Boundary layer transition determination for periodic and static flows using phase-averaged pressure data

Abstract

the power required. Additionally the transition affects the resistance of components to boundary layer separation.As seen in Fig. 1 on the example of a flat plate, the boundary layer is characterised by a laminar part near the leading edge which has low turbulence and wall shear stress. At some point, turbulence in the boundary layer is no longer damped, and the turbulence increases along a "transition region" until the boundary layer is fully turbulent. Compared to the laminar region, the turbulent region has higher recovery temperature, wall heat flux, wall shear stress, and higher RMS values of wall static pressure, Pitot pressure and heat flux (Schlichting and Gersten 2006). These characteristics can be used to detect transition, and the detection of boundary layer transition can be performed by a large number of methods (Nitsche and Brunn 2006), most of which work for transition positions which are invariant with time or quasi-steady with regard to the surface temperature distribution, for example: --Skin friction measurements by piezoelectric sensors (Nitsche and Brunn 2006) or oil films for steady flows (Tanner and Blows 1976) or unsteady flows (Schülein 2014). --Measurements of the transition position by sublimation (Velkoff et al. 1971) or China-clay (Richards and Burstall 1945). --Measurements of the wall recovery temperature by thermocouples, temperature-sensitive paint, temperaturesensitive liquid crystals or infrared cameras (Peake et al. 1977). --Measurements of the wall heat transfer by thermocouples, infrared cameras, or direct measurements by hotfilm anemometers (Schultz and Jones 1973). --Measurements of the boundary layer velocity profile by hot-wire anemometers, Pitot tube or PIV (Nitsche and Brunn 2006).Abstract A method of boundary layer transition measurement is presented for wind tunnel models instrumented with surface pressure taps. The measurement relies on taking a number of theoretically identical measurements at different times and then analysing the standard deviation of the pressures. Due to the slight unsteady movement of the transition position, a peak in the standard deviation of pressure σ C P peak is found at the transition position, and this is correlated with measurements of the transition position with an infrared camera and hot-film anemometers. In contrast to microphone measurements, it is shown that the transition detection works for data which have been low-pass filtered with a cut-off of 1 Hz. The application to static and dynamic transition measurements on static and periodically pitching helicopter rotor blade airfoils at Mach 0.3-0.5 is demonstrated.

Single-shot pressure-sensitive paint lifetime measurements on fast rotating blades using an optimized double-shutter technique

“…Pr 3 for laminar and turbulent boundary layers (see Eckert, 1962) at a Prandtl number of Pr = 0.72 for air, this temperature difference is ∼ 0.3 K (according to Eq. 4).…”

Laminar‐turbulent transition detection on airfoils by high‐frequency microphone measurements

“…For many years, experimental studies have been devoted to laminar‐turbulent transition, to understand in detail the transition process and to generate transition detection databases for computational validations and improvements. Some of the most common experimental methods are using pressure transducers, hot film sensors, and infrared thermography . Although there are many experiments conducted in literature for detection of the laminar‐turbulent transition in aircraft applications, research on wind turbine blades are comparably rare.…”

Unsteady boundary-layer transition measurements and computations on a rotating blade under cyclic pitch conditions

“…According to Richter et al (2016) and Gardner et al (2017), the DIT peak locations are equivalent to the positions corresponding to 50% turbulence intermittency. The DIT peak location is also at the position where the cycle-to-cycle RMS pressure transducer signal peaks (according to the so-called c p method by Gardner and Richter 2015), and therefore considered as the transition position (x∕c) tr in this work. A comparison of the peak positions in Fig.…”

“…To compare with DIT results, the c p method was applied according to Gardner and Richter (2015). The detected phases corresponding to boundary-layer transition at the respective sensor positions are marked by open symbols in Fig.…”

“…Measurements of unsteady boundary-layer transition on rotor blades for the validation of transition models are a challenging task. Many experiments reduce the complexity of a rotor setup by investigating boundary-layer transition on periodically pitching airfoils equipped with locally installed fast-response hot-film sensors (Lorber and Carta 1992) or dynamic pressure transducers (Gardner and Richter 2015). The application of these techniques in the rotating frame demands the laborious effort of integrating the sensor into the model, yields results at only discrete locations, and possibly disturbs the boundary-layer flow.…”