Momentum Flux Measurements Research Articles

Fetch effects on air-sea momentum transfer at very high wind speeds

ABSTRACT Accurate estimates of momentum flux through the air-sea interface at very high wind speeds are important for predicting tropical cyclone intensities. To estimate the air-sea momentum flux under the long-fetch condition (20 m fetch) at very high wind speeds using a laboratory tank, a simple momentum flux measurement method using only four water-level gauges is conducted based on the momentum budget method. The air-water momentum flux under long-fetch conditions at very high wind speeds was measured in a typhoon simulation tank at the Research Institute of Applied Mechanics, Kyushu University. The verification was performed using previous values estimated by the eddy correlation method in a typhoon simulation tank at Kyoto University, Japan. The results showed good correlation between the values of momentum flux measured by the present momentum budget method and the other methods. The drag coefficient at very high wind speeds (41 m/s) under long-fetch conditions leveled off, as well as under the short-fetch condition (4.5 m and 6.5 m fetch). Moreover, a very weak relationship was found between the drag coefficient and the fetch. Since the maximum fetch in the present laboratory experiments is 20 m, the future field observation with the longer fetch condition will be needed for applying the results to oceans.

Momentum fluxes from airborne wind measurements in three cumulus cases over land

Abstract. Measurements of wind and momentum fluxes are not typically at the centre of field studies on (shallow) cumulus convection, but the mesoscale organization of convection is likely closely tied to patterns in wind. This study combines in situ high-frequency turbulence measurements from a gust probe onboard a Cessna aircraft with downward profiling Doppler wind lidar (DWL) measurements onboard a Falcon aircraft to study variability in the wind profile and momentum fluxes in regions of convection. The dual-aircraft measurements were made during three prototype flights in shallow convective regimes over German agricultural areas (two of which had hilly topography, one flat) in late spring 2019, including forced cumulus humilis under weak winds and “popcorn” cumuli during stronger wind and wind shear after front passages. All flights show pronounced meso-gamma (2–20 km) scale variability in the wind, with the largest wind variance (on the order of 2–4 m2 s−2) towards cloud base and in the cloud layer on flights with large vertical wind shear. The wind and wind variance profiles measured in situ and by lidar compare very well, despite the DWL's coarse (∼ 8 km) horizontal footprint. This highlights the presence of wind fluctuations on scales larger than a few kilometres and that wind lidars can be used more deliberately in field studies to map (mesoscale) flows. Cloudy transects are associated with more than twice the momentum flux compared with cloud-free transects. The contribution of the updraft to the total momentum flux, typically one-third to two-thirds, is far less than the typical contribution of the updraft to buoyancy flux. Even on the same flight day, momentum flux profiles can differ per track, with one case of counter-gradient momentum transport when the updraft does carry substantial momentum flux. Scales beyond 1 km contribute significantly to the momentum flux and there is clear evidence for compensating flux contributions across scales. The results demonstrate that momentum flux profiles and their variability require understanding of motions across a range of scales, with non-negligible contributions of the clear-sky fluxes and of mesoscales that are likely coupled to the convection.

Local momentum flux measurement: An effective way for GDI spray targeting in flash boiling conditions

