Subsonic Velocities Research Articles

Experimental studies of laminar-turbulent transition on a body of revolution

The focus of the present paper is hydrodynamic stability and transition to turbulence on an axisymmetric body. The objective is to trace the evolution of perturbed flow close to the surface of experimental model with increase of the angle of attack starting with zero incidence. In what follows, we briefly summarize our wind-tunnel data on this topic which were obtained at low subsonic velocities through hot-wire measurements and flow visualization. As is found, in conditions of laminar boundary-layer separation and flow instability, even small variations of the body incidence may have a profound effect on the flow pattern.

Passive control of deep cavity shear layer flow at subsonic speed

Passive control of the flow over a deep cavity at low subsonic velocity is considered in the present paper. The cavity length-to-depth aspect ratio is L/H = 0.2. particle image velocimetry (PIV) measurements characterized the flow over the cavity and show the influence of the control method on the cavity shear layer development. It is found that both the “cylinder” and the “shaped cylinder”, placed upstream from the cavity leading edge, result in the suppression of the aero-acoustic coupling and highly reduce the cavity noise. It should be noted that the vortical structures impinge at almost the same location near the cavity downstream corner with and without passive control. The present study allows to identify an innovative passive flow control method of cavity resonance. Indeed, the use of a “shaped cylinder” presents similar suppression of the cavity resonance as with the “cylinder” but with less impact on the cavity flow. The “shaped cylinder” results in a smaller shear layer growth than the cylinder. Velocity deficiency and turbulence levels are less pronounced using the “shaped cylinder”. The “cylinder” tends to diffuse the vorticity in the cavity shear layer and thus the location of the maximum vorticity is more affected as compared to the “shaped cylinder” control. The fact that the “shaped cylinder” is capable of suppressing the cavity resonance, despite the vortex shedding and the high frequency forcing being suppressed, is of high interest from fundamental and applied points of view.

Modeling of gas flows in radial micro-nozzles

In the present paper, results of an experimental and numerical study of gas flows in radial micro-nozzles are reported. The radial micro-nozzle consists of two disks spaced apart by a less than one millimeter distance. To the inlet of the radial nozzle, a gas under high pressure is supplied; through the nozzle, the pressurized gas is ejected into ambient space. In the present study, we analyzed the flows in which the gas at the inlet to the micro-nozzle had a supersonic velocity. It was shown that, on the condition that the micro-nozzle width was smaller than some critical value, then, due to the wall friction force, a pseudo-shock arose in the micro-nozzle. In the pseudo-shock, the gas underwent deceleration to a subsonic velocity. A satisfactory agreement between the calculated and experimental data was achieved.

Filament formation in wind–cloud interactions– II. Clouds with turbulent density, velocity, and magnetic fields

We present a set of numerical experiments designed to systematically investigate how turbulence and magnetic fields influence the morphology, energetics, and dynamics of filaments produced in wind-cloud interactions. We cover 3D magnetohydrodynamic systems of supersonic winds impacting clouds with turbulent density, velocity, and magnetic fields. We find that log-normal density distributions aid shock propagation through clouds, increasing their velocity dispersion and producing filaments with expanded cross sections and highly-magnetised knots and sub-filaments. In self-consistently turbulent scenarios the ratio of filament to initial cloud magnetic energy densities is ~1. The effect of Gaussian velocity fields is bound to the turbulence Mach number: Supersonic velocities trigger a rapid cloud expansion; subsonic velocities only have a minor impact. The role of turbulent magnetic fields depends on their tension and is similar to the effect of radiative losses: the stronger the magnetic field or the softer the gas equation of state, the greater the magnetic shielding at wind-filament interfaces and the suppression of Kelvin-Helmholtz instabilities. Overall, we show that including turbulence and magnetic fields is crucial to understanding cold gas entrainment in multi-phase winds. While cloud porosity and supersonic turbulence enhance the acceleration of clouds, magnetic shielding protects them from ablation and causes Rayleigh-Taylor-driven sub-filamentation. Wind-swept clouds in turbulent models reach distances ~15-20 times their core radius and acquire bulk speeds ~0.3-0.4 of the wind speed in one cloud-crushing time, which are three times larger than in non-turbulent models. In all simulations the ratio of turbulent magnetic to kinetic energy densities asymptotes at ~0.1-0.4, and convergence of all relevant dynamical properties requires at least 64 cells per cloud radius.

