Complicated Flow Physics Research Articles

BEM modelling of a hydrodynamic lifting surface working in ground proximity

Present paper aims to assess the performance and limits of boundary element method for the reliable simulation of ground effect phenomena on hydrodynamic lifting surfaces. Benefitting from the ground effect, especially in extreme ground proximity, suggests a promising concept for lifting bodies. On the other hand, the more pronounced the ground effect, the more complicated flow physics it exhibits, making the problem harder to handle numerically. In this study, a potential-based boundary element method (BEM) is applied to the flow past a 3D rectangular wing within a wide range of ground clearances corresponding to out of ground effect (OGE), in ground effect (GE) and extreme ground effect (EGE) conditions. Both positive and negative angles of attack are tested to examine the lift and downforce producing cases. The potential flow around the airfoil is computed using the mixed constant-strength source and constant-strength dipole based panel method. The results show that BEM successfully predicts the lift force variation for a wing working in ground proximity (GE). However, approaching the bounds of extreme ground effect is seen to introduce a progressive decrease of the fidelity of the method. A similar tendency is observed in the downforce generating conditions, which is also an important scenario for ground vehicle aerodynamics. BEM remarkably overestimates the magnitude of the downforce when the ground clearance reaches up to the EGE conditions, still, it produces quite satisfactory results for the rest of the tested configurations. The pressure distributions are also provided to examine the flow field in the aforementioned cases. Overall, the results lead us to conclude that BEM stands as a robust and reliable prediction tool for ground effect on 3D wings but the extreme ground effect condition represents the limit of its accuracy range.

Multiscale pore-fracture hybrid pore network modeling for drainage in tight carbonate

Tight carbonate possesses both fractures and multiscale pores. Flow through tight carbonate rocks involves several length scales and the complicated flow physics. Capturing and modeling the entire void space into a single pore network model for drainage is challenging. Here, we present a novel method for modeling of a multiscale pore-fracture hybrid pore network, which enables various void spaces at multiple length scales and the flow characteristics in them to be included. Pores and fractures in a low-resolution CT image are extracted by medial axis-maximal ball algorithm and medial surface algorithm, respectively, to generate a pore-fracture hybrid pore network. Then, a statistics-based small-scale stochastic pore network is established in the CT image solid domains to represent the micropores unresolved by the image. The two pore networks are organically integrated into a multiscale hybrid pore network by cross-scale connections. The statistics of small-scale pore network are determined by experimental permeability. We study the effects of the small-scale pore network density on connectivity, single- and two-phase flow properties of the model to obtain the robust parameter. The influence of fractures on two-phase flow properties is analyzed. Results show the significance of incorporating fractures and micropores in the model. The model is validated via comparison between simulation result and experimental data. This method can accurately, efficiently, conveniently construct multiscale pore-fracture hybrid pore network of tight carbonate rock for evaluating the drainage characteristics.

Parallel fully implicit chemical potential-based modeling of unconventional shale gas reservoirs

The design of novel mathematical models and state-of-the-art simulators for unconventional shale gas reservoirs plays an increasingly important role in the current energy mix of the world’s growing energy demand. In this paper, we present a robust and scalable framework, which is based on the chemical potential-based modeling and the fully implicit method (FIM), to model and simulate this highly nonlinear flow transport problem on a large scale. In the proposed approach, a thermodynamically consistent mathematical model of gas flow in shale formations, which employs the gas density and the chemical potential gradient, is developed to satisfy the second law of thermodynamics and meanwhile guarantee the energy dissipation property. And then the family of fully implicit finite element algorithms is utilized to accurately capture the complicated flow physics behind the transport process in shale media on high resolution grids. In particular, our approach further enhances the numerical formulation by proposing the family of Newton–Krylov methods for efficiently computing, and the parallel implementation of the simulator is achieved by using the domain decomposition technique. Numerical experiments are presented to demonstrate the robustness and parallel scalability of the solution strategies for several interesting shale gas flow problems in two or three dimensions. With the proposed parallel method, large-scale reservoir simulation can be obtained on the Shaheen-II supercomputer with up to 40960 processors, which enables to achieve a good strong scalability by saving more than 90 percent of computing time, when the problem size is enlarged to hundreds of millions of degrees of freedom.

