Prediction of air blast mitigation in an array of rigid obstacles using smoothed particle hydrodynamics
The mitigation of blast waves propagating in air and interacting with rigid barriers and obstacles is numerically investigated using the mesh-free smoothed particle hydrodynamics method. A novel virtual boundary particle procedure with a skewed gradient wall boundary treatment is applied at the interfaces between air and rigid bodies. This procedure is validated with closed-form solutions for strong and weak shock reflection from rigid surfaces, supersonic flows over a wedge, formation of reflected, transverse, and Mach stem shocks, and also earlier experiments on interaction of a blast wave with concrete blocks. The mitigation of the overpressure and impulse transmitted to the protected structure due to an array of rigid obstacles of different shapes placed in the path of the blast wave is thereafter determined and discussed in the context of the existing experimental and numerical studies. It is shown that blockages having the shape of a right facing triangle or square placed in tandem or staggered provide better mitigation. The influence of the distance between the blockage array and protected structure is assessed, and the incorporation of a gap in the blockages is shown to improve the mitigation. The mechanisms responsible for the attenuation of air blast are identified through the simulations.
Simulation of laminar confined flow pasta circular cylinder with integral wake splitter involving heat transfer
A finite element method is used to study the effect of flow past a circular cylinder with an integral wake splitter. A fractional step algorithm is employed to solve the Navier-Stokes and Energy equations with a Galerkin weighted residual formulation. The vortex shedding process is simulated and the effect of splitter addition on the time period of shedding is studied at a Reynolds number of 200 and a blockage ratio of 0.25. The effect of splitter and the Strouhal number and heat transfer augmentation per unit pressure drop has been investigated.
No-slip boundary condition in finite-size dissipative particle dynamics
Dissipative particle dynamics (DPD) is an efficient, particle based mesoscopic numerical scheme to simulate dynamics of complex fluids and micro-flows, with spatio-temporal scales in the range of micrometers and microseconds. While the traditional DPD method treated particles as point masses, a modified DPD scheme was introduced recently [W. Pan, I.V. Pivkin, G.E. Karniadakis, Single-particle hydrodynamics in DPD: a new formulation, Europhysics Letters 84 (2008) 10012] by including transverse forces between finite sized particles in addition to the central forces of the standard DPD.The capability of a DPD scheme to solve confined wall bounded flows, depends on its ability to model the flow boundaries and effectively impose the classical no-slip boundary condition. Previous simulations with the modified DPD scheme used boundary conditions from the traditional DPD schemes, resorting to the velocity reversal of re-inserted particles which cross the solid wall. In the present work, a new method is proposed to impose no-slip or tunable slip boundary condition by controlling the non-central dissipative components in the modified DPD scheme. The solid wall is modeled in such a way that the fluid particles feel the presence of a continuous wall rather than a few discrete frozen particles as in conventional wall models. The fluid particles interact with the walls using a modified central repulsive potential to reduce the spurious density fluctuations. Several different benchmark problems (Poiseuille flow, lid-driven cavity and flow past circular cylinder) were solved using the new approach to demonstrate its validity. © 2012 Elsevier Inc.
A mechanistic model for embryo size prediction at boiling incipience: ‘Work of formation’ based approach
The initial size of the embryo, which is formed at the inception of boiling, plays a vital role in the accurate prediction of component scale wall boiling phenomenon. Embryo size predictions are typically calculated using the classical theory of nucleation. However, in recent times, the predictive capability of this theory was found to have limitations. Hence, there is need for a more fundamental and mechanistic model to overcome some of the drawbacks. In this paper, we propose a ‘work of formation’ based model for the embryo formation. This model is mechanistic and includes a Van der Waals based real gas treatment for the vapour. It also incorporates Lewins surface tension model that is a function of the boiling-nucleus size. The present model also accounts for the boiling occurrence in the presence of undissolved nanobubbles on the surface. The embryo formation model has been extensively tested for both low and high pressures, horizontal and vertical test section orientation, and for different surfaces and fluids. The energy required for the embryo formation was found to be higher, when the initial gas bubble is intact compared to when the gas bubble diffuses into the embryo. Some of the contradictory claims on the suitability of classical theory of nucleation to nanosurfaces have been tested. From the present embryo formation model, the physics of nucleation such as, the effect of pressure fluctuations and energy dissipation mechanisms involved in the formation is explained.
Suboptimal control of circular cylinder wakes using Van der Pol oscillator
In the present study, we develop an energetically efficient suboptimal open-loop strategy to control the wake behind a circular cylinder in the laminar regime. The open-loop suboptimal controller is designed to resemble the feedback integral controller with reference to its dynamical behaviour. Energetic efficiency is measured using the power loss coefficient. The Van der Pol model for the evolution of lift force on the cylinder is chosen as the reduced-order model for the development of an open-loop suboptimal controller. The parameter estimation of the low- dimensional model is carried out using the results from the continuum based Navier - Stokes simulations. It is shown that a subspace identification method can be used to model the relationship between the inputs to the reduced-order model and the inputs to the higher-order computational fluid dynamic model. The development of the suboptimal control is realised by means of solving suitably formulated optimal tracking and regulator problems using the Pontryagin's minimum principle. The resultant controller is found to be energetically efficient and also successful in the control of vortex shedding.
A dissipative particle dynamics study of a flexible filament in confined shear flow
In this paper, we investigate the dynamics of a tethered flexible filament due to fluid flow inside a microchannel. We use the finite sized dissipative particle dynamics (FDPD) approach to model this problem. The flexible filament is modeled as a bead-spring system with both extensional and flexural rigidity. The influence of flow rate and bending stiffness on the filament dynamics is studied in terms of the different conformational modes obtained. The competing effects of the hydrodynamic force and elastic force in the presence of Brownian thermal effects of comparable order influence the mode shapes of the filament. The dynamics of the filament motions are further analyzed using proper orthogonal decomposition. An important consequence of the dynamics of the filament is that it causes cross-flow in the micro-channel, which could potentially be exploited in micro-mixing and pumping applications. The cross stream fluid transport is observed to be more pronounced for higher bending stiffness.
