Turbulence Enhancement Research Articles

Large-eddy simulation of tip clearance cavitating flow around a hydrofoil

Large-eddy simulations of tip clearance cavitating flow between a National Advisory Committee for Aeronautics 0012 hydrofoil and the endwall have been performed to investigate the effects of cavitation on vortical structures, turbulence statistics and hydrofoil performance within the tip clearance region. The wall-adapting local eddy-viscosity model is employed to compute the subgrid-scale stress, and the Zwart-Gerber-Belamri cavitation model is used to predict the cavitating flow. The simulation results show that cavitation has insignificant effects on the tip clearance flow structures and the wandering motion of the tip leakage vortex (TLV). Detailed analysis of turbulence statistics indicates that the turbulent kinetic energy and pressure fluctuation inside the TLV core are significantly increased under the effect of cavitation. In addition, it is found that cavitation enhances the unstable wall-normal motion of the TLV, leading to the enhancement of turbulence near the TLV core. The hydrofoil performance affected by cavitation is also investigated, showing that the lift and drag forces acting on the hydrofoil is substantially reduced due to the cavitation.

Influence of swirl, tumble and squish flows on combustion characteristics and emissions in internal combustion engine- review

This study gives an overview of available literature on flow patterns such as swirl, tumble and squish in internal combustion engines and their impacts of turbulence enhancement, combustion efficiency and emission reduction. Characteristics of in-cylinder flows are summarized. Different design approaches to generate these flows such as directed ports, helical ports, valve shrouding and masking, modifying piston surface, flow blockages and vanes are described. How turbulence produced by swirl, tumble and squish flows are discussed. Effects of the organized flows on combustion parameters and exhaust emission are outlined. This review reveals that the recent investigations on the swirl, tumble and squish flows are generally related to improving in-cylinder turbulence. Thus, more experimental and numerical studies including the impacts of this organized flows on turbulence production, combustion behavior and pollutant formation inside the cylinder are needed.

Reduction or Enhancement of Stellarator Turbulence by Impurities

A systematic study of the impact of impurities on the turbulent heat fluxes is presented for the stellarator Wendelstein 7-X (W7-X) and, for comparison, the Large Helical Device and ITER. By means of nonlinear multispecies gyrokinetic simulations, it is shown that impurities, depending on the sign of their density gradient, can significantly enhance or reduce turbulent ion heat losses. For the relevant scenario of turbulence reduction, an optimal impurity concentration that minimizes the ion heat diffusivity emerges as a universal feature. This result demonstrates the potential of impurities for controlling turbulence and accessing enhanced confinement regimes in fusion plasmas and, in particular, in W7-X. Published by the American Physical Society 2024

Investigation of turbulence and interfacial exchange features of the gap area within the fully developed Shallow-Submerged canopy flow

The flow patterns within a longitudinal gap area formed by discontinuous distributions of submerged canopy, as well as the momentum and mass exchange characteristics between the gap area and the overlying free-flow, were studied using high-resolution Large Eddy Simulation (LES). The gap area is located within the fully developed region of submerged canopy flow. The simulations considered four aspect ratios of the gap area, L/h (ranging from 1 to 4), where L and h represent the span of the gap area and the height of the canopy, respectively, and two canopy densities, ϕ = 0.08 and 0.15. Results indicate that recirculation vortices appear only at ϕ = 0.15 and penetrate the downstream canopy patches to varying extents, with the distance of invasion decreasing as L/h increases. Influenced by the recirculation vortices, the bed shear stress in the downstream part of the gap area and the initial section of the downstream canopy patch is significantly increased compared to the fully developed region of the upstream canopy patch. Within the parameter range covered, vertical penetration of the mixing layer into the gap area always occurs, consistently falling into the shear layer growth regime, with significant enhancement of turbulence within the gap area even at L/h = 1. The high momentum entrainment elevates longitudinal velocity and turbulence intensity in the upper corner regions of the downstream canopy patch’s leading edge, combined with localized increases in bed shear stress, potentially destabilizing plants at the forefront of the downstream canopy patch. Although the total momentum exchange across the interface between the gap area and the upper free-flow layer is approximately independent of L/h, larger canopy densities lead to stronger momentum transport, and turbulent transport always dominates. Mass exchange generally increases with L/h, with more efficient vertical mass exchange in ϕ = 0.15 cases at L/h = 3 and 4.

