Yongmann M. Chung

Unsteady Heat Transfer Analysis of an Impinging Jet

Unsteady heat transfer caused by a confined impinging jet is studied using direct numerical simulation (DNS). The time-dependent compressible Navier-Stokes equations are solved using high-order numerical schemes together with high-fidelity numerical boundary conditions. A sixth-order compact finite difference scheme is employed for spatial discretization while a third-order explicit Runge-Kutta method is adopted for temporal integration. Extensive spatial and temporal resolution tests have been performed to ensure accurate numerical solutions. The simulations cover several Reynolds numbers and two nozzle-to-plate distances. The instantaneous flow fields and heat transfer distributions are found to be highly unsteady and oscillatory in nature, even at relatively low Reynolds numbers. The fluctuation of the stagnation or impingement Nusselt number, for example, can be as high as 20 percent of the time-mean value. The correlation between the vortex structures and the unsteady heat transfer is carefully examined. It is shown that the fluctuations in the stagnation heat transfer are mainly caused by impingement of the primary vortices originating from the jet nozzle exit. The quasi-periodic nature of the generation of the primary vortices due to the Kelvin-Helmholtz instability is behind the nearly periodic fluctuation in impingement heat transfer, although more chaotic and non-linear fluctuations are observed with increasing Reynolds numbers. The Nusselt number distribution away from the impingement point, on the other hand, is influenced by the secondary vortices which arise due to the interaction between the primary vortices and the wall jets. The unsteady vortex separation from the wall in the higher Reynolds number cases leads to a local minimum and a secondary maximum in the Nusselt number distribution. These are due to the changes in the thermal layer thickness accompanying the unsteady flow structures.

Numerical study of momentum and heat transfer in unsteady impinging jets

Direct numerical simulations of an unsteady impinging jet are performed to study momentum and heat transfer characteristics. The unsteady compressible Navier–Stokes equations are solved using a high-order finite difference method with non-reflecting boundary conditions. It is found that the impingement heat transfer is very unsteady and the unsteadiness is caused by the primary vortices emanating from the jet nozzle. These primary vortices dominate the impinging jet flow as they approach the wall. Detailed analysis of the instantaneous flow and temperature fields is performed, showing that the location of primary vortices significantly affects the stagnation Nusselt number. Spatio-temporal behaviour of the heat transfer is analysed, with instantaneous Cf and Nu variations showing the correlation between the local heat transfer and the flow field. Near the secondary vortices, the breakdown of the Reynolds analogy is observed.

Comparative Study of Inflow Conditions for Spatially Evolving Simulation

A new spatiotemporal inflow condition is devised and evaluated with other methods, i.e., temporal, phase jittering and amplitude jittering, and random noise. These methods are validated by testing a large-eddy simulation of turbulent channel flow. Computational results are presented to disclose the ability of inflow conditions to capture the turbulent statistics with correct phase information and dynamics. The present spatiotemporal inflow condition is found to be generally satisfactory in CPU time and data management.

Effectiveness of active flow control for turbulent skin friction drag reduction

The effectiveness of the opposition control method proposed by Choi et al. [J. Fluid Mech. 262, 75 (1994)] has been studied using direct numerical simulations. In this study, the effects of the amplitude and the phase of wall blowing and suction control input were considered separately. It is found that the amplitude of wall blowing and suction as well as the detection plane location played an important role in active control for skin-friction drag reduction. By changing the amplitude, a substantial drag reduction was achieved for all detection plane locations considered, and the efficiency of the opposition control was also improved. When the control was effective, the drag reduction was proportional to the wall blowing and suction strength. There existed a maximum wall blowing and suction strength, beyond which the opposition control became less effective or even unstable. Turbulence characteristics affected by various wall blowing and suction parameters were analyzed to understand the underlying mechanisms for drag reduction. The wall-normal velocity and vorticity fluctuations showed a strong correlation with drag reduction. © 2011 American Institute of Physics

