Mathematical Problems in Engineering

Volume 2017 (2017), Article ID 9795286, 8 pages

https://doi.org/10.1155/2017/9795286

## Initial Velocity Effect on Acceleration Fall of a Spherical Particle through Still Fluid

Engineering College, Ocean University of China, Qingdao, China

Correspondence should be addressed to Bingchen Liang

Received 10 October 2016; Accepted 4 January 2017; Published 6 February 2017

Academic Editor: Sébastien Poncet

Copyright © 2017 Zegao Yin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A spherical particle’s acceleration fall through still fluid was investigated analytically and experimentally using the Basset-Boussinesq-Oseen equation. The relationship between drag coefficient and Reynolds number was studied, and various parameters in the drag coefficient equation were obtained with respect to the small, medium, and large Reynolds number zones. Next, some equations were used to derive the finite fall time and distance equations in terms of certain assumptions. A simple experiment was conducted to measure the fall time and distance for a spherical particle falling through still water. Sets of experimental data were used to validate the relationship between fall velocity, time, and distance. Finally, the initial velocity effect on the total fall time and distance was discussed with different terminal Reynolds numbers, and it was determined that the initial velocity plays a more important role in the falling motion for small terminal Reynolds numbers than for large terminal Reynolds number scenarios.

#### 1. Introduction

Since the middle of the 19th century, a solid particle falling in gases or liquids has been investigated in many fields, such as hydraulics and chemistry. Considerable attention has been given to the steady-state motion of a spherical particle falling in incompressible Newtonian fluids, especially for the relationship between drag coefficient and Reynolds number . Stokes [1] neglected the inertia item in the Navier-Stokes equations and proposed an inversely proportional relationship between and with scenarios for the first time. Then, the inertia item was taken into account by Oseen [2, 3], and an improved relationship was proposed for scenarios. Then, the perturbation theory was used to obtain a formula for scenarios [4, 5]. A more recent study on correlations with can be found in the works of Chester et al., Clift et al., and Liao and Julien [6–9]. It should be noted that most of these aforementioned correlations had little deviations [10].

In contrast to the steady-state fall of a particle, an unsteady-state fall has an acceleration process before it reaches the terminal fall velocity. Chang and Yen [11] discussed the importance of added mass and the Basset force of a sphere falling for a low sphere-to-fluid density ratio and low . Jalaal et al. [12] used the homotopy perturbation method to derive the fall velocity, acceleration, and distance equations of a spherical particle falling unsteadily in a Newtonian fluid, ignoring the effect of the Basset force. Guo [13] analysed the acceleration fall behaviour of a sphere through still fluid for arbitrary Reynolds numbers and presented a simple solution for fall distance and velocity. Yin et al. [14] neglected the Basset force effect and experimentally investigated a spherical particle acceleration fall through still water with large .

The objective of our study was to investigate the initial velocity effect on a spherical particle acceleration fall through still fluid analytically and experimentally. In this study, the relationship between and was investigated, and different parameters in the equation were obtained with respect to small, medium, and large zones. Then, the finite fall time and distance equations were derived in terms of some assumptions by using the results reported by Guo [13]. Furthermore, a simple experiment was conducted to measure the fall time and distance for a spherical particle falling through still water. In addition, sets of experimental data were used to validate the relationship between fall velocity, time, and distance. The initial velocity effect on fall time and distance was discussed with different terminal Reynolds numbers. Finally, the conclusions are summarized.

#### 2. Kinetic Equation of an Acceleration Fall

##### 2.1. Equation Derivations

Concerning a spherical particle falling with acceleration through still fluid, the classic Basset-Boussinesq-Oseen (BBO) equation has often been used to depict the behaviour of the fall [3, 11–13, 15–17].where the left-hand side of (1) is the inertia of a spherical particle. The first, second, third, and fourth terms on the right-hand side are its submerged weight, drag, added mass force, and Basset history force, respectively. is the spherical particle mass, is the fall velocity, is the time, is the mass of fluid for the same volume of spherical particle, is the gravitational acceleration, is the fluid density, is the diameter of spherical particle, is the drag coefficient, is the classic added mass coefficient, is the classic Basset force coefficient, , is the dynamic viscosity of fluid, and is the dummy variable for integration.

The submerged weight is given by subtracting the buoyancy force from the particle weight, and it is well accepted that this term plays an important role in the total force [9]. The contribution of drag force is decided by to a great extent. Considerable attention has been given to its value with different (), in which is the kinematic viscosity of fluid [1–3, 7, 11–13]. In terms of , it can be classified as small, medium, and large numbers, and the corresponding critical values are assumed to be and [13, 18], where is defined as the first critical velocity between small and medium zones, is defined as the second critical value between medium and large zones, and their corresponding times were defined as and , respectively. The relationship between and can be expressed as the following general expression with Rubey’s drag law [19]:

For small in a laminar zone (), and [1]. For large in a turbulent zone (), and , which agreed well with the experimental data over a wide range of Reynolds numbers [13]. As for medium in a transitional zone (), Lapple and Shepherd [20] stated that decreases with increasing :

The range of deviation in (3) is −8% to 5% [7]. Using (3), results for range from 0.1 to 1000 at a step of 0.1 were obtained, and (3) was fitted as

The correlation coefficient is 0.997. Therefore, (4) can be used to depict the relationship between and in a transitional zone without causing major error.

