This study investigates the influence of aspect ratio on the aerodynamic characteristics of a finite-length square cylinder with free ends using computational fluid dynamics simulations at a Reynolds number of 6000. Aspect ratios from 12.5 to 2 are analyzed, revealing crucial insights into aerodynamic behavior under varying conditions. At an aspect ratio near 8, a major wake reorganization occurs, marking the dominant transition in the wake structure. Higher aspect ratios result in periodic shedding of coherent tip and spanwise vortices, forming a von K & aacute;rm & aacute;n vortex street. As the aspect ratio decreases, tip vortices migrate toward the leeward side, disrupting spanwise vortices and reducing vortex shedding. Below an aspect ratio of 8, steady loops of tip and spanwise vortices form, suppressing periodic shedding. The drag coefficient peaks at an aspect ratio of 12.5 and declines with decreasing aspect ratio, while the root mean square lift coefficient reaches zero below an aspect ratio of 6.5. The Strouhal number indicates no periodic vortex shedding below an aspect ratio of 4.5. Pressure distribution analysis reveals intricate patterns and changes in high-pressure regions. These findings highlight the crucial role of aspect ratio in aerodynamic behaviors, providing valuable insights for optimizing square cylinder designs in various engineering applications.

Reinforcement learning offers a promising path for reducing the energy footprint of server cooling systems. This study develops a Multi-Agent Deep Deterministic Policy Gradient (MADDPG) framework for the thermal management of a 2U air-cooled server. By assigning an independent agent to each fan and employing a centralized critic, the framework learns cooperative control strategies that eliminate redundant cooling. The agents’ learning is guided by a novel physics-informed reward function that divides the server’s thermal headroom into distinct operational zones, adding penalties to mitigate fan vibrations while dynamically balancing energy efficiency and thermal safety. To validate generalization, the MADDPG algorithm is trained in a simulation environment and subsequently deployed on experimental mock-up servers. A total of five configurations and power maps are used for validation. Each fan agent relies solely on local temperatures of its state space, while the centralized critic receives the global state of the server during training to penalize redundant cooling actions. The MADDPG controller reduced fan energy consumption by an average of 31.4 % compared to a conventional fan-table controller, while maintaining all component temperatures below their critical thresholds. The results also revealed that performance is highly dependent on server layout, with energy savings ranging from 43.8 % in centrally-located CPU configurations to 20.5 % when CPUs are at the chassis extremes, highlighting the importance of hardware-aware control policies.

Bistable wake switching is identified experimentally and numerically in an elliptical airfoil undergoing combined heaving and pitching in quiescent fluid. A history-dependent “remnant vortex impingement” mechanism drives the instability: A lingering secondary leading-edge vortex (LEV) from the preceding stroke triggers premature detachment of the nascent LEV, whose subsequent merger with the trailing-edge vortex forms an obliquely ejecting wake dipole. Aperiodic reversal of dipole direction switches the lift asymmetry despite symmetric kinematics, manifesting as nonlinear spectral sidebands in the lift spectrum. Between switching events, a vortex-splitting process facilitates a metastable symmetric reverse Kármán vortex street until disrupted by ambient disturbances.

Flapping-wing aerodynamics governs the flight performance of insects, birds, and bioinspired micro air vehicles operating in low-Reynolds-number regimes. Unlike classical steady aerodynamics, flapping flight is dominated by unsteady vortex dynamics, including leading-edge vortex stabilization, rotational lift, wake capture, and wing–wake interactions. Over the past two decades, extensive experimental, numerical, and theoretical studies have improved understanding of these mechanisms, while advances in kinematic optimization, bioinspired wing geometry, and data-driven modeling have expanded the design space of flapping-wing micro air vehicles. Despite these developments, existing research remains fragmented across aerodynamic mechanisms, kinematic control, wing morphology, wake topology, and emerging artificial intelligence-based modeling approaches, limiting the development of unified physical insight and aerodynamic design strategies. This review synthesizes advances in flapping-wing aerodynamics through a mechanism-centered framework that adopts vortex dynamics and wake topology as the unifying physical perspective linking flapping kinematics to aerodynamic force generation. The review first examines the fundamental unsteady aerodynamic mechanisms governing lift and thrust production, followed by analyses of kinematic parameters, bioinspired wing geometry, and tandem-wing and environmental interaction effects. Wake topology classification and vortex-interaction physics are then discussed to clarify how flow organization governs aerodynamic loading. Finally, developments in data-driven and artificial intelligence-enabled modeling and control are reviewed to highlight how data-driven methods can complement physics-based understanding. By integrating aerodynamic mechanisms, kinematics, wing design, wake dynamics, and data-driven approaches within a unified framework, this review provides a physically grounded synthesis and insights for the aerodynamic design, kinematic optimization, and control of next-generation bioinspired micro air vehicles.

