Cryogenic Wind Tunnel Research Articles

Numerical investigation on the tracer followability of nitrogen droplets over the airfoil in cryogenic wind tunnel

Cryogenic wind tunnels (CWT) achieve high Reynolds number aerodynamic performance measurements by cooling the medium with liquid nitrogen. Utilization of naturally-occurring liquid nitrogen droplets as tracer particles for non-intrusive measurement applications in CWT shows great potential in the previous work. However, the droplet motion characteristics over the airfoil remain to be investigated, and the followability evaluation approach is lacking. Accordingly, an Euler-Lagrange model for transonic particle flow over the NACA0012–64 airfoil was established. The effect of flow characteristics on the particle motion was explored. The results reveal that the flow separation and shock wave phenomenon over the airfoil causing velocity deviation between airflow and droplet. The shock wave is the main deviation driver, and flow separation causing trace blind zones. Droplets larger than 10 μm fail to effectively follow the variation of airflow. The blind zone enlarges with angle of attack (AoA), particularly when AoA exceeding 10 degrees. Moreover, Stokes number is found to be inadequate for characterizing the followability of tracer particles in turbulent flow. Consequently, an evaluation method based on the Basset-Boussinesq-Oseen equation was developed. This research provides theoretical and technical support for advancing the use of nitrogen droplets as tracer particles in CWT.

Theoretical analysis of the initial nucleation characteristics of trace water vapor frosting on a cryogenic surface

In certain extreme conditions characterized by ultra-low water vapor content and ultra-low temperatures (e.g. cryogenic wind tunnel), trace water vapor frosting can also pose significant hazards. Considering the crucial role of initial nucleation on subsequent frosting processes, this study firstly investigated and compared the nucleation characteristics of frosting at different water vapor contents (0.1−1000 ppmv) from humid air to trace water vapor based on classical nucleation theory. The preferred nucleation phase diagrams and critical nucleation conditions were analyzed. Sensitivity of the nucleation mass transfer rate, as well as the frosting pathways were further identified. Results show that the nucleation characteristics of trace water vapor frosting (<100 ppmv) is significantly different from that of humid air frosting (>1000 ppmv). Trace water vapor frosting is more inclined towards desublimation nucleation due to its lower nucleation temperature and larger critical contact angle. The critical contact angles for 1000 ppmv, 100 ppmv and 0.1 ppmv are 49°, 75° and 120°, respectively. Furthermore, trace water vapor nucleation requires a greater subcooling degree, has a smaller critical nucleation radius, and is highly sensitive to surface contact angle and further-subcooling degree. At a surface contact angle of 120°, the nucleation subcooling degree of 0.1 ppmv is 2.9 K greater than that of 1000 ppmv. This study helps understanding the nucleation characteristics under different water vapor content conditions, which indicates that reducing subcooling degree and increasing contact angle are more effective anti-frosting methods than reducing water vapor content for trace water vapor frosting.

A complete environmental wind tunnel for studying the evolution of snow accumulations under the effects of wind, rain, and heat

Given the escalating frequency of extreme cold weather worldwide, snow-induced building failures have increased substantially, especially in large-span space structures. The reason for engineering accidents lies in a lack of understanding of the intricate process of the mechanism of full-period evolution of snow “accumulation-melt-crystallization- accumulation” under the coupling actions of wind, rain, and heat. It is significant to develop a comprehensive and accurate environmental wind tunnel for conducting snow-relevant experiments under the multi-factors, which are barely studied. Hence, this paper proposes and establishes an experimental system called “Simulator of Natural Action of Wind-Rain-Heat-Snow for Space Structures (SNOW).” The system consists of an atmospheric boundary layer cryogenic wind tunnel subsystem, snowfall simulation subsystem, rain simulation subsystem, solar and building heating simulation subsystem, and high-precision multi-task monitoring and control subsystem. The facility can provide a 0–20 m/s stable wind field, −15 °C–0 °C low-temperature conditions, and a 2 m × 2 m experimental section. The calibrations of each subsystem were conducted in SNOW to ensure each component can successfully provide basic conditions for snow-relevant experiments. The SNOW can accurately simulate the entire process of natural snowfall and snow evolution under the coupling effects of wind, rain, and heat.

