Mark R. Loewen

Simultaneous particle image velocimetry and infrared imagery of microscale breaking waves

We report the results from a laboratory investigation in which microscale breaking waves were detected using an infrared (IR) imager and two-dimensional (2-D) velocity fields were simultaneously measured using particle image velocimetry (PIV). In addition, the local heat transfer velocity was measured using the controlled flux technique. To the best of our knowledge these are the first measurements of the instantaneous 2-D velocity fields generated beneath microscale breaking waves. Careful measurements of the water surface profile enabled us to make accurate estimates of the near-surface velocities using PIV. Previous experiments have shown that behind the leading edge of a microscale breaker the cool skin layer is disrupted creating a thermal signature in the IR image [Jessup et al., J. Geophys. Res. 102, 23145 (1997)]. The simultaneously sampled IR images and PIV data enabled us to show that these disruptions or wakes are typically produced by a series of vortices that form behind the leading edge of the breaker. When the vortices are first formed they are very strong and coherent but as time passes, and they move from the crest region to the back face of the wave, they become weaker and less coherent. The near-surface vorticity was correlated with both the fractional area coverage of microscale breaking waves and the local heat transfer velocity. The strong correlations provide convincing evidence that the wakes produced by microscale breaking waves are regions of high near-surface vorticity that are in turn responsible for enhancing air–water heat transfer rates.

Characteristics of the wind drift layer and microscale breaking waves

An experimental study, investigating the mean flow and turbulence in the wind drift layer formed beneath short wind waves was conducted. The degree to which these flows resemble the flows that occur in boundary layers adjacent to solid walls (i.e. wall-layers) was examined. Simultaneous DPIV (digital particle image velocimetry) and infrared imagery were used to investigate these near-surface flows at a fetch of 5.5 m and wind speeds from 4.5 to 11 m s −1 . These conditions produced short steep waves with dominant wavelengths from 6 cm to 18 cm. The mean velocity profiles in the wind drift layer were found to be logarithmic and the flow was hydrodynamically smooth at all wind speeds. The rate of dissipation of turbulent kinetic energy was determined to be significantly greater in magnitude than would occur in a comparable wall-layer. Microscale breaking waves were detected using the DPIV data and the characteristics of breaking and non-breaking waves were compared. The percentage of microscale breaking waves increased abruptly from 11 % to 80 % as the wind speed increased from 4.5 to 7.4 m s −1 and then gradually increased to 90 % as the wind speed increased to 11 m s −1 . At a depth of 1 mm, the rate of dissipation was 1.7 to 3.2 times greater beneath microscale breaking waves compared to non-breaking waves. In the crest–trough region beneath microscale breaking waves, 40 % to 50 % of the dissipation was associated with wave breaking. These results demonstrated that the enhanced near-surface turbulence in the wind drift layer was the result of microscale wave breaking. It was determined that the rate of dissipation of turbulent kinetic energy due to wave breaking is a function of depth, friction velocity, wave height and phase speed as proposed by Terray et al. (1996). Vertical profiles of the rate of dissipation showed that beneath microscale breaking waves there were two distinct layers. Immediately beneath the surface, the dissipation decayed as ζ −0.7 and below this in the second layer it decayed as ζ −2 . The enhanced turbulence associated with microscale wave breaking was found to extend to a depth of approximately one significant wave height. The only similarity between the flows in these wind drift layers and wall-layers is that in both cases the mean velocity profiles are logarithmic. The fact that microscale breaking waves were responsible for 40 %–50 % of the near-surface turbulence supports the premise that microscale breaking waves play a significant role in enhancing the transfer of gas and heat across the air–sea interface.

Shallow turbulent wakes behind bed-mounted cylinders in open channels

Flow patterns in the shallow turbulent near-wakes behind bed-mounted cylinders are investigated on smooth and rough beds. A wall wake analysis revealed that the flows in the region away from the bed are similar and well described by the plane wake equation. Wall wake similarity was also observed for turbulent kinetic energy and primary Reynolds stress for moderate to deeply submerged cylinders.Wake analyses in the horizontal plane revealed that similarity of mean velocity profiles across the flow exist in the near-wake region at all elevations for slightly submerged and surface piercing cylinders; and at all elevations below the object height close to moderate and deeply submerged cylinders. For slightly submerged and surface piercing cylinders similarity was observed in the near-wake region between the non-dimensional velocity profiles across the wake and the plane wake equation. However, the similarity of the velocity profiles improves if a modified transverse length scale is used for shallow near-wake flows.

Detecting microscale breaking waves

Microscale breaking waves are short wind waves that break without entraining air. Experiments have shown that microscale breaking waves are on average steeper than non-breaking waves, that they generate warm turbulent wakes that are visible in infrared (IR) images and that they have high values of vorticity in their crests. Based on this knowledge, we compared three independent methods for detecting microscale breaking waves. The three methods identify or detect microscale breaking waves when a threshold value is exceeded. The wave slope, areal extent of the thermal wake and the variance of the vorticity in the crest region are the threshold parameters that are used by the three detection methods. Comparison of the breaking percentages predicted by the different methods indicates that the method that utilizes the variance of the vorticity is the most accurate. It predicts that at a fetch of 5.5 m the percentages of microscale breaking waves are 9%, 78% and 90% at wind speeds of 4.5, 7.4 and 11 m s−1, respectively.