Oscillation mechanism and predictive model of explosion load for natural gas in confined tube

Gas explosion in confined space often leads to significant pressure oscillation. It is widely recognized that structural damage can be severe when the oscillation frequency of the load resonates with the natural vibration frequency of the structure. To reveal the oscillation mechanism of gas explosion load, the experiment of gas explosion was conducted in a large-scale confined tube with the length of 30 m, and the explosion process was numerically analyzed using FLACS. The results show that the essential cause of oscillation effect is the reflection of the pressure wave. In addition, due to the difference in the propagation path of the pressure wave, the load oscillation frequency at the middle position of the tunnel is twice that at the end position. The average sound velocity can be used to calculate the oscillation frequency of overpressure accurately, and the error is less than 15%. The instability of the flame surface and the increase of flame turbulence caused by the interaction between the pressure wave and the flame surface are the main contributors to the increase in overpressure and amplitude. The overpressure peaks calculated by the existing flame instability model and turbulence disturbance model are 31.7% and 34.7% lower than the numerical results, respectively. The turbulence factor model established in this work can describe the turbulence enhancement effect caused by flame instability and oscillatory load, and the difference between the theoretical and numerical results is only 4.6%. In the theoretical derivation of the overpressure model, an improved model of dynamic turbulence factor is established, which can describe the enhancement effect of turbulence factor caused by flame instability and self-turbulence. Based on the one-dimensional propagation theory of pressure wave, the oscillatory effect of the load is derived to calculate the frequency and amplitude of pressure oscillation. The average error of amplitude and frequency is less than 20%.

A novel concept for performance enhancement of immersion-cooled data center servers

Efficient thermal management of highly densified data centers is important for carbon-neutral strategies. To this end, this work experimentally explored the most promising single-phase immersion cooling (SPIC) systems. Moreover, it proposed an active performance enhancement technology for SPIC systems. The results indicate that increasing the coolant immersion height helps to improve the SPIC performance, and the resulting increase in flow resistance is negligible. Increasing the volume flow rate improves heat transfer performance while also increasing flow resistance. Moreover, there exists a critical volume flow rate (i.e., 8 L/min) beyond which fails to significantly enhance the SPIC performance. The cooling water temperature is positively correlated with the device temperature but negatively correlated with flow resistance. The micropump-based flow turbulence enhancement and the fan-based flow redistribution techniques reduce the maximum CPU surface temperature by 6.1 % and 5.0 %, respectively. Compared to the micropump method, the fan-based technique degrades the thermal uniformity of coolants but does not significantly increase flow resistance. The economic benefit of the micropump-based strategy is greater for upstream CPUs than downstream CPUs, while the fan-based method is more economically advantageous for downstream CPUs. Moreover, the micropump-based strategy outperforms the fan-based method and demonstrates a 21.39 % increase in energy-economic performance.

Large eddy simulation of turbulent transport and thermal enhancement in a steam-cooled ribbed channel