Prediction model of aircraft hinge moment: Compressed sensing based on proper orthogonal decomposition

By hinge moment, we mean the aerodynamic torque exerted on the rudder shaft by the airflow passing through the aircraft control surface, with obtaining high-precision results often relying on wind tunnel tests. Due to the complex aerodynamic balance insulation and installation errors that must be considered in cryogenic wind tunnels, the main method for calculating hinge moments is to directly integrate surface pressure distribution information. However, it is usually difficult to arrange enough pressure taps, resulting in the accuracy failing to meet expectations. Combining the sparse wind tunnel test data and low-precision computational fluid dynamics results, this paper introduces the compressed sensing based on proper orthogonal decomposition (CS-POD) method and presents the sub-Ma model and the full-Ma model for predicting hinge moments. The number of sensors and sensor positions are determined based on the sparsity of the numerical simulations and basis functions. Then, the CS algorithm solves the basis coefficients. Finally, the hinge moments are obtained by integrating the reconstruction pressure distribution which is calculated by linearly combining the basis functions and basis coefficients. The result shows that the full-Ma model exhibits higher prediction accuracy with approximately five sensors under subsonic and transonic cases, reducing the relative error of the sub-Ma model by 2–10 times, even at high angles of attack. The mean reconstruction accuracy for the hinge moments is 97.6%, and for the normal forces, it is 94.3%. Therefore, adding relevant terms when the number of samples is small can effectively improve modeling accuracy.

Numerical investigation of nitrogen spontaneous condensation over airfoils in cryogenic wind tunnel

Cryogenic wind tunnels (CWT) provide the aerodynamic tests that occur at high Reynolds numbers by operating under cryogenic environment to meet the requirements of hypersonic flight simulation. However, condensation in cryogenic environment near the limited condition destroys the reliability of the test measurement results. Accordingly, research on condensation in CWT is imperative for devising effective control strategies. This study introduces a transonic non-equilibrium condensation model, tailored to evaluate the complex phenomena of nitrogen condensation over airfoils. The model was applied to assess condensation effects on the NACA 0012–64 airfoil and the RAE 2822 airfoil under various operating conditions. Furthermore, the study examined the effect of the angle of attack and airfoil shape on the condensation process. The result show that nitrogen condensation near the airfoil is observable only under operating conditions within the pre-condensation zone. Operating conditions of CWT can be tailored to prevent condensation with specific airfoil profiles. The effect of condensation on the pressure coefficient includes advancing the position of the pressure jump and increasing the pressure coefficient of the airfoil intermediate region, particularly within the 20 %–70 % chord length span on the airfoil's upper surface. This primarily affects the measurement of lift coefficients in CWT. In the operating conditions of this study, the lift coefficient exhibits a maximum deviation of 31.4 %, and the drag coefficient shows a maximum deviation of 2.4 %. The transonic spontaneous condensation model offers an approach to evaluate condensation's impact on airfoil tests within CWT. The findings introduce a methodological framework for effectively controlling condensation phenomena.

Analysis of frost layer growth behavior on cryogenic surfaces in ultralow dew point environments by experimental tests and numerical simulations

Maintaining an ultralow dew point is essential for the operation of transonic cryogenic wind tunnels, as it significantly influences aerodynamic characteristics. Even in extremely dry conditions, minute quantities of frost can form on cryogenic surfaces, which compromises data quality. Conducting experiments on frost desublimation in such ultralow dew point environments poses significant challenges due to the demanding experimental conditions. Therefore, this study aimed to explore the desublimation characteristics of cryogenic surfaces in ultralow dew point environments and to address the challenges of experimental investigations under such extreme conditions. Initially, the study experimentally investigated frost formation by assessing the impact of various factors, such as temperature and water vapor content. It was observed that the type of frost formed depended on the water vapor content, and altering the surface temperature influenced the growth region of the frost crystals. The irregular growth of the frost layer on low-temperature surfaces was evident, as temperature markedly affected both the growth rate and the diffusion direction of frost formation. Moreover, the process of trace water frost formation was simulated using a dispersive multiphase model, predicting the frost layer's thickness. The numerical results suggest that the frost layer is micrometers thick, with a minimum thickness of 20 μm. A nonlinear relationship was identified between frost growth and dew point level. This research lays the groundwork for a deeper understanding of desublimation characteristics in ultralow dew point environments and for further investigation of low-temperature heat and mass transfer phenomena.

