Physical study of the non-equilibrium development of a turbulent thermal boundary layer

The direct numerical simulation of a non-equilibrium turbulent heat transfer case is performed in a channel flow, where non-equilibrium is induced by a step change in surface temperature. The domain is thus made of two parts in the streamwise direction. Upstream, the flow is turbulent, homogeneous in temperature and the channel walls are adiabatic. The inflow conditions are extracted from a recycling plane located further downstream, so that a fully developed turbulent adiabatic flow reaches the second part. In the domain located downstream, isothermal boundary conditions are prescribed at the walls. The boundary layer, initially at equilibrium, is perturbed by the abrupt change of boundary conditions, and a non-equilibrium transient phase is observed until, further downstream, the flow reaches a new equilibrium state, presenting a fully developed thermal boundary layer. The work aims at identifying the non-equilibrium effects that are expected to be encountered in comparable flows, while providing the means to understand them. In particular, the study allows for the identification of an inner region of the developing boundary layer where several quantities are at equilibrium. Other quantities, instead, exhibit a behaviour of their own, especially in proximity to the leading edge. The analysis is supported by mean and root-mean-square profiles of temperature and velocity, as well as by budgets of first- and second-order moment balance equations for the enthalpy and momentum turbulent fields.

SODAR STUDIES OF THE MONSOON TROUGH BOUNDARY LAYER AT JODHPUR (INDIA)

A monostatic sodar was set up at Jodhpur, the extreme end of the monsoon trot*, to study the thermal boundary layer up to a height of 700 m. This effort was a part of the co-ordinated multi institutional project to study the monsoon dynamics. The usual structures of thermal plumes, ground based stable layers, elevated/multi- layers with or without undulations and dot echoes were seen. However, erosion of the inversion layer normally observed in the morning in the form of a rising layer over land areas was absent all through the period of observation from June to August 1990. In the paper, a study of the observed data in relation to the rainfall activity has been made. A preliminary examination shows that sodar structures may provide addi• tional information, not available normally through the conventional meteorological tools.

Energy balance of the airflow boundary layer in the brake disc ventilation

Airflow through the brake disc ventilation causes the formation of a boundary layer at the walls. It affects both the dynamic processes related to air exchange in the space between the walls and thermal processes associated with air insulation of the heated surfaces of the ventilation ducts. The present paper aims to develop a model for calculating plane airflow in a ventilation duct in polar coordinates. Using the Navier-Stokes equations and the equations of the energy balance of the airflow boundary layer, we succeeded in determining the elements that affect the intensity of changes in the air masses in the boundary layer and the elements that are responsible for the thermal conductivity of the thermal boundary layer of the airflow. Besides, we obtained an energy balance equation, which takes into account the enthalpy and thermodynamic parameters of the thermal boundary layer, as well as found the possibilities of influencing the heat exchange processes by minimizing factors of the heat-insulating boundary layer. Finally, we specified the dependence of the boundary layer temperature on the temperature of the walls of the brake disc ventilation. The obtained dependences lay the ground for formulating variants of the influence on the heat-insulating boundary layer of the airflow, namely, the design of a forced air supply system at different angles of attack into the ventilation cavity of the brake disc or the manufacture of ventilation ducts with complex geometry.