Pulsating flow, as an active heat transfer enhancement technique, is expected to further improve thermal performance. To demonstrate the potential value of pulsating flow in blade cooling, a numerical investigation is conducted on the flow and heat transfer in a steam-cooled ribbed channel under steady and pulsating inflow at Re = 12000. LES, based on OpenFOAM-8, is employed and precursor simulation method is used to generate fully developed turbulent inflow. Three operating conditions, steady inflow (StF = 0), low frequency pulsation (StF = 0.02) and medium frequency pulsation (StF = 0.08), are studied. The features under steady inflow condition (StF = 0) are firstly discussed, indicating that, due to the entry effect, the ribbed channel can be divided into two regions with different vortex motion behaviors. The ejections induced by vortex evolution enhance turbulent and energy transport between the sidewall and mainstream. Two cases of pulsating inflow are then carried out, finding that stronger ejections appear and low heat transfer area is significantly improved. Corresponding to low frequency pulsation (StF = 0.02) and medium frequency pulsation (StF = 0.08), the Nusselt number has an increase of 3.511 % and 14.242 %, respectively, and friction factor has a rise of 3.934 % and 8.253 %, compared with steady inflow. The thermal performance factor is increased by 11.262 % at StF = 0.08, indicating better cooling performance than the rise of 2.188 % at StF = 0.02. It is suggested that pulsating flow with medium frequency pulsation has positive performance in a steam-cooled ribbed channel.

Experimental performance analysis of methanol adsorption in granular activated carbon packed bed through design of a double pipe heat exchanger with longitudinal fins

Activated carbon (AC) is essential for its exceptional adsorption properties and high surface area in various applications. Adsorption/desorption (A/D) processes with AC can be exothermic or endothermic, necessitating improved heat exchanger (HE) designs due to AC's low thermal conductivity. This study presents an A/D HE design with longitudinal fins in the sorption bed to enhance heat transfer during methanol A/D in AC. Bed heating/cooling uses water flowing through pipes inside the HE. Experimental investigations focus on thermal characteristics and A/D efficiency under diverse conditions, comparing results to a base non-finned HE design. Various experiments measured HE shell temperatures at different water flow rates while varying the sorption bed condition (vacuumed, filled with air, or methanol). The finned HE design showed a 1.8 °C increase in maximum bed surface temperature and a 3-h, 40-min reduction in process time compared to the non-finned HE. Subsequent tests with the finned HE at different water flow rates (5, 10, and 15 l/min) demonstrated superior heat transfer at 15 l/min due to turbulent flow enhancement. Additionally, adsorption ceased at a bed temperature of 54 °C. These findings highlight the efficacy of finned HE for optimizing methanol A/D processes, offering insights into improving heat transfer in AC-based systems.

Simulation of the effect of the structure and location of non-closed droplet microchannel on flow/ thermal performance

In order to investigate the effects of geometric parameters and flow modes on the fluid flow morphology, resistance and heat transfer effect in the non-closed droplet pin fin array, in this manuscript, a numerical simulation has been conducted under Re = 100–700 with different pin fin heights (Hp = 0.3 mm, 0.5 mm, 0.7 mm) and pin fin densities (β = 0.046, 0.059, 0.072), with different flow directions under heat flux (10 W/cm2 – 50 W/cm2) studied as well. It is discovered that increasing the height and density of the non-closed droplet pin fin increases the interference with the fluid, resulting in a greater flow resistance phenomenon. However, the enhancement of turbulence not only exacerbates the flow instability in microchannels, but also further promotes the mixing of hot and cold fluids, making the internal temperature distribution more uniform. In which way, the effect of optimizing heat transfer performance achieves. In addition, as q > 20 W/cm2, the overall heat transfer capacity of flow direction B (the tip of the pin fin as upstream fluid direction) is better than that of flow direction A (the cavity of the pin fin upstream as upstream fluid direction). When q = 30 W/cm2, there is a process transferring from single-phase flow to two-phase flow along with the direction of working fluid flow in the microchannel. Under this condition, the local maximum heat transfer coefficient is 15,502.4 W/(m2·K) in flow direction A and 16,094.8 W/(m2·K) in flow direction B. Furthermore, the comprehensive evaluation concludes that the channel flow/thermal performance is the best with Hp = 0.7 mm, β = 0.046.

