Isothermal Mixed Layer Research Articles

Direct numerical simulation of turbulent structures and asymmetric properties in the supersonic non-isothermal mixing layer

By direct numerical simulations, the non-isothermal effects on turbulent structures and asymmetric properties are investigated in the spatially developing supersonic mixing layers with high convective Mach numbers (Mc > 0.6). Hot air is blown in the high-speed stream, and cold air is added on the low-speed side. Two non-isothermal simulations with different temperature gradients are conducted and compared with the isothermal mixing layer. The self-similar model of the spatially developing supersonic turbulent mixing layer is analyzed to reveal the physical mechanisms for the asymmetry of non-isothermal mixing layers. The supersonic mixing layer is characterized by diverse vortices and unsteady shocklets, which increase in the initial shear layer and then decrease in the self-similar turbulent region. Also, the mixing layer is asymmetric between the high- and low-speed streams, and the shear layer center skews toward the low-speed side with more vortices and less shocklets, which is attributed to the streamwise momentum gradient. The effects of temperature gradients enhance the flow instability and accelerate the growth of vortices and shocklets in the initial mixing layer. Nevertheless, the turbulent structures are attenuated in the fully developed region, as the viscous dissipation is augmented and turbulence decays more strongly. In addition, the streamwise momentum gradient is reduced in the non-isothermal mixing layers. The vortical structures suffer from stronger attenuation on the cold side, while the shocklets are more significantly reduced on the hot side. Thus, the skewness of the shear layer center toward the low-speed side is reduced, and the mixing layer asymmetry is attenuated.

Direct numerical simulation of turbulence anisotropy and growth rate in the supersonic non-isothermal mixing layer

By direct numerical simulations, non-isothermal effects on turbulence anisotropy and growth rate are investigated in three-dimensional spatially developing supersonic mixing layers with high convective Mach numbers (Mc > 0.6). Hot air is blown into the high-speed stream, and cold air is added on the low-speed side. Two non-isothermal simulations with different temperature gradients are conducted and compared with the isothermal mixing layer. The Reynolds stress transport is analyzed to reveal the underlying modulation mechanisms by temperature gradients. The supersonic mixing layer is significantly anisotropic, and the streamwise turbulent intensity is larger than the transverse and spanwise turbulent intensities. The non-isothermal effects enhance the energy transfer from the streamwise Reynolds stress to transverse and spanwise Reynolds stresses in the initial shear layer, and the mixing layer anisotropy is intensely attenuated, which is attributed to the increment of the flow instability. Thus, the shear layer growth is initially accelerated, and the supersonic mixing layer is destabilized. However, the fluid viscosity and the viscous dissipation are enhanced, and the Reynolds stresses decay more strongly in the fully developed region. The transverse and spanwise turbulent intensities decrease more than the streamwise turbulent intensity due to the attenuation of the correlation between pressure and dilatation fluctuations. Consequently, the shear layer turbulence anisotropy is augmented with stronger three-dimensionality. As the Reynolds shear stress is reduced and the entrainment of irrotational fluid from the free streams into the shear layer is attenuated, the mixing layer growth rate is decreased in the self-similar turbulence.

A Priori Assessment of Subgrid-Scale Models and Numerical Error in Forced Convective Flow at High Prandtl Numbers

An extensive a priori analysis has been carried out on data from Direct numerical simulation of fully developed heated turbulent pipe flow at high molecular Prandtl numbers Pr=10/20, testing three popular modelling candidates for subgrid-scale closure in Large-Eddy simulation (LES). Aside from assessing the models’ capabilities to describe quantitatively the unresolved turbulent fluxes, a special focus is also put on the role of the numerical error, which arises from the discretization of the filtered advective fluxes on a coarse LES grid. The present analysis extends here previous studies on subgrid-scale momentum transport in a isothermal mixing layer and channel flow carried out by Brandt (J Numer Methods Fluids 51: 635–657, 2006) and Vreman et al. (J Eng Math 29: 299–327, 1995), respectively, to the subgrid-scale transport of heat at high Prandtl numbers. The statistical dependence between the individual contributions (resolved, subgrid-scale, numerical discretization error) constituting the filtered advective flux divergence in the LES formulation is investigated as well, in terms of corresponding cross-correlations. The sensitivity of the tested sgs-models to a grid refinement is further examined performing also a posteriori LES, where the basically more sophisticated candidates turn out to be more demanding in terms of required grid resolution.

