Transition Over C4 Leading Edge and Measurement of Intermittency Factor Using PDF of Hot-Wire Signal

Abstract

The variation of intermittency factors in the transition region of a C4 leading edge flat plate is measured at three incidence angles in a low-turbulence free stream. During the determination of intermittency factor, the threshold value of the detector function and the validity of conditional averaging are verified by a method based on the direct application of PDF of the hot-wire output. As the angle of incidence is increased, the transition progressively moves through all the three modes on the suction surface: at zero incidence the bypass transition, at 2 deg incidence the natural transition, and at 4 deg incidence the separated-flow transition occur, respectively. All three modes of transition exhibited the chordwise intermittency factor variation in accordance with Narasimha’s universal intermittency distribution; thus, the method based on spot production rate is applicable to all the three modes of transition. In the transition zone of the attached boundary layers, the conditionally averaged interturbulent profiles are fuller than the Blasius profile, while the conditionally averaged turbulent profiles follow a logarithmic profile with a variable additive parameter.

The Effect of Reynolds Number on the Behaviour of a Leading Edge Separation Bubble

“…The excellent match between the theoretical curve and the experimental results is obtained shows that Narasimha's assumption of spot production in an extremely narrow band around the onset position holds good for the separated flow transition also. This confirms Hirsch, 1995 andMalkiel andMayle, 1995) that the spot production rate is a logical parameter to describe the y peak distribution in a separated flow transition also. However, as suggested in the above mentioned papers the parameters are orders of magnitude larger than those in the attached flows.…”

“…The measurements of intermittency factors through the leading edge separation bubbles reported by Hazarika and Hirsch (1995) and Malkiel and Mayle (1995) show that the peak intermittency factor variation follows the universal distribution of Narasimha (1985) derived for transition of boundary layers over flat plates. Both these investigations were conducted in long separation bubbles.…”

“…The mean velocity and the RMS of fluctuating velocity are measured at a large number of stations for all the six test cases. The development of U and u through the separation bubble and the recovery region for the test case E41'05 in the as measured form was presented before (case D in Hazarika and Hirsch, 1995). In this paper the profiles of U and u are presented for the case E4T08 in Fig.…”

“…The small value is due to the short length of highly accelerating then retarding region between the stagnation point and the separation point where the boundary layer develops before separation. It has been shown earlier (Hazarika and Hirsch, 1995) that the spot production rate in a separated shear layer is several orders of magnitude higher than those in an attached shear layer. Mayle's (1991) correlation between spot production rate and Ree s has been confirmed by Malkiel and Mayle (1995) The data point for E4T03 does not give results suggested by this equation.…”

“…The stored data are later analysed for intermittency factor (y) and conditionally averaged to obtain the mean and the fluctuating velocities in nonturbulent and turbulent portions using a new method based on the probability density function (PDF). The procedure of this method is given in Hazarika and Hirsch (1995).…”