Experimental and numerical investigation on turbulent convection enhancement in minichannel heat sink with staggered V-shaped pin fins

With the continuous advancements in new energy vehicle technology, the insulated gate bipolar transistor (IGBT) thermal properties have a direct impact on power system reliability. In this article, a minichannel heat sink with staggered V-shaped pin fins (SVPF) is investigated through both experimental and numerical approaches. Initially, the heat transfer performance of SVPF and conventional minichannel heat sink was analyzed using the CFD numerical simulation method. Subsequently, numerical and experimental validation was carried out by assessing the Nusselt number and turbulence characteristics of SVPF. Finally, an in-depth analysis was conducted to investigate the effects of SVPF geometric parameters on the turbulent characteristics and heat transfer performance of the minichannel heat sink. The results demonstrate that SVPF exhibits superior hydrothermal performance, which could maintain better heat transfer performance while simultaneously reducing the friction factor of minichannels. These findings provide valuable insights for the design of three-dimensional pin fins minichannels heat sink.

Numerical analysis of turbulent flow and heat transfer enhancement using V-shaped grooves mounted on the rotary kiln’s outer walls

Rotary kilns have been widely employed in various industrial uses, especially the cement production. This article deals with enhancing the thermal performance of a rotary kiln duct with V-shaped grooves mounted on the outer wall. Four V-shaped grooves with different depths h/D ranging from 0.1 to 0.4 were designed. The Reynolds Averaged Navier–Stokes equations (RANS) of two-dimensional steady-state flow are used to model the governing flow equations by using the finite volume approach (FVM) in FLUENT. k-ε standard, k-ε Realizable, k-ω SST and k-ε RNG turbulence models of the RANS approach and the k -ω SST model has been adopted to validate CFD results. In this study, the numerical results have revealed that the increase in groove depth decrease the temperature of the rotary kiln’s outer wall than the smooth walls and gives the largest Nu number, especially for the groove with h/D =0.3 and 0.4 depths.

Turbulence enhancement in body force opposed flows

Turbulence may be strongly influenced by near-wall nonuniform body forces. A widely encountered example is buoyancy acting in mixed convection flows. Here we use direct numerical simulations to present two key ideas: First we show that introducing opposing body forces whilst maintaining the pressure gradient does not cause key turbulence characteristics to change. It follows that the concept of “apparent Reynolds number” introduced by He [] to explain laminarization can now be extended to explain the opposite phenomena of turbulence enhancement by opposing body forces. This new understanding has allowed turbulent shear stress and skin friction to be easily predicted simply from the nonuniform body force profile and reference data despite the nonequilibrium nature of the flow. Second, we analyze the flow in the context of turbulence regeneration cycle and provide evidence that complements the recent study from Jiménez [], which suggests that streaks are more decoupled from the regeneration cycle than previously thought. Published by the American Physical Society 2024

Gyrokinetic simulation of pedestal degradation correlated with enhanced magnetic turbulence in a DIII-D ELMy H-mode discharge

Gyrokinetic simulation of a dedicated pedestal density ramping-up discharge on DIII-D can reproduce the enhancement of magnetic turbulence in the pedestal, which is identified to be caused by micro-tearing modes (MTMs). An increase of MTM amplitude results in higher electron thermal diffusivity, consistent with experimentally observed lower electron temperature gradient and degraded pedestal height. Gyrokinetic simulation identifies the major cause of MTM enhancement to be the increase of collisionality, which has a significant impact on the MTM intensity and is beyond the description of any (quasi-)linear theory.

