Vertical Plume Research Articles

Large-eddy simulation of passive scalar dispersion in an urban-like canopy

AbstractResults from large-eddy simulations of short-range dispersion of a passive scalar from a point source release in an urban-like canopy are presented. The computational domain is that of a variable height array of buildings immersed in a pressure-driven, turbulent flow with a roughness Reynolds number ${\mathit{Re}}_{\tau } = 433$. A comparative study of several cases shows the changes in plume behaviour for different mean flow directions and source locations. The analysis of the results focuses on utilizing the high-fidelity datasets to examine the three-dimensional flow field and scalar plume structure. The detailed solution of the flow and scalar fields within the canopy allows for a direct assessment of the impact of local features of the building array geometry. The staggered, skewed and aligned arrangements of the buildings with respect to the oncoming flow were shown to affect plume development. Additional post-processing quantified this development through parameters fundamental to reduced-order Gaussian dispersion models. The parameters include measures of concentration decay with distance from the source as well as plume trajectory and spread. The horizontal plume trajectory and width were found to be more sensitive to source location variations, and hence local geometric features, than vertical plume parameters.

Non-hydrostatic modeling of cohesive sediment transport associated with a subglacial buoyant jet in glacial fjords: A process-oriented approach

Fine sediment transport produced by a subglacial freshwater discharge is simulated with a 2D non-hydrostatic model. The circulation pattern revealed a buoyant jet issuing from the aperture representing the subglacial tunnel, a vertically buoyant plume and a surface gravity current forming part of an estuarine circulation. Momentum-dominated experiments are more sensitive to the presence of suspended sediment in the discharge. At low concentrations, the sediment stays in the vertical plume and surface gravity current, and its concentration is progressively decreased by mixing but no settling is observed through the water column. At high concentrations, the sediment settles in the far field and is transported back to the near field by the landward estuarine current. Sediment settled from the surface layer through convective sedimentation, a process that was more effective than flocculation to transport sediment vertically, and showed vertical velocities faster than 1.0×10-2ms−1. Implications of these results are discussed.

Temperature Measurement of Air Flow Using Fluorescent Mists Combined with Two-Color LIF

This study proposes a two-color laser-induced fluorescence (LIF) technique for measuring the temperature distribution of an airflow by spraying a mist of a fluorescent dye. The mist is generated by using propylene glycol, the vapor pressure of which is much lower than that of water, as the solvent of the fluorescent dyes. A supersonic moisture chamber is used as the atomizer for seeding the tracer particles to be visualized. The proposed technique is applied to the measurement of the temperature distribution in a thermal vertical buoyant plume. The proposed two-color LIF thermometry is found to be very effective for the study of such a thermal structure, and it is well suited for measuring the temperature field of an airflow.

A new approach to characterize emission contributions from area sources during optical remote sensing technique testing

In the method termed “Other Test Method-10,” the U.S. Environmental Protection Agency has proposed a method to quantify emissions from nonpoint sources by the use of vertical radial plume mapping (VRPM) technique. The surface area of the emitting source and the degree to which the different zones of the emitting source are contributing to the VRPM computed emissions are often unknown. The objective of this study was to investigate and present an approach to quantify the unknown emitting surface area that is contributing to VRPM measured emissions. Currently a preexisting model known as the “multiple linear regression model,” which is described in Thoma et al. (2009), is used for quantifying the unknown surface area. The method investigated and presented in this paper utilized tracer tests to collect data and develop a model much like that described in Thoma et al. (2009). However, unlike the study used for development of the multiple linear regression model, this study is considered a very limited study due to the low number of pollutant releases performed (seven total releases). It was found through this limited study that the location of an emitting source impacts VRPM computed emissions exponentially, rather than linearly (i.e., the impact that an emitting source has on VRPM measurements decreases exponentially with increasing distances between the emitting source and the VRPM plane). The data from the field tracer tests were used to suggest a multiple exponential regression model. The findings of this study, however, are based on a very small number of tracer tests. More tracer tests performed during all types of climatic conditions, terrain conditions, and different emissions geometries are still needed to better understand the variation of capture efficiency with emitting source location. This study provides a step toward such an objective. Implications The findings of this study will aid in the advancement of the VRPM technique. In particular, the contribution of this study is to propose a slight improvement in how the area contributing to flux is determined during VRPM campaigns. This will reduce some of the technique's inherent uncertainties when it is employed to estimate emissions from an area source under nonideal conditions.

