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In deep stormwater tunnel systems (DSTSs), entrapped air pockets are prone to pressurization and deformation during rapid filling, resulting in pressure surges that threaten system security. This study investigates the effects of structural and inflow parameters on peak pressures. To simulate the pressure surges of entrapped air pockets, a rigid-column model was developed for a simplified DSTS configuration comprising two shafts and one tunnel. The global sensitivities of the air pocket, system structure, and inflow parameters were calculated using the Extended Fourier Amplitude Sensitivity Test (EFAST), which employed indicators of the maximum pressure and relative increment. The results indicate that parameters related to the shape and initial state of the air pocket exert a substantial and direct effect on pressure surges in rapid filling, whereas tunnel system parameters tend to exhibit minimal influence. Notably, compared to the total inflow discharge, the flow difference between two shafts imposes a more significant and direct impact. For the maximum pressure of air pocket, the initial pressure and the maximum water level height of shafts demonstrate more pronounced effects. The proposed sensitivity analysis could be integrated into methodologies for system safety assessment, while the rigid-column model may be extended to accommodate multiple shafts and air pockets. [ABSTRACT FROM AUTHOR]

A rectangular cylinder, with an aspect ratio of 5, is a widely used bluff body in engineering practice. It undergoes intricate dynamical behavior in response to minute alterations in the flow angle of attack (α). These modifications invariably precipitate the failure of wake control for classical flow control methods with various α values. In this study, global linear instability, adjoint method, and sensitivity analysis are employed to identify the optimal position for flow control. It is found that the sensitive region gradually transitions from the leeward side to the downwind side of the model as α and Reynolds number (Re) increase. So, we set up airflow orifices for flow control in both positions. Jet flow control on the leeward side effectively inhibits vortex shedding (α ≤ 2°). High-order dynamic mode decomposition is employed to reveal the inherent mechanism of control. Suction control on the downside effectively mitigates the shear layer separation phenomenon induced by the altered spatial structure associated with higher α. A novel zero-net-mass-flux wake control, bionics-based breathe-valve control (BVC), is proposed to optimize the control effect. BVC is applicable for various α and Re, with optimal effectiveness achievable through jet velocity adjustments. The prediction-control approach in this investigation provides a targeted method to mitigate flow-induced vibration. [ABSTRACT FROM AUTHOR]

The current investigation is based on the impact of the nanoparticle shape on the micropolar hybrid nanofluid flow in a vertical plate. Furthermore, the aim of this investigation is to optimize the skin friction as well as the Nusselt number using a statistical approach known as "Response Surface Methodology" (RSM). The micropolar hybrid nanofluid is considered due to its enhanced thermal properties likely the Hamilton–Crosser thermal conductivity, Gharesim model viscosity, etc. A suitable similarity rule is adopted for the transformation of the designed model into ordinary and then solved numerically utilizing the shooting-based Runge–Kutta fourth-order technique. The simulated results of diversified parameters are presented through graphs. Furthermore, RSM is employed to design and develop a mathematical model to get an optimized hear transfer rate along with the rate of shear stress. The required components are carefully selected, and the corresponding responses are recorded. The collected data is subsequently employed in constructing a response surface through regression analysis. This process allows for the determination of optimal conditions to enhance heat transfer, which is then confirmed through analysis of variance testing. However, the major outcomes of the study are; for the case of suction with increasing particle concentrations, the shear rate, hear transfer rate, and couple stress coefficients are enhanced significantly. Furthermore, the non-Newtonian parameter and the magnetic parameter also favor in enhancing the rate coefficients. [ABSTRACT FROM AUTHOR]

Slurry transportation via pipelines has garnered growing attention across various industries worldwide, thanks to its efficiency and environmental friendliness. It has emerged as a vital tool for conveying significant volumes of raw phosphate material from extraction points to industrial plants, where it is processed into fertilizers. Yet, optimal and secure pipeline operations necessitate the careful calibration of several physical parameters and their interplay to minimize energy losses. A thorough exploration of the flow pressure drop and the various factors that influence it constitutes a crucial step in attaining this goal. The computational fluid dynamics techniques required to simulate three-dimensional slurry pipe flows pose formidable challenges, primarily due to their high computational costs. Furthermore, numerical solutions for slurry flows are frequently subject to uncertainties arising from the initial and boundary conditions in the mathematical models employed. In this study, we propose the use of polynomial chaos expansions to estimate the uncertainty inherent in the desired slurry flow and perform a sensitivity analysis of flow energy efficiency. In this framework, five parameters are considered as random variables with a given probability distribution over a prescribed range of investigation. The uncertainty is then propagated through the two-phase flow model to statistically quantify their effect on the results. Our findings reveal that variations in slurry velocity and particle size play a pivotal role in determining energy efficiency. Therefore, controlling these factors represents a critical step in ensuring the efficient and safe transportation of slurry through pipelines. [ABSTRACT FROM AUTHOR]

