Danesh K. Tafti

Optimal control and simulation of multidimensional crystallization processes

The optimal batch control of a multidimensional crystallization process is investigated. A high resolution algorithm is used to simulate the multidimensional crystal size distribution under the operations defined by two optimal control trajectories. It is shown that a subtle change in the optimal control objective can have a very large effect on the crystal size and shape distribution of the product crystals. The effect of spatial variation is investigated using a compartmental model. The effect of differing numbers of compartments on the size and shape distribution of the product crystals is investigated. It is shown that the crystal size distribution can be very different along the height of the crystallizer and that a solution concentration gradient exists due to imperfect mixing. The nucleation rate can be significantly larger at the bottom of the crystallizer and the growth rate can be much larger at the top. The high resolution method provides high simulation accuracy and fast speed, with the ability to solve large numbers of highly nonlinear coupled multidimensional partial differential equations over a wide range of length scales. A parallel programming implementation results in simulation times that are short enough for using the simulation program to compute optimal control trajectories.

STUDY OF DISCRETE TEST FILTERS AND FINITE DIFFERENCE APPROXIMATIONS FOR THE DYNAMIC SUBGRID-SCALE STRESS MODEL

This paper investigates the effects of discrete test filters and finite‐difference approximations for large‐eddy simulations using the dynamic subgrid‐scale stress model. Discrete explicit test filters based on finite‐difference formulations have been constructed and the characteristics of their transfer function are studied. Several definitions of the scaling factor are investigated in the context of the discrete test filters. Two test filters, one based on a discrete representation of the top‐hat filter (A), and another based on a high‐order filtering operation (C) are evaluated in simulations of the turbulent channel flow at Reτ=180. It is found that filter A calculates a higher turbulent viscosity than filter C, which behaves more like a cutoff filter. For the same test filtering operation, the results are found to be sensitive to the ratio of the characteristic lengths of the test and grid filters. By testing two approximations to the convection terms based on second‐order central difference and a no...

Flow transition in a multilouvered fin array

Abstract The paper describes the detailed transition mechanism from steady to unsteady flow in a multilouvered fin geometry. The initial instability appears in the wake of the exit louver at a Reynolds number of 400 with a characteristic non-dimensional frequency (based on inflow velocity and louver pitch) of 0.84. Between a Reynolds number of 700 and 800, power spectra in the interior of the array indicate an increase in energy in the vicinity of the first harmonic of the initial exit wake instability. By a Reynolds number of 900, free shear layer or Kelvin–Helmholtz type instabilities develop on the leading edge shear layers of louvers near the exit. These instabilities have a characteristic non-dimensional frequency of 1.7. As the Reynolds number increases further, instabilities move upstream into the array. By a Reynolds number of 1300, most of the louvers exhibit unsteadiness, except for the entrance louver and the first two louvers following it. By this Reynolds number, the flow in the downstream half of the array exhibits a chaotic behavior.

Flow efficiency in multi-louvered fins

The paper studies the effect of Reynolds number, fin pitch, louver thickness, and louver angle on flow efficiency in multi-louvered fins. Results show that flow efficiency is strongly dependent on geometrical parameters, especially at low Reynolds numbers. Flow efficiency increases with Reynolds number and louver angle, while decreasing with fin pitch and thickness ratio. A characteristic flow efficiency length scale ratio is identified based on geometrical and first-order hydrodynamic effects, which together with numerical results is used to develop a general correlation for flow efficiency. Comparisons show that the correlation represents more than 95% of numerical predictions within a 10% error band, and 80% of predictions within a 5% error band over a wide range of geometrical and hydrodynamic conditions.

Computations of flow and heat transfer in parallel-plate fin heat exchangers on the CM-5: effects of flow unsteadiness and three-dimensionality

Abstract An accurate computational method for the calculation of flow and heat transfer in compact heat exchangers is developed and implemented on the massively parallel Connection Machine, CM-5. In this method, the unsteady Navier-Stokes and energy equations are solved. The current study shows that the inclusion of flow unsteadiness plays a very important role in the accurate prediction of heat exchanger performance quantities of interest such as the Colburn j factor and the friction factor. It is also shown that at higher Reynolds numbers, the additional effect introduced by the intrinsic three-dimensionality of the flow plays another important role in determining the overall heat exchanger performance. Therefore, the above two effects have to be taken into account in order to accurately predict heat exchanger performance.

A numerical study of the effects of spanwise rotation on turbulent channel flow

The effects of spanwise rotation on the large‐scale structures in a turbulent channel flow are numerically simulated by integrating the filtered Navier–Stokes equations. A finite‐difference technique is used to retain the generality for complex geometries and developing flows to be considered in the future. The computed results are consistent with previous flow visualization data and measurements. It is seen that for the presently considered low Reynolds number, the flow significantly laminarizes on the stable side even for low rotation rates. Cellular spanwise structures are observed to develop as a result of the unstable interaction between mean shear and Coriolis forces. The statistics of these roll cells and the underlying turbulence are analyzed and presented in this paper. It is observed that the roll cells make a significant contribution to total turbulent quantities and also aid in the transport of underlying turbulence.

