Surface-averaged Pressure Research Articles

Numerical simulation of impact of supersonic molecular beam injection on edge localized modes

A three-field model with the impact of supersonic molecular beam injection (SMBI) based on the BOUT++ code is built to simulate edge localized modes (ELMs). Different parameters of SMBI are explored to find an optimal SMBI scenario for ELM mitigation. The linear simulations show that the growth rate of peeling-ballooning mode is reduced by SMBI. The reduction amplitude of the growth rate is increased when the amplitude or width of SMBI is increased, and when SMBI is deposited at the top, bottom and middle of the pedestal, the reduction amplitude increases successively. The nonlinear simulations show that the ELM size is reduced by SMBI. The reduction amplitude of the ELM size is increased when the amplitude or width of SMBI is increased, and when SMBI is deposited at the bottom, top and middle of the pedestal, the reduction amplitude increases successively. Surface-averaged pressure profiles and filamentary structures are analyzed when the ELMs erupt. Deep deposition of SMBI such as at the top and middle of the pedestal reduces the inward collapse amplitude of the pressure profiles, which can improve the confinement efficiency during ELMs. Shallow deposition of SMBI such as at the middle and bottom of the pedestal reduces the outer extent of the filamentary structures, which can slow down the erosion of plasma-facing components caused by ELMs. In conclusion, shallow deposition of SMBI with sufficient amplitude and width can meet the needs of ELM mitigation.

Optimization of RANS turbulence models using genetic algorithms to improve the prediction of wind pressure coefficients on low-rise buildings

Being associated with natural ventilation, the pressure distribution on surfaces is relevant for energy consumption, thermal comfort, and air quality in buildings. The aim of this work is to present a simulation-based optimization methodology to recalibrate the closure coefficients of Reynolds-averaged Navier-Stokes (RANS) turbulence models in order to improve the prediction of wind surface-averaged pressure coefficients on a wide range of isolated low-rise buildings. To accomplish this, genetic algorithms and Computational Fluid Dynamics (CFD) simulations are dynamically coupled to find the closure coefficients set which minimize the CFD prediction error regarding wind-tunnel experimental data. The methodology is applied to two turbulence models, the renormalization group k-epsilon model (RNG) and the Spalart-Allmaras model (SA), considering as target cases buildings with different roof types (flat, gable and hip) and wind incidence angles. In order to show the strength of the novel optimal sets of closure coefficients obtained, an exhaustive validation is performed over other low-rise buildings (52 new cases) which were not calibrated against. Results validate using the optimal sets because the recalibrated RNG and SA models decrease the prediction error between 11-64% and 8–45%, respectively, regarding using the standard ones.

Non-isothermal Transport of Multi-phase Fluids in Porous Media. Constitutive Equations

We develop constitutive equations for multi-component, multi-phase, macro-scale flow in a porous medium exposed to temperature-, composition-, and pressure -gradients. The porous medium is non-deformable. We define the pressure and the composition of the representative elementary volume (REV) in terms of the volume and surface averaged pressure and the saturation, and the respective driving forces from these variables. New contributions due to varying porosity or surface tension offer explanations for non-Darcy behavior. The interaction of a thermal and mechanical driving forces give thermal osmosis. An experimental program is suggested to verify Onsager symmetry in the transport coefficients.

Computational modeling of natural ventilation in low-rise non-rectangular floor-plan buildings

Natural ventilation (NV) is a relevant passive strategy for the design of buildings in seek of energy savings and the improvement of the indoor air quality and the thermal comfort. The main aim of this work is to present a comprehensive NV modeling study of a non-rectangular floor-plan dwelling. Given the arbitrary shape of the building, recourse is made to computational fluid dynamics (CFD) to determine the surface-averaged pressure coefficients ( $$\overline {{C_p}} $$ ). The CFD model was calibrated to match experimental data from an extensive wind tunnel database for low-rise buildings. Then, $$\overline {{C_p}} $$ computation via CFD is used to feed the building performance simulation software EnergyPlus, in replacement of the built-in Swami and Chandra parametric model that is only valid for estimating $$\overline {{C_p}} $$ in rectangular floor-plan buildings. This computational tool is used to investigate the effect of NV on the thermal performance and the airflow rate in a social housing located in the Argentine Littoral region. Simulation results of the considered building show that NV enables to reduce even more than 65% of the cooling degree-hours. Furthermore, regarding to the $$\overline {{C_p}} $$ source (either CFD or Swami and Chandra’s), it is also found that this data has a considerable effect on the airflow rates, but a little effect on the thermal performance.

