## 3 Results and Discussion
If the air resistance can be reduced by designing an inlet tip with an acute angle, the degree of isokinetic sampling of the sampling probe will be further enhanced. Figure 6: Velocity distribution of sampling probe at free-stream velocity of 5 m/s: (**a**) circular inlet; and (**b**) ellipsoidal inlet. Figure 7 shows the simulation results obtained from viewing the pressure distribution around the inlet tip in the major-axis direction. Figure 7a,b reveal the pressure distributions of the circular and ellipsoidal inlets, respectively. Relatively high pressure is formed around the inlet tip owing to the low flow velocity, and the dynamic pressure increases toward the inlet tip. A pressure field extends to the center line in the circular inlet but is only formed locally around the tip of the ellipsoidal inlet, indicating that the effect of the pressure field can be minimized if the length of the major axis is sufficiently long. Figure 7: Pressure distribution of sampling probe at a free-stream velocity of 5 m/s: (**a**) circular inlet; and (**b**) ellipsoidal inlet. Figure 8 shows the velocity distribution obtained by dividing the sampling probe inlet into front (Line 1), inlet (Line 2), and rear (Line 3) sections, as shown in Figure 3, and setting the length of the lines to 2 cm in the major-axis direction. Figure 8a,b show the velocity distribution when the free-stream velocity is 5 m/s; it can be seen that the velocity represented by Line 1 is constant regardless of the sampling probe type, while that of Line 2 increases toward the sampling probe wall and then decreases to 0 at the inlet tip marked in Figure 3, similarly to the velocity of the flow field around the sampling probe inlet where flow suction actually occurs. Then, the velocity represented by Line 3 decreases to 0 on the sampling probe wall owing to the viscosity effect, and a velocity boundary layer is developed with a smooth curve. For the circular inlet with an identical radius, the difference between the maximum and minimum flow velocities is 0.56 m/s in the \(-\)0.0042-0.0042 m range in which actual suction in the sampling probe occurs, as represented by Line 2. For the ellipsoidal inlet, the difference between the maximum and minimum flow velocities is 0.61 m/s in the \(-\)0.00735-0.00735 m range. Hence, the difference between the two inlets is 0.05 m/s, which is negligible because it accounts for only 1.05% of the 5 m/s free-stream velocity. Figure 8c,d show the flow velocity distribution around the sampling probe at 10 m/s. In Line 2, the flow velocity decreases further at the inlet tip of the sampling probe compared with that of the 5 m/s condition. Figure 8e,f display the flow velocity distribution around the sampling probe at 15 m/s; similarly, the flow velocity again decreases at the inlet tip of the sampling probe, which becomes more noticeable as the free-stream velocity increases. In addition, as the flow velocities of Line 1 and Line 2 are similar, it can be concluded that the sampling probe suctions the free-stream velocity without distortion; compared with the circular inlet, the ellipsoidal inlet gives rise to a 0.92% more similar flow velocity as the free stream, which is a trivial difference. Figure 8: _Cont._  
Figure 9 shows the aspiration ratio of the circular and ellipsoidal sampling probes at flow velocities of 5, 10, and 15 m/s. To compare the two inlet shapes, 31,000 particles were uniformly injected at the inlet of the flow field. The aspiration ratio tended to exceed 1 as the flow velocity and particle size increase; in all cases, the aspiration ratio of the ellipsoidal inlet was closer to 1 than that of the circular inlet. This result implies that isokinetic sampling of aerosols in the air is possible when an ellipsoidal nozzle is used. Figure 9a shows the aspiration ratio when the flow velocity is 5 m/s, where the mean aspiration ratios of the circular and ellipsoidal inlets have a 4.1% relative difference with respect to a ratio of 1.