Ronald Calhoun

Turbulent flow over a wavy surface: Neutral case

In many geophysical flows the passage of a turbulent fluid over a wavy surface significantly affects quantities of interest, such as momentum flux, mixing, and transport of scalars. Large-eddy simulation is used here to investigate neutrally stratified flow over a wavy bed. A wavy surface tends to organize the turbulent flow by imposing its wavelength and phase on the near-wall field. Both instantaneous turbulent structures and turbulence statistics provide evidence of this organization. For example, a vortex identification method applied to the instantaneous flow field reveals streamwise vortices that are locked into a phase relationship with the wavy boundary. These vortices may be linked to a Gortier instability mechanism caused by the wavy wall. Statistical analyses are also presented to gain insights into the effect of the wavy surface. In particular, we compare the effects of three different hill heights. Finally, a brief analysis of the components of drag is given.

Virtual Towers Using Coherent Doppler Lidar during the Joint Urban 2003 Dispersion Experiment

Abstract During the Joint Urban 2003 (JU2003) atmospheric field experiment in Oklahoma City, Oklahoma, of July 2003, lidar teams from Arizona State University and the Army Research Laboratory collaborated to perform intersecting range–height indicator scans. Because a single lidar measures radial winds, that is, the dot product of the wind vector with a unit vector pointing along the lidar beam, the data from two lidars viewing from different directions can be combined to produce horizontal velocity vectors. Analysis programs were written to retrieve horizontal velocity vectors for a series of eight vertical profiles to the southwest (approximately upwind) of the downtown urban core. This technique has the following unique characteristics that make it well suited for urban meteorology studies: 1) continuous vertical profiles from far above the building heights to down into the street canyons can be measured and 2) the profiles can extend to very near the ground without a loss of accuracy (assuming clear l...

The canopy horizontal array turbulence study

The Canopy Horizontal Array Turbulence Study (CHATS) took place in spring 2007 and is the third in the series of Horizontal Array Turbulence Study (HATS) experiments. The HATS experiments have been instrumental in testing and developing subfilterscale (SFS) models for large-eddy simulation (LES) of planetary boundary layer (PBL) turbulence. The CHATS campaign took place in a deciduous walnut orchard near Dixon, California, and was designed to examine the impacts of vegetation on SFS turbulence. Measurements were collected both prior to and following leafout to capture the impact of leaves on the turbulence, stratification, and scalar source/sink distribution. CHATS utilized crosswind arrays of fast-response instrumentation to investigate the impact of the canopy-imposed distribution of momentum extraction and scalar sources on SFS transport of momentum, energy, and three scalars. To directly test and link with PBL parameterizations of canopy-modified turbulent exchange, CHATS also included a 30-m profile ...

Retrieval of Microscale Wind and Temperature Fields from Single- and Dual-Doppler Lidar Data

Dual-Doppler lidar observations are used to assess the accuracy of single-Doppler retrievals of microscale wind and temperature fields in a shear-driven convective boundary layer. The retrieval algorithm, which is based on four-dimensional variational data assimilation, is applied by using dual- and single-Doppler lidar data that are acquired during the Joint Urban 2003 field experiment. The velocity field that was retrieved using single-Doppler data is compared directly with radial velocities that were measured by a second noncollocated lidar. Dual-Doppler retrievals are also performed and then compared with the singleDoppler retrieval. The linear correlation coefficient and rms deviation between the single-Doppler retrieval and the observations from the second lidar are found to be 0.94 and 1.2 m s 1 , respectively. The high correlation is mainly the result of good agreement in the mean vertical structure as observed by the two lidars. Comparisons between the single- and dual-Doppler retrieval indicate that the single-Doppler retrieval underestimates the magnitude of fluctuations in the crossbeam direction. Vertical profiles of horizontally averaged correlations between the single- and dual-Doppler retrievals also show a marginal correlation (0.4–0.8) between one of the horizontal velocity components. Again, this suggests that the retrieval algorithm has difficulty estimating the crossbeam component from single-Doppler data.

Coplanar Doppler Lidar Retrieval of Rotors from T-REX

Abstract Dual-Doppler analysis of data from two coherent lidars during the Terrain-Induced Rotor Experiment (T-REX) allows the retrieval of flow structures, such as vortices, during mountain-wave events. The spatial and temporal resolution of this approach is sufficient to identify and track vortical motions on an elevated, cross-barrier plane in clear air. Assimilation routines or additional constraints such as two-dimensional continuity are not required. A relatively simple and quick least squares method forms the basis of the retrieval. Vortices are shown to evolve and advect in the flow field, allowing analysis of their behavior in the mountain–wave–boundary layer system. The locations, magnitudes, and evolution of the vortices can be studied through calculated fields of velocity, vorticity, streamlines, and swirl. Generally, observations suggest two classes of vortical motions: rotors and small-scale vortical structures. These two structures differ in scale and behavior. The level of coordination of ...

