The “Bag Breakup” Spume Droplet Generation Mechanism at High Winds. Part II: Contribution to Momentum and Enthalpy Transfer

AbstractIn Part I of this study, we used high-speed video to identify “bag breakup” fragmentation as the dominant mechanism by which spume droplets are generated at gale-force and hurricane wind speeds. We also constructed a spray generation function (SGF) for the bag-breakup mechanism. The distinctive feature of this new SGF is the presence of giant (~1000 μm) droplets, which may significantly intensify the exchange between the atmosphere and the ocean. In this paper, Part II, we estimate the contribution of the bag-breakup mechanism to the momentum and enthalpy fluxes, which are known to strongly affect the development and maintenance of hurricanes. We consider three contributions to the spray-mediated aerodynamic drag: 1) “bags” as obstacles before fragmentation, 2) acceleration of droplets by the wind in the course of their production, and 3) stable stratification of the marine atmospheric boundary layer due to levitating droplets. Taking into account all of these contributions indicates a peaking dependence of the aerodynamic drag coefficient on the wind speed, which confirms the results of field and laboratory measurements. The contribution of the spray-mediated flux to the ocean-to-atmosphere moist enthalpy is also estimated using the concept of “reentrant spray,” and the equation for the enthalpy flux from a single droplet to the atmosphere is derived from microphysical equations. Our estimates show that a noticeable increase in the enthalpy exchange coefficient at winds exceeding 30–35 m s−1 is due to the enhancement of the exchange processes caused by the presence of giant droplets originating from bag-breakup fragmentation.

Download Full-text

A new moving model test method for the measurement of aerodynamic drag coefficient of high-speed trains based on machine vision

Proceedings of the Institution of Mechanical Engineers Part F Journal of Rail and Rapid Transit ◽

10.1177/0954409717731233 ◽

2017 ◽

Vol 232 (5) ◽

pp. 1425-1436 ◽

Cited By ~ 1

Author(s):

Zhiwei Li ◽

Mingzhi Yang ◽

Sha Huang ◽

Dan Zhou

Keyword(s):

Machine Vision ◽

Drag Coefficient ◽

Model Test ◽

High Speed ◽

Aerodynamic Drag ◽

Full Scale ◽

Test Method ◽

Test Accuracy ◽

Friction Resistance ◽

Aerodynamic Drag Coefficient

A moving model test method has been proposed to measure the aerodynamic drag coefficient of a high-speed train based on machine vision technology. The total resistance can be expressed as the track friction resistance and the aerodynamic drag according to Davis equation. Cameras are set on one side of the track to capture the pictures of the train, from which the line marks on the side surface of the train are extracted and analyzed to calculate the speed and acceleration of the train. According to Newton’s second law, the aerodynamic drag coefficient can be resolved through multiple tests at different train speeds. Comparisons are carried out with the full-scale coasting test, wind tunnel test, and numerical simulation; good agreement is obtained between the moving model test and the full-scale field coasting test with difference within 1.51%, which verifies that the method proposed in this paper is feasible and reliable. This method can accurately simulate the relative movement between the train, air, and ground. The non-contact measurement characteristic will increase the test accuracy, providing a new experimental method for the aerodynamic measurement.

Download Full-text

First air–sea gas exchange laboratory study at hurricane wind speeds

Ocean Science Discussions ◽

10.5194/osd-10-1971-2013 ◽

2013 ◽

Vol 10 (6) ◽

pp. 1971-1996

Author(s):

K. E. Krall ◽

B. Jähne

Keyword(s):

Wave Breaking ◽

High Speed ◽

Wind Wave ◽

Breaking Waves ◽

Gas Transfer ◽

Transfer Velocity ◽

Wind Speeds ◽

Hurricane Wind ◽

Wide Range ◽

Enhanced Surface

Abstract. In a pilot study conducted in October and November 2011, air–sea gas transfer velocities of the two sparingly soluble trace gases hexafluorobenzene and 1,4-difluorobenzene were measured in the unique High-Speed Wind-Wave Tank at Kyoto University, Japan. This air–sea interaction facility is capable of producing hurricane strength wind speeds of up to u10=67 m s−1. This constitutes the first lab study of gas transfer at such high wind speeds. The measured transfer velocities k600 spanned two orders of magnitude, lying between 11 cm h−1 and 1180 cm h−1 with the latter being the highest ever measured wind induced gas transfer velocity. The measured gas transfer velocities are in agreement with the only available dataset at hurricane wind speeds (McNeil and D'Asaro, 2007). The disproportionately large increase of the transfer velocities found at highest wind speeds indicates a new regime of air–sea gas transfer, which is characterized by strong wave breaking, enhanced turbulence and bubble cloud entrainment. It was found that tracers spanning a wide range of solubilities and diffusivities are needed to separate the effects of enhanced surface area and turbulence due to breaking waves from the effects of bubble and spray mediated gas transfer.

