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Flow Field Measurements by Particle Image Velocimetry (PIV) Techniques
Published in Je-Chin Han, Lesley M. Wright, Experimental Methods in Heat Transfer and Fluid Mechanics, 2020
The Stokes number represents the interaction between a foreign particle and its surrounding fluid. Non-dimensionally, the ratio of the particle’s inertial force to the fluid’s inertial force is represented with the Stokes number. More commonly, the Stokes number is presented as the ratio of the relaxation time of the particle compared to the characteristic time of the surrounding fluid. If the particle motion is independent of the fluid motion (the movement of the particle is not influenced by the fluid), both the relaxation time of the particle and the inertial force of the particle are large. In the extreme case of fixed foreign particles, the Stokes number approaches infinity. On the other hand, if the particle is identically following the motion of the fluid, it is not exerting unnatural force on the fluid, and its relaxation time approaches zero. Therefore, the Stokes number approaches zero. For PIV applications, it is necessary to select the seed particles so their Stokes number approaches zero, and they are neutrally buoyant in the surrounding fluid. Generally, a particle is acceptable if the Stokes number is less than 0.1; under such a condition, the error associated with deviation of the particle from the fluid is less than 1% [5].
Diffusion and convective transport of particles
Published in S. Mostafa Ghiaasiaan, Convective Heat and Mass Transfer, 2018
The Stokes number for a particle is defined as the ratio between the stopping distance of the particle and the characteristic dimension of the flow field or the object on which the particle may deposit Stk=lsdl=ρPdP2uP,018μl.
Airborne Particle Counting and Environmental Monitoring
Published in Thomas A. Barber, Control of Particulate Matter Contamination in Healthcare Manufacturing, 1999
The situation shown in Figure 9.15 is pertinent with regard to sampling of turbulent air (Agarawal and Liu 1980). In the figure, St Is the Stokes number. The Stokes number is the product of the particle relaxation time (time for a particle to respond to a change In air velocity and/or direction) and the particle velocity divided by the diameter of the sample tube inlet: () St=tV/R
Anisotropic turbulent flow model effect on the prediction of the erosion rate of the micro particulate flow in the elbow
Published in Particulate Science and Technology, 2021
Sina Bahmani, Hamid Reza Nazif
As previously stated, the particle motion path can be a useful parameter in justifying the amount of erosion (see Figure 7). The Stokes number is defined as the ratio of the particle relaxation time to the fluid time scale. The Stokes number indicates the degree to which the particles follow the flow. For Stokes numbers, much smaller than 0.1 particles follow the flow perfectly. For Stokes numbers more than1, the motion of particles is completely independent of the flow of fluid. According to the results of the present study, the Stokes number is in the range of 0.01-30, which indicates the adherence of particles in parts of the secondary flow. Due to the results of modeling secondary flow and the presence of strong secondary flow, such as Dean flow, it has been observed that in parts of the elbow where there is a strong secondary flow, a very low Stokes number has been reported, which indicates the high compliance of particles with these flows. The presence of these flows was proved to increase the velocity of the fluid in the bend. Therefore, it has increased the velocity of particles in the elbow. Therefore, this event has increased erosion. The validation is based on the experimental data of Vieira et al. (2016), for better understanding the validation and result in Table 5 the experimental data of Vieira et al. have been shown.
Investigation of erosion phenomena and influencing factors due to the presence of solid particles in the flow: a review
Published in International Journal of Ambient Energy, 2021
Suhas M Shinde, Pradeep A Patil, Virendra K Bhojwani
Also, Wang and Shirazi (Wang and Shirazi 2003) proposed that the fluctuations in secondary flows, turbulent flow, and squeeze film play important roles in erosion prediction. However, it is rarely concerned that how the secondary flow and squeeze film affects the erosion phenomena. Kai Wang et al. (Wang et al. 2017) considers stokes number (Stk) as a factor responsible for erosion phenomena. Stokes number is a dimensionless number characterising the behaviour of particles postponed in a fluid flow. The Stokes number is defined as the ratio of the characteristic time of a particle to a characteristic time of the flow. A particle with a low Stokes number follows fluid streamlines resulting in insignificant erosion, while a particle with a large Stokes number is subjugated by its inertia and continues along its original trajectory resulting in impacts on walls in bend pipes and these behaviour further results in significant erosion [refer to Figure 14].
High-order overset grid method for detecting particle impaction on a cylinder in a cross flow
Published in International Journal of Computational Fluid Dynamics, 2019
Jørgen R. Aarnes, Nils E. L. Haugen, Helge I. Andersson
A fluid flow will be deflected by the object, and particles in the flow will experience a drag force. This force will accelerate the particles along the fluid trajectory, leading particles away from the bluff body. The particle Stokes number, , where and are particle and fluid time scales, respectively (details in Section 4), can be considered a measure of particle inertia. Hence, particles with a small Stokes number follow the flow to a larger extent than particles with a large Stokes number. By using potential flow theory to compute the flow past a circular cylinder, Israel and Rosner (1982) determined a relation for the impaction efficiency as a function of the Stokes number. The predictions by Israel and Rosner (1982) are inaccurate in predicting particle impactions for flows where the viscous boundary layer of the cylinder plays a significant role. This is because potential flow theory assumes inviscid flow. In particular, this theory is insufficient at predicting impactions for particles with small Stokes numbers, and for moderate Reynolds number flows. Here, the Reynolds number is defined as , where is the mean flow velocity, D is the diameter of a cylinder (the bluff body in the flow) and ν is the kinematic viscosity of the fluid.