High-speed Flow Research Articles

Numerical modeling of the non-equilibrium condensation in multi-component flows through the high expansion ratio nozzles

Excessive flow expansion in the thruster nozzles reduces the flow temperature and pressure. This provides conditions for the condensation phenomena in the nozzle, which can affect the thrust performance of thrusters. In this paper, the condensation effect of water components in the decomposition products of hydrogen peroxide in a typical thruster nozzle is modeled. To describe the state and properties of the real gas mixture, the Peng-Robinson equation of state is used. Moreover, the effects of H2O2 concentration and nozzle profile on the condensation and nozzle performance parameters are studied. The results show that by more flow expansion and crossing the saturated state of condensable components, the thrust coefficient improves up to 6%. This improvement decreases by increasing the H2O2 concentration in the reaction chamber for a fixed nozzle expansion ratio. Due to the small growth of droplets during the free molecular regime of the droplet growth in the high-speed flow, the non-equilibrium condensation continues until the nozzle outflow. In addition, although all the profiles have the same expansion ratio for nucleation start, increasing the nozzle length results in larger nano-droplets, more liquid mass fraction formation up to 4%, and moving the Wilson point to the further upstream sections in the nozzle.

A particle-resolved direct numerical simulation method for the compressible gas flow and arbitrary shape solid moving with a uniform framework

Compressible particle-resolved direct numerical simulations (PR-DNS) are widely used in explosion-driven dispersion of particles simulations, multiphase turbulence modelling, and stage separation for two-stage-to-orbit vehicles. The direct forcing immersed boundary method (IBM) is a promising method and widely applied in low speed flow while there is few research regarding compressible flows. We developed a novel IBM to resolve supersonic and hypersonic gas flows interacting with irregularly shaped multi-body particle. The main innovation is that current method can solve the interaction of particles and high-speed fluids, particle translation and rotation, and collision among complex-shaped particles within a uniform framework. Specially, high conservation and computation consumption are strictly satisfied, which is critical for resolving the high speed compressible flow feature. To avoid the non-physical flow penetration around particle surface, an special iterative algorithm is specially derived to handle the coupling force between the gas and particles. The magnitude of the velocity difference error could be reduced by 6–8 orders compared to that of a previous method. Additionally, aerodynamic force integration was achieved using the momentum equation to ensure momentum conservation for two-phase coupling. A high-efficiency cell-type identification method for each step was proposed, and mapping among LPs and cells was used again to select the immersed cells. As for the collision force calculation, the complex shape of a particle was represented by a cloud of LPs and the mapping of LPs and cells was used to reduce the complexity of the algorithm for contact searching. The repetitive use of the mapping relationship could reduce the internal memory and improve the efficiency of the proposed algorithm. Moreover, various verification cases were conducted to evaluate the simulation performance of the proposed algorithm, including two- and three-dimensional moving and motionless particles with regular and complex shapes interacting with high-speed flow. Specifically, an experiment involving a shock passing through a sphere was designed and conducted to provide high-precision data. The corresponding results of the large-scale numerical simulation agree well with those obtained experimentally. The current method supports flow simulations at a particle-resolved scale in engineering.

Infrared thermography techniques for boundary layer state visualisation

The rapid decarbonisation of the power generation and aviation sectors will require a move away from incremental development, exposing designers and researchers to the risk of unexpected results from uncertainty in boundary layer state. This problem already exists for parts developed with fully turbulent assumptions, but in novel design spaces the risk increases for both real components, where previous knowledge of similar designs may be inapplicable, and particularly in experimental testing of scaled models, where reducing Reynolds number can result in a drastic change in flow topology that skews the conclusions of a test. Computational methods struggle to reliably predict boundary layer state so experimental techniques for diagnosing boundary layer state are needed. Infrared thermography (IR) is a non-invasive technique that offers simple, fast visualisation of boundary layer state with no additional instrumentation. IR is relatively uncommon in the literature and there is minimal information available on the best practices for its use. This paper aims to encourage the adoption of IR as a diagnostic tool by demonstrating routes for optimisation and pointing out pitfalls to avoid. A low-order model is developed and used to predict how the signal-to-noise ratio (SNR) of an IR visualisation changes depending on the thermal design of the test piece. It is shown that in low-speed flows with active heating from the surface the SNR is maximised through a suitable choice of surface insulation, while in high-speed flows, where passive temperature differences are used, there is a crossover between heat transfer and recovery temperature effects that results in an SNR of zero, an effect that can arise in both steady-state and transient experiments. Experimental validation of the 1D model in both flow regimes is shown alongside two case studies on the use of IR in sub-scale testing where uncertainty in boundary layer state results in critical differences from the full-scale flow.