Concerning the added mass force term, it deserves a brief description. One important property of the solid-fluid interaction is that the solid is not entirely free to move within the fluid. As the solid particle moves from one position to the other, an equal volume of fluid must move in the opposite direction. So if the mass of solid particle is accelerated, the mass of an equal volume of fluid must also be accelerated. And this concept is referred to as the added mass [9]. In (1), is equal to 1/2 for the creeping motion after the integration of the potential unsteady pressure over the sphere’s surface [21]. Hamilton and Lindell [22] also concluded that is consistently experimentally close to 0.5 for values up to 35000, by neglecting the sphere acceleration effect. In contrast, increases with because of flow separation [9]. Odar [23] further correlated using an instantaneous acceleration modulus; nevertheless, the modulus is arduous to obtain explicitly.

The Basset force contribution to total force was determined by the density ratio to a considerable extent, where is the density of the particle. It was found that the Basset force is always important in the calculations for scenarios. For some heavy particles in air such as , the Basset force plays an insignificant role in the total force. The Basset force contribution for depends on the type of particles and on the frequency of the fluid velocity fluctuations [24]. Particular attention has been devoted to the importance of the Basset force compared to the other hydrodynamic forces [25]. Lawrence and Weinbaum [26] investigated the unsteady force on a sphere at low Reynolds number and pointed out that the Basset force for an arbitrary velocity contained a new memory integral whose kernel differed from the classical behaviour derived by Basset [15]. Sobral et al. [27] considered the unsteady motion of a rigid spherical particle in fluid flows and found that its motion is significantly affected by the Basset force on the early stages of the motion and on the approach to the steady state. Due mainly to the complexity of the Basset force, a series of numerical methods were employed to resolve it, such as the Laplace transform method [28], the “window model” [29], and fractional-derivative approach [30]. However, its computation was far more expensive than other items in the BBO equation because of the dramatic demands of computational time and memory. In contrast to the numerical solutions, its analytical solution has advantages including simplicity and reliability. However, the analytic solutions of a spherical particle falling in still water were still not found. Attempts to resolve this dilemma were made for the theoretical solution of the Basset force and BBO equation with a simplified method. Guo [13] assumed to be a constant to separate it from the definite integral and as in the Basset force item. He empirically combined the added mass force and Basset force into one integrated term, ( is the integrated added mass coefficient), and the fall velocity was expressed as where , , , , , is the initial time, and is the initial velocity. The terminal fall velocity, , can be deduced with (5) for :

Using (5) with or , and can be expressed as

The fall distance was expressed as

Assuming an initial fall velocity of zero, Guo [13] derived the following relationship between fall distance and fall velocity:where is the inverse hyperbolic function.

##### 2.2. Finite Values of Acceleration Fall Time and Distance

For , the fall distance in (8) approaches an infinite value. In order to simplify the study, the finite values of fall time and distance were expressed with the following assumptions.

For scenarios, the acceleration fall process was assumed to be completed when the fall velocity reached 99% of the terminal fall velocity. Applying (6) and to (5) gives the corresponding time as follows:

Substituting (10) into (8) gives

For scenarios, the acceleration fall process was assumed to be completed when the fall velocity reached 101% of the terminal fall velocity. Applying (6) and to (5) gives the corresponding time as follows:

Substituting (12) into (8) gives

Note that the scenarios of or are not addressed in our study because they are very close to the terminal fall velocity.

#### 3. Validation by Experimental Data

In order to validate (9), two data sets of a spherical particle acceleration falling through still fluid from Allen [31] and Moorman [32] were used. Allen [31] measured a steel sphere falling in a rectangular water tank; its depth, length, and width were 28 cm, 11.5 cm, and 3 cm, respectively. The water temperature was 17.8°C, , mm, , cm/s, and the terminal Reynolds number . Moorman’s (1955) tests were conducted in an oil tank with mm, kg/m^{3}, kg/m^{3}, m^{2}s^{−1}, , cm/s, and . The fall velocities and distances of the experiments were plotted, and their comparisons with (9) were shown in Figure 1. Note that and for , and for , for , and , , and for as reported by Guo [13] because of its good validation with Allen’s [31] and Moorman’s [32] test data. Figure 1 shows that the results from (9) for Allen’s [31] scenario and the experimental data agreed well. As can be seen from Figure 1, the results from (9) are slightly overestimated according to the experiment with Moorman's scenarios [32].