P-wave tomography of the western United States: Insight into the Yellowstone hotspot and the Juan de Fuca slab

We used 190,947 high-quality P-wave arrival times from 8421 local earthquakes and 1,098,022 precise travel-time residuals from 6470 teleseismic events recorded by the EarthScope/USArray transportable array to determine a detailed three-dimensional P-wave velocity model of the crust and mantle down to 1000km depth under the western United States (US). Our tomography revealed strong heterogeneities in the crust and upper mantle under the western US. Prominent high-velocity anomalies are imaged beneath Idaho Batholith, central Colorado Plateau, Cascadian subduction zone, stable North American Craton, Transverse Ranges, and Southern Sierra Nevada. Prominent low-velocity anomalies are imaged at depths of 0–200km beneath Snake River Plain, which may represent a small-scale convection beneath the western US. The low-velocity structure deviates variably from a narrow vertical plume conduit extending down to ∼1000km depth, suggesting that the Yellowstone hotspot may have a lower-mantle origin. The Juan de Fuca slab is imaged as a dipping high-velocity anomaly under the western US. The slab geometry and its subducted depth vary in the north-south direction. In the southern parts the slab may have subducted down to >600km depth. A “slab hole” is revealed beneath Oregon, which shows up as a low-velocity anomaly at depths of ∼100 to 300km. The formation of the slab hole may be related to the Newberry magmatism. The removal of flat subducted Farallon slab may have triggered the vigorous magmatism in the Basin and Range and southern part of Rocky Mountains and also resulted in the uplift of the Colorado Plateau and Rocky Mountains.

Properties of Sarychev sulphate aerosols over the Arctic

Aerosols from the Sarychev Peak volcano entered the Arctic region less than a week after the strongest SO2eruption on June 15 and 16, 2009 and had, by the first week in July, spread out over the entire Arctic region. These predominantly stratospheric aerosols were determined to be sub‐micron in size and inferred to be composed of sulphates produced from the condensation of SO2gases emitted during the eruption. Average (500 nm) Sarychev‐induced stratospheric optical depths (SOD) over the Polar Environmental Atmospheric Research Laboratory (PEARL) at Eureka (Nunavut, Canada) were found to be between 0.03 and 0.05 during the months of July and August, 2009. This estimate, derived from sunphotometry and integrated lidar backscatter profiles was consistent with averages derived from lidar estimates over Ny‐Ålesund (Spitsbergen). The Sarychev SOD e‐folding time at Eureka, deduced from lidar profiles, was found to be approximately 4 months relative to a regression start date of July 27. These profiles initially revealed the presence of multiple Sarychev plumes between the tropopause and about 17 km altitude. After about two months, the complex vertical plume structures had collapsed into fewer, more homogeneous plumes located near the tropopause. It was found that the noisy character of daytime backscatter returns induced an artifactual minimum in the temporal, pan‐Arctic, CALIOP SOD response to Sarychev sulphates. A depolarization ratio discrimination criterion was used to separate the CALIOP stratospheric layer class into a low depolarization subclass which was more representative of Sarychev sulphates. Post‐SAT (post Sarychev Arrival Time) retrievals of the fine mode effective radius (reff,f) and the logarithmic standard deviation for two Eureka sites and Thule (Greenland) were all close to 0.25 μm and 1.6 respectively. The stratospheric analogue to the columnar reff,f average was estimated to be reff,f(+) = 0.29 μm for Eureka data. Stratospheric, Raman lidar retrievals at Ny‐Ålesund, yielded a post‐SAT average of reff,f(+) = 0.27 μm. These results are ∼50% larger than the background stratospheric‐aerosol value. They are also about a factor of two larger than modeling values used in recent publications or about a factor of five larger in terms of (per particle) backscatter cross section.

Wildfire and the atmosphere: Modelling the chemical and dynamic interactions at the regional scale

