Lesson 3: Properties of Fluids
In this lesson, we will study the types of fluid properties involved in aerodynamics such as viscosity, compressibility, pathlines and streamlines. This will include a study of the nature of air in motion and patterns of airflow. Patterns of airflow are important in establishing typical aerodynamic flow patterns. Major patterns include:
- A main, streamlined flow,
- A vortex or large circulatory flow,
- A boundary layer with rapid changes of fluid speed near to surfaces,
- A Wake or ragged disturbed flow left downstream of an object, and
- Shock waves, which occur if the flow exceeds the speed of sound.
Section 1 - Introduction to the Fluid
When we refer to fluids in the study of aerodynamics, this includes both gases and liquids. The term “fluids” is used because the motion of objects in air and in water obeys identical laws until their speed approaches the speed of sound. In the discussion of fluids, concepts such as pressure and phenomena such as the static behavior of water are applicable to the instrumentation used in wind tunnels on airplanes.
PHASES
There are three phases appearing all around us – solid, liquid, and gaseous. A solid substance offers resistance to a change of shape. For example, to bend a steel rod, you must use force. If you let it go, it snaps back to its original position unless deforming occurred.
If you bend the steel rod more, it remains in a new shape, or it may be subject to breakage. In comparison, liquids and gases do not offer resistance to a change in shape.
There are clear differences between liquids and gases. A gas such as air fills a container uniformly and does not have a free surface like that of a lake. Here, certain physical properties are obviously different. However, in the streamline patterns of gases and liquids such as the lines of flow by which we can visualize the motion of air or water around an automobile or a submarine, or an airplane are indistinguishable. Therefore, both liquids and gases are called fluids.
PROPERTIES OF LIQUIDS
The properties of liquids and gases including color, taste, and viscosity depend on their varied molecular structures.
The molecules of liquids are much more dense or tightly packed than those of gases. This state results in restricted motion. In contrast, a gas molecule or atom moves freely in space until it bumps into another one that deflects or interrupts its course. This great difference in packing and motion is reflected in the density, ρ, or mass per unit volume of the two kinds of substances.
The dynamic viscosity, µ, is a measure of the internal friction of fluids. The kinematic viscosity, v, is the ratio of dynamic viscosity to density, or:
µ/ ρ
In the flow of incompressible fluids, dynamic viscosity and density are constant. The kinematic viscosity is also a constant.
We can obtain dimensionless ratios by dividing the value of a given property by the value of the same property for air. For example, consider that water is about 800 times as dense as air, and liquid mercury which is actually a metal, is 11,000 times as dense. This relatively tight packing of their molecules makes liquids nearly incompressible; meaning, an application of pressure does not change the volume of a liquid. Air and other gases can easily be compressed. For example, when you use a bicycle pump, you are actually compressing air.
Molecules in large numbers within a system can be viewed as tiny bits of matter packed closely together. This allows for a continuum, which is a model of gases and liquids assuming that liquids are indivisible.
The continuum of indivisible fluids governs our daily lives. When you watch the wind move clouds or you drink a glass of water, you are regarding fluids as continuous substances. The molecular structure determines the particular properties of the fluid.
To illustrate the huge number of molecules involved in a fluid process, we can look at an example from Sir James Jeans as written in his book “An Introduction to the Kinetic Theory of Gases.” In this example, Sir Jeans demonstrates that flow such as that about a wing should be treated as laws of mechanics rather than the laws of molecular interactions.
His example is as follows: There are about 3 x 1019 molecules of “air,” consisting mostly of nitrogen and oxygen, per cubic centimeter in a room at normal pressure and temperature. In one breath, we inhale close to 0.4 liter (1 liter equals approximately 1 quart), or 400cm3, containing about 1022molecules. The entire atmosphere of the earth is made up about 1044 molecules.
Therefore, the ratio of the total number of molecules in the atmosphere to the number of molecules in one breath or 1044/1022 = 1044-22 = 1022. This number is equal to the number of molecules in that breath.
If we assume that the atmosphere has been well mixed by turbulent motion so that all molecules have scattered in the course of the last two thousand years, we can conclude that every time we breathe we inhale at least one molecule of the dying breath that Julius Caesar exhaled as he was murdered in the Roman senate house in 44 B.C! Amazing!
