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The effects of on-road disturbances on the aerodynamic drag are attracting attention in order to accurately evaluate the fuel efficiency of an automobile on a road. The present study investigated the effects of cornering motion on automobile aerodynamics, especially focusing on the aerodynamic drag. Using a towing tank facility, measurements of the fluid-dynamic force acting on Ahmed models during steady-state cornering were conducted in water. The investigation included Ahmed models with slant angles θ = 25° and 35°, reproducing the wake structures of two different types of automobiles. The drag increase due to steady-state cornering motion was experimentally measured, and showed good agreement with previous numerical research, with the measurements conducted at a Reynolds number of 6 × 105, based on the model length. The Ahmed model with θ = 35° showed a greater drag increase due to the steady-state cornering motion than that with θ = 25°, and it reached 15% of the total drag at a corner with a radius that was 10 times the vehicle length. The results indicated that the effect of the cornering motion on the automobile aerodynamics would be more important, depending on the type of automobile and its wake characteristics.

Coastal areas often experience high population density and intense human activity owing to the considerable value of the ocean. Therefore, devices for monitoring marine disasters are crucial for ensuring the safety of human life. Herein, we develop hemispherical spring origami (SO) triboelectric nanogenerators (TENGs) (HSO-TENGs) for self-powered ocean wave monitoring. Optimization is performed using two approaches. First, swing machine experiments are conducted to investigate the monitoring performance of the HSO-TENGs regarding wave height and period with satisfactory accuracy. To increase power generation and monitoring accuracy, the internal inertia and centroid of gravity of the HSO-TENGs are optimized with respect to the structural parameters (i.e., magnet weight, hammer height, and external swing arm length). Second, numerical simulations are performed using the smoothed-particle hydrodynamics (SPH) method to determine the most suitable fixed condition for the HSO-TENGs for sensing wave changes. Subsequently, wave tank experiments are conducted on the HSO-TENGs to determine their ability to sense wave height, period, frequency, and direction. Tests related to supplying other sensors are also conducted. Eventually, the ability of the HSO-TENGs to monitor wave direction and spreading parameters is investigated in a numerical SPH circular wave tank. The results prove that the optimized HSO-TENGs can achieve powering and sensing through the same device.

AbstractWake vortices associated with a significant aerodynamic drag increase in a simplified notchback car model at a certain yaw angle and flow resulting in these vortices are identified in this study. In comparison with previous studies that only indicated the characteristic flow attachment on the leeward rear-end corner of the target car model under a significant drag increase, this study clarified the changes in the wake vortices in the target car model. Computational fluid dynamics simulations at typical yaw angles before and after the drag increase were performed. Two vortex visualization methods that depict the isosurface of the dimensionless vorticity and the vortex core lines of low-pressure vortices with swirling motions were applied. As results, four vortices were identified near the region where the surface pressure of the base decreases. These vortices were formed near the leeward side and center of the base and became stronger as the drag increased significantly. Therefore, these vortices are considered to contribute to the drag increase, in addition to enhancing the Coanda effect of the attached flow along the leeward rear-end corner. The flows associated with the four vortices were also identified by visualizing the streamlines passing through their vortex core lines. These findings will be useful in the suppression of significant aerodynamic drag increase at specific yaw angles.Graphical abstract

Spatial and temporal information on oceanic flow is fundamental to oceanography and crucial for marine-related social activities. This study attempts to describe the short-term surface flow variation in the area south of the Lombok Strait in the northern summer using the hourly Himawari-8 sea surface temperature (SST). Although the uncertainty of this temperature is relatively high (about 0.6 ∘ C), it could be used to discuss the flow variation with high spatial resolution because sufficient SST differences are found between the areas north and south of the strait. The maximum cross-correlation (MCC) method is used to estimate the surface velocity. The Himawari-8 SST clearly shows Flores Sea water intruding into the Indian Ocean with the high-SST water forming a warm thermal plume on a tidal cycle. This thermal plume flows southward at a speed of about 2 m / s . The Himawari-8 SST indicates a southward flow from the Lombok Strait to the Indian Ocean, which blocks the South Java Current flowing eastward along the southern coast of Nusa Tenggara. Although the satellite data is limited to the surface, we found it useful for understanding the spatial and temporal variations in the surface flow field.