Single-phase immersion cooling (SPIC) has emerged as a promising solution to address the escalating thermal demands of data centers and high-performance computing (HPC). This review consolidates recent advances in single-phase immersion cooling system, including discussions on dielectric fluids, cooling architectures, and intelligent control frameworks, with emphasis on experimental validation and system-level deployment. Progress in dielectric fluid is also addressed in details. The common dielectric fluid includes synthetic hydrocarbons and nanofluid-enhanced oils that were examined in association with thermal performance and compatibility with electronic components. At the control frontier, models subject to predictive control, digital twins, and reinforcement learning have enabled real-time optimization, predictive fault detection, and adaptive energy management. In parallel, sustainability metrics such as Carbon Usage Effectiveness (CUE), Water Usage Effectiveness (WUE), Renewable Energy Factor (REF), and Life Cycle Assessment (LCA) extend evaluation beyond traditional power usage effectiveness (PUE), incorporating water footprint, carbon impact, and waste heat recovery. Comparative assessments confirm SPIC's operational stability and scalability, but also highlight persistent challenges in coolant reliability, system retrofitting, and certification standards. By bridging the fluid/thermal science, intelligent control, and sustainability metrics, this review provides a comprehensive framework for advancing SPIC as a cornerstone of sustainable, scalable data center cooling.

This study investigates the influence of the Strouhal number on the thrust and propulsive efficiency of two-dimensional tandem elliptical flapping airfoils through numerical simulations validated by in-house experiments at a Reynolds number of 5000. The front and rear airfoils, identical in shape and size, are aligned with the freestream, with the rear airfoil positioned one chord length behind the front. The Strouhal number was varied from 0.2 to 0.7, while the geometric effective angle of attack amplitudes at the pivot points were set to 10, 15, and 20 degrees. Results show that the rear airfoil has minimal effect on the time-averaged thrust of the front airfoil, but the wake of the front airfoil strongly influences the flow field and thrust of the rear one. The thrust produced by the rear airfoil increases with the Strouhal number up to a critical value of 0.4, then decreases, with larger angles of attack producing higher thrust. The growth of the leading-edge vortex on the rear airfoil is controlled by the residual vortex beneath the front airfoil and the shear layer shed from the upper surface of the front airfoil. The residual vortex enhances the local flow angle and increases thrust, while the shear layer intermittently reduces it. When the shear layer passes over the rear airfoil, the local flow angle rises sharply, leading to rapid vortex growth and greater thrust, particularly at a 15-degree angle of attack. These results demonstrate the aerodynamic advantages of tandem flapping configurations under optimized unsteady flow interactions.

This study explores the aerodynamic performance of a wing and propeller system through experiments and numerical simulations. Three configurations, including a single tip propeller and four distributed propellers with the same or alternative rotational directions, are tested. The findings demonstrate that the distributed propulsion technique enhances wing performance by generating higher lift but may also increase drag and reduce cruising efficiency. The effects of propeller slipstream on pressure distribution, lift distribution, and boundary layer separation are analyzed, providing insights into the underlying mechanisms. The presence of a single tip propeller improves overall performance, whereas four propellers distributed in front of the leading edge delay stall and enhance the lift-to-drag ratio at high angles of attack. At these high angles of attack, an interesting phenomenon occurs in which the propeller slipstream is deflected upward toward the upper surface of the wing. This upward deflection of the slipstream plays a crucial role in suppressing separation of the boundary layer above the wing. This phenomenon effectively delays stall and significantly enhances the overall aerodynamic performance of the wing.

