SURFACE PRESSURE MEASUREMENTS ON AN AEROFOIL IN TRANSONIC FLOW
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 …show more content…
From figure 7 we can see that this value is, M∞=0.732.
Transonic flow occurs when ‘there is mixed sub and supersonic local flow in the same flow field.’ (Mason, 2006) This generally occurs when free-stream Mach number is in the range of M=0.7-1.2. The local region of supersonic flow is generally ‘terminated’ by a normal shockwave resulting in the flow slowing down to subsonic speeds.
Figure 8 below shows the typical progression of shockwaves as Mach number increases. At some critical Mach number (0.72 in the case of Figure 8), the flow becomes sonic at a single point on the upper surface of the aerofoil. This point is where the flow reaches its highest local velocity. As seen in the figure, increasing the Mach number further, results in the development of an area of supersonic flow.Increasing the Mach number further again then moves the shockwave toward the trailing edge of the aerofoil and a normal shockwave will develop on the lower surface of the aerofoil. As seen in figure 8, approaching very close to Mach 1, the shockwaves move to the trailing edge of the aerofoil. For M>1, the flow behaves as expected for supersonic flow with a shockwave forming at the leading edge of the aerofoil.
Figure 9-Progression of shockwaves with increasing Mach number (H.H.Hurt, 1965)
In normal subsonic flow, the drag is composed of 3 components-skin friction drag,