Free Stream Temperature Research Articles

Improvement of transition prediction model in hypersonic boundary layer based on field inversion and machine learning framework

The classical four-equation γ−Reθ transition model has presented excellent accuracy in low-speed boundary layer transition prediction. However, once the incoming flow reaches hypersonic speed, the original model is no longer applicable due to the compressibility problem and the appearance of multiple instability modes. Recently, there has been widespread interest in data-driven modeling for quantifying uncertainty or improving model prediction accuracy. In this paper, a data-driven framework based on field inversion and machine learning is performed to extend the prediction capability of the original γ−Reθ transition model for the hypersonic boundary layer transition. First, the iterative regularized ensemble Kalman filter method is applied to obtain the spatial distribution of the perturbation correction term β for the switching function Fonset1, and the effectiveness of this method is initially verified in the hypersonic flat plate case. Then, the random forest algorithm is adopted to construct a mapping from the average flow features to β. The generalizability of the well-trained learning model is fully validated in the blunt cone cases with different unit Reynolds numbers, free-stream flow temperature, and bluntness. The simulation results indicate that the performance of the original γ−Reθ transition model in the hypersonic boundary layer transition prediction is significantly improved, and the boundary layer transition onset location and the length of transition zone can be correctly obtained. In addition, the machine learning model investigates the importance of the input features and confirms that the effective length scale plays a significant role in the numerical simulation of the hypersonic boundary layer transition.

Computational analysis and evaluation of thermal comfort environment of a space by reducing the inside air temperature

The tents are used as temporary shelters and evenly they sometimes become a home for a number of years when the returns to permanent shelter is very slow or unaffordable for any person so that the comfort of these shelters are very much essential like home. This paper is primarily concerned with identifying the objectives of reducing the inside air temperature of shelter using different retro-reflective materials. In the present work computational fluid dynamics analysis have been performed for a shelter under the summer climatic conditions to reduce the inside air temperature of shelter using different retro-reflective materials like polyvinyl chloride, wood-fibre, polystyrene, aerogel and phenolic foam. For CFD analysis free stream temperature of air is set 311 K taken with constant heat flux of 900 W/m2 solar radiations is used on roof, second order upwind discretization scheme is used to approximate the solution for momentum and pressure. Result show that that minimum temperature variation of 4.8 degreesK by using retro-reflective material as phenolic-foam and 18.74 % temperature percentage variation compared with other reflective materials.

Temperature recovery factor for gaseous nitrogen flow in a microtube

For gas flow over a flat plate, the temperature recovery factor is defined as a fraction of free-stream total temperature rise recovered at the wall or is the ratio of the actual temperature rise at the wall to the maximum possible temperature rise in the free-stream. Its value depends on Prandtl number and is independent of Mach number. The gas velocity and temperature are determined by the temperature recovery factor using the adiabatic wall temperature. It is an important parameter to be used to evaluate the gas velocity and temperature indirectly. In the present study, for internal flow, a methodology was introduced to estimate the temperature recovery factor numerically based on the total and bulk temperature for both laminar and turbulent flow regions in a microtube. The numerical simulations are based on the arbitrary Lagrangian-Eulerian method. The compressible momentum and energy equations for an ideal gas were solved to obtain the temperature recovery factor. To further validate the temperature recovery factor from numerical results, experiments were conducted using stainless steel microtubes. Although it is not a direct comparison, the gas bulk temperature and Mach number that were determined by the temperature recovery factor were compared with the experimental results and they were in good agreement.