The first coastal acoustic tomography (CAT) experiment site of the Neko-Seto Channel was revisited to elucidate the propagation and generation characteristics of the M2 and M4 tidal currents with a second CAT experiment, which was conducted from 3–6 April 2018 (local time). Two-dimensional flow fields of the M2 and M4 tidal currents and the residual current were reconstructed using a coast-fitting inversion model with the reciprocal travel-time data of five acoustic stations. The M2 tidal current is a progressive-type wave that propagates eastward at a speed of 0.7 ms−1, much slower than expected for free progressive tides in this region (19 ms−1). The M4 nonlinear current constructed an out-of-phase relationship between the western and eastern halves of the tomography domain, implying the generation of standing-type waves. Such nonlinear processes led to flood- and ebb-dominant tidal current asymmetries for the western and eastern halves of the model domain, respectively. The two-day mean residual currents constructed a northeastward current with a maximum speed of 0.3 ms−1 in the western half of the model domain and a clockwise rotation in the eastern half. The averaged inversion errors were 0.03 ms−1, significantly smaller than the amplitude of the aforementioned currents.

AbstractThis paper proposes a method to identify low-pressure vortices with swirling motions around a vehicle by considering the two-dimensional pressure minimum. The existing sectional-pressure-minimum-and-swirl method combined with the finite difference method can be used to identify low-pressure vortices with swirling motions in homogeneous isotropic turbulence. To apply this method to the flow field around a vehicle, a method that extends the existing method to the finite volume method on unstructured grids and prevents the fragmentation of the vortex core lines was developed. To verify the proposed method, it was applied to the von Kármán vortices of the square cylinder on the unstructured grids. The results indicate that the von Kármán vortices, which involve low-pressure vortices with swirling motions, could be effectively captured using the proposed method. Finally, the proposed method was applied to the flow field around a vehicle. Compared with the existing method, the proposed method could better prevent the fragmentation of the vortex core lines. In addition, four known vortex structures around the vehicle could be identified by using the proposed method in combination with the isosurface method. Compared to other generally used methods in the field of vehicle aerodynamics, the proposed method could better identify the vortex core lines within a few minutes. These results demonstrate that the proposed method is effective for identifying the vortices around a vehicle.Graphic abstract

Reciprocal acoustic transmission and coastal acoustic tomography (CAT) is a powerful tool for measuring tidal currents in shallow coastal water, especially if data assimilation is employed. In previous CAT data assimilation studies, the ensemble Kalman filter (EnKF) has been implemented to assimilate observed path-averaged velocity, but ad-hoc procedures called localization and inflation, which compensate for issues associated with using ensemble approximation, were not always implemented. In this study, EnKF is applied to assimilate the path-averaged currents obtained from a reciprocal acoustic transmission experiment conducted at Mihara-Seto in the Seto Inland Sea, Japan, with four acoustic stations in 2020 to reconstruct spatiotemporal variations of tidal currents at the observation site. We executed EnKF with several combinations of different values of the inflation, localization, and the number of ensemble members. The resulting data assimilated velocity reconstructions are compared with acoustic Doppler current profiling (ADCP) results. The results show that data assimilation with EnKF improved the velocity reproduction compared with the model prediction and that implementing covariance inflation contributed to additional improvements. The covariance localization did not improve the results in our case. The best result in terms of fractional error variance (FEV) between the ADCP velocity was obtained from the case with 980 ensemble members with a covariance inflation of 1.01; the FEV was 7.9%. The case of 98 ensemble members with a covariance inflation of 1.01 resulted in similar performance; the FEV value was 8.2%. Thus, with the covariance inflation, the number of ensemble member used in previous CAT studies are reasonable. In the study, we also clarified the reason for the high-frequency variation in the observed path-averaged currents in a preliminary experiment; the path-averaged currents had captured the spatiotemporal variation of vortex generation associated with island wakes. The reciprocal acoustic transmission with EnKF can capture short-period variation over a long period; thus, it can be used in studies of coastal physical processes with various time scales.

