What is a Fluid

Generally speaking, a fluid can be a liquid or a gas, where an important difference is in the equation of state that provides a relationship among the pressure, temperature, and mass density. Gases, of course, are compressible in the simplest case an ideal gas law provides the equation of state for a multicomponent mixture as 

In addition to the equation of state, it will be necessary to describe other thermodynamic properties of the fluid. These include specific heat, enthalpy, entropy, and free energy. For ideal gases the thermodynamic properties usually depend on temperature and mixture composition, with very little pressure dependence. Most descriptions of fluid behavior also depend on transport properties, including viscosity, thermal conductivity, and diffusion coefficients. These properties generally depend on temperature, pressure, and mixture composition. 

In this book we consider only fluids that are isotropic, meaning that the fluid properties are independent of direction. By contrast, solids can readily have spatially oriented properties. Consider, for example, a common material like graphite, whose molecular structure has strongly oriented layers. Both mechanical and thermal properties are vastly different normal to and parallel to the layers. While ordinary fluids exhibit no such properties, it is possible to have anisotropic fluids. For example, long-chain polymeric fluids can exhibit properties that are oriented relative to the flow directions. 

Velocity is a relatively simple and intuitive concept for a solid body. Because a fluid is continuously deformable, however, defining its velocity takes a bit more care. At a molecular scale a fluid is a collection of particles. In principle, one can describe the velocity of a fluid in terms of the velocities of each molecule in the fluid. Obviously this would be impractical owing to the extreme numbers of molecules that would have to be considered. Instead, it is appropriate to use a velocity field that represents an the average fluid velocity at every point within the fluid. 

We focus our attention on a packet of fluid, or a fluid particle, whose size is small compared to the length scales over which the macroscopic velocity varies in a particular flow situation, yet large compared to molecular scales. Consider air at room temperature and atmospheric pressure. Using the ideal-gas equation of state, it is easily determined that there are approximately 2.5 x 107 molecules in a cube that measures one micrometer on each side. For an ordinary fluid mechanics problem, velocity fields rarely need to be resolved to dimensions as small as a micrometer. Yet, there are an enormous number of molecules within such a small volume. This means that representing the fluid velocity as continuum field using an average of the molecular velocities is an excellent approximation. 

---

What is a fluid It isn t a solid, but what is a solid Perhaps it is easier to define these materials in terms of how they respond (i.e., deform or flow) when subjected to an applied force in a specific situation such as the simple shear situation illustrated in Fig. 3-1 (which is virtually identical to Fig. 1-1). We envision the material contained between two infinite parallel plates, the bottom one being fixed and the top one subject to an applied force parallel to the plate, which is free to move in its plane. The material is assumed to adhere to the plates, and its properties can be classified by the way the top plate responds when the force is applied. 

What is a non-Newtonian fluid Describe the principal types of behaviour exhibited by these fluids. The viscosity of a non-Newtonian fluid changes with the rate of shear according to the approximate relationship ... 

You are given a liquid and are asked to find its viscosity. Its density is known to be 0.97 g/cm3. You place the fluid in an open vessel to which a 20 cm long vertical tube with an inside diameter of 2 mm, is attached to the bottom. When the depth of the liquid in the container is 6 cm, you find that it drains out through the tube at a rate of 2.5cm3/s. If the diameter of the open vessel is much larger than that of the tube and friction loss from the vessel to the tube is negligible, what is the fluid viscosity ... 

What is a supercritical fluid Why are supercritical fluids useful in solid-liquid extraction ... 

Air too is a fluid that exerts a small buoyant effect on any object it surrounds At ordinary conditions the density of air is 1 2 x 10 g/ml, a in ver accurate weighings it is necessary to take into account the buoyant effect of the displaced air, that is, we must calculate what the weight of an object would have been had the weighing been done in vacuo where there would be no buoyant effect. We can make this correction, which involves both the object and the weights, as follows (see Figure 7-3)... 

In order to introduce basic equations and quantities, a preliminary survey is made in Section II of the statistical mechanics foundations of the structural theories of fluids. In particular, the definitions of the structural functions and their relationships with thermodynamic quantities, as the internal energy, the pressure, and the isothermal compressibility, are briefly recalled together with the exact equations that relate them to the interparticular potential. We take advantage of the survey of these quantities to introduce what is a natural constraint, namely, the thermodynamic consistency. 

Why would you want to do supercritical fluid cleaning In fact, what is a supercritical fluid (SCF) Is it a supercold, mysterious fluid that physicists study What is it good for What can you dissolve in it organics, salts, particles, resins Why not just use water and add a little detergent These are some of the questions which have been discussed by authors in previous chapters and are reviewed in this chapter. 

C What is Newtonian fluid Is water a Newtonian fluid ... 

Camphor, a saturated monoketone from the Asian camphor tree, is used as a moth repellent and as a constituent of embalming fluid, among other things. If camphor has = 152, what is a likely molecular formula How many rings does camphor have ... 

Imagine for a moment what would happen if the pH of the blood were to become too acidic or too basic. Blood is a fluid that contains water and dissolved electrolytes, a variety of cells, including the red blood cells responsible for oxygen transport, and many different proteins. These proteins include fibrinogen, which is involved in the clotting reaction immunoglobulins, which protect us from disease and albumins, which carry hydrophobic molecules in the blood. 

Gas flows steadily and at low speed past a semi-infinite flat plate. The fluid properties are constant. Far from the plate the gas has a velocity U parallel to the plate, a temperature T, and a species concentration Cg. The temperature of the plate surface and species concentration there are constant, and the pressure is everywhere uniform. For what fluid property condition is there an analogy between the spatial distribution of temperature, concentration, and velocity component parallel to the plate Is the fluid property condition realistic, and what is a statement of the analogy ... 

Have you ever bitten into a piece of aluminum foil If you have, and you had fillings in your teeth, then you ll recall that it was a very unpleasant experience. What you really did was create a battery by joining two dissimilar metals through an electrolyte. An electrolyte is a fluid through which an electric... 

---