Fuel injection and spray development are crucial processes for gasoline direct injected engines as an extremely accurate control of mixture formation is mandatory in order to attain high thermal efficiency targets while limiting pollutant emissions formation (mainly soot and NOx). In particular detecting the actual spray targeting is challenging as the spray evolution is significantly affected by the wide range of injection operating conditions typically implemented in GDI engines to allow a proper combustion process control. Peculiar injection strategies (rail pressure level, number of injection events) and combustion chamber pressure/temperature levels can result in flash boiling conditions, in which both the atomization process is enhanced and the spray structure is heavily modified with respect to the nominal one, with the different jets possibly collapsing in one single plume. The spray structure experimental analysis is problematic in flash boiling conditions, being conventional optical techniques as imaging and phase Doppler anemometry often unable to provide quantitative data about the inner spray structure, particularly very close to the nozzle. In this study, the actual spray evolution in flashing and non-flashing conditions was investigated by imaging and by momentum local distribution measurement, a methodology based on the spray-impact force method applied to small portions of the fluid structure to detect the local contribution of both the liquid and the gaseous phases to the jet momentum flux. The resulting momentum flux maps can be obtained very close to the nozzle and are not affected by the injector and test chamber operating conditions, offering a significant insight in the jet-to-jet interaction and spray evolution in flash boiling conditions. The obtained results evidenced the progressive modification of the spray structure of a 6-hole symmetrical GDI nozzle from standard to flare flash-boiling conditions, passing from transitional state. The plumes fingerprint gets larger when the ambient to saturation pressure ratio decreases and, when flare flash-boiling occurs, the plumes interaction becomes so intense that the spray structure collapses with a strong reduction of the global cone angle and an increased tip penetration. The obtained results confirmed momentum distribution maps to be an effective way to quantitatively analyze the actual spray targeting, the hole-to-hole flow distribution and the overall spray evolution under a wide range of operating conditions, overcoming severe limitations of other diagnostics like spray visualization and phase Doppler anemometry.

Characterisation of Internal Flow Conditions in GDI Injectors by Means of Spray-Hole-Individual Mass Flow Rate and Momentum Flux Measurements

Characterisation of Internal Flow Conditions in GDI Injectors by Means of Spray-Hole-Individual Mass Flow Rate and Momentum Flux Measurements

Observation of Gravity Waves at the Tropical Tropopause Using Superpressure Balloons

AbstractTropical gravity wave activity is investigated using measurements of momentum fluxes gathered during Strateole‐2 superpressure balloon flights. The data set consists of eight balloon flights performed in the deep tropics from November 2019 to February 2020. The flights lasted for 2–3 months each, and in‐situ meteorological data were collected every 30 s. The relation between gravity waves and deep convection is investigated using geostationary satellite data from the NOAA/NCEP GPM_MERGIR satellite data product, at 1 h resolution. The amplitude of gravity wave momentum fluxes shows a clear dependence on the distance to the nearest convective system, with a strong decay as distance to convection increases. The largest momentum‐flux values (>mPa) are only found less than 200 km away from deep convection. The sensitivity of the wave flux to distance from convection is stronger for high frequency gravity waves (periods shorter than 60 min). Lower frequency waves tend to a non‐zero, background value away from convection, supporting some background value in gravity‐wave drag parameterizations. On the other hand, the wide range of momentum flux values observed close to the convection emphasizes the intermittent nature of the gravity‐wave source. The large scale variation of gravity‐wave intermittency within the equatorial belt is also studied. The results highlight spatial variations of gravity wave activity, with the highest momentum flux recorded over land.

Using momentum flux measurements to determine the injection rate of a commercial Urea Water Solution injector

The Selective Catalytic Reduction (SCR) technology allows the transformation of the Nitrous Oxide emissions present in exhaust gases into gaseous nitrogen and water. For a proper operation of the SCR, a urea-water solution (UWS) injector must dose an adequate amount of liquid into the exhaust pipe in order to avoid deposit formation and to guarantee the SCR system efficiency. This task requires the knowledge of the performance of the injector. Then, the goal of this work is to study the hydraulic performance of a UWS injector, by means of measuring the spray momentum flux in order to understand the influence of different variables as injected fluid, injection pressure, counter pressure and cooling temperature of the injector on the flow characteristics. The tested injector was cooled at three different temperatures, 60, 90 and 120 °C, the injection pressure of the UWS was set at 5, 7 and 9 bar, with counter pressures of 750, 900, 1000 and 2000 mbar for the two tested fluids, water and UWS. The measurements were carried out using an experimental facility developed at CMT-Motores Térmicos for the determination of spray momentum flux, where a piezoelectric pressure sensor was located near the nozzle exit of the injector, which measures the impact force of the spray. Additionally, the proposed methodology allowed to determine the injected mass flow and to capture the transient injection events, such as the opening and closing stages. Moreover, mass flow rate measurements of the injector were performed under the same operating conditions, determining the influence of the injection pressure, cooling temperature, counter pressure and fluid properties. Regarding the pressure, the tendency was as expected, the higher the injection pressure the higher the Momentum flux and flow rate. Results showed that an increment of the cooling temperature of the injector induces the appearance of flash boiling conditions, having an impact on the total injected mass and momentum flux, changing the behaviour of the spray. For the same conditions, water has a higher momentum flux than the UWS due to differences in fluid properties and velocity at the nozzle exit.

