Design of multiscale inhomogeneous grids for the generation of atmospheric boundary layer logarithmic mean flows
Creators: name:  T. HURET 
name:   G. TANGUY 
name:   C. VASSILICOS 
name:   L. JACQUIN 
name:   Q. GALLAS 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLIGHT PHYSICS
WIND TUNNEL
MEASUREMENT TECHNIQUE

description: The use of wind tunnel facilities for the experimental generation of flows representative of Atmospheric Boundary Layer (ABL) still requires a trial-and-error based design process (Counihan 1969, Cook 1978). A new type of passive grids Multiscale Inhomogeneous Grids (MIGs) is suggested as a promising solution for tailoring both mean flow profile and turbulence intensity profiles in wind tunnel, based on recent theoretical and experimental results from fractal grids literature (Zheng et al. 2018). The MIG grids are structured by parallel vertical bars of non-uniform spacing and width, enabling to create a non-uniform blockage ratio over the full-section. Within the perspective of reproducing ABL flow characteristics, the objective of this paper consists in demonstrating theoretically and experimentally the possibility of using the MIG grids to reproduce logarithmic mean flows over a full section of the wind tunnel. This flow type is characteristic of the Surface Layer (SL) in neutral ABL conditions (negligible vertical heat transfer) (Kaimal Finnigan 1994). The reproduction of turbulent intensity profile, another essential characteristic of the ABL, will be the next step ahead of the current study. A predictive model for the downstream evolution of turbulent intensity is under development and will benefit from the current experimental works. First, a grid design algorithm was developed and implemented to generate MIG grids for a large variety of arbitrary desired mean flows, based on analytical models adapted from the literature (McCarthy 1964, Roach 1987). It was applied to the generation of several logarithmic flow profiles, characterized by their equivalent artificial roughness length h_0, normalized by the considered wind tunnel height H. Our first experimental measurements downstream of generated MIG grids were conducted within the SCL-PIV subsonic wind tunnel at ONERA Lille, with a square test section of H=0.3 m and velocities up to 30 m/s. The experimental measurements by Hot-Film Anemometry and Particle Image Velocimetry show that MIG grids can successfully generate logarithmic mean velocity profiles up to artificial roughness of h_0/H=10^(-3), relevant for the reproduction of high roughness ABL configurations. However, the artificially generated flows still suffer from two discrepancies with full-scale ABL surface layers, both consistent with the literature on regular grids (Lau Baines 1968, Cook 1978). The first one appears in the direct vicinity of the wall surface, as a mean flow overshoot disturbance attributed to the upstream boundary layer influence. An experimentally explored solution is the extension of the previous grid design model to arbitrary non-uniform flows with wall boundary layers. The second discrepancy between our results with full-scale ABL surface layer is the fast decay of turbulence to intensities lower than required. The addition of our turbulent intensity prediction model into the current grid design algorithm is expected to mitigate this phenomenon for future grids. Finally, the last part of this work is dedicated to the experimental investigation of the flow produced by the combination of a MIG grid with a roughness fetch. Such configuration has the objective of both (i) regulating the mean flow in the wall vicinity by fulfilling the Jensens roughness similarity criterion (Jensen Franck 1953) and (ii) reinforcing the downstream production of turbulence. The comparison of this configuration with full-scale turbulence models and turbulence generation devices will guide the future use of our turbulent intensity model within our grid design process.