Prediction of wind pressure coefficients on building surfaces using artificial neural networks

Knowing the pressure coefficient on building surfaces is important for the evaluation of wind loads and natural ventilation. The main objective of this paper is to present and to validate a computational modeling approach to accurately predict the mean wind pressure coefficient on the surfaces of flat-, gable- and hip-roofed rectangular buildings. This approach makes use of artificial neural network (ANN) to estimate the surface-average pressure coefficient for each wall and roof according to the building geometry and the wind angle. Three separate ANN models were developed, one for each roof type, and trained using an experimental database. Applied to a wide variety of buildings, the current ANN models were proved to be considerably more accurate than the commonly used parametric equations for the estimation of pressure coefficients. The proposed ANN-based methodology is as general and versatile as to be easily expanded to buildings with different shapes as well as to be coupled to building performance simulation and airflow network programs.

Flow-induced vibration in the compressible cavity flow

The cavity plays an important role in the fuel-air mixing and combustion stability inside the hypersonic scramjet. However, the high levels of time-dependent loading resulting from the supersonic cavity flow can cause intense structural vibration even damage. Experiments and numerical simulations were performed to understand the complex fluid-structure interaction in this paper. A cantilever plate with a cavity was installed as a splitter plate in the supersonic mixing layer wind tunnel. The response displacements of this cantilever plate were measured by a non-intrusive laser vibrometer. Large eddy simulation (LES) was applied to calculate the aerodynamic loading. Results show that the St number of time-dependent surface-averaged pressure difference agrees well with semi-empirical relation of Heller used to predict the resonance mode. The cantilever plate exhibits a directly dependent response to self-oscillation of supersonic cavity flow. Measurement results of displacement indicate that the vibration shape of this plate is dominantly two-dimensional.

Experimental and numerical investigation of bubble augmented waterjet propulsion

This contribution presents experimental and numerical investigations of the concept jet propulsion augmentation using bubble injection. A half-3D (D-shaped cylindrical configuration to enable optimal visualizations) divergent-convergent nozzle was designed, built, and used for extensive experiments under different air injection conditions and thrust measurement schemes. The design, optimization, and analysis were conducted using numerical simulations. The more advanced model was based on a two-way coupling between an Eulerian description of the flow field and a Lagrangian tracking of the injected bubbles using our Surface Averaged Pressure (SAP) model. The numerical results compare very favorably with nozzle experiments and both experiments and simulations validation the thrust augmentation concept. For a properly designed nozzle and air injection system, air injection produces net thrust augmentation, which increases with the rate of bubble injection. Doubling of thrust was measured for a 50% air injection rate. This beneficial effect remains at 50% after account for liquid pump additional work to overcome increased pressure by air injection.

Effects of Flow Patterns on Aerodynamic Forces of a Square Cylinder at Incidence

ABSTRACTThe pressure distributions around a square cylinder in a crossflow were experimentally studied in a wind tunnel. The subject of study was conventional, but the results presented new findings. The experiments were performed by using a home-made linear pressure scanner. The ranges of Reynolds number and incidence angle were 2 × 104- 9.4 × 104and 0° - 45°, respectively. According to the topological flow patterns, the flows around the square cylinder at incidence showed three characteristic regimes: The subcritical, supercritical, and wedge flows. A critical incidence angle αcri= 15° separated the regimes of subcritical and supercritical modes. The results of current study provided information regarding the effects of the topological flow patterns on surface pressure distribution, drag, and lift characteristics. The pressure distributions, drag, and lift presented different characteristics in different characteristic flow regimes and had close correlations with the flows. At the critical incidence angle 15° which separated the subcritical and supercritical regimes, the surface-averaged pressure coefficient on each face displayed local extreme value—The drag coefficient attained a minimum of 1.6, the lateral force coefficient reached a maximum of 0.9. The appearance of the minimum drag at the critical incidence angle was attributed to the reduction of wake width which was induced by two surface flow phenomena: (1) reattachment of the separated boundary layer on the lateral surface facing windward at the critical incidence angle and (2) flow pattern change on the lateral surface facing leeward.