Evolution of Runner Forces during Simultaneous Pump-Trip Transient Process of Two Pump-Turbines

Abstract To ensure no cavitation and water column separation in pump-turbines, the suction heights of pump-turbines in high-head pumped-storage hydropower stations have to be decreased to about –100 m, which leads to high pressure in the draft-tube. During the pump-trip transient process of pump-turbines, the positive water hammer wave in draft-tube superposing the initial high pressure may cause an imbalance of rotating parts and even a lifting-up accident. To quantify the runner forces, two typical transient processes, with and without guide-vane closure after the simultaneous power-off of the two pump-turbines in one hydraulic system, were simulated by using the one-dimensional and three-dimensional coupled (1D-3D) computational fluid dynamics (CFD) method. The results show that the maximum upward axial force on the runner during the process without guide-vane closure (runaway) is significantly higher than that during the process with guide-vane closure. Two significant maximum upward axial forces were observed during the runaway process, and they were around the zero and the valley speed moments. The main causes are the pressure decrease in the runner’s high-pressure zone owing to high-speed bypass flows, and the spiral-case negative water-hammer wave and draft-tube positive water-hammer wave caused by discharge rise. However, the draft-tube positive water hammer wave superposing the high pressure did not cause the maximum axial force. The above two moments of highest axial forces during the runaway process and the proper guide-vane closure rule deserve special attention in the design and operation phases.

Research on the cooling mechanism of low-temperature liquid jet mixing in high-temperature and high-speed airflow

The multi-stage water spray cooling system of the space launch site involves a complex gas–liquid two-phase mixed cooling problem in the process of spraying water onto the rocket engine gas jet. To investigate the complex multiphase flow field of the interaction between the airflow and liquid water jet, the Mixture multiphase flow model is adopted, which is coupled with the vaporization equation and component transport model of liquid water. The computational fluid dynamics method is used to numerically analyze the mixing and cooling mechanism of low-temperature liquid jet in high-temperature and high-speed gas flow. The results show that mixing low-temperature liquid jets in high-temperature and high-speed airflow can cause the exchange of momentum and energy, resulting in pressure loss and temperature reduction in the gas phase flow field. Due to the acceleration of transverse airflow, the diffusion flow range of the jet is significantly increased, and violent vaporization occurs, which further consume a large amount of airflow energy. The mixing cooling effect of transverse airflow and liquid water jet is directly related to the mass flow rate ratio of the jet to airflow. In addition, directly designing liquid water jet orifices on the solid wall of the guiding device can achieve the effect of improving the gas flow field environment. This conclusion can provide theoretical reference for the thermal protection design of ground launch devices in space launch site.

Variations of flow patterns and pressure pulsations in variable speed pump-turbine during start-up process

Abstract In recent years, variable speed pumped-storage (VSPS) technology has become a hotspot of research and application due to its excellent power grid regulation capabilities. Since the speed of a VSPS unit is changeable, when switching operating conditions, more stable operating trajectories can be chosen to reduce vibration and damage. The simulations of transient processes of VSPS units normally use 1D method, which cannot judge the quality of operating conditions because of no flow pattern and pressure pulsation information. Using 3D CFD simulation based on OpenFOAM, we analyse the flow patterns and pressure pulsations when a VSPS unit with a full size frequency converter (FSFC) is in the turbine start-up process. The typical operating points in the variable speed transient trajectories obtained from 1D simulation are studied. It is found that in the initial stage of start-up, high-speed circulation flow arises in the vaneless space, leading to flow separation and large pressure pulsation. The results can provide a reference for the operation of VSPS power stations, so as to avoid operating conditions with adverse flow patterns and pressure pulsations.

