Transitional Flow Research Articles

Direct Numerical Simulation of Tandem-Wing Aerodynamic Interactions at Low Reynolds Number

A three-dimensional direct numerical simulation was conducted to investigate the vortex–wing interactions through two NACA 0012 stationary wings placed in tandem at a low Reynolds number Re=10,000. The aerodynamic characteristics and three-dimensional flow structures were analyzed for the tandem wings. The back wing disturbed by the upstream vortices gained an evident increase in aerodynamic performance, where the advantage is related to the suppression of the large-scale vortex formation near the trailing edge. The Liutex method was applied to visualize the vortical structures for investigating the three-dimensional evolution and instability when interacting with the back wing. The upstream wake triggered dual-secondary vortices and intensified the secondary instability on the back wing. The induced vortices contributed to the lift enhancement because they provided an extra low-pressure region when propagating downstream on the suction side of the back wing. Because of the three-dimensional destabilization, the vortex interaction in the evolution process accelerated the transition and injection of the high-momentum flow into the boundary layer attached to the back wing, energizing the turbulent boundary layer and eliminating the large-scale separation near the trailing edge. This study provided a new perspective on the enhanced aerodynamic performance of tandem layout.

Experimental study on influences of steps on hypersonic boundary layer transition at different angles-of-attack

Experimental study on influences of steps on hypersonic boundary layer transition at different angles-of-attack

Interwell interference model of horizontal wells in shale gas reservoirs based on multi-connected boundary element method

Due to the wide application of closely spaced multi-well horizontal pads for developing unconventional gas reservoirs, interference between wells becomes a significant concern. Communication between wells mainly occurs through natural fractures. However, previous studies have found that interwell communication through natural fractures is varied, and non-communication also appears in the mid and late stages of production due to natural fracture closure. This study proposes a boundary element method for coupling multi-connected regions for the first time. Using this method, we coupled multiple flow fields to establish dual-well models with various connectivity conditions of the stimulated reservoir volume (SRV) region. These models also take into consideration of adsorption and desorption mechanism of natural gas as well as the impact of fracturing fluid retention.The study found that when considering the non-communication of SRV regions between multi-well horizontal pads, the transient behavior of the targeted well exhibits a transitional flow stage occurring before the well interference flow stage. In addition, sensitivity analysis shows that the well spacing and production regime, as well as the connectivity conditions of the SRV region, affect the timing of interwell interference. Meanwhile, the productivity of the two wells, reservoir properties, and fracturing operations affect the intensity of interwell interference.

Acoustic Resonance in an Annular Cavity with Axial Transit Flow

Acoustic Resonance in an Annular Cavity with Axial Transit Flow

Flow Response of a Laminar Shock–Boundary Layer Interaction to Prescribed Surface Motions

The response of an impinging laminar shock–boundary layer interaction (SBLI) to prescribed surface motions is investigated numerically at M∞=2, Rea=120,000, and a shock-associated pressure ratio of 1.5. Parametric sweeps over classical standing and traveling mode shapes are considered at low-, moderate-, and high-frequency conditions for fluid–structure interaction. The specific values are chosen based on compliant panel deformations and frequencies reported in the literature. At low surface oscillation frequencies, the SBLI responds to the deformations in a quasi-steady fashion, with standing wave forcing displaying both breathing and sloshing of the separated region depending on structural mode shape. As the oscillation frequency is increased, the flow transitions to an unsteady response with pronounced separation bubble undulations in time. Higher-order modes and frequencies lead to the largest reductions in the time-mean separation bubble size, more so with traveling surface waves. Modal decompositions show that pressure fluctuations, which arise due to dynamic interaction with the surface, persist downstream and increase in amplitude with the surface oscillation frequency.