Effects of thermophoresis on high-pressure binary-species boundary layers with uniform and non-uniform compositions

Direct numerical simulations of high-pressure binary-species temporal boundary layers are performed to investigate the flow physics for three situations: (1) uniform and equal composition, (2) uniform but unequal compositions and (3) non-uniform composition. Both colder- and hotter-wall situations compared with the free stream are simulated. The working fluid is a nitrogen/methane mixture. The analysis is performed at a case-specific self-similar state. Even when the initial composition is uniform, the methane mean mass fraction decreases near the colder wall, whereas it increases near the hotter wall and the mass fraction fluctuates in the entire boundary layer. Analysis of the species-mass diffusion balance and flow structures reveal that both mass-fraction variation and fluctuations are induced by the Soret effect. When the initial composition is non-uniform and the wall is colder, the methane mean mass fraction monotonically increases from the wall akin to its initial profile. However, when the wall is hotter the mean mass fraction decreases near the wall in contrast to its initial profile, a fact traced through the species-mass diffusion balance to the Soret effect being large and enriching methane near the wall. In contrast, the direction of the Soret flux is opposite for the colder wall, thus keeping the methane concentration small. Although the initial magnitude of the difference between the wall and free-stream temperature is the same in all cases, the situation is not symmetric between colder- and hotter-wall cases; the flow structure exhibits much smaller scales when the wall is hotter than when the wall is colder.

Heat transfer and behavior of the Reynolds stress in Mach 6 boundary layer transition induced by first-mode oblique waves

This paper investigates a Mach 6 oblique breakdown via direct numerical simulation in conjunction with stability and quadrant analyses. Particular emphasis is placed on, first, the heat transfer and mean flow distortion in the near-wall and outer transitional boundary layer, and, second, the flow events that are responsible for the production of the Reynolds stress. The energy budget reveals that enhancement of viscous dissipation due to mean flow distortion dominates the heat transfer overshoot, while the dissipation due to fluctuations is lesser but not negligible. Downstream of the location of the peak mean heat flux, the wall temperature gradient (non-dimensionalized by the freestream temperature and local boundary layer thickness) varies little, owing to the occurrence of breakdown and the establishment of self-similarity. Renormalized by the boundary layer thickness, a new correlation of the Stanton number shows no overshoot or difference between the original overshoot region and the turbulent region, which indicates the possibility of similarity once breakdown has occurred. In the outer region, enhanced advective heat exchange strongly reshapes the mean temperature profile. Because of successive modal growth and nonlinear saturation, the contributions of the primary oblique mode, streak mode, and a superharmonic to the outer advective heat transfer are found to compete near the location of the peak heat flux. From the perspective of fluid motions, quadrant analysis highlights the evenly and broadly distributed joint probability density function (PDF) of the fluctuating velocities during transition, which results in overproduction of the Reynolds stress, while the PDF is concentrated around zero in the turbulent region. The flow event Q2 (ejection) overtakes Q4 (sweep) in the outer boundary layer of the transitional region, mainly owing to the primary mode, while the two events become attenuated and nearly achieve balance when transition is complete.

Influence of thermal effects on the cavitation of orifices in nuclear power plants

Orifices are widely used as throttling devices in nuclear power plants. The pressure drop is not the only driving parameter of cavitation when the fluid is operating at its critical temperature, and the thermal effects cannot be neglected owing to the phase change. This study aims to extend a cavitation model to investigate the impact of thermal effects by modeling the water cavitation flow in an orifice. The thermal effects on the mass transfer process are considered using the Zwart–Gerber–Belamri cavitation model, and the effect of the two-phase flow on the turbulence viscosity is considered in the turbulence model. The numerical results reveal that the period of the variation in vapor volume is shorter under thermal cavitation than under isothermal cavitation. The pressure downstream of the orifice fluctuates with the heat transfer during thermal cavitation. The thermal cavitation model can reproduce the instability downstream of the orifice, which is caused by cavity shedding. The shedding cavities become mushy and frothy during thermal cavitation. The thermal effect term plays a dominant role during vaporization. The influence of thermal effects is positively correlated with the vapor volume fraction, and as the free-stream temperature and velocity increase, the ratio of the thermal effects term to the pressure difference effect term increases during the phase change process.