The dynamical response of turbulent flow to bed deformation under submerged discharge and plane vertical impinging jets

Abstract On the conditions of discharge under sluice gate and vertical jets, the research on the dynamical response of hydraulic elements to bed deformation is much concerned in the field of high-speed flow and hydraulic dredging but not be solved yet. Research achievement from numerical simulation are drawn as follows: (1) Under sluice gate discharge condition, the response of hydraulic elements to bed deformation is that the bed is deforming in the direction of the mechanical energy loss, the rate of the bed deformation and the mechanical energy loss near the local scour hole and the horizontal distance from the reattachment point to the maximum scour depth are all continuously varying with the scouring of time and synchronously approaching to its own certain value. (2) Under vertical jets discharge condition, the response of hydraulic elements to bed deformation is that the rate of bed deformation and the pressure of the maximum scour depth is monotonous continuously decreasing with the scouring of time and synchronously approaching to its own certain value.

Self-supervised transformers for turbulent flow time series

There has been a rapid advancement in deep learning models for diverse research fields and, more recently, in fluid dynamics. This study presents self-supervised transformers' deep learning for complex turbulent flow signals across various test problems. Self-supervision aims to leverage the ability to extract meaningful representations from sparse flow time-series data to improve the transformer model accuracy and computational efficiency. Two high-speed flow cases are considered: a supersonic compression ramp and shock-boundary layer interaction over a statically deformed surface. Several training scenarios are investigated across the two different supersonic configurations. The training data concern wall pressure fluctuations due to their importance in aerodynamics, aeroelasticity, noise, and acoustic fatigue. The results provide insight into transformers, self-supervision, and deep learning with application to complex time series. The architecture is extendable to other research domains where time series data are essential.

Lattice Boltzmann simulation for wake interactions of aligned wind turbines using actuator line model with turbine control

A wind turbine wake causes a decrease in wind speed and an increase in turbulence intensity. The wind turbine wake interaction is essential for predicting the power output of a wind farm consisting of many wind turbines. This research proposes a CFD method able to reproduce wake interactions and power outputs of multiple wind turbines with high speed and accuracy. Large eddy simulations with the lattice Boltzmann method are used for fluid calculations, specifically for large-scale CFD simulations. The wind turbines are represented using an actuator line model. Optimal power generation efficiency is achieved by controlling the rotor speed and blade pitch angle. Large-scale simulations of eight aligned wind turbines are conducted using 1.75 billion grid points and 40 GPUs. We compare two cases with and without control to investigate the effect of turbine control on wake and power output. Both the instantaneous and mean streamwise velocities confirm that the turbine control reduces the wake velocity deficit of the downwind wind turbine. High-speed inflow of wind to the downstream turbines augments their power output. With implementation of turbine control, the power outputs of the downstream turbines agree well with the observation data obtained in an earlier study. The results demonstrate the importance of controlling the rotational speed and pitch angle for actuator line simulations.

Damage effect of underwater explosion bubble on concrete gravity dam

The underwater explosion bubble accounts for half of the total explosive energy and has been found to induce serious structural damage to ship structures. However, the existing research has commonly ignored the damage effect of an underwater explosion bubble on a concrete gravity dam. The current study aims to narrow this gap through high-fidelity numerical simulations and scale model tests. A fully-coupled numerical model is established first and is validated by small-scale centrifugal model tests. The bubble dynamics of underwater explosions near the perpendicular dam structure have been explored. The main focus is then on the sequential damage effects of the underwater explosion shock wave, bubble pulse, bubble jet, and bubble collapse on a concrete gravity dam. It is found that the shock wave causes a first attack, inducing tensile cracks in the vulnerable part of the dam. Subsequently, the bubble pulse generates a secondary attack, facilitating the penetration of the tensile crack throughout the entire dam structure. Additionally, the bubble jet driving the high-speed fluid flow and the bubble collapse causing the uplifted free water surface, united with the upstream reservoir water with huge hydrostatic pressure, largely accelerate the fracture of the dam, then facilitate the fractured dam to fall downstream leaving a breach with certain widths in the dam’s upper part, and finally speed up the discharge of the upstream reservoir water through the breach. It also finds that the underwater explosion bubble can generate no new cracks in the gravity dam, even in the pre-damaged area as long as there are no obvious pre-cracks by the shock wave. That is, the shock wave is the key trigger for dam failure, without which the dam failure will not happen, while the bubble, united with the upstream reservoir water, accelerates the process of dam failure.

