Turbulence Generation Mechanisms Research Articles

Numerical Simulation of the Small Vortices in the Intake and Compression Processes of an Engine.

The turbulence-generation mechanism in the intake and compression processes of an engine with a square cylinder is investigated by performing a three-dimensional numerical simulation. Emphasis is placed on the influences of the intake turbulence and the compression effect on the TDC turbulence. The compressible Navier-Stokes equations are solved without any explicit turbulence models. The employed numerical algorithm is an extended version of the ICE method, and the third-order upwind scheme is employed for the convective terms. The obtained computational results agreed well with the experimental visualization using freon as the working fluid under the Mach-number condition over 0.5. It is shown that the drastic transition to turbulence near TDC is grasped by computations using this numerical method.

Turbulent stark effect: Possible observational manifestations in solar flares

For a variety of turbulence generation mechanisms in solar flares, an analysis is made of the mutual orientation of the electric field vectors of high-frequency (HF) and low-frequency (LF) plasma oscillations. Examination has been carried out separately for the three separate structural parts of a flare: the energy release region, flare loops, and the chromospheric flare itself. The results obtained are used to draw the conclusion that the most favorable conditions for gaps to appear on hydrogen line profiles and consequently for HF turbulence diagnostics exist in the lower parts of the flare loops. In order to detect the LF turbulence it is useful to carry out spectral observations in the flare ribbon region in the chromosphere.

Fluctuating dynamo models

A modification of mean-field type models for dynamo action in helical turbulent fluids by the inclusion of local fluctuations of the growth rate of the unstable large scale magnetic field modes exhibits an intermittent temporal behaviour of the magnetic field. The fluctuating dynamo models offer a possible explanation of the various periods of minimal activity of the sun's magnetic field as a result of the very nature of its turbulent generation mechanism.

Structures responsible for rapid fading of medium frequency radio reflections from the day-time E-layer

The steerable beam Bribie Island radar (152°E, 27°S) operating at a frequency of 1.98 MHz was used to obtain data relevant to reflection conditions near 100 km altitude on 7 days during June–October 1982. The rapid signal fading commonly observed is primarily due to transient reflectors with lifetimes of a few seconds, often seen up to angles of 20° from the zenith. Longer lived moving reflectors (presumed to be sporadic-E clouds) also play a part. Certain properties of the transient reflectors are consistent with a turbulent generation mechanism. However, any theory of their origin must explain why, for about a third of the time, they tend to occur preferentially to the north and east of the observing site. A direct comparison of velocities using Doppler and spaced antenna drifts methods shows reasonable agreement when the data is averaged over quarter hour periods. However, conclusions by previous workers, on the basis of observations of motions of diffraction patterns, that the ionospheric structure responsible for the diffraction pattern observed on the ground is undulations of the isoionic contours by gravity waves, is not supported by a detailed analysis of the data.

Interfacial Stability in Channel Flow

Interfacial stability in the special case of superposed turbulent layers with a zero velocity difference at the interface was examined. In contrast with previous investigations in which the primary mechanism of turbulence generation was interfacial shear, the primary source of turbulence was bottom boundary shear. Interfacial stability was defined in terms of two dimensionless parameters: the Keulegan number and a Richardson number based on the shear velocity and the maximum change in density across the flow. Laboratory flume data and field data taken downstream of thermal power plants on several rivers confirmed the hypothesis.

Time dependent mixing in a salt wedge estuary

Measurements of the profiles of salinity and velocity at a station in the Duwamish River estuary indicate that the turbulent mixing through the density interface is highly time dependent with the most intense mixing occurring at the maximum current speed during the ebb. Additional measurements of the turbulent kinetic energy and the turbulent flux of salt by the eddy correlation technique verify that interpretation. The vertical turbulent salt flux and the turbulent kinetic energy vary by an order of magnitude over the tidal cycle and approximately half of the total vertical salt transport takes place during a short time centered about the maximum ebb. Different mechanisms of turbulence generation operate at various times during the tidal cycle and the intense mixing periods are shown to coincide with conditions favorable for the formation of an internal hydraulic jump.

Mixed-Layer Depth Model Based on Turbulent Energetics

Abstract Mixed layer depths are predicted using an entrainment equation with conservation equations. The entrainment equation is based on the turbulent kinetic energy equation for the mixed layer. The atmosphere is idealized as having temperatures, humidities and winds constant with height in the boundary layer with a step discontinuity marking the top of the mixed layer. This model is tested with mixed layer depth observations made during the 1953 Great Plains experiment, the 1967 Australian Wangara experiment, and the 1972 Puerto Rican tropical experiment. Model calculations of inversion rise and mixed layer depth offer good agreement with the observations. It is found that none of the turbulence generation and loss mechanisms for the mixed layer (such as buoyancy, wind shear and gravity waves) should be neglected a priori.

Turbulence energy balance in reacting turbulent flows

The turbulence accompanying combustion and the propagation of detonation waves in gases has been studied theoretically and experimentally in many papers [1–8]. The attention of researchers has been concentrated on essential questions like how the turbulent flow field interacts with the kinetics of the chemical reaction and to what extent the process of chemical change is intensified, and how the turbulence itself is deformed by the heat released and the accompanying expansion of the gases. The various mechanisms proposed for these phenomena are based on various hypotheses concerning the structure of the combusion zone and the determinative stage of the interaction of the turbulence with the chemical-reaction kinetics. The mechanism of turbulence generation by combustion proposed in a number of papers [3–6] is based on the observation in turbulent flow of a weakly curved flickering laminar flame. This gives rise to a nonuniform flow field of the gas, part of the energy of which goes over into the energy of turbulent fluctuations. Other authors [7, 8] considered the turbulence field to interact with the chemical-reaction kinetics via a volume mechanism and suggested a criterion of turbulence intensification based on certain physical considerations, e.g., the condition for the intensification of thermogaskinetic oscillations proposed by Rayleigh [9]. In the present paper the problem is analyzed by introducing Kolmogorov's general equation for the turbulence energy balance in reacting turbulent flows [10]. In accordance with, this equation the turbulence energy can vary due to energy exchange between the turbulent motion and the mean gas flow as a result of the work on turbulent mass transport in the acceleration field of the mean flow, and due to the effect of pressure fluctuations on the rate of thermal expansion from the chemical reaction. Each of these effects is considered and analyzed.

Internal waves in the atmosphere from high-resolution radar measurements

A radar sounding system developed by Richter has a height resolution (about 1 meter) capable of resolving the detailed structure of small features in the atmosphere that were never before seen. Two distinctly different types of wave phenomena characterize many of the records. One type is a long-period internal wave. The mechanism of generation is discussed, and waves observed on the radar are compared with theory. They are shown to represent the fundamental mode of gravity waves for a stable lower troposphere. The other wave type is much shorter in period and typically shows a cusped structure like a breaking wave. Radar observations are compared with simultaneous wind and temperature soundings, and we conclude that wind shear is unquestionably the generation mechanism for the second type. We present evidence that untrapped waves above the region of instability indicate a flow of energy toward the shear zone, and we calculate the energy flux. The relation of these waves to atmospheric stability is described; their size is deduced from the observations, and their potential as a mechanism for generation of turbulence at the high-frequency end of the atmospheric turbulence spectrum is discussed.