A Na density lidar method and measurements of turbulence to 105 km at the Andes Lidar Observatory

This study presents a method of measuring atmospheric turbulence from resonance-fluorescence lidar observations of atmospheric Na layer density fluctuations, which is used to characterize turbulent eddy diffusion (kzzTurb) from 85 km to 105 km altitude over the Andes Lidar Observatory (ALO) in Cerro Pachón, Chile, (30.3°S, 70.7°W) at timescales between 12 seconds and 78 seconds. Na lidar measurements of heat, momentum, and constituent fluxes at turbulence scales have traditionally been limited to the 85–100 km region due to signal limitations; however, the ”Na density” method described herein offers a 3x signal-to-noise ratio improvement over Doppler heat flux measurements, enabling determination of eddy diffusion to higher altitudes and at shorter timescales. The presented mean turbulent eddy diffusion profile is determined from 115 h of zenith data acquired over 25 nights via the Na density method. Results are supplemented by off-zenith data for all 25 nights, which is used to examine atmospheric stability. Mean kzzTurb over the timescales analyzed ranged between 1.2×105 cm2s−1 and 5.7×105 cm2s−1 in the 85–100 km region and increased from a minimum at 98 km to a maximum of 4.5×106 cm2s−1 near 105 km altitude. The mean profile is compared with the Doppler method for (i) the same ALO dataset discussed herein and (ii) vertical-only ALO data between 85 km and 100 km altitude. The critical region (95–110 km) of diffusive coupling in the mesosphere and lower thermosphere (MLT) differs in forcing uniquely with a dominance of wind shear and associated instabilities. Source investigations pertaining to the induced transport difference from the critical to the lower MLT region (80–95 km), where the effects of damped atmospheric gravity waves and convective instabilities are well-established, require an ability to gather data on the variability of turbulence with altitude, latitude, and season, as well as atmospheric wave activity.

Influence of aging of a diesel injector on multiple injection strategies

Multiple injection strategies have contributed to the improvement of thermal processes in Internal Combustion Engines. On this matter, the correct performance of the injector is a critical factor in the implementation of these techniques, as changes in the behavior of the injector could affect the injection development, altering the combustion process and heat transformation efficiency. Thus, the present study analyzes the development of multiple injection events in a diesel injector aged due to partial obstruction of the holes of the nozzle. To this end, a multi-hole piezoelectric injector was employed, in which the aging of the injector is evaluated through momentum flux measurements for each hole. Afterwards, rate of injection tests with pilot and post injection strategies are performed, and the results are compared to the injection events made by the same injector before aging. The results provided by the momentum flux experiments confirmed the partial obstruction of two holes. Furthermore, rate of injection comparisons showed that the injector had a lower steady-state rate of injection after aging and required a longer time to end the injection. These phenomena affected the multiple injection events, as the extended time required to end the injection reduced the hydraulic dwell time between pilot/post injection and the main injection. Furthermore, for short dwell times, this phenomenon led to the unification of the main and post injection, resulting in a large single injection. Hence, not only an increase in fuel consumption would appear, but the thermal benefits of implementing the post injection would be lost.

