Stagnation Pressure Research Articles

Precise Fluid-Solid Simulation of Human Left Ventricle along with Aortic Valve during Systole

This paper presents an accurate blood flow model with tissue deformation of the human left ventricle, including the aortic valve. A two-way fluid-solid Interaction (FSI) algorithm is employed to simulate the performance of the human left ventricle during systole. The initial geometry of the left ventricle is extracted from CT scan images of a healthy person. The simulation results produced the systolic anterior motion of the Left Ventricle (LV) identical with the CT scan images at later times during systole. Besides, the numerical results for left ventricular volume change, maximum blood velocity at the aortic valve, and its maximum opening are in good agreement with physiological data. Although no clear image of the aortic valve is apparent in CT images, the FSI simulation predicted the maximum opening of the aortic valve to be 4.38 cm2 which is consistent with physiological observation on a healthy individual. As an application of the above algorithm, a model of Hypertrophic Cardiomyopathy (HCM) or septal wall thickening disease is constructed and studied during systole. This simulation provides an understanding of heart performance under HCM conditions. According to the simulation outcomes, the mitral valve approaches the septal wall under HCM due to the change in pressure gradient and the drag force on the mitral valve. This blockage of the LV blood passage by the mitral valve results in stagnation pressure loss and weaker hearth pumping power. Therefore, the maximum opening of the aortic valve, in this case, is 2.28 cm2, which is much lower than the physiological range, indicating the drastic effect of HCM on the performance of the aortic valve and systolic performance.

Bayesian Mass Averaging in Rigs and Engines

Abstract This paper introduces the Bayesian mass average and details its computation. Owing to the complexity of flow in an engine and the limited instrumentation and the precision of the sensor apparatus used, it is difficult to rigorously calculate mass averages. Building upon related work, this paper views any thermodynamic quantity’s spatial variation at an axial plane in an engine (or a rig) as a Gaussian random field. In cases where the mass flowrate is constant in the circumferential direction but can be expressed via a polynomial or spline radially, this paper presents an analytical calculation of the Bayesian mass average. In cases where the mass flowrate itself can be expressed as a Gaussian random field, a sampling procedure is presented to calculate the Bayesian mass average. Examples of the calculation of the Bayesian mass average for temperature are presented, including with a real engine case study where velocity profiles are inferred from stagnation pressure measurements.

Maximum Wall Stress on a Smooth Flat Plate Under Planar Jet Impingement

Abstract This paper investigates the maximum wall shear stress value τmax and its location xmax as measured on a smooth flat plate impinged upon by a normal planar jet. τmax and xmax are found to be closely related to the stagnation pressure Ps and the half-width of the mean wall pressure profile bpw. The measurements were made by two different techniques: a Stanton probe and oil film interferometry. The maximum wall shear stress location xmax is found to be independent of the jet Reynolds number. At a small nozzle-to-plate distance H≲6 Djet, xmax is related to the jet slot width as xmax≈1.1Djet. At a large nozzle-to-plate distance H≳6 Djet, the maximum wall shear stress location is related to the mean wall pressure half-width as xmax≈1.4 bpw. A new Reynolds number, referred to as the stagnation Reynolds number, is defined as Res=def2bpwPs/ρ/ν, where ρ is the fluid density and ν is the kinematic viscosity. The maximum wall shear stress is found to be strongly influenced by the stagnation Reynolds number, and the dependence as measured by Stanton probes is approximated by a power law of τmax/Ps≈0.38/Res0.38. The solution of the laminar flow equations in the Appendix gives an alternate relation for τmax, which is in better agreement with the oil film interferometry measurements. Dimensional analysis is performed to gain insight into the empirical findings.