A complicated coupling system performs fuel filling operation of a ship on the sea that involves tank sloshing and filling impact. This research elaborated on the sloshing load and flow characteristics of the variable mass tank and revealed its physical process through numerical simulations to provide safety assurance for fuel filling. First, this work clarified the difference between the numerical theory of variable mass tank sloshing and that of ordinary sloshing. On this foundation, a numerical model was developed for a three-dimensional variable mass tank sloshing. In addition, the model was validated by comparing the results of the previous experiments. Second, the sloshing pressure and internal free surface of the tank were monitored by numerical simulations. At the same time, a series of sensitivity analyses were carried out against the sloshing period, filling rate, and sloshing amplitude. Finally, the baffle was added to the sloshing tank to study the influence of internal barrier structure on the sloshing flow and pressure characteristics. The following conclusions were obtained from this work: Fuel filling caused an increase in liquid mass, suppressing sloshing and increasing the tank load. Overall, the sloshing load increased with the filling rate. Moreover, the impact of internal barrier structure on sloshing is discussed in this study, and it was discovered that the sloshing effect could be efficiently suppressed by the barrier structure. [ABSTRACT FROM AUTHOR]

For large public buildings, a siphon drainage system facilitates good on building safety and operation. However, conventional siphon drainage systems are designed based on full pipe flow, which is suitable only under specific criteria and cannot assess the detailed flow characteristics from priming to the entire siphon formation process. In this paper, a full-size experimental platform for a single outlet siphon drainage system is proposed to research the flow characteristics of the siphon drainage. Then, a full-scale numerical model of a siphon drainage system is established and the process of the siphon drainage is simulated by the volume of fluid model. The accuracy of the numerical model is verified by comparing its results with the experimental data. Moreover, a sensitivity analysis of different pipeline structure parameters on siphon priming time is presented, which shows that a larger length and diameter of the suspension pipe and a larger riser pipe diameter prolong the siphon formation time. Finally, theoretical formulas are derived to describe the mathematical relationships between the suspension pipe length and maximum displacement and the suspension pipe length and siphon priming time. [ABSTRACT FROM AUTHOR]

The usage of direct numerical simulations for research of turbulent combustion for space propulsion applications is explored. With this goal in mind, the combustion near the injection of a fuel-rich methane–oxygen flame at 20 bar is simulated using a massively parallelized solver. The statistical properties of the relevant physical fields are examined to study interactions between turbulence and combustion. This analysis is complemented by an investigation and quantification of the error sources in direct numerical simulations of turbulent diffusion flame. A method to estimate the statistical error is derived based on the classical inference theory. In addition, critical resolution criteria are discussed using a mesh sensitivity analysis. [ABSTRACT FROM AUTHOR]

Perturbations of flow control parameters may yield a significant alteration in the boundary layer stability. Based on the previously established parameter-associated sensitivity, the present work derives the optimal minor parameter perturbation analytically under the constraint of base flow energy variation. Specifically, the steady blowing-suction factor and the generalized Hartree parameter are examined at Mach number 4.5 to stabilize the mode S. Good agreement between the linear stability theory calculation, sensitivity theory, and Lagrangian approach is achieved for the optimal parametric state. The optimal state occurs if the contribution of the base velocity distortion has the greatest advantage over the temperature counterpart. Contributions of various physical sources to the growth rate behave similarly and collapse onto one correlation if normalized by the maximum, particularly for the major four: advection, mean shear, base temperature gradient, and pressure gradient. When the parameter perturbation further becomes finite, the optimal state is found on the constraint border of control parameters. Although the favorable pressure gradient and wall suction stabilize the broadband mode S, an unusual opposite tendency may occur for a single-frequency disturbance. In this unusual parametric range, positive contributions of both the major and minor physical sources to the growth rate are promoted. The contributive increase in major and minor sources are attributed to the enhancement of mean shear and viscous effect, respectively. Whether the parametric influence is stabilization or destabilization is intrinsically determined by the sensitivities, and the intermediate process is analyzed. Finally, given the modification to the critical Reynolds number, the input control parameter perturbation is inversely obtained and verified. [ABSTRACT FROM AUTHOR]