Comparison of some upwind-biased high-order formulations with a second-order central-difference scheme for time integration of the incompressible Navier-Stokes equations

Abstract The paper compares high-order finite-difference discretizations with a second-order central-difference scheme for the time integration of the incompressible Navier-Stokes equations. Conservative and non-conservative forms of the convection terms are discretized using fifth-order accurate upwind-biased based approximations. The high-order non-conservative treatment of the convection terms exhibits the best accuracy at high resolutions but deteriorates rapidly as the resolution decreases. The high-order conservative treatment of the convection terms, despite the second-order finite-volume operator, exhibits high-order accuracy but does not show any advantage over the non-conservative formulation. Combinations of the divergence and gradient operators based on second and fourth-order accurate central-difference approximations are used to construct high-order Laplacians in the pressure equation. There is little evidence that the high-order treatment of the pressure equation adds sufficiently to the overall accuracy of the scheme to justify the extra computational effort associated with it. Simulations of turbulent channel flow indicate that the second-order central-difference scheme resolves the turbulent spectrum better than the high-order upwind schemes.

Efficient parallel CFD-DEM simulations using OpenMP

The paper describes parallelization strategies for the Discrete Element Method (DEM) used for simulating dense particulate systems coupled to Computational Fluid Dynamics (CFD). While the field equations of CFD are best parallelized by spatial domain decomposition techniques, the N-body particulate phase is best parallelized over the number of particles. When the two are coupled together, both modes are needed for efficient parallelization. It is shown that under these requirements, OpenMP thread based parallelization has advantages over MPI processes. Two representative examples, fairly typical of dense fluid-particulate systems are investigated, including the validation of the DEM-CFD and thermal-DEM implementation with experiments. Fluidized bed calculations are performed on beds with uniform particle loading, parallelized with MPI and OpenMP. It is shown that as the number of processing cores and the number of particles increase, the communication overhead of building ghost particle lists at processor boundaries dominates time to solution, and OpenMP which does not require this step is about twice as fast as MPI. In rotary kiln heat transfer calculations, which are characterized by spatially non-uniform particle distributions, the low overhead of switching the parallelization mode in OpenMP eliminates the load imbalances, but introduces increased overheads in fetching non-local data. In spite of this, it is shown that OpenMP is between 50-90% faster than MPI. OpenMP parallelization of a CFD-DEM code.Validation of the CFD-DEM code.OpenMP has clear advantage over MPI for dynamic load imbalance case of fluid-particulate system.Parallel performance comparison using fluidized bed and rotary kiln simulations.Full scale rotary kiln simulation using OpenMP parallel code.

Passive energy recapture in jellyfish contributes to propulsive advantage over other metazoans

Significance Jellyfish have the ability to bloom and take over perturbed ecosystems, but this is counterintuitive because jellyfish are described as inefficient swimmers and rely on direct contact with prey to feed. To understand how jellyfish can outcompete effective visual hunters, such as fish, we investigate the energetics of propulsion. We find that jellyfish exhibit a unique mechanism of passive energy recapture, which can reduce metabolic energy demand by swimming muscles. Contrary to prevailing views, this contributes to jellyfish being one of the most energetically efficient propulsors on the planet. These results demonstrate a physical basis for the ecological success of medusan swimmers despite their simple body plan and have implications for bioinspired design, where low-energy propulsion is required. Gelatinous zooplankton populations are well known for their ability to take over perturbed ecosystems. The ability of these animals to outcompete and functionally replace fish that exhibit an effective visual predatory mode is counterintuitive because jellyfish are described as inefficient swimmers that must rely on direct contact with prey to feed. We show that jellyfish exhibit a unique mechanism of passive energy recapture, which is exploited to allow them to travel 30% further each swimming cycle, thereby reducing metabolic energy demand by swimming muscles. By accounting for large interspecific differences in net metabolic rates, we demonstrate, contrary to prevailing views, that the jellyfish (Aurelia aurita) is one of the most energetically efficient propulsors on the planet, exhibiting a cost of transport (joules per kilogram per meter) lower than other metazoans. We estimate that reduced metabolic demand by passive energy recapture improves the cost of transport by 48%, allowing jellyfish to achieve the large sizes required for sufficient prey encounters. Pressure calculations, using both computational fluid dynamics and a newly developed method from empirical velocity field measurements, demonstrate that this extra thrust results from positive pressure created by a vortex ring underneath the bell during the refilling phase of swimming. These results demonstrate a physical basis for the ecological success of medusan swimmers despite their simple body plan. Results from this study also have implications for bioinspired design, where low-energy propulsion is required.

Effect of Wing Flexibility on Lift and Thrust Production in Flapping Flight

In bird and insect flight, wing deformation plays an important role in aerodynamic performance. The wing deformation is produced by neuromuscular control and/or by aeroelastic effects. The focus of the current study is to evaluate the effects of wing deformation by coupling a large-eddy simulation solver with a linear elastic membrane model. Different membrane prestresses are investigated to give a desired camber in response to the aerodynamic pressure. All simulations are carried out at Re = 10, 000 for forward flight with an advance ratio of 0.5. The results show that the camber introduced by a flexible wing increases the thrust and lift production considerably. An analysis of flow structure reveals that, for flexible wings, the leading-edge vortex stays attached on the top surface of the wing and glides along the camber and covers a major part of the wing, which results in high force production. On the other hand, for rigid wings, the leading-edge vortex lifts off from the surface resulting in lower force production. Further, introduction of the camber also increases the force component contributing to thrust, leading to a high thrust-to-lift ratio. In comparison to a rigid wing, a 40% increase in thrust is observed for the low-prestress case, which results in a camber of 0.25 chord. Further, the results also show that the wing with high spanwise prestress and low chordwise prestress offers better performance both in terms of force production and uniformity in the force-induced cambering.