Transonic Shock Buffet Flowfield Assessment Using Various Dynamic Mode Decomposition Techniques

Dynamic mode decomposition (DMD) is a powerful data-driven modal decomposition technique that extracts spatiotemporal coherent structures: a useful process in flow diagnostics and future state estimation of complex nonlinear flow phenomena. Transonic shock buffet is a complicated phenomenon, and modal decomposition techniques such as DMD provide significant insight into its complicated flow physics; but, often, flowfield data are corrupted because of various sources of noise due to the presence of outliers or the absence of critical data components. Therefore, noise corruption renders the modal decomposition inaccurate, and thereby not useful. In this paper, two sources of noise have been considered: simple white noise, and complex salt-and-pepper-type spurious noise. Various DMD techniques including standard DMD, forward–backward DMD, total-least-squares DMD, higher-order DMD, and robust DMD have been implemented. Their effectiveness and limitations in countering noise corruption have been investigated systematically. In the case of white noise corruption, forward–backward DMD, total-least-squares DMD, and higher-order DMD capture the buffet frequency and growth rate with sufficient accuracy, whereas the latter outperforms the other two when the noise variance level is above 5%. In the case of spurious noise, robust DMD handles noise corruption efficiently, with surprisingly high values of pixel corruption of up to 30%.

Flow Patterns of Self-Sustained Oscillations in Fluidic Diverters

A fluidic oscillator is a device capable of transforming a steady input flow into an oscillatory output using solely fluid dynamic principles and without requiring any moving components. Although the basic operation principles of fluidic oscillator are known to some extent, the unsteady and turbulent nature of its internal flow remains difficult to understand. The current study aims to further clarify the complicated flow physics involved in fluidic oscillators by investigating a device based on the fluid diverter principle. Spatiotemporally resolved velocity and pressure data obtained from a large-eddy simulation of the internal flow are decomposed into the proper orthogonal modes. The finite-time Lyapunov exponents are computed on the flowfield reconstructed using the proper orthogonal modes to perform a Lagrangian analysis of the switching mechanism. It is found that four dominant modes are sufficient to describe the jet switching. Strong coherent structures are found to separate the flow going through the oscillator from secondary flows through the feedback tube. The deflection of the jet inside the device is found to depend only on the pressure difference across the control ports, whereas the change of this pressure difference is mediated by the flows in the feedback tube.

Bound-preserving inexact Newton algorithms on parallel computers for wormhole propagation in porous media

Simulating wormhole propagation during reactive dissolution of carbonates is often significantly challenging, because of the high nonlinearity of the governing equations with complex fluid physics. High resolution grids are often required to represent the complex geological heterogeneity, which demands massively parallel computers with a large number of processors. Herein, we present a parallel and scalable simulator on parallel computers for the fully implicit solution of the wormhole propagation model. Our approach is based on a family of mixed finite element methods for the spatial discretization and the implicit Backward Euler scheme for the temporal integration, to handle the combination of complicated flow physics and high resolution grids in their full complexity. Moreover, the active-set reduced-space method, as a class of bound-preserving inexact Newton algorithms, is proposed for the resultant nonlinear system of equations, to guarantee the nonlinear consistency of the fully implicit discretization in a monolithic way and to ensure the boundedness requirement of the solution. Numerical experiments are presented to demonstrate the effectiveness and efficiency of the proposed simulator for a set of heterogeneous medium problems. Large-scale results are provided to show the scalability for reservoir simulation with hundreds of millions of unknowns by using several thousand processors.

