Turbulent Flow Characteristics Research Articles

Investigation of the single-phase immersion cold plate amid PAO-4 and Noah@3000A – An experimental approach and its numerical verification

This study investigates improving traditional immersion cooling systems by employing cold plates in single-phase immersion cooling with dielectric fluids (Noah@3000 A and PAO-4). Experimental and CFD methods explore various cold plate structures in a 1 U server with a TTV heat source. Flow rates range from 1 to 3 LPM, and configurations include side-open, fully enclosed, and dual-open designs. Results show that incorporating cold plates reduces fluid bypass, enhancing thermal performance. High-viscosity dielectric oil as a coolant yields substantial improvements (36.1% and 41.6% at low and high flow rates) compared to traditional systems. Thermal resistance decreases with higher flow rates, more notably for Noah@3000A. Immersing the cold plate in liquid enhances convective heat transfer, reducing thermal resistances by 7.2% and 9.4% for Noah@3000A and PAO-4 at low flow rates. Smaller fin pitch consistently lowers resistance by eliminating bypass. Outlet configuration significantly influences thermal resistance, with Enclosed cold plates showing the least performance for PAO-4 and dual-open cold plate yields the worst performance with Noah@3000A. Differences in heat transfer behavior between PAO-4 and Noah@3000A are linked to laminar and turbulent flow characteristics. These findings offer insights into optimizing immersion cooling systems, emphasizing the impact of fluid choice and cold plate design on thermal performance.

Evaluation of flow and flame characteristics of turbulent bluff-body CH4-H2 flame using LES-FPV approach

ABSTRACT This study investigated the flow and flame structure of a bluff-body stabilised CH4-H2 flame using the large eddy simulation (LES) and the flamelet progress variable (FPV) model. The performance of three subgrid-scale (SGS) LES models, i.e. the standard Smagorinsky model (SSM), the wall-adapting local eddy viscosity (WALE) model, and the dynamic Smagorinsky model (DSM) in predicting turbulent and reactive flow characteristics was assessed. Parameters, namely y+ , Pope index (M), vorticity field, and resolved turbulent kinetic energy (kt ), were analysed to determine the predictive capability of the three models. The results demonstrated that DSM and WALE successfully resolved 80% of kt for chosen grid configurations, whereas SSM exhibited discrepancies. The overall trends captured by all three models matched well with the experimental trend with DSM and WALE outperforming SSM in predicting reacting flow fields in the recirculation zone, while SSM produced better predictions in the regions away from the recirculating zone.

Turbulent flow characteristics over rough permeable and impermeable gravel-bed stream- an experimental study

Abstract The study aims to presents flow heterogeneity over rough permeable and impermeable gravel-bed stream. The rough permeable stream is prepared by laying multiple layers of gravel, whereas its impermeable counterpart is presented by a resin-casted gravel-bed. In general, a common approach can be found in literature to mimic an impermeable bed by laying single layer of gravels, therefore some results are compared with single layer gravel-bed stream keeping the hydraulic conditions same. An acoustic Doppler velocimeter was used for flow measurements whereas; double averaging (DA) methodology was adopted for data analysis. The larger flow penetration depth and intense flow mixing in permeable gravel-bed infer sufficient impetus for organized flow turbulence and damping of DA Reynolds shear stresses whereas; the wall-blocking in resin-casted gravel-bed prevents fluid infiltration which leads to follow the linear stress profile away from the crest level. The damping of DA Reynolds shear stress (RSS) is compensated by enhanced DA form-induced shear stress (FISS). The results are further analysed under the light of the energy budget to characterize the mass-momentum exchange as it penetrates the subsurface layers. The energy budget indicates negative pressure energy diffusion rates corroborating gain in turbulence production in the permeable gravel-bed stream.