Sensitivity Analysis of Frost Deposition in Turbulent Flow over a Cold Plate using Direct Numerical Simulation

We use a recently-developed, turbulence-informed physics-based model to study frost buildup over a flat plate subjected to turbulent flow. The plate temperature is held fixed but varied across simulations in the range −10∘C≤Tp*≤−5∘C. Similarly, the free-stream temperature and humidity ratio are also held fixed but varied across simulations in the range 0∘C≤T∞*≤20∘C and 3.77×10−3≤w˜∞≤1.47×10−2, respectively. We find the Nusselt and Sherwood numbers, when scaled by the frost surface to free-stream temperature and humidity ratio difference, respectively, to become independent of free-stream temperature and humidity ratio as well as plate temperature. The scaling can be used in conjunction with the frost layer conservation equations of mass and energy to provide an estimate for the temporal evolution of the frost layer under turbulent flow conditions for a range of plate temperatures as well as free-stream temperatures and humidity ratios. For an initial shear Reynolds number of Re=180, we find the heat and mass transfer rates to be enhanced by about a factor of three compared to laminar flow conditions. We additionally conduct a sensitivity analysis on the frost layer density and thermal conductivity empirical correlations, as well as the frost layer Schmidt number. The latter parameterizes lateral frost diffusion and must be obtained from experiments. For the cases considered, we find the frost buildup to be only sensitive to the frost density correlation.

High resolved Optical Emission Spectroscopy as accurate physics methodology for plasma freestream temperature characterization

In this work the hypersonic plasma flow generated by SCIROCCO hypersonic facility has been experimentally characterized as test case, using a high resolved Optical Emission Spectroscopy (OES) during a high enthalpy test campaign. The CIRA methodology to measure roto-translational and NO vibrational temperatures of a free jet via analyses performed in the Ultra Violet spectral range has been improved and a comparison between experimental data and Computational Fluid Dynamics (CFD) simulations has been carried out. The revised methodology estimates the roto-translational and vibrational temperatures using high definition spectroscopy carried out at conditions of high values of pressure and enthalpy.In particular, attention is mainly focused on the NO emission band at Non-Local Thermal Equilibrium (NLTE) conditions in the hypersonic flight regime which means equilibrium between the molecular rotational and translational degrees of freedom but not between the roto-translational and vibrational degrees of freedom. Three tests at high enthalpy hypersonic conditions (23.8, 24.0 and 26.0 MJ/kg at a total pressure of about 2.5 bars) have been investigated and experimental data have been compared with numerical simulations in order to verify sensitivity of the method in estimating the small variation in NO roto-translational and vibrational temperatures by changing slightly the total enthalpy fluid-dynamic conditions. The current work presents a high definition spectral method which can be used to determine the temperature of NLTE hypersonic free jets, and may serve as a benchmark in determining the accuracy of various numerical models dedicated to these flow regimes.

The flow of MHD Maxwell liquid over an extending surface with variable free stream temperature

AbstractThe aim of this article is to investigate the steady two‐dimensional flow of a magnetohydrodynamics Maxwell liquid over a stretched exterior having prescribed exterior hotness in the presence of changeable free stream temperature. Here the Maxwell liquid replica reflects non‐Newtonian liquid behavior. By means of similarity alterations, the leading differential equations which are partial in nature are altered to differential equations that are ordinary in nature and subsequently numerical solutions are received via a shooting scheme. The motion and heat transport characteristics for the leading parameters is scrutinized and talked about elaborately using their graphical demonstration. It is established that the liquid speed reduces with escalating magnetic parameter. The consequences of growing Maxwell parameter are to hold back the speed of liquid. An increment in heat movement speed has been observed for enhancing the temperature ratio and Maxwell parameters.

Aerothermodynamics of a sphere in a monatomic gas based onab initiointeratomic potentials over a wide range of gas rarefaction: transonic, supersonic and hypersonic flows