Frictional moving contact problem for a layer indented by a rigid cylindrical punch

In this study, frictional moving contact problem for a rigid cylindrical punch and an elastic layer is considered. The punch is subjected to concentrated normal and tangential force, and moves steadily with a constant subsonic velocity on the boundary. The problem is reduced to a singular integral equation of the second kind, in which the contact stress and the contact area are the unknowns, and it is treated using Fourier transforms and the boundary conditions for the problem. The numerical solution of the singular integral equation is obtained by using the Gauss–Jacobi integration formulas. Numerical results for the contact stress and the contact area are given. The results show that with increasing values of relative moving velocity, contact width between the moving punch and the layer increases, whereas contact stress decreases.

Numerical Study on Flow and Cooling Characteristics for Supersonic Film Cooling

ABSTRACTWith the booming performances of the gas turbine engine, the turbine vane of the gas turbine engine experiences more extreme thermal environment with supersonic flows. The film cooling applied in the supersonic flow condition has essential difference from that used in the subsonic flow condition in the flow characteristics and cooling effectiveness. This article focused on the film cooling of two parallel flows (primary flow and coolant flow) with supersonic or subsonic velocity, respectively. The results show that: on the condition of supersonic primary flow and subsonic coolant flow, the coolant flow with lower momentum is sheared and dragged by the higher momentum primary flow because of the viscous property of fluid． At the meantime, the thermal and momentum of the primary flow transfers into the coolant flow rapidly． It causes the great damage of the film coverage, and the decrease of the cooling effectiveness dramatically． In contrast, on the condition of supersonic primary flow and supersonic coolant flow, the film coverage of the supersonic coolant flow can last further far than that of the subsonic coolant flow on the same blowing ratio． The turbulence kinetic energy seems to be depressed by the shorten of velocity difference of two supersonic flow. Therefore, the cooling effectiveness is enhanced by 45% for the supersonic primary and coolant flow.

On the error of the time–pulse method of measuring air consumption in mines

The derivation of a formula for the time during which a sound signal propagates between two given points A and B in a stationary gas flow is considered. It is shown that the gas flow changes the signal reception time by a quantity proportional to the consumption, regardless of the detailed velocity profile. The difference between the reception time of signals from point B to the point A and vice versa is proportional to air consumption with high accuracy. It is shown that the relative error of the obtained formula does not exceed the squared maximum Mach number in the gas flow. This allows measurement of the consumption of gas moving in a mine with an arbitrary stationary subsonic velocity field.

Experimental investigation of the machine-free method of temperature separation of air flows based on the energy separation effect in a compressible boundary layer

The process of energy (temperature) separation in an airflow is experimentally investigated in a device consisting of two coaxial channels with heat-conducting walls. The air flows at a supersonic velocity along the inner channel and at a subsonic velocity in the outer channel. The initial total temperatures of the flows are the same. Heat transfer arises due to the energy separation effect in the boundary layer of the compressible flow at Pr≠1. The parameters varied in the process of investigation are the initial flow temperature, the supersonic flow velocity (Mach number), the mass fraction of the subsonic flow, the scheme of the flow organization in the device, and the presence/absence of heat transfer intensifiers in the subsonic channel. In all the regimes considered the subsonic flow cooling and the supersonic flow heating were fixed. The total pressure of the subsonic flow was almost conserved in the maximum cooling regimes. An engineering method of calculating the temperature separation in the similar devices is proposed. The expressions for the limiting values of the flow heating and cooling corresponding to the energy separation using this technique are obtained. The flow parameters that have an effect on the temperature separation efficiency are determined.

