Aerodynamic Admittance Research on Wide-body Flat Steel Box Girder
Abstract: Aerodynamic admittance is a key parameter affecting the analysis accuracy of the bridge buffeting response. However, few articles have covered the issue of the wide-body flat box girder with much smaller depth-width ratio (such as 1/12 studied in this paper). Therefore, to explore the real buffeting force of the wide-body flat box girder, experiments of the static force coefficients and aerodynamic admittance are carried out in wind tunnel. Wooden segmental model has a scale ratio of 1/60 to an actual 42 m wide suspension bridge girder. Using the high-frequency-force-balance (HFFB) equipment fixed with the segmental model under the conditions of different wind speeds and different wind attack angles, wind power spectrums of the buffeting force are measured and the variation of aerodynamic admittance parameters is analyzed. The results show that the aerodynamic admittance of the box girder measured in the experiment with this so much smaller depth-width ratio differs from the corresponding classical Sears expression. Some inspiration can be presented for the future study of the buffeting response for this kind of much wider bridge with the similar ratio in this paper.
Numerical and Experimental Studies on Grid-Generated Turbulence in Wind Tunnel
The grid-generated turbulence in wind tunnel is commonly applied in aerodynamic performance experiments of various industrial structures. However, the characteristics and application conditions of the grid-generated turbulence in the wind tunnel are often affected by the grid shape and the test position, respectively. Therefore, in order to clarify the influence of the grid shape and the test position on the grid-generated turbulence in wind tunnel, a computational fluid dynamics (CFD) numerical study on the turbulence generated by two different grids was presented herein, with the wind tunnel experimental validation. Firstly, time histories of the wind speed at 19 monitoring points behind the grid in wind tunnel were collected. Secondly, the entire turbulent field generated by two different grids was simulated using CFD. Then, the characteristics of the turbulence generated by the grids in wind tunnel were investigated. Finally, the accuracy of the CFD simulations compared with the wind tunnel test was studied. Results show that the grid blockage and the interference effect lead to a significant increase of 200% in the turbulence intensity near the grid. Moreover, the grid-generated turbulence conforms to the isotropic assumption only after a specific distance of 2.0-4.0 m from the grid. In addition, CFD errors are generally within 15%–20% of the corresponding measurements in the wind tunnel. This study provides guidance for selecting the installation location of the structure model in a wind tunnel.
Turbulent buoyant jets into stratified or flowing ambient fluids
Theoretical and experimental studies were made on two classes of buoyant jet problems, namely: 1) an inclined, round buoyant yet in a stagnant environment with linear density-stratification; 2) a round buoyant jet in a uniform cross stream of homogenous density. Using the integral technique of analysis, assuming similarity, predictions can be made for jet trajectory, widths, and dilution ratios, in a density-stratified or flowing environment. Such information is of great importance in the design of disposal systems for sewage effluent into the ocean or waste gases into the atmosphere. The present study of a buoyant jet in a stagnant environment has extended the Morton type of analysis to cover the effect of the initial angle of discharge. Numerical solutions have been presented for a range of initial conditions. Laboratory experiments were conducted for photographic observations of the trajectories of dyed jets. In general the observed jet forms agreed well with the calculated trajectories and nominal half widths when the value of the entrainment coefficient was taken to be α = 0.082, as previously suggested by Morton. The problem of a buoyant jet in a uniform cross stream was analyzed by assuming an entrainment mechanism based upon the vector difference between the characteristic jet velocity and the ambient velocity. The effect of the unbalanced pressure field on the sides of the jet flow was approximated by a gross drag term. Laboratory flume experiments with sinking jets which are directly analogous to buoyant jets were performed. Salt solutions were injected into fresh water at the free surface in a flume. The jet trajectories, dilution ratios and jet half widths were determined by conductivity measurements. The entrainment coefficient, α, and drag coefficient, Cd, were found from the observed jet trajectories and dilution ratios. In the ten cases studied where jet Froude number ranged from 10 to 80 and velocity ratio (jet: current) K from 4 to 16, α varied from 0.4 to 0.5 and Cd from 1.7 to 0.1. The jet mixing motion for distance within 250D was found to be dominated by the self-generated turbulence, rather than the free-stream turbulence. Similarity of concentration profiles has also been discussed.