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Jet impingement is widely used for forced-convection heat transfer applications and knowledge about its flow structure and heat transfer rate on a static surface are well established. However, the characteristics of jet impingement on an oscillating surface are relatively unknown. This study experimentally investigates the effect of surface oscillation on the fluid dynamics and heat transfer of an unconfined turbulent impinging jet. The Reynolds numbers of the axisymmetric jet are 5000 and 10,000, based on the jet-nozzle exit diameter, and the surface is placed at nominal standoff distances of 2 and 5 diameters from the jet-nozzle exit. The surface oscillates in a direction parallel to the jet axis at frequencies of 20 Hz and 50 Hz and at a peak-to-peak displacement amplitude of 0.2 times the jet-nozzle exit diameter. The phase-average and mean flow characteristics at six phases through the surface oscillation cycle, and the steady-state mean heat transfer rate at the oscillating surface, are determined using particle image velocimetry and infrared thermography respectively. These are analyzed and compared with the mean flow and heat transfer characteristics for jet impingement on a static surface. Surface oscillation directly affects the mean axial jet velocities and thence the mean radial velocities, and this effect is greater at locations in the flow-field closer to the surface. This gives rise to lower mean axial and radial strain rates in the impingement region and lower turbulence intensities in the wall-jet region when compared with those for a static surface. The frictional interaction between the impinging jet and oscillating surface induces higher surface temperatures than those on a static surface. These factors reduce the heat transfer rate for jet impingement on an oscillating surface when compared with that on a static surface. The reduction is greater in the impingement region than in the wall-jet region with the stagnation point Nusselt number for an

High-resolution large-eddy simulations (LES) are performed for an incompressible turbulent circular jet impinging upon a vibrating heated wall supplied with a constant heat flux. The present work serves to understand the flow dynamics and thermal characteristics of a turbulent jet under highly dynamic flow and geometric conditions. The baseline circular vibrating-wall jet impingement configuration undergoes a forced vibration in the wall-normal direction at the frequency, f = 100 Hz. The jet Reynolds number is Re=DVb/νRe=DVb/ν = 23,000 and the nozzle-exit is at y/D = 2 where the wall vibrates between 0 and 0.5D with amplitude of vibration, A = 0.25D. The configuration is assembled through validation of sub-systems, in particular the method for generating the turbulent jet inflow and the baseline circular jet impingement configuration. Both time-mean and phase-averaged results are presented. The mean radial velocity increases upon positive displacement of the wall and decreases upon negative displacement but this correlation changes with increased radial distance from the stagnation point. Vortical structures are shown to play a major role in convective heat transfer even under the vibrating conditions of the impingement wall. Periodic shifts in the secondary Nusselt number peak are observed that depend upon the travelling eddy location and strength of large-eddy structures. Enhancement in heat transfer is seen in the stagnation region but this beneficial effect of vibration on heat transfer is confined to the impingement region, r/D < 1.5.

The design and performance characteristics of a mass flow control valve to generate unsteady pulsating flows are presented in this paper. As the majority of the naturally occurring flows are pulsatile in nature, it becomes imperative to simulate these flows for their experimental analysis. The proposed flow control valve is capable of producing controlled, periodic pulsating mass flows having sinusoidal, triangular, and trapezoidal waveforms to be used for fluid dynamics research. The design of the valve, while being simple, allows for generation of pulsatile flows over wide range of amplitudes and frequencies. Distinct waveforms for the output flows, at two amplitudes and frequencies, are demonstrated experimentally, and compared with the conceptual design.

The fluid dynamics and heat transfer characteristics of a turbulent round jet are modelled numerically using Reynolds- Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES). Meshes with varying degrees of coarseness, with both radial and axial refinements are investigated. Discretization is carried out using the finite volume method. The jet configurations are chosen to enable validation against well-established experimental jet-impingement heat-transfer studies, particularly that of Cooper et al. [1]. The Reynolds number studied is 23000. The height of discharge from the impingement wall is fixed at twice the jet diameter. The work critically examines the effect of Reynolds number, standoff distance and helps to ascertain the relative merits of various turbulence models, by comparing turbulent statistics and the Nusselt number distributions. The present work is carried out as a preliminary validation, in a wider study intended to determine the thermofluidic behaviour of jets impinging upon an oscillating surface.

The introduction of vibrations to a horizontal plate can induce turbulence in the flow field adjacent to the plate under certain combinations of amplitudes and frequency. It is also known that beyond a threshold level of heating, the convective flow field over a heated plate will transition to turbulence. The characterization of the flow field with turbulence in the domain is of paramount importance to ensure a realistic simulation of the flow physics. In the present study, a computational analysis is carried out to characterize the flow regime over a transversely vibrating flat plate (unheated and heated) into laminar or turbulent. The range of frequency and amplitudes of vibrations considered for this analysis are 0-150 Hz and 0-2 mm respectively. Three different models viz. laminar; Reynolds Averaged Navier-Stokes (RANS) approach with k-? SST model; and Large-Eddy Simulations (LES) approach with dynamic Smagorinsky-Lilly model are employed, and the results of local, time and space averaged wall shear stress, and the Nusselt number are compared. It has been found that under conditions with unheated or heated vibrating plate, the wall shear stress predictions by all three models are in good agreement with each other with a maximum deviation of 9.5 %. However, when the predictions of local Nusselt number on the heated vibrating plate are compared, it is found that the laminar and LES predictions are in good agreement with each other; the k-? SST model predictions deviate significantly from the other two models.

The paper describes the effect of oscillation of an impingement surface under a partially confined slot jet. Both the flow characteristics and the heat transfer characteristics are explored and presented. The numerical conditions were as follows: Reynolds number Re = 1000-2000, dimensionless distance between the nozzle and the impingement surface h/B = 4.0, surface oscillation frequency, f = 0-100 Hz and the amplitude a = 1.0 mm. The experimental data of Ichimiya and Yoshida [5] are taken as a baseline case and the present numerical results are compared with the experimental studies. The results compare well with the experimental work. The present numerical work serves as the basis for a wider study intended to optimise the thermofluidic characteristics of jets impinging upon oscillating surfaces.

Convective heat transfer studies of impinging jets have received considerable attention over the years, due to their high local, and averaged heat transfer coefficients. In the present work, highly resolved Large Eddy Simulations (LES) are carried out for a turbulent impinging round jet with structured meshes of varying resolution, with both radial and axial refinements. The Reynolds number studied is 23000. The height of discharge from the impingement wall is 2 times the jet diameter. A particular focus of the current research is to study the role of turbulent structures in jet impingement heat transfer. Second order turbulence statistics are computed and validated against established experimental work. Preliminary validation work was carried out by Natarajan et al. [8]. The present study is a continuation of this research, the focus now being upon the turbulence statistics. The work also critically examines the effect of Reynolds number, standoff distance and reveals the role of turbulence in enhancing heat transfer rate, by comparing turbulent statistics and the Nusselt number distributions.