Distributed propulsion (DP) systems have gained attention in the design of unmanned aerial vehicles (UAVs) for their potential to enhance flight performance and operational versatility. By dividing thrust generation across smaller distributed propellers positioned along the wing, these systems enhance aerodynamic performance, particularly during low-speed operations such as takeoff and landing. Although DP systems primarily enhance lift during critical flight phases, their contribution to aerodynamic efficiency during cruising is less pronounced. This study provides a detailed examination of the aerodynamic effects of propeller size and elevation in DP systems during climbing, cruising, and transitional phases through numerical simulations. Employing both structured and unstructured meshes, along with a sliding mesh technique for propeller rotation, the shear stress transport (SST) k-omega model is adjusted and validated against experimental data to enhance simulation accuracy. Findings revealed that although DP systems increase lift coefficients, they also elevate drag coefficients, leading to a net reduction in the lift-to-drag ratio. Notably, smaller propeller sizes demonstrated improvements in lift-to-drag ratio compared with the baseline wing configuration. To optimize the system's performance, particular emphasis is placed on raising the leading-edge propellers three-quarters of the propeller radius above the airfoil. This adjustment accelerates the propeller slipstream above the upper airfoil surface, enhancing overall aerodynamic efficiency.

This study investigates the near-wake flow dynamics and their influence on the aerodynamic thrust characteristics of a two-dimensional flapping wing with leading edge tubercles, comparing it to a smooth leading edge (baseline) counterpart under forward flight conditions. Using the numerical simulations, validated with in-house experiments, the aerodynamic performance is analyzed across a range of non-dimensional flapping frequencies (Strouhal numbers, St = 0.2–0.6) with a Reynolds number of 5000 and a maximum effective angle of attack of 15°. The tubercle geometry, characterized by a sinusoidal amplitude of 0.125c and a wavelength of 0.5c (c: chord length), consistently underperforms the baseline wing in both time-averaged and transient thrust generation. This disparity arises from distinct wake dynamics. While the baseline wing sustains thrust through the formation of coherent leading edge vortices, the tubercle wing generates counter-rotating vortex pairs (CRVPs) that lift away from the surface. The CRVP liftoff leads to the formation of secondary structures, including residual CRVPs and hairpin vortices, which impede thrust recovery. Moreover, the residual CRVPs form complex loops that interact with the wing surface, creating an unfavorable flow environment that exacerbates thrust loss. At higher flapping frequencies, these interactions intensify, revealing loop-like vortex formations along the wingspan, further reducing pressure differentials essential for thrust generation. A critical decline in thrust performance is observed for both wings beyond St = 0.5. These findings provide detailed insights into the wake dynamics, highlighting the inherent aerodynamic limitations imposed by the tubercle geometry in forward flight applications.

The aim of this paper is twofold: firstly, to examine the influence of fin arrangement on liquid mixing performance, and secondly, to gain insights on the flow pattern, principally on transverse flow induced by the presence of different fin arrangements, in the mixing channel. Hence, this paper numerically analyzed the liquid mixing performance of a T-mixer with fins in two distinct arrangements, namely, in-line and staggered. The effects of fin arrangements on the liquid flow characteristics in the mixing channel are assessed, and it can be profoundly indicated that the induced transverse flow tends to promote better liquid mixing. Comparatively, a mixer with a staggered fin arrangement yields superior mixing performance than that of an in-line fin arrangement, owing to a greater transverse flow. This forces the bulk of the liquid to flow alternately through both halves of the mixing channel. Supplementary to the better mixing performance attained, a mixer with staggered fins also yields a lower pressure drop. When fins are placed perpendicular to the axial flow direction (i.e., θ = 0^0 ), the pressure drop yielded by an in-line fin arrangement is remarkably high (> 60 times that of the basic T-mixer and > 6 times that of the staggered fin arrangement). The superiority of the staggered fins on liquid mixing is also observed for flows at different Reynolds numbers, ranging from 0.1 to 100.