Active Valve Resonant Pulse Combustor for Pressure Gain Combustion Applications

This work is an experimental demonstration of a novel resonant pulse combustor technology for which sustained pressure gain can be achieved at thermal equilibrium. The resonant pulse combustor employs an actively driven air inlet valve with passively modulated fuel injection such that the device operating frequency is specified by the valve set-point frequency. This configuration enables more control over the combustor operation compared to passive valve and valveless resonant pulse combustors and enables the durability necessary for consideration in practical applications. Combustor performance is characterized with measured stagnation pressure gain as a function of stagnation temperature and fuel mass flux. Steady-state stagnation pressure ratios between 1.005 and 1.025 are demonstrated for gaseous (ethylene) and liquid (gasoline) fuels at temperature ratios from 2.8 to 4.5. Baseline performance measurements for a well-studied passive valve resonant pulse combustor are presented for comparison. The proposed active valve technology provides a direct path for translation from research-type devices to practical gas turbine applications by addressing key challenges inherent of existing resonant pulse combustor technologies.

Second law analysis of aerodynamic characteristics with flow temperature variations of simple angle and compound angle full coverage film cooling

The present investigation employs entropy generation to quantify aerodynamic characteristics with flow temperature variations, and the associated second law losses, which are present within a film cooled boundary layer produced by a unique full-coverage compound angle hole configuration. With this arrangement, an alternating sign for the compound angle β is employed from one streamwise row of holes to another, such that compound angle β is +30° in one row of holes, followed by −30° in the next row of holes. Entropy change distributions, entropy generation distributions, and mass-averaged overall exergy destruction from this compound angle arrangement are compared to results from a simple angle arrangement with β = 0°. These quantities are determined from flow field measurements of local flow temperature variations, and local stagnation pressure variations with film cooling. Within the investigation, full-coverage film cooling blowing ratio values range from 2.9 to 6.0, mainstream Reynolds numbers Rems range from 127,000 to 142,000, and effusion flow Reynolds numbers range from about 7000 to approximately 13,000. Distributions of local entropy generation and mass-averaged overall exergy destruction (both relative to freestream flow) are determined using three analysis approaches, which account for local flow temperature variations only, local flow stagnation pressure variations only, and variations of stagnation pressure and local flow temperature together. Similar qualitative trends of local entropy generation for the three different types of data are observed, which mean that temperature and stagnation pressure within the flow experience similar rearrangements by secondary advection and diffusion phenomena for the two film cooling arrangements. From a quantitative perspective, values associated with flow temperature variations only, are consistently larger than values associated with stagnation pressure variations only. Important local variations of local entropy generation are observed for the compound angle arrangement, which are due to skewed and non-symmetric film distributions, along with the associated tilted horseshoe-shaped vortex, which forms around each film coolant concentration.

Observations on the deformation of metal microspheres in shock-driven polymer flows

Solid particles can be fragmented by a fast-moving fluid if their velocity difference is great enough, such as during the atmospheric entry of meteoroids or the shock compression of engineered particulate composites. The extent of particle deformation and breakup in such systems is poorly understood because the necessary extreme conditions make observation difficult and data scarce. To meet this need, experiments combining ultrafast synchrotron-based radiography with plate impact loading were performed at the dynamic compression sector at the advanced photon source. Metal microspheres of several densities and strengths (Au, Ta, and W) were placed inside a polymer matrix. A planar shock wave was then produced in the polymer by the impact of a gun-launched flyer plate. X-ray images of the resulting flow were collected at ∼150 ns intervals. These images document the progression of particle deformation across a range of flow conditions and particle materials. They show that the extent of deformation is sensitive to the ratio of drag stress to particle strength. The deforming particle's shape is determined by the initial shock–particle interaction, fluid stagnation pressure, and vorticity, each acting on its own timescale. A set of scaling relationships is presented to capture these observations and enable comparison with prior hydrodynamic data. The result is a framework for predicting the conditions under which strong particles are severely deformed by a shock-driven flow.

