DEN 302 Applied Aerodynamics
SURFACE PRESSURE MEASUREMENTS ON AN AEROFOIL IN TRANSONIC FLOW
Abstract
The objective of this exercise is to measure the pressure distribution across the surface on an aerofoil in a wind tunnel. The aerofoil is tested under several different Mach numbers from subsonic to supercritical. The purpose of measuring the pressure distributions is to assess the validity of the Prandtl-Glauert law and to discuss the changing chracteristics of the flow as the Mach number increases from subsonic to transonic.
As a result of the experiment and computation of data, the aerofoil was found to have a critical Mach number of M=0.732. Below this freestream Mach number the Prandtl-Glauert law predicted results very
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However it is deemed reasonably accurate for cases such as this in which thin aerofoils are tested at small incidence. The law does not hold in super-critical flow when local regions of supersonic flow and shockwaves appear.
The value of the critical pressure coefficient, Cp*, according to local sonic conditions is calculated by:
Cp*=10.7M∞25+M∞263.5-1forγ=7/5 (6)
The co-ordinates for the NACA 0012 section are as follows:
Figure 1-Co-ordinates for aerofoil (Motallebi, 2012)
Results
Given atmospheric conditions of:
Patm=30.65 in-Hg
Tatm=21°C
The following results were achieved:
Figure 2-Pressure coefficient vs x/c for M=0.83566
Figure 3-Pressure coefficient vs x/c for M=0.83119
Figure 4-Pressure coefficient vs x/c for M=0.79367
Figure 5-Pressure coefficient vs x/c for M=0.71798
Figure 6-Pressure coefficient vs x/c for M=0.59547
Figure 7-Pressure coefficient vs x/c for M=0.44456
Figure 8-Cp* and Cpminvs Mach Number
From figure 7 the critical Mach number is
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