Wake Size Research Articles

Exploring Near-Wake Structures: Bio-Inspired Convergent-Divergent Riblets on Circular Bluff Bodies

Abstract Convergent-divergent (C-D) riblets represent a pioneering rough surface inspired by the skin of fast-swimming sharks and flight feathers of the birds. In this study, we applied convergent-divergent riblets to the outer surface of a circular cylinder to examine their influence on the near-wake flow structure behind the cylinder. Various experimental techniques, such as particle image velocimetry (PIV), hot-wire anemometer, and a load cell for force measurement, were employed for this study. The experimental measurements have been conducted at different Reynolds numbers ranging from (6–12) ×103. The findings from force measurement indicate a decrease in the mean drag coefficient of around 14%, 15%, 11%, and 12% obtained by riblets at Re = 6000, 7500, 10,000, and 12,000, respectively. The time-averaged flow field including streamlines, velocity field, Reynolds shear stress, vorticity, and turbulent kinetic energy were measured. The measurements suggest that the wake's size has decreased by using convergent-divergent riblets. Furthermore, data obtained from the hot-wire anemometer reveal that these riblets alter the vortex shedding frequency to a higher value and impact the amplitude peak of the power spectrum at various Re numbers.

CFD study of heat transfer in power‐law fluids over a corrugated cylinder

AbstractThe computational study of power‐law fluid flow, along with heat transfer attributes over a corrugated heated cylinder, is explored using ANSYS FLUENT (Version 18.0). Fluids power‐law indices fall in the range of 0.25 ≤ n ≤ 1.5, and the Reynolds number spans in the range of 1 ≤ ReN ≤ 40. The flow is two‐dimensional, steady, and laminar. A wide range of Prandtl numbers (0.7 ≤ PrN ≤ 500) is used to cover the most industrially applied fluids. A domain height of 135Dh is used. A grid with the smallest element size of 0.04 m and 135,914 nodes was used. Flow and heat transfer attributes were studied using streamlines, isotherms, and local and average Nusselt numbers. The average Nusselt number increases with ReN and/or PrN. The heat transfer rate is significantly lower in dilatant fluids and higher in pseudoplastic fluids than in Newtonian fluids. The onset of wake formation behind the cylinder takes place at ReN = 10. The increase in Reynolds number (ReN) and power‐law index (n) causes an increase in wake size. Heat transfer increases with the Reynolds number and/or decrease in the power‐law index. The enhancement in heat transfer due to corrugation is studied in detail in terms of average Nusselt number, which has not been studied for arched corrugated cylinder, even for Newtonian fluids in low Reynolds number range. A Nusselt number correlation is also developed for the given ranges of conditions.

Experimental investigation of characteristics of turbulence induced by multiple oscillating grids

Experiments were conducted to systematically investigate the nearly homogeneous turbulence generated by multiple oscillating grids. Altogether 54 runs were completed by employing five different grids. For each run, four to nine layers of grids were installed, which were operated with different frequencies, strokes and grid spacings. The particle image velocimetry was used to obtain flow velocity characteristics. Instantaneous flow measurements reveal that the flow structure varies depending on the grid configuration. For regular grids, turbulent wakes of nearly uniform size were generally observed. In comparison, for double-scale grids, the size of wakes varied with the bar width. In the near-grid region, the horizontal distributions of Reynolds stresses are strongly influenced by the grid geometry, showing obvious inhomogeneity especially for wide bars. The inhomogeneity reduces as the distance from the grid increases. At the midplane between two neighboring grids, the flow characteristics appear similar for all grid configurations. The size of the region where the turbulence appears nearly homogeneous is affected by the grid spacing and stroke. Generally, the double-scale grid can produce a greater homogeneous region than the regular grid. This study shows that the decay law of turbulence intensity developed for a single oscillating grid can be successfully extended to multiple oscillating grids. In addition, a theoretical interpretation of the decay law is presented by considering the variation in the velocity and size of wakes when evolving away from the grid.