Thermal-hydraulic performance of printed circuit heat exchangers with various channel shapes under rolling conditions

The floating liquefied natural gas (LNG) facilities are located in deep oceans, where the harsh ocean conditions cause the facilities to sway and tilt. The rolling conditions in floating LNG systems affect the uniformity of gas–liquid distribution in heat exchangers. Thermal and hydraulic performances of printed circuit heat exchangers (PCHEs) with straight, zigzag, and trapezoidal channels are investigated under static and rolling conditions. The results reveal that all channels enhance heat transfer near the pseudo-critical point of LNG at a rolling period of 2 s and a rolling amplitude of 15°. The Nusselt number for the PCHE with a zigzag channel increases by 32.2% and 72.5% compared to those with straight and trapezoidal channels. The highest increase in comprehensive performance is obtained for the zigzag channel with an evaluation index of 1.23 under the rolling condition. Compared with under the static condition, the Nusselt number and Fanning friction factor for the zigzag channel increase by 12% and 28% under the rolling condition. The thermal performance is weakened by the nonuniform flow velocity and improved by the enhancement of flow turbulence. The thermal–hydraulic performance increases with the rolling period from 1 s to 3 s and the rolling amplitude from 15° to 45°. The maximum improvement of 65.3% and 97.2% in Nusselt number and Fanning friction factors is observed at a rolling period of 1 s and a rolling amplitude of 45°. The methods to suppress the deterioration of heat transfer in microfluidic channels under rolling conditions are proposed to satisfy the requirement of LNG with low-resistance and high-efficiency microfluidic structure.

Experimental and numerical study on flow field characteristics of a combustion chamber with double-stage counter-rotating swirlers

To discover a good predictive turbulent model and reveal the mechanism of turbulent mixing enhancement, a new combustion chamber equipped with a double-stage counter-rotating swirler with vane angles of both stages as 45° is proposed and the flow field characteristics are explored by combining 2D-PIV experiment and Reynolds averaged numerical simulation (RANS). Firstly, the performance of various turbulence models in predicting the flow field characteristics is evaluated by comparing them with experimental results. The study concludes that the RNG k-ε turbulence model exhibits superior performance in predicting velocity distribution, recirculation zone length, and vorticity distribution. Hence, it is selected to analyze the influence of the Reynolds number (Re) of swirlers on the flow field characteristics. In this study, seven Re numbers ranged from 5425 to 54,245 were selected for analysis. Through comparison, it appears that changes in Re number have a strengthening effect on the flow field at various locations, including the recirculation zone, wall surface, and shear layer. This suggests that Re number has an influence on turbulence or boundary layer behavior. Interestingly, there seems to be a critical value for Re number of 21,698, which may be related to a competition between Re number and wall effects.

Mechanisms and characteristics of wall skin friction reduction by boundary layer injection under hypervelocity inflow conditions

In this paper, the influences of boundary layer injection on skin friction reduction are numerically studied using Reynolds-averaged Navier-Stokes equations. The methodology is first validated by comparing the numerical results with the existing experimental data. Studies on the effects of inert gas boundary layer injection reveal that boundary layer injection forms a gas film cooling effect, which reduces skin friction through low viscosity and low temperature effects. However, the enhancement of turbulence in the boundary layer by the jet weakens this effect For fuel boundary layer injection, boundary layer combustion creates a low-density environment in the boundary layer, reducing the momentum transport of the fluid to the wall and significantly enhancing the drag reduction effect. Based on this, the drag reduction effect of boundary layer combustion is found to decrease significantly under mainstream heat release conditions, mainly due to the large-scale heat release in the outer edge of the boundary layer, which puts the fluid in a high-temperature and high-pressure state as a whole, and boundary layer combustion cannot establish a “barrier” for momentum transport in a low-density region.

Effect of Small-Scale Topographical Variations and Fetch from Roughness Elements on the Stable Boundary Layer Turbulence Statistics

Understanding the influence of roughness and terrain slope on stable boundary layer turbulence is challenging. This is investigated using observations collected from October to November of 2018 during the Stable Atmospheric Variability ANd Transport (SAVANT) field campaign conducted in a shallow sloping Midwestern field. We analyze the turbulence velocity scale and its variation with the mean wind speed using observations up to 10–20 m on four meteorological towers located along a shallow gully. The roughness length for momentum over this complex terrain varied with wind direction from 0.0049 m to a maximum of 0.12 m for winds coming through deciduous trees present in the field. The variation of the turbulence velocity with wind speed shows a transition from a weak wind regime to a stronger wind regime, as reported by past studies. This transition is not observed for winds coming from the tree area, where turbulence is enhanced even for weak wind speeds. For weak stratification and stronger winds, the turbulent velocity scale increased with an increase in roughness while the terrain slope is seen to have a weak influence. The sizes of the dominant turbulent eddies seen from the vertical velocity power spectra are observed to be larger for winds coming through the tree area. The turbulence enhancement by the trees is found to be strong within a fetch distance of 7 times the tree height and not observable at 16 times of the tree height.