This study proposes a new physics-constrained data-driven approach for sensitivity analysis and uncertainty quantification of large-scale chaotic Partial Differential Equations (PDEs). Unlike conventional sensitivity analysis, the proposed approach can manipulate the unsteady sensitivity function (i.e., tangent) for PDE-constrained optimizations. In this new approach, high-dimensional governing equations from physical space are transformed into an unphysical space (i.e., Hilbert space) to develop a closure model in the form of a Reduced-Order Model (ROM). This closure model is derived explicitly from the governing equations to set strong constraints on manifolds in Hilbert space. Afterward, a new data sampling method is proposed to build a data-driven approach for this framework. A series of least squares minimizations are set in the form of a novel auto-encoder system to solve this closure model. To compute sensitivities, least-squares shadowing minimization is applied to the ROM. It is shown that the proposed approach can capture sensitivities for large-scale chaotic dynamical systems, where finite difference approximations fail. [ABSTRACT FROM AUTHOR]

The compressible-boundary-layer stability can be considerably influenced by base flow distortion. The distortion may originate from perturbations of flow parameters, such as the Mach number. In this paper, sensitivities of the boundary layer stability to certain flow parameters are derived analytically by utilizing the homotopy analysis (with codes shared), in conjunction with a direct-adjoint stability theory. The sensitivities can be categorized according to the routes the distortion evolves. Route I is that parameters distort the base flow (Sensitivity A), which, in turn, affect the eigenvalue of the linear stability equation (Sensitivity B). Route II gives rise to the effects of flow parameters onto eigenvalues caused by direct perturbation of the linear operators (Sensitivity C). Results indicate that Sensitivity A is characterized by the only peak found on the sensitivity profile that corresponds to the maximum gradient of base flow; for Sensitivity B, production terms, e.g., the mean-shear terms, are found to be significant, while for Sensitivity C, which is rarely discussed in existing literature, the pressure gradient terms in the momentum equations are dominant in affecting the stability via route II. Furthermore, route II can be more significant than route I. Having examined the variation of the mean shear gradient, d (ρ ¯ d u ¯ / d y) / d y , near the critical layer yc, it is proven that the sensitivity of the eigenvalue to the velocity or temperature distortion is negative at yc under certain assumptions, particularly for the temperature-relevant sensitivity that has hardly been discussed before. [ABSTRACT FROM AUTHOR]

A machine learning control (MLC) is proposed based on the explorative gradient method (EGM) for the optimization and sensitivity analysis of actuation parameters. This technique is applied to reduce the drag of a square-back Ahmed body at a Reynolds number Re = 1.7 × 105. The MLC system consists of pulsed blowing along the periphery of the base, 25 pressure taps distributed on the vertical base of the body, and an EGM controller for unsupervised searching for the best control law. The parameter search space contains the excitation frequency fe, duty cycle α, and flow rate blowing coefficient Cm. It is demonstrated that the MLC may cut short the searching process significantly, requiring only about 100 test runs and achieving 13% base pressure recovery with a drag reduction of 11%. Extensive flow measurements are performed with and without control to understand the underlying flow physics. The converged control law achieves fluidic boat tailing and, meanwhile, eliminates the wake bistability. Such simultaneous achievements have never been reported before. A machine-learned response model is proposed to link the control parameters with the cost function. A sensitivity analysis based on this model unveils that the control performance is sensitive to fe and α but less so to Cm. The result suggests that a small sacrifice on performance will give a huge return on actuation power saving, which may provide important guidance on future drag reduction studies as well as engineering applications. [ABSTRACT FROM AUTHOR]

Compared to the drawbacks of traditional experimental and numerical methods for predicting bubble migration, such as high experimental costs and complex simulation operations, the data-driven approach of using deep neural network algorithms can provide an alternative method. The objective of this paper is to construct a two-branch deep neural network (TBDNN) model in order to improve the high-fidelity bubble migration results and further reduce dependence on the quantity of experimental data. A TBDNN model is obtained by embedding the features of the Kelvin impulse into a basic deep neural network (BDNN) system. The results show that compared to the original BDNN model, TBDNN performs much better in accurately predicting bubble migration based on the same amount of training data. Using the TBDNN model, the critical condition of bubble oscillation at a fixed location can be detected under the influence of boundary properties (normalized stiffness and mass) and bubble standoff. Furthermore, the initial position of the bubble and normalized stiffness of boundaries have a positive correlation with bubble migration, whereas normalized mass has a negative impact. It was found that the normalized mass of boundaries plays the most important role in affecting bubble migration compared to the standoff and stiffness when using the method of variable sensitivity analysis. [ABSTRACT FROM AUTHOR]

