Aerodynamic and hydrodynamic properties in food engineering

1. AERODYNAMIC AND
HYDRODYNAMIC
PROPERTIES
RAMYA M
M.Tech Food Technology
CFDT, TANUVAS, Chennai - 52.
ramya2798@gmail.com

2. INTRODUCTION
 In food handling and processing, air and water is
used as carrier for separation and transportation
 Fluid flow or fluid mechanics find increasingly wide
application in handling and processing.

3. AERODYNAMIC PROPERTIES
 The airflow or ‘flow field’ around an object produces
‘aerodynamic’ forces and moments. The interaction
between solid particles and the moving fluid
determines the forces applied to the particles.
 Two important aerodynamic properties
Drag coefficient
Terminal velocity

4. DRAG COEFFICIENT
 It is used to quantify drag or resistance of an object
in a fluid environment such as air or water.
 It is a dimensionless quantity.
 Drag coefficient is always associated with surface
area of an object.
 The shape of the body and the angle of attack
determine the type of drag.
Eg : Sky diver

5.  The pressure of the upper side of the object is less
than that of lower side.
 The pressures on it can be divided into a force that
tends to lift the object and a force that will retard its
movement or drag its movement down.
 In addition to these forces normal to the surface of the
object, there are shear stresses, τ, acting tangential to the
surface in the direction of flow and resulting from
frictional effects.

6.  For drag coefficient,
 Fluid resistance to movement of particle is called
Drag force

7. DRAG COEFFICIENT OF FLAT PLATE
 If a flat plate or circular disk is placed normal to the
flow, the total drag will contain negligible frictional
drag and doesn’t change with Reynold number.

8. DRAG COEFFICIENT VS. REYNOLDS NUMBER
 At very low reynold number (<0.2), stroke law is
applicable. The inertia forces may be neglected
and those viscosity along considered

9.  The drag coefficient of the material and its resistance to
air flow depend upon,
(1) the bed thickness of the material,
(2) type, shape and size of grain,
(3) the air velocity and
(4) orientation and packing of the material.

10. TERMINAL VELOCITY OR SETTLING VELOCITY
 The maximum velocity that an object will obtain in a
free fall situation.
 In free fall, the object will attain a constant terminal
velocity Vt at which, where acceleration will be zero.
 Gravitational force = buoyant force + drag force

11.  The speed at which particles settles depands on
particle properties : D, shape, density
fluid properties : density, viscosity, Re.

12.  When settling velocity
is low (d<0.1 mm) flow
around the particle as it
falls smoothly follow
the form of the sphere.
 When settling velocity
is high (d>0.1 mm) flow
separates from the
sphere and a wake of
eddy develops.

13.  Settling velocity is temperature dependent because
fluid density and viscosity vary with temperature.
 Under steady state condition, where terminal
velocity has been achieved, if the particles density
is greater than fluid density, then the particles
motion will be downward.
 If particles density is smaller than the fluid density,
then the particle will be rise.
Eg: grading of fruit by water & separation of foreign
particle in grain by air.

14. MEASUREMENT OF TERMINAL VELOCITY
 The set up usually consists of a
vertical air column, which is
blown from the bottom and
passes through the screen.
 The screen uniformly distributes
the air velocity. The blower
maintains variable speed.
 When grains are allowed to drop
into the column, initially they
attains acceleration, once the
velocity is adjusted they fall to
the bottom with a constant
velocity. This constant velocity is
termed as terminal velocity

15. HYDRODYNAMIC PROPERTIES
 Important characteristics for hydraulic transport,
handling and sorting of agricultural products.
 Water- lower energy consumption and less
damage.
 Properties - Flow rate, velocity, Viscosity, density,
specific gravity, pressure, vapour pressure,
temperature, surface tension, compressibility, type
of flow (laminar or turbulent) and some
dimensionless number for fluids.

16. FLUIDS
 Fluids – liquid, gas or vapour.
 No definite shape
 It undergoes continuous deformation when
subjected to a shear force
 Major characteristics which influence the dynamic
properties - viscosity, density, pressure.

17. VISCOSITY
 Viscosity – measure of its resistance to flow
 It is defined as the shear stress require to produce
unit rate of shear deformation.

18. KINEMATIC VISCOSITY
 Depends upon density and absolute or kinematic
viscosity of fluids.
 Ratio of dynamic viscosity and density of fluid is
kinematic viscosity

19. REYNOLDS NUMBER
 Ratio of inertial force and viscous force is called
Reynolds number (Re)
 Dimensionless number.
 Its is a function of fluid density, flow velocity, pipe
diameter and fluid viscosity

20. TYPES OF FLOW
 Laminar flow Re < 2000
 Transition flow Re 2000 - 4000
 Turbulent flow Re > 4000
Eg: A bed of river, a blood in veins

21. REYNOLD'S EXPERIMENT
 The types of flow is demonstrated by injecting a
very thin stream of colour fluid having the same
density of water into a large transparent tube
through which water is flowing.

22.  The type of flow α velocity of the liquid
for laminar flow
flow is parallel to the bottom of glass tube
hf α V
For turbulent flow,
hf α 𝑉𝑛
n = 1.75 to 2

23. APPLICATIONS
 In food processing, aerodynamic and hydrodynamic
properties play an important role and must be
known for optimum design and operation of the
equipment
 Pneumatic Separation of foreign materials from
seeds, grains, potato.
 Conveying and handling of grains, small & large
fruits
 Hydraulic handling of apples, cherries, mango&
potatoes etc.

24. REFERENCE:
 Richard L Stroshine. 2004. Physical properties of
agricultural materials and food products. 10 -
Aerodynamic and hydrodynamic properties. Pg.No:
257-264.
 Developments in agricultural engineering. Volume
8, 1986. 14 – Aerodynamic and hydrodynamic
properties and phenomena. Pg.No: 284-351

25. THANK YOU