Tunnel Flow Research Articles

Experimental comparison on the smoke movement in horizontal and downhill tunnel fires under subcritical ventilation

In longitudinally ventilated tunnel fires, subcritical ventilation is widely used in the early stage of accidents to maintain smoke stratification. Due to the stack effect, the smoke movement in inclined tunnels is different from that in horizontal tunnels. In this study, a series of brine-water experiments are carried out to compare the characteristics of buoyant contaminants flow in horizontal and downhill tunnels under subcritical ventilation, which has not been experimentally investigated. The results suggest that subcritical ventilation is an effective longitudinal ventilation strategy in the early stage of horizontal tunnel fires, as it can retain the smoke stratification and an acceptable back-layering flow. Moreover, the critical ventilation condition can be resumed after the ventilation pressure increases to the critical value. However, if subcritical ventilation is first adopted in downhill tunnel fires, no stable back-layering flow will be observed, the buoyant contaminants consistently flow out from the higher exit, ultimately leading to the establishment of a “stack-dominated flow regime”. Under this circumstance, even if the ventilation pressure increases to the critical value or slightly higher, the buoyant contaminants upstream of the source still cannot be pushed back. Moreover, the reduced gravity of the smoke in the tunnel increases as the ventilation pressure rises. Therefore, subcritical ventilation is not suitable for downhill tunnel fires. The study is helpful for understanding the smoke movement in horizontal and inclined tunnel fires under subcritical ventilation.

Investigation of aerodynamic effects of rotation on 6-DOF motion of flat square plate based on free flight experiments

Plate-like debris is common windborne debris in windstorms. Although the 3-degree-of-freedom (3-DOF) motion (two translational DOFs and one rotational DOF) of flat plates has been widely investigated in previous studies, the 6-DOF motion is more likely to occur in real storms. We investigate the 6-DOF motion by performing free flight experiments of a square plate with different initial orientations under uniforms flow in a wind tunnel. Two cameras are used to record the flight motion of the plate. Then the recordings are processed in an image analysis software to estimate the kinematics of the plate. We first investigate the characteristics of the rotational motion. According to the rotational characteristics, the plate trajectories are categorized into four flight patterns: flutter, tumbling, transition, and quasi-translation. Then we explore the rotational effects on the aerodynamics. We propose a hypothesis that the magnitude and direction of the rotational forces depend on the angular velocity and the relative wind velocity. We also propose a hypothesis that the rotational moment has negligible influences on the direction of the instantaneous moment. These hypotheses are supported by the experimental results. These verified hypotheses may be helpful for modelling the aerodynamics of the 6-DOF motion of flat plates.

The Meioflume: A New System for Observing the Interstitial Behavior of Meiofauna.

Meiofauna (benthic invertebrates<1 mm in size) facilitate sediment biogeochemical cycling, alter sediment microbial community structure, and serve as an important trophic link between benthic micro- and macrofauna, yet the behaviors that mechanistically link individuals to their ecological effects are largely unknown. Meiofauna are small and sediments are opaque, making observing the in situ activities of these animals challenging. We developed the Meioflume, a small, acrylic flow tunnel filled with grains of cryolite, a transparent sand analog, to simulate the in situ conditions experienced by meiofauna in an observable lab environment. The Meioflume has a working area (28.57 mm×10.16 mm×1 mm) that is small enough to quickly locate fauna and clearly observe behavior but large enough that animals are not tightly confined. When connected to a syringe press, the Meioflume can produce low velocity flows consistently and evenly across the width of its working area while retaining the contents. To demonstrate its functionality in observing the behavior of meiofauna, we placed individual meiofaunal animals (a protodrilid annelid, a harpacticoid copepod, and a platyhelminth flatworm) in Meioflumes and filmed their behavioral response to a sudden initiation of porewater flow. All animals were clearly visible within the flume and could be observed responding to the onset of flow. The design and construction of the Meioflume make it an accessible, affordable tool for researchers. This experimental system could be modified to address many questions in meiofaunal ecology, such as studying behavior in response to chemical cues, allowing us to observe meiofaunal behaviors to better understand their ecological effects.