Preflight and Postflight Thermal/Structural Analysis of BOLT-2: The Holden Mission

BOLT-2: The Holden Mission, a follow-on to the BOLT experiment, was designed to measure the characteristics of turbulent flow in the hypersonic flight regime. This paper summarizes preflight transient thermal and structural analyses for the predicted flight trajectories that led to the selection of suitable materials for the flight experiment. Following the successful flight in March 2022, the preflight analysis methods, along with temperature measurements from the flight, were used to conduct a postflight thermal analysis based on the as-flown trajectory that attempted to match the flight data. Conservatism in the preflight design analysis was compared to the flight measurements. Databases of computed laminar and turbulent heating were generated as part of this effort that can aid in future analysis of the experimental data. The effects of modeling transitional heating during ascent were quantified when compared to a fully turbulent heating assumption. A set of estimated three-dimensional surface temperature distributions have been computed and were found to be a good match to the available flight data. Postflight structural modeling was used to estimate the differential thermal expansion at the nosetip/frustum joint interface.

A new probability-distribution scale synthetic eddy method for large eddy simulation of wind loads

The accurate generation of inflow turbulence in large eddy simulation (LES) plays a crucial role in assessing the wind loads on buildings. To faithfully recreate turbulent flow fields, the inflow turbulence must accurately replicate vortex structures. In this study, we propose a novel method for generating turbulent inflow called the probability-distribution scale synthetic eddy method (PDSSEM). PDSSEM utilizes a random generation of eddy positions, with the length of each eddy scale determined by its probability density function (PDF). This approach ensures that PDSSEM maintains consistency in the mean wind speed profile, turbulence integral length, turbulence intensity profile, and wind speed spectra of the atmospheric boundary layer (ABL). The PDSSEM method can be more easily applied compared with multi-scale synthetic eddy method (MSSEM), making it highly versatile and adaptable. The self-sustaining characteristics of the flow field generated by PDSSEM are validated, revealing abundant vortex structures and good agreement with targeted values for turbulence flow characteristics. Furthermore, comparing the wind pressures of a tall building with the experimental data, the accuracy and feasibility of PDSSEM are comprehensively demonstrated. Thus, it indicates the great potential of the proposed method for broader computational wind engineering applications, including wind load evaluation and flow pattern investigation.

A Computational Investigation of the Influence of Seafloor Conditions on the Turbulent Flow Characteristics of an Autonomous Underwater Vehicle

AbstractThe effect of the seabed on the hydrodynamics of three-dimensional autonomous underwater vehicles (AUVs) varies according to the physical conditions of the place where AUVs interact with the environmental conditions. This study examines the hydrodynamics of an AUV resembling a torpedo model while taking the influence of the seabed surface as a function of the dimensionless distances (G/D) between the torpedo and the seabed. Reynolds numbers, varying from 1 × 104 to 8 × 104, were considered. These Reynolds numbers were associated with various seabed distances falling within 0.25 ≤ G/D ≤ 1.5. To perform the simulations, governing equations were utilized and incorporated with the k–ω SST turbulence model. It has been observed that when AUVs or torpedo models operate in close proximity to the seabed surface, several key hydrodynamic parameters and flow characteristics are affected. These include the pressure coefficient (Cp), drag coefficient (CD), overall flow structures, maneuverability, and performance of the torpedo model. As the AUV or torpedo model approaches the seabed surface, the symmetrical flow pattern deteriorates. This deterioration is associated with changes in vortical flow structures under the influence of seabed surfaces. Additionally, the intensity of the shear stress (τ) near the seabed surface gradually increases as the AUV or torpedo model gets closer to it. In summary, the proximity of AUVs or torpedo models to the seabed surface causes disruptions in the flow patterns, increased shear stress, and alterations in key hydrodynamic parameters, ultimately affecting the system's performance and behavior.