Aerothermodynamic characteristics of a sphere such as drag and energy transfer coefficients are calculated for Mach numbers varying from 1 to 10 over a wide range of the gas rarefaction degree spanning the free molecular, transitional and near hydrodynamic flow regimes. The effects of major factors determining the gas flow are studied using the direct simulation Monte Carlo method. To reveal the effect of gas species, the simulations are performed based onab initiointeratomic potentials for helium, neon, argon and krypton, as well as based on the hard sphere model. The impact of the accommodation coefficients is evaluated by applying the Cercignani–Lampis model of gas–surface interaction. The calculations are carried out for several values of the free stream and sphere temperatures. It is found that the effects of gas species on the drag and energy transfer coefficients are approximately 3 % and 6 %, respectively. The accommodation coefficients in the Cercignani–Lampis model strongly affect all aerothermodynamic characteristics. The drag and energy transfer coefficients calculated for different accommodation coefficients vary within 30 % and 200 %, respectively. It is found that the variation of the tangential momentum and normal energy accommodation coefficients can induce an increase of the drag coefficient compared to the case of diffuse gas–surface interaction. In hypersonic flows, the drag coefficient varies within 30 % when the sphere temperature varies from the free stream temperature to the stagnation temperature. The drag and energy transfer coefficients are found to be non-monotonic functions of the free stream temperature.

Wall-cooling effects on secondary instabilities of Mack mode disturbances at Mach 6

In hypersonic boundary layers, Mack modes play a crucial role in flow instability, whose secondary instability is a hot research topic. Since hypersonic flight vehicles will probably work under high-stagnation temperature conditions, which significantly affect the aerodynamic heating calculation and aero-thermal protection design of hypersonic vehicles, it is necessary to compare the primary and secondary instabilities in high-stagnation temperature boundary layers and that in the Boeing/AFOSR Mach 6 quiet tunnel (BAM6QT). Herein, wall-cooling is adopted in order not to consider chemical reactions. With the same freestream temperature of 100 K, two Mach 6 boundary layers with the wall temperature of 20 and 600 K, corresponding to the cooled wall condition and the quiet wind tunnel condition, respectively, are chosen to conduct the linear/non-linear stability and the secondary instability analysis. Our results show that the most dangerous Mack mode originates from a fast discrete mode in the present cooled-wall flow and the most dangerous Mack mode is born from the slow discrete modes in BAM6QT boundary layers. Furthermore, when the primary amplitude of Mack mode disturbances is large, the fundamental resonance always dominates the secondary instability, resulting in steady streaky structures that have the largest amplitude in the spectrum. In addition, the present results point out that the distribution of the eigenfunctions of the fundamental modes and subharmonic modes are significantly different under various wall-temperatures. What is more, different ratios of wall temperature to incoming flow temperature have changed the spanwise wave-angle of the secondary disturbances.

Start-Up Analysis of a Hypersonic Short-Duration Facility

This paper analyzes the nozzle start-up of the von Kármán Institute Longshot, an impulse hypersonic gun tunnel, using a combination of the Lagrangian L1D code (used to model the initial compression process of the test gas into a reservoir by an inertial piston) and unsteady Reynolds-averaged Navier–Stokes simulations to model the flow expansion through the appended nozzle. The influence of initial nozzle flow conditions on the duration of the start-up is investigated based on various simulated experiments, and it is verified that the shortest establishment time is obtained for the lowest initial pressure. The applicability of the numerical scheme is proved through a comparison of the inlet and outlet total and static conditions against experimental results at relevant conditions. Besides, the right running and left running shocks constituting the start-up shock system are tracked through the nozzle, and their propagation velocities are reported. Momentum and thermal boundary-layer establishment times are reported based on the temperature and velocity profiles predicted at the nozzle exit. Finally, the flow establishment time is compared against several criteria available in the literature. Additionally, new establishment and quasi-steady behavior criteria are presented, derived from the freestream static pressure and temperature evolution.

Divergent channel flow of Casson fluid and heat transfer with suction/blowing and viscous dissipation: Existence of boundary layer

The boundary layer existence in divergent porous channel of non-Newtonian Casson fluid with heat transfer in presence of suction/blowing and viscous dissipation is studied. Nonlinear coupled ODEs are obtained from governing PDEs and the conditions under which boundary layer structure for Casson fluid exists by controlling backflow are explored. Numerical solutions are also obtained using “bvp4c”, a MATLAB package. The study reveals that if mass suction exceeds a certain amount which is dependent on Casson parameter then only boundary layer flow is possible and as Casson parameter increases, the requirement of mass suction for boundary layer flow reduces. In addition, rheological characters of Casson fluid and viscous dissipation have a major impact on temperature distribution and due to these, the temperature falls lower than free stream temperature within the boundary layer region.