Enhanced methodology for thermal management area assessment of metro lines

The article analyzes the operation of metro lines with doubletrack and single-track tunnels in the Russian Federation. Simultaneous activation of traffic on single-track and double-track tunnels affects the management of thermal and humidity parameters of the air environment on metro lines. Currently, there is limited experience in using shared lines. The study conducted an analysis of the features in the formation of thermal and ventilation regimes in double-track and single-track subway tunnels. It also examined the temperature regime of the station when mixing air flows from a double-track tunnel between stations and the flow from one of the stations. An algorithm has been defined for controlling the thermal and humidity parameters of the environment in underground structures. Measures are proposed to minimize the consequences of combining air flows and normalize microclimate parameters. The analysis of air mass movement revealed that the air temperature at the station located beyond the junction point of the double-track tunnel section and single-track tunnels is influenced by circulation flows in single-track tunnels, as well as the flow rate and temperature of the air coming from the double-track tunnel.

Influence of the porosity pattern on the aerodynamics of a square-shaped fly-swatter

The evolution of the normal aerodynamic coefficient of 19 configurations of square plates with various porosity patterns, ranging from solid plate to homogeneous porous plate, is experimentally characterized. The variation of the porosity pattern is obtained by partially covering the holes of a commercial fly-swatter using adhesive tape. Evolution of the normal aerodynamic coefficient is assessed from the measurement of the angular position of the porous plate, placed as a freely rotating pendulum swept by a flow in a wind tunnel. These angular measurements are also supported by particle image velocimetry (PIV) measurements of the structure of the wake. We show that the porosity pattern determines whether or not an abrupt stall occurs. In particular, the details of the porosity pattern on the edges of the plate are decisive for the existence of abrupt stall.

Apatinib has anti-tumor effects and induces autophagy in lung cancer cells with high expression of VEGFR-2.

This study investigated the inhibitory effect of apatinib on lung cancer cells with high expression of vascular endothelial growth factor-2 (VEGFR-2) and on inducing cellular autophagy and drug resistance. The expression of VEGFR-2 was detected using western blotting and RT-PCR. Cell proliferation was measured using the CCK8 and colony formation assays. The cell apoptosis rate was determined using flow cytometry and tunnel assay. Cellular autophagy was detected by measuring the expression of LC3-II using Western blotting and cellular immunofluorescence. The inhibitory effect of apatinib on lung cancer cells and transplanted tumors was observed after treatment with the autophagy inhibitor chloroquine. Apatinib dose-dependently inhibited the proliferation of H1975 and H446 cells; it induced apoptosis via the PARP and caspase-3 pathways in H1975 and H446 cells and effectively inhibited the growth of transplanted tumors. Apatinib induced autophagy in a dose-dependent manner in H1975 and H446 cells. The inhibitory effect of apatinib on cells and the promotion of apoptosis were significantly enhanced after treatment with chloroquine. Immunohistochemistry showed that combining apatinib with chloroquine could reduce the expression of CD31 and Ki67 and increase the expression of caspase-3. Apatinib inhibits proliferation and induces apoptosis in H1975 and H1446 lung cancer cells with high VEGFR2 expression and autophagy in H1975 and H446 cells.

Supersonic Wind Tunnel Flow Characterization Using Planar Laser CO2 Rayleigh Scattering

A complementary experimental and computational study was performed to characterize the test section flowfield of a variable-Mach-number supersonic wind tunnel. A planar laser Rayleigh scattering technique using CO2 nanocrystals was used to capture cross-sectional flow visualizations at various test section streamwise locations. Reynolds-averaged Navier–Stokes simulations of the wind tunnel flow were performed using a k-ω shear stress transport turbulence model to compare against the flow visualizations. Mach 3.20 and 3.55 flow conditions were investigated in the wind tunnel with an empty test section. The Rayleigh signal profile near the wind tunnel walls was compared against boundary-layer thickness data from shadowgraph visualizations, pitot survey measurements, and numerical simulation predictions to guide the interpretation of the scattering results. A brief discussion regarding the nature and potential source of certain unexpected scattering patterns in the images was provided. The most distinguishable flowfield characteristics observed in the Rayleigh scattering visualizations were the asymmetric thickness of the boundary layers along the test section walls and the appearance of two floor lobes near the flow-path corners. The results of the numerical simulations showed favorable agreement with the experimental data and were used to provide insight into the mechanism that caused the boundary-layer features observed. This study demonstrated the usefulness of CO2 Rayleigh scattering to obtain flow visualization data of great value to better understand flow quality in supersonic wind tunnels with two-dimensional nozzles.