Experimental study of frosting characteristics of trace water vapor under cryogenic wind tunnel conditions

In cryogenic wind tunnels, the working fluid mixed with trace amounts of water vapor may form frost on the test model, which could affect the testing results. To investigate the frosting phenomenon under trace water condition, an experimental system is built for visualizing the frosting process on a cryogenic surface. The effects of variable factors on frost formation are analyzed focusing on the growth characteristics of frost layer. Under trace water and cryogenic surface conditions, the water vapor desublimates into crystals without obvious condensation. As the surface temperature increases from −183 ℃ to −123 ℃, the frost becomes thicker and more porous due to the thinner outer boundary layer. When the water vapor content rises from 30 ppm to 500 ppm, the frost morphology transforms from a flat layer to frost clumps with visible dendritic structure. Additionally, the decrease in nitrogen flow temperature and the increases in ambient pressure and surface roughness of the cryogenic surface are conducive to the development of the branches on the frost crystal, which accelerate the growth of the frost layer. Furthermore, a dimensionless correlation is proposed for predicting the frost thickness under cryogenic and trace water conditions, with an overall error within ±20 %.

Frost layer growth behavior study on a cryogenic surface with water vapor content

Frosting under cryogenic conditions is common in liquefied natural gas (LNG), cryogenic containers, cryogenic wind tunnels, cryopump and other engineering fields, but the corresponding researches are still insufficient compared with that under general-low temperature conditions. Especially, research on extra-low-content water vapor frosting under cryogenic conditions stays almost blank. A visualization experiment system was built, based on which cryogenic frosting experiments of water-vapor-carrying nitrogen gas flow with different water vapor contents (120 ppmv ≤ ωin ≤ 780 ppmv) considering the cooling process (from 290 K to 100 K) were carried out. Images through the whole frosting period were recorded and image processing was executed. The evolution of the frost surface was captured and the translation velocity was calculated. Physical properties of the frost layer including thickness (as large as 1.75 mm at 2 h) and roughness (up to 76.2 μm) were obtained and compared with humid air frosting. The frost layer growth pattern transition from needle-like vertical growth to clump-grass-like circular rotational growth was observed in the rapid cooling stage. The results reveal differences in frost formation mechanisms and characteristics from general-low to cryogenic temperatures and can provide a quantitative reference for mass and heat transfer analysis in practical frosting circumstances.

Aeroacoustic testing on a full aircraft model at high Reynolds numbers in the European Transonic Windtunnel

This paper presents an end-to-end approach for the assessment of pressurized and cryogenic wind tunnel measurements of an EMBRAER scaled full model close to real-world Reynolds numbers. The choice of microphones, measurement parameters, the design of the array, and the selection of flow parameters are discussed. Different wind tunnel conditions are proposed which allow separating the influence of the Reynolds number from the Mach number, as well as the influence of slotted and closed test sections. The paper provides three-dimensional beamforming results with CLEAN-SC deconvolution, the selection of regions of interest, and the corresponding source spectra. The results suggest that slotted test sections have little influence on the beamforming results compared to closed test sections and that the Reynolds number has a profound, non-linear impact on the aeroacoustic emission that lessens with increasing Reynolds number. Further, sources show a non-linear Mach number dependency at constant Reynolds number but are self-similar in the observed Mach number range. The findings suggest that it is possible to study real-world phenomena on small-scale full models at real-world Reynolds numbers, which enable further investigations in the future such as the directivity of sources.