Momentum flux measurements in the airflow over wind-generated surface waves

The air–sea momentum exchanges in the presence of surface waves play an integral role in coupling the atmosphere and the ocean. In the current study, we present a detailed laboratory investigation of the momentum fluxes over wind-generated waves. Experiments were performed in the large wind-wave facility at the Air–Sea Interaction Laboratory of the University of Delaware. Airflow velocity measurements were acquired above wind waves using a combination of particle image velocimetry and laser-induced fluorescence techniques. The momentum budget is examined using a wave-following orthogonal curvilinear coordinate system. In the wave boundary layer, the phase-averaged turbulent stress is intense (weak) and positive downwind (upwind) of the crests. The wave-induced stress is also positive on the windward and leeward sides of wave crests but with asymmetric intensities. These regions of positive wave stress are intertwined with regions of negative wave stress just above wave crests and downwind of wave troughs. Likewise, at the interface, the viscous stress exhibits along-wave phase-locked variations with maxima upwind of the wave crests. As a general trend, the mean profiles of the wave-induced stress decrease to a negative minimum from a near-zero value far from the surface and then increase rapidly to a positive value near the interface where the turbulent stress is reduced. Far away from the surface, however, the turbulent stress is nearly equal to the total stress. Very close to the surface, in the viscous sublayer, the wave and turbulent stresses vanish, and therefore the stress is supported by the viscosity.

Aging of a Multi-Hole Diesel Injector and Its Effect on the Rate of Injection

<div class="section abstract"><div class="htmlview paragraph">In order to comply with the increasingly restrictive limits of emissions and fuel consumption, researches are focusing on improving the efficiency of combustion engines. In this area, the aging of the injector and its effect on the injection development is not entirely analyzed. In this work, the rate of injection of a diesel injector at different stages of its lifetime is analyzed. To this end, a multi-hole piezoelectric injector was employed, comparing the injection rate measured at the beginning of its lifetime to the rate provided by the injector after aging, maintaining the same boundary conditions in both measurements. Injection pressures up to 200 MPa were used throughout the experiments. The results showed that the steady-state rate of injection was lower after the injector aged. Furthermore, the injector took a longer time to close the needle and end the injection, in comparison with the measurements done at earlier stages of its lifetime. To explain this phenomenon, measurements of momentum flux for each injector hole were done, and results showed that two holes were partially obstructed. Thus, the presence of coking in the nozzle of the injector was considered as the cause of the differences between stages seen in the rate of injection. In this sense, a reduction of the effective outlet holes area due to deposits would restrict the flow rate. Moreover, the pressure in the nozzle due to the contained fuel is extended, leading to a longer time required by the needle to descend and end the injection. Then, aging of the injector was attributed to deposits in the nozzle holes. This phenomenon could result in an inhomogeneous spray distribution, which should be taken into account when designing the combustion process.</div></div>

Impingement heat transfer to the synthetic jet issuing from a nozzle with an oscillating cross section

This experimental study is focused on a rectangular synthetic (zero-net-mass-flux) jet impinging on a wall. The working fluid is air. A novel variant of a synthetic jet actuator (SJA) is proposed. Following biomimetic principles, the actuator incorporates a nozzle in which the cross-sectional area periodically oscillates during the driven cycle. Two independent actuating systems are used to drive the main SJA diaphragm (as seen in common SJAs with rigid nozzles) and the oscillating nozzle walls (unique to this study). Namely, the diaphragm is driven electrodynamically, and the nozzle walls are driven using a pair of piezoelectric transducers. The operating frequency of 62 Hz is selected by tuning both systems at their resonances. The nozzle slot width oscillates around an average value of approximately 6 mm. Four experimental methods were used: phase-locked visualization of the oscillating nozzle lips, direct measurement of the jet momentum flux using a precision scale, hot-wire anemometry, and an evaluation of the heat transfer coefficients on the exposed wall using heat flux sensors. The local distribution of the heat transfer coefficient on the wall (Nusselt number in a dimensionless form) indicated the three-dimensional character of the flow field with an axis-switching effect. It was concluded that the momentum flux and the heat transfer rate could be enhanced by the phase delay between the diaphragm and nozzle cycles. The reason is the fact that the nozzle cross-sectional area during the extrusion stroke is smaller than the area during the suction, thus the SJ velocity and momentum flux are promoted during extrusion while the losses are reduced during suction. The maximal effect was found at the phase shift of 270° for the ratio of the cross-section areas during the extrusion and suction of 0.74.