On smoke layer thickness in short naturally ventilated tunnels with arched roof

Understanding the characteristics of smoke layer thickness is vital for accurate fire detection, efficient smoke exhaust and safe evacuation in tunnel fire. Nevertheless, smoke flow in tunnels with arched ceiling may show different characteristics from the tunnels with flat roof which is usually studied. Theoretical analysis was conducted on the smoke flow characteristics and thickness under the curved roof of natural ventilation tunnel, and further, numerical simulations were performed to elucidate and verify the mechanism. The results show that smoke mass flow rate is positively correlated with heat release rate(HRR) and section radius. However, the smoke layer thickness is not significantly affected by HRR, and is positively related to the section radius. Based on the mass conservation, a practical prediction model for smoke layer thickness under arched ceiling was derived, and an empirical equation for estimating the total smoke mass flow rate was also proposed. The results presented fairly good agreement with simulations and experimental data from previous works.

CFD-based simulation study of dust transport law and air age in tunnel under different ventilation methods.

To solve the problem of high-concentration dust pollution in a bored tunnel, we conducted a simulation study on the dust transport law and air age of the wind flow in a bored tunnel under different ventilation methods. Air age was innovatively introduced as an index for evaluating tunnel air quality. The results show that dust pollution is serious under conditions of press-in ventilation, which is unfavorable to personnel operations. Following the installation of an on-board dust-removal fan, an effective dust-control air curtain forms in the tunnel, and the high-concentration dust is essentially controlled within the range of Z = 13m from the working face. The dust concentration in the working area on the left side of the tunnel is CD < 200mg/m3, and the dust-control effect is obvious. At the same time, the air age on both sides of the tunnel is reduced by 35.5% following the use of the on-board dust-removal fan. Taking into account dust control by ventilation and dust removal by fan, spraying dust reduction measures are added, and we developed automated wind-mist synergistic wet high-frequency oscillation dust-capturing technology for tunnel boring. This could effectively improve the problem of high levels of coal dust pollution in tunnels.

Water spray effects on fire smoke temperature distribution beneath a symmetrical V-shaped tunnel ceiling

The coupling effects of the double-slope stack effect, the thermal buoyancy induced by the V-shaped tunnel fire and the drag force generated by the water spray system alter fire smoke flow regulation compared to fire smoke flow in a horizontal tunnel. By numerical simulation, this work investigates the effect of water spray flow rate (0 L/min – 600 L/min), atomization angle (0–150°) and the length within fire source and the grade change point (0–120 m) on smoke temperature distribution beneath a symmetrical V-shaped ceiling. The findings indicate that the cooling effect on smoke temperature can be significantly improved by increasing the rate of water spray flow, while water spray increasing angle and tunnel slope have little contribution to the cooling smoke temperature. Thus an improved ceiling smoke temperature distribution model is proposed by importing the flow rate, angle coefficients and tunnel slope. As the fire source distance from the grade change point increases, stack effect on fire smoke flow could be categorized into three zones, which are the double-slope control zone, the transition zone and the single-slope control zone, which causes that downstream smoke temperature become higher gradually than the upstream one, while the smoke temperature difference tends to stabilize at the single-slope control stage. Finally, a global ceiling smoke temperature distribution in the symmetrical V-shaped tunnel installed a water spray system is established by introducing fire source distance coefficients.

Martian flow characterization using tunable diode laser absorption spectroscopy, in high enthalpy facilities

This research aims at analyzing thermo-chemical properties of the hypersonic high-enthalpy flow in the L2K wind tunnel, situated in Köln at the German Aerospace Center (DLR). In the L2K wind tunnel, Martian atmosphere can be created, and the facility can simulate heat load conditions encountered during atmospheric entry of Martian missions. The focus of this project is the analysis of the non-intrusive experimental technique “Tunable Diode Laser Absorption Spectroscopy” (TDLAS), based on line of sight absorption spectroscopy, and applied to hypersonic flow. A simplified Martian atmosphere (97% CO2 and 3% N2) was used. A new interpretation for CO-TDLAS experimental technique applied to hypersonic wind tunnel flow analysis was developed. Numerical simulations with the DLR-TAU non-equilibrium flow solver were used as support of this analysis, and match between simulations and experiments was observed. Flow speed and absorption line’s width were measured, and the knowledge of L2K’s flow structure was extended.