Download Full-text

Aerodynamic Drag Coefficient for Various Reentry Configurations at High Speed

24th AIAA Applied Aerodynamics Conference ◽

10.2514/6.2006-3173 ◽

2006 ◽

Cited By ~ 3

Author(s):

Rakhab Mehta

Keyword(s):

Drag Coefficient ◽

High Speed ◽

Aerodynamic Drag ◽

Aerodynamic Drag Coefficient

Download Full-text

First laboratory study of air–sea gas exchange at hurricane wind speeds

Ocean Science ◽

10.5194/os-10-257-2014 ◽

2014 ◽

Vol 10 (2) ◽

pp. 257-265 ◽

Cited By ~ 20

Author(s):

K. E. Krall ◽

B. Jähne

Keyword(s):

Wave Breaking ◽

High Speed ◽

Wind Wave ◽

Gas Transfer ◽

High Wind ◽

Transfer Velocity ◽

Data Set ◽

Wind Speeds ◽

Bubble Cloud ◽

Hurricane Wind

Abstract. In a pilot study conducted in October and November 2011, air–sea gas transfer velocities of the two sparingly soluble trace gases hexafluorobenzene and 1,4-difluorobenzene were measured in the unique high-speed wind-wave tank at Kyoto University, Japan. This air–sea interaction facility is capable of producing hurricane strength wind speeds of up to u10 =67 m s−1. This constitutes the first lab study of gas transfer at such high wind speeds. The measured transfer velocities k600 spanned two orders of magnitude, lying between 11 cm h−1 and 1180 cm h−1 with the latter being the highest ever measured wind-induced gas transfer velocity. The measured gas transfer velocities are in agreement with the only available data set at hurricane wind speeds (McNeil and D'Asaro, 2007). The disproportionately large increase of the transfer velocities found at highest wind speeds indicates a new regime of air–sea gas transfer, which is characterized by strong wave breaking, enhanced turbulence and bubble cloud entrainment.

Download Full-text

Non-linear resonant instability of short surface waves as the first stage “bag-breakup” process at the air-sea interface at high winds

10.5194/egusphere-egu2020-7591 ◽

2020 ◽

Author(s):

Dmitry Kozlov ◽

Yulia Troitskaya

Keyword(s):

Fluid Mechanics ◽

Small Scale ◽

Viscous Effects ◽

Dominant Mechanism ◽

Oblique Waves ◽

Wind Speeds ◽

Hurricane Wind ◽

Resonant Instability ◽

Non Linear ◽

High Winds

The recent experimental study [1], [2] identify &#8216;&#8216;bag breakup&#8217;&#8217; fragmentation as the dominant mechanism by which spume droplets are generated at hurricane wind speeds. These droplets can significantly affect the exchanging processes in the air-ocean boundary layer. In order to estimate spray-mediated heat, momentum and mass fluxes we need not only reliable experimental data, but a theoretical model of this process. The &#8220;bag-breakup&#8221; fragmentation is a strongly non-linear process, and we focus only on its first stage which includes the small-scale elevation of the water surface.Our model of the bag&#8217;s initiation is based on a weak nonlinear interaction of a longitudinal surface wave and two oblique waves propagating at equal and opposite angles to the flow as it was done in [3], [4]. All of these waves have the same critical layer where cross velocities of oblique waves become infinite making inviscid analysis invalid. So we took into account viscous effects. As a result, it has been established that for a piecewise continuous velocity profile explosive growth of wave amplitudes is possible at the wind speeds exceeding the critical one.The present model let us find the dependency of &#8220;bag&#8217;s&#8221; transverse size on the wind speed and estimate its lifetime.&#160;&#160;AcknowledgementsThis work was supported by the RSF project 19-17-00209 and the RFBR projects 19-05-00249, 19-35-90053, 18-05-00265.References:<ol><li>Troitskaya, Y. et al. Bag-breakup fragmentation as the dominant mechanism of sea-spray production in high winds. Sci. Rep. 7, 1614 (2017).</li> <li>Troitskaya, Y. et al. The &#8220;Bag Breakup&#8221; Spume Droplet Generation Mechanism at High Winds. Part I: Spray Generation Function. J. Phys. Oceanogr., 48, 2167&#8211;2188 (2018).</li> <li>A. Craik. Non-linear resonant instability in boundary layers// Journal of Fluid Mechanics. 50, 393-413 (1971).</li> <li>A. Craik. Resonant gravity-wave interactions in a shear flow// Journal of Fluid Mechanics. 34, 531-549 (1968).</li> </ol>