Amplification factor transport equation modelling of Mack mode disturbances in hypersonic boundary layers

In the hypersonic boundary layer, the streamwise instability modes are mainly the Mack first mode and the Mack second mode. After the Mach number exceeds 4.0, the Mack second mode becomes the dominant instability mode for boundary layer transition. Currently, there is no complete analytical method to establish an amplification factor transport equation for the second mode. In order to improve the application of the amplification factor transport (AFT) model in hypersonic boundary layer transition prediction, this paper employs boundary layer similarity solutions to conduct stability analysis on the second mode and introduces a function to calculate the growth rate of disturbances. Additionally, a transport equation for the amplification factor of the second mode is formulated. For the non-local variables appearing in the equation, a local calculation method is provided. Combined with the intermittency factor transport equation, a transition prediction model for the second mode is formed. Hypersonic flat plate, wedge, and flared wedge are selected for model validation. The computed results show good agreement with standard stability analysis or experimental results, demonstrating the rationality of the transition model and its high prediction accuracy and reliability.

Coupling Upscaled Discrete Fracture Matrix and Apparent Permeability Modelling in DFNWORKS for Shale Reservoir Simulation

Modelling non-Darcy flow behaviour in shale rocks, composed of nanometer-sized pores and multi-scale fracture networks, is crucial for various subsurface energy applications. However, incorporating multiple physical mechanisms across numerous scales is not trivial. This work proposes an improved and practical upscaling workflow for coupling an Upscaled Discrete Fracture Matrix (UDFM) model and a pressure-dependent apparent permeability (Kapp) model to capture the effects of non-Darcy flow in multi-scale fractured shale reservoirs.First, a 3D DFN is upscaled into octree-refined continuum meshes, where equivalent rock parameters and rock-fluid functions are defined using the UDFM approach. Then, the flow simulation is coupled with a pressure-dependent Kapp updating scheme using an existing Kapp model and a multiple-restart technique. The effects of non-Darcy flow mechanisms (e.g., slip flow, transitional flow, Knudsen diffusion) are captured. The constructed models are then used to study the impacts of fracture network connectivity and pressure interference on production. The results of this new approach are compared against those obtained from another commercial package while preserving the advantages of DFNWORKS. Neglecting non-Darcy flow behaviours could significantly underestimate gas production and water recovery. It is illustrated that the nanoscale flow mechanisms help to enhance matrix-matrix and matrix-fracture flow. The constructed models are also utilized to study the effects of disconnected or isolated fractures, pressure interference, water retention, and shut-in durations on well performance. The proposed flexible strategies can be adopted in other commercial/open-source fractured-porous-media subsurface-flow simulation frameworks.

Streamwise-Elongated Sinusoidal Roughness Elements with Enhanced Laminarizing Effect on Three-Dimensional Boundary Layer

As a laminar flow control device for delaying the crossflow-induced transition of a three-dimensional boundary layer, sinusoidal roughness elements (SREs) are placed in a Falkner–Skan–Cooke boundary layer, and the resultant laminarizing effect is numerically investigated in comparison with discrete roughness elements (DREs). Because SREs are elongated in the streamwise direction and designed to avoid flow tripping, the critical height of SREs is much higher than that of DREs. Moreover, the wake flow behind SREs efficiently generates and sustains crossflow vortices that are not dangerously unstable against secondary instabilities but able to strongly distort the mean crossflow profile into a less unstable one. By measuring this mean flow distortion by SREs and DREs, the laminarizing effect is compared among them. It is shown that the effect of SREs is higher than that of DREs and can be enhanced by choosing the appropriate height, angle, and wavelength depending on the local boundary-layer profile.

Newton-Krylov continuation of amplitude-modulated rotating waves in sheared annular electroconvection.