The combined electro-osmotic and pressure-driven flows (PDFs) have pronounced impacts on the solute transport in permeable porous media, particularly mixing and separation processes. However, the relationship between the physical properties of the permeable porous media and the combined electro-osmotic and PDFs still needs further investigation. This study focuses on the transport of a neutral nonreacting solute in a channel with permeable porous walls under the combined effects of electro-osmotic and PDFs. With the aid of perturbation theory and asymptotic analysis, the equivalent one-dimensional equations governing the solute concentrations in the channel and permeable porous medium under the combined velocity are derived. Based on this, an exact analytical expression relating the dispersion coefficient with the physical properties of the permeable porous medium and the combined flow is obtained. The model parameters exerting the most influence on the results are identified through sensitivity analysis. The proposed model is compared and validated with several previously developed models in the literature. The findings of this study can pave the way for the quantitatively design of solute transport through microporous coatings and porous microfluidic membranes. [ABSTRACT FROM AUTHOR]

Reynolds Averaged Navier Stokes (RANS) closures assume that the state of the turbulent flow field can be uniquely described using a finite set of tensors. However, these tensors are averaged statistics, and thus, varied turbulent flow fields with different internal structuring can correspond to the same values for this finite tensor basis. This causes the modeled initial value problem governing turbulence evolution to be ill-posed. This non-unique specification of the initial turbulent flow field may lead to non-unique evolution for the real turbulent flow at this level of description, introducing epistemic uncertainty in the RANS modeling problem. In this investigation, we show that for homogeneous turbulence, there is a range of possible evolution of a real turbulent flow field if only a finite set of local tensors are specified. Furthermore, this range can include diametrically opposite behavior of the flow. The dependence of this uncertainty is analyzed and quantified for different mean velocity gradients and at different flow parameters. It is exhibited that this uncertainty is engendered by the linear mechanisms in turbulence physics. The magnitude of sensitivity of flow evolution on flow history is explained, using the spectral space structure of the linear instabilities manifested for different mean flows. [ABSTRACT FROM AUTHOR]

Numerical simulations and sensitivity analysis are carried out regarding control vortex shedding from a circular cylinder using small rotating and non-rotating control cylinders. Prediction of the changes in unsteady wake, instability growth rate, and frequency produced by the control devices is presented in the framework of sensitivity analysis to steady field force. In the case of the rotating cylinders, this prediction relies on the observation that the small control cylinders introduce a steady force whose direction is not aligned to the base flow direction. This external force direction depends on the control cylinders rotation rates having an additional parameter to attain flow control. The evaluation of the control cylinders rotation rate within the sensitivity analysis framework is a novel contribution in this work. The results of the sensitivity analysis to a steady force are then compared to direct numerical simulation computations of the flow fields serving as supporting data for discussion. [ABSTRACT FROM AUTHOR]

Two models of the flow over and through an immersed, vegetated layer are examined to study the onset of instability waves across the layer and to assess the effect of mild variations in the mean flow and in the drag force exerted by the canopy onto the frequency and growth rate of the monami instability. One of the two models, based on the use of Darcy's equation, with a tensorial permeability, within the canopy is more robust than the other (which uses a scalar drag coefficient), i.e., it is less sensitive to the inevitable imperfections or approximations in the input data. [ABSTRACT FROM AUTHOR]

We study the full three-dimensional instability mechanism past a hemispherical roughness element immersed in a laminar Blasius boundary layer. The inherent threedimensional flow pattern beyond the Hopf bifurcation is characterized by coherent vortical structures usually called hairpin vortices. Direct numerical simulation results are used to analyze the formation and the shedding of hairpin vortices inside the shear layer. The first bifurcation is investigated by global-stability tools. We show the spatial structure of the linear direct and adjoint global eigenmodes of the linearized Navier-Stokes equations and use the structural-sensitivity field to locate the region where the instability mechanism acts. The core of this instability is found to be symmetric and spatially localized in the region immediately downstream of the roughness element. The effect of the variation of the ratio between the obstacle height k and the boundary layer thickness δ* k is also considered. The resulting bifurcation scenario is found to agree well with previous experimental investigations. A limit regime for k/δ*k < 1.5 is attained where the critical Reynolds number is almost constant, Rek ≈ 580. This result indicates that, in these conditions, the only important parameter identifying the bifurcation is the unperturbed (i.e., without the roughness element) velocity slope at the wall. [ABSTRACT FROM AUTHOR]