Free flow over and under cylindrical weir-gates with a flow extender

ABSTRACT The complex field of free flow around a cylindrical weir – gate with and without a flow extender is investigated experimentally. The effects of the gap between the channel bed and the cylinder on the discharge characteristics of the cylindrical weir-gate are studied. It is observed that the ratio of the gate to weir discharge increased non-linearly with the normalized opening height. Four triangular downstream extensions of the cylinder oriented at, θ = 0, 10, 20, and 30 degrees were tested, and the minimum underflow to overflow discharge ratio was observed for θ = 10°. The highest improvement in discharge coefficients was 16% ± 6% for extender orientation angles of θ = 10°. The effects of opening height and extender angle on flow streamlines, pressure distribution, vortex strength, turbulent kinetic energy, and Reynolds stresses were investigated. A region of negative pressure was observed near the weir crest and the peak negative pressure increased with the opening height. The size and orientation of downstream wakes were measured, and it was found that the length of wakes varies from 0.30 to 1.06 times the diameter of the cylinder. The experiments showed that for a constant normalized opening height, the vortex center point formed further downstream as the upstream water head increased.

A simplified geospace model for satellite design

This study introduced a simplified, integrated computational model to estimate the drag coefficient and surface charging within the plasma wake of Low Earth Orbit (LEO) spacecraft influenced by various solar activities. The model constituted four core modules: a solver for the solar wind-magnetosphere interaction, a magnetosphere-ionosphere coupling for electric field mapping, an ionization model for the F-layer of the ionosphere, and an estimation model for excess satellite drag and plasma wake size. The initial module adopted a simplified 1D ideal Magnetohydrodynamic (MHD) model while incorporating magnetosphere-ionosphere interactions to derive electric field data near Earth’s boundary. A simplified ionospheric model for the F-layer was implemented to compute the O+ and electron densities. Subsequently, the spacecraft drag coefficient and the plasma wake size were determined for varying solar wind speed inputs corresponding to quiet, intense, and extreme space weather conditions. In the case of extreme conditions with a solar wind speed of 1000 km/s, the results indicated that the excess satellite drag coefficient and plasma wake size increased to around ten percent.

Optimization of Co-Flow Jet Parameters for Ahmed Body Application

This study evaluates the drag reduction strategy of suction and blowing on idealize automotive vehicle, Ahmed Body. Optimization approach is adapted in order to analyse the effect of slot location, momentum coefficient and slot angle on the vehicle which experiencing drag. Despite all the efforts that have been done to reduce the Ahmed body drag using various active flow control system, most of the drag reduction were only less than 15%. A 25° Ahmed body with build in co-flow jet is modelled using a CAD software. The flow around the Ahmed body is simulated at Reynolds number based on length Re = 4.29 × 106. The governing equation were solve using an open source software package, which has been validated against experimental data. Pressure Implicit with Splitting of Operator (PISO) algorithm is applied to solve the equation. The outcome of the simulation are varies depending on the variables. Some show a decrease in drag while there are also that actually increase the drag of the system. This are caused by the suction and blowing slots that effect the surrounding air flow whether it is reducing or increasing the wake size downstream of the body. The result shows the momentum coefficient and location of both suction and blowing jet played an important role of manipulating the flow around the body and reducing the drag. The velocity contours indicated that the key to drag reduction is by using 40 m/s jet velocity, placement of suction and blowing away from each other.

Aerothermodynamic analysis and rear wake assessment of shock wave interference over blunt leading edge at Mach 6.5

A detailed numerical study of shock-wave interference on a cylindrical blunt leading edge in hypersonic flow is carried out to reveal the effect of shock-shock interaction on peak heating and blunt body aerodynamics. This study is unique in that it examines the effect of interactions on rear wake formation and aerodynamic forces acting on the blunt body. Six different shock wave interference patterns described by Edney are studied for a freestream Mach number of 6.5. Compressible Reynolds-averaged Navier–Stokes equations are solved using finite volume method to obtain accurate prediction of the flowfield and aerodynamic loads. Hugoniot jump conditions are imposed in the inlet boundary to realize oblique shock of desired strength to interact with the detached shock at specific location. Numerical predictions are in good agreement with reported experimental measurements. The results obtained in this study reveals that the type of shock-shock interaction pattern can significantly alter the characteristics of the rear wake. Comparisons to undisturbed flow conditions reveal that Type II to VI interactions lead to an increase in wake size, whereas Type I interaction shows a marginal reduction. These changes in wake size are attributed to modifications in the forebody boundary layer induced by the shock-shock interactions. In the case of Type I interaction, however, the transmitted wave interacting with the rear wake is found to be responsible for the marginal reduction in wake size. This study also shows that changes to the rear wake structure caused by the change in interaction type can affect aerodynamic loads. Type VI interaction recorded a maximum drag coefficient of 2.96, whereas Type IV interaction yielded a maximum lift coefficient of 0.992. These findings demonstrate the potential for dynamically adjusting the control forces of a flying body by manipulating shock interference.