Evolution of laminar flow disturbances behind a step on a surface generated by its localized vibrations

The formation and development of hydrodynamic disturbances generated by low-frequency vibrations of a local region on a flat plate behind a rectangular step in separated flow is investigated in a wind tunnel. The results are obtained at a small subsonic flow velocity using the hot-wire anemometry. It is established that the wall vibrations induce separation zone disturbances representing streaky structures accompanied by wave oscillation packets. Laminar boundary layer separation favors the wave packet growth followed by wall flow turbulization.

Numerical and experimental investigation of the means for reducing the aeroacoustic loads in an extended rectangular cavity at subsonic and transonic freestream velocities

The results of a comprehensive investigation including numerical calculations and experiments with models in a wind tunnel and a vehicle under flight conditions aiming to find the ways of reducing the pressure fluctuation levels in an extended cavity at subsonic and transonic freestream velocities are presented. It is shown that the reduction of these loads can be achieved using the means which have demonstrated their effectiveness for cavities with the open-type flows, for example, a permeable deflector and the bevelling of the rear wall but only in the case of a given combination of their geometric parameters. The mechanisms of the action of these devices on the flow, thanks to which the intensity of the wave disturbances generated by the rear wall is reduced, the instability wave growth in the mixing layer behind the deflector is limited, and the fluctuation level in the cavity decreases, are investigated. The results of numerical investigations of the flow in a cavity with a permeable deflector are apparently among the first.

Modification of flow perturbations in a laminar separation bubble by heat transfer

Laminar boundary layer separation in conditions of localized heat transfer is investigated at low subsonic velocity through wind-tunnel measurements and linear stability analysis. A backward-facing step flow is subjected to a stationary temperature variation generated by Peltier elements installed on the test model directly downstream of the separation line. The experimental and theoretical data clarify the response of velocity disturbances in the separation region to the temperature variation, the latter appearing primarily as a modifier of the initial wave spectrum of the amplifying separated layer oscillations.

Effects of perfluoroalkylsilane molecular assembly on flow induced voltage generated by doped silicon wafers

We report the effects of surface modifications on (i) Seebeck coefficients and (ii) flow induced voltage generations of the n-type (n-Si) and p-type (p-Si) silicon wafers. The surfaces of n-Si and p-Si wafers were coated with 1H,1H,2H,2H- perfluorooctyltrichlorosilane (FOTS) molecules. The surface modified n-Si and p-Si of size 12 mm×4 mm were mounted on the π/4 angle inclined experimental mount, and nitrogen gas was flown over the inclined surface at the subsonic velocities, 5.3, 10.61, 15.91, 21.22, and 26.52 ms−1, and the voltage difference between the lead and rear ends of pristine and surface modified n-Si and p-Si was measured. The experimental results and theoretical relations are presented. The flow induced voltage generation is caused by the interplay between the Bernoulli flow and Seebeck effect. The flow-voltage response results show that the half coated and full coated n-Si and p-Si wafers generate more voltage than that of the uncoated at a given velocity. The band theory reveals that the flow of nitrogen gas accumulates charge carriers at the FOTS self assembled monolayer (SAM)—silicon interfaces, which resulted in the more voltage generation by full and half coated ni-Si and p-Si surfaces than that of the pristine surface. The enhanced voltage generations and high sensitivities are caused by an effective increase of the gradient of Fermi Energy (EF) (Seebeck coefficient) due to FOTS SAM coatings. Because of that the FOTS SAM modified n-Si and p-Si are become highly sensitive to nitrogen gas flow.