In the last few years, many researchers have noted that regions of recirculating flow often exhibit particularly high sensitivity to spatially localized feedback. We explore the flow through a T-shaped pipe bifurcation--a simple and ubiquitous, but generally poorly understood flow configuration--and provide a complex example of the relation between recirculation and sensitivity. When Re ≥ 320, a phenomenon resembling vortex breakdown occurs in four locations in the junction, with internal stagnation points appearing on vortex axes and causing flow reversal. The structure of the recirculation is similar to the traditional bubble-type breakdown. These recirculation regions are highly sensitive to spatially localized feedback in the linearized Navier-Stokes operator. The flow separation at the corners of the "T," however, does not exhibit this kind of sensitivity. We focus our analysis on the Reynolds number of 560, near the first Hopf bifurcation of the flow. [ABSTRACT FROM AUTHOR]

We use adjoint-based gradients to analyze the sensitivity of the drag force on a square cylinder. At Re = 40, the flow settles down to a steady state. The quantity of interest in the adjoint formulation is the steady asymptotic value of drag reached after the initial transient, whose sensitivity is computed solving a steady adjoint problem from knowledge of the stable base solution. At Re = 100, the flow develops to the timeperiodic, vortex-shedding state. The quantity of interest is rather the time-averaged mean drag, whose sensitivity is computed integrating backwards in time an unsteady adjoint problem from knowledge of the entire history of the vortex-shedding solution. Such theoretical frameworks allow us to identify the sensitive regions without computing the actually controlled states, and provide a relevant and systematic guideline on where in the flow to insert a secondary control cylinder in the attempt to reduce drag, as established from comparisons with dedicated numerical simulations of the two-cylinder system. For the unsteady case at Re = 100, we also compute an approximation to the mean drag sensitivity solving a steady adjoint problem from knowledge of only the mean flow solution, and show the approach to carry valuable information in view of guiding relevant control strategy, besides reducing tremendously the related numerical effort. An extension of this simplified framework to turbulent flow regime is examined revisiting the widely benchmarked flow at Reynolds number Re = 22 000, the theoretical predictions obtained in the frame of unsteady Reynoldsaveraged Navier-Stokes modeling being consistent with experimental data from the literature. Application of the various sensitivity frameworks to alternative control objectives such as increasing the lift and reducing the fluctuating drag and lift is also discussed and illustrated with a few selected examples. [ABSTRACT FROM AUTHOR]

This article presents a quasi-laminar stability approach to identify in high-Reynolds number flows the dominant low-frequencies and to design passive control means to shift these frequencies. The approach is based on a global linear stability analysis of mean-flows, which correspond to the time-average of the unsteady flows. Contrary to the previous work by Meliga et al. ["Sensitivity of 2-D turbulent flow past a Dshaped cylinder using global stability," Phys. Fluids 24, 061701 (2012)], we use the linearized Navier-Stokes equations based solely on the molecular viscosity (leaving aside any turbulence model and any eddy viscosity) to extract the least stable direct and adjoint global modes of the flow. Then, we compute the frequency sensitivity maps of these modes, so as to predict before hand where a small control cylinder optimally shifts the frequency of the flow. In the case of the D-shaped cylinder studied by Parezanovi'c and Cadot [J. Fluid Mech. 693, 115 (2012)], we show that the present approach well captures the frequency of the flow and recovers accurately the frequency control maps obtained experimentally. The results are close to those already obtained by Meliga et al., who used a more complex approach in which turbulence models played a central role. The present approach is simpler and may be applied to a broader range of flows since it is tractable as soon as mean-flows -- which can be obtained either numerically from simulations (Direct Numerical Simulation (DNS), Large Eddy Simulation (LES), unsteady Reynolds-Averaged- Navier-Stokes (RANS), steady RANS) or from experimental measurements (Particle Image Velocimetry - PIV)--are available. We also discuss how the influence of the control cylinder on the mean-flow may be more accurately predicted by determining an eddy-viscosity from numerical simulations or experimental measurements. From a technical point of view, we finally show how an existing compressible numerical simulation code may be used in a black-box manner to extract the global modes and sensitivity maps. [ABSTRACT FROM AUTHOR]

Gravitationally unstable, transient, diffusive boundary layers play an important role in carbon dioxide sequestration. Though the linear stability of these layers has been studied extensively, there is wide disagreement in the results, and it is not clear which methodology best reflects the physics of the instability. We demonstrate that this disagreement stems from an inherent sensitivity of the problem to how perturbation growth is measured. During an initial transient period, the concentration and velocity fields exhibit different growth rates and these rates depend on the norm used to measure perturbation amplitude. This sensitivity decreases at late times as perturbations converge to dominant quasi-steady eigenmodes. Therefore, we characterize the linear regime by measuring the duration of the initial transient period, and we interpret the convergence process by examining the growth rates and non-orthogonality of the quasi-steady eigenmodes. To judge the relevance of various methodologies and perturbation structures to physical systems, we demonstrate that every perturbation has a maximum allowable initial amplitude above which the sum of the base-state and perturbation produces unphysical negative concentrations. We then perform direct numerical simulations to demonstrate that optimal perturbations considered in previous studies cannot support finite initial amplitudes. Consequently, convection in physical systems is more likely triggered by 'sub-optimal' perturbations that support finite initial amplitudes. [ABSTRACT FROM AUTHOR]

