Kamal Viswanath

A new approach for conjugate heat transfer problems using immersed boundary method for curvilinear grid based solvers

Abstract Use of immersed boundary method (IBM) based techniques have helped considerably in easing the grid generation process in flows involving complex geometries and/or large boundary movements. Body fitting grid based techniques still, however, are advantageous in terms of accuracy and efficiency. In this work, we have developed an IBM scheme applicable to curvilinear coordinates, aiming at taking advantage of both the methodologies. The framework uses efficient algorithms for search, locate, and interpolate operations. A new method of implementing the conjugate heat transfer (CHT) boundary condition is proposed which is a direct extension of the method used for other boundary conditions and does not involve any complex interpolations like previous CHT implementations using IBM. The developed scheme is shown to be applicable to complex geometries on curvilinear grids, while also being very efficient, consuming less than 1% of the total simulation time per time-step. Very good scalability on massive computations is demonstrated using strong scaling study up to 1024 cores. Detailed code verification process is undertaken to show that the method is second-order accurate for both the velocity and temperature fields for all the boundary conditions considered. Further, validation studies involving uniform flow over stationary and oscillating cylinders are carried out to demonstrate the accuracy of the developed method. Lastly, simulations are performed to study flow and conjugate heat transfer through thick-walled micro-channels using body-fitted background grids and the results are shown to be in excellent agreement with previously published results.

Effect of Frontal Gusts on Forward Flapping Flight

The response of a rigid flapping wing in forward flight, at Re = 10,000, subjected to frontal gusts has been investigated. The phasing and duration of the gusts and their impact on the various unsteady mechanisms are analyzed within a single flapping cycle. The gust is characterized by a step function with integral length scale much larger than that of the physical dimension of the micro air vehicle and with time scale much smaller than the flapping time period. The instantaneous lift and thrust profiles were observed to be influenced by a combination of the effective angle of attack, wing rotation, and the leading-edge vortex structures existing in the flow at any given time, with the leading-edge vortices themselves being influenced by the duration and magnitude of the change in effective angle of attack. Frontal gusts applied during the downstroke accelerated the development of the flow, resulting in the formation and detachment of multiple leading-edge vortices on the wing surface that increased the lift and thrust, illustrating the importance of the leading-edge vortex dynamics to force production. The effect of the gust is observed to be diminished when it occurs during rapid supination of the wing. The lift and thrust profiles are found to react in a similar fashion for gusts applied during the downstroke, whereas they experienced opposite effects during the upstroke. During the upstroke, force characteristics are shown to primarily react to effective angle-of-attack changes more than to changes in flow structures.

Climbing Flight of a Fruit Bat Deconstructed

The climbing flight flapping kinematics obtained on a fruit bat, Cynopterus brachyotis, is deconstructed using proper orthogonal decomposition to shed light on the functional kinematic modes which enables bat flight. Mode 1 and cumulative mode 1 + 2, compared against the native kinematic, is simulated in a Immersed Boundary CFD framework to observe the effect these modes have on the unsteady transient mechanisms of the flow produced by the flapping wings. From the obtained data, the bat exhibits fine control of its mechanics by actively varying wing camber, wing area, torsional rotation of the wing, forward and backward translational sweep of the wing, and wing conformation to dictate the fluid dynamics. As is common in flapping flight, the primary force generation is through the attached unsteady vortices on the wing surface. Mode 1 is seen to act as the power stroke dumping energy into the fluid. Mode 2 adds the mechanism which maneuvers the flow structures to enable forward flight.

Effect of Frontal Gusts on Flexible Wings in Forward Flapping Flight

The response of a flexible wing and a rigid wing in flapping forward flight, at Re=10,000, subjected to frontal gusts is investigated. The impact of a gust of chosen phasing and duration on the various unsteady mechanisms are analyzed within a single flapping cycle. The gust is characterized by a step function with integral length scale much larger than that of the physical dimension of the MAV and with time scale much smaller than the flapping time period. The instantaneous lift and thrust were observed to be influenced by a combination of the effective angle of attack, chosen kinematics, transient Leading Edge Vortex(LEV) structures, and the dynamic cambering in the case of the membrane wing. The effect of the gust on the lift profile is observed to have opposite effects for the rigid vs. flexible case while the thrust behaves in a similar profile. Nomenclature C = Airfoil chord length CL = Coefficient of lift CT = Coefficient of thrust h = wing thickness J = Advance ratio; U ∞/U f ; Ratio of the flight velocity to the flapping velocity Nξ = pre-stress along the chord Nζ = pre-stress along the span Nξζ = pre-stress in shear p = Pressure Re = Reynolds number; UfC/ν Re t = Inverse of the turbulent viscosity U∞ = Free stream velocity; Forward flight velocity Uf = Flapping velocity; 2 �fR w = Out of plane deformation x = Physical space coordinate

Flow Statistics and Noise of Ideally Expanded Supersonic Rectangular and Circular Jets

Simulations of an ideally expanded supersonic jet for two operating temperatures are conducted with a low aspect ratio of two rectangular nozzle, and an equivalent diameter circular nozzle of the same design Mach number. The emphasis is on accurately resolving the near-field data, capturing the far-field acoustics, and examining the flow asymmetry of the rectangular jets. The simulated acoustic data of the cold and heated jets are validated against experimentally recorded sound-pressure-level spectra with very good agreement. Rectangular nozzle metrics are calculated in two orthogonal, axial spanning planes to determine the impact of the nozzle exit asymmetry, and then they are compared with the circular jet. The shock cell structure, the jet core length, and the axial distribution of the flow at similar radial distances (normalized by the major, minor, or circular radial lengths) are observed to be different between the two planes of the rectangular jet and relative to the circular jet. The dynamics of t...