A New Loss Generation Body Force Model for Fan/Compressor Blade Rows: Application to Uniform and Non-Uniform Inflow in Rotor 67

Abstract Despite advances in computational power, the cost of time-accurate flows in axial compressor and fan stages with spatially non-uniform inflow is still too high for design-stage use in industry. Body force modeling reduces the computation time to practical levels, mainly by reducing the problem to a steady one. These computations are important to determine efficiency penalties associated with non-uniform inflows. Previous studies of body force methods have, in most cases, relied on computations with the presence of the blades to calibrate loss models. In some recent studies, uncalibrated models have been used, but such models can drop off in accuracy at conditions where separation would occur on the blade surfaces. In this paper, a neural-network-based loss model introduced in a recent paper by the authors is implemented for NASA rotor 67 for both uniform and non-uniform inflow conditions. For uniform inflow, the spanwise trend of entropy variation is generally captured with the new body force model. Although there are discrepancies at some span fractions, the present model generally predicts the compressor’s isentropic efficiency to within 3% compared to bladed Reynolds-averaged Navier–Stokes simulations. For non-uniform inflow, we consider a stagnation pressure profile representative of boundary layer ingestion. The results show that the region of maximum entropy generation is captured by the present model and the prediction of isentropic efficiency penalty due to the non-uniform inflow is only 0.2 points less than that determined from bladed computations.

Heat Transfer of Turbulent Gaseous Flow in Microtubes With Constant Wall Temperature

Abstract In this paper, we report on experimental results to measure the total temperature of nitrogen gas at the inlet and outlet of microtubes with constant wall temperature and to quantitatively determine the heat transfer rates. Experiments were conducted with nitrogen gas flowing in a stainless steel microtube with a diameter of 524 μm and a copper microtube with a diameter of 537 μm. The temperature differences between the inlet and the wall were maintained at 3, 5, and 10 K by circulating water around the inlet and the wall. The stagnation pressures were also controlled so that the flow, with atmospheric back pressure, could reach Reynolds numbers as high as 26,000. To measure the total temperature, a polystyrene tube with a thermally insulated exterior wall containing six plastic baffles was attached to the outlet. Heat transfer rates were obtained from the gas enthalpy difference by using the pressures and the total temperatures measured at the inlet and outlet. Heat transfer rates were also compared with those obtained from the ideal gas enthalpy using the measured total temperatures and from the Nusselt number of incompressible flows. It was found that the measured total temperature at the microtube outlet was higher than the wall temperature. Also, the heat transfer rates calculated from the total temperature difference were higher than the values obtained from the incompressible flow theory.

An optimisation method for the cold-spray process: On the nozzle geometry

Currently, the cold-spray process, or simply cold spray, is an extensively used technique in coating applications. The low temperature of the deposition process is the distinctive feature that makes it suitable for many additive manufacturing activities such as repair and restoration of damaged components. The reliability of the coatings is strongly dependent on the velocity of the powder during its impact on the target surface. Spraying conditions such as the pressure and temperature of the carrier gas and the geometry of the nozzle control the acceleration of the powder particles. Consequently, there is an increasing interest in the optimisation of nozzle geometry so as to maximise the acceleration of the particles through the nozzle path that they follow. In contrast with various extant approaches to achieve this aim (finite element modelling, experimental approach, and analytical methods), an alternative model based on the one-dimensional isentropic theory that accounts for the dynamics of the dilute two-phase flow was developed in this study. First, an analysis of the common hypotheses used to obtain the equation of motion of the particle was carried out. Subsequently, with the new insights revealed from the previous analysis, a new theoretical model for the optimisation of the divergent part of the nozzle was performed considering a geometric angle restriction. This model is based on the numerical integration of the equation of motion of the particle, ensuring the maximisation of the particle drag force by means of the Lagrange multiplier method. Once the analytical model is formulated, a set of curves describing the optimal geometric parameters for different conditions is obtained. Moreover, some optimal geometries are presented demonstrating the low influence of the angle restriction. Additionally, the inversely proportional relationship between stagnation pressure and temperature is revealed.