The stability properties of the wake past an oblate spheroidal bubble held fixed in a uniform stream are studied in the framework of a global linear analysis. In line with previous studies, provided the geometric aspect ratio of the bubble, χ, is large enough, the wake is found to be unstable only within a finite range of Reynolds number, Re. The neutral curves corresponding to the occurrence of the first two unstable modes are determined over a wide range of the (χ, Re) domain and the structure of the modes encountered along the two branches of each neutral curve is discussed. Then, using an adjoint-based approach, a series of sensitivity analyses of the flow past the bubble is carried out in the spirit of recent studies devoted to two-dimensional and axisymmetric rigid bodies. The regions of the flow most sensitive to an external forcing are found to be concentrated in the core or at the periphery of the standing eddy, as already observed with bluff bodies at the surface of which the flow obeys a no-slip condition. However, since the shear-free condition allows the fluid to slip along the bubble surface, the rear half of this surface turns out to be also significantly sensitive to disturbances originating in the shear stress, a finding which may be related to the well-known influence of surfactants on the structure and stability properties of the flow past bubbles rising in water. [ABSTRACT FROM AUTHOR]

Slender bodies of revolution placed at high angles of attack will produce a steady asymmetric vortex pair. This behavior is determined by the spatial instability of the flow. The asymmetric vortex pair is very sensitive to small disturbances near the tip. This paper presents a simple side force alleviation method without a power unit for slender bodies at high angles of attack. The method uses the self-excited oscillation (flutter) of a small cantilever flag as the excitation source on the cone tip. An experimental study is performed to investigate the characteristics of the asymmetric side forces of a 20° cone-cylinder slender body with and without a small flutter flag. Pressure distribution at eight cross sections of the cone forebody is measured at different free-stream velocities and different angles of attack. The process demonstrates that the excitation provided by the fluttering flag can change the asymmetric bistable flow state to a well symmetric one at high angles of attack, and that the corresponding side force and yaw moment will significantly decrease. The flutter strength of the flag will be enhanced by increasing the free-stream velocity. The effectiveness of the side force alleviation method will also be increased. [ABSTRACT FROM AUTHOR]

The laminar flow in two-dimensional diffusers may produce either symmetric or nonsymmetric steady solutions, depending on the value of the Reynolds number as compared with some critical value. The stability properties of the flow are studied in the context of linear theory. In this context, a sensitivity analysis of the flow instability is carried out with respect to perturbations that may be produced by a realistic passive control, thus providing qualitative hints and quantitative information for the control design. Following the so-obtained information, a passive control is built by introducing a small cylinder in the flow with the aim of stabilizing the unstable symmetric flow configuration in the diffuser. The effectiveness of this control is finally assessed by direct numerical simulation. It is shown that the introduction of the cylinder, placed following the indications of the linear sensitivity analysis in the stable asymmetric flow configuration, allows a steady completely symmetric or less asymmetric flow to be recovered. The flow transient between the uncontrolled asymmetric solution and the symmetric controlled one is analyzed in terms of streamlines and vorticity evolution; the effects of the cylinder introduction on flow dissipation are also assessed. [ABSTRACT FROM AUTHOR]

We use adjoint-based gradients to analyze the sensitivity of turbulent wake past a D-shaped cylinder at Re = 13000. We assess the ability of a much smaller control cylinder in altering the shedding frequency, as predicted by the eigenfrequency of the most unstable global mode to the mean flow. This allows performing beforehand identification of the sensitive regions, i.e., without computing the actually controlled states. Our results obtained in the frame of 2-D, unsteady Reynolds-averaged Navier-Stokes compare favorably with experimental data reported by Parezanovic and Cadot [J. Fluid Mech. 693, 115 (2012)] and suggest that the control cylinder acts primarily through a local modification of the mean flow profiles. [ABSTRACT FROM AUTHOR]

