Examination of Reynolds number effect on the development of round jet flow

In this study, the Reynolds number effect on the development of round jet flow is presented. The jet is produced from a smoothly contracting round nozzle and the flow structure is controlled by varying the air blower speed in order to obtain various Reynolds numbers (Re). The flow Reynolds number considered varies between 1140 and 9117. Mean velocity measurements were taken using hot-wire probe at different axial and lateral distances (0≤x/d≤50, where x is the downstream distance and d is the nozzle diameter) for the jet flow and at for 0≤x/d≤30 in long pipe attached to the nozzle. Measurements reveal that Reynolds number dictate the potential core length such that the higher the Reynolds number, the lower the potential core which is a measure of mixing of jet and ambient fluid. It shows that further away from the jet exit section, potential core decreases as Reynolds number increases, the velocity profile has a top hat shape very close to the nozzle exit and the shape is independent of Reynolds number. It is found that potential core extends up to x/d=8 for Reynolds number of 1140 as against conventional near field 0≤x/d≤6. This may suggest effect of very low Reynolds number. However, further investigation is required to ascertain this at extremely low Reynolds numbers. It is also observed that further away from the jet exit section, the higher the downstream distance, the higher the jet half-width (R1/2). Furthermore, the flow in the pipe shows almost constant value of normalised axial centerline velocity for a longer distance and this clearly indicates that there is mass redistribution rather than entrainment of ambient fluid. Overall, the Reynolds number controls the magnitude rather than the wavelength of the oscillation

Numerical Prediction of the Second Peak in the Nusselt Number Distribution from an Impinging Round Jet

The results of numerical simulations of a single impinging round jet, using different numerical parameters are presented. To simulate the heat transfer in industrial drying with arrays of different jets the heat transfer for a single round jet (Re=23000 based on jet’s diameter and bulk velocity and the dimensionless jet’s outlet to target wall distance= 2) is used as a test case to validate the numerical model. The distribution of the Nusselt-number serves as a benchmark and the computational cost with regard to CPU-time and memory requirements should be minimal. To accurately predict the intensity and position of the secondary peak from an impinging flow, different approaches for turbulence modeling are considered and their results are compared with data from the literature. The influence of the grid size and the grid shape is analyzed and the grid-independent solution is determined. The results using different implementations of the SST k-omega model, as the best compromise between the computational cost and accuracy are compared. Low Re damping modification in the implementation of SST K-ω has an important role in the prediction of the secondary peak. Good results can be achieved with a coarse grid, as long as the boundary region is appropriately resolved. Polyhedral grids produce good quality results with lower memory requirements and cell numbers as well as shorter run times.

Time Resolved PIV Measurements of a Slot Lobed Jet Issuing Into a Crossflow

The near field mixing phenomenon created by a round jet with three slot lobes exhausting into a crossflow are investigated at a velocity ratio of 0.5. Time-resolved particle image velocimetry measurements provide instantaneous velocity fields of the slotted jet in crossflow, allowing for evaluation of the first and second order turbulent statistics in two perpendicular planes of interest. The independently controlled jet exit and crossflow inlet are first characterized extensively to confirm the velocity ratio and anticipated momentum exchanges. Spanwise and transverse mean velocity profiles reveal that the interaction of the three slot lobes and the center round jet primarily occur in the immediate jet exit region, though residual effects are also found in the wake. Evaluation of the Reynold stresses aims to quantify the near region mixing between the jets collated geometric features and their interaction with the crossflow. Frequency analysis reveals that low-frequency harmonics in the wake region provide greater energy contributions than that of the higher-frequency harmonics found along the leading edge shear layer. This behavior is attributed to the low velocity ratio, where the freestream velocity is twice as large as the jet exit velocity. The experimental data and observations herein serve analogous computational modeling efforts for the slotted jet in crossflow at low velocity ratios, with ample information to inform necessary boundary conditions, fluid properties, and flow fields for validation.

Control of a Round Jet Intermittency and Transition to Turbulence by Means of an Annular Synthetic Jet

This paper deals with active control of a continuous jet issuing from a long pipe nozzle by means of a concentrically placed annular synthetic jet. The experiments in air cover regimes of laminar, transitional, and turbulent main jet flows (Reynolds number ranges 1082–5181). The velocity profiles (time-mean and fluctuation components) of unforced and forced jets were measured using hot-wire anemometry. Six flow regimes are distinguished, and their parameter map is proposed. The possibility of turbulence reduction by forcing in transitional jets is demonstrated, and the maximal effect is revealed at Re = 2555, where the ratio of the turbulence intensities of the forced and unforced jets is decreased up to 0.45.