Large‐eddy simulation of plume dispersion in a turbulent boundary layer flow generated by a dynamically controlled recycling method

AbstractWhen conducting large‐eddy simulations (LESs) of plume dispersion in the atmosphere, crucial issue is to prescribe time‐dependent turbulent inflow data. Therefore, several techniques for driving LESs have been proposed. For example, in the original recycling (OR) method developed by Kataoka and Mizuno (Wind and Structures, 2002, 5, 379–392), a mean wind profile is prescribed at the inlet boundary, the only fluctuating components extracted at the downstream position are recycled to the inlet boundary. Although the basic turbulence characteristics are reproduced with a short development section, it is difficult to generate target turbulent fluctuations consistent with realistic atmospheric turbulence. In this study, we proposed a dynamically controlled recycling (DCR) method that is a simple extension of the OR procedure. In this method, the magnitude of turbulent fluctuations is dynamically controlled to match with the target turbulent boundary layer (TBL) flow using a turbulence enhancement coefficient based on the ratio of the target turbulence statistics to the computed ones. When compared to the recommended data of Engineering Science Data Unit (ESDU) 85020, the turbulence characteristics generated by our proposed method were quantitatively reproduced well. Furthermore, the spanwise and vertical plume spreads were also simulated well. It is concluded that the DCR method successfully simulates plume dispersion in neutral TBL flows.

Experimental study on turbulent characteristics of dispersed bubbly flow in a narrow rectangular channel by two-phase PIV

Narrow rectangular channels are widely used in high heat flux applications, due to the high heat transfer efficiency and compact structure. However, a rare understanding was known about the two-phase turbulent characteristics. In this study, an experimental investigation of bubble and turbulent characteristics of dispersed bubbly flow in the narrow rectangular channel was carried out by using the high-speed camera and two-phase particle image velocimetry. The bubble size effect was studied independently using the continuous spectrum bubble generator. According to the results, the bubble size distribution conforms to a Weibull distribution, and the wall-peak void fraction distributions were found. Both turbulent enhancement and suppression were observed in current conditions. Large bubbles enhance turbulence intensity, while small bubbles suppress it. The eddy viscosity ratio ϕ characterizes the modification of turbulence intensity by bubbles, with a critical value of ϕ=1.

Particle-resolved direct numerical simulation of particle-laden turbulence modulation with high Stokes number monodisperse spheres

Graphic processing units (GPU)-accelerated direct numerical simulations of particle–fluid systems based on lattice Boltzmann method combined with discrete element method are successfully performed. This method is validated to be accurate and efficient in the four-way coupling simulation of particle–fluid systems. Particle-laden homogeneous isotropic turbulence under particle size dp=30.1η (the Kolmogorov length scale), a wide range of solid volume fraction ϕV=2%−20% and particle Stokes number St=503.87−50 387 are systematically studied. The additional energy dissipation rate εaddi induced by particles is quantitatively calculated. One criterion for turbulence enhancement or attenuation based on energy balance is proposed and successfully used in different particle-laden turbulence. The relative magnitude of two-way coupling rate ψ and particle-induced dissipation εaddi is the direct factor determining the modulation effect on turbulence. Turbulence laden with high Stokes number particles is enhanced. Turbulence laden with high volume fraction particles transited rapidly from enhancement to attenuation, which is attributed to the more rapid decay of ψ relative to εaddi in the freely decaying turbulence.