Base Bleed Flow Control Tool for Circular Cylinders with three Side-By-Side Arrangements in Shallow Water

In this study, Particulate Image Velocimetry (PIV) was utilized to determine the flow characteristics of three side-by-side circular cylinders with a base bleed. The height of the water was set as hW=20 mm which has characteristics of shallow water and flow images were captured from an elevation of hL/hW= 0.5 (mid-plane of water height). The freestream velocity of water and circular cylinder diameters were chosen as U∞=0.125 m/sec and D=40 mm, respectively which are the corresponding Reynolds number of ReD=5000. Three different gap/diameter ratios between the cylinders were tested as G/D=1.0, 1.25, and 1.5 throughout the experiments. As a passive flow control technique, base bleed slots located through the cylinders were used with the dimensionless width ratio of B/D= 0.05 to 0.25 with 0.05 increment. As a result of the study, the base bleed was found out an effective way to control unsteady wake. Besides, whereas the increasing height of VSP results in further decrement turbulence statistics, Increasing the width of base bleed leads to a reduction of maximum Reynolds shear stress concentration. Nevertheless, jet deflection flow cannot be avoided for the G/D=1.25 while, flow deflection is avoided with all base bleed case the gap ratio increases to G/D=1.5. The wake size of each cylinder is almost the same for G/D=1.5.

BIO22: Effects of Leakage Flows around TAVI models on Thrombus Formation by Optical Visualization Method with Various Geometries in Vitro

Background: There is a possibility of leakage (PVL; Paravalvular leak) flow around TAVI (Transcatheter Aortic Valve Implantation) and a high risk of thrombus formation. To find out the mechanism of thrombus formation by this leakage flow is important issue. In our previous studies, effects of the high shear rate on the thrombus formation rate at wall on orifice flow were examined by observation of thrombus formation using laser sheet, and it was found that the thrombus formation rates increase with the high shear rates. In this study, observation of thrombus formation around TAVI models including leakage flows by laser sheet in vitro with various geometries. Method: Experimental objects are orifice pipe flow and the leakage models with (Fig.1) for observation of thrombus formation. Each leakage size h is 0mm, 0.5mm, 1mm respectively with 20 mm diameter with stenosis to simulate valve part. The target flow filed with fresh blood plasma of pigs is observed with high speed CCD camera by laser sheet. Once the thrombus formation starts, the scattering wave can be detected by this system. The movies for three kinds of geometries and two flow rates (3 and 5l/min). Results & Conclusions: As for visualization results, typical distribution of thrombus formation area at h= 0, 0.5, 1 mm is shown in Fig.2. Due to the increasing of leakage size, the thrombus formation area at the outer wall decreases. In case of leakage size h=0.5 and 1 mm, these areas correspond to wake size at the back side of the stenosis. Figure 3 shows the thrombus formation rate by evaluation of the thrombus area with changing the leakage size and geometries. It was found that the thrombus formation rate increases with the leakage size, and there are differences between the two geometries. These findings related thrombus formation area were also evaluated from the leakage flow rates in the CFD (Computational Fluid Dynamics) results. And it was also found that this formation rates increase with leakage flow rate obtained by CFD. In conclusion, there is a high possibility to generate thrombus formation with larger leakage flow for TAVI. In future, the effects of valve motion should be investigated.

Decreasing trend of tropical cyclone-induced ocean warming in recent decades

Tropical cyclones (TCs) can influence climate by pumping heat into the ocean, yet little is known about how the TC-induced ocean heat uptake (OHU) has changed in recent decades. Based on ocean reanalysis, we calculated OHU and found a significant decline of TC-induced OHU from 1982 to 2018. If all the ocean heat gain is balanced by poleward heat transport, approximately 15% of peak ocean heat transport would have been reduced during the study period. The decreasing trend of OHU is consistent with the enhanced ocean stratification, the shallowed mixed layer depth and the reduced cold wake size. The reduction of OHU primarily occurs in the Northwest Pacific, where the shortened TC lifespan contributes as well. Furthermore, the decline of OHU might offset about 28% of the upper ocean warming in the subtropical Northwest Pacific.