The Influence of the Barrier Layer on SST Response during Tropical Cyclone Wind Forcing Using Idealized Experiments

AbstractMultiple studies have shown that reduced sea surface temperature (SST) cooling occurs under tropical cyclones (TCs) where a fresh surface layer and subsurface halocline exist. Reduced SST cooling in these scenarios has been attributed to a barrier layer, an upper-ocean feature in the tropical global oceans in which a halocline resides within the isothermal mixed layer. Because upper-ocean stratification theoretically reduces ocean mixing induced by winds, the barrier layer is thought to reduce SST cooling during TC passage, sustaining heat and moisture fluxes into the storm. This research examines how both the inclusion of salinity and upper-ocean salinity stratification influences SST cooling for a variety of upper-ocean thermal regimes using one-dimensional (1D) ocean mixed layer (OML) models. The Kraus–Turner, Price–Weller–Pinkel, and Pollard–Rhines–Thompson 1D OML schemes are used to examine SST cooling and OML deepening during 30 m s−1 wind forcing (~category 1 TC) for both temperature-only and temperature–salinity stratification cases. Generally, the inclusion of salinity (a barrier layer) reduces SST cooling for all temperature regimes. However, results suggest that SST cooling sensitivities exist depending on thermal regime, salinity stratification, and the 1D OML model used. Upper-ocean thermal and haline characteristics are put into context of SST cooling with the creation of a barrier layer baroclinic wave speed to emphasize the influence of salinity stratification on upper-ocean response under TC wind forcing.

Seasonal and interannual variation in the cross‐equatorial meridional currents observed in the eastern Indian Ocean

Observations from moored subsurface acoustic Doppler current profilers (ADCPs) have revealed the structure of cross‐equatorial meridional currents in the eastern equatorial Indian Ocean. Using observational data longer than 8 years, we present observations of the seasonal and interannual variation in the cross‐equatorial currents in the 2000s. In line with earlier observations and numerical simulations, there were significant subsurface currents that displayed seasonal variation. The annual mean condition was dominated by currents during the boreal summer monsoon in which the Ekman transports are southward on both sides of the equator. The vertical structure of the mean meridional current displayed evidence of a shallow “equatorial roll,” consisting of northward surface flow and southward subsurface flow above the base of the isothermal mixed layer. The seasonal variation in upper 120 m meridional transport can be explained by wind‐driven cross‐equatorial Ekman/Sverdrup transport. Interannual variability was higher in the winter monsoon period than in the summer monsoon. The interannual variation could be related to the Pacific El Niño/Southern Oscillation (ENSO). ENSO‐induced variation in the southeasterly trade wind could modulate meridional Sverdrup transport. Implications of these observational results for meridional heat transport are discussed.

Numerical Study of Mach Number and Thermal Effects on Sound Radiation by a Mixing Layer

Mach number and thermal effects on the mechanisms of sound generation and propagation are investigated in spatially evolving two-dimensional isothermal and non-isothermal mixing layers at Mach number ranging from 0.2 to 0.4 and Reynolds number of 400. A characteristic-based formulation is used to solve by direct numerical simulation the compressible Navier-Stokes equations using high-order schemes. The radiated sound is directly computed in a domain that includes both the near-field aerodynamic source region and the far-field sound propagation. In the isothermal mixing layer, Mach number effects may be identified in the acoustic field through an increase of the directivity associated with the non-compactness of the acoustic sources. Baroclinic instability effects may be recognized in the non-isothermal mixing layer, as the presence of counter-rotating vorticity layers, the resulting acoustic sources being found less efficient. An analysis based on the acoustic analogy shows that the directivity increase with the Mach number can be associated with the emergence of density fluctuations of weak amplitude but very efficient in terms of noise generation at shallow angle. This influence, combined with convection and refraction effects, is found to shape the acoustic wavefront pattern depending on the Mach number.

Topographic effects on the thermal structure of Himalayan glacial lakes: Observations and numerical simulation of wind

The thermal structure of two Himalayan glacial lakes, Tsho Rolpa and Imja in the eastern Nepal, was examined by observations in the pre-monsoon seasons of 1996 and 1997. Tsho Rolpa had an isothermal mixed layer at water depths of 0–20 m, whereas Imja did not possess such an isothermal layer. This difference in the thermal structure is explained by the condition that diurnal valley winds, producing wind-driven currents, blow strongly near the water surface of Tsho Rolpa, but very weakly near that of Imja. The wind observations above or near the end moraine indicated that a daily wind system of strong, diurnal valley winds and weak, nocturnal mountain winds is common to the lakes. It was suggested that with respect to the valley winds, the weak winds near the surface of Imja result from topographic screening effects of the upwind dead-ice zone and end moraine 20–25 m higher than the water surface. In order to ascertain the topographic effects, three-dimensional numerical simulation of airflow was carried out by making topographic models of actual size in the calculation domain, corresponding to Tsho Rolpa and Imja and their surrounding topography. The simulation revealed that, when winds blow at constant velocities of 1–5 m s −1 at 2 m above the points corresponding to the weather stations, the wind velocity at 2 m above the water surface for Imja is 33–42% smaller than for Tsho Rolpa. With increasing heights of the end moraine and dead-ice zone, the wind velocity near the lake surface efficiently decreased by decreasing the lake length from 3.1 or 2.2 km to 1.2 km.

