In-Flight Investigation of Transition Under Turbulent Conditions on a Laminar Wing Glove

Abstract

Flight experiments carried out with a motorized glider yield insights into the aerodynamic processes acting on a laminar wing section under varying on-coming flow conditions. Measurement data are obtained on a laminar wing glove equipped with pressure transducers and wall microphones to detect transition and investigate the boundarylayer instabilities. The on-coming flow, including the instantaneous angle of attack and the turbulence characteristics, is measured with hot-wire sensors. The flight tests were carried out under calm and moderately turbulent conditions. Characteristic results for both on-coming flow conditions are presented and differences are analyzed. Selected comparisons of the experimental results with steady boundary-layer and linear stability computations are provided. The effects observed in the experiments cannot be described solely by quasi-steady theory. Unsteady changes of the pressure distribution are identified as one source for the modification of the transition development. A conclusion on the observed superimposed effect of increased levels of atmospheric small-scale turbulence on boundary-layer receptivity cannot yet be drawn from the present experiments. Nomenclature= turbulent kinetic energy density spectrum, m∕s 2 ∕m −1 E xx = one-dimensional energy density spectrum, m∕s 2 ∕m −1 F = dimensionless stream function, -k = wave number, 1∕m k x = streamwise wave number, -k z = spanwise wave number, -L I = integral length scale, m L ref = reference length, m l k = Kolmogorov length scale, m m = dimensionless pressure gradient, -N = N factor, -P = pressure, Pa P = mean pressure, Pa p 0 = pressure fluctuation, Pa q 0 = dimensionless disturbance vector, -q = dimensionless disturbance amplitude function, -Re = Reynolds number, -Tu xy = two-component turbulence intensity, -U = velocity vector U; V; W T , m∕s U = mean velocity vector U; V; W T , m∕s U e = edge velocity, m∕s U eff = effective velocity, m∕s U ref = reference velocity, m∕s U TAS = true airspeed (U ∞ ), m∕s U ∞ = freestream velocity, m∕s u 0 = fluctuation velocity vector u 0 ; v 0 ; w 0 T , m∕s x = streamwise coordinate, m y = wall-normal coordinate, m z = spanwise coordinate, m α = angle of attack, deg β = sideslip angle, deg ε = energy dissipation rate, m 2 ∕s 3 η = dimensionless Falkner-Skan variable, -θ = probe axis inclination to the flow, deg ν = kinematic viscosity, m 2 ∕s ρ = air density, kg∕m 3 Ψ = stream function, m 2 ∕s ψ = yaw angle, deg ψ = wire slant angle for θ equal to zero, deg ψ eff = effective yaw angle, deg ω = angular frequency, 1∕s

Small-Scale Atmospheric Turbulence and Its Impact on Laminar-to-Turbulent Transition

“…The leading edge, the thickness and the camber of the original airfoil have been modified as described in [1] to compensate for the additional weight and drag due to the glove (airfoil geometry parameters can be found in [2]). Previous investigations showed that the cross-flow velocity on the center section of the glove is less than 2% of the freestream [1] and that the flow can be considered two-dimensional [15]. The laminar-turbulent transition process taking place along the pressure side of the airfoil is investigated with a streamwise array of 20 electret microphones (Sennheiser KE 4-211-2) mounted behind holes of 0.3 mm diameter.…”

“…Note that after the measuring bridge circuits, a low-pass filter with a bandwidth of 3.8 kHz is applied to the signals, which limits the upper range of frequencies detected. The X-wire probe is calibrated using the effective angle calibration introduced in [35] and described in detail in [15]. The anemometer outputs are not corrected for temperature changes with respect to the calibration conditions on the ground (temperature difference of around 10 to 20 • C), as none of the analytical correction formulas tested were able to provide satisfying precision [15].…”

“…The X-wire probe is calibrated using the effective angle calibration introduced in [35] and described in detail in [15]. The anemometer outputs are not corrected for temperature changes with respect to the calibration conditions on the ground (temperature difference of around 10 to 20 • C), as none of the analytical correction formulas tested were able to provide satisfying precision [15]. However, the fluctuations around the time averaged inflow velocity and angle of attack can be precisely measured without correcting for ambient temperature changes (see [15] for more information on the precision retained).…”

“…Various wind-tunnel investigations have also been conducted for different turbulence levels (see for instance [13,14]). Recently, the laminar-turbulent transition was studied in flight for different levels of atmospheric turbulence [15] ranging from calm to moderately turbulent conditions (see Sec. III.A for a detailed definition of the turbulent ranges).…”

“…However, this effect superimposes on those of increased levels of atmospheric small-scale turbulence that enter the boundary layer through receptivity and provide initial amplitudes of disturbances. Therefore, as their effects are combined, a separation between these two driving phenomena is difficult to obtain solely with flight measurements [15]. Wind tunnel and computational fluid dynamic investigations enable the separate study of these different scales, but they require a complete description of the turbulent inflow characteristics to correctly model the inflow conditions.…”