We present an approach for studying the primary, secondary, and tertiary flow transitions in sheared annular electroconvection. In particular, we describe a Newton-Krylov method based on time integration for the computation of rotating waves and amplitude-modulated rotating waves, and for the continuation of these flows as a parameter of the system is varied. The method exploits the rotational nature of the flows and requires only a time-stepping code of the model differential equations, i.e., it does not require an explicit code for the discretization of the linearized equations. The linear stability of the solutions is computed to identify the parameter values at which the transitions occur. We apply the method to a model of electroconvection that simulates the flow of a liquid crystal film in the smectic A phase suspended between two annular electrodes and subjected to an electric potential difference and a radial shear. Due to the layered structure of the smectic A phase, the fluid can be treated as two-dimensional (2D) and is modeled using the 2D incompressible Navier-Stokes equationscoupled with an equationfor charge continuity. The system is a close analog to laboratory-scale geophysical fluid experiments and thus represents an ideal system in which to apply the method before its application to these other systems that exhibit similar flow transitions. In the model for electroconvection, we identify the parameter values at which the primary transition from steady axisymmetric flow to rotating waves occurs, as well as at which the secondary transition from the rotating waves to amplitude-modulated rotating waves occurs. In addition, we locate the tertiary transition, which corresponds to a transition from the amplitude-modulated waves to a three-frequency flow. Of particular interest is that the method also finds a period-doubling bifurcation from the amplitude-modulated rotating waves and a subsequent transition from the flow resulting from this bifurcation.

The flow regimes of two parallel-connected chambers respectively with a mechanical extractor and a buoyancy source

This research studies the ventilation flow patterns and density stratification in two parallel-connected chambers which respectively have a mechanical extractor and a localized buoyancy source. Two parallel-connected chambers are denoted as the forced and extracted chambers. The forced chamber having a continuous localized buoyancy source is located upstream, and the extracted chamber having a mechanical extractor is located downstream. Two chambers are interconnected by an internal connection opening and are connected to the exterior in a parallel manner through their own connection openings. The effects of the adjoining chamber having a mechanical extractor on the ventilation flow patterns and density stratification in the space are investigated in this study. The flow driving forces include the buoyant and inertial forces in this ventilated space. The buoyant force results from the buoyancy source, and the inertial force is due to the mechanical extractor. This research uses the theoretical analysis approach and the laboratory reduced-scale model to investigate this problem. This paper focuses on the ventilation flow patterns in the steady state and discusses different flow regimes due to the adjoining extracted chamber effect. The internal connection opening level and orientation between two chambers are discussed in this study. Three flow regimes, the small (or reduced), transition, and large (or enhanced) flow regimes, are found in two parallel-connected chambers and there is a range of critical flow rates to determine the flow regime when the internal connection opening is located in a partition wall as investigated in this research. If the internal connection opening is located in a horizontal ceiling, a clear-cut critical flow rate is expected and only two flow regimes exist. This study discusses the characteristics of different flow regimes in the space of two parallel-connected chambers. The paper presents three derived theoretical models and analogous salt-bath experimental results for three flow regimes, and comparisons between them show reasonable agreement.

Moist air permeability characteristics of flexible contact gaskets used in refrigerators

Flexible contact gasket is a significant component affecting the sealing performance of the refrigerator. However, few studies were conducted on the permeability characteristics of moist air through refrigerator gaskets. The purpose of this study is to explore the permeability characteristics including permeation paths size, flow type, modeling and temperature-humidity-pressure-structure effect. The characteristic scale of the interface of permeation paths is observed as 10−5 m in the free-state by atomic force microscope. The equation of Knudsen number and contact stress relation is established to judgment the flow types. By comparing the critical and simulated contact stresses, it is inferred that moist air simultaneously occurs slip, transition and free-molecular flows. A multi-scale coupling model of permeation rate is constructed to obtain the apparent permeability coefficient characterizing the permeability intensity. The air pressure difference affects the permeability characteristics in the form of direct driving force and changing the contact stress of the interfaces. Free-molecular flow mainly occurs under the negative pressure condition with inhalation coefficient of 2.26 × 10−5∼2.84 × 10−5 m2, while the slip flow mainly occurs under the positive pressure conditions with exhalation coefficient of 2.87 × 10−5∼4.65 × 10−5 m2. The essence of enhancing the sealing performance of structural improvement is to increase the contact stress and length of permeation path. By increasing the height, increasing the inclination angle, adding the auxiliary edges and adding an auxiliary airbag, the inhalation coefficient of moist air, the exhalation coefficient of moist air and the inhalation coefficient of water vapor are decreased by 54.82 %, 51.46 % and 66.04 %.

