CEE 310 Fluid Mechanics

   Lab 1  Fluid Properties
 

I. Purpose

    1. To observe examples of fluid behavior.
    2. To become familiar with the properties of fluids and their measurement.
    3. Devices:  Hydrometer
                     Capillary Tubes and Cenco-DuNouy Interfacial Tensionmeter
                    Falling Sphere Viscosity Measurement and Cannon-Fenske Viscometer,
 
 
 

II. Density and Specific Gravity -- Hydrometer

     Density and specific gravity are typically measured with a hydrometer.  A hydrometer works on the principle that when a body is immersed in a liquid it is buoyed up with a force equal to the weight of the liquid displaced.  The hydrometers used in this lab have been calibrated such that the specific gravity can be read directly from the paper scale located inside the glass neck.
 
 

III Surface Tension

        Surface tension is the physical property of a fluid that causes the capillary attraction or repulsion of two distinct fluids in contact with a solid.  In this case one of the fluids will be water and the other fluid will be air.

A. Capillary Tubes

        Because of surface tension, water in contact with a solid experiences an attractive force.  When a tube is partly immersed in water, the water inside the tube is drawn upwards by surface tension forces.  This is also called capillary rise. A rack of small tubes is partly immersed in a tray of water.  Notices that the water rises higher in the smaller tubes.  Note: This is a demonstration of the phenomena; no measurements are necessary.
 

B. Cenco-DuNouy Interfacial Tensionmeter

 This instrument uses a small metal ring of known perimeter and wire diameter.  When immersed in the liquid the scale is set to zero since there is no force.  Then the ring is slowly lifted above the water surface.  Water in contact with the ring also rises.  The force required to lift the ring is proportional to the surface tension of the fluid.  When the ring finally breaks free of the water surface the scale measures the surface tension value for the fluid directly in dynes/cm. Try this experiment with pure water.  Then add one drop of soap to the water and repeat the experiment.
 
 
 

IV. Viscosity

     Viscosity is the property of a fluid that resists the action of a shear force.  Viscosity results from molecular action and cohesion in the fluid.  As the temperature of a gas increases, the viscosity will typically increase.  In contrast, as the temperature of a liquid increases, the viscosity will typically decrease. There are several ways to measure viscosity, the simplest being a measurement of the terminal velocity of a falling sphere.  This is suitable for fluids of high viscosity, but not for fluids of low viscosity such as water.

     A. Falling Sphere Viscosity Measurement.

      The viscosity of a liquid can be determined by measuring the terminal velocity of a sphere (a steel ball bearing) falling freely in the liquid.  Once a constant velocity is established, a force of gravity downward is balanced with the buoyant force and the drag force which are upwards.  Expressions for these forces are:

  Gravitational force is

  Buoyant force is

  From the Stoke’s Law for laminar flow, the drag force is

  Using the force balance, i.e.

   one can solve for the viscosity of the fluid:

                
    Where,
             rs = density of the sphere (steel) = 7.86 g/cm3
             rf  = density of the fluid
             r = radius of the sphere
             Vt = terminal velocity
             m = viscosity of the fluid

        To determine the terminal velocity, a imaging frame grabber system is used to take consecutive images. The intervals between each image is 0.1 sec. The following is the processed image that contains the double exposure of the particle.

 

    B. Cannon-Fenske Viscometer

 The Cannon-Fenske Viscometer is another device which can be used to measure the viscosity of a liquid.  This device is particularly useful with low-viscosity fluids.  The kinematic viscosity is determined by measuring the time for a volume of fluid to pass through a small tube.  The kinematic viscosity is directly proportional to the time, related by a calibration factor.  We will use a size 100 model V920 viscometer to measure the kinematic viscosity of water.  With a measurement of the time, t, the kinematic viscosity is given by:

               n = k t

where the calibration factor is, k = 0.01556 centistokes/s

Note that 1 centistoke = 0.01 cm2/s.