乱流域におけるマイクロチューブを流れるガスの温度回復係数の推定

In this paper, a temperature recovery factor defined as the ratio of the actual temperature rise at the wall to the maximum possible temperature rise were estimated from experimentally measured wall temperatures and pressures to determine the velocity and temperature of gas through an adiabatic microtube. The experiments were carried out using two pairs of stainless steel microtubes with D=528.4 and 754.2 μm. One of a pair was tested for measurements of local pressures to obtain local Mach numbers and bulk temperatures. And the other of a pair was tested for measurements of local wall temperatures. The stagnation pressure with the atmospheric back pressure was chosen in such a way that the outlet flow reached sufficiently choked. Local total and bulk temperatures were obtained with total and static enthalpies determined with the measured stagnation & local pressures, stagnation temperatures, mass flow rates by real gas manner. In order to estimate the temperature recovery factor, the measured wall temperature and the obtained total temperature were normalized by bulk temperature and represented as Mach numbers. The estimated values of the temperature recovery factor were independent of the Mach number. The present estimated temperature recovery factor was compared with numerical and experimental ones obtained with the assumption of an ideal gas.

Training Cost Reduction of Physics-Informed Neural Networks for Airfoil Aerodynamic Performance Prediction

This study works on how to simulate a flow field in a data-driven manner using a deep learning method called Physics-Informed Neural Networks (PINNs). This method trains a neural network, which inputs time and space and outputs the corresponding flow quantities by satisfying the Navier-Stokes equations, boundary conditions, and initial conditions. Then it can quickly reproduce a flow field in any time and space. The application of PINNs to engineering problems is an area for further investigation since PINNs need to be re-trained whenever the boundary conditions change, i.e., the training cost should be as low as possible. Hence, this study considers how to reduce the training cost of PINNs for an airfoil by adaptively arranging the collocation points. At present, this paper examines PINNs for a laminar flat plate as a preliminary step. The results show that, for qualitatively accurate PINNs, it is influential to consider the stagnation pressure at the leading edge of the plate as a constraint in the PINNs loss function.

Estimation of surface roughness from friction loss of gaseous flow though stainless steel microtubes

Effects of surface roughness on microchannel gas flow are significant since the surface roughness of a microchnnel is relatively large compared to conventional size tubes. Therefore, in this study, the effects of surface roughness on friction factors of nitrogen gas flow through stainless steel microtubes experimentally investigated under atmospheric back pressure and variable inlet pressure. The experiments were carried out with six different diameters stainless steel microtubes whose diameters range from 256 to 867 μm. The average values of Mach number and Fanning & Darcy friction factors between the inlet and outlet considering decrease in gas temperature were obtained from measured stagnation temperatures & pressures and mass flow rates. The effects of surface roughness on the compressibility effect were described with Fanning and Darcy friction factor correlations as a function of Mach number. The arithmetic mean height of the surface (Ra) of the microtube was also measured with a 3D laser scanning confocal microscope for profilometry. Comparing the measured Ra and the equivalent sand grain surface roughness (Ks) determined from Colebrook-White correlation, a correlation between Ra and Ks was obtained.

Micrometer-sized droplets from liquid helium jets at low stagnation pressures

Droplets and droplet beams produced from the breakup of micrometer-sized liquid helium jets in vacuum were studied in this work, advancing into previously unexplored regimes of low stagnation pressures. Using a 5 μm orifice, the droplet beam shows surprisingly diverse characteristics at increasing nozzle pressures from 0.6 to 100 bar: a well-collimated beam at low stagnation pressures, a spray at some intermediate values, and a less-collimated beam at high pressures. Focusing on a nozzle stagnation of 0.6 bar and 2.7 K, we highlight the spectrum of jet disturbances, resulting in different droplet beam behaviors. On some occasions, we observed uniformly sized and equidistant droplets with diameters ranging from 11 up to more than 25 μm and separations from 15 to 100 μm. From simple estimates using the ratio between the droplet separations and diameters, we determined the disturbance frequencies benchmarking the production of repeatable targets for future experiments with superfluid helium droplets. Further analysis of the droplet beam behavior at farther distances from the nozzle revealed that the droplet diameter grew downstream up to 22 μm from an initial value of 13 μm, while their aspect ratio decreased from 1.33 to 1.16. These results indicate that droplet coagulation and superfluidity both influence the droplet beam up to several hundreds of millimeters after the nozzle exit.