An experimental investigation of jet flow on a stepped chute

In this paper we report on the results from an experimental study conducted to investigate the properties of the turbulent two-phase flow that occurs on a stepped chute in the jet flow regime. Simultaneous measurements of the void fraction, bubble sizes and bubble velocities were made using a fiber-optic probe. The flow was found to be fully developed by the twelfth step and as expected the head loss along the step was found to be equal to the step height. When the discharge was increased by 40% the average void fraction decreased by 8%, the depth of flow decreases by 9% and the mean velocity increased by 17%. Estimates of the turbulence intensity were computed using the mean and RMS bubble velocities. The turbulence was found to be very intense with values of the turbulence intensity varying from approximately 0.25 to 0.6. A strong negative correlation was observed between the turbulence intensity and the average bubble diameter, i.e., higher turbulence intensities produced smaller bubbles. Hinzes (J. AlChe. 1, 1955, 289) theory was used to show that Reynolds stresses are responsible for the break-up of bubbles and that the magnitudes of the observed values of the turbulence intensity were consistent with the observed bubble sizes.

Mean Flow in a Submerged Hydraulic Jump with Baffle Blocks

AbstractThe flow in a submerged hydraulic jump with three-dimensional (3D) baffle blocks was experimentally studied. An incoming jet with varying supercritical Froude numbers was deflected with one or two rows of 3D baffle blocks for different downstream water depths. Depending on the submergence factor, two flow regimes were observed for which the time-averaged 3D velocity field was measured with an acoustic Doppler velocimeter (ADV). Measurements were conducted at different stations located both upstream and downstream of the blocks and above the crest of the blocks. Velocity measurements were made in a plane located at the center of the middle block (i.e., the centerplane of the flume) and, also, in a plane located at the midpoint between the blocks (off-centerplane). The mean flow pattern was found to be significantly different for the two flow regimes. In the deflected surface jet (DSJ) regime, which occurred at low submergence factors, the major flow features were a small surface roller upstream of ...

Physical oceanography: inside whitecaps.

Innovative experiments have provided new insights into how bubbles are created by breaking waves. These findings might ultimately lead to more accurate models of global climate.

Infrared remote sensing of microscale breaking waves and near-surface flow fields

We report the results from a series of wind-wave flume experiments, in which microscale breaking waves were detected using an infrared (IR) imager and two-dimensional velocity fields were measured simultaneously using particle image velocimetry (PIV). Careful measurements of the water surface profile enabled us to make accurate estimates of the near-surface velocities using PIV. We show that strong vortices are generated behind the leading edge of microscale breaking waves. These vortices disrupt the cool skin layer, which appears as wake in the IR image. The near-surface vorticity was correlated with the fractional area coverage of microscale breaking waves providing convincing evidence that the wakes produced by microscale breaking waves are regions of high near-surface vorticity.

Interrelationship Between Nutrients and Chlorophyll-a in an Urban Stormwater Lake During the Ice-covered Period

Urban stormwater lakes in cold regions are ice-covered for substantial parts of the winter. It has long been considered that the ice-covered period is the “dormant season,” during which ecological processes are inactive. However, little is known about this period due to the historical focus on the open-water season. Recent pioneering research on ice-covered natural lakes has suggested that some critical ecological processes play out on the ice. The objective of this study was to investigate the active processes in ice-covered stormwater lakes. Data collected during a two-year field measurement program at a stormwater lake located in Edmonton, Alberta, Canada were analyzed. The lake was covered by ice from November to mid-April of the following year. The mean value of chlorophyll-a during the ice-covered period was 22.09% of the mean value for the open-water season, suggesting that primary productivity under ice can be important. Nitrogen and phosphorus were remarkably higher during the ice-covered period, while dissolved organic carbon showed little seasonal variation. Under ice-covered conditions, the total phosphorus was the major nutrient controlling the ratio of total nitrogen to total phosphorus, and a significant positive correlation existed between total phosphorus and chlorophyll-a when the ratio was smaller than 10. The results provide preliminary evidence of the critical nutrient processes in the Stormwater Lake during the ice-covered period.

The Impact of Surface Contamination on the Flow Beneath Wind-Generated Waves

We report on an experimental study conducted to investigate flow characteristics in the near-surface layer beneath clean and surfactant-contaminated water surfaces in the presence of wind. The two-dimensional velocity field beneath the water surface was measured using particle image velocimetry. The water surface temperature measurements were made simultaneously using infrared imagery. The results show the existence of the viscous sublayer beneath both clean and contaminated water surfaces. Within the viscous sublayer in contaminated water, the mean streamwise velocity is 25–30% larger and the mean streamwise velocity gradients are more than a factor of two larger compared to that beneath clean water surfaces.Copyright