Coastal acoustic tomography (CAT), which measures path-averaged currents from reciprocal acoustic transmission experiments and reconstructs velocity fields from the multiple path-averaged current data, is useful for monitoring tidal currents in coastal shallow water, especially if data assimilation is employed. Previous CAT data assimilation studies have focused on state estimation problems, i.e., the reconstruction of tidal currents and following dynamical discussion. In this study, we investigate the use of path-averaged currents in a boundary control problem. Specifically, we aim to use the observed path-averaged currents to determine the parameters of a numerical ocean model, which were tidal amplitudes and phases as the open boundary conditions in this study. We investigate two methods: using the ensemble Kalman filter (EnKF) results and a linearization approach called model Green’s function method. Both calibration methods decreased the amplitudes of tidal constituents at the open boundaries. We compare the model performance between the model predictions with and without the calibration of the open boundary conditions. The model predictions with the calibrated open boundary conditions improved the agreement with the observed path-averaged current. We also implemented the sequential updates of EnKF with the two calibrated open boundary conditions. The EnKF results with the independently calibrated two open boundary conditions improved the agreement with the comparison data obtained by acoustic Doppler current profiler measurement compared with the original EnKF result with the initial open boundary conditions.

The growing populations of coastal-based city in Indonesia, such as Banten, Jakarta, and Bandar Lampung which located in Sunda Strait area are heavily affect the ocean environment quality due to increasing land-based pollution, with growing evidences of heavy metals were founded in many previous research. Further research about spatial and temporal distributions of heavy metal pollution due to ocean current are presented in this paper. Furthermore, the effect of heavy metal contamination in the ocean water to photosynthesis rate of dominant species algae populating Sunda Strait are estimated. It is estimated that a single source of lead heavy metal can spread to five kilometers radius and reduce the algae photosynthesis rate until 100% in just four weeks. Both numerical model for north-west and south-east monsoon seasons also show high concentration of suspended solid, which could also decrease algae photosynthesis rate because of less sunlight penetration. Considering fishery as one of the main economic resource in Sunda Strait area, this numerical predictions are important for coastal area management plan, environmental protection program or at least to avoid economical loss.

This study aims to study the distribution of contaminants in rivers that flow into the Caribbean Sea using chlorophyll-a (Chl-a) and suspended sediment (SS) as markers and ALOS AVNIR-2 satellite sensor data. The Haina River (HN) and Ozama and Isabela Rivers (OZ-IS) that flow through the city of Santo Domingo, the capital of the Dominican Republic, were chosen. First, in situ spectral reflectance/Chl-a and SS datasets obtained from these rivers were acquired in March 2011 (case A: with no rain influence) and June 2011 (case B: with rain influence), and the estimation algorithm of Chl-a and SS using AVNIR-2 data was developed from the datasets. Moreover, the developed algorithm was applied to AVNIR-2 data in November 2010 for case A and August 2010 for case B. Results revealed that for Chl-a and SS estimations under cases A and B conditions, the reflectance ratio of AVNIR-2 band 4 and band 3 (AV4/AV3) and the reflectance of AVNIR-2 band 4 (AV4) were effective. The Chl-a and SS mapping results obtained using AVNIR-2 data corresponded with the field survey results. Finally, an outline of the distribution of contaminants at the mouth of the river that flows into the Caribbean Sea was obtained for both rivers in cases A and B.