This article explores the aerodynamic performance of a two-dimensional elliptical airfoil undergoing sinusoidal heaving and asymmetric pitching motions in forward-flight conditions for the Strouhal number (St) range of 0.1–0.6. The study employs numerical simulations and water tunnel experiments to investigate the effects of non-zero pitch angular offset angles (θoffset) while maintaining a fixed Reynolds number of 5000 and an effective angle of attack amplitude of 15° at the pivot location. The θoffset is varied from −15° to +15° at 5° intervals. The present research shows that these parameters significantly impact the leading-edge effective angle of attack, flow velocity, and the formation of high-pressure regions, which are crucial factors in thrust and lift generation throughout the flapping cycle. Moreover, the pitch angle determines whether the resultant force favors thrust or lift. It is observed that the cyclic time-averaged lift consistently increases with θoffset, surpassing symmetric cases (θoffset = 0°). Conversely, the cyclic time-averaged thrust is lower for non-zero θoffset values. Increasing St enhances both cyclic time-averaged thrust and lift up to the respective critical Sts, after which their performance declines. Notably, the critical St of cyclic time-averaged lift exceeds that of cyclic time-averaged thrust; interestingly, their values are invariant with θoffset. Moreover, in the conditions where thrust efficiency maximizes, lift efficiency attains a minimum value and vice versa. So, depending upon the application, one needs to suitably select the pitch angular offset and flapping frequency to maximize thrust or lift performance.

In this article, two-dimensional numerical simulations are performed to investigate the effectiveness of a hinged splitter plate for manipulating the unsteady laminar wake regime of tandem circular cylinders (TCCs) at a pitch ratio of G/D = 5 and Reynolds number of Re = UD/upsilon = 100, where G is the distance between the centers of the cylinders having diameter D, U is the free-stream velocity, and upsilon is the kinematic viscosity of the fluid. These simulations are conducted using the in-house developed flexible forcing immersed boundary-one-step simplified lattice Boltzmann method (FFIB-OSLBM) solver. The splitter plate is hinged to the upstream cylinder's rear base point (HSPU) that pitches at amplitudes theta(m) (10 degrees-20 degrees) and non-dimensional frequencies St(f )(0.1-0.4). The plate length L-f/D is varied between 0 and 1. These pitching parameters substantially influence the wake topologies, vortex-interaction modes, pressure distribution, and flow-induced forces on the cylinders. Moreover, the TCC-HSPU combination exhibits four different wake patterns. In Type-I, regular vortex shedding occurs, with the upstream cylinder vortex (UCV) dominating the plate vortex (PV) in the cylinder gap region. The stronger and bigger PV in Type-II forms parallel vortex streets. In Type-III, the PV becomes strong enough to prevent the shedding of UCV. Finally, in Type-IV, the PV attains its maximum strength, and its interaction with UCV forms a new vortex that dominates the cylinder gap region. Among them, Type-II and Type-III regimes are associated with a lower range of drag force on TCC-HSPU configurations. In the considered parametric space, the TCC-HSPU arrangement achieves a maximum drag reduction of 43% compared to the TCC when L-f/D = 1.00, St(f) = 0.20, and theta(m) = 15 degrees.

The aeroacoustic study of the cavity flow is closely related to the stealth technology of the modern military flight vehicle and has attracted intensive attention from both the research community and industry since the 1950s. Traditionally the cavity flows were studied with experimental or computational fluid mechanics techniques. However, due to the great progress in computational hardware capability and the development of artificial neural network algorithms, machine learning technology has widely been applied to various areas of fluid mechanics. In this paper, a CFD data -based cavity flow ML surrogate model has been proposed. The development of the model began with the extraction of the dataset from the CFD result, an RNN algorithm called LSTM was then applied to predict the pressure fluctuations in the cavity flow domain, transforming the prediction to the SPL spectra and comparing it with wind tunnel data. Three test points in the frequency domain were selected to assess the prediction accuracy of the LSTM algorithm by comparing the results with the CFD results of the same case. The results show that the present proposed surrogate model is highly efficient and reasonably accurate in predicting pressure fluctuations, thus is very suitable for engineering applications during the initial concept design stage, during which time is critical.