A compact open boundary treatment for a pure thermal plume by direct numerical simulation

This study introduces a novel open boundary treatment method for simulating thermal plumes in natural convection, utilizing a combination of the Perfectly Matched Layer (PML) and local one-dimensional inviscid (LODI) methods within a compressible flow solver for Direct Numerical Simulation (DNS). This innovative approach significantly reduces the size of the computational domain yet effectively captures the complex transition from laminar to turbulent flow. Validation of the proposed methodology includes comparisons with existing empirical formulations, experimental data, and well-resolved DNS results, incorporating both time-averaged and spectral analyses. Additionally, the buoyancy-induced laminar-turbulent transition is investigated to enhance understanding of the dynamics and behaviors of thermal plumes. The findings offer insights into optimizing computational efficiency without compromising the accuracy needed to analyze intricate fluid dynamics in thermal engineering applications.

Stability Transition in Gap Expansion-Driven Interfacial Flow.

We investigate interfacial instability in a lifting Hele-Shaw cell by experiments and theory. We characterize the unexplored transition from stable to unstable patterns under a wide range of controlling parameters. Surprisingly, we find that the perturbation growth rate-based criterion for the onset of instability from linear stability theory is too strict by over 3 orders of magnitude. To reconcile this striking discrepancy, we propose a new criterion based on perturbation amplitude, which is in excellent agreement with the experimental results. We further show that the fingering pattern evolves to produce a hierarchical fluid structure and derive a theoretical equation to predict the fingering evolution.

Variability and predictability of a reduced-order land–atmosphere coupled model

Abstract. This study delves into the predictability of atmospheric blocking, zonal, and transition patterns utilizing a simplified coupled model. This model, implemented in Python, emulates midlatitude atmospheric dynamics with a two-layer quasi-geostrophic channel atmosphere on a β plane, encompassing simplified land effects. Initially, we comprehensively scrutinize the model's responses to environmental parameters like solar radiation, surface friction, and atmosphere–ground heat exchange. Our findings confirm that the model faithfully replicates real-world Earth-like flow regimes, establishing a robust foundation for further analysis. Subsequently, employing Gaussian mixture clustering, we successfully delineate distinct blocking, zonal, and transition flow regimes, unveiling their dependencies on surface friction. To gauge predictability and persistence, we compute the averaged local Lyapunov exponents for each regime. Our investigation uncovers the presence of zonal, blocking, and transition regimes, particularly under conditions of reduced surface friction. As surface friction increases further, the system transitions to a state characterized by two blocking regimes and a transition regime. Intriguingly, periodic behavior emerges under specific surface friction values, returning to patterns observed under low friction coefficients. A model resolution increase impacts the system in such a way that only two regimes are then obtained with the clustering: the transition phase disappears and the predictability drops to roughly 2 d for both of the remaining regimes. In accordance with previous research findings, our study underscores the fact that when all three regimes coexist, zonal patterns exhibit a more extended predictability horizon compared to blocking patterns. Remarkably, transition patterns exhibit reduced predictability when coexisting with the other regimes. In addition, within a specified range of surface friction values where two blocking regimes are found, it is observed that blocked atmospheric situations in the west of the applied topography are marked by instabilities and reduced predictability in contrast to the blockings appearing on the eastern side of the topography.

