Important Similarity Parameters Research Articles

Аномальный отклик стратифицированной среды на объемное тепловыделение

A linear stationary problem of the response of a semi-bounded stably stratified fluid/gaseous medium to volumetric spatially inhomogeneous heat release in the Boussinesq approximation is theoretically investigated. The most important similarity parameters are the analogue of the Rayleigh number and the aspect ratio of the source of buoyancy. For a perturbation in the form of a single horizontal harmonic, an analytical solution has been found that makes it possible to analyze a number of significant regularities. The nontrivial possibility of an intense hydrothermodynamic response to a weak heat release at a certain ratio of the mentioned parameters is found.

Anomalous response of a stratified medium to volume heat release

A linear stationary problem of the response of a semi-bounded stably stratified fluid/gaseous medium to volumetric spatially inhomogeneous heat release in the Boussinesq approximation is theoretically investigated. The most important similarity parameters are the analogue of the Rayleigh number and the aspect ratio of the source of buoyancy. For a perturbation in the form of a single horizontal harmonic, an analytical solution has been found that makes it possible to analyze a number of significant regularities. The nontrivial possibility of an intense hydrothermodynamic response to a weak heat release at a certain ratio of the mentioned parameters is found. Keywords: stratified medium, volume heat release, convection, density flows, analytical solution, intense response, instability.

Space-charge induced particle reflection between hybrid AC/DC biased electrodes

Space-charge limited current flow between DC biased electrodes is a widely applicable problem in many areas of physics. Recently, radio-frequency biasing, together with DC self-bias formation, has been studied as a new concept for the extraction of charged particles from an upstream plasma source. Here, we compare particle extraction between systems using this hybrid AC/DC biasing, with conventional DC biased electrodes, and identify important similarity parameters. The injection current first leading to particle reflection strongly depends on the applied AC frequency and voltage magnitude, as well as the initial particle injection velocity, and is in general lower than the DC case. For injection currents above the AC limit, the system becomes unstable, and self-excited space-charge oscillations are generated. A critical parameter is the ratio of the average particle transit time between the electrodes to the AC period, γ = t L/T. As long as γ ≫ 1, the onset of particle reflection can be sufficiently delayed that the extracted current approaches the DC limit.

Wind tunnel test of snow loads on a stepped flat roof using different granular materials

An accurate prediction of wind-induced redistribution of snow load on roof surfaces is vital to structural design. To represent the pattern of snow distribution caused by snowdrift in wind tunnel test, appropriate modeling particles should be selected. The particle density is the key to determine the values of several important similarity parameters. In this study, the redistribution of snow load on a stepped flat roof was simulated by means of wind tunnel test using low-density saw wood ash, medium-density polyfoam, and high-density silica sand, respectively. To ensure the comparability of the test results of the three modeling particles, the wind tunnel test results for comparison were performed under almost the same conditions of dimensionless wind velocity and dimensionless time. Then, the results of the present study were compared with those from field observations of prototypes in previous studies. The effects of wind duration, wind velocity, and roof span on the redistribution of snow on roof surfaces were investigated. The characteristics of erosion/deposition range and the location of maximum quantities of erosion/deposition under independent effects of wind duration, wind velocity, and roof span were also studied.

Thermal surface signatures of ship propeller wakes in stratified waters

When a ship moves in temperature stratified water, e.g., in the ocean diurnal thermocline, the propeller(s) as well as the turbulent boundary layer of the hull mix the surface water with underlying colder fluid. As a result, when observed from above, a temperature “wake signature” of ∼1–2 °C may be detected at the water surface. To quantify this phenomenon, theoretical modeling and physical experiments were conducted. The dominant processes responsible for thermal wake generation were identified and parameterized. Most important similarity parameters were derived and estimates for wake signature contrast were made. To verify model predictions, scaled experiments were conducted, with the water surface temperature measured using a sensitive infrared camera. Comparison of laboratory measurements with model estimates has shown satisfactory agreement, both qualitative and quantitatively. Estimates for ocean ship-wake scenarios are also given, which are supported by available field observations.

