Flow Characteristics of Transitional Boundary Layers on an Airfoil in Wakes

Abstract

Transition characteristics of a boundary layer on a NACA0012 airfoil are investigated by measuring unsteady velocity using hot wire anemometry. The airfoil is installed in the incoming wake generated by an airfoil aligned in tandem with zero angle of attack. Reynolds number based on the airfoil chord varies from 2.0×105 to 6.0×105; distance between two airfoils varies from 0.25 to 1.0 of the chord length. To measure skin friction coefficient identifying the transition onset and completion, an extended wall law is devised to accommodate transitional flows with pressure gradient and nonuniform inflows. Variations of the skin friction are quite similar to that of the flat plate boundary layer in the uniform turbulent inflow of high intensity. Measured velocity profiles are coincident with families generated by the modified wall law in the range up to y+=40. Turbulence intensity of the incoming wake shifts the onset location of transition upstream. The transitional region becomes longer as the airfoils approach one another and the Reynolds number increases. The mean velocity profile gradually varies from a laminar to logarithmic one during the transition. The maximum values of rms velocity fluctuations are located near y+=15-20. A strong positive skewness of velocity fluctuation is observed at the onset of transition and the overall rms level of velocity fluctuation reaches 3.0–3.5 in wall units. The database obtained will be useful in developing and evaluating turbulence models and computational schemes for transitional boundary layer. [S0098-2202(00)01603-5]

Direct Noise Computation of the Turbulent Flow Around a Zero-Incidence Airfoil

“…From y ' 10 to around y ' 80, the profile approximately follows the standard logarithmic law u 1= ln y B established for a zero-pressure-gradient boundary layer (dashed line in Fig. 3, with 1= and B taking the standard values of 2.44 and 5.2), albeit being slightly above, which is consistent with the adverse pressure gradient faced by the boundary layer and is also observed in the experimental data of Lee and Kang [34]. It is in fact possible to derive a similar logarithmic law governing the mean velocity in the overlap region when the pressure …”

“…This distribution is shown in Fig. 2 and is compared with experimental values obtained by Lee and Kang [34] for a NACA 0012 airfoil at a chord-based Reynolds number of 600,000 and a Mach number of M 0:09. A good agreement is found between the computational and experimental results.…”

Modelling of natural and bypass transition in aerodynamics

“…Two cases ware used for the comparison of the both transition models. Firstly, the numerical simulation of the flow around the NACA0012 airfoil investigated by Lee and Kang [7] at the moderate The multi-block structured grid with the block overlapping was used together with the combined C-H grid for the simulation by the EARSM model with the algebraic transition model. The structured quadrilateral grid was applied for the k-k L -Z model.…”

“…The modified algebraic transition model of Straka and PĜíhoda [3] connected with EARSM turbulence model of Hellsten [9] and the k-k L -Z model of Walter and Cokljat [10] were tested by means of the incompressible flow around airfoils at relatively low free-stream turbulence, partly the flow around the NACA0012 airfoil by Lee and Kang [7] at the moderate free-stream turbulence Tu = 0.3 % and partly the flow around the XIS40MOD airfoil by Würz [8] at very low free-stream turbulence Tu = 0.012 % in regimes with the transition in attached and separated flows. The algebraic model proposed initially for modelling of the bypass transition gives acceptable results as for the flow at low free-stream turbulence.…”

“…Both models were used for simulation of the incompressible flow around airfoils, namely the NACA0012 airfoil measured by Lee and Kang [7] at Tu = 0.3 % and the XIS40MOD airfoil investigated by Würz [8] at Tu = 0.012 %. In addition, various transition criteria were compared for the flat-plate flow at low freestream turbulence level.…”