Sloshing dynamics of shallow water tanks: Modal characteristics of hydraulic jumps
The dynamics of slosh induced wave systems in shallow water tanks is analyzed for various excitation conditions. Shake table experiments have been systematically performed to understand the complex interaction of multi-wave system under harmonic excitation. From the experiments, it was observed that, for relatively large excitation amplitudes, the hydraulic jumps emanated around the resonance region. The hydraulic jump phenomenon is further explored for different tank aspect ratios, i.e, 2.5 ≤L∕B≤ 4.038. To establish the frequency bounds for hydraulic jumps, excitation amplitude and frequency are demarcated over the range of 0.841 ≤β≤ 1.628 and liquid depth range of 0.034 ≤h∕L≤0.069. The experimental bounds are juxtaposed with the theoretical bounds to analyze the margins present in hydraulic jumps. Although, the theoretical bound is independent of liquid depth, experimental observations clearly indicate a strong dependency.
Numerical investigation of membrane oxygenation using sub-channel analysis
Purpose: Better membrane oxygenators need to be developed to enable efficient gas exchange between venous blood and air. Design/methodology/approach: Optimal design and analysis of such devices are achieved through mathematical modeling tools such as computational fluid dynamics (CFD). In this study, a control volume-based one-dimensional (1D) sub-channel analysis code is developed to analyze the gas exchange between the hollow fiber bundle and the venous blood. DIANA computer code, which is popular with the thermal hydraulic analysis of sub-channels in nuclear reactors, was suitably modified to solve the conservation equations for the blood oxygenators. The gas exchange between the tube-side fluid and the shell-side venous blood is modeled by solving mass, momentum and species conservation equations. Findings: Simulations using sub-channel analysis are performed for the first time. As the DIANA-based approach is well known in rod bundle heat transfer, it is applied to membrane oxygenators. After detailed validations, the artificial membrane oxygenator is analyzed for different bundle sizes (L/W) and bundle porosity (epsilon) values, and oxygen saturation levels are predicted along the bundle. The present sub-channel analysis is found to be reasonably accurate and computationally efficient when compared to conventional CFD calculations. Research limitations/implications: This approach is promising and has far-reaching ramifications to connect and extend a well-known rod bundle heat transfer algorithm to a membrane oxygenator community. As a variety of devices need to be analyzed, simplified approaches will be attractive. Although the 1D nature of the simulations facilitates handling complexity, it cannot easily compete with expensive and detailed CFD calculations. Practical implications: This work has high practical value and impacts the design community directly. Detailed numerical simulations can be validated and benchmarked for future membrane oxygenator designs. Social implications: Future membrane oxygenators can be designed and analyzed easily and efficiently. Originality/value: The DIANA algorithm is popularly used in sub-channel analysis codes in rod bundle heat transfer. This efficient approach is being implemented into membrane oxygenator community for the first time.
Numerical prediction of dryout in a 19 rod bundle under the effect of eccentricity and blockage
An accurate estimation of dryout power and its location (zcr) is central to the safety of nuclear reactors. In the present study, a sub-channel analysis code is developed by extending the standard single phase DIANA algorithm to two phase flow conditions. The mass, momentum and energy conservation equations are solved, using a mixture model, which is validated against available experimental data. Numerical simulations are performed to determine the dryout location for a circular 19 rod bundle, in conjunction with a film thickness model. In critical sub-channels, a sudden jump in wall temperature was noticed at the dryout location. The effect of eccentricity(e) on the dryout location was investigated in the range of 0.0⩽e⩽0.7, under different operating conditions. It was observed that, eccentricity causes flow maldistribution in different sub-channels, and in turn affects the dryout location. For low inlet mass fluxes, sub-channels which never experienced dryout (for e=0.0) were found to experience dryout (for e<0.0) The effect of blockage (b) was also systematically studied for 0.0⩽b⩽0.3. In flow regimes with higher vapor quality, the blockage leads to a disturbance in the continuous liquid film, resulting in an early occurrence of dryout. Two types of axial power distribution (APD) viz uniform and sinusoidal heat flux imposition were numerically simulated. The latter was found to delay the occurrence of dryout, compared to the former.
A skewed kernel approach for the simulation of shocks using SPH
A wide variety of jump discontinuities, such as shock fronts, are abound in high-speed flows. An accurate approximation of these fronts may require higher order techniques either under mesh-based methods or mesh-free methods. In the latter class, the smoothed particle hydrodynamics (SPH) is becoming popular as a promising method. However, the standard approach in SPH (like any other discrete methods) can result in highly diffusive solutions because of the inevitable use of artificial viscosity to suppress numerical oscillations. On the other hand, the SPH formulation allows innovative ways to model complicated phenomena. In this paper, we introduce the novel idea of a skewed Gaussian kernel, to improve the shock capturing capability in high speed flows. Here, the standard Gaussian kernel function is modified, and its ‘shape’ is altered with a predesigned tunable skewness parameter, while the basic unity property of the kernel function is still preserved. The SPH with the proposed skewed Gaussian kernel is then applied on a number of benchmark problems in computational fluid dynamics, featuring shocks. The simulations have shown significantly better shock capture through the skewed kernel approach as against the standard techniques, with almost no increase in computational time. Copyright © 2016 John Wiley & Sons, Ltd.