Numerical Study on Flow Field and Heat Transfer Enhancement in a Cooling Channel With Novel Ribs Based on Battery Thermal Management System

Turbulent flow and heat transfer characteristics in a cooling channel with novel ribs have been numerically studied to improve thermal efficiency inside future electric vehicle batteries. In this study, the essential ratios of channel length, rib height, and rib pitch were all set to 6.67, 0.13, and 8.66, respectively. The realizable turbulent kinetic energy - epsilon model was used for the numerical simulation work under the Reynolds number varied from 10,000 to 50,000. The results showed that the heat transfer enhancement on a surface depended on the secondary flows due to the ribs. The enhancement of the wavy-winglet rib was approximately 9.8%−32% compared to other rib cases. Nevertheless, the improved heat transfer came at the expense of a higher pressure penalty. Above 29.7% of the friction factor for the wavy-winglet rib was obtained, compared to orthogonal ribs. In addition, the wavy-winglet rib gave the best thermal-hydraulic performance of the designs explored.

Prediction and comparison of aerodynamic performance and flow control for smooth and wavy tube banks in crossflow at subcritical Reynolds numbers using a 3D RANS approach

This paper investigates the effects of introducing wavy cylinders of appropriate wavelength into tube bundles for the passive control of flow-induced vibration. The Reynolds-averaged Navier–Stokes approach RANS was used to study the turbulent flow characteristics within staggered tube bundles having transverse and longitudinal pitch-to-diameter ratios of 3.8 and 2.1, respectively. Two wavy models are tested with different wavelength-to-mean diameter ratios, namely: Model-A with [Formula: see text] and Model-B with [Formula: see text], and their performances are compared with those of a smooth circular tube bundle Model-C ([Formula: see text]), in a range of subcritical Reynolds numbers [Formula: see text]. Mean velocity and pressure fields as well as aerodynamic coefficients are presented and compared. According to the obtained results, the wavy tube bundle with [Formula: see text] exhibits the best performance, with slightly smaller [Formula: see text] values than the smooth tube bundle. However, results show that the wavy model with [Formula: see text] is clearly the worst with an increase of [Formula: see text] up to 40%. Numerical outcomes also reveal that the interaction of neighboring rows and columns affects the fluctuation of [Formula: see text] values relative to each tube position. Moreover, [Formula: see text] evolutions depend significantly on the variation of Re as well, where drag reduction is more substantial by increasing Re. Thus, wavy-shaped tubes with optimum parameters could be more suitable for flow control by enhancing the secondary flow, turbulence and fluid mixing.

A numerical simulation of turbulent flow characteristics through periodic grooves of varying curvatures

A numerical simulation of turbulent flow characteristics through periodic grooves of varying curvatures

Structure and role of the pressure Hessian in regions of strong vorticity in turbulence

Amplification of velocity gradients, a key feature of turbulent flows, is affected by the non-local character of the incompressible fluid equations expressed by the second derivative (Hessian) of the pressure field. By analysing the structure of the flow in regions where the vorticity is the highest, we propose an approximate expression for the pressure Hessian in terms of the local vorticity, consistent with the existence of intense vortex tubes. Contrary to the often used simplification of an isotropic form for the pressure Hessian, which in effect inhibits vortex stretching, the proposed approximate form of the pressure Hessian enables much stronger vortex stretching. The prediction of the approximation proposed here is validated with results of direct numerical simulations of turbulent flows.

Characteristics of turbulent flow around bridge abutments in the presence of vegetation in channel bed under ice-covered flow conditions

During winter, ice cover frequently forms on the water surface of rivers with vegetated channel beds in cold regions. The investigation of the impacts of both ice cover and channel bed vegetation on flow structures around bridge abutments is essential for engineers to gain a comprehensive understanding of the complex interactions occurring in such a situation. In the present study, the flow structure around a rectangular bridge abutment in the presence of vegetation under ice-covered conditions has been studied. Considering different vegetation densities by arranging vegetation elements in square and staggered configurations, this study incorporates the influence of ice covers with different roughness, namely smooth and rough ice cover. Key turbulence parameters, including turbulence intensity, Reynolds shear stress (RSS), and turbulent kinetic energy (TKE), are also examined based on laboratory experiments. Results show that the shape of velocity profiles for flow over a vegetated channel bed changes from an S-shaped curve under an open flow condition to a convex shape under ice-covered conditions. The effects of an ice cover and vegetation on the flow around the bridge abutment create unpredictable turbulence intensity patterns. Under a rough-covered flow condition, there appears a larger area with negative Reynolds shear stress (RSS) downstream of the abutment. Turbulent kinetic energy (TKE) under ice-covered conditions has substantially lower magnitudes than in open flow conditions.