Download Full-text

The Impacts of Gustiness on Air–Sea Momentum Flux

Fluids ◽

10.3390/fluids6100336 ◽

2021 ◽

Vol 6 (10) ◽

pp. 336

Author(s):

Meng Lyu ◽

Henry Potter ◽

Clarence O. Collins

Keyword(s):

Drag Coefficient ◽

Momentum Flux ◽

Aerodynamic Drag ◽

Critical Threshold ◽

Wind Speeds ◽

The Earth ◽

Momentum Fluxes ◽

Aerodynamic Drag Coefficient ◽

Turbulent Spectra ◽

The Impact

The exchange of momentum across the air–sea boundary is an integral component of the earth system and its parametrization is essential for climate and weather models. This study focuses on the impact of gustiness on the momentum flux using three months of direct flux observations from a moored surface buoy. Gustiness, which quantifies the fluctuations of wind speed and direction, is shown to impact air–sea momentum fluxes. First, we put forward a new gustiness formula that simultaneously evaluates the impact of fluctuations in wind direction and speed. A critical threshold is established using a cumulative density function to classify runs as either gusty or non-gusty. We find that, during runs classified as gusty, the aerodynamic drag coefficient is increased up to 57% when compared to their non-gusty counterparts. This is caused by a correlated increase in vertical fluctuations during gusty conditions and explains variability in the drag coefficient for wind speeds up to 20 m/s. This increase in energy is connected with horizontal fluctuations through turbulent interactions between peaks in the turbulent spectra coincident with peaks in the wave spectra. We discus two potential mechanistic explanations. The results of this study will help improve the representation of gustiness in momentum flux parameterizations leading to more accurate ocean models.

Download Full-text

Design and Aerodynamic Assessment of Diffuser with Longitudinal Separators for Underbody of Sedan

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.215-216.1033 ◽

2012 ◽

Vol 215-216 ◽

pp. 1033-1037 ◽

Cited By ~ 2

Author(s):

Xing Jun Hu ◽

Rui Zhang ◽

Jian Ye ◽

Xu Yan ◽

Zhi Ming Zhao

Keyword(s):

Relative Position ◽

High Speed ◽

Aerodynamic Drag ◽

Lift Coefficient ◽

Great Influence ◽

Aerodynamic Characteristics ◽

Aerodynamic Coefficient ◽

Aerodynamic Drag Coefficient ◽

Diffuser Angle ◽

High Speed Vehicle

The aerodynamic characteristics have a great influence on the steering stability and the fuel economics of a high speed vehicle. The diffuser located at the aft part of a car underbody is one of the most important aerodynamic add-on devices. The parameters of the diffuser, including the diffuser angle, the number and the relative position of longitudinal separator (LS), the shape of the end plate and etc, will affect the underbody flow and the wake. Here, diffuser with longitudinal separator of different number and relative position was investigated. Numerical simulation was used to study the aerodynamic characteristics of a simplified sedan with different diffuser of longitudinal separator. The study found aerodynamic coefficient of the car changes little when we change the relative position of diffuser's longitudinal separator. Besides, we also found that increasing the number of the diffuser's longitudinal separator will increase the vehicle's aerodynamic drag coefficient and reduce the vehicle's lift coefficient.

