ALBERT

Description: A simple modification is introduced into the integral model (IM) CorJet in an effort to predict better the characteristics of negatively buoyant jets (NBJ) discharged in a stationary ambient. Although this modification was developed for the CorJet model, it can be applied to every IM which employs the entrainment hypothesis. The detrainment of fluid from the main flow is taken into account by inserting a coefficient “p” into the conservation equations of volume, buoyancy and tracer mass flux. This coefficient expresses the ratio of the specific mass flux of the detrained fluid to the net specific mass flux entrained to the NBJ. The value of p is assumed constant along the jet trajectory and up to the maximum jet height, becoming zero thereafter. Results show that the modified CorJet model (MCM) predicts reasonably well experimental data from the literature and data from experiments performed in this work. The optimal value of p and therefore the detrained fluid from the main NBJ flow was found to decrease as the jet initial densimetric Froude number increases.

Description: This paper explores the effects of droplet size on droplet intrusion and subsequent transport in sub-surface oil spills. In an inverted laboratory set-up, negatively buoyant glass beads were released continuously into a quiescent linearly stratified ambient to simulate buoyant oil droplets in a rising multiphase plume. Settled particles collected from the bottom of the tank exhibited a radial Gaussian distribution, consistent with their having been vertically well mixed in the intrusion layer, and a spatial variance that increased monotonically with decreasing particle size. A new typology was proposed to describe plume structure based on the normalized particle slip velocity \(U_{N} =u_s /(BN)^{1/4}\) , where \(u_s \) is the particle slip velocity, \(B\) is the plume’s kinematic buoyancy flux, and \(N\) is the ambient stratification frequency. For \(U_N \le 1.4\) particles detrain from the plume, but only those with smaller slip velocity \((U_N \le 0.3)\) intrude. An analytical model assuming well-mixed particle distributions within the intrusion layer was derived to predict the standard deviation of the particle distribution, \(\sigma _r =\sqrt{\frac{0.9-0.38(U_N )^{0.24}}{\pi }}\frac{B^{3/8}}{N^{5/8}u_s ^{1/2}}\) and predictions were found to agree well with experimental values of \(\sigma _{r}\) . Experiments with beads of multiple sizes also suggested that the interaction between two particle groups had minimal effect on their radial particle spread. Because chemical dispersants have been used to reduce oil droplet size, this study contributes to one measure of dispersant effectiveness. Results are illustrated using conditions taken from the ‘Deep Spill’ field experiment and the recent Deepwater Horizon oil spill.

Description: The study presents experimental results of coherent structures and their interactions in a smooth open channel flow based on measurement of instantaneous two-dimensional velocity vectors with particle image velocimetry. The sampled data were analyzed through techniques of ensemble average, vortex extraction, and proper orthogonal decomposition (POD). Redistribution of turbulent kinetic energy is observed in the near-surface region. The spanwise vortices, which are closely related to hairpin vortices, exhibit a clear dependence on Reynolds number of the flow. Hairpin vortex packets and long streamwise vortices are identified as typical large-scale and super-scale coherent structures, respectively, and their interaction is revealed by examining the relationship between the population density of spanwise vortices and the coefficient functions of the first POD mode. Interactions between large-scale and super-scale structures have been recognized to support the hypothesis of closed-loop feedback cycle.

Description: The effect on the flow over a street canyon (lateral length/height, L/h \(=\) 30) of using either 3D (cube) or 2D (rectangular block) upstream roughness arrays, of the same height as the canyon, has been studied for two streamwise canyon width to height aspect ratios (AR \(=\) W/h) of 1 and 3, in a wind tunnel using Particle Image Velocimetry. The mean streamwise velocity, shear stress, turbulent intensities and length scales, together with shear layer boundaries and mass fluxes across the canyon opening are presented for different combinations of skimming and wake-interference regimes using different upstream roughness and canyon configurations. These results show significant trends with canyon aspect ratio and roughness array plan area packing density \((\uplambda _{\mathrm{p}})\) with respect to 2D and 3D configurations. The mean streamwise velocity for configurations of equal \(\uplambda _{\mathrm{p}}\) is higher in 3D than 2D configurations, while the spatially averaged shear stress is shown to be lower in 3D than 2D configurations. The relative contribution to the total turbulent kinetic energy (TKE) demonstrates that staggered and aligned arrays or 2D and 3D arrays do not produce similar profiles of TKE. Finally, the integral length scale is larger in 2D cases than 3D cases of equal \(\uplambda _{\mathrm{p}}\) . Urban air quality is a significant concern for human health. By investigating the influence of upstream roughness on canyon flow one can determine which cases or flow regimes in both the upstream roughness and canyon will result in decreased ventilation and negatively effect the air quality of urban areas. From the present work decreased ventilation occurs in the skimming flow regime and is lowest in the case of upstream 2D bar roughness with \(\uplambda _{\mathrm{p}} = 50~\%\) and canyon AR \(=\) 1.

Description: This paper presents laboratory experiments of wave-driven hydrodynamics in a three-dimensional laboratory model of constructed coastal wetlands. The simulated wetland plants were placed on the tops of conically-shaped mounds, such that the laboratory model was dynamically similar to marsh mounds constructed in Dalehite Cove in Galveston Bay, Texas. Three marsh mounds were placed in the three-dimensional wave basin of the Haynes Coastal Engineering Laboratory at Texas A&M University, with the center of the central wetland mound located in the center of the tank along a plane of symmetry in the alongshore direction. The experiments included two water depths, corresponding to emergent and submerged vegetation, and four wave conditions, typical of wind-driven waves and ocean swell. The wave conditions were designed so that the waves would break on the offshore slope of the wetland mounds, sending a strong swash current through the vegetated patches. Three different spacings between the wetland mounds were tested. To understand the effects of vegetation, all experiments were repeated with and without simulated plants. Measurements were made throughout the nearshore region surrounding the wetland mounds using a dense array of acoustic Doppler velocimeters and capacitance wave gauges. These data were analyzed to quantify the significant wave height, phase average wave field and phase lags, wave energy dissipation over the vegetated patches, mean surface water levels, and the near-bottom current field. The significant wave height and energy dissipation results demonstrated that the bathymetry is the dominant mechanism for wave attenuation for this design. The presence of plants primarily increases the rate of wave attenuation through the vegetation and causes a blockage effect on flow through the vegetation. The nearshore circulation is most evident in the water level and velocity data. In the narrowest mound spacing, flow is obstructed in the channel between mounds by the mound slope and forced over the wetlands. The close mound spacing also retains water in the nearshore, resulting in a large setup and lower flows through the channel. As the spacing increases, flow is less obstructed in the channel. This allows for more refraction of waves off the mounds and deflection of flow around the plant patches, yielding higher recirculating flow through the channel between mounds. An optimal balance of unobstructed flow in the channel, wave dissipation over the mounds, and modest setup in the nearshore results when the edge-to-edge plant spacing is equal to the mound base diameter.