Features of an underexpanded pulsed impact gas-dispersed jet with a high particle concentration

We have reported on the results of a numerical simulation of the inflow of an underexpanded pulsed gas-dispersed jet with a high particle concentration onto a rigid obstacle unbounded in the transverse direction. The characteristic features of such interaction, in particular, the anomalous formation of the shock-wave structure of the two-phase flow at the subsonic velocity of the carrier gas and the evolution of self-sustained oscillations, have been investigated.

Investigating the instability of the torus around the blackhole in the presence of the radial and angular velocity perturbations

We have performed the numerical simulation of the perturbed relativistic torus around the black hole by solving the general relativistic hydrodynamics equations on the equatorial plane. We have monitored the different simple models by applying the radial and angular velocity perturbations to the steady state torus around the black hole. The instabilities are triggered by adding the subsonic velocity fields over the radial and angular velocities of the stable torus. We have found the disk instability so-called the Papaloizou-Pringle instability and excited modes of the Quasi-Periodic Oscillations (QPOs) in the thin disk approximation case. The significant oscillations and oscillation modes in the mass accretion rates are observed around the rotating black hole with a radial velocity perturbation. The angular velocity perturbation can not create the significant variations and QPOs in the nonselfgravitating relativistic torus around the nonrotating and rotating black holes. The results observed from the radial velocity perturbation might be used to explain the Papaloizou-Pringle instability observed in $NGC$ $1068$

On the role of outer boundary condition in the collapse of molecular clouds

In this paper, we investigate the influence of the outer boundary condition on the collapse of dense molecular clouds. Observational data confirm subsonic inward contraction both before and after the formation of a central protostar. Here, we study the problem of steady, spherical accretion of gas onto a protostar with the polytropic equation of state. Our model include self-gravity of the cloud and has an open outer boundary condition in which the velocity is no longer zero there. Thus, matter continuously drifts across this outer boundary. Since we study the early protostellar cloud evolution, the central protostar is highly less massive than the surrounding cloud. We assume the cloud radius is very large and impose a finite, subsonic velocity at the cloud’s outer boundary and ignore magnetic field effects entirely. Our assumptions, while highly idealized, show supersonic infall is confined to the small central region of cloud.

Mode transition of fuel control test in a dual-mode combustor

Ignition combustion test in dual-mode combustor with fuel flow rate change was carried out in a direct-connected wind tunnel facility that supplied airflow with total temperature of 880K, total pressure of 8atm and Mach number of 2.0. According to the wall pressure of combustor measured through the test, the distribution of other airflow parameters in the combustor was obtained by calculating with a one-dimensional analysis method, and the mode transition process in the condition of fuel flow rate change was studied. In addition, the position that the airflow in the isolator accessed to the subsonic velocity and the location of “choked thermal throat” at the combustor downstream were determined by analyzing of pressure distribution in the combustor. The results indicated that sole hydrogen combustion brought a supersonic combustion mode, then combined combustion of hydrogen and kerosene induced a subsonic combustion mode and combustion-induced pressure disturbance had been spread to the entrance to the combustor. In cases of sole kerosene combustion and the fuel equivalence ratio being kept at 0.53 and 0.61 respectively, both were in the subsonic combustion mode. Gradual increment of the fuel equivalence ratio was accompanied by enlarging subsonic combustion area. In case of combined combustion of hydrogen and kerosene, and improving kerosene flow rate, supersonic combustion mode was transformed to a subsonic one in a gradual manner.

Uniqueness for a seismic inverse source problem modeling a subsonic rupture

ABSTRACTWe consider an inverse source problem for an inhomogeneous wave equation with discrete-in-time sources, modeling a seismic rupture. The inverse source problem, with an arbitrary source term on the right-hand side of the wave equation, is not uniquely solvable. Here we formulate conditions on the source term that allow us to show uniqueness and that provide a reasonable model for the application of interest. We assume that the source term is supported on a finite set of times and that the support in space moves with subsonic velocity. Moreover, we assume that the spatial part of the source is singular on a hypersurface, an application being a seismic rupture along a fault plane. Given data collected over time on a detection surface that encloses the spatial projection of the support of the source, we show how to recover the times and locations of sources microlocally and then reconstruct the smooth part of the source assuming that it is the same at each source location.