Download Full-text

CFD Simulation of a Hyperloop Capsule Inside a Low-Pressure Environment Using an Aerodynamic Compressor as Propulsion and Drag Reduction Method

Applied Sciences ◽

10.3390/app11093934 ◽

2021 ◽

Vol 11 (9) ◽

pp. 3934

Author(s):

Federico Lluesma-Rodríguez ◽

Temoatzin González ◽

Sergio Hoyas

Keyword(s):

Shock Waves ◽

Power Consumption ◽

High Speed ◽

Cfd Simulation ◽

Aerodynamic Drag ◽

Flow Behavior ◽

Critical Conditions ◽

Vehicle Speed ◽

Blockage Ratio ◽

The Cost

One of the most restrictive conditions in ground transportation at high speeds is aerodynamic drag. This is even more problematic when running inside a tunnel, where compressible phenomena such as wave propagation, shock waves, or flow blocking can happen. Considering Evacuated-Tube Trains (ETTs) or hyperloops, these effects appear during the whole route, as they always operate in a closed environment. Then, one of the concerns is the size of the tunnel, as it directly affects the cost of the infrastructure. When the tube size decreases with a constant section of the vehicle, the power consumption increases exponentially, as the Kantrowitz limit is surpassed. This can be mitigated when adding a compressor to the vehicle as a means of propulsion. The turbomachinery increases the pressure of part of the air faced by the vehicle, thus delaying the critical conditions on surrounding flow. With tunnels using a blockage ratio of 0.5 or higher, the reported reduction in the power consumption is 70%. Additionally, the induced pressure in front of the capsule became a negligible effect. The analysis of the flow shows that the compressor can remove the shock waves downstream and thus allows operation above the Kantrowitz limit. Actually, for a vehicle speed of 700 km/h, the case without a compressor reaches critical conditions at a blockage ratio of 0.18, which is a tunnel even smaller than those used for High-Speed Rails (0.23). When aerodynamic propulsion is used, sonic Mach numbers are reached above a blockage ratio of 0.5. A direct effect is that cases with turbomachinery can operate in tunnels with blockage ratios even 2.8 times higher than the non-compressor cases, enabling a considerable reduction in the size of the tunnel without affecting the performance. This work, after conducting bibliographic research, presents the geometry, mesh, and setup. Later, results for the flow without compressor are shown. Finally, it is discussed how the addition of the compressor improves the flow behavior and power consumption of the case.

Download Full-text

Topographic Speed-Up Effects and Observed Roof Damage on Bermuda following Hurricane Fabian (2003)

Weather and Forecasting ◽

10.1175/waf-d-12-00050.1 ◽

2013 ◽

Vol 28 (1) ◽

pp. 159-174 ◽

Cited By ~ 13

Author(s):

Craig Miller ◽

Michael Gibbons ◽

Kyle Beatty ◽

Auguste Boissonnade

Keyword(s):

Surface Roughness ◽

Wind Speed ◽

Surface Wind ◽

Open Water ◽

Clear Trend ◽

Wind Speeds ◽

Research Division ◽

Hurricane Wind ◽

Layer Flow ◽

Observed Damage

Abstract In this study the impacts of the topography of Bermuda on the damage patterns observed following the passage of Hurricane Fabian over the island on 5 September 2003 are considered. Using a linearized model of atmospheric boundary layer flow over low-slope topography that also incorporates a model for changes of surface roughness, sets of directionally dependent wind speed adjustment factors were calculated for the island of Bermuda. These factors were then used in combination with a time-stepping model for the open water wind field of Hurricane Fabian derived from the Hurricane Research Division Real-Time Hurricane Wind Analysis System (H*Wind) surface wind analyses to calculate the maximum 1-min mean wind speed at locations across the island for the following conditions: open water, roughness changes only, and topography and roughness changes combined. Comparison of the modeled 1-min mean wind speeds and directions with observations from a site on the southeast coast of Bermuda showed good agreement between the two sets of values. Maximum open water wind speeds across the entire island showed very little variation and were of category 2 strength on the Saffir–Simpson scale. While the effects of surface roughness changes on the modeled wind speeds showed very little correlation with the observed damage, the effect of the underlying topography led to maximum modeled wind speeds of category 4 strength being reached in highly localized areas on the island. Furthermore, the observed damage was found to be very well correlated with these regions of topographically enhanced wind speeds, with a very clear trend of increasing damage with increasing wind speeds.

Download Full-text