This study focuses on the improvement of the natural convection heat dissipation performance of a fully closed cabinet. Both experiments and simulations are performed for eight different cases to examine the effect of the heat sink with chimney effect, baffle, ceiling and ceiling angle with different perforation. A heat sink is introduced to form dual chimneys to separate/facilitate the airflow circulation in the interior and exterior of the cabinet. The heat generation is 90 Watts and the exterior ambiance is around 25 degrees C. The chimney effect of the heat sink with baffle improved the airflow circulation by reducing 11-14% of the thermal resistance (TR) than the reference case. TR was found to increase with the increase in baffle height and decrease with the increase of perforation and ceiling angle, respectively. The overall analysis recommends the optimized parameters namely, baffle length 175 mm, 20 mm opening height under the baffle, length of ceiling plate to be 75 mm with 20% perforation, ceiling height 75 mm with 20% perforation and 27-degree ceiling angle. The optimized design reduced the TR by 17.9% than the base case standardizing the natural convection potential that can enhance the airflow and heat transfer.

An experimental time-resolved particle-image velocimetry study was conducted on wavy cylinders possessing wavelength (λ) and amplitude (a) combinations that are significantly different from earlier studies at Re Dm = 2700. Results show that vortex formation length increases as the wavelength decreases from λ/D m = 2.4 to 1.2, but decreases when the latter decreases to λ/D m = 0.6. Amplitude increments lead to significant vortex formation length growths and reductions at the saddles/nodes of λ/D m = 2.4 and 1.2 wavy cylinders, respectively. In contrast, λ/D m = 0.6 wavy cylinders produce significantly shorter vortex formation lengths like a baseline cylinder, regardless of amplitude. Regular reversed flow “lobes” are observed for λ/D m = 2.4 and 1.2 wavy cylinders, but not λ/D m = 0.6 ones, which lead to variations in the spanwise vortex formation lengths. Proper orthogonal decomposition (POD) analysis shows that only a/D m = 0.4, λ/D m = 0.6 wavy cylinder has the same vortex-shedding frequency as the baseline cylinder. Other POD results also demonstrate that the vortex-shedding behaviour between λ/D m = 0.6 wavy and baseline cylinder is very similar. The present study shows that there exists a minimal wavelength below which that coherent streamwise vortices will not be produced and wake control benefits of wavy cylinders will be considerably reduced.

The discharge of atmospheric-pressure plasma jet (APPJ) applied on the water was studied by varying an air gap between a powered electrode and the water, and the alternating current power. It was found that the streamers were converged by the air flow to form a streamer bridge to allow current flow across the air gap to transition from a dielectric barrier discharge (DBD) mode to a streamer mode. Because the AC power alternates periodically, this study proposed an analysis without the parameter of time by the voltage amplitude to power characteristic. It was found that the longer distance required higher voltage amplitude and power to transition. Prior research of plasma rarely mentioned the real power and the reactive power, which are essential for electric devices. Because different discharge mode produces different species, this study proposed a simple method to analyze and predict discharge modes based on the power factor, which presents the relationship of the real power and the reactive power. It was found that the streamer mode had a higher power factor than the DBD mode, and thus the power factor decides the discharge mode. Therefore, a power factor analysis can be a systematic basis to produce the required discharge by changing the equivalent capacitance and resistance to change the reactive power and real power. This study proposed a simple method to analyze periodic discharge by the intensified charge-coupled device (ICCD). A period of the AC was divided into twelve timings. For each timing, each shot was configured to have the minimum exposure time to avoid the afterimage. Thousands of shots were overlapped into one snapshot to increase the fluorescence and make such snapshot more representative for the timing. Snapshots show that the streamer bridge was only formed in specific timings periodically.

This article reports an investigation on the impacts of pivot locations and pitching angular offsets on the two-dimensional flapping elliptic airfoil's time-averaged and transient aerodynamic force coefficients. The study is carried out using ANSYS FLUENT solver and water tunnel experiments. Three different pivot point locations and pitch angular offsets are considered while keeping the flapping frequency fixed at 0.32, pitching amplitude at 30° and Reynolds number at 5000. The flapping airfoil generates a non-zero time-averaged lift for a non-zero pitch angular offset due to asymmetry kinematics in the downstroke and upstroke of a flapping cycle at all pivot locations. Furthermore, the time-averaged lift performance of the flapping airfoil is substantially improved at every pivot point locations for increased pitch angular offset. However, the time-averaged thrust generally deteriorates with the pitch angular offset, except at small pitch angular offset values. We have further identified the optimum pivot location and pitch angular offset for superior aerodynamic performances at the considered parametric range of investigation.

