Post on 24-Dec-2015
transcript
Lecture #17Boundary LayerMeasurements
Boundary layer Thickness* Displacement ThicknessMomentum 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.
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.
Momentum Thickness
The rate of mass flow across an element of the boundary layer is ( u dy) and the mass has a momentum ( u2 dy ) The same mass outside the boundary layer has the momentum ( u ue dy)
is a measure of the reduction in momentum transport in the Boundary Layer
Empirical Equations of Laminar Boundary Layer Parameters
• Boundary Layer Thickness
• Momentum Thickness
• Displacement Thickness
• Skin Friction Coefficient
Empirical Equations of Turbulent Boundary Layer Parameters
• Boundary Layer Thickness
• Momentum Thickness
• Displacement Thickness
• Skin Friction Coefficient
Mean and Fluctuating Streamwise Velocity Components
• Velocity in streamwise direction
• Instantaneous, mean 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
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.