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AOE 3054 Flow Visualization

• Relevant to experiments 1, 4 and 7.• General background to one of the most

basic techniques of wind tunnel testing • Reading: Lab manual, experiment 1. Also

Barlow, “Low Speed Wind Tunnel Testing” (on reserve).

What is flow visualization?

• The art of making an invisible flow visible.

•

• A key diagnostic tool for engineers.

This Class

• An overview of a few of the many flow visualization techniques– How do you use them?– What is seen?– Advantages, disadvantages?– Examples– Variations

What are the various techniques?

• Methods for visualizing flow fields1. 2.

• Methods of visualizing surface flows3. 4.

Methods for Visualizing Flow Fields1. Smoke or Dye Injection

Smoke (for air flows)

Dye (for water flows)

How?

(add labels)

1. Smoke or Dye Injection

Dye in water - NACA 65A015, Re = 7000, zero degrees angle of attack.

What is seen?

Note

1. Smoke or Dye Injection

Kerosine vapor in air. Sphere at 1.6m/s. (Experiment 1)

Advantages/Disadvantages••••

1. Smoke or Dye InjectionVariations

(a) …

(b) …

LEX vortex bursting on a 1/48th

scale F18 model visualized by injecting dye in water.

Spiral vortices formed on a 20cm-wide spinning cone at 2.9m/s. Boundary layer is revealed.

1. Smoke or Dye InjectionVariations

(c) …

Picture shows European Starling in 9m/s air flow. Filament lines are revealed.

http://www.biology.leeds.ac.uk/staff/jmvr/Flight/fvbzwjm.htm

Virginia Tech – Miami, 2003Crop spraying

Yachts in the Volvo Ocean Race in Cape Town

Methods for Visualizing Flow Fields2. Bubble Techniques(a) Air: Helium bubble technique

How?

What is seen?

(add labels)

2a. Helium bubble techniqueExamples

(b) Flow over a wing tip. Bubbles visualized using pulsed laser (note dotted paths)

Virginia Tech AOE Dept.

(a) Wind-tunnel test of a 1:20 scale model of the space shuttle

Sage Action Incorporated

2a. Helium bubble technique Example

Advantages/Disadvantages•

•

•

•

(c) Helicopter in forward flight

NASA

Methods for Visualizing Flow Fields2. Bubble Techniques

(b) Water: Hydrogen bubble technique

How?

What is seen? Depends on mode of operation:

Fluid line:

2(b) Hydrogen bubble technique

Examples

(a) Flow downstream of a circular cylinder

Cylinder

Platinum wire

Prongs supporting wire

(b) Flow in an inlet region of a circular pipe (water, flow velocity 6cm/s, 27mm pipe, Re=1600)

Japan Society of Mechanical Engineers: Visualized Flow, Pergamon Press, New York, 1988

Armfield Ltd. UK

2(b) Hydrogen bubble technique

Advantages/Disadvantages•••

Examples

(c) Turbulent flow in a rectangular duct (pulsed/insulated)

Armfield Ltd. UK

Methods for Visualizing Flow Fields2. Bubble Techniques

Variations

Turbulent separation over a rectangular block (water flow)

Methods for Visualizing Surface Flows3. Surface Oil Flow Visualization

How?

What is seen?

Skin-frictionlines

Separation ‘bubble’(Separation followed by Immediate reattachment)

Drips of oil!

Flow

3. Surface Oil Flow Visualization

Sep. Line

Sep. Line

(a) Flow over a stalled airfoilExamples

3. Surface Oil Flow Visualization

Advantages/Disadvantages••••

Examples(b) 50 c.c. Land Speed Record Vehicle Model Wind Tunnel Oil Flow Visualization (Sideslip Angle, β = 12°). Variation: use of dark pigment (not TiO2) Photo courtesy of

Eugene Heim

Methods for Visualizing Surface Flows4. Tufts

How?

What is seen?

Advantages/Disadvantages•••

4. Tufts

Examples

(a) Flow over a Clark Yshowing stalled and unstalled behavior.

(Experiment 7)

Flow

Flow

4. Tufts

(c) …

Flow over a transport aircraft wing.

University of Washington Aeronautical Laboratories

(b) Yarn tufts. Flow over the main sail of a model yacht

WB Sails Ltd.

Mast

Flow

4. Tufts(d) In flight tuft visualization

4. Tufts

Variations

Matt Orr, MS Thesis, Virginia Tech, 2000