AERODYNAMICS CHARACTERISTICS DEVELOPMENT of TARUMANAGARA ECO-VEHICLE (MOBIL IRIT UNIVERSITAS TARUMANAGARA) USING CFD METHOD

Vehicles experience the forces of aerodynamics at various degrees and magnitude. The impact of aerodynamic must be considered because it affects the efficiency of the object. Because aerodynamic forces highly affect a vehicle’s efficiency, improvement in aerodynamics characteristics will improve the vehicles efficiency. However, the aerodynamic characteristic involves vehicle profile, dimensions. Designers now have the option of using a CFD software to simulate and calculate the aerodynamic properties of an object before production. The 2 objects on this research are an improved version of Mobil Irit Universitas Tarumanagara body by modifying its length therefore reducing the angle of the body, the body is named 3a and 3b. CFD simulation of body 3a and 3b is used to determine whether the flow characteristic is improved by the length modification of the object, and will be done using ANSYS Workbench R1 Academics Version at 5 different inlet velocity (1,39 m/s; 2,78 m/s; 4,17 m/s; 5,56 m/s; and 12,5 m/s) and will use a 3D simulation with single phase fluid. Subject 3a has 42699 nodes 234941 elements, subject 3b has 49535 nodes 272509 elements. The result shows a reduction in wake size at the back of subject 3b, whereas wake size increased in subject 3a. The drag coefficient result in reduction on 3b and an increase in 3a.

Acoustic preliminary design of a low-noise fan stage considering a variable-area nozzle and variable-pitch rotor blades

A low-noise low-pressure ultra-high-bypass-ratio fan stage to be implemented in the next generation of aircraft engines is described and evaluated acoustically with semi-empirical and analytical methods suited for preliminary design. As expected, good reduction potentials are observed for the jet noise and fan tonal noise components when the UHBR design is compared to current fans in service. However, concerns are identified for fan broadband noise, which are attributed to the off-design operation of the UHBR fan too close from its stability limit. By unloading the fan and thus reducing the size of the rotor wakes, the variable-area nozzle provides a substantial fan broadband noise reduction with a nozzle opened by around 15% from its design value. Alternatively, with the variable-pitch fan, closing the rotor blades by roughly 5° turns out to be an even more effective method to reduce fan noise, as the unloading mechanism is combined with a stronger tilting of the rotor wakes and a lower intra-stage flow Mach number. Opening the nozzle or closing the blades beyond the setting that provides the best fan efficiency is not recommended as the acoustic benefit progressively vanishes, whereas technical feasibility becomes more challenging. Finally, the presence of one of these systems may allow for the design of a low-solidity rotor, with a smaller contribution from the rotor wakes and thus a weaker fan noise emission.

Hydrodynamic effects of non-uniform feed spacer structures on energy loss and mass transfer in spiral wound module

To find a balance between feed flow rate and energy penalty, the hydrodynamic effects of non-uniform feed spacer structures on energy loss and mass transfer in spiral wound reverse osmosis (RO) module were numerically simulated by the CFD method through adopting dumbbell-shaped filament. The user-defined function (UDF) was employed to satisfy the permeability boundary conditions of membrane surfaces. The hydrodynamic effects of different filament diameter ratios (DR, d2/d1) as well as the spacer mesh angle α were studied. The simulation results showed that the spacer configuration greatly affected the energy loss and mass transfer in the spiral wound module. The pressure drop rate increased with increasing space mesh angle and a mesh angle between 60° and 120° was recommended for bearable pressure drop. The specific helicity was introduced to measure the vorticity transported with the flow. The negative region of specific helicity can be used to represent the wake size easily. The feed spacer with varied cross-section has the potential to reduce the specific energy consumption. When DR decreases from 1.0 to 0.45, the wake length becomes longer, and the pressure drop rate decreases significantly by about 58.1%. By increasing the Reynolds number from 50 to 200, the averaged wall shear stress increases by more than 5.6 times, and the averaged permeate water flux increases by 18.2%. Thus, the recommended value of feed flow Re was about 200. This detailed numerical study is of great significance to improve the understanding of concentration polarization and energy loss suppression by the optimized feed spacer configuration.