EXACT SOLUTIONS FOR VIBRATING RECTANGULAR MEMBRANES PLACED IN A VERTICAL PLANE

This technical note presents the exact solutions for the vibration frequencies of rectangular membranes placed in a vertical plane. The membranes are fixed on the top edge and side edges. The bottom edge may be either free and carrying a uniformly distributed mass, or fixed. It is found that the width of the membrane b and lateral tension ratio c appears in the exact solution in a combined form [Formula: see text]. This important similarity parameter implies that a larger width has the same frequencies as lowered lateral tension, therefore, this parameter greatly reduces the number of tables. For the vibrating vertical membranes, we find that when ξ increases generally, the frequencies decrease. For small ξ (small width or large lateral tension ratio), the horizontal modes are all n = 1, or a half-sine wave in the horizontal direction. For large ξ, the horizontal modes increase with each increased frequency. The fundamental frequencies always correspond to n = 1.

A Laboratory Study of the Urban Heat Island in a Calm and Stably Stratified Environment. Part I: Temperature Field

An extensive and systematic water-tank study was performed to simulate the urban heat island under a calm and stably stratified environment. The objective was to examine the mean-temperature field, mixing height, and heat-island intensity as functions of surface heating rates, heat-island sizes, and ambient temperature gradients. The scaling parameters for the temperature field associated with the heat-island plume are the diameter D, surface heating rate H0, and Brunt–Väisälä frequency N of the ambient stratification. The induced plume was turbulent, and the Froude number was found to be the most important similarity parameter. The differences between low- and high-aspect-ratio plumes are discussed, and simple theoretical models are proposed for low-aspect-ratio plumes. The experimental results generally confirm the theoretical predictions and agree reasonably well with field observations, in spite of several limitations of the laboratory simulation. The mean-temperature distributions are found to have a universal shape that is a function of location only. The results are described in two papers. In this paper, the temperature fields are described. Part II describes the velocity fields and develops analytical models that apply to low-aspect-ratio plumes.

A comparative study of the different criteria for the physical modeling of buoyant plume rise in a neutral atmosphere

Some of the proposed criteria for physical modeling of rising stack and chimney plumes in the atmosphere are evaluated on the basis of our wind tunnel experiments. The observed jet and plume rises in a uniform cross flow follow the well-known 1 3 and the 2 3 power laws, respectively. These become independent of the stack effluent Reynolds number ( Re s ) only when Re s exceeds a certain minimum or critical value ( Re c ) of about 2000 and 600, respectively, for momentum jets and buoyant plumes. It is necessary to ensure a large enough Reynolds number ( Re s > Re c ) and a large enough effluent to ambient velocity ratio ( W s / U ⩾ 1.5), or the momentum flux ratio, in order to avoid any adverse effects of stack wake and downwash on plume rise in the model. The most important similarity parameter is the buoyancy-length scale to the stack diameter ratio ( l b / D ), which must be matched in the model and the prototype situations. The other parameters, such as the density ratio, the effluent Froude number and the velocity or momentum ration, may be slightly mismatched (say, within a factor of 2), if necessary, for buoyancy-dominated plumes.

Properties of Magnetically Balanced Arcs

Experimental results on steady magnetically balanced arcs in supersonic external flows are presented and analyzed, along with investigations elsewhere in subsonic flows, to determine the important similarity parameters. The studies indicate that the steady arc has a central core, impervious to the external flow, within which differential Lorentz forces generate a circulation analogous to that generated by gravity forces in nonuniformly heated gas in a horizontal tube. The conservation equations indicate that a “Lorentz convection parameter,” analogous to the Grashof number in free convection phenomena, is an important similarity parameter over a significant range of arc variables. A “characteristic velocity” for the core circulation is indicated. The similarity parameters that provide the interpolation between the properties of subsonic and supersonic balanced arcs indicate the approximations permissible for a first-order solution to the internal circulation. It is inferred from the results that to a first approximation the thickness of the enthalpy boundary layer varies linearly with the reciprocal of the Reynolds number of the external flow.