Aerodynamic investigation of the turbulent flow past a louvered-fin-and-tube automotive heat exchanger

Turbulent flow past a radiator contributes to noise emission when interacting with a downstream cooling fan. This study examines turbulent flow characteristics downstream of a radiator, focusing on turbulence production, dissipation, non-homogeneity, and anisotropy. We assess turbulence models based on isotropy and homogeneity for noise prediction in real applications. Results show significant non-homogeneity due to the radiator grid’s wakes, extending downstream to a region where the fan would be located. Flow in this region trends toward isotropy, especially when considering a downstream circularly-holed wood panel mimicking the flow contraction caused by a fan casing. The radiator core’s fins and louvers break down vortical structures more than generating large eddies, as reported in studies involving heat exchangers with larger gaps between cooling pipes. The von Kármán turbulence model approximates the experimental spectrum, but caution is needed with rapid flow acceleration. This investigation enhances our understanding of turbulence characteristics in the vicinity of radiators with implications for noise spectrum prediction and modeling in such systems.

Performance evaluation and enhancement of turbulent flow and convective heat transfer characteristics for turbine blade internal cooling

For turbine blade internal cooling, the performance of turbulent flow and convective heat transfer in the cooling channel is numerically investigated via the computational fluid dynamics method, where the cooling channel adopts the combination of transverse rib arrangements and round-edged rib configurations or the combination of oblique rib arrangements and round-edged rib configurations. Under these two combination conditions, the simulation result shows that in the wide Reynolds number range of Re = 20 000–80 000, the heat transfer enhancement and the flow loss reduction, such as the larger normalized Nusselt number of Nu/Nu0 and smaller normalized friction factor of f/f0, are simultaneously realized by the oblique rather than transverse rib arrangement. In the oblique rib channel, the relationship between turbulent flow and convective heat transfer is for the first time revealed by the relationship between secondary vortices and turbulent kinetic energies. Based on the relation between secondary vortices and turbulent kinetic energies, the overall performance of turbulent flow and convective heat transfer for the oblique rib channel is first optimized by various normalized rib height, e/Dh, and rib spacing, p/e, and then evaluated by both overall performance factors of OPT1 and OPT2. Both OPT1 and OPT2 are larger at e/Dh = 0.062 and p/e = 15.00 in the wide range of e/Dh = 0.047–0.101 and p/e = 10.00–15.00. As a result of the comprehensive evaluation of OPT1 and OPT2, the combination of e/Dh = 0.062 and p/e = 15.00 is eventually employed by the oblique rib channel.

The Effect of Mechanical Circulatory Support on Blood Flow in the Ascending Aorta: A Combined Experimental and Computational Study.

Percutaneous mechanical circulatory support (MCS) devices are designed for short-term treatment in cases of acute decompensated heart failure as a bridge to transplant or recovery. Some of the known complications of MCS treatments are related to their hemodynamics in the aorta. The current study investigates the effect of MCS on the aortic flow. The study uses combined experimental and numerical methods to delineate complex flow structures. Particle image velocimetry (PIV) is used to capture the vortical and turbulent flow characteristics in a glass model of the human aorta. Computational fluid dynamics (CFD) analyses are used to complete the 3D flow in the aorta. Three specific MCS configurations are examined: a suction pump with a counterclockwise (CCW) rotating impeller, a suction pump with a clockwise (CW) rotating impeller, and a discharge pump with a straight jet. These models were examined under varying flow rates (1-2.5 L/min). The results show that the pump configuration strongly influences the flow in the thoracic aorta. The rotating impeller of the suction pump induces a dominant swirling flow in the aorta. The swirling flow distributes the incoming jet and reduces the turbulent intensity near the aortic valve and in the aorta. In addition, at high flow rates, the local vortices formed near the pump are washed downstream toward the aortic arch. Specifically, an MCS device with a CCW rotating impeller induces a non-physiological CCW helical flow in the descending aorta (which is opposite to the natural helical flow), while CW swirl combines better with the natural helical flow.