The sensitivity to steady local disturbances of the turbulent wake over a 3D blunt body with an axisymmetric trailing edge is investigated at Re = 2.1 × 104. The flow presents a favored m = 2 azimuthal periodicity due to the mounting of the body. It is proved that two asymmetric flows cohabit and generate a statistically symmetric natural flow. Topology shifts are random but are visible after a large number of global mode periods. The statistical symmetry is highly sensitive to any steady antisymmetric disturbance. As a consequence, depending on its position, a small control cylinder (considered as a disturbance with m = 1 azimuthal periodicity) in the close wake fixes the flow to one asymmetric topology affecting shedding activity as well as drag. On the contrary, a thin control ring (i.e., m = 0 disturbance) preserves the mean flow symmetry and has a greater impact on global mode frequency and drag. Whatever the disturbance, the sensitivity of the wake to control devices is concentrated into the mixing layers; the observed effects seem to depend more upon their local turbulent characteristics than on the global inviscid dynamics of vorticity. [ABSTRACT FROM AUTHOR]

A dynamic framework for hybrid Reynolds-averaged Navier-Stokes (RANS)-large eddy simulation (LES) modeling is proposed, wherein the RANS-to-LES transition parameter is adjusted to maintain continuity in turbulence production. The model is applied for temporally developing plane channel flow at varying Reynolds numbers with different initial turbulence intensity and grid resolution. On sufficiently fine grids, dynamic hybrid RANS-LES model (DHRL) yields similar results to LES simulations. On coarse grids, DHRL activates RANS mode in the log layer, thus improving the mean flow predictions. The model framework addresses grid sensitivity issues observed in other hybrid RANS-LES approaches and may be used with any desired combination of LES and RANS basis models. [ABSTRACT FROM AUTHOR]

The Kolmogorov's 4/5 law is often considered as the sole exact relationship of inertial range statistics. Its asymptotic character, however, has been evidenced, investigating the finite Reynolds number (FRN) effect for the third-order structure function S3(r) (e.g., for longitudinal velocity increments with r separation length) using variants of the Kármán-Howarth equation in physical space. Similar semi-empirical fits were proposed for the maximum of the normalized structure function, C3 = -maxrS3(r)/(&eh;r), expressing C3 - 4/5 as a power law of the Taylor-based Reynolds number. One of the most complete studies in this domain is by Antonia and Burratini [J. Fluid Mech. 550, 175 (2006)]. Considering that these studies are based on a model for the unsteady second-order structure function S2(r,t), with no explicit model for the third-order structure function itself, we propose to revisit the FRN effect by a spectral approach, in the line of Qian [Phys. Rev. E 55, 337 (1997), Phys. Rev. E 60, 3409 (1999)]. The spectral transfer term T(k,t), from which S3(r,t) is derived by an exact quadrature, is directly calculated by solving the Lin equation for the energy spectrum E(k,t), closed by a standard triadic (or three-point) theory, here Eddy Damped Quasi Normal Markovian. We show that the best spectral approach to the FRN effect is found by separately investigating the negative (largest scales) and positive (smaller scales) bumps of the transfer term, and not only by looking at the maximum of the spectral flux or [formula]. In the forced case, previous results are well reproduced, with Reynolds numbers as high as Reλ = 5 000 to nearly recover the 4/5 value. In the free decay case, the general trend is recovered as well, with an even higher value of Reλ = 50 000, but the EDQNM plots are systematically below those in Antonia and Burattini [J. Fluid Mech. 550, 175 (2006)]. This is explained by the sensitivity to initial data for E(k) in solving the Lin equation at moderate Reynolds numbers. Accordingly, an ad hoc initialization yields results consistent with the experimental spectrum measurements of Comte-Bellot and Corrsin [J. Fluid Mech. 48(2), 273 (1971)], from which S3(r) are recalculated. Present results show that the dispersion observed in existing data at low Reynolds number may be due to sensitivity to initial spectrum shape, a feature of the flow which is not under control in most of laboratory experiments. [ABSTRACT FROM AUTHOR]

The paper investigates the sensitivity of numerically computed flow fields to uncertainties in thermodynamic models for complex organic fluids. Precisely, the focus is on the propagation of uncertainties introduced by some popular thermodynamic models to the numerical results of a computational fluid dynamics solver for flows of molecularly complex gases close to saturation conditions (dense gas flows). A tensorial-expanded chaos collocation method is used to perform both a priori and a posteriori tests on the output data generated by thermodynamic models for dense gases with uncertain input parameters. A priori tests check the sensitivity of each equation of state to uncertain input data via some reference thermodynamic outputs, such as the saturation curve and the critical isotherm. A posteriori tests investigate how the uncertainties propagate to the computed field properties and aerodynamic coefficients for a flow around an airfoil placed into a transonic dense gas stream. [ABSTRACT FROM AUTHOR]

