The mean velocity profiles at hole exit have been found to be highly sensitive to the film cooling length. Film cooling with short holes (L/D=2.3 in figures) tends to have a great deal of its coolant mass distributed towards the upstream portion of and along the edges of the hole at the exit plane. At entrance to the hole, a separation bubble forms that causes this malproportioned coolant mass distribution.
Film cooling through long lengths (L/D=7.0 in figures) though is characterized by more uniform profiles or those slightly favoring the downstream portion of the hole at exit.
The disturbance at hole entrance found with short-hole cases is also present but is attenuated by the additional hole length. The allows the flow to recover to a more "fully-developed" or parabolic exit flow. The coolant-to-freestream blowing rate, and associated coolant momentum, influences the exit profiles as well. Hole exit mean velocity (Ulocal/Ubulk) profiles for film cooling through short and long lengths (L/D=2.3 a nd 7.0) and at low and high (VR=0.5 and 1.0) blowing rates are provided below.