Chapter 5 VISCOUS FLOW: PIPERS AND CHANNELS

    Chapter 3: the fluid was considered frictionless, or in some cases losses were assumed or computed without probing into their underlying causes.

This chapter deals with real fluids, i.e., situations in which irreversibilities are important.

Viscosity is the fluid property that causes shear stresses in a moving fluid; it is also one means by which irreversibilities or losses are developed. In turbulent flows random fluid motions, superposed on the average, create apparent shear stresses that are more important than those due to viscous shear.

This chapter:    concept of the Reynolds number developed characteristics that distinguish laminar from turbulent flow are presented, and the categorization of flows into internal versus external is established concentrates on internal-flow cases.

 steady, laminar, incompressible flows are first developed, since the losses can be computed analytically.

5.1 LAMINAR AND TURBULENT FLOWS; INTERNAL AND EXTERNAL FLOWS

  Laminar flow: fluid moves in layers, or laminas, one layer gliding smoothly over an adjacent layer with only a molecular interchange of momentum.

 Any tendencies toward instability and turbulence are damped out by viscous shear forces that resist relative motion of adjacent fluid layers.

Turbulent flow has very erratic motion of fluid particles, with a violent transverse interchange of momentum.

    The nature of the flow, i.e., whether laminar or turbulent, and its relative position along a scale indicating the relative importance of turbulent to laminar tendencies are indicated by the

Reynolds number.

This section: the concept of the Reynolds number Sec. 3.5: an equation of motion was developed with the assumption that the fluid is frictionless, i.e., that the viscosity is zero. More general equations that include viscosity have been developed by including shear stresses.

These equations (see Sec. 5.2) are complicated nonlinear partial differential equations for which no general solution has been obtained.

  1) Two flow cases are said to be dynamically similar when They are geometrically similar, i.e., corresponding linear dimensions have a constant ratio. 2) The corresponding streamlines are geometrically similar, or pressures at corresponding points have a constant ratio. In considering two geometrically similar flow situations, Reynolds deduced that they would be dynamically similar if the general differential equations describing their flow were identical. By changing the units of mass, length, and time in one set of equations and determining the condition that must be satisfied to make them identical to the original equations, Reynolds found that the dimensionless group ulρ/μ must be the same for both cases. The quantity u is a characteristic velocity, l a characteristic length, ρ the mass density, and μ the viscosity. This group, or parameter, is now called the Reynolds number R, (5.1.1)

 To determine the significance of the dimensionless group, Reynolds conducted his experiments on flow of water through glass tubes (Fig. 5.1)  A glass tube was mounted horizontally with one end in a tank and a valve on the opposite end. A smooth bellmouth entrance was attached to the upstream end, with a dye jet so arranged that a fine stream of dye could be ejected at any point in front of the bellmouth. Reynolds took the average velocity V as characteristic velocity and the diameter of tube D as characteristic length, so that R = VDρ/μ. Figure 5.1 Reynolds apparatus

     For small flows the dye stream moved as a straight line through the tube, showing that the flow was laminar. As the flow rate increased, the Reynolds number increased, since D, ρ, μ, were constant and V was directly proportional to the rate of flow.

With increasing discharge a condition was reached at which the dye stream wavered and then suddenly broke up and was diffused throughout the tube. The flow had changed to turbulent flow with its violent interchange of momentum that had completely disrupted the orderly movement of laminar flow.

By careful manipulation Reynolds was able to obtain a value R = 12,000 before turbulence set in.

A later investigator, using Reynolds' original equipment, obtained a value of 40,000 by allowing the water to stand in the tank for several days before the experiment and by taking precautions to avoid vibrations of the water or equipment.

These numbers, referred to as the

Reynolds upper critical numbers

, have no practical significance in that the ordinary pipe installation has irregularities that cause turbulent flow at a much smaller value of the Reynolds number.

    Starting with turbulent flow in the glass tube, Reynolds found that it always becomes laminar when the velocity is reduced to make R less than 2000. This is the Reynolds lower critical number for pipe flow and is of practical importance.

With the usual piping installation, the flow will change from laminar to turbulent in the range of Reynolds numbers from 2000 to 4000. For the purpose of this treatment it is assumed that the change occurs at R = 2000.

In laminar flow the losses are directly proportional to the average velocity, while in turbulent flow the losses are proportional to the velocity to a power varying from 1.7 to 2.0. There are many Reynolds numbers in use today in addition to the one for straight round tubes. For example, the motion of a sphere through a fluid can be characterized by uDρ/μ, in which U is the velocity of sphere, D is the diameter of sphere, and ρ and μ are the fluid density and viscosity.

     The

nature

of a given flow of an incompressible fluid is characterized by its Reynolds number.