As inkjet printing technology is increasingly applied in a broader array of applications, careful characterization of its method of use is critical due to its inherent sensitivity. A common operational mode in inkjet technology known as drop-on-demand ejection is used as a way to deliver a controlled quantity of material to a precise location on a target. This method of operation allows ejection of individual or a sequence (burst) of drops based on a timed trigger event. This work presents an examination of sequences of drops as they are ejected, indicating a number of phenomena that must be considered when designing a drop-on-demand inkjet system. These phenomena appear to be driven by differences between the first ejected drop in a burst and those that follow it and result in a break-down of the linear relationship expected between driving amplitude and drop mass. This first drop, as quantified by high-speed videography and subsequent image analysis, can be different in morphology, trajectory, velocity, and volume from subsequent drops within a burst. These findings were confirmed orthogonally by both volume and mass measurement techniques which allowed quantitation down to single drops. [ABSTRACT FROM AUTHOR]

We show that some statistical properties of forced two-dimensional turbulence have an important sensitivity to the form of large-scale dissipation, which is required to damp the inverse cascade. We consider three models of large-scale dissipation: linear 'Ekman' drag, nonlinear quadratic drag, and scale-selective hypo-drag that damps only low-wavenumber modes. In all cases, the statistically steady vorticity field is dominated by almost axisymmetric vortices, and the probability density function of vorticity is non-Gaussian. However, in the case of linear and quadratic drag, we find that the velocity statistics is close to Gaussian, with non-negligible contribution coming from the background turbulent flow. On the other hand, with hypo-drag, the probability density function of velocity is non-Gaussian and is predominantly determined by the properties of the vortices. With hypo-drag, the relative positions of the vortices and the exponential distribution of the vortex extremum are important factors responsible for the non-Gaussian velocity statistics. [ABSTRACT FROM AUTHOR]

The three-dimensional stability dynamics of a separation bubble over a flat plate has been studied in both linear and nonlinear conditions. Using a global eigenvalue analysis, two centrifugal global modes are identified: an asymptotically unstable three-dimensional weakly growing mode which appears to be originated by a Rayleigh instability; a marginally stable three-dimensional steady mode which is originated by a convective Gortler instability. Direct numerical simulations show that both modes play a role in the route to transition toward the turbulent flow. A structural sensitivity analysis is used to investigate the mechanism of selection of the path toward transition when small perturbations are considered. Finally, a scenario of transition via Gortler modes breakdown is studied in detail, revealing the formation of trains of hairpin vortices in streamwise succession. [ABSTRACT FROM AUTHOR]

In the present work, a simple proportional feedback control is designed to suppress the vortex-shedding instability in the wake of a prototype bluff-body flow, i.e., the flow around a square cylinder confined in a channel with an incoming Poiseuille flow. Actuation is provided by two jets localized on the cylinder surface and velocity sensors are used for feedback control. This particular configuration is a pretext to propose a more general strategy for designing a controller, which is independent of the type of actuation and sensors. The method is based on the linear stability analysis of the flow, carried out on the unstable steady solution of the equations, which is also the target flow of the control. The idea is to use sensitivity analysis to predict the displacement in the complex plane of some selected eigenvalues, found by the linear stability analysis of the flow, as a function of the control design parameters. In this paper, it is shown that the information provided by only sensitivity analysis carried out on the uncontrolled system is not sufficient to design a controller which stabilizes the flow. Therefore, the control is designed iteratively by successive linearizations. Apart from possible constraints, the position of the sensors, the direction along which velocity is measured, and the feedback coefficients are outputs of the design procedure. The proposed strategy leads to a successful control up to a Reynolds number which is at least twice as large as the critical one for the primary instability, using only one velocity sensor. [ABSTRACT FROM AUTHOR]

The quantum kinetic chemical reaction model proposed by Bird for the direct simulation Monte Carlo method is based on collision kinetics with no assumed Arrhenius-related parameters. It demonstrates an excellent agreement with the best estimates for thermal reaction rates coefficients and with two-temperature nonequilibrium rate coefficients for high-temperature air reactions. This paper investigates this model further, concentrating on the non-thermal reaction cross sections as a function of collision energy, and compares its predictions with those of the earlier total collision energy model, also by Bird, as well as with available quasi-classical trajectory cross section predictions (this paper also publishes for the first time a table of these computed reaction cross sections). A rarefied hypersonic flow over a cylinder is used to examine the sensitivity of the number of exchange reactions to the differences in the two models under a strongly nonequilibrium velocity distribution. [ABSTRACT FROM AUTHOR]