3D Numerical Simulation and Structural Optimization for a MEMS Skin Friction Sensor in Hypersonic Flow

A skin friction sensor is a three-dimensional MEMS sensor specially designed for measuring the skin friction of hypersonic vehicle models. The accuracy of skin friction measurement under hypersonic laminar flow conditions is closely related to the fabrication and micro-assembly accuracy of MEMS skin friction sensors. In order to achieve accurate skin friction measurement, high-precision linear laser scanning ranging, multi-axis precision drive, and 3D reconstruction algorithms are investigated; a MEMS skin friction sensor micro-assembly height error measurement system is developed; and the MEMS skin friction sensor micro-assembly height error control method is carried out. The results show that the micro-assembly height error measurement of MEMS skin friction sensors achieves an accuracy of up to 2 μm. The height errors of the MEMS skin friction sensor were controlled within −8 μm to +10 μm after error control. The angular errors were controlled within the range of 0.05–0.25°, significantly improving micro-assembly accuracy in the height direction of the MEMS skin friction sensor. The results of hypersonic wind tunnel tests indicate that the deviation in the accuracy of the MEMS skin friction sensors after applying height error control is about 5%, and the deviation from the theoretical value is 8.51%, which indicates that height error control lays the foundation for improving the accuracy of skin friction measurement under hypersonic conditions.

A Relation between flow structure and local skin friction on the wall surface of the clustered linear aerospike nozzle models was investigated by conducting cold flow experiments using a skin friction sensor which has been developed in this study. The skin friction sensor is capable of measuring the local skin friction value on the wall. Measured skin friction value was compared to some existing prediction model and results found that the local skin friction value becomes drastically different from the one predicted by existing models. In cases of over-expansion condition skin friction was drastically increased by oblique shock wave. In cases of under-expansion condition skin friction was increased which was caused by interference of jet boundary of cell-nozzle flows jetted from adjoined cellnozzle.

This study presents an application of the naphthalene sublimation technique to examine the manner in which solid fuels utilized in air-breathing engines may be expected to regress under various flow conditions. A counterflow burner rig was used to exhibit the increase in the sublimation rate of naphthalene when subjected to greater levels of shear stress and surface temperatures. Additionally, skin friction sensor data was taken using Ahmic Aerospace’s supersonic wind tunnel, with a naphthalene coated flat plate instrumented with three skin friction sensors installed in the wall of the wind tunnel. It was found that wall shear decreased an average of 11% across the three sensors over the course of four Mach 2.4 wind tunnel runs. This indicates that naphthalene sublimation can be used as an effective tool to study changes in surface roughness and skin friction at supersonic Mach numbers. Future work with Ahmic’s skin friction sensors and thermocouples embedded within cast naphthalene will be conducted in Virginia Tech’s Hypersonic Wind Tunnel at the Advanced Power and Propulsion Laboratory. This will allow for further definition of the skin friction experienced by naphthalene, which can be used to characterize the skin friction experienced by solid fuels in similar flow scenarios. The naphthalene sublimation technique provides analogous data to study the rate and manner of regression of solid fuels which can also be utilized to refine computer models of surface regression.

This investigation concerns the design, build, and testing of a new class of skin friction sensors capable of performing favorably in high-speed, high-enthalpy flow conditions, such as those experienced by atmospheric re-entry vehicles, scramjets, jet engines, material testing, and industrial processes. Fully understanding and optimizing these complex flows requires knowledge of the relevant aerodynamic properties, which, in turn, requires numerical and analytical modeling as well as reliable diagnostic instrumentation. A rigorous, multistep approach was developed to systematically test the present skin friction sensors through various facilities, where flow enthalpy and run duration were progressively increased, culminating in simulated scramjet flight conditions at the Air Force Research Laboratory (AFRL). At AFRL, nominal Mach 2.0 to 2.2 flows were studied at stagnation pressures and temperatures ranging from 172 to 995 kPa (25 to 144 psia) and 294 to 1000 K (530 to 1800 °R), respectively. The skin friction sensors made static wall shear stress measurements between 94 and 600 Pa (1.96 and 12.5 psf) and skin friction coefficient measurements between 0.00158 and 0.00280. In addition, the sensors were able to clearly indicate the transient flow conditions of a full scramjet combustion operability cycle to include shock train movement, separation zones, and reverse flow along the isolator wall. The sensor demonstrated full functionality under sustained high-enthalpy flow conditions upward of 1053 kJ/kg (453 Btu/lbm). Issues with pressure disturbance impingement on the sensor were identified and discussed. The total uncertainty of the present skin friction sensor was determined to be ±8.7% at 95% confidence. The experimental wall shear data demonstrated good agreement with simple, flat-plate analytical estimations. Numerical estimations of the scramjet flow path resulted in favorable results for steady coldand hot-flow conditions, but had to be refined to handle the various fueling injection schemes with burning in the downstream combustor and surface roughness models.