Feasibility investigation of non-equilibrium condensing nitrogen droplets as tracer particles in cryogenic wind tunnels

Application of high-precision non-intrusive flow field measurement techniques in transonic cryogenic wind tunnels (CWT) is confined to the lack of suitable tracer particles. The present study explored the feasibility of utilizing non-equilibrium condensing nitrogen droplets as tracer particles. A numerical model of the nitrogen non-equilibrium condensation was established. Accordingly, the non-equilibrium condensation process of nitrogen droplets and feasibility analysis of tracing were investigated. The results reveal that the occurrence of nitrogen droplets as well as the droplet concentration and mean radius can be regulated in the nucleation zone and droplet growth zone by varying gas temperature or pressure. Furthermore, the feasibility of employing nitrogen droplets as tracer particles in the working conditions of CWT is confirmed through the analysis of tracking capability, response time, concentration, and duration of droplet traceability. Formulas correlating used to calculate duration of droplet traceability have been proposed. This study proved the potential application of condensing nitrogen droplets as tracer particles in CWT.

Разработка и апробация методики численного моделирования аэроупругого состояния крыла на основе методов конечных элементов и контрольных объемов

The paper considers a current trend in designing structural elements for the modern airliners. Calculation of the aerodynamic loads acting on the aircraft in flight is of primary interest at the initial design and development stages. Based on the experimental data, a CAD model was developed, and a computational grid for the CFD solver and the finite element model were constructed. The proposed technique for determining external loads on the airframe is based on the coupled solution of the aerodynamics and strength dynamic models. When using this technique, the serial data exchange between the solvers was actually implemented making it possible to simulate various periodic processes, including the structural vibrations. This work studies aerodynamic characteristics and eigenforms of the loaded wing using the example of the wing blowing calculated repetition in a cryogenic wind tunnel. The purpose of the work is to develop and test a technique for numerical simulation of the wing aeroelastic state, which allows performing calculations to determine the loads on both the lightly loaded elements and the critical units, including the wing box and the empennage. The results obtained testify to applicability of the developed technique at the transonic speeds, as well as a good convergence of the mathematical simulation results with the experimental data.

Real-time mass detection of trace water vapor frosting on a cryogenic surface based on quartz crystal Microbalance (QCM) sensor

Trace water vapor remaining in a large-scale cryogenic wind tunnel may form an extremely thin frost layer on the scaled model, significantly affecting the aerodynamic characteristics and degrading the data quality. Developing a high-precision detection method for trace water vapor frosting can be very helpful in revealing the underlying mechanisms. In this work, utilizing the relationship between deposited mass and frequency variation of Quartz Crystal Microbalance (QCM), an ultra-sensitive (nanogram level) mass sensor for cryogenic and flow conditions was developed. Furthermore, after investigating the effects of different parameters (flow velocity, pressure, temperature and mass) on frequency, the real-time mass detection of trace water vapor frosting under cryogenic and flow conditions was achieved for the first time. The results show that the effect of flow velocity or pressure on frequency is almost negligible compared to that of temperature or mass. Although temperature (from 290 K to 140 K) has a significant effect on frequency, by using the relationship between the measurement surface and the reference surface, the separate effect of mass on frequency can be obtained and the real-time mass detection is achieved. The calculated total deposited mass by trace water vapor frosting is about 4.64 μg/cm2 when the water vapor content, lowest surface temperature, pressure, velocity and frosting time are 186.5 ppb, 140 K, 0.1 MPa(G), 1.0 m/s, 160 min, respectively. This self-designed device and the analysis results of this work can be used as a reference for the quantitative measurement of trace water frosting as well as the phase change process of other trace substances in cryogenic and flow environments.