This study investigates drag reduction capability of naturally occurring-oscillating axial secondary flow (ASF) induced by helical-corrugated surface in Taylor–Couette flow (TCFHelical) for three values of pitch to wavelength ratios (P* = 1, 2, and 3) and amplitude to wavelength ratio(A*) of 0.25. As reported in Razzak et al. [“Numerical study of Taylor Couette flow with longitudinal corrugated surface,” Phys. Fluids 32(5), 053606 (2020)], emergence of naturally occurring-oscillating ASF induced by longitudinal-corrugated surface in TCF (TCFLongitudinal) and increasing trend on its magnitude with Reynolds number (Re) results in the occurrence of drag reduction. This has motivated us to study the possibility of enhancing drag reduction by maintaining a consistently increasing trend with Re in the magnitude of naturally occurring-oscillating ASF induced by the helical-corrugated surface on the stationary outer cylinder in TCF. From flow structures, steady ASF with non-zero mean is observed at Re = 60, which suppresses the strength of azimuthal vorticities for Re > 85, and contributed to the occurrence of drag reduction. As Re is increased to 100, 90, and 85 for P* = 1, 2, and 3, respectively, the formation of periodic oscillating ASF with non-zero mean and its increasing trend in magnitude with Re suppresses azimuthal vorticities further, which contributes to the maximum drag reduction of 13%. For Re > 165, 145, and 140 for P* = 1, 2, and 3, respectively, non-periodic oscillating ASF is observed, and its magnitude remains nearly unchanged or decreases slightly with Re, which results in the suppression effect of azimuthal vortices to be weaker. This results in the decrease in the drag reduction. Oscillating ASF observed in TCFHelical is found to occur at earlier Re, and it is stronger than that of TCFLongitudinal, which contributes to the occurrence of higher drag reduction in TCFHelical.

In this article, the effect of the pivot point location on the thrust performance of a two-dimensional sinusoidal flapping elliptic airfoil in a forward flight condition is investigated using numerical simulations and in-house water tunnel experiments. On the chord line, three different pivot locations at a distance of 0.25c, 0.5c, and 0.75c from the leading edge of the airfoil are considered, where c is the chord length of the airfoil. The flapping frequency and effective angle of attack are varied to investigate the propulsive performance of the airfoil at a Reynolds number of 5000. It is noticed that a modification in the pivot location significantly influences the linear velocity distribution, the evolution of the leading-edge vortex, and the near wake region on the airfoil. Consequently, both the transient and time-averaged thrust coefficient of the flapping airfoil is considerably affected. In addition, we have observed when the flapping frequency is increased, the time-averaged thrust coefficient of the airfoil tends to increase up to a critical Strouhal number and deteriorates thereafter. The same trend of time-averaged thrust coefficient is seen at all considered pivot locations and effective angle of attacks. Our finding suggests, at the high flapping frequency, the formation of rotation induced adverse suction region around the airfoil and delay in the shedding of the leading edge vortex developed in the previous flapping stroke are the primary sources, attributing to the thrust deterioration of the flapping airfoil with symmetric pivot location 0.5c. On the other hand, the thrust degrading effects at the two asymmetric pivot locations, 0.25c and 0.75c, are triggered by the adverse suction regions induced by asymmetric airfoil-surface velocity distribution as well as airfoil-wake vortices interaction. Moreover, the thrust degradation can be postponed to a higher critical Strouhal number if the airfoil pivot location is set near the leading edge and higher amplitude of effective angle of attack is followed. Besides, we found that the airfoil propulsive efficiency is affected due to a change in the aerodynamic power co-efficient with the modification of the pivot location. Furthermore, our observation concludes that the pivot location at 0.25c from the leading edge has maximum time-averaged thrust and propulsive efficiency performances at least for the range of pivot locations, flapping frequencies, and effective angle of attacks examined here.