Compared with a laminar boundary layer, a turbulent boundary layer generally has
Answer: A
Turbulent mixing carries high-momentum fluid towards the wall, which steepens the velocity gradient and raises friction, but that same momentum lets the layer run further against a rising pressure before separating. Designers trade one against the other.
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Flow separation occurs when the boundary layer encounters
Answer: B
Where pressure rises in the flow direction the slowest fluid near the wall is brought to rest and then reversed, and the layer lifts off the surface. Separation costs lift and adds pressure drag, and at the extreme it is what causes an airfoil to stall.
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Skin friction drag arises from
Answer: C
It is the integral of the wall shear stress over the wetted area, so it depends on whether the boundary layer is laminar or turbulent and on how much surface is exposed. Pressure acting normal to the surface gives form drag instead.
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Pressure drag on a bluff body is largely a consequence of
Answer: D
When the flow separates, pressure recovery over the rear of the body is lost and the resulting fore-and-aft pressure difference dominates the drag. Streamlining works precisely by delaying separation so that pressure is recovered.
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Aerodynamic coefficients such as lift coefficient are useful because they
Answer: A
Dividing the force by dynamic pressure and a reference area leaves a dimensionless number that depends chiefly on shape, angle of attack, Mach number and Reynolds number. That is what lets wind tunnel results on a model be applied to a full-size aircraft.
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For two flows to be dynamically similar, they must match in
Answer: B
Geometric similarity plus equality of the governing groups, chiefly Reynolds and Mach number, guarantees that the non-dimensional flow fields are identical. Matching both at once in a tunnel is often impossible, which is the central difficulty of experimental aerodynamics.
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Lift coefficient is defined as lift divided by
Answer: C
The reference area is normally the wing planform area, and it is a convention rather than the true area over which the pressure acts. Consistency of reference matters more than its physical meaning, since coefficients are only comparable when defined the same way.
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The lift-to-drag ratio of an aircraft is a measure of its
Answer: D
A high ratio means much lift is obtained for little drag, which sets the glide angle and strongly influences range. It is a property of the aerodynamic design and varies with angle of attack, reaching a maximum at one particular value.
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In the International Standard Atmosphere, temperature in the troposphere
Answer: A
The standard lapse rate is a fall of about 6.5 kelvin per kilometre up to the tropopause, above which the model holds temperature constant through the lower stratosphere. Since the speed of sound follows temperature, it falls with height as well.
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Air density in the standard atmosphere
Answer: B
Both pressure and density fall with height, which is why the same true airspeed produces less dynamic pressure and less lift as an aircraft climbs. The equation of state ties density to pressure and temperature together.
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