3.1.1. Properties of Fluid

Today I will just have a recap of the previous lectures. Then we will go for the two concepts that prevail in fluid mechanics: microscopic and macroscopic. Then we will discuss fluid properties like density, specific volume, specific gravity, and specific weight.

In this chunk, the focus is on the introduction to fluid properties. The lecture outlines that today's discussion will recap earlier topics before introducing key fluid properties. Properties such as density, specific volume, specific gravity, and specific weight are fundamental to understanding fluid behavior.

Think of fluid properties like different attributes of a fruit. Just as an apple has weight (mass), size (volume), and density (how heavy it feels for its size), fluids also have similar properties that help us understand their behavior in real-world applications like water flowing in pipes.

Then we will discuss what is the effect of temperature and pressures on the viscosities, then the surface tensions.

This chunk introduces the two main perspectives in fluid mechanics: microscopic and macroscopic. The microscopic view examines individual molecules and their interactions, while the macroscopic view focuses on the bulk behavior of fluids. The effects of temperature and pressure on viscosity and surface tension are key topics since they affect how fluids flow and behave under different conditions.

Imagine a crowded concert hall where individuals (molecules) move differently depending on space (temperature) and crowding (pressure). When it’s hot (high temperature), people may move more freely compared to when it's packed (high pressure), just as fluids change viscosity with temperature.

If I consider a very tiny volume, and if I consider that sample volumes, I will have the density as mass per unit volume.

Density is defined as the mass of a fluid divided by its volume. When considering very small sample volumes of a fluid, it’s important to recognize that the randomness of molecular motion can cause variations. As one increases the sample volume, the density stabilizes and begins to reflect true fluid characteristics.

Think of density like mixing rice in a container. If you only have a few grains, the density is hard to estimate. But as you fill the container (increase volume), you get a better sense of how compact or dense the rice is, similar to how fluid density behaves in larger samples.

Specific Gravity is the ratio of density of substance to density of a well-known substance (water considered here).

Specific gravity compares the density of a fluid to water's density. This ratio indicates how heavy a substance is relative to water; for instance, a specific gravity greater than 1 means a liquid is denser than water, while less than 1 indicates it is lighter.

Consider a buoy in water. If the buoy (less dense) floats and a rock (denser) sinks, the rock has a higher specific gravity compared to the water. This concept helps to determine whether materials will float or sink in liquids.

Specific weight is the weight of unit volume of a substance.

Specific weight is defined as the weight of a unit volume of fluid, incorporating the effects of gravity. This property is essential for understanding fluid forces acting in situations such as buoyancy in fluids and is calculated by multiplying density by gravitational acceleration.

Think of how a sponge (with water) feels heavier when submerged in water compared to when it’s dry. The added weight from the water can be understood through specific weight, illustrating how much the sponge weighs for every unit of space it occupies.

Now let us go to the very interesting concept, the Newton's laws of viscosities.

Newton's laws of viscosity describe how fluids resist flow (viscosity) and how this resistance relates to the velocity gradients. The shear stress is proportional to the rate of change of velocity within the fluid. The coefficient of viscosity is a measure of this resistance, varying from fluid to fluid based on their molecular properties.

Imagine spreading honey on toast. The thickness and stickiness of honey (viscosity) determine how easily it flows. The faster you spread it, the more resistance (shear stress) you feel. This principle helps us understand how different fluids behave under various flow conditions.

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