Direct numerical simulation of hypersonic boundary layer transition over a lifting-body model HyTRV

Direct numerical simulation (DNS) of transition over a hypersonic lifting body model HyTRV developed by China Aerodynamics Research and Development Center is performed. The free-stream parameters are: the free-stream Mach number is 6, the unit Reynolds number is 10000/mm, the free-stream temperature is 79 K, the angle of attack is 0, and the wall temperature is 300 K. Weak random blowing-and-suction perturbations in the leading range are used to trigger the transition. A high order finite-difference code OpenCFD developed by the authors is used for the simulation, and grid convergence test shows that the transition locations are grid-convergence. DNS results show that transition occurs in central area of the lower surface and the concaved region of the upper surface, and the transition regions are also the streamline convergence regions. The transition mechanisms in different regions are investigated by using the spectrum and POD analysis.

Nonequilibrium effects in high enthalpy gas flows expanding through nozzles

An approach based on the direct simulation Monte Carlo method is proposed to model a core flow in a converging–diverging nozzle. The area of applicability of this approach is defined by the Boltzmann equation, which allows fully kinetic models that accurately capture thermal and chemical nonequilibrium to be applied to gas flows where the flow regime rapidly changes from continuum to transitional. The approach is validated through comparison with available experimental data. The examination of nonequilibrium and reaction rate effects for Caltech's T5 shock tunnel condition has shown little impact of nonequilibrium but demonstrated significant sensitivity of nitric oxide (NO) density to all exchange reaction and NO recombination rates. The use of the most recent theoretical and experimental rates results in a factor of two lower NO density at the nozzle exit as compared to the conventional Park rates, which indicates that re-visiting of the latter may be necessary. Multi-parametric sensitivity study of T5 conditions has not provided an explanation for a large drop in free-stream temperature and NO density over time, under constant flow velocity, observed recently in T5. Modeling of High Enthalpy Shock Tunnel Göttingen conditions has demonstrated considerable nonequilibrium between vibrational modes of N2, NO, and O2; it has also shown that the vibration–dissociation coupling strongly influences mole fractions of NO and O2.

Investigation of thermal adiabatic boundary condition on semitransparent wall in combined radiation and natural convection

Two thermal adiabatic boundary conditions arise on the semitransparent window (wall made of glass material) owing to the fact that whether semitransparent window allows the energy to leave the system by radiation mode of heat transfer or not. It is assumed that the energy does not leave by the conduction mode of heat transfer, due to the low thermal conductivity of semitransparent material. This does mean that the semitransparent window may behave as only conductive adiabatic or combined conductive and radiative adiabatic In the present work, the above two thermal adiabatic boundary conditions have been investigated for natural convection in a cavity, whose left vertical wall has been divided into upper and lower parts in the ratio 4:6. The upper section is a semitransparent window, while the lower section is an isothermal wall at a temperature of 296 K. A collimated beam is irradiated on the semitransparent window with values 0, 100, 500, and 1,000 at an azimuthal angle 135°. The cavity is heated from the bottom by convective heating with free stream temperature 305 K and heat transfer coefficient 50 While the right wall is also isothermal at the same temperature as of lower left wall and the upper wall is adiabatic. All walls are opaque for radiation except the semitransparent window. The results reveal that the dynamics of both the vortices inside the cavity change drastically with irradiation values and also with the boundary conditions on the semitransparent window. The temperature and Nusselt numbers increase many folds inside the cavity for combined conductive and radiative adiabatic condition than only conductive adiabatic condition on the semitransparent window.

