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Rolf Kaiser and Evgeni Fedorovich

Abstract

A model of the atmospheric convective boundary layer (CBL) is realized in the thermally stratified wind tunnel of the Institute of Hydrology and Water Resources, University of Karlsruhe. Further experimental results from this model are presented. The wind tunnel with a test section 10 m long, 1.5 m wide, and 1.5 m high allows one to generate a quasi-stationary, horizontally evolving CBL, characterized by convective Richardson numbers RiΔT up to 10 and RiN up to 20, with the bottom shear/buoyancy dynamic ratio u */w * in the range of 0.2 to 0.5. The convective regime in the tunnel is dominated by bottom-up forcings. Effects of entrainment in the simulated CBL play a secondary role.

The spectra of turbulence in the wind tunnel flow are calculated from high-resolution velocity component and temperature time series, simultaneously measured using laser Doppler and resistance-wire technique, respectively. The spectra from the mixed core of the CBL and in the entrainment zone display pronounced inertial subranges. The ratio of vertical to horizontal velocity spectra in these subranges is within the interval from 1.3 to 2, which is slightly larger than could be expected for purely isotropic turbulence. Different maxima in the production ranges of the spectra are related to dominant turbulence scales in the wind tunnel flow. The energy-containing ranges of the wind tunnel spectra exhibit plateaulike shapes resulting from modification of the turbulence production by the flow shear. The comparison with atmospheric spectra and spectral data from water tank and large eddy simulation studies of the CBL suggests that the turbulence spectral regime in the tunnel flow has much in common with its atmospheric prototype.

The turbulence kinetic energy dissipation rate and the destruction rate of temperature fluctuations are evaluated based on the relationships resulting from the Kolmogorov theory for the inertial-subrange spectra. The dissipation rates obtained are within the scatter ranges of data from atmospheric measurements and model studies of convection. High dissipation values in the lower portion of the simulated CBL are indicative of the shear enhancement of turbulence production in the wind tunnel convective flow.

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Evgeni Fedorovich, Rolf Kaiser, Matthias Rau, and Erich Plate

Abstract

Experiments on simulating the atmospheric convective boundary layer (CBL), capped by a temperature inversion and affected by surface shear, were carried out in the thermally stratified wind tunnel of the Institute of Hydrology and Water Resources, University of Karlsruhe. The tunnel is of the closed-circuit type, with a test section 10 m long, 1.5 m wide, and 1.5 m high. The return section of the tunnel is subdivided into 10 layers, each driven by its own fan and heating system. By this means, velocity and temperature profiles can be preshaped at the inlet of the test section, which allows for the reproduction of developed CBL over comparatively short fetches. The bottom heating is controlled to produce the constant heat flux through the floor of the test section. The flow velocity components in the tunnel are measured with a laser Doppler system; for temperature measurements, the resistance-wire technique is employed.

A quasi-stationary, horizontally evolving CBL was reproduced in the tunnel, with convective Richardson numbers RiΔT and RiN up to 10 and 20, respectively, and the shear/buoyancy dynamic ratio u */w * in the range of 0.2–0.5. Within the employed modeling approach, means and other statistics of the flow were calculated by temporal averaging. Deardorff mixed-layer scaling was used as a framework for processing and interpreting the experimental results. The comparison of the wind tunnel data with results of atmospheric, water tank, and numerical studies of the CBL shows the crucial dependence of the turbulence statistics in the upper part of the layer on the parameters of entrainment, as well as the modification of the CBL turbulence regime by the surface shear.

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