Experimental study on the characterization of the self-starting capability of a single and double-stage Savonius turbine

The Savonius hydrokinetic turbine is a viable alternative for rural electrification despite its relatively low power coefficient. There are many ways to improve power performance, including the use of multiple-stage turbines. Although many studies have investigated the effects of multi-staging, few have captured their effect on three performance parameters combined: power efficiency, flow structure, and self-starting capability. Therefore, the current study explored and analyzed the extent of these parameters using a multi-stage conventional Savonius turbine in a wind tunnel facility with a varying flow speed of 5 m/s to 9 m/s. The findings suggested that adding a stage increased the power coefficient to a maximum of 138% at 5 m/s, but reached a plateau as the wind speed increased to 9 m/s. The double-stage was discovered to reduce the self-starting speed of the turbine at all rotor angles for self-starting capabilities. The coefficient of static torque results indicated that the maximum static torque for the single and double-stage configurations occurred at 45° and 45°/135° rotor angles, respectively. The qualitative study revealed that the double-stage turbine has a larger wake size at higher wind speeds, resulting in a reduced wake intensity at the rear side of the rotor. The reduced torque variation caused by the additional stage has improved the overall performance of the turbine, making the double-stage configuration more preferable than the single-stage, particularly for low speed, high depth, and narrow river applications.

Investigating the flow field dynamics of transonic shock buffet using particle image velocimetry

An experimental investigation was performed in order to better understand the transonic shock buffet phenomenon and determine the dominant flow interactions at specific flow conditions. A rigid wing in the shape of an OAT15A airfoil was placed in the Trisonic Wind Tunnel Munich, where both the Mach number and the angle of attack were varied between 0.65<M∞<0.77\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.65< M_\\infty < 0.77$$\\end{document} and 3.8<α<6.3∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3.8< \\alpha < 6.3^\\circ$$\\end{document} respectively. With the use of high-speed imaging, high-quality optics and state-of-the-art laser equipment, highly resolved velocity field measurements were obtained via particle image velocimetry, where the streamwise and vertical velocity components were computed over the suction side of the wing center plane. It was shown that sustained buffet first occurs at M∞≥0.74\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_\\infty \\ge 0.74$$\\end{document} when maintaining the angle of attack constant at α=5.8∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha =5.8^\\circ$$\\end{document}. Similarly, an increase in α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha$$\\end{document} for a fixed M∞=0.74\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_\\infty =0.74$$\\end{document} also led to the development of shock buffet. Instantaneous snapshots confirmed the presence of a recirculation region downstream of the moving shock, where an increase in the wake size was confirmed when the shock was located most upstream. Streamwise correlations were also computed near the airfoil’s upper surface in order to extract the characteristic convective velocity of flow structures. The convective velocity appeared to increase with streamwise distance, ranging on average between 50 and 150 m/s. Overall, these time-resolved velocity field measurements allow for the investigation of the flow dynamics during shock buffet and highlight the independent effect of Mach number and angle of attack on this complex phenomenon.

Steady flow of power-law fluids past a slotted circular cylinder at low Reynolds number

Steady laminar flow past a slotted circular cylinder was investigated for non-Newtonian power-law fluids at the low Reynolds number (Re) range (5 ⩽ Re ⩽ 40). Flow simulation was carried out for shear-thinning fluids with their power-law indices (n) varying from 0.2 to 1 (n = 0.2, 0.4, 0.6, 0.8, and 1). The normal (case A) and the slotted (case B) circular cylindrical geometries were considered, where the slit was placed between the front and the base pressure stagnation points. A finite volume method was used to calculate the flow field. The flow characteristics, such as flow separation angles, wake size, coefficients of pressure (Cp), and drag (CD), were studied for different Re and n values. For all n values, the slotted cylinder effectively delayed the flow separation. It showed much better pressure recovery than the normal cylinder due to the interaction between the self-bleed from the slit exit to the cylinder wake. The vorticity of this bleed influenced the wake's vorticity, and an increase of 3%–26.4% in higher maximum surface vorticity was reported for the slotted cylinder. An increase of 0.7%–6.5% in the bubble length was observed for the normal cylinder due to early flow separation. An enhanced pressure recovery across the slotted cylinder resulted in a significant drop in the pressure drag with 0.2%–4.56% reduction in the overall drag coefficient.

Turbulent Structures in Gap Flow

A computational study was carried out to investigate the effect of the gap between the body and the bed on the wake characteristics in shallow flows. A sharp-edged bluff body with two different clearances from the bed is employed in this investigation. The transient three-dimensional governing Navier-Stokes equations are numerically solved using a finite volume formulation with improved delayed detached-eddy simulation as a turbulence model. The volume of fluid approach is used in the current study as a free-surface modelling technique for locating and tracking the free surface. It is found that the existence and size of the gap influence the wake size and the location where the free surface is restored to its original and flat shape. This study also reveals that the fluid structures lose their symmetry about the center of the wake as the gap size increases.