Assessment of particle image velocimetry applied to high-speed organic vapor flows

Compressible flows of fluids whose thermophysical properties are related by complex equations are quantitatively and can be qualitatively different from high-speed flows of ideal gases. Nonideal compressible fluid dynamics (NICFD) is concerned with these fluid flows, which are relevant in many processes and power and propulsion systems. Typically, NICFD effects occur if the fluid is an organic compound and its vapor state is close to the vapor–liquid critical point, at high-reduced temperature and pressure (even supercritical). Current design and analysis of devices operating in the nonideal compressible regime demand for validated simulation software, characterized in terms of uncertainty. Moreover, experiments are needed to further validate related theory. Experimental data are limited as generating and measuring these flows is challenging given their high pressure or temperature or both. In addition, flows of organic compounds can be flammable, can thermally decompose, and sealing may demand for special materials. Recently, more research has been devoted to the measurement of these flows using both intrusive and less intrusive techniques relying on optical access and lasers. The transparency and refractive properties of these dense vapors pose additional problems. The ORCHID (organic Rankine cycle hybrid integrated device) at the Aerospace Propulsion and Power Laboratory of Delft University of Technology is a closed-loop facility, used to generate a continuous nonideal supersonic flow of siloxane MM with the vapor at 4bar\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ extrm{bar}}}$$\\end{document} and 220 °C at the inlet of the test section. Within this work, we have employed particle image velocimetry for the first time to obtain the velocity field in a de Laval nozzle in such flows. Measured velocity fields (expanded uncertainty within 1.1% of the maximum velocity) have been compared with those resulting from a CFD simulation. The comparison between experimental and simulated data is satisfactory, with deviation ranging from 0.1 to 10 % from the throat to the outlet, respectively. This discrepancy is attributed to hardware limitations, which will be overcome in the future experiments. The feasibility of PIV with uncontrolled but fixed seeding density to measure high-speed vapors of organic vapors has been demonstrated, and future experimental campaigns will target flows for which nonideal effects are more pronounced, other paradigmatic configurations, and improvements to the measurement techniques.

Short-term traffic flow prediction via weight optimization of composite models

ABSTRACT Accurate prediction of peak-hour traffic flow is crucial for congestion management. Neural network models have varying performance in different domains, limiting their effectiveness in traffic data. To address this, we propose the SSA-BiLSTM-BP-Elman ensemble model. It integrates bi-directional long short-term memory (BiLSTM), back propagation (BP) neural network, and Elman network. The model uses the sparrow search algorithm (SSA) to optimize the weight distribution to improve the prediction accuracy. Initially, the BiLSTM, BP, and Elman models are parameter-optimized by the Bayesian approach. SSA then assigns unique weights to each model based on the characteristics of the extracted data features. Results from an application to UK high-speed traffic flow data show that SSA significantly improves the accuracy of model predictions. This integrated model effectively utilizes the strengths of each model by assigning appropriate weighting coefficients by SSA, thus improving the overall prediction performance.

ANALISIS ALIRAN UDARA PADA PIPA KOMPOR BURNER

A used oil burner stove is a type of stove that uses used oil as fuel to produce fire and heat which is used in the cooking process. This concept is more commonly found in areas with limited access to conventional energy sources such as electricity or gas. Apart from that, there is a blower which has an important function to maintain sufficient and high-speed air flow towards the combustion axis, thereby helping to increase oil burning efficiency. This test was carried out using Solidworks software with the CFD or Computational Fluid Dynamic feature which has the function of analyzing fluid flow on an object by entering the required parameters. In this paper, a burner stove pipe with an initial cross-sectional area of 2" inches and a final cross-sectional area of 1/2" inches is used where an initial velocity input is given with independent variables of 2 m/s, 4 m/s and 6 m/s and the output is obtained or The results on the stove burner pipe were 17.9 m/s, 37.3 m/s and 54.6 m/s.