Direct numerical simulation of nanoparticle coagulation in a temporal mixing layer via a moment method

Direct numerical simulations of coagulating aerosols in two-dimensional, incompressible, iso-thermal mixing layers are performed. The evolution of the particle field is obtained by utilizing a moment method to approximate the aerosol general dynamic equation. We use a moment method which assumes a lognormal function for the particle size distribution and requires the knowledge of only the first three moments. This approach is advantageous in that the number of equations which are solved is greatly reduced. A Damköhler number is defined to represent the ratio of convection to coagulation time scales. Simulations are performed for three flows: Damköhler numbers of 0.2, 1, and 2. The spatio-temporal evolution of the first three moments along with the mean diameter and standard deviation are discussed.

K-1415 反応乱流混合層のラージ・エディ・シミュレーション : 発熱とシアーの効果(G05-4 PTV計測と混合層)(G05 流体工学部門一般講演)

Effects of the heat release and the fluid shear on the nonpremixed reaction processes are studied by applying a three-dimensional large eddy simulation (LES) to a compressible, spatially developing nonpremixed reacting plane mixing layer. The fluid shear is generated by the streamwise and spanwise inlet velocity differences between the fuel and oxidizer. SGS turbulence and combustion models used are the dynamic Smagorinsky model (DSM) and the scale similarity filtered reaction rate model (SSFRRM), respectively. The results show that the heat release and the spanwise fluid shear promote the mixing between the fuel and oxidizer and increase the production. Although Reynolds stress and fuel flux are usually positive in the isothermal mixing layer except, for the region near the inlet, these values become negative in certain regions away from the inlet due to the heat release and the spanwise fluid shear.

Nonpremixed combustion in turbulent mixing layers part 1: flame characteristics

Experiments on recirculating diffusion flames are reported, the focus of the investigation being the interaction between the combustion process and the coherent structures formed naturally in the turbulent mixing layer bounding the recirculation zone. The experimental configuration was that of a rearward-facing step with distributed injection of gaseous fuel into the recirculation zone. Over a wide range of conditions the primary combustion activity was found to be located almost entirely in the mixing layer, commencing at some distance from the step as a lifted flame. In this regime the flame was produced by the quasi-periodic inflamation of coherent structures formed in the non-reacting part of the mixing layer. These were similar to the structures formed in isothermal mixing layers and appeared to have undergone the mixing transition well upstream of the point at which they inflamed. When the lifted flame was stabilized close to the step attachment occurred intermittently over part of the span. The periods during which the flame was attached to the step in this way were characterized by an absence of coherent-structure formation. The average lift height of the flame was sensitive to the chemical composition of the fuel and the pattern of the fuel distribution within the recirculation zone but was also found to be a function of the ratio of the fuel flow rate to the air velocity. Analysis of this behavior suggests that the stabilization mechanism was controlled by mixing-related rather than kinetic factors.

Transient climate response to external forcing on 100–104 year time scales part 1: Experiments with globally averaged, coupled, atmosphere and ocean energy balance models

We present the results of a study of the sensitivity of the transient surface temperature response to a step function solar constant increase using a globally averaged energy balance climate model which is coupled to two different globally averaged thermodynamic ocean models. The first ocean model is a box model, in which the ocean is divided into a small number of isothermal reservoirs. Experiments with this model demonstrate some circumstances in which it is important to treat the mixed layer and deep ocean separately rather than lumping them into a single reservoir, as has been the practice in a number of recent studies using coupled atmosphere‐ocean‐cryosphere models over 104 year time scales. The second ocean model has an isothermal mixed layer which is coupled to an advective‐diffusive deep ocean having a continuous vertical temperature profile. We permit the thermal diffusivity K and the advection velocity w to vary vertically and introduce a variety of plausible feedbacks between K and the vertical temperature gradient, and between w and the perturbation mixed layer wanning. We also permit the bottom water source temperature to change by a prescribed fraction of the mixed layer response. The transient surface response is relatively insensitive to changes in K and w for the nonfeedback cases, but is more sensitive to individual feedback mechanisms. When multiple feedback processes are permitted a complex transient surface temperature response is possible. When applied with a periodic solar constant forcing, these feedback processes can either significantly reduce or increase the amplitude of the surface temperature response compared to the no‐feedback case.