3.1

3.1  TYPES OF FLOW

Fluid flow can refer of an ideal fluid and real fluid.

Ideal fluid: fluid that has no viscosity, flow in  a straight conduit, all particles move in parallel lines with equally velocity.

Real fluid : the real fluid, the velocity adjacent to the wall will be zero, it will increase rapidly within short distance from the wall and produce a velocity profile.

                                       IDEAL FLUID                      REAL FLUID

3.1.1 INCOMPRESSIBLE FLUID FLOW OR COMPRESSIBLE FLUID FLOW

 It is the flow that is being considered in this case, and not the fluid. The gases are, in general, quite compressible; yet flows of gases can often be treated as incompressible flows. A simple, and quite important, example of this is flow of air in air-conditioning ducts. For our purposes in this course, a flow will be considered as incompressible if its density is constant. This will often be the case in the problems treated here. But we note that there are some flows exhibiting variable density, and which can still be analyzed accurately as incompressible.

3.1.2 STEADY FLOW OR UNSTEADY FLOW

One of the most important, and often easiest to recognize, distinctions is that associated with steady and unsteady flow. In the most general case all flow properties depend on time; for example the functional dependence of pressure at any point (x, y, z) at any instant might be denoted p(x, y, z, t). This suggests the following: Definition 2.8 If all properties of a flow are independent of time, then the flow is steady; otherwise, it is unsteady. Real physical flows essentially always exhibit some degree of unsteadiness, but in many situations the time dependence may be sufficiently weak (slow) to justify a steady-state analysis, which in such a case would often be termed a quasi-steady analysis. It is also worth mentioning that the term transient arises often in fluid dynamics, just as it does in many other branches of the physical sciences. Clearly, a transient flow is time dependent, but the converse is not necessarily true. Transient behavior does not persist for “long times.” In particular, a flow may exhibit a certain type of behavior, say oscillatory, for a few seconds, after which it might become steady. On the other hand, time-dependent (unsteady) behavior is generally persistent, but it may be generically similar for all time after an initial transient state; i.e., the qualitative nature of the behavior may be fixed even though the detailed motion changes with time. Such a flow is often termed stationary. Examples of these flow situations are depicted in Fig. 2.13 in terms of their time series.

3.1.3      UNIFORM FLOW

A uniform flow is one in which all velocity vectors are identical (in both direction and magnitude) at every point of the flow for any given instant of time. Flows for which this is not true are said to be non-uniform.