Eye of the Storm: Observing Hurricanes with a Small Unmanned Aircraft System

AbstractUnique data from seven flights of the Coyote small unmanned aircraft system (sUAS) were collected in Hurricanes Maria (2017) and Michael (2018). Using NOAA’s P-3 reconnaissance aircraft as a deployment vehicle, the sUAS collected high-frequency (>1 Hz) measurements in the turbulent boundary layer of hurricane eyewalls, including measurements of wind speed, wind direction, pressure, temperature, moisture, and sea surface temperature, which are valuable for advancing knowledge of hurricane structure and the process of hurricane intensification. This study presents an overview of the sUAS system and preliminary analyses that were enabled by these unique data. Among the most notable results are measurements of turbulence kinetic energy and momentum flux for the first time at low levels (<150 m) in a hurricane eyewall. At higher altitudes and lower wind speeds, where data were collected from previous flights of the NOAA P-3, the Coyote sUAS momentum flux values are encouragingly similar, thus demonstrating the ability of an sUAS to measure important turbulence properties in hurricane boundary layers. Analyses from a large-eddy simulation (LES) are used to place the Coyote measurements into context of the complicated high-wind eyewall region. Thermodynamic data are also used to evaluate the operational HWRF model, showing a cool, dry, and thermodynamically unstable bias near the surface. Preliminary data assimilation experiments also show how sUAS data can be used to improve analyses of storm structure. These results highlight the potential of sUAS operations in hurricanes and suggest opportunities for future work using these promising new observing platforms.

Evaluation of hole-specific injection rate based on momentum flux measurement in GDI systems

A novel methodology for the estimation of the mass flow rate delivered by each hole of a multi-hole GDI nozzle is presented and discussed in this paper. The proposed method is based on the measurement of the impact force of each jet and it is able to evaluate the individual hole injection rate and injected mass using the relationship between the spray momentum flux and the mass flow rate. Three different nozzles are tested, one featuring equal hole diameters and two with differentiated hole-to-hole dimensions. Firstly, results are validated in terms of total injection rate, comparing the sum of the individual hole flow rates with the signal from a Zeuch-based flow meter. Secondly, outcomes are compared with the direct measurement of the injected quantity by means of a special device able to collect and weight the fuel delivered from each hole.Results evidence an excellent agreement in terms of mass flow rate dynamic profile as the proposed method is able to detect both opening and closure transients and the static flow rate. The proposed method proved to be able to capture the dynamic mass flow rate in transient conditions, i.e. very short injections, overcoming a limitation of the methodology proposed in a previous study. Moreover, in terms of injected mass, the results show a percentage error lower than 5% for medium and long energizing times and a maximum error of 9.5% for short injections in ballistic operating conditions.

Simultaneous Measurements of Local Axial and Radial Momentum Fluxes near a Radial Wall of a Helicon Source

Simultaneous Measurements of Local Axial and Radial Momentum Fluxes near a Radial Wall of a Helicon Source

Characterization of Impingement Heat/Mass Transfer to the Synthetic Jet Generated by a Biomimetic Actuator

Two biomimetic synthetic jet (SJ) actuators were designed, manufactured, and tested under conditions of a jet impingement onto a wall. Nozzles of the actuators were formed by a flexible diaphragm rim, the working fluid was air, and the operating frequencies were chosen near the resonance at 65 Hz and 69 Hz. Four experimental methods were used: phase-locked visualization of the oscillating nozzle lips, jet momentum flux measurement using a precision scale, hot-wire anemometry, and mass transfer measurement using the naphthalene sublimation technique. The results demonstrated possibilities of the proposed actuators to cause a desired heat/mass transfer distribution on the exposed wall. It was concluded that the heat/mass transfer rate was commensurable with a conventional continuous impinging jets (IJs) at the same Reynolds numbers.