For large values of R: One or all of the terms in the numerator are large compared with the denominator  large expanse of fluid, high velocity, great density, extremely small viscosity, or combinations of these extremes.

The numerator terms are related to acceleration or deceleration of the fluid; the denominator term is the cause of viscous shear forces 

inertial forces

, or to forces set up by the Reynolds number parameter can also be considered as a ratio of inertial to viscous forces.

Large R indicates a highly turbulent flow with losses proportional to the square of the velocity. The turbulence may be

fine-scale

a great many small eddies that rapidly convert mechanical energy into irreversibilities through viscous action; or it may be , composed of

large-scale

, like the huge vortices and swirls in a river or gusts in the atmosphere. The large eddies generate smaller eddies, which in turn create fine-scale turbulence. Turbulent flow may be thought of as a smooth, possibly uniform flow, with a secondary flow superposed on it. In general, the intensity of turbulence increases as the Reynolds number increases.

  For intermediate values of R both viscous and inertial effects are important, and changes in viscosity change the velocity distribution and the resistance to flow. For the same R, two geometrically similar closed-conduit systems (one, say, twice the size of the other) will have the same ratio of losses to velocity head. The Reynolds number provides a means of using experimental results with one fluid to predict results in a similar case with another fluid.

Internal and External Flows

  Another method of categorizing flows is by examining the extent of the flow field.  Internal flow involves flow in a bounded region, as the name implies.

 External flow involves fluid in an unbounded region in which the focus of attention is on the flow pattern over a body immersed in the fluid. The motion of a real fluid is influenced significantly by the presence of the boundary.

  Fluid particles at the wall remain at rest in contact with the wall. In the flow field a strong velocity gradient exists in the vicinity of the wall, a region referred to as the boundary layer.

A retarding shear force is applied to the fluid at the wall, the boundary layer being a region of significant shear stresses.

  This chapter deals with flows constrained by walls in which the boundary effect is likely to extend through the entire flow.

The boundary influence is easily visualized at the entrance to a pipe from a reservoir (Fig. 5.2). Figure 5.2 Entrance zone of pipeline.

 At section A-A, near a well-rounded entrance, the velocity profile is almost uniform over the cross section. The action of the wall shearing stress is to slow down the fluid near the wall. As a consequence of continuity, the velocity must increase in the central region. Beyond a transitional length L' the velocity profile is fixed since the boundary influence has extended to the pipe centerline. The transition length is a function of the Reynolds number; for laminar flow Langhaar [2] developed the theoretical formula    In turbulent flow the boundary layer grows more rapidly and the transition length is considerably shorter than given by Eq. (5.1.2). (5.1.2) In external flows, with an object in an unbounded fluid, the frictional effects are confined to the boundary layer next to the body (examples: golf ball in flight, an airfoil, and a boat) The fully developed velocity profile, presented in Fig. 5.2 for an internal flow, is unlikely to exist in external flows. Typically interest is focused on drag forces on the object or the lift characteristics developed on the body by the particular flow pattern.

5.2 NAVIER-STOKES EQUATIONS

   The equations of motion for a real fluid can be developed from consideration of the forces acting on a small element of the fluid, including the shear stresses generated by fluid motion and viscosity.

The derivation of these equations, called the

Navier-Stokes equations

, is beyond the scope of this treatment. They ire listed, however, for the sake of completeness, and many of the developments of this chapter could be made directly from them.

First, Newton's law of viscosity, Eq. (1.1.1), for one-dimensional laminar flow can be generalized to three-dimensional flow (Stokes' law of viscosity)  (5.2.1) The first subscript of the shear stress is the normal direction to the face over which the stress component is acting. The second subscript is the direction of the stress component.

   Chap. 3: in developing the Euler and energy equations, z was taken as the vertical coordinate, so that z was a measure of potential energy per unit weight.

In dealing with problems in Chaps. 5 and 8 it is convenient to allow the x, y, z system of right-angular coordinates to take on any arbitrary orientation.

Since gravity, the only body force considered, always acts vertically downward, h is taken as a coordinate which is positive vertically upward; then dh/dx is the cosine of the angle between the x axis and the h axis, and similarly for the y and z axes.

 When the Navier-Stokes equations are limited to incompressible fluids, they become (5.2.2) (5.2.3) (5.2.4)  in which v is the kinematic viscosity, assumed to be constant, d/dt is differentiation with respect to the motion  and  For a nonviscous fluid, the Navier-Stokes equations reduce to the Euler equations of motion in three dimensions, (8.2.3) to (8.2.5).

 For one-dimensional flow of a real fluid in the l direction (Fig. 5.3) with h vertically upward and y normal to l (v = 0, w = 0, du/dl = 0), the Navier-Stokes equations reduce to  For steady flow

Figure 4.4

Model tests showing the influence of a bulbous bow on bow wave