Near-field plume-surface interaction and regolith erosion and dispersal during the lunar landing

A rocket plume impinging on the lunar surface when a lunar lander approaches a landing site can cause significant dust dispersal. This study investigated the near-field rocket plume-lunar surface interaction and subsequent regolith erosion and particle dispersal. These subjects are challenging because of the complicated flow physics associated with the inherently multi-physics multi-scale problem, and the special lunar conditions, characterized by micro-gravity, near-vacuum, extreme dryness, and the unique properties of the regolith. Gas expansion into the near-vacuum lunar condition compared to exhaust gas under terrestrial circumstances varies not only in the shape of plume but also in the pressure profile on the surface. To understand the effect of surface erosion on flow characteristics, in conjunction with the finite volume method of plume impingement of a rocket nozzle, the Roberts erosion model was introduced for the influx mass flow rate of dust particles based on excess shear stress. The particulate phase was then handled in a Lagrangian framework using the discrete phase model. A parametric study on erosion rate was also conducted to examine the effect of particle density, particle diameter, Mach number, and hover altitude. Additionally, the maximum speed and inclined angle of the particles from the surface were computed for various particle diameters and hover altitudes. The resulting information about the pressure and heat flux distribution on lunar module components can be used for engineering design. Finally, high-fidelity simulations of particles eroded from the surface indicated that several scenarios may occur depending on particle diameters, grain-inclined angles from the surface, and hover altitudes.

Simulation intricacies of open-cell metal foam fin subjected to convective flow

An exact 3D digital image of 10 PPI open-cell metal foam matrix used in this study has been generated through X-ray micro-computed tomography. The precise foam geometrical model, in combination with the commercial computational software, facilitates synthesizing typical thermal applications of metal foam subjected to convective fluid flow and creates unique opportunity of probing some of the finer fluid flow aspects usually beyond the scope of most of the experimental investigations. Simulation of fluid flow through 10 PPI open porous metal foam subjected to constant terminal temperature has been carried out and validated a priori with the experimental data. The convective flow simulation, being computationally intensive, has been carried out in a small piece of metal foam to eventually extrapolate the data over the original length. The practical limitations of foam modelling leading to the deviations between experimental data and the predicted results have been explained in details. Subsequently, the temperature and flow fields have been probed to understand the complicated flow physics around the metal foam strut. This study gives a better insight of the state of the art research and development in the field of high porosity open-cell metal foam. The computational predictions have also been compared at length with the analytical outcomes based on the simple cubic model of metal foam. This work, in all, provides the end-user an overview of the associated intricacies while considering the computational, experimental and simplified models for open-cell metal foams when used as extended heat transfer surfaces or fins.

Intrinsic features of flow past three square prisms in side-by-side arrangement

An investigation on the flow around three side-by-side square prisms can provide a better understanding of complicated flow physics associated with multiple, closely spaced structures in which more than one gap flow is involved. In this paper, the flow around three side-by-side square prisms at a Reynolds number$Re=150$is studied systematically at$L/W=1.1{-}9.0$, where$L$is the prism centre-to-centre spacing and$W$is the prism width. Five distinct flow structures and their ranges are identified, viz. base-bleed flow ($L/W<1.4$), flip-flopping flow$(1.4<L/W<2.1)$, symmetrically biased beat flow$(2.1<L/W<2.6)$, non-biased beat flow$(2.6<L/W<7.25)$and weak interaction flow$(7.25<L/W<9.0)$. Physical aspects of each flow regime, such as vortex structures, vortex dynamics, gap-flow behaviours, shedding frequencies and fluid forces, are discussed in detail. A secondary (beat) frequency other than the Strouhal frequency (primary frequency) is observed in the symmetrically biased and non-biased beat flows, associated with the beat-like modulation in$C_{L}$-peak or amplitude, where$C_{L}$is the lift force coefficient. Here we reveal the generic and intrinsic origin of the secondary frequency, establishing its connections with the phase lag between the two shear-layer sheddings from the two sides of a gap. When the two sheddings are in phase, no viscous force acts at the interface (i.e. at the centreline of the gap) of the two sheddings, resulting in the largest fluctuations in streamwise momentum, streamwise velocity and pressure; the maximum$C_{L}$amplitude thus features the in-phase shedding. Conversely, when the two sheddings are antiphase, a viscous force exists at the interface of the two sheddings and restricts the momentum fluctuation through the gap, yielding a minimum$C_{L}$amplitude. When the phase relationship between the two sheddings changes from in phase to antiphase, the extra viscous force acting at the interface becomes larger and causes the$C_{L}$amplitude to change from a maximum to a minimum.