METHOD FOR DETERMINING ADIABATIC FILM EFFECTIVENESS IN PRESENCE OF THERMAL BOUNDARY LAYER

In this paper we present a new method for determining adiabatic film effectiveness in film-cooling experiments with non-uniform inlet temperature distributions, in particular the situation of an inlet thermal boundary layer. This might arise in a quasi-steady experiment due to loss of heat from the mainstream flow to the inlet contraction walls, for example. In this situation the thermal boundary layer would be time varying. Adiabatic film effectiveness is generally normalised by the difference between mainstream and coolant gas temperatures. Most importantly these temperatures are generally assumed to be spatially—and, possibly temporally—uniform at the system inlet. In experiments with non-uniform inlet temperature, the relevant hot-gas temperature for a particular point of interest on a surface is not easily determined, being a complex function of both the inlet temperature profile and the flow-field between the inlet and the point of interest. In this situation, adiabatic film effectiveness cannot be uniquely defined using conventional processing techniques. We solve this problem by introducing the concept of equivalent mainstream effectiveness, a non-dimensional temperature for the mainstream that can be used to represent the thermal boundary layer profile at the inlet plane, or the effective temperature of the mainstream gas—which we refer to as the equivalent mainstream temperature—entrained into the mixing layer affecting the wall temperature at a particular point of interest.

Flow visualization around cylinder under surface discharge action in the still atmosphere

The paper presents the universal gas-discharge system for surface discharge generation on a cylinder body providing PIV experiments and shadow studies. The system enables the flow visualization around cylinder, discharge power consumption measurements and of temperature fields on the cylindrical surface recording. Under surface discharge action on cylindrical surface in the quiescent air, the flow accompanied by the formation of a near-wall vortex structure and a set of the radially-oriented jets is visualized. The observed jets leave the thermal boundary layer and are able to influence to the gas areas located far away. The presented results indicate the effectiveness of the surface discharges use to control gas-dynamic, thermo-physical and mass transfer processes in the vicinity of streamlined bodies such as cylinders.

Magnetic bubbles dynamics and heat transfer characteristics under influence of non-uniform magnetic fields

The computational study of transient immiscible and incompressible two-phase flows is one of the most common and desirable way for investigation of engineering phenomena and physics science. In the previous studies, generally bubbles current have been used as an active method for increasing heat transfer, however, due to existence of hydraulic boundary layers, the bubbles were not able to cross over this layer to thinning the thermal boundary layer and consequently the efficiency of this method was not very considerable. In this study, by considering potential of magnetic field, the effect of co-applying of external non uniform magnetic field and magnetic bubbles in enhancing the heat transfer efficiency in a 3-D tube has been investigated. The computational model consisted of the Navier–Stokes equation for liquid phase and VOF model for interface tracking are carried out by OpenFOAM. The external magnetic field has been considered non-uniform and time dependent. The results predicted that magnetic bubbles and external magnetic field due to their effect on thermal boundary layer increased significantly heat transfer and Nusselt number. Furthermore, results indicated magnetic bubbles can act as an active torbulators in the flow field and can be applied for increasing recirculation and secondary flow in the flow field. The average temperature and magnetic field over times for different cases have been discussed in the results.

Falkner–Skan Equation with Heat Transfer: A New Stochastic Numerical Approach

In this study, a new computing model is developed using the strength of feedforward neural networks with the Levenberg–Marquardt method- (NN-BLMM-) based backpropagation technique. It is used to find a solution for the nonlinear system obtained from the governing equations of Falkner–Skan with heat transfer (FSE-HT). Moreover, the partial differential equations (PDEs) for the unsteady squeezing flow of heat and mass transfer of the viscous fluid are converted into ordinary differential equations (ODEs) with the help of similarity transformation. A dataset for the proposed NN-BLMM-based model is generated in different scenarios by a variation of various embedding parameters, Deborah number ( β ) and Prandtl number (Pr). The training (TR), testing (TS), and validation (VD) of the NN-BLMM model are evaluated in the generated scenarios to compare the obtained results with the reference results. For the fluidic system convergence analysis, a number of metrics such as the mean square error (MSE), error histogram (EH), and regression (RG) plots are utilized for measuring the effectiveness and performance of the NN-BLMM infrastructure model. The experiments showed that comparisons between the results of the proposed model and the reference results match in terms of convergence up to E-05 to E-10. This proves the validity of the NN-BLMM model. Furthermore, the results demonstrated that there is an increase in the velocity profile and a decrease in the thickness of the thermal boundary layer by increasing the Deborah number. Also, the thickness of the thermal boundary layer is decreased by increasing the Prandtl number.