Research of the Stagnation Pressure Increasing Possibilities in a High-Speed Flow

Research of the Stagnation Pressure Increasing Possibilities in a High-Speed Flow

Predicting the Unstart of Scramjet Engine Air Intakes through CFD Simulation

Scramjet is an air breathing engine in which the combustion takes in a supersonic air stream by generating high pressure. Wedge shaped strut plays a major role in the scramjet engine to mix and compress air at high speeds, igniting the air fuel mixture and sustaining the flame. But the geometric profile of wedge in which the air gets blocked partially or completely is unanswered. Therefore, the effect of blockage on intake characteristics is investigated in this present study. Numerical investigations are carried out using ANSYS Fluent at Mach 5 to study the changes in static pressure, stagnation pressure, temperature and mass flow rate for different percentage of blockage which is varied from 10% to 45%. Unstart-free air intake for scramjet engine is ensured by keeping the percentage blockage of fuel injection struts between 10% and 35%. The Mach number at supersonic regime until 35% of blockage turns suddenly into subsonic conditions afterwards for further increase in blockage percentage. Moreover, as the blockage is further increased beyond 35%, bow shock is observed ahead of the cowl and the flow through the intake is subsonic, thereby indicating ‘unstart’ intake. Based on this geometry in air intake, smooth gas dynamic compression or ramming effect is obtained up to 35% of blockage.

A review of the bird impact process and validation of the SPH impact model for aircraft structures

The aim of this paper is to study the phenomenon of bird strike during each phase of the impact and to present a numerical model for its prediction in order to develop the best practices to make a structural component resistant to bird strike. To this end, the Smooth Particle Hydrodynamics (SPH) bird model is developed and validated based on experiments by Barber and Wilbeck and two papers by Guida et al. The experiments considered the impact of small birds on rigid flat panels. Guida et al. developed a 8 lb bird model to predict the impact on a deformable small leading-edge bay and on a full-scale leading edge. The hydrodynamic theory is applied to determine the shock pressure, the shock equation of state, the stagnation pressure and the steady-state equation for water with different porosities. Subsequently, the bird structure is analyzed for different bird geometries and target models. This analysis allowed to design critical components of an aircraft structure, such as the leading edge of the C27J aircraft tail cone in compliance with current aviation airworthiness requirements.

Simulation and experimental study on the influence of needle-free jet injection nozzle structure on injection performance

Needle-free jet injection is an excellent alternative to conventional injection, which can reduce patients’ fear of needle and needle injury and reduce the probability of cross contamination. It is highly appropriate for the administration of small doses of medicines, such as vaccines and insulin. The structure of the needle-free jet injector nozzle determines the properties of the jet, and the optimized nozzle can improve injection performance. This study used the realizable k-ε turbulence model to study the effects of different nozzle cone angles and nozzle lengths on the injection effect. The peak stagnation pressure, jet velocity, and peak turbulence intensity were selected as the primary research parameters. We achieved a better nozzle structure with further analysis, and experiments were performed to verify the accuracy of the simulation model. The simulation results show that in the five nozzles with different taper settings, the nozzle with a smaller taper has higher jet velocity and peak stagnation pressure at the nozzle orifice and within a specific taper range. On the contrary, the nozzle with a smaller taper has a lower jet velocity at 4 mm away from the nozzle orifice. The effect of the nozzle length on peak stagnation pressure is limited and shows no apparent law, while the effect on the turbulence intensity is evident. The longer the nozzle length is, the less turbulence intensity is. At the same time, we also studied the influence of different viscosity liquids and different elastic coefficients of the driving spring on the injection performance. The jet impact force experiment was carried out. The results show that the simulation results are highly consistent with the experimental results, which verify the accuracy of the CFD model. The results provide ideas for optimizing the nozzle structure of commercial needle-free jet injector and help realize efficient drug delivery.