A study of streamline geometries in subsonic and supersonic regions of compressible flow fields

The geometrical properties of streamlines, such as the curvatures, directions and positions, are studied in steady inviscid compressible flow fields via differential geometry theories and conservation laws. The influences of the streamline geometries on the flow speeds and pressures are also identified and discussed. By transforming the streamlines to fill the domain and satisfy the boundary conditions, a unified geometry-based solver, the streamline transformation method, is proposed for both subsonic and supersonic regions. The governing equations and boundary conditions along streamlines and shock waves are also derived. This method is verified by numerical results of three typical flow fields, including the subsonic channel flow, the supersonic downstream of attached shock waves and especially the subsonic/supersonic downstream of detached bow shock waves. Both two-dimensional planar and axisymmetric flow fields are considered. Compared with the results from computational fluid dynamics, good agreements are achieved by this method, while fewer computational resources, by an order of magnitude, are consumed. Features of these flow fields are also analysed from a geometrical perspective, such as flow speeds and pressures deviated by the wall curvatures, and three-dimensional effects in the after-shock flow fields. For a hyperbolic-shaped bow shock wave, the stand-off distances and the transitions from subsonic to supersonic regions are also discussed. As indicated by the accuracy, efficiency and applicability in a wide range of flow speeds, the streamline transformation method would be a potential candidate for the theoretical analysis and inverse design of high-speed flow fields, especially where the subsonic regions exist downstream of strong shock waves.

Aerodynamic breakup of gel suspension droplets loaded with aluminum particles

The persistent and frequent space explorations are affected by the performance of propellants, among which the closely related gel propellants have been widely used due to their unique characteristics. Currently, research efforts primarily concentrate on the metallization and atomized combustion of gel propellants, emphasizing the combustion performance of metallized gel propellants. However, little attention has been given to the secondary breakup process that occurs just before combustion. By utilizing high-speed flow visualization technology, this study examines the key aerodynamic breakup process of gel suspension droplets loaded with micro aluminum particles, experimentally and theoretically investigating the droplet dynamics induced by particle volume fraction. The experimental confirmation of the three breakup behaviors exhibited by gel suspension droplets, namely oscillation mode, bag-stamen breakup, and hemline breakup, is presented along with the construction of a phase diagram. The impact of heterogeneous effects resulting from an increase in particle volume fraction on the critical breakup Weber number is revealed, with a concentration of 9 % as the turning point. Within the range of moderate Weber numbers, the tail flick deformation of suspension droplets in airflow is observed and reported for the first time. At different particle concentrations, there keeps a linear correlation between the dimensionless tail length and Weber number. Spherical suspension droplets will deform into thin disks in the airflow, with a nearly constant deformation rate regardless of changes in particle concentration. Additionally, the relationships between characteristic times and droplet parameters are discussed through statistical analysis of the results. As Ohnesorge number and particle volume fraction increase, the characteristic times both show a monotonically increasing trend. Moreover, a model of unstable growth rate associated with the particle volume fraction is established through linear instability analysis, accurately predicting the variation in critical Weber number for gel suspension droplet breakup. The congruence between our theoretical framework and experimental findings not only enhances comprehension of the impact of micro aluminum particles on the secondary breakup of gel suspension droplets but also provides valuable guidance for the aerospace applications of metallized gel propellants.

Aerodynamic interactions of blunt bodies free-flying in hypersonic flow

This paper takes a new look at how the aerodynamic interactions of multiple bodies in high-speed flow affect their motion behaviors. The influence of the body shape and orientation on aerodynamic and stability behavior in the case of shock–shock and wake–shock interactions is the focus of this publication. Experiments were performed in the hypersonic wind tunnel H2K at the German Aerospace Center (DLR) in Cologne. Free-flight tests with tandem arrangements of spheres and cubes were performed with a synchronized dropping of both objects at various initial conditions of relative streamwise and vertical distance as well as pitch angle. A high-speed stereo-tracking captured the model motions during free-flight, and high-speed schlieren videography provided documentation of the flow topology. Based on the measured 6-degrees-of-freedom (6DoF) motion data, aerodynamic coefficients were determined. As a result, the final lateral velocity of trailing cubes is found to be many times greater than that of spheres regarding shock-wave surfing. For rotating cubes, the results showed that stable shock-wave surfing can become possible over an increasingly wide range of initial positions. This study has identified that the trailing drag coefficient of two axially aligned objects varies strongly with their relative streamwise distance. Furthermore, it was shown that the wake is a region of stability for downstream objects.Graphical abstract