Boundary Layer Analysis of a Transonic High-Pressure Turbine Vane Using Ultra-Fast-Response Temperature-Sensitive Paint

Abstract The focus of this article is the impact of surface roughness on the boundary layer caused by a 7YSZ thermal barrier coating (TBC). Experimental investigations are conducted on a NGV installed inside the wind tunnel for Straight Cascades Göttingen (EGG). The shape of the vane has been altered in a way that eliminates the influence of TBC's thickness. Therefore, it is expected that only the surface roughness is influencing the location of the separation and boundary layer transition. The transition next to the roughness can also be affected by positive and negative pressure gradients, separation, and interacting shocks. The impact of TBC on the turbulent wedges' appearance, separation bubble's position and length, and transition location is examined in this study. This research, combined with prior investigations, provides a comprehensive understanding of a turbine vane's aerothermodynamics. To investigate unsteady flow phenomena on a TBC-coated NGV, ultra-fast-response temperature-sensitive paint (iTSP) is utilized. This dataset will serve as a reference point for developing new turbine vane designs that include TBC and extensive cooling. Furthermore, the findings will be employed as a benchmark for improving numerical models.

Experimental investigation on seeping gas film effectiveness in supersonic flow downstream of a porous injector

Based on seepage flow through porous media, the seeping gas film cooling method is an effective means to protect large areas of optical windows and other hot components of hypersonic vehicles. Here, the pressure-sensitive paint (PSP) technique was applied and experiments were conducted on seeping gas film effectiveness downstream of a porous injector. A Mach 3 wind tunnel was used to explore the influence of different cooling gas blowing ratios, gas types, and incoming boundary-layer conditions on film effectiveness. Results show film effectiveness decreases monotonically along the downstream wall; however, with blowing ratio increases the film effectiveness increases linearly. Once the blowing ratio exceeds 0.4% film effectiveness ascends nonlinearly, indicating that the contribution to film effectiveness per unit mass flow-rate promotion gradually decreases. In contrast, the non-uniformity of the film coverage on the downstream wall in the spanwise direction becomes more significant at a higher blowing ratio. For the same blowing ratio, helium, with a low molecular weight has a higher film effectiveness compared to nitrogen and carbon dioxide. With blowing ratio less than 0.4 %, the film effectiveness downstream of the porous injector under laminar flow conditions is almost twice that of turbulent flow. However, with blowing ratio above 0.5 %, the growth rate of the film effectiveness decreases dramatically to 50 %. Preliminary analysis suggests that this is caused by the complete transition of the laminar boundary layer to turbulent flow after passing through the porous wall under high blowing ratios, where the mixing effect of turbulence is fully manifest.

Clogging transition of granular flow in porous structures

The clogging transition of granular flow through a disordered porous structure is investigated by three-dimensional discrete element simulations. Results reveal the existence of an intermittent flow state, which has been already observed for active particles or agitated grains, but is absent in single-pore flows of passive particles. Interestingly, the analysis of the intermittent dynamics shows similar attributes to the bottleneck flow of active grains, i.e., exponential distribution of flowing intervals and power-law tails for the clogging times. Precisely, based on the exponent of the power-law tail, a clogging phase diagram for porous structures is proposed in the plane of the scaled pore size D/dp and the friction coefficient μs. Then, we demonstrate that the intermittency arises from a delay in the dissipation of particles' kinetic energy after clogging imposed by the porous structure. Finally, the maximum achievable particle flow rate driven by gravity is predicted from the Froude number Frm, which linearly scales with the pore size and the friction coefficient. Published by the American Physical Society 2024

Instability and transition control by steady local blowing/suction in a hypersonic boundary layer