Abstract Forest fires release significant amounts of trace gases and aerosols into the atmosphere. Depending on meteorological conditions, fire emissions can efficiently reduce air quality and visibility, even far away from emission sources. In 2005, an arson forest fire burned nearly 700 ha near Lancon-de-Provence, southeast France. This paper explores the impact of this Mediterranean fire on the atmospheric dynamics and chemistry downwind of the burning region. The fire smoke plume was observed by the MODIS-AQUA instrument several kilometres downwind of the burning area out of the Mediterranean coast. Signatures of the fire plume on air pollutants were measured at surface stations in southeastern France by the air quality network AtmoPACA. Ground-based measurements revealed unusually high concentrations of aerosols and a well marked depletion of ozone concentrations on the day of the fire. The Lancon-de-Provence fire propagation was successfully simulated by the semi-physical fire spread model ForeFire. ForeFire provided the burnt area at high temporal and spatial resolutions. The burnt areas were scaled to compute the fire heat and water vapour fluxes in the three-dimensional meso-scale non-hydrostatic meteorological model MesoNH. The simulated fire plume kept confined in the boundary layer with high values of turbulent kinetic energy. The plume was advected several kilometres downwind of the ignition area by the Mistral winds in accordance with the MODIS and AtmoPACA observations. The vertical plume development was found to be more sensitive to the sensible heat flux than to the fire released moisture. The burnt area information is also used to compute emissions of a fire aerosol-like tracer and gaseous pollutants, using emission factors for Mediterranean vegetation. The coupled model simulated high concentrations of the fire aerosol-like tracer downwind of the burning zone at the right timing compared to ground-based measurements. A chemical reaction mechanism was coupled on-line to the MesoNH model to account for gaseous chemistry evolution in the fire plume. High levels of ozone precursors (NO x , CO) were simulated in the smoke plume which led to the depletion of ozone levels above and downwind of the burning zone. This depletion of ozone was indeed observed at ground-based stations but with a higher impact than simulated. The difference may be explained by the simplified design of the model with no anthropogenic sources and no interaction of the smoke aerosols with the photolysis rates. Ozone production was modelled tens of kilometres downwind of the ignition zone out of the coast.

Methane emissions from 20 landfills across the United States using vertical radial plume mapping

Landfill fugitive methane emissions were quantified as a function of climate type and cover type at 20 landfills using U.S. Environmental Protection Agency (EPA) Other Test Method (OTM)-10 vertical radial plume mapping (VRPM) with tunable diode lasers (TDLs). The VRPM data were initially collected as g CH4/sec emission rates and subsequently converted to g CH4/m2/day rates using two recently published approaches. The first was based upon field tracer releases of methane or acetylene and multiple linear regression analysis (MLRM). The second was a virtual computer model that was based upon the Industrial Source Complex (ISC3) and Pasquill plume stability class models (PSCMs). Calculated emission results in g CH4/m2/day for each measured VRPM with the two approaches agreed well (r 2 = 0.93). The VRPM data were obtained from the working face, temporary soil, intermediate soil, and final soil or synthetic covers. The data show that methane emissions to the atmosphere are a function of climate and cover type. Humid subtropical climates exhibited the highest emissions for all cover types at 207, 127, 102, and 32 g CH4/m2/day, for working face (no cover), temporary, intermediate, and final cover, respectively. Humid continental warm summers showed 67, 51, and 27 g CH4/m2/day for temporary, intermediate, and final covers. Humid continental cool summers were 135, 40, and 26 g CH4/m2/day for the working face, intermediate, and final covers. Mediterranean climates were examined for intermediate and final covers only and found to be 11 and 6 g CH4/m2/day, respectively, whereas semiarid climates showed 85, 11, 3.7, and 2.7 g CH4/m2/day for working face, temporary, intermediate, and final covers. A closed, synthetically capped landfill covered with soil and vegetation with a gas collection system in a humid continental warm summer climate gave mostly background methane readings and average emission rates of only 0.09 g CH4/m2/day flux when measurable. Implications The OTM-10 method is being proposed by EPA to quantify surface methane emissions from landfill covers. This study of 20 landfills across the United States was done to determine the efficacy of using OTM-10 for this purpose. Two recently published models were used to evaluate the methane flux results found with VRPM optical remote sensing. The results should provide a sense of the practicality of the method, its limitations at landfills, and the impact of climate upon the cover's methane flux. Measured field data may assist landfill owners in refining previously modeled methane emission factor default values.