Unsteady Heat Transfer and Pressure Measurements on the Airfoils of a Rotating Transonic Turbine With Multiple Cooling Configurations

Measurements are presented for a high-pressure transonic turbine stage operating at design-corrected conditions with forward and aft purge flow and blade film cooling in a short-duration blowdown facility. Four different film-cooling configurations are investigated: simple cylindrical-shaped holes, diffusing fan-shaped holes, an advanced-shaped hole, and uncooled blades. A rainbow turbine approach is used so each of the four blade types comprises a wedge of the overall bladed disk and is investigated simultaneously at identical speed and vane exit conditions. Double-sided Kapton heat-flux gauges are installed at midspan on all three film-cooled blade types, and single-sided Pyrex heat-flux gauges are installed on the uncooled blades. Kulite pressure transducers are installed at midspan on cooled blades with round and fan-shaped cooling holes. Experimental results are presented both as time-averaged values and as time-accurate ensemble-averages. In addition, the results of a steady Reynolds-averaged Navier–Stokes computational fluid dynamics (RANS CFD) computation are compared to the time-averaged data. The computational and experimental results show that the cooled blades reduce heat transfer into the blade significantly from the uncooled case, but the overall differences in heat transfer among the three cooling configurations are small. This challenges previous conclusions for simplified geometries that show shaped cooling holes outperforming cylindrical holes by a great margin. It suggests that the more complicated flow physics associated with an airfoil operating in an engine-representative environment reduces the effectiveness of the shaped cooling holes. Time-accurate comparisons provide some insight into the complicated interactions that are driving these flows and make it difficult to characterize cooling benefits.

Wireless Telemetric Data Acquisition and Real-Time Control for a High Measurement-Density Internal Heat Transfer Experiment

This paper describes a wireless data transmission system for a large-scale rotating experiment to investigate the heat transfer in a three-passage serpentine test section. Patterned after the NASA HOST program, the current experiment extends the data set to larger aspect ratios including 1:2, 1:4, and 1:6. As with HOST, heat transfer is measured using the heated segments technique, and the serpentine test section spins at rotation numbers representative of engine conditions. Rotating experiments are essential for capturing the representative operating conditions and complicated flow physics that must be understood to advance internal cooling technology for high aspect ratio configurations. There are challenges associated with controlling the operating parameters and collecting accurate data for high measurement-density rotating experiments. This experiment requires that 140 copper panels be held at a constant temperature by independently controlling and recording the power supplied to a separate heater on each panel. This means there must be 140 temperature measurements, 140 pairs of heater power leads, enough power to drive all of these heaters, and data recording capacity left over to measure fluid temperatures and pressures. Traditional methods of transferring rotating signals to the stationary frame of reference (like slip rings) are widely implemented but have practical limitations in the quantity of transferrable signals and the electrical current capacity of the individual channels. Alternatively, wireless transmission techniques were first developed decades ago, but their practical use has been limited by onboard power delivery requirements and cost. This paper describes the development of a new data transmission and control system that takes advantage of improvements in inexpensive electronics to create a battery-powered and microprocessor controlled system for acquisition, storage, control, and wireless communication. These components are assembled as an integral part of the rotating mechanical hardware. By handling high-fidelity microcircuit signal conditioning, data acquisition, feedback control, and data storage in the rotating frame and transmitting the results wirelessly, this system provides high measurement density and active feedback control that would have been impractical with a conventional slip-ring approach. The design and construction of the wireless control system for one full sidewall of the three-serpentine passage is described in detail. Its capability and functionality is demonstrated with operational data. It will be demonstrated that while all of the components in this system are readily available, the unique combination of this technology opens up a new world of measurement capabilities.