Harmonic effects in atomistic phase interactions between phonons and dislocations moving at relativistic velocities

The observation of distinct velocity ‘plateaus’ below the upper limits of the sonic velocity have frustrated many in the scientific community studying high-velocity dislocation dynamics. Liebfried and Frank derived the well-established elastic models of dislocation motion, showing dislocation core energy approaches a relativistically infinite level at the limiting sonic velocity due to a singularity. Eshelby predicted the possibility of a single stable ‘transonic’ velocity with the Peierls-Nabarro model, however he could not provide a physical mechanism to explain the acceleration from the sub-sonic velocity limit. Weiner proposed a linear elastic model where the atomic masses within a dislocation move in a coordinated (or harmonic) manner due to coupled momentum transfer, and used this to predict the limiting velocity at 0K. In modern times, detailed phonon dissipation models have been developed to predict the dislocation velocity relationships at higher temperatures. However, few studies have been performed to show how phonon interactions with dislocations influence the atomistic behaviour. This paper presents a conceptual model based on the coupled motion of multiple atoms in the dislocation core. When compared with molecular dynamics simulations, the model predicts key qualitative and quantitative metrics, such as the lower and upper limiting dislocation velocities. Detailed atomistic analysis confirms the in-phase coordination of atoms that become disrupted by interactions with dislocations at both limiting velocities. The model provides a physically realistic mechanism that is capable of explaining the observation of subsonic and transonic velocity plateaus.

The formation and evolution of reconnection-driven, slow-mode shocks in a partially ionised plasma

The role of slow-mode MHD shocks in magnetic reconnection is one of great importance for energy conversion and transport, but in many astrophysical plasmas the plasma is not fully ionised. In this paper, we investigate, using numerical simulations, the role of collisional coupling between a proton-electron charge-neutral fluid and a neutral hydrogen fluid for the 1D Riemann problem initiated in a constant pressure and density background state by a discontinuity in the magnetic field. This system, in the MHD limit, is characterised by two waves: a fast-mode rarefaction wave that drives a flow towards a slow-mode MHD shock. The system evolves through four stage: initiation, weak coupling, intermediate coupling and a quasi steady state. The initial stages are characterised by an over-pressured neutral region that expands with characteristics of a blast wave. In the later stages, the system tends towards a self-similar solution where the main drift velocity is concentrated in the thin region of the shock front. Due to the nature of the system, the neutral fluid is overpressured by the shock when compared to a purely hydrodynamic shock which results in the neutral fluid expanding to form the shock precursor. The thickness of the shockfront once it has formed proportional to the ionisation fraction to the power -1.2, which is a smaller exponent than would be naively expected from simple scaling arguments. One interesting result is that the shock front is a continuous transition of the physical variables for sub-sonic velocity upstream of the shock front (a c-shock) to a sharp jump in the physical variables followed by a relaxation to the downstream values for supersonic upstream velocity (a j-shock). The frictional heating that results from the velocity drift across the shock front can amount to approximately two per cent of the reference magnetic energy.

Nonlinear Elastic Plate in a Flow of Gas: Recent Results and Conjectures

We give a survey of recent results on flow-structure interactions modeled by a modified wave equation coupled at an interface with equations of nonlinear elasticity. Both subsonic and supersonic flow velocities are considered. The focus of the discussion here is on the interesting mathematical aspects of physical phenomena occurring in aeroelasticity, such as flutter and divergence. This leads to a partial differential equation treatment of issues such as well-posedness of finite energy solutions, and long-time (asymptotic) behavior. The latter includes theory of asymptotic stability, convergence to equilibria, and to global attracting sets. We complete the discussion with several well known observations and conjectures based on experimental/numerical studies.