Modelling of high-speed engine flows using a space-marching method

A computationally efficient mathematical model was developed to analyze high-speed engine flows. The inlet and nozzle flows were modelled using the space-marching method to automatically solve the shock reflection and nozzle plume flow. The combustor flow was treated using a quasi-one-dimensional model that considered the rise in pressure of the isolator owing to the shock train. The resulting model was compared with the experimental results. Three test cases were used to validate the model's accuracy: 1) a hydrogen-fuelled combustor experiment, 2) an integrated inlet/combustor configuration experiment, and 3) a single-expansion-ramp nozzle flow experiment. The results showed that the space-marching method predicted the pressure profiles of the inlet flow and nozzle plume flow with reasonable accuracy. The computational time for the inlet and nozzle flows merely takes several minutes on a CPU with 3.6 GHz. The quasi-one-dimensional model calculated the fuel ignition and pressure rise in the combustor with a high computational efficiency of several seconds. The integration of the space-marching method and quasi-one-dimensional method can be developed as a powerful tool for fast evaluations of high-speed engine performance.

Mechanism and deterioration pattern of sandstone surrounding rock voiding at bottom of heavy-haul railway tunnel

This study combines laboratory experiments and discrete element simulation methods to analyze the mechanism and deterioration patterns of sandstone surrounding rock voiding the bottom of a heavy-haul railway tunnel. It is based on previously acquired measurement data from optical fiber grating sensors installed in the Taihangshan Mountain Tunnel of the Wari Railway. By incorporating rock particle wastage rate results, a method for calculating the peak strength and elastic modulus attenuation of surrounding rock is proposed. Research indicates that the operation of heavy-haul trains leads to an instantaneous increase in the dynamic water pressure on the bottom rock ranging 144.4–390.0%, resulting in high-speed water flow eroding the rock. After 1–2 years of operation, the bottom water and soil pressures increase by 526.5% and 390.0%, respectively. Focusing on sandstone surrounding rock with high observability, laboratory experiments were conducted to monitor the degradation stages of infiltration, particle loss, and voiding of rock under the action of dynamic water flow. The impact of water flow on the “cone-shaped” bottom rock deformation was also clarified. The extent of rock deterioration and voiding was determined using miniature water and soil pressure sensors in conjunction with discrete element numerical simulations. The measured rock particle loss was used as a criterion. Finally, a fitting approach is derived to calculate the peak strength and elastic modulus attenuation of surrounding rock, gaining insight into and providing a reference for the maintenance and disposal measures for the bottom operation of heavy-haul railway tunnels.

TEMPERATURE FACTOR EFFECT ON PULSATING HYDROGEN COMBUSTION IN A HIGH-SPEED AIR FLOW

The results of a numerical study of the combustion of hydrogen jets injected into a highspeed air flow before a sudden channel expansion are presented. It is shown that under constant input conditions, a pulsating combustion mode is realized in the channel, which is maintained when the wall temperature increases from 300 to 900 K. An increase in the temperature factor leads to a decrease in the oscillation frequency and amplitude.

Instantaneous thermometry imaging using two-photon laser-induced fluorescence.

Measuring temperature in complex two-phase flows is crucial for understanding the dynamics of heat and mass transfer. In this Letter, we introduce a novel, to the best of our knowledge, optical approach based on the combination of two-photon laser-induced fluorescence (2p-LIF) imaging and two-color laser-induced fluorescence (2CLIF) for instantaneous temperature mapping of complex liquid media. Using Kiton Red (KR) and Rhodamine 560 (R560), a temperature sensitivity of 1.54%/∘C has been achieved over a range of 17-60°C. The monitoring of two-dimensional transient temperature dynamics in the heating and degassing of water shows the efficiency of the 2p-2CLIF. This new approach contributes to the toolkit of optical temperature measurement techniques, providing a robust solution for studying transient scattering media and high-speed two-phase flows.