Robust Airfoil Shape Optimization Using Design For Six Sigma

Introduction B ECAUSE of the rapid development of computer technologies, tremendous achievement has been made in the computational fluid dynamics (CFD) area during the past few decades. Currently, CFD techniques are widely used not only to simulate and understand complicated flow physics, but also to optimize the aerodynamic shape of air vehicles that enables maximum performance at desired operating conditions. One of the most popular aerodynamic shape optimization algorithms currently available is the gradient-based optimization technique in which a specified objective function is minimized. Sensitivities, the gradients of the objective function with respect to design variables, are used to update the design variables in the direction that the magnitude of the objective function can be reduced in a systematic way. However, application of the aerodynamic shape optimization has been mostly made to single-point optimizations1−4 in which the optimal shape is determined for a specific flight condition. Even though the single-point design optimization guarantees good performance at the specified design point, practical experience indicates that it can lead to a dramatically inferior performance even at a slightly off-design operating condition caused by local gusts or atmospheric disturbances frequently encountered in actual flights. This problem may also occur due to errors involved in manufacturing, which cannot be completely avoided in a practical sense. A straightforward method of avoiding this difficulty is to consider multiple flight conditions simultaneously by constructing the objec-

An investigation of the heat transfer and static pressure on the over-tip casing wall of an axial turbine operating at engine representative flow conditions. (I). Time-mean results

The over-tip casing of the high-pressure turbine in a modern gas turbine engine is subjected to strong convective heat transfer that can lead to thermally induced failure (burnout) of this component. However, the complicated flow physics in this region is dominated by the close proximity of the moving turbine blades, which gives rise to significant temporal variations at the blade-passing frequency. The understanding of the physical processes that control the casing metal temperature is still limited and this fact has significant implications for the turbine design strategy. A series of experiments has been performed that seeks to address some of these important issues. This article reports the measurements of time-mean heat transfer and time-mean static pressure that have been made on the over-tip casing of a transonic axial-flow turbine operating at flow conditions that are representative of those found in modern gas turbine engines. Time-resolved measurements of these flow variables (that reveal the details of the blade-tip/casing interaction physics) are presented in a companion paper. The nozzle guide vane exit flow conditions in these experiments were a Mach number of 0.93 and a Reynolds number of 2.7 × 10 6 based on nozzle guide vane mid-height axial chord. The axial and circumferential distributions of heat transfer rate, adiabatic wall temperature, Nusselt number and static pressure are presented. The data reveal large axial variations in the wall heat flux and adiabatic wall temperature that are shown to be primarily associated with the reduction in flow stagnation temperature through the blade row. The heat flux falls by a factor of 6 (from 120 to 20 kW/m 2). In contrast, the Nusselt number falls by just 36% between the rotor inlet plane and 80% rotor axial chord; additionally, this drop is near to linear from 20% to 80% rotor axial chord. The circumferential variations in heat transfer rate are small, implying that the nozzle guide vanes do not produce a strong variation in casing boundary layer properties in the region measured. The casing static pressure measurements follow trends that can be expected from the blade loading distribution, with maximum values immediately upstream of the rotor inlet plane, and then a decreasing trend with axial position as the flow is turned and accelerated in the relative frame of reference. The time-mean static pressure measurements on the casing wall also reveal distinct circumferential variations that are small in comparison to the large pressure gradient in the axial direction.

Computational prediction of airfoil dynamic stall

The term dynamic stall refers to unsteady flow separation occurring on aerodynamic bodies, such as airfoils and wings, which execute an unsteady motion. The prediction of dynamic stall is important for flight vehicle, turbomachinery, and wind turbine applications. Due to the complicated flow physics of the dynamic stall phenomenon the industry has been forced to use empirical methods for its prediction. However, recent progress in computational methods and the tremendous increase in computing power has made possible the use of the full fluid dynamic governing equations for dynamic stall investigation and prediction in the design process. It is the objective of this review to present the major approaches and results obtained in recent years and to point out existing deficiencies and possibilities for improvements. To this end, potential flow, boundary layer, viscous–inviscid interaction, and Navier–Stokes methods are described. The most commonly used numerical schemes for their solution are briefly described. Turbulence models used for the computation of high Reynolds number turbulent flows, which are of primary interest to industry, are presented. The impact of transition from laminar to turbulent flow on the dynamic stall phenomenon is discussed and currently available methods for its prediction are summarized. The main computational results obtained for airfoil and wing dynamic stall and comparisons with available experimental measurements are presented. The review concludes with a discussion of existing deficiencies and possibilities for future improvements.