resources, J.-H.Y.; Data curation, M.-C.C.; Writing--original draft preparation, M.-C.C.; Writing--review and editing, D.-S.K. and J.-H.N.; Visualization, M.-C.C.; Supervision, J.-H.N.; Project administration, D.-S.K. and J.-H.N.; Funding acquisition, J.-H.N. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20181110200170).  
**Data Availability Statement:** Not applicable.  
The authors declare no conflict of interest.