## 4 Discussion  
The results of the measurements of the flow velocity field in a minichannel downstream of the nanofibrous filter show that the non-uniformity of the velocity vector profile downstream of the filter can occur due to turbulent vortices generated in the channel, also for low Reynolds numbers. This effect can be caused by the inhomogeneity of the nanofibrous filter, i.e., the nonuniform thickness of the filter and the non-even distribution of the pores in it. The second cause of turbulence can be the presence of a relatively large boundary layer compared to the channel cross section. Due to the small cross section of the channel, the boundary layer can be estimated to be about 20% of the channel width. Such a large area occupied by the boundary layer can lead to a turbulence downstream of the nanofibrous filter, besides a low Reynolds number, which was estimated to be smaller than 1300, for the investigated flow velocities. These results indicate that it is necessary to take into account the problem of the boundary layer when designing the nanofibrous filters in a minichannel. In order to obtain a more uniform gas flow in the channel, the produced nonwoven nanofibrous filter should be uniform over its entire area, which requires further research on improving this technology.  
The inhomogeneity of the fibrous filter structure, which is a natural effect of the production process, was investigated by [PERSON] et al. [93]. The authors observed a local higher particle penetration in the zones of the higher porosity of the filter. From their theoretical considerations and experiments resulted that the penetration can be smaller for nanofibrous filter media. The effect of the inhomogeneity of the fibrous filter structure was recently investigated by [PERSON] et al. [78]. The numerical simulations carried out for the nanofibrous filter with a mean fiber diameter of about 300 nm, 42 um filter thickness,and 93% porosity showed that the gas flow velocity through the filter medium and at its outlet had high non-uniformity. There were regions in the filter in which the local velocity magnitude was about four times larger than the mean velocity in the channel. Investigations of the inhomogeneity of electrospun fibers were carried out, for example, by [PERSON] and [PERSON] in 2011 [94] or [PERSON] and [PERSON], 2015 [95], but those results refer to water filtration. This problem will require further investigation. Another problem is the reduction in the random motion of particles in minichannels, which is particularly important in liquid flowing in a microchannel. Such a motion, also known as Gaussian noise, had been considered by [PERSON] [96] for a channel with Y and T junctions. This noise distorts the information about the flow of conveying liquid. The image filtration can improve the quality of measurements.  
In the current paper, the effect of filter inhomogeneity on local differences in the flow velocity vector was observed using the PIV measurements. This inhomogeneity caused a turbulence in the whole cross section of the minichannel downstream of the nanofibrous filter, besides a low Reynolds number (!1300).  
A similar non-uniform flow velocity in front and downstream of commercial pleated filters in large channels was observed by [PERSON] et al. [86], which also used the PIV method. A flow of smaller non-uniformity was obtained at larger distances from the filter; however, a certain flow recirculation was observed directly behind a pleat, for all investigated pleated media, regardless of the pleat geometry.