Modification to a turbulent inflow generation method for boundary-layer flows

NUMERICAL simulations of turbulent boundary layers require inflow/outflow boundary conditions. Downstream flow is particularly sensitive to the inlet boundary condition; it is necessary to provide a realistic, coherent series of time-varying velocity components to avoid wasteful and potentially costly readjustment behavior. Simple periodic boundary conditions (where downstream flow is reapplied at the inlet), while suitable for channel or pipe flow simulations, are poorly suited to spatially developing flows such as flat-plate boundary layers [1]. Lund et al. [2] (LWS) developed a quasi-periodic approach using an accurate scaling technique. This method used recycling of the downstream data to provide the inlet boundary condition on the inflow simulation (illustrated in Fig. 1). It has been successfully applied in both incompressible and compressible boundary-layer simulations [3–5]. Despite the wealth of publications that have successfully applied this method, a number of studies [5–10] have indicated that some aspects of LWS method can prove difficult to implement. Hurdles include spurious periodicity, error accumulation, and initial conditions. The main objective of this Technical Note is to propose simple modification to the original LWS formulation to address these issues, and also to avoid use of the 99% boundary-layer thickness.

Initial relaxation of spatially evolving turbulent channel flow with blowing and suction

A direct numerical simulation is performed to examine the initial relaxation of spatially evolving turbulent channel flow subjected to uniform wall blowing and suction. Just beyond the entrance section, a uniform blowing is applied at the lower wall and a uniform suction at the upper. The relaxation processes of the mean velocity, turbulence intensities, Reynolds shear stresses, and vorticity fluctuations after the sudden wall blowing and suction are scrutinized. The effect of wall blowing and suction on the location and strength of the streamwise vortices is significant. The relaxation associated with suction is much slower than that with blowing. The initial relaxation is evaluated in terms of the turbulent kinetic energy transport. Blowing enhances the intercomponent energy transfer between the Reynolds stresses, whereas suction suppresses it.

Unsteady laminar flow and convective heat transfer in a sharp 180° bend

Unsteady laminar flow and heat transfer in a sharp 180 bend is studied numerically to investigate a convective heat transfer regime of especial relevance to electronic systems. Due to the high geometrical aspect ratios occurring in the practical application, two-dimensional unsteady simulations are considered. The two-dimensionality assumption adopted is validated by three-dimensional test simulations. Unsteady heat transfer simulations are performed for 50 6 Re 6 1000. Results show that the flow remains steady until Re � 600. In this steady regime, the re-attachment length increases gradually with the Reynolds number. For Re > 600, the flow becomes unsteady with large-scale vortices emanating from the sharp edge dominating the flow field. Flow oscillation causes a substantial reduction in the re-attachment length and a dramatic heat transfer enhancement. As the vortices move downstream, the Nusselt number along the wall oscillates significantly. The correlation between the flow structure and the heat transfer is found to be strong. 2002 Elsevier Science Inc. All rights reserved.

Assessment of Periodic Flow Assumption for Unsteady Heat Transfer in Grooved Channels

Numerical studies of unsteady heat transfer in grooved channel flows are made. The flows are of special relevance to electronic systems. Predictions suggest a commonly used periodic flow assumption (for modeling rows of similar electronic components) may not be valid over a significant system extent. It is found that the downstream flow development is strongly dependent on geometry. @DOI: 10.1115/1.1833371#

Turbulent Flow and Scalar Mixing of a Coaxial Injector Having Two Fluid Jets

The turbulent flow and scalar mixing in a coaxial injector having two fluid jets were investigated by using large eddy simulation (LES). Several recess lengths, which are the retracted distance of the inner jet in a coaxial injector, were considered to analyze the geometric effects on the scalar mixing for nonreacting variable–density flows at a constant Reynolds number. The LES results showed the detailed description of flow structures and the changes of turbulent fluctuations caused by the recess. The flow structures in the near field of the injection plane were modified by the recess. When the inner jet was recessed, the development of turbulent kinetic energy became faster than that of the nonrecessed case. Also, it was found that a combination of outer–inner jets with a large density difference is better to complete the flow mixing within shortened streamwise length.

Large-Eddy Simulation of Complex Internal Flows

Large-eddy simulation for a complex internal flow is performed. An idealised electronic system cooling flow is considered. The flow is highly oscillatory and flow separation and reattachment takes place in several regions. Comparison is made with Laser Doppler Anemometry (LDA) measurements along with Unsteady Reynolds Averaged Navier-Stokes (URANS) computations. LES results show good performance and agree well with available experimental data.