Mach and Reynolds number effects on transonic buffet on the XRF-1 transport aircraft wing at flight Reynolds number

This work provides an overview of aerodynamic data acquired in the European Transonic Windtunnel using an XRF-1 transport aircraft configuration both at cruise conditions and at the edges of the flight envelope. The goals and design of the wind tunnel test were described, highlighting the use of the cryogenic wind tunnel’s capability to isolate the effects of M_{infty }, {text{Re}}_{infty } and the dynamic pressure q/E. The resulting dataset includes an aerodynamic baseline characterization of the full span model with vertical and horizontal tailplanes and without engine nacelles. The effects of different inflow conditions were studied using data from continuous polars, evaluating the changes in aeroelastic deformation which are proportional to q/E and the influence of M_{infty } and {text{Re}}_{infty } on the shock position. Off-design data was analyzed at the lowest and highest measured Mach numbers of 0.84 and 0.90, respectively. Wing lower surface flow and underside shock motion was analyzed at negative angles of attack using {{c}_{text{p}}} distribution and unsteady pressure transducer fluctuation data, identifying significant upstream displacement of the shock close to the leading edge. Wing upper-side flow and the shock motion near buffet onset and beyond was analyzed using unsteady pressure data from point transducers and unsteady pressure-sensitive paint (PSP) measurements. Buffet occurs at lower angles of attack at high Mach number, and without clearly defined lift break. Spectral contents at the acquired data points in the buffet range suggest broadband fluctuations at Strouhal numbers between 0.2 and 0.6, which is consistent with recent literature. The spanwise shock propagation velocities were determined independently via analysis of unsteady PSP and pressure transducers to be in the range between u_{text{s}} / u_{infty } = 0.24 and 0.32, which is similarly in line with published datasets using other swept wing aircraft models.

A Review on the Precise Control of the Liquid Nitrogen Supplying System in Transonic Cryogenic Wind Tunnel

A Review on the Precise Control of the Liquid Nitrogen Supplying System in Transonic Cryogenic Wind Tunnel

Time-resolved pressure-sensitive paint measurements for cryogenic wind tunnel tests

A first time-resolved pressure-sensitive paint (PSP) test campaign at the European Transonic Wind Tunnel (ETW) was conducted within the research initiative “Unsteady flow and interaction phenomena at High Speed Stall conditions”. One of the objectives of this wind tunnel campaign was to resolve time-series of surface pressure distributions caused by complex 3-D buffet phenomena on a full-span airplane model XRF-1 transport aircraft configuration. At higher angle-of-attack and high Mach number, pressure fluctuations with a frequency of several hundred Hertz are expected to occur on the main wing and the horizontal tail plane (HTP) caused by the buffet effect. To capture the expected buffet phenomena by PSP, the German Aerospace Center developed a time-resolved PSP measurement and data acquisition system as well as a post-processing method for measurements in ETW. The measurements were conducted on the main wing and HTP simultaneously, with a camera frame rate of up to 2 kHz. The transonic buffet phenomena were observed at the flight relevant Reynolds number Re = 12.9 × 106 and Re = 25.0 × 106. The time-varying surface pressure distribution on the model was successfully captured by PSP. The time-series and spectra of both PSP and pressure transducer data match very well.

Design and validation of a single-jack variable Mach number nozzle in a cryogenic transonic wind tunnel

Design and validation of a single-jack variable Mach number nozzle in a cryogenic transonic wind tunnel

Experimental and numerical study of the liquid nitrogen accumulator based on two-phase thermal stratification

Accurate control and rapid regulation of the liquid nitrogen supplying pressure are the basis for the total temperature operation of the cryogenic wind tunnel. The pressure evolution during the discharging process and external pressurization in an accumulator are measured experimentally. The thermal boundary conditions of the tank during the test are mathematically obtained with the consideration of frosting and convective heat transfer. A numerical framework of two-phase flow based on the volume of fluid (VOF) and the heat and mass transfer model is carried out. The computed results with the accommodation coefficient of β = 10−6 in the Lee model are closer to the measured values than using the original value of 0.1, and the laminar model is more suitable for the numerical study of the gas-liquid transfer in the tank than the turbulent model. The growing dynamic process of gas-liquid interface and temperature distribution during the entire test is achieved. Thermal stratification is formed though the coming liquid nitrogen is highly subcooled. The liquid level has not been stable immediately but grows slowly when the pressure reaches the set value. The simulated temperature evolution, distribution, and the final volume fraction of gaseous nitrogen are satisfactorily consistent with the experiments. The results show that the numerical model can simulate the complex heat and mass transfer characteristics inside the cryogenic accumulator and provide theoretical guidance for injection pressure control.