Effect of wall curvature on heat transfer and hydrodynamics in a ribbed cooling passage

Simplified rectangular ribbed cooling passages with a flat wall are extensively considered in exploring the internal cooling features of turbine blades, but the realistic blade has a twisted shape inherently. The effects induced by the curved wall have not been clarified in detail. In this work, adopting a verified v2f turbulence model, numerical investigations are completed to evaluate the effects of the curved wall on the internal cooling characteristics of a ribbed channel. Adopting the unified ribbed channel, flat, convex, and concave walls with distinct curvatures are comprehensively evaluated and compared in a wide Re range for the turbulent flow and heat transfer features as well as the flow and thermal performance. It is found that using the flat wall, ribs can typically induce recirculation vortices having a two-dimensional nature. In contrast, the curved wall significantly contributes to the counter-rotating vortex pairs on the spanwise plane. Combined with recirculation vortices offered by the ribs, the turbulent flow of the cooling channel with the curved wall has a remarkable three-dimensional feature. Hence, the turbulent activity and fluid mixing are enhanced greatly along with the raised heat transfer enhancement and friction loss. Particularly, the convex wall with a curvature of K = 4 provides 28.6 % higher heat transfer performance (Nu/Nu0) but 88.4 % higher resistance (f/f0) than the flat wall. Considering the overall cooling performance, the concave wall with a relatively small curvature is suggested with an improvement of up to 32.8 % concerning the factor (Nu/Nu0)/(f/f0) and 9.5 % on (Nu/Nu0)/(f/f0)1/3. Finally, it is highlighted that considering the effect of the wall curvature, the current study stimulates the mechanistic understanding and provides a design guideline for high-performance blade internal cooling.

Assessment on natural ventilation and pollutant dispersion around sunken squares by transient simulation

Sunken square is a semi-open public space that has been gaining popularity in urban planning and designs. Its semi-open design is beneficial to ventilation of the connected underground spaces, but it is also subject to potential exposure to ground-level traffic pollutants. To evaluate the semi-open design’s influence on the balance between underground ventilation efficiency and the pollutant exposure, this study uses large eddy simulation (LES) to simulate turbulent wind flow characteristics and pollutant dispersion in a typical sunken square. The sunken square is connected to two indoor spaces with single-sided opening, and the design features of staircase and arcade are further tested for their influences on the wind flow field. The air change rate per hour (ACH, h−1) is adopted to quantify the ventilation efficiency of the two connected indoor spaces. Results show that the staircases amplify wind velocities for the indoor spaces but also bring higher pollutant concentrations inside together. Moreover, the design of the arcade at the opening prompts the vortex to shift towards the entrance, leading to a heightened concentration of pollutants. Meanwhile, the effective ACH is mostly contributed by the turbulent diffusion (ACHt), while the mean flow (ACHm) has much less effect on the ventilation of the indoor spaces. These findings of this study provide references for sunken square design.

