Air Flow Visualization

Question 1

Movement of liquids and gases in response to forces

Answer 1

fluid flow

Question 2

The property of a fluid that describes its resistance to shear deformation or internal friction.

Answer 2

viscosity (μ)

Question 3

Measures how strongly fluid layers resist sliding past each other

Answer 3

viscosity (μ)

Question 4

Measures internal resistance of fluid to shear

Answer 4

dynamic viscosity (μ)

Question 5

Measures the resistive flow of a fluid when no external force is applied except the influence of weight

Answer 5

kinematic viscosity (ν)

Question 6

• Viscosity effects are ignored.
• Used in high Reynolds number flows where inertial forces dominate.
• Provides quick estimation of aerodynamic forces.

Answer 6

inviscid flow

Question 7

• Viscosity effects are important.
• Internal friction and shear stresses affect the flow.
• Important near solid surfaces.

Answer 7

viscous flow

Question 8

• Fluid particles move in smooth parallel layers.
• Minimal mixing between layers.
• Occurs at low Reynolds numbers.

Answer 8

laminar flow

Question 9

• Fluid motion is irregular and chaotic.
• Strong mixing between layers.
• Occurs at high Reynolds numbers.

Answer 9

turbulent flow

Question 10

The thin region of fluid close to a surface where viscous effects are significant

Answer 10

boundary layer

Question 11

• Velocity changes from zero at the wall to the free-stream velocity.
• Caused by the no-slip condition, where the fluid sticks to the surface.

Answer 11

boundary layer

Question 12

A measure of the ratio of inertia forces to viscous forces

Answer 12

Reynolds number

Question 13

Can be used to assess how comparable the aerodynamic flows are in a body and its scaled counterpart

Answer 13

Reynolds number

Question 14

Can be used to determine whether the boundary layer is turbulent or laminar in whole or in part

Answer 14

Reynolds number

Question 15

Shows how fluid velocity changes from the wall to the free stream

Answer 15

velocity profile

Question 16

At the wall: velocity = _ (no-slip condition)

Answer 16

0

Question 17

Away from the wall: velocity _ until it reaches free-stream velocity.

Answer 17

increases

Question 18

Characteristics:
- Smooth and orderly velocity distribution.
- Velocity increases gradually from the wall.

Answer 18

Laminar Boundary Layer Velocity Profile

Question 19

Characteristics:
- Velocity profile is fuller and steeper near the wall.
- Strong mixing of fluid layers.

Answer 19

Turbulent Boundary Layer Velocity Profile

Question 20

A point in the flow where it changes or “transitions” from laminar flow to turbulent flow

Answer 20

transition point

Question 21

Its location in the chord is indicated by the critical chord x_cr derived from the critical Reynolds number.

Answer 21

transition point

Question 22

• Studies focus on shear forces, pressure forces, and heating loads that a fluid flow exerts on a solid surface.
• Flow pattern visualization near or on the surface aids these studies.
• Surface coating methods are commonly used, where a material interacts with the flow to provide visible data.
• Interaction types: mechanical, chemical, or thermal.

Answer 22

surface flow patterns

Question 23

Standard technique for decades in wind tunnel studies

Answer 23

oil film visualization

Question 24

Method:
- Surface coated with thin oil layer containing finely powdered pigment.
- When airflow starts, oil moves with the air, leaving pigment streaks on the surface.

Answer 24

oil film visualization

Question 25

Purpose: - Streak patterns indicate flow direction near the surface. - Specific features in the pattern reveal flow separation or flow reattachment points.

Answer 25

oil film visualization

Question 26

Method: Short tufts attached to the surface of interest

Answer 26

wall tufts

Question 27

Tufts align with local flow direction

Answer 27

Laminar, attached flow

Question 28

Tufts show unsteady motion

Answer 28

Unsteady or turbulent flow

Question 29

Method: Coating surface with sublimating materials (e.g., naphthalene or azobenzene)

Answer 29

mass transfer

Question 29

Tufts exhibit violent motion or tend to lift off the surface

Answer 29

Separated flow

Question 30

Mechanism: Material evaporates into the airflow, visualizing mass transfer between the surface and fluid

Answer 30

mass transfer

Question 31

- Luminescent dye (luminophore) dispersed in oxygen-permeable binder. - Excitation: Illuminated with short-wavelength light (blue or UV). - Mechanism: Collisions between luminophores and oxygen reduce emitted light intensity. - Output: Light intensity variations reveal wall pressure distribution.

Answer 31

pressure sensitive paint (PSP)

Question 32

- Similar principle as PSP but without oxygen quenching. - Measures wall temperature distribution instead of pressure.

Answer 32

temperature sensitive paint (TSP)

Question 33

Injection of Foreign Materials for Flow Visualization

Answer 33

tracer particles

Question 34

Method: Introduce a visible material into the flow. - Air flows: commonly use smoke - Water flows: commonly use dye

Answer 34

tracer particles

Question 35

Purpose: - Track specific flow lines downstream of the injection point. - Fill a flow region to visualize boundaries and geometric extension of the flow regime. - Particularly useful for visualizing flow separation.

Answer 35

tracer particles

Question 36

Common Techniques: - Smoke visualization - Dye visualization in water

Answer 36

tracer particles

Question 37

Includes steam, vapor, mist, and aerosols, not just combustion products

Answer 37

smoke

Question 38

Desired tracer properties: - Neutrally buoyant (follows the flow accurately) - Non-toxic (safe for use) - Low mixing with the main fluid (maintains clear visualization of flow paths)

Answer 38

smoke visualization

Question 39

- neutral buoyancy - high stability against mixing - good visibility

Answer 39

visualization by dye in water

Question 40

Principle: Fluid density affects its refractive index. Application: Variations in refractive index reveal density changes due to: - Compressibility - Thermal effects - Mixing of fluids with different densities

Answer 40

optical methods

Question 41

Common uses: High-speed wind tunnels, ballistic ranges, shock tubes. Common Techniques: - Shadowgraph - Schlieren

Answer 41

optical methods

Question 42

Simplest optical flow visualization technique (first explored by Dvorak)

Answer 42

Shadowgraph

Question 43

Setup: Only requires a light source; no lenses or mirrors needed. Principle: Variations in refractive index produce visible shadow patterns. Applications: - Open-air visualization using sunlight. Examples: - Rising warm air - Benzene vapor projections - Shock wave visualization

Answer 43

Shadowgraph

Question 44

Enhanced optical technique derived from Shadowgraph, more sensitive to density changes

Answer 44

Schlieren system

Question 45

Principle: Detects first-derivative of density, making it more sensitive than Shadowgraph Setup: - Parallel light beam is converged by a lens or spherical/parabolic mirror (“Schlieren head”). - Knife edge at focal point blocks part of light, creating contrast based on density gradients. - Camera focuses the flow field onto a recording plane to eliminate unwanted shadows. Application: Visualization of fine density variations in gases (e.g., shock waves, thermal flows).

Answer 45

Schlieren system