Observation of turbulent dispersion of artificially released SO<sub>2</sub> puffs with UV cameras

Abstract. In atmospheric tracer experiments, a substance is released into the turbulent atmospheric flow to study the dispersion parameters of the atmosphere. That can be done by observing the substance's concentration distribution downwind of the source. Past experiments have suffered from the fact that observations were only made at a few discrete locations and/or at low time resolution. The Comtessa project (Camera Observation and Modelling of 4-D Tracer Dispersion in the Atmosphere) is the first attempt at using ultraviolet (UV) camera observations to sample the three-dimensional (3-D) concentration distribution in the atmospheric boundary layer at high spatial and temporal resolution. For this, during a three-week campaign in Norway in July 2017, sulfur dioxide (SO2), a nearly passive tracer, was artificially released in continuous plumes and nearly instantaneous puffs from a 9 m high tower. Column-integrated SO2 concentrations were observed with six UV SO2 cameras with sampling rates of several hertz and a spatial resolution of a few centimetres. The atmospheric flow was characterised by eddy covariance measurements of heat and momentum fluxes at the release mast and two additional towers. By measuring simultaneously with six UV cameras positioned in a half circle around the release point, we could collect a data set of spatially and temporally resolved tracer column densities from six different directions, allowing a tomographic reconstruction of the 3-D concentration field. However, due to unfavourable cloudy conditions on all measurement days and their restrictive effect on the SO2 camera technique, the presented data set is limited to case studies. In this paper, we present a feasibility study demonstrating that the turbulent dispersion parameters can be retrieved from images of artificially released puffs, although the presented data set does not allow for an in-depth analysis of the obtained parameters. The 3-D trajectories of the centre of mass of the puffs were reconstructed enabling both a direct determination of the centre of mass meandering and a scaling of the image pixel dimension to the position of the puff. The latter made it possible to retrieve the temporal evolution of the puff spread projected to the image plane. The puff spread is a direct measure of the relative dispersion process. Combining meandering and relative dispersion, the absolute dispersion could be retrieved. The turbulent dispersion in the vertical is then used to estimate the effective source size, source timescale and the Lagrangian integral time. In principle, the Richardson–Obukhov constant of relative dispersion in the inertial subrange could be also obtained, but the observation time was not sufficiently long in comparison to the source timescale to allow an observation of this dispersion range. While the feasibility of the methodology to measure turbulent dispersion could be demonstrated, a larger data set with a larger number of cloud-free puff releases and longer observation times of each puff will be recorded in future studies to give a solid estimate for the turbulent dispersion under a variety of stability conditions.

The effect of cavitation on the estimation of fuel injection rates based on momentum flux measurements

A study was conducted to determine how geometry-induced cavitation affects the reliability of injection rate estimates based on momentum flux measurements. Experiments were performed with two hydro-grinding diesel injector nozzles (Do = 190 µm), one having a cylindrical nozzle orifice that has a strong tendency to induce cavitation, and the other having a convergent nozzle orifice that tends to suppress cavitation. In addition, near-field microscopic images were captured to investigate the influence of cavitation on the liquid jet at the nozzle exit. The momentum flux measurements and near-field imaging were conducted at ambient pressure, using injection pressures of up to 2500 bar. When estimating the injection rate based on momentum flux measurements, failure to account for cavitation caused the integrated injection rate to be overestimated around 9% for the cavitating (cylindrical) nozzle but not for the non-cavitating (convergent) nozzle. We therefore recommend that cavitation should be accounted for when estimating injection rates for cavitating nozzles but not for non-cavitating nozzles. Further, we show that the tendency of a given nozzle orifice to induce cavitation can be evaluated by measuring the nozzle’s momentum flux under the assumption of zero cavitation and then comparing the estimated mass of injected fuel to the weighed mass.

