Lesson 3: Properties of Fluids
Section 2 - The Fluid and Fluid Mechanics
FLUID MECHANICS
Fluid mechanics is defined as the behavior of fluids at rest and in motion. Since fluid flow is critical to aerodynamics, we can apply principles of fluid mechanics to our study.
Here, we will introduce and re-introduce concepts briefly discussed in earlier lessons.
Viscosity - There are basically three states of matter - solid, liquid, and gas. H2O is commonly called "ice" in the solid state, "water" in the liquid state, and "water vapor" in the gaseous state.
Assume one has a piece of ice and side forces are applied to it referred to as shearing forces. Very large forces are needed to deform or break it. The solid has a very high internal friction or resistance to shearing. The word for internal friction is viscosity and for a solid its value is generally very large.
Liquids and gases are considered to be fluids since they behave differently from a solid. Imagine two layers of water or air. If shear forces are applied to these layers, one discovers a substantial and sustained relative motion of the layers with the air layers sliding faster over one another than the water layers. However, the fact that a shear force must be applied to deform the fluids indicates that they also possess internal friction.
Water, under normal temperatures, is about fifty times more viscous than air. Ice is 5 x 1016 times more viscous than air. We can conclude that, in general, solids have extremely high viscosities whereas fluids have low viscosities.
Under the category of fluids, liquids generally possess higher viscosities than gases. Air, of primary interest in aerodynamics, has a relatively small viscosity, and in some theories, it is described as a perfect fluid-one that has zero viscosity or is "inviscid."
Therefore, even a small viscosity of air (or internal friction) has important effects on an airplane in terms of lift and drag.
Compressibility - All fluids are compressible which means density increases under increasing pressure, but liquids are generally highly incompressible compared with gases. Even gases may be treated as incompressible provided the flow speeds involved are not great.
For subsonic flow over an airplane at about 150 m/sec, air may be treated as incompressible, meaning there is no change in density throughout the flow. At higher speeds the effects of compressibility must be taken into account.
STEADY FLOW
Let us now talk about steady flow compared with unsteady flow. Of notable importance in understanding fluid movements about an object is the concept of a "steady flow."
On a windy day a person calls the wind steady if where he stands it blows constantly from the same direction at a constant speed. If, however, the speed or direction changes, the wind is "gusty" or unsteady. In a similar manner the flow of a fluid about an object is steady if its velocity (speed and direction) at each point in the flow remains constant. The velocity here does not need to be the same at all points in the fluid.
To consider this further, Figure 2 presents the fluid flow of air about a house on a windy day at one instant of time and figure 3 shows the flow an instant of time later. One sees that this flow is unsteady. There are many areas where the flow pattern is different; the streamlines are changing their position and shape with time. Particle pathlines and streamlines for this flow are not equivalent.
Figure 22 shows a nicely "streamlined" body in contrast to the bluff-shaped house in a wind tunnel. At time to the tunnel is not running and no air is flowing. At time t1 the tunnel is started and air begins flowing about the body; the flow develops further at time t2 and finally reaches a constant pattern at time t3.
The flow appears unchanged at time t4 and time t5. When the flow starts, it passes through an unsteady transient state. Particle, pathlines and streamlines are not the same.
From time t3 onwards a steady flow is established. Streamlines appear fixed in position with respect to the body. A particle P shown on a streamline at time t3 moves downstream along that streamline as shown at times t4 and t5. The particle pathline coincides with the streamline.
