D. E. Neff

TURBULENCE STRUCTURE IN A STRATIFIED BOUNDARY LAYER UNDER STABLE CONDITIONS

Turbulence structure in stably stratified boundary layers isexperimentally investigated by using a thermally stratified wind tunnel. Astably stratified flow is created by heating the wind tunnel airflow to atemperature of about 50 °C and by cooling the test-section floor to asurface temperature of about 3 °C. In order to study the effect ofbuoyancy on turbulent boundary layers for a wide range of stability, thevelocity and temperature fluctuations are measured simultaneously at adownwind position of 23.5 m from the tunnel entrance, where the boundarylayer is fully developed. The Reynolds number, Reδ, ranges from 3.14× 104 to 1.27 × 105, and the bulk Richardson number, Riδ,ranges from 0 to 1.33. Stable stratification rapidly suppresses thefluctuations of streamwise velocity and temperature as well as the verticalvelocity fluctuation. Momentum and heat fluxes are also significantlydecreased with increasing stability and become nearly zero in the lowest partof the boundary layer with strong stability. The vertical profiles ofturbulence quantities exhibit different behaviour in three distinct stabilityregimes, the neutral flows, the stratified flows with weak stability(Riδ = 0.12, 0.20) and those with strong stability (Riδ= 0.39,0.47, 1.33). Of these, the two regimes of stratified flows clearly showdifferent vertical profiles of the local gradient Richardson number Ri,separated by the critical Richardson number Ri cr of about 0.25. Moreover,turbulence quantities in stable conditions are well correlated with Ri.

Wind-tunnel modeling of hill and vegetation influence on wind power availability

Abstract Forested hills and ridges pose a number of significant technical and environmental issues for siting wind turbines. The change in the wind profile across the top of the hill due to the presence or absence of trees and differences in roughness characteristics must be understood in order to develop accurate energy estimates. A three phase research program was undertaken to examine wind flow over tree covered hills and ridges. The first phase of this work was a review of the known effects on wind flow due to individual trees, forest stands, and forest clearings. The second phase of the work involved wind tunnel measurements of hill-top wind speed profiles as a function of surface roughness, hill shape, and hill slope. The third phase examined the behavior of wind floor over a scale model of a proposed wind energy site in the United States, New England region.

Mean wind and turbulence characteristics due to induction effects near wind turbine rotors

Abstract The objective of this study was to place a model wind turbine into a wind tunnel and measure the wind characteristics in the vicinity of the spinning rotor for a variety of flow conditions. A 0.53 meter diameter model wind turbine was placed in the Meteorological Wind Tunnel facility at Colorado State University. Four different approach flow conditions were studied. These were two different mean wind speeds (6 and 7.6 m/s) and two different turbulence conditions (0.1% and 1.5% intensity). For each of these test conditions the three-dimensional wind field was measured between 3 rotor diameters upwind to 1 2 rotor diameter downwind. The rotor power coefficient versus tip speed ratio was also obtained.

Numerical modelling of water spray barriers for dispersing dense gases

A depth-integrated numerical model is used to calculate the behavior of heavy and cold gas clouds subjected to the diluting influence of a water spray curtain. The model includes the physics of a cold cloud heated by the ground which entrains moist air. Entrainment velocities are influenced by cloud stratification. Laboratory water-curtain trials are predicted accurately. The model reveals the influence of curtain placement, width of spray, spray strength and thermal effects on the efficacy of the water curtain to mitigate potential hazards.

Wind-tunnel simulation of field dispersion tests (by the U.K. health and safety executive) of water-spray curtains

Field trials of water-spray curtain tests performed by the (British) Health and Safety Executive and designated by HSE 41 and HSE 46 were modeled at a scale ratio of 1:28.9 in an atmospheric boundary-layer wind tunnel. The tests confirmed that dense clouds of carbon dioxide gas were significantly diluted by additional air entrained by the water curtain. Linear and logarithmic scatter diagrams of concentrations measured at equivalent scaled points produced correlations of 0.87 and 0.97, respectively. Results confirm the use of Froude-number modeling of both the dense gas cloud and water spray droplet fluid dynamics.

Upstream and lateral wind turbine wake effects on nearby wind turbine performance

Abstract A selection of different 1:50 scale rotor blades were evaluated with dynamometers, force balances, and wake measurements to select a rotor model which correctly simulates the full-scale behavior of an actual wind mill. A wind-tunnel measurement program was then carried out on a set of five dynamic (operating) wind mills placed at various heights and orrientations to one another. The interdependence of wind-turbine performance on such spacing was determined.

Dispersion of vapor from liquid natural gas spills— Evaluation of simulation in a meteorological wind tunnel: Five-cubic-meter China Lake spill series

Abstract A series of six-cubic-meter liquid natural gas (LNG) spills were performed in 1978 at the China Lake Naval Weapons Center, CA. A parallel set of modeled spills were simulated in meteorological wind-tunnel facilities to provide field-test planning information, to extend the value of the limited set of field measurements carried out, and to evaluate the concept of physical modeling of LNG plume dispersion as a predictive hazard analysis tool.

Physical Modeling of Forty Cubic Meter LNG Spills At China Lake, California

In many parts of the world there is the perception that current and planned liquid natural gas(LNG) operations and facilities present an unacceptable risk to the public. Hence, the Division of Environmental Control Technology, Department of Energy, and the Gas Research Institute, USA have supported a series of tests on liquid natural gas spilled in amounts of five and forty cubic meters onto a pond at China Lake Naval Weapons Center, California. A parallel wind tunnel model program has been performed in the meteorological wind tunnels of Colorado State University to provide field test planning information, to extend the value of a limited set of field measurements, and to validate the concept of physical modeling of LNG plume dispersion as a predictive hazard analysis tool. The measurement results described herein provided a foundation for instrument placement and interpretation of terrain effects during the 40 m3 field experiments. Wind tunnel laboratory measurements permits a degree of control of safety, meteorological, source and site variables not often feasible or economic at full scale.

Analysis of vapor barrier experiments to evaluate their effectiveness as a means to mitigate HF cloud concentrations

Abstract Accidental release of Hydrogen Fluoride (HF) can result in initially dense, highly reactive and corrosive gas clouds. These clouds will typically contain a mixture of gases, aerosols and droplets which can be transported significant distances before lower hazard levels of HF concentration are reached. Previous related field and laboratory experiments have been analyzed to estimate the effectiveness of barrier devices. The experiments were examined to determine their relevance to Hydrogen Fluoride spill scenarios. Wind tunnel and field data were compared where possible to validate the laboratory experiments. Barrier influence on peak concentrations, cloud arrival time, peak concentration arrival time, and cloud departure time were determined. These data were used to develop entrainment models to incorporate into integral and depth averaged numerical models. The models were then run to examine barrier performance for a typical Hydrogen Fluoride spill for a wide range of vapor barrier heights, spill sizes, meteorological conditions and release configurations. Finally the results of the data analysis and numerical sensitivity study were interpreted and expressed in a form useful to evaluate the efficiency of vapor barrier mitigation devices.

LABORATORY SIMULATION OF LIQUID NATURAL GAS VAPOR DISPERSION OVER LAND OR WATER

The primary purpose of this paper is to show through basic similarity analysis and comparisons of model and full-scale data that atmospheric transport of dense, cold natural gas clouds can be physically modeled in “meteorological” wind tunnels for a range of real boundary conditions which have great practical importance with respect to liquid natural gas (LNG) spill hazard analysis.