Lecture 6 - Fluid Mechanics Forces

Question 1

What do fluid mechanics forces result from? (give equation)

Answer 1

The action of pressure and shear stresses on the object.

F_–> = int_Swet [tau_–>dS] - int_Swet[p*n_–>dS]

Question 2

S_wet - ?
tau_–> - ?
p - ?
n_–> - ?

Answer 2

S_wet - wetted surface area
tau_–> - viscous stress vector on the object
p - static pressure
n_–> - outward pointing surface normal vector

Question 3

Why are the Navier-Stokes equations nonlinear?

Answer 3

Convective acceleration in the equations

Question 4

Pressure forces are created… - ?

Answer 4

at the surface of a body due to collisions between molecules of the fluid and the surface of the body.

Question 5

Shearing forces - ?

Answer 5

produced by fluid viscosity,
provide information how well momentum is transferred between adjacent layers of the fluid.

Question 6

Lift coefficient

Answer 6

C_L = L/q_inf * S * l

Question 7

Drag coefficient

Answer 7

C_D = D/q_infSl

Question 8

Moment coefficient

Answer 8

C_M = M/q_infSl

Question 9

q_inf - ?

Answer 9

1/2 * rho_inf * V_inf^2

Question 10

S - ?
l - ?

Answer 10

S- reference area
l - reference length

Question 11

what do capital letters for forces represent?

Answer 11

3D Geometries

Question 12

what is an aerofoil?

Answer 12

wings with infinite span

Question 13

Sources of drag for bird wings or fins and fish tails etc?

Answer 13

Skin Friction
Pressure drag

Question 14

What causes skin friction?

Answer 14

shear stresses in the boundary layer

Question 15

What causes pressure drag?

Answer 15

Wake - a front back asymmetric flow pattern caused by the
flow separation. Occurs on blunt bodies or aerofoils when the
angle of attack is too large.

Downwash - vortex induced velocity. It affects the effective
angle of attack.

Question 16

What is a streamlined body?

Answer 16

Drag is dominated by a frictional component.

Question 17

What is a bluff body?

Answer 17

When pressure forces dominate drag.

Question 18

What is von Karman vortex street?

Answer 18

Wake consisting of a procession of equal-strength vortices, equally
spaced but alternating in sign.

Occurring behind bluff bodies, including flat plates normal to the
flow direction.

In which the vortices move downstream at a speed less than the
freestream velocity (the reduction in speed is inversely
proportional to the streamwise distance separating alternate
vortices).

Giving rise to an alternating lift force (changes sign + or −) on
the cylinder at the same frequency as that of the vortex
shedding

Question 19

What is strouhal number?

Answer 19

St = eta * D/ V_inf