The efficacy of steady large-amplitude blowing/suction on instability and transition control for a hypersonic flat plate boundary layer with Mach number 5.86 is investigated systematically. The influence of the blowing/suction flux and amplitude on instability is examined through direct numerical simulation and resolvent analysis. When a relatively small flux is used, the two-dimensional instability critical frequency that distinguishes the promotion/suppression mode effect closely aligns with the synchronisation frequency. For the oblique wave, as the spanwise wavenumber increases, the suppression effects would become weaker and the mode suppression bandwidth diminishes/increases in general in the blowing/suction control. Increasing the blowing/suction flux can effectively broaden the frequency bandwidth of disturbance suppression. The influence of amplitude on disturbance suppression is weak in a scenario of constant flux. To gain a deeper insight into disturbance suppression mechanism, momentum potential theory (MPT) and kinetic energy budget analysis are further employed in analysing disturbance evolution with and without control. When the disturbance is suppressed, the blowing induces the transport of certain acoustic components along the compression wave out of the boundary layer, whereas the suction does not. The velocity fluctuations are derived from the momentum fluctuations of the MPT. Compared with the momentum fluctuations, the evolutions indicated by each component's velocity fluctuations greatly facilitate the investigations of the acoustic nature of the second mode. The rapid variation of disturbance amplitude near the blowing is caused by the oscillations of the acoustic component and phase speed differences between vortical and thermal components. Kinetic energy budget analysis is performed to address the non-parallel effect of the boundary layer introduced by blowing/suction, which tends to suppress disturbances near the blowing. Moreover, viscous effects leading to energy dissipation are identified to be stronger in regions where the boundary layer is rapidly thickening. Finally, it is demonstrated that a flat plate boundary layer transition triggered by a random disturbance can be delayed by a blowing/suction combination control. The resolvent analysis further demonstrates that disturbances with frequencies that dominate the early transition stage are dampened in the controlled base flow.

Controlling hypersonic boundary layer transition with localized cooling and metasurface treatments

This study investigates a novel method to control hypersonic boundary layer transition using a combined local cooling and local metasurface treatment. The method’s effectiveness was investigated on a 5-degree half-angle blunt wedge with a nose radius of 0.0254 mm at a freestream Mach number of 6.0 using direct numerical simulations and linear stability theory. We explored four cases: (i) adiabatic baseline case, (ii) locally cooled case, (iii) local metasurface case, and (iv) combined local cooling-local metasurface case. Results showed that the combined local cooling-local metasurface treatment significantly reduced both wall pressure disturbance amplitude and the density perturbation amplitude around the sonic line, indicating a potential for controlling hypersonic boundary layer transition. In the local cooling-local metasurface case, the disturbance amplitude at the end of the computational domain was 270 times lower than the baseline case. The study also examined the impact of Reynolds numbers, ranging from 25.59 million per meter to 32.80 million per meter. Unsteady simulations revealed that the Reynolds number had a negligible effect on the local cooling-local metasurface performance, indicating that the proposed method applies to a wide range of flight conditions.

Joint mining of fluid knowledge and multi-sensor data for gas–water two-phase flow status monitoring and evolution analysis

Gas-water two-phase flow process widely exists in petroleum, chemical and other industrial processes, which is intricate, manifested by dynamic evolution of fluid structures and real-time fluctuation of flow parameters. Detailed description and real-time monitoring of flow status are of great significance to safe operation of industry processes. In this paper, a monitoring framework jointly mining the fluid knowledge and latent property behind multi-sensor data is proposed to address the issues of uncertainty and sample scarcity in transitional flow statuses, which enables real-time monitoring on structural evolution and parameter change of flow process. Firstly, the shared iterative algorithm with multi-task weight discovery is proposed to establish the relative semantic description system to characterize flow process knowledge by mining the relationship between multi-sensor data and different fluid semantic properties, which is helpful for interpretable monitoring of flow status structure and parameter information. Secondly, latent properties mined from multi-sensor data by sparse exponential discriminant analysis contribute to flow characterization completeness and discrimination. Thirdly, the process knowledge and latent information of flow status is transformed into different degrees of describable semantic and latent properties, which obtain relative representations of flow status in the joint description space realized by rank support vector machine. The results demonstrate that the method enables precise monitoring and detailed description of typical and uncertain flow statuses in the absence of transitional flow status modeling samples.