Flow over immersed bodies

Question 1

What are aerodynamic forces due to?

Answer 1

Aerodynamic forces are due to pressure and shear stress. The integral of pressure/shear stress distrubition is an aerodynamic force.

Question 2

How can aerodynamic coefficients be measured?

Answer 2

a) In the wind tunnel.

1) With a mean and fluctuating part (standard deviation), aerodynamic coefficients can be measured with their fluctuations.

2) Wind loads may be computed through integration of the pressure field when wall shear stress contribution may be considered negligible

b) Numerical analysis (CFD)

c) Full scale measurements

Question 3

What are aerodynamic coefficients function of?

Answer 3

CF = f(Re, Minf, Iu, ε/l)
- Reynolds number
- Mach
- Free stream turbulence intensity
- Surface rougness

Similitude of all the parameters is impracticable, so in some operating conditions the dependancy of some parameters is negligible.

Question 4

What are the two regions in the flow field?

Answer 4

1) Region affected by no-slip condition, which means that the velocity of the flow is equal to the velocity of the body (boundary layer)

2) Region that viscous affects are negligible (potential flow)

Question 5

What are the viscous effects?

Answer 5

In presence of the velocity gradient (relative velocity between the body and flow), fluid elements located in the moving fluid move with a velocity and generally undergo a change in shape (angular deformation).

τ = μdu/dz

Question 6

What are the potential flow assumptions?

Answer 6

1) Fluid is inviscid
2) Fluid is incompressible

With this assumption, Bernoull Equation can be used

Question 7

What is a stream line?

Answer 7

A steamline is a line that is everywhere tangent to the velocity field. For a steady flow, streamlines are fixed lines in a flow field which show the paths the fluid particles follow.

Question 8

What is the boundary layer?

Answer 8

Boundary layer is similar to the boundary layer created by the atmosphere. At the surface, the no-slip condition is true and a velocity gradient is created up to a point δ, where the velocity is equal to the flow velocity U.

Question 9

How does the boundary layer change with Re number?

Answer 9

In large reynold numbers, the streamlines deflection is very slight.

Question 10

What are the available solutions for BL?

Answer 10

The available solutions are given by Prandlt/Blasius, by using a similarity variable, for steady, 2D, negligible gravitational effect flow.

Question 11

What is turbulence?

Answer 11

Turbulence develops as an instability of laminar flow. The turbulent flow shows high diffusivity. At high Re numbers, the flow transitions from laminar to turbulent flow.

Mixing processes and heat and mass transfer processes are considerably enhanced in tubulent flow compared to laminar flow. This is due to the macroscopic scale of the randomness in turbulent flow.

Question 12

What does the critical reynolds number on a flat plate depend on?

Answer 12

It depends on the rougness and the incoming turbulents. Transition occures over a region and not at a specific location. The boundary layer is increased with turbulent flow.

Question 13

What is the Boundary Layer transition affected by?

Answer 13

It is affected by the pressure distribution in the external flow, the body roughness and the turbulence intensity in the external flow.

Question 14

How can the velocity profile be approximated?

Answer 14

1) Logarithmic law profile
2) Power law

Question 15

What is a bluff body?

Answer 15

A bluff body is a body, where separation occurs in a flow.

Question 16

What can affect the separation position?

Answer 16

1) Reynolds number
2) Roughnes
3) Turbulence intensity

Question 17

Discuss drag on bluff bodies.

Answer 17

For streamlined bodies where BL separation does not occur, the drag is mainly due to friction drag and it is larger when the BL than when the flow is turbulent. If the BL is laminar and separation occurs, the drag force depends also by the wake width and is usuall greater than when the BL is laminar.

Question 18

What are the ESDU definitions for streamlined and bluff bodies?

Answer 18

1) A streamlined body is defined as a body for which the major contribution to the drag force results from the viscous effects or skin friction

2) A bluff body is defined as a body for which the major contribution to the drag force is due to pressure forces arising from ceparation of the boundary layer.

Question 19

How can we increase Re,cr?

Answer 19

Increased roughness and tubulence intensity can allow to simulate higher Reynolds Number condition.

Question 20

What are the aerodynamic forces features?

Answer 20

Aerodynamic forces features are:
1) Mean vlaue
2) Fluctuating part (vortex shedding, buffeting)
3) Interaction with the body motion (galloping, flutter)

Question 21

What is buffeting?

Answer 21

Buffeting is the variation in the force due to the incoming flow.

Question 22

What is vortex shedding?

Answer 22

Steady flow past a blunt object may produce an oscillating Karman vortex strees behind the object. The shedding of vortices from the body exerts a periodic force on the body, that if the frequency is right, can cause the body to oscillate

St = fsD/U

Question 23

Discuss vortex shedding forces.

Answer 23

Due to the alternating vortex wake, the oscillations in lift force occur at fs (avg lift = 0), and oscillations in drag force at twice the frequency of fs.

Question 24

How can the structure response to wind forces be computed?

Answer 24

It can be computed in the frequency domain.
1) Given the wind spectrum
2) Multiply with an aerodynamic admittance (filter) is created to take into account the larger effects
3) Given the input of the force
4) Multiply with the the Frequency Response Function
4) Output