Closure to “Numerical investigation of air demand by the free surface tunnel flows” by WANGRU WEI, JUN DENG and WEILIN XU, J. Hydraulic Res. 59(1), 2021, 158–165, https://doi.org/10.1080/00221686.2020.1744747

Closure to “Numerical investigation of air demand by the free surface tunnel flows” by WANGRU WEI, JUN DENG and WEILIN XU, J. Hydraulic Res. 59(1), 2021, 158–165, https://doi.org/10.1080/00221686.2020.1744747

Improving the technology for processing the aggregated data flow of a secure corporate multiservice communication network

This paper considers the process of dynamic reservation of the channel resource of a secure corporate multi-service communication network. It has been established that the processes of building and functioning of the schemes of a secure corporate multi-service communication network and improving the quality of the implementation of its main work processes involve the evaluation and dynamic reservation of channel resources for incoming aggregated data flows of the network. The model of dynamic reservation of the channel resource of the aggregated data stream of the secure corporate multi-service communication network was built and proposed. The proposed model makes it possible to set the quantitative values of the reserved channel resource for different service methods depending on the number of component flows in the total aggregated data flow of the VPN tunnel. It was established that an increase in the density of the aggregated data stream requires an increase in the reserved channel resource. At the same time, its value is influenced by the way of servicing the aggregated data flow in the VPN tunnel of the secure corporate multi-service communication network. Application of the isolated service method gives a gain in the allocated resource for the channel reserve from 10 to 20 percent compared to the group service method for IR and video telephony. This is due to the more flexible management process of the border router's incoming data storage buffer under the isolated service mode. The model of dynamic reservation of the channel resource of the secure corporate multi-service communication network reported in this paper could be used in the improvement of existing and development of new structures of the secure corporate multi-service communication network. The consequence of such an improvement is a reduction in the delay time for the processing of incoming data packets in the specified network

Dynamic Parameter Calibration of an Analytical Model to Predict Transient Groundwater Inflow into a Tunnel

The use of either analytical or numerical models of groundwater inflow into a tunnel, ignoring the excavation process, will result in inaccurate prediction. More researchers have begun to study prediction methods with temporal and spatial variables, in accompaniment with the tunnel excavation progress. The hydrogeological parameters significantly affect tunnel inflow prediction; thus, it is always worthy to carry out an inversion analysis when the inflow rate observation is available for the excavated part of a tunnel. Based on a transient analytical model and the Trust Region Reflection (TRR) algorithm, this paper proposes a Dynamic Parameter Calibration (DPC) method to sequentially optimize parameters for a tunnel that is divided into several sectors. The results of two case studies indicate that the fitting effects by DPC are significantly improved compared with the empirical trial-and-error method, indicating good optimizations. The proposed scheme can conduct calibration simultaneously for multiple tunnel sectors and also several optimizations for the same sector. The parameter optimization results reflect the lithologic heterogeneity of different strata. Parameter sensitivity analysis proves that the hydraulic conductivity K has a greater influence on water inflow calculation than the specific storage coefficient S.

Universal Velocity Statistics in Decaying Turbulence.

In turbulent flows, kinetic energy is transferred from large spatial scales to small ones, where it is converted to heat by viscosity. For strong turbulence, i.e., high Reynolds numbers, Kolmogorov conjectured in 1941 that this energy transfer is dominated by inertial forces at intermediate spatial scales. Since Kolmogorov's conjecture, the velocity difference statistics in this so-called inertial range have been expected to follow universal power laws for which theoretical predictions have been refined over the years. Here we present experimental results over an unprecedented range of Reynolds numbers in a well-controlled wind tunnel flow produced in the Max Planck Variable Density Turbulence Tunnel. We find that the measured second-order velocity difference statistics become independent of the Reynolds number, suggesting a universal behavior of decaying turbulence. However, we do not observe power laws even at the highest Reynolds number, i.e., at turbulence levels otherwise only attainable in atmospheric flows. Our results point to a Reynolds number-independent logarithmic correction to the classical power law for decaying turbulence that calls for theoretical understanding.