Mechanisms of trace water vapor desublimation over airfoil in transonic cryogenic wind tunnels

Cryogenic wind tunnels can provide a larger operating Reynolds number compared to conventional ones. However, the internationally known “moisture contamination problem” resulting from the residual trace water vapor desublimation in the cryogenic wind tunnel may affect the accuracy of the aerodynamic data. Due to extreme difficulties in experiments, the detailed trace water desublimation characteristics remain unrevealed. An Euler–Euler two-phase flow model based on classical nucleation theory and a droplet growth model were established to predict the trace water vapor spontaneous desublimation in the nitrogen flow over airfoil in a transonic cryogenic wind tunnel. The proposed model was validated by experimental data obtained from the literature and showed good agreements. The verified model was applied to 0.152-m National Advisory Committee for Aeronautics 0012 airfoil in a Langley 0.3-m transonic cryogenic wind tunnel under a series of operating conditions. The simulated results reveal that the water vapor desublimation process in a transonic cryogenic wind tunnel can be divided into two patterns by a critical region. In pattern I, over the upper critical total temperature, the water vapor desublimates quickly during the rapid expansion of gas flow over the airfoil surface. In pattern II, below the lower critical total temperature, the water vapor is sufficiently supercooled and desublimates completely into small ice particles in the free-stream flow. For pattern I, the desublimation characteristics were analyzed in detail, and the influences of the desublimation process on the aerodynamic data were quantitatively evaluated, which can provide theoretical guidance for the practical operation of transonic cryogenic wind tunnels.

Temperature measurement based on the magneto-optic Kerr effect in Ni nanofilms

Thermometry is essential in laboratory and industry settings. We propose a noninvasive temperature measurement method based on the superparamagnetic properties of nickel-plated films. When the nickel (Ni) film is uniformly heated from 100 K to 300 K, we observe that the polarization angle of the reflected polarized light decreases correspondingly. Compared with optical temperature measurement, this method introduces magnetic physical quantity, which increases the penetration of temperature measurement. Compared with magnetic temperature measurement, this method introduces optical physical quantity, which greatly increases the amount of information contained in the measurement channel. On this basis, we introduce the characteristic matrix of membrane system, and the improved the mean field theory which can interpret superparamagnetism. Our experiment has demonstrated that the above theoretical model is feasible from low temperature to normal temperature. The thin film sensor element provides the necessary temperature information for cryogenic wind tunnel design and hypersonic vehicle surface model design in real time

Experimental investigation of snow drifting on flat roofs during snowfall: Impact of roof span and snowfall intensity

Owing to the controllability and reliability, wind tunnel experiment is considered as a powerful and effective approach for the investigation of snow drifting. Most of the previous experimental studies were mainly concerned with snow drifting on roofs without snowfall, in which a layer of particles was uniformly paved on the roof surface to model the initial snow cover. Hence, a cryogenic wind tunnel experiment using artificial snow particles was conducted to investigate snow drifting on flat roofs during snowfall. The similarity requirement related to snowfall intensity is derived according to the similarities of snow distribution pattern and the development of snow transport. Based on the results of snow distributions and transport rates for different roof spans and snowfall intensities, the features of snow drifting under snowfall conditions are analyzed in detail. The concurrence of snowfall is further revealed to greatly affect the development of snow drifting on flat roofs and thereby result in a reduction in the required fetch for reaching the saturated state. And the typical distribution patterns for snow drifting on flat roofs with and without snowfall are also summarized. Besides, the snow transport rate is found to increase along the roof surface before the achievement of the saturated state, whereas a slight decrease in the degree of snow erosion with roof span is observed. But the location of the crest formed near the windward edge is nearly unaffected by roof span as well as snowfall intensity. The results also indicate obvious linear characteristics in the relationship between the degree of erosion on the roof surface and snowfall intensity. And the normalized development process of snow transport before reaching the saturated state is nearly independent of snowfall intensity, although both the saturated transport rate and the required fetch for saturated drifting are strongly affected in the meantime.