The studies on vascular flows have increased in the last decade. In this work; we have focused on the effects of flow model and blood rheology on hemodynamics for a real-subject scan using Computed Tomography Angiography (CTA) during numerical solutions. Various vascular flow studies using Newtonian or non-Newtonian blood models were presented in the literature with laminar or turbulent flow assumptions. In this study; six different turbulent models (Realizable k-[Formula: see text], Standard k-[Formula: see text], SST k-[Formula: see text], Standard k-[Formula: see text], Transition k-kl-[Formula: see text], Transition SST) were compared to laminar flow to show whether turbulent flow solution is necessary. Blood rheology was investigated by using five different non-Newtonian models (Carreau, Herschel–Bulkley, Carreau–Yasuda, Casson, Power-Law) in addition to Newtonian model to indicate whether non-Newtonian blood assumptions is necessary. The In vivo boundary conditions were utilized by the UDF code which defines the real-patient cardiac cycle obtained by Echocardiography (ECHO) to present hemodynamics in the study. The results show that laminar flow well matched with the four turbulent models and two models shows by 4.8% and 19.5% differences in Wall Shear Stress (WSS) according to laminar flow. When the blood rheology was investigated, results revealed significant differences in WSS by 25.7%, 8.7%, 22.4%, 12.3%, and 32.5% for the non-Newtonian models in the given order, respectively, compared to Newtonian assumption. It concluded that laminar flow solution could be effective instead of solving turbulent flows in terms of computational cost, however, non-Newtonian blood effects could be considered to determine critical hemodynamics levels in a normal aortic arc.

High heat fluxes occurring in modern electrical systems require direct cooling methods in which the electrical components come in direct contact with the cooling fluid. Hence electrically non-conductive fluids with thermal properties inferior to the ones of water have to be applied. In order to assess the cooling performance of such fluids a physically-derived figure of merit (FOM) is presented based on the ratio of the achieved heat flux and the required pumping power in an exemplary cooling application under laminar flow conditions. This cooling application is the stator cooling of an electrical engine through two free-surface impinging jets, each one aimed axially on each coil end of the stator teeth. Due to the high viscosities of most cooling fluids combined with small geometrical measures of modern electrical components, the assumption of a laminar flow is often justified. The FOM is derived analytically through generally accepted correlation functions and solely a function of the thermal fluid properties. Hence it is suitable for any other cooling configuration with laminar external flow conditions. The results of an experimental study of the considered cooling case are used to verify and further enhance the FOM.

New pressure and skin friction measurements on a cylinder were made and compared with analytical and computational modeling. The measurements were made in what we call the stagnation region, such as exists on the leading edge of a turbine airfoil. The new skin friction measurements were made with an oil film interferometry technique that has been used in earlier turbine cascade studies, and the accuracy of the technique is demonstrated by comparisons with the analytical and computational modeling. The analytical model is an exact solution for laminar stagnation point or Hiemenz flow, and it is based solely on a reference velocity and a length scale, both which describe the free stream acceleration and pressure gradient in the stagnation region. Comparisons were made between the analytical model and measurements of preasure and skin friction from other cylinder studies in the open literature. These comparisons were used to demonstrate that the analytical model accounts for variations in the free stream pressure gradient and acceleration of the flow due to blockage, separation location, and compressibility. The simplicity of the analytical model allows one to quickly characterize stagnation region aerodynamics based only on a pressure profile, which can be used, for example, to assess the computational grid density in the stagnation region of a turbine airfoil. This basic cylinder experiment along with the analytical modeling are shown to be particularly effective for the validation of skin friction techniques, and this study serves to validate the OFI method used in earlier cascade studies.

Aerodynamic characteristics of a guided rocket projectile are numerically simulated. Simulations are performed in a range of flight conditions using a commercial CFD software FLUENT, and variations of lift coefficients and drag coefficients with Mach numbers and angles of attack were demonstrated. The numerical results also indicate that the angle of canard deflection has significant influence on the flow field near tailfins.

This paper focuses on the convective heat transfer characteristics of Fe3O4-water magnetic nanofluids under laminar and turbulent conditions. After verifying the accuracy of the experimental apparatus, the effects of magnetic field strength, concentration, Reynolds number and temperature on the convective heat transfer coefficient have been studied. The convective heat transfer characteristics of nanofluids under laminar and turbulent flow conditions were studied in depth, and the influence of each factor on the heat transfer coefficient was analyzed by orthogonal experimental design method. Under the laminar flow conditions, the convective heat transfer of magnetic nanofluids performed best when the Reynolds number was 2000, the magnetic field strength was 600, the temperature was 30?C, and the concentration was 2%. The convective heat transfer coefficient, h, increased by 3.96% than the distilled water in the same conditions. In turbulent state, the convective heat transfer of magnetic nanofluids performed the best when the Reynolds number was 6000, the magnetic field strength was 600, the temperature was 40?C, and the concentration was 2%. The h increased by 11.31% than the distilled water in the same Reynolds number and the magnetic field strength conditions.