Experimental and numerical study on the effects of sweep angle on cavitation around a wedge-section hydrofoil

The influence of sweep angles on cavitation characteristics and mechanisms of a wedge-section hydrofoil is investigated experimentally and numerically. Four hydrofoils with sweep angles of 0° (straight), 30°, 45°, and 60° are considered across a range of cavitation numbers from 2.00 to 0.48 and angle of attack (AOA) of 0°, 5°, 10°, and 15°. Two high-speed cameras are used to visualize the cavitation flow in a high-speed cavitation tunnel. The numerical simulation is conducted using unsteady Reynolds-averaged Navier–Stokes equations through OpenFOAM. At an AOA of 0° and 5°, vortex cavitation first appears in the wake region of all models at a cavitation number of 0.98. However, at higher AOA values of 10° and 15°, tip-vortex cavitation (TVC) begins first for the straight foil, and this is followed by sheet and wake cavitation. In contrast, the swept foil does not succumb to TVC. Instead, as the sweep angle increases, sheet cavitation develops into root leading-edge vortex cavitation (LEVC). The inclination angle of the LEVC from the leading edge is observed to be between 6° and 15°, depending on the sweep angle, and it is independent of the cavitation number and AOA. The wake vortex changes from eddy vortex shedding at the wake region of the straight hydrofoil into two root trailing-edge vortices as the sweep angle increases. The swept hydrofoil reduces the average cavity volume by more than 45% compared with the straight foil. The lift coefficient of the straight hydrofoil increases as sheet cavitation is generated and reaches a maximum value of 0.6 when cavitation covers the suction side of the hydrofoil before dropping sharply when it extends to the wake region. However, there is only minor deterioration in the lift coefficient of the 60° swept-angle foil when the cavitation occurs. The drag coefficient reduces when cavitation forms for both foils. However, the drag coefficient of the swept hydrofoil is lower than that of the straight foil. These findings offer valuable insight into the design and optimization of foils for various applications where cavitation affects their performance and stability.

A new insight into wormhole propagation prediction in carbonate matrix acidizing based on experimental approaches

Carbonate acidizing is a common practice for production/injection enhancement in damaged oil/gas wells by creating conductive flow tunnels called wormholes. Previous linear acid flood studies are mostly performed on core plugs limited to the Darcy scale with a strong dependency on lab results for upscaling techniques. In this study, a novel scale-free approach of wormhole propagation inside linear porous media for small to very large core diameters is introduced. The model associates the wormhole tip velocity and wall fluid loss to the injection rate using novel flow correlations in terms of core diameter and wormhole geometries (wormhole density, length and diameter). These correlations are constructed based on the results obtained from a large number of 3D-FEM (Finite Element Method) simulations of water flow utilizing the porous rock with predefined dominant cylindrical wormholes. The application of the model is easy and quick without complex or time-consuming calculations even for large rock diameters up to 28 in (71 cm) which simultaneously predicts the acid response curve by matching with the acid coreflood experiments on specific acid/rock systems. Also, the output matching parameters for each acid/rock system can be directly used for larger core diameters without extra-large scale acid flood experiments. In the experimental parts of this study, acid flooding is performed on various rock types of four Iranian carbonate reservoir layers to find their corresponding acid response curves and optimum flow conditions required to design stimulation programs. Besides, the collected data bank could help to enrich the data pool required to validate the developed model practically. The proposed model is validated by literature model results and also experimental data obtained in our laboratory and those presented in the literature and the results showed that good agreement is obtained in regression analysis with R2 = 0.953, MSE = 0.046452, RMSE = 0.215528 and MAPE = 24.11357.

Flow field control for 2-meter high-speed free-jet wind tunnel

The 2-meter High-speed Free-jet Wind Tunnel (2 m HFWT) is China's first large-scale open-jet trisonic wind tunnel. Compared to traditional closed high-speed wind tunnels, this wind tunnel is endowed with remarkable advantages of ample test chamber space, less interference from the tunnel wall, flexible model support mode, and adjustable continuous variation of the Mach number. Nevertheless, its unique structure makes traditional wind tunnel control methods difficult to apply and brings significant challenges to wind tunnel flow field control. In this paper, a flow field control system is designed for the 2 m HFWT by comprehensively using advanced control technologies such as neural network, gain scheduling, feedforward control, and adaptive control. Through practical application tests, it is proved that the proposed control system successfully solves the problem of high-precision flow field control under continual depletion of storage tank pressure, and realizes distinctive functions of adaptive static pressure matching and continuously varying Mach number at supersonic speed. In addition, due to the application of workflow technology, the flow field control system can flexibly adapt to the implementation of tests of different types and operation conditions, thus fully satisfying the needs of conducting various conventional and special tests in the 2 m HFWT.

