Lecture #17
Boundary Layer
Measurements
d Boundary layer Thickness
d* Displacement Thickness
q Momentum Thickness
Objectives of this Lab
• To obtain a better understanding of the
characteristics of a boundary layer over an
airfoil.
• To gain an understanding of the value of using a
constant temperature hot wire anemometry
(CTA) to acquire boundary layer measurements.
• Analyze both a laminar and turbulent boundary
layer.
• Compare data with boundary layer data for a flat
plate.
Instantaneous and Averaged
Boundary Layer Profiles
Recall NSF Film
Boundary Layer Thickness
Boundary layer Thickness
Boundary layer thickness is generally defined as
the height above the surface where the velocity
reaches 99% of the freestream velocity.
Displacement Thickness
Laminar B.L.
Momentum Thickness
The rate of mass flow across an element of the boundary
layer is (r u dy) and the mass has a momentum (r u2 dy )
The same mass outside the boundary layer has the
momentum (r u ue dy)
Q is a measure of the reduction in
momentum transport in the Boundary Layer
Empirical Equations of Laminar
Boundary Layer Parameters
• Boundary Layer
Thickness
• Displacement
Thickness
• Momentum Thickness
• Skin Friction
Coefficient
Empirical Equations of Turbulent
Boundary Layer Parameters
• Boundary Layer
Thickness
• Displacement
Thickness
• Momentum Thickness
• Skin Friction
Coefficient
Skin Friction Coefficient
Mean and Fluctuating Streamwise
Velocity Components
• Velocity in streamwise • Instantaneous, mean
direction
and RMS velocities
From Bertin and
Smith Reference 1
Hot Wire Calibration
• Determine relationship between Hot Wire Output
Voltage, E and the Actual Mean Flow Velocity
• King’s Law
Hot Wire Anemometer
Thermal Anemometry
•
•
•
•
Frequency Response: > 200 Kilohertz
Size: 5mm in diameter by 2mm long
Velocity range
Component Resolution: Hot wires can be used to resolve
one , two and three components of flow field velocity.
• Accuracy: Hot wires are very repeatable. Accuracy is
really a function of how closely the calibration conditions
are reproduced. Velocities can be measured to within <
2% of the actual velocity.
• Resolution: One part in 10,000 can be accomplished
with a hot wire system.
Influence of Adverse Pressure
Gradient
Adverse Pressure Gradient dp/dx>0 can cause
flow separation
Turbulent Boundary Layer
Measurements over a Flat Plate
From White Reference 2
References
• 1. White, F. M., Viscous Fluid Flow,
McGraw-Hill, New York, 1991.
• 2. Bertin , J. J., and Smith, M. L.,
Aerodynamics for Engineers, Prentice
Hall, Englewood Cliffs, New Jersey, 1989
• Schlichting, H., Boundary Layer Theory, 7th
edition, McGraw-Hill, New York, 1979.