Uncertainty in airflow rate calculations due to the use of surface-averaged pressure coefficients

Abstract Mean wind pressure coefficients ( C p ) are key input parameters for air infiltration and ventilation studies. However, building energy simulation and stand-alone airflow network programs usually only provide and/or use a limited amount of C p data, which are based on several assumptions. An important assumption consists of using surface-averaged C p values instead of local C p values with a high resolution in space. This paper provides information on the uncertainty in the calculated airflow rate due to the use of surface-averaged C p data. The study is performed using published empirical data on pressure coefficients obtained from extensive wind tunnel experiments. The uncertainty is assessed based on the comparison of the airflow rate ( ϕ ) calculated using the surface-averaged C p values ( ϕ AV ) and the airflow rate calculated using local C p values ( ϕ LOC ). The results indicate that the uncertainty with a confidence interval of 95% is high: 0.23 ϕ AV ϕ LOC ϕ AV . In cases with the largest surface-averaged Δ C p , the underestimation or overestimation is smaller but not negligible: 0.52 ϕ AV ϕ LOC ϕ AV . These results provide boundaries for future improvements in C p data quality, and new developments can be evaluated by comparison with the uncertainty of the current methods.

Numerical simulation of the flow in a rotating disk filtration module

The characterization of the flow inside an experimental flat membrane module with a smooth rotating disk was performed. The module consists of a disk rotating at speeds up to 3000 rpm inside a cylindrical housing equipped with a stationary circular flat membrane. The characterization was carried out by using a finite volume CFD software with the κ- omega turbulence model and results of the range of rotation speeds 300 ≤ Ω ≤ 20000 rpm were compared with experimental and theoretical data reported in previous studies. The simulations suggest high permeate fluxes for the device due to large average shear stresses on the membrane and the absence of stagnant zones inside the module, which are desirable features to avoid membrane fouling processes. The simulations show an overall good agreement with theoretical results based on the main assumption that the wall shear stress on the membrane and on the disk can be predicted using modified correlations for rotating flow over a stationary wall and for flow induced by a rotating disk, respectively and with experimental pressure measurements. It has been found that the flow rate imposed at the inlet of the module has an important effect on the pressure distribution. At the membrane some discrepancies were found between the results obtained with the simulations and with the theoretical approach because of the limitations of the assumptions, especially at low rotating speeds for which the effect of the flow through the module becomes important. The correlations relating the disk rotation rate with the surface averaged pressure and the shear stress on the membrane were determined.

Scaling of Tip Vortex Cavitation Inception Noise With a Bubble Dynamics Model Accounting for Nuclei Size Distribution

The acoustic pressure generated by cavitation inception in a tip vortex flow was simulated in water containing a realistic bubble nuclei size distribution using a surface-averaged pressure (SAP) spherical bubble dynamics model. The flow field was obtained by the Reynolds-averaged Navier–Stokes computations for three geometrically similar scales of a finite-span elliptic hydrofoil. An “acoustic” criterion, which defines cavitation inception as the flow condition at which the number of acoustical “peaks” above a pre-selected pressure level exceeds a reference number per unit time, was applied to the three scales. It was found that the scaling of cavitation inception depended on the reference values (pressure amplitude and number of peaks) selected. Scaling effects (i.e., deviation from the classical σi∝Re0.4) increase as the reference inception criteria become more stringent (lower threshold pressures and less number of peaks). Larger scales tend to detect more cavitation inception events per unit time than obtained by classical scaling because a relatively larger number of nuclei are excited by the tip vortex at the larger scale due to simultaneous increase of the nuclei capture area and of the size of the vortex core. The average nuclei size in the nuclei distribution was also found to have an important impact on cavitation inception number. Scaling effects (i.e., deviation from classical expressions) become more important as the average nuclei size decreases.

1446 3 圧力 2 流体モデルによる気泡流中を伝わる衝撃波の数値シミュレーション

The shock wave propagation in a bubbly liquid is numerically studied with a two-fluid and three-pressure model of averaged equations for two-phase bubbly flow. A surface average pressure at the bubble wall is introduced in addition to the volume average pressures for gas and liquid phases so as to deal with the local high pressure caused by the cavitation bubble collapse in high-speed bubbly flows. The governing equations are solved with MacCormack scheme. The influence of initial void fraction on the propagation of shock wave is clarified. Furthermore, it is shown that the difference of the propagation characteristics between the compression and expansion waves appears to be conspicuous more and more as the initial void fraction increase.