Comparing a 3-d printed hemispherical-head and Rankine-body probe shapes for very low speed flush air data system (FADS) measurements

This study investigates the feasibility of using Flush Air Data Sensing (FADS) System technology for air data measurements at the very low-airspeeds, where many Unmanned Aerial Vehicles (UAVs) operate. FADS is a non-intrusive alternative to pitot probes, where the vehicle nosecone, wing leading edge, or other aerodynamic surfaces are configured with multiple pressure-ports distributed along the windward face. Although FADS technology has been used for a variety of high-speed aircraft, FADS has never been applied to very low-airspeed flight regimes. This study reports on wind tunnel tests of two 3-D printed shapes: 1) a cylindrical body with a hemispherical head, and 2) a Rankine-Body. These body shapes can act as a vehicle analog to a wide range of three-dimensional shapes and account for both blunt leading edge and trailing after body flow characteristics. For this study the "probes" were printed with 5 pressure ports and the associated flow channels aligned at 0o, +22.5o and +45o direction-angles along the vertical centerlines of the models. Sensed pressure data were curve-fit, developing quasi-potential flow calibration models for each probe, with coefficients compiled as a function of geometric angle-of-attack and tunnel airspeed. The calibration models account for end-to-end systematic effects, including the mounting sting flow compression, up wash, and tunnel blockage. Using the derived calibration models and the sensed pressure data, the effective angles-of-attack were re-calculated using the well-known "Triples" algorithm. The associated airspeed and dynamic pressure are estimated from the sensed pressure data using non-linear regression. The resulting estimates are compared to the tunnel reference conditions. Generally, both probe shapes performed well, with the redundant 5-port arrangement allowing for significant noise rejection. Both probes achieved RMS airspeed errors of less than 5%, angle-of-attack errors less than 1 deg., and dynamic pressure errors of less than 12 pascals, across airspeeds ranging from 5 to 25 m/sec. The sensed Airdata measurements at the lowest airspeeds (5 m/sec), exhibited similar accuracy to those sensed at the highest airspeeds (25 m/sec), verifying the applicability of FADS technology to very low airspeed flight regimes.

Semi-empirical models for predicting stable water inflow and influence radius of a tunnel considering non-Darcian effect

How to accurately predict water inflow and relevant groundwater influence of a tunnel are classical scientific issues in the field of hydrogeology, which are also of great significance to the construction and operation of underground engineering. This issue is usually investigated based on the assumption of the linear Darcy's law. However, the tunnel inflow is usually featured by high velocities or high pressures, especially in karst regions, which allows the flow regime to easily develop into non-Darcian flow regime. The impact of the non-Darcian effect on predicting water inflow and influence radius of a tunnel has not yet been systematically examined. Here, we numerically investigated the prediction of stable water inflow and influence radius under non-Darcian flow regime. By numerically solving the continuity equation of seepage flow incorporating the nonlinear Forchheimer law, a series of numerical simulations were conducted for stable water inflow into a tunnel by varying key controlling parameters. The quantitative dependencies of stable water inflow on these parameters were figured out, based on which a semi-empirical stable water inflow model was developed considering the non-Darcian effect. Meanwhile, the magnitude of the influence radius was investigated by statistical analysis of 59 actual engineering cases and estimated using existing empirical formulas. Based on these preliminary explorations, a semi-empirical model for estimating influence radius was concluded from massive simulations. The validities of our proposed semi-empirical models for stable water inflow and influence radius were further numerically verified. Comparative study and application in a real engineering case showed that compared with our proposed models, most existing empirical methods predicted a larger stable water inflow and a smaller influence range of groundwater. Besides, comparative results of proposed models with omitting non-Darcian term indicated that neglecting non-Darcian effect would overestimate both the stable water inflow and influence radius. These findings and established models would be useful for assessing stable water inflow and influence radius of a tunnel under non-Darcian flow regime.