Numerical Simulation of Strongly Stratified Flow Over a Three-Dimensional Hill

A numerical study of stably stratified flow over a three-dimensional hill is presented. Large-eddy simulation is used here to examine in detail the laboratory experimental flows described in the landmark work of Hunt and Snyder about stratified flow over a hill. The flow is linearly stratified and U∞/Nh is varied from 0.2 to 1.0. Here N and U∞ are the buoyancy frequency and freestream velocity respectively, and h is the height of the hill. The Reynolds number based on the hill height is varied from 365 to 2968. The characteristic flow patterns at various values of U∞/Nh have been obtained and they are in good agreement with earlier theoretical and experimental results. It is shown that the flow field cannot be predicted by Drazins theory when recirculation exists at the leeside of the hill even at U∞Nh ≪ 1. The wake structure agrees well with a two-dimensional wake assumption when U∞/Nh ≪ 1 but lee waves start to influence the wake structure as U∞/Nh increases. The dividing-streamline heights obtained in the simulation are in accordance with experimental results and Sheppards formula. The energy loss along the dividing streamline due to friction/turbulence approximately offsets the energy gained from pressure field. When lee waves are present, linear theory always underestimates the amplitude and overestimates the wavelength of three-dimensional lee waves. The simulated variations of drag coefficients with the parameterK (=ND/π U∞) are qualitatively consistent with experimental data and linear theory. Here D is the depth of the tank.

Flow around a Complex Building: Comparisons between Experiments and a Reynolds-Averaged Navier–Stokes Approach

Abstract An experiment investigating flow around a single complex building was performed in 2000. Sonic anemometers were placed around the building, and two-dimensional wind velocities were recorded. An energy-budget and wind-measuring station was located upstream to provide stability and inflow conditions. In general, the sonic anemometers were located in a horizontal plane around the building at a height of 2.6 m above the ground. However, at the upwind wind station, two levels of the wind were measured. The resulting database can be sampled to produce mean wind fields associated with specific wind directions such as 210°, 225°, and 240°. The data are available generally and should be useful for testing computational fluid dynamical models for flow around a building. An in-house Reynolds-averaged Navier–Stokes approach was used to compare with the mean wind fields for the predominant wind directions. The numerical model assumed neutral flow and included effects from a complex array of trees in the vicin...

Scopes and Challenges of Dual-Doppler Lidar Wind Measurements - An Error Analysis

AbstractPulsed Doppler lidars are powerful tools for long-range, high-resolution measurements of radial wind velocities. With the development of commercial Doppler lidars and the reduction of acquisition costs, dual-Doppler lidar systems will be become increasingly accessible in upcoming years. This study reviews the most common dual-Doppler techniques, describes the setup of a highly synchronized long-range dual-Doppler lidar system, and discusses extensively the different kinds of errors connected with this complex measurement technique. Sources of errors and their interactions are traced through the retrieval algorithm, including errors from single-Doppler lidar and those occurring from a combination of instruments related to various parameters, such as relative beam angles, time and spatial scales of the scan pattern, and atmospheric conditions.

Nocturnal Low-Level-Jet-Dominated Atmospheric Boundary Layer Observed by a Doppler Lidar Over Oklahoma City during JU2003

Abstract Boundary layer wind data observed by a Doppler lidar and sonic anemometers during the mornings of three intensive observational periods (IOP2, IOP3, and IOP7) of the Joint Urban 2003 (JU2003) field experiment are analyzed to extract the mean and turbulent characteristics of airflow over Oklahoma City, Oklahoma. A strong nocturnal low-level jet (LLJ) dominated the flow in the boundary layer over the measurement domain from midnight to the morning hours. Lidar scans through the LLJ taken after sunrise indicate that the LLJ elevation shows a gradual increase of 25–100 m over the urban area relative to that over the upstream suburban area. The mean wind speed beneath the jet over the urban area is about 10%–15% slower than that over the suburban area. Sonic anemometer observations combined with Doppler lidar observations in the urban and suburban areas are also analyzed to investigate the boundary layer turbulence production in the LLJ-dominated atmospheric boundary layer. The turbulence kinetic ener...

Three-Dimensional Wind Retrieval: Application of MUSCAT to Dual-Doppler Lidar

Abstract During the field campaign of the Terrain-induced Rotor Experiment (T-REX) in the spring of 2006, Doppler lidar measurements were taken in the complex terrain of the Californian Owens Valley for six weeks. While fast three-dimensional (3D) wind analysis from measured radial wind components is well established for dual weather radars, only the feasibility was shown for dual-Doppler lidars. A computationally inexpensive, variational analysis method developed for multiple-Doppler radar measurements over complex terrain was applied. The general flow pattern of the 19 derived 3D wind fields is slightly smoothed in time and space because of lidar scan duration and analysis algorithm. The comparison of extracted wind profiles to profiles from radiosondes and wind profiler reveals differences of wind speed and direction of less than 1.1 m s−1 and 3°, on average, with standard deviations not exceeding 2.7 m s−1 and 27°, respectively. Standard velocity–azimuth display (VAD) retrieval method provided higher ...