ME 305 FLUID MECHANICS I
MEASUREMENT OF FLUID PROPERTIES
PREPARATION: In this course, you will conduct the experiments at the Fluid Mechanics Laboratory, by yourselves, without any help or instruction from the teaching assistants. You must read the lab sheet thoroughly and understand what you are expected to do (and why) for each experiment, before coming to the lab. At the end of each experiment, you will have to do certain calculations, present and plot (when asked) your results on the provided report sheets attached to the end of the lab sheet. The experiment and the report-writing will all take place in the lab within the time allocated to your group (total: 1 hour). You will not have any time to study the lab sheet during the lab hour, if you have not done so before. Therefore, you must come to the lab fully prepared. Although you perform the experiments as a group, each person will submit a separate report (not a single group report) at the end of the lab hour. There will be no “group study” in writing the reports – everyone will prepare his/her report individually using the data he/she recorded during the experiments.
For Experiment 1, you must bring a calculator to the laboratory. You must also have your watch or a timer (you will record time in one of the experiments).
MEASUREMENT OF THE DENSITY OF A LIQUID
The density of a liquid is to be measured using a hydrometer.
A hydrometer uses the principle of buoyancy to determine the specific gravity of a liquid. Here, the weight of the hydrometer (set by the metal spheres in its bulb) is balanced by the buoyancy force exerted by the liquid in which it is immersed. The buoyancy force is the weight of the liquid displaced by the solid. Figure 1.1 presents the working principle of a hydrometer. In this sketch, a hydrometer is shown submerged in two different liquids. The stem of the hydrometer has a cross-sectional area of A. If the liquid is distilled water (Figure 1.1a), then its specific gravity will be 1.0. At equilibrium,
W = ρw g V
* revised by M. Erdal, October 2011
s = 1.0
s = 1.0
marked off to
indicate s of
(b) Any liquid
(a) Distilled water
Figure 1.1 A hydrometer in distilled water and in another liquid where W is the weight of the hydrometer, ρW is the density of the distilled water, g is the gravitational acceleration ( 9.81 m/s2 ) and V is the volume of the submerged part of the hydrometer in distilled water . The position of the distilled water surface is marked on the stem of the hydrometer to indicate the reference specific gravity. When the hydrometer is now floated in another liquid with a specific gravity of s, as shown in Figure 1.1b, the equation for the vertical equilibrium becomes
W = ρ g (V − A h )
where ρ indicates the density of the second liquid. Note that h would be measured as a negative value for liquids lighter than water, i.e. when the position of the liquid level on hydrometer is above the reference level (s = 1.0). Combining Equations (1.1) and (1.2) and solving for s yields
(V − A h )
The liquid position on the stem may then be marked off to read the liquid specific gravity, s (Figure 1.1b).
1.1.3 Experimental Procedure
The hydrometer to be used in the measurement is shown in Figure 1.2. On the hydrometer scale (Figure 1.2 (a)), the level 1000 refers to the water level, s = 1.0. The remaining levels above are scaled relative to s = 1.0, e.g. the level 700 corresponds to s = 0.7 and so on. In the lab test set-up, you will
find the hydrometer already inserted in the dark liquid with the unknown viscosity, as shown in the Figure 1.2 (c).
(a) Hydrometer Scale
(c) Hydrometer inserted in liquid
Figure 1.2. The hydrometer in the Fluid...
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