Page 50 - Fluid mechanics, heat transfer, and mass transfer

FLOW PHENOMENA 27

. Above what Reynolds numbers (for flow past a sphere),
flow separation takes place?
& N Re 20.
. What is a stagnation point?
& In flow past an immersed body, boundary layer
separation starts at the front center of the body where
the fluid velocity will be zero. This point is called the
stagnation point. Boundary layer growth begins at
this point and continues over the surface until the
layer separates.
Drag coefficients for different shaped objects.
FIGURE 2.6
. What is stagnation pressure?
Note: All objects have the same projected (frontal) area. Drag
2
coefficient C d ¼ F D /0.5rAV , where A is the projected area. & Stagnation pressure is the pressure at a stagnation
point in a fluid flow, where the kinetic energy is
converted into pressure energy. It is the sum of the
many different scales being shed in a turbulent wake
dynamic pressure and static pressure at the stagnation
from the body.
point.
& Each of these flow regimes produce a different
amount of drag on the sphere. . Illustrate the relationship between drag coefficient and
Reynolds number.
& Comparing different downstream shapes, it can be
& At low Reynolds numbers, involving laminar flow,
noted that the downstream shape can be modified to
reduce drag. drag coefficients are high. With increase in N Re
drag coefficients decrease sharply and under fully
. What are the different types of drag and lift?
turbulent conditions they remain constant with
& There are two basic types of drag as explained below.
further increases in N Re .
& For the case of fluid friction inside conduits, the
& Figure 2.7 illustrates this point.
transfer of momentum perpendicular to the surface
. What are the common additives used to reduce drag in
results in a tangential shear stress or drag on the
pipeline flow?
smooth surface parallel to the direction of flow. This
& Carboxymethyl cellulose (CMC), methyl methacry-
force exerted by the fluid on the solid in the direction
late, or other polymer solutions.
of flow is called skin friction or skin or wall drag.
. What is Coanda effect?
& For any surface in contact with a flowing fluid, skin
& The Coanda effect is the tendency of a stream of fluid
friction will exist. In addition to skin friction, if the
to follow the contour of a surface.
fluid is not flowing parallel to the surface but must
change direction to pass around a solid body such as a . What is magnetohydrodynamics (MHD)?
sphere, significant additional frictional losses will & The studyofflow of electrically conducting fluidsin a
occur. This phenomenon, called form drag,isin magnetic field is called magnetohydrodynamics.
addition to the skin drag in the boundary layer. & The phenomenon arising from a coupling between
& Drag is also classified into the following types: either electric fields or electric discharges and ve-
➢ Surface friction causes viscous drag. locity fields is called MHD.
➢ Turbulence causes pressure drag. & For example, gas–liquid flows can be altered by
➢ Deflected flow causes lift. electric field effects.
➢ Deflected flow causes induced drag. & The simplest example of an electrically conducting
. Define drag coefficient. fluid is a liquid metal, for example, mercury or liquid
sodium. However, the major use of MHD is in plasma
& Drag coefficient,C d , is a dimensionless numbergiven
physics. (A plasma is a hot, ionized gas containing
by
electrons and ions.)
C d ¼ F D =0:5rAV 2 & There are two serious technological applications of
¼ drag force=ðprojected area velocity headÞ; MHD that may both becomevery important in future.
First, strong magnetic fields may be used to confine
ð2:20Þ
rings or columns of hot plasma that might be held in
where F D is the drag force, r is the fluid density, V is place long enough for thermonuclear fusion to occur
the fluid velocity, and A is the projected area of the and for net power to be generated. In the second
solid object. application, which is directed toward a similar goal,