Study on characteristics of fluid-flow and heat transfer in the torsional flow heat exchanger with drop-shaped tube

Adopting enhanced tube is an effective way to enhance the performance of a shell-and-tube heat exchanger. In this paper, a drop-shaped tube with stream?lined cross-section was used to enhance heat transfer of the torsional flow heat exchanger. The characteristics of the fluid-flow and heat transfer in torsional flow heat exchanger with drop-shaped tube were studied numerically, considering three kinds of axis ratios (a/b=1.5, 1.8, 2) of the tube. The reliability of numerical results was verified through experimental results. The results indicate that the wake size of the streamlined drop-shaped tube is smaller than that of the conventional smooth tube, and the drop-shaped tube reduces the flow dead zone in torsional flow heat exchanger. As the axis ratio of a/b increases, the shell side Nusselt number and comprehensive performance increase, due to enhancement of the turbulence kinetic energy of the transition section. When the axis ratio is 2, the Nusselt number is increased by 12.44-18.99%, and the comprehensiveness is increased by 13.27-19.2%, compared with the torsional flow heat exchanger with the smooth tube. The quantitative analysis of the velocity indicates that the relative magnitude and proportion of transverse velocity components of fluid are important factors affecting the thermal-hydraulic performance of torsional flow heat exchanger.

Recent tropical cyclone changes inferred from ocean surface temperature cold wakes

It has been challenging to detect trends of tropical cyclone (TC) properties due to temporal heterogeneities and short duration of the direct observations. TCs impact the ocean surface temperature by creating cold wakes as a “fingerprint”. Here we infer changes of the lifetime maximum intensity (LMI), size and integrated kinetic energy from the cold wakes for the period 1982–2019. We find a globally enhanced local cold wake amplitude 3 days after the LMI of − 0.12 ± 0.04 °C per decade whereas the cold wake size does not show any significant change. Multivariate regression models based on the observed ocean cooling, the TC translation speed and the ocean mixed layer depth are applied to infer LMI and TC size. The inferred annual mean global LMI has increased by 1.0 ± 0.7 m s−1 per decade. This inferred trend is between that found for two directly observed data sets. However, the TC size and the TC destructive potential measured by the integrated kinetic energy, have not altered significantly. This analysis provides new independent and indirect evidence of recent TC LMI increases, but a stable size and integrated kinetic energy.

Flow and heat transfer in the wake of a triangular arrangement of spheres

This research work seeks to investigate the influence of spacing and heat transfer on the wake behavior of a triangular arrangement of spheres. Four experimental configurations have been investigated at three Reynolds numbers, Re1 = 350, Re2 = 700, and Re3 = 1050. Two isothermal cases were investigated with spacing between the spheres of zero and one sphere diameter, and two cases were investigated with an applied heat flux at the same spacing conditions. The time resolved particle image velocimetry results revealed various flow phenomena including flow separations, von Karman vortex shedding, and Kelvin–Helmholtz instabilities. The turbulent statistics reveal the effect of proximity and heat transfer on the time averaged values of the wake size, turbulent strengths, and Reynolds shear stress in the wake of each sphere, namely, the laminarization effects from the addition of heat and the suppression of the lead sphere wake from the proximity of the trailing spheres. These results are complemented by the application of proper orthogonal decomposition (POD) to the flow fields, which extracts the coherent structures from the flow. The modes that describe the coherent structures are extracted and described in detail, which provide further insight into effects of the experimental conditions on the temporal behavior of the flow. Many of the low order modes are found to be associated in pairs, corresponding to asymmetric structures or advection of a given structure downstream. The capability of POD to produce reduced order models of the flow is then utilized to facilitate vortex identification analysis. A turbulent kinetic energy based mode truncation criteria, which has been found to enhance vortex identification capability, is applied to select the POD modes and temporal coefficients to be used in the reduced order modeling. The reconstructed velocity fields are then analyzed with vortex identification algorithms to extract the vortex cores and boundaries. The combination of these approaches allows the study of the effect of proximity and heat transfer on the vortex characteristics, such as size, strength, and distribution.