Mechanical Properties of Fluids Test - 8

The viscosity of liquids decreases with increase in temperature. The cohesive forces between molecules of liquids decrease as temperature increases. At high temperature the molecules of liquids have high energy and overcome strong cohesive forces and move freely. Hence, viscosity of liquids decreases with increase in termperature.

Drag acts as an opposing force to acceleration. Drag will never increase velocity, but whilst the forces acting to move the object are greater than the drag or resistive forces then the object will continue to accelerate, thus increasing velocity. Simplified, drag is caused by the object colliding with air particles. As the object moves faster, the object will come into contact with more particles and this will increase the effect of drag. Drag decreases acceleration and therefore velocity, but an increase in velocity will increase drag forces.

According to kinetic theory of gases viscosity in gases increases with increase in temperature because increase in kinetic energy will increase the frequency of collision between the molecules.

Velocity gradient = dv/dx
dv = 10 cm/s
dx = 0.1 cm
dv/dx = 10/0.1 = 100cm/s cm

Stoke’s Law calculates the drag force on the sphere as it travels through a fluid.
F = 6 πrηv

The Reynolds Number (Re) is proportional to the ratio between the momentum of a flowing fluid and its viscosity. The lower Re, the more viscosity governs the flow behavior, so the more it sticks to a surface, and the flow is laminar. The higher the Re, the more its velocity governs, so the less it sticks to surface and the flow is turbulent. Laminar flow is straight lines. Turbulent flow is eddies and swirling vortexes.