Characteristics of turbulent flow in 3-D pools in the presence of submerged rigid vegetation in channel bed

Characteristics of turbulent flow in 3-D pools in the presence of submerged rigid vegetation in channel bed

Numerical analysis of turbulent flow characteristics with the influence of speed ratio in a double-sided cavity

The present study numerically investigates the two-dimensional steady incompressible turbulent flow characteristics in an enclosed cavity. The finite volume method (FVM) is used to discretize the governing equations, and k-ε turbulence models are adopted to predict the flow characteristics. The turbulent flow behavior is studied by varying the speed ratio (0.05 ≤ S ≤ 1.0), aspect ratio (0.5 ≤ K ≤ 2.0), and Reynolds number (1 × 104 ≤ Re ≤ 2 × 105). The flow characteristics are analyzed using stream function (ψ), Reynolds stresses (u'v'), and turbulent quantities. Results show the Reynolds number and speed ratio significantly influence the formation of vortices over the selected range of operating parameters. With the speed ratio, the turbulent kinetic energy reduces considerably by increasing the Reynolds number and aspect ratio. Similarly, for S = 0.05 and K = 0.5, the turbulent kinetic energy and dissipation rate are decreased by 89.16% and 42.28%, respectively. When Re is increased from 1 × 104 to 2 × 105, the turbulent viscosity increases by 92.10%. By comparing the results, average turbulent quantities are decreased by increasing the flow parameters.•Turbulent flow behavior is investigated by using the FVM near-wall treatment approach.One of the unique parameters called speed ratio is emphasized.•Contours of turbulence kinetic energy, dissipation, and viscosity are examined.•The average intensity of turbulent quantities is decreased by increasing the speed ratio.

Analysis of the particle characteristics of solid-liquid turbulent flow by triaxial vibration statistic and machine learning methods

The real-time monitoring of particle information in turbulent flow is critical for safe and efficient production. A new method based on the fusion of particle-wall free collision and triaxial vibration method are proposed to measure particle information in turbulent flow. The characteristic frequency band of the triaxial vibration response excited by the particle-wall collision is successfully characterized by time-domain statistical, Zoom-FFT and STFT analysis, etc. Further, SVM and CNN were used to evaluate the performance of the particle characterization and analysis methods, high recognition accuracy were obtained for whether particles were present (98.48%), the flow velocities (85%), and the particle sizes (96.67%), which also proved the validity of the triaxial vibration response analysis method. Finally, a particle-wall collision energy model expressed as Energycollision=a1∙m+a2∙p2+b1∙m+b2∙p+c1∙m+c2 was established for solid-liquid flow, and the average error rates for each vibration direction were 11.40% (x-direction), 3.17% (y-direction), 13.92% (z-direction) and 7.90% (multidirectional joint).

Experimental investigations of the fire behavior of composites impacted by kerosene/air turbulent flame

In this research, we conducted an experimental analysis of a Kerosene/air burner designed according to the FAA's proposed ISO 2685 standard. The burner serves as a vital tool for generating flames and burnt gases, simulating real-world conditions relevant to fire safety studies. The core objective of this study was to comprehensively characterize the burner's behavior. Through a series of experiments, we investigated the influence of the equivalence ratio on several key parameters, including the spatial distribution of gas temperature (monitored using thermocouples), heat flux (measured with heat flux gauges), and the composition of gas emission species. Notably, our findings revealed a significant correlation between the equivalence ratio and these parameters. Specifically, we observed that as the equivalence ratio increased, so did the flame temperature, heat flux, and heat release rate. This increase continued until a critical equivalence ratio value of 1.03 was reached, resulting in a flame temperature of 1101.5°C and a heat flux of 140 kW/m2. Furthermore, our study encompassed a pyrolysis test on various composite materials, including Carbon-phenolic, Carbon-BMI, and Carbon-PEKK. The comparative analysis of these materials highlighted the superiority of Carbon-PEKK, which exhibited the lowest mass loss, the highest back-face temperature without experiencing significant material delamination, and the lowest concentration of gas emission species. Importantly, our investigation also delved into the influence of the equivalence ratio on the flow's turbulence characteristics, a critical factor in comprehending the burner's performance and its interactions with materials in the context of fire safety scenarios. These findings contribute to a better understanding of combustion dynamics, which is invaluable for enhancing fire safety measures and developing more efficient flame-generating systems.