Significance of thermal radiation and thermo‐physical properties on the modeling of sodium droplet explosion with different reaction rates

AbstractThe temperature of explosive substances (i.e., coolants) in the nuclear industries are necessary to ensure safety. The present study is focused on the analysis of the significance of thermal radiation and variable thermo‐physical properties on thermal stability characteristics of sodium droplet use as a coolant in the design of nuclear facilities. It is pertinent to note that the inclusion of heat loss by convection and heat generated by the reaction is based on Newton's law of cooling and Arrhenius kinetic equation, respectively, with the P‐1 approximation radiative heat loss model. The emerging mathematical model is a highly nonlinear ordinary differential equation. The model is solved using the classical fourth‐order Runge‐Kutta scheme. Consequently, the analysis of the results of the present study reveals the quantitative impacts of the thermal radiative model and variable thermo‐physical properties on the temperature of the sodium droplet. The results reveal that the combined inclusion of variable thermophysical properties and thermal radiation significantly reduces the droplet temperature. The temperature of the coolant increases with an increase in the reactants' numerical exponent and thermal radiation reduces the temperature of the sodium droplet at the initial temperature with the reverse effect at free stream temperature.

Experimental and numerical evaluation of thermodynamic effect on NACA0015 hydrofoil cavitation in hot water

In this study, the cavitation in hot water, which implies tight interaction of thermodynamic effect, phase change phenomena, and flow behavior, was studied by a combination of experiment and numerical simulation. The experiment in water up to 90°C was performed in the high temperature and high-pressure water tunnel with NACA0015 as a cavitator. The temperature inside the cavity was measured using the high-accuracy thermistor probe. According to the result, the temperature depression in the cavity was increased proportionally with the increase of freestream temperature. The inverse thermodynamic effect was observed with the increase of cavity length when temperature increased. The maximum temperature depression of about 0.41°C was measured in the water at around 90°C. The temperature drop was reasonably captured in simulation by coupling our simplified thermodynamic model with our cavitation model and governing equations. The tendency of temperature depression in the cavity agreed well with experimental data under different flow conditions.

Uncertainty Analysis of Inflow Parameters for Mid-Lift-to-Drag Vehicle Transition

The boundary-layer transition and aerothermal environment for mid-lift-to-drag-ratio entry vehicles are highly complex and influenced by several factors. The uncertainty and sensitivity of transition and aeroheating predictions on such vehicles with respect to the inflow parameters are investigated in this paper. Five inflow parameters (freestream Mach number, freestream pressure, freestream temperature, wall temperature, and angle of attack) are chosen as the uncertainty variables. The transition predictions are obtained using Reynolds-averaged Navier–Stokes simulations with the -- transition model. The point-collocation nonintrusive polynomial chaos method is employed to describe and propagate the uncertainty of the output quantities of interests. Moreover, the Sobol index is adopted to quantify the contribution of each variable to the total uncertainty. Our results reveal that small changes (less than 3.4%) in the inflow parameters could yield roughly 10% uncertainty in the transition lengths and more than 15% uncertainty in the transitional aeroheating and skin friction. The freestream temperature and angle of attack strongly influence the transitional aeroheating. The angle of attack, freestream pressure, and freestream temperature are the major uncertainty contributors to the skin friction. The freestream pressure and angle of attack significantly impact the transition onset and length.

Dimensionless Model of Frost Roughness on Cold Flat Plate Under Forced Convection

Laboratory experiments were conducted to generate empirical dimensionless correlations for estimating surface roughness of frost formed on a cold flat plate under forced convection. Environmental variables, including wall temperature , air velocity , relative humidity , and air temperature , were considered when generating the correlations. The test conditions were chosen to simulate the environmental conditions experienced by cold-soaked fuel frost formation on the upper surface of airplane wings. Experimental conditions included wall temperatures from to , freestream temperatures from 8 to , relative humidities from 60 to 91%, and air velocities from 0.5 to . The root-mean-square height, skewness, and equivalent sand-grain roughness height of frost were correlated as functions of the Reynolds number, absolute humidity, and dimensionless temperatures. The comparison between the correlation of frost equivalent sand-grain roughness height in this study and a previous model revealed the importance of test surface geometry on frost roughness formation.