Design and optimization of aerospike nozzle for rotating detonation engine

Rotating detonation engine (RDE) is increasingly viewed as a viable technology to improve the performance of propulsion devices. At now, the design of RDE faces challenges when integrated with aerospike nozzle due to the large pressure fluctuations of rotating detonation waves. To optimize the nozzle configuration for thrust generation, the current work investigates the performance of RDE combined with various types of aerospike nozzles. The method of designing optimal aerospike nozzle for RDE with annulus combustion chamber is proposed. A series of three-dimensional numerical simulations are conducted to obtain the thrust performance of nozzles based on open-source software OpenFOAM. The flow-field structure, pressure gain in combustor and thrust performance of RDE are investigated. The results suggest that although the instantaneous flow-field in RDE is time-varying, the averaged plume field can be viewed as axisymmetric under-expanded supersonic jets. It's reasonable to use the time-averaged value of physical quantities for nozzle design. For nozzle with inward-bend cowl lip and isentropic ramp surface, the supersonic flow in nozzle undergoes approximate isentropic expansion with minimum loss. The flow at nozzle exit is basically parallel to chamber axis, generating the optimal thrust. This indicates that the aerospike nozzle is a good match for efficient expansion of rotating detonation products for propulsion. Moreover, the stagnation pressure gain in combustor has been greatly improved with the constriction nozzle throat. This suggests that in order to elevate the chamber pressure, the convergence section with nozzle throat should be adopted in RDE just as in traditional rocket engines.

Experimentally validated pore-scale numerical analysis for high-temperature (>700°C), high-efficiency (>90%) volumetric solar receivers

• Proposes an innovative, high-temperature semi-transparent packed-bed solar receiver. • Hits SunShot target with one of the highest efficiencies, 92% for T outlet ≥ 700 °C. • Develops a 3-D pore-scale model which couples the optical and thermo-fluid analyses. • Minimises reflection loss via an optical baffle and the pore-scale arrangement. • Reports new experimental validation test data for heat transfer in packed bed. Concentrated solar thermal/power (CST/P) systems are evolving to operate at higher temperatures (i.e., above 700 °C) to drive advanced cycles and thermochemical processes. The development of high temperature, high efficiency solar receivers—as the highest temperature component in a CST/P system—represents a critical technical challenge in this evolution. Commercial solar receivers have been limited to operation below 600 °C due to (1) the historic trade-off between operation temperature and efficiency, and (2) the operating temperature limits of the liquid conventional heat transfer fluids. This paper investigates a gas-phase, packed-bed receiver, which is capable of addressing these limitations. To date, this novel design has shown promise for this application, but it has only been modelled on the macro-scale (i.e., a 1-D model). To obtain a more detailed understanding of this type of receiver, this paper presents the results of a novel 3-D pore-scale analysis of the proposed receiver to characterise the optical-thermo-fluid behaviour. To ensure the validity of this approach, experimental tests were conducted for a complex packed bed of spheres. The 3-D pore-scale analysis enables the development of innovative mechanisms to reduce radiosity losses. It also provides a deeper level of insight regarding the stagnation pressure loss caused by the inlet air-jet flows. The final results demonstrated that the pore-scale modifications of the design can yield an optimised packed-bed receiver with a maximum efficiency of ∼ 92 % (for a hot air outlet temperature well above 700 °C). This optimised receiver design represents a new pathway towards high-temperature solar receivers, which can operate at very high efficiency, to enable CST/P systems to utilise advanced power cycles.

Qualification and reusability of long and short fibre-reinforced ultra-refractory composites for aerospace thermal protection systems

Qualification and reusability campaigns were performed on ultra-high temperature ceramic matrix composites (UHTCMCs) made of a ZrB2-SiC matrix with short/long carbon fibre to assess their performance as thermal protection systems. Heat fluxes and stagnation pressures were set following those of reference re-entry missions. An interdisciplinary analysis followed by combining infrared measurements of the thermal response, estimation of surface properties and microstructural characterization. Compared to conventional C/SiC composites, the increased erosion resistance of the new UHTCMCs was assessed in ultra-hot regime: although these experienced much higher thermal loads, they underwent minimal erosion and preserved their structural stability even after multiple exposures.