Sensitivity of surface drag predictions using Monin–Obukhov similarity theory in non-ideal flow conditions

Turbulent flux measurements in the atmospheric surface layer (ASL), acquired from a vertical tower of sonic anemometers, are used to investigate the transient behavior of the drag coefficient over a relatively flat, homogeneous, desert playa surface. The goal of the study is to quantify the extent to which Monin–Obukhov similarity theory (MOST) successfully predicts the empirical drag coefficient when the data are not necessarily restricted to idealized conditions. Here, “idealized” refers to the conditions under which MOST is assumed to be valid, namely that the ASL exhibits statistically stationary behavior and stresses are constant with height. In order to quantify the agreement between the empirical and theoretical drag coefficients, uncertainty bands as a function of time of day were calculated for each. These uncertainty bands account for random errors and nonstationarity effects in the momentum flux, horizontal wind velocity, and stability parameter, as well as uncertainties associated with the model parameters appearing in MOST, i.e., the von Karman constant and aerodynamic roughness length. Results show that the average uncertainties in the empirical and theoretical drag coefficients are about $$\pm \,30\%$$ and $$\pm \,20\%$$ , respectively. In the case of the former, the major contribution lies from uncertainty in the momentum flux measurements; while, the latter is dominated by uncertainty in the von Karman constant. Joint probability density functions reveal that large differences between the empirical and theoretical drag coefficient have a high probability of occurrence when the ASL is nonstationary and/or exhibits a non-constant stress layer.

Direct Measurements of Momentum Flux and Dissipative Heating in the Surface Layer of Tropical Cyclones During Landfalls

AbstractThis study analyzes high‐frequency wind data collected by research towers in the surface layer of Typhoons Hagupit (2008) and Chanthu (2010) to investigate the characteristics of the momentum flux, turbulent kinetic energy (TKE), drag coefficient, and dissipative heating (DH) during landfalls. It is found that the momentum flux TKE and DH increase with the wind speed up to the maximum observed wind speed (~40 m/s), in agreement with previous studies that presented eddy correlation flux data in a similar condition but with lower maximum observed wind speed. However, the momentum flux, TKE, drag coefficient, and DH are found to be substantially larger in Typhoon Chanthu (2010) than those in Typhoon Hagupit (2008) at a given wind speed, likely due to much rougher surface conditions surrounding the tower deployed in Typhoon Chanthu (2010). Furthermore, the DH is calculated using two different methods: (1) based on surface‐layer theory and (2) based on the standard turbulent spectra method. It is found that the first method tends to overestimate the value of DH compared to the second method, and the overestimation of the DH by the first method is much smaller over rougher underlying surface than over the smoother underlying surface. Our analysis shows that the magnitude of the DH over land is as large as the sensible heat flux (~100 W/m2) previously observed over the ocean, which should not be neglected in numerical models simulating tropical cyclones during landfalls.

A Synthetic Jet Issuing From a Bio-Inspired Actuator With an Oscillating Nozzle Lip

A novel variant of a synthetic jet actuator (SJA) has been designed, manufactured, and tested. The novelty consists in a bio-inspired nozzle whose oscillating lip is formed by a flexible diaphragm rim. The working fluid is air, and the operating frequency is 65 Hz. The proposed SJA was tested by three experimental methods: phase-locked visualization of the nozzle lips, hot-wire anemometry, and momentum flux measurement using a precision scale. The results demonstrate advantages of the proposed SJA, namely, an increase in the momentum flux by 18% compared with that of a conventional SJA.