Abstract
Scientists and engineers continue to develop new and improved diagnostic techniques to investigate fluid dynamics and combustion. This is driven by the desire to better understand flow phenomena, provide information for computational validation, and develop more accurate computational models. There are several characteristics that would be representative of the perfect diagnostic technique, but there are five that are relevant to our current discussion:
Nonintrusive: Techniques in which the measurement apparatus is located outside the flow and does not significantly perturb the flow. This eliminates concerns that the researcher may have that the probe disturbs the flow field which is being measured.
Multipoint: Techniques capable of capturing multiple points (i.e. in a plane for instance) simultaneously allow large regions in the flow field to be interrogated. Not only is this cost-effective for expensive large wind tunnel tests, but also it is desirable in order to characterize structures in a flow field.
Scalable: Many times diagnostic techniques are developed that perform well in the laboratory environment but cannot be scaled up to operate in large facilities. For measurement of properties such as velocity, however, it is desirable to utilize the diagnostic in large-scale facilities so that nondimensional parameters can be better matched between the model and prototype.
Instantaneous: As one investigates flow phenomena of research interest, generally there is a desire to resolve the instantaneous property fluctuations in time. This is due to the fact that average measurements sometimes mask the fluctuating flow phenomena, or desired turbulence quantities are defined by multiple velocity component fluctuations.
Multi-property: For investigations into combustion and compressible fluid dynamics there is a desire to measure more than one thermodynamic property simultaneously. Other than the velocity field, it is often important to measure the temperature, density, species concentrations and pressure simultaneously in order to better understand the physics of the observed flow phenomena and compare the results with computational models.
Over the last decade there has been an interest in developing laser diagnostic techniques that use molecular/atomic absorption filters to modify the scattered spectrum of light in order to measure properties in a flow field. In molecular/atomic filter diagnostics, the filter - which contains within a glass cell selected vapour-phase molecules or atoms (e.g., I2 or Hg) - is placed in front of the detector to modify the frequency spectrum of radiation scattered by flow field constituents (i.e., molecules/atoms and/or particles). Depending on the application, molecular/atomic filter based techniques have been used to improve flow visualizations, measure the velocity field, or measure other thermodynamic properties such as temperature, pressure and density. The articles contained in this special issue cover a broad range of topics associated with the development of these techniques such as: the accuracy of the technique and mitigation of sources of uncertainty, application of the technique to complex flows and large facilities, extension of the technique to measure multiple velocity components and turbulence quantities, and measurement of thermodynamic properties utilizing molecular Rayleigh scattering. The overall goal of these articles is to help the reader understand the current state-of-knowledge of these techniques and aid those who are considering molecular/atomic filtered based diagnostics for their application.
The hard work and dedication of the authors and reviewers who made this special issue possible is greatly appreciated. Thanks also go to the editors and staff of Institute of Physics Publishing for supporting this initiative.