Measuring Trace Gas Emission from Multi-Distributed Sources Using Vertical Radial Plume Mapping (VRPM) and Backward Lagrangian Stochastic (bLS) Techniques

Two micrometeorological techniques for measuring trace gas emission rates from distributed area sources were evaluated using a variety of synthetic area sources. The vertical radial plume mapping (VRPM) and the backward Lagrangian stochastic (bLS) techniques with an open-path optical spectroscopic sensor were evaluated for relative accuracy for multiple emission-source and sensor configurations. The relative accuracy was calculated by dividing the measured emission rate by the actual emission rate; thus, a relative accuracy of 1.0 represents a perfect measure. For a single area emission source, the VRPM technique yielded a somewhat high relative accuracy of 1.38 ± 0.28. The bLS technique resulted in a relative accuracy close to unity, 0.98 ± 0.24. Relative accuracies for dual source emissions for the VRPM and bLS techniques were somewhat similar to single source emissions, 1.23 ± 0.17 and 0.94 ± 0.24, respectively. When the bLS technique was used with vertical point concentrations, the relative accuracy was unacceptably low.

LDV measurements and analysis of gas and particulate phase velocity profiles in a vertical jet plume in a 2D bubbling fluidized bed Part II: Mass and momentum transport

A Laser Doppler Velocimetry (LDV) technique was implemented to simultaneously measure the gas and particulate phase velocities in a high-speed jet plume in a two-dimensional (2D) bubbling fluidized bed. The gas and particulate phase velocity profiles are presented and analyzed. This includes similarity profile scaling as well as volume fraction, mass flow, and momentum transport calculations for the two phases. Furthermore, applying the Eulerian equation of motion to the particulate phase with the measured velocity profiles, the bed particle drag coefficient is recovered and is found to be consistent with the established empirical value.

LDV measurements and analysis of gas and particulate phase velocity profiles in a vertical jet plume in a 2D bubbling fluidized bed Part III: The effect of fluidization

In the present work, the effect of emulsion fluidization on the gas and particle phase transport in a high-speed gas jet plume in a bubbling bed is examined. Implementing a Laser Doppler Velocimetry (LDV) technique, the gas and particulate phase velocities of a gas jet injected into a bubbling bed of 838μm high-density polyethylene (HDPE) micropellets were measured. These velocity profiles were used to determine the volume fraction, mass flow, and momentum transport rate for each phase at various axial locations as well as an average particle drag coefficient in the measurement region of the plume. Maintaining consistent gas jet inlet conditions, the HDPE emulsion fluidization was varied from a spouted bed to 50% beyond minimum fluidization. In addition to the integral flow values, centerline phase velocity scaling is also examined. The scaling is based on the development of similarity velocity profiles for the two phases. Additional experiments to investigate this idea were carried out with 1) the particle emulsion and fluidization level is held constant while the jet inlet velocity is varied and 2) the jet inlet velocity and fluidization ratio is held constant while the bed particles are varied.

LDV measurements and analysis of gas and particulate phase velocity profiles in a vertical jet plume in a 2D bubbling fluidized bed: Part I: A two-phase LDV measurement technique

A laser Doppler Velocimetry (LDV) technique has been developed to simultaneously measure the gas and particulate phase velocities in a high-speed jet plume in a 2D bubbling fluidized bed. The laser, optics, and signal processing filters were configured to eliminate problematic laser-beam intensity fluctuations, which can contaminate Doppler signals in optically dense flows. In order to avoid damaging the optical access windows, the high-speed gas jet was seeded with small ice crystals. LDV bursts from the bed particles and gas tracer ice crystals were simultaneously recorded. The Doppler signals from the tracer crystals and bed particles were differentiated based on their burst intensity and coincidence to yield the particulate and gas phase velocities at a given location within the jet plume. Example gas and particulate phase velocity profiles and the associated measurement uncertainties are presented.

Sensitivity Study of Simulation Parameters Controlling CO2 Trapping Mechanisms in Saline Formations

The primary purpose of this study is to understand quantitative characteristics of mobile, residual, and dissolved CO2 trapping mechanisms within ranges of systematic variations in different geologic and hydrologic parameters. For this purpose, we conducted an extensive suite of numerical simulations to evaluate the sensitivities included in these parameters. We generated two-dimensional numerical models representing subsurface porous media with various permutations of vertical and horizontal permeability (kv and kh), porosity (\({\phi}\)), maximum residual CO2 saturation (\({S_{\rm gr}^{\max}}\)), and brine density (ρbr). Simulation results indicate that residual CO2 trapping increases proportionally to \({k_{\rm v}, k_{\rm h}, S_{\rm gr}^{\max}}\) and ρbr but is inversely proportional to \({\phi.}\) In addition, the amount of dissolution-trapped CO2 increases with kv and kh, but does not vary with \({\phi }\) , and decreases with \({S_{\rm gr}^{\max}}\) and ρbr. Additionally, the distance of buoyancy-driven CO2 migration increases proportionally to kv and ρbr only and is inversely proportional to \({k_{\rm h}, \phi }\) , and \({S_{\rm gr}^{\max}}\) . These complex behaviors occur because the chosen sensitivity parameters perturb the distances of vertical and horizontal CO2 plume migration, pore volume size, and fraction of trapped CO2 in both pores and formation fluids. Finally, in an effort to characterize complex relationships among residual CO2 trapping and buoyancy-driven CO2 migration, we quantified three characteristic zones. Zone I, expressing the variations of \({S_{\rm gr}^{\max}}\) and kh, represents the optimized conditions for geologic CO2 sequestration. Zone II, showing the variation of \({\phi}\) , would be preferred for secure CO2 sequestration since CO2 has less potential to escape from the target formation. In zone III, both residual CO2 trapping and buoyancy-driven migration distance increase with kv and ρbr.