The converter transformer is subjected to AC/DC composite voltage during operation, and the sealed and time-varying internal state makes its electric field distribution and charge accumulation unable to be monitored in real-time experiments. In this paper, aiming at the influence of bubbles in the oil passage of the converter transformer on charge accumulation before discharge, a simulation model in a laminar flow environment is established, and four different calculation conditions are set to simulate the charge accumulation in 1 s. It is found that under laminar flow conditions, the trapped bubbles on the insulation paper wall play an obvious role in intensifying the charge accumulation in transformer oil, and the extreme range of charge density will increase by about 104 times. Bubbles aggravate the electric field distortion, and the insulation strength of bubbles is lower, which becomes the weak link of insulation. In the laminar flow environment, the oil flow will take away part of the accumulated charge in the oil, but in the case of trapped bubbles, the charge accumulation in the insulating paper will increase from the order of 10−2 to 10−1. In the case of no bubbles, the transformer oil layer flow will increase the charge accumulation in the insulation paper by 4–5 orders of magnitude. Therefore, it can be seen that the flow of transformer oil will increase the deterioration level of insulation paper. And when the transformer oil is already in the laminar flow state, the influence of laminar flow velocity on charge accumulation is not obvious. The research results in this paper provide a time-varying simulation reference state for the charge accumulation problem that cannot be measured experimentally under normal charged operation conditions, and we obtain quantitative numerical results, which can provide a valuable reference for the study of transformer operation and insulation discharge characteristics.

Transitional flow has a significant impact on vehicles operating at supersonic and hypersonic speeds. An economic way to simulate this problem is to use computational fluid dynamics (CFD) codes. However, not all CFD codes can solve transitional flows. This paper examines the ability of the Spalart–Allmaras one-equation BCM (SA-BCM) transitional model to solve hypersonic transitional flow, implemented in the open-source CFD code Eilmer. Its performance is validated via existing wind tunnel data. Eight different hypersonic flow conditions are applied. A flat plate model is built for the numerical tests. The results indicate that the existing SA-BCM model is sensitive to the freestream turbulence intensity and the grid size. It is not accurate in all the test cases, though the transitional length can be matched by tuning the freestream intensity. This is likely due to the intermittency term of the SA-BCM model not being appropriately calibrated for high-velocity flow, though if the model can be recalibrated it may be able to solve the general high-velocity flows. Although the current SA-BCM model is only accurate under certain flow conditions after one calibration process, it remains attractive to CFD applications. As a one-equation model, the SA-BCM model runs much faster than multiple-equation flow models.

The study presents wind-tunnel measurements of surface static pressures, equilibrium temperatures, and skin friction downstream of tangential slot injection into a thick turbulent hypersonic boundary layer from two modified slot configurations. The data are compared with results obtained for baseline configurations reported by Cary and Hefner (1970, 1972) to determine whether simple modifications to the slot configuration can produce improved cooling effectiveness and skin friction reduction. The baseline slot configurations are simply modified by thickening the slot lip and by elevating the location of the slot exit above the flat plate. Although the results indicate that simple modifications of the baseline slot configurations can enhance the skin friction reductions obtained with tangential slot injection, slot base drag estimates show that neither of the modifications will lessen the impact of the systems penalties for collecting, ducting, and injecting the slot air.

A beam-continuum model is used to calculate the hypersonic flow past a flat plate at zero angle of incidence. The hypersonic freestream is modeled by a zero temperature molecular beam. The beam molecules are converted into continuum molecules after experiencing collisions with other continuum or with reflected molecules. Navier-Stokes terms are used to describe the behavior of the continuum molecules with extra terms to account for the addition of mass, momentum, and energy through collisions with beam molecules. Thin layer approximations are made. The resulting initial value problem is solved numerically by the Dufort-Frankel centered difference method. Comparison with experimental density profile measurements and purely continuum theoretical calculations shows much improvement over continuum models in the maximum density ratio and the thickness of the shock structure in the merged layer region. The skin friction, heat transfer coefficients, and surface pressure at the plate surface are compared with results from continuum models and experiments.

Computational fluid dynamics (CFD) is a powerful tool for analyzing the performance of heat exchangers. However, such an approach may be often limited by unaffordable computational time. In this paper, a multiscale CFD capable of accurately and efficiently prediction of the heat transfer of compact heat exchangers is presented. This methodology is based on a small-scale CFD analysis of a single tube and a small element of the compact heat exchanger, and it is able to predict the thermal performance of an entire heat exchanger in a wide range of inlet conditions, with a reduced computational time. The proposed up-scaling procedure makes use of specific approximations and correlations derived from the CFD model and literature, in order to consider the typical phenomena occurring in compact heat exchangers under laminar flow conditions. Results demonstrate an excellent accuracy when compared to experimental data (discrepancies <4.3%). This novel up-scaling method may have a strong impact on modeling and design strategy of compact heat exchangers in the industrial field.

The application of a heated thin metallic film to the measurement of mean skin friction in laminar and turbulent flow on a flat plate, circular cylinder and in an annular tunnel is described. The manner in which transition and separation are detected with this instrument is illustrated by reference to tests on a circular cylinder. The influence of the ambient air temperature and the gauge temperature on the behaviour of the instrument is analysed and it is shown that a reliable element can be constructed, capable of being moved from one location to another, while retaining its calibration. An element, calibrated on an oscillating flat plate, is used to obtain the spectral density of skin friction in a turbulent boundary layer.