Boundary Layer and Drag Questions and Answers

Consider the following statements about the Reynolds number:

1. It compares inertial with viscous forces.

2. Matching it between model and full scale is a central difficulty in wind-tunnel testing.

3. Transition on a smooth flat plate occurs at a fixed value of about five hundred thousand.

Which of the statements given above is or are correct?

Match each boundary-layer quantity in List I with its meaning in List II.

List I

A. Displacement thickness

B. Momentum thickness

C. Shape factor

D. Wall shear stress

List II

1. Directly related to the friction drag accumulated so far

2. The outward shift of the external streamlines

3. Viscosity times the velocity gradient at the surface

4. The ratio of the first two thicknesses, watched as a separation indicator

Select the correct match.

Match each flow-control device in List I with how it works in List II.

List I

A. Vortex generator

B. Boundary-layer suction

C. Golf ball dimple

D. Streamlining

List II

1. Removes the slowest fluid through the surface

2. Stirs faster air down towards the wall

3. Tapers the rear so the pressure rise is gentle

4. Trips the layer turbulent to bring on the drag crisis

Select the correct match.

Match each body in List I with the drag component that dominates it in List II.

List I

A. A flat plate aligned with the flow

B. A circular cylinder in crossflow

C. A well-streamlined strut

D. A parachute canopy

List II

1. Almost entirely pressure drag from a very broad wake

2. Almost entirely skin friction

3. Pressure drag, from separation near the shoulder

4. Mostly skin friction, with a modest pressure contribution

Select the correct match.

Match each Blasius laminar result in List I with its value in List II.

List I

A. Boundary layer thickness coefficient

B. Displacement thickness coefficient

C. Momentum thickness coefficient

D. Shape factor

List II

1. 1.721

2. 5.0

3. 2.59

4. 0.664

Select the correct match.

Match each surface condition in List I with its consequence in List II.

List I

A. Roughness buried in the laminar sublayer

B. Roughness protruding through the sublayer

C. A favourable pressure gradient

D. A steep adverse pressure gradient

List II

1. Skin friction rises sharply

2. The surface behaves as hydraulically smooth

3. Separation

4. Transition is delayed and the layer stays attached

Select the correct match.

All of the following delay boundary-layer separation EXCEPT

Which of the following is NOT a consequence of a boundary layer going turbulent?

Which statement about the drag crisis is INCORRECT?

Which of these does NOT reduce the drag of a bluff body?