I’m studying an interesting physics problem which involves the introduction of a secondary fluid jet into the cross flow of a primary fluid, also known as the Jet-In-Cross-Flow phenomena (JICF). A salient feature in the JICF problem is understanding the physical mechanisms which govern the depth of penetration of the secondary fluid jet, the trajectory (shape) of the jet, and also the rate at which mixing between the primary and secondary jet occurs. Knowledge of the characteristics which govern the mixing/entrainment rate of the fluid mixture, and the penetration depth will allow for development of ideas which can increase the efficiency of fuel delivery systems in aerospace propulsion systems like turbojet and scramjet engines.
Examples of JICF problems include the plume produced in a chimney, when smoke is released into the air while the wind is blowing in a direction perpendicular (or at some angle) to the plume. So I’m studying chimney plumes for my research. Well sorta. The same physics which govern the trajectory, penetration, and mixing of chimney smoke injected into air governs fuel jets-in-cross-flows in air-breathing engines. A defining characteristic of the JICF problem is the momentum flux ratio, denoted as J in literature. It’s the ratio of the dynamic pressure of the jet to that of the main flow. A small J means a flatter jet with a lower penetration depth. A large J means a higher penetration of the fuel jet.
The challenge in scramjet JICF problems is that the jets and cross flow are sonic or supersonic. Supersonic flows (those travelling at greater than the speed of sound) produce shocks in the flow structure, adding to the interesting fluid phenomena. In order to study problems involving supersonic jets-in-cross-flow, either experimental testing in a supersonic wind tunnel or numerical/computational fluid dynamics (CFD) are required.
Aerospace Propulsion 101
In aerospace propulsion there are several types of systems which produce thrust in aircraft or spacecraft including propeller driven engines, turbo-jet engines, high speed propulsion engines (ramjets/scramjets) and rocket engines. Most aerospace engines fall under either of two categories: air-breathing engines or rocket engines. For combustion (and thrust production) to occur, an engine needs to convert the chemical energy of a fuel-oxidizer reaction to mechanical energy (which produces thrust and causes the vehicle to move). Air-breathing engines take oxidizer from the air via an inlet and injects fuel into the airstream in a combustion chamber to produce thrust. These can only operate in an atmosphere of air. Rocket engines contain the propellant combination and thus can operate in the vacuum of space as well.
So what are scramjets anyways?
Supersonic combustion ramjets (or scramjets) are a class of air-breathing engines which take in air through an inlet, slows the air-stream down in a diffuser but keeps it supersonic, injects fuel into the stream, burning the propellant mixture, and expanding it through a nozzle to produce thrust (i.e. make the vehicle move). This is necessary when a vehicle is travelling really, really, fast because of the temperatures and pressures involved with high speed (supersonic) flight. A Mach 5 scramjet cruising at around 35,000 feet could make the trip from New York City to Los Angeles in a bit less than an hour!
Currently, scramjets are at best experimental test vehicles; several have been test flown including NASA’s X-43 which briefly flew at Mach 12 in 2004 and DARPA’s X-51 Waverider, which flew at Mach 5.1 just last year (2013). Applications in scramjet technology include transportation to outer space. Well that’s my primary interest and fascination with them anyways. In theory, coupling a scramjet engine with a rocket system could tremendously reduce the cost of transportation to space in comparison with conventional re-usable rocket systems. I did say in theory. These concepts have yet to be demonstrated practically.
What exactly am I doing?
As a researcher, I dig up and study the work others in the past have done to understand the physics of the JICF problem, as well as attempt to come up with new ideas efficient means of fuel jet penetration and mixing. Currently, while I’m finding, reading and studying previous works, I’m also involved with a team of staff members on constructing a small supersonic wind tunnel to study the behavior of supersonic jets in cross flows. Eventually, I hope to understand the physics of the JICF problem well enough to produce a new, unique solution to creating a more efficient means of fuel delivery mechanism in scramjet engines. And then I’ll graduate! 🙂