V. V. Golub

Self-ignition of a fuel gas upon pulsed efflux into an oxidative medium

It is demonstrated that hydrogen can exhibit self-ignition when a starting shock wave, on which the temperature increases above the stagnation temperature, appears in front of a cold expanding gas jet. This heating leads to the ignition of the hydrogen-air mixture formed at the contact surface. The results of numerical simulations show that, at a gas pressure in the vessel on the order of 300–600 bar, the intensity of a shock wave formed in air is sufficient to produce self-ignition of the hydrogen-air mixture formed behind the front of the jet of compressed hydrogen.

Laminar Superlayer in a Turbulent Boundary Layer

We confirm the existence of a sharp boundary between the turbulent and non-turbulent fluid at the outer region of a zero-pressure-gradient turbulent boundary layer at a low Reynolds number. For the first time, using the Tomographic Particle Image Velocimetry technique, we determine mean statistical parameters of the boundary: its thickness and relative velocity jump.

The Shock-Wave Mechanism of Spontaneous Ignition of Hydrogen under Conditions of Sudden Efflux from Reservoir at High Pressure

Analysis is made of the conditions of spontaneous ignition of hydrogen as a result of emergence of a starting shock wave in air before an expanding cold flow of gas. The rise of temperature behind the shock wave causes ignition of the mixture of combustible gas with air, which forms on the contact surface. The condition for spontaneous ignition is the sufficient time of residence of mixture at high temperature for mixing and ignition. The calculations of spontaneous ignition of hydrogen jet are based on a model which takes into account the gasdynamic transport of viscous gas, the kinetics of oxidation of hydrogen, multicomponent diffusion, and thermal conductivity. The range of pressures is determined in a reservoir, during whose depressurization the shock wave forming in air exhibits intensity sufficient for igniting the hydrogen-air mixture behind the front of propagating jet of compressed hydrogen. Results of analysis are given of the dependence of conditions of ignition on the pressure of hydrogen in the reservoir, on the size of the outlet opening, and on the initial temperature of hydrogen and air.

The formation of vortex shocks in a 3D subsonic flow behind a weak shock wave emerging from a channel

The structure of a 3D subsonic flow behind a diffracted shock wave was studied by experimental and numerical methods for the incident shock wave Mach numbers M0 close to unity. It is established that vortex shocks appear in the flow behind the diffracted shock wave even when M0 decreases to 1.04, which is much lower than the threshold Mach number obtained analytically for a 2D automodel case. The time interval from the outflow start to the local supersonic zone formation, as well as the experimentally measured time of appearance of the first vortex shock, increase with decreasing M0.

Increasing the force with which a shock wave discharged from the channel acts on an obstacle by way of converting a normal pressure shock to a system of oblique shocks

The results are given of experimental and numerical investigations of the effect produced on an obstacle by shock waves discharged from channels of different cross-sectional shapes (circle, square, cross). The pressure distribution on an obstacle mounted normally to the flow axis is measured. The experimental results are compared to the data of numerical calculation for determining the optimal modes as regards the duration of calculation and the cell size that produce the least difference between the experimental and numerical data. Calculations are performed of the gas flow behind a shock wave discharged from a channel of X-shaped cross section, and the distribution of pressure and temperature over the obstacle surface is plotted. It is found that the force with which a flow acts on an obstacle when discharged from a channel of X-shaped cross section is much greater than in the case of being discharged from a channel of round or square cross section. Shadow photographs show that this is due to the reduction of the loss of total pressure in the flow because of the conversion of the normal pressure shock to a system of oblique shocks.

Three-Dimensional Effects and the Interaction between an Obstacle and a Shock Wave Discharged from a Channel

The results are given of experimental and numerical investigations of the total pressure across a stagnation shock in the flow behind a shock wave discharged from open ends of round and square channels. It is found that the loss of total pressure behind a stagnation wave before an obstacle when discharged from a square channel exceeds that in the case of a round channel. The observed effect is attributed to the fact that, when a shock wave is discharged from a square channel, the flow separation occurs at a greater angle, which is accompanied by an increase in the Mach number of the flow.

Flow Expansion behind a Shock Wave Discharged from a Channel

The results are given of experimental and numerical investigations of the structure of flow behind a shock wave discharged from open ends of round and square channels. It is demonstrated that the expansion regions arising in a flow behind a diffracted wave are characterized by a larger volume and a higher expansion ratio than those arising in a stationary underexpanded jet with the same value of the Mach number of flow at the channel exit section.

Decreasing the action of a flow behind a weak shock wave emerging from a channel upon an obstacle

It is experimentally demonstrated that, if the flow behind shock waves outgoing from a channel is subsonic, the action of this flow upon an obstacle can be decreased by changing the shape of the output channel cross section. The results of numerical modeling of the experimental oscillograms show that, by replacing a round channel with a cross-shaped one, the excess action upon the obstacle in a quasi-stationary stage can be reduced by half. This decrease is related to energy dissipation in a three-dimensional flow as a result of the interference of rarefaction and compression waves. The interference arises due to mass redistribution between various symmetry axes in the gaseous medium and due to the flow energy losses for the vortex formation at the channel edges, which are revealed by schlieren photographs.

Sliding electric arc discharge as a means of aircraft trajectory control

The dynamics of sliding electric arc discharge and the formation of shock waves in the stages of leader motion and the electric arc development in a supersonic air flow behind the shock wave have been studied for an initial pressure of 0.09–0.5 atm (bar). The air flow in the discharge was imaged using an optical system comprising a shadow device (IAB-458), an optical interference attachment (RP-452), and a modified ruby laser (OGM-20) producing 10–15 output pulses per pumping pulse. Stable initiation of sliding electric arc discharge takes place in a supersonic air flow behind the shock waves with 1.7<M<3.4. This discharge produces shock waves leading to separation of the boundary layer and to an increase in the pressure at the surface. These shock waves can be used for modifying gasdynamics in the air flow streamlining the surface and for controlling the motion of an aircraft.

Growth of the total pressure recovery factor in a flow behind a shock wave emerging from a channel with concave corners in cross section

Experimental data on the interaction of an obstacle with a shock wave emerging from a square channel with convex and concave corners in the cross section are considered. The flow structure was imaged by high-speed photography of shadow patterns. The total pressure losses in the flow were determined by measuring pressure at an obstacle oriented perpendicularly to the flow. It is established that the total pressure losses as a result of the flow drag upon efflux from the channel with X-shaped cross section are significantly lower as compared to the case of a channel with a round or square cross section.