Gas Dynamics and Turbulence Laboratory: Research

For a list of GDTL publications please visit the principal investigators' Google Scholar profiles:

Mo Samimy's Google Scholar profile

Nathan Webb's Google Scholar profile

The research at The Ohio State University’s Gas Dynamics and Turbulence Laboratory (GDTL) is based on understanding flow physics and control of high-speed and high Reynolds number flows of interest in propulsion and aerodynamic applications. A primary focus of the GDTL is in free shear layers, which are present in many flows of interest in applications. This class of flows, which develops away from surfaces that would impose a no-slip boundary condition, is ubiquitous in practical applications. They include, for example, jets, cavity flows, wakes behind vehicles, and separated flows over solid surfaces. The existence of large-scale structures in turbulent flows in general, and free shear flows in particular, have been known for a long time. This came to focus with two seminal discoveries in free shear flows, which occurred in the 1960s and 70s. The first discovery was the finding that free shear flows, which have vorticity distribution that contains a maximum (or a velocity distribution with an inflection point), are unstable to small perturbations over a wide range of frequencies. This instability is called the Kelvin-Helmholtz instability or the inviscid instability at sufficiently high Reynolds numbers. The second discovery was the existence of coherent large-scale structures in free shear layers, even in very high Reynolds number flows.

These two initial findings followed by additional findings found that the dynamics of large-scale structures dominate important processes, such as entrainment and mixing, momentum transport, and noise generation. These findings motivated extensive research activities in the 1970s and 80s with the purpose of controlling these flows. Tremendous progress in the use of instability based active flow control was made in the 1970s and 80s. The experimental research focused almost exclusively on low-speed and low Reynolds number flows (e.g., ReD < 50,000 in jets). As the speed and the Reynolds number of the flow increases, so does the background noise and instability frequencies. Actuators must therefore provide excitation signals of much higher amplitude and frequency (two opposing requirements), thus imposing a very significant demand on mechanical and acoustic actuators. As a result, there was practically no experimental work in the active control of high-speed and high Reynolds number jets, with only a few exceptions.

We have developed a class of plasma actuators, called Localized Arc Filament Plasma Actuators (LAFPAs), that can provide local thermal perturbations with high amplitude and high frequency for high-speed and high Reynolds number flow control. These actuators’ frequency, phase, and duty cycle are controlled independently, allowing several of these actuators to be used to excite not only the free shear-layer instability, but also, for example, various azimuthal modes in axisymmetric free shear layers. We have successfully used these actuators in several high-speed and high Reynolds number flows, such as in jets, twin jets, shock/boundary-layer interactions, and sub- and supersonic cavity flows. Recently, we have also used different types of plasma actuators (NS-DBD), which still produce localized thermal perturbations, but are more suitable for spatially distributed actuation, to control flow over airfoils in both static and dynamic stall conditions. These actuators not only provide means to control various flows over a large range of Reynolds numbers and Mach numbers, they also provide a precise phase/time reference, which provides exceptional help in exploring and better understanding of flow physics. There are downloadable publications on all of these topics in our publications page.

In addition to the actuators developed for flow control, GDTL possesses significant diagnostic capabilities, and flow facilities (ARC facilities webpage). The current active projects include:

• Control of rectangular, supersonic twin jets for noise and pressure fluctuation mitigation in next-generation tactical aircraft
• Control of separated flows over airfoil in both static and dynamic settings in both fixed wing and rotorcraft applications