Localized precipitation and runoff on Mars

We use the Mars Regional Atmospheric Modeling System (MRAMS) to simulate lake storms on Mars, finding that intense localized precipitation will occur for lake size >=10^3 km^2. Mars has a low-density atmosphere, so deep convection can be triggered by small amounts of latent heat release. In our reference simulation, the buoyant plume lifts vapor above condensation level, forming a 20km-high optically-thick cloud. Ice grains grow to 200 microns radius and fall near (or in) the lake at mean rates up to 1.5 mm/hr water equivalent (maximum rates up to 6 mm/hr water equivalent). Because atmospheric temperatures outside the surface layer are always well below 273K, supersaturation and condensation begin at low altitudes above lakes on Mars. In contrast to Earth lake-effect storms, lake storms on Mars involve continuous precipitation, and their vertical velocities and plume heights exceed those of tropical thunderstorms on Earth. Convection does not reach above the planetary boundary layer for lakes <<10^3 km^2 or for atmospheric pressure >O(10^2) mbar. Instead, vapor is advected downwind with little cloud formation. Precipitation occurs as snow, and the daytime radiative forcing at the land surface due to plume vapor and storm clouds is too small to melt snow directly (<+10 W/m^2). However, if orbital conditions are favorable, then the snow may be seasonally unstable to melting and produce runoff to form channels. We calculate the probability of melting by running thermal models over all possible orbital conditions and weighting their outcomes by probabilities given by Laskar et al., 2004. We determine that for an equatorial vapor source, sunlight 15% fainter than at present, and snowpack with albedo 0.28 (0.35), melting may occur with 4%(0.1%) probability. This rises to 56%(12%) if the ancient greenhouse effect was modestly (6K) greater than today.

Tracer concentration profiles measured in central London as part of the REPARTEE campaign

Abstract. There have been relatively few tracer experiments carried out that have looked at vertical plume spread in urban areas. In this paper we present results from two tracer (cyclic perfluorocarbon) experiments carried out in 2006 and 2007 in central London centred on the BT Tower as part of the REPARTEE (Regent's Park and Tower Environmental Experiment) campaign. The height of the tower gives a unique opportunity to study vertical dispersion profiles and transport times in central London. Vertical gradients are contrasted with the relevant Pasquill stability classes. Estimation of lateral advection and vertical mixing times are made and compared with previous measurements. Data are then compared with a simple operational dispersion model and contrasted with data taken in central London as part of the DAPPLE campaign. This correlates dosage with non-dimensionalised distance from source. Such analyses illustrate the feasibility of the use of these empirical correlations over these prescribed distances in central London.

Field-case simulation of CO2 -plume migration using vertical-equilibrium models

When injected in deep saline aquifers, CO2 moves radially away from the injection well and progressively higher in the formation because of buoyancy forces. Analyzes have shown that after the injection period, CO2 will potentially migrate over several kilometers in the horizontal direction but only tens of meters in the vertical direction, limited by the aquifer caprock [1, 2]. Because of the large horizontal plume dimensions, three-dimensional numerical simulations of the plume migration over long periods of time are computationally intensive. Thus, to get results within a reasonable time frame, one is typically forced to use coarse meshes and long time steps which result in inaccurate results because of numerical errors in resolving the plume tip. Given the large aspect ratio between the vertical and horizontal plume dimensions, it is reasonable to approximate the CO2 migration using vertically averaged models. Such models can, in many cases, be more accurate than coarse three-dimensional computations. In particular, models based on vertical equilibrium (VE) [3] are attractive to simulate the long-term fate of CO2 sequestered into deep saline aquifers. The reduced spatial dimensionality resulting from the vertical integration ensures that the computational performance of VE models exceeds the performance of standard three-dimensional models. Thus, VE models are suitable to study the long-time and large-scale behavior of plumes in real large-scale CO2-injection projects [4, 1, 2, 5]. We investigate the use of VE models to simulate CO2 migration in a real large-scale eld case based on data from the Sleipner site in the North Sea. We discuss the potential and limitations of VE models and show how VE models can be used to give reliable estimates of long-term CO2 migration. In particular, we focus on a VE formulation that incorporates the aquifer geometry and heterogeneity, and that considers the eects of hydrodynamic and residual trapping. We compare the results of VE simulations with standard reservoir simulation tools on test cases and discuss their advantages and limitations and show how, provided that certain conditions are met, they can be used to give reliable estimates of long-term CO2 migration.

Mixing Evaluation of a Vertical Bubble-driven Plume with Various Types of Air Injection Nozzles

Mixing Evaluation of a Vertical Bubble-driven Plume with Various Types of Air Injection Nozzles

Numerical simulation of turbulent dispersion on a two-way facing traffic road

A Large Eddy Simulation (LES) model was developed to investigate vehicle-induced turbulence and its effect on turbulent dispersion. The instantaneous behaviour of vehicle exhausts and the time-space averaged distributions of flow and concentration statistics on the roadway were estimated. The patterns of mean flow are significantly different between the one-way and the two-way traffic cases. In the vehicle passing area, the standard deviations of the spanwise and the vertical fluctuation velocities are two times larger than the maximum of the mean velocity. The relationship between the growth rate of vertical plume width and the background cross flow was also investigated.

Vertical release of hydrogen in a partially enclosed compartment: Role of wind and buoyancy

The natural and wind driven mixing and dispersion of hydrogen released in an accidental manner in a partially enclosed compartment with two vents is investigated using theoretical tools. A simple analytical model is constructed to predict the entrainment of air in a buoyant turbulent hydrogen plume and the properties of the resulting two-layer stratification that drives the flow through the vents. Air flows in through vents below the position of neutral buoyancy and exits from vents above it. CFD simulations are conducted in a full-scale geometry to confirm the physical phenomena and to compare with the analytical results. Analytical results are also compared with experimental data from a ¼ scale two-car residential garage. The analytical model is used to understand the important physical processes involved during hydrogen release as a vertical plume, and dispersion in a compartment with vents at multiple levels, with and without a steady wind. Parametric studies are conducted to study the effect of hydrogen release rate on location of the interface between the two layers and hydrogen volume fraction in the upper layer. Analytical model results indicate that for a given hydrogen release rate, the hydrogen concentration in the upper layer reaches a maximum under wind conditions that oppose the buoyancy induced flow, and that this maximum value can be as much as 70% higher than the case with no wind effects. Results also indicate that blowing outdoor air into the lower vent is an effective strategy for reducing the flammable volume of hydrogen gases in a compartment, following an accidental release.

Sheared bioconvection in a horizontal tube

The recent interest in using microorganisms for biofuels is motivation enough to study bioconvection and cell dispersion in tubes subject to imposed flow. To optimize light and nutrient uptake, many microorganisms swim in directions biased by environmental cues (e.g. phototaxis in algae and chemotaxis in bacteria). Such taxes inevitably lead to accumulations of cells, which, as many microorganisms have a density different to the fluid, can induce hydrodynamic instabilites. The large-scale fluid flow and spectacular patterns that arise are termed bioconvection. However, the extent to which bioconvection is affected or suppressed by an imposed fluid flow and how bioconvection influences the mean flow profile and cell transport are open questions. This experimental study is the first to address these issues by quantifying the patterns due to suspensions of the gravitactic and gyrotactic green biflagellate alga Chlamydomonas in horizontal tubes subject to an imposed flow. With no flow, the dependence of the dominant pattern wavelength at pattern onset on cell concentration is established for three different tube diameters. For small imposed flows, the vertical plumes of cells are observed merely to bow in the direction of flow. For sufficiently high flow rates, the plumes progressively fragment into piecewise linear diagonal plumes, unexpectedly inclined at constant angles and translating at fixed speeds. The pattern wavelength generally grows with flow rate, with transitions at critical rates that depend on concentration. Even at high imposed flow rates, bioconvection is not wholly suppressed and perturbs the flow field.