Wind tunnel study of various stable boundary layers: The Effects of Temperature Profiles on SBL Flow Structures

1. Introduction : The stable boundary layer (SBL) is difficult to describe and model. The turbulence structure and transport process of SBL with a wide variety of stability have not yet been fully clarified, because of the difficulties of measurement and the complexity associated with unsteadiness and non-uniformity. In the present study, we have developed a simulation method for SBL by using a thermally stratified wind tunnel. To produce thermally stratified flows, the tunnel is equipped with two independent temperature systems which consist of an air-flow heating unit and a floor temperature controlling unit. We have investigated the turbulence phenomena of SBLs for a wide range of stability, particularly focusing on the effects of various vertical temperature profiles on the flow characteristics of SBLs.

2. SBLs with a polynomial temperature profile (Part 1) : A stably stratified flow is created by heating the wind tunnel airflow (40Ž) and by cooling the test-section floor (10Ž). The Reynolds number, Re_ƒÂ, based on the boundary layer thickness, ƒÂ, ranges from (2-5.3)x10⁴ and the bulk Richardson number, Ri_ƒÂ(=(g/ƒ¦o)E(ƒ¦_‡-ƒ¦_s)ƒÂ/‚t_‡² ), ranges from 0 to 1.29. Here, ‚t_‡ and ƒ¦_‡ are the ambient air speed and temperature, ƒ¦_s and ƒ¦_o are the surface and reference temperatures. Stable stratification rapidly suppresses the fluctuations of velocity and temperature. Momentum and heat fluxes are also decreased with increasing stability and become nearly zero in the low part of the boundary layer with strong stability. The vertical profiles of turbulence quantities exhibit different behavior in two distinct stability regimes of the SBL flows with weak and strong stabilities. From the flow visualization, waves due to the Kelvin-Helmholtz instability can be observed locally and intermittently in a SBL flow with strong stability. Moreover, the cross-spectral analysis of velocity and temperature fluctuations suggests that internal waves also occur in the lower part of boundary layer with very strong stability.

3. SBLs with a linear temperature profile (Part 2) : A stably stratified flow with linear temperature profile is created by heating the wind tunnel airflow and by cooling the test-section floor (8Ž). A preshaping vertical profile of temperature is set with the air-flow heating unit, giving a linear temperature gradient of 12Ž to 72Ž in the range of z=0-60cm (1Ž/cm) and a gradient of 0.33Ž/cm in the upper range of z=60-120cm. The Reynolds number ranges from (3-5.3)x10⁴ and the bulk Richardson number ranges from 0 to 1.15. The vertical profile of temperature variance for the weak stability case is remarkably different from that for the SBL(part 1). From the flow visualization, Kelvin-Helmholtz waves can be observed very frequently in the lower part of boundary layer in a SBL flow with strong stability. Moreover, wavy motions driven by buoyancy (internal waves) can also be observed in the upper part of boundary layer with very strong stability.