* Particle size and shape: Smaller particles settle slower than larger particles, and irregular shapes settle slower than spherical shapes.
* Particle density (specific gravity): Denser particles settle faster than less dense particles.
* Fluid density: The denser the fluid, the slower the settling velocity.
* Fluid viscosity: Higher viscosity fluids result in slower settling velocities.
Specific gravity is the ratio of the density of a substance to the density of a reference substance (usually water at 4°C). It's directly related to the particle density, and thus influences the settling velocity.
Here's how specific gravity influences settling velocity:
1. Higher specific gravity: Particles with higher specific gravity (meaning they are denser than the fluid) will settle faster. This is because the gravitational force acting on the particle is stronger, overcoming the drag force from the fluid more effectively.
2. Lower specific gravity: Particles with lower specific gravity (meaning they are less dense than the fluid) will settle slower. The gravitational force acting on the particle is weaker, and the drag force from the fluid has a greater effect.
Calculating Settling Velocity:
There are several formulas to calculate settling velocity, but the most common one for spherical particles is Stokes' Law:
```
v = (2 * g * (ρ_p - ρ_f) * r^2) / (9 * η)
```
Where:
* v is the settling velocity (m/s)
* g is the acceleration due to gravity (9.81 m/s²)
* ρ_p is the density of the particle (kg/m³)
* ρ_f is the density of the fluid (kg/m³)
* r is the radius of the particle (m)
* η is the dynamic viscosity of the fluid (Pa·s)
Specific gravity (SG) relates to particle density (ρ_p) by the following equation:
```
SG = ρ_p / ρ_w
```
Where:
* ρ_w is the density of water (1000 kg/m³)
Therefore, by knowing the specific gravity of the solid particles, you can calculate the particle density and use it in Stokes' Law to estimate the settling velocity.
Important Note: Stokes' Law is only valid for spherical particles and for low Reynolds numbers